Revista Española de
Vol. VII, núm. 3, pp. 363‑371
Victoria, B. C., Canadá
With reference to forthcoming series of papers, attention is drawn to some problems which affect all the South American countries when planktonic foraminifera are applied to their Tertiary biostratigraphy. In particular, modern age determinations differ appreciably from those which formerly seemed firmly established in the literature. Much of the “Oligocene” is now recognized as really Miocene, some ‘Upper Miocene” becomes Pliocene, and some “Pliocene” is raised into the Pleistocene.
Con referencia a los artículos siguientes, se llama la atención sobre ciertos problemas que afectan a todos los países suramericanos cuando se aplican los foraminíferos planctónicos a la bioestratigrafía de sus formaciones terciarias. En particular, las determinaciones modernas de edad son apreciablemente distintas de aquellas que en su día se establecieron sin lugar a dudas. Gran parte del “Oligoceno” es ahora reconocido como realmente Mioceno, una parte del “Mioceno superior” pertenece en realidad al Plioceno, y ciertos niveles del “Plioceno” se trasladaron al Pleistoceno.
The following notes arise from a project to compile a symposium of the applications of planktonic foraminifera to the Tertiary biostratigraphy of South America. The organization of such a compilation was suggested by Prof. F. Perconig to the present writer, who was then in Caracas, several years ago. It seemed to be an excellent idea, on the one hand as an incentive to closer cooperation between micropaleontologists in the numerous countries of South America, on the other hand as an indication to our colleagues elsewhere of the validity on our continent of the zonal schemes already established in other regions.
For a combination of personal and professional reasons, initiation of the project was deferred until 1971, when a circular letter was sent to colleagues known to be interested in the theme. The initial response was enthusiastically in favour of the proposal. By further correspondence agreement was reached on the basic format of a historical summary of the studies already made in each country followed by a review of the technical aspects of particular interest there. Prof. Perconig expressed willingness to print the symposium in a single issue of the Revista Española de Micropaleontología.
At this point, however, difficulties became prominent which had been disregarded during the excitement of starting a new project. Despite their geographic propinquity, the countries of South America differ in many respects pertinent to the contemplated symposium. First and foremost, their status during Tertiary time varied between a portion of a stable craton and a region undergoing powerful deformation and division into disconnected basins. The extent of marine Tertiary beds, and especially those of deeper facies suited to the study of planktonic foraminifera, varies from none whatever to thick and extensive deposits. In the oilfield regions these beds have been thoroughly studied in the subsurface but elsewhere they are only known from natural outcrops, which may be sparse and deeply weathered. The degree of advancement of Tertiary micropaleontology varies from country to country, depending partly on economic incentives, partly on the personal interests of leading stratigraphers. Even in well‑studied areas unsolved problems of paleoclimatic nature exist, related perhaps to ocean currents or perhaps to shifts and rotations of crustal plates, and it would be premature to publish existing data without deeper study of their significance. Yet another problem is that several Tertiary basins transgress the political boundaries between countries, so that their study could most satisfactorily be performed by an international committee: a procedure easy to recommend but difficult to implement.
For the reasons indicated, different rates of progress could be anticipated in preparing the chapters of the compilation. In point of fact the outcome was that by the end of 1972 only one contribution (that of Brazil) was complete and in other countries the status ranged from well advanced to still under discussion. Until now only the manuscripts for Colombia and Venezuela were also complete. In consequence, the studies of Brazil, Colombia and Venezuela will appear in these pages and those of other countries will, it is hoped, appear in future issues.
Collaborators in the correspondence on this project agreed that certain problems affect all the countries equally, and that they should be explained in an introductory article to avoid repetition in different words in every chapter of the symposium. In principle this still seems a sound proposal and the following paragraphs implement is. The main point at issue is that the time scale based on planktonic foraminifera differs appreciably from the Tertiary age determinations which seemed to be firmly fixed in such classic works as Steinmann’s “Geología del Perú” (1930), Liddle’s “Geology of Venezuela and Trinidad” (1928, 1946), and their counterparts in other countries.
During the past twenty years the application of planktonic foraminifera to Tertiary stratigraphy has changed from a little‑known study by a few specialists into a recognized tool for making exact correlations and precise age determinations. It has led to fundamental revisions and corrections of long‑accepted correlations and age attributions among the classical Tertiary stages in the whole of western Europe. In turn, because the sequence of these European stages (as traditionally interpreted) was the basis for the international Tertiary time scale, the revisions have led to corresponding changes in age determinations which seemed to be established beyond doubt around the entire world. A representative example of the scope of the revisions is as follows:
The zonation of Tertiary strata by means of planktonic foraminifera was first applied (on a wide scale) more or less simultaneously in the oilfields of the Soviet Union (see Glaessner, 1937; Subbotina, 1953) and Trinidad (see Brönnimann, 1952; Bolli, 1974). Grimsdale (1951) presented a study at the III World Petroleum Congress which focussed the attention of biostratigraphers on this promising field of investigation. Bolli (1957) expanded the Trinidad studies into a detailed zonation from the Paleocene into the Miocene and refined the taxonomy of the index species. Attempts to identify the Caribbean zones in Europe were made by Drooger (1956), Bolli (1959) and Stainforth (1959, 1960). These essays by individuals led to the coordinated efforts of the Committee on Mediterranean Neogene Stratigraphy which, as reported at its meetings in 1959 (Vienna), 1961 (Madrid), 1964 (Berne), 1967 (Bologna) and 1971 (Lyon), has played a major role in progressively establishing the present status of planktonic foraminiferal biostratigraphy.
Authors of the papers here published, on South American aspects of this theme, base their age determinations on the planktonic foraminiferal zones. In certain cases, where long‑established age determinations have remained in force, the changes may be so marked as to cause confusion. Nevertheless, this abrupt revision is considered desirable as a step towards more exact understanding of the geological history of the continent. Each country has its own particular problems, but some points which crop up more persistently than others are:
1) The base of the Paleocene coincides with extinction of the Globotruncanidae, the costate and striate Heterohelicidae, and the genus Rugoglobigerina. Under this definition the Danian Stage is Paleocene despite being referred to the Late Cretaceous in earlier studies.
2) The base of the Eocene has still not been firmly established in Europe, but a preference seems to be growing for equating it with the base of the llerdian Stage (see Pomerol, 1969). If this should become the generally accepted level, the base of the Globorotalia subbotinae ( = G. rex) Zone will serve to define the base of the Eocene.
3) The threefold division of the Eocene is somewhat arbitrary because the type Middle Eo cene ( = Lutetian Stage of the Paris Basin, defined there by a prominent limestone) is difficult to trace away from its type region. Among the planktonic assemblages two natural datum planes offer a convenient definition of the mid‑Eocene interval, namely an incursion of distinctive new genera at the base (e. g. Hantkenina, Globigerinatheka, Clavigerinella) and the abrupt extinction of spinose forms at the top. Nevertheless, these boundaries do not necessarily coincide with the Middle Eocene as defined by other criteria.
4) The Eocene/Oligocene boundary has resisted the modern revisions because its classical definition coincides, for all practical purposes, with a level of worldwide extinction of such distinctive planktonlc foraminifera as the family Hantkeninidae, the genus Globigerinatheka and the species Globorotalia cerroazulensis (sensu lato).
5) The effect of one of the strongest revisions is that the scope of the Oligocene, as formerly recognized in America, has been markedly reduced. The assertion was even made, with ample documentation by Eames et al. (1962), that in much of the American region no marine beds of Oligocene age are preserved. Subsequently the “Globigerinoides Datum” has received wide acceptance as an international guide to the Oligocene/Miocene boundary, with the result that widespread Oligocene beds are again recognized in America. Nevertheless, the Oligocene thus defined corresponds only to the “Lower Oligocene” of much earlier literature, while beds formerly treated as “Middle and Upper Oligocene” are today transferred to the Miocene.
6) The term “Globigerinoides Datum” originated in the Committee on Mediterranean Neogene Stratigraphy and its acceptance has varied between an useful but somewhat indefinite level (see, for instance, Cita, 1968) and a reliable, easily recognizable isochron as asserted, for instance, by Blow (1969, p. 201, 202, 223, 224). Nominally it is defined by the first appearance of supplementary apertures on the spiral side of small Globigerina species. Specimens thus distinguished have been referred to Globigerinoides apertasuturalis Jenkins 1960 and Globigerinoides quadrilobatus primordius Blow and Banner 1962 and considered ancestral to the diversified Neogene forms of Globigerinoides.
It has been assumed that, as in other lineages, the evolutionary step from Globigerina to Globigerinoides occurred simultaneously around the world. Unfortunately, however, there is growing evidence that this was not strictly true. Whereas Bolli (1957) and Blow (1969) record the earliest appearance of Globigerinoides within the short life‑range of Globorotalia kugleri, numerous records now exist of small Globigerinoides at earlier levels where presence of Globigerina ciperoensis (s.s.) and occasionally Globorotalia opima opima is indicative of Oligocene age. Such occurrences are recorded in Australia and New Zealand (Jenkins, 1960, 1965, 1971) France (Caralp, Valeton and Vigneaux, 1965), Italy (Borsetti, in Cita, 1968), the mid‑American region (Poag, 1972; Seiglie, 1973; and undocumented cases known to the writer), Colombia (H. Duque Caro, personal communication), and doubtless elsewhere.
Evidently, from these records, the level of first appearance of the genus should not be used rigidly to define the Globigerinoides Datum in its capacity as guide to the Oligocene /Miocene boundary. Conceptually more satisfactory for this purpose is the level at which Globigerinoides starts its explosive development into one of the most persistent and conspicuous genera in Neogene assemblages. For present purposes it suffices to mention that the problem exists and may require an arbitrary solution in certain areas.
7) The lowering of the Oligocene/Miocene boundary in America has resulted in a complete re‑evaluation of the Neogene. Currently the Lower/Middle Miocene boundary is equated with the “Orbulina Datum” and the Middle/Upper Miocene boundary is placed near the extinction of Globorotalia fohsi. In other words, the “Middle Oligocene” and “Upper Oligocene” of American authors prior to about 1955‑1960 are respectively equivalent to the Lower Miocene and Middel Miocene of today’s usage.
8) Absorption of much of the former “Oligocene” results in assigning to the Miocene a disproportionate thickness of the American Neogene, from which it becomes logical to suspect that certain beds assigned to the Upper Miocene might really belong to the Pliocene. Applications of planktonic foraminiferal zonation have verified this, a good example being the Playa Rica Formation of Ecuador (compare Stainforth, 1948 and Blow and Banner, 1967).
9) Likewise it might be expected that some formations treated as Pliocene would prove to be Pleistocene, as has indeed proved to be the case for the Cabo Blanco Group of Venezuela. Formerly there was reluctance to believe that orogenically deformed beds (as in Venezuela) or greatly elevated deposits (as in northern Peru) could be as young as Pleistocene, but the concept of plate tectonics now provides an explanation of such phenomena.
* * *
Documentation of the progressive revisions of Tertiary stratigraphy is voluminous and no attempt can be made here to provide a complete bibliography. A concise and amply documented summary of the European aspects was prepared by Berggren (1971). Key studies of planktonic foraminiferal biostratigraphy are those of Cati et al. (1968) and Blow (1969). As to the South American aspects, the case of Venezuela is representative and has been explained by Stainforth (1969).
It is a pleasure to acknowledge the active cooperation of Sra. Gloria Guevara de Rojas (Quito) and of Drs. Pedro J. Bermúdez (Caracas), Estebán Boltovskoy (Buenos Aires), Hermann Duque Caro (Bogotá), Max A. Furrer (Caracas), Frederico W. Lange (Rio de Janeiro), Norberto Malumián (Buenos Aires), Isao Noguti (Brazil), Rubén Martinez P. (Santiago de Chile), Francisco Paula de Medeiros (Rio de Janeiro), Enrico Perconig (Madrid), Jacques Sigal (Paris) and Johannes C. Troelsen (Brazil).
Bolli, H. M.
1957a The genera Globigerina and Globorotalia in the Paleocene‑Lower Eocene Lizard Springs Formation of Trinidad, B. W. I. U. S. Nat. Mus. Bull., n.° 215, pp. 61‑81. 1957b Pianktonic foraminif era from the Oligo‑Miocene Cipero and Lengua formations of Trinidad, B. W. I. U. S. Nat. Mus. Bull., n°. 215, pp. 97‑123.
Cati, F. y
otros (21 coautores)
1968 Biostratigrafia del Neo gene mediterraneo basata sui foraminiferi pianctonici. Soc. Geol. Italia Boll., vol. 87, pp. 491‑503. (Se resumió en castellano e inglés en Revista Española de Micropaleontología, vol. 1, número 1, pp. 103‑111, 1969.)
1960 Current status of transatlantic Oligo‑Miocene correlation by means of planktonic foraminifera. Revista Micropaléont., vol. 2, n°. 4, pp. 219‑230. 1969 Ages of Upper Tertiary and Quaternary formations in Venezuela (Edades de formaciones del Terciario superior y del Cuaternario de Venezuela). Asoc. Venez. Geol. Min. Petrol., Bol. inform, vol. 12, n°. 4, pp. 75‑90.
1953 Iskopaemye foraminifery SSSR; Globifjerinidae, Hantkeninidae i Globorotaliidae. [Foraminíferos fóssiles de la URSS; Globigerinidae, Hantkeninidae y Globorotaliidae]. Vses. Neft. Nauchno‑Issled. Geol. Razved. Inst. Trudy, n°. 76 (n. s.), 296 p. (Traducción en inglés emitido en 1971 por Collett’s Press Ltd., London y Wellingborough, 321 p.).
Manuscrito recibido el 31‑VII‑1973.