THE IMPACT OF PERSISTENT BACKWATER DYNAMICS ON THE LITHOLOGY AND ARCHITECTURE OF FLUVIO-DELTAIC CHANNEL BELTS (Invited Presentation)

Spatial changes in flow and sediment transport dynamics, as well as channel kinematics have been observed where rivers transition from the normal flow zone to the backwater zone. In deep rivers with low bed slopes (eg. the modern Mississippi River, USA) the backwater zone can extended for hundreds of kilometers upstream of the river mouth. We evaluate the impact of persistent backwater effects on the composition and architecture of fluvio-deltaic channel deposits constructed over thousands to millions of years, over large spatial scales on the order of tens of thousands of square kilometers.

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 backwater 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. Bar stratigraphy shows a coincident increase in the thickness of suspension-dominated heterolithic deposits, transitioning from roughly 20-30% of the bar thickness in the upstream reaches to 100% in the lower backwater zone. Coupled with drastically reduced lateral migration rates in this zone, this results in narrow channel belt deposits that are dominated by fine sand and mud deposits. Dimensionless comparisons show that these trends are similar in other channel belts of modern and ancient river systems.

This connection between planform geometries, lithology and stratigraphic architecture of deposits is a valuable tool for reconstructing paleo-environment of deposition in ancient fluvio-deltaic systems and characterizing reservoir properties from remotely-sensed data with limited coverage. Furthermore, as buried channel deposits delineate pathways for contaminant transport, fresh-water dispersal to oceans and salt-water intrusion associated with rising sea-levels in large river deltas today, characterization of downstream trends in reservoir potential of these deposits is now critical to protecting the natural resources of coastal communities.