Shelf-to-basin architecture and facies variability of a Cretaceous intrashelf basin in the northwest Gulf of Mexico
Sitgreaves, Jeffrey Robert
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The geomorphic expression of intrashelf basin systems (ISBs) and their associated facies patterns is extremely subtle, with shelf-to-basin dip angles that can average 0.3° across the slope profile. This presents an issue to stratigraphers working to understand facies variability at the reservoir-scale because the changes in stratal geometries at the shelf-to-basin transition will occur beneath the resolution of conventional subsurface datasets. Exposures along the Pecos River Canyon provide a unique opportunity to observe the transition from grain-dominated facies of the ramp crest into planktonic foraminifera mudstones/wackestones of the intrashelf basin. For this study, 475 m of detailed sections were collected at five localities and integrated with a high-resolution 3D digital outcrop model (DOM) to document the relationship between vertical facies successions and stratal geometries of the intrashelf basin profile. The high-resolution DOM provides the ability to accurately interpret the subtle depositional dips of the shelf-to-ISB profile that range from less than 0.1° to 0.7°. The development of the differential topography and facies changes associated with the formation of the Maverick ISB is attributed to differential sediment accumulation rates between active rudist-skeletal shoal formation versus deeper-water foraminiferal mudstones of the basin-center. Rudist bank deposition early in the Albian 6 Composite Sequence formed the positive topographic relief (1-3m) that led to the localization of rapid shallow-water sediment accumulation. After the development of subtle topographic expression, ensuing changes in relative sea-level promoted the development of ISB margins that were dominated by rudist faunal assemblages. The development of the ISB margin increasingly led to the differentiation between the grain-dominated facies along the margin and deposition of globigerinid mudstones in the basin-center. The extensive and largely undeformed exposures along the lower Pecos River Canyon and adjacent Amistad Reservoir provide clear evidence of the constructional differential-accumulation-driven formation for the Maverick ISB. Similar constructional models are likely for the East Texas and Fort Stockton ISBs on the Texas Comanche Shelf. Similar constructional progressions have been called on for the Bab intrashelf basin and the Natih-E Formation in the Cretaceous of the Middle East.