AAPG Annual Convention and Exhibition

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Spatial Trends in Stratal Architectures Across the Backwater Transition in Lowland Rivers


A few key questions inevitably arise while mapping seismically-imaged channel bodies from past landscapes. Where was the shoreline? Can the planform of channel bodies help us predict the reservoir characteristics of the preserved deposits? In this talk, we will discuss how the back-water length scale in rivers defines systematic changes in stratal architecture across the transition from normal flow to the back-water influenced zone. A spatial reduction in bed material flux is observed where rivers transition from normal flow to the back-water influenced zone, as the river flow begins to feel the effect of sea-level in the receiving basin. The back-water length, scaled by characteristic flow depth divided by water surface slope, is a characteristic length scale of all rivers entering a receiving basin and is best distinguished in deep lowland rivers with shallow gradient. 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 transition zone to nearly equal to the channel width downstream of the transition zone. This variation in width of the channel belt is tied to the decreased lateral mobility of the channel downstream of the back-water transition. The thickness of bank-attached bar deposits, collected from USACE cores in 110 cross-sections, was used as a proxy for channel depth from Cairo to Head of Passes. Thickness trends reveal that bank-attached bars thicken from approximately 20m upstream of the transition to 45m just above Head of Passes, while decreased lateral migration results in less extensive bar deposits. A comparison of 4 different channel belts from the Rhine-Meuse and Mississippi systems is presented. For these two systems, we present a method that uses the backwater length to non-dimensionalize the geometries of channel belts with disparate scales. We apply this scaling to the Mio-Pliocene Mississippi Delta system, imaged in an industry seismic volume over Breton sound, to estimate distance to the shoreline. Our estimates are tested against independent reconstructions of paleo-geography for the area. Results indicate that this method can be a powerful tool for reconstructing paleo-environment of deposition and characterizing reservoir architecture in ancient seismically-imaged channel belts.