THE BEARTOOTH MOUNTAINS MONTANA AND WYOMING: TRAVERSING FOUR BILLION YEARS OF EARTH HISTORY, CLASSIC EXPOSURES OF GEOLOGIC PROCESSES, CONTRIBUTIONS TO THE HISTORY OF GEOLOGIC THOUGHT, AND PUBLIC POLICY ISSUES

A geologic trip around the Beartooth Mountains, MT and WY, encompasses a 4 billion year record of Earth history, classic examples of Earth processes, and locations of great controversies in geologic thought. Starting in the NW near Livingston, MT, Laramide-style faults define the northern margin of the Beartooth Mountains. The adjacent Stillwater Complex is a layered mafic-ultramafic magmatic body that preserves classic igneous textures, provides evidence of a periodically replenished magma chamber, and is host to mineral exploitation from WWII era chrome mines to the only PGE mine in operation in the U.S. To the east, south of Red Lodge, Rock Creek occupies a classic U-shaped glacial valley, and the surrounding high plateau hosts periglacial features. Archean rocks exposed in the high Beartooths include pendants of metasupracrustal rocks with detrital zircons as old as 4.0 Ga and tonalitic gneisses dated at 3.5-3.2 Ga that are intruded by voluminous calc-alkaline magmatic rocks dated at 2.8 Ga. These outcrops address a) “the granite controversy” with arguments about granitization v. magmatic processes, and the question b) “when did plate tectonics begin on Earth?” Over the Beartooth Highway heading west, the Great Unconformity is encountered between the 2.8 Ga granites and the Cambrian Flathead sandstone with overlying strata that preserve Devonian fish fossils. Further west, the Eocene Absaroka volcanics overlie the basement rocks, and these are host to a) the controversial Crown Butte Au-Cu deposits just outside of Yellowstone National Park, and b) the famous Heart Mountain detachment system that has inspired debate about mechanisms of gravity faulting. The northern margin of Yellowstone National Park is underlain by a >2.8 Ga low-grade metasedimentary sequence of turbidites with well-preserved sedimentary structures. Quaternary deposits of the Yellowstone caldera eruptive units of the Huckleberry Ridge and Mesa Falls tuffs, along with extinct travertine deposits are exposed near Gardiner, MT. The final northward leg passes through Paradise Valley of the Yellowstone River, where active Quaternary faults and Pinedale and Bull Lake glacial deposits are exposed. An amazing journey through geologic space and time, with many sites of interest for expert and novice geoscientists alike!