Organic and isotope geochemistry of the Early Cretaceous rift sequence in the Camamu Basin, Brazil: paleolimnological inferences and source rock models

The Camamu Basin, located at the eastern Brazilian continental margin, is characterized by a thick Early Cretaceous organic-rich rift sequence. The integration of bulk geochemical, isotopic and biomarker data with paleontological and geological information provided the basis for a better assessment of rift lake evolution (hydrological regime, redox potential, salinity, etc.) and its impact on source rock development. Pyrolysis and visual kerogen analyses reveal the dominance of lipid-rich algal (bacterial?) type I kerogen through the entire rift sequence. Paleolimnological reconstruction suggests that the lower rift sequence (Morro do Barro Formation, Neocomian) was deposited in a fault-bounded deep lake with fresh to brackish water, stable water column stratification and water bottom anoxia. High hydrogen indices (700-900 mg HC/gTOC) and C-13-depleted carbon isotopic composition of the organic matter (delta(13)C values around -30%(0)) might reflect enhanced organic preservation, the recycling of light CH4 generated by methanogenesis, and the incorporation of isotopically light lipid-rich bacterial biomass. On the other hand, the upper rift sequence (Rio de Contas Formation, Barremian) was probably deposited within a broader/shallower lake with a higher frequency of water-column overturn and a deeper thermocline. High organic carbon contents and strong C-13 enrichment of the organic matter (delta(13)C excursions of up to -23%(0)) are interpreted to have resulted from enhanced primary productivity triggered by an increased input/recycling of nutrients favored by lake morphology and humid climate. (C) 2002 Elsevier Science Ltd. All rights reserved.