Abstract

The Elk Mountain district is at the northwest end of the Medicine Bow Range, where trend changes from northerly in the Southern Rockies to westerly in the Central Rockies of Wyoming. Deformation of pre-Cambrian crystallines and 17,000 feet of predominantly shaly Paleozoic and Mesozoic beds proceeded during and after the deposition of many thousand feet of Upper Cretaceous and Fort Union beds in the basin to the north. Folds trend generally north, and their axial planes dip west. The following unusual features are described and discussed in terms of genesis:

(1) Two westward-dipping thrusts pass laterally northward into west-striking tears, which cut across folds; deformation in a block on one side of a tear was more or less independent of that in the other block. (2) A thrust cuts diagonally across a major syncline and anticline. (3) Marked rotatory movement is shown by a northeast-striking tear, which terminates upward at a thrust, and by several smaller tears cutting across slices between thrusts. (4) A fault having the characteristics of a subsequent shear thrust is probably a combined break and erosion thrust. (5) A bedding thrust in the bentonite-bearing Mowry shale of the gentle west flank of an anticline crosses the axis and offsets the axial plane.

Oil and gas possibilities of the district and the possible effects of bedding thrusts on oil traps are discussed.

The structural pattern of the district and surrounding region (Pl. 2) diverges greatly from that produced by uniform stress applied to uniform rocks. This is attributed to effective compression acting in different directions and to the influence of such features as bedding, schistosity, contacts of igneous bodies, and faults in the pre-Cambrian.

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