Sediment dispersal and organic carbon preservation in a dynamic mudstone-dominated system, Juana Lopez Member, Mancos Shale

摘要

A balance between primary production, rates of sediment accumulation or dilution, and biological or diagenetic destruction has long been considered a key control on organic carbon preservation in modern offshore marine environments. Additionally, current understanding of sediment transport processes in offshore environments has advanced in the last decade to include variable energy and dynamic mechanisms, requiring a re-evaluation of ancient deposits in these systems. The Juana Lopez Member of the Mancos Shale preserves organic carbon-rich mudstone interbedded and interlaminated with sandstone that records high energy traction flow conditions. Core, outcrop and geochemical data from the Juana Lopez Member were used to elucidate sediment provenance and processes controlling organic carbon preservation and distribution in this mudstone-dominated system. Five dominant lithofacies with varying grain size, sedimentary fabrics, composition and grain origins were differentiated and were deposited in three main environments: the prodelta, fringe zone and low angle offshore ramp. Basal deposits of the Juana Lopez Member consist of siliceous sandstone-dominated, heterolithic deposits with characteristic sedimentary structures (for example, current ripples and normal grading) that indicate offshore-directed underflows, or hyperpycnites, delivered from the updip Ferron/Frontier deltaic system. In the upper portion of the Juana Lopez Member, a compositional change to biogenic carbonate-rich sandstone and mudstone is interpreted to be as a result of increased accommodation in central Utah (USA), associated base-level rise and shoreline-parallel sediment transport. Non-parallel laminated, organic carbon-rich mudstone is preserved throughout the Juana Lopez Member. Depositional fabrics and trace element signatures suggest that these deposits are the result of dynamic conditions at the sea floor and in the oxic to suboxic water column, further challenging the notion that organic-bea