PRODELTA FORMATION ON THE ATCHAFALAYA SHELF, LOUISIANA

Kasten cores, vibracores, and CHIRP subbottom profiles collected from the Atchafalaya inner shelf (<20 m water depth) and adjacent Atchafalaya Bay (15-7 m water depth) are used to examine the development of a muddy prodelta since the system became a significant distributary of the Mississippi River (~30% of total discharge). Cross-shelf CHIRP transects indicate the prodelta is a lens-shaped deposit that is thickest (~2.5 m) seaward of the 45-km-long shell reef at the mouth of Atchafalaya Bay and pinches out at about the 8 m isobath. Eight kasten and three vibracore sites were analyzed for ²¹⁰Pb to determine the geochronology of this sediment deposit over the last century. Maximum sediment accumulation (>10 cm/y) is confined to 3-5 m water depth adjacent to the Atchafalaya shelf dredge channel. Rates decrease east-west to 2-5 cm/y westward along the zone of maximum prodelta thickness. Rates decrease offshore until a zone of relict deltaic shoals covered by a thin (<50 cm), discontinuous modern mud is found from 10-18 m water depth. X-radiography indicates the prodelta stratigraphy varies on a cm-scale, interbedded sand and silty clays near the dredge channel, fining to laminated silts and clays alongshore, and massive to burrowed silty clays offshore. Relict shoal sediments offshore are a heterogeneous mix of reworked deltaic sands, in-situ floodplain and delta front lithologies. A muddy shell hash that reflects extensive shell reef development prior to the main phase of Mississippi capture in the 20th century underlies the prodelta strata. The interbedded sands and muds near the dredge channel contain up to 30% sand intervals, indicating this region is a potential future site for land building.