ENVIRONMENT RECONSTRUCTION FOR A PART OF THE GLEN ROSE LIMESTONE, CENTRAL TEXAS

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SUMMARY

An approximately 50 ft. stratigraphic section of the Lower Cretaceous Glen Rose Limestone was sampled at 24 separate localities in central Texas from the standpoint of reconstructing the depositional environment. Among these samples 199 representative specimens were selected and subjected to point-count analyses, X-ray analyses, and insoluble residue determinations. Statistical analysis of the accumulated data with an IBM 7090 computer yielded the following facies: Corbula facies—characterized by a relative abundance of thick-shelled ostracods, thin-walled miliolid foraminifers, and the small pelecypod Corbula martinae; steinkern facies—characterized by large mollusc steinkerns, cellular mollusc shells and the foraminifer Orbitolina; mudstone facies—typified by less than 10% sand-sized grains; and mixed particle facies—characterized by worn skeletal and nonskeletal carbonate grains. The mud-stone facies subsequently was divided into two subfacies: (1) lime mudstones—characterized by delicate skeletal constituents and lime mud, and (2) marly mud-stones—consisting of a mixture of lime mud and terrigenous clay-sized material. Similarly the mixed particle facies was divided into four subfacies: skeletal calcarenites—characterized by sand-sized skeletal debris; skeletal wackestones—consisting of sand-sized skeletal particles floating in a mud matrix; nonskeletal calcarenites—characterized by nonskeletal carbonate grains; and nonskeletal wackestones—consisting of nonskeletal carbonate grains floating in a mud matrix. In addition limey sandstone, dolomite, and stromatolite facies were distinguished on the basis of relatively obvious textures and compositions.

The attributes of these facies as evidenced by the 199 statistically analyzed specimens were then used to assign each of the additional samples (350) to a particular facies and to identify the distribution of these facies in the field. A reconstruction of the depositional environment was made for each facies, and the following depositional history was interpreted from the resulting facies pattern.

The lowermost beds of the unit, consisting largely of stromatolites and nonskeletal calcarenites, are interpreted as representing deposition in very shallow, probably intertidal, waters, Following the deposition of these beds, the depth of water increased

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