ABSTRACT

Paleogeographic and lithofacies reconstruction suggest a braided stream-alluvial fan origin for the coarse grained Plio-Pleistocene Citronelle formation of southeast Louisiana.

Plio-Pleistocene topography controlled lithofacies distribution. Gravels are concentrated in paleovalleys while divided are covered with sand. Plio-Pleistocene streams are associated with modern rivers, although the Pearl River did not flow in its present location and the Amite flowed to the southwest of its modern course.

Gravel deposits are concentrated in eight trends which are interpreted as: (a) large fans that formed near the heads of major valleys, (b) small fans that formed where small streams entered major valleys or crossed the downthrown (south) side of normal faults, and (c) braided stream deposits that formed downstream from fans. Within these trends, three types of gravel deposits were recognized. Massive (Type I) gravels originated as thick longitudinal bars deposited on fan surfaces or as basal fan gravels. Channel fill gravels (Type II) were deposited in channels near the heads of fans. Interbedded sands and gravels (Type III) are interpreted as a series of longitudinal transverse and linguoid bars that formed in braided streams downstream from fans.

The sudden availability of coarse bedload material for Citronelle streams suggests that the Citronelle and its equivalents a blanket of coalescing alluvial fan and braided stream deposits that stretches from the Upper Mississippi Embayment to the Middle Atlantic Coastal Plain, may reflect Plio-Pleistocene epierogenic uplift in the continental interior.