INTERNATIONAL UNION OF GEOLOGICAL SCIENCES
COMMISSION ON STRATIGRAPHY
COMMITTEE ON MEDITERRANEAN NEOGENE STRATIGRAPHY
PROCEEDINGS OF THE THIRD SESSION IN BERNE
8-13 June 1964
C. W. DROOGER[1]
Table 8
Prior to the actual zonings established during the last decade, several reports already indicated the probable stratigraphic value of planktonic Foraminifera for long-distance correlations of the Miocene. One of the most interesting contributions is that of Le Roy (1948, 1952) on faunas of Sumatra, since it stressed the importance of the Orbulina-surface as a datum plane within the Miocene. Underlying this plane Le Roy found a Globigerinoides Zone, and within the range of Orbulina a lower Zone with Globorotalia canariensis and Globigerinoides irregularis, and a higher one characterized by Globorotalia barisanensis.
Research on Sumatra was not continued, but the papers by Cushman & Stainforth (1945) and Cushman & Renz (1947) on the faunas of the Cipero and Sainte Croix Formations of Trinidad initiated a wealth of publications, which made the southern part of the Caribbean the classical area of planktonic zoning of the Oligo-Miocene. From the onset the placing of the Oligocene-Miocene boundary changed continuously; for this reason the "Oligocene" has to be considered as well as the Miocene.
Cushman and his co-authors recognized four zones, viz. from bottom to top, the cf. concinna (=ciperoensis Bolli), dissimilis, insueta and fohsi Zones. Since much controversy existed and still exists in the literature about the generic names of some of the characteristic species, they had better be left out of the zonal names.
During the following years it was mainly the work of Brönnimann and Bolli that further refined the zoning in Trinidad. As a result about ten successive zones were recognized around 1954-55.
From this time onward two trends of research ensued. In one it is tried to recognize the Trinidad zones in other parts of the world, the other showed a continued elaboration of the zoning in the type area. As to the latter, Bolli (1957) gave a detailed account of the ranges of all planktonic species of Trinidad and he added three more zones to the existing ten. Blow (1959), working in the same area, brought the total up to 17 zones (and subzones).
According to Eames, Banner, Blow & Clarke (1962) all these zones overlie the Oligocene-Miocene boundary. Based on data from other areas, mainly Tanganyika, these authors recognize below this assumed boundary an oligocaenica (= sellii) Zone, which, according to them, comprises the major part of the Oligocene interval, and which is lacking in nearly the entire Caribbean area.
Thus a total of 18 zones is the most refined subdivision of the Oligo-Miocene, the Upper Miocene and Pliocene being hardly touched, if at all.
If one attempts to review the efforts of recognition of the Caribbean zones in other areas, many difficulties appear, part of which may be considered as inherent to the group of planktonic Foraminifera.
R. M. Stainforth[2]
Drooger's review reveals both the strengths and weaknesses of inter-regional Miocene correlations by means of planktonic Foraminifera.
I agree with the remarks on the dangers of fine splitting of species groups, and especially on the formal naming of minutely different forms. Distinctions, even on the generic level, have often been based on slight differences in the bulla (Catapsydracinae), on the shape of pores, and similar hair-splitting differences. From these have been derived lineage diagrams which at best are subjective and at worst are misleading by their implication that the fine varieties must succeed one another in the order indicated.
For long-range correlations I pin my faith on the boldly defined lineages in which the whole test, the individual chambers, and the apertural features all show progressive and obvious change. The triloba-to-universa development is a prime example. The sequences barisanensis-fohsi-lobata-robusta and Globigerinoides grimsdalei-Sphaeroidinella-Sphaeroidinellosis are equally useful, but their distribution is far less global.
Also climatic control is rightly stressed. The case of Orbulina appearing as late as the Vindobonian in the Vienna basin, may be an example. Certainly many forms characteristic of particular levels in the mid-American region are rare to unknown in Europe and elsewhere (e.g. kugleri, insueta, fohsi s.l., menardii). Temperature differences related to latitude provide an obvious explanation. Coastal Peru and Ecuador, though straddling the equator, appear to have been a cool-water province in Mid-Tertiary time, just as they are today, and here also the forms just noted are scarce to absent.
Nevertheless, key Antillean forms have been recorded in Europe. Increased attention to their European ranges is highly desirable, especially in areas where a linkage is possible with the types of particular stages, as for instance in the subsurface of the Aquitaine Basin. Apart from the Orbulina lineage and the levels of first or strong appearance of Globoquadrina (especially atispira) and the Globigerinoides triloba group, some others are worth mentioning. They are Globigerina ampliapertura (to be distinguished from similar forms; its linkage with the Eocene centralis needs to be studied in Europe; validity of pseudoampliapertura needs adjudication), G. ciperoensis ciperoensis (the form with large gaping umbilicus, to be distinguished from the longer-ranging angustiumbilicata), C. ciperoensis angulisuturalis (readily distinguished, and short-lived), G. dissimilis (not to be confused with other bullate forms) and G. mayeri (its lower limit has similar significance to that of triloba and altispira; its upper limit may become in Europe a datum comparable in importance to the fohsi/menardii datum used in America).
Finally, there are prior synonyms for two of the names used several times by Drooger. They are Globigerinoides sicanus De Stephani 1951 for G. bisphericus Todd 1954, and Globigerina sellii (Borsetti) 1959 for G. oligocaenica Blow & Banner 1962.