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Abstract

Abstract: The Williams Ranch Member of the upper Cutoff Formation in the Guadalupe and Delaware Mountains, west Texas, U.S.A., consists of six offlapping lithologic units. The deposits formed during carbonate turbidite deposition across a drowned Early Permian carbonate platform. They have an areal extent of more than 20,000 km2 and reach a maximum thickness of at least 113 m. At the terminal margin of the older platform, the carbonate turbidites were partially redistributed by mass-transport events (MTEs) onto the slope and basin floor. Deposits formed during individual mass-transport events (MTE bodies) comprise the bulk of the Williams Ranch Member basinward from the drowned margin for at least 28 km along a transect oblique to depositional dip. MTE bodies are interbedded with undeformed carbonate turbidites and contain soft-sediment folds, faults, and extensional and shortening lineations, as well as termination surfaces (beds terminated from above and/or below). Turbidite deposition and subsequent mass transport caused general basinward thickening of the Williams Ranch Member from the drowned margin, where the Cutoff Formation is missing, to the basin floor.

Deposition responded to, and modified, inherited bathymetric relief. Compared to isopach thins, isopach thicks formed in bathymetric lows and locally formed bathymetric highs. Isopach thicks contain more undeformed strata and show more soft-sediment folds. These relationships suggest better preservation of strata in structurally controlled inherited bathymetric lows. In general, MTE bodies are preferentially deposited in these paleobathymetric lows. A minimum of six vertically stacked MTE bodies are recognized in the main study area with thicknesses ranging from less than one to tens of meters. MTE bodies show a general S-to-SSE paleotransport direction, with significant local variation, reflecting either underlying bathymetric relief and/or different source locations. Repeated MTEs resulted in a reduction of the overall basin gradient and created local positive bathymetry. Sand fairways and ponded sheet deposits in the overlying Brushy Canyon Formation are focused in bathymetric lows, and sands thin and onlap onto bathymetric highs.

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