Abstract: Facies, Diagenesis, and Porosity Development in Lower Cretaceous Bank Complex, Edwards Limestone, North-Central Texas
Ralph S. Kerr
A carbonate-bank complex developed on the structurally positive Belton high during deposition of the Edwards Formation 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 on the south.
Early stages of bank development are well exposed near Belton, Texas, and exhibit a progradational vertical facies succession. This sequence also represents the lateral facies tract, and consists of (1) plant-rich skeletal wackestone of the open-marine Comanche Peak Formation at the base, (2) alternate skeletal grainstone and burrowed wackestone deposited as mobile grain shoals, (3) rudist bafflestone associated with patch reefs, (4) skeletal grainstone of an exposed-beach complex, (5) dolomitized wackestone deposited in a restricted lagoon, and (6) intertidal burrowed and supratidal laminated mudstone capping the sequence. Paleocaliche, desiccation cracks, and algal boundstone suggest subaerial exposure of this uppermost unit.
The major diagenetic changes occurred early, and predate regional exposure of the bank complex at the end of Edwards deposition. Most calcite cementation is early and associated with local meteoric water tables, where finely crystalline bladed crusts, syntaxial overgrowths, solution-cavity fill, and medium to coarsely crystalline equant calcite were precipitated. Early magnesian calcite bladed crusts also formed in the marine- or mixing-zone environments. Late-stage calcite cementation was limited to coarsely crystalline equant calcite.
Most of the dolomite is of multistage freshwater mixing origin. Dolomitization of carbonate muds and calcite cements occurred early, in association with local meteoric water tables, or late, as the result of a meteoric groundwater system. Finely crystalline anhedral dolomite in lagoonal and tidal flat facies is of hypersaline origin. Final porosity is a function of primary depositional fabric and subsequent diagenetic alteration. The shoreface and foreshore facies of the beach complex have up to 35% (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% (estimated) moldic and intercrystalline porosity owing to (1) dissolution of aragonitic allochems, and (2) partial dolomitization and subsequent dissolution of the carbonate mud.
AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas