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By R. M. Stainforth [2]

Reference is made to Dr. H. H. Renz’ article in the June 1961 Boletín Informativo entitled “Correlation of geologic formations in Venezuela”. I am not concerned at the moment with formational names, but with the ages assigned to certain units.

Dr. Renz observes: “In reading the stratigraphic table it should be kept in mind that in the western hemisphere, particularly in the Caribbean area, the position of the contacts between the Cretaceous and Tertiary, and between the Oligocene and Miocene, are not fully established and, thus, are subject to personal interpretation.” With this statement as justification he places the Cretaceous/Tertiary and Oligocene/Miocene boundaries essentially where they were in his well-known paper of 1942.

In my opinion enough concrete evidence has been put forward in the last dozen years to justify the appreciable adjustments already made to these boundaries by other authors. It is true that many of our faunal units have moved up and down the geologic time scale at the whim of various authors, and some skepticism is thereby justified regarding proposed new revisions. However, intensified studies of the ranges of planktonic foraminifera have presented us with a tool valid for direct intercontinental correlations, and modern correlations within the Upper Cretaceous and Tertiary are definitely firmer and less liable to change than they used to be. The Danian stage, for instance, is readily identifiable and there is no reason to hyphenate it with the Maestrichtian as does Dr. Renz.

Regarding the Cretaceous/Tertiary boundary, the problem is not, as Dr. Renz implies, one of correlation of the Caribbean region with other areas. A standard sequence of zonally significant Globotruncanidae and Globorotaliidae has been established, by reference to which interregional correlations can readily be made. The real problem is agreement on where the Cretaceous/Tertiary boundary shall be placed within this sequence.

In the past the Danian stage has often been treated as transitional, with strong post-Cretaceous affinities in its lack of ammonites, belemnites, and rudistid and inoceramid mollusks, but with certain pre-Tertiary affinities in the lingering presence of Cretaceous types of bryozoa, brachiopods, pelecypods and echinoderms. In recent years it has been demonstrated that the typical Cretaceous planktonic foraminifera, especially the Globotruncanidae and Heterohelicidae, die out completely and abruptly at the end of the Maestrichtian. They are replaced in the Danian by a new suite, not highly distinctive but clearly a forerunner of the Globorotalia-Globigerina assemblages which dominated the Lower Tertiary. Also it has been shown that the level between extinction of the dinosaurs and appearance of the placental mammals corresponds to the base of the Danian. These new lines of evidence strengthen the case for equating the Maestrichtian/Danian boundary with the Cretaceous/ Tertiary boundary.

A symposium on this topic was held at the Geological Congress in Copenhagen last year. Detailed evidence of the smaller and larger marine invertebrates of four continents was presented and of the laid vertebrates of North America, as well as less detailed reference to still other faunas. Five of the papers are inconclusive, mainly because of lack of critical faunas in the areas studied, Of the remaining 17 authors, 15 indicate the Maestrichtian/Danian level as the Cretaceous/Tertiary boundary, most of them in decisive terms. Of the remaining two, Lys writing on Madagascar treats the Danian as a “zone de passage ou transition”, but seems to be following previous usage rather than re-assessing the evidence, Only Yanshin, writing on Russia, combines a thoughtful discussion with the opinion that the Danian may as well be considered late Cretaceous as early Tertiary.

It is evident from this that here in Venezuela, if we wish to conform with majority opinion, we should recognize the Danian stage and put it at the base of the Tertiary sequence. In western Venezuela this would require lowering the Mito Juan/Guasare contact, and in eastern Venezuela a greater proportion of the Vidoño would become Tertiary,

Regarding the Oligocene Miocene boundary, Dr. Renz places it at the top of the range of Globorotalia fohsi (s. l.), and this seems indefensible. The G. fohsi zone is recognizable in western Europe by direct matching of ranges of planktonic index foraminifera, and it corresponds quite closely to the Burdigalian stage, The overlying zone of Globorotalia menardii (s. l.) is equally readily identified as the Vindobonian (Helvetian-Tortonian) stage. In other words, the level which Dr. Renz recommends as the Oligocene/Miocene contact is actually the Lower/Middle Miocene boundary. The placement of the Aquitanian stage, whether Upper Oligocene or Lower Miocene, has no bearing whatever on this correlation.

I do agree with Dr, Renz that identification of the Oligocene/Miocene boundary in the Caribbean region is difficult. So few authorities now consider the Aquitanian as possibly Oligocene that the real problem is identification of the Chattian/Aquitanian boundary. (This is unfortunate for us because the top of the Aquitanian is well defined by the rapid evolution of Porticulosphaera spp. culminating in the ubiquitous Orbulinas of the Burdigalian) The evidence of planktonic foraminifera alone is inconclusive, but taken in conjunction with that of the larger foraminifera, especially the Miogypsinidae, it seems that the Aquitanian must be carried down to include the zone of C. dissimilis (despite its long allocation to the mid-Oligocene)

For practical purposes the base of the C. dissimilis zone would be a good ar–bitrary choice for the base of the Caribbean Miocene. it is readily defined by the explosive advent of the Globigerinoides triloba clan, a group more tolerant of shallow-water conditions than most plankton. Generally speaking, if a bed contains any plankton and is within the stratigraphic range of C. triloba (s.l), that species will be present. Consequently separation of Oligocene from Miocene could rest on presence or absence of this index, and the negative evidence would usually be valid, providing the material studied contained some plankton.

Dr. Renz recognizes the possibility that some day what he has labelled Middle and Upper Oligocene will, have to be put in the Lower Miocene. We differ in that I think the time to do it is now, while we are all in the mood to scrutinize Venezuelan stratigraphy. The Oligocene/Miocene boundary here proposed is arbitrary, but it is easily recognized and comes much closer to the factual evidence than the top of the G. fohsi zone. Dr. Eames in London has fired a couple of shots across our bows, warning that he plans to eradicate the Oligocene from the Caribbean sequence. When we have perused and analysed his promised book, the picture may become clearer.


BERGGREN, W. A., 1960: “Paleogene biostratigraphy and planktonic foraminifera of the SW, Soviet Union”, University of Stockholm, Contributions in Geology, vol. VI, N° 5

BOLLI, H. M., 1960 (1959): “Planktonic foraminifera as index fossils in Trinidad, West Indies. and their value for worldwide stratigraphic correlation”, Eclog Geol. Helv,, vol. 52, N° 2, p. 627-637

EAMES, F. E, et al., 1960: “Mid-Tertiary stratigraphical paleontology” Nature, vol. 185, N° 4711, p. 447-448; also Idem, vol. 187, N° 4738, p. 679-680

RENZ, H. H., 1942. “Stratigraphy of northern South America, Trinidad and Barbados”, 8th Am, Sci. Congr, Proc., vol. 4, p. 513-571

âÄ”, 1961: “Correlation of geologic formations in Venezuela”, A.V.G.M.P, Bol. Inf., vol. 4, N° 6 p. 199- 203

STAINFORTH, R. M., 1960: Current status of transatlantic Oligo/Miocene correlations by means of planktonic foraminifera”, Rev. Micropaléont, vol. 2, n° 4, p. 219- 230

SYMPOSIUM, 1960: “The Cretaceous/Tertiary boundary”, Internat. Geol. Congr., rept. of 21st Session, Part V

[1] Manuscript received 24 July 1961. Published with the permission of Creole Petroleum Corporation.

[2] Geologist, Creole Petroleum Corporation.