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Abstract
AAPG Bulletin, V. 84, No. 3 (March 2000), P. 360-375.
Laramide Structures in Basement and Cover of the Beartooth Uplift
Near Red Lodge, Montana1Donald U. Wise2
©Copyright 2000. The American Association of Petroleum Geologists. All rights
reserved.
1Manuscript received August 20, 1998; revised manuscript received June 21,
1999; final acceptance September 3, 1999.
2Department of Geosciences, Franklin and Marshall College, Lancaster, PA
17604-3003; e-mail: [email protected]
The Yellowstone Bighorn Research Association field camp at Red Lodge served as a base for
work on this project on and off over the last 40 years. Members of that association, as
well as Alan and Helen Weaver of Red Lodge, were of great assistance. Much of the work in
the 1970s and 1980s was done at the University of Massachusetts at Amherst. Final
manuscript preparation was as a research associate at Franklin and Marshall College.
Grateful acknowledgment is given for the sharpening and refining of ideas and data by a
host of friends, associates, colleagues, and former students, all of whom managed to
overcome their natural shyness to highlight many inherent problems. John Bartley, Edward
Beutner, Peter DeCelles, Richard Hoppin, Neil Mancktelow, and Walter Snyder provided
helpful reviews; however, none of the above should be held culpable for the final product.
ABSTRACT
Ramp mechanisms associated with decoupling of a 6-7-km-thick basement slab may have
been responsible for progressively more and more horizontal components of thrusting of the
northeast corner of the Beartooth uplift near Red Lodge, Montana. As part of a nearly
right-angle corner of the uplift, two apparent tear faults bound a 7-km-long block of
Laramide mountain-front structures. New roadcuts and a deep well through basement refine
geometry of range overthrusting and show that these apparent tear faults are really
pivoting normal faults that cut frontal thrust structures on either side of an uplifted
corner flap. A ship's prow analogy of late-stage horizontal thrust motion is proposed with
the "bow wave" causing uplift and rotation of the corner flap. Volumetric
adjustments associated with late-stage stuffing of basin material beneath frontal thrusts
plus deeper duplexing of basement beneath the uplift helped define final details of range
geometry, a mechanism probably applicable elsewhere in the middle Rocky Mountains.
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