The area of study is located on the north flank of the northeast-southwest trending Blue Mountains Anticlinorium in north-central Oregon. The primary geologic problems investigated were: 1) the age and stratigraphic position of leaf-bearing tufts mapped as Oligo-Miocene John Day Formation but surrounded by andesitic basalts of the Eo-Oligocene Clarno Formation, 2) the age of "olivine basalt" mapped as John Day Formation by some workers but referred to the Clarno Formation by others, and 3) the petroleum potential of the Fossil area.
To resolve these three problems I mapped a 25 square mile area surrounding Fossil at 1 :24,000 scale. This detailed mapping revealed that the leaf-bearing shales of the John Day Formation are exposed along the axes of two narrow synclines separated from John Day Formation outcrops to the north by a breached dome, herein named the Fossil Dome. The John Day leaf-bearing shales directly overly Clarno andesitic basalt.
Petrography and x-ray diffraction of rocks mapped by Robinson (1975) as John Day Formation "olivine basalt" demonstrate that it is a rhyodacite porphyry containing pigeonite and a high alkali content. In mesoscopic view the rhyodacite porphyry resembles and is probably genetically related to rhyodacite flows and intrusions on Black Butte to the west.
A high-titania olivine basalt dike found at the western edge of the map area is similar to basalt described by Robinson (1969) as part of the John Day Formation. However, this basalt crosscuts John Day stratigraphy and was dated at 15.5 +/- 0.8 M.Y. In addition, an x-ray diffraction pattern of the basalt shows that its titania occurs in ulvospinel whereas titania in Robinson's (1969) basalts occurs in ilmenite. These factors suggest that this basalt is a feeder dike for Grande Ronde Basalts of the Columbia River Basalt Group.
Oil and gas shows in an oil well and water wells drilled in and near Fossil and a soil hydrocarbon anomaly reported by Sidle and Richers (1985) suggest the possible presence of petroleum in the subsurface. Besides the Fossil Dome, several other possible petroleum traps are present.
Lanny H. Fisk
H. Paul Buchheim
Ivan G. Holmes
Master of Science (MS)
Year Degree Awarded
Date (Title Page)
Library of Congress/MESH Subject Headings
Geology -- Oregon -- Wheeler County
Loma Linda University Libraries
This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.
Riseley, Patrick W., "Geology in the Vicinity of the City of Fossil, Wheeler County, North-Central Oregon" (1989). Loma Linda University Electronic Theses, Dissertations & Projects. 1011.
Loma Linda University Electronic Theses and Dissertations
Loma Linda University. Del E. Webb Memorial Library. University Archives