Middle Devonian Orogeny in Western North America?: A Fish and Other Fossils

Abstract

Middle Devonian orogeny in western North America is suggested by unconformities in widely separated areas along the continental margin, and is consistent with a variety of geologic information. In the Shasta Lake region of northern California the Copley Greenstone and Balaklala Rhyolite, which contained an isolated arthrodire plate, are inferred to be of Eifel age. These units underlie the Late Eifel age Kennett Formation. An early Paleozoic sequence in the Gazelle-Callahan area to the north includes fossiliferous beds as young as the Ems to Middle Devonian. Age of metamorphism of rocks known a short distance to the west is about 380 m.y., which could represent an Eifel age event. An angular unconformity and orogenic interval is known at the northern end of the Sierra Nevada between fossiliferous Silurian and Mississippian age beds. In the southern Sierra Nevada, an unconformity exists between fossiliferous Late Devonian beds and unfossiliferous beds of older Paleozoic age. In the eastern Sierra Nevada, rocks of probable Permian age rest unconformably on rocks of possible Ordovician or Silurian age at one locality. Evidence for possible Middle Devonian orogeny is also known from Northwest Washington (Givet age beds resting unconformably on basement complex) and from southeastern Alaska (Givet age beds resting on Silurian and older rocks; Lower Devonian beds overlain with possible unconformity by beds of Givet and Eifel [?] age). A clastic wedge, of Givet age at its base and possibly derived from the west, is known from Northeastern British Columbia. Widespread disconformities of Ems and Eifel age on the western edge of the adjacent platform are perhaps unrelated to Middle Devonian orogeny. These scattered data may indicate the presence of a Late Eifel age orogenic episode which involved the so-called Klamath-Sierran island arc and parts of other inferred arcs located in western Canada and southeastern Alaska. If so, this orogenic interval is considerably older than the Antler event, which centered in Latest Devonian-Mississippian time in a more eastern continental slope environment.