Correlation of Cretaceous Blackleaf and Frontier Formations in Parts of Madison and Beaverhead Counties, Montana: ABSTRACT

Abstract

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) inmore » 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.« less