The term "alluvial architecture" refers to the spatial arrangement and stratigraphic relationships between fixed channel belts (shoestring sands) and finer grained intervening flood-basin deposits. Overbank subenvironments associated with an active channel belt of the Mississippi River, including the levee, crevasses, and splay deposits, the upper point bar, and the abandoned channels, were cored using a standard vibracorer and Giddings Rig Soil probe. This study proposes that two end-member types of transition zones exist between channel belts and flood basins at an instant in time for a simple, sine-wave-shaped stream meandering in a fixed channel belt: (1) a levee to backswamp transition zone (type A), and (2) an upper point bar to backswamp transition zone (type B). T pe A transition zones, which consist of interstratified crevasse channel-fills, crevasse splay sheet sands, and fine-grained material of the natural levee, have greater potential for preservation than do type B transitions because crevasse splay deposits may extend for kilometers out into flood basins (type A) whereas preexisting point-bar deposits are destroyed during channel migration within the channel belt (type B). In type B transition zones, a sharp, erosional contact between the backswamp and upper point-bar subenvironments exists because of lateral migration of the point bar through preexisting facies within the fixed channel belt.

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