GSA Field Guide 1: Colorado and adjacent areas by David R. Lageson, Alan Lester, Bruce Trudgill

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780-801. , Jr, 1977, Independence Mountain thrust fault, North Park basin, Colorado: Contributions to Geology, v. 16, p. 1-15. , 1991, Tectonic relationships of the southeastern Wind River Range, southwestern Sweetwater Uplift, and Rawlins Uplift, Wyoming: Report of Investigation No. 47, Geological Survey of Wyoming, 24 p. , Geologic history of the Colorado Front Range: Rocky Mountain Association of Geologists Field Trip Guidebook, 189 p. , 1957, Tectonics of eastern flank and foothills of Front Range: American Association of Petroleum Geologists Bulletin, v.

The aptly named Troublesome Formation is well dated by mammal fossils and volcanic ash with fission track ages between 20 and 13 Ma (Izett and Barclay, 1973). To the west near Kremmling, it blankets the Precambrian rocks in the hanging wall of the Williams Range thrust. This suggests that the Troublesome Formation may have been deposited in localized basins caused by backsliding on this thrust fault. Fold axes in the Troublesome Formation parallel the Williams Range thrust and are defined by rotated strata with slight to locally steep (80°) dips (Izett and Barclay, 1973).

Unlike the higher angle faults of the eastern margin of the Front Range, the Williams Range thrust is low angle to nearly horizontal in most places. At this locality, the Montezuma stock, a monzogranite porphyry with an age of 38-39 Ma (Marvin and others, 1989; McDowell, 1971) domed Williams Range thrust along normal faults, forming a thrust window (Fig. 6; Ulrich, 1963). A basement overhang of at least 9 kilometers is indicated by the distance from the thrust window to the frontal exposure of the thrust to the west combined with the angle of the thrust plane and the thickness of the sedimentary section.