Figure 15. The position of major delta lobes on the gulf coast during the previous 

 25,000 years. (A) Late Wisconsin, 25,000 - 20,000 yr B. P. (B) Late Wisconsin, 15,000 

 yr B. P. (C) Early Holocene, 12,000 - 10,000 yr B. P. (0) Present, 5,000 - 1,000 yr B. 

 P. SL = relative sea level. 



has sunk beneath the marine waters. 

 Scruton (1960) referred to this stage of 

 the delta cycle as the destructional 

 phase. Thus, in a relatively short period 

 of geologic time both land gain and land 

 loss occur, a function of the stage of the 

 normal delta cycle. The initial phase of 

 delta progradation is characterized by 

 formation of coastal marshes associated 

 with the advancing delta.^ Coastal marshes 

 deteriorate when a delta lobe is aban- 

 doned, and a new delta cycle begins else- 

 where. 



Figure 17, a satellite image of the 

 eastern portion of the Mississippi Delta 

 Plain, shows several delta lobes in 

 different stages of construction and 

 destruction. The oldest shown on this 

 image is the St. Bernard Delta, a delta 



lobe that was actively prograding some 

 3,000 years before present. This delta 

 lobe remained active for approximately 

 1,200 years, forming a broad, coastal 

 marshland along the eastern deltaic plain. 



Approximately 1,800 years ago, the 

 Lafourche channel began its progradation. 

 In the St. Bernard Delta, deprived of its 

 sediment load, marine processes and 

 subsidence (primarily compaction) became 

 dominant. The Lafourche distributary 



gradually increased its sediment yield and 

 within 1,000 years built out a major delta 

 lobe west of the modern or Balize Delta. 

 During this time the St. Bernard Delta 

 continued to be dominated by marine 

 processes and subsidence. Marine waters 

 began to intrude into the formerly fresh- 

 water marshes, and marshland deterioration 



17 



