A ‘NEW” CHARACTERIZATION OF DEFORMED PALEOGENE VOLCANICS FROM THE PAISLEY HILLS, SOUTH-CENTRAL OREGON

We have begun to characterize a little-known occurrence of Paleogene volcanics in the Paisley Hills and surrounding area of south-central Oregon. The area was first mapped by Walker (1963), who split the strata into a lower mafic section and an upper siliceous section, but did not describe or name the units. Walker used this two-fold division to correlate these rocks to the Clarno and overlying John Day Formations that crop out 150 km to the north. Subsequent work relied upon the detailed thesis mapping of Muntzert (1969).

The lower mafic section contains interbedded basalt flows, breccias, basaltic to andesitic mudflows, local dacite flows, and intrusive rhyolitic to basaltic plugs. The combined thickness of this section probably exceeds 3 km along the Chewaucan River and possibly 5 km on the east side of the range. Muntzert dated a quartz monzonite stock that cuts the lowest (?) part of this unit at 33.6 Ma (K/Ar). These mafic rocks form the core of the Paisley Hills. The overlying siliceous section is comprised of at least ten, variably-welded, quartz-bearing, crystal-lithic tuffs. The combined thickness of the upper unit is greater than 600 m. A similar succession of 15-degree east-dipping tuffs is found on both sides of the core, where they lie with angular unconformity upon the mafic section. New U/Pb ages on zircon from the silicious tuffs place the base at about 27 Ma, with stratigraphically higher tuffs at 25.5 and 25 Ma. The two younger tuffs also have additional age peaks at 26.5 and 28.5 Ma.

There is evidence for both west-directed and north-directed contractional folding of the mafic section, possibly prior to quartz monzonite intrusion at 34 Ma. This folding pre-dates deposition of the silicic tuffs beginning at ~27 Ma. The similar ~15-degree east dips for the silicic tuffs on both sides of the range suggest post-25 Ma domino-style extension on west-dipping normal faults. Basin-and-Range normal faults of Late Miocene-Recent age appear to overprint all earlier events, and are mostly east-dipping. As currently dated, the >34-25 Ma Paisley Hills stratigraphy is equivalent to the 40-25 Ma John Day Fm. (i.e., no Clarno ages yet), the McCulley Ranch through Cedar Pass Volcanics near Cedarville, CA (40-24 Ma), and the Roxy Fm. volcanics (35-25? Ma) in the Western Cascades. Sampling is underway to further refine the age span at Paisley.