--> --> The Fate of the Mississippi River Sediment During the Last Glacio-eustatic Cycle: a Volumetric Quantification

AAPG Southwest Section Annual Convention

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The Fate of the Mississippi River Sediment During the Last Glacio-eustatic Cycle: a Volumetric Quantification

Abstract

Paralic sedimentary deposits contain almost 40% of the hydrocarbon reservoirs and continue to be important for the industry, and even more so by its key link in understanding sediment fate into a source-to-sink perspective. In order to achieve a greater understanding of sediment dispersal during transgression and regression associated with river deltas, the last such cycle of the Mississippi River delta is studied herein. Previously published sediment thickness maps and borehole data of the Mississippi valley, delta, and shelf deposits have been compiled to discover how the Mississippi locus of deposition fluctuated since the Last Glacial Maximum (LGM). This investigation also compared calculated river derived sediment volumes during the last 20ky to deduce the volume and rate of sediment that bypassed the shelf onto the slope. At the LGM (19ka) when the sea level was 120 metres below present, the Mississippi River lay entrenched into the shelf and it built its deltas on the shelf margin. As the base level rose rapidly subsequent to the LGM, the river filled its valleys with sediment in the ensuing retrogradation and formed depocentres outside the valley. As the sea levelled, the river’s deltas prograded back outside of its valleys. Digitizing and employing previously published data showed that shelf depocentres migrated widely from the outer to the inner shelf following sea level rise and had thicknesses varying between 5–500 metres. This investigation seeks to test the various hypotheses concerning the locus of deposition during each of the stages of sea level rise. To this end, the sediment deposited in the valley and delta has been mapped and the volume quantified for each of these stages. Recently calculated values for the Mississippi River water discharge since the LGM, estimated to have fluctuated to as much as eight times (160,000 m3s-1) that of the present, allowed for the enumeration of sediment discharge through time. Using the rate of sea level rise to plot the location of the shoreline with the inferred past river mouth and the delta through time, the rate of volumetric change of the mapped on shelf deposits was calculated with respect to time. By deducting this rate of volumetric change of mapped on shelf deposits from the computed sediment discharge through time, the rate and volume of off shelf deposition since the Last Glacial Maximum was quantified, displaying a higher rate of deposition before the retreat of the shelf margin deltas.