ROCKY MOUNTAINS IN MESOZOIC TIME 



293 



EARLY AND MID-CRETACEOUS OROGENY 



The Fernie strata in Alberta appear to grade into the overlying Koote- 

 nay formation of Lower Cretaceous age. The Kootenay consists of 

 alternating sandstone and dark shale with many coal beds, perhaps all of 

 nonmarine origin, and decreases in thickness from west to east. Its greatest 

 thickness is 5000 feet. The presence of thick sandstone and conglomerate 

 beds in the Kootenay is indicative of further uplift of the land to the west, 

 and the presence of granite pebbles in the conglomerates indicates that 

 erosion and differential movement by Kootenay time had so far proceeded 

 as to lead to the uncovering of deep-seated plutonic masses. 



Near the south end of the Wasatch Mountains in the Cedar Hills, Gun- 

 nison plateau, and Sanpete Valley are immense, coarse deposits probably 

 of early Late Cretaceous age. They make up the basal part of the Indianola 

 group (Spieker, 1946). See Fig. 22.16 and Chapter 22. Since the con- 

 glomerates, sandstones, and shales, together with some higher fossiliferous 

 marine beds, are a lithologic unit, Spieker believes that all the deposit is a 

 consecutive response to an uplift, and therefore that the orogeny occurred 

 at the beginning of late Cretaceous time. Because of the thick deposit of 

 Mid-Cretaceous age in the Cedar Hills, and the information obtained 

 there about the disturbance, the orogeny will be called after them, namely, 

 the Cedar Hills orogeny. 



The elastics of the Indianola group are coarsest toward the west in the 

 Cedar Hills (Schoff, 1937) and the Gunnison plateau. They grade east- 

 ward into the Mancos shale at the east front of the Wasatch plateau. The 

 greatest thickness known is 15,000 feet in the Cedar Hills. The belt of 

 intense deformation lay west of the Cedar Hills, because in the Cedar 

 Hills the conglomerates rest conformably upon the underlying Upper 

 Jurassic shales (Spieker, 1946). 



The belt of the Cedar Hills orogeny must have extended from southern 

 Nevada northward through Utah to eastern Idaho. In southern Nevada, 

 the Overton fanglomerate in the Muddy Mountains is of early late Cre- 

 taceous age (Hewett, 1931), and rests in angular unconformity on 

 folded and thrust-faulted Mesozoic rocks, the youngest of which are Juras- 

 sic in age (Longwell, 1928, 1936). The Overton fanglomerate and the 



angular unconformity are believed to mark the Cedar Hills orogeny in 

 southern Nevada, and the inference has been made that the belt of 

 orogeny extended continuously between southern Nevada and central 

 Utah. 



North of the Cedar Hills in north-central Utah, a coarse conglomerate, 

 the Kelvin, is probably the equivalent of the lower Indianola conglom- 

 erates. It is about 200 feet thick and grades eastward into finer sediments. 

 The uplift lay immediately west of the present Wasatch Mountains, and 

 Permian cherts and Pennsylvanian quartzites in the uplift furnished most 

 of the pebbles of the conglomerate. The site of conglomerate accumula- 

 tion became a trough of subsidence, and in the Colorado epoch of late 

 Cretaceous time, over 5000 feet of strata collected in it. Volcanoes nearby 

 emitted dust which collected as tuff in the lower part of the sequence 

 (the Aspen formation); then sandstones with numerous oysters and 

 fresh-water shales and sandstones with coal seams accumulated alternately. 

 Several conglomerates in the Colorado series mark continued unrest to 

 the west. See the paleotectonic map, Plate 12. 



Mansfield (1927) believes that the Lower Cretaceous Gannett group 

 in southeastern Idaho, with its several coarse sandstones and conglom- 

 erates, signifies a sharp uplift in the land to the west as a reflection of the 

 intense Nevadan orogeny still farther west. Probably this uplift in the 

 Utah trough area was a forerunner to the main orogeny which resulted 

 in the deposition of about 3000 feet of coarse debris of early Late Cre- 

 taceous age, the Wayan formation (Read and Brown, 1937) unconform- 

 ably on the Gannett. 



About 200 miles northwest of southeastern Idaho in southwestern Mon- 

 tana, a Cretaceous sequence is present, but has not yet been well worked 

 out. In places below beds of Aspen ( Colorado epoch ) aspect, and above 

 the Lower Cretaceous Kootenai elastics is a pebble and cobble con- 

 glomerate. Although these beds may be part of the Kootenai, fossil evi- 

 dence is lacking and they may be early Late Cretaceous. If so, the belt 

 of Mid-Cretaceous orogeny may have extended northward to western 

 Montana. 



Since no Jurassic and Cretaceous beds were deposited in the Mesozoic 

 geanticlinal area along the east side of which the Cedar Hills orogeny oc- 



