Geologic Field Trips in Northern Oregon and Southern Washington Classic Reprint

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 Port land lies the north-south trending Cascade Range, 0 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 geol09y 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 ge0109y 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 Iithospheric 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.

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