Reinterpretation of Depositional Processes in a Classic Flysch Sequence (Pennsylvanian Jackfork Group), Ouachita Mountains, Arkansas and Oklahoma: Discussion

Abstract

The Pennsylvanian Jackfork Group in the Ouachita Mountains of Arkansas and Oklahoma has conventionally been interpreted by many workers, including us, as a classic flysch sequence dominated by turbidites in a submarine fan setting; however, normal size grading and Bouma sequences, indicative of turbidite deposition, are essentially absent in these sandstone beds. They 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, a planar clast fabric, lateral pinch-out geometries, moderate to high detrital matrix (up to 25%), sigmoidal deformation (duplex) structures, and contorted layers. All these features indicate sand mplacement by debris flows (mass flows) and slumps. Mud matrix in these sandstones was sufficient to provide cohesive strength to the flow. Discrete units of current ripples and horizontal laminae have been interpreted to represent traction processes associated with bottom-current reworking. The dominance of sandy debris-flow and slump deposits (nearly 70% at DeGray Spillway section) and bottom-current reworked deposits (40% at Kiamichi Mountain section), and the lack of turbidites in the Jackfork Group have led us to propose a slope setting. Our rejection of a submarine fan setting has important implications for predicting sand-body geometry and continuity because deposits of fluidal turbidity currents in fans are laterally more continuous than those of plastic debris flows and slumps on slopes. A turbidite-dominated fan model would predict an outer fan environment with laterally continuous, sheetlike sandstones for the Jackfork Group in southern Oklahoma and western Arkansas, whereas a debris-flow/slump model would predict predominantly a slope environment with disconne ted sandstone bodies for the same area.