INTRODUCTION 



During the Holocene, the broad Mississippi River deltaic plain was built by "delta 

 switching" (Figure I). This fundamental land-building mechanism resulted in a net 

 progradation of the shoreline over the past 6,000 to 8,000 years. The depositional history 

 consists of construction and abandonment of large and complex delta lobes on a time 

 scale of about 1,000 years for each major sedimentation event. During the regressive 

 phase of a delta lobe's history, local progradation of the shoreline and the building of new 

 marshland are maximized. Domination by fluvial processes over marine processes, as is 

 the case in the Mississippi Belize delta lobe, results in rapid progradation of 

 distributaries and associated facies, causing a complicated channel geometry. Between 

 and along the flanks of major feeder channels relatively thin wedges of rapidly deposited 

 sediments create bay fills, which are initiated, fill the bay with marshlands, and 

 deteriorate to an open-bay condition once again on a time frame of generally less than 

 200 years. At some point, however, the major delivery system diverts sediment and 

 water through a more efficient and generally shorter route to the receiving basin. As 

 diversion takes place the formerly active lobe is starved of sediment. The effects of 

 sediment dewatering and compaction, as well as regional subsidence associated with 

 northern Gulf of Mexico depocenter, become dominant and a phase of rapid land loss is 

 initiated. Since there is generally only one major locus of deposition or active delta lobe 

 along the coast at any given time, the remaining coastal areas ore in various stages of 

 retreat, depending on their relative ages. In deltas such as the Mississippi which have 

 been constructed by deposition of dominantly fine-grained sediment in a receiving basin 

 with low wave-current energy, the coastline is always in a state of dynamic change. 



The modern Belize "delta lobe has been the locus of Mississippi River deposition for 

 the past 600 to 800 years. This delta-building event has resulted in a thick sequence of 

 both subaerial and subaqueous sediments that have prograded onto the Continental 

 Shelf. Because of this extensive progradation and other geological factors, the modern 

 river course has reduced its gradient and general flow efficiency to a point that upstream 

 diversion is favored. Fisk (1952) predicted abandonment in favor of the more efficient 

 Atchafalaya River course by the mid-1970's if the diversion were not controlled. From 

 the point at which the two rivers meet, north of Baton Rouge, the Atchafalaya course to 

 the sea is 307 km shorter and, therefore, is favored by its steeper gradient. Although 

 Mississippi River flow down the Atchafalaya course has been documented as far in the 

 past as the 1500's (Fisk 1952), it was not until the early I950's that significant quantities 

 of sediment started to arrive at the coast. Shiemon (1975) and Roberts et al. (1980) 

 discussed the basin-filling phase prior to the arrival of abundant prodelta clays in 

 Atchafalaya Bay and along the downdrift coasts. At this time a new phase of delta 

 building in the Mississippi River delta complex was initiated, and areas that have 

 experienced coastal retreat for literally hundreds of years entered a new era of coastal 

 accretion. This paper describes the early stages of Atchafalaya delta growth and the 

 implications of this event with reference to Louisiana's problems of land loss and coastal 

 retreat. 



DELTA HISTORY 



Delta development in Atchafalaya Bay can be divided into two major stages, 

 subaqueous and subaerial. The subaqueous phase was initiated as deposition in the 

 intricate network of lakes and swamps of the Atchafalaya Basin reached a point such 

 that sediments were fluxed through the system to the coast. This natural catchment 



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