Cyclic Sedimentation in the Mississippi River Deltaic Plain

ABSTRACT A major characteristic of modern Mississippi River sediments is the orderly repetition of depositional events. This cyclic repetition consists of alternations of detrital and nondetrital deposition. Each major deltaic lobe is composed of a detrital lens or complex of lenses bounded on all sides by essentially nondetrital sediments indigenous to the basin of deposition. Examples of major cycles are provided by the modern and pre-modern lobate deltas. A shift in the point source of sediment supply is responsible for the abandonment of an active delta and initiation of a second cycle related to the new point source. The abandoned delta, deprived of nourishment, undergoes coastal retreat and inundation due to continuing subsidence. During this process, reworked and in situ deposits accumulate over the detrital lens, forming the bounding component of the cycle. The pre-modern deltas, varying in time of abandonment, afford a natural laboratory for the study of these capping accumulations. Two such examples, the St. Bernard and Sale-Cypremort deltas, are presented. Subdeltas or crevasses are scaled down versions of the major deltaic cycle and can be used as a model. Because of their smaller size and shorter duration, the processes of deposition and facies relationships of the detrital component can be more easily studied than in the major deltaic lobes. Vertical and lateral distribution of environmentally controlled facies within a deltaic mass are the result of the cyclic nature of sedimentation and delta growth. Some possible facies relationships are explored in a hypothetical sequence of overlapping cycles and compared with an actual vertical section taken at Fort Jackson, Louisiana.