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CLARK, CHRISTIAN J., Reservoirs, Inc., Houston, TX; ARNOLD H. BOUMA, and GIAN A. CONSTANTINE, Department Of Geology & Geophysics, Louisiana State University, Baton Rouge, LA

Abstract: Interpretation of Turbidites of the Lower Atoka Formation, Arkansas

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 is comprised of intercalated sandstone and shale layers with varying bed thickness; and Facies III is comprised of shale beds which are different from one another. These shale beds occasionally contain layers of thin-bedded silty sandstones which represent slumps or debris flow deposits. Stacking patterns within the six sequences are variable with upward-thickening, upward-thinning, and mixed sequences being evident. It is inferred that the sandstone sequences represent lowstand and transgressive time deposition in a channelized mid-fan to sheet sands in an outer fan setting. The thick shales represent either highstand deposition or more likely, lateral overbank deposition due to avulsion of the centers of major sand deposition, or top fill of a channel.

The sandstones show a variety of sedimentary structures, such as amalgamated contacts, apparent foresets, some graded bedding, current ripples, climbing ripples, 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 prove to be the best means of interpreting the deposits, and their setting. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana