An isotopic and geochemical study of climatic and oceanographic factors influencing organic-matter preservation during the late Cretaceous in Sergipe Basin, Brazil.

摘要

Mesozoic evolution of the Sergipe Basin, in northeastern Brazil is linked to rifting of Gondwanaland and opening of the South Atlantic Ocean. At the Votorantim Quarry, located in the State of Sergipe, about 70 meters of Upper Cretaceous marine strata are well exposed and contain more than 300 hemipelagic limestone/marlstone couplets (Sergipe couplets) deposited in a carbonate ramp during the open ocean stage of basin evolution. The Votorantim section is late/early to early/middle Turonian in age and is part of the Sapucari Member of the Cotinguiba Formation. The depositional periodicities of the Sergipe couplets vary between 1 to 10 kyr and probably reflect equatorial climate dynamics under Cretaceous greenhouse conditions.; The primary depositional nature of Sergipe couplets was demonstrated using integrated isotopic and geochemical techniques. Titanium-normalized concentration of Fe, Mg, Mn, and Sr indicated that diagenetic unmixing was minimal. Abundances of organic carbon (C{dollar}sb{lcub}rm org{rcub}{dollar}) and carbonate carbon (C{dollar}sb{lcub}rm carb{rcub}{dollar}) suggest that the Sergipe couplets were formed primarily by variations in carbonate productivity. Alternating dry and wet climatic intervals controlled the input of carbonate from shallow water banks through the sensitivity of carbonate producers to water turbidity. Limestone deposition occurred during the dry periods under clear water conditions. Three intervals with elevated concentration of C{dollar}sb{lcub}rm org{rcub}{dollar} were interpreted to result from enhanced primary productivity. Two of these intervals were characterized by positive excursions in the isotopic composition of C{dollar}sb{lcub}rm org{rcub}{dollar} probably resulting from productivity blooms. Biogeochemical cycles during early diagenesis were assessed using the isotopic composition of various sulfur species. The combined use of pyrolysis-gas chromatography/mass spectrometry and petrographic techniques provided new insights into the mechanism of organic matter sulfurization and kerogen formation.