PLEISTOCENE STRATIGRAPHY AND CHRONOLOGY OF THE LOWER YELLOWSTONE BASIN, NORTH AMERICA

The Pleistocene stratigraphic record in the Lower Yellowstone basin, in eastern Montana, consists of a variety of lithofacies associated with geomorphic features and vertebrate fossils. Middle Pleistocene or possibly Sangamonian age gravels form high terraces in the Yellowstone River Valley. Some of these deposits, such as the Doeden gravels, contain Illinoian/Sangamonian(?) age fossil vertebrates, including ground sloths, mammoth, mastodon, horses, musk ox, and giant short-faced bear. Dates on tephra and calcrete from within the Yellowstone River and Tongue River valleys indicate that these high terraces were formed in the Middle Pleistocene or early Late Pleistocene. Lower terrace gravels at Glendive, Montana, contain mammoth fossils dated to 20,470 +/-80 RCYBP (Beta-155642), while gravels along Beaver Creek, near Wibaux, Montana, contain tusk fragments dated to 26,000 +/- 120 RCYBP (SR-6086). Upland settings within the lower Yellowstone drainage indicate that Late Glacial environments were associated with intervals of aeolian deposition interrupted by periods of increased landscape stability and soil formation. Pedogenic features consist of secondary carbonates perhaps linked to arid climates as well as well-developed A-horizons possibly connected to wetter or cooler climates. Upland silts within the South Fork of Deer Creek overlie bedrock and contain buried A-horizons and secondary carbonates. The silts contain the remains of a mammoth, with dates on collagen of 11,500 +/- 80 RCYBP (Beta-102031) and 12,330 +/- 50 RCYBP (SR-5576). Stratigraphic sequences south of the Yellowstone River, at OTL ridge (south of Glendive, Montana), contain buried A-horizons developed within aeolian silts dated to between 9,330 +/- 80 RCYBP (Beta-155708) and 11,415 +/- 35 RCYBP (SR-6089). These upland lithostratigraphic sequences can be correlated with the Aggie Brown Member of the Oahe Formation. The lower Yellowstone Valley contains fluvial gravels that form Middle and Late Pleistocene-age terraces. Vertebrate fossils recovered from the lower terraces provide dates of about 26,000-20,000 RCYBP. Upland silts contain mammoth remains dated to about 12,000 RCYBP. Soil forming episodes occurred close to the Pleistocene-Holocene transition, generally within the interval of 11,000-9,000 RCYBP.