Geochemical and Micropaleontological Characterization of Lacustrine and Marine Hypersaline Environments from Brazilian Sedimentary Basins

Abstract

Geological studies integrated with paleogeographical, geochemical, and micropaleontological evidence, suggest that part of the organic-rich Permian and Lower Cretaceous to middle Albian successions in the Brazilian sedimentary basins were deposited in marine and lacustrine to marine hypersaline environments. Biological marker and micropaleontological data can be used to characterize four distinct hypersaline systems: 1. a Late Permian epicontinental hypersaline system (Irati Formation, Parana basin), composed of calcareous black shales that contain abundant lipid-rich organic matter (total organic carbon [TOC] up to 24%); 2. an Early Cretaceous, shallow lacustrine, hypersaline system composed of thick, organic-rich, calcareous black shales (TOC up to 9%) containing lipid-rich organic matter; 3. an Aptian, shallow marine evaporitic system (proto-oceanic gulf) mainly composed of marlstones, carbonates, and calcareous black shales containing abundant lipid-rich amorphous organic matter (TOC up to 14%); and 4. a late Aptian-middle Albian, marine semi-restricted, dominantly neritic, hypersaline system, composed of marlstones and calcareous black shales containing abundant lipid-rich algal organic matter (TOC up to 12%). The main molecular features diagnostic of these environments are: phytane more abundant than pristane, low abundances of diasteranes and tricyclic terpanes, high abundances of gammacerane, s-hopanes, s-carotane, regular C27 to C29 steranes, regular C25 isoprenoid and squalane, Ts<Tm, and C35s-hopanes more abundant than C34s-hopanes. End_Page 17------------------------- The micropaleontological features that characterize these systems are the occurrence of abundant low-diversity communities representing opportunistic species, and common occurrence of millimeter-thick strata with census populations, representing mass mortality events during short-term environmental turnovers.