LANDSCAPE CONTEXT OF THE LINDSAY MAMMOTH, EASTERN MONTANA: STRATIGRAPHY, GEOMORPHOLOGY, AND GEOCHRONOLOGY

Stratigraphic and geomorphic investigations related to the Lindsay mammoth provide information on local and regional physical environments, and can be used to evaluate Late Quaternary landscape dynamics on the northern Great Plains. The nearly complete skeleton of a mammoth (cf. Mammuthus columbi) was recovered about 12.4 km northwest of Glendive, in an upland divide between the drainages of the South Fork of Deer Creek and Spring Creek (Deer Creek Church USGS 7.5 minute quadrangle). In this upland area north of the Yellowstone River, the Tertiary bedrock is overlain by loess deposits containing paleosols. The mammoth remains were found embedded in these silts. In places the sedimentary matrix enclosing the top of the mammoth bones contains higher levels of carbonates and is overlain by a buried soil A-horizon. Locally, gravels interpreted as remnants of older alluvial terraces also overlie Tertiary bedrock. Direct radiocarbon measurements of the mammoth bones and correlations with other regional geomorphic and stratigraphic localities lead to the conclusion that the mammoth is older than 12,000 RCYBP. If the mammoth remains are older than 12,000 RCYBP then it is older than both the Younger Dryas, which ranges from about 10,900-9,800 RCYBP, as well as Clovis artifact assemblages, which have a proposed range of either 11,500-10,900 or 11,050-10,800 RCYBP. Although the Lindsay locality is situated in an unglaciated region of the northern Plains, glacial lakes extended into the valleys of Redwater River and the Yellowstone River. Glacial ice advanced nearly to Glendive, where mammoth remains dated to around 20,500 RCYBP were recovered in alluvial gravels overlain by silts. Gravels at Wibaux contain tusk dated to about 26,000 RCYBP. South of the Yellowstone, uplands silts contain buried soils dated from 11,415-9,300 RCYBP. Based on these data, it can be proposed that the alluvial gravels were deposited during the Last Glacial Maximum, followed by incision which formed terraces. Loess, containing the Lindsay mammoth remains, began to accumulate in upland areas prior to 12,000 RCYBP. Deposition of wind-blown sediments continued throughout the late Pleistocene and early Holocene interrupted by several intervals of landscape stability correlated with the Younger Dryas.