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GEOLOGY OF THE EAGLE-CIRCLE DISTRICT,

ALASKA

By J. B. MERTIE, Jr.

INTRODUCTION

LOCATION OF THE AREA

The district here designated the Eagle-Circle district lies between meridians 140° 45' and 144° 15' west longitude and parallels 64° 30' and 66° north latitude and comprises a nearly square area of about 10,000 square miles. (See fig. 1.) The Yukon River runs diagonally across the area from the southeast corner to the northwest corner. This area is geographically the connecting link between the Fortymile mining district, to the southeast, and the Circle mining district, to the northwest. It embraces the Seventymile district and the American Creek, Fourth of July Creek, and Woodchopper Creek precincts.

PREVIOUS WORK

This region has interested prospectors and geologists for many years. Gold placer mining was begun in the Fortymile district in 1887 and in the Circle district in 1894, and mining operations have continued in these two districts up to the present day. In view of their productiveness it was but natural that the attention of prospectors should be directed to the intervening area south of the Yukon. A great deal of prospecting has been done, and in the aggregate a considerable amount of gold has been recovered in the last 25 or 30 years, although no large mining camps have so far been established. The Geological Survey, also, has been greatly interested in this intervening area because of its potential importance to the mining industry and its unique geologic features.

The area considered in the present report may logically be divided into two parts; one, which comprises some 40 per cent of the total, lies northeast of the Yukon, and the other lies southwest of the Yukon. The northeastern part is unmapped, both topographically and geologically, except for the work of the international boundary survey along the international boundary and the observations of

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and geologic maps and to correlate the facts of mining and geology to prospectors and miners the obvious benefits of good topographic is concentrated in the environs of mining districts, in order to give

Much of the initial work of the Geological Survey in a new region

reconnaissance scale of 1:250,000. almost completely, both topographically and geologically, on the fifths, because of its potential economic significance, has been mapped workers who have traversed the Yukon. The southwestern three

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FIGURE 1.-Index map showing location of area covered by this report

for a better understanding of the processes of mineralization. This procedure, of course, is most desirable; but geologically it is often disappointing, because regions in which intensive mineralization has occurred are likely to be regions of more than ordinarily complex geology. Under such circumstances the geologist undertakes to decipher the geologic record under the least favorable conditions, and instead of working from the simple to the complex he is obliged to work in the reverse order. The area here considered is an excellent example of this condition. The southwestern part is a region composed largely of metamorphic rocks, and although a considerable amount of geologic work has been done there, the total results are somewhat disappointing. The northeastern part contains geologic formations of the same general age as those south of the river, in a relatively unmetamorphosed condition, but it is practically an unknown land so far as the geologist is concerned. The answers to numerous unsettled geologic problems mentioned in this paper will ultimately be found through study of the hitherto neglected territory north of the Yukon.

The earliest geologic notes on this stretch of the Yukon, from Circle to the international boundary, were made in 1888 by McConnell, in the course of an exploratory trip from the Mackenzie River across to the Porcupine, down that stream to Fort Yukon, and up the Yukon to the confluence of the Pelly and Lewes Rivers. Ogilvie, also in 1888, made a traverse up the Tatonduk River and over to the Porcupine, taking some geologic notes upon the rocks in the Tatonduk Valley. In 1889 Russell : made a rapid trip up the Yukon River from St. Michael to the headwaters. His observations, however, were more concerned with the surficial features of the region than with the hard-rock geology.

Systematic geologic work in this region was first attempted in 1896, when a Geological Survey party under the leadership of J. E. Spurr * traversed the Yukon from its headwaters as far downstream as Nulato, visiting on the way the mining camps contiguous to the river. The report embodying the results of this expedition is of great interest in so far as it relates to the history of exploration and early mining activities, and many of the geologic observations also are useful. Three of Spurr's formation names, Birch Creek“ series,” Rampart “series," and Tahkandit “ series," have been redefined to fit later knowledge of the stratigraphic and lithologic conditions and remain in common usage, these being now designated, respectively, Birch Creek schist, Rampart group, and Tahkandit limestone.

1 McConnell, R. G., Report on an exploration in the Yukon and Mackenzie Basins, Northwest Territory : Canada Geol. Survey Ann. Rept., new ser., vol. 74, pp. 134-139D, 1890.

2 Ogilvie, William, Exploratory survey of part of the Lewes, Tatonduk, Porcupine, Trout, Peel, and Mackenzie Rivers, Canada Interior Dept., 1890.

* Russell, 1. C., Notes on the surface geology of Alaska : Geol. Soc. America Bull., vol. 1, pp. 99–156, 1890.

* Spurr, J. E., Geology of the Yukon gold district, Alaska : U. S. Geol. Survey Eighteenth Ann. Rept., pt. 3, pp. 87–392, 1898.

The first topographic mapping in this region was done in 1898, when E. C. Barnard made a reconnaissance topographic map of the Fortymile quadrangle. The northern part of this quadrangle includes the Yukon River and contiguous territory from the international boundary downstream to the Tatonduk River, as well as the Seventymile River and American Creek Basins, and this map remains still a suitable base for geologic mapping in the southeast corner of the area under consideration.

Some further geologic observations were made in 1899 by Brooks,5 in the course of a traverse from Lynn Canal to Eagle.

The accumulation of detailed stratigraphic data was begun in 1902 by Collier, who traversed the Yukon from Dawson, Yukon Territory, to the Yukon Delta, giving particular attention and study to the Mesozoic and Tertiary coal-bearing terranes. During 1903 Arthur Hollick visited the same general region, in order to make more extensive collections of fossil plants from the coal-bearing beds.

In 1903 Prindle began a systematic study of the geology and mineral resources of the Yukon-Tanana region, which continued intermittently until 1911. Coincidentally with this geologic work, a reconnaissance topographic map of the Circle quadrangle south of the Yukon was prepared in 1903, 1904, 1905, and 1908, by T. G. Gerdine, D. C. Witherspoon, R. B. Oliver, J. W. Bagley, and G. T. Ford. Prindle, in these years, outlined the fundamental geologic features of this region and laid a lasting geologic foundation for subsequent work. Prindle's traverse of 1903 from Eagle to Fairbanks by way of Circle and the traverse in 1911 by Prindle and the writer both cover a part of the area here described and form a part of the groundwork for the following publications:

Prindle, L. M., The gold placers of the Fortymile, Birch Creek, and Fairbanks regions, Alaska: U. S. Geol. Survey Bull. 251, 1905.

Prindle, L. M., The Yukon-Tanana region, Alaska--Description of Circle quadrangle: U. S. Geol. Survey Bull. 295, 1906.

Prindle, L. M., The Fortymile quadrangle, Yukon-Tanana region, Alaska : U. S. Geol. Survey Bull. 375, 1909.

Prindle, L. M., and Mertie, J. B., jr., Gold placers between Woodchopper and Fourth of July Creeks, upper Yukon River : U. S. Geol. Survey Bull, 520, pp. 201-210, 1912.

Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska : U. S. Geol. Survey Bull. 538, 1913.

5 Brooks, A. H., A reconnaissance from Pyramid Harbor to Eagle City, Alaska, including a description of the copper deposits of the upper White and Tanana Rivers : U. S. Geol. Survey Twenty-first Ann. Rept., pt. 2, pp. 331-391, 1900.

• Collier, A. J., Coal resources of the Yukon Basin : U. S. Geol. Survey Bull. 218, 1903. 7 Brooks, A. H., and Kindle, E. M., Paleozoic and associated rocks of the upper Yukon, Alaska : Geol. Soc. America Bull., vol. 19, pp. 255–314, 1908.

An important contribution to the geology of this area was made in 1906 by Brooks and Kindle,? who made a boat traverse from the international boundary to Circle, spending about two months in a study of the section exposed along the river. Their paper not only contained additional stratigraphic data of great value but also attempted for the first time a general correlation of the geologic terranes of interior Alaska.

In 1911 and 1912 the International Boundary Survey prepared a topographic map embracing a strip 2 to 3 miles wide on each side of the boundary from the Yukon River to the Arctic Ocean. A geologic survey along the boundary was also made, and for this purpose the strip was divided into a northern section extending from the Porcupine River to the Arctic and a southern section extending from the Yukon River to the Porcupine. The southern part of the southern section is within the area here described. The geology of the southern section was studied by Cairnes, representing the Geological Survey of Canada. One of the interesting results of Cairnes's work was the discovery in this area of a well-developed sequence of lower Paleozoic rocks, including rocks of Cambrian age.

The triangle bounded on the east by the international boundary, on the northwest by the Porcupine River, and on the southwest by the Yukon River contains unquestionably the most complete Paleozoic section in Alaska, but it is as yet almost unexplored. Wishing, if possible, to obtain additional data on this Paleozoic section, A. H. Brooks, chief Alaskan geologist of the United States Geological Survey, in 1915 dispatched Eliot Blackwelder upon a boat trip from the international boundary to Circle. Blackwelder spent perhaps a month in this work, and from the data so collected he prepared a geologic report on this river section. His report was never published but was available to the writer, both in the field and in the office, and was of great assistance in the preparation of the present report.

Further paleontologic field work was done in 1918 by G. H. Girty, of the Geological Survey, who made extensive fossil collections from the type Mississippian and Permian localities along the Yukon at Calico Bluff and at the mouth of the Nation River, respectively.

SCOPE OF THE PRESENT REPORT

During the summer of 1925 the writer, assisted by M. M. Knechtel, made another boat traverse from the international boundary to Circle, for the purpose of correlating the work of earlier investigators.

$ Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers : Canada Geol. Survey Mem. 67, 1914.

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