Upper Pennsylvanian and Lower Permian Shelf-to-Basin Facies Architecture and Trends, Eastern Shelf of the Southern Midland Basin, West Texas,

Our study documents the shelf, shelf-edge, slope, and basin-floor depositional facies characteristics, stratigraphic variations, and sedimentation trends of the Missourian Canyon Group and Virgilian–Wolfcampian Cisco Group across the southern Eastern Shelf and the adjacent Midland Basin. The Canyon Group (base Palo Pinto Limestone to top Home Creek Limestone) consists of an aggradational carbonate bank succession having locally prominent reef facies. Similar reef facies continued to accumulate during early Cisco sedimentation. The bank/reef interval, largely equivalent in age to the Horseshoe Atoll complex, is as much as 1,540 ft (469 m) thick in northeastern Coke County and forms an irregular, but distinct, shelf margin throughout the eastern part of the study area. Reef buildups are generally aligned at the margin but also occur as local pinnacles in the platform interior. Canyon basin-floor facies are equivalent to the lower part of the Cline shale (“Wolfcamp D”) and consist primarily of dark, organic-rich (>2 percent organics) mudrocks. The overlying Cisco section comprises a series of 13 mudrock, limestone, and sandstone cycles (top Home Creek Limestone to top Coleman Junction Limestone), correlated from outcrop, that collectively form a progradational succession extending from the eastern edge (Bunger Limestone) to the central part of the study area (Coleman Junction Limestone). The top of the Home Creek Limestone coincides with a regional downlap surface for the progradational Virgilian lower Cisco shelf strata. Progressive upward decrease in height of shelf-margin clinoforms indicates that accommodation had markedly decreased during deposition of the upper Cisco Group. The Pennsylvanian–Permian (Virgilian–Wolfcampian) boundary is at the top of the Cline shale in the basin and slope provinces and just above the Crystal Falls Limestone in the shelf area. The thickness of the Wolfcampian section is regionally consistent at the shelf (~700 to 850 ft [~213 to 259 m]), expands markedly basinward to as much as 3,500 ft (1,067 m) into a regional depocenter of high accommodation and high sediment influx associated with slope sedimentation, and then thins into the basin proper (to <500 ft [152 m]). Slope facies closest to Virgilian and lower Wolfcampian shelf margins are dominantly siliciclastic mudrocks and sandstone turbidite deposits in channel-levee complexes. The same facies of the upper Wolfcampian section are dominated by calcareous mudrocks and allochthonous carbonates as superimposed debris-flow deposits. The Wolfcampian (upper Cisco) basin system to the west comprises (1) primarily siliciclastic strata occurring as unconfined, thin turbidites and hemipelagic mudrocks in the lower part of the Wolfcampian succession and (2) carbonate debris-flow deposits and turbidites in the upper part. In contrast, the Virgilian (lower Cisco) basin succession constitutes mostly organic-rich mudrocks of the upper Cline shale. Depositional cycles of the Virgilian and Wolfcampian (Cisco) 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. Shelf-edge systems of the 13 depositional cycles generally trend north–northeastward, subregionally coincide as stacked composite successions, and record periods of prolonged shelf-edge stability.