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Sedimentology and Stratigraphy of a Carbonate Slope System Along a Drowned Margin; Cutoff Formation, Guadalupe Mountains, West Texas


The Cutoff Formation consists of carbonate and siliciclastic sediments that were shed across the drowned Victorio Peak – Bone Spring margin and Delaware Basin floor during late Leonardian to early Guadalupian time. Here we focus on developing an understanding of how the drowned Late Leonardian (L6) margin influenced the stratigraphic architecture of draping Cutoff strata. This emphasis clarified the styles of deposition of organic rich siltstones and sandstones identified in the Bone Spring Play on the basin floor. The 7 km oblique-dip Cutoff outcrop on the Western Escarpment of the Guadalupe Mountains from the L6 margin basinward has a shelf-to-basin relief of 400 meters, with dip angles ranging from 12-14 degrees near the draped L6 margin to 1 degree at the toe of slope. Measured sections through 16 oblique-to-dip aligned canyons and cliff draws were taken at 400 meter spacing. Real Time Kinematics (RTK) GPS was used to trace key stratigraphic contacts and surfaces at centimeter scale resolution. We integrated this dataset with airborne LIDAR surveys, gigapan photomosaics. Aerial pans and hand-held gamma ray augmented the mapping. Additional laboratory work includes: petrographic analysis of 170 thin sections, x-ray diffraction, and rock eval. pyrolysis. Processes of accretion, bypass, and erosion are focused at varying positions relative to the drowned L6 margin. Near the edge of the L6 margin, processes of erosion are dominant. In this area, unconformable contacts and channel forms convolute the one dimensional stacking pattern of carbonate and siliciclastic facies. Downdip of the margin, there is a progressive transition from erosional to depositional regimes. Sediments that have bypassed the margin accumulate in the form of kilometer wide channel complexes and debris flows. Further downdip, near the toe of slope, accretionary processes are dominant. Mass transport complexes, debris flows, and fine sandstones accumulate between conformable contacts. Basin restricted strata in the Cutoff Formation reveal a newfound record of argillacous and siliciclastic sediments that potentially fed basin floor reservoirs of the Bone Spring play. We will present these observations in the context of sequence stratigraphic models developed for the shelf-equivalent Lower San Andres composite sequence in order link processes occurring in this deepwater system to base level changes on the shelf.