Biostratigraphic and paleoenvironmental inferences for the Late Cretaceous (Coniacian–Maastrichtian) of the São Paulo Plateau (DSDP Site 356) using calcareous nannofossils, ostracods, and geochemical data

Abstract:

Micropaleontological studies conducted in the drill cores of the Deep Sea Drilling Project (DSDP) in the 70s and 80s in the South Atlantic are considered important references, especially in sites such as the São Paulo Plateau. Petroliferous activity in Brazil have been developed in deep water Cretaceous deposits, making it necessary to reinterpret previously established biostratigraphic zonations for these areas. This work presents biostratigraphic and paleoenvironmental considerations based on the study of calcareous nannofossils, ostracods, x-ray fluorescence (XRF), total organic carbon (TOC), and total sulfur (S). The data were acquired from 66 samples from the basal section of Site 356, DSDP Leg 39, performed by the DSDP, in the southeastern portion of the São Paulo Plateau. A total of 140 nannofossil taxa and 34 ostracod species were identified. The nannofossil assemblages were diverse and abundant, with their preservation varying from moderate to good. In contrast, ostracods were rare and poorly preserved. Both ostracod and nannofossil species found show that there is a progressive increase in water depth towards the top of the drill core. Five calcareous nannofossils biozones were identified, ranging in age from the middle? to late Coniacian to the latest Maastrichtian, with the top of the section being of Danian age: UC 10, UC 11, UC 15 (c, d, e), UC19, and UC 20 (a, b, c, d). Two ostracod species of biostratigraphic value (Dutoitella mimica and Phacorhabdotus subtridentus) reinforce the ages identified. XRF analysis using Terr10/Ca, Zr/Rb, K/Al, Ti/Al, Mn/Al, Mn/S, V/Al indicate an increase in water depth towards the top, as well as two low oxygenation events at the base of the section and a possible record of a volcanic tephra. TOC and total S values reinforce the presence of low oxygenation at the base. These data, and the low abundance of nannofossils recorded in these samples, probably are result of an anoxic event occurring at that interval, which could also suggest that these are important hydrocarbon source rocks for the plateau. [IODP/CAPES 8888.091703/2014-01].