Dating the onset of Miocene crustal extension in southwest Montana, northwest Wyoming, and adjacent Idaho
- Univ. of California, Berkeley, CA (United States). Dept. of Integrative Biology
- Univ. of Pittsburgh, PA (United States). Dept. of Earth and Planetary Sciences
- Lazy Ranch, Wise River, MT (United States)
For at least three decades a regional unconformity of middle Miocene age -- dubbed the Mid-Tertiary Unconformity -- has been recognized in the intermontane-basin deposits of adjacent WY, MT, and ID. However, constraining the age of the erosional event solely by radiometric means has proven difficult because datable tephras generally are scarce (except locally) and too fine-grained. On the other hand, biostratigraphic and magnetostratigraphic studies have yielded results that bracket the unconformity between 17 and 18 ma in Jackson Hole and just north of Yellowstone Park (Burbank and Barnosky, 1990). Whether the unconformity is time-transgressive from east to west remains an open question and is being addressed by biochronologic and magnetostratigraphic studies of a particularly complete stratigraphic sequence near Leadore, Idaho. Preliminary indications are that this part of the Idaho-Montana border was a depositional basin throughout most of the Miocene, covered with a large fresh-water lake during parts of the Arikareean (ca. 29--20 Ma) and saline lakes during parts of the Hemingfordian (ca. 20--16.8 Ma). Then sometime near the transition between Hemingfordian and Barstovian (ca. 16.8 to 15.0 Ma), just after the local development of the Mid-Tertiary Unconformity, local uplift increased the influx of clastic sediments, including debris flows, into the basin. Hence in general character the sedimentation patterns (but not the sediment types) along the Idaho-Montana border are identical to those in the eastern most Rockies at similar latitudes, both areas indicating that extensional block faulting and uplift started near the Hemingfordian-Barstovian boundary.
- OSTI ID:
- 5292024
- Report Number(s):
- CONF-9305259-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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