Illustrations
Figure Page
Flow-sheet
of laboratory treatment of field-samples
Figure 1(A): Spot-map as submitted by the field-geologist
Figure 1(B): Corresponding palaeontological spot-map, using symbols as defined below
Figure 1(C): Stratigraphic column derived from field geology and washed residue studies
Figure 2(A): Unmodified form of distribution chart
Figure 2(B): Species-grouped according to their zonal ranges on the original chart (Figure 2(A))
Figure 2(C): Distribution chart representing the data of Figure 2(A) modified to emphasize the faunae assemblage of each zone
Figure 3: Illustrating the integration of local stratigraphic columns
Figure 4(A): Correlation and classification of sediments in the Naparima Basin of Trinidad
Figure 4(B): Correlation and classification of sediments in the Naparima Basin of Trinidad
Table 1: Ranges of some planktonic foraminifera in the Naparima Basin
Figure 5(A): Initial interpretation based on reconnaissance study of outcrops and fossils
Figure 5(B): Revised interpretation incorporating zonation based on planktonic foraminifera
Figure 5(C): Interpretation based on extended field studies, well evidence, and detailed micropalaeontology
Figure 6: Cyclical repetition of facies in the Miocene of Trinidad
Figure 7: Facies-pattern over a buried ridge. (Based on the Talara formation of Peru.)
Figure 8: The depositional distory of the Mal Paso formation in N. W. Peru
Range chart of genera of planktonic foraminifera
Figure 9(A): Frequency ratios of selected ecologically restricted species
Figure 9(B): Hypothetical dip-section showing the levels at which different faunules are dominant
Figure 10: Species-concept diagrams
I. PREAMBLE
The science of palaeontology is largely academic in its
scope. Though fascinating to the layman as much as to the expert scientist, the
reconstruction of ancient creatures and their environment is seldom applicable
to material problems of human welfare. The location of aquifers, of
structurally sound or unsound bases for large structures, of beds of coal, and
similar problems of structural geology may be simplified by attention to the
fossils present in neighbouring rock exposures, but usually the solution can be
found without their aid.
In sharp contrast to the general case, palaeontology is
applied extensively and indispensably in solving the structural problems
associated with the discovery of petroleum. The need arose for a method of
subdividing great thicknesses of lithologically uniform beds and of
distinguishing between similar sediments known to lie both above and below
certain oil sands. Palaeontology has filled this need, since the vertical
ranges of fossils are limited and a given assemblage of forms can only occur
within a fixed zone. A drawback to the practical use of most fossils lies in
their limited availability: surface mapping is commonly necessary in areas of
thick alluvial cover or deeply weathered sediments, and subsurface samples
consist of the bit-cuttings and occasional cores, only a few inches in diameter,
collected in the course of drilling a well. Under these conditions the finding
of normal-sized molluscs, echinoids and other familiar megafossils becomes a
matter of chance. Recourse must therefore be made to microscopic fossils so
small that a representative suite may reasonably be expected in a few ounces of
rock from a given formation, whether the sample be from an outcrop, a test-pit,
an auger hole or a deep well.
Following up the promising results of a few pioneer students[1],
several oil companies in the United States began to experiment with
micropalaeontological correlation in the second decade of this century. The
value of the results proved so high that by now the absence of a
micropalaeontological laboratory within the geological department of an oil
company is unusual. A voluminous literature now exists, devoted exclusively to
the classification and
distribution of fossil Foraminifera, ostracods, conodonts, and minute members
of other phyla.
The expanding role of applied micropalaeontology may be followed
in a series of presidential addresses[2]
to the American geological institutions, since every few years the president
has chosen to review the current state of petroleum geology and ancillary
sciences. At first the new tool was regarded with hope tempered with misgiving.
The similarity of many Recent and Tertiary microfaunas was held by some to
nullify the value of microfossils in zonation, but experience proved these
pessimists wrong. Micropalaeontology thus became an accepted branch of
petroleum geology, ranking equally with field geology, geophysical prospecting,
and electric well logging in its contributions to the solution of structural
and stratigraphical problems encountered in the search for oil.
For many years zonation and correlation were the only goals
sought by practising micropalaeontologists, and their work was considered
adequate if they could recognize the “Marginulina zone”, the “Heterostegina
zone” and similar units. A more recent development has been the application of
micropalaeontology to regional palaeogeography, a line of research which
naturally followed the trend of search for oil in stratigraphical traps. The
great East Texas field[3],
in particular, showed that faulted and anticlinal structures were not necessary
for major accumulations of oil. The isolation of porous beds could be due to
depositional factors such as differential current-sorting, offshore gradation
of sands into clays, reef developments or unconformable capping of porous by
non-porous beds. Areas in which these conditions may be expected at depth can
be deduced only from palaeogeographical studies which reveal the ancient
shorelines and axes of intermittent uplift, the trends along which most
interfingering of different types of sediment may be expected below the surface.
The necessity for palaeogeographical research in the
location of new petroliferous territory has been mentioned, and the application
of micropalaeontology suggested as a more accurate control than the
lithological basis of most existing palaeogeographical studies, in the more
recent surveys of the status of petroleum geology. Microfaunal assemblages are
closely controlled by ecological factors. Slight changes in temperature, depth,
salinity, and bottom conditions may cause a complete change of a marine
microfauna, though insufficient to alter the megascopic aspect of the sea-floor
sediments. Published data on the ecological distribution of Recent
microorganisms are limited, though growing rapidly, but are already adequate
for tolerably accurate reconstruction of the life-environment of most
microfossil assemblages. The typical features are known of the living
assemblages characteristic of fresh-water, brackish, littoral, neritic, and
abyssal facies in different regions of the earth and by a process of
extrapolation the ecological significance of extinct forms may be deduced.
Published literature on the application of micropalaeontology to
palaeogeography is still in its infancy, though significant reports exist in
the confidential files of the major oil companies.
The purpose of the present paper is to discuss the methods
by which the results of micropalaeontological studies may be used to the best
effect in complementing petroleum geology as a whole. Laboratory technique and
micropalaeontological zonation are treated more briefly than the application of
micropalaeontology to palaeogeography, since the two former topics have already
been treated extensively in the literature.
To maintain continuity of the text all references are given
separately in an annotated bibliography at the end of the paper.
II. LABORATORY TECHNIQUES
The four main steps in accumulating micropalaeontological
data are as follows:
(i) collecting of rock samples
(ii) processing of rock samples
(iii) examination and identification of the microfaunas encountered
(iv) classification and interpretation of the results obtained
As has been stated, the main theme of this paper is the
final item, but inasmuch as valid interpretation requires reliable groundwork
it is necessary to include a brief discussion of the manipulative techniques in
relation to the final quality of the interpretive results.[4]
(i) Collecting of rock samples
In general, surface samples are collected by the field
geologists and subsurface samples by the well geologists, but the
palaeontologist should make himself familiar with the techniques in force and
should be prepared to suggest modifications which may improve his own final
results.
(a) Surface samples[5]
Surface samples are collected from natural exposures such as
sea-cliffs and stream banks, from artificial exposures such as road-beds and
foundations for new buildings, from test-pits and from auger-holes. Samples of
these different types are variably affected by weathering and surface
contamination, and the palaeontologist should ascertain that the samples
submitted to him suffer as little as possible in these respects.
Optimum size of samples is a matter of opinion, but in
general they should be as large as is reasonably possible. The writer favours
between one and two pounds from each locality. Exceptionally, and only in areas
where the zonal succession of microfaunas is so well known as to be routinised
on a semi-statistical basis, much smaller samples of uniform bulk are used.
The spacing of collecting localities is a matter of economic
significance. Over-wide spacing may fail to reveal an important zone or zonule,
whereas over-close spacing may lead to the same interpretation as sampling on a
broader grid, thus wasting time, money and effort. The palaeontologist should be
alert to detect fault in either of these respects, basing his recommendations
on the complexity of structure and facies-variation apparent from preliminary
and progressive studies of an area.
As soon as the broad outlines of structure and stratigraphy
have become apparent the palaeontologist should choose control sections for the
collection of very closely spaced samples on which to base his faunal
distribution charts. As far as possible these control sections should be
aligned on simple structures and should give over-lapping coverage of the whole
stratigraphical column locally exposed, including lateral variants of the
different formations.
(b) Subsurface samples[6]
Subsurface samples are obtained from exploitation and
exploration wells. Drill samples from seismograph shot-holes are a special
case. The most satisfactory samples for palaeontological research are cores
because they are free from contamination and give an accurate record of the
sediments at depth. However, extensive coring is costly and for this reason the
bit-cuttings are commonly used for routine micropalaeontological studies of
well sections. The main drawback to bit-cuttings is that as a rotary well is
drilled steadily deeper there is inevitably much mixing of freshly drilled rock
chips with material caving from higher up the hole or lagging in the
mud-stream. There is similar, but far less severe, contamination of cuttings in
cable-tool (percussion) wells. The resulting heterogeneity introduces an
element of doubt into any conclusions based on the micropalaeontology of
cuttings.
For simple correlation of neighbouring wells, when once a
standard zonation has been established by coring or extrapolation from surface
samples, cuttings are entirely satisfactory for recognizing the tops of the successive
zones penetrated by a well. In routine work of this type the spacing routine
varies, but one sample from each twenty to thirty feet of hole is adequate for
most normal purposes. Usually within a single oil-pool there is a constant
relationship between changes in lithology, microfauna and electric parameters.
The microfaunas are useful in checking the normality or abnormality of a well
section while drilling is actually in progress, but for final precise
correlation the electric (Schlumberger) log is often more satisfactory.
The palaeontologist concerned should pay special attention
to exploratory (‘wildcat’) wells drilled outside the limits of proven structure
and stratigraphical sequence. A coring programme should be prepared, designed
in the first instance to demonstrate the anticipated zonal succession but
subject to adjustment as the well is drilled. In the early exploration of an
area strategically taken cores are invaluable and the full exploitation of
numerous pools has been retarded by failure to recognize this fact.
Control sections are as important in the subsurface as at
the surface, especially in the deeper beds which do not crop out in the
vicinity. Adequate sample coverage, preferably by continuous coring, should be
organized in favorably placed wells.
(ii) Processing of samples
For microscopical study it is necessary to reduce the rock
samples to fine residues or, in the case of limestones and hard sandstones, to
prepare thin or polished sections. The preparation of rock sections is a standard
laboratory routine which needs no further comment here but some remarks are
made below on the preparation of washed residues.
The preparation of fine residues from the softer
sediments—mostly clays, shales, silts and unconsolidated sands—requires the two
stages of disintegration and sieving (or decantation). Disintegration may be
accomplished by mere soaking in water, as with the colloidal clays, or it may
call for lengthy manual processes in the cases of indurated or non-porous
rocks. Faintly argillaceous samples react gradually to plain water or a weak
electrolyte such as sodium carbonate solution. Porous but non-argillaceous
samples may be disrupted by alternate freezing and thawing after soaking in
water, or by the crystallization of sodium thiosulphate after soaking in a
strong solution of this salt. The disintegration of non-porous,
non-argillaceous rocks cannot always be achieved satisfactorily for the purpose
of extracting microfossils, and recourse to sectioning may be required.
Exceptionally, in a brittle rock in which they are not too tightly bound to the
matrix, the microfossils may be freed by careful crushing with pestle and
mortar, by using light vertical blows with avoidance of rotary grinding.
Various mechanical aids to disintegration are available, such as ball-mills,
pressure bombs and supersonic vibrators[7].
After disintegration of a sample, its microfossils are
concentrated from the raw residue by decantation or by washing through sieves
with a stream of water. The objective is to remove both the finer colloidal and
silty matter and the coarser rock particles. The grades of sieves used vary
with the type of fauna under study but most foraminifera and ostracods pass a
30-mesh sieve and are retained by a 100-mesh sieve. The action of a jet of
water may be destructive of fragile faunas and in such cases the slower process
of decantation is recommended. An alternative is to place the raw residue in a
bag of suitably fine cloth and tie it over the orifice of a slowly running
water faucet. The wet residue obtained by one or other of these processes is
spread on a dish and dried by gentle heating.
A branch of micropalaeontology of rising importance is the
study of fossil plant spores. The specialized technique used in their
extraction utilizes the fact that these minute fossils are remarkably inert and
remain unaffected by chemical processes which completely digest the inorganic
matter in which they are embedded. Hydrofluoric acid is the reagent most used
in the preparation of spore concentrates.
Siliceous microfossils may be extracted from calcareous
matrices by treatment with dilute acid. Best results are obtained by the slow
application of very weak acid, since violent effervescence is destructive of
such delicate organisms as radiolaria.
(iii) Examination and identification of microfaunas[8]
The most important phase of micropalaeontological technique
is the scrutiny and analysis of the residues and sections prepared from the
rock samples received at the laboratory. Points to record are: the type of
microfaunal assemblage; the presence of microfossils of zonal value; the
micro-lithology of the sample. A flexible system of filing and recording
samples and data must be established and its efficiency will have a marked
effect on the final interpretive results obtained.
As received from the processing department, washed residues
consist of finely divided rock matter which usually contains microscopic
fossils. The micropalaeontologist should examine both the rock matter and the
fossil content but commercial practice varies as to the details of presentation
of samples. In some cases the palaeontologist studies the washed residues and
either merely makes written records or himself picks out a representative
microfossil assemblage, placing the pickings in suitable slides. In other
laboratories junior assistants pick the fauna and pass their slides on to the
palaeontologist along with glass vials containing cuts of the residues. In some
well routinised laboratories only the slides picked by the junior assistants
are examined. When practicable the middle method is recommended, since it frees
the micropalaeontologist from much purely manipulative work and gives him
correspondingly more time for interpretive research. The tedium of picking
microfaunas out of washed residues may be reduced, under suitable conditions of
preservation, by a process of flotation in some medium such as carbon
tetrachloride.
The closeness of scrutiny given to individual prepared
residues depends on the problem in hand and the phase of development reached in
the study of the area concerned. In the case of a well drilled to exploit a
structure already intensively drilled and studied, the palaeontologist does
little more than check the expected appearance of changes in microfauna and
microlithology, but in the initial exploration of an area greater detail is
required. Until analytical study has established which microfossils are most
significant in correlation, it is requisite to make a detailed faunal list for
each sample and to note any lithological features which may assist in defining
stratigraphical subdivisions. After the establishment of faunal zones and
zonules, the first objective of exploratory studies, the same records will be
used in palaeogeographical research and for this reason notes should be made on
the grade and degree of sorting of sands, presence of two-phase sediments,
occurrence of glauconite, and all similar minor clues to depositional history.
The preparation of faunal lists for all samples taken in
exploratory surveys has been recommended above, but the question might be
raised of correct identification and nomenclature of microfossils in a
previously unstudied area. The difficulty is dealt with simply, by grouping the
organisms encountered under appropriate generic names and referring to their
different species by numbers. This local classification is controlled by a
collection of type-slides housing chosen specimens of all the species, each
labelled with genus-name, species-number and sample-number. Genus names[9]
are best used in the simplest sense possible without leading to an unwieldy
number of species referred to any one of them: as instances it may be adequate
to classify the ostracods encountered simply as Ostracod 1, Ostracod 2, et
seq. without referring to actual genera, or
to include several verneuilinid genera of foraminifera under the working name Gaudryina. This simplification is considered to be advisable
for its expediency, its reduction of the chance of introducing synonyms, and
its reduction of personal factors in a laboratory employing several
palaeontologists. If material from several disconnected areas is under study it
may prove convenient to prepare separate type-collections, in which case prefix
letters are used to avoid confusion between species from different areas. The
identification of microfossils in a correct, academic sense is of no benefit
during the exploratory phase of micropalaeontological studies. It does become
important in a later phase of research when regional correlation and palaeo-ecology
are under study, and at this stage a cross-index may be prepared showing that Marginulina
A-4 and Vaginulina C-6 are both actually Dentalina nonexistens Fulano, and so forth.
The value of the type-collection is increased by maintaining
a card-index to record the stratigraphical and geographical ranges of all the
species. Whenever a species is found higher or lower than it was previously
known, or when a new area is studied and yields species already in the
type-collection, corresponding entries are made in the card-index, which thus
becomes a useful supplement to formal distribution charts.
In recording microfaunal lists a note should be made of the
relative frequency of different species. There are two reasons for this. One is
that certain assemblages sometimes persist through a great thickness of uniform
strata with no marked change in their constituent species but a variation in
the particular species dominant at different levels. In such cases zonation is
based on faunal statistics rather than on the actual ranges of the different
species encountered. The second reason for recording frequency is that
palaeo-ecological deductions are based on the species dominant in microfossil
assemblages much more than on the scarce, sporadically-appearing forms. For instance
a few Globigerinas are a normal
constituent of any marine microfauna, not excluding beach sands and lagoonal
deposits, but a flood of Globigerinas is strongly suggestive of a rather deep open sea environment. The
symbols —(flood), • (abundant), O(common) and X (scarce) are used in many
laboratories and are more distinctive than the initials F, A, C, and S
sometimes used.
(iv) Filing of records and samples
A major commercial laboratory may receive samples at a rate
of several hundred daily. These must be processed, examined, recorded, and
stored in an orderly manner or confusion will result. Use of a continuous
series of ‘laboratory numbers’ for the identification of samples (in the order
of receipt) is recommended as requiring a minimum amount of clerical work and
hence giving minimum scope for errors in copying sample data. The flow-sheet
below shows the essential steps involved in handling the samples for which
washed residues are to be prepared: with slight modification it is applicable
for special samples requiring thin-section preparation, heavy residue study,
chemical analysis, megafossil extraction, and so forth.
Flow-sheet of laboratory treatment of field-samples
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Received at
Laboratory
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Field Sample
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Tag with field number, locality details, etc.
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Preliminary
Recording
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Field Sample, Placed in a metal bowl and
examined
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Tag. Lab. Number entered, brief lithological
description written on back. (Code letters may be used to indicate process
needed—WR, washed residue; TS, thin-section; M, megafossils, etc.
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Control-book. Lists lab. nos. in numerical
order with corresponding sample data, a card-index is maintained, based on
control-book, to give cross-reference between field and lab. numbers.
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 to processing room
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Actual Processing
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Field sample (WR) soaked in water
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  Tag clipped to
side of bowl
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Half sample may be stored for future
reference, filed in order of lab. numbers if this routine is followed.
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Washed residue prepared from field sample
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Tag used to identify residue while drying
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Original tag to files (check on labelling
errors)
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Record Stage of
Recording
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Dried residue placed in envelope or other
container marked with lab. number
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Sample card prepared showing field and lab.
Numbers and lithological description as on the tag, with blank spaces for
further data
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Microscopic Study
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Residue examined by palaeontologist. Slide
picked if required
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Sample card used for entry of the microscopic
data
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Sample cards to files
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        Residues and
slides to storage cabinets
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The flow-sheet is recommended for a busy laboratory handling
many samples from numerous sources. It may be unnecessarily complex for a small
laboratory. Furthermore, this system is primarily designed for use with
extensive field surveys, to avoid the tedium and fruitful source of error
inherent in repeated copying of the field geologists’ sample numbers and
topographical data. In a laboratory primarily concerned with well samples, a
ready-made basis for orderly filing and recording exists in the fact that wells
are usually numbered consecutively and samples from each well are received in
order of regularly increasing depth. Nevertheless, the use of laboratory
numbers for well samples has some advantages such as a greater ease of
labelling slides and small vials. Printed tag-labels and sample cards and
uniform sizes of residue containers and microfossil slides are recommended for
ease of operation. Description of gross lithology by the palaeontologist
concerned is not strictly necessary but is recommended to improve liaison
between field and laboratory: washed residues may give a false impression of
their source, especially if appreciable amounts of colloidal clay were
originally present.
Instead of cards for recording the details of well samples a
strip-log may be used. The term is almost self-explanatory and applies to a
strip of paper marked with a central half-inch column. Conventional symbols are
used for plotting the microlithology of the samples in the column and written
annotations on fauna and lithology are entered on the wide margins. Strip-logs
on a scale of 1:5000 or 1:2500 are much more convenient for the comparison of
well sections than sheafs of written notes.
ROUTINE SAMPLE REPORT
Field Geologist: H. R. Gotham Area:
upper Hamster River
Field No. Lab.
No.
HRG-81 Microlithology: grey
glauconitic micaceous shale 19157
Microfauna: Rotalia
B-3 (0), Buliminidae (X),
ostracods (X), echinoid fragments (X)
Determination:
Rosetta
formation, Rotalia B-3 zone
HRG-82 19158
etc.,
etc.
o r
WELL No. DP-18 SAMPLE
RANGE:
Cores:
------
Cuttings:
1080-1460’
1080-1170’ Microlithology: bluish
calcareous clay
Microfauna: very
rich and diversified;
Gastropod 18 (O)
Determination: Concourse
formation, probably lower part
1170-1320’ Microlithology: coarse
heterogeneous sand (plus cavings)
Microfauna: weak
as above, probably all cavings
Determination: Vanguard
sand
1320-1460’
etc.,
etc.
The routine outlined in the foregoing pages for treating,
studying and recording microfaunal material embodies the essential requirements
for a full interpretive analysis of the material. Every laboratory has its own
problems of climate, availability of equipment and man power, dominant rock
type in the samples received, and so on. The flow-sheet of basic routine varies
correspondingly from laboratory to laboratory as modifications are introduced
to allow for these local factors. No attempt is made here to go into details of
these useful modifications, but it is stressed that the test of efficient
routine lies in the three points:
Prompt availability of micropalaeontological analyses of all
samples received at the laboratory.
Ready availability of old sample records (largely governed by an
efficient filing system).
Completeness of records, so that no further reference to stored
samples and slides is normally necessary when queries are received.
(v) Summarization of sample records
Up to this point the routine of preparing and recording
individual samples achieved by issuing two types of sample reports, Routine and
Non-routine.
A routine typescript report is made for each batch of
samples received from a given source—field geologist or well—the contents being
copied directly from sample cards. Copies are issued to all interested parties
and one is kept in the laboratory files. Typical forms of layout are shown
below.
Non-routine reports include progress and final reports on
research projects, notes on the implications of new discoveries, desirable
revisions of stratigraphical columns, and so forth. Also falling in this category
are the summary reports made on completion of wells and field surveys;
according to circumstances these may contain much or little interpretive
reasoning.
III. INTERPRETIVE TECHNIQUES
In the application of micropalaeontology to geological
problems, the validity and completeness of interpretive analysis depends on
making the most efficient use of the
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Requisite
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Optional but
recommended
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Washed residue (or rock section) of each sample
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A portion of each sample, for future comparisons and
checks
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Sample card showing source and analytical data for each
sample
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Microfossil slide(s) for each sample
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Type-collection(s) of microfossil species
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File of routine and non-routine reports as under (v) above
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Cross-index between source of samples and storage system
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Card-index showing ranges of all microfossil species
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Figure 1(A): Spot-map as submitted by the field-geologist
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Figure 1(B): Corresponding palaeontological spot-map, using symbols as defined below
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Figure 1(C): Stratigraphic column derived from field geology and washed
residue studies
samples on which studies are based. For this reason some
space has been devoted above to outlining a routine appropriate for a
laboratory engaged in micropalaeontological research as well as workaday
correlation. Modification of this routine may be desirable for special local
conditions, but the final effect should be completeness and ready availability
of the following items:
These are the basic tools of research; the products of
interpretive research—distribution charts, spot-maps, palaeogeographical maps
and textual matter—will be filed alongside them.
Any scientific research is a controlled advance from the
known to the unknown, an elucidation of successively farther reaching
relationships. The normal pattern of progress in micropalaeontological research
is:
(i) establishment of local biostratigraphical columns for each
of several small areas,
(ii) integration of the initial results to give a
biostratigraphical column of regional application,
(iii) dating the units of the column in terms of the standard
geological time scale,
(iv) palaeo-ecological studies leading to palaeogeographical
reconstruction.
Analytical methods applied in the separate phases of
research are discussed below.
(i) The establishment of local biostratigraphical
columns
(a) Studies based on surface samples
On completion of a field survey the palaeontologist possesses a set of analytical records of the samples sent in for study. He has probably noticed that they fall within a limited number of groups with distinguishing characteristics but he does not know the significance of this variation. The clue to the relationship of faunal and lithological changes lies in the preparation of a spot-map.
The draughting of a spot-map begins with a blank topographical map submitted by the field geologist and marked with all sample localities and major structural features. The palaeontologist marks his data for the individual samples against their positions as spotted on the map. A system of symbols or code letters is used to distinguish between the different faunal groups already noted, with written annotations for minor variations of possible significance. More often than not, the resulting picture shows a pattern of relationship between microfaunal groups and structural position of the samples. From this relationship the local biostratigraphical column may be deduced: this process is illustrated on Figure 1 by a simple hypothetical spot-map and the basic zonation derived from it.
It is now possible to arrange the samples in their correct
stratigraphical order and to prepare a distribution-chart of the vertical ranges of microfossil species. The
function of this chart is to reveal greater detail of the faunal
characteristics of the zones already
Figure 2(A): Unmodified form of distribution chart
recognized and also to establish further zonal subdivisions
for use in detailed correlation with neighbouring areas. The significance and
validity of a distribution-chart are directly proportional to the closeness of
spacing of the samples entered on it, for which reason the spot-map should be
examined critically before deciding which lines of samples to use. The possible
desirability of first collecting some additional samples to fill in gaps on the
map should be considered. It is preferable to avoid great lateral spread of the
samples plotted, in case of facies change or unsuspected local structures: as a
safeguard, distribution charts may be made for several parallel lines of samples
and may be unified later if they show no discrepancies.
The construction of a distribution-chart is purely
mechanical. Squared paper is used with two axes marked at right-angles. Along
one axis sample-numbers are written in stratigraphical order and along the
other the names of all microfossil species listed in the microfaunas of the
selected samples. On a large chart ease of plotting is increased by first
making out the species list in alphabetical order. The sample-cards are taken
in order and their microfaunal records entered in the appropriate column of the
chart,
ZONE OF LOWEST
RECORDED OCCURRENCE
|
1
|
Anomalina
1, 4, 5
Bolivina 1, 2
Bulimina 1, 2
Cibicides 1
Uvigerina 1
Vaginulina 1, 2
|
|
|
|
|
|
|
2
|
Bulimina
7
Uvigerina 2
|
Ammobolculites
2
Ammodiscus 3
Anomalina 6
Bathysiphon 2
Bulimina 6
Textularia 2
Trochammina 1
Verneuilina 1
|
|
|
|
|
|
3
|
|
Ammobolculites
1
Ammodiscus 2
Bathysiphon 1
Bulimina 5
Textularia 2
Vaginulina 3
|
Allomorphina
1
|
|
|
|
|
4
|
|
|
Bulimina
3
|
Bolivina
4
|
|
|
|
5
|
|
Ammodiscus
1
|
|
|
Bolivina
3
Cibicides 3
Siphogenerina 1, 2
|
|
|
6
|
Bulimina
4
|
|
|
|
Anomalina
2, 3
Cibicides 2
Siphogenerina 3, 4
Uvigerina 5
Virgulina 1
|
Uvigerina 3, 4
|
Figure 2(B): Species-grouped
according to their zonal ranges on the original chart (Figure 2(A))
using a frequency symbol, as recorded, opposite the name of
each species listed. The first draught of the chart is of the form shown on
Figure 2(A), a hypothetical example.
A chart of the type described contains all the information
required but can be used more readily if the species are arranged in a
stratigraphical order, thus emphasizing the assemblages characteristic of
successive layers. Before attempting this, the chart should be checked for
levels of marked faunal change (other than those derived from the spot-map). A
simple method is to put symbols or coloured marks against the highest and
lowest occurrences of all the species on the chart: some zonal significance may
then be attached to any level at which a concentration of such marks is seen. After recognition of the zonal divisions a second simple process gives the species list in order of stratigraphical significance: a square grid is prepared as depicted on Figure 2(B) and each species is marked in the square appropriate to its recorded range. A revised distribution chart may now be plotted with the species listed in order from top to bottom of successive columns of the grid just described. For convenient reference an alphabetical key may be provided on the opposite side of the chart, the final version thus having the form shown in Figure 2(C). It is usual to use the name of a characteristic fossil rather than a number designating each zone. Referring again to Figure 2(C), Zone 1 might be named the “Bolivina 1” zone. Zone 2 the “Bulimina 6” zone, and so
on.
(b) Studies based on subsurface samples
In principle the erection of a microfaunal zonation is the
same for a set of well samples as for surface samples, but in practice some
allowance must be made for the difference in source.
The fact that well samples are labelled with the depth from
which they were collected obviates the need for any equivalent of a spot-map. A
distribution chart may be prepared by plotting faunal records against a depth
scale, and it will give a zonation valid for a single well. However, the results
will be more acceptable if based on the joint evidence of several neighbouring
wells and, according to local conditions, discrepancies may or may not be
apparent when the faunal zonations of these separate wells are laid side by
side. If there should be signs of structural elimination or repetition of
section, unconformity, or lateral change of facies the probable cause should be
ascertained by use of electric logs, drillers’ logs, rate-of-penetration charts
and other records. A composite chart may then be compiled to show the entire
zonal scheme applicable to the subsurface beds of the area considered.
Due allowance must be made, in plotting subsurface
distribution charts, for the type of samples used. The ideal case is based on
continuous cores, since no question of contamination then arises, provided that
drilling mud was carefully removed before the samples were processed. More
often the bulk of the section is represented by bit-cuttings and in this case
contamination may give rise to serious error. In general, the abrupt appearance
of a species in fair abundance in the residues of the cuttings is a reliable
indication of the top of its range, but if its frequency dwindles downwards
this may signify passage below the base of its range or may represent the
actual distribution of the species. The significance of sporadic microfossils
in bit-cuttings varies with local conditions, most of all with the degree to
which the formations penetrated tend to cave into the hole. It may be noted
that if and as caving continues the volume of contaminatory rock matter
increases rapidly with each unit increase in hole diameter. Wells in Trinidad
frequently pass through beds of mudflow, a highly colloidal, hydrophilic clay
full of heterogeneous microfossils, and in such cases contamination is so
severe as almost to obliterate the autochthonous faunas in cuttings for several
hundred feet below the mudflow band: yet other cases could be cited in which
contamination due to caving is negligible. There is, in fact, no valid generalisation
covering the problem of contamination of well cuttings; it is a local problem
to be
Figure 2(C): Distribution chart
representing the data of Figure 2(A) modified to emphasize the faunae
assemblage of each zone
solved by accumulated experience qualified by information
gained from cores and extrapolations to surface sections.
(ii) Integration of local biostratigraphical
columns
Micropalaeontological studies in a region are first applied
in a detailed manner in some limited area chosen arbitrarily for its ease of
accessibility, discovery of oil nearby, excellence of surface exposures, or
some comparable reason. As geological exploration proceeds parallel studies are
made on related, but possibly disconnected, areas and it becomes desirable to erect
a master zonation which will indicate the units of regional extent, those which
are localised, and what relationship exists between synchronous local variants.
Superficially there might appear to be little difficulty in
compressing several detailed distribution charts into a single one which will
retain the major features common to all. Actually there exist major
difficulties and sources of error which need to be recognized and assessed. The
prime source of error is failure to distinguish between time-controlled and
facies-controlled changes in microfauna. A living community of micro-organisms
exists in a state of delicate ecological balance with its environment. Any
slight change in temperature, salinity, turbidity, turbulence, or other factor
included in the collective term facies must cause slight modification of the
fauna: a major ecological change may cause almost complete reconstitution of
the assemblage. In contrast to this intimate relationship between facies and
faunal constitution, time-controlled changes in microfaunas are slight in
relation to the fine subdivisions used in local zonation. Evolutionary trends
are evident in certain phyla but in application are better suited to the
identification of epochs or stages than of minor zones. Exceptionally the time
of blending of two previously isolated faunal provinces may be marked by an
abrupt appearance of foreign forms
[10]
without any general change in facies-type.
Since facies-variation affects microfaunas much more
intimately and more profoundly than any non-ecological progressive time effect,
it follows that all local changes of fossil microfaunas, in the vertical as
well as the lateral sense, are best treated as symptoms of ecological change
with no intrinsic time significance. They only acquire age-determinative value
when the facies-pattern of the sediments has been established and has been tied
in to a time-grid based on non-ecological factors. In order to illustrate the
term facies-pattern the flank of any
normal marine basin may be considered: at a given instant a sequence of
distinct but laterally intergradational facies must exist, representative of
deep offshore, shallow offshore, inshore, littoral, and brackish to terrestrial
milieux. As long as sea-floor depths remain unchanged the gradational
boundaries between facies-provinces remain fixed and the corresponding
sediments show consistent lateral change but little vertical change. More
usually, the flank of the basin is subsiding or emerging and there is a
corresponding lateral shift of facies provinces: over a period of time the
different facies-provinces overstep one another’s initial boundaries to extents
and in directions governed by the diastrophic and depositional history of the
basin. The resulting complex of interfingering and overlapping sediments
differing in lithology and fauna is the facies-pattern of the region. It is a
four-dimensional continuum in which the time dimension bears a determinable,
but not necessarily simple, relationship to the spatial configuration. The
smaller the area under study, the less error is made in assuming coincidence
between time planes and lithologically defined horizons: the greater the area,
the less desirable is this assumption.
The spatial relationships of different facies within a block
of sediments are more readily apparent than temporal factors, for the former
are indicated by gross physical features of rock type and bulk fauna whereas
the latter must be determined by reference to specialised fossil groups, not
necessarily conspicuous, or to mineral analyses. Consequently in first attempts
to compile a regional stratigraphical chart it is recommended that lithological
correlation be used in establishing a framework. A broad correspondence is to
be expected between major faunal variants and major lithological units, though
more variability is to be expected in the microfaunas because in life they were
sensitive to ecological changes too slight to affect the processes of
sedimentation. On its completion, the regional chart should be checked for homogeneity:
lateral and vertical gradations are to be expected but should show parallelism
between lithological and faunal trends. Of special importance are the mixed
facies gradation between two or more ‘pure’ facies: the original evidence may
indicate lateral equivalence of a richly fossiliferous clay in one sector with
a barren sand in another, but the correlation remains inferential unless
physical interfingering or gradation through weakly fossiliferous silts can be
demonstrated in the intervening area.
In order to establish the temporal setting of a regional
facies-pattern, attention must be paid to non-ecological variables in the
sediments. Of greatest importance from the micropalaeontological point of view
are minute remains of floating organisms (immune from sea-floor facies effects)
and of free-swimming organisms (which could avoid hostile environments). The
floating organisms which occur most frequently as microfossils are the
planktonic (pelagic) foraminifera and radiolaria: less frequent and rarely used
in commercial work are coccoliths and other flagellates, diatoms and
Calpeonellidae. Swimming organisms include ostracods and fish (as teeth, scales
and otoliths, possibly also as conodonts and scolecodonts), certain crinoids
(as dissociated ossicles) and possibly other obscure phyla. Pollen and spores
from terrestrial plants have been used successfully in establishing zonation of
continental beds and in inter-correlation of marine and continental beds. It
may be noted that planktonic forms such as the graptolites, cephalopods, and
stalkless crinoids are the basis of the most reliable long-range megafossil
correlations, whereas correlations based on benthonic megafossils may be in
error, reflecting only a similarity of facies.
(Complementary to palaeontological variables which are
independent of local facies and therefore suitable as time-indicators, the
variations of the mineral content of sediments reflect changes in the type of
source-rock supplying detritus. Such changes are induced by progressive erosion
or by igneous activity and metamorphism associated with orogeny. The usual
method of studying mineral variation is based on the ‘heavy minerals’ which do
not float in bromoform. Counts are made of the grains of different constituents
in the heavy residues and, after analysis of the results, consistent
fluctuations of mineral frequency provide datum-points on a time scale.
Rittenhouse recently introduced the concept of hydraulic value to account for the different lateral spread of
mineral grains derived from a single source but differing in size, shape and
specific gravity.
[11]
In the
past, failure to allow for this factor has been comparable to confusing
ecological and temporal factors in microfaunas, but recent work incorporating
the concept of hydraulic value has given valuable results. Although
Figure 3: Illustrating the
integration of local stratigraphic columns
(1) Correlation of Four Columns on the Basis of Thickness, Lithology arid Facies-Faunas of the Beds
(2) Correlation as Above with Superimposed Zones Based
on Planktonic Microfossils, Evolutionary Sequences, Heavy Minerals, and Other
Facies-Free Indices
(3) Data as Above Plotted Against a Scale of Time-Units Based on the Zones.
few micropalaeontologists have time to spare for the tedious
routine of quantitative heavy-residue analysis, they should be familiar with
the type of results obtainable, because in barren sands fringing marine
deposits it may be very helpful to extrapolate microfossil zones parallel to
the mineral zones.)
To determine the time dimension applicable to a chart of
regional zonation is tantamount to preparing a distribution chart of the
facies-free microfossils and using it as an overlay on the chart based on
benthonic facies faunas. The necessary data are available on the local
distribution charts discussed above under III-(i). In the writer’s experience
planktonic foraminifera, radiolaria, and ostracods are the most reliable
time-markers, in the order stated, for Tertiary and Cretaceous studies. In
older rocks planktonic foraminifera are absent and emphasis has been placed on
the ostracods. The time-grid based on floating and free-swimming forms may be
supplemented by the evidence of evolutionary trends in the benthonic genera.
[12]
The larger foraminifera (Orbitoididae, Discocyclinidae, Fusulinidae, etc.) show
regular trends of development in their embryonic chambers and general form and
in regional correlation possess a great value limited only by their restriction
to near-shore facies. Certain smaller foraminifera show evolutionary trends
proved empirically to be valid in age-determination over great distances. Still
other smaller foraminifera may show a provincialised evolutionary trend, such
as a tendency of rebuilds to develop aberrant evolute forms at certain levels.
By attention to the microfossil groups emphasized above a
sequence of time-significant subdivisions may be recognized in each area of the
original study. These new zones may be superimposed on the regional chart as
first draughted and they provide the time dimension inherent in the regional
facies-pattern. If they indicate parallelism between facies units and time
units stable deposition, with sedimentary infill compensated by general
subsidence, is indicated. If there is obliquity or transection of time-planes
relative to layers of uniform facies regression or transgression is indicated.
Absence of any time-significant zone must indicate a diastem or unconformity,
unless its absence draws attention to some previously unsuspected structural
anomaly.
Figure 3 illustrates the preceding remarks on the basis of a hypothetical example.
(iii) Dating the units of a regional
stratigraphical column in terms of the standard geological time scale
In the course of geological studies extending back for more
than a century, various phyla, genera, and species of fossils have been shown
to be restricted to short intervals within the geological column. In studies of
a new region special search is made for marker fossils of this type and, when
found, they provide the basis for dating the units of the local succession. In
general, the older the rocks, the more reliable are long-range correlations
based on index fossils. The principle of such correlation is sound but there
are sources of error to be known and avoided; viz.:
(a) The non-absolute aspect of the geological time
scale
Time, “like an ever-rolling stream”, is the axis against
which the history of the earth is measured. The standard geological time scale
is a form of calendar, now established on a quantitative basis as a result of
studies of radioactive minerals.
[13]
This time scale is an objective reality but is almost always used subjectively.
The more precise are attempts to delimit minor time subdivisions, the greater
is the disagreement among stratigraphers, simply because no man can be
omniprescient and each individual stresses the branch of knowledge in which he
specializes. The indices for subdivision of the geological time scale are
almost always fossils and the intention has been to make the major breaks in
the time scale correspond to major breaks in faunal trends. However, it does
not follow that an abrupt change of terrestrial life must coincide with a
change in the deep marine fauna, nor that such a change in one hemisphere
should be equally pronounced in the other. Consequently such questions as the
correct designation of correlatives of the Maestrichtian, Danian, and Montian
stages or the exact correlation of the North American and Russian Permian
stages remain unsettled, controversial issues.
A secondary means of correlation is the study of cycles of
sedimentation and diastrophism.
[14]
Certain clearly marked events such as the Laramide orogeny and the Jacksonian
transgression left their imprint in the sediments over vast regions of the
earth. Correlation on this basis must be applied with discretion as the basal
beds of an unconformable sequence may vary in age owing to slow encroachment
over a land mass. Errors may result (as in Peru and Ecuador) from attempting to
correlate unconformities in a mobile geosyncline with those of an adjacent
taphrogenic province.
[15]
Some authors support the theory of eustatic changes of sea level as the
underlying cause of major marine transgressions and regressions, but there are
few cases in which the physical evidence conforms completely with this
hypothesis.
[16]
(b) The unfortunate choice of some type stages
The concept of a stage as the smallest regionally
recognizable time-stratigraphical unit dates back to a time when very little
attention was paid to facies-control, of faunas. There was a strong tendency to
assume different ages for different faunas and as a result some of the stage
names now in use include overlapping units partly based on synchronous but environmentally
different faunas. A desideratum is an agreed series of multiple type units
designed to exemplify the variation of faunas in accepted stages under
different environmental and climatic conditions. The following quotations
illustrate modern views on the subjects of the preceding paragraphs.
Kleinpell 1938 “The
synchronization of stratal units as small as stages in areas as geographically
remote as Europe and California may well be considered extremely hazardous in
the light of present knowledge. Similarly synchronization between either of
these areas and the intermediate Caribbean area is also hardly to be considered
reliable within the limits of a stage or zone.”
[17]
Arkell 1933 “…few
of the geologists working in other countries at first adopted d’Orbigny’s
stages, for with their more detailed local knowledge they were unable to
recognize any such ten divisions which might have been ‘delineated by nature
with bold strokes across the whole earth’…”
[18]
Davies 1934 “While
Eras, Periods and Epochs can be recognized over most of the world (allowing
some elasticity in correlation for epochs) correlation of Ages (sic) over wide
areas is dangerous at present. To write of ‘Bartonian’ or ‘Burdigalian’ as if
they could be confidently identified in lands far away from their type locality
is inadvisable.“
[19]
Schenck 1935 “(in
Europe)…only in local areas is there an appreciable faunal break between the
Eocene and Oligocene. The species from the Upper Eocene Barton clay of England
are closely related to the lower Tongrian ones from Belgium, The ‘great faunal
break’ at the close of the Oligocene is a myth, both in Europe and America. The
difficulty, moreover, of separating the Cretaceous from the Tertiary is
classic. How to distinguish the Miocene from the Pliocene or how to divide the
Pliocene from the Pleistocene have never been stated to universal
satisfaction.”
[20]
Hedberg 1937 “Terms
of the standard time-scale (Oligocene, early Eocene, etc.) are convenient as a
means of expressing opinions on the approximate or relative geological age of
sediments. Their value lies particularly in the fact that they are in use
throughout the whole world. However, since there is much difference as to their
scope even in the region where most of them originated (Western Europe) it is
impossible that they should have any very exact significance in northern South
America.”
[21]
(c) Lingering errors
An erroneous age-determination, once published, is never
completely eradicated from the literature. Kugler gives a good example in
reference to the Palaeocene (midway) Soldado formation
[22]
exposed on Soldado Rock near Trinidad. In 1912 Maury described the fauna and
recognized its affinities to the Midway assemblages of the southeastern United
States. In 1928 Liddle, owing to his mistranslation of an intervening paper in
German by Kugler, confused Palaeocene and Upper Eocene beds on the islet. In
1929 Maury corrected Liddle’s error but nevertheless in 1934 Shimer quoted the
faulty passage of Liddle. In 1935 Maury corrected Shimer but in the same year
Schuchert drew on Liddle’s erroneous version for one of his famous treatises on
palaeogeography. Kugler’s paper of 1938, dedicated to the memory of the lately
deceased Carlotta J. Maury, reiterated her correct initial statement and
pointed out the series of erroneous references, but with the error already
appearing in well-known compilations there is little doubt that future authors
will perpetuate it.
Schuchert (1935)
[23]
accepted determinations of the Oceanic formation of Barbados as Miocene,
overlooking the more up-to-date demonstrations of its Middle Eocene to Middle
Oligocene age: likewise he treated the San Fernando formation of Trinidad as
Oligocene despite long-standing recognition of its Eocene age. These errors are
incorporated in his treatment of the diastrophic history of the Antillean
region and are likely to be disseminated by subsequent authors,
The moral to be drawn from such examples is that an
age-determination based on fossils should only be made or accepted after
checking the validity of age-assessment of the fauna used as a standard.
Circular reasoning is to be avoided—of the type: “fauna (a) from country A is
questionably Miocene: fauna (b) from country B is the same as fauna (a): faunas (c) and (d) from countries
C and D are almost identical with (b) so must be Miocene: fauna (a) is
typically Miocene by comparison with (b), (c) and (d)”. Spread over several
publications issued at leisurely intervals the argument is not readily
recognized as specious and examples of this type do exist.
[24]
The molluscan faunas of Panama have long been an accepted standard for the
younger Tertiary of the Caribbean region, but current studies are revealing
long-concealed errors in their age-assessments.
[25]
(d) Failure to distinguish between ecological and
temporal factor
[26]
Although not so sensitive as micro-organisms, the larger
marine invertebrates are limited as to habitat and, in part, are structurally
specialized for life in certain environments—robust or sessile in areas of
heavy wave action, discoidal in areas of rapid sedimentation, and so on. A
megafauna specialized for life in a certain habitat might persist with little
change for a considerable period: in the fossil state it would be erroneous to
consider all occurrences of such a fauna synchronous.
A case in point is the so-called Hannatoma fauna of mid-Tertiary molluscs in northern South
America.
[27]
In its first published description it was referred to the Middle Oligocene of
Peru and Ecuador.
[28]
Subsequent
discoveries of the same basic fauna in Colombia and Ecuador were at first
regarded as synchronous since they were found in thick otherwise almost barren,
brackish-water sediments and no reason was seen to doubt their synchroneity
with the southerly examples. Then, as local studies were incorporated into
regional compilations, an anomaly arose in that beds with the Hannatoma fauna fell in an interval which on grounds of marine
faunas and regional history must be regarded as Upper Eocene. In a symposium on
this anomaly the joint opinion of the contributors is that the principal
species of the Hannatoma fauna
were long-ranging forms specialized for life in a brackish-water habitat and
are not suitable fossils for long-range correlation.
A comparable case has arisen from the failure of some
authors to recognize that on account of uniform facies, the foraminiferal
faunas (mid-Tertiary) of the continental shelf of northern South America
include many persistent forms. A clearly defined zonation based on planktonic
species is applicable over this whole region, but several authors have
published erroneous correlations based on unreliable, long-ranging benthonic
species.
iv) Palaeo-ecological studies and
palaeogeographical reconstruction
The methods discussed up to this point have been almost
mechanical, largely capable of application by an intelligent person with little
geological knowledge, The recognition of age-significant fossils and their
application to dating a succession certainly do call for palaeontological
knowledge, both of the literature and of fossil assemblages. However, palaeontologists have been
split into the ‘palaeo-biologists’ and the ‘biostratigraphers’ and those of the
former group, more concerned with the morphology, phylogeny and classification
of fossils than with their status as symptoms of earth history, possess the
knowledge requisite for the type of correlation so far discussed.
[29]
To progress beyond correlation into the realm of
palaeogeography the ‘biostratigraphical-altitude must be adopted. Constant
attention must be given to the problem of how fossils may best be applied to
interpreting the earth’s history with its complex cycles of subsidence and
emergence, its changing climates and its shifting continental margins. The
problem passes from static analysis of a three-dimensional space continuum to
dynamic analysis of a four-dimensional space-time continuum, and coincident
with this development it becomes less easy to lay down set rules of procedure
in reaching the desired result. An analogy with modern detective fiction is
tempting: Father Brown, Dr. Thorndyke, Mr. Nero Wolfe, M. Poirot, and their
many fictional colleagues can step beyond the stolid police routine of
classifying clues, and the successful palaeogeographer must likewise develop
some degree of intuition in picking on the key to a complex problem. A less
fanciful indication is given by the term introduced by Cram—“controlled
imagination”.
[30]
The first clue to the palaeogeography of a region is given by charts of the type shown on Figure 3. From these can be seen certain trends of transgression, stability or regression. There is, however, a random factor in the data because of the arbitrary choice of the areas selected for detailed study. The section may, by chance, be representative for the region but equally it may chance to be atypical and to omit or overemphasize certain features. It is at least a starting point for interpretive studies.
The main lines of approach in applying micropalaeontology to
palaeogeography are:
(a) implications of lateral changes
(b) palaeo-ecological deductions
(c) tests of rationality
(d) preparation of palaeogeographical
maps
Discussion of these four topics follows.
(a) Implications of lateral changes
[31]
Stratigraphical correlations prepared for application to
structural problems are based on the recognition of identical or closely
similar sequences of rock types and fossil faunas at several suitably spaced
localities. The literature dealing with local problems, such as the structure
and stratigraphy of a single oilfield or pool, is far more voluminous than that
covering the depositional histories of regional tracts. In consequence, there
exists something of a cult for establishing ‘marker-beds’ and emphasizing their
continuity.
In palaeogeographical research the emphasis is placed
differently, since lateral discontinuities and gradations often provide the key
to understanding some phase of depositional history. A widespread and uniform
bed may on occasion indicate an event of regional significance, yet examples
will be cited below which show such an interpretation to be false and
misleading.
In the world as we know it today lateral change from one
environment to another is a normal effect of physiographic and climatic
variation. Davies (1934) recognizes some eighteen marine provinces and six
terrestrial provinces characterised by distinctive faunal associations. Each of
these major provinces could be broken into innumerable sub-provinces governed
by the accident of local ecological conditions. An obvious example is any coast
line where different types of rock have been exposed to marine erosion. The
harder rocks form cliffs and headlands with boulder beaches at their base; the
softer rocks are eroded into bays and inlets with sand or mud-flat beaches.
Islets, sand bars and spits, dunes, lagoons, coral reefs, mangrove swamps and
other similar features may be present. Where streams and rivers reach the sea
the coast is modified into estuaries or deltas. Viewed ecologically a varied
coast line such as this presents a series of intergradational facies units.
Focal points could be chosen where peculiarities of environment have induced
the co-existence of specialized organisms, such as robust sessile forms on an
exposed rocky shore or burrowers and mud-ingesters on mudflats. In between
these foci of pure facies type the lateral merging of different physical
extremes is matched by a gradation of one faunule into another.
At right angles to the coastline there is a gradation from
the landward facies to the deep oceanic environment. The main stages are:
Terrestrial
and fresh-water facies
Brackish facies
Eulittoral
(intertidal) facies
Sublittoral
facies
Deep
sea (archibenthic, abyssal) facies
The physical processes normally active in these different
environments vary from one to another, causing variation in sedimentary type.
The eulittoral, intertidal facies, for example, is characteristically very
heterogeneous in rock-type, whereas the deep sea facies is typically fine
textured and homogeneous over vast areas. Local sedimentary type is largely a
function of erosion and of transport of particles as governed by Stokes’ law,
so that physiography, marine currents and availability of mineral matter are
controlling factors, modified by such effects as electrolytic flocculation of
colloids at river mouths, bio-chemical precipitation and growth of organic
reefs. Marine life is affected directly by these physico-chemical factors and
also by variations of temperature, salinity, stagnancy, turbidity, aeration and
similar effects which are not reflected in the inorganic sediments. Most of these
ecological variants are direct functions of sea depth, which is therefore
commonly used as a reference datum in comparing different facies. Although each
main depth zone differs from the others in conspicuous aspects of sedimentary
type and faunal community, no sharp boundaries usually separate them: there are
intermediate transitional zones in which intermingling and gradation of
characters are observable.
The rocks which form the basis for micropalaeontological
research were formed under similar conditions to the sediments of Recent sea
floors. That some rocks were laid down in deep water, others in shallow water,
is reflected in their lithology and fossil content, and lateral gradation from
one fossil facies to another is normal, as in Recent marine sediments.
(b) Palaeo-ecological deductions
Palaeo-ecology is the study of fossil communities in
relation to their environment. It is the branch of palaeogeography in which
lateral and vertical changes among different types of animal and plant
assemblages are used to establish ancient climatic changes, drainage patterns,
emergences and subsidences of sea floors, and related palaeogeographical
subjects which are reflected more clearly in their effects on fossil organisms
than in their influence on the mechanics of sedimentation.
The prime axiom of palaeo-ecology is that specialization of
a fossil fauna indicates the same environmental control as would lead to the
development of a similarly specialized unit among Recent faunas. A frequently
cited example is that coral reefs in the modern seas only flourish in warm,
clear water of normal salinity, and it is assumed that fossil coral reefs are
an index of similar marine conditions. A fossil fauna which lived in fresh
water may be identified by its specialization along the same trends as a Recent
fresh-water fauna, e.g. molluscs thin-shelled and smooth, charophytes present,
admixture of terrestrial matter, especially plant spores in abundance. A means
of direct check on the temperatures of ancient environments is being developed
by Urey, the basic method being to determine the carbon-isotope ratios in the
calcium carbonate of fossils and from these figures to deduce the original
temperature at which the shell was formed.
[32]
Organisms vary in their degree of tolerance of environmental
changes. The prefixes ‘eury’ and ‘steno’, implying respectively broad and
slight tolerance, are used in descriptive terms such as eurybathic, euryhaline,
stenohygric, stenothermal. In palaeo-ecology the most significant fossils are
those of stenobathic, stenothermal and stenohaline organisms, since they can be
used as indices of former conditions of sea-depth, water temperature, and
salinity.
Our knowledge of the ecology of living marine
micro-organisms, applicable to palaeo-ecology is drawn from the results of
oceanographical research. Since the cruise of the British research ship H. M.
S. “Challenger” in 1873-1876, many vessels of various nationalities have made
cruises, either world wide in scope or limited to some special region such as the
polar seas, with the purpose of collecting organic material and
physico-chemical data for the advance of oceanographical research Some of the better known research ships
have been the “Dana” (Denmark), “Meteor” (Germany), “Discovery” (Great
Britain), “Willebrord Snellius” (Holland), “Maud” and “Fram” (Norway),
“Albatross”, “Carnegie”, “Catalyst” and “E. W. Scripps” (U.S.A.).
[33]
The knowledge of life in the open oceans provided by these ships has been
supplemented by studies of the faunas of coastal waters made by various
shore-based institutions.
[34]
Up to the present, the sea floor at the outer edge of the continental shelf and
the upper edge of the continental slope has not been investigated as thoroughly
as the shallower and deeper regions which flank this province.
The ecology of terrestrial organisms can be studied without
the elaborate preparations necessary for an oceanographical cruise, but
palaeo-ecology is so largely concerned with marine sediments that knowledge of
Recent terrestrial life is seldom significant. An exception lies in the use of
plant spores as indicators of climate. Between the poles and the equator there
is a well marked floral zonation: among trees in the northern hemisphere the
birch, the conifers, the oaks, the palms and the mango typify provinces
successively farther from the pole. During the Pleistocene epoch advances and
retreats of the polar ice-cap caused parallel movements of the plant
communities Evidence of this is to
be found in the spore content of the glacial and interglacial sediments. It may
be noted that the results of such studies match the palaeo-ecological evidence
of foraminifera in cores from the Atlantic ocean floor, and jointly they add
greatly to the preciseness of our knowledge of correlation of the interglacial
periods formerly based on gross physical features such as leached surfaces in
glacial tills and changes of provenance of erratic boulders.
[35]
An aspect of marine life which militates against positive
palaeo-ecological deductions is the tendency of specialization among all animal
phyla for life in all variants of the marine environment. (Plants are more
limited because photosynthesis is only operative down to the maximum depth of
penetration of sunlight.) Consequently it is exceptional for any major phylum of
animals to be restricted to a particular environment: it is usually necessary
to descend to the level of genus, or even species, in designating index-forms
of special habitats.
This adaptiveness of marine animals reduces the number of
broad generalisations which can be made on the palaeo-ecological significance
of fossils. In bio-stratigraphy the significance of fossils can be expressed by
many generalisations of the type, “The graptolites and trilobites are index
fossils for the Palaeozoic, ” but few of these dicta have palaeo-ecological
counterparts. Of relatively few orders is it even true to say that they are
exclusively marine, since specialization has permitted entry into brackish and
fresh-water environments. Among the larger organisms, this difficulty can
partly be overcome by considering morphological specialization rather than
systematic classification of the animals in a given community, since the larger
marine invertebrates frequently show a mechanistic adaptation of their
body-form to fit them for a certain environment. The corals in areas of heavy
surf are compact sphaeroidal forms but their close relatives in quiet water
develop delicate ramose and flabelliform shapes. The decapod crustaceans
include armadillo-like forms (Emerita)
which can withstand the buffeting of waves on sandy beaches, compressed forms (Petrolisthes) which can elude enemies by backing into crevices
among rocks, small planktonic forms (Lucifer), free-swimming forms with oar-like limbs (Portunus), deep-water species with long slender limbs and
appendages (Nematocarcinus, Aristaeus), and the amphibious hermit crabs (Eupagurus) which use acquired gastropod shells instead of
having chitinous armour of their own. (Even the sessile barnacles are not
distant relatives of the mobile decapods, and their resemblance to the
molluscan limpets is a typical case of homoeomorphy due to environmental
specialization.) Similar examples of wide adaptation to specialized conditions
could be cited for the molluscs, the echinoids, the worms or the fishes. Hence
an assemblage of marine megafossils may reveal its ecological status merely by
the morphological features of the organisms present. An abundance of
thick-shelled calcareous fossils, regardless of individual orders, genera, and
species, is a fairly sure sign of deposition in a warm shallow sea—shallow
because of the high concentration of sunlight necessary for photo-synthetic
production of a high carbonate-ion concentration, warm because the rate of the
reaction of lime precipitation is very slow in cool water.
Micro-organisms, among which a linear dimension of half an
inch is exceptional, rarely show any mechanistic adaptation of their hard parts
appropriate to their chosen milieu. The foraminifera of tropical coastal waters
are often dominated by discoidal genera such as Amphisorus, Amphistegina. Peneroplis, etc., the
shape of which doubtless prevents the burial which would befall spherical forms
of the same mass; but other externally similar discoidal forms such as Cyclammina and Robulus, are typical of rather deep offshore water. The planktonic
Globigerinidae have very thin inflated tests, yet so do many non-planktonic
foraminifera. The Upper Cretaceous Gümbelinas were evidently planktonic but only differ
superficially from the benthonic Bolivinas. Among Recent foraminifera there are many baffling instances of
species closely similar in size, appearance and relationship but demonstrably
different in their optimum habitats. For example, the genus Elphidium is commonly regarded as an index of warm coastal
waters, yet the species E. incertum
is restricted to a subarctic habitat: the genus Nonion includes species restricted to inshore waters, such
as N. scapha, and others typical
of ocean-floor faunas, such as N. pompilioides. Uvigerina peregrina, only known from cool water, differs only in detail
from other Uvigerinas which live
in warm water. As a basis for ecological deduction it is therefore preferable
to emphasize the over-all aspect of a microfauna rather than the presence of
any particular species.
Among the assemblages of micro-organisms which typify
special environments are the following:
PLANTS
Chara is an algal
genus of fresh-water habit, characterised by an ability to secrete lime in its
tissues. An abundance of its oögonia, ellipsoidal spore-cases with spiral
ridges encircling them, is a clue to fresh-water sediments.
The calcareous algae are usually recognized as megafossils,
and under optimum conditions they may be a main component of organic
limestones. However, under less favorable conditions they may be present only
as a minor element in a fossil community, not readily recognizable except in
washed residues. They are symptomatic of warm shallow sea water.
The spores of terrestrial plants are liable to occur in
abundance in continental deposits. In lesser quantities they may occur in
marine sediments, since they were subject to aeolian distribution. They are of
value as climatological indices, at least within the geological range of Recent
floras.
RADIOLARIA
The radiolaria are exclusively marine. Because the
radiolarian oozes of the Recent oceans are of abyssal origin their fossil
counterparts have often been claimed as deep-sea sediments. More recently
authors have tended to challenge this belief, and the term ‘pseudabyssal’ has
been applied to radiolarites of shallow origin. In western North America a
relationship has been claimed between vulcanism, an excess of silica in the
neighbouring sea, and a rich Tertiary radiolarian fauna. In western South
America Tertiary sediments with a microfauna consisting almost entirely of
radiolaria have been claimed to mark the paths of cold marine currents.
[36]
OSTRACODS
Literature on the ecology of ostracods is scanty, but it is
a matter of observation that the more ornate species with angular and sculptured
tests are marine, whereas the fresh-water types are simple bean-shaped forms
with no pronounced irregularities of shape and only a low degree of surface
ornamentation. Ostracods are not usually present in deep sea sediments.
[37]
FORAMINIFERA
More detailed studies have been made of the ecology of
foraminifera than of any other order of micro-organisms, and some consistent
relationships have been indicated between the presence of certain forms and the
prevailing ecological conditions,
Some of the assemblages most useful in palaeo-ecology are described
below.
An association with abundant discoidal calcareous
foraminifera of large size is typical of shallow near-shore waters in tropical
and subtropical latitudes. In Recent examples the most typical families are the
Camerinidae, Peneroplidae, Alveolineliidae, Amphisteginidae, Calcarinidae,
Cymbaloporidae and Planorbulinidae. Extinct families which appear to have lived
under similar conditions include the Orbitoididae, Discocyclinidae and
Miogypsinidae. To some extent the depth-control within this group is due to commensality with algae.
An assemblage similar to the last often typifies offshore
reefs, the main difference being a greater diversity of fauna, in particular a
greater abundance of planktonic and benthonic smaller foraminifera of open sea
character and often an admixture of sessile micro-organisms such as the
foraminifera Carpenteria and Rupertia and the bryozoan Cupularia.
In general a microfauna with a small number of foraminiferal
species, thirty or less is representative of a shallow environment, regardless
of its richness in individuals The
shallower inshore waters are more subject to annual and diurnal changes of
temperature, reduction of salinity by river discharge, physical effects of
storm currents and tides than the waters of the open sea. The majority of
foraminiferal species cannot tolerate such a wide range of conditions. In warm
shallow waters such as the coasts of the Caribbean Sea, the western Atlantic
and the southern Mediterranean a profusion of Elphidium spp. and Streblus (‘Rotalia’. auct.) beccarii is usual. In cooler waters, such as the west coast
of America excepting the Panamic province, the Discorbinae and Buliminidae of
the genera Bolivina, Bulimina and Uvigerina dominate the micro-faunas. The Miliolidae and certain species of Nonion and Nonionella also tend to occur in abundance in the shallow coastal environment.
Streblus beccarii, in
particular, and many species of Elphidium are notably euryhaline. As well as in coastal waters of normal
salinity they can thrive in a subsaline environment such as Lake Maracaibo
[38]
or in the supersaline waters such as Lago Enriquillo
[39]
in Hispaniola.
A distinctive assemblage known from numerous fossil
occurrences, though apparently not recorded among Recent faunules, consists
almost exclusively of large simple arenaceous foraminifera of the genera Bathysiphon, Hormosina, Cyclammina, Haplophragmoides, Trochammina, Gaudryina s. l., Clavulina s. l., Verneuilina, Valvulina, and others. Development of such a fauna seems to be closely related
to turbidity of sea water, since the examples known represent a piedmont facies
(Chaudiere and Nariva formations of Trinidad, Capiricual member of Santa Inés
formation of Venezuela, Mal Paso formation of Peru, and the alpine Flysch),
deposition at the mouth of a large delta (Cruse formation of Trinidad) or
re-deposition of mudflow exudations and slumped clays (Ste. Croix formation, in
part, of Trinidad, Socorro formation, in part, of Ecuador).
[40]
It is feasible that the excessive turbidity of these environments inhibits
photosynthesis and thus leads to too low a concentration of carbonate ion for
the viability of calcareous organisms. It is also possible that in the presence
of a surplus of clay colloids base-exchange reactions reduce the available
calcium ion.
Another assemblage dominated by arenaceous foraminifera
exists under stagnant, usually brackish conditions. In this instance the
species are small, usually with flimsily constructed tests. Ammobaculites, Trochammina, and Haplophragmoides
are genera of wide distribution represented in this environment by diminutive
species, and there are other genera which seem to be restricted to a brackish
habitat, such as Leptodermella, Lagunculina, Ammoastuta, and Trochamminita. Such
a fauna is known from bayous and salt marshes in the Mississippi delta, from
Miocene deltaic sediments and from a Recent salt marsh in Trinidad, and also in
the Mississippi area in shallow marine facies under stagnant, poorly oxidised
conditions.
Sometimes associated with tiny arenaceous foraminifera in
the brackish facies is a suite of small calcareous forms with the genera Buliminella, Nonion,
and Nonionella dominant. A
peculiarity in several instances is a yellowish to russet orange coloration
which suggests that an appreciable content of ferric salts is present in the
shell material. This calcareous faunule has been noted in the Recent
Mississippi delta
[41]
, Pliocene
estuarine beds in the Nile valley, and Miocene deltaic beds in Venezuela and
Trinidad.
[42]
In tropical America some Upper Oligocene and Lower Miocene
microfaunas are dominated by costate Siphogenerinas of the S. transversa species-group. These assemblages appear to represent
deposition in warm shallow water. In Ecuador the greatest concentration of
these Oligo-Miocene Siphogenerinas
seems to be at the intergradation between deposits of the continental shelf and
the outer edge of an advancing fan of terrigenous sediments. In Recent faunas
species of the same group have been recorded off various Pacific and West
Indian islands, partly at depths as shallow as ten meters.
An abundance of Lagenidae and Buliminidae, both represented
by numerous genera and species, is a sign of rather deep water. This is
especially the case if they are accompanied by species of Gyroidina, Epistomina, Cassidulina, Laticarinina, Lagena
and other genera more typical of deep than of shallow water. Under certain
conditions, not yet fully understood, the buliminid genera Bolivina (B. costata, B. floridana, etc.) and
Uvigerina (U. pygmaea, etc.), and the lagenid genus Robulus (R. calcar, etc.) may dominate an inshore faunule.
The planktonic foraminifera (sometimes less accurately
called the pelagic foraminifera) live in the surface layers of the oceans and
are dispersed by currents, only sinking to the sea floor at death. Consequently
they are not valid as indicators of sea floor conditions. In the modern oceans
‘Globigerina oozes’, almost entirely composed of the tests of foraminifera, are
typical of abyssal depths, but it is fallacious to deduce from this that any
sediment of like composition must be of deep water origin. At any place where
strong surface current sets towards an island or continental coast an abundance
of planktonic
foraminifera will be found in the littoral deposits: this is exemplified by
shallow-water Globigerina limestones in
some of the Pacific islands.
[43]
In an area where warm and cold currents intermingle the death rate of the
planktonic foraminifera may be high: this appears to explain an abundance of Globigerinoides
sacculifer, etc., in sub-littoral Miocene
beds of northwest Ecuador, where the ancestral Humboldt current met the warmer
waters of the Caribbean province. The two cases cited are exceptions, however,
and if a gradient can be demonstrated along which increasing abundance of
planktonic foraminifera corresponds to increasing distance from a coastline,
then increasing sea depth may be assumed: the Eo-Oligocene Naparima Basin of
Trinidad provides a good example.
The presence of a rich planktonic microfauna in a sediment
in the absence of any benthonic fauna commonly signifies a euxinic facies
[44]
(Pons Euxinus = the Black Sea). This
usually implies the presence of a silled basin in which the bottom water layers
stagnated and became charged with toxic hydrogen sulphide. Normal benthonic
life was thus inhibited, though the nekton and plankton were unaffected. In the
Caucasus Lower Tertiary marls with a fauna of Globigerinas, Gümbelinas, Globorotalias, and Hantkeninas are interbedded with marls carrying a rich benthonic
fauna in addition to these planktonic genera. This appears to be an example of
euxinic conditions, as do the Cretaceous Globigerina and Gümbelina limestones of the Andean geosyncline, Mexico, and elsewhere. Modern
examples are the Black Sea itself and some of the Norwegian fjords.
In general the planktonic foraminifera thrive-best in warm
waters. Globorotalia menardii is
considered an exceptionally strong index for warm conditions. Globigerina
pachyderma, however, is an exception
indicative of cool to cold water. Frequency ratios of these and other species
have been used to establish temperature fluctuations in the late Quaternary,
based on submarine cores from the floor of the north Atlantic Ocean.
[45]
DWARFED MICROFAUNAS
Dwarfing is commonly related to abnormal salinity. High
salinity, such as results from solar evaporation of an isolated lagoon, may
cause the effect, or it may result from low salinity in an area of strong
fluvial discharge.
[46]
BARREN SAMPLES
Sedimentary samples may be barren of fossils for the primary
reason of representing a sterile environment or the secondary reason of
non-preservation. Secondary barrenness is suspected in the presence of abundant
products of chemical weathering (gypsum, selenite, limonite, laterite,
jarosite, etc.). If there is no such evidence, and especially if similar
sediments nearby do carry a fauna, some life-inhibiting ecological factor must
be suspected. Possibilities to be assessed in individual cases include physical
effects such as abnormally high or low temperatures; chemical effects such as
abnormal concentrations of chloride, sulphide, or carbonate ion; and mechanical
effects such as turbulence, scouring, or dumping of sediments. A wide range of
diurnal or annual fluctuation of ecological factors is also highly inimical to
micro-organic life.
MEGAFOSSIL FRAGMENTS
Several authors have shown by localised examples that
comminuted fragments of megafossils can be identified as to genus and even
species. If a particular megafossil has known ecological significance its
recognition from fragments in washed residues may be used to supplement other
palaeo-ecological data.
EXTINCT ORGANISMS
The mode of life of some extinct organisms has been found by
experience to match the generalisations applicable to their living descendents.
For instance an abundance of plant spores has been typical of continental
deposits since the dawn of plant life; ornate ostracods have occurred in marine
but not in fresh-water facies at least since the Cretaceous.
In the case of foraminifera it has been mentioned above that
closely similar species may pertain to entirely different environments, hence
reliable deductions cannot be drawn from superficial resemblance between a
certain fossil species and a Recent form of limited habitat. More reliable is
the process of identifying facies-provinces from the areal distribution of
different types of microfaunal assemblage and then deducing the
palaeo-ecological status of individual species. The two species Hantkenina
alabamensis Cushman and Uvigerina (Uvigerinella) carapitana Hedberg may
be cited as representative examples.
[47]
The former has a world-wide range in Upper Eocene sediments and occurs in a
variety of marine facies. It might, therefore, be taken as either a eurybathic
benthonic species or as a planktonic form. The question is decided by
considering the euxinic-facies marls in the Upper Eocene of the Caucasus, where
this species occurs in association with a purely planktonic assemblage of
Globigerinidae and Globorotaliidae and hence must itself be assumed to have
been planktonic in habit. Uvigerina carapitana is an Upper Oligocene species recorded in several
countries of South America and the Antilles. Along with many other Buliminidae
of the same age it is restricted to the sublittoral (neritic) facies, not
appearing in the shallower littoral facies nor in the deeper archibenthic
Globigerina marls. Furthermore, in Ecuador U. carapitana is not a common species except at the inner margin
of the neritic facies, just beyond the outer margin of a sheet of encroaching
littoral sediments which gradually filled in the basin of deposition. This
extinct species has thus been established by inferential reasoning as a
significant palaeo-ecological index.
In other cases it becomes apparent that certain microfossils
were subject to ecological control of their distribution but the controlling
factor is not readily apparent. The Upper Cretaceous genus Siphogenerinoides may occur as one element in a richly diversified
neritic-facies microfauna, as in the Pazul Basin of northwest Peru, yet in
adjacent beds of similar lithology this genus may be abundant to the exclusion
of almost all other foraminifera. Evidently Siphogenerinoides could endure an extreme of some ecological factor,
such as stagnancy, which left no tangible record in the sediments. The genus Epistomina shows a similar uneven distribution in the Jurassic
and Lower Cretaceous of northern Germany.
[48]
SUMMARY
Palaeo-ecological deductions regarding a fossil microfauna
are based on recognition of specialization corresponding to that of a Recent
fauna of known environment. If the degree of specialization is high, reliable
conclusions may be drawn from a single sample. In general it is more
satisfactory to study a group of related samples, to ensure that their apparent
environmental features blend into a rational facies-pattern. Some examples from
the literature are mentioned in the bibliography.
[49]
(c) Tests of rationality
No version of the depositional history of a region is
acceptable if it contains irrational elements. To take a simple example, if the
Upper Eocene should be construed as an emergent phase represented by
continental and marginal marine facies, it would be irrational to postulate a
small centrally placed abyssal facies of the same age. The evidence leading to
such a conclusion would need to be re-assessed to determine the cause of
discordancy. Faulty age determination, incorrect structural interpretation,
reworking of faunas, confusion of sample data, or some comparable reason
usually comes to light when such cases are investigated.
A palaeogeographical reconstruction is considered rational
if it postulates only the normal processes of sedimentation and if their
products show a normal intergradation. In a given region during any geological
epoch there tended to be some fundamental pattern underlying the detailed
sequence of events: such patterns included periodic isostatic adjustments
during a mountain-building phase, block-fault foundering, and slow epeirogenic
uplift or subsidence. A palaeogeographical reconstruction which conforms to
such a pattern is rational but one which claims an implausible mixture or
alternation of such basic features is irrational.
In northwest Ecuador, in thick jungle almost inaccessible
except along the main rivers, sections on several parallel streams indicated a
monocline in Lower Oligocene shales. Microfaunas indicated a semi-barren,
euxinic facies. On one river an algal limestone was mapped, dipping
concordantly with the shales, and on field evidence it was taken as an
interbedded lens. However, in palaeogeographical terms such a relationship,
with no signs of intergradation, appears irrational and requires checking. In
this particular case the limestone was later proved to be Upper Oligocene in
age, a relic of a landslip which had fortuitously left a large block inclined
at the same angle as the monoclinal Lower Oligocene shales.
In northern Peru, isopach studies appeared to indicate
transgression of a certain formation over a sculptured erosional surface of
older rocks. The geomorphology of the reconstructed surface seemed logical,
hard sandstones forming fault-scarp ridges and soft shales marking depressions.
The deeper-water facies in the transgressive formation were confined to its
thickest parts, its thinner parts carrying only shallow-water facies-faunas.
The palaeogeographical picture was rational except for one point, namely the
very high vertical relief postulated on an erosional surface which would more
naturally be a peneplane. Recognition of this irrationality led to a critical
review, whereby it was established that block-faulting along the lines of the
pre-existing pattern had been renewed during the deposition of the
transgressive formation. This explained not only the mechanistic irrationality,
but also an abnormally abrupt change from a shallow-water to a deep-water
facies, the apparent development of some orbitoidal reefs in deep water, and
other palaeo-ecological peculiarities of the transgressive formation.
In Barbados the Eocene and Oligocene sediments show a
succession upwards of poorly sorted piedmont-facies sediments, radiolaria beds,
Globigerina marls, and neritic-facies
clays.
[50]
On the assumption that the radiolaria beds represent an abyssal facies it has
been claimed that during the Eocene epoch the sea-floor underwent rapid
downbuckling from a shallow level to depths of 5000 to 6000 meters and then
rose again. On mechanical grounds such profound and rapid movement appears
implausible (though one may not deny it dogmatically until island arcs are more
fully understood)
[51]
. The
apparent irrationality may be removed by claiming that radiolaria beds are not
necessarily evidence of abyssal deposition.
Usually palaeogeographic deductions which might be termed
freakish, requiring the supposition of a special set of conditions, are suspect
in a check of rationality. Nevertheless local peculiarities do now exist (as in
the Sargasso Sea, the Bay of Fundy and the Baltic Sea) and they must have
existed in the course of geological time. In Trinidad the Oligocene Ste. Croix
beds are closely similar in lithology and fauna to the Brasso formation
(Esmeralda member). The main difference is that the Brasso is a peripheral
formation of the Naparima Basin, representing the sublittoral facies and
grading offshore into the deep-water Cipero marls, whereas the Ste. Croix beds
are lenses of shallow-water sediments situated out in the basin and completely enveloped by the Cipero marls (see Figure 4(A)). The explanation of this phenomenon must rank
Figure 4(A): Correlation and classification of
sediments in the Naparima Basin of Trinidad
as freakish, yet it seems well authenticated. It is that
anticlinal ridges were pushed up from the floor of the basin and provided
platforms on which colonies of shallow-facies organisms could settle. Such a
process conforms with the basic pattern in Trinidad (lateral compaction due to
gravity collapse on the sloping Guiana shield) and it is worthy of note that
sand lenses in the Ste. Croix beds are lag sediments, of which the only likely
source is submarine extrusion of mudflow.
[52]
A different type of irrationality arises from the uncritical
assumption that because some particular sector has been studied in more detail
than others it is necessarily typical of the region as a whole. Examples are
cited from Trinidad and Ecuador. In the former case the large Fyzabad oil field
was taken as a standard of reference when various outlying smaller pools were
developed. It so happens that the Miocene Cruse formation at Fyzabad shows only
one cycle of deltaic deposition, whereas in some of the newer pools the main
cycle is broken into three sub-cycles. Solely because of the preconception
based on Fyzabad, the cyclical repetition of lithology and faunules was for
some time taken to indicate overthrusting and an entirely erroneous concept was
formed of the geological conditions. In the Manta area of Ecuador outcrops are
limited by a thick mantle of Pleistocene Tablazo beds, and consequently the
coastal cliffs have been over-emphasized as the key to subsurface geology
inland. This led to a faulty interpretation of structure and stratigraphy which
was only dispelled by the drilling of a deep well. In both examples incorrect
assessment of structure and stratigraphy caused incorrect interpretation of
palaeogeography, and the basic reason was unjustified extrapolation from a
known to an unknown sector.
(d) Presentation of palaeogeographical data
[53]
The aim of palaeogeographical studies is to discern how, in
the course of geological time, the outlines and physiography of lands and seas
have been changed by the effects of orogeny, epeirogeny, and taphrogeny and by
the constant processes of weathering, erosion and sedimentation. The ideal
basis for such studies is an area with abundant outcrops and closely spaced,
carefully sampled wells, simple structure, a stratigraphical succession readily
subdivisible in terms of facies and age, and evidence as to the nature of any
beds removed by erosion. With such a groundwork it is possible to follow the
broad pattern of sedimentary cycles and orogenic phases and the fine pattern of
their effects at different localities. In practice the ideal is never attained
and some degree of extrapolation becomes necessary to cover gaps due to
erosion, inaccessibility or alteration of beds, lack of index criteria, or
undecipherable structure.
Supposing that the available evidence has been sifted and
analysed and the palaeogeographer has formed definite ideas on the sequence of
geological events, the problem which next arises is to present his conclusions
in a graphic form readily assimilable by other geologists. This is by no means
a simple matter since the palaeogeographical concept is a four-dimensional
continuum of space relationships varying in respect to a time axis. To register
the space relationships at any chosen instant in geological time requires some
form of perspective drawing (block-diagram, fence-diagram or serial sections)
or contour drawing as a base, with superimposed colours or symbols to denote
different faunal or lithological facies. Any attempt to indicate the course of
changing events, of which the phase represented is only an arbitrary choice, is
likely to result in a diagram so complicated as to be unintelligible.
Consequently it is usual to employ a serial technique in graphic representation
of palaeogeographical conclusions. Points in time are selected, either as
representing extreme fluctuations in the trend of geological events or as being
accurately determinable on faunal grounds at widespread sections, and a facies
diagram is prepared corresponding to each of these time points. When viewed in
order these serial diagrams indicate the trends of geographical change in
geological time.
An alternative method, applicable to the simpler
palaeogeographical processes such as infill of an embayment, is to mark
successive positions of facies-province boundaries on a single map. Boundary
lines corresponding to three or four phases of the process may be indicated,
each line being numbered to indicate which phase it represents. Bands of colour
along the boundaries give a key to the expansion and shrinkage of different
facies-provinces, and arrows may be used to emphasize the direction of shift.
Comprehension of a complex chain of palaeogeographical
events may be ensured by preparing a simplified perspective drawing to
emphasize the main trends, after which local anomalies and peculiarities may be
depicted on the appropriate serial diagrams. In general, it seems more
acceptable to present palaeogeographical diagrams in a simple but lucid form
than to lose clarity in an attempt to present complex details which might
better be discussed textually. Various types of graphic representation are used
in illustration of other sections of this paper.
IV. EXAMPLES OF PALAEOGEOGRAPHICAL DEDUCTIONS BASED
ON MICROFAUNAS
In the following pages some examples are given to
demonstrate the palaeo-geographical significance of fossil microfaunas. They
are all, except the last, based on first-hand studies by the writer while
employed by various oil companies. The purpose of these examples is to
illustrate methods of analysis and interpretation rather than to publicise
local data, hence the writer has felt free to ignore details of local
importance but irrelevant to the main theme. This attitude is further justified
by the fact that much of the basic information used must, while it remains
unpublished, be regarded as the confidential property of the oil companies
concerned.
EXAMPLE 1 A
basinal flank which subsided at a rate equal to the rate of accumulation of
sediments, so that sea-floor conditions remained stable.
(Based
on the Naparima Basin of Trinidad)
The area considered shows three parallel belts, each
typified by a distinctive rock-type. The northernmost consists of clastic
sediments grading irregularly upwards from conglomeratic basal beds, through
grits and sandstones, to muddy, sandy shales. These comprise the Nariva
formation and its various members. The middle strip is dominated by clay-shales
but the lithology is variable, locally including limestones, conglomerates,
glauconitic beds, orbitoidal sandstones, and both calcareous and non-calcareous
clays and shales. The older, more variable, parts comprise the San Fernando
formation and the younger, more uniformly shaly, parts belong to the Brasso
formation. These two strips are peripheral to a large central area occupied by
highly calcareous marls of the Cipero formation and other subdivisions of the
Naparima marl group.
Megafossil evidence indicates that each of the three
lithologically distinct groups has an age range from Upper Eocene to Upper
Oligocene. Hence clearly, since their deposition was synchronous, their
differences must be due to environmental factors. The Nariva formation is
practically barren but its physical characteristics, such as coarse average
grade, conglomeratic layers and ripple marks, denote a littoral environment.
The San Fernando fauna includes large orbitoids, irregular echinoids, various
molluscs, crustaceans, calcareous algae, and other fossils which jointly suggest
a sub-littoral environment. The Brasso likewise contains molluscs of inshore
type. The Naparima marls contain very few megafossils but the molluscs Thyasira and Pleurophopsis in the Cipero formation indicate a cold, therefore
in tropical latitudes presumably a deep, environment.
Figure 4(B) is a correlation table of the Naparima Basin sediments based on field geology and megafossils. The evidence cited is imperfect because of lack of fossils in the Nariva and the lower parts of the Naparima marls. Even if structural conditions were less complex in the Naparima Basin it would be difficult to make a more closely knit interpretation of stratigraphical relationships because of the great vertical thicknesses of uniform lithology and the abrupt lateral changes of rock-type. The published literature on the basin proves the difficulty of making an exact interpretation on a basis of field geology and megapalaeontology. Perusal of the studies of Wall and Sawkins (1860),
B) Distribution of Age-Significant Megafossils and
Larger Foraminifera:
Note the much more precise correlation in Diagram A) based on microfossils.
1. Derived megafossils as young
as Paleocene in boulders.
2. Upper Eocene molluscs,
echinoids, larger foraminifera, etc.
3. Middle Oligocene molluscs and
larger foraminifera.
4. Upper and late Middle Oligocene
megafossils and larger foraminifera
5. Upper Oligocene and Miocene
molluscs.
6. Upper Oligocene molluscs of
deep-water type.
Figure 4(B): Correlation and
classification of sediments in the Naparima Basin of Trinidad
Guppy (1866-1893), Waring (1926), Illing (1928), Liddle
(1923), Lehner (1936) and Kugler (1936) shows the gradual realization of
lateral equivalence, but it was not until Renz (1942) utilised the results
of intensified microfaunal research that a coherent and detailed interpretation
began to emerge.
[54]
The fossil microfaunas of the Naparima Basin provide
additional stratigraphical information of three categories, viz., zonation
based on planktonic species, palaeo-ecology based on the lateral variation of
the assemblages, and positive evidence of lateral transition shown by
intermingling of facies-controlled faunas.
Zonation
[55]
The Naparima marls contain exceedingly abundant planktonic
foraminifera. Most of the species had a limited life range and are therefore
applicable to a zonal scheme, as tabulated below. The oldest marls (Lizard
Springs and Navet formations) contain in their faunas certain planktonic
foraminifera (Globorotalia velascoensis,
Hantkenina mexicana, H.
aragonensis, and others) indicative of an
age range from Upper Cretaceous to Middle Eocene. Of more immediate interest
are the younger beds (Hospital Hill and Cipero marls, Lengua calcareous clay)
in which six zones are clearly defined by the ranges of Hantkenina
primitiva, Globigerina concinna, G. dissimilis, Globigerinatella insueta,
Globorotalia fohsi, and G.
menardii. These species appeared and
vanished in succession with only slight overlap in their ranges. Usually one or
the other of them is present in a representative sample, but in their absence
numerous other planktonic species are valid as zonal indicators.
Table
1: Ranges of some planktonic foraminifera in
the Naparima Basin
|
|
Upper
|
Oligocene
|
Lower
|
|
|
Eocene
|
Lower
|
Middle
|
Upper
|
Miocene
|
|
|
Hantkenina primitiva
|
XXXXXXX
|
|
|
|
|
|
|
|
Globorotalia cerroazulensis
|
xxxxxxxxx
|
|
|
|
|
|
|
|
Globigerina dissimilis
|
xxxxxxxxx
|
xxxxxxxxxxxxx
|
XXXXXX
|
|
|
|
|
|
Globigerina insueta
|
|
XXXXXXXXXX
|
|
|
|
|
|
|
Globigerinatella insueta
|
|
|
|
XXXXXX
|
|
|
|
|
Globigerinoides sacculiferus
|
|
|
|
xxxxxxxx
|
xxxxx
|
|
|
|
Globigerinoides conglobatus
|
|
|
|
xxxxxxxx
|
xxxxx
|
|
|
|
Globorotalia mayeri
|
|
|
|
xxxxxxxx
|
xxxxx
|
|
|
|
Globigerina grimsdalei
|
|
|
|
xxxxxxxx
|
xxxxx
|
xxxxxxxxx
|
|
|
Globigerinoides rubrus
|
|
|
|
xxxxxxxx
|
xxxxx
|
xxxxxxxxx
|
|
|
Globorotalia praemenardii
|
|
|
|
xxxxxxxx
|
xxxxx
|
xxxxxxxxx
|
|
|
Globorotalia fohsi
|
|
|
|
|
XXXX
|
|
|
|
Candorbulina universa
|
|
|
|
|
xxxxx
|
xxxxxxxxx
|
|
|
Globorotalia menardii
|
|
|
|
|
|
XXXXXXX
|
|
|
Sphaeroidinella dehiscens
|
|
|
|
|
|
xxxxxxxxx
|
|
|
Globigerinita naparimensis
|
|
|
|
|
|
xxxxxxxxx
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Variations of average frequency are
not shown. The larger symbols (XXX) indicate the name-species of the six zones.
The same zones are demonstrable by changes in the benthonic
microfaunas of the marls, but in attempting to establish correlation between
the deep-water marls and the sublittoral deposits the benthonic forms are of little
value. This is because ecological differences led to the development of such
entirely different benthonic assemblages that, while it may be difficult to
distinguish Upper Eocene and Middle Oligocene faunas within the San Fernando formation, it requires
almost an act of faith to believe that Lower Oligocene assemblages from the San
Fernando and Cipero formations actually lived in synchronous and contiguous
deposits. However, attention to the planktonic foraminifera shows the same
distribution of species in the sublittoral facies as in the marls. In the San
Fernando formation the zones of Hantkenina primitiva and Globigerina concinna are easily recognizable and in the Brasso the
characteristic species of the younger zones appear.
In the littoral-facies Nariva formation, sensu stricto, microfaunas are sparse and consist largely of
arenaceous benthonic foraminifera. However, the succession of sandy beds is
locally broken by thin marly beds with weak, but zonally determinable, faunas
of San Fernando type. Along the line of the main change of facies it is locally
possible to observe physical intergradation of the Nariva into an offshore
facies of determinable zone. At Bon Accord the basal Nariva conglomerate has a
shaly matrix in which Hantkenina primitiva has been noted.
Palaeo-ecology
The Naparima marls are fossil versions of the Globigerina-oozes of the modern ocean floors. The Cipero
formation has been discussed in detail in the literature and shown to match the
definition of a Globigerina-ooze
as a sediment consisting, to at least 25 percent of its dried weight, of
unbroken foraminifera with a ratio of more than 25 to 1 of planktonic to
benthonic specimens. In the Cipero faunas the frequency ratios of species and
specimens referrable to different benthonic families of foraminifera match most
closely the figures for Recent faunas living below 300 meters, as recorded on
sea-floor profiles off California and in the Gulf of Mexico. The uniform
composition of the Naparima marl microfaunas over wide areas and through great
thicknesses indicates long-standing ecological stability, a criterion of deep
rather than shallow marine conditions.
[56]
The microfaunas of the San Fernando formation confirm the
evidence of variable lithology and of megafossils as to its moderately
shallow-water environment. The type of microfauna varies considerably in
adjacent beds and planktonic foraminifera are not as conspicuous as in the
Naparima marls. The ‘larger foraminifera’, species of Operculinoides, Lepidocyclina and Asterocyclina, are well
represented in the faunas, locally in rock-forming abundance. The Brasso
formation also contains orbitoidal layers near its base, but the strongest
indication of its shallow-water origin is an abundance of costate
Siphogenerinas in its microfaunas.
The Nariva facies-province was largely a sterile
environment, but in the muddy shales of the upper Nariva a microfauna of large
simple arenaceous foraminifera (Eggerella,
Bathysiphon, etc.) is present. As
noted earlier, such assemblages are taken to indicate a marine environment in
which turbidity is the dominant ecological factor.
Positive evidence of lateral transition
On a large-scale map the concentric facies-provinces of the
Naparima marls, San Fernando/Brasso and Nariva formations stand out clearly,
but the local details of lateral transition are less easy to discern. Natural
outcrops are rare. Deep weathering and tropical vegetation prevent access and
force the field geologist to rely on auger surveys supplemented by test pits.
The resulting lithological map is not a reliable record of changes of facies.
In one sector it may clearly show a facies change, e. g. from Nariva sand to
Brasso calcareous clay, but in another it may show only uniform marly clay
where actually the Cipero and San Fernando formations interfinger.
Washed residue studies provide a means of precise detection
of changing facies In each of the
three main provinces the micro-faunas contain certain ‘facies-markers’,
foraminiferal species which persisted throughout the deposition of that facies
but were ecologically inhibited from migrating laterally out of their own
province. In the Nariva Eggerella sp. is
the main facies marker, supplemented by short-ranging foraminifera and
ostracods at certain levels. In the San Fernando and Brasso formations the
facies markers include Vaginulina elegans mexicana, Pseudoglandulina conica, Uvigerina curta-group, Plectofrondicularia miocenica-group, Frondicularia tenuissima, Siphogenerina transversa, Ceratobulimina evoluta, Cibicides trinitatensis, Lingulina grimsdalei and many others. A few of the many facies markers
typical of the Cipero and older Naparima marls are Ammolagena clavata, Bolivinopsis (? Spiroplectammina) clotho/cubensis,
Cibicides cicatricosa, Dentalina
havanensis, a group of the Ellipsoidinidae,
Karreriella subcylindrica, and Planulina
illingi.
Examination of washed residues from a line of auger samples
taken in the strike across a facies boundary usually shows a lateral change of
microfauna as in the example below.
NORTH SOUTH
Brasso formation Transition
beds Cipero
formation
|
Pure Brasso microfauna
|
Brasso assemblages with sprinkling of Cipero facies
markers
|
Cipero assemblages with sprinkling of Brasso facies
markers
|
Pure Cipero fauna
|
Occasionally the transitional beds show faunal features
other than simple admixture. A case in point is provided by beds in lateral
transition between the Nariva and Cipero formations. These beds tend to contain
a fauna composed only of indeterminate arenaceous foraminifera and Nodosaria spp., although Nodosaria is never a dominant genus in either of the parent
facies. (Direct lateral passage from the Nariva to the marl facies without
intervention of the San Fernando/Brasso facies is a feature only locally
demonstrable but of general significance. It implies that the Nariva facies was
controlled by some factor independent of sea depth, a condition satisfied by
treating it as a turbid-water province.)
Résumé
By study of their microfaunas, individual samples of the sediments of the Naparima Basin can usually be diagnosed as to zonal position and palaeo-ecological status. The microfaunal zonation gives an accurate key to the complex structures of the basin, and when the structure is understood its effects may be eliminated before the palaeo-ecological data are applied to palaeogeographical reconstruction. Figure 4(A) shows such a reconstruction, in which microfaunal details have been used to supplement the megascopical data on which Figure 4-B was based.
In descriptive terms, littoral, sublittoral, and
archibenthic facies-provinces have existed side by side through the Upper
Eocene and Oligocene. The positions of the three provinces remained stable,
with only minor encroachment of one over another: this indicates steady
subsidence of the basin at a rate equal to the rate of accumulation of
sediments. A marginal turbid-water facies, attributed to rapid erosion and
re-deposition of the Central Range landmass, at times overstepped the
sublittoral facies and merged laterally into the archibenthic facies.
Supplementary note.
The Ste. Croix beds are shown on the diagrams, but they are not mentioned in
the text because they are peculiar to the Naparima Basin and have no bearing on
the general theme of this example. They are lenses of shallow-water sediments
enclosed in the deep-water Cipero marls and they owe their existence to local
upwarping of the floor of the basin.
[57]
EXAMPLE 2 A
basinal flank on which the rate of accumulation of sediments differed from the
rate of subsidence.
(Based
on the Borbón Basin of northwest Ecuador)
Present understanding of the palaeogeography of the Borbón
Basin was achieved in three stages. Reconnaissance field geology supported by
megafaunal evidence appeared to indicate widespread unconformity at a
mid-Oligocene level. Then zonation by planktonic foraminifera showed that over
much of the area time planes transect the ‘unconformity’, which is in fact a
surface of abrupt facies change. Finally the samples from more detailed field
surveys and test wells were analysed and gave a fairly exact understanding of
the depositional history.
[58]
The region is difficult of access, being covered by tropical forest, and the initial reconnaissance survey was limited to the courses of the main-rivers. By extrapolating across the unsurveyed areas a map was prepared which seemed to show an Oligo-Miocene group of sands, silts and orbitoidal limestones resting unconformably on an Eo-Oligocene group of marine shales. Around the margin of the basin angular unconformity was demonstrable and in central portions the projected plane of unconformity matched a sharp change from shales to coarse sands. The basic relationship then assumed is shown on Figure 5(A).
FIGURE 5: STRATIGRAPHY OF THE BORBÓN BASIN, ECUADOR
Figure 5(A): Initial interpretation based on
reconnaissance study of outcrops and fossils
Preliminary examination of the microfaunas of samples
collected on the reconnaissance survey appeared to confirm this interpretation,
as the assemblages in the marine shales (Zapallo, Pambil and Viche formations)
differed in kind and in preservational aspect from the faunas of the overlying
Angostura and Onzole formations. A routine analysis of the faunas was made with
a view to establishing microfaunal zones. To assist in regional correlation
special attention was paid to the planktonic foraminifera and it was found that
the sequence of Zapallo, Pambil, and Viche shales corresponded closely to the
zones of Hantkenina primitiva, Globigerina
concinna, G. dissimilis, Globigerinatella insueta, and Globorotalia fohsi as defined in Trinidad. Attention was next paid to
the presumed transgressive formations and the surprising discovery came to
light that these
sediments also fell within the zones of Globigerinatella
insueta and Globorotalia fohsi. This could only mean that the Angostura sands and Viche shales were synchronous but environmentally different deposits. The sharp lithological break between them was not a plane of unconformity but merely the physical expression of gradual encroachment of a littoral over a sublittoral facies. Figure 5-B depicts the revised concept of age relationships between the various formations.
Figure 5(B): Revised interpretation
incorporating zonation based on planktonic foraminifera
The reconnaissance survey was followed by more detailed
mapping Special efforts were made
to obtain samples along the Angostura/Viche and Onzole/Viche contacts. Test
wells provided subsurface material. Microfaunal analysis of the new material
showed many details, minor in themselves but of cogent value in interpretation
of the facies pattern. Points of this type were: observation of the restriction
of gerinella carapitana and Trifarina sp. to beds transitional between the Viche and the
Onzole; development of a zonule rich in Amphistegina lessoni at the outer fringe of the Angostura province;
abundance of Siphogenerina transversa group in the Viche only near its transitional contact with the
Angostura. Beds of the Globigerina concinna and Globigerinatella insueta zones are in unconformable contact around the edges of the basin; towards the centre of the basin highly foraminiferal shales of the two zones are separated by a band of ashy shales with a meagre fauna of long ranging (tolerant) forms; still farther out the two zones become part of a sequence of marine shales with no marked break in lithology or in benthonic microfauna. Collation of all these palaeo-ecological data against a time grid based on the foraminiferal zonation led to a detailed reconstruction of the facies pattern, depicted on Figure 5(C).
Expressed in terms of depositional history. Figure 5(C)
indicates the following sequence of events:
(1) A normal sedimentary basin existed in Upper Eocene and Lower
Oligocene time It is mostly
represented by marine shales (Zapallo and Pambil) but marginal conditions are
shown by orbitoidal reefs in the Playa Rica sands.
(2) At mid-Oligocene time an orogenic spasm raised and tilted
the margin of the basin. Volcanic activity at this time seems to be indicated
by offshore deposition of ashy shales and by temporary disappearance of the
rich sublittoral microfauna.
(3) General subsidence led to development of a marginal littoral
facies (Angostura) transgressive over eroded Lower Oligocene and older rocks.
Meanwhile normal sublittoral conditions were restored in the off shore areas,
as shown by the deposition of the Viche formation, ecologically similar to the
Zapallo and Pambil shales.
Figure 5(C): Interpretation based
on extended field studies, well evidence, and detailed micropalaeontology
(4) Locally the rate of subsidence exceeded the rate of
sedimentation, with the result that the sublittoral facies encroached over the
littoral facies.
(5) As the rate of subsidence dwindled the process was reversed.
A marginal facies consisting of foreset beds (Angostura) and topset beds
(Onzole) advanced steadily into the basin. By Lower Miocene time subsidence had
ceased and infill of the basin was completed by sediments of the Onzole type.
EXAMPLE 2-A Lateral
equivalence of two dissimilar formations inferred and later demonstrated.
(Based
on the Angostura and Viche formations of Ecuador.)
As a digression from the generalized study of the Borbón
Basin a very clearcut case of lateral equivalence of dissimilar formations is
here extracted.
The Angostura formation consists largely of coarse sand
characterized by an abundance of euhedral grains of various volcanic minerals.
Local variants are conglomerates and cream algal limestones. Shell beds occur
at certain levels. The micro-fauna consists of white foraminifera, among which
the most conspicuous are species of Valvulineria, Bolivina, Robulus, and Cibicides.
The Viche formation consists of massive uniform clay-shales
with few megafossils. The microfauna consists largely of cream to brown
foraminifera among which species of Cassidulina, Gyroidina, Epistomina, and the nodosarian genera are conspicuous along
with many others which barely appear in the Angostura assemblages.
Superficially these two formations have almost no features in common, yet a detailed study has shown them to be intimate lateral equivalents representing the littoral and sublittoral facies in a single sedimentary basin. The first clue to their possible equivalence was field evidence that they both lay between the Pambil and Onzole formations (see Figure 5). Confirmation was given by their content of planktonic foraminifera, both of them proving to include the zones of Globigerinatella insueta and Globorotalia fohsi. Direct evidence was later obtained by demonstration
of a facies boundary marked by intermingling of faunas and by development of
specialised faunules, such as the Siphogenerina transversa, Uvigerinella carapitana, and Amphistegina lessoni assemblages, not known elsewhere in the basin.
EXAMPLE 3 Cyclical
repetition of facies
(Based
on Miocene deltaic sediments in Trinidad.)
Probably the finest example of rhythmical or cyclical
repetition of a sequence of different facies is provided by the Pennsylvanian
cyclothems of Illinois, but their existence is demonstrated with little
recourse to micropalaeontology.
[59]
The Miocene deltaic sediments of southern Trinidad present a less impressive
example, but it is of some interest here because microfaunal studies aid in
understanding its lithological expression.
In the Naparima Basin the Oligocene Cipero marls were
followed by the Lower Miocene Lengua calcareous clay. The Lengua was, in
essence, a transitional unit which heralded and finally merged into a massive
fan of deltaic sediments advancing from the Orinoco drainage system. These
deltaic beds, the Cruse and Forest formations of current stratigraphy,
eventually filled and obliterated the Naparima Basin and were succeeded by
brackish- and fresh-water beds of the Morne l’Enfer and La Brea formations.
Superimposed on the main marine-to-nonmarine cycle is a series of sub-cycles, not equally developed in all areas. In the Fyzabad sector only one subcycle is readily recognizable within the Cruse formation but in other oil pools the same formation shows a triple subdivision. Figure 6 shows at the left (schematically and not to scale) the
Figure 6: Cyclical repetition of facies in the
Miocene of Trinidad
lithological and electrical logs of a well drilled through
the Lower Forest, Cruse and Lengua formations in a pool where the subcycles are
well defined. The thrice repeated sequence of colloidal clay, silt and porous
sand is clearly marked in the Cruse. The Lower Forest shows the basal clay
followed by the silt of a fourth subcycle but the pattern peters out in the
Upper Forest. Microfaunal analysis of cores from the Cruse and Lower Forest
shows three foraminiferal assemblages, each of which is practically confined to
one of the three types of sediment. The colloidal clays carry a rich fauna
composed almost exclusively of large robust species of the arenaceous genera Bathysiphon, Hormosina, Valvulina, Cyclammina, Haplophragmoides, etc. The silts are largely barren but occasional
layers are rich in tiny flimsy-tested arenaceous species of Haplophragmoides, Ammobaculites, Trochammina, Textularia, Valvulina, etc. The sands also tend to be barren but if a fauna is present it
consists of small calcareous species of Uvigerina, Buliminella, Nonion, Nonionella, Robulus, etc. The writer made some computations of relative frequency of these three faunules at different levels, averaged over a group of neighbouring wells, and found a definite relationship between faunal statistics and position in a subcycle. This is shown graphically on the right of Figure 6.
A local, non-regional cause for the repetition of facies
seems to be indicated by the consistency of each of the three faunules and by
the lateral variability of the cyclical pattern. The problem has not yet been
fully analysed but it appears probable that the colloidal clays represent
periodic isostatic adjustment in the form of subsidence balanced by extensive
extrusion of mudflow from the core of the Southern Range anticlinorium. On this
basis the silts, become foreset beds and the sands topset beds of fans of
deltaic sediments advancing after each subsidence. The incompetent Naparima
marls typically deform in a plastic manner under sedimentary or gravitational
load.
[60]
In the literature on the Mississippi delta, built out over a more competent
substratum, no clear parallel has been found to the Miocene subcycles of
Trinidad.
There is practical value in recognition that the repetition
of facies is a palaeo-ecological effect. For instance it prevents faulty
interpretation in which nonexistent faulting might be introduced and it
facilitates correlation between pools which differ in details of subsurface
lithology.
EXAMPLE 4 Facies-pattern
over a buried ridge
(Based
on examples in the Talara area of Peru)
In general the existence of a buried ridge is most simply
shown by isopach studies. In the example chosen, complex block-faulting has
reduced the value of uncompensated isopach studies, but microfaunal variations
within the overlapping sediments show a direct relationship to the original
buried topography.
During a mid-Eocene interval there was a phase of uplift,
block-faulting and strong erosion. The general effect was peneplanation but the
hard Pariñas sandstone, interbedded between thick shale formations, resisted
erosion and gave rise to numerous ridges. When subsidence led to marine
transgression by the Talara formation these ridges in the sea floor caused
development of a distinctive facies-pattern.
[61]
Figure 7 is a schematic representation of the distribution of facies in the Talara formation. Transgression is first indicated by the appearance of conglomeratic sands with an orbitoidal fauna (Discocyclina, Amphistegina). These beds give place to shallow-water silts with a fauna rich in
small foraminifera but limited as to species: only the five species “Trochammina” samanica,
Hopkinsina talarana, Buliminella
quemadana, Cassidulina cladra and Valvulineria perversa occur consistently and in abundance. Both units thin
towards the ridge, the silts usually overstepping the sands but often failing
to cover the crestal strip. The silts are followed by marine shales which are
appreciably thinner over the crest of the buried ridge than on its flanks. The
thinner, frequently sandy, shales
Figure 7: Facies-pattern over a buried ridge. (Based
on the Talara formation of Peru.)
over the crest carry a shallow-facies fauna consisting of a
flood of the single species Valvulineria duboisi with sparse specimens of Lepidocyclina and Operculinoides. In their thicker outlying parts the shales carry an
entirely different fauna, a rich highly diversified foraminiferal assemblage
including large species of Robulus,
Globigerina, Nodosaria, Pseudoglandulina, Bulimina,
etc., indicative of rather deep water. Faunal gradation is demonstrable between
the two extremes, the elements suggestive of deep water becoming scarcer
towards the ridge and the shallow water indices disappearing from the faunas in
the reverse direction. The deep-water assemblage grades upwards as well as
laterally, into a faunule of intermediate type indicative of shallowing by
sedimentary infill, a process which culminated with deposition of flaggy
sandstones conformably above the shales.
Recognition of the basic facies-pattern is of practical
importance in detecting subsurface faults. Two oil wells may both pass directly
from Talara shale down into Pariñas sandstone. In one of them the crestal
facies is well developed, hence a normal unconformable contact may be assumed.
In the other the deep-water facies is present but neither of the basal
transgressive facies appears below it, hence elimination by faulting of part of
the section appears probable.
EXAMPLE 5 A
piedmont facies
(Based
on the Danian Mal Paso formation of Peru)
A piedmont facies, also known as a flysch or dump-deposit
facies, is developed where large-scale structural activity has caused products
of erosion of an uplifted landmass to be deposited in concentrated fashion in a
neighbouring sea. The mountain-building phase of geosynclinal activity is
marked by piedmont facies such as the alpine Flysch or the Nariva and Chaudiere
formations of Trinidad. Piedmont facies may also develop in grabens and
half-grabens formed during taphrogeny, if erosion of the up-faulted blocks is
rapid; examples are the Mal Paso formation of Peru and the Socorro formation of
Ecuador. As an example for discussion the Mal Paso formation has been chosen
because micropalaeontology has played an essential role in demonstrating its
depositional history.
[62]
The depositional environment of the Mal Paso formation is
indicated by four inter-linked peculiarities, viz.:
(i) The
formation is covered by Tertiary rocks. Well evidence near Negritos shows an
almost rectilinear boundary between two provinces, in one of which the Mal Paso
thickness is of the order of 5000 to 8000 feet but in the other seldom exceeds
500 feet This condition is
suggestive of a buried fault. On physical evidence alone the faulting might
have ante-dated completion of Mal Paso deposition or it might have post-dated
it with heavy pre-Tertiary erosion on the upthrown side. Examination of
microfaunas indicates a zonule, rich in Eponides huaynai and Robulus rotulatus, which only occurs at the top of the Mal Paso and is
uniformly developed on both sides of the fault. This is clear evidence that
faulting occurred either before or during Mal Paso deposition, not later.
(ii) In
the downfaulted, but not in the upfaulted, province the base of the Mal Paso
contains a thick poorly sorted, heterogeneous conglomerate. This rests on Upper
Cretaceous shales of the Redondo formation. The Redondo is complete and not
appreciably eroded, since the same sequence of four foraminiferal zones based
on an evolutionary set of Siphogenerinoides
spp. is present below the Mal Paso as at the type locality some twenty miles
away. Yet a plentiful component in the basal Mal Paso conglomerate consists of
angular pebbles of soft shale, clearly not far travelled and shown by their
microfauna to come from all levels within the Redondo formation. The seeming
paradox is most simply explained by assuming that the conglomerate is derived
from erosion of the upthrown scarp. This assumption is supported by conditions
on the upthrown side of the fault, where the upper Mal Paso rests unconformably
on deeply eroded Redondo shale or on still older rocks, fragments of which also
occur in the conglomerate.
(iii) At its type locality (Pazul) the Redondo shale
passes transitionally upwards into the sandy Monte Grande formation. The latter
carries an impoverished microfauna but significant in it are the planktonic
foraminifera Globigerina rugosa and Gümbelina
globulosa. In the Mal Paso these two
species are present only in the shales just above the basal conglomerate. Hence
in a time sense the basal Mal Paso conglomerate, lying between the zones of Siphogenerinoides
parva and Globigerina rugosa, corresponds merely to an ephemeral transition. This
evidence accords better with a concept of abrupt graben-faulting and rapid
erosion than with the more usual explanation of a massive conglomerate as a
sign of erosion during a lengthy period of emergence.
(iv) Except for foraminifera the Mal Paso is
practically barren of any fossils. Foraminifera are abundant, but the fauna is
of a restricted type completely dominated by large arenaceous species of the
genera Bathysiphon, Pelosina, Haplophragmoides, Cyclammina, Spiroplectammina, Trochammina, etc. Such calcareous species as are present are
very scarce except in the Eponides huaynai-Robulus rotulatus zonule at the top of the formation.
A
controlling factor in the development of microfaunas consisting dominantly of
large arenaceous foraminifera, as inferred from numerous fossil examples, is
turbidity of the water in their domain. The mechanism may simply be that
turbidity inhibits photosynthesis, lack of photosynthesis precludes growth of
calcareous organisms, and absence of competitive calcareous foraminifera
stimulates the growth of the arenaceous genera. In the case of the Mal Paso,
dominance of the arenaceous genera corresponded to the phase of active erosion
and rapid deposition, concomitant with highly turbid water; the calcareous
forms only became common in the final phase when erosion on one side and
sedimentary infill on the other had reduced the topographic gradient across the
fault and had converted the graben into a normal marine basin.
Figure 8 depicts stages in the depositional history of the Mal Paso sediments, based on the foregoing discussion.
EXAMPLE 6 Effect
of regional subsidence on different facies-provinces
(Based
on the Pliocene sediments of Egypt)
Structural movements on a regional scale are reflected in
all the facies-provinces of a region. Epeirogenic uplift is marked by a general
shallowing of facies-type, subsidence by a deepening. Tilting may cause both
effects simultaneously. A point to stress is that vertical changes of
microfauna induced by the local effects of movement are liable to vary
considerably from place to place. Lack of index-species may prevent a
bio-stratigraphical approach, but the palaeo-ecological evidence of movement at
numerous localities provides a basis for correlation.
In Egypt three isolated sections reveal evidence of
post-Miocene subsidence. They are separated by wide areas denuded of Pliocene
sediments by subsequent erosion, so that direct correlation is not possible.
The three sections are entirely distinct, lithologically and faunally, but on
palaeogeographical grounds there seems to be no reasonable doubt as to their
correlation.
[63]
Figure 8: The depositional distory of the Mal Paso
formation in N. W. Peru
In the Western Desert and Libya the Miocene sediments are
fringing limestones with a shallow marine microfauna of Elphidium, Rotalia
and Discorbis and in the higher
layers the semi-littoral Borelis melo. Locally there are remnants of overlying beds, finer-textured
sediments with a much more diversified microfauna, indicative of definitely
deeper water.
Some 500 miles to the east, in the Nile valley at Cairo and
farther upstream, an incised valley in Eocene limestone is filled by Pliocene
beds. The microfauna is an assemblage of tiny ferruginous Buliminellas, Nonions,
Nonionellas, etc. strongly
suggestive of estuarine conditions. Drowning of the pre-Nile valley is
indicated.
The Gulf of Suez is a deep narrow rift filled with Lower
Miocene marine sediments followed by Middle Miocene evaporites. Some 250 miles
southeast of Cairo, in the southerly part of the Gulf, marine sediments are
known above the evaporites. They contain a microfauna which includes genera and
species-groups conspicuously different from any known in the Miocene of the
Gulf or in the Mio-Pliocene of the Mediterranean coast. It is tentatively
assumed that in post-Miocene subsidence a portal was opened corresponding to
the present Red Sea, and that the unfamiliar species are migrants by this route
from the Indian Ocean.
To reiterate, viewed factually the three sections have
almost nothing in common, but viewed palaeogeographically they provide evidence
of a widespread post-Miocene subsidence.
EXAMPLE 7 Organic
reefs (bioherms)
Fringing and barrier reefs, built up by colonial organisms
such as corals, algae or rudistids, have tended to form in shallow water
peripheral to large marine basins. If oil should be generated within the
basinal sediments it would tend to migrate updip towards the porous reefal
mass. Consequently, it is not surprising that some of the world’s most prolific
oilfields, such as the Middle East fields and more recent discoveries in Canada
and Texas, should be fossil reefs.
[64]
In North America the discoveries in Alberta have led to
vigorous investigation into the oil possibilities of other neglected or
undiscovered reefal trends. Interesting new methods are being developed for
making palaeogeographical maps which will reveal trends favourable to the
growth of bioherms. However, since most of this research is concerned with
Palaeozoic rocks micropalaeontology finds little application. In the Middle
East, in contrast, the reefs are of Upper Cretaceous and Tertiary age and a
study of microfaunal facies-variation has given the key to exact understanding
of the depositional history of the reefs. As the rocks involved are mostly
limestones, a technique has been developed for the recognition of facies-types
from their characteristics in thin section.
F. R. S. Henson, the protagonist of these studies in the
Middle East, has shown that, by noting the direction of interdigitation and
overlap of different facies, the status of a reef may be determined relative to
cycles of regression and transgression.
[65]
Henson recognizes the co-existence of five principal facies during the growth
of a reef. In order of encounter along a traverse from land to sea they are:
Facies Type
of microfauna
Littoral
clastic facies: variable,
not genetically related to the reef.
Back-reef
facies: an
assemblage of Miliolidae, Peneroplidae and Alveonellidae is typical; also some
Planorbulinidae, Rotaliidae, Amphisteginidae and Orbitolinidae may be present.
Reef-core
facies: dominantly
reef-building organisms (corals, algae, bryozoa, rudistids); foraminifera rare,
mostly sessile forms such as Rupertia
and Homotrema, with thick-shelled
Amphistegina and Rotalia.
Fore-reef
facies: foraminifera
large, robust and abundant; include (according to age) Nummulitidae,
Orbitoididae, Miogypsinidae, Peneroplidae, Alveolinidae, Rotaliidae,
Orbitolinidae and Lituolidae. Echinoid fragments are also plentiful.
Basinal
facies: a
normal marine assemblage of Globigerinidae and small benthonic foraminifera.
V. APPENDIX ON PLANKTONIC FORAMINIFERA
Possibly many genera of foraminifera are planktonic in the
early stages of the microspheric generation, but there are very few which
complete their life-cycle in the surface waters of the oceans. Despite the
small number of genera and species, the planktonic foraminifera are conspicuous
in both Recent and fossil sediments by reason of the countless millions of
individuals co-existent under favourable conditions. The Cipero marl of
Trinidad is a fossil Globigerina-ooze in
which the tests of planktonic foraminifera constitute from a third to a quarter
of the total bulk of sediment.
In the course of Cretaceous and Tertiary time there was a
steady advent and extinction of different planktonic species. Their mode of
life resulted in almost worldwide distribution in every type of marine facies.
Several genera, such as Gümbelina, Globotruncana, Globorotalia and Hantkenina, followed
evolutionary trends in the development of species. These factors render the
group of the utmost importance as precise indices of geological age.
[66]
This fact has been utilised in local studies, where
Range chart of genera of planktonic
foraminifera
complexity or monotony of facies has hindered zonation on
the basis of benthonic species, and it has been applied in regional correlation
in the mid-American countries and in direct correlation of the mid-American and
Middle East Tertiaries. There is scope for a concerted effort by
micropalaeontologists towards establishment of world-wide correlations, but
progress towards this end has been slow.
The chart indicates the geological ranges of genera known or
suspected to be planktonic in habit. As is evident from the chart, certain
generic combinations suffice to determine the age of an assemblage of
planktonic foraminifera: a Gümbelina-Globotruncana fauna is diagnostic for Upper Cretaceous age, for
instance, whereas an Orbulina-Globigerinoides fauna denotes the Upper Tertiary
or younger. By attention to species a still finer time-grid may be established,
as is indicated in the discussion of different genera which follows.
Globigerina
[67]
In the literature the speciation of Globigerina is in such a confused state that in recent years the
authors of many faunal papers have merely recorded the presence of the genus
and have not figured nor attempted to determine the different species present.
The late renowned J. A. Cushman, whose work was a stimulus to all students of
foraminifera, usually followed this practice, which is unfortunate for its
retardation of knowledge of the geographical and geological ranges of the
various species.
Most of the species of Globigerina have certain conspicuous features in common, such as
a globose test, inflated chambers and large ventral apertures into an open
umbilicus, and they are therefore less easy to separate than the diversified
species of benthonic genera such as Bulimina or Cibicides. Nevertheless close scrutiny will show that a normal post-Cretaceous
microfauna of deep marine facies contains five or six species of Globigerina, each of which is consistent in its special
characteristics. The chamber arrangement of Globigerina is a trochoid spiral
and a basic feature of each species is the form into which this spire has been
modified. In plan view it may be trigonal, quadrate, pentagonal, or polygonal.
TRIGONAL
QUADRATE PENTAGONAL
HEPTAGONAL
(G.
canariensis) (G.
venezuelana) (G. triloculinoides) (G.
altispira) (G.
cretacea var. eggeri)
In side view the spire may be flattened, moderately high, or
lofty, viz.:
DEPRESSED LOW LOFTY
(G.
compressa) (G.
concinna) (G.
bradyi)
In each species the individual chambers possess the same
shape—globular, reniform, appressed or digitate, as the case may be—and they
are arranged in a regular manner. The rate of increase of chamber size is a
very constant feature of each species and according as it is low, as in G.
venezuelana, or high, as in G.
triloculinoides, two species with a
basically similar spire may look entirely different. Certain species show
individual peculiarities such as a supplementary chamber bridging the apertures
in G. dissimilis, the presence of
extra chamberlets in G. venezuelana
and the bizarre adult chambers of G. fistulosa and G. digitata. It has been claimed that each species is consistent in the surface
density of pores visible in the shell under high magnification, and that
differentiation of species can be made on this basis alone.
[68]
The various features which distinguish one species of Globigerina from another only become marked in the adult test.
Juvenile specimens are much less easy to distinguish and may readily be
confused with related genera such as Globigerinoides and the orbulines unless growth-series are carefully
picked out.
The sequence of species of Globigerina in the geological column shows few evolutionary
features. This may indicate diffusion from various centres of the species which
successively, and usually abruptly, became dominant in the planktonic faunas.
Differential migration has certainly affected the geographical spread of the
genus, as is shown by examples such as the extinction of G. concinna in the mid-Oligocene of the Caribbean region but
persistence of the same species into the Miocene of the Mediterranean region.
Because of the lack of marked evolutionary trends the
establishment of life-ranges of the Globigerinas must be studied empirically. Foraminifera have been referred to the
genus from rocks as old as the Pennsylvanian (G. seminolensis)
[69]
,
Permian (G. permica)
[70]
and Jurassic (G. lobata)
[71]
but it is only in the Lower Cretaceous that forms are found which may fairly be
considered planktonic on morphological and palaeo-ecological grounds. These
Cretaceous species are almost all low spires of globular chambers, five to
seven per whorl. Only in the late Cretaceous are more globose forms seen, such
as G. mckannai
[72]
foreshadowing the dominance of this group in the Tertiary assemblages. In the
Eocene many species have a hispid or pustulose shell surface, a character of
scarce development in the post-Eocene Globigerinas. A finely cancellate shell surface is an almost
invariable feature of the Tertiary species and is related to an array of fine
radiating spines; whether this specialization was an evolutionary development
during the Eocene, or whether the habitually poorer preservation of Cretaceous
faunas has prevented its observation, is a subject not yet clarified.
Globigerinoides
[73]
This genus differs from Globigerina in the presence of supplementary
apertures on the dorsal side of the adult test, in addition to the normal
ventrally-placed globigerinid apertures. The genus was only separated from Globigerina by Cushman in 1927, by which time most of the known
species had already been listed under Globigerina. The type description employs the phrase “numerous
large supplementary apertures”, and in stratigraphical studies the word “large”
should be stressed because, although Globigerinas with loosely attached chambers have been recorded in
the Eocene, true Globigerinoides
with large lunate dorsal apertures do not appear until later in the Tertiary.
There is an abrupt and explosive development of such species, coincident with
the first appearance of orbuline genera, in the mid-Tertiary of regions as
widely separated as the mid-Americas, the Middle East and the Far East. In the
geological time-scale as locally applied the “Orbulina-surface” (LeRoy, 1948)
[74]
has been variously classified as from mid-Oligocene to mid-Miocene in age, but
it appears reasonable to claim that this faunal datum is a valid index of
contemporaneity and that the discrepancies arise from former acceptance of less
reliable indices.
The ancestral form of Globigerinoides is probably G. trilobus
[75]
,
from which arose the involute G. conglobatus
[76]
,
the lofty-spired G. rubrus
[77]
,
the deformed G. sacculiferus
[78]
and other basically similar species, most of which have persisted through the
Tertiary into Recent faunas.
Orbuline genera
A spherical test covered by radiating spines is ideally
suited to a micro-organism of planktonic habit. Many of the radiolaria are so
constructed. In the mid-Tertiary spherical Globigerinidae appeared in which the
main apertures were suppressed and a globular adult chamber almost completely
enveloped the trochoid embryo. Orbulina universa and Candorbulina universa
[79]
(usually separated by systematists, though possibly synonymous) rapidly
achieved world-wide distribution and have persisted unchanged from the Upper
Oligocene to the Recent. A more complex genus (Globigerinatella)
[80]
with sinuous chamberlets covering the sutures appeared in the Caribbean region
just before Orbulina but was
short-lived, as was the similar Lower Miocene genus Globigerinita
[81]
.
Still earlier, in the Upper Eocene, a species developed in which eleven or
twelve chambers are visible externally, arranged in a regular trochoid manner
but so shaped that they conform with the spherical contours of the test; this
is Globigerina mexicana
[82]
,
recorded from Mexico, the West Indies, Spain, Switzerland and the Middle East.
Nautiliform genera (Globigerinella, etc.)
Certain globigerinid species show development from a
compressed trochoid initial stage to a bisymmetrical nautiloid coil of adult
chambers. Species of this type appeared as early as the Lower Cretaceous (Globigerinella
cushmani)
[83]
.
In the Upper Cretaceous of Arkansas Biglobigerinella
[84]
is recorded, a genus in which the adult chambers tend to split into
symmetrically placed pairs. The genus Globigerinelloides
[85]
is another Upper Cretaceous variant typified by semi-evolute coiling. In the
Middle Eocene of Peru and Ecuador simple globose species of Globigerinella made a short-lived appearance. From late Oligocene
to Recent G. aequilateralis
[86]
has been widely distributed. Lack of any records of nautiliform species in the
Lower Eocene and Lower Oligocene suggests repeated evolution from trochoid
stock rather than persistence of an unbroken line of them since the Cretaceous.
On the basis of type figures Hastigerina
[87]
,
Thomson 1876, may be a prior synonym of Globigerinella, Cushman 1927
[88]
,
but the possibility needs to be checked by a comparison of types.
Hastigerinella
This is a planktonic genus which may be considered a
derivative from Globigerinella by
elongation of the adult chambers into club-shaped and finger-shaped forms. This
development started with some small Cretaceous species (H.
subcretacea
[89]
, H.
simplex
[90]
) and gave
rise to a group of Eocene species which are among the largest planktonic
foraminifera. In Peru and Ecuador an evolutionary series of species runs through
the Middle and Upper Eocene and Lower Oligocene but available data do not
suffice to establish the regional scale of the evolutionary pattern. In washed
residues specimens of Hastigerinella are usually found broken into individual
chambers.
Hantkenina
The basic form of this genus is a compressed nautiliform
coil of four to six chambers, each of which is prolonged radially or carries a
prominent radial spine. In the oldest species (subgenus Aragonella) the chambers are apiculate. These are followed by
species (subgenus Applinella) in
which a stout spine is present, centrally placed in the juvenile chambers but
tending to move nearer the anterior suture in adult chambers. In the next
youngest subgenus (Hantkenina s.
s.) the spines are more distinct, no longer related to radial prolongation of
the chambers, and are close to or enveloped by the anterior suture. End-forms
are the subgenera Hantkeninella.
which shows ontogenetic recapitulation of the Applinella-Hantkenina s. s. evolution, and Cribrohantkenina, an inflated form with a subglobular final chamber.
The apertures of successive subgenera show evolutionary development from a
simple arched slit through a tripartite slit with projecting lips to a complex
aperture of large subcircular pores.
The rapid evolution of Hantkenina, its demonstrably planktonic mode of life, and its
world-wide distribution make its subgenera valuable age-indicators. The oldest
species are Lower Eocene and the youngest are Upper Eocene.
[91]
The abrupt extinction of the genus shortly after the appearance of the highly
specialized forms H. bermudezi
and H. supersuturalis suggests
phylogerontism.
[92]
Records
exist of the usually Upper Eocene H. alabamensis in the basal Oligocene of the Gulf Coast, but
competent authorities have asserted that the specimens are allochthonous.
[93]
Globorotaliidae (Globotruncana, Globorotalia, etc.)
This family appears to have developed from Globigerina by the appearance of two peripheral keels (Globotruncana). The primitive Globorotalias evolved from Globotruncana by suppression of one keel and they gave rise in
their turn to more advanced forms, at first with an acute periphery but later
with no relic of their keeled ancestry.
Globotruncana has
been divided into an evolutionary sequence of species which in the successive
stages of the Cretaceous show an increasingly lofty spire and the development
of the double keel to maximum prominence in the lower Senonian, then reduction
of this feature in the upper Senonian. No Danian species are known.
[94]
The evolutionary pattern in Globorotalia is not so clear-cut, though several lines of
development may be recognized and are partly segregated under the names Turborotalia, Truncorotalia
[95]
,
Globorotalites, Rotalipora
[96]
,
Thalmanninella
[97]
and Cribrogloborotalia
[98]
.
During the long life-range of Globorotalia s. l. there has been an oscillation
in the dominant type. Non-umbilicate, ventrally conical forms, for instance,
are normal in Upper Eocene and Plio-Pleistocene faunas but seldom occur in
Oligo-Miocene faunas. Tumid forms with an open pit between ventral projections
of the chambers are characteristic of the late Cretaceous and Palaeocene.
Species with a hispid, spinose or papillate shell do not appear in the
post-Eocene faunas.
[99]
Gümbelininae
In the Upper Cretaceous of many parts of the world species
of the hetero-helicid genus Gümbelina
are found in great numbers. Their mode of occurrence is the same as Globotruncana and the genus appears to have been planktonic in
habit.
[100]
The basic form of Gümbelina is a simple biserial set of globular chambers. Smooth simple forms
persisted during the Lower and Middle Cretaceous but in the Santonian they
began to show specialization. During the Campanian and Maestrichtian highly
specialized forms became conspicuous, some possessing heavily striate or costate
tests, others developing an angular, quadrate cross section. Still others, by
the growth of additional adult chambers, led to the appearance of the
multiserial genera Ventilabrella,
Pseudotextularia and Planoglobulina. The specialized species and genera died out
abruptly at the end of the Maestrichtian and in Danian and younger faunas only
small, simple Gümbelinas are found, never in abundance.
[101]
Pulleniatina
This genus is distinguished from other Globigerinidae by the
non-cancellate, almost porcellanous, shell of its adult chambers. The chambers
follow a quadrate coil, non-umbilicate and strongly inflated ventrally. The
adult chambers tend to encroach successively farther over the flattened dorsal
surface. Usually the genus is regarded as monotypic, based on P.
obliquiloculata (Parker and Jones, 1865)
but study of the literature
[102]
suggests that Globigerina inflata
is synonymous and hence that the genotype should be designated P.
inflata (d’Orbigny, 1839)
[103]
.
It is a geologically young genus, first appearing in the late Miocene and
persisting into present day faunas.
Sphaeroidinella
This is an involute development from Globigerina in which
the later chambers envelop the earlier ones. Externally three chambers are
usually visible in a trochoid coil so modified that the test has a smooth ovoid
outline. The aperture is a broad slit corresponding to the umbilicus of Globigerina. The shell surface is cancellate in the juvenile
stage but tends to become vitreous in mature individuals. The genus is found in
Recent faunas and dates back to the Lower Miocene.
[104]
VI. APPENDIX ON STATISTICAL METHODS
Statistical analysis of microfaunas has found three main
applications in micropalaeontology, viz.:
(1) Determination of
palaeo-ecological status of fossil faunas by study of the frequency-ratios of
species and genera.
(2) Study of statistical variations of
fauna in relation to stratigraphic level.
(3) Checking the validity of species.
(1) Frequency-ratios of
species and genera in relation to their environment
Analysis of Recent marine microfaunas has shown a definite
variation in the dominance of different foraminiferal families in samples from
different habitats. Tentative assumption that similar variations in fossil
microfaunas implied similar facies-control resulted in plausible
palaeogeographical reconstructions in accord with other evidence. Consequently
the statistical approach has become a recognized means of palaeo-ecological
determination.
The simplest method is to make a systematic analysis of a
chosen micro-fauna and then to express the number of species per family as a
percentage of the total number of foraminiferal species recorded. A comparison
of the figures with those recorded among Recent faunas gives an indication of
the depth and temperature favoured by such an assemblage.
The method above is simple but it gives a distorted
impression of any fauna containing floods of certain facies-controlled species.
In the Cipero marl of Trinidad, for instance, the Lagenidae provide one-fifth
of the number of foraminiferal species and the Globigerinidae only
one-twentieth, but by number of individual specimens the Lagenidae form less
than one-thousandth of the fauna whereas the Globigerinidae form over
nine-tenths. To allow for such factors it is desirable to make counts or computations
from which the frequency-ratios of specimens, rather than species, per family
may be assessed.
In a mixture of components of which the true mean
proportions vary between 95 percent and .0001 percent of the whole it is
difficult to obtain reliable figures for the minor ingredients on a basis of
actual counts. If c samples are counted and P1, P2, P3,…Pc are the individual
percentages recorded for a certain component, then the standard error of Pm, the arithmetic mean
percentage, is:
If small samples are counted and Pm is low, then the probable error is too
high to be acceptable. If large samples are counted the probable error is
reduced but the process becomes very lengthy. The writer therefore prefers a
simple system of computation in which the samples are analysed, each species is
marked by a symbol as being scarce, common or abundant. Values of 1, 10 and 100
are ascribed to these symbols and the frequency ratios of different families
are based on summation of the figures allotted to their species. Intermediate
values such as ‘fairly common’ (5) and ‘almost abundant’ (50) may be
introduced. The method is subjective but gives consistent results when applied
by an experienced worker. It is advisable to check by some actual counts the
ratios which correspond to one’s own personal assessment of the relative
abundance of different species: modifications such as 1:4:8:40:100 or
1:6:20:100:150 may be found more accurate than the scale of 1:5:10:50:100 used
as an example.
The probable environment of a fossil microfauna is
determined by comparing its statistical composition with those of Recent
assemblages (Lowman, Natland, Norton, Parker, Phleger, etc.; see bibliography).
[105]
A sequence of samples may be used to show a trend of changing facies.
(2) Statistical variations related to stratigraphical
level
Gradual vertical changes in a fossil fauna may have been
caused by a slow palaeo-ecological change or by evolutionary change of its
components. An ecological cause may be demonstrable by detailed statistical
analysis of a set of samples, as outlined in the foregoing section. A simpler
method is to use the “oscillation chart” of Israelsky.
[106]
The basis of this approach is selection of a few microfossil species which are
common in one but scarce in others of a series of intergradational
facies-variants. Israelsky used a
Figure 9(A): Frequency ratios of selected
ecologically restricted species.
well in Louisiana as an example and chose five groups of
calcareous foraminifera as indices, including:
Group I
(brackish water)
‘Rotalia’ beccarii;
Elphidium spp.
Group II
(shallow marine)
Quinqueloculina spp.; Virgulinella spp.
Nonion scapha-group; Nonionella spp.
Buliminella curta; Reussella spinulosa
Group III
(intermediate)
Discorbis spp.; Eponides antillarum
Cibicides concentricus
Group IV
(intermediate)
Robulus spp.; Amphistegina lessoni
Cibicides carsteni
Group V
(fairly deep marine)
Cancris sagra; Lagena
spp.
Siphonina sp.; Casidulinidae
Gyroidina soldani-group
For analytical purposes a unit volume of sediment is taken
from each of a set of equally spaced samples. After processing, the number of
specimens of each index species in each sample is counted, all other species
being ignored. The figures are totalled for each of the five groups, expressed
as percentages and plotted against a depth scale.
Fig 9(A) (after Israelsky) gives the idealized form of an
oscillation chart showing a point of maximum subsidence between two emergent
phases. Fig. 6, used earlier to demonstrate cyclical repetition of facies in
Trinidad, is another example. Use of these charts is clearly valid for
correlation of dissimilar well sections by assuming that points of maximum
subsidence or of corresponding cyclical position lie on a time-plane—see Fig. 9-B
(also after Israelsky). They might also be applied to recognition of
unconformities or faults in cases of faunal discontinuity, such as Group-I and
Group-V faunas in contiguous layers.
Figure 9(B): Hypothetical
dip-section showing the levels at which different faunules are dominant.
The heavy dashed line joins points of deepest facies
in-.the wells and is valid for time correlation. The more obvious correlation
along the oblique bands of uniform facies is not valid in a chronologic sense.
(adapted after Israelsky 1949: American Association of Petroleum Geologists,
Bulletin Bull., XXXIII, pp. 95, 98).
Frequency-ratios may be applied to the zonation of thick
uniform sections. The Miocene Globigerina-marls
of the Gulf of Suez are a case in point, only divisible into two somewhat
arbitrary zones on the basis of species-ranges. Tromp has shown a trend of
change in the frequency-ratios of different globigerinid genera, Globigerina becoming commoner while Globigerinella and Globigerinoides become scarce in the younger beds.
[107]
The figures are tabulated below:
Proportion per
genus of total globigerinid fauna
Lower Middle Upper Basal
Miocene Miocene Miocene Pliocene
Globigerina 43
± 2% 75
± 15% 80
± 17% 91%
Globigerinella 24
± 5% 11
± 9% 10
± 8% 1%
Globigerinoides 33
± 7% 15
± 7% 10
± 5% 7%
Orbulina <2% <2% mainly<2% 1%
The method has been successfully applied in well
correlations but its value in routine studies is offset by the laborious
process of counting: Tromp mentions a count of 1,492,416 Globigerinidae in a single
100-gram sample.
The
evolutionary change of a species may be discernible without recourse to
detailed measurement. An example is the change from the thickset Bulimina
jacksonensis to the slender B. sculptilis at the Eocene/Oligocene boundary in the Caribbean
region. However, measurements of specimens from successively higher layers may
reveal an almost imperceptible change in the mean form and thus provide a
method of fine zonation. The diagram below shows the variations in length of Bolivina
marginata and B. m. multicostata in successive zones of the Miocene of Florida.
[108]
Mean, minimum and
Length in millimeters
BOLIVINA
MARGINATA BOLIVINA
MARGINATA MULTICOSTATA
maximum lengths all show a tendency to reduce with
decreasing age. (These particular data are open to criticism because of the
small number of specimens counted and failure to separate microspheric and
megalospheric individuals, but they serve for an example.)
Hofker has used a statistical approach to evolutionary
subdivision of the family Globigerinidae. He states: “A very reliable and,
within a given species, very constant character is the porosity of the test.
Not only is the nature of the pores very constant within a given species but
also, in a successive series, the pores are often much finer in the
geologically older forms than in the more recent ones. In order to have a clear
and accurate way of expressing the nature of the porosity in different species
I have invented the ‘pore-index’. A drawing, as accurate as possible, is made
of the surface of the test of some of the last-formed chambers, viewed in a
clarifier (rhicinus oil) under a magnification of about 500x. A one-centimetre
square opening is cut in a thick piece of paper, and this square is
superimposed on the drawing at several places in order to obtain an average
count of the pores found within this area. The average number of pores is
recorded and their average diameter is added to the expression. The nature of
the porosity is indicated in the following manner: 15-2, 500´. The first figure represents the average
number of pores per sq. cm.; the second gives the average diameter of the pores
in mm., both at a magnification of 500x.”
[109]
No comments by other students on the validity of Hofker’s pore-index have yet
been noted, but it appears to offer a statistical basis for classification of
the stratigraphically important but systematically difficult family
Globigerinidae.
Bolli has shown a relationship between stratigraphical level
and the ratio of dextrally coiled to sinistrally coiled individuals of certain
foraminifera.
[110]
In general
he notes that in the oldest assemblages the two forms are present in equal
amounts, but each species tends to select either dextral or sinistral coiling
so that in the younger assemblages only a few individuals differ from the rest.
He also notes cases of abrupt change in the dominant direction of coiling, a
curious phenomenon which, though difficult to explain, can be applied in zonal
studies.
Carter, working on foraminiferal faunas in the English Pliocene,
has shown that the bulk-distribution of foraminifera shows the same pattern of
frequency-fluctuation as is shown by sieve-analysis statistics of the inorganic
sediments.
[111]
The
implication of this parallelism is that the bulk of the microfauna is exogenous.
However, certain individual species of foraminifera show an independent
frequency-pattern and these are presumed to be indigenous elements of the
fauna.
[112]
(3) Statistical checks on the validity of species
The concept of a fossil species dates back through the
centuries to a time when fossils were regarded as individual curiosities,
variously held to be relics of the Flood or the Creation or mere tricks of
Nature.
[113]
As
palaeontology became a systematic study the status of fossils as the remains of
once-living organisms was established and eventually a belief in their
evolutionary development became general. Nevertheless, fossil species continued
to be defined in morphological terms and specimens were identified on a
subjective basis of idiomorphy with designated types.
[114]
Only in recent years has the subjective approach to
speciation been challenged The
basis for the challenge is the zoological definition of a living species as an
interbreeding natural population. A natural population is not a suite of living
idiomorphs: the adult individuals vary in size and shape and the individuals of
different growth-stages vary markedly in form. The human male gives a
sufficient example, the height of adults varying about a mean of five to six
feet between gerontic extremes near three and nine feet, while the head-to-body
ratio of the infant is much higher than that of the adult. A population can be
defined in statistical terms based on the mean dimensions and the degree of
variability of the component parts of its individuals. Small samples may be
compared statistically and the probability of their derivation from the same
species (population) may be assessed objectively. The methods and formulas are
those normal in the study of random samples, but care must be taken to study
different growth-stages and sexual variants separately.
Figure 10: Species-concept diagrams
It is reasonable to claim that an objective definition of a
fossil species is preferable to the subjective method of comparison of specimens
but in practice certain difficulties arise. One is that it cannot be
established that certain fossils were actually or potentially, interbreeding;
another is that non-preservation, current sorting, palaeo-ecological factors or
incomplete collecting may render statistical analysis of a collection
non-representative of the original population. A more philosophical difficulty
is that a species must be regarded as a unit in the stream of life. A
population is a temporary cross-section of this continuum and the
neo-zoologists are, in a sense, fortunate in having only one cross-section to
consider. The problem of the palaeontologist is expressed graphically on Fig.
10 where populations of an evolving species are indicated at hemeras T1, T2, T3, etc., in geological
time. The T1- and T8-populations are
completely distinct and would be determined by subjective comparison as
different palaeontological species, whereas the T4- and T5-groups scarcely differ. The basic problem is subdivision
(classification) of a continuum into discrete portions: the process is
unnatural but it is a human failing that tabulated facts and figures are more
readily comprehensible and transmissible than abstruse and nebulous
descriptions. All that need be stated here is that a quantitative statistical
method is better justified than the usual qualitative subjective approach to
the definition of fossil species.
The foregoing remarks apply to fossils in general. As
applied to micro-fossils in particular, one point which stands out clearly is
that this group is ideally suited to statistical analysis. Rich populations
need no special search and it is a simple matter to obtain a sequence of them
in known stratigraphical order. There are, however, very few papers extant in
which a correct statistical approach has been made to speciation of
microfossils. In classifying the Fusulinidae various dimensions and ratios have
been used but multivariate analysis of the apparently different species Triticites
collus and T. caccus has indicated that they do not differ significantly:
this seems to imply over-fine splitting of species within the family. Study of
ostracods has given statistical confirmation of the assumption that a
discontinuous series of size variants represents successive moult-stages in a
single species. Scatter diagrams have been used to show different species of a
foraminiferal genus within a fauna (though the possibility of dimorphism does
not appear to be eliminated in some of the examples seen). A detailed attempt
was made to employ statistics in defining several species of Elphidium from Pacific Coast faunas, but the conclusions have
since been declared invalid because of faulty mathematical technique.
[115]
Negative evidence of the importance of statistical control
of the designation of species lies in the over-elaborate subdivision of most
foraminiferal genera. Nonion, for
instance, is a simple nautiloid genus of which the species vary mainly in the
ratio of maximum to minimum diameters, the ratio of axial thickness to mean
diameter, the number of chambers and the bluntness of sharpness of the
periphery. If the species of Nonion
are treated as mere geometrical variations about a mean form some fifty forms
may be visualized, but at least a hundred and fifty have been described in the
literature. Similarly with Quinqueloculina a generous estimate of eighty variants can be visualized, based on
length-to-breadth ratio, cross-sectional outline and size, but some two hundred
smooth species have been described: as late as in 1949 seven species were
described as new from a single Eocene fauna in Louisiana.
[116]
* * * *
* * *
* *
*
VII. ANNOTATED BIBLIOGRAPHY
In this section references to the literature are given in an
order corresponding to the arrangement of the preceding text. [Ed: In this
text digitized in 2003, notes below relate to endnote numbers in the text. The
original, typewritten text provided only page references in the bibliography.
We have retained relevant notes and made reference to particular text; any
inaccuracy is ours. Please inform us.] The
list is not exhaustive but it includes the source-references of specific points
cited and representative general references many of which include
bibliographies. Most of the papers cited are American, a fact partly due to the
writer’s limited library facilities in Peru but nevertheless indicative of the
great volume of micropalaeontological literature sponsored, directly or
indirectly, by the American oil industry.
† Ref: no objection to
use, Wallie Rasmussen, Exxon-Mobil legal department, 23 Nov 2002.
[1] R. D. Reed,
American Association of Petroleum Geologists, Bulletin vol. 15,
pp. 731-754. 1931.
C. Croneis, American Association of Petroleum Geologists, Bulletin vol. 25,
pp. 1219-1225. 1941.
[2] 1928: J. J. Galloway,
Journal of Paleontology vol. 2, pp. 225-227.
1932: F. Lahee, Geological Society of America, Bulletin vol. 43, p. 960.
1933: R. S. Bassler, Geological Society of America, Bulletin, vol. 44,
pp. 269, 279, 284.
1938: J. A. Cushman, Geological Society of America, Bulletin vol. 49,
pp. 363-365.
1941: C. Croneis, American Association of Petroleum Geologists, Bulletin vol.
25, pp. 1243-1250.
1946: J. R. Sandidge, American Association of Petroleum Geologists, Bulletin
vol. 30, pp. 1088-1094.
1946: C. Stock, Geological Society of America, Bulletin vol. 57,
pp. 320-322.
1947: H. V. Howe, American Association of Petroleum Geologists, Bulletin vol.
31, pp. 713-730.
[3] H. E. Minor and
M. A. Hanna in “Stratigraphic type oilfields”, published by American
Association of Petroleum Geologists, 1941. pp. 600-640.
[4] H. G. Schenck and
B. C. Adams, Journal of Paleontology vol. 17, pp. 554-5. 1943.
M. F. Glaessner, “Principles of Micropalaeontology” by M. F. Glaessner,
Melbourne University Press, pp. 35-51, 1945.
L. W. LeRoy and H. M. Crain, “Subsurface Geologic Methods” by L. W. LeRoy and
H. M. Grain, Colorado School of Mines, 1st Edition., pp. 85, 86, 1949.
[5] F. H. Lahee,
“Field Geology”, 4th ed., chapter 14. McGraw-Hill Co., Inc., New York. 1941.
[6] L. W. LeRoy and
H. M. Crain, “Subsurface Geologic Methods” by L. W. LeRoy and H. M. Grain,
Colorado School of Mines, 1st Edition.,
pp. 296-298. 1949
[7] W. Wetzel, Erdöl
und Kohle, vol. 3, no. 5, pp. 212-214. Hamburg; 1950.
[8] K. Zittel, ed. C.
R. Eastman, “Text-book of Palaeontology”, Macmillan & Co., London 1927.
M. F. Glaessner, “Principles of Micropalaeontology” by M. F. Glaessner,
Melbourne University Press, pp. 8-31. 1945.
L. W. LeRoy and H. M. Grain, “Subsurface Geologic Methods” by L. W. LeRoy and
H. M. Grain, Colorado School of Mines, 1st Edition., pp. 58-84, 1949.
[9] Calcareous
algae:
J. H. Johnson, Journal of Paleontology vol. 19, pp. 350-354. 1945.
Calpeonellidae:
G. Colom, Sociedad Española de Historia Natural, Boletín, tomo 34,
pp. 379-388. Madrid, 1934.
Charophytes:
R. E. Peck, Journal of Paleontology vol. 8, pp. 83-119: 1934.
Foraminifera:
J. A. Cushman, “Foraminifera: their classification and economic use”, 4th ed.,
Harvard University Press, 1948.
B. F. Ellis and A. R. Messina, “Catalogue of Foraminifera”, American Museum of
Natural History, New York 1940 et seq.
J. J. Galloway, “A manual of foraminifera”, Bloomington, Ind., 1933.
M. F. Glaessner, “Principles of Micropalaeontology” by M. F. Glaessner,
Melbourne University Press, pp. 55-93. 1945.
Megafossil fragments:
H. L. Geis, Journal of Paleontology vol. 10, pp. 427-448. 1936.
J. S. Smiser, Journal of Paleontology vol. 5, pp. 293-295. 1931.
Journal
of Paleontology vol. 7, pp. 123-163. 1933.
Ostracods:
C. L. Cooper, Journal of Paleontology vol. 16, pp. 764-776. 1942.
Otoliths:
R. B. Campbell, Journal of Paleontology vol. 3, pp. 254-279. 1929.
Radiolaria:
E. Aberdeen, Journal of Paleontology vol. 14, pp. 127-139. 1940.
[10] An example is the
presumed appearance of Indian Ocean elements in a microfauna of Mediterranean
stock in the Pliocene of Egypt. See R. M. Stainforth, Journal of Paleontology
vol. 23, pp. 420,421. 1949.
[11] G. Rittenhouse,
Geological Society of America, Bulletin vol. 54, pp. 1725-1780. 1943.
[12] Orbitoidal
foraminifera:
J. J. Galloway, Journal of Paleontology vol. 2, pp. 45-69, 1928.
(This paper is at fault in not making due allowance for known age-ranges. For
example the Cretaceous Lepidorbitoides and Pseudorbitoides are
classed as descendents of the Eocene Helicolepidina.)
R. W. Barker anti T. F. Grimsdale, Journal of Paleontology vol. 10,
pp. 231-247. 1936.
Nummulites:
W. á Wengen, Hendelingen van het vierde Nederlandsch-Indisch
Natuurwetenschappelijk Congres, pp. 448-466. Java, 1926.
Bulimina spp.
B. C. Adams, 6th Pacific Science Congress, Proceedings vol. 2,
pp. 665-670. University of California Press, 1940.
Planktonic foraminifera:
See under Section V below. [Ed: notes 67 et seq.]
[13] C. S. Piggott and W. D.
Urry, Geological Society of America, Bulletin vol. 53, pp. 1187-1210.
1942.
[14] Symposium,
International Geological Congress, Report of 18th Session, part 12. London,
1950.
[15] F. G. Snyder and H. E.
Wheeler, Journal of Geology vol. 55, pp. 146-159. 1947.
M. Gignoux, “Géologie stratigraphique”, pp. 25, 502, 530. Masson et Cie.,
Paris, 1950.
A. A. Olsson, Bulletins of American Paleontology, vol. 17, no. 62, p. 11
etc. 1930.
(Olsson wrongly correlates the basal Talara unconformity—Middle Eocene—of the
taphrogenic province of western Peru with the Jacksonian transgression—Upper
Eocene—of the Andean geosyncline.)
[16] P. H. Kuenen, “Marine
Geology”, pp. 532-551. Wiley & Sons, New York, 1950.
[17] R. M. Kleinpell,
“Miocene Stratigraphy of California”, p. 98. Published by Amer. Ass. Petr.
Geol., 1938
[18] W. J. Arkell, “The
Jurassic system in Great Britain”. Oxford University Press, 1933.
[19] A. M. Davies, “Tertiary
faunas”, vol. 2, p. 159. Murby &c Sons, London, 1934.
[20] H. G. Schenck, American
Association of Petroleum Geologists, Bulletin vol. 19, p. 529. 1935.
[21] H. D. Hedberg,
Geological Society of America, Bulletin vol. 48, p. 1976. 1937.
(A pertinent statement, too lengthy to quote in full, is made by M. Kay,
American Association of Petroleum Geologists, Bulletin vol. 31,
pp. 163-166. 1946.)
[22] 1912. C. J. Maury,
Acad. Nat. Sci. Philadelphia, Jour. vol. 15, pp. 28-31.
1928. R. A. Liddle, “The geology of Venezuela and Trinidad”, pp. 223-225.
Fort Worth, Texas.
1929. C. J. Maury, Journal of Geology vol. 37, p. 177.
1934. H. W. Shimer, Geological Society of America, Bulletin vol. 35,
p. 909.
1935. C. J. Maury, Science, vol. 82, no. 2122, p. 192.
1935. C. Schuchert, “Historical geology of the Antillean-Caribbean region”,
p. 701. Wiley & Sons, New York
1938. H. G. Kugler, Ministerio de Fomento, Boletín de geología y minería, tomo
2, nos. 2-4, English ed., pp. 204-224. Caracas, Venezuela.
[23] C. Schuchert, 1935, as
above.
[24] F. A. Schilder,
Schweizerischen Päleontologischen Gesellschaft, Abhandlungen, Band 62. 1939.
(An attempt to recognize evolutionary trends in the cypraeid molluscs, at fault
due to earlier confusion of Oligocene and Miocene limestones in Trinidad.)
[25] W. P. Woodring and T.
F. Thompson, American Association of Petroleum Geologists, Bulletin vol. 33,
pp. 223-247. 1949.
[26] J. E. Eaton, American
Association of Petroleum Geologists, Bulletin vol. 15, pp. 367-384. 1931.
[27] Symposium, Journal of
Paleontology vol. 23, pp. 145-160. 1933.
[28] J. B. Dorr, Journal of
Paleontology vol. 7, pp. 432-438. 1949.
E. S. Franklin, Journal of Paleontology vol. 18, pp. 301-319. 1944.
(For comments on these papers see H. H. Renz, Geological Society of America,
Memoir 32, pp. 86, 87, 1948 and R. M. Stainforth, Journal of Paleontology
vol. 22, pp. 144, 145. 1948.)
[29] J. M. Weller et al. Journal of Paleontology vol. 21, pp. 570-575.
1947.
vol.
22, pp. 264-269. 1948.
vol.
23, pp. 677-679. 1949.
[30] I. H. Cram, American
Association of Petroleum Geologists, Bulletin vol. 29, p. 861. 1945.
[31] A. M. Davies, “Tertiary
faunas”, vol. 2, map (frontispiece) and chapter 1. Murby & Sons, London,
1934
Symposium, “Sedimentary facies in geologic history”, Geological Society of
America, Memoir 39. 1949.
L. W. LeRoy and H. M. Crain, “Subsurface Geologic Methods” by L. W. LeRoy and
H. M. Grain, Colorado School of Mines, 1st Edition., pp. 28-40. 1949.
W. H. Twenhofel, Journal of Paleontology vol. 8, pp. 456-468. 1934.
T. L. Bailey, Geological Society of America, Bulletin vol. 46,
pp. 489-502. 1935.
R. M. Stainforth, American Association of Petroleum Geologists, Bulletin vol.
32, pp. 1313-1318. 1948.
A. C. Ellisor, American Association of Petroleum Geologists, Bulletin vol. 24,.
pp. 435-475. 1940.
S. W. Lowman, American Association of Petroleum Geologists, Bulletin vol. 33,
pp. 1944-1947. 1949.
[32] H. C. Urey et al.,
Geological Society of America, Bulletin vol. 62, pp. 399-416. 1951.
[33] “Albatross” (U. S. A.)
J. M. Flint, U. S. National Museum, Report for 1897, pp. 249-349. 1899.
J. A. Cushman, idem, Bulletin 161, parts
1-3. 1942.
“Challenger”
(Great Britain)
Report on the scientific results of the voyage of H. M. S. “Challenger” during
the years 1873-76. H. M. Stationery Office, London, 1884.
(separate volumes for each phylum)
“Meteor”
(Germany)
Deutschen Atlantischen Expedition aud dem Forsungs- und Vermessungsschiff
“Meteor”, 1925-27; Wissenschaftliche Ergebnisse. Berlin and Leipzig.
[34] Belgium:
J. A. Cushman, Institut Royal des Sciences Naturelles de Belgique, Mémoire 111.
1949.
England:
E. Heron-Allen and A. Earland, Royal
Microscopical Society, Journal for 1911, pp. 436-448. 1911.
(a summary of several earlier papers)
Scotland:
E. Heron-Allen and A. Earland, Linnéan
Society, Transactions, 2nd series. Zoology, vol. 2, part 13, pp. 197-300.
1916.
North Atlantic:
F. B. Phleger, Geological Society of
America, Bulletin vol. 50, pp. 1395-1422. 1939
Geological
Society of America, Bulletin vol. 53, pp. 1073-1098. 1942
West Atlantic:
F L Parker, Museum of Comparative Zoology,
Bulletin, vol. 100, no. 2, pp. 213-241. Cambridge, Mass., 1948.
Antillean-Caribbean region:
R. D. Norton, Scripps Institute of
Oceanography Bulletin, Technical series, vol. 2, no. 9. 1930.
S. W. Lowman, American Association of Petroleum Geologists, Bulletin vol. 33,
pp. 1950-1963. 1949.
P. J. Bermudez, Sociedad Cubana de Historia Natural, Memorias, vol. 19, no. 3.
1949.
P. Bronnimann, Naturforschungs Gesellschaft, Verhandlungen. Band 60,
pp. 180-185. Basel, 1949.
East Pacific:
M. L. Natland, Scripps Institute of
Oceanography, Bulletin, technical series, vol. 3, no. 10. 1933.
J. A. Cushman, op. cit., vol. 1,
no. 10. 1927.
North Pacific:
J A. Cushman, U. S. National Museum,
Bulletin 71, parts 1-6. 1910-1917.
F. W. Millett, Royal Microscopical Society, Journal, numerous short papers.
1898-1904.
Australasia:
W. J. Parr, Royal Society of Victoria,
Proceedings, vol. 44, pp. 1-14, 218-234. Melbourne, 1932.
East Africa:
E. Heron-Allen and A. Earland, Zoological
Society, Transactions, vol. 20, part 12, pp. 363-383; part 17,
pp. 543-794. London, 1914-1915.
Mediterranean Sea:
H. Sidebottom, Manchester Literary and
Philosophical Society, Memoirs and Proceedings, vols. 48-54, many short papers.
1904-1910.
South Atlantic:
E. Heron-Allen and A. Earland, Royal
Microscopical Society, Journal, series 3, vol. 49, pp. 102-108, 324-334;
vol. 50, pp. 38-45; vol. 52, pp. 253-261. London, 1929-1932.
Antarctica:
F. Chapman, W. J. Parr, et al, Australasian
Antarctic Expedition 1911-1914, Scientific Reports, Series C, Zoology and
Botany. Sydney 1937.
W. J. Parr, B. A. N. Z. Antarctic Research Expedition, 1929-1931: Reports,
series B, vol. 5, pt. 6, pp. 233-392. 1950.
Brackish-water provinces:
H. D. Hedberg, Journal of Paleontology vol.
8, pp. 469-476, 1934.
H. Hiltermann, Erdöl und Kohle, vol. 2, pp. 4-8. Hamburg, 1949.
[35] F. B. Phleger and W. A.
Hamilton, Geological Society of America, Bulletin, vol. 57, pp. 961-963.
1946.
E. S. Deevey, American Journal of Science, vol. 237, pp. 691-724. 1939.
L. L. Ray, Geological Society of America, Bulletin vol. 51, pp. 1851-1918.
1940.
[36] [Ed: see also
general references given under notes 8 and 9.]
W. G. Woolnough, American Association of Petroleum Geologists, Bulletin vol.
26, pp. 765-792. 1942.
D. Andrusov, Práce Státneho Geologického Ustavu sosit 25, pp. 132-135.
Bratislava, 1950.
[37] [Ed: see also
general references given under notes 8 and 9.]
H. B. Ward and G. C. Whipple, “Fresh-water biology” , chapter 24, Wiley &:
Sons, New-York, 1945.
[38] [Ed: see also
general references given under notes 8 and 9.]
H. D. Hedberg, Journal of Paleontology vol. 8, pp. 469-476. 1934.
[39] [Ed: see also
general references given under notes 8 and 9.]
P. J. Bermudez, Sociedad Cubana de Historia Natural, Memorias, vol. 19, no. 3,
p. 315. 1950.
[40] [Ed: see also
general references given under notes 8 and 9.]
H. H. Renz, 8th American Scientific Congress, Proceedings, Geological Sciences,
General geology, pp. 528, 529. University of California Press, 1942.
H D Hedberg and A. Pyre, American Association of Petroleum Geologists, Bulletin
vol. 28, pp. 20-24. 1944.
D. L. Frizzell, Journal of Paleontology vol. 17, pp. 331-353. 1943.
T. Lipparini, Servicio Geologico d’ltalia, Bolletino, vol. 71 (1947, ‘48 &
‘49), nota 11, pp. 5, 6. Rome, 1951.
R. M. Stainforth, American Association of Petroleum Geologists, Bulletin vol.
32, pp. 1308, 1317. 1948.
R. Grill, 18th International Geological Congress, Report, part 15, pp. 51
(abstract), 59. London, 1950.
[41] [Ed: see also
general references given under notes 8 and 9.]
S. W. Lowman, American Association of Petroleum Geologists, Bulletin vol. 33,
pp. 1950-1963. 1949.
[42] [Ed: see also
general references given under notes 8 and 9.]
J. A. Cushman and P. Bronnimann, Cushman Laboratory for Foraminiferal
Research, Contributions vol. 24, pp. 15-21; 37-42. 1948.
[43] [Ed: see also
general references given under notes 8 and 9.]
G. W. Crickmay, H. S. Ladd and J. E. Hoffmeister, Geological Society of
America, Bulletin vol. 52, pp. 79-106. 1941.
[44] [Ed: see also
general references given under notes 8 and 9.]
M. F. Glaessner, “Principles of Micropalaeontology” by M. F. Glaessner,
Melbourne University Press, pp. 193, 194. 1945.
[45] [Ed: see also
general references given under notes 8 and 9.]
F. B. Phleger, Geological Society of America, Bulletin vol. 50,
pp. 1395-142Z. 1939.
Geological
Society of America, Bulletin vol. 52, pp. 1073-1098. 1942.
F. B. Phleger and W. A. Hamilton, Geological Society of America, Bulletin vol.
57, pp. 951-966. 1946.
[46] [Ed: see also
general references given under notes 8 and 9.]
Symposium, Journal of Paleontology vol. 18, pp. 51-70. 1944.
[47] [Ed: see also
general references given under notes 8 and 9.]
M. F. Glaessner, “Principles of Micropalaeontology” by M. F. Glaessner,
Melbourne University Press, p. 193. 1945.
P. Bronnimann, Journal of Paleontology vol. 24, p. 397. 1950.
[48] [Ed: see also
general references given under notes 8 and 9.]
M. F. Glaessner, “Principles of Micropalaeontology” by M. F. Glaessner,
Melbourne University Press, p. 185. 1945.
[49] Examples of
palaeo-ecological reconstruction
A. Examples not based primarily on microfossils:
Diatomites and fish-beds, Algeria:
R. V. Anderson, Journal of Geology vol. 41, pp. 673-698. 1933.
Pennsylvanian of Missouri
W. F. Bailey, Journal of Paleontology vol.
9, pp. 482-602. 1935.
Gulf Coast Tertiary:
M. Bornhauser, American Association of
Petroleum Geologists, Bulletin vol. 31, pp. 698-712. 1947.
The Permian:
C.C. Dunbar, Geological Society of America,
Bulletin vol. 52, pp. 313-332. 1941.
Miocene of California:
J E Eaton, U. S. Grant and H. B. Allen,
American Association of Petroleum Geologists, Bulletin vol. 25,
pp. 193-262. 1941.
Silurian reef, Illinois:
H. A. Lowenstam, “Structure of typical
American ‘ Oilfields”, pp. 153-158. Published by Amer. Ass. Petr. Geol.,
1948.
Gulf Coast Tertiary:
W. G. Meyer, American Association of
Petroleum Geologists, Bulletin vol. 23, pp. 197-209. 1939.
Permian of Arizona:
D. Nicol. Journal of Paleontology vol. 18,
pp. 553-557. 1944.
Crinoidal limestone:
T. G. Payne, American Association of
Petroleum Geologists, Bulletin vol. 26, pp. 1697-1770. 1942.
Cretaceous ammonites:
G. Scott, Journal of Paleontology vol. 14,
pp. 299-323. 1940.
Bryozoa:
L. W. Stach, Journal of Geology vol. 44,
pp. 60-65. 1936.
East Indies:
J. F. H. Umbgrove, American Association of
Petroleum Geologists, Bulletin vol. 22, pp. 1-78. 1938.
Barred basins, pseudabyssal sediments:
W G Woolnough, American Association of
Petroleum Geologists, Bulletin vol. 21, pp. 1101-1157. 1937.
American
Association of Petroleum Geologists, Bulletin vol. 26, pp. 765-792. 1942.
General:
Series: “Report of the committee on marine
ecology Issued annually by the National Research Council, Washington,
D. C., 1941-1951.
Symposium, Journal of Paleontology vol. 9, pp. 63-81 92-147,72-283. 1935.
H. G. Schenck, Journal of Paleontology vol. 2 pp. 158-165 1928.
T. W. Vaughan, Geological Society of America, Bulletin vol. 51,
pp. 433-4o8. 1940.
B. Examples based on microfaunas:
Liassic foraminifera, England:
T. Barnard, 18th International Geological
Congress, Report, part 15, pp. 39-41. London, 1950.
Eocene foraminifera, Washington State:
R. S. Beck, Journal of Paleontology vol.
17, pp. 584-614. 1943.
Fossil pollen:
W. S. Cooper, Journal of Geology vol. 50,
pp. 981-994. 1942.
Submarine Tertiary off New England:
J. A. Cushman, Geological Society of
America, Bulletin vol. 47, pp. 413-440. 1936.
Orbitoidal foraminifera, India:
L. M. Davies, 6th Pacific Science Congress,
Proceedings, vol. 2, pp. 483-501. University of California Press, 1940.
Oligo-Miocene foraminifera, California:
R. M. Kleinpell, “Miocene stratigraphy of
California”, pp. 11-19, 79-87. Published by Amer. Ass. Petr. Geol., 1938.
Tertiary and Recent microfaunas, Gulf Coast:
S. W. Lowman, American Association of
Petroleum Geologists, Bulletin vol. 33, pp. 1971-1983. 1949.
Tertiary foraminifera, Washington State:
W. W. Rau, Journal of Paleontology vol. 22,
pp. 153-155. 1948.
Oligo-Miocene foraminifera, Venezuela:
H. H. Renz, Geological Society of America,
Memoir 32, p. 40 et seq. 1948.
Tertiary microfaunas, Barbados:
A. Senn, Eclogae Geologicae Helveticae,
vol. 40, pp. 200-222. 1948.
Tertiary microfaunas of Ecuador:
R. M. Stainforth, Journal of Paleontology
vol. 22, pp. 135-139. 1948.
Radiolarites, Turkey:
S. W. Tromp, Journal of Geology vol. 56,
pp. 492-494. 1948.
Gault foraminifera, England:
M. H. Khan, Geological Magazine, vol. 87,
pp. 175-180, London, 1950.
(NOTE: additional references in which a quantitative approach is used are
given under section VI below. [Ed: see notes 105 et seq.])
[50] A. Senn, Eclogae
Geologicae Helveticae, vol. 40, no. 2, pp. 199-222. 1948.
[51] H. H. Hess, American
Philosophical Society, Proceedings, vol. 79. 1938.
J. L. Rich, Geological Society of America, Bulletin vol. 62,
pp. 1193-1196. 1951.
[52] C. C. Wilson, American
Association of Petroleum Geologists, Bulletin vol. 24, pp. 2102-2125.
1940.
[53] L. W. LeRoy and H. M.
Crain, “Subsurface Geologic Methods” by L. W. LeRoy and H. M. Grain, Colorado
School of Mines, 1st Edition., pp. 595-681. 1949.
T. H. Bower, American Association of Petroleum Geologists, Bulletin vol. 31,
pp. 340-349. 1947.
T. B. Haites. idem, pp. 777-778.
Representative examples of serial maps are to be found in Schuchert’s
well-known treatises, Weeks’ “Palaeogeography of South America” (American
Association of Petroleum Geologists, Bulletin, vol. 31, pp. 1194-1229) and
other readily accessible papers.
[54] 1860 G. P. Wall
and J. G. Sawkins, “Report on the geology of Trinidad”, H. M. Stationery
Office, London.
1866-99 R. J. L. Guppy, Quart. Jour. Geol. Soc., vol. 22, pp. 570-590;
vol. 48, pp. 519-541, and other papers, many of them first printed in
ephemeral local publications but later reprinted by G. D. Harris in Bulletins
of American Paleontology, vol. 8, no. 35. 1921.
1928 V. C. Illing, Quart. Jour. Geol. Soc., vol. 84, pp. 1-56.
1928 R. A. Liddle, “The geology of Venezuela and Trinidad”. Fort Worth, Texas.
1935 E. Lehner, Annales de l’Office Nationale des Combustibles Liquides, no. 4,
pp. 691-730. Paris.
1936 H. G. Kugler, American Association of Petroleum Geologists, Bulletin vol.
20, pp. 1439-1453.
1942 H. H. Renz, 8th American Science Congress, Proceedings, vol. 4,
p. 540 et seq. University of
California Press
1948 R. M. Stainforth, American Association of Petroleum Geologists, Bulletin
vol. 32, pp. 1306-1308, 1313-1319.
[55] Renz 1942 and
Stainforth 1948 as above.
J. A. Cushman and R. M. Stainforth, Cushman Laboratory for Foraminiferal
Research, Special Papers 14, pp. 3-12.
1945.
J. A. Cushman and H. H. Renz, Cushman Laboratory for Foraminiferal Research,
Special Papers 22, pp. 1-3. 1947.
P. Bronnimann, Cushman Foundation for Foraminiferal Research, Contributions
vol. 1, pp. 80-82, 1950; vol. 2, pp. 16-18, 1951
Journal
of Paleontology vol. 24, pp. 397-420. 1950.
H. Bolli, Cushman Foundation for Foraminiferal Research, Contributions vol. 1,
pp. 82-89. 1950.
[56] R. M. Stainforth,
American Association of Petroleum Geologists, Bulletin vol. 32,
pp. 1319-13Z7. 1948.
[57] Stainforth 1948, as
above, pp. 1318-1319.
[58] R. M. Stainforth,
Journal of Paleontology vol. 22, pp. 140-144. 1948.
[59] H. R. Wanless and F. P.
Shepard, Geological Society of America, Bulletin vol. 47, pp. 1177-1206.
1936. (Includes an extensive bibliography on cyclical sedimentation.)
[60] C. C. Wilson, American
Association of Petroleum Geologists, Bulletin vol. 24, pp. 2115-2121, etc.
1940.
R. M. Stainforth, American Association of Petroleum Geologists, Bulletin vol.
32, p. 1319. 1948.
[61] B. Stone, Cushman
Foundation for Foraminiferal Research, Contributions, ? 1952 (in press).
[62] D. L. Frizzell, Journal
of Paleontology vol. 17, pp. 331-353. 1943.
[63] R. M. Stainforth,
Journal of Paleontology vol. 23, pp. 420, 421. 1949.
[64] Symposium (including
Henson 1950 as below), American Association of Petroleum Geologists, Bulletin
vol. 34, pp. 181-312. 1950.
“Bibliography of organic reefs, bioherms and biostromes”, issued gratis by the
Seismograph Service Corporation, Tulsa, Okla., 1950.
[65] F. R. S. Henson,
American Association of Petroleum Geologists, Bulletin vol. 34,
pp. 215-238. 1950.
[66] T. F. Grimsdale, Third
World Petroleum Congress, Proceedings, vol. 1. Leyden, 1951.
R. M. Stainforth Journal of Paleontology vol. 22, pp. 114-125. 1948.
L. W. LeRoy, Journal of Paleontology vol. 22, pp. 500, 501. 1948.
[67] Globigerina
G. canariensis
A. d’Orbigny in “Histoire Naturelle des Iles Canaries by P Barker-Webb and S.
Bérthelot, tome 2, part 2, p. 133, pl. 2, figs. 10-12. Paris, 1839.
G. venezuelana
D. Hedberg, Journal of Paleontology vol. 11, p. 681, pl. 92, fig. 7. 1937.
G. altispira
J. A. Cushman and P. W. Jarvis, Cushman Laboratory for Foraminiferal Research,
Contributions, vol. 12, p. 5, pl. 1, figs. 13, 14. 1936.
G. triloculinoides
H. J. Plummer, Texas Univ. Bulletin, Bureau of Economic Geology, no. 2644,
p. 134, pl. 8, fig. 10. 1926.
G. cretacea var. eggeri
E. Heron-Allen and A. Earland, British Antarctic (“Terra Nova”) Expedition
1910, Natural History Report, Zoology, vol. 6, no. 2, p. 188, pl. 7, figs.
6-8. British Museum (N. H.), 1922.
(this name is an invalid synonym of G. eggeri Rhumbler)
G. compressa
H J. Plummer, as for G. triloculinoides above, p. 135, pl. 8, fig. 11.
1926.
G. concinna
A. E. Reuss, Kaiserlichen Akadamie der Wissenschaften,
Mathematisch-Naturwissenschaftliche Classe, Denkschriften, Bd. 1, p. 373,
pl. 47, fig. 8. Vienna, 1850.
G. bradyi
H Wiesner in E. von Drygalski, Deutsche Süd-Polar Expedition, 1901-1903, Bd.
20, p. 133. Berlin and Leipzig, 1931.
G. dissimilis
J A. Cushman and P. J. Bermudez, Cushman Laboratory for Foraminiferal Research,
Contributions vol. 13, p. 25, pl. 3, figs. 4-6, 1937.
G. fistulosa
R J Schubert, Geologische Reichsanst., Verhandlungen, p. 323, tf. 2,
p. 324. Vienna, 1910.
G. digitata
H. B. Brady, Quarterly Journal of Microscopical Science, vol. 19 (n. s.),
p. 286. London, 1879.
[68] J. Hofker, The
Micropaleontologist, vol. 4, pp. 16, 17, American Museum of Natural
History, 1950.
[69] B. H. Harlton, Journal
of Paleontology vol. 1, p. 24, pl. 5, fig. 7. 1927.
[70] A. de Gregorio, Annales
Géologiques de le Paléontologie, livre 52, p. 48, pl. 20, figs. 1, 2.
Palermo, 1930.
[71] O. Terquem, “Cinquiéme
mémoire sur les foraminifères du systéme Oölithique de la zone á Ammonites
parkinsoni de Fontoy (Moselle)” p. 364, pl. 41, figs. 6-9. Metz, France,
1883.
[72] M. P. White, Journal of
Paleontology vol. 2, p. 194, pl. 27, fig. 16. 1928.
[73] J. A. Cushman, Cushman
Laboratory for Foraminiferal Research, Contributions vol. 3, p. 87. 1927.
[74] L. W. LeRoy, Journal of
Paleontology vol. 22, pp. 500-508. 1948.
[75] A. E. Reuss, Kaiserliche
Akademie der Wissenschaften, Mathematisch-Naturgewissenschaftliche Classe,
Denkschriften, Bd. 1, p. 374, pl. 47, fig. 11. Vienna, 1850.
[76] H. B. Brady, Quarterly
Journal of Microscopical Science, vol. 19 (n. s.), p. 286. London,
1879.
[77] A. d’Orbigny, in Ramon
de la Sagra, “Histoire physique et naturelle de l’Ile de Cuba” p. 82, pl.
4, figs. 12-14. Paris, 1839.
[78] H. B. Brady, The
Geological Magazine, vol. 4 (n. s.), decade 2, p. 535. London, 1877.
[79] J. A. Cushman and A. L.
Dorsey, Cushman Laboratory for Foraminiferal Research, Contributions vol. 16,
pp. 40-42. 1940.
L. W. LeRoy, Journal of Paleontology vol. 22, pp. 500-508. 1948.
[80] J. A. Cushman and R. M.
Stainforth, Cushman Laboratory for Foraminiferal Research, Special Papers 14,
pp. 68, 69, pl. 13, figs. 7-9. 1945.
P. Bronnimann, CFC vol. 1, pp. 80-82. 1950.
[81] P. Bronnimann, CFC vol.
2, pp. 16-18. 1951.
[82] J. A. Cushman, Cushman
Laboratory for Foraminiferal Research, Contributions vol. 1, p. 6, pl. 1,
fig. 8. 1925.
T. F. Grimsdale, Third World Petroleum Congress, Proceedings, vol. 1. Leyden,
1951.
M. R. de Gaona and G. Colom, Estudios Geologicos, no. 12, p. 379, etc.,
Instituto de Investigaciones Geologicos “Lucas Mallada”, Madrid, 1950.
P. J. Bermudez, Cushman Laboratory for Foraminiferal Research, Special Papers
25, p. 279. 1949.
H. Bolli, Eclogae Geologicae Helveticae, vol. 43, no. 2, p. 113, 1951.
[83] H. Tappan, Journal of
Paleontology vol. 17, p. 513, pl. 83, fig. 5, 1943.
[84] C. G. Lalicker, Journal
of Paleontology vol. 22, p. 624, pl. 92, figs. 1-3. 1948.
[85] J. A. Cushman and A.
ten Dam, Cushman Laboratory for Foraminiferal Research, Contributions vol. 24,
p. 42, pl. 8, figs. 4-6. 1948.
[86] H. B. Brady, Quarterly
Journal of Microscopical Science, vol. 19 (n. s.), p. 285, London,
1879.
[87] W. Thomson (in J. Murray),
Royal Society, Proceedings, vol. 24, p. 534. London, 1876.
[88] J. A. Cushman, Cushman
Laboratory for Foraminiferal Research, Contributions vol. 3, p. 87. 1927.
[89] H. Tappan, Journal of
Paleontology vol. 17, p. 513, pl. 83, fig. 4. 1943.
[90] A. L. Morrow, Journal
of Paleontology vol. 8, p. 198, pl. 30, fig. 6. 1934.
[91] H. E. Thalmann, Eclogae
Geologicae Helveticae, vol. 25, pp. 287-292. 1932
Stanford University Publications, University series, geological science, vol.
3, no. 1, pp. 1-24. 1942.
American Journal of Science, vol. 240, pp. 802-820. 1942.
Geological Society of America, Bulletin vol. 57, pp. 1236, 1237. 1946.
P. Bronnimann, Journal of Paleontology vol. 24, pp. 397-420. 1950.
T. F. Grimsdale, Annals and Magazine of Natural History, ser. 12, vol. 4, pp. 292-294.
1951.
[92] P. Bronnimann, as
above.
[93] H. alabamensis, disputed records in the basal Oligocene:
H. V. Howe, Journal of Paleontology vol. 2, pp. 173-176. 1928.
B. Stone, Journal of Paleontology vol. 23, p. 157. 1949.
[94] O. Renz, Eclogae
Geologicae Helveticae, vol. 29, pp. 1-149. 1936.
H. Bolli, Journal of Paleontology vol. 25, pp. 187-199. 1951.
[95] Globorotalia s. s.; Turborotalia; Truncorotalia
J. A. Cushman and P. J. Bermudez, Cushman Laboratory for Foraminiferal
Research, Contributions vol. 25, pp. 26-45, pl. 5-8. 1949.
[96] F. Brotzen, Sveriges
Geologiska Undersökning, Avhandlingar, ser. C. no. 451 (årsb. 36, no. 8),
pp. 31, 32. Stockholm, 1942.
[97] J. Sigal, Institut
Français de Pétrole, Revue, vol. 3, No. 4, p. 101. Paris, 1948.
[98] J. A. Cushman and P. J.
Bermudez, Cushman Laboratory for Foraminiferal Research, Contributions vol. 12,
p. 36. 1936
[99] Critique of the
subgenera: T. F. Grimsdale, Third World Petroleum Congress, proceedings, vol.
1, Leyden, 1951.
[100] M. F. Glaessner, “Principles
of Micropalaeontology” by M. F. Glaessner, Melbourne University Press,
p. 193, 1945.
[101] N. L. Thomas and E. M. Rice,
Journal of Paleontology vol. 1, pp. 141-144. 1927.
J. A. Cushman, U. S. Geological Survey, Professional Paper 206, pp. 12,
103-109, pl. 44-46. 1946.
[102] J. A. Cushman, Cushman
Laboratory for Foraminiferal Research, Contributions vol. 3, p. 90. 1927.
[103] A. d’Orbigny in “Histoire
Naturelle des Iles Canaries” by P Barker-Webb and S. Bérthelot, tome 2, pt. 2,
Zoology, p. 134, pl. 2, figs. 7-9. Paris, 1839.
[104] J. A. Cushman, Cushman
Laboratory for Foraminiferal Research, Contributions vol. 3, p. 90. 1927.
[105] a) In Recent microfaunas
from various environments:
The Skagerak
H. Höglund, Zoologiska Bidrag fran Uppsala, Bd. Z6. 1947.
Gulf of Mexico
S. W. Lowman, American Association of
Petroleum Geologists, Bulletin vol. 33, pp. 1950-1963. 1949.
Florida, Bahamas
M. L. Natland, Scripps Institute of
Oceanography, Bulletins, Technical series, vol. 3, no. 10, pp. 225-230.
1933.
California
R. D. Norton, op. cit., vol. 2, no. .9, pp. 331-388. 1930.
Eastern seaboard of the United States
F. L. Parker, Museum of Comparative
Zoology, Bulletin, vol. 100, no. 2, pp. 213-241. Cambridge, Mass., 1948.
Plankton of deep-sea cores
F. B. Phleger, Geological Society of
America, Bulletin vol. 50, pp. 1395-1422. 1939;
vol. 53, pp. 1073-1098. 1942;
vol. 57, pp. 951-966. 1946.
Marsh Bay, Massachusetts
F B. Phleger and W. R. Walton, American
Journal of Science, vol. 248, pp. 274-294. 1950.
Red Sea
R. Said, CFC vol. 1, pp. 9-29. 1950.
Equatorial Atlantic Ocean
W. Schott, Deutsche Atlantische Expedition
“Meteor”, 1925-27, Wissenschaftliche Ergebnisse, Bd. 3, Teil 3. Berlin and
Leipzig, 1935.
(b) In fossil microfaunas, with deductions as to environment:
Eocene of Washington State
R. S. Beck, Journal of Paleontology vol.
17, pp. 587-590. 1943.
Oligocene and Pleistocene of Italy
C. Emiliani, Journal of Paleontology vol.
24. pp. 485-491. 1950.
Oligo-Miocene of California
R. M. Kleinpell, “Miocene stratigraphy of
California”, fig. 5. Published by Amer. Ass. Petr. Geol., 1938.
Miocene of Jamaica
D K. Palmer, Bulletins of American
Paleontology, vol. 29, no. 115, pp. 11-15. 1945.
Pleistocene of Atlantic sea-floor.
F. B. Phleger, as under (a) above.
Eo-Oligocene of Washington State
W. W. Rau, Journal of Paleontology vol. 22,
pp. 153-155. 1948.
Oligocene and Miocene of Venezuela
H. H. Renz, Geological Society of America,
Memoir 32, pp. 40-76. 1948.
Oligocene of Trinidad
R. M. Stainforth, American Association of
Petroleum Geologists, Bulletin vol. 32, pp. 1319-1327., 1948.
[106] M. Israelsky, American
Association of Petroleum Geologists, Bulletin vol. 33, p. 92-98. 1949.
[107] S. W. Tromp, Journal of
Paleontology vol. 23, pp. 223-224; 673-676. 1949.
[108] J. A. Cushman and R. Todd,
Cushman Laboratory for Foraminiferal Research, Contributions vol. 17,
pp. 29-31. 1941
[109] J. Hofker, The
Micropaleontologist, vol. 4, pp. 16-17. American Museum of Natural
History, 1950.
[110] H. Bolli, CFC vol. 1,
pp. 82-89. 1950.
[111] D. J. Carter, Geological
Magazine, vol. 88, no. 4, pp. 236-248. 1951.
[112] Not cited in the text:
Morphological evolution of Globorotalia
K. Schmid, Eclogae Geologicae Helveticae,
vol. 27, pp. 45-143. 1934.
Foraminifera
J. A. Cushman and R. W. Harris, Cushman
Laboratory for Foraminiferal Research, Contributions vol. 2, pp. 92-94.
1927.
Morphological evolution of Bolivinoides
H. Hiltermann and W. Koch, Geologische
Jahrbuch, Bd. 64, pp. 595-632. Hannover/Celle, 1950.
Foraminifera
C. Emiliani, Rivista Italiana di
paleontologia e Stratigrafia. Anno 15, fasc. 1, pp. 1-15. Milan 1949.
Pliocene foraminifera, Japan
Y. Takayanagi, Tohoku University, Institute
of Geology and Paleontology, Short Papers, no. 2, pp. 23-26. 1950
[113] B. L. Clark, Journal of
Paleontology vol. 19, pp. 158-172. 1945.
N. D. Newell, Journal of Paleontology vol. 22, pp. 225-232. 1948.
[114] B. Burma, Journal of
Paleontology vol. 22, pp. 725-761. 1948.
Journal of Paleontology vol. 23, pp. 95-103. 1949.
C. Emiliani, Journal of Paleontology vol. 24, pp. 485-491. 1950.
[115] Burma 1948, 1949 and Emiliani
1950 as above.
C. L. Cooper, Journal of Paleontology vol. 19, pp. 368-375. 1945.
M. G. Rutten, Journal of Paleontology vol. 9, pp. 527-545. 1935.
D. Nicol, Journal of Paleontology vol. 18, pp. 172-185. 1944. (criticized
by Burma - 1948, pp. 737-741 - and Emiliani - 1950, pp. 488, 489).
[116] K. M. Hussey, Journal of
Paleontology vol. 23, pp. 120, 121. 1949.
