ABSTRACT

The Lower Atoka Formation along Highway 5, northwest of Jacksonville, Arkansas, consists of an alternating sequence of turbiditic sandstones and shales. Six sandstone and shale sequences can be distinguished, separated by thick units of shale. These six can be grouped into three distinct facies. Facies I is comprised of stacked, amalgamated, thick-to-very-thick-bedded, massive sandstones; Facies II of intercalated sandstone and shale layers with varying bed thickness; and Facies III of shale beds which are different from one another. These shale beds occasionally contain layers of thin-bedded silty sandstones that represent slumps or debris flow deposits. Stacking patterns within the six sequences are variable with upward-thickening, upward-thinning, and mixed sequences present. The sandstone sequences represent lowstand and transgressive deposits in a channelized mid-fan to sheet sands in an outer fan setting. The thick shales represent either highstand system tracts, lateral overbank deposition due to avulsion of the centers of major sand deposition, or top fill of a channel forming during lowstand.

The sandstones show a variety of sedimentary structures, such as amalgamated contacts, graded bedding, current ripples, climbing ripples, apparent cross-beds, and parallel laminations. None are uniquely indicative as to the location on the submarine fan. Comparison with literature and outcrops of similar fine-grained turbidites proved the best means of interpreting the deposits and their setting.