Print this pageSources of Organic Matter and the Paleoenvironment of Neocomian Lacustrine Source Rocks from the Potiguar Basin (Brazil)
SANTOS NETO, E. V., and J. M. HAYES,
The Neocomian (Early Cretaceous) organic-rich shales of the Pendencia Formation are the lacustrine source rocks of the Potiguar Basin, Northeastern Brazil. Those sediments were deposited during the rift phase of the Brazilian marginal basins. Carbon isotopic compositions of saturated hydrocarbons were obtained from core samples drilled in a thermally immature part of the section. Isotopic compositions of n-alkanes vary systematically with carbon number. At most depths in the section the range is about 8 o/oo, indicating a significant diversity of sources. The pattern of the variation is consistent with depth, requiring that the diverse community of source organisms persisted over time. Hopanes are strongly depleted in {13}C (to delta{13}C approx.= -55.6 o/oo vs. PDB) indicating that methane was both produced and consumed and suggesting that the water column was well stratified. Both beta- and gamma-carotane are present. Their isotopic compositions differ, suggesting inputs from halophilic algae and from halophilic heterotrophic bacteria. The Pendencia paleolake apparently had a stable photic zone occupied by both algae and cyanobacteria. Bottom waters and/or sediments were anaerobic and recycling of CH[4] was favored.