Retrospective of Brazilian marine palynostratigraphy with emphasis on Cretaceous dinoflagellates

Abstract

As concerns the Brazilian Cretaceous, palynostratigraphy – palynology applied to biostratigraphy – was mainly based on terrestrial palynomorphs (mostly spores and pollen grains) until about a decade ago. This was due to the following factors: (1) for several decades, Brazilian petroleum exploration was focused on the Recôncavo Basin, where the sediment filling is essentially nonmarine; (2) some other basins, both of marine and nonmarine character, have yielded reasonable results through the application of traditional palynostratigraphy based only on terrestrial palynomorphs; (3) pioneer foreign palynologists (e.g., H. Müller), who came to Brazil to train the earliest native biostratigraphers at PETROBRAS (Brazilian state-owned oil company), were all experts in spores and pollen grains; (4) marine Cretaceous rocks suitable to palynological study are poorly exposed in Brazilian onshore areas, so hampering investigations by Brazilian academicists who had no access to subsurface data. The last factor was particularly crucial in affecting the development of the Brazilian marine Cretaceous palynology. A striking contrast is noticed as the situation is compared to the study of Brazilian Devonian marine palynomorphs – mostly scolecodonts, chitinozoans and microphytoplankton. Here, investigations began as early as the forties and fifties (1940/1950’s), supplied by abundant materials collected from the extensive outcrop areas on the borders of the great Paleozoic intracratonic basins, particularly the Paraná and Amazonas basins. During the sixties, PETROBRAS extended oil exploration to the Brazilian continental shelf (offshore areas). Nevertheless, at that time the company did not invest immediately in marine palinostratigraphy, because it was believed that biostratigraphic schemes based on foraminifers and calcareous nannofossils would be more efficient and reliable than palynology in Cretaceous marine sequences. This belief changed only in the seventies, when the commercial oil fields were discovered in the Campos Basin. The oil reservoirs identified at that time were within the Macaé Formation, a rock unit deposited in a carbonate shelf environment under restricted (hypersaline), shallow marine conditions. These environmental conditions were certainly hostile to the development and post-mortem preservation of foraminifers and calcareous nannofossil-producing algae. In result, no more than two or three biozones could be identified in the Macaé carbonate section on the basis of such organisms. Besides, carbonate shelf sediments, subject to only minor terrigenous input, are usually poor in terrestrial palynomorphs. After some initial attempts it was soon concluded that traditional palynostratigraphy, based mainly on terrestrial palynomorphs, was unsuitable to the zonation of the Macaé formation. On the other hand, the palynological content of some Macaé strata consists mainly (up to 100%) of such marine palynomorphs as dinoflagellates, acritarchs, and palynoforaminifers. Consequently, PETROBRAS recognized the importance of the development of a zonal framework based on these organisms. The first zonation was erected in 1976, using one acritarch and three dinoflagellate species to subdivide the Macaé Formation. Since the eighties, marine Cretaceous palynostratigraphy has made significant advances mainly due to the use of dinoflagellates. Hundreds of Cretaceous dinoflagellate index species have been introduced in PETROBRAS databanks, and more detailed zonal schemes incorporating several dinoflagellate-defined bio-horizons and zones are becoming widely applicable to all basins of the Brazilian continental margin.