Stratigraphy, petrology, and provenance of Cenomanian to Turonian Frontier Formation near Lima, southwest Montana
The Frontier Formation in the study area is subdivided into three informal lithofacies, all of which were deposited in fluvial environments: a mixed clastic lower unit (1500 ft), a fine-grained medial unit (5000 ft), and a mixed clastic upper unit (800 ft). In the lower unit, sandstones contain as much as 50% total feldspar and volcanic rock fragments; conglomerates are rich in quartzite clasts derived from the Belt Supergroup and contain as much as 15% silicic volcanic and 2% limestone clasts. Mudstone, siltstone, shale, and coal are interbedded with the sandstones. The fine-grained medial unit contains nonvolcanic mudstones and siltstones and minor sandstones and pebble conglomerates rich in chert and limestone lithic detritus. The mixed clastic upper unit contains abundant porcellanite and bentonite; sandstone interbeds contain plagioclase and limestone lithic grains. Sources for Frontier detritus include Proterozoic, upper Paleozoic, and Mesozoic sedimentary and volcanic rocks to the south and west. The Frontier was deposited during a depositional cycle prior to development of the Laramide Blacktail-Snowcrest uplift. Exposures of the Frontier near Lima appear to be among the thickest in the region and were probably deposited near the axis of a foredeep in front of the advancing Cordilleran thrust belt.
- Research Organization:
- Geological Survey, Denver, CO (USA)
- OSTI ID:
- 6148164
- Report Number(s):
- CONF-890404-
- Journal Information:
- AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States), Journal Name: AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States) Vol. 73:3; ISSN AABUD
- Country of Publication:
- United States
- Language:
- English
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CARBONACEOUS MATERIALS
CARBONATE ROCKS
COAL
CONGLOMERATES
ENERGY SOURCES
FEDERAL REGION VIII
FOSSIL FUELS
FUELS
GEOLOGIC DEPOSITS
GEOLOGIC FORMATIONS
GEOLOGY
LIMESTONE
MATERIALS
MINERAL RESOURCES
MONTANA
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PETROLEUM DEPOSITS
RESERVOIR ROCK
RESOURCES
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SANDSTONES
SEDIMENTARY ROCKS
SHALES
SILTSTONES
STRATIGRAPHY
USA