Michael H. Gardner1
(1) Colorado School of Mines, Golden, CO
ABSTRACT: Initiation Growth and Retreat Phases of Slope and Basin Deposition, Brushy Canyon Formation, West Texas
The Brushy Canyon and lower part of the Cherry Canyon Formations represent the lowstand systems tract of a composite third-order depositional sequence. Organic-rich siltstone intervals bound this 350-m thick clastic succession across the 55-km outcrop belt. The deposit architecture is best described in terms of initiation, growth and retreat phases of slope and basin deposition.
Sediment dispersal is focused in the Initiation Phase with deposits forming small elongate basin-floor fans with minor slope deposits. Efficient sediment bypass reflects a fixed high-gradient profile provided by underlying paleotopography. This produces unorganized cycle stacking patterns but sandstone pinch-outs promote stratigraphic traps. Basin-floor sands deposited from predominately high-density flows are commonly dewatered and form numerous distributary channelforms encased by sheets that pinch out into siltstone to fill paleotopography and reduce the gradient.
The Growth Phase records the main phase of slope and basin deposition reflecting high sediment flux from multiple and dispersed sources. Successive fourth-order fan cycles offlap but consist of fifth-order cycles that stack to equilibrate sediment volumes across the profile. The slope is an active bypass site, but sheet and channelform sandbodies increase in shelfward-stepping cycles. Over-spilling from multiple filled channels produces sheets that encase channelforms and increase the basin-floor sand percent. The decreasing gradient corresponds to increased channelform size and interchannel organic-richness.
Significant sand deposition on the slope reflects less efficient bypass during the Retreat Phase. Thick organic-rich and fine-grained slope deposits contain a high proportion of low-concentration flows that encase km-scale channelform sandbodies. Low gradients produce slope-centered sediment thicks with sandy basin-floor equivalents. Basin-floor channelforms encased in layered sheets show a dominant retrofit architecture that reflects under-filling. These mounded complexes control km-scale compensational patterns.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado