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Abstract: Distributary Development of Shoe-String Sand Bodies Within the Backswamp, Northern Mississippi Alluvial Valley


The backswamp paradigm of a fluvial environment is commonly that of a thick, homogeneous clay which fines with, distance from the channel and forms a flat surface gently sloping away from the natural levee and downvalley. Within the Mississippi Alluvial Valley this is an incomplete paradigm because numerous distributaries are present within the backswamp. These distributary channels usually begin as crevasse splays and a few of the larger ones have become avulsions of the Mississippi River, forming new meander belts. Pemiscot Bayou, the largest of 4 major distributary systems in the upper Mississippi Alluvial Valley, captured up to 25% of the Mississippi discharge 3-4 ka and flowed back into the Mississippi channel further downstream. It never became an avulsion, has ceased to be an active distributary, and the channel has largely infilled. While active, the bayou developed a meander belt 1.5-5 km wide with all of the deposits associated, with a meandering system such as point bar, natural levee and other overbank deposits, as well as channel-fill of meander cutoffs. The distributary channel was 600 m wide and 12 m deep near its exit from the Mississippi channel. Fifty-eight kilometers downvalley the channel was reduced to a 200-300 m width and a 7-9 m depth. Overbank silt, up to 2 m thick and 2 km wide, was deposited as sheet flow and buried backswamp clay and point-bar deposits. Laminated fluvial silt and fine sand and lacustrine clay infilled the abandoned channel. Thus the fluvial architecture of the Mississippi River backswamp includes shoe-string sand bodies 10 m thick, up to 5 km wide, and tens of km long within backswamp clay and overbank silt.

AAPG Search and Discovery Article #90944©1997 AAPG Mid-Continent Section Meeting, Oklahoma City, Oklahoma