Development of a Modern Subaqueous Mud Delta on the Atchafalaya Shelf, Louisiana
Mead A. Allison, Ciara F. Neill, 2003. "Development of a Modern Subaqueous Mud Delta on the Atchafalaya Shelf, Louisiana", Siltstones, Mudstones and Shales: Depositional Processes and Characteristics, Erik D. Scott, Arnold H. Bouma, William R. Bryant
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Sediment cores and high-resolution CHIRP seismic data were collected on the inner shelf adjacent to Atchafalaya Bay, Louisiana to examine the evolution of the newly forming mud delta associated with the Atchafalaya River lobe of the Mississippi deltaic plain. 210Pb accumulation rates from sediment cores show maximum sedimentation rates (10-20 cm/yr) are concentrated on the innermost shelf (<6 m water depth) immediately seaward of the Pt. Au Fer shell reef at the bay mouth. Rates decrease rapidly offshore to 8-10 m water depth, where seismic profiles show modern deposits pinch out adjacent to shoals formed by erosional remnants of older Holocene deltaic deposits. Alongshore, rates remain relatively high to the west (along the chenier coast of West Louisiana) following the trend of coastal currents. The wedge-shaped prodelta reaches 2.5 m in thickness and is gas-charged adjacent to the Atchafalaya dredge channel on the shelf. In areas where accumulation rates exceed ~1 cm/yr, the prodelta muds take the form of cm-scale interlaminations of silty sands (proximal) or silts (distal) and clayey silt layers. At accumulation rates of ~0.5-1 cm/yr, primary fabric is partially destroyed by macrofaunal burrows. In seaward areas, 10-25 m water depth, where modern sediments are accumulating but rates are low, clayey silt and silty clay deposits are completely homogenized by burrowing activity. Sediment dispersal paths can be traced seaward of the bay mouth and westward, along the direction of prevailing coastal currents, using coarse silt content of the mud delta. This indicates that coarse silts behave as individual particles and are preferentially sorted, while fine-medium silts are contained within flocs with clay-size mineral grains, and show no preferential sorting downdrift from the source.
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Siltstones, mudstones and shales have been studied mainly with regard to clay mineralogy and general transport-deposition. Recent studies on deepwater deposits from cores and outcrops have shown that fluid flow properties of deepwater reservoirs are greatly affected by the presence of finer-grained deposits in the reservoir. Initial analysis indicates that the majority of these finer grained deposits have a large silt component and are closer to siltstones rather than mudstones, commonly called shales To date, little attention has been given to their characteristics resulting from different depositional processes.