During its early stages of development, the Atchafalaya River began to construct a new delta 

 in Grand Lake (the Atchafalaya Basin), north of Morgan City (Figure 9). By 1952, the Grand 

 Lake region was nearly full of sediments and the lower Atchafalaya River south of Morgan City 

 began to transport muds into Atchafalaya Bay. By 1972, the lower Atchafalaya River began to 

 transport sand into the bay and a new subaerial delta was born. Several papers have been writ- 

 ten on the Atchafalaya Deltas (Shlemon and Gagliano 1972; Shlemon 1975; Rouse et al. 1978; 

 Gagliano et al. 1981; van Heerden 1983; Roberts 1986; and van Heerden and Roberts 1988). 



Maximum Development of Deltaic Plain Complex 



The total amount of new land formed by the Mississippi River during the past 7,000 years was 

 about 14,000 mi 2 (Figure 10). 



Stages in Development of the Deltaic Sequence of Sediments 



Each of the four large deltas which make up the Mississippi River Deltaic Plain complex is 

 underlain by very characteristic vertical sequences of sediments. The stages in the construction of 

 deltaic sequences were discussed by LeBlanc (1972) and are summarized by six diagrams shown on 

 Figures 11A 11B, and 11C. 



Partial Destruction of the Deltaic Plain Complex 



Geologists have been aware for over 50 years that the loss of coastal deltaic marshes, and the 

 transgression of the marine environments over these marshes, can occur as a result of channel 

 diversion upstream of the delta. Russell and Russell (1939), LeBlanc (1972, 1977), and Morgan 

 (1973) discussed the processes of delta shifts, loss of land, and marine transgressions. However, 

 the detailed documentation of these processes and partial destruction of deltaic marshes did not 

 occur until research geologists with the Louisiana Geological Survey, under the direction of State 

 Geologist G.C. Groat, began a new phase of research on the delta complex. The best papers on 

 this topic were published since 1985 by the following members of the Survey research staff: Shea 

 Penland, J.R. Suter, Ron Boyd, and R.A McBride of the Louisiana Geological Survey. Penland 

 et al. (1987) and Penland and Boyd (1985) described and illustrated three important stages in the 

 transgressive depositional history of an abandoned Mississippi River Deltaic Plain complex (Figure 

 12). For additional information on the processes of loss of land, the reader should study these 

 papers and also an earlier paper by J.P. Morgan (1973). 



Total Amount of Land Lost by Transgression 



Figure 13 shows that about 7,000 mi 2 of coastal land have been lost by natural, transgressive 

 marine processes during the past 7,000 years. 



SUMMARY 



Research on modern deltas and ancient delta deposits during the past 110 years has established 

 the basic principles of deltaic sedimentation (LeBlanc 1975). The natural processes of upstream 

 river diversions and the abandonment of deltas are now well documented and understood by 

 geologists. It is also well documented that once a delta is abandoned, its sediments continue to 

 compact and subside, under perfectly natural conditions, and this permits the marine environments 

 to transgress over the abandoned delta marshes and a great loss of coastal land results. 



Forty-four of the 51 significant papers on the Mississippi River Deltaic Plain Complex cited 

 herein were written by professors of geology at LSU and geologists who received their geological 



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