Devonian rocks of the Yukon-Porcupine Rivers area and their tectonic relation to other Devonian sequences in Alaska

Abstract

Devonian rocks along the Yukon River near the Alaska-Yukon boundary comprise 250 feet of limestone and shale and 800 feet of chert and siliceous shale, all referred to the McCann Hill Chert of Early to Late Devonian age; about 3,000 feet of non-marine chert-pebble conglomerate, graywacke, and shale of the Nation River Formation (Late Devonian); and about 3,000 feet of an unnamed chert and siliceous shale formation, of Late Devonian to Early or Late Mississippian age. The McCann Hill Chert apparently rests disconformably on graptolitic shale of the Road River Formation that has a succession of graptolite zones ranging in age from Ordovician to possibly Early Devonian. The Devonian succession along the Porcupine River, 170 miles north of the Yukon River locality, consists of 600 feet of Salmon-trout Limestone, overlain by 200 feet of unnamed shale and about 500 feet of unnamed dolomite. The Salmontrout Limestone rests accordantly and probably conformably on graptolitic shale with Monograptus hercynicus of Late Silurian or Early Devonian age, and the dolomite is overlain unconformably by limestone and shale of Carboniferous age. Tentaculitid faunas from the top of the Salmontrout Limestone and from the basal limestone and shale member of the McCann Hill Chert are similar and indicate that the units are partly correlative. These faunas and others throughout the Salmontrout resemble those from the Lower Devonian of Bohemia, indicating that the Salmontrout Limestone and the limestone and shale member of the McCann Hill Chert are somewhat older than previously reported. The faunas, and those from the underlying graptolitic shales, also suggest that sedimentation in this area of Alaska was nearly continuous from Late Silurian into the Earliest Devonian, and that there is little record of Caledonian earth movements. The stratigraphy of the Porcupine River area is not as yet well enough established to determine if rocks equivalent in age to the Nation River Formation are present, were once present and have since been eroded, or were never deposited. Pillow basalt and andesitic breccia, although closely associated with Devonian limestone in the western part of the Yukon-Porcupine Rivers area, seem only structurally related and of different age. Devonian rocks in southern and southeastern Alaska consist of several thousand feet of graywacke, shale and volcanic rocks deposited in the northern Cordilleran geosyncline. A combination of strike-slip and thrust faults of Tertiary and Mesozoic age may have since displaced the Devonian geosynclinal rocks relatively northward into central Alaska.