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Shelf-to-Basin Architecture and Depositional Trends, Missourian-Wolfcampian Strata of the Eastern Shelf of the Southern Midland Basin, West Texas


Regional Upper Pennsylvanian-Lower Permian depositional features of the northern Eastern Shelf are well documented by Frank Brown and others; however, the facies architecture of the southern half of this major petroleum province has been incompletely examined. Our regional chronostratigraphic synthesis documents the shelf, shelf margin, slope, and basinal depositional-facies characteristics, stratigraphic variations, and sedimentation trends of the Missourian Canyon Group and Virgilian-Wolfcampian Cisco Group across the southern Eastern Shelf, and to a lesser extent, the adjacent Midland Basin.

The Canyon Group (base Palo Pinto Limestone to top Home Creek Limestone) consists of a carbonate shelf succession with locally prominent reef facies. The shelf/reef interval, largely equivalent in age to the Horseshoe reef complex, forms an irregular shelf margin throughout the eastern part of the study area. Reef buildups are aligned at the margin but also occur as local pinnacles in the shelf interior.

The overlying Cisco section comprises a cyclic series of 14 mudrock, limestone, and sandstone deposits that form a progradational succession extending from the eastern edge to the central part of the study area. Progressive upward variation in shelf-margin clinoform configuration indicates that accommodation had markedly decreased during deposition of the upper Cisco Group. The Pennsylvanian-Permian boundary is near the top of the Cline shale (“Wolfcamp D”) in the basin province. Thickness of the Wolfcamp section is regionally consistent at the shelf (~700 to ~850 ft), expands dramatically basinward into an area of high accommodation and abundant sedimentation (~3,500 ft) associated with the slope systems, and thins in the basin proper (<700 ft). Slope facies closest to Virgilian and lower Wolfcampian shelf margins are dominantly siliciclastic mudrocks and sandstone debris-flow deposits. The same facies of the upper Wolfcampian slope/basin systems to the west comprise (1) siliciclastics near the base occurring as unconfined, thin turbidites, and (2) carbonates as debris-flow facies in the upper part.

Depositional cycles of the Virgilian and Wolfcampian shelf are dominantly transgressive limestones interstratified with highstand fluvial-deltaic and lowstand incised-valley-fill sandstones and mudrocks. Alternating thickened transgressive shelf-edge limestone systems and lowstand shelf-edge deltaic systems were deposited along the margin of the deepening basin.