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Mid-Tertiary Diastrophism in Northern South America

by R. M. Stainforth
Creole Petroleum Corporation,
Caracas, Venezuela

Prepared for presentation at the


Trinidad, 1965

by permission of Creole Petroleum Corporation


In the marine Tertiary basins which ring the Brazilian Shield, stratigraphers and paleontologists have long recognized a certain rhythm of subsidence and emergence. Development of the planktonic foraminiferal zones as a basis for precise correlation on a regional scale enables us to integrate these local diastrophic patterns into a regional whole. This is attempted in summary fashion for the mid-Eocene to mid-Miocene interval. Two cycles of sedimentation are recognized, each starting with strong marine transgression over a previously uplifted and eroded surface and continuing with basinal infill until regressive conditions developed. The basal beds of the two cycles are of Upper Eocene and Lower Miocene (C. dissimilis Zone) age. No evidence is seen for a break in deposition at the end of the Eocene, hence presence of Oligocene beds in the sequence is postulated.



1    Sketch map of northern South America

2    Charts of sea-depth vs. time, PERÚ and ECUADOR

3    Charts of sea-depth vs. time, NORTHWEST VENEZUELA (Falcón Basin)

4    Charts of sea-depth vs. time, EASTERN VENEZUELA BASIN

5    South flank of Eastern Venezuela Basin: highly generalized diagram showing formational sequence

6    Charts of sea-depth vs. time, TRINIDAD

7    Faunal distribution in the San Fernando Formation of Trinidad

8    Comparison of charts of sea-depth vs. time in different sectors of northern South America

9    Idealized chart of sea-depth vs. time in the geosynclinal basins of northern South America


Stratigraphers and paleontologists have long recognized certain cycles of sedimentation, some localized and some of regional extent, in the Tertiary marine basins of northern South America. They reflect rather closely the diastrophism, or crustal movements, of the region. Downwarping of the earth’s crust is indicated by marine transgression; upwarping is reflected in regression and, if pronounced, in erosional truncation.

In recent years the value of planktonic foraminifera in precise regional correlation of the marine Tertiaries has been established. The purpose of this paper is to apply this precise tool to a comparison of the sedimentational, hence the diastrophic, patterns in the different basins. It is concluded that a regional rhythm is apparent, which affected surprisingly uniformly the mobile geosynclinal basins extending around the Brazilian Shield[1] from Perú to Trinidad. One well defined cycle of subsidence and emergence corresponds to the interval from the Upper Eocene up through the Globigerina ciperoensis Zone. A second corresponds to the zones of Catapsydrax dissimilis, Globigerinatella insueta, and Globorotalia fohsi. Regional uniformity is less apparent in the Globorotalia menardii Zone. An important corollary is that sedimentation was continuous on a regional scale from the Upper Eocene into the post-Eocene Globigerina ciperoensis Zone, hence the latter must, at least in part, be Oligocene in age.

Although this compilation is largely based on published literature, the writer has worked as an oil-company paleontologist and geologist in all the sectors discussed, and has drawn on his own knowledge where necessary.


The paper is concerned with those Tertiary basins of northern South America which were sites of marine sedimentation during most of the interval from Upper Eocene to mid-Miocene

time. The areas and localities mentioned are shown on a sketch map (Fig. 1). It may be noted that the extremes of northwest Perú and Trinidad lie some 1700 miles apart in a direct line. Measured along their curvilinear trend, the basins discussed extend for some 2300 miles around the Brazilian Shield.

The principal content of this paper is a set of charts showing changes of sea-depth relative to zonal level (Figs, 2,3,4,6, individual basins; Fig. 8, compilation of individual charts; Fig. 9, idealized regional chart). The text is primarily a commentary on these charts. Some points to note regarding their construction and interpretation are as follows:

Zonal divisions

Prior to 1957 five major zones, based on ranges of planktonic foraminifera, were recognized in the post-Eocene sequence of Trinidad and subsequently throughout the Caribbean region. In 1957 Bolli (-a, p. 98-102) erected a finer zonation in Trinidad, in which the same interval was divided into thirteen subdivisions. This refined scheme also seems to be valid on a regional scale. Unfortunately, however, the present compilation rests largely on studies completed before Bolli’s work was published, and the important new species and subspecies which he defined are not listed in them. The older system of five zones is therefore used, and its correspondence to the refined scheme is as follows:

Zones of this paper

Zones of Bolli (1957-a)

Globorotalia menardii

G. menardii
G. mayeri

Globorotalia fohsi

G. fohsi robusta
G. fohsi lobata
G. fohsi fohsi
G fohsi barisanensis

Globigerinatella insueta

G. insueta
G. stainforthi

Catapsydrax dissimilis

G. dissimilis
G. kugleri

Globigerina ciperoensis

G. c. ciperensis
G. o. opima
G. ampliapertura

For the sake of brevity the zones are nominated herein by only the trivial names of the zonal indices, e.g. “dissimilis Zone”. The formal names of the Upper Eocene zones are likewise abbreviated to “Hantkenina Zone”, which corresponds largely to Bolli’s Globorotalia cocoaensis Zone (1957-b) but is not exclusive of slightly older beds.

Determination of sea-depth

Estimates of ancient sea-depths can best be based on the paleoecology of fossilized foraminiferal assemblages: Firstly, the gross aspect of the fauna may be revealing, e.g.

very deep water :

planktonic species strongly predominant

deep to medium depth :

rich and highly diversified benthonic fauna; planktonics plentiful but not dominant

shallow offshore :

rich but not diversified; three or four species may make up the bulk of the fauna; non-foraminiferal microfossils may be abundant

shallow, reefal to sub-reefal :

orbitoids abundant, usually associated with sessile organisms

shallow, inshore :

few genera (e.g. Streblus, Elphidium, miliolids) could survive in this variable environment, but they may be present in abundance.

Secondly, by reference to ecologic limitations of individual genera and species, or their analogues in Recent faunas, rather precise depth limits can be estimated for fossil assemblages. The writer has been greatly assisted by inclusion of such studies in some of the papers cited (e.g. H.H. Renz, 1948; Petters and Sarmiento, 1956).

Lithology is also an important key to depth of deposition, especially in barren beds. In general coarse clastic sediments indicate inshore environments, but care must be taken not to confuse primary sandstones and conglomerates (shallow water) with secondary turbidites (re-deposited in deep water). The anglicized word “wildflysch” is used in this paper to denote exceptionally coarse turbidites in the form of slumped clays full of exotic blocks of heterogeneous rock formations.

Relation of sea-depth to crustal movements

It needs emphasizing that fluctuations of sea-depth are not a direct indication of crustal movements. At any given moment in geologic time (t):

sea-depth (Ds) = crustal depth (Dc) - thickness of sediments (T). Rate of change of sea-depth is therefore expressed by

dDs/dt = dDc/dt - dT/dt

If dT/dt (rate of sedimentation) exceeds dDc/dt (rate of crustal subsidence), then dDs/dt becomes negative. In other words, shallowing of facies can occur even though the crust is subsiding steadily. The key to such cases is that, since the sedimentation-rate is high, a remarkable thickness of shallow-facies beds has accumulated. The Naricual Formation of eastern Venezuela and the Zapotal Formation of southwest Ecuador appear to be examples of this situation.

(Introduction of mathematical symbols may appear unnecessary in demonstrating an obvious truth. However, by postulating physically reasonable relationships such as dT/dt inversely proportional to Ds2, and then integrating, some significant relationships can be developed between thicknesses of sedimentary intervals and rates of crustal and sea-floor movements).

Positive inferences which can be drawn from the diagrams presented here (Figs. 2,3,4,6) are:

1)   Increasing sea-depth is a sure sign of crustal subsidence

2)   Physical unconformity is almost invariably a sign of preceding crustal uplift, which has elevated underlying beds into the zone of subaërial erosion. The only exception is the rare case where submarine tilting has led to slumping, hence to unconformable relationships in deepwater beds. The Cipero/Lengua formational contact in Trinidad appears to represent such conditions

3)   The optimum sector of a basin for these studies is the inner margin. Along the axis, deep sedimentation persists and sedimentary cycles are obscure. In the outer periphery, persistence of shallow to non-marine facies, development of numerous unconformities, and excessive erosion combine to obscure the diastrophic history.

No allowance is made herein for eustatic changes of sea-level, which even if discernible would be trivial in comparison with the great crustal movements concerned in this study. Possibly in the late Miocene and Pliocene a eustatic rise of sea-level accounted for widespread marine invasion of northern South America, but that interval is not treated in the present paper.


In the discussions of individual charts, certain “principal references” are listed. These are the papers which make direct reference to foraminiferal faunas and, in most cases, to the planktonic zones. The lists exclude many well known works on general stratigraphy, but these can be located in the bibliographies of the authors cited. Other papers are referred to selectively in support of specific points raised in the text. All references are listed alphabetically by authors at the end of this paper.

The charts (Figs. 2, 3, 4, 6)

In the discussion which follows, the sea-depth versus time charts are treated in clockwise order from Perú to Trinidad, and they are believed to be self-explanatory. The following points should be noted regarding the symbolism employed.

Environmental subdivisions are denoted by

d -

deep to very deep marine

m -

medium-depth marine


this datum, separating the deeper from the shallower facies, is marked by a vertical line on the charts

s -

shallow marine

c -

continental, non marine

T - denotes prominent turbidities in the deep-water beds

In one or two places a double line represents differences between the axial and peripheral provinces of a basin.

In the stratigraphic columns alongside, unconformities and transitional relationships are shown conventionally. Formation and group names are in capitals, members and lesser units in lower case script.



Principal references: Cushman and Stone, 1947, 1949; Weiss, 1955; Stainforth, 1955.

The area in question is the coastal belt west of the Andes in westernmost Perú, from the Sechura Desert in the south to Tumbes in the north. It is the southernmost part of a chain of basins, similarly located in Ecuador and Colombia, which some authors have termed the “Bolívar Geosyncline” (see Nygren, 1950).

The Upper Eocene includes the Verdun orbitoidal grits, which are extensively unconformable on Cretaceous and older Eocene rocks. They pass transitionally upwards into deep-water shales of the Chira and Cone Hill formations, which are separated by the conglomeratic sandstones, with brackish-water mollusks, of the Mirador Formation. Index fossils of the Hantkenina Zone persist to the truncated top of the Cone Hill shales, but appearance of Globigerina venezuelana (of Weiss, 1955; possibly G. rohri Bolli) and Bulimina sculptilis in the highest beds may indicate transitional passage to the ciperoensis Zone, now eroded away.

Except for this inferential evidence, the ciperoensis Zone is unknown. The next youngest unit is the Mancora Formation, composed of grits and sands with a pronounced basal unconformity. They pass transitionally upwards into the Heath shales of moderately deep marine character. On evidence of rather sparse planktonic foraminifera (G. dissimilis, G. fohsi-group, etc.) supported by benthonic foraminifera and mollusks of age-diagnostic value, the Mancora-Heath interval embraces the dissimilis, insueta and fohsi zones.

Regressive conditions are indicated by appearance of interbedded sands in the upper Heath shales, which pass transitionally up into molluscan sandstones of the Zorritos Formation. Index foraminifera of the menardii Zone (G. menardii, Sphaeroidinella dehiscens are only present at post-Zorritos levels.


Principal references: Stainforth, 1948, 1949; Stone, 1949; Marks, 1951; Cushman and Stainforth, 1951; Hofker, 1956; Small, 1962.

The following account of the Progreso Basin is generalized and refers to an arbitrary axial section. Around the flanks the deeper marine shales (Seca, Dos Bocas) merge laterally into almost continuous sequences of the shallower deposits (Zapotal, Progreso).

As is also partly true in northwest Perú, sedimentation persisted without a break from the Middle to the Upper Eocene. The most widespread Upper Eocene formation is the Seca (or Jusá) shales of similar deep-water character to the Chira-Cone Hill shales of Perú, and likewise containing index fossils of the Hantkenina Zone.

Late Eocene regression is foreshadowed by intertonguing of the Ancon Point Formation, of deltaic aspect, followed upwards by the thick Zapotal Formation composed of conglomerates, sandstones with brackish-water mollusks, and non-marine red beds and coaly clays. The occasional marine shales interbedded with the Zapotal mostly contain foraminifera indicative of the Hantkenina Zone, such as Hantkenina alabamensis, Hastigerinella eocanica and Bulimina jacksonensis. However, a change of both planktonic and benthonic index-species indicates that the upper few hundred feet of the formation belong to the ciperoensis Zone.

The conspicuous regressive phase ends with a new marine transgression, of which the earliest beds are the San Pedro sandstones containing Miogypsina (Miolepidocyclina) ecuadorensis These pass upwards into the Dos Bocas shales of open-sea character. Presence of Catapsydrax stainforthi, Globigerinatella insueta and Globorotalia fohsi fohsi, as well as age-diagnostic benthonic foraminifera and mollusks, indicates that the transgression lasted through the dissimilis insueta and fohsi zones.

Without sharp lithologic change the Dos Bocas Formation alters up-section becoming gradually coarser in texture and showing a change in facies-faunas through sublittoral Streblus-Nonionella and Amphistegina-rich assemblages to a fauna of brackish aspect with only Streblus spp. and ostracods. Authors have variously included this regressive phase in the Dos Bocas Formation or given it local names, which include Bajada, Subibaja and Zacachun, but the preferred designation is Progreso Formation. It extends from the fohsi Zone into the menardii Zone.


Principal references: Stainforth, 1948; Cushman and Stainforth, 1951; Hofker, 1956.

More complete data are available for the northerly Borbón Basin than for the southerly Progreso Basin, and the sequences of events along both periphery and axis can be described. Allowance has been made for Stainforth’s inclusion (1948) in “Globigerina dissimilis” of the form now separated as Catapsydrax (or Globigerinita stainforthi (or incrusta) which ranges up through most of the insueta Zone.

The Upper Eocene rests unconformably on Cretaceous cherts or older metamorphics. Orbitoidal limestones are locally present at the base, but the interval is largely represented by the Zapallo Formation of rather deep marine shales. These usually contain the typical planktonic foraminifera of the Hantkenina Zone (except in their radiolarian facies, which need not be discussed here).

In the axial sector the Zapallo shales continue up with no great change into the Pambíl Formation, which contains the index foraminifera of the ciperoensis Zone. Along the basinal margin, however, the two shale units are separated by the Playa Rica sands, which are prominent inter-beds of reefal facies, rich in orbitoids. Stainforth (1948, p. 134) notes that, although their fauna as a whole is typically Oligocene, a puzzling point is the abundance of Pliolepidina tobleri usually considered an established Eocene marker. Cole (1963-a, p. 32) has suggested that a misidentification is involved, but P. tobleri is too distinctive for this to seem probable. Other possible explanations are: 1) that the orbitoidal beds do in fact extend back into the Eocene; or 2) that progressive reworking of reefs in the same site has produced a mixed fauna; or 3) that this case justifies Cole’s claim (1960-1963) that Pliolepidina is an aberrant form produced by fusion of two or more embryos, and is liable to appear in lepidocycline assemblages regardless of stratigraphic level. These are speculative remarks, and the point deserves further critical study by those in a position to re-sample the sections in the Telembí area.

The reefal beds are overlain by marine Pambíl shales as in the axial sector, but along the basin margin these are truncated with angular unconformity by the lower Angostura sands. Lenses of the Cupa limestone, with Miogypsina (Miolepidocyclina) ecuadorensis date the base of these transgressive beds as mainly dissimilis Zone. Out in the basin there is no apparent break in sedimentation, but beds of the ciperoensis and dissimilis zones are separated by highly tuffaceous shales and ash beds Chumundé Formation). This fact, coupled with an abundance of euhedral mineral grains in the Angostura sands, suggests a brief period of volcanic uplift nearby to the east.

Except around the periphery, where deposition of sands persisted, the lower Angostura sands pass transitionally up into the Viche shales of rather deep marine character. The contact is strongly diachronous, reflecting the slow submergence of the basinal flank and corresponding outward spread of the deep marine facies. Maximum marine invasion corresponded to the top insueta Zone or basal fohsi Zone, after which the process was reversed. The upper Angostura became regressive, moving slowly basinward over the upper Viche shales,[2] In the wake of the foreset Angostura sands came a sheet of fine-textured topset beds of shallow marine character, namely the Onzole Formation. The Angostura/Onzole contact is diachronous and roughly parallel to the Viche/Angostura contact.

The Viche, Angostura and Onzole formations all contain planktonic index-foraminifera, though naturally the open-sea Viche shales are richest, and the dissimilis, insueta, fohsi and menardii zones are recognizable. Their obliquity to the formational units, each of which contains a rather uniform benthonic microfauna, is an object lesson on why facies-controlled faunas should not be applied directly to chronologic correlation.


Principal references: Redmond, 1953; Bürgl, Barrios & Röström, 1955; Petters & Sarmiento, 1956; Becker & Dusenbury, 1958; O. Renz, 1960; Bürgl, 1961; de Porta, 1962.

The Tertiary geologic history of Colombia is complex and incompletely documented. hence the remarks below are generalized. Pertinent statements may well be cited, in translation, from the two latest papers listed above. De Porta states (1962, p. 8): “The few works which have been published on the stratigraphy of Colombia are out of date and need fundamental revision”, while Bürgl (1961, p. 170) comments that: “In general it is not possible to speak of regional marine transgressions and regressions, because a transgression in a certain area is matched by a simultaneous regression in another.” Over much of the country the Tertiaries are dominantly continental and can not be correlated with the planktonic foraminiferal zones.

Marine Tertiaries are best developed in the northwest, and the published studies refer especially to the sector south of Barranquilla. Petters and Sarmiento describe in detail the Carmen-Zambrano section, nearly 8000 feet thick and extending from the top of the Hantkenina Zone up into the menardii Zone. The sequence is broken into nine local zones which, from the data provided, can be correlated with the regional planktonic zones (see de Porta, 1962, p. 20). Details of the lithology and biofacies are given and are combined on a time/sea-depth chart. For purposes of this compilation the presentation could not be improved, but it must be noted that the authors echo Bürgl’s remark (above), commenting that this section is not representative for all parts of the basin.

In the Carmen-Zambrano section deepening is indicated through the upper ciperoensis Zone to a maximum in the lower dissimilis Zone, and from then on a fairly steady shallowing is seen through the upper dissimilis, insueta, fohsi and lower menardii zones. This culminates in appearance of brackish facies in the upper menardii Zone, where Streblus beccarii and miliolids are the dominant foraminifera.

Bürgl (1961, p. 173-175) synthesizes the available data in a tabulation of tectonic movements of the Colombian Andes from the Upper Cretaceous through the Tertiary. Of significance for the present paper is the following segment (translated).

The reasons for placing the dissimilis Zone at the end of the lower cycle, rather than the beginning of the upper cycle, are not given in detail. Bürgl (p. 174, translated) states: “In many parts of the marine Tertiary, the Zone of Catapsydrax dissimilis and also at times the zones below were eroded and the Upper Oligocene (zones of Siphogenerina transversa, S. basispinata and Robulus wallacei) rests discordantly on the Eocene.” Likewise Petters and Sarmiento state (p. 14) that in some sections the insueta Zone (their “Upper Oligocene”) rests directly on the ciperoensis Zone (their “Lower Oligocene”).

To the writer it seems feasible that emergence and erosion occurred at the end of ciperoensis time. Subsidence then commenced and transgressive beds of the dissimilis-insueta sequence encroached slowly over the eroded surface. In the outer peripheral parts of the basin, insueta Zone beds might well rest on truncated ciperoensis or Hantkenina Zone beds, but absence of the dissimilis Zone would represent non-deposition, not removal by erosion. This is exactly the case in the Borbón Basin of Ecuador (see Fig. 2).

Stage (p. 175)

Zone (P. 173)


Diastrophic events (p. 175)



U. Aquitanian

menardii (base)



transgression followed by regression




movements of major intensity; volcanic intrusions

L. Aquitanian


U. Eocene

M. Eocene (part)








movements of major intensity; volcanic intrusions and extrusions

J.L. Lamb (private report) re-studied the Carmen-Zambrano section, attempting to establish there the finer zonation of Bolli (l957-a). He expressed the opinion that the diagnostic elements of the ciperoensis Zone are too sparse and incomplete to be considered an authigenic fauna, and suggested that they are reworked in the base of the dissimilis Zone. Despite the conflict between this opinion and the assertion by Petters and Sarmiento (p. 11-14) of a continuous, gradational sequence up from the Hantkenina Zone, both are in accord as to strong marine transgression at the base of the dissimilis Zone.

Furthermore in another part of Colombia, namely the isolated Guajira Peninsula in the northeast, O. Renz (1960, p. 338-343), Fig. 9) has demonstrated the gradual transgression of Miogypsina-bearing conglomerates and limestones over deeply eroded pre-Tertiary rocks. These basal beds belong to the dissimilis and higher zones, and they grade up diachronously into marine shales of the insueta and fohsi zones (see Becker and Dusenbury, 1958).

Also in the Guajira Peninsula Renz (p. 336, 337) refers to fossiliferous reef limestones of Upper Eocene age, which are strongly unconformable on older rocks. Renz refers some of the limited outcrops to the Paleocene, but currently they are all regarded as Upper Eocene (see Stainforth, 1962).

Nygren (1950) has described the western coastal belt (“Bolívar Geosyncline”) in highly generalized terms. In his tabulation of the formational sequence (p. 2004) he notes northward transgression of the Upper Eocene, apparently continuous deposition from Eocene to Oligocene, unconformity of “Middle” on “Lower Oligocene”, and continuous deposition from “Middle” to “Upper Oligocene”. In current usage, his “Lower”, “Middle” and “Upper Oligocene” would correspond respectively to the ciperoensis, dissimilis and insueta-fohsi zones.

Because of the limited data and the uncertainties indicated, no time versus sea-depth chart is submitted for Colombia.


Principal references: H.H. Renz, 1948; Blow, 1959; Kavanagh de Petzall, 1959; Wheeler, 1963; Gamero & Díaz de Gamero, 1963.

The Falcón Basin of northwest Venezuela underwent a complex history in Eocene through Miocene time (see Wheeler, 1963; p. 62-65). The studies cited above include exceptionally detailed descriptions and analyses of small sectors, but these must be combined with Wheeler’s more generalized paper in assessing the diastrophism of the basin as a whole.

In the Agua Salada sector to the east, medium-depth foraminiferal shales are dominant in the sequence of Cerro Misión, Guacharaca, San Lorenzo and lower Pozón formations. Interruptions of regressive character occur at the top of the Cerro Misión (Cerro Campana reefal limestones) and in the upper Guacharaca sands which grade up into the El Salto (sand) Member of the San Lorenzo. The base of the Pozón Formation (Policarpio Member) is marked by highly glauconitic beds slightly unconformable on the San Lorenzo. In the uppermost part of the Pozón Formation the microfaunas become less diversified, and beds rich in Amphistegina are followed by sandy clays with Streblus-Elphidium assemblages of brackish aspect. However, this final regression is mainly evident in post-menardii Zone beds.

At the western end of the basin the basal beds of the Upper Eocene Agua Negra Group are of shallow marine character (Sta. Rita conglomerates, Churuguarita reefal limestones) and rest with angular unconformity on older Eocene formations. They are followed transitionally by the Jarillal shales of rather deep character. Appearance of sandstones (El Paraíso) marks the base of the Oligocene, but these again give place to highly foraminiferal shales of the upper Paraíso and Pecaya formations. A strong tongue of mixed lignitic, mottled, conglomeratic and sandy beds (Castillo Formation), with brackish to shallow marine faunas, next appears, but passes transitionally upwards into shales of the Agua Clara Formation, which contains sparse interbeds of limestone and sandstone. The top of the sequence is marked by rapid transition through shallow marine formations to the largely non-marine deposits of the La Puerta Formation.

In the center of the basin the formational sequence is similar but, as might be expected, all units show somewhat deeper character than their equivalents to the west. The basal El Paraíso sands are less prominent and the Pedregoso Formation, composed of interbedded marine shales, orbitoidal limestones and molluscan sandstones, replaces the more brackish Castillo. A regressive phase follows Agua Clara deposition, but shallow marine facies persist through the interval represented by non-marine beds to the west.

In summary of the three representative sequences discussed, they show certain features in common, which presumably reflect the diastrophism of the basin even though masked by localized depositional patterns. Expressed in terms of the planktonic zones, they are as follows. The oldest is a regressive phase at the base of the ciperoensis Zone. Both Renz (1948, p. 30, 38) and Wheeler (1963, p. 44 ) indicate a conformable and transitional contact, with the regressive conditions starting to appear within the Upper Eocene (Hantkenina Zone) interval. The second is a regressive phase at the top of the dissimilis Zone. The third event is a regression in late insueta early fohsi time, expressed by the Cerro Pelado Formation and Policarpio Member. In all three sectors the deepest marine facies correspond to the mid-Hantkenina Zone, upper ciperoensis/lower dissimilis zones, and mid-insueta Zone.


Principal references: Quarfoth & Caudri, 1961; H.H. Renz, 1962; Lamb & De Sisto, 1963; Lagaaij, 1963; Lamb, 1964-a,-b; Salvador, 1964 (esp. Figs. 1-7); Peirson, 1964; Lamb & Sulek, 1965.

The Eastern Venezuela Basin is favored by nature for detailed analysis of its geologic history. Outcrops are extensive in the mountains to the north and drilling for oil has provided abundant subsurface data in the plains to the south. In broad terms, the north flank was spasmodically raised and thrust southwards; the orogenic spasms were reflected in the axial sediments; the south flank subsided slowly in epeirogenic fashion and was hardly affected by the orogenic movements.

On Figure 4 representative plots of sea-depth versus time are depicted for the north flank and axis of the basin. It must be emphasized that both these sectors moved southwards as the northern borderland expanded by orogenic uplift. By menardii time, for instance, the depositional axis followed a trend which in Hantkenina time had been high on the south flank of the basin. This explains why two separate sectors must be used to depict the older and the younger zones of the north flank, and why the axial diagram is a composite of three sectors.

The north-flank facies of the three lower zones have only escaped erosion in the Guárico sector. Here the Upper Eocene Peñas Blancas reefal limestones rest unconformably on Paleocene to mid-Eocene beds. The age of the Peñas Blancas is established by presence of Pliolepidina tobleri in rock-forming abundance, largely in their attitude of growth, associated with occasional megafossils such as Echinolampas ovum-serpenti and Tubulostium leptostoma clymenioides (the latter fide Rutsch, 1939, p. 239). The limestones are overlain conformably by the thick, deep marine Roblecito shales of Eo-O1igocene age (see Quarfoth and Caudri, 1961; Lagaaij, 1963, p. l99-201; Peirson, 1964), which in turn are followed transitionally by marginal marine sandstones of

the Naricual Formation, which represents the dissimilis Zone. The former seaway into western Venezuela had now closed, and the Guárico Sub-basin was almost land-locked. Consequently it filled rapidly with terrigenous sediments and the sea retreated eastwards into the Maturín Sub-basin. The great composite thickness of the Naricual, Quebradón and Quiamare formations attests to great crustal subsidence during dissimilis-insueta-fohsi time, but the sediments give no reflection of discrete diastrophic events.

In northwest Monagas, however, the equivalent interval is represented by marine formations in which significant fluctuations of sea-depth are observable. The central column on Figure 4 represents the sequence encountered in the subsurface north of the Pirital Thrust Fault, especially to the north of the Jusepín-Santa Bárbara oilfields. On this diagram the Naricual Formation, already mentioned, is shown as grading up into marine Carapita shales. Strictly the Naricual Formation here is not a north-flank unit, but the indicated deepening from dissimilis to insueta time is presumptively correct for more northerly beds which have been destroyed by erosion in the mountains. The Carapita shales, of open marine character, represent the upper dissimilis Zone and the insueta Zone. The sequence is interrupted by a strong unconformity at the base of the fohsi Zone, partly as an intraformational break within the Carapita, partly as a basal unconformity of the upper Carapita on pre-Carapita wilts (see Lamb and De Sisto, 1963, p. 274, Figs. 3,4,5; Lamb and Sulek, 1965). Marine shales of the insueta Zone are truncated or eliminated at this level. The basal beds of the fohsi Zone have a nearshore aspect, being sandy and carrying faunas with Amphistegina and Archaias prominent, but they grade up into shales of deeper aspect.

Higher in the section a second strong unconformity appears, where truncated Carapita shales are overlain by shallow marine sandy shales of the La Pica Formation, which falls within the menardii Zone. The La Pica represents the final marine invasion of the basin and passes conformably up into the non-marine Las Piedras Formation of Mio-Pliocene age.

The right-hand column of Figure 4, representing the basin axis, is a composite of three sectors of northern Monagas. The sequence from the Hantkenina Zone to the base of the dissimilis Zone is best displayed in the outcrops along Río Aragua, 25 miles northwest of Maturín, which have been described in some detail by Lamb (1964-a). An uninterrupted Eocene sequence is represented by the Caratas Formation, an alternation of massive shales and sandstones. The Upper Eocene portion (Hantkenina Zone) is shaly and is overlain directly in this section by the Los Jabillos sandstone. (In the subsurface to the south, shales of the basal ciperoensis Zone intervene, with a Globigerina ampliapertura fauna). Transitionally above lie silty shales of the Areo Formation, highly foraminiferal with plentiful plankton of the ciperoensis Zone. At the top is a conglomeratic sandstone for which Lamb (p. 117) offers two alternative explanations. The present writer prefers to accept it as a turbidite, genetically related to the Nariva Formation of Trinidad. The overlying Carapita shales are less silty, but their microfauna is similar to the Areo except for species diagnostic of the dissimilis Zone.

The axial facies from the upper ciperoensis Zone to the upper fohsi Zone are well known from the subsurface of the Quiriquire oilfield and the area to the west, south of the Pirital Thrust Fault. The glauconitic silty shales of the Areo Formation contain a rather shallow Uvigerina-Nonionella facies-fauna. They pass conformably up into colloidal shales of the basal Carapita Formation, which locally contain abundant planktonic foraminifera dominated by G. c. ciperoensis These shales are only present in a few wells, arid the reason is elimination by erosion prior to unconformable overlap by the dissimilis Zone portion of the Carapita. The basal beds above the unconformity carry a shallow facies-fauna of Buliminellas and small arenaceous foraminifera, but this is rapidly replaced upwards by a rich diversified deep-water assemblage. Globorotalia kugleri is seldom encountered, a fact which may indicate a condensed or incomplete section at the base of the dissimilis Zone.

The Carapita shales extend up without change into the insueta Zone. The same rich deep-water microfauna persists and locally approaches the abyssal “Cipero” facies (white preservation, profusion of plankton). Near the top of the zone the faunas become more restricted, a symptom of shallowing which corresponds to the uplift of the north flank.

In the overlying fohsi Zone the faunas return abruptly to the deep facies, and the lenticular sands (Cachipo Member) developed at this level are undoubtedly turbidites. They are full of rock fragments and reworked microfossils distinctive of various Cretaceous to Eocene formations now exposed in the mountains to the north. It is visualized that, under compression, the basin floor ruptured and the peripheral portion assumed an unstable, up-thrusted position while the axial segment subsided.

In the higher parts of the fohsi Zone a gradual shallowing is evident, culminating in brackish-water beds with Streblus beccarii and Miliammina fusca. Though discernible in the Quiriquire area, this topmost part of the Carapita Formation can best be studied farther west, to the south of the Tacat oil field. There the regression (which corresponds to the angular unconformity already noted in the north flank) is marked by transitional sandy beds conformable between the shaly Carapita and La Pica formations, in the base of the menardii Zone.

The south flank of the basin is not shown on Figure 4 because it was not involved in the spasmodic orogeny of the northern sectors. During the Upper Eocene this broad shelf was tilted down to the north (basin axis) and up to the south (Guiana Shield). This led simultaneously to transgression north of the hinge line and regression south of it, and also to deep erosion and peneplanation of the uptilted portion. From Oligocene to mid-Miocene time the area subsided slowly without further tilting. Marine sediments slowly encroached southwards until shales of the shallow marine to medium depth Freites Formation extended at least as far south as the present Orinoco River. At that level regressive conditions set in, and infill of the basin was completed by the La Pica-Las Piedras deposits already mentioned.

A highly generalized north-south section through the south flank is given in Figure 5, mainly to show the great differences in sedimentary sequence from one sector to another. Because of these differences, no single plot of seadepth versus time would be representative of this province.

TRINIDAD (Figure 6)

Principal references: Cushman and Stainforth, 1945; Stainforth, 1948; Kugler, 1953; Bolli, 1957-a, -b; Suter, 1960.

In central Trinidad there is evidence of four diastrophically controlled cycles of deposition. The first starts with basal conglomerates of the San Fernando Formation, which consists of various shallow marine members including sandstones, reefal limestones, and orbitoidal beds. The higher parts are dark foraminiferal clays and shales (though traditionally called “silts”) of deeper water aspect. These bridge the Hantkenina-ciperoensis zonal contact with no change of lithology or overall faunal content, except for the few species diagnostic of the two zones. Slumped masses of Cretaceous and Paleocene rocks indicate the instability of the newly uplifted rim of the basin. These upper beds pass transitionally up into light-colored marls of the lower Cipero Formation, of very deep marine origin.

A second strong phase of uplift and erosion to the north is marked by a great flood of turbidite deposits in the dissimilis Zone. They form the Nariva Formation, which includes spectacular wildflysch containing gigantic slumped blocks. In the writer’s opinion, however, the more northerly grits and basal conglomerates of the Nariva may reasonably be regarded as inshore facies of a marine transgression. The Nariva passes transitionally upwards into marine shales of the Brasso Formation, which are of moderately deep origin in the upper dissimilis Zone and insueta Zone. Locally orbitoidal beds at the base of the Brasso are strongly unconformable on Cretaceous rocks.

Shallowing at the base of the fohsi Zone is indicated by rather abrupt appearance within the Brasso shales of sub-reefal limestones (Machapure, Biche).

The upper Brasso beds are overlain with slight unconformity by sub-reefal limestones, local conglomerates, and associated shallow marine beds of the Tamana Formation, which represents the menardii Zone.

In southern Trinidad the Naparima Group of light-colored marls extends up without a break from the Upper Cretaceous to the lower Miocene. These marls are fossilized Globigerina-oozes of archibenthic to abyssal origin. The Hantkenina Zone is represented by the upper Navet Formation and the ciperoensis through fohsi zones by the Cipero Formation. The menardii Zone is represented by the Lengua Formation, still of deep-water character, but transitional up into the Cruse clays which herald advance of a fan of terrigenous (Cruse-Forest) deposits into the basin.

The Naparima marls in themselves tell us nothing of the diastrophic history of the basin. However, they are interrupted at three levels by prominent lenticular turbidites and slumped blocks, which must reflect orogenic uplifts of the basinal rim. The dissimilis Zone is marked by south-pointing tongues of the Nariva turbidites, also by isolated slumped masses exemplified by the Flat Rock beds. In the fohsi Zone the petroliferous Retrench and Herrera sands are turbidites and the Ste. Croix limestones are slumped masses. At the base of the menardii Zone the Río Claro boulder-bed is a coarse turbidite, locally a “wildflysch”, and is roughly correlative with finer turbidites of the Karamat Formation of the Southern Range.

The three turbidite levels clearly correspond to the uplifts associated with regressive sedimentation, already demonstrated in central Trinidad. The fact that no similar turbidites are recorded in the basinal uppermost Eocene marls may simply result from lack of data. These beds are only known in very limited outcrops and have seldom been penetrated by wells.

Note on Age and Status of San Fernando Formation

The writer rejects the lengthy argument put forward by Eames et al. (1962, p. 41-42, 70, 77-81) in support of a great hiatus within the San Fernando Formation. The two main points on which those authors elaborate are: 1) they consider the genus Pliolepidina represented by P. tobleri to be a Miocene index-fossil; and 2) they designate zones in the Oligocene of East Africa based on planktonic species unrecorded in Trinidad. On this basis they postulate that the Oligocene is missing in Trinidad and that the rich Upper Eocene faunas of the Mount Moriah silts are reworked en masse in beds of actually Miocene (Aquitanian) age.

To the writer it is completely unrealistic to postulate a post-Eocene age for Pliolepidina tobleri. On empirical evidence throughout the Caribbean region, this species invariably occurs associated with accepted index-fossils of Upper Eocene age. A key occurrence is in the Peñas Blancas limestones of north-central Venezuela, which represent an algal-orbitoidal community which grew in place in quiet water. The limestones contain virtually no clastic material and the delicate flanges of the orbitoids are perfectly preserved. Reworking of the contained fossils cannot be contemplated. Yet the most abundant species is P. tobleri (fide T.W. Vaughan) in association with discocyclines and occasional specimens of Tubulostium leptostoma clymenioides (fide R. Rutsch) and Echinolampas ovum-serpentis (fide H.D. Hedberg). The one possible exceptional case, in Ecuador, is incompletely documented (regrettably by fault of the present writer) and cannot validly be used to contradict the mass of evidence indicating the Upper Eocene age of P. tobleri.

Regarding the concept of mass reworking of the microfaunas of the Mount Moriah “silts”, this could not be upheld by any paleontologist who had worked intimately with these beds. They are part of a complex of inter-grading shallow, medium-depth and deep-water environments in which corresponding facies-controlled faunules existed. This faunal facies-variation is still readily apparent and matches the changes of lithologic texture within the formation. Bulk reworking would have destroyed these delicate transitional relationships.

Certainly reworked faunas exist in the San Fernando Formation, as in most strongly transgressive beds, but they occur haphazardly and dwindle away up-section, in parallel with the amount of megascopic slumped and re-deposited detritus. In two continuously cored boreholes at Vistabella and in the formerly well exposed sequence at Point Bontour, the faunal succession in the upper San Fernando Formation (i.e. the Mount Moriah “silts”) could be expressed diagrammatically as on Figure 7. Twenty-odd years ago the writer had a roving assignment to sample every new road cut and building foundation dug in the San Fernando area and to make foraminiferal studies of the samples. No cases are recalled in which the deeper marine Mount Moriah beds could not be referred to one zone or the other by the scheme here illustrated. The faunal change in the smaller foraminifera is slight, but abrupt and consistent, and involves both extinction and advent of species.

In the writer’s opinion any distributional pattern such as is shown on Figure 7 can only be interpreted as a mixture of indigenous foraminifera (solid bars) and reworked species (so indicated). If the Upper Eocene species were also reworked, they would. not be so consistently present, they would not disappear abruptly at the zonal boundary, and they would not be replaced at exactly that level by a group of new species.

If the foregoing interpretation is correct, deposition was continuous up from shallower to deeper facies within the Upper Eocene portion of the San Fernando Formation and from Eocene to post-Eocene in its deeper facies (the Mount Moriah Member), and postulation of a great unconformable hiatus within the formation is unjustified. There could be minor gaps caused by basinward slumping in the course of sedimentation (cf. the Flat Rock beds in the Cipero type section), but not a major gap of diastrophic origin. The San Fernando area is undeniably complex, and for clarification one might naturally refer to less complex sectors of the same basin. These are available in eastern Venezuela, as described in the preceding section, and in both the Roblecito and Caratas formations the upward passage from Hantkenina Zone to ciperoensis Zone occurs within a sequence of shales of uniform open-sea character.

The writer ventures to extend this digression by commenting on the lithology and nomenclature of the San Fernando Formation. Trinidad has been anomalous in that, while physically part of America, it has been an enclave of European geologists. Consequently the stratigraphic nomenclature of the island is strongly affected by the longstanding European tradition of separating rock units on the basis of their age. A clear-cut example is the current definition of the San Fernando Formation as “the Upper Eocene deposits of the San Fernando area” (Kugler, 1956, p. 95). My own copy of this lexicon has “Ugh!” pencilled at the side, because modern codes of stratigraphic nomenclature forbid application of age-factors to the

designation of rock-stratigraphic units.[3] Both H.H. Renz (1942, p. 547) and Bolli (1957-a, p. 98, 100) state openly that, lithologically, the Mount Moriah Silt Member of the San Fernando Formation extends up into the ciperoensis Zone, yet they include these upper beds in the Cipero Formation simply to preserve the (false) concept of two formations separated by the Eocene/Oligocene boundary. Eames et al. (loc. cit.) separate an undefined “true San Fernando Formation” from the Mount Moriah silts purely on their concept of age relationships.

In the writer’s opinion the situation would be greatly clarified by divorcing the lithostratigraphic nomenclature from age considerations. Guppy (1866, p. 571-572) introduced the name “San Fernando beds” casually for fossiliferous beds of shallow aspect distinct from the deep-water Naparima marls. In 1892 (p. 521-524), although he dropped the name, he gave more details and pointed out the difference between “these shallow-water beds of the Naparima Series” (p. 523) and the “true solidified Globigerina-ooze, corresponding with that now found in the abysmal depths of the ocean” (p.  528). Modern authors have revived Guppy’s name, in the form of “San Fernando Formation”, in essentially his original sense, though introducing the objectionable time concepts already mentioned. To accord with modern usage the definition should be purely lithologic, and would in essence confine the San Fernando Formation to distinctive beds, of markedly shallower character, encountered between the deep-water Navet and Cipero marls. Individual beds of conglomerate, limestone and sandstone may be named as members or lentils, as has been done in the past (e.g., Mount Moriah, Vistabella, Morne Roche). However, it would be desirable to eliminate some existing duplication, exemplified by use of the name Mount Moriah to designate conglomerates, sandstones and “silts” in different parts of the formation. Use of the formational name for a subordinate member, as in the term “San Fernando conglomerate” introduced by Eames et al. without definition, is considered objectionable under modern codes of nomenclature.


On Figure 8 the individual charts, already discussed, of sea-depth versus time are reproduced side by side. If the central column (NW. Venezuela) is ignored, the other four show points of similarity which are surprising in view of their wide geographic spread. Features discernible on a regional scale are:

1)   Strong transgression at the base of the Hantkenina Zone. Typically shallow-water beds, often orbitoidal reefs, are strongly unconformable on older beds and pass up into deep-water shales. Only along basinal axes is this transgression inconspicuous.

2)   Persistence of deep-water shales from the Hantkenina Zone into the ciperoensis Zone. In three of the four sectors temporary regression is marked by interbedded sandstones within this interval, but these phases do not coincide.

3)   Strong regression in the upper ciperoensis Zone, marked either by upward transition into non-marine beds or by erosional hiatus at the base of the dissimilis Zone.

4)   Strong and sustained transgression at the base of the dissimilis Zone. This is marked in Perú and Ecuador by coarse clastics and reefal beds grading up into open-marine shales.

      The unconformable basal beds may be as young as the insueta Zone in the outer parts of the basin. In Trinidad the most obvious symptom of this episode is the flood of turbidites and wildflysch representative of the dissimilis Zone. However, the basal unconformity of the Brasso/Nariva is significant, and the allochthonous masses of the Ste. Croix limestones are interpreted as relics of the peripheral transgressive beds, which subsequently slumped towards the basin axis. In the Quiriquire sector of eastern Venezuela unconformable truncation of the ciperoensis Zone is followed by a shallow-to-deep sequence in the dissimilis Zone. Farther west the course of events is masked by deposition of almost non-marine coal-swamp deposits (Naricual), but their great thickness is proof of steady subsidence during dissimilis time.

5)   Deepening, or persistence of deep-water facies, from the dissimilis into the insueta Zone.

6)   In general terms, a gradual shallowing from the insueta Zone up through the fohsi and menardii zones. In Ecuador this process is straightforward. In eastern Venezuela and Trinidad it is interrupted by two transgressive spasms. In Perú the shallowing proceeds through the fohsi Zone, but a new transgression is seen in the menardii Zone.

The conclusion seems unavoidable that the pattern of sedimentation in the basins of Perú, Ecuador, eastern Venezuela and Trinidad expresses response to a regional diastrophic rhythm. Although the data on Colombia are inadequate for depiction, they are in accord with the postulate of major marine transgressions in Hantkenina and dissimilis-insueta time. An interplay of crustal movements is visualized between the Brazilian Shield craton and the flanking geosynclines.

The case of northwest Venezuela (Falcón Basin), as depicted on Figure 8, may seem so exceptional as to discredit the concept of uniform regional diastrophism. Although the Hantkenina Zone transgression is well defined, the basal dissimilis Zone transgression is not apparent, and the upper dissimilis/lower insueta interval is regressive instead of transgressive. Yet these anomalies do not actually conflict with the concept of a regional rhythm in the mobile Andean geosynclines.

From late Cretaceous through Tertiary time a borderland of folded mountains arose, in the manner described in texts on geosynclinal theory, and encircled the Brazilian Shield. The mountains of northern Venezuela and Trinidad are part of this system, likewise the Andes of Colombia, Ecuador and Perú, but the Mérida Andes of Venezuela are not. The present area of Lake Maracaibo persisted into early Tertiary time as a rigid block which, for reasons not fully explained, resisted incorporation into the folded borderland. Only in Miocene time did the suppressed crustal stresses overcome the resistance and force up the gigantic horst-block of the Mérida Andes. The present Maracaibo-Falcón Basin covers the formerly stable province, hence its divergence from the rhythm of the folded mountains is accountable.

To summarize the foregoing discussion, Figure 9 presents an idealized chart of sea-depth versus time in the basins of Perú, Ecuador, eastern Venezuela and Trinidad. Deductions as to regional diastrophism are expressed below, as drawn from this figure.

Phase I is a cycle of sedimentation corresponding to the Hantkenina-ciperoensis interval. Marine transgression was triggered by downwarping of basinal provinces. Corresponding uplift of inter-basinal areas led to accelerated erosion, hence to build-out of tongues of clastic sediments. The resulting regressive phase is registered at appreciably different levels in different sectors, hence cannot be attributed to a single diastrophic event: rather it reflects temporary excess of rate of sedimentation over rate of crustal subsidence. The cycle ended with crustal uplift, as demonstrated by extensive erosion of the ciperoensis Zone before the next cycle commenced.

Phase II is a cycle of similar character extending from the dissimilis Zone into the fohsi Zone. It likewise began with subsidence of the basinal provinces, but this process was apparently milder and more prolonged than in Phase I. Evidence to this effect is 1) the diachronous basal unconformity, extending into the insueta Zone in peripheral areas, 2) absence of clastic tongues such as are present in Phase I, and 3) persistence of non-marine conditions through the dissimilis Zone in much of eastern Venezuela. Maximum crustal subsidence was achieved in the insueta Zone, after which the seas became shallower through fohsi time. In the west (Perú, Ecuador) the regression was gradual, and might reflect sedimentary infill rather than crustal uplift. In the east (Venezuela, Trinidad) two accompanying phases of crustal uplift are indicated by unconformities basal to the fohsi and menardii zones.

Phase III, the menardii interval, is not marked by any consistent regional pattern. In Perú there is a mild transgression. In Ecuador gentle regression continues from the fohsi Zone into the menardii Zone with no interruption. In the eastern Venezuela-Trinidad basin sharp marine transgression is quickly followed by a regression which extends into post-menardii levels. The suggested inference is that the prolonged compressive orogeny, which built the geosynclinal mountains and basins of the region, was dwindling away during fohsi time and ceasing to be a controlling factor in the patterns of sedimentation.


In the foregoing discussion the conclusion is reached that sedimentation was continuous on a regional scale from the Hantkenina Zone into the ciperoensis Zone. The zonal boundary corresponded to a phase of maximum spread of the sea, near the midpoint of a sedimentary cycle. Though other sectors were stressed in establishing the regional diastrophic pattern, evidence from Colombia and NW. Venezuela fully accords with these conclusions.

The Hantkenina Zone is Upper Eocene. Its fauna consistently contains such index-fossils as Tubulostium, Asterocyclina, Hantkenina, Globorotalia cerroazulensis (= G. cocoaensis), Bulimina jacksonensis and many others. Equally certainly the ciperoensis Zone is post-Eocene. None of the recognized Eocene markers survive above the zonal boundary and new forms replace them, notably in the orbitoidal and planktonic foraminifera.

If these statements are correct, it is irrefutable that Oligocene time is represented by all or the lower parts of the ciperoensis Zone. For that zone to be entirely Miocene, a depositional or erosional hiatus would be requisite at exactly the level which regional evidence shows to be a time of maximum marine invasion. This is not a geologically reasonable postulate.

The writer is therefore unable to accept the claim set forth by Eames et al. (1962) that the Oligocene is absent in all the sectors of northern South America herein described. Their lines of argument are primarily paleontologic, based on long-range matching of fossil faunas, and the reader must judge if this procedure is more or less reliable than the paleogeographic approach adopted in this paper. However, brief comments appear justified on certain forms which are of key importance in the reasoning of Eames et al.

a) Eames et al. (p. 50-52) regard the genus Pliolepidina as restricted to the Miocene (Aquitanian-Burdigalian) on evidence of its occurrence in the Far East. Consequently they treat any American fauna containing P. tobleri as Aquitanian, and they dismiss its invariable accompaniment of accepted Eocene species as the result of wholesale reworking (p. 35-49). To the writer this is as illogical as asserting that only one soldier in the company is keeping step. On the empirical evidence alone, without introducing Cole’s biological explanation (1960-1963) of the status of Pliolepidina, P. tobleri is an index-species of the American Upper Eocene.

b) Eames et al. (p. 67-71) lay great stress on the general absence in America of two zones of planktonic foraminifera which, in East Africa, separate the equivalents of the Hantkenina and ciperoensis zones of the present paper. Their explanation is a regional hiatus, equally pronounced in the abyssal marls of Barbados as in the shallow marine facies of eastern Venezuela (Eames et al., Fig. 5). A simpler explanation is that the distinctive East African species were geographically confined by the climatic provincialism which affects distribution of all planktonic foraminifera. Occasional presence in Caribbean faunas of at least one of these species, namely Globigerina oligocaenica (= G. sellii as admitted by Eames et al. (p. 35, 49, 71), supports the concept that it was a rare migrant into American waters.

c) Eames et al. (p. 27-29) maintain that appearance of Miogypsina (sensu stricto excluding Miogypsinella indicates the base of the Miocene (Aquitanian). In the writer’s experience Miogypsina does not occur below the dissimilis Zone in the American region. Eames et al. (p. 76) mention two supposed records of Miogypsina s.s. from the ciperoensis Zone, but fail to specify their limitations. One from Trinidad (after Kugler, 1954, p. 411) is highly dubious as, although Kugler uses the phrase: “The Globigerina cf. concinna Zone includes the oldest Miogypsina s. str. bearing sediments in Trinidad…,” he fails to substantiate it. His specific references to Miogypsina records are all in the allochthonous Ste. Croix limestones, now accepted as falling within the dissimilis-insueta zonal interval (Higgins, in Suter, 1960, p. 139, 140). Kugler’s reference in this context to the “rich orbitoid fauna” of the ciperoensis Zone is puzzling and apparently unwarranted, as both Vaughan and Cole (1941) and Caudri (in Stainforth, 1948, p. 1309, 1310) failed to record miogypsinids at this level, although their studies embraced material from all three areas of exposure — Vistabella, Point Bontour and the Flat Rock slump mass (“Bamboo clay”). The second case cited by Eames et al. is from Carriacou (Grenadines), where joint occurrence of Miogypsina and Globorotalia opima is claimed (after Martin-Kaye, 1958) However, this record is discredited by a qualifying phrase in the original paper (p. 398), viz. “Unfortunately the precise localities of Lehner’s samples cannot now be ascertained.”

In all cases where Miogypsina occurs in a well established sequence in the American Tertiaries, the level is within the dissimilis-insueta interval or occasionally higher. Many of these cases are cited by Eames et al. (p. 33-47). Also important is the negative evidence of absence of Miogypsina in such richly orbitoidal facies of the ciperoensis Zone as the Antigua Formation, the Playa Rica Formation of Ecuador, and their correlatives. Furthermore, Neorotalias of the N. mexicana-group are typical of the faunas at this level and, since they are considered ancestral to the Miogypsinidae (Barker and Grimsdale, 1937), the joint occurrence of their specialized descendants should not be expected.

In short, in America as in the Old World there is an interval between extinction of Eocene faunas and first appearance of Miogypsina s.s. This interval is virtually equivalent to the Oligocene and in America it corresponds within close limits to the ciperoensis Zone.


BARKER, R.W. & GRIMSDALE T.F., 1937 Studies of Mexican fossil foraminifera. Ann. Mag. Nat. Hist., ser. 10, vol. 19, p. 161-178

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—, 1957-b Planktonic foraminifera from the Eocene Navet and San Fernando formations of Trinidad, B.W.I. U.S. Nat. Mus., Bull. 215, p. 155-172

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—, 1962 Embryonic chambers and the subgenera of Lepidocyclina. Bull. Am, Pal., vol. 44, no. 200

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—, 1963-b Analysis of Lepidocyclina radiata (Martin) Bull. Am. Pal., vol. 46, no. 208

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—, 1892 The Tertiary microzoic formations of Trinidad, West Indies. Geol. Soc., Quart. Jour., vol. 48, p. 519-541

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—, 1954 The Miocene/Oligocene boundary in the Caribbean region. Geol. Mag., vol. 91, no. 5, p. 410-413

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[1] The literature on Venezuela and Trinidad customarily refers to the craton as the Guiana Shield (Escudo de Guayana), but the term Brazilian Shield seems more appropriate to the scope of this paper.

[2] Individual names would be desirable for the upper and lower tongues of “Angostura” sands, but it is beyond the scope of this paper to rectify the existing nomenclature.

[3] Under stimulus of the International Subcommission on Stratigraphic Terminology, numerous countries in recent years have published stratigraphic codes or statements of principles in which the separation of lithostratigraphy from biostratigraphy is generally recognized and stressed. Pertinent references include: “Stratigraphic classification and terminology”, Int. Subcom. Strat. Term., 1961 (XXI Session, Int. Geol. Congr., Copenhagen); “Code of stratigraphic nomenclature”, Am. Comm. Strat. Nomencl., 1961 (Bull. Am. Ass., Petr. Geol., vol. 11.5, no. 5); “Código de nomenclatura estratigráfica”, Mexico, 1961 (sponsored by three major groups of Mexican geologists); “Codice di nomenclatura stratigrafica secondo i Nord-Americani” (1962) and “Un altro codice di nomenclatura stratigrafica” (1963) (Riv. Ital. Paleont., vol. 68, no. 1; vol. 69, no. 3, translations sponsored by the editors); “Principes de classification et de nomenclature stratigraphiques”, Comité Français de Stratig., 1962 (G. Demarcq, Paris).