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| Absaroka Volcanics | |
|---|---|
| Name | Absaroka Volcanics |
| Location | Montana, Wyoming, United States |
| Range | Absaroka Range |
| Type | Stratovolcanic field |
| Age | Eocene |
| Last eruption | Eocene |
Absaroka Volcanics The Absaroka Volcanics represent a voluminous Eocene volcanic province centered on the Absaroka Range of Montana and Wyoming, United States. The complex produced extensive andesitic to rhyolitic lavas and pyroclastic deposits that interact with contemporaneous sedimentary basins such as the Bighorn Basin and Wind River Basin. Studies by institutions including the United States Geological Survey, University of Wyoming, and Montana Bureau of Mines and Geology have integrated field mapping, geochemistry, and geochronology to constrain its evolution.
The volcanic pile crops out across the Yellowstone National Park vicinity, the Shoshone National Forest, and the Gallatin National Forest, extending from eastern Idaho margins into central Montana and northwestern South Dakota. It overlies and intertongues with Paleogene strata of the Fort Union Formation, Willwood Formation, and Bridger Formation along margins of the Powder River Basin and Denver Basin. Regional mapping by the U.S. Geological Survey and academic groups at the California Institute of Technology and Columbia University correlate units across structural domains controlled by the Rocky Mountain Front and Laramide uplifts near the Beartooth Mountains. The province abuts thrust systems tied to the Laramide Orogeny and extends toward volcanic centers documented in the Cascade Range and Sierra Nevada for comparative volcanology.
Volcanic lithologies encompass porphyritic andesite, dacite, rhyodacite, rhyolite, tuff, breccia, and ignimbrite, with subordinate basaltic andesite. Mineral assemblages include plagioclase, hornblende, biotite, orthoclase, magnetite, and accessory zircon crystals used for geochronology. Geochemical datasets from laboratories at Massachusetts Institute of Technology, Stanford University, and the Smithsonian Institution show calc-alkaline affinities, trace-element signatures, and isotopic ratios that compare to volcanic arc suites recognized at Mount Shasta, Mount St. Helens, and the Cascades Volcanic Arc. Petrological studies reference experimental work from Carnegie Institution for Science and thermobarometry approaches developed at University of California, Berkeley.
Stratigraphic frameworks integrate region-wide columns correlated with biostratigraphy from fossils in the Bridger Formation, radiometric ages from zircon U-Pb and ^40Ar/^39Ar dates provided by laboratories at the Geological Society of America-affiliated institutions. Ages center on the early to middle Eocene (~56–38 Ma), overlapping epochs studied in the Green River Formation and the Wasatchian biochron. Key tuff units serve as marker horizons in cross-sections tied to work by researchers from Princeton University, University of Michigan, and Yale University. Stratigraphic relationships record synvolcanic sedimentation in fluvial and lacustrine settings contemporaneous with the Eocene Thermal Maximum documented by paleoclimate studies at Lamont–Doherty Earth Observatory.
Eruption styles ranged from effusive lava flows to explosive ignimbritic eruptions and dome-forming events, comparable to processes at Mount Hood, Mount Adams, and Santorini. Pyroclastic density currents deposited extensive ignimbrites studied with techniques from the American Geophysical Union and sedimentologic models originating at the University of Colorado Boulder. Lava-dome collapse, sector collapse, and lahar generation affected paleodrainages like the ancestral Yellowstone River and Clarks Fork of the Yellowstone River. Volcanic facies mapping by teams from Utah State University and Montana State University documents transition zones from proximal breccia to distal ash-fall beds correlated with climatic perturbations assessed by paleoenvironmental researchers at the University of Arizona.
The Absaroka volcanic episode is tied to regional plate interactions following the Laramide Orogeny, influenced by subduction dynamics along the western margin of North America and intraplate extension related to the formation of the Rocky Mountains. Paleogeographic reconstructions using data from the Paleogeography and Paleoclimatology community and mapping by the Smithsonian Institution situate the province within Eocene topography adjacent to the Sevier Orogeny-modified landscape. Faulting associated with the Beartooth Lake fault and basin subsidence in the Bighorn Basin controlled magma ascent and vent distribution, themes explored in tectonic syntheses by the National Academy of Sciences and researchers at the University of Texas at Austin.
Hydrothermal alteration zones within the volcanic complex host mineralization, including epithermal veins with gold, silver, and native mercury, and porphyry-style copper occurrences documented in reports by the Montana Bureau of Mines and Geology and mining histories connected to Butte, Montana regional metallogeny. Investigations by the U.S. Bureau of Mines and economic geologists at Virginia Polytechnic Institute and State University document sulfide assemblages, alteration halos, and alteration-mineral vectors used in exploration models similar to those applied at Homestake Mine and Carlin Trend. Geothermal potential linked to Eocene heat flow has been assessed by the Department of Energy and academic teams from Idaho National Laboratory.
Interbedded sedimentary units adjacent to the volcanic deposits preserve paleobotanical and vertebrate assemblages including fossil floras, mammals, and freshwater faunas comparable to assemblages from the Bridgerian and Wasatchian North American land mammal ages. Paleontological collections housed at the American Museum of Natural History, Smithsonian Institution National Museum of Natural History, and Denver Museum of Nature & Science contain specimens that help constrain paleoecology altered by volcanic ash-fall events similar to biotic responses recorded at La Brea Tar Pits-era contexts and Green River Formation fishes. Paleoenvironmental work by the University of Kansas and Paleontological Society uses taphonomy, palynology, and stable-isotope datasets to reconstruct habitats impacted by volcanism and Eocene climate trends identified by the International Geosphere-Biosphere Programme.
Category:Volcanic fields Category:Eocene volcanism Category:Geology of Montana Category:Geology of Wyoming