LAKE MONROE, POSSIBLE FLUVIAL RESPONSE OF THE OUACHITA RIVER TO PLEISTOCENE ARKANSAS RIVER AGGRADATION

Lake Monroe in the Lower Mississippi Alluvial valley of eastern Louisiana was first reported by R.T. Saucier in 1970 based on an anomalously low gradient of an Ouachita River terrace. Cores support the existence of this paleolake. It is approximately 70 km long, 17 km to 30 km wide, and lake fill is up to 8 m thick. In the upstream portion of the lake gray clay and sandy clay, typically (2.5Y 5/1), is derived from the Ouachita River which flows through Paleozoic sandstones and shales in the Ouachita Mountains. The central portion of the lake is filled with both gray and red clays. The basal clay is gray and also derived from the Ouachita River. The upper clay is red, typically (7.5YR 4/3), and is derived from the Arkansas River which flows through the Mesozoic red beds in the Great Plains. The change from gray to red clay suggests that there was an avulsion of the Arkansas River into the central portion of the lake after the lake formed. Fill in the southern portion of the lake is dominated by red clay because of the propinquity to both courses of the Arkansas River. The maximum age of the lake is approximately 30 ky, based on OSL dates of Macon Ridge, which is the eastern lake margin. The Holocene Ouachita River meander belt has incised into the lake sediment. Three hypotheses for the lake have been proposed. First, the Monroe uplift may have ponded both the Arkansas and Ouachita rivers. However, the mapped uplift is located upstream of the southern lake margin. Second, is aggradation of the Mississippi River, damming both the Ouachita and Arkansas rivers at the southern margin of Macon Ridge. However, no high Mississippi River surface is present at this locality and no unequivocal lake sediment was found near the hypothesized dam. Third, is damming of the smaller Ouachita River by aggradation of an Arkansas River course. This seems to be the most likely origin for the lake based on distribution of the lake sediment.