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STRUCTURAL GEOLOGY OF NORTH AMERICA 



Triassic Spearfish shales that principally underlie it, and also as the Race- 

 track (Darton and Paige, 1925; O'Harra, 1933). 



The Precambrian rocks consist of highly folded schists intricately in- 

 vaded in the southern hills by large and small masses of granite. Laramide 

 plutons intrude the Precambrian in the northern part, where the great 

 Homestake gold mine is located, and, as some believe, are responsible for 

 the ore deposits in large part. 



General Characteristics of Powder River Basin 



West of the Rlack Hills and between them and the Rig Horn Range is 

 the broad Powder River basin, floored by the Cretaceous, Paleocene, and 

 Eocene beds. The Early Tertiary deposits are 10,000 feet thick in the 

 deepest part of the basin, as indicated by seismic prospecting, and over 

 most of the basin no reversals of dip, i.e., gentle anticlines or synclines, 

 have been found. Only along the east flank of the Big Horns do any folds 

 occur. Consult U.S. Geological Survey Preliminary Map 33. The very 

 productive Salt Creek anticline is at the southern end of this belt. Darton 

 estimates the strata were uplifted 9000 feet in the Black Hills, so the 

 structural relief between the bottom of the Powder River basin and the 

 top of the Black Hills is in the neighborhood of 20,000 feet. 



Age of Uplift 



The age of the uplift can be only approximated, because no Paleocene 

 or Eocene overlaps exist. Those deposits of Laramide age that might have 

 been in part derived from the Black Hills are now in surrounding areas 

 fairly distant from the uplift and separated from it by a wide Cretaceous 

 belt of outcrop. The doming could have started in latest Cretaceous 

 time, with the deposition of the Fox Hills and Lance beds in the Powder 

 River basin and around the north and northeast ends; and the distribu- 

 tion of the Fort Union and Wasatch beds, partly around the uplift and 

 especially to the northeast, seem to indicate that the uplift had occurred 

 and was furnishing some of the sediments that were accumulating. 



Post-Laramide History 



By early Oligocene time, erosion had trenched the uplift almost as 

 deeply as now, and a mountain and valley surface of at least 1500 feet 



relief existed. Then the regimen of erosion changed to one of aggradation 

 coincident with the change through central Wyoming and the Great 

 Plains, and even in the early, deep valleys of the Black Hills, lower Oligo- 

 cene beds began to accumulate ( Darton and Paige, 1925 ) . Deposition in 

 these mountainous valleys lagged until middle Oligocene, whereas it was 

 taking place in early Oligocene on the Great Plains ( Fillman, 1929 ) . The 

 sediments may have reached such a thickness that all but the highest 

 features of the range were buried, judging from the elevation of the 

 White River beds to the east of the uplift, but if so they have since been 

 removed within the hills except in small, protected patches. With the 

 renewal of erosion, a drainage pattern, in details slightly at variance with 

 the old, has failed to clean out all the Oligocene deposits, and has left 

 them in places, forming low ridges and also extending down nearly to 

 present valley bottoms. The surface upon which the middle Oligocene 

 deposits accumulated in the hills has been called the Mountain Meadow 

 (Fillman, 1929). 



The Great Plains on the east of the Black Hills are covered by several 

 formations ranging in age from lower Oligocene to Pliocene, and within 

 this group are several disconformities. Some geologists have related the 

 disconformities to uplifts in the Black Hills, but Mackin ( 1947 ) believes 

 that the dominant form was a graded surface — erosional in the uplift and 

 depositional on the peripheral regions. With regional uplift, in mid-Ter- 

 tiary time, and associated change in climate to aridity, the graded surface 

 was dissected to produce the landforms of today. 



SWEETWATER RANGE 



Extending westward from the north side of the Hanna basin to the 

 southeast end of the Wind River Range is a series of hills, most of which 

 are islands of Precambrian rock in Miocene and Oligocene sediments. Suf- 

 ficient Paleozoic and Mesozoic strata are also exposed to indicate that 

 the Precambrian islands demarcate the position of the core of a former 

 great range, extending in general in an east-west direction through central 

 Wyoming (Fig. 24.12). It probably was traversed obliquely by several 

 sharp folds that trended in a northwest direction and which cast the bor- 



