18. 



The Triassic sediments were spread in the shape of a wing of a butterflv 

 over the Rocky Mountain states (Plate 9). The site of greatest subsidence 

 was along the east margin of the former Paleozoic geosyncline, or along 

 the Wasatch line, where marine waters entered and considerable limestone 

 was deposited, such as the ammonite bearing Thaynes formation of north- 

 ern Utah and southeastern Idaho. The geanticline which started to emerge 

 in Permian time became more pronounced in Triassic time, but still a 

 wide shallow connection existed with the Pacific. It seems also that a 

 southwestern passage to the Pacific existed. East of the marine deposits 

 the sediments are mostly of flood-plain origin and are deep red. They 

 are now known to extend northeasterly over part of the Williston basin. 

 They overlapped the edges of the Ancestral Rockies but did not bury 

 them completely. 



ROCKY MOUNTAINS 

 IN MESOZOIC TIME 



TRIASSIC GEOGRAPHY 



The seaways that had existed in the Paleozoic miogeosyncline were 



i considerably changed during Mesozoic time, and a wide belt of land 



'gradually rose in the middle of the old geosyncline to separate two troughs 



of sedimentation. The western trough as recounted in Chapter 17 was 



filled with more than 30,000 feet of interbedded sediments and volcanics 



and was subjected to repeated orogeny. The eastern trough was filled with 



marine and nonmarine beds with only a trace of volcanic material. The 



| eastern was fairly stable with disturbance reaching orogenic proportions 



only in the late Mesozoic along the border of the geanticline. 



EARLY JURASSIC GEOGRAPHY 



The folio of the U.S. Geological Survey, Paleotectonic Maps of the 

 Jurassic System is taken here as a guide and should be referred to for 

 details of the distribution, thickness, and lithology of the several time divi- 

 sions of the system in the United States (McKee et al., 1956). Four major 

 units are recognized which from oldest to youngest are labeled A, B, C, 

 and D. The two oldest which include strata of Lias, Bajocian, Bathonian, 

 and Callovian ages are represented in Plate 12. They include the well- 

 known continental sandstone formations, Nugget, Navajo, and Kayenta, 

 and the marine limestone and shale formations, the Twin Creek, Gypsum 

 Springs, Lower Sundance, Sawtooth, Carmel, etc. 



The Cordilleran geanticline became continuous by Early Jurassic time 

 and joined with a large emergent area of the southwestern states and 

 Mexico. 



In Middle Jurassic time an irregular island in western Montana was 

 uplifted. It is known as the Sweetgrass arch and stretched from Great 

 Falls northward to the Canadian border. About and over it unconformities 

 occur which involve the Jurassic Sawtooth, Rierdon, Swift, and Morrison 

 formations and Early Cretaceous Kootenai formation. See Fig. 18.1. The 

 lowest of the formations, the Sawtooth, is sandstone, siltstone, sandy lime- 



291 



