336 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



anticlinal deposts of Mesozoic age in their central and eastern parts. 

 Refer again to the paleotectonic maps of Chapter 3. 



The Ephraim conglomerate marks the first vigorous uplift of the geanti- 

 cline to the west, and the age of the conglomerate, according to Mansfield 

 (1927), is Early Cretaceous, but according to W. L. Stokes (personal 

 communication) may be latest Jurassic. Somewhat later, but still in early 

 Cretaceous time, the Bechler conglomerate was washed eastward from 

 the westward-lying geanticline. 



Colorado Phase 



The orogenic deposits of the Colorado phase ( Fig. 22.3 north and north- 

 east of Jackson are the Frontier formation, Cody shale, Bacon Ridge 

 sandstone, and the Coaly Sequence (Love, 1956a,b). They make up a 

 series of clastic deposits about 5000 feet thick. East of Evanston the 

 Frontier formation and Hilliard shale are about 9000 feet thick. These 

 deposits undoubtedly attest the rise of adjacent land on the west, but for 

 most of the length of the deformed belt it is impossible to identify any 

 structures there that were formed at this time. The Taylor and Ogden 

 thrusts predate the Willard thrusting, which is probably Montana in age, 

 so they may be structures formed as the west-lying land was elevated. 



Montana Phase (Early Laramide) 



The deformed belt of western Wyoming and southeastern Idaho is 

 noted for a number of thrust faults, the main ones of which are shown on 

 Fig. 22.4. They have all moved eastward, or at the north end of the belt 

 northeastward, and in places a number of sheets are stacked on each 

 other in imbricate fashion. These probably formed during late Montana or 

 early Paleocene time. 



The Bannock thrust was first detailed by Mansfield (1927) as shown in 

 Fig. 22.10. A sheet of wide proportions was postulated to have moved 

 eastward over 40 miles and to have been folded and eroded such that a 

 large window occurs in it. Later work by geologists of Standard Oil 

 Company of California and the U.S. Geological Survey indicates that 

 several imbricate thrust sheets are involved and that the interpretation 

 of one single sheet is not correct. 



The Absaroka thrust has been traced the entire length of the belt and 

 is an integral part of the frontal structure of the central and southern 

 parts. To the north it runs back of the Darby thrust, presumably of the 

 same age. Also on the north end a complex of thrust sheets, one par- 

 ticularly of considerable extent, the St. John, overrides the Absaroka in 

 the Snake River Range. It may belong to the Paleocene or Eocene phase 

 of deformation. 



The youngest strata involved in the Bannock thrusting are Lower 

 Cretaceous Gannett. The Frontier formation of Colorado age is deformed 

 within the Absaroka and Darby sheets. The Absaroka overrides the Ada- 

 ville beds near Kemmerer. 



The foredeep beds deposited during the Montana epoch attained a 

 thickness of over 7000 feet in the Jackson area, and their deposition 

 climaxed in the Harebell conglomerate of Beltian boulders, cobbles and 

 pebbles ( Love, 1956a ) . At the extreme southern end of the belt the Echo 

 Canyon conglomerate and related deposits accumulated at about the same 

 time (Williams and Madsen, 1959). The manner and route of long- 

 distance transit of the Beltian cobbles of the Harebell conglomerate 

 from closest Beltian outcrops 200 miles to the northwest are a mys- 

 tery. 



Since the Adaville is overridden by the Absaroka thrust, the de- 

 formation, at least here along the front of the belt of deformation, 

 carried on into late Montana time and possibly into early Paleocene. 

 The Paleocene Hoback formation was deposited in a foredeep (see 

 map, Fig. 22.5), and it is possible that the foredeep occurred in 

 response to the thrusting, and that the thrusting is therefore related to 

 the Hoback formation rather than to the late Montana sediments. The 

 thrusting in the southern end of the belt is pre-Knight, and Veatch ( 1907) 

 had presumed it to be post-Almy, but a recent revision of the stratig- 

 raphy and mapping in the Fossil basin (Tracy and Oriel, 1959) shows 

 the thrusting there to be pre-Evanston. The lower part of the Evanston 

 is latest Cretaceous, and hence the thrusting is Late Cretaceous. De- 

 formation in and around Fossil basin continued through the Paleocene, 

 however, as indicated by the conglomerates and unconformities in the 

 Evanston and Almy. 



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