Abstract: Edwards Group, Surface and Subsurface, Central Texas

By

Peter R. Rose

In the subsurface of south-central Texas the Lower Cretaceous Edwards Group consists of about 400 to about 600 feet of porous dolomite and limestone that accumulated on the Comanche shelf as shallow marine, intertidal, and supratidal deposits in the lee of the Stuart City Reef. The Edwards is conformable with the Glen Rose Formation below and disconformable with the Georgetown Formation above.

At the surface in the eastern Edwards Plateau of Central Texas the Edwards Group consists of about 400 to about 650 feet of dolomite and limestone similar to that of the subsurface Kainer and Person Formations; it thickens southward from the Central Texas Platform and grades into rudist limestone of the Devils River Formation. The Edwards is divided into the Fort Terrett Formation (new) below and the Segovia Formation (new) above, separated by a widely traceable marine interval, the Burt Ranch Member (new), at the base of the Segovia.

Fredericksburg and Washita rocks in Texas represent nine major depositional environments, which were controlled by circulation patterns of marine waters, water depth, and terrigenous material in sea waters. Each environment can be recognized in the rock record by a combination of mineralogy, faunal content, sedimentary structures, and tectonic setting. The nine environments are: open deep marine; open shelf; open shallow marine, moderate to high-wave energy; open shallow marine, low-wave energy; restricted shallow marine; tidal flat; euxinic evaporitic shelf basin; evaporite-dominated supratidal flat; coastal terrigenous.

From northeast to southwest, Edwards and equivalent units form two complete carbonate facies complexes. The lower (Fredericksburg) complex reflects extremely shallow water, high salinities, and low subsidence rates. The upper (Lower Washita) complex is a facies assemblage more like the standard carbonate model, with low-angle clinoforms along the subsiding basin margins and increasing dolomite toward the interior of the Central Texas Platform.

Through Fredericksburg and Washita time, facies tracts retreated onto higher parts of the Central Texas Platform as it was progressively inundated during deposition of three units of successively deeper-water sediments.

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