Revised stratigraphic framework for the Cutoff Formation and implications for deepwater systems modified by large-scale inflections in slope angle below the shelf break




Hurd, Greg

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Near times of ocean anoxic events or the peaks of long-term transgressions, carbonate margins may be drowned below a backstepping platform, and preserved as a submarine edifice which demonstrates hundreds-of-meters of vertical relief above the basin floor and forms a large-scale inflection (LSI) in slope angle. This study uses outcrops in the Guadalupe Mountains Region of West Texas, a robust suite of subsurface data from the northern Delaware Basin, and previously published literature in order to illustrate how LSIs influence patterns of sediment dispersal and accumulation in carbonate systems. Outcrops of the Lower – Upper Permian Cutoff Formation reveal that processes of channelization, bypass, and slope failure associated with a drowned carbonate margin (LSI) contributed to a dramatic thickening of carbonate, shale, and sandstone strata in the basinward direction. Correlation of this unit to the equivalent shelf system reveals that bedrock channels incising the LSI served as conduits for turbidity currents throughout the transgression and much of the ensuing highstand of a 2-4 m.y. composite sequence (PCS9). In the latest highstand, an increase in the production of carbonate mud by the active shelf system contributed to the complete filling of bedrock channels and the accumulation of carbonate mud-dominated mass-transport deposits basinward of the LSI. Correlation of this shelf-to-slope framework to basinal strata of the Bone Spring and Avalon trends in the subsurface reveals a landward shift in the locus of deposition downdip of the LSI across the northern Delaware Basin which was concomitant with the transition to patterns of sediment dispersal dominated by mass-transport events. Strata associated with turbidite systems accumulate mainly near the center of the basin, and demonstrate basinward-stepping geometries along shallow slope gradients downdip of the LSI, while demonstrating aggradational geometries along steep slopes. Strata associated with mass-transport deposits accumulate near the LSI on both shallow and steep slopes. Previous studies in other basins have documented LSIs which exhibit dimensions and stratal patterns comparable to the relict platform margin in the Delaware Basin. Insights developed from this study can help to improve exploration activities in the Delaware Basin, and other basins with analogous deepwater systems.


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