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Description
The tectonomagmatic evolution of the northwestern margin of the Basin and Range Province is well expressed by the mafic-to-intermediate-to-silicic volcanic stratigraphy and structure of the south Warner Valley in southern Oregon. The Warner Valley, a north-south trending Basin-Range extensional graben, exposes hundreds of meters of Java flows and ignirnbrites, which include Oligocene-Miocene calc-alkaline-to-rnildly alkaline volcanics, mid-Miocene tholeiitic eruptions of Steens basalt, and Miocene-to-Recent eruptions of impressive basaltic andesite, trachyandesite, and trachydacite flows and tuffs, rhyolitic ignimbrites, and high-alumina olivine tholeiites {HAOT). The mid-Miocene genesis of the voluminous Columbia River Basalt Group (CRBG) began with the eruption of Steens basalt in southeastern Oregon, and was followed by effusive intermediate-to-silicic volcanism related to Basin-Range extension. Previously, the only geologic map for the Warner Valley area is the 1 :250,000-scale reconnaissance map of Walker and Reppening (1968). The field area spans six USGS I :24,000-scale quadrangles and contains a diverse volcanic sequence spanning an age range of - 20 m.y. This entire succession has been faulted and offset by both NW- and subordinate NE-trending normal faults. The NE-trending faults are associated with Basin-and-Range extension that began in the late Miocene to early Pliocene, whereas the NW-trending structures form a slightly older set of faults associated with the Eugene-Denio fault zone. The footwall of the Warner Valley is delineated by Basin-and-Range boundary faults with over 600 meters of offset, and the valley floor contains smaller nonnal faults bounding tilted fault blocks with gentle slopes (5-10°). The focus of this study was to produce a detailed field map across six 1 :24,000-scale quadrangles in the southern Warner Valley, and to describe in detail the field characteristics of the volcanic succession, with the intent of making a contribution to the sparse stratigraphic and petrologic database evident in this part of eastern Oregon.
