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Abstract: Deep-Water Sandstones of the Pennsylvanian Jackfork Group, Ouachita Mountains: A Slope-Dominated System

G. Shanmugam, R. J. Moiola

Conventionally, the Pennsylvanian Jackfork Group in the Ouachita Mountains of Arkansas and Oklahoma has 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, are in reality composed of multiple events emplaced by debris flows and slumps, and commonly reworked by bottom currents. Normal size grading and Bouma sequences are essentially absent in these sandstones. Most sandstone beds appear "massive" (i.e., structureless) in outcrop, but when slabbed and examined reveal diagnostic internal features. These beds exhibit sharp upper bedding contacts, inverse size grading, floating mudstone clasts, planar clast fabric, contorted layers, fluid escape structures, and moderate to high matrix content. All these features are indicative of sand emplacement by debris flows (mass flows) or slumps. The dominance of debris-flow 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 have led us to propose a slope-dominated system. Conventional submarine fan models, designed for turbidite systems, are not applicable to the debris-flow emplaced and bottom-current reworked sandstones of the Jackfork Group. This unconventional model has direct implication for sand-body geometry and continuity because deposits of fluidal (suspension) turbidity currents are laterally more continuous than those of plasti (en masse) debris flows.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994