Counter-Intuitive Braid Deposits in from Active Channel Belt

Dustin E. Ward, April D. Moreno, Neal Alexandrowicz, Jordan Garrett, and John Holbrook
Department of Geology, University of Texas at Arlington, Arlington, TX

Few studies have focused on the subject of lithofacies heterogeneity, architecture and general processes at the scale of channel belts. Hence, information on the subject is limited. We addressed this by studying a portion of the Missouri River floodplain along a 59mi section of undammed and unchannelized river in the Missouri National Recreational River corridor. A Corp of Engineers map from 1892 reveals a braided section of river that migrated south approximately 1km to its current location. A N-S cross section of the area was constructed based on bore-holes placed at semi-regular intervals modulated to record particular topographic features. Bore-hole sediments were sampled and logged at 10cm intervals. Surface expression of the braided system mimics deposits of a smaller low-sinuosity meandering river with multiple amalgamated open meander loops and small outer channel fills. Surficial exposures reveal 45% channel and 55% bar deposits by area. Upper channel deposits are primarily mixtures of clays, loams and sands and channel bases are defined by coarse sands. The average thalweg depth of the modern Missouri river is about 9m, and is a reasonable maximum estimate of channel belt thickness. Depth of the fine-grained channel fill components are 2-3m with underlying sections filled with permeable sand. Potential interconnectivity between sandy bars through the lower sandy channel fills appears high.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas