Laramide Structures in Basement and Cover of the Beartooth Uplift Near Red Lodge, Montana¹

Donald 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.