Terrigenous Clastic Depositional Systems

Cultural Resources Investigation for Item M-178.0 to 173.2-R, Mississippi River Levees, Louisiana

“…The lower part of a point bar is deposited by lateral accretion and the upper few meters accumulates by overbank sedimentation. Approximately 4 to 6 m (13 to 20 ft) of the natural levee deposits of point bar deposits overlie the upper point bar deposits (Saucier 1974;Galloway and Hobday 1983).…”

“…Eventually, the cutbank is oversteepened to the point that it caves into the river. When this occurs, the channel laterally shifts the cutbank and simultaneously deposits sand and silt onto the opposite convex bank, called the "point bar," of the river channel (Fisk 1947;Flores et al 1985;Galloway and Hobday 1983).…”

“…Only the finer suspended clay is transported by unconfined floodwatero into the backswamp of the flood basin. The silt and sand accumulates incrementally with each flood to build low, wedged-shaped ridges, called "natural levees," paralleling the river banks which slowly decreases in elevation away from the river (Galloway and Hobday 1983;Farrell 1989;Flores et al 1985).…”

“…The natural levees of the smaller crevasse distributaries consist of stiff gray clay containing a small percentage of silt and fine sand. They contain abundant plant roots and these are sometimes, but not always, oxidized (Galloway and Hobday 1983;Farrell 1989;Flores et al 1985).…”

“…As a result, natural levees contain massive, buried weathering zones containing iron oxides, carbonate nodules, and iron oxide concretions. These characteristics reflect subaerial weathering and soil formation during periods of subaerial exposure of natural levees between flood events (Fisk 1947;Galloway and Hobday 1983).…”