Devonian of Washington, Oregon and Western British Columbia

Abstract Devonian sequences of the Pacific Northwest are either of eugeosynclinal type or transitional between the eugeosyncline and the miogeosyncline. The most westerly exposures are in north-western Washington State about 80 miles from the present-day western land margin of the continent. Here, Middle and Upper Devonian limestone reef complexes occur interbedded with shales, siltstones, greywackes and volcanic rocks. At one locality the Devonian rocks appear to lie unconformably on a basement complex of amphibolites and diorites. Early Pennsylvanian rocks overlie the Devonian sequence with apparent conformity. Two hundred and fifty miles to the east, in northeastern Washington, Upper Devonian limestone reef complexes interbedded in argillite, sandstone and conglomerate occur in one area. They are thought to be transitional between the eugeosyncline and miogeosyncline. Argillites and limestone bodies of probable Silurian age underlie the Devonian. Overlying rocks are unknown. Two small outcrops of Middle Devonian limestone reef complexes are known in the Suplee area of east-central Oregon and they also may be transitional betwen the eugeosyncline and mio-geosyncline. One of these limestone outcrops is associated with argillite, radiolarian chert, chert grit and sandstone. The base of the Devonian sequence is not exposed. It is overlain with apparent conformity by sediments of Mississippian or early Pennsylvanian age. A reported Devonian occurrence near Grants Pass in southwestern Oregon could not be verified. No definitely identified outcrops of Devonian age are known in western British Columbia, but in two areas of the southwestern part of the province limestone clasts in younger formations contain possible Devonian or Mississippian fossils.