Late Pleistocene evolution of the lower Mississippi River valley, southern Missouri to Arkansas
The Mississippi River served as the principal conduit for meltwaters discharging from the southern margins of North American ice sheets. The northern half of the Mississippi River alluvial valley in Missouri and Arkansas can be subdivided into two distinct channel courses, the Western and Eastern Lowlands, which are separated by Crowleys Ridge. The Eastern Lowlands contain two entry points, the Bell City–Oran and Thebes Gaps; the latter is a narrow bedrock gorge that contains the modern river. This paper summarizes a model for evolution of channel courses in the Western and Eastern Lowlands during the Illinoian and Wisconsin glacial stages. For the Western Lowlands, stratigraphic relations between loess units and subjacent fluvial deposits show that major continuous terrace surfaces, or valley trains, are Illinoian or late Wisconsin in age. Investigations of tributary valley fills further show the Western Lowlands were deeply incised during the early to middle Wisconsin stage, then aggraded during the late Wisconsin stage to such an extent that tributary valleys were backfilled with slackwater deposits. Moreover, analysis of loess thinning and fining trends shows that early to middle Wisconsin Roxana Silt and the late Wisconsin Peoria Loess covers on Western Lowlands valley trains were derived from deflation of valley trains in the Eastern Lowlands. We conclude that the Western Lowlands were the primary Mississippi River course during Illinoian deglaciation but were abandoned shortly thereafter, then episodically reoccupied during the late Wisconsin stage. For the Eastern Lowlands course, valley trains of early to middle Wisconsin age are not preserved, with the exception of a small remnant where the Mississippi River broke through a gap in the southern margins of Crowleys Ridge and reentered the Western Lowlands. All valley trains in the Eastern Lowlands are late Wisconsin age. A series of discontinuous terrace surfaces in the Bell City–Oran Gap represent dissected remnants of the primary late Wisconsin full glacial and early late glacial course of the Mississippi River or combined Mississippi and Ohio Rivers, whereas younger continuous braided stream surfaces in the Bell City–Oran and Thebes Gap courses represent valley trains of latest Wisconsin and earliest Holocene age. Hence, the Bell City–Oran Gap was the primary course during the entire Wisconsin stage, and the Thebes Gap was permanently occupied in earliest Holocene time. As much as 5 m difference in elevation between successive late Wisconsin terrace surfaces of the Eastern Lowlands indicates that valley trains were rapidly constructed, incised, and abandoned in response to fluctuations in the delivery of meltwater and outwash from the former ice margin. Moreover, a variety of data indicates that deglacial meltwater floods were commonly of sufficient magnitude to overflow from the primary course in the Bell City–Oran Gap. Throughout the late Wisconsin substage, floodwaters overflowed into the Western Lowlands, constructed a series of secondary valley trains, and filled tributary valleys with slackwater deposits. During the latest Wisconsin stage, perhaps in association with the drainage of Glacial Lake Agassiz, floodwaters overflowed into the Western Lowlands and broke through the Thebes Gap. The Charleston fan represents the initial deposits of the Thebes Gap course. Following deposition of a large crevasse splay at the Western Lowlands entrance, that course was no longer active, but flow was routed through the Thebes Gap during periods of high-magnitude flooding until the Bell City–Oran Gap was abandoned and the Thebes Gap course was permanently occupied during earliest Holocene time.