GEOLOGIC CONTEXT OF A PALEOINDIAN DISTRICT ON THE EASTERN PLAINS – OZARK PLATEAU BOUNDARY IN SOUTHWEST MISSOURI

One of the richest, best-preserved, Paleoindian districts in the country is in the lower Sac River valley in southwest Missouri on the boundary between the Eastern Plains and the Ozark Plateaus. Understanding the geologic context, history of landscape evolution, and paleoenvironmental change in the Sac River valley forms a rational basis for targeting not only Paleoindian site discovery, research and management, but also guiding the search for possible cultural deposits that pre-date the Clovis period.

Previous survey of lower Sac River cutbanks documented 13 Paleoindian deposits that have yielded a wide array of geographically diverse, temporally diagnostic, Paleoindian projectile points over the last 30 years. Excavations and geoarchaeological assessments at two locations, the Big Eddy and Montgomery sites, along with examination of cutbank stratigraphy, demonstrate that in the lower Sac River valley, Paleoindian deposits, in places stratified and multicomponent, are well preserved deep within a T2 terrace sediment assemblage of the Rodgers Shelter member. Paleoindian deposits are likely far more extensive then currently documented. Only a fraction of the lower Sac River stream bank consists of actively eroding cutbanks, but three-quarters of the cutbank length that exposes the T2 terrace sediment assemblage also exposes prehistoric cultural deposits of various periods. Nearly half of these cutbanks with cultural deposits expose Paleoindian deposits in the early submember, with more than one Paleoindian component at four locations.

The sediment assemblage that hosts Paleoindian and possible pre-Clovis cultural deposits consists of an upward fining alluvial sequence of upper point bar and floodplain overbank deposits that dates between about 13,000 and 10,000 yrs BP. Floodplain aggradation provided a context for both burial and relatively intact preservation of the early cultural deposits. The early submember conformably overlies paleo-braid-bars and channel swales of a braided Late Wisconsin Sac River. It is the better-drained braid-bar position over which Paleoindian deposits tend to be found, and where possible pre-Clovis deposits should be sought. The early submember is buried by two thick increments of terrace veneer deposits that comprise two additional middle and late Holocene submembers.