WYOMING ROCKIES 



381 



Cretaceous Sea and that some of the last of the marine deposits, such as the 

 basal Medicine Bow and Lewis, and even possibly the Mesaverde, may be 

 locally derived. 



Diagram No. 2, represents conditions as they may have existed during the 

 deposition of the late Lower Eocene (Wind River?). The Paleocene-Eocene (?) 

 succession has been steeply upturned adjacent to the highlands and the large 

 thrust fault has brought the Precambrian in contact with these rocks and they 

 have been truncated by erosion in the vicinity of the highlands. The coarse 

 conglomerates, derived chiefly from the Precambrian, lie with marked angular 

 discordance upon older rocks along the margin of the Basin. These late 

 Lower Eocene (Wind River?) rocks become finer textured and the pronounced 

 angular discordance between them and the underlying rocks disappears as 

 they are traced basinward. 



Following the deposition of the Eocene (Wind River?) rocks the region 

 was subjected to moderate folding. 



It is apparent that if any Oligocene rocks were laid down in the Basin they 

 were largely or entirely removed before the deposition of mid-Miocene rocks. 

 To the south mid-Miocene rocks rest unconformably upon Cretaceous and 

 older rocks. Following the deposition of the mid-Miocene (Browns Park) the 

 region was subjected to considerable disturbance. A notable feature of this 

 disturbance was wide-spread normal faulting. Available evidence indicates that 

 this disturbance took place in the late Miocene time. It is believed that the 

 region suffered rather extensive uplift during this disturbance. It is probable 

 that the numerous normal faults common to the Basin were formed at this time. 

 Regional evidence indicates that the area was blanketed with sediments during 

 early Pliocene time. The question of the time of the superposition of the North 

 Platte River across the Basin and elsewhere has interested the writer for many 

 years. Until evidence to the contrary is forthcoming, it is concluded that the 

 present course of the North Platte River was established upon the Lower 

 Pliocene surface following regional uplift with some tilting. This uplift began 

 the present cycle of erosion. 



Sweetwater Arch 

 * 



Hanna Basin 





Fiqureno.1 Early Paleocene 



Fiqure ho. Z Eocene (Wind River f) 



Medicine Bow River 



Fiquneno.3 Recent Front face of diagram 

 alonq north-south line one mile east of TroublesomecreeK. 



LATE TERTIARY DOWNFAULTING OF SWEETWATER RANGE 



Along the north and south margins of the exposed Precambrian core 

 of the Laramide Sweetwater Range normal faults of Late Tertiary age 

 have been recognized which have resulted in the downward displace- 

 ment of the core area some 2500 to 3000 feet ( Blackstone, 1951 ) . See 

 Fig. 24.12. Near the volcanic necks of the Rattlesnake anticline the normal 

 faults can be dated as post-middle Miocene. 



Since the Late Tertiary downfaulting follows approximately the same 

 pattern as the late Eocene, Oligocene, and early Miocene sagging or 



Trace of thrust 



Preb' 



'■• ,'" Paleocene- Early Eocene ?^« '^^- : - - 



<Gret) 



Fiqure no.4 Recent Front face of diaqram 

 alonq north-soutn line midway between Troublesome CrocK 

 and Austin CreeK. M.-R for Miocene -Pliocene. 



Fig. 24.15. Development of the north flank of the Hanna Basin. Reproduced from Knight, 1951. 



