. The ecology of delta marshes of coastal Louisiana : a community profile . 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 prodelta 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 r
. The ecology of delta marshes of coastal Louisiana : a community profile . 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 prodelta 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
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