[sup 40]Ar/[sup 39]Ar ages of Challis volcanic rocks and the initiation of Tertiary sedimentary basins in southwestern Montana

Journal Article · Fri Oct 01 04:00:00 EDT 1993 · Mountain Geologist; (United States)

OSTI ID:5289981

M'Gonigle, J W ^[1]; Dalrymple, G B ^[2]

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1. Geological Survey, Denver, CO (United States)
2. Geological Survey, Menlo Park, CA (United States)

[sup 40]Ar/[sup 39]Ar ages on single sanidine crystals from rhyolitic tuffs and ash flow tuffs within the uppermost and lowermost parts of the volcanic sequence of the Horse Prairie and Medicine Lodge topographic basins, southwestern Montana, show that these volcanic rocks were emplaced between about 48.8[+-]0.2 Ma and 45.9[+-]0.2 Ma, and are correlative with the Eocene Challis Volcanic Group of central Idaho. Sanidine ages on tuffs at the base of the Tertiary lacustrine, paludal, and fluvial sedimentary sequence, which unconformably overlies the volcanic sequence, suggest that sedimentation within an ancestral sedimentary basin that predated the development of the modern Horse Prairie and Medicine Lodge basins began in the middle Eocene. 22 refs., 3 figs., 2 tabs.

• OSTI ID: 5289981
• Journal Information: Mountain Geologist; (United States), Journal Name: Mountain Geologist; (United States) Vol. 30:4; ISSN MOGEA2; ISSN 0027-254X
• Country of Publication: United States
• Language: English

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