USING PETRIFIED WOOD TO RESOLVE LONG-TERM CLIMATIC CHANGE FROM THE PAYNE CLIFFS FORMATION TO THE WESTERN CASCADES GROUP (EOCENE TO MIOCENE), JACKSON COUNTY, OREGON

Petrified wood specimens were collected from five distinct sites along a northeast transect parallel to the 10^o to 15^o dip of the Payne Cliffs Formation and from the Western Cascades Group in southwestern Oregon. Petrified wood specimens were identified by examining vascular structures on transverse surfaces with a 20x hand lens; microscopic study of thin-sections was also used to further identify selected specimens.

The oldest site yielded 305 specimens of petrified wood from sandstones and conglomerates of the basal part of the Payne Cliffs Formation (Middle to Late Eocene) with only 6 percent of the dicots exhibiting distinct growth rings and none having ring porous or semi-ring porous wood. The upper part of Payne Cliffs Formation (Late Eocene) provided 278 specimens, with 66 percent of the dicots exhibiting distinct growth rings. In addition, two specimens of Palmoxylon were collected from this locality. The third site yielded 521 petrified wood specimens from Late Eocene to Early Oligocene volcaniclastics with 88 percent of the dicots exhibiting distinct growth rings. This site also yielded two specimens of Cibotium oregonensis (tree fern) and one specimen of Palmoxylon. The fourth site provided 218 petrified wood specimens from volcaniclastics of the middle part of the Western Cascades Group (mid-Oligocene) with 97 percent of the dicots exhibiting distinct growth rings. Finally, the youngest site yielded 254 specimens from volcaniclastic sediments of the upper part of the Western Cascades Group (Late Oligocene to Early Miocene) with all dicots exhibiting distinct growth rings and 59 percent having ring porous or semi-ring porous wood. Finally, this study further supports the age of this site with a ⁴⁰Ar/³⁹Ar plateau age date of 24.09 ± 0.24 Ma from plagioclase crystals in a tuff.

The increase over time in the percentages of dicot specimens with distinct growth rings and with ring porous and semi-ring porous wood from these five localities suggests an overall climatic shift from tropical (Middle Eocene) to 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.