1999 Abstract: Belopolsky and Droxler
Uppermost Pleistocene Transgressive Coralgal Reefs on the Edge of the South Texas Shelf: Analogs for Reefal Reservoirs Buried in Siliciclastic Shelves
Andrei
V. Belopolsky
Department of Geology and Geophysics
Rice University
P.O. Box 1892
Houston, Texas 77251
André
W. Droxler
Department of Geology and Geophysics
Rice University
P.O. Box 1892
Houston, Texas 77251
Abstract
Southern Bank is one among 20 coralgal reefs of latest Pleistocene age occurring on the edge of the southern Texas shelf between the Rio Grande and Brazos/Colorado lowstand deltas. These drowned reefs, partially buried in Holocene clay-rich deposits, referred to on the Texas shelf as "the mud blanket," are cropping out today on the sea floor in water depths ranging from 58 to 82 m. Description and interpretation of published single-channel seismic lines crossing several banks and recently acquired multi- channel high-resolution seismic profiles in the area of Southern Bank demonstrate that the coralgal edifices are partially buried in clays and twice as thick as the average 20 m of relief exposed on the sea floor. The seismic images illustrate well the establishment, growth, and demise of the reefs.
The relationship between the coralgal reef edifice and the underlying and surrounding siliciclastic sediments, as well as the morphology and the internal structure of the reef package itself, is also imaged by the multi-channel high-resolution seismic data set. Southern Bank established itself on a north-trending topographic high formed during lowstand siliciclastic deposition. The asymmetric morphology of Southern Bank is typical of a reef edifice with well- defined reef, fore-reef, and back-reef geometries. The reef core is characterized by chaotic reflectors, the back-reef facies by sub-parallel gently dipping reflectors, and the fore-reef facies by steeply dipping discontinuous reflectors. The results of our study and radiocarbon dates of coral material collected in previous studies (Bright and Rezak 1976) show that the reefs were established sometime after 18,000 years BP (21,500 calendar years BP) and stopped accreting at about 10,600 years BP (12,300 calendar years BP). These 30- to 50-m-thick coralgal edifices were constructed at maximum rates of reef growth during the first 7,000 years of the last glacial to interglacial sea-level transgression. The timing of reef demise falls either within or at the end of the Younger Dryas, a ~1,000-year-long time interval when the rates of sea-level rise first slowed down considerably, possibly dropped by several meters, and then significantly accelerated. Subsequent to the drowning of the reefs, Holocene clay-rich deposits partially buried the coralgal edifice. Recognizing the relationship between the siliciclastic lowstand substratum and the overlying transgressive reefal edifice has direct implications for hydrocarbon exploration, especially in locating a series of transgressive carbonate reservoirs in a mixed carbonate/ siliciclastic setting.
Because the coralgal edifices were constructed during an interval of unusually high rates of sea-level rise at the upper limit of known coral reef growth potential, these edifices are expected to be characterized by high primary porosity and permeability. Because only the crests of the coralgal buildups were possibly exposed for a short interval prior to their final drowning, the occlusion of pore spaces through cementation had to be minimal. In this context, the coralgal banks along the South Texas shelf could become, therefore, excellent hydrocarbon reservoirs. Moreover, the overlying clay deposits could potentially serve as a tight seal.