Jesse J. Melick1, Michael H. Gardner1
(1) Colorado School of Mines, Golden, CO
ABSTRACT: Criteria for Defining the Base-of-Slope for Deep Water Clastic Systems, Upper Brushy Canyon Formation, West Texas
Two detailed outcrop studies across a 5-km oblique-dip segment of the upper Brushy Canyon Formation in the central Delaware Mountains document the sedimentology of base-of-slope (BOS) and proximal basin-floor (PBF) deposits. Positioned 25 to 30 km from an antecedent carbonate ramp margin, the upper Brushy Canyon consists of two fourth-order fan cycles (fans 6-7) bracketed by the thickest siltstone wedges of the Brushy Canyon third-order sequence. Sandstone content sharply increases at the north end of the study, 25-km from a ramp margin, where inferred BOS deposits of each fan show the most significant facies change and sand percent increase. Although BOS channels show more offset making complexes wider, they are generally smaller than either PBF or slope equivalents. The principal sediment bodies are sheet and channel sandbodies. Bypass deposits commonly base complexes and reflect significant down-profile transmission of sediment.
A reduced gradient at the BOS is inferred from small, active and mobile channels filled with more diverse facies and sandier overbank deposits (60- 85%) reflecting increased out of channel deposition. Thin sandstone-poor intervals precede channelization. Multistory and multilateral channel complexes are smaller (10- 20%) but contain less sand (85%) with coarser and more mixed energy lags preferentially preserved along margins. By contrast, PBF channelform sandbodies represent large retrofit complexes. Multiple episodes of re-incision of older channel deposits produce a large depression filled with more vertically stacked channel fills of thick-bedded amalgamated sandstone. Effective sediment trapping within the retrofit depression produces a 10% increase in channel sand percent and 20% decrease in overbank percent deposits (50% - 70%) relative to BOS deposits.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado