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Reconstructing the Depositional Conditions Associated with Bank-Attached Bars in Submarine Channels of the Upper Brushy Canyon Formation, West Texas

Fernandes, Anjali M.*1; Petter, Andrew 2; Mohrig, David 1; Steel, Ronald 1
(1) Dept. of Geological Sciences, University of Texas at Austin, Austin, TX.
(2) St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN.

The Brushy Canyon Formation, a predominantly fine-grained turbidite system, was deposited on the slope and basin floor of the late Paleozoic Delaware Basin. Our project focuses on resolving intra-channel sediment sorting within upper-slope channel deposits, and comparing these deposits to channel fills on the proximal basin floor.

The depositional facies on the upper slope fall into two broad classes: A) open-channel facies associated with bypass of sediment to deeper water; and B) channel-filling facies associated with bed aggradation and significant loss of channel relief. Deposits accumulating during bypass are interpreted to be eddy bars located in bank-attached zones of flow separation. These deposits are characterized by packages of steeply inclined beds composed of planar-stratified, trough cross-stratified or sub- to super-critically climbing rippled deposits, with abundant mud drapes (D50=110µm). The channel-filling deposits form thick-bedded, sometimes gravel-rich, sandstone bodies which are structureless or which possess stratification associated with migrating dunes and intra-channel barforms(D50=156µm). On the proximal basin floor, the channel-filling sandstones (D50=110µm) are dominated by stratification associated with trains of dunes climbing at sub- to super-critical angles, indicating high rates of deposition from suspension.

Grain-size analyses show that particles in the 200-400µm range are common in the channel-filling deposits of upper-slope channels, but are poorly represented in the upper-slope eddy bars and the channel fills on the proximal basin floor. The eddy bars and basin-floor channel fills primarily consist of particles finer than 200µm, which we interpret as the size fraction that was fully-suspended on the upper slope. This size fraction dominates the eddy-bar deposits because only fully suspended particles can be advected into the bank-attached zones of flow separation in significant volumes. We will synthesize depositional styles and grain-size data in order to: 1) produce a facies model for thick bank-attached bar deposits built in zones of flow separation associated with planform irregularity in submarine channels, 2) estimate flow velocities and current thicknesses; and 3) assess sediment sorting and storage between channels on the upper slope and the proximal basin floor.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California