502 



Tusmar Plateau 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Sevier Plat. 



Hurricane fault zone 



Fig. 31.10. Upper diagram: the Tushar and Seviur faults of the High Plateaus in Utah, after 

 Eardley and Buetner, 1934. 



Lower diagram: the Hurricane fault in Uinkaret plateau, northern Arizona, after Koons, 1945. 

 Im, Moencopi fm; CK, Kaibab Is.; Ct, Toroweap fm.; Ch, Hermit sh.; Cc, Supai ss. 



recent past renewed volcanic activity has formed a single, small cone 

 and lava flow. 



Wasatch Range 



The late Cenozoic high-angle faulting along the west front of the 

 Wasatch Range and the faults of the ranges immediately westward have 

 already been described as part of the Rasin and Range province. The belt 

 of great trenches includes these faults. 



Western Wyoming and Southeastern Idaho 



Superposed on the Laramide structures of western Wyoming and south- 

 eastern Idaho are several northward-trending high-angle faults that have 

 helped delineate and deepen the major intermontane valleys. Since the 

 later structures parallel the earlier in northern Utah, southeastern Idaho, 

 and southwestern Wyoming, the two have not been clearly distinguised; 

 but toward the northern end of the belt in connection with the Snake 

 River, Hoback, and Teton ranges, the Laramide structures veer northwest- 

 ward, and the later high-angle faults cut across them at acute to right 

 angles. A distinctive basin fill is also a result of the faulting, and helps 

 distinguish the older from the younger. 



A straight and youthful-appearing fault scarp occurs along the east 





side of Rear Lake in northern Utah and southeastern Idaho. It is re- 

 sponsible for the Rear Lake depression ( Mansfield, 1927 ) . 



Star Valley in western Wyoming and its northward continuation in 

 Grand Valley and Swan Valley between the Caribou and Snake River 

 ranges is blocked out on one side and in places on both sides by- 

 faults of late Miocene and early Pliocene age. See cross section of 

 Fig. 31.11. 



An extensive graded surface had been eroded by middle Miocene time, 

 and remnants of it still exist at elevations of 8500 to 9500 feet, especially 

 in the Gros Ventre and Wind River ranges to the east. Rlackwelder 

 (1915) has called it the Union Pass surface. The main drainage lines of 

 the present, except where affected by later faulting, had been established 

 in and across the Laramide folds and thrust sheets by this time. Then the 

 region was broadly uplifted, the streams rejuvenated, and the surface 

 deeply dissected. The transverse and longitudinal canyons and valley 

 were eroded as deep as today and in the same position. These include the 

 Snake River Canyon through the Snake River Range and the Hoback 

 Canyon through the Hoback Range. Following the dissection of the Union 

 Pass surface, normal faulting occurred as depicted in the series of dia 

 grams of Fig. 31.12. In Grand Valley, west of the Snake River Range, the 

 faulting and consequent deposition occurred in two episodes, and at 

 unconformity was produced between two divisions of the valley fill. The 

 sediments more than filled the graben and accumulated on the prefault 

 ing surface to elevations above the fault scaqD, and the canyons tributan 

 to the graben that had previously been eroded in the Union Pass surface 

 were flooded with debris. Toward the heads of these canyons, coars< 

 material accumulated to elevations of 8500 feet. Volcanic activity accom 

 panied the deposition of the valley fill, and much tuffaceous materia 

 was contributed to the deposits, and some thick sills split the basin beds 

 Then another cycle of erosion followed, and the Rlack Rock surface wa 

 cut at about 7500 feet. It was also a pediment that flanked the grabei : 

 valley, and it beveled both the basin fill and the bedrock. The stream 

 were again rejuvenated, perhaps several times, and the present valley 

 about 1000 below the Rlack Rock surface were eroded. The old fanH 



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