BIOMARKER ANALYSIS OF SOLVENT EXTRACTABLE ORGANIC MATTER FROM THE LATE NEOPROTEROZOIC KWAGUNT FORMATION, CHUAR GROUP (~800-742 MA), GRAND CANYON
The aliphatic fraction of the extractable organic matter contained a diverse array of biomarkers including n-alkanes, monomethyl alkanes, acyclic isoprenoids, branched alkanes with quaternary carbon atoms and polycyclic terpanes (bi-, tri-, tetra-, penta-). The data indicate the extracted hydrocarbons are syngenetic and thermally mature. Three general stratigraphic trends are observed. i) Biomarker abundance and TOC is lower during the positive δ13Corg isotopic excursions suggesting lower productivity. ii) Samples that are stratigraphically above the isotopic excursion, near the base of the Walcott, have highly variable biomarker abundances that suggest rapid changes in environmental conditions. Higher concentrations of gammacerane during this interval indicate restricted circulation and elevated salinity. iii) The OM abundance gradually decreases at the top of the Walcott Member before the onset of the Sturtian snowball Earth episode. The diversity and abundance of hopanes, including a homologous series of C30-C35 diahopanes, and elevated abundances of rearranged C27-C29 steranes are indicative of clay-mediated diagenesis.
These results suggest that organic matter production varied greatly during shifting climate conditions associated with the isotopic anomaly. The variations in compound class abundances also suggest a fluctuating redox state and salinity in the water column.