deltaic coastal plain which has a total 

 area of 28,568 km^ of which 23,900 km^ is 

 exposed above the sea surface (subaerial) 

 (Coleman 1976). 



In one of its earlier channels the 

 river built the Sale-Cypremont Delta along 

 the western flanks of the present 

 Mississippi River Delta Plain. In 



approximately 1,200 years an extensive 

 coastal marshland emerged before the river 

 switched its course to another locus of 

 deposition, the Cocodrie system. A 



similar sequence of events continued, and 

 site of deposition was 

 new del ta lobe began a 

 buildout. This process 

 each delta completing a 



with time this 

 abandoned and a 

 period of active 

 has continued. 



cycle of progradation that requires 

 approximately 1,000 - 1,500 years. 



Over approximately the last 500 

 years, the most recent delta cycle has 



formed the modern birdfoot or Balize Delta 

 (Figure 16). The modern delta has nearly 

 completed its progradation cycle, and in 

 the recent past a new distributary, the 

 Atchafalaya River, began tapping off a 

 portion of the Mississippi River's water 

 and sediment discharge. A new delta is 

 beginning its progradational phase (Van 

 Heerden and Roberts 1980; Wells et al. 

 1982). 



In each progradational phase of the 

 delta cycle, broad coastal marshes are 

 constructed. Scruton (1960) referred to 

 this as the constructional phase. 

 However, once the river begins to abandon 

 its major deposition site, the unconsoli- 

 dated mass of deltaic sediments is immedi- 

 ately subjected to marine reworking pro- 

 cesses and subsidence. Waves and coastal 

 currents, and subsidence result in pro- 

 gressive inundation of the marshes, and 

 within a few thousand years the delta lobe 



Figure 14. Location of major buried river channels fomied during the Wisconsin glacial 

 period (after Fisk 1954). 



16 



