Roger N. Wagerle¹, Michael H. Gardner¹

(1) Colorado School of Mines, Golden, CO

ABSTRACT: Architecture of Retrofit Submarine Channel Complexes

Retrofit channel complexes represent a common lower slope and basin-floor channelform architecture in the Brushy Canyon Formation of West Texas. Retrofitting occurs when younger channel complex deposits are confined within a larger erosional depression that incises older channel complex deposits to form an even larger km-scale channelform sandbody. Because the erosional depression confines sedimentation to the previous site of channelization a large and mounded sediment body is constructed. These large channelforms promote both fan-scale compensation and large-scale compactional deformation. Proximal basin-floor deposits of the middle Brushy Canyon at Colleen Canyon illustrate the 3D facies architecture of a 45-m thick and 1.2 km-wide retrofit channelform sandbody.

Both the cross sectional geometry, map pattern, and fill of retrofit complexes reflect a chronology of early deposition of multiple channel complexes, erosion of the older channel deposits to form a larger erosional depression, and filling of the depression by smaller more sinuous channels. A significant period of bypass and erosion produces a large depression that truncates multiple older and more widely distributed complexes. The preserved deposits of older channel complexes form topography that confines fine-grained intrachannel deposits that flank the smaller axial channels. Such intrachannel deposits are absent in channel complexes that contain sand-rich overbank successions. Sediment trapped in intrachannel areas of the depression enrich organic matter in fine-grained overbank deposits that flank retrofit sandbodies. Low preservation potential in the eroded interior preferentially preserves channel lag deposits to the lower part and margins of the sandbody. The lag deposits and more heterolithic fills of channel fills preceding erosion produce more heterogeneity along sandbody margins. The smaller channels within the depression are more sinuous, with aggradational fills that form sandy plugs with nearly vertical channel margins.

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