Stratigraphic and geochronologic contexts of mammoth (Mammuthus) and other Pleistocene fauna, Upper Missouri Basin (northern Great Plains and Rocky Mountains), U.S.A.
Introduction
Fossil remains of mammoth (Mammuthus) have been recovered from a variety of stratigraphic and chronologic contexts in the Upper Missouri River Basin. Situated in the western interior of North America (Fig. 1), the Upper Missouri region consists of the watersheds of the Missouri and Yellowstone Rivers upstream from their confluence (Fig. 2). Mammoth fossils have been found in both unglaciated and glaciated areas of the Northern Great Plains as well as in valley deposits within the Rocky Mountains. Depositional environments associated with the stratigraphic contexts include paludal-lacustrine and swamp-bog settings, caves and rockshelters, alluvial deposits and fluvial terraces, uplands associated with eolian deposition and paleosols, and debris flows. Where stratigraphic and chronologic indicators are available, they seem to show that the mammoth remains date to the middle and late Pleistocene.
Section snippets
Mammoth localities
An inventory of mammoth remains recovered from the Upper Missouri River Basin is presented in Table 1. The fossil localities containing mammoth remains and other Pleistocene fauna can be organized by geographic area, principally by regional drainage basins. These regions include the headwaters of the Missouri River, the glacial Lake Great Falls Basin, the Missouri River system east of Great Falls, and the drainage of the Yellowstone River (Fig. 2).
Paludal–lacustrine, swamp-bog settings
At the Merrell Locality (Fig. 2), swamp-bog and paludal–lacustrine deposits (designated as stratum B, Albanese et al., 1995; Hill, 1999b) contain mammoth remains. Radiocarbon dates associated with these sediments range in age from >42,000 to 32,000 14C BP (Hill, 1999b; Table 2) (Fig. 15, Fig. 16). Thus the fauna from these deposits appears to date to the middle Wisconsinan (the pre-Last Glacial Maximum interstadial, isotope stage 3).
In the vicinity of the Sun River, west of Augusta (Fig. 7),
Chronology of mammoth localities
The chronological position of some deposits containing Pleistocene faunal assemblages from the Upper Missouri River Basin can be inferred using stratigraphic relationships and chronometric dating. Remains of Mammuthus, Equus, and Camelops occur in alluvium below pre-Illinoian till in the Milk River area (Fullerton and Colton, 1986). The Doeden gravels are possibly Illinoian or Sangamonian in age, based on K-Ar, U-series and radiocarbon dating of regional stratigraphic sequences and geomorphic
Summary and discussion
Fossil vertebrate localities from the Upper Missouri Basin provide information on the paleobiotic character of the Rocky Mountains and western interior Plains primarily from the late Pleistocene (the Wisconsinan). Some fossils appear to date to the middle Pleistocene. The localities contain evidence for a variety of large extinct herbivores and some carnivores. The list of extinct animals includes mammoths, ground sloths (both Jefferson's and Harlan's), American mastodon, horse, camel, bison,
Acknowledgements
I am especially grateful to John Storer for providing the opportunity to contribute this paper that is based on a presentation from the Third International Mammoth Conference in Dawson City, Yukon Territory. Tag Rittel kindly permitted the studies at Blacktail Cave, undertaken in collaboration with excavations directed by Les Davis and field supervised by Dave Batten. The fossils from the Doeden collection from Miles City were donated to Montana State University by Kathy Doeden. Mike Wilson
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