MODERN SHELF CLINOFORMS IN THE MISSISSIPPI-ATCHAFALAYA DELTA

Recent high-resolution seismic and coring studies of the Mississippi delta on the continental shelf of Louisiana have shown strong contrasts in stratigraphic architecture between the main Plaquemine (Balize) lobe and the newly forming Atchafalaya lobe. The latter is a distributary that empties into Atchafalaya Bay 150 km west of the main lobe and has received ~30% of the total Mississippi water discharge since 1963. The main lobe at SW Pass is a seaward-thinning prodelta mud wedge in greater than 40 m of water that is up to 10 m thick and has an offshore gradient (1:300 to 1:1300) that is dependent on orientation relative to the shelf edge. The shelf seaward of Atchafalaya Bay is forming a progradational clinoform mud sequence landward of the 10 m isobath over a low gradient (1:1800) inner shelf surface. Peak Pb-210 sediment accumulation rates (3-4 cm/yr) are found along the foreset area of the sigmoidal clinoform, decreasing to only 1-2 cm on the topset area (1-4 m water depth). Maximum clinoform thickness is only 3 m. Interbedded stratigraphy and seasonably variable Be-7 deposition rates suggest that initial riverine sediment supplied to the topset area is partly redistributed to the clinoform front, perhaps in the form of fluid mud gravity flows created in the waning phase of winter cold front resuspension events. This example suggests modern subaqueous mud clinoforms are not confined to high-energy, river-dominated shelves (Amazon, Ganges-Brahmaputra, Fly), but can also form in the early phases of lower-energy systems when sediment accumulation is focused in shallow water.