QUATERNARY STRATIGRAPHIC AND GEOMORPHIC CONTEXTS IN THE NORTHERN ROCKY MOUNTAINS AND UPPER MISSOURI BASIN

Lithostratigraphic sequences and landscape features in the Rocky Mountains of western Montana and the adjacent Great Plains provide an indication of Quaternary physical and biotic paleonvironmental conditions. Although some Middle Pleistocene and older Cenozoic sequences are available, late Pleistocene strata have been more extensively studied, providing geoecological information pertaining to the time before and during the Last Glacial Maximum (LGM) and the last glacial to interglacial transition. Within the Rocky Mountains, lithostratigraphic sequences containing paleobiotic evidence (fossils, artifacts) are found in valley basins, upland-foothill contexts, and caves. Lacustrine, fluvial, and debris flow deposits from Centennial Valley contain extinct vertebrate remains. Radiocarbon and luminescence measurements appear to demonstrate that these deposits can be assigned to the LGM or earlier times. Deposits from Blacktail Cave also contain vertebrate remains dating from before the LGM to the Pleistocene-Holocene boundary. Late-glacial and post-glacial stratigraphic sequences from Sheep Rock Springs, Indian Creek, MacHaffie, and the Sun River are dated by radiocarbon measurements as well as tephras, artifacts, and biotic remains. On the Great Plains adjacent to the Rocky Mountains, stratigraphic sequences are situated along the Missouri and Yellowstone Rivers and their tributaries. Within the Yellowstone basin geomorphic and stratigraphic contexts can be used to develop a model of landscape evolution. Tephra, fluvial, and eolian deposits and paleosols are associated with extinct Pleistocene fauna. Within the upper Missouri basin, fluvial, alluvial, eolian, lacustrine, and glacial (till) deposits have been dated by the presence of extinct fauna, and radiocarbon and luminescence measurements. Examples include dated stratigraphic sequences in the Great Falls region along the Missouri River and deposits exposed along the Marias River. Based on these temporal constraints, correlations between the Rocky Mountains and Plains contexts can be used to develop models linking the dynamics of landscape evolution and Quaternary biotic communities for the mountains and adjacent plains.