Structure and Sedimentological History of the Overbank Deposits of a Mississippi River Point Bar

Abstract

ABSTRACT Six structure morphotypes were recognized from the overbank deposits of Plaquemine Point, a point bar on the Mississippi River. They included ripple drift laminations, trough cross-stratification, massive structures, horizontal laminations, parallel laminations, and distorted laminations. The probable hydrodynamic conditions responsible for producing these morphotypes were established from existing literature. Utilizing sediment properties, process-form relationships, probable hydrodynamic conditions and vertical changes in structure suites, the sequence of structures was correlated with a flood cycle of the river. The sequence comprised (from bottom to top): 1) initial overbank flooding in lower flow regime and formation of ripple drift lamination, 2) high turbulence during rising fl od and formation of deformational structures, 3) upper flow regime during maximum flood and formation of horizontal laminations (erosion occurred at this stage), 4) lower flow regime in the falling stage of flood and formation of small scale trough cross-stratification, 5) formation of clay drapes. Each vertical sequence could be recognized by either complete or partial preservation of the sedimentary structures. An attempted correlation of the preserved structure sequences (seven in all) and the flood hydrographs of the river showed that not all the floods were recorded in the portion of the deposit studied.