G. R. Mansfield — Triassic and Jurassic Formations. 53 



Aet. VI. — Triassic and Jurassic Formations in south- 

 eastern Idaho and neighboring regions; by Geoege 



EOGEES MAXSFIELD. 1 



Introduction. 



In the course of the Geological Survey's examination 

 of the Idaho phosphate field two areas have been studied 

 in considerable detail. The first is the Fort Hall Indian 

 Reservation; the second is a group of seven quad- 

 rangles ranged along or near the state line northward 

 from the southeastern corner of Idaho. The southern- 

 most member of the group is the Montpelier, a 30-minute 

 quadrangle. The other six are 15-minute quadrangles, 

 namely, Slug Creek, Crow Creek, Lanes Creek, Free- 

 dom, Henry, and Cranes Flat. The location of these 

 areas and of other districts mentioned below is shown 

 in fig. 1. 



Triassic and Jurassic rocks are exposed in most of this 

 country. During the progress of the work, and with 

 increasing knowledge, changes in the classification and 

 nomenclature of these rocks became necessary. It is 

 the purpose of the present paper to discuss these changes, 

 leaving in large measure the details of the formations 

 themselves to a forthcoming extended report 2 on the 

 same quadrangles above mentioned. 



Triassic System. 



Occurrence and subdivisions. — The Triassic system is 

 well developed in southeastern Idaho and is exposed in 

 much territory, particularly in the Montpelier quadrangle 

 and the four quadrangles to the north. In the Henry 

 and Cranes Flat quadrangles it occupies smaller areas. 



In the seven quadrangles three formations or groups, 

 aggregating 5,350 feet in thickness, are distinguished and 

 assigned to the Lower Triassic series, namely the Wood- 

 side shale, Thaynes group, and Timothy sandstone. In 

 addition, three other formations of uncertain age and 

 with a combined thickness of 520 feet are considered 

 provisionally Triassic. These are respectively the 

 Higham grit, Deadman limestone, and Wood shale. In 

 the Fort Hall Indian Reservation the Thaynes group is 



1 Published by permission of the Director of the U. S. Geological Survey. 



2 Mansfield, G. E. : The geography, geology, and mineral resources of part 

 of southeastern Idaho, U. S. Geol. Survey Prof. Paper (in preparation). 



