MANSFIELD: MESOZOIC OF FORT HALL RESERVATION 35 



THAYNES GROUP 



The Thaynes limestone, which it is here proposed to call the 

 Thaynes group, takes its name from Thaynes Canyon, in the 

 Park City Mining District, Utah. In northeastern Utah and in 

 the Slug Creek and Montpelier districts of southeastern Idaho 

 the Thaynes forms platy, calcareous shales and brown weathering 

 limestones with a massive limestone at the top. In that district 

 the Thaynes limestone is about 2,000 feet thick. In the Fort 

 Hall Indian Reservation the formation shows a marked tendency 

 to differentiate into several units that can be mapped separately. 

 These beds have there a thickness of 3,600 feet, yet according to 

 G. H. Girty 4 fossils similar to those of the upper limestone were 

 found by C. L. Breger in shales underlying the Ankareh shale in 

 Montpelier Canyon, Montpelier quadrangle. Thus the thicker 

 group in this district occupies the same stratigraphic interval as 

 the Thaynes limestone farther southeast. It has been found 

 advisable to subdivide the group into three formations, the Ross 

 limestone at the base, the Fort Hall formation, and the Portneuf 

 limestone. 



Ross limestone. The Ross limestone takes its name from Ross 

 Fork Creek, in the upper waters of which this limestone is well 

 exposed. The base of the formation lies conformably upon the 

 Woodside shale and is marked by the "Meekoceras beds" recog- 

 nized by the Hayden Survey and referred to the Triassic and later 

 referred by Hyatt and Smith 5 to the Lower Triassic. 



The Meekoceras zone consists of gray to reddish brown lime- 

 stones about 50 feet thick containing numerous Ammonites the 

 chambered shells of which appear on the weathered surface of the 

 rock. In the Fort Hall Indian Reservation the fossils do not 

 weather out so readily and the horizon is not so conspicuous as in 

 the Slug Creek quadrangle farther east. The Tyrolites and 

 Columbites zones, which have been recognized by Smith in the 

 region of Paris, Idaho, 250 and 275 feet, respectively, above the 



4 Personal communication. 



5 Hyatt, A., and Smith, J. P. The Triassic cephalopod genera of America. 

 U. S. Geol. Survey Prof. Paper 40: 17-19. 1905. 



