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

Abstract

Associate Editor -Piret Plink-Bj€ orklund ABSTRACT 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-bearing mudstone is deposited solely through suspension settling in anoxic waters. Punctuated delivery of organic carbon laden sediment from mixed terrestrial and marine sources resulted in an event-bed style of organic carbon deposition and preservation.

Detailed facies analysis of Cenomanian–Turonian organic‐rich mudstones: Implications for depositional controls on source rocks

“…One of the controls on the abundance of preserved organic matter in the study interval appears to be a balance between low dilution by siliciclastic material and periodic, rapid input of sediment to bury and preserve organic carbon‐rich beds. A similar phenomenon is documented in the offshore deposits of the Mancos Shale in DeReuil and Birgenheier (). In the lower 2WS Formation, bottom currents rework and concentrate organic carbon present in biogenic aggregates and pellets, while periodic wave‐enhanced and current‐enhanced flow events rapidly bury and preserve these beds.…”

Effects of Anisotropy and Saturation on Geomechanical Behavior of Mudstone

“…The Mancos Shale is a detrital silica‐ and clay‐rich mudstone unit, deposited during the Late Cretaceous (Turonian to Campanian). Detailed descriptions of the geological context of the Mancos Shale can be found in Birgenheier et al () and DeReuil and Birgenheier (). Calcareous mudstone samples from the Agrio Formation were taken from outcrop cores drilled in the Neuquén Basin, Argentina, from a detailed section measured at El Portón outcrop locality.…”

“…Facies analysis provided a wholistic description of sample heterogeneity. More details on the procedure for the lithofacies analysis are outlined and detailed in Birgenheier et al () and DeReuil and Birgenheier (). Analysis was completed in order to ensure that a broad range of intrinsic bedding plane anisotropy and compositional, textural, and fabric heterogeneity was tested at a variety of orientations, saturation levels, and confining pressures.…”

Decoding the origins and sources of clay minerals in the Upper Cretaceous Tununk Shale of south‐central Utah: Implications for the pursuit of climate and burial histories

“…The complex sources and dispersal processes (and pathways) of fine-grained sediments in marine environments, as revealed by sedimentologic and petrographic analysis Taylor and Macquaker, 2014;Harazim and McIlroy, 2015;Lazar et al, 2015;Li et al, 2015;Wilson and Schieber, 2015;Schieber, 2016a;Li and Schieber, 2018a;DeReuil and Birgenheier, 2019), pose challenges for the interpretation of climate signals from clay mineral assemblages of ancient marine mudstone successions (Clayton et al, 1999;Thiry, 2000). The complex sources and dispersal processes (and pathways) of fine-grained sediments in marine environments, as revealed by sedimentologic and petrographic analysis Taylor and Macquaker, 2014;Harazim and McIlroy, 2015;Lazar et al, 2015;Li et al, 2015;Wilson and Schieber, 2015;Schieber, 2016a;Li and Schieber, 2018a;DeReuil and Birgenheier, 2019), pose challenges for the interpretation of climate signals from clay mineral assemblages of ancient marine mudstone successions (Clayton et al, 1999;Thiry, 2000).…”

“…Recent advances in the understanding of the provenance and depositional processes of fine-grained sediments, however, indicate that clay minerals in mudstones can have a wide variety of origins. The complex sources and dispersal processes (and pathways) of fine-grained sediments in marine environments, as revealed by sedimentologic and petrographic analysis Taylor and Macquaker, 2014;Harazim and McIlroy, 2015;Lazar et al, 2015;Li et al, 2015;Wilson and Schieber, 2015;Schieber, 2016a;Li and Schieber, 2018a;DeReuil and Birgenheier, 2019), pose challenges for the interpretation of climate signals from clay mineral assemblages of ancient marine mudstone successions (Clayton et al, 1999;Thiry, 2000). The common occurrence of silt to sand-sized clay-dominated composite particles (aggregates) of multiple origins in both ancient and modern finegrained deposits (Macquaker et al, 2010;Plint et al, 2012;Schieber, 2016b;Laycock et al, 2017;Shchepetkina et al, 2018;Li and Schieber, 2018b) strongly suggests that the origin of clay minerals in fine-grained sedimentary successions is a much more complex issue than commonly presumed.…”