ABSTRACT

The Pennsylvanian Jackfork Group in the Ouachita Mountains of Arkansas and Oklahoma had conventionally been considered a classic example of a turbidite sequence deposited in a submarine-fan setting. However, the apparent "Bouma turbidite sequences" in these strata, which were used as evidence for single-event turbidity current deposition, were in reality deposited by multiple events, including debris flows and slumps. These were then commonly reworked by bottom currents. Normal size grading and Bouma sequences are essentially absent in these sandstone beds, which appear "massive" (i.e., structureless) in outcrop, but when slabbed reveal diagnostic internal features. These beds exhibit sharp and irregular upper bedding contacts, inverse size grading, floating mudstone clasts, planar clast fabric, lateral pinch-out geometries, moderate to high matrix content (up to 25%), contorted layers, and fluid escape structures. All these features are indicative of sand emplacement by debris flows and slumps. The dominance of debris-flow and slump deposits (nearly 70%) and the lack of turbidites have led us to propose a slope-dominated paleoenvironment for these sediments. Conventional submarine-fan models, designed for turbidite-dominated systems, are not applicable to the debris-flow and slump-emplaced sandstone beds of the Jackfork Group.