Abstract

The upper Laramie River Valley is at the junction of the Medicine Bow Range and the Colorado Front Range. Pre-Cambrian crystallines and 8000 feet of predominantly shaly Paleozoic and Mesozoic beds are sharply folded and faulted. The valley, a structural trough trending north to northwest, is bounded on both sides for the most of its length by outward-dipping thrust faults. Near the south end, where the trough is deepest, its width is less than the respective slips of the bounding faults. Erosion must have cut away the front of one or both of the advancing thrust blocks during the interval of deformation in order to permit them to reach their present positions.

The following unusual features are described and discussed in terms of genesis: (1) A northeast-dipping thrust carries an anticline onto the adjacent anticline to the southwest and produces thrusting of younger rocks onto older. In the fault zone are slice blocks younger than both formations bounding the fault zone. (2) East of this thrust is a north-striking high-angle fault with downthrow on the east. The fault terminates against the thrust to the south and dies out in a monocline to the north. (3) A high-angle fault shows several reversals of relative vertical movements of blocks. (4) Several high- and low-angle faults turn in depth toward stratification and do not affect the crystallines. (5) Pre-Cambrian and younger rocks occur in relations that permit deductions on the influence of pre-Cambrian lithology on Laramide structures.

Application of criteria to distinguish underthrust from overthrust faults leads to inconsistent results. It is concluded that features in the mapped area of a kind that have been advanced as criteria show only differential relative movements produced by east-west compression acting on a vertically and horizontally nonuniform crust.

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