Geology of Oregon and Washington and Its Relation to Occurrence of Oil and Gas

Abstract

Oregon and Washington contain the following topographic provinces, each with distinctive geological features: Coast Range, Cascade Mountains, Puget Sound-Willamette trough, Okanogan Highlands, Blue Mountains, Klamath Mountains, Olympic Mountains, and Columbia Plateau. The northern Cascades of Washington, the Okanogan Highlands, the Klamath Mountains and a part of the Blue Mountains are composed largely of thick sections of folded Paleozoic and early Mesozoic marine sediments which have been intensely metamorphosed and deformed. Associated with these are granites and other intrusive rocks. The formations in eastern Oregon and Washington and in the Cascade Mountains constitute the base on which rests a thick series of Tertiary lavas and associated lacustrine and fluviatile ediments. The core of the Olympic Mountains consists of greatly indurated but not metamorphosed sedimentary rocks, possibly late Mesozoic in age, which are bordered on the north, east, and south by 3,000 feet of Eocene volcanics and about 8,000 feet of later Tertiary marine sediments. Granites are absent in this area. The Coast Range from the Olympic Mountains southward to the Klamath Mountains is composed entirely of folded and locally faulted Tertiary lavas and sediments ranging from 10,000 to 20,000 feet thick. The basement rocks are unknown but the lower part of the Tertiary section everywhere consists of 3,000 to 5,000 feet of basic volcanics largely submarine in origin. Crustal movements of late Miocene age produced northwest-southeast folds in Washington, and north-south folds in the Coast Range of Oregon. Major north-south upwarps and downfolds at the close of the Pliocene were superimposed on the earlier structures with the resultant development of the Coast Range, Cascade Mountains, and the intervening trough. Indications of oil in the form of very small seepages occur in the Tertiary formations west of the Cascade Mountains. There is no conclusive evidence that commercial amounts of oil are present in these formations although drilling tests under geologic control are considered warranted. The metamorphic rocks, granites, lavas and continental sediments in eastern Oregon and Washington, and in the northern Cascade Mountains are considered as being unfavorable for the commercial occurrence of oil.