DATING MIDDLE AND LATE WISCONSIN EPISODE STRATIGRAPHIC SEQUENCES IN THE ROCKIES AND MISSOURI PLATEAU: LUMINESCENCE AND RADIOCARBON CHRONOLOGIES

Optically stimulated luminesence (OSL) and radiocarbon (14C) dates from stratigraphic sequences in the Rocky Mountains and the western interior Plains provide a chronologic framework that can be related to glacial-interglacial climate change, and late Pleistocene palentological and archaeological contexts. In the Montana Rockies, a pre-Last Glacial Maximum (LGM) fossil-bearing sequence has been dated using OSL and 14C in Centennial Valley, while pre-LGM fossil-bearing strata have been dated using 14C at Blacktail Cave. A mammoth from Box Creek (south of Fort Peck) also is pre-LGM (ca. 33,200 14C B.P.). Radiocarbon dates associated with the LGM include fossil-bearing deposits in the Centennial Valley, as well as vertebrate localities on the Plains. For example, mammoth collagen from terrace gravels at Glendive date to 20,470 ± 80 14C B.P. Preliminary OSL dates for laminated lacustrine silts and overlying sands at Holter Lake indicate the youngest interval of glacial lake Great Falls has an age of around 15,000-13,000 OSL B.P. (UW355 and UW356). This suggests that a lobe of the Laurentide glacier blocked the Missouri River during the later part of the Wisconsin Episode, which has implications for the regional deglaciation chronology and post-LGM paleoecological dynamics. Dates near the Pleistocene-Holocene boundary are available from Blacktail Cave (11,240-10,270 ± 115 14C B.P.), as well as for the Lindsay mammoth (11,925 to 9,490 14C B.P.). The eolian silts containing the mammoth can be correlated with the Aggie Brown Member of the Oahe Formation. Overlying paleosols reflect intervals of stabilization which elsewhere have been related to moist-cool late Pleistocene and early Holocene climates, and increasingly arid middle Holocene (“Altithermal”) conditions. Evidence for this wetter-cooler climatic interval may also be present in upland sequences, where bulk carbon dates indicate soil-forming conditions at around 9,560 and 9,330 B.P.