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

Southwest Section AAPG Annual Convention

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

Abstract

Regional depositional features of the northern half of the Eastern Shelf are well documented in the seminal works of Brown et al. (1987, 1990) and others; however, the facies architecture of the southern half of this major petroleum province has been only incompletely examined in published local field studies. 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 platform succession with locally prominent reef facies that accumulated during early Cisco sedimentation. The platform/reef interval, largely equivalent in age to the Horseshoe reef complex, is as much as ∼1,530 ft thick in northeastern Coke County and forms an irregular, but distinct, platform margin throughout the eastern part of the study area. Reef buildups are generally aligned at the margin but also occur in local pinnacles in the platform interior.

The overlying Cisco section comprises a cyclic series of 14 mudrock, limestone, and sandstone deposits (top Home Creek to top Coleman Junction Limestone), correlated from outcrop by Brown et al. (1990), 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 shelf strata. Progressive upward variation in shelf-margin clinoform configuration 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 (“Wolfcamp D”) in the basin and slope provinces, and it occurs just above the Crystal Falls Limestone in the shelf area (based on outcrop paleontologic studies). 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 then 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 in channel-levee complexes. The same facies of the upper Wolfcampian slope and basin systems to the west comprise both (1) siliciclastics near the base occurring as unconfined, thin turbidites, and (2) carbonates as debris-flow facies within blocky gamma-ray log signatures 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. Shelf-edge systems of the 14 depositional cycles generally trend north-northeastward and subregionally coincide (i.e., stack) in the north half of the study area. In contrast, many of the shelf edges closest to the southern tip of the Central Basin Platform (Ozona Arch) abruptly project westward, perhaps reflecting the underlying east–west structural fabric in this area.

Brown, L. F., Jr., R. F. Solis Iriarte, and D. A. Johns, 1987, Regional stratigraphic cross sections, Upper Pennsylvanian and Lower Permian strata (Virgilian and Wolfcampian Series), north-central Texas: The University of Texas at Austin, Bureau of Economic Geology, 26 p. + oversized plates.

Brown, L. F., Jr., R. F. Solis Iriarte, and D. A. Johns, 1990, Regional depositional systems tracts, paleogeography, and sequence stratigraphy, Upper Pennsylvanian and Lower Permian strata, north-and west-central Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 197, 116 p. + oversized plates.