Petrified wood of southwestern Oregon: Implications for Cenozoic climate change

Over 1900 petrified wood specimens were collected from six localities spanning the Eocene to Miocene along a northeast transect parallel to the dip of the Payne Cliffs Formation and Western Cascades Group in southwestern Oregon. This study also presents new 40Ar/39Ar plateau age data for Cenozoic deposits in southern Oregon. Lower to Middle Eocene deposits yielded 305 specimens of petrified wood from sandstones and conglomerates of the basal part of the Payne Cliffs Formation with only 6% of dicotyledons exhibiting distinct growth rings and none having ring porous or semi-ring porous wood. Middle Eocene exposures just stratigraphically above the first locality produced 278 specimens from the lower to the middle part of the Payne Cliffs Formation, with 66% of the dicotyledons exhibiting distinct growth rings. Two specimens of Palmoxylon were also collected from sediments at this locality. An Upper Eocene exposure produced 792 petrified wood specimens from volcaniclastic sediments with 88% of the dicotyledons exhibiting distinct growth rings. Sediments at this locality also produced one specimen of Cibotium oregonensis (Oregon tree fern) and several specimens of Palmoxylon. Middle Oligocene deposits yielded 218 petrified wood specimens from volcaniclastic sediments of the middle part of the Western Cascades Group with 97% of the dicotyledons exhibiting distinct growth rings. An Upper Oligocene exposure yielded 254 specimens from volcaniclastic sediments of the upper part of the Western Cascades Group with all dicotyledons exhibiting distinct growth rings and 59% having ring porous or semi-ring porous wood. Further, this study establishes a 40Ar/39Ar plateau age date of 24.09±0.24Ma from plagioclase crystals in a tuffaceous sandy to pebbly siltstone at this locality. Finally, Lower Miocene rocks yielded 101 specimens, including 20 dicotyledons, of which 70% were ring porous or semi-ring porous, with most specimens consisting of gymnosperms. The increase over time in the percentages of dicotyledon specimens with distinct growth rings and with ring porous and semi-ring porous wood from this study suggests an overall climatic shift from tropical (Early Eocene) to cool temperate (Early Miocene) in southwestern Oregon. These results are consistent with a similar climatic shift evidenced by paleoecological reconstructions for the Eocene to Miocene of the John Day Fossil Beds in central Oregon.