MODERN MISSISSIPPI RIVER SUBDELTAS 

 A Dry Cypress Bayou Complex 

 B Grand Liord Complex 

 C West Boy Complex 



r- ,- D Cubits Gap Complex 



i-- _ j^ ^.)i£^\?' E Baplisle Collette Complex 

 '^i?^ 1 i^f*''«'"s*>,j/~F ^Garden Island Bay Compli 



^10?/r 





u Sect CSI LSU 



Figure 18. Six subdeltas of the modern 

 Mississippi Balize Delta recognized from 

 maps and sediment analysis. Dates 

 indicate year of crevasse opening (Wells 

 et al. 1982). 



In cross section, the prodel ta clays 

 constitute the base of the sequence 

 (Figure 19b). The lowermost clay marks 

 the first introduction of sediment into 

 the bay. Above the prodel ta clays are 

 the coarser-grained silts and sands that 

 form the delta front environment. These 

 sandy deposits are laid down immediately 

 in front of the advancing river mouth. 

 Once active sedimentation ceases in the 

 crevasse system, compaction and retreat 

 dominate. For a time marsh growth can 

 keep pace with compaction, but eventually 

 large bays tend to develop, and the 

 shoreline retreats rapidly. Small 



beaches accumulate near the major 

 distributaries where coarser-grained 

 sediment is available for reworking. 

 Oyster reefs may find a foothold along the 

 old channel margins of the submerged levee 

 ridges . 



Historic maps of one of these 

 crevasses, Cubits Gap, can be used 

 to illustrate a cycle of delta building 

 and abandonment. Figure 20 shows the 

 sequential development of the Cubits Gap 

 crevasse. The 1838 map was surveyed 

 prior to the break and shows a narrow, 

 natural levee separating the Mississippi 

 River from the shallow Bay Rondo. 



The idealized sequence is shown in 

 the plan view in Figure 19. The crevasse 

 initiates as a break in the major distrib- 

 utary levee in the vicinity of point A. 

 During the early formative years coarse- 

 grained sediments are deposited in the 

 immediate vicinity of the break. With 

 time new channels fonn, bifurcate and 

 reunite, forming an intricate pattern of 

 distributaries. Later, some distributar- 

 ies are abandoned and become inactive. 

 When a systematic channel pattern 

 develops, the bay fill front advances 

 rapidly into the bay, resulting in the 

 deposition of a sheet of relatively coarse 

 sediment thickening locally near the 

 channels. Seaward of the active channel 

 mouths, fine-grained sediments settle out 

 in deposits commonly referred to as 

 prodel ta clays. Other parts of the 

 crevasse system which have been abandoned 

 or dre deprived of a continuing sediment 

 supply compact rapidly, and many areas 

 tend to open up and revert to shallow 

 marine bays . 



In 1862 a ditch excavated by the 

 daughters of an oyster fishennan na:;ied 

 Cubit to allow passage by shallow draft 

 boats caused the crevasse break. The 

 original ditch was about 120 m wide; the 

 flood of 1862 enlarged the opening, and by 

 1868 the the break was 740 m wide. 



By 1834 the map shows the initial 

 buildout of a complex series of 

 distributary channels that had deposited 

 relatively coarse sediment near the break. 

 Note also the shoaling in the bay caused 

 by subaqueous deposition of the 

 finer-grained deposits. The map of 1905 

 shows that many of the major 

 distributaries had developed and that 

 rapid progradation had taken place in the 

 11-year period since 1884. 



A major portion of the crevasse had 

 been constructed by 1922; some small bays 

 were already beginning to open up, 

 indicating that some parts of the crevasse 

 system were being deprived of sediments. 

 The 1946 map shows that sedimentation was 



2U 



