Organic and isotopic geochemical study of the middle Proterozoic Nonesuch Formation, North American Midcontinent Rift.

摘要

he approximately 1.1 Ga Nonesuch Formation is an important datum for organic geochemical and stable isotopic studies of Precambrian rocks because it contains relatively high concentrations of organic matter that have not been highly altered thermally. The purpose of this research is to reconstruct aspects of Proterozoic microbiota and diagenesis based on whole-rock geochemistry, biomarker analysis, and stable isotopic composition of organic carbon and total sulfur.;A logarithmic relationship between organic carbon and total sulfur (dominantly pyrite sulfur) is interpreted to result from heterotrophic respiration of sulfate-reducing bacteria in a marine paleoenvironment. The degree of pyritization of iron suggests that Nonesuch sediments were deposited under oxygenated bottom waters. The overall high ratio of total sulfur to organic carbon is interpreted to be the result of efficient pyrite formation driven by deposition of highly metabolizable organic matter, absence of bioturbation, and relatively high concentrations of reactive iron.;Diverse biosynthetic pathways of algae and bacteria are suggested by a wide range of molecular fossils (biomarkers) identified in saturated-hydrocarbon fractions of bitumen extracted from unmineralized Nonesuch strata. Pentacyclic triterpanes, steranes, monomethyl alkanes, acyclic isoprenoids, alkylcyclohexanes, and n-alkanes are identified and are interpreted to derive from eukaryotic and prokaryotic sources. Stereochemical configurations and ratios of components suggest that thermal maturity of the Nonesuch Formation is within the zone of petroleum generation and preservation.;Isotopic compositions of organic carbon and total sulfur provide a record of early diagenesis in Nonesuch sediments. Isotopic composition of organic carbon is