Wax Lake Delta: Depositional Architecture, Delta Evolution, and Impacts beyond Atchafalaya Bay
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NO PRESENTATION AVAILABLE
H.H. Roberts, C. Li, N. Walker, and E. D’Sa hrober3@lsu.edu
Coastal Studies Institute, Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803
Abstract
The Wax Lake and Atchafalaya Deltas together reflect an important event in the Holocene history of coastal Louisiana, switching of Mississippi River discharge to the Atchafalaya River and delta‐building at a new site along the coast. Although the diversion of Mississippi River water and sediment down the Atchafalaya course is controlled to around 30%, the Wax Lake and Atchafalaya bayhead deltas represent the embryonic stage of a new major Mississippi River delta lobe. Recent geophysical and vibracoring studies of the relatively unmodified Wax Lake delta indicate that during the spring flood when deposition is maximized, Atchafalaya Bay is full of freshwater and deltabuilding takes place in an unstratified water body by turbulent jet flow. Time‐series air photographs (1973‐2002) indicate that the delta evolved to its present configuration by progradation and compensational stacking of sand‐rich bar complexes (1‐1.5 m thick, 1.3.5 km long, and 1‐2 km wide). As bar complexes evolve through extension and aggregation, flow divergence occurs forming new bar complexes adjacent to and seaward of the abandoned complex. Subaqueous levees weld bar complexes as channel extension occurs. Side‐scan sonar and core data suggest density underflow deposition may be taking place. However, an instrumented tripod placed at an active channel mouth (spring 2006) did not detect such flows during the monitoring period. The Wax Lake delta is composed of ~ 70% sand‐sized sediment. Sands represent < 20% of the Atchafalaya River sediment load. The remaining suspended load is transported out of the bay to distances as far as 75 km offshore when aided by cold front passages in the early spring. Fine‐grained sediments (pro‐delta facies) form low angle clinoforms that mark seaward progradation on the inner shelf opposite Atchafalaya Bay. When advected westward with the dominant inner shelf longshore drift these finegrained sediments are responsible for the prograding mudflats along the eastern chenier plain and thick fluid mud deposits on the inner shelf.
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