MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA 



503 



a* 







Fig. 31.11. Late Tertiary faulting near Alpine, Idaho, and Wyoming, and its relation to the 

 laramide structure. After Bayless, 19*7. Cgv, Gross Ventre formation; €b, Boysen limestone; Ob, 

 Bighorn dolomite; Dd, Darby formation; Cmb, Madison and Brazer limestone; Cw, Wells forma- 



tion; Pp, Phosphoria formation; Trd, Dinwoody formation; Trw, Woodside formation; Trt, Thaynes 

 formation; Ted, Camp Davis conglomerate (upper Miocene or lower Pliocene). 



carps that had been buried by the basin deposits were partly, but con- 

 jpicuously, exhumed below the Black Rock surface. The one along the 

 ivest side of Grand Valley has all the physiographic features of a youthful 

 jault scarp, yet is a fault-line scarp. 

 j Jackson Hole, between the Teton and Gros Ventre ranges, is the result 



i downdropping along the Teton fault (Horberg, 1938; Love, 1956a). 



"he Union Pass surface is believed to have been broken and rotated so 



hat it passes below the valley fill on the Gros Ventre side, and has been 

 ilevated and tilted westward on the Teton side. The basin deposits, largely 

 conglomerates, tuffs, and lavas, may have been folded somewhat after 



Reposition, but this aspect of the history is not clear. 



The discordant relations of the Grand Valley, Hoback, and Teton faults 

 to the Laramide structures in map view are shown in Fig. 31.13. 



Southwestern Montana and Central Idaho 



Fresh fault scarplets occur along the west base of the Madison and 

 the Tendoy ranges of southwestern Montana, and major fault scarps occur 

 along the east faces of the Blacktail and Ruby ranges, and the northwest 

 face of the Bitterroot Rane;e. 



The northeast face of the Lemhi Range in Idaho is thought to be. in 

 part at least, a fault scarp. There may be others, but these are the only 

 ones that the writer has seen. Although not yet studied in detail, these 



