ABANDONED VALLEYS IN WESTERN MISSOURI: IMPLICATIONS FOR LANDSCAPE EVOLUTION OF THE NORTHWEST OZARK PLATEAU BORDERLAND

At least four abandoned valleys in western Missouri are present in western Missouri; they predate the incision of modern streams, and they partly have controlled the development of modern stream piracies, principally in the Horse Creek (Sac River) and McCarty-Clear Creek (Osage River) drainage basins. Although an abandoned valley previously has been reported locally, and structural lineaments have been discussed by previous investigators and are shown on provisional geologic maps, new analyses of shuttle radar topography mapping (SRTM) and digital elevation model (DEM) imagery, together with provisional field studies, reveals the striking lateral continuity of these features. The ages and precise origins of the valleys, however, remain obscure. Regardless of the origin, their record of preservation and subsequent landscape evolution seemingly indicates a late phase of uplift in the western Ozarks.

Newly named features include El Dorado Valley (previously reported as the McCarty-Horse Creek lineament), which is 40 km long and approximately 2.5 km wide; Walnut Creek interfluve, which branches off from El Dorado Valley; Wilson Branch Valley, which may have connected with the Walnut Creek interfluve; and Rocky Hollow, a possible tributary of Wilson Branch Valley. All of these features run in a roughly north-south direction in eastern Bates, southwestern Cedar, eastern Vernon, and northeastern Barton counties. All cut Pennsylvanian strata. Pleistocene and Quaternary stream valleys are superposed on the paleovalleys, but the margins of the paleovalleys are mostly linear and not extensively eroded, unlike the irregularly eroded zero-edge of the Pennsylvanian onlap. This suggests a relatively recent, possibly Neogene, age and a fluvial origin. Alternative hypotheses include parallel faults or fractures or exhumed Pennsylvanian paleovalleys.

Other paleovalleys include the Collins-Humansville Valley (previously referred to at the Humansville-Weaubleau Creek lineament), which stretches from near Collins to Bolivar; it follows a structural lineament that records Pennsylvanian incision into Mississippian and lower Ordovician carbonates, and Roscoe Valley in southern St. Clair County, which remains largely unknown. Consequently, there may be different ages and origins for all of these features.