378 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



eastern part are the Cedar Ridge fault and the McComb anticline. The 

 fault trends northwestward, and the northeast side is down about 1000 

 feet. It cuts the youngest rocks in the area, and is therefore post-Oligo- 

 cene. 



The McComb anticline is a complex structure and is associated with 

 southward thrusting of Copper Mountain. Thrusting is also indicated in 

 connection with the Cedar Ridge fault. According to Tourtelot and Nace: 



At the west end of Cedar Ridge, and on the south side of the Cedar Ridge 

 fault, over 1,000 feet of Upper Cretaceous beds, nearly vertical or slighdy over- 

 turned away from the Big Horn Mountains, are overlain by a thick sequence of 

 boulder beds in the Lysite member of the Wind River formation. The over- 

 turning of the Upper Cretaceous beds may be explained by the passage of a 

 thrust sheet of older rocks from the north over them, or by the presence of a 

 thrust sheet just to the north that did not override the Upper Cretaceous rocks 

 but strongly deformed the beds beyond the point of its farthest advance. In 

 addition, as Love points out, there is not enough room for a normal section 

 between the southward-dipping Paleozoic formations and the overturned Upper 

 Cretaceous beds standing about a mile to the south. It is believed that the 

 boulder beds in Cedar Ridge were derived from a thrust sheet which moved 

 southward from the Big Horn Mountains. Knight has postulated a similar origin 

 for boulder beds of this type in the Crooks Mountain area, where the sole of the 

 thrust mass, from which the boulder beds were derived, is exposed. If the 

 boulder beds in the Wind River formation on Cedar Ridge were deposited 

 as erosion products of a thrust sheet, the thrusting must have occurred in 

 Wasatchian (early Eocene) time. These Wasatchian and also younger rocks 

 were cut by the Cedar Ridge fault during or after Oligocene time. 



The sequence of major diastrophic events that affected rocks in the north- 

 eastern part of the Wind River Basin is summarized as follows: 



1. Mountain building during or at the end of late Cretaceous time. 



2. Thrust faulting from the north in Wasatchian time along the southern 

 margin of the Big Horn Mountains and the south side of the Owl Creek Moun- 

 tains. 



3. Localized gentle folding after the close of Bridgerian time along the south- 

 ern margin of the Big Horn Mountains. 



4. Normal faulting during or after Oligocene time along the south end of the 

 Big Horn Mountains and the south side of the Owl Creek Mountains. 



The Cenozoic history of the north flank of the Sweetwater Range and 

 the south flank of the Wind River basin is portrayed in a series of block 

 diagrams by S. H. Knight, reproduced in Fig. 24.13. 



HANNA BASIN 



The Hanna basin is bounded on the west by the Rawlins uplift, the 

 north by the Sweetwater uplift, the south by the Medicine Row Range, 

 but on the east it merges with the northwest end of the Laramie basin. 

 Retween the Laramie basin and the Hanna basin is the Carbon basin, 

 through which the two were once continuous but are now separated by 

 Laramide anticlines. The Saddleback Hills anticline separates the Carbon 

 basin from the Hanna, and the Medicine Row and associated anticlines 

 separate the Carbon from the Laramie. These anticlines are rather sharp 

 and extend northerly from the broad north end of the Medicine Bow 

 Range. The Hanna basin is fairly circular and, although not so large as 

 the other basins of Wyoming, it carries a very thick succession of beds. 

 Paleozoic, Mesozoic, and lower Tertiary formations are over 35,000 feet 

 thick, with Upper Cretaceous, Paleocene, and Eocene accounting for 

 most of the accumulation. The succession is very important because it re- 

 cords better than elsewhere the several episodes of deformation in this 

 part of Wyoming. The formations listed by Dobbin, Bowen, and Hoots 

 ( 1929 ) are as follows : 



North Park fm. (Miocene?) 



Unconformity 

 Hanna fm. (early Eocene) 



Unconformity 

 Ferris fm. (lower part is uppermost Cretaceous) 

 Medicine Bow fm. (uppermost Cretaceous) 

 Lewis sh. ^ 



Mesaverde fm. 

 Steele sh. 



Niobrara fm. I Upper 

 Carlile sh. Cretaceous 



Frontier fm. 

 Mowry sh. 

 Thermopolis sh. 



Cloverly fm. (Lower Cretaceous) 

 Morrison fm. (Upper? Jurassic) 

 Chugwater (Triassic) 

 Embar(?) fm. (Permian) 

 Tensleep (Pennsylvanian) 

 Probably pre-Pennsylvanian beds 



0-400 feet 



7000 



6500 



4000-6200 



3300 



2200-2700 



4000-5000 



700 



400 



725 



120 



180 



128 



350 



1300 



150 



250 



? 



