摘要：Retrospective of Brazilian marine palynostratigraphy with emphasis on Cretaceous dinoflagellates.
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.
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