A LOWER JURASSIC FLORA FROM THE UPPER MATA- 

 NUSKA VALLEY, ALASKA. 



By F. H. Knowlton, 



Custodian of Mesozoic Plants, United States National Museum. 



The material upon which this paper is based was collected by Dr. 

 George C. Martin, R. M. Overbeck, and J. B, Mertie, jr., of the 

 United States Geological Survey, during the field season of 1913. 

 The general location of the fossil collections, together with the avail- 

 able structural and stratigraphio data, is described in the following 

 notes, which Doctor Martin has kindly prepared : 



The fossil plants were obtained from four localities, all situated in a small area 

 lying south of the east fork of Boulder Creek and nbrth of Antliracite Ridge. These 

 localities are in the eastern part of the Talkeetna Mountains, or in the upper Mata- 

 nuska Valley, Boulder Creek flowing westward into Chickaloon River, which is the 

 largest northern tributary of the Matanuska. 



The Lower Jurassic rocks of the Matanuska Valley and adjacent areas have a wide 

 geograpliic extent and constitute a very important stratigraphio and structural ele- 

 ment in the geology of the eastern Talkeetna Mountains. These rocks were described 

 by Paige and Knopf as "lower Middle Jurassic rocks" (including only the volcanic 

 members as described in the text, but the whole of the "Middle Jurassic andesitic 

 greenstones, etc.," as represented on the map). They were tentatively correlated by 

 Brooks with the Skwentna- group, which he referred to the lower Middle Jurassic on 

 the basis of Paige and Knopf's assignment of their Matanuska Valley rocks. They 

 include the "Lower Jurassic rocks" described by Martin and Katz,^ and by Martin 

 and Mertie.* 



These rocks include lavas, agglomerates, breccias, and tuffs, interbedded with 

 lesser volumes of sandstone and shale. They are composed chiefly of water-laid vol- 

 canic detritus. Their thickness is probably several thousand feet, but can not be 

 accurately estimated on account of the complex structure and the lack of recognizable 

 horizons. The position of the plant-bearing beds within the formation has not been 

 determined. These rocks carry also an abundant marine invertebrate fauna which 

 Stanton regards " as probably of Lower Jurassic age, and as probably equivalent to that 



■Paige, Sidney, and Knopf, Adolph., Geologic reconnaissance in the Matanuska and Talkeetna basins, 

 Alaska, U. S. Geol. Survey Bull. 327, 1907, pp. 1&-19. 



» Brooks, Alfred H., The Mount McKinley region, Alaska, U. S. Geol. Survey Prof. Paper 70, 1911, 

 pp. 85-87. 



^ Martin, G. C, and Katz, F. J., Geology and coal fields of the lower Matanuska Valley, U. S. Geol. 

 Survey Bull. 500, 1912, pp. 29-32. 



« Martin, G. C, and Mertie, J. B., jr., Geology of the upper Matanuska district, U. S. Geol. Survey 

 Bull, (in preparation). 



5 U. S. Geol. Survey Bull. 500, p. 31. 



Proceedings U. S. National Museum, Vol. 51— No. 2158. 



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