Past valley widening and recent creep beside the Lower Mississippi Valley

Tertiary strata are offset from the bluff at several places beside the broad valley of the Mississippi River in Mississippi. Most displaced blocks lie hidden under river water and floodplain, but the bluff at Vicksburg presents a partially exposed example. Block displacements there illustrate a mechanism of valley widening that operated before the geomorphic regime of the present. These displacements represent spreading of strata toward the valley in response to reductions of lateral confinement accompanying heavy river erosion nearby. Flat slip surfaces that developed in clayey strata during block displacements have facilitated recent movements as the meandering river impinged on the bluff. The foundations of two bridges at Vicksburg have experienced such creep-like movements in recent years. Localized plastic deformation has complicated the geologic picture near the bridges. Deformation features are confined to a single bed of marly limestone and the underlying layer of clayey marl. This plastically deformed couplet is contained within the much thicker assemblage of displaced blocks having horst-and-graben features. Plastic deformation of a bed that has subsequently been indurated to marly limestone indicates an origin well back in time, perhaps in the Oligocene. The structural fabric of the plastically deformed unit, however, indicates movement to the west and suggests some relation to the widening Pleistocene valley.