Thesis Defense - Fall 2009 - Michelle Dooley

Geologic Mapping and Petrochemical Stratigraphy of Southern Warner Valley, Southern Oregon, USA

Michelle Dooley
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Vic Camp

ABSTRACT
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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 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 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 1:24,000-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 normal faults bounding tilted fault blocks with gentle slopes (5-10o).
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.