Geology of the Eastern Part of the Alaska Range and Adjacent Area GEOLOGICAL SURVEY BULLETIN 989-D "Geology of the Eastern Part of the Alaska Range and Adjacent Areas," by Fred H. Mofflt, presents a carefully considered summary of many years of geologic investigation in this part of Alaska by the geologist most eminently qualified to do so. Mr. Moffit's first work in Alaska for the Federal Geological Survey was in 1903, and with the exception of two brief intervals (1917-18; 1933) field work in Alaska was continuous each season until 1944. Compulsory retirement age was reached in April of 1944, but a special tempo­ rary appointment was granted to enable him to complete some important work on hand, and he did not retire from active service until January 1, 1945. After retiring, Mr. Moffit continued work on this manuscript and herein pre­ sents an integrated and informative discussion of the geology of this interesting and important section of Alaska. Here he has pooled the information gleaned from many years of field investigations and drawn together his vast knowledge of this area, which heretofore appeared in many Survey bulletins and separate chapters in the bulletins on the mineral resources of Alaska. One needs only a few minutes' study of the pamphlet "Publications of the Geological Survey" to appreciate the scope of the foundation for this report, and future students of this area will be ever grateful for the time and effort which Mr. Moffit put into the preparation of this important contribution to the knowl­ edge of the geology of Alaska. W. H. BRADLEY, Chief Geologist. 63 A CONTRIBUTION TO ALASKAN GEOLOGY BY FRED H. MOFFIT ABSTRACT This paper describes the geology of a part of the Alaska Range, extending from the Delta River to the international boundary between Alaska and Canada, and of an additional area that includes part of the Wrangell Mountains and the upper Copper River valley. The Alaska Range is one of the youngest yet one of the most outstanding physiographic features of the Territory. It includes Mount McKinley, the highest peak of the North American continent, and many other peaks of excep­ tional height and is noteworthy for the ruggedness of its outlines, its many snow-capped summits, and its numerous glaciers, the sources of many glacial streams. Eastward from Mount McKinley the range becomes lower 'and less rugged and finally loses its identity as a structural feature or merges with the Coast Range mountains north of Mount St. Elias. The Alaska Range and the Wrangell Mountains are comparable in age, both developed largely in Tertiary time. The Alaska Range owes its height and length to folding, faulting, and elevation of the land; but the mountains of the Wrangell group are due to the piling up of lavas and tuff beds. The forms of both mountain systems were much modified by the erosion of streams and glacial ice. The rocks are partly sedimentary and partly igneous in origin, ranging in age from pre-Cambrian to Recent and in degree of inetamorphisin from schist or gneiss to unaltered rock. The. oldest and most highly altered rocks, derived chiefly from siliceous and argillaceous sedimentary beds intruded by granitic bodies, occur on the north, side of the Alaska Range and are correlated tenta­ tively with the Birch Creek schist (pre-Cambrian) of the Yukon and Tanana River valleys. The oldest Paleozoic rocks are the Middle and Upper Devonian sequence of the Tetlin and Chisaua areas, which include conspicuous limestone units and locally are interbedded with lava flows. Rocks of early Carboniferous age are lava flows and sedimentary beds tliat are dominantly argillaceous but locally calcareous and contain one or more massive limestones. These rocks,- which include the Chisna formation, have been referred to the early Carbonif­ erous or possibly the Devonian, but the assignment is not certain. The Devon­ ian rocks are restricted to the Nut/otin Mountains area, where they are asso­ ciated with Permian rocks that consist dominantly of lava flows and calcareous tuffs but include a large amount of fossiliferous limestone and subordinate shale. Upper Triassic limestone and shale were deposited unconformably on the older rocks and in turn were elevated above the sea, folded, and partly eroded in early 65 66 CONTRIBUTIONS TO ALASKAN GEOLOGY Jurassic (?) time. Then a great thickness of Upper Jurassic and Lower Cre­ taceous marine deposits, possibly separated from each other by an unconformity, was deposited on the remains of the Upper Triassic and older rocks. The vol- canism that characterized the Permian apparently continued into early Meso­ zoic time. More important in many ways was the intrusion of great bodies of granitic and dioritic rock that accompanied the mountain-building disturb­ ances of the Late Mesozoic time and continued into the Tertiary. The rocks deposited in Late Jurassic and Early Cretaceous time, consisting largely of thin-bedded shale and argillite or sandstone with important conglomerate mem­ bers, were folded and eroded in Late Cretaceous time and overlaid locally by Upper Cretaceous terrigenous deposits. Throughout Cenozoic time most of Alaska, with the exception of some coastal areas, has stood above the level of the sea and has been subjected to continuous subaerial erosion. In the late Eocene, mountain-building forces elevated the Alaska Range. At about the same time were laid down fresh-water gravels, sands, and peat beds, which became the coal measures, and shortly thereafter, still in the Tertiary, a renewed uplift of the range brought about the deposition of other thick fresh-water sediments the Nenana gravel. Concurrently with the laying down of the coal measures, the outpouring of the Wrangell lava began. Tertiary history was ended with climatic changes that resulted in the development of Pleistocene glaciers, which left deposits of morainal material and outwash gravels and played an important part in shaping the topography of the Alaska Range and the Wraugell Mountains as they are today. At the height of glaciation the entire Copper River basin was filled with ice to a depth of many hundreds of feet. The north side of the Alaska Range, however, was less favorably situated for the accumulation of ice and shows relatively little glaciation. The glaciers of the present day seem to be in retreat. Metallic minerals associated with the late Mesozoic and early Tertiary granitic intrusions are those of gold, copper, antimony, and molybdenum. Most of the gold production has conie from the placer mines of the Chistochina district, where gold valued at nearly $3,000,000, has been derived largely from the gravels of Slate Creek and Miller Gulch, and from a lode deposit exploited as the Nabesna Gold mine, from which gold valued by the management at $1,869,376 had been produced when the mine was closed in 1940. No metals other than gold have been produced in the eastern Alaska Range, though known low-grade prospects may be of future commercial value. INTRODUCTION This paper describes the geology of the Alaska Range from the Delta River to the international boundary, which separates the Terri­ tory of Alaska from the Yukon Territory of Canada. It also de­ scribes some adjacent area, most of \vhich is in the northern part of the valley of the Copper River (fig. 18). The Alaska Range is a major physiographic feature of the Terri­ tory of Alaska. It is noteworthy because of its great relief, as it in­ cludes many lofty mountains, among them Mount McKinley, the highest peak of the North American Continent. The Range is also important geologically, as it includes many geologic formations of widely differing ages and presents interesting problems in stratig­ raphy, structure, and geologic history, many of which are not yet solved. GEOLOGY OF THE EASTERN PART OF THE ALASKA RANGE 67 140° 132° FIGURE 18. Index map of Alaska showing location of the eastern Alaska Range . and adjacent area described in this paper. ' GEOLOGIC AND TOPOGRAPHIC SURVEYS The men who made the early explorations in the eastern part of the Alaska Range contributed more to knowledge of the geography of the region than to its geology. They were of necessity occupied with the difficulties of travel, which left little time in the short mild season for geologic observation of more than the obvious features. Their contribution was essentially geographic, yet they laid a founda­ tion for the work of later geologists. The first known white man to cross the eastern Alaska Range was Lieut. H. T. Alien, who traversed the mountains from Suslota Lake to Tetlin Lake and the Tanana River in 1885. This was a journey, undertaken for the War Department, from Prince William Sound to St. Michael by way of the Copper, Tanana, Yukon, and Koyukuk Rivers a journey that is outstanding in the history of Alaskan travel. Six years later (1891) Lieut. Frederick Schwatka and C. W. Hayes ascended the White River and crossed Skolai Pass to the Copper River drainage basin, taking a route around or across the southeast end of the range. In 1898, and again in 1899, W. J. Peters and A. H. Brooks visited the eastern part of the Alaska Range. Both expedi­ tions were sponsored wholly by the U. S. Geological Survey. In the 6& CONTRIBUTIONS TO ALASKAN GEOLOGY first year they descended the Tanana River, thus skirting the north­ east side of the range, and in the second they crossed the passes at the head of the White and Nabesna Rivers and descended the latter stream, thus crossing the range by a route to the southeast of that used by Alien. Only a few weeks later Oscar Rohn made his way from the Chitina valley by the Nizina and Chisana Glaciers to the head of the Chisana River, and thence across Cooper Pass and up the valley of either Jack or Platinum Creek to the Copper River side of the Wrangell Mountains.