--> Abstract: Preservation of Cross-bedding by Carbonate Grains in a Quartz Arenite, Childress Member, Whitehorse Group, Permian Basin, Texas, USA, by J. O. Jones; #90987 (1993).

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JONES, JAMES O., The University of Texas at San Antonio, Geology Department, San Antonio, TX

Preservation of Cross-bedding by Carbonate Grains in a Quartz Arenite, Childress Member, Whitehorse Group, Permian Basin, Texas, USA

The Childress Member of the Whitehorse Group is a two part unit in outcrop, quartz arenite in the lower and gypsum in the upper. However, the lower unit superficially appears to be massive gypsum beds similar to the upper unit. It is cemented with gypsum. Occasionally medium-grained carbonate grains, ooids and well rounded medium-grained quartz particles are preserved on lee slopes of cross-bedding in very fine-grained sandstone and silt of the lower unit. Dissolution of gypsum cement often leaves the larger particles standing in relief on weathered cross-sectional surfaces of beds in the lower unit. Internal structures of the quartz arenite are usually not preserved without the carbonate and quartz grains. Upon total removal of the gypsum cement the quartz sandstone becomes a friable sand with nostructures. The upper unit is massive gypsum with interbedded laminations of algal mats, very little sand in present. Gypsum cement in the lower unit probably was derived from partial dissolution of the upper unit and provides a poikilotopic fabric.

Occasionally small symmetrical ripple marks and large linguoid ripples are preserved on bedding planes. Cross-sections of linguoid ripples are preserved and commonly show reactivation surfaces. Large cross-stratification, up to 1.5 meters, is preserved by carbonate and well rounded quartz grains on bedding surfaces. Carbonate pebbles and very coarse-grained quartz fragments are present at the toe of some lee slopes. The Childress Member was deposited on a mud-rich siliciclastic tidal flat and includes well preserved tidal channels.

The Childress Member has been traced from Fisher County into Cottle County, a distance of over 170 km. It caps buttes and mesas and is exposed along east-facing scarps in the western part of the counties. It is part of the up-dip edge of westward dipping strata of the region.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.