PALEOZOIC CORDILLERAN GEOSYNCLINE 



71 



Mississippian Basins 



Major miogeosynclinal deposits extend from the Big Snowy basin of 



: Montana in a fairly narrow trough southward through eastern Idaho 



into Utah and then southwesterly into southern Nevada. The greatest 



thickness is reached in the Lemhi and Lost River ranges of Idaho (Figs. 



'6.6 and 6.11). 



Characteristic formations of the trough are shown in Fig. 6.16. In 

 summary of the strata of the eastern trough it may be said that they 

 consist mostly of limestones, but that the limestones grade into a thick 

 shale (now argillite) section in Idaho, which may savor of the eugeosyn- 

 cline. Also the Manning Canyon shale of western Utah is thick (1100 

 feet) and marks the transition from the Mississippian to the Pennsyl- 

 jjvanian. For references see Scholten (1957), Morris (1957), and Gilluly 

 •i(1932). 



The change from shelf to miogeosyncline is shown in Figs. 6.11 and 

 6.17. The Raft River geanticline just southwest of the Montana-Idaho 

 ^boundary is well illustrated in Fig. 6.11. 



Antler Orogenic and Post-Orogenic Stratigraphy 



Coarse elastics in places 10,000 feet thick were spread eastward and 

 westward from the Antler orogenic belt, and overlap the pre-existing 

 : 3ugeosynclinal, transitional, and miogeosynclinal assemblages. According 

 to Roberts et al, (1958): 



The lithologic character of the overlap assemblage is variable from place to 

 ilace, and different names have been applied to correlative beds. In the east, 

 he Eureka-Carlin sequence includes the Chainman shale, Diamond Peak 

 formation, Ely limestone, Carbon Ridge, and Garden Valley formations of the 

 iureka area, and correlative formations in the Carlin area. In the west, the 

 Antler sequence includes the Battle formation, Highway limestone, Ander Peak 

 iimestone, and Edna Mountain formation. Because of local variations in source 

 ireas, in conditions of deposition, and subsequent history of these rocks, it is 

 mpossible to make precise correlations of the units in the different sequences. 

 Regional lithologic similarities indicate, however, that similar environmental 

 jjonditions prevailed over broad areas. The Havallah formation of the Sonoma 

 md East ranges was probably laid down 50-100 miles west of the orogenic 

 |)elt and was thrust eastward into juxtaposition with the Antler sequence during 

 vlesozoic orogeny. It therefore has had a somewhat different history and is 



MISSISSIPPIAN 



Fig. 6.6. Thickness and paleographic map of the Mississippian. A-S.G. AR. is Apishapo- 

 Sierra Grande arch. Uncompahgre and Colorado uplifts first became emergent in latest Missis- 

 sippian, and developed into major ranges in Early Pennsvlvanian. 



