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 laminations, unidirectional cross-laminations, parallel and wavy laminations, distorted layers and burrowed oxidized silty sands. Channel fill deposits consisted of alternating 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. Two types of interdistributary bay deposits were recognized, 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 two 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 form a valuable tool for use in interpreting palaeo-environments in ancient sedimentary rocks.

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