Abstract

Biomarker distributions are reported for Neoproterozoic units in the Kwagunt Formation of the Chuar Group, Grand Canyon, and the Red Pine Shale and the Gilbert Peak shale of the Uinta Mountain Group, Utah. According to biomarker parameters, sample organic matter is not significantly biodegraded and most samples are in the post mature range. Several samples contain an unusual enrichment in 5α(H), 14α(H), 17α(H)- C₂₇ sterane 20S and 20R relative to the 5α(H), 14β(H), 17β(H)- C₂₇ steranes and relative to C₂₈ and C₂₉ sterane isomers. The ααα C₂₇ sterane enrichment is inconsistent with other sterane and hopanes maturity parameters and may derive from a non-indigenous component, or from unique sterol precursors. Other major classes of compounds in these units are branched and cycloalkylalkanes which elute between the C₁₆ – C₃₅ n-alkanes. The monomethyl alkanes generally follow n-alkane distributions and do not co-occur with chain hydrocarbons having more complex branch patterns or functional groups (i.e. diethyl-, cyclopentyl-, and cyclohexyl- alkanes). Monomethyl alkanes are linked with cyanobacteria in geologic samples and modern environments (Shiea and others, 1990) and are taken to indicate cycanobaterial input in several samples of the Red Pine Shale and the Kwagunt Formation. 2-methylhopanoids detected in the Red Pine shale provide additional evidence of cyanobacterial input. The diethyl- and cycloalkylalkane series encountered in other samples of the Red Pine Shale, Gilbert Peak shale and Kwagunt Formation indicate the presence chemolithoautotrophic bacteria. Sterane distributions offer limited information on eukaryotic input. Molecular fossils from a Pre-Triassic form of dinoflagellates and marine algae are detected in a sample of the Kwagunt formation. C₂₇ sterane enrichment in samples of the Gilbert Peak shale, Red Pine Shale and Kwagunt Formation suggest restricted diversity dominated by a singular algal class such as the Rhodophyceae.