Progradation of Atchafalaya Delta, one of the most dynamic geological events of the century, has produced a sizable new sand body on the Louisiana coast. Evolution of depositional environments in Atchafalaya Bay has been determined from analysis of sediment cores and bathymetric surveys. Use of X-ray radiography has made possible recognition of a number of subenvironments within major environments. Atchafalaya Delta exhibits all the sedimentary environments recorded in earlier Mississippi delta lobes. However, excellent stratigraphic control and current knowledge of the processes of deposition in Atchafalaya Bay make it possible to better link process-response in this than in other Mississippi subdeltas. In contrast to the modem Mississippi subdelta, the Atchafalaya should prograde more rapidly, form thinner sand bodies, and eventually cover a wider area, much like the Lafourche, St. Bernard, and Teche delta lobes. <...>