PALEOAMBIENTE DEPOSICIONAL E ORIGEM DA MATÉRIA ORGÂNICA DOS FOLHELHOS BETUMINOSOS DA FORMAÇÃO IPUBI, PORÇÕES LESTE E SUDESTE DA BACIA DO ARARIPE, NE DO BRASIL: EVIDÊNCIA DE N-ALCANOS E ISOPRENOIDES

Paleoenvironmental conditions from cretaceous black shales of Ipubi Formation (Araripe Basin-NE Brasil) are continuously debated. The main point of controversy is if these rocks were deposited in a lacustrine or restricted marine environment. The present study was developed in these black shales, by saturated biomarkers analyses (n-alkanes and isoprenoids) to discuss about the paleoenvironment of deposition of those shales. The n-alkanes predominance between n-C13 and n-C19 reveal algalic contribution. However an influence of terrestrial organism is also observed by the presence of n-C27, n-C28, and n-C29. The Pr/Ph ratio and Pr/Ph vs. (Pr+n-C17)/(Fi+n-C18) diagram indicated both anoxic and hypersaline conditions during the deposition of those shales. The Pr/n-C17 vs. Ph/n-C18 diagram also suggests reducing conditions, and classify an algalic contribution as marine-transitional source. Thus, this study indicates that the investigated shales were deposited in reducing and hypersaline conditions in a restricted marine to transitional environment.