Connecting the Backwater Dynamics of Large Rivers to the Composition and Shapes of Channel Belts in the Coastal Zone

Abstract

Three-dimensional seismic data is often used to visualize the interior of thick sediment accumulations on shelf margins. Channel belts, the composite deposits left behind by laterally mobile channels, are easy to identify in sculpted seismic volumes that image fluvio-deltaic deposits. One key question inevitably arises while mapping channel belt deposits in seismic volumes- what can the shapes of the acoustically imaged deposits tell us about their lithology? A spatial change in bed material flux is observed where rivers at low discharge transition from normal flow to the backwater zone, where the river responds to the standing body of water in the ocean. Here, we connect the backwater transition to systematic changes in the shapes and composition of time-integrated channel deposits. Measurements along the Holocene Mississippi Channel Belt from Cairo to Head of Passes show a dramatic reduction in the width of the channel belt from roughly 20 times the channel width upstream of the backwater transition zone to nearly equal to the channel width downstream of the transition zone. This variation in width of the channel belt is coincident with a decreased lateral mobility of the channel downstream of the back-water transition. Data from 860 cores reveal that bank-attached bars thicken from approximately 20m upstream of the transition to 40m just above Head of Passes, and show a dramatic reduction in the contribution of bedload to bar deposits over the same interval. Coupled with drastically reduced lateral migration rates in this zone, this results in less extensive bar deposits that were primarily geometries of deposits is a simple and powerful tool for reconstructing paleo-environment of constructed from suspended load. This connection between depositional patterns and planform deposition and characterizing reservoir architecture in ancient seismically-imaged channel belts, where limited data exists. A dimensionless scaling approach shows that these geometric trends are transferrable to channel belts of other river systems with different sizes.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017