New Data on Tertiary Tectonism in Blue Mountains REgion, North-Central Oregon, as Determined from 1 Kirkpatrick Well

Thomas P. Fox, Stephen P. Reidel

The Standard of California 1 Kirkpatrick well, near Condon, Oregon, penetrated 2,440 ft of Columbia River Basalt (CRB), 4,255 ft of John Day Formation-Clarno Formation(?), and was abandoned after penetrating 2,031 ft of Mesozoic sedimentary rocks. The CRB section is entirely Grande Ronde Basalt, and probably contains all four magnetostratigraphic units. Prineville and Picture Gorge basalts, which crop out less than 15 mi away, are absent. Below the CRB are the volcanic rocks of the John Day Formation. Below the ash-flow tuff member 'A' are 560 ft of either John Day or Clarno Formation which, on the basis of petrography, isotopic age dates, and flow compositions, we interpret as a previously unrecognized part of the John Day Formation. Part of a 28 m.y.-old rhyolitic intru ion is interpreted to occur in the Mesozoic rocks and lower John Day Formation-Clarno Formation(?) section.

The Tertiary stratigraphy at the well appears to be controlled by uplift of the Blue Mountains and by local deformation. The John Day Member 'A' occurs at about 2,000 ft elevation near Fossil, Oregon, and in the well at about 3,300 ft below MSL; this suggests that over 5,300 ft of uplift occurred over the last 37 m.y. in the Blue Mountains, relative to the Kirkpatrick area. The area coincides with a steep gravity gradient along the north flank of the Blue Mountains (Riddihough, 1984), which suggests a fault. Uplift began during the Clarno as indicated by paleocurrent directions in pre-Clarno rocks near Heppner, Oregon, and continued beyond the Miocene, controlling the CRB and producing the north-dipping homocline.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.