Abstract

The Five Springs Creek area comprises a portion of the northwest flank of the Bighorn Mountains. The marginal structures along which the range uplift occurred are the Five Springs thrust along the base of the range front and a faulted monocline 1 mi to the northeast.

The Five Springs thrust dips 15° NE. along the front. Toward the southeast the fault turns east and then northeast across the front and into the basement rocks; the dip increases to 45° N. and then vertical where it becomes a left-hand strikeslip fault. Toward the northwest, however, the fault turns west into the Bighorn basin and becomes a left-hand strike-slip fault. This unusual trace, and the drag and movement senses along the tear faults at either end indicate that the northwest part of the thrust is under-thrust while the southeast part is overthrust.

The overturned sedimentary rocks of the upper plate just south of Five Springs Creek are capped by a small recumbent flap fold. North of the creek, the area is covered by debris from the breaking up of the flat upper limb of the flap fold by gravitational sliding. Impetus for the formation of these collapse structures was supplied by the outward movement of the upper plate as the uplift rose above the adjoining basin supplemented by under thrusting.

Excellent exposures across the faulted monocline yielded convincing evidence that the basement rocks adjusted by block tilting. Rotation was demonstrated by changes in attitude of originally flat-lying Precambrian pegmatites and relict sheeting and by a system of early Laramide orthogonal vertical northwest and northeast fractures. The sharp tilting between blocks took place in narrow zones of intense deformation by small displacements along closely spaced fractures.

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