Donna S. Anderson¹, Mary M. Carr¹, Michael H. Gardner¹

(1) Colorado School of Mines, Golden, CO

ABSTRACT: Paleotopography and Gradient Controls on Deepwater Sandstone Architecture, Lower Brushy Canyon Formation, central Delaware Mountains, Texas

The facies architecture of the basal 110-m of a 900-m succession of deepwater clastics is controlled by underlying paleotopography that produces knickpoints across a slope-to-basin profile and progressively decreases depositional gradient. Positioned 32 km from a shelf margin, basal clastic deposits in the Colleen Canyon study area show diverse sediment transport directions and an upward increase in sand percent and channelization. Paleotopography produces >100-m change in thickness and conspicuous facies changes across the 10-km2 study area. Decreasing gradient produces an upward increase in the size and abundance of channels with progressively more fixed trends. Composite geologic maps of 11 fifth-order cycles (5-20 m thick) that comprise three fourth-order fan cycles (30-50 m thick) illustrate these patterns.

The oldest Fan 1 deposits record sand dispersal primarily from the SW. Sandstones of the upper two 5th-order cycles form dewatered sheets that lack channels and that pinch out northward into coeval siltstone. Where sandstone is absent, organic-rich siltstone drapes paleohighs. Fan 2 sand-dispersal is from the WNW with a significant component from SW. Amalgamated, layered and channelized sandstone sheets encase channel complexes with trends that parallel the sandstone pinchouts of the lower two fifth-order cycles. These sandstones pinchout into coeval siltstone at progressively more northerly positions relative to those of Fan 1. Channels evolve from isolated, braided single-story bodies to multiple, amalgamated multistory bodies with high degrees of re-incision. Fan 3 sand-dispersal is dominantly from the NW across the paleohigh that controlled the underlying sandstone pinchouts. The basal 5th-order cycle contains the northernmost sandstone pinchout and anneals this paleotopography. Broad, shallow anastomosed channel complexes dissect amalgamated sandstone sheets to produce the highest sand percent (70 to 99%) in the interval.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado