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Course Notes 40: Deep-Water Sandstones, Brushy Canyon Formation, West Texas

Copyright 1999 AAPG. All rights reserved.

Course Notes 40: Deep-Water Sandstones, Brushy Canyon Formation, West Texas

R.T. Beaubouef, C. Rossen, F.B. Zelt, M.D. Sullivan, D.C. Mohrig, G D.C. Jennette


Exceptional oblique-dip exposures of submarine fan complexes of the Brushy Canyon Fm. allow reconstruction of channel geometries and reservoir architecture from the slope to the basin floor. The Brushy Canyon consists of 1,500 ft. of basinally restricted sandstones and siltstones that onlap older carbonate slope deposits at the NW margin of the Delaware Basin. This succession represents a lowstand sequence set comprised of higher frequency sequences that were deposited in the basin during subaerial exposure and bypass of the adjacent carbonate shelf. Progradational sequence stacking patterns reflect changing position and character of the slope as it evolved from a relict, carbonate margin, to a constructional, siltstone-dominated slope. Lowstand fan systems tracts consist of sharp-based, laterally extensive, sand-prone basin floor deposits and large, sand-filled channels encased in siltstones on the slope. The abandonment phase of each sequence (lowstand wedge-trans- gressive systems tract) consists of basinward-thinning siltstones that drape the basin floor fans. The slope-to-basin distribution of lithofacies is attributed to a three stage cycle of: 1) erosion, mass wasting, and sand bypass on the slope with concurrent deposition from sand-rich flows on the basin floor, 2) progressive back-filling of feeder channels with variable fill during waning stages of deposition, and 3) cessation of sand delivery to the basin and deposition of laterally-extensive silt-stone wedges. Paleocurrents and channel distributions indicate SE-E sediment transport from the NW basin margin via closely spaced point sources.

Slope-to-basin variations in channel size, geometry and fill are related to variations in the degree of bypass associated with channels and the timing of channel backfill. On the slope, major feeder channels are deeply incised into thick laminated siltstones, have simple margins, and are vertically stacked due to proximity of fixed point sources. The channel fills are highly variable in character, reflecting deposition from both lower and higher-energy flows during late-stage backfilling. At the toe of slope, sandstones occur in nested, multi-story channel complexes not confined by single, master erosion surfaces. Channel bases are commonly marked by lenticular, coarse-grained lags deposited from high-energy bypassing flows. Channel fills are complex, and indicate repeated episodes of erosion, bypass, and backfilli, with thick-bedded sandstones concentrated in channel axes and thin-bedded sandstones and siltstones preferentially preserved along channel margins. In down-fan, more aggradational settings, lags are absent. Channels are smaller, less complex, and simply filled with thick-bedded amalgamated sandstones. These channels are relatively short-lived features that were rapidly plugged by high-energy flows. In distal, predominantly nonchannelized areas of the basin floor, sandstones form laterally extensive sheets that are broadly lenticular as a result of minor erosion, depositional mounding, and compensational stacking patterns.


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