ALASKA AND THE YUKON 



617 



unit includes the Kushtaka, Stillwater, and Tokun formations in the Katalla 

 district, and the Kultheith formation in the Yakataga and Malaspina districts. 

 It is not represented in the exposed Tertiary sequence of the Lituya district. 



The middle unit, formed in middle Oligocene to approximately middle 

 Miocene time, is characterized by massive concretionary mudstone and silt- 

 stone, believed to have been deposited in moderately deep water, in part in a 

 reducing environment. Local volcanic activity is indicated by interbedded 

 .marine tuff and agglomerate. This unit is highly organic at some places, and 

 many of the known indications of petroleum in the Katalla and Yakataga 

 districts are associated with it. The unit includes the lower and middle parts 

 of the Katalla formation in the Katalla district, the Poul Creek formation in 

 the Yakataga district, and the basal part of the exposed Tertiary sequence in 

 the Lituya district. It is absent in the exposed Tertiary sequence in the 

 Malaspina district, where the early and late Tertiary units are in unconformable 

 contact. 



The youngest unit, deposited during the time interval from middle or 

 late Miocene to late Pliocene or possibly earliest Pleistocene, consists of shallow 

 marine sandstone and siltstone interbedded with marine tillite ("conglomeratic" 

 sandy mudstone). The marine invertebrate fauna, on the whole, indicates 

 considerably colder water than in earlier Tertiary time, and the marine glacial 

 deposits indicate rigorous glaciation of adjacent land areas. This unit is 

 represented by the upper part of the Katalla formation in the Katalla district, by 

 the Yakataga formation in the Yakataga and Malaspina districts, by the upper 

 part of the unnamed sequence in the Lituya district, and by strata exposed on 

 Middleton Island, a small island in the Gulf of Alaska 80 miles southwest of 

 Cordova (D. J. Miller, 1959). 



Structure. The structure and orogenic history of the Gulf of Alaska 

 Tertiary Province is described by D. J. Miller (1959) as follows (Fig. 

 39.9): ' 



In late Tertiary or early Pleistocene time the Chugach-St. Elias Mountain 

 chain was uplifted along an arcuate northward-dipping fault system, and 

 the bordering belt of Tertiary sedimentary rocks was folded and displaced 

 along many high-angle thrust faults. The largest of these faults, the Chugach-St. 

 Elias fault, has been traced along the southern front of the Chugach and 

 St. Elias mountains from the delta of the Copper River to Yakutat Bay, a 

 distance of 180 miles. This fault, which dips 30°-60°N., is estimated to have 

 a stratigraphic throw of not less than 10,000 feet. In the Lituya district the 

 Fairweather fault, lying in a great trench at the base of the Fairweather Range, 

 bounds the Tertiary province. 



The major thrust faults and grain of folding in the Tertiary rocks in general 

 parallel the trend of the bordering fault system along the Chugach-St. Elias 

 front; the intensity of folding and magnitude of displacement along faults 

 increases toward the mountain front. Transverse trends in the western part 



Fig. 39.9. Generalized geologic map of Gulf of Alaska Tertiary Province, after Miller et al., 1959. 

 Q, lowland area covered by ice or unconsolidated deposits of Quaternary age; possible underlain 

 by sedimentary rocks of Tertiary age. T, sedimentary rocks of Tertiary age. M, metamorphosed 

 sedimentary rocks and volcanic rocks of Mesozoic and older ? age. 



of the Katalla district apparently are related to the northward-trending Ragged 

 Mountain fault that exposes the pre-Tertiarv basement rocks. In the Katalla 

 district the folds are typically of small amplitude, tightly compressed, and 

 asymmetric, the axial planes being inclined to the west or north. 



In the Yakataga district three belts of differing structural pattern are re 

 nized: In the belt nearest the Chugach-St. Elias fault the Tertian rocks 

 show intense minor folding with much overturning, and are displaced along 

 many northward-dipping high-angle thrust faults, which in general arc .sub- 

 parallel to the axial planes of the folds. In the intermediate belt the folds arc 

 of small amplitude but relatively long, and are less tightly compressed and 

 more widely spaced. The belt nearest the coast is characterized by broad 

 synclines and narrow, tightly pinched, asymmetric, longitudinally faulted 

 anticlines. 



In the Malaspina district, faulting and uplift predominated over Folding 

 during the late Cenozoic orogeny, for the youngest Tertiary strata are only 

 broadly folded or gentlv tilted. At least two earlier stages of deformation and 

 uplift within the Tertiary period are recorded bv angular unconformities 

 within the Kulthieth and Yakataga formations, and by overlap of the upper 



