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Abstract: Facies, Diagenesis, and Porosity Development in a Lower Cretaceous Bank Complex, Edwards Limestone, North Central Texas
Ralph S. Kerr (1)
An Edwards carbonate bank complex developed on the structurally positive Belton high in north central Texas. Mobile carbonate sand bars, rudist reefs, and beaches of the shallow-water bank differ strikingly from time-equivalent deeper water muds of the Tyler basin to the north, and supratidal dolomites and evaporites of the central Texas platform to the south.
Early stages of bank development are well-exposed near Belton, Texas, and exhibit the following progradational vertical facies succession: open marine basin, shallow grain shoal, rudist path reef, exposed beach, restricted lagoon, and intertidal-supratidal mud flats. Paleocaliche, desiccation cracks, and algal boundstone suggest subaerial exposure of the uppermost unit.
The major diagenetic changes occurred early, and predate regional exposure of the bank complex at the end of Edwards time. Most calcite cementation is early, and associated with local meteoric water tables, where fine crystalline bladed crusts, syntaxial overgrowths, solution-cavity fill, and medium to coarse crystalline equant calcite were precipitated. Early Mg-calcite bladed crusts also formed in the marine or mixing zone environments. Late-stage calcite cementation was limited to coarse crystalline equant calcite.
Most of the dolomite is of multi-stage fresh water mixing origin. Dolomitization of lime muds and calcite cements occurred early, in association with local meteoric water tables, and late, as the result of a meteoric ground water system. Fine crystalline anhedral dolomite in lagoonal and tidal flat facies is of hypersaline origin.
The shoreface and foreshore facies of the beach complex have up to 35 percent (thin section estimated) moldic and interparticle porosity as a result of (1) high original interparticle porosity, (2) early cementation to reduce compaction, (3) aragonite allochem dissolution to produce moldic porosity, (4) subaerial exposure to reduce further cementation, and (5) an early seal to retard fluid migration. The lagoonal and tidal flat facies have up to 20 percent (estimated) moldic and intercrystalline porosity due to (1) dissolution of aragonitic allochems, and (2) partial dolomitization and subsequent dissolution of the lime mud.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
(1) Shell Oil Company, Houston, Texas
Copyright © 1999 by The Gulf Coast Association of Geological Societies