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Geologic Field Trips in Northern Oregon and Southern Washington (Classic Reprint)

Geologic Field Trips in Northern Oregon and Southern Washington (Classic Reprint)

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Excerpt from Geologic Field Trips in Northern Oregon and Southern Washington

The geology of Oregon is as varied and fascinating as that of any state of the Union. East of Portland lies the north-south trending Cascade Range, a snow-capped volcanic pile. Farther east lie flood basalts, batholithic highs, and Basin and Range terrain. West of the Cascades the interaction of land, sea, and volcanism has left a challenging geologic record.

In tune with geologic developments around the world, the plate tectonic model is finding increasing application to the geology of the State of Oregon. The imbricate thrust complex of Mesozoic rocks in southwestern Oregon has been attributed to sea-floor spreading, and recently the enigmas associated with the Colebrooke Schist have begun to yield to analysis in terms of the "new tectonics." Recent work in the Tertiary units about the periphery of the Klamath Mountains may soon lead to an even more accurate appraisal of the age and nature of the later stages of plate tectonic activity in that area. Closer to home, the Coast Range is largely unmapped and is ripe for reinterpretation in terms of the new tectonics.

The fault blocks, basalt flows, and ignimbrites of southeastern Oregon are undergoing re-evaluation in terms of some of the subtler aspects of sea-floor spreading. Tensional shearing related to deep-seated upwelling may account for the tapping of basaltic magma at depth and for the regional development of block faulting. Similar mechanisms may account for the extensive flood basalts of the Columbia River Group to the north. Both may be part of a larger picture in which late Miocene re-orientations of plate tectonic features profoundly affected the geology of much of the Pacific Basin and the world.

Going back in time, the early and middle Tertiary andesites of the Cascades Range and the John Day Basin may be interpreted as volcanic material produced by differential melting of a lithospheric plate related to an ancestral Pacific Basin rise system. Still further back in time, many of the Paleozoic exposures of northeastern Oregon may represent structural slabs of oceanic rock brought together in a subduction zone by sea-floor spreading. Viewed in this light, our failure to relate the various units stratigraphically is not only understandable, it is to be expected. With the present day stratigraphic techniques, perhaps our aim should be to understand the structural relationships of the units and to reserve final stratigraphic synthesis until later.

Practical use of the geologists' knowledge of Oregon is increasing as cities and counties become more aware of the influence of mass movement, seismic activity, and ground water on urban development. Similarly, various governing bodies and segments of private industry are integrating more and more geologic data into their planning process. Concern for ecology is foremost in the minds of the people, and a knowledge of geology is basic to an understanding of the environment.

Geology in Oregon has outgrown its historical role as just a basic tool of research and production for the mineral industry, although this will always be one of its important functions. Geology in Oregon today is an integral part of land-use planning, energy resource appraisals, power plant siting, and water management. It is responsible for the body of knowledge which surrounds a potential new source of almost pollution-free power within the state - geothermal steam.

As shown by the variety of the field trips presented here, it is evident that there is room in Oregon for specialists in all aspects of geology. Field Trip 1 views the Cretaceous marine strata and Tertiary volcanic and volcaniclastic deposits of central Oregon. Field Trips 2 and 3 respectively investigate the early Tertiary and middle Tertiary records of western Oregon. Trips 4 and 6 are aimed primarily at the geology of the Columbia River Group and associated units.
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