Cyclic, Shoaling-Carbonate Banks in the Lower Glen Rose Formation (Cretaceous), South Texas
The lower Glen Rose Formation in the subsurface of South Texas contains three shoaling-upward cycles which culminated as high-energy banks, bars, and biogenic reefs over the Pearsall Arch. The shoaling cycles can be correlated over an area that is approximately 125 km (78 mi) long and lies parallel to and approximately 70 km (43 mi) seaward of the early Cretaceous shoreline.
Each cycle is represented by three major depositional environments: open-marine shelf, shoal-water banks and reefs, and protected lagoon. The open-marine shelf deposits are characterized by mud-rich carbonates with varied, but few, fauna and, in places, by interbedded clastics. The shoal-water banks and bars consist of grain-supported carbonates with abundant and varied fauna. High-energy patch reefs, consisting of corals, stromatoporoids and caprinids in a boundstone texture, cap the grainstone deposits. Protected lagoonal deposits, characterized by mud-rich carbonates with low faunal abundance and diversity, overlie the shoaling banks and reefs and indicate seaward progradation of each cyle. Progradation was hatted as open-marine shelf deposits of the succeeding cycle transgressed the lagoonal environment.
Diagenesis of the grainstones has resulted in an overall loss of porosity with burial. Porosity can be attributed to both (I) the preservation of primary interparticle pore space which survived due to early meteoric cements that provided framework support and (2) secondary moldic porosity created by the dissolution of aragonite allochems.
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Carbonate buildups have long been a focus of intense geological study. An underlying reason is the importance of carbonate buildups as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record created by carbonate buildups with emphasis on lithofacies, stratigraphy of buildups and their surrounding deposits, geometry, “reef”-building and sediment-producing organisms, and diagenesis and porosity evolution