Controls on mixed carbonate-siliciclastic slope morphology, early Permian, northern Delaware Basin, U.S.A.

Lower to Middle Permian strata in the Delaware Basin in southeast New Mexico and West Texas present a unique system to assess spatial and temporal variations in carbonate slopes as well as controls on variation. Detailed subsurface gamma-ray well-log mapping and visualization using approximately 8000 densely spaced well logs in the northern portion of the Delaware Basin aided in correlating and characterizing Wolfcampian and Leonardian (L1-L6 composite sequences) slope profiles. The Wolfcampian strata form irregular slope profiles ranging from 2 to 10°. L1-L2 composite sequences develop comparatively steeper aggradational to retrogradational slopes with maximum mapped gradients between 15 and 25°, platform to basin relief of 920–1100 m, and slope widths of 10–20 km. The L3-L6 composite sequences denote a shift to platform and margin progradation. However these intervals exhibit variable progradational profiles in the along the western slopes with maximum slope gradients between 2 and 10°, progradation aggradation ratios ranging approximately from 10 to 100, wider slope widths reaching over 30 km, and platform to basin relief ranging from 500 to 900 m while northern slopes exhibited more continuous steep gradients between 10 and 20°, progradation-aggradation ratios of 0–13, slope widths of 10–15 km, and average approximate platform to basin relief of 900 m. The broad shift from aggradation-retrogradation to progradation during the Leonardian correlates with low-order sea-level change previously noted from outcrop studies. The eustatic shift likely influenced platform accommodation, driving a regional transition to progradation during low-order highstand. The variation in Leonardian progradation rates is significantly influenced by underlying Pennsylvanian carbonate buildups generating large-scale inflections in the slope and toe-of-slope environments. The underlying feature generated lower platform to basin relief and wider slopes to the west, promoting progradation, and formed narrower slopes with greater platform to basin relief to the north, likely inhibiting rapid progradation. Bottom currents and subsequent drift deposition along the western slopes also changed slope angle and geometry, promoting progradation over and around the features in subsequent units. Also, bottom currents possibly swept sediments from toe of slope environments in the north where drifts aren't present, inhibiting progradation and driving aggradation. The documented spatial and temporal variations in slope profiles may exert significant influence on transport, routing, and confinement of platform-top and slope-derived sediments to the deeper basin, imparting control on basinal stratigraphic architecture. As such, this type of characterization should provide a predictive tool for assessing reservoir and non-reservoir architecture in the slope and re-sedimented basinal deposits in both the Permian Basin and other analogous basins.