WYOMING ROCKIES 



3S3 



the overlying Green River as shown in the generalized columnar section 

 [Fig. 24.5]. 



The Washakie Basin is a shallow syncline lying on the east side of the Rock 

 Springs uplift. Along the north and east sides of the basin, the dip of the beds 

 ranges from 3° to 5° toward the center, whereas along the west and southwest 

 sides it ranges from 8° to 12°. In the large central area of the basin the rocks lie 

 nearly flat. This essentially uniform synclinal structure is broken only along the 

 Wyoming-Colorado line, where a west-trending fault zone forms the southern 

 boundary of the basin. This zone, which appears to be an eastward extension of 

 the structural lines in the Hiawatha gas field, is described in an earlier report. 

 The faults of this zone fall into two broad groups, according to their general 

 direction of strike. The faults of the dominant group strike generally westward, 

 whereas, those of the other group strike more nearly northward or northwest- 

 ward. The faults of the first group are, in general, the older as they are cut by 

 faults of the second group. Moreover, the faults of the second group cut beds of 

 the Browns Park formation, whereas the other faults in most places do not. 

 However, some of the larger east-west faults were apparendy active during the 

 I second stage of faulting, because locally, as along Cherokee Ridge, they also cut 

 the Browns Park formation. 



The rocks in the fault zone are folded into several synclines, the axes of 

 which are parallel to the dominant, westward-striking faults. Most of the folds 

 are rather gentle, having dips that range from 3° to 7°, though locally, as in the 

 vicinity of Baggs, the beds dip as much as 16°. 



GREEN RIVER BASIN 

 ': 



The Green River basin (also referred to as the Bridger basin) is 

 bounded on the south by the great Uinta anticline, on the west by the 

 central Rockies of western Wyoming, on the east by the Rock Springs 

 uplift, and on the north and northeast by the Wind River and Gros Ventre 

 ranges. The extreme northern end of the basin is a wedge between the 

 southwestward thrust Gros Ventre Range and the eastward thrust Hoback 

 Range, and is drained by the Hoback River, a tributary of the Snake 

 River, and hence locally known as the Hoback basin. The Green River 

 drains the rest of the Green River basin. Refer to Wyoming Geological 

 Association Guidebook, Tenth Ann. Field Conference, 1955. 



The evolution of the Green River basin has been depicted in Chapter 

 22. See Figs. 22.4 to 22.6 and 24.16. The relation of the Uinta anticline to 

 the basin is shown in Fig. 24.17. From the diagram it may be seen that 

 considerable arching and erosion preceded the deposition of the Green 



Fig. 24.16. Isopach map of 

 Mesaverde group, Wyoming, 

 Utah, and Colorado. Courtesy 

 John Burger. The site of the 

 present Rock Springs uplift was 

 a basin in Mesaverde time. 



River formation, but from other locations along the north flank of the 

 Uintas the major and sharp rise, involving high-angle thrusting, followed 

 the Green River. Regarding the graded surfaces exhibited on the north 

 flank of the Uinta Mountains and extending far out into the Green River 

 Basin Bradley says: 



Long, narrow remnants of four old erosion surfaces slope gently northward 

 from the north flank of the Uinta Range and truncate the upturned edges of 

 hard and soft beds. The Gilbert Peak erosion surface, which is the highest and 

 oldest of these surfaces, once extended from the crest of the range at an altitude 

 of about 13,000 feet to the center of the Green River Basin. Because un- 

 disturbed remnants of this surface have gradients ranging from about 400 feet 

 to the mile near the crest of the range to 55 feet to the mile 35 miles out in the 

 basin, because island mounts of limestone rise rather abruptly from it. and be- 

 cause it apparendy never had a soil mantle but is covered in most places by 

 conglomerate, this surface is interpreted as a pediment formed in a semiarid or 

 arid climate. At the time the Gilbert Peak surface was cut the Green River 

 Basin was filled to a greater depth than now with Eocene sedimentary rocks. 

 The Gilbert Peak erosion surface truncated these rocks at very low angles and 

 extended northward across them as a continuous plain. On this plain the master 



