WYOMING ROCKIES 





in close proximity to the Beartooth thrust. They were intruded before 

 the thrusting took place and have been cut and displaced by the fault 

 or tears associated with it. See Lodgepole intrusive, Fig. 24.3. The in- 

 trusions are in the form of small sills principally in the Cambrian strata, 

 nearly horizontal sheetlike masses not far below the Cambrian strata in 

 the Precambrian and laccoliths. The latter are found near the mountain 

 front where the Nye-Bowler lineament is closest. 



The northwest end of the Beartooth Range and hills in the vicinity of 

 Livingston, Montana, are structurally complex. The northward flowing 

 Yellowstone River bounds the range on the west, but extending north- 

 westward beyond are low mountains that link with the Bridger Range. 

 The northeast front of the Beartooth Range is generally bounded by a 

 low-angle thrust dipping into the range, and the thrust sheet has moved 

 northeastward. In the Livingston area, however, several thrust sheets 

 from in front of the main mountain block have moved southward and 

 have been resisted by a corner of the "North Snowy block" (Lammers, 

 1937). See upper cross section, Fig. 24.3. The thrusting may have been 

 preceded by a stage of folding and erosion which could correspond with 

 the post-Lance and pre-Fort Union unconformity (Skeels, 1939). The 

 thrusting itself may correspond to the post-Fort Union and pre- Wasatch 

 unconformity in the Livingston basin. See discussion of the Beartooth 

 thrust in Chapter 23. 



Foose ( 1960 ) has treated the Beartooth Bange as a rectangular block 

 j primarily elevated above adjacent basins and secondarily affected in 

 places by horizontal transport of its marginal rock masses. At the north- 

 |east (Bear Lodge) corner the vertical structural relief is 15,000-20,000 

 feet, and in the absence of confinement, he concludes that the mountain 

 mass has moved outward on the adjacent basin as much as 10,000 feet. 

 The movement was facilitated by such secondary structures as bent high- 

 angle faults, tear faults, and imbricate thrusts. 



OWL CREEK AND WASHAKIE MOUNTAINS 



Rattlesnake Mountain west of Cody and other smaller topographic fea- 

 tures continue the Beartootii uplift southward, but on the west great 



accumulations of volcanics compose the mountain mass and extend south- 

 ward for about 50 miles, where the Owl Creek Mountains appe.tr. The 

 volcanics spread northwestward, over considerable areas of sedimentary 

 rock, and lay up on the southwest flank of the Beartooths. They form the 

 Absaroka Range (lower section in Fig. 24.4). From under the volcanics a 

 large asymmetrical anticline, the Owl Creek Mountains, appears, which 

 extends generally eastward, and in places at least, is overthrust south 

 ward. See upper section in Fig. 24.4. The large anticline is broken b) 

 many faults and rendered further complex by small folds (Fanshaw, 

 1939). The shelf facies of Paleozoic, Triassic, and Jurassic rocks is essen- 

 tially the same here as in the Big Horn and Wind River ranges. It is 

 probable that the structures of the Owl Creek Mountains extend north- 

 westward under the Absaroka volcanics so as to lie west of Rattlesnake 

 Mountain and the Beartooth plateau, but the volcanics cover most of die 

 area and little is known of the underlying rocks or structure. 



Wise (1961) recognizes a primary vertical uplift of about 20,000 feet 

 of the Owl Creek block, then gravity sliding of Mississippian strata away 

 from the crest of uplift, where kegstone-like graben exist. The sliding 

 toward the Wind Biver basin was unrestrained, and the lower extremities 

 of the thrust sheets are much brecciated and grade into conglomerate 

 lenses of the Eocene Wind River formation. 



South of the Owl Creek Mountains and between it and the Wind River 

 Range is the Wind River basin, which contains an instructive sequence of 

 orogenic sediments. They are tabulated in Fig. 24.5. At the west end ot 

 the Owl Creeks and at the south end of the Absarokas is die Washaki. 

 Range, which has been studied in considerable detail by Love I 1939). His 

 account is representative of the Laramide history of the Wind River basin 

 and adjoining ranges and is abstracted with minor changes as follows 



The Owl Creek Mountains and the Washakie Range were folded and prob- 

 ably faulted at the close of Lance time and before the beginning of Fort Union 

 deposition in the area to the northeast; the granitic core of the Washakie Range 

 was exposed and being eroded when the upper part of the Fort Union forma- 

 tion was being deposited in the southwestern portion of the Big Horn basin: at 

 the close of Fort Union time there was additional folding and probably faulting 

 along the margins of the Owl Creek and Washakie ranges; the Pinyon con- 

 glomerate was deposited in the northwestern part of the Wind River basin dur- 



