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Petrography and Geochemistry of Dolomites in the Lower Cretaceous Edwards Formation in Taylor County, Texas

C. L. Welch
Texas Tech University, Dept. of Geology, Lubbock, TX

The Lower Cretaceous Edwards Formation in Taylor County, Texas is composed of two tidal flat capped parasequences. The individual parasequences are 10 to 30 ft thick and consist of subtidal skeletal wackestones/packstones capped with laminated and mud cracked dolomitized mudstones. In some of the parasequences the underlying subtidal wackestones/packstones have been subjected to two stages of dolomitization. The first stage of dolomitization replaced the lime mud matrix with microcrystalline dolomite (<10 мm). The second stage of dolomitization consists of 10–150 мm dolomite rhombs, which together with equant calcite fill or partially fill fossil moldic porosity. Based on previous stratigraphic work, both stages of dolomite must have formed very early and at a shallow depth of burial.

This study uses petrographic and geochemical data to determine the origin of the dolomitization and the effects of dolomitization on porosity formation and destruction in the Edwards Limestone. Carbonate petrography and scanning electron microscopy are used to identify the carbonate facies and to aid in the determination of the diagenetic sequence. б18O and б13C isotopic compositions of the dolomitized subtidal packstones above the lower tidal flat (б18O = –3.657 to –.911 PDB, б13C = 0.982 to 2.439 PDB) have a marine signature, whereas both tidal flats (б18O = –0.915 to –0.175 PDB, б13C = 0.605 to 1.915 PDB) have a more hypersaline to marine signature when compared to Lower Cretaceous marine (б18O = – 4.415 PDB, б13C = 2.419 PDB) and meteoric (б13O = –8.809 PDB, б13C = 1.072 PDB) standards developed in this study.

AAPG Search and Discovery Article #90905©2001 AAPG Southwest Section Meeting, Dallas, Texas