PRELIMINARY GEOLOGIC MAPPING AND PETROCHEMICAL STRATIGRAPHY ACROSS THE WARNER VALLEY, SOUTHERN OREGON

The tectonomagmatic evolution of the northwestern margin of the Basin and Range Province is well expressed by the largely bimodal volcanic stratigraphy and structure of the south Warner Valley in southern Oregon. 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 bimodal volcanism related to Basin-Range extension. The Warner Valley, a north-south trending Basin-Range extensional graben, exposes hundreds of meters of lava flows and ignimbrites, which include Oligocene-Miocene calc-alkaline-to-mildly alkaline volcanics, mid-Miocene tholeiitic eruptions of Steens basalt, and Miocene-to-Recent eruptions of impressive rhyolitic ignimbrites and high-alumina olivine tholeiites (HAOT). Currently, the only geologic map for the Warner Valley area is the 1:250,000-scale reconnaissance map of Walker and Reppening (1965). The field area spans six USGS 1:24,000-scale quadrangles. It 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 footwall of the Warner Valley is delineated by boundary faults with over 600 meters of offset, whereas the valley floor contains smaller normal faults with small offsets bounding tilted fault blocks with gentle slopes (5-10o). Here, we describe our recent field mapping in the Warner Valley together with major- and trace-element geochemistry of the volcanic succession with the intent of making a contribution to the sparse stratigraphic and petrologic database in this part of Oregon.