Abstract: Reevaluation of Morrowan-Atokan Series Boundary in Northwestern Arkansas and Northeastern Oklahoma

Patrick K. Sutherland, Robert C. Grayson, Jr., Grant D. Zimbrick

In the area of the type Morrowan Series in Washington County, Arkansas, the boundary of that series with the overlying Atokan traditionally has been placed at the base of the first cliff-forming sandstone above the Kessler Limestone Member of the Bloyd Formation. This horizon is the arbitrary location of the base of the Atoka Formation and the top of the Trace Creek Shale Member of the Bloyd Formation. Sandstones locally as thick as 20 ft (6 m) within the Trace Creek Shale in Washington County have been ignored. The assumption has been that deposition is regionally continuous from the Bloyd to the Atoka Formation in this area although it has been recognized that the base of the Atoka Formation becomes a regionally truncating surface within 20 mi (32 km) west, in Oklahoma. /P>

New field evidence indicates that (1) the Trace Creek Shale in Washington County, Arkansas, is a facies of the lower part of the Atoka Formation in Oklahoma and (2) the regionally truncating unconformity at the base of the Atoka Formation in Oklahoma passes eastward not into a horizon at the base of the cliff-forming Atoka sandstones but into one at or near the base of the underlying Trace Creek Shale.

The presence of a regionally significant hiatus at or near the base of the Trace Creek Shale in Washington County, Arkansas, and Adair County, Oklahoma, also is indicated by a gap in the conodont faunal succession, as compared with other, more complete, stratigraphic sequences in the Ardmore basin and in the frontal Ouachitas in southern Oklahoma. Conodont faunas from as few as 10 (3 m) above the base of the Trace Creek Shale correlate with occurrences in the Atokan Series in southern Oklahoma, whereas those in the underlying Kessler Limestone Member of the Bloyd Formation are Morrowan in age. Consequently, the Morrowan-Atokan boundary in northwestern Arkansas and northeastern Oklahoma should be redefined to conform with these new data.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma