U-PB AND COCCOLITH AGES FROM THE OREGON COAST RANGE - HOOKING SILETZIA UP TO THE GEOMAGNETIC POLARITY TIME SCALE

The Oregon Coast Range contains a tremendous thickness of Paleocene and Eocene oceanic tholeiitic and alkalic basalts thought to represent an oceanic terrane accreted to North America in the Eocene. It has been difficult to date these volcanic rocks due to alteration and lack of suitable mineral phases. We dated the Siletz River Volcanics (SRV) and younger flows by obtaining U-Pb ages from zircons (Zr), and we compare our ages to previously determined magnetic polarity and coccolith ages. Near Roseburg, reversed polarity pillow basalt is interbedded with coccolith-bearing sediments referrable to Zones CP 8/9 and 10 (C 24r, ~55-53.4 Ma), consistent with recently reported Ar/Ar ages of D. Pyle and R. Duncan. In the type area and in the Tillamook Highlands to the north, the SRV is interbedded with coccolith-bearing sediments referrable to CP11. A sparse Zr fraction from tuffaceous CP11 interbeds near the top of the SRV gives a concordant age of 52±1 Ma, within CP11. Other fractions show inheritance. Rhyolitic ash flow tuff near the top of the SRV section contains a single zircon population with an age of 50 Ma. Tyee Formation turbidites (CP12a,b) overlie the type SRV. Detrital Zr populations from the mixed polarity Tyee formation in the Roseburg area have a young age peak at 49 Ma, consistent with the CP12a/b boundary and the C22n/r boundary in the GPTS. Tuffaceous sediments of the overlying middle Eocene Yamhill Fm. contain a tuff bed with a concordant Zr U-Pb age of 46.5±0.2 Ma, consistent with its CP13-14a coccolith zones. A composite mafic sill swarm (mostly Zr barren) intrudes the Yamhill and is cut by dikes of the younger Tillamook Volcanics. The reversed polarity Tillamook Volcanics contain rhyolite near the top of the section with a-U-Pb age of 41.5±0.5 Ma, based on a single concordant fraction. This is consistent with previous ages and CP14a coccoliths at the base of the subaerial flows, suggesting eruption during Chron C18 or 19r. Magmatism continued with the eruption of the Yachats basalt and the basalt of Cascade Head between about 37 and 33 Ma, based on earlier Ar/Ar ages. One of the sills cutting the Yamhill gives a concordant Zr U-Pb age of 35.3±0.6 Ma. Together, the ages and paleontologic zonation are consistent and allow us to correlate mafic magmatism of Siletzia with Eocene reorganization of plate motion in the NE Pacific.