60 G. R. Mansfield — Triassic and 



Fort Hall Indian Reservation. — In 1913 the writer 

 and G. H. Girty made a joint stndy of the Triassic forma- 

 tions of the Fort Hall Indian Reservation. The Wood- 

 side shale was found to be somewhat thinner than in the 

 Montpelier district, 900 feet, but the Thaynes much 

 thicker, about 3,650 feet. The Thaynes proved to be a 

 group divisible into three formations, each with a dis- 

 tinctive fauna. These formations have been described 

 and named 13 but only the uppermost, the Portneuf lime- 

 stone, need be mentioned here. Following the previous 

 usage in southeastern Idaho, the beds between the 

 massive Portneuf limestone and the conglomerate sup- 

 posed to mark the base of the Nugget were assigned to 

 the Ankareh, but these comprised 800 feet of sugary, 

 yellow sandstones, without significant shales. From 

 their stratigraphic position, however, they were called 

 "Ankareh sandstone.'' The basal conglomerate of the 

 Nugget as then interpreted was found to be about 500 feet 

 thick, succeeded above by well defined limestone and shale 

 members, below the main sandstone. The aggregate 

 thickness was apparently as much as 2,400 feet. These 

 four members were also differentiated and named. 



Later ivork, southeastern Idaho. — 1914-16 the inter- 

 pretations used in the Fort Hall Indian Reservation 

 were carried eastward into the Cranes Flat, Lanes Creek, 

 and Freedom quadrangles and southward into the Mont- 

 pelier quadrangle. In the Lanes Creek and Freedom 

 quadrangles the Portneuf limestone of the Thaynes was 

 found divisible into two massive limestone members with 

 a well defined red bed member between. The "Ankareh 

 sandstone" of the Fort Hall Indian Reservation and the 

 four members of the Nugget above mentioned were all 

 present and readily recognizable. The limestone mem- 

 ber above the basal conglomerate of the Nugget, however, 

 was locally reddish with shaly tendencies in its lower 

 part. 



In the Montpelier quadrangle east and northeast of 

 Montpelier the subdivisions of the region to the north 

 are still recognizable. The uppermost member of the 

 Portneuf limestone, however, has dwindled in thickness 

 from about 400 feet in the Freedom quadrangle to about 

 50 feet in the home Canyon section near Montpelier, but 



13 Mansfield, G. E. : Subdivisions of the Thaynes limestone and Nugget 

 sandstone, Mesozoic, in the Fort Hall Indian Eeservation, Idaho, Jour. 

 Washington Acad. Scl, vol. 6, No. 2, pp. 31-42, 1916. 



