14 



SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, lf>19. 



CRKTACEOUS SYSTEM. 



KOOTENAI (1) FORMATION. 



That part of the Kootenai ( ?) penetrated by 

 the drill in the Vananda well consists in de- 

 scending order of 40 feet of sandstone, 19 feet 

 of limestone, and 98 feet of red and white shale 

 in alternate layers. There is no information 

 available as to whether these beds are of fresh 

 water or marme origin; in lithologic aspect, 

 however, the beds, especially the red and white 

 shales, are similar to the fresh-water Kootenai 

 (Lower Cretaceous) of the Great Falls field. 

 In that field the Kootenai as described by 

 Fisher ' is 400 to 500 feet thick and consists 

 predominantly of clay, with thin intercalated 

 beds of sandstone and limestone, but contains 

 valuable beds of coal in the lower part and a 

 basal sandstone from which building stone is 

 obtained. 



COLORADO SHALE. 



The Colorado shale is the oldest formation 

 exposed in this area. It consists of dark-gray 

 to black fissile shale, with some thin bands of 

 sandstone and sandy shale in its upper part, 

 and according to the record of the Vananda 

 well has more or less sandstone and a thin bed 

 of limestone at the base. The shale contains 

 large calcareous concretions in which fossils 

 occur; some of the sandstone bands are also 

 distinctly fossiliferous. The formation has a 

 total thickness, as interpreted from the log of 

 the Vananda well, of about 2,300 feet. It 

 probably includes the time equivalents of both 

 the Benton and Niobrara formations. In this 

 part of Montana there appears to be no recog- 

 nizable lithologic distinction between these two 

 formations, and they are therefore grouped 

 together under the name Colorado shale. It 

 may be, however, that detailed paleontologic 

 work would show the existence of a Niobrara 

 fauna in the upper part of the Colorado shale, 

 even though no lithologic distinction exists. 

 Weed ^ and Stanton and Hatcher ^ have 

 already pointed out a general resemblance be- 

 tween the famia of the upper part of the Colo- 

 rado in the vicinity of Fort Benton and the 

 fauna of the Niobrara. It might be possible, 



1 Fisher, C. A., Geology of the Great Falls coal field, Mont.: U. S. 

 Geol. Survey Bull. 356, pp. 22, .■50-35, 1909. 



2 Weed, W. H., U. S. Geol. Survey Gcol. Atlas, Fort Benton folio 

 (No. 55), p. 2, 1899. 



' Stanton, T. W., and Hatcher, J. B., Geology and paleontology of the 

 Judith River beds: V. S. Geol, Survey Bull. 257, pp. 63, 64, 1905. 



therefore, on purely paleontologic grounds to 

 separate the Colorado of this area into the 

 Benton and Niobrara, as in Nebraska, Colorado, 

 South Dakota, and Wyoming. 



In the easternmost areas just mentioned the 

 Niobrara formation consists chiefly of lime- 

 stone; farther west it is a calcareous buff shale; 

 and apparently stiU farther west it loses its 

 calcareous aspect, becoming a typical clay 

 shale that is somewhat arenaceous. This is the 

 sort of change to be expected in a formation of 

 this kind when traced toward the source of its 

 constituent materials. 



The Colorado differs from the overlying 

 Claggett in being darker and more fissile and in 

 becoming only slightly plastic when wet; be- 

 cause of this last characteristic the local term 

 "gumbo" is not generally applied to it. 



The Colorado occupies an area of about 350 

 square miles in the central part of the Porcu- 

 pine dome, which lies principally west and 

 south of the eastern part of this area. It also 

 occupies another small dome extending west- 

 ward from a point a few miles east of Mussel- 

 shell River. Except in the small area west of 

 the Musselshell, where the Eagle can be recog- 

 nized, the Colorado and Claggett have not been 

 differentiated on the accompanying map. 



EAGLE SANDSTONE. 



The Eagle, which forms a prominent and 

 persistent lithologic unit farther west, is repre- 

 sented in this area by a buff medium-grained 

 sandstone consisting of quartz, feldspar, and 

 black chert with a small amount of mica in- 

 closed in a calcareous matrix. It ranges from 

 a knife-edge to about 20 feet in thickness. The 

 sandstone is recognizable as a distinct unit at 

 the western margm of the area, from which 

 its outcrop has been traced northwestward and 

 southwestward for many miles. In these more 

 western areas the formation ranges from 200 to 

 300 feet in thickness, but it thins eastward, so 

 that near the east side of T. 15 N., R. 29 E., 

 where the sandstone is exposed in a clean-cut 

 face, it is only 21 feet thick, with marine shale 

 both above and below. From this place the 

 formation can be traced eastward for about 2 

 miles, and in that distance the sandstone is seen 

 to become more and more interlaminated with 

 thin bands of shale in its lower part. The 

 sandstone passes under an old river terrace 



