Oregon Geology: Sixth Edition

Coast Range and Continental Margin

Physiographic map (After Loy, et al., eds., 2001) 213 Coast Range and Continental Margin Past and Present As with other provinces, the geologic examination of the Coast Range began with the exploration for minerals, the collection of fossils, and mapping. The details of unknown regions had to be filled in before a picture emerged. Joseph Diller’s map folios in the late 1800s and early l900s provided a regional overview as did the stratigraphic study by Ralph Arnold and Harold Hannibal in 1913. Close to the same time oil, gas, and mineral potentials were being assessed by Chester Washburne and Joseph Pardee. A considerable step forward was initiated with a cooperative mapping and stratigraphic project begun in the 1940s between the U.S. Geological Survey and the Oregon Department of Geology and Mineral Industries (DOGAMI). As part of an investigation for petroleum resources, three Survey geologists W. C. Warren, Hans Norbisrath, and Rex Grivetti worked in the northwest counties. Their efforts were followed by the 1949 publications on the central Coast Range by Harold Vokes and Parke Snavely, also with the Survey. Ewart Baldwin’s first revision of Cenozoic stratigraphy appeared about the same time. First advanced in the 1920s, the theory of plate tectonics, which now dominates West Coast geology , only gained acceptance some 40 years later after research by many geologists and oceanographers confirmed the notion of large moving crustal plates. The concept crossed the continent, where its significance for western North America was ushered in with Robert Dott’s “Implications for sea floor spreading” along with Tanya Atwater’s papers on tectonics and Cenozoic evolution. William Dickinson’s tectonic models of the Paleozoic were published in the late 1960s and early 1970s. Landscape of the Coast Range and Continental Margin Moderately high moun­ tains, terraces, rocky headlands , sandy beaches, and an offshore shelf and slope all contribute to the complex topography of the Coast Range and continental margin. From the Olympic Peninsula of Washington, the province continues southward to the Middle Fork of the Coquille River and from the edge of the Willamette Valley west to the base of the continental slope. The marine influence on western Oregon is responsible for the warmest average winter temperatures , the coolest summers, and the greatest precipitation of any region in the state. Rainfall in excess of 100 inches a year has molded the landscape with intricate stream patterns and dense forests. Because of extensive erosion on the western slope, the crest of the range is offset to the east. The highest elevations are Marys Peak at 4,097 feet, Trask Mountain at 3,423 feet, Sugarloaf Moun­ tain at 3,415 feet, and Saddle Mountain at 3,283 feet. Most rivers of the Coast Range are moderate in size, and only the Columbia and Umpqua cut entirely across the province. The notable west-flowing Nehalem, Wilson, Siletz, Yaquina, Alsea, Siuslaw, Coos, and Coquille rivers end in broad tidal estuaries . Of those streams flowing into the Willamette Valley, the Long Tom, Marys, Luckiamute, Yamhill, and Tualatin have the greatest discharge and the longest watersheds. Along the Pacific strand, abrupt headlands are punctuated by bays, estuaries, pocket beaches, sand dunes, and spits. The distinctive terraced promon­ tory at Cape Blanco extends into the Pacific Ocean as Oregon’s most westerly point. Offshore, the province continues across the continental shelf and slope. 214 Oregon Geology Overview What is now coastal Oregon had its beginnings in the late Paleocene to early Eocene, with the construction of a volcanic seamount chain along a spreading center between tectonic plates. Transported as a large igneous province, the seamounts of Siletzia collided with and were accreted to the North American plate, forming the basement of the offshore shelf, Coast Range, Willamette Valley, and Western Cascades. Once incorporated around 50 million years ago, Siletzia rotated in a clockwise direction and shifted northward. As a broad plain and marine shelf west of the emerging Cascade volcanic arc, the subsiding Coast Range slab (Siletzia) accumulated thick From the 1940s through the early 2000s, Parke Snavely’s geologic examination of the Cenozoic onshore and offshore rocks established the basis for unraveling the tectonic processes of the Pacific continental margin. Born in Yakima, Washington, Snavely completed his graduate work at the University of California, Los Angeles. In a 50-year career, he held positions in both the Pacific Marine Geology and the Western Regional Geology branches of the U.S.G...