Heart Mountain and South Fork Thrusts, Park County, Wyoming

Abstract

The Heart Mountain thrust sheet of northwestern Wyoming is traceable from Clark Fork Valley southward across Sunlight Basin and across the North and South forks of the Shoshone River. If it continues still farther southward into the northwestern part of the Wind River Basin, as appears possible, its linear extent is more than 90 miles. The thrust sheet moved eastward a distance of more than 36 miles, much of which was across the surface of the land. The South Fork thrust is beneath, and is older than, the Heart Mountain thrust. The rocks of the South Fork overthrust sheet are sedimentary formations of Jurassic and Cretaceous age, whereas those of the Heart Mountain thrust sheet are limestones and dolomites of Paleozoic age. A trough-like fold of the South Fork thrust sheet, which appears to have been downfolded after the thrusting, lies in the valley of the South Fork of the Shoshone River. It is 8 miles in length and bounded at both ends by transverse faults. The rocks in the trough have been folded into a syncline and a recumbent anticline presumably formed during the emplacement of the thrust. Northeastward from the South Fork of the Shoshone, the thrust extends as a low-angle fault into the Shoshone Reservoir, where it is tho ght that the inclination and trend change abruptly, and that the fault thence continues to the northwest up Rattlesnake Valley as a high-angle shear fault. On the basis of structural deformation, the Wasatch formation of this region is divisible into two units. The emplacement of the South Fork thrust followed the deposition of the earlier unit and the emplacement of the Heart Mountain thrust followed the deposition of the later unit. After the emplacement and partial erosion of the Heart Mountain thrust sheet the tuffaceous sediments and volcanic rocks comprising the early basic breccia of the region were deposited. Vertebrate fossils from beds below the Heart Mountain thrust and others from beds above the thrust indicate that the thrusting took place near the close of the lower Eocene. The South Fork thrust was formed some time earlier in the Eocene.