Sedimentology of the Upper Pennsylvanian organic-rich Cline Shale, Midland Basin: From gravity flows to pelagic suspension fallout

Although deep-water fine-grained sedimentary rocks comprise approximately two-thirds of the stratigraphic record, the transportation and depositional processes are poorly understood compared with their shelf counterparts. This study reports the range of fine-grained sedimentary rock lithofacies, transport, and depositional processes and cyclicities recorded in deep-water deposits on the basis of three continuous cored wells from the Upper Pennsylvanian Cline Shale, Midland Basin, USA, with the goal of elucidating general principles that can inform synthesis depositional models for deep-water fine-grained sedimentary systems. A combination of sedimentological, petrographic and bulk geochemical analysis has defined seven lithofacies that stack in a repeated pattern to constitute ca 8 to 20 m thick composite cycle sets. The lower unit of each composite cycle set is characterized by basal siliciclastic-rich lithofacies interpreted to record dilute, low-density turbidity currents, potentially derived from hyperpycnal input which grade upward into carbonate-rich lithofacies interpreted as debris-flow deposits or pelagic suspension deposits. The upper unit of each composite cycle set is characterized by basal carbonate-rich lithofacies interpreted as debris-flow deposits or pelagic suspension deposits which grades upward into siliciclastic-rich lithofacies interpreted to record dilute, low-density turbidity currents, potentially derived from hyperpycnal input. The cyclicities recorded in the Cline Shale are believed to be controlled by high-amplitude glacioeustatic sea-level fluctuation (mean: ca 100 m). The siliciclastic-rich lithofacies, potentially derived from hyperpycnal turbidity flows, were deposited during sea-level lowstand, when siliciclastic sediment-transport systems extended across the wide Eastern Shelf and deltas developed at the shelf margin. Carbonate-rich lithofacies are interpreted to be deposited from debris flows or pelagic suspension fallout during sea-level highstand when carbonate platforms were developed on the surrounding shelves. The prevalence of sediment density flow deposits, even in distal basin floor environments, challenges the conventional model that deep-water fine-grained sedimentary rocks are dominantly background sedimentation.

Automated summary

This study investigates the deep-water fine-grained sedimentary rocks in the Upper Pennsylvanian Cline Shale, Midland Basin, USA, through sedimentological, petrographic and bulk geochemical analysis of three cored wells. The findings reveal a repeated pattern of lithofacies stacking to form composite cycle sets, controlled by high-amplitude glacioeustatic sea-level fluctuation. The presence of sediment density flow deposits challenges the conventional model of deep-water sedimentation as dominantly background sedimentation.