334 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Cm Mkfo* f 



dl c - 



)n<i 



to 



d9 e 



LIMA PKS. 



IDAHO I MONTANA 



\XMS W 



10, ooo'- 



Fig. 22.8. Cross section of thrusts in southwestern Montana and adjacent Idaho, after Drexler, 

 McUsic, and Kildal, Master's theses, University of Michigan. Cm, Madison formation; Ct, Tensleep 

 sandstone; Cp, Phosphoria formation; Rd, Dinwoody formation, 'Rw, Woodside formation; 'St, 



Miocene-Pliocene Phase 



A fairly extensive episode of erosion followed the deposition of the 

 Cook Ranch beds, and in the Blacktail Range southeast of Dillon, tilting 

 and the overlap of younger beds seem to indicate the inception of block 

 faulting. This would have occurred in latest Oligocene or earliest Miocene 

 time. Then volcanism broke out anew at the north end of Blacktail Range 

 and extensively in the Snake River Valley, Yellowstone Park, and the Co- 

 lumbia Plateau. Deposition of lower Miocene Blacktail Deer Creek beds 

 and associated basalts, tuffs, and agglomerates resulted in the Upper Sage 

 Creek area, along the northwest flank of the Snowcrest Range, and in the 

 Ruby Reservoir basin. 



Then followed erosion to an extensive surface of moderate relief. In 

 places the pre-Sage Creek surface may have been reexhumed and become 

 coextensive with this new post-Blacktail surface, which is present now in 

 summit areas of the Blacktail Range. There, lower Miocene basalts and 

 tuffaceous beds are beveled. 



An episode of block faulting is clearly recorded in the Ruby Reservoir 

 basin following the deposition of the Blacktail beds, and then in the down- 

 faulted basin, the upper Miocene and lower Pliocene Madison Valley beds 

 accumulated. 



Seo /eve/ 



Thaynes formation; Js, Sawtooth formation; Jr, Rierdon formation; Jm, Morrison formation; Kk, 

 Kootenay formation; Kbr, Bear River formation; Ka, Aspen formation; Tc, Paleocene (?) conglomer- 

 ate; Tvr, Rhyolite flows. Section 1, Index map, Fig. 22.1. 



Pliocene and Quaternary Faulting and Erosion 



Regional uplift, in places possibly accompanied by more block faulting, 

 and the erosion of extensive pediments followed. The pediments on the 

 northwest side of Snowcrest Range are the most extensively and perfectly 

 developed. The pediments on basin beds of the back valleys in Beaver- 

 head Range (graben valleys) are of this age. In valleys like Beaverhead 

 River, Blacktail Creek, and Sweetwater, downfaulting was so extensive 

 that alluvial aprons were deposited along the base of the fault scarps. 



A third episode of block faulting resulted in alluviation in places, and in 

 others of gentle uplift and dissection of the pediments. Two episodes of 

 glaciation in the Beaverhead Range are recorded, one probably occurring 

 before dissection of the pediments, and one afterward. 



Block faulting at the front of the Tendoy and Madison ranges has con- 

 tinued in modern times. 



SOUTHEASTERN IDAHO AND WESTERN WYOMING 



Latest Jurassic and Early Cretaceous Phase 



Like the southwestern Montana Rockies, those of southeastern Idaho 

 and western Wyoming contain Paleozoic formations of geosynclinal thick- 

 nesses on the west, of shelf thicknesses on the east, and of marginal ge- 



