--> Patterns of Sediment Dispersal and Accumulation Basinward of a Large-Scale Inflection in Slope Angle; Cutoff Fm. and Avalon Trend, Delaware Basin

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Patterns of Sediment Dispersal and Accumulation Basinward of a Large-Scale Inflection in Slope Angle; Cutoff Fm. and Avalon Trend, Delaware Basin

Abstract

When carbonate platforms drown and backstep, this process may form a large-scale inflection (LSI) in slope angle below the shelf break, which significantly influences the geometry of the slope profile and patterns of sediment dispersal. In the northwest Delaware Basin, processes of channelization and slope failure associated with an LSI created by a drowned late Leonardian platform contributed to a dramatic basinward thickening of carbonate and siliceous strata comprising the Cutoff Formation during a 2-4 m.y. sequence (Permian Composite 9, PCS9). This study incorporates a new stratigraphic framework for the Cutoff Formation in the Guadalupe Mountains in order to correlate the exposed shelf and slope systems of PCS9 to the subsurface Avalon Trend. The 7 km oblique-dip outcrop is positioned just east of an extensive well log dataset in the northern Delaware Basin (500 wells). Handheld gamma ray logs collected along the outcrop were integrated with measured sections, XRD analyses, and rock eval. to calibrate facies to log-signatures. Subsurface mapping benefitted from close well spacing (<1-8 km) and previous outcrop-to-subsurface studies in the area. The proposed framework identifies PCS9 as a 500 – 700 ft. interval of mixed carbonate and siliceous facies on the toe-of-slope and basin floor. This succession demonstrates apparent onlap to a landward-thickening carbonate interval, interpreted as the drowned platform-LSI. During the transgression and much of the ensuing highstand (L7 – G4 LST), bedrock channels incising the LSI served as conduits for sediment gravity flows and depocenters for draping siliceous mudstones. Well-to-well correlations suggest that the percentage of carbonate comprising the L7 – G4 LST decreases significantly basinward from the toe-of-slope, as these deposits decrease in thickness and pinch-out into siliceous mudstones. Sandstones deposited during intermittent periods of platform exposure (G4 LST) also demonstrate fan-shaped geometries, but thicken basinward from the toe-of-slope. In the latest highstand (G4 HST), an increase in supply of carbonate mud from the active platform resulted in the filling of bedrock channels near the LSI, downslope shedding of mass-transport complexes, and widespread accumulation of tabular carbonate deposits across the basin-floor. New depositional models for the Cutoff-Avalon system may improve the understanding of reservoir architecture in this heavily explored, but poorly understood play.