Deposition of Prograding Carbonate Sand Shoals and Their Subsequent Diagenesis--Lower Glen Rose (Cretaceous), South Texas: ABSTRACT

Abstract

End_Page 546------------------------------The lower Glen Rose in southwest Texas is a widely explored, but oil- and gas-barren, carbonate sequence (200 to 300 m thick) that was deposited on a broad, shallow-marine shelf. Three cyclic, shoal-water complexes, consisting of high-energy grainstone and coral-stromatoporoid-caprinid boundstone, developed over the Pearsall arch of south Texas. Facies distributions, determined from core and electrical logs, show that these linear complexes trend east-west for at least 125 km, and are located about 80 km inland of the Cretaceous shelf edge and 70 km seaward of the Cretaceous shoreline. The shoal-water, cyclic sequences change upward from: (1) sandy, fossiliferous mudstone/wackestone deposited in an open-shelf environment; (2) echinoid-mollusk and oncolite-caprinid packstone deposited in intertidal shoals and subtidal grain flats; (3) coated-grain and echinoid-mollusk grainstone deposited in sand flats, tidal channels, spits, and bars; and (4) coral-stromatoporoid-caprinid boundstone and packstone deposited as patch reefs and flanking deposits. Lagoonal deposits, consisting of toucasiid-oyster-miliolid wackestone, boundstone, and mudstone enclose each of the shoal-water sequences, and indicate successive seaward progradations, interrupted by transgressions of open-shelf facies. The patch reefs may have prograded out across the shelf and formed the initial buildup of he Stuart City shelf margin. Four gradational phases of grainstone diagenesis have caused almost continuous loss of porosity during burial. Micritic envelopes and isopachous crusts are early submarine cements. Next, development of an extensive meteoric-water system during burial to shallow depths led to dissolution or neomorphism of aragonite and precipitation of equant, isopachous cement, syntaxial cement, and nonferroan, equant calcite. As burial increased, subsurface brine displaced the original connate fluids and caused complex cementation and replacement by zoned ferroan and nonferroan calcite, lutecite and megaquartz, anhydrite, and saddle dolomite. The highest porosity is found in mappable facies of shoal-water grainstone. End_of_Article - Last_Page 547------------