WELCH, CYNTHIA L., Texas Tech University, Dept. of Geology, Lubbock, TX

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

The lower Cretaceous Edwards Formation in Taylor County, Texas is composed of tidal flat capped parasequences. The individual parasequences are 10' to 30' thick and consist of subtidal skeletal wackestonespackstones capped with laminated and mud cracked dolomudstones. In some of the parasequences the underlying subtidal wackestonespackstones have been subjected to two stages of dolomitization. The first stage of dolomitization replaced the lime mud matrix with microcrystalline-dolomite. The second stage of dolomitization consists of 10-150 µm dolomite rhombs, which together with equant calcite fill or partially fill fossil moldic porosity. Both stages of dolomite must have formed very early and at a shallow depth of burial, because the burial depth had to be less than 1000 feet.

This study will use 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 will be used to identify the carbonate facies, to help determine the diagenetic sequence, and to determine where to sample the limestone and dolomite for geochemical analysis. The geochemistry will consist of dO¹⁸ and dC¹³ isotopic, trace element (Na, Mg, Mn, Sr, Ca, and Fe), and X-ray diffraction analyses in both phases of dolomitization as well as the equant calcite. With these data an attempt will be made to determine: 1) the environment, 2) the timing, and 3) the composition of the fluids that dolomitized the matrix and precipitated the pore filling dolomite and equant calcite.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid