KERANS, CHARLES, Bureau of Economic Geology, University of Texas at Austin, Austin, TX, and PAUL M. HARRIS, Chevron Petroleum Technology Company, La Habra, CA, and JONES, JAMES O., University of Texas at San Antonio, San Antonio, TX

ABSTRACT: Gypsum of the Lower Whitehorse Group, Eastern Shelf, Permian Basin, Texas: A Fine-Grained Sandstone

Basal Whitehorse rocks have been mapped on the Texas Atlas sheets for Childress County and southward to Fisher County as the Childress Dolomite. However, near the Childress and Cottle counties line, the dolomite pinches out and, at nearly the same horizon, is a massive and resistant gypsum and sandstone unit about 7.0 m thick. The upper 3.5 m are principally massive gypsum, the lower 3.5 m are gypsum cemented fine to very fine-grained sandstone.

Of particular interest is the lower 3.5 m. It is cemented with diagenetically derived gypsum, probably from the overlying massive gypsum unit. Sedimentary structures are generally absent throughout the sandstone. At a few localities, excellent features are preserved: large cross-stratification up to 1.5 m, symmetrical and linguoid ripple marks, and smaller cross-bedding. All features preserved and visible on outcrop are the result of larger grains of well-rounded quartz, oolites, carbonate fragments, and chert. Dissolution of gypsum leaves the sandstone friable, and structures are seldom preserved.

Larger cross-stratification is part of megaripples or dunes. Symmetrical ripples develop on dunes. Possible tidal channels may be part of the unit. Therefore, the basal Whitehorse sandstone was deposited on a mud-rich siliciclastic tidal flat.

AAPG Search and Discovery Article #90994©1993 AAPG Southwest Section Meeting, Fort Worth, Texas, February 21-23, 1993.