Mississippi River sedimentation is dominated by the process of delta switching. Upstream distributary diversion during the Holocene Epoch periodically shifted the depocenter of Mississippi River sedimentation, producing a sequence of four abandoned shallow-water delta complexes on the Louisiana coast. Abandonment and the cessation of active distributary sedimentation result in subsidence, creating a rapid coastal transgression in each abandoned delta complex.

In its destructional phase, the delta evolves through a sequence of three stages, each associated with distinct transgressive sedimentary environments. This evolutionary sequence begins with an erosional deltaic headland and flanking barrier stage, backed by restricted interdistributary bays. In the following transgressive barrier island arc stage, the barrier island encloses an open intra-deltaic lagoon. Long-term sea-level transgression eventually causes the destruction of the subaerial barrier and the development of an inner-shelf sand sheet and shoal.

Figure

Coarse-grained sediment dispersal following delta abandonment is characterized by reworking of distributary sand bodies into transgressive coastal barrier systems. Barrier orientation to the dominant wave approach controls the pattern of longshore sediment dispersal. Sediment transported offshore during frontal and tropical cyclone passage forms the inner-shelf sheet sand. Fine-grained sediments accumulate in a variety of subsiding back-barrier environments and on the continental shelf.

If the validity of the model proposed for Louisiana holds true for older shallow-water Mississippi-type deltas, an idealized stratigraphic record for this category of transgressive deltaic environments should contain the following vertical sequence: a thin bay facies overlain by tidal inlet and/or recurved barrier sands that grade updip into a thickening lagoonal facies overlain by extensive washover sands. The uppermost unit would consist of shallow marine sands with an overlying cap of continental-shelf muds.

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