Correlation of Cretaceous Blackleaf and Frontier Formations in parts of Madison and Beaverhead Counties, Montana
The Albian Blackleaf Formation and the Cenomanian lower part of the Frontier Formation in Madison and Beaverhead Counties, Montana, lie within or east of the southwestern Montana fold-and-thrust belt. They were deposited near the western edge of the foreland basin complex prior to thrusting. Eleven stratigraphic sections are displayed and correlated using the Stratigraphic Report Graphic (SRG), a VAX-11-780-based graphic display program that plots lithology and associated geologic data. The Blackleaf Formation ranges from 425 to 600 m (1400 to 2000 ft) in thickness and is subdivided into four mappable lithofacies units: lower transitional clastic (unit 1), lower mudstone shale (unit 2), upper clastic (unit 3), and upper volcaniclastic (unit 4). Unit 1 conformably overlies the upper carbonate member (gastropod limestone) of the Kootenai Formation. This basal unit ranges from 13 to 87 m (43 to 285 ft) in thickness and consists of mudstone, quartz- and chert-rich sandstone, and limestone deposited in fluvial, lacustrine, and shallow marine environments. Unit 2 ranges from 17 to 70 m (56 to 230 ft) in thickness and consists predominantly of mudstone, shale, and siltstone deposited in lagoonal to shallow marine environments. Unit 3 ranges from 4 to 76 m (13 to 249 ft) in thickness and consists of quartz-rich sandstone bodies and interbedded mudstone deposited in fluvial, deltaic, and shallow marine environments. Unit 4 ranges from 270 to 360 m (886 to 1181 ft) in thickness and consists of porcellanitic mudstone, shale, and subordinate lithic sandstone deposited in fluvial, lacustrine, and shallow marine environments. Unit 4 of the Blackleaf Formation is overlain by the Frontier Formation. The Frontier reaches a maximum thickness of at least 420 m (1375 ft) and is composed of lithic sandstone, conglomerate, and subordinate mudstone and shale deposited in fluvial environments.
- Research Organization:
- Geological Survey, Denver, CO
- OSTI ID:
- 7042481
- Report Number(s):
- CONF-8609129-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 70:8; Conference: AAPG Rocky Mountain Section meeting, Casper, WY, USA, 7 Sep 1986
- Country of Publication:
- United States
- Language:
- English
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