A. CAMBRIAN LIMESTONE AND ASSOCIATED ROCKS ON THE NORTH BANK OF THE YUKON RIVER JUST NORTH OF CALICO BLUFF Looking upstream. Shows thin plate of Upper Cambrian limestone dipping toward the river in center and at left. Middle Cambrian limestone along top of ridge at right. Photograph by Eliot Blackwelder. B. CRINOIDS IN CRYSTALLINE LIMESTONE OF SILURIAN (?) AGE FROM WEST SLOPE OF LOOP MOUNTAIN, YUKON TERRITORY About four-fifths natural size. B. WOODCHOPPER VOLCANICS ALONG THE NORTHEAST BANK OF THE YUKON RIVER BELOW THE MOUTH OF WOODCHOPPER CREEK B. ELLIPSOIDAL GREENSTONE IN WOODCHOPPER VOLCANICS ON THE NORTH BANK OF THE YUKON RIVER ABOVE WOODCHOPPER Photograph by Eliot Blackwelder. A. UNDIFFERENTIATED PALEOZOIC GREENSTONE IN BLUFF ON THE WEST SIDE OF THE YUKON RIVER AT THE MOUTH OF MISSION CREEK B. LOWER MISSISSIPPIAN VOLCANIC ROCKS ALONG THE EAST BANK OF THE YUKON RIVER ABOVE CIRCLE by Smith and the writer 52 into the valleys of the Alatna and Noatak Rivers. The Skajit limestone, which is believed to extend nearly or quite all the way across northern Alaska, is now believed to be mainly of late middle Silurian or early upper Silurian age. Above the Skajit limestone but below the Devonian sequence in northern Alaska lies another group of rocks which is composed mainly of noncalcareous beds but contains one or more prominent beds of limestone; this is believed to be of upper Silurian age. Similarly, along the Porcupine River, in northern Alaska, Kindle 53 found both middle and upper Silurian rocks, the middle Silurian consisting of a massive magnesian limestone and the upper Silurian of graptolite-bearing shale. In general, the upper Silurian is more widespread in Alaska than the middle Silurian, though the latter is more conspicuous on account of its massive limestones. In addition to their occurrence in northern Alaska, upper Silurian rocks are found in Seward Peninsula, in the Nixon Fork country southwest of Lake Minchumina, at several places in the Yukon-Tanana region, along the international boundary north of the Yukon, and at a large number of localities in southeastern Alaska. The rocks along the Yukon that are assigned to the upper Silurian (?) are believed to be correlative with this general upper Silurian horizon or horizons. DEVONIAN SYSTEM The Devonian system is represented along the Yukon between the international boundary and Circle by a group of rocks of late Middle Devonian age, herein named the Woodchopper volcanics, and by another group of rocks, also called late Middle Devonian, to which no formational name has been applied. The typical Middle Devonian rocks and fauna of interior Alaska are found along the international boundary north of the Yukon but have not been separately mapped. Lower Devonian rocks have nowhere been found in Alaska. MIDDLE DEVONIAN SERIES WOODCHOPPER VOLCANICS DISTRIBUTION The late Middle Devonian rocks to which the name Woodchopper volcanics is here applied crop out along both banks of the Yukon Smith, P. S., and Mertie, J. B., Jr., Geology and geography of northwestern Alaska: U. S. Geol. Survey Bull. 815, pp. 124-132, 1930. Kindle, E. M., Geologic reconnaissance of the Porcupine Valley, Alaska: Geol. Soc. America Bull., vol. 19, pp. 325-327, 1908. |