Morrow Group and Lower Atoka Formation of Arkansas

Abstract

The Morrow group and the Atoka formation represent the lower Pennsylvanian and the lower part of the middle Pennsylvanian, respectively, of the Ozark Highlands of Arkansas and Oklahoma. The Morrow group has previously been divided into two formations--the Hale formation below and the Bloyd shale above. In the past, two members of the Bloyd shale have also been recognized and mapped--the Brentwood limestone member and the Kessler limestone lentil. The Hale formation has been regarded as having a highly variable composition, but study shows that it is divisible into two distinct parts, for which names are proposed in this paper--a lower, shaly, more or less calcareous, silty sandstone unit, the Cane Hill member, and an upper, massive, variously sandy, cross-bedded, pock-mar ed limestone unit, the Prairie Grove member. The Bloyd shale contains a coal-bearing terrestrial unit above the Brentwood limestone member to which the name Woolsey member is applied. The base of the Atoka formation is clearly and prominently marked over a wide area by a bench-forming, silty, thin-bedded marine sandstone unit, named in this paper the Greenland sandstone member. The facies fossils Taonurus and Scalarituba in the Greenland member and certain beds higher in the Atoka differentiate those strata locally from the Morrow.