Sedimentary Structures: Mississippi River Deltaic Plain: ABSTRACT

Abstract

Minor sedimentary structures were studied in cores and exposures from the deltaic and marginal deltaic plains of the Mississippi River. Selected active environments were sampled, and the occurrence of sedimentary structures from each was recorded. Individual structures were found to occur in more than one environment; however, suites of structures were characteristic. Within the study area the following twelve depositional environments have been investigated: shelf, prodelta, delta front (distal bar, distributary mouth bar, channel, and subaqueous levee), subaerial levee, marsh and swamp, interdistributary bay, mudflat, and fresh-water lake. Shelf deposits consisted of: (1) fine-grained clastics, burrowed and showing parallel laminations and (2) marine organic debris. Prodelta deposits are similar to clayey shelf deposits, but contain lenticular and parallel lamination with finely divided plant inclusions. The delta front is a complex of sub-environments constituting the advancing locus of active deposition of the prograding delta. The sloping seaward margin on this zone--the distal bar--exhibits current structures such as trough cross-laminations and current ripples as well as parallel and lenticular laminations, wave ripples, and burrows. The silty and sandy distributary mouth bar is characterized by a variety of small-scale, multi-directional cross-laminations and gas-heave structures. Channel deposits exhibit trough c oss-laminations, scour and fill, and distorted laminations, whereas subaqueous levees contain abundant ripple and unidirectional cross-laminations, parallel, wavy, and distorted laminations. In addition, subaerial levees are burrowed and oxidized. Marsh and swamp deposits are distinguished by abundant plant remains, burrows, and parallel laminations. Lenticular laminations, wave ripples, burrows, shell, and plant remains are characteristic of both interdistributary bay and fresh-water lakes. The mudflat assemblage of structures includes lenticular laminations, current and wave ripples, burrows, and shell remains. Not only are associations of sedimentary structures End_Page 521------------------------------ important in recognizing individual environments of deposition; equally important is the association of the environments to one another. This relationship must be understood for correct paleogeographic reconstruction of ancient deltaic deposits. The manner in which a sequence of marine and deltaic deposits might accumulate in a segment of a basin with resulting stratigraphic relationships of associated environmentally determined facies, is illustrated. End_of_Article - Last_Page 522------------