ABSTRACT: Evolution of Sedimentary Architecture and Surface Morphology: Atchafalaya and Wax Lake Deltas (1973-1994)
ROBERTS, HARRY H., ROB CUNNINGHAM, NAN WALKER, and SUSAN MAJERSKY
The abnormally high flood of 1973 initiated the subaerial phase of delta development in Atchafalaya Bay by scouring sands stored in the lower Atchafalaya River channel and depositing this sediment in the bay. Peak flows in 1973 exceeded 700,000 cfs (19,824 cms) at Simmesport and 600,000 cfs (16,992 cms) at Morgan City, about twice the average peak flood levels at these sites. Since 1973 deltas have dramatically expanded from both lower Atchafalaya River and Wax Lake Outlets and now comprise 1258 km2 of new land above the mean low tide level. Data from 1994 indicate that the Atchafalaya delta accounts for 77 km2 while the Wax Lake delta has an area of 48.8 km2. These two deltas have prograded as silt/sand-rich wedges composed of numerous bifurcating channels separating sandy lobes. Sedimentologically, both deltas display the typical coarsening upward trend, but with a very thin (<1 m) prodelta facies above the highly burrowed, and shell-rich bay bottom sediments. However, the deltas do have differences. The Wax Lake delta is still in the channel elongation-bifurcation and bar formation phase while the Atchafalaya delta is experiencing channel abandonment, lobe fusion, and by-passing of sediment to the continental shelf (via the dredged navigation channel). Rate of Wax Lake delta growth has increased since about 1980 so that subaerial exposure now rivals the Atchafalaya delta. Zones of scour between the two emerging deltas and around their perimeters has decreased between 1981-1994 and sedimentation on the shelf and along downdrift coasts is becoming significant.
AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana