ABSTRACT

The Jackfork Group in the Ouachita Mountains of Arkansas is an Upper Mississippian flysch succession that displays readily distinguishable lithofacies. The frontal Ouachitas contain a preponderance of disturbed beds which are interbedded with turbidite sandstones. These disturbed and disrupted beds developed at the foot of a northern unstable slope, the resultant bedding dependent upon the type of material involved, its state of consolidation, and the distance moved. Information gathered concerning the progressive changes from one type of sedimentary structure to another gives a qualitative measure of downslope distance. Muds which undergo submarine deformation upon a sloping sea floor first become flowage shales which change to blocky mudstones with continued failure. Failure of plas ic muds and cohesive sandstone progressively form contorted, chaotic, and rubble bedding. Deformation of muds and sands form slurried bedding and breccias. Movement of flowing, creeping, or sliding clean sand may have occurred but is not considered to be a type of disruptive bedding. The deformation of mobilized, liquefied sand under some sediment cover produced load structures, sandstone dikes, and intrastratal flow structure.

The Ouachita trough possibly consisted of an offsetting series of unstable shale slopes deepening southward to a gentle, west-dipping abyssal plain. The muds and channel sands upon the slope frequently slumped southwards, sometimes flowing out over the abyssal floor at right angles to basinal turbidites flowing westward down the trough axis. Belts where disturbed bedding comprises the greatest percentage of the entire section define the unstable margins of the flysch trough while the highest sand-shale ratios parallel the axis of the trough.