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Shelf-to-Basin Architecture and Facies Variability of a Cretaceous Intrashelf Basin in the Northwest Gulf of Mexico


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 across a platform because the changes in stratal geometries at the shelf-to-basin transition often occur beneath the resolution of conventional subsurface datasets. In order to address this issue, outcrops that capture the shelf-to-basin transition from the late Albian (Cretaceous) Maverick Intrashelf Basin were mapped along a 15 km dip-oriented transect on the lower Pecos River. This study takes a quantitative approach to characterize the relationships between dip angles, paleobathymetry, and sediment production along the shelf-to-ISB profile. The goal of this research is to provide detailed documentation of an intrashelf basin that can function as an outcrop analog for carbonate shelf exploration and fields producing in similar settings. 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. Over 800 photographs were captured and geographically located using Differential Global Navigation Satellite System (DGNSS) methods to model the vertical canyon walls, that vary in height from 90-115 m across the 15 km transect. The high-resolution DOM provides the ability to accurately interpret the subtle depositional dips of the shelf-to-ISB profile that range from 0.1° to 1°. We show that the development of the Maverick Intrashelf Basin is attributed to the constructional nature of the carbonate factory. Differential sediment accumulation rates between rudist-skeletal shoals and deep-water foraminiferal mudstones of the basin-center created the necessary topography for ISB development. The early growth of rudist faunal assemblages formed the subtle topographic expression that promoted the continued rapid sedimentation of the ISB margins. This constructional model for ISB development is also applicable to the East Texas and Fort Stockton Basins on the Texas Comanche Shelf, and similar constructional interpretations have been proposed for the Bab Intrashelf Basin and the Natih-E Formation of the Middle East. Ultimately, such models help provide an improved understanding of intrashelf basin development, which has global implications for carbonate exploration and field development on large land-attached carbonate shelf systems.