Deposition of prograding carbonate sand shoals and their subsequent diagenesis, lower Glen Rose (Cretaceous), South Texas
Creators: name: Bay, Annell Russell
Publisher: name: The University of Texas at Austin
Year: 1980
Subjects: subject: Carbonate rocks
subject: Carbonate rocks
subject: Facies (Geology)--Texas
subject: Geology--Texas
subject: Diagenesis--Texas

description: The Glen Rose Formation is a thick (300 to 900 m), carbonate unit that was deposited on a broad, shallow-marine shelf during the early Cretaceous. Three cyclic, shoal-water complexes of the lower Glen Rose Formation developed over the Pearsall Arch in Frio and Medina Counties of South Texas. These complexes, consisting of high-energy grainstone and coral-stromatoporoid-caprinid boundstone and packstone, trend east-west for at least 125 km and are located approximately 80 km inland of the Cretaceous shelf edge and 70 km seaward of the Cretaceous shoreline. Three major, depositional environments have been recognized within the lower Glen Rose of South Texas: open shelf, shoal-water complex, and protected lagoon. The facies and their respective depositional environments, from the base to the top of the upward-coarsening sequences are (1) sandy mudstone/wackestone, fossiliferous quartzarenite, mollusc-miliolid wackestone, and echinoid-mollusc wackestone deposited in an open-shelf environment; (2) echinoid-mollusc and oncolite-caprinid packstone deposited in intertidal shoals and subtidal grainflats; (3) coated-grain and echinoid-mollusc grainstone deposited in sandflats, tidal channels, spits, and bars; (4) coral-stromatoporoid-caprinid boundstone and packstone deposited as patch reefs and associated flanking debris. Toucasiid-oyster-miliolid wackestone, toucasiid boundstone, and burrowed and laminated mudstone deposited in shallow-water, protected lagoons and tidal mudflats interfinger with upward-coarsening sequences on the landward side. The lagoonal deposits overlying the shoal-water sequence indicate the seaward progradation of the complex. Eventually each complex was transgressed by open-shelf facies and the cycle began again. The patch reefs and associated biostromes may have prograded out across the shelf and formed the initial build-up of the Stuart City shelf margin. A petrographic study of grainstone diagenesis indicates there are four gradational phases of diagenesis: (1) early submarine, (2) early mixing-zone and meteoric, (3) late meteoric-to-early subsurface, and (4) late subsurface. Early cementation in the marine environment is evidenced by micrite envelopes and isopachous crusts around some allochems. Subaerial exposure of bars and spits allowed early invasion of meteoric and mixing-zone water as indicated by aragonite dissolution or neomorphism to calcite; precipitation of finely crystalline, equant, isopachous cement; syntaxial cement; and medium crystalline, nonferroan, equant, calcite cement. During early burial of the rocks coarser crystals of nonferroan, equant calcite precipitated, indicating continual flushing with meteoric water. Later burial of rocks to the subsurface lead to the growth of zoned ferroan and nonferroan calcite, replacement of mollusc shells by lutecite and megaquartz, precipitation of euhedral quartz overgrowths, anhydrite precipitation and replacement, precipitation of baroque dolomite, and stylolitization. Petrography of the mud-supported facies indicates there is replacement of micrite by both limpid and inclusion-rich dolomite. The limpid dolomite is interpreted to form in a schizohaline environment whereas the inclusion-rich dolomite formed in both schizohaline and hypersaline environments.