Subsurface geologic data reveal that uppermost Canadian carbonate rock units previously designated as the Smithville and Black Rock Formations in northeastern Arkansas are intertonguing lithofacies of the Powell formation, which trends east-west across northern Arkansas, and do not overlie the Powell as tabular formations. Therefore, these units are considered members of the Powell.

The Powell of northeastern Arkansas was deposited in and marginal to a transgressing epeiric sea along the hinge line of the Reelfoot basin, which lay to the east. Transgression, coupled with subsidence, resulted in Black Rock wedging to the northwest, onlapping the Smithville. Numerous minor regressions superimposed interfingering upon the overall transgressive pattern. Influx of quartz sand terminated Powell deposition, initiating deposition of the Everton Formation without interrupting sedimentation.

Constantly shifting environments resulted in complex intertonguing of carbonate lithofacies, with nonmarine lithofacies in the west progressively giving way to marine lithofacies toward the east. The Powell of northwestern and north-central Arkansas is characterized by nonfossiliferous, unburrowed cryptalgalaminate dolostone deposited above the strand on occasionally inundated algal mudflats. The Smithville Formation of northeastern Arkansas is characterized by fenestral carbonates and an abundant gastropod fauna, deposited above the strand on frequently inundated algal mudflats. The Black Formation is characterized by two distinct marine lithofacies: burrow-mottled cryptalgalaminates, deposited below the strand but above wave-base in a low-energy, schizohaline environment; and abunda tly fossiliferous sponge-bearing biomicrudite, deposited below the strand but above wave-base in a moderately agitated, normally saline marine environment.

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