ABSTRACT

Paleolake Monroe occupied the central Ouachita River Valley from the Saline River confluence to the Monroe area during late Pleistocene and early Holocene time, draining approximately 5000 B.P. Post-draining deformation of the lake basin indicates that this area is undergoing active localized uplift along east-west zones. The largest of these ridges (at Sterlington, LA), which rises over 30 ft (approximately 10 m) above the main lake-floor level, formed the barrier for the last stage of the lake. Breaching of this barrier by headward sapping of Bayou de Loutre feeder streams drained the lake. Smaller uplifts to the south are inferred to have created the original lake basin, with the later uplifts raising the original shorelines far above the later lake levels. It is the last stage of the lake that produced the most pronounced lake terraces.

Late in the history of the lake, Bayou Bartholomew was captured by a lake basin tributary stream and then provided the sediment source for the extensive beach terraces along the southeast margins of the lake. Variations in the current elevations of beach terraces around the lake indicate that there have been post-draining differential uplifts of more than 5 ft (1.5 m) within the southern end of the lake. An abrupt vertical offset of the valley floor just south of Felsenthal, AR indicates a post-draining differential uplift to the north of nearly 10 ft (3 m). It is interpreted that this area is experiencing significant neotectonic deformation, and that much of the geomorphology of the area is a direct result of this deformation.