BIOMARKER ANALYSIS OF SOLVENT EXTRACTABLE ORGANIC MATTER FROM THE LATE NEOPROTEROZOIC KWAGUNT FORMATION, CHUAR GROUP (~800-742 MA), GRAND CANYON

Twenty seven predominantly siliclastic sediment samples from the Awatubi and Walcott Members of the Neoproterozoic Kwagunt Formation were solvent extracted and analyzed for hydrocarbon biomarkers using metastable reaction monitoring gas chromatography-mass spectrometry (MRM-GCMS). These samples mark a probable continuous record of Neoproterozoic deposition along an intracratonic rift basin located within 10°N and S of the equator that predates the breakup of Rodinia and the onset of the Sturtian snowball Earth event (750-700 Ma). Deposition occurred in a shallow, subtidal to intertidal-supertidal marine environment. The Kwagunt Formation records a large positive δ¹³C_TOC excursion of up to 13‰ that is globally correlated.

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 δ¹³C_org 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.