Lower permian sequence stratigraphy of the western Delaware Basin margin, Sierra Diablo, West Texas
Fitchen, William Mills
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The Sierra Diablo provide unique exposures of Lower Permian carbonate platforms, in which sequence stratigraphic analysis based on three-dimensional stratal geometries can be integrated with conventional one-dimensional cycle stacking pattern analysis. The sequence framework in the Sierra Diablo includes 3 middle through upper Wolfcampian high-frequency sequences (HFS) and 6 Leonardian HFS (1.6 ma average duration). This framework highlights both the regional predictability of HFS attributes, as well as the systematic local variability of HFS attributes caused by non-eustatic factors. The Wolfcampian HFS comprise an updip siliciclastic unit (Powwow Formation; 0-75 m), which onlaps a widespread angular unconformity, and a downdip, platform carbonate unit (main body of the Hueco Group; 420 m thick). The carbonate unit contains two middle Wolfcampian HFS (mW1-mW2), which compose a progradational composite sequence (CS), and a backstepped late Wolfcampian HFS (uWl). 270 m of Wolfcampian platform margin strata are truncated by a subaerial-to-submarine unconformity. The unconformity is onlapped by breccias along the toe-of-slope and exhibits a 43 m paleokarst profile along the platform top. Submarine erosion is attributed to platform margin slumping, probably initiated in the early late Wolfcampian following platform margin backstepping. The Leonardian sequences comprise the Victorio Peak (platform facies; 160-220 m) and Bone Spring Formations (platform margin to basin facies; 40-230 m). These include the L1-L6 HFS, which compose two larger-scale CS. The lower CS consists of lowstand (L1), transgressive (L2-L3) and highstand (L4) HFS sets; the upper CS contains transgressive (L5) and highstand (L6) HFS sets. Lowstand and highstand HFS exhibit high positive progradation/aggradation ratios, seaward-stepping cycles, low facies diversity, a seaward shift in the position of maximum accommodation, toplap below seaward-dipping sequence boundaries, and greater potential for karst development along sequence boundaries. Transgressive sequences exhibit low positive to negative progradation/aggradation ratios, landward-stepping to vertically-stacked cycles, high facies diversity, a landward shift in the position of maximum accommodation, and more common outer platform/margin reef development. Along-strike variability in stacking patterns is best developed in transgressive HFS, and is attributed to spatial variations in accommodation, antecedent topography, differential compactional, energy regime (related to wind direction, headland-bight shoreline trends, and shelf paleobathymetry), and sediment accumulation rates.