CENTRAL ROCKIES 



337 



BEAR RIVER 

 VALLEY Cb 



Eighteen miles north of Montpelier, Idaho Section 3 



Snowdrift Mtn^ & CR0W CREEK VALLtY 



5.0OO' 



Sect /on 4 



NOUNAN VALLEY 

 Til "*< v 



v5eve^7 miles north of Montpelier, Idaho 



Js Jp 



Fig. 22.10. Cross sections of the Central Rockies in southeastern Idaho, after Mansfield, 1927. 

 Cq, Brigham quartzite; Ogc, Garden City Is.; D, Devonian Three Forks or Jefferson Is.; Cm, 

 Madison Is.; Cb, Brazer Is.; Cw, Wells quartzite; Cpa and Cpb, Phosphoria fm.; "Ew, Woodside 



The Rannock and Willard thrusts are presumed to have formed in 

 Montana time, the same as the Absaroka, but they might be older. 



Figure 22.11 shows thrusting during the deposition of the Echo 

 Canyon conglomerate, but this is an inferred structure. 



Paleocene Phase (Mid-Laramide) 



Figure 22.5 illustrates deposits and uplifts along the east front of the 

 i Central Rockies in Paleocene time. The major sediment accumulation was 

 the continental Hoback formation made up of about 15,000 feet of sand- 

 stone, siltstone, and shale. A few thin limestone and conglomerate beds 

 jare also present (Dorr, 1958). 



Sedimentation was very rapid, probably beginning and accelerating in 

 Torrejonian, culminating during late Torrejonian, then decelerating during 

 Tiffanian, Clarkforkian, and Graybullian times prior to a late phase of orogeny. 

 ,Sediment was derived locally from western, mid-Laramide highlands which 

 began to rise in the early Torrejonian; the uplift culminated between the middle 

 and end of the Torrejonian. Orogenic phases were relatively brief but intense. 

 The area of deposition was much lower, forested, temperate, humid, locally 



Miles 



3 



sh.; lit, Thaynes Group; 'Sty, Timothy sandstone; "Eh, Higham grit; 16, Deadman Is.; Jn, 

 Nugget ss.; Jtc, Twin Creek fm.; Jkb, Beckwith fm.; Jp, Preuss ss.; Js, Stump ss.; Kge, Ephram 

 conglomerate of the Gannett group. 



swampy with some lakes, and largely inhabited by a forest-dwelling mam- 

 malian fauna (Door, 1958). 



The Cliff Creek thrust sheet (Jackson thrust of Fig. 22.9) overrides 

 the Hoback formation. It is overlapped by the Eocene Pass Peak forma- 

 tion, so probably is a last phase of the deformation during the Paleocene 

 which resulted in the deposition of the Hoback beds. 



The Uinta uplift appeared first in Paleocene time. The Currant Creek 

 conglomerate, which had previously been related to the Montana Price 

 River of central Utah, is now regarded as Paleocene by Bissell (1959). It 

 rests unconformably on older strata, and postdates the Deer Creek- 

 Strawberry thrusts. 



The linear uplift and basin (Fossil basin) east and north of Evanston 

 are developments during latest Montana-Paleocene time. 



Eocene Phase (Late Laramide) 



At the north end of the deformed belt of western Wyoming the Pass 

 Peak conglomerate was deposited on the Hoback formation and older 



