Minor Sedimentary Structures in a Prograding Distributary: ABSTRACT

Abstract

Minor sedimentary structures were studied in cores taken at the mouth of a small prograding distributary within the Mississippi River delta. The mouth of Johnson's Pass in Garden Island Bay was mapped and the following environments were recognized: subaerial and subaqueous natural levee, channel, distributary mouth bar, interdistributary bay, and marsh. Oriented, undisturbed cores were taken from each environment. These cores were split, dried, and photographed and the types of minor sedimentary structures within each environment were tabulated. Natural levee deposits contained abundant current ripple bedding, unidirectional cross-laminations, parallel and wavy laminations, distorted layers and burrowed oxidized silty sands, whereas channel fill deposits consisted of alte nating beds of clay and silt containing trough cross-laminations, scour and fill structures, and distorted layers. The distributary mouth bar, composed predominantly of silt and sand, is characterized by a variety of small-scale multi-directional cross-laminations and air-heave structures. Three types of interdistributary bay deposits were recognized; highly burrowed interbedded silt and clay, homogeneous clay with scattered brackish-water fauna, and a predominantly clay section with thin parallel and lenticular laminations and ripple marks. The structures within these three types are a reflection of availability of coarse detritus. Marsh deposits are characterized by the abundance of peat, carbonaceous clays, calcareous nodules, and root disturbances. Each environment is characterized by a distinct assemblage of structures. These assemblages can be used to interpret paleoenvironments in ancient sedimentary rocks. End_of_Article - Last_Page 353------------