DESCRIPTION OF THE ROCKS 691 



(Unconformity ?) Feet 



Thin-bedded limestone, with some intercalated shale, probably 



- including Nizina limestone. Contains OrMculoidea ?, Halo- 

 Ma cf. superba, Myophoria f, Pecten, Tropites, Juvavites ?, 

 Geratites ?, and Arcestes ? 500-3,000 ? 



( Conformity. ) 



Chitistone limestone (massive gray limestone, with Terebrat- 

 ula, Spirifefina, Avicula. HaloMa cf. superba, Gryplicea f, 

 Myophoria, Pecten, Hinnites ?, Pleuvomya, Tropites t, and 

 Arcestes) 300-1,200 ? 



(Conformity ?) 

 Triassic or Permian : 



Nikolai greenstone (basaltic lava flows) 6,500 ? 



(Underlain conformity (?) by Carboniferous beds.) 



The basal formation of the undoubted Upper Triassic rocks of the 

 Chitina Valley is the Chitistone limestone, which, as exposed in the type 

 section in the Nizina Valley, 6 is a massive bluish gray limestone, 1,800 

 or 2,000 feet thick, which rests with apparent conformity (see plate 26, 

 figure 1 ) on the volcanic beds of the Nikolai greenstone and which is over- 

 laiu also with apparent conformity (see plate 26, figure 2) by the thin- 

 bedded Nizina limestone which is described below. 



On the tributaries of Kuskulana and Kotsina rivers, in the western part 

 of the Chitina Valley, the Chitistone limestone seems to be reduced to a 

 single massive plate which is, in general, not more than 300 to 500 feet 

 thick. The Chitistone limestone thus has apparently thinned toward the 

 west, while the overlying Nizina limestone has grown thicker. The ex- 

 posures in the intervening district have not been studied in detail, so it is 

 unfortunately not known whether there is a gradual change from the one 

 section to the other. It is possible that the apparent westward thinning 

 of the Chitistone limestone may be due to the cutting out of parts of the 

 formation, in some places by faults and in other places by unconformity, 

 and the apparent westward thickening of the overlying Nizina limestone 

 may be caused by structural repetition of the beds (see plate 27, figure 2). 

 If the apparent westward thinning of the Chitistone limestone actually 

 exists as a purely stratigraphic feature, it may be due to variation in the 

 volume of the original sediments, the limestone having been deposited in 

 the form of a wedge-shaped plate thinning westward ; or it may be due to 

 variation in the character of the original sediments, the massive limestone 

 beds of the eastern area grading westward into thin-bedded limestone and 

 shale. 



°F. H. Moffit and S. R. Capps : Geology and mineral resources of the Nizina district. 

 Alaska. U. S. Geol. Survey Bull., No. 448, 1911, pp. 21-28. 



