SEDIMENTOLOGY AND FACIES ARCHITECTURE OF OVERBANK DEPOSITS OF THE MISSISSIPPI RIVER, FALSE RIVER REGION, LOUISIANA

Abstract

This detailed study of the sedimentology and facies architecture of three overbank subenvironments which occur marginal to a meander belt in the lower Mississippi River Valley leads to the following conclusions: 1) backswamp, levee and splay deposits can be subdivided into units related to the establishment of the associated channel belt; 2) cycles related to avulsion, levee progradation, splay progradation and abandonment, and sheet-flood events are preserved in flood basin deposits; and 3) flood basin facies prograde basinward as the levee builds upward over time along a channel belt margin. The establishment of the channel belt is divided into four phases: 1) a pre-avulsion stage, 2) an avulsion stage, 3) an early channel belt, and 4) a late channel belt stage. In the study area, the flood basin sequence affiliated with the avulsion and establishment of the channel belt (30 m) has a maximum thickness of about 10 m. The avulsion event is probably recorded in the sedimentary sequence as the lithologic change from blue clay with detrital organie debris deposited in standing water to sheet-flood silts and sands related to levee and splay progradation during incipient formation of the channel belt. Surficial levee deposits (silty sand unit) exhibit an overall coarsening-upward sequence (2.5 m) that reflects meander bend migration toward the sampling site during the late stage of channel belt development. Surficial splay deposits (silty sand unit) initially coarsen upward during the progradational phase and then fine upward as the splay is abandoned (3 m). Individual flood cycles (mm-cm) occur as small-scale fining-upward rhythmites with poorly preserved stratification in all sub-environments.