--> --> Lowstand Deltas and Incised Valleys of the Tannehill Sandstone (Cisco Group) of the Southern Eastern Shelf of the Permian Basin, West Texas

AAPG Southwest Section Annual Convention

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Lowstand Deltas and Incised Valleys of the Tannehill Sandstone (Cisco Group) of the Southern Eastern Shelf of the Permian Basin, West Texas

Abstract

Depositional cycles of Permo-Pennsylvanian (Virgilian and Wolfcampian) Cisco Group strata of the Eastern Shelf of the Permian Basin 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 deposits, equivalent to the valley-fill systems, were also deposited along the margin of the deepening basin. This study focuses on the lower Wolfcampian Tannehill Sandstone as an example of the areal and shelf-to-basin stratigraphic expression of one of these valley-fill/ lowstand delta systems. In Nolan, Taylor, Coke, and Runnels counties, the Tannehill Sandstone occurs in two areal configurations: (1) narrow, slightly curvilinear belts that extend southwestward and due east-west across this study area and (2) local, digitate depositional trends. The first configuration represents lowstand valley-fill systems that locally incise the underlying Saddle Creek Limestone. These facies are 20-50 ft thick, blocky to upward coarsening, characterized by consistently low resistivity values and high SP/low GR values, and are restricted to on-shelf areas. Down-dip equivalents of these sandstones with digitate areal configurations are marked by similar well-log values but in contrast are 60- 100 ft thick, upward-coarsening, blocky, and digitate. Moreover, they partially overlap and extend about 8-12 mi basinward of the Saddle Creek shelf edge and record deposition of lowstand shelf-edge deltas fed by the incised-valley systems. Digitate areal facies were also deposited in the on-shelf area and are primarily upward coarsening and represent thinner (10-30 ft thick) highstand deltas. Incised-valley-fill, lowstand-delta, and highstand-delta sandstone facies are significant hydrocarbon producers in the Eastern Shelf. Mapping of shelf-edge lowstand deltas of the Tannehill and other Cisco Group sandstones record an aspect of the self-edge evolution in the Eastern Shelf. These deltas formed depositional platforms over which the next younger limestone (in this case, the Stockwether Limestone) transgressed across the shelf. As a result, the western (basinward) limit of the lowstand-delta platforms marks the shelf edge of these overlying limestones.