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STRUCTURAL GEOLOGY OF NORTH AMERICA 



downwarping, we may assume that the normal faults are the late result 

 of the subsiding process. In the early stages gentle flexing must have oc- 

 curred, but later ruptures broke along the sides of the subsiding block. 



Tertiary faults have been recognized in several places in central Wyo- 

 ming, but as yet the extent of the system has not been very well delimited. 

 The pattern of subsidence is outlined under the next heading. 



LARAMIDE PATTERN AND CENOZOIC STAGES IN 

 THE SWEETWATER RANGE REGION 



It is evident by inspection of the Geological Map of Wyoming, from 

 which Fig. 24.12 is taken, that the structures trend in two directions, 

 northwesterly and westerly. From this it may be concluded that two 

 phases of Laramide orogeny occurred. The relations of Casper Mountain 

 to the Emigrant Gap anticline might be taken to indicate that the north- 

 westerly trending structures are the older, and that the east-west trending 

 structures have been superposed on the northwesterly. However, on 

 stratigraphic grounds Love ( 1954 ) lists the following succession of events : 



1. At the close of Cretaceous the broad Sweetwater Range arch rose. 



2. The arch continued to rise during the Paleocene. 



3. At the close of the earliest Eocene the thrusting along the south flank 

 of the Sweetwater Range occurred. 



4. At the close of Oligocene time the gentle northwesterly trending folds 

 of central Wyoming developed. The settling of the Sweetwater Range 

 was taking place. 



5. In post-middle Pliocene and pre-Pleistocene time: 



large-scale block faults developed in many parts of Wyoming; the floor of 

 Jackson Hole dropped several thousand feet; the southern end of the Wind 

 River Mountains collapsed; the central arch of the Granite Mountains 

 (Sweetwater Ridge) dropped several thousand feet; local areas west of the 

 east margins of the Sierra Madre, Medicine Bow, and Laramie mountains 

 were down-dropped; part of the Rawlins uplift collapsed and a broad west- 

 trending anticline formed south of Rawlins; a large area southeast of the 

 Hartville uplift was downfaulted; the southern end of the Big Horn Moun- 

 tains probably collapsed at this time (Love, 1954). 



RAWLINS UPLIFT 



The Rawlins uplift (see Tectonic Map of the United States) is one of 

 fairly sharp and high structural relief. It stands perhaps 40,000 feet above 

 the crystalline floor of the Hanna basin. It is complicated by folds and 

 thrust faults, and one of the thrusts has brought Precambrian rock in 

 contact with the Mesaverde. This thrust is located just west of the city of 

 Rawlins, and the movement of the overriding sheet is toward the west 

 and southwest. The uplift dips generally northward, toward the Sweet- 

 water uplift, which has been thrust southward against and over it. 



WASHAKIE BASIN 



Bradley (1945) has reported on the Washakie basin, and his summary 

 is as follows: 



The synclinal structure of the Washakie Basin has given rise to a bold, 

 outward-facing, encircling escarpment, developed on beds in the Green River 

 formation that are more resistant to erosion than other beds in the section. 

 Along the northern margin of the basin this rim rises 600 to 700 feet above the 

 country to the north, and is known as Laney Rim. Southward the escarpment 

 increases in height, and locally on each side of the basin its crest stands about 

 1,200 feet above the surrounding terrane. Along the southwestern margin of the 

 basin the rim is known as the Kinney Rim. The southward facing escarpment at 

 the southern margin of the basin is broken by stream valleys at many places, 

 and is generally lower. Near the head of Powder Wash, just south of the 

 Wyoming-Colorado boundary line, where the escarpment is low, there is a nar- 

 row, southward extension of the Basin, which is clearly shown by the outcrop 

 pattern of the Green River formation. This panhandle is expressed topographi- 

 cally by a pair of outward-facing escarpments that rise to an altitude of more 

 than 8,000 feet at Lookout Mountain. 



The rocks in the Washakie Basin are divided into four main units, from bot- 

 tom to top, the Wasatch, Green River, and Bridger formations of Eocene age, 

 and the Browns Park formation of probable Miocene age. In the broadest and 

 simplest terms, the Green River formation is a huge lens of relatively fine 

 grained fluviatile sandy mudstone that formerly filled a huge intermontane 

 basin far larger than the Washakie Basin. The mudstone is divided into two 

 formations: (1) the Wasatch formation, below the lens of Green River forma- 

 tion, and (2) the Bridger formation above. The sedimentary history of the in- 

 termontane basin was complicated, however, by changes in the level of the 

 lake, which resulted in an intertonguing relationship between the Wasatch and 



