Abstract:
The thesis area is located in north-central Oregon approximately 20 km west of the town of Mitchell. The rocks exposed in the area belong to the Clarno Formation. This study supports the informal reclassification of the Clarno to Group status, composed of an Upper and Lower Clarno Formation. An angular unconformity separates the two Formations. The Lower Clarno Formation is composed of a lower Ross Flat Member dominated by epiclastic sedimentary rocks and an upper Heflin Creek Member composed primarily of pyroxene andesite lavas. The Upper Clarno Formation consists of two conformable units, a lower Q,uartz-bearing Andesite Unit and an upper Pyroxene Andesite Unit. A thin sequence of rhyolite flows is found near the top of the upper unit. Andesite, dacite and rhyolite rocks intrude the lavas and sediments
of the Group. When plotted on a Peacock diagram the Clarno rocks are found to be calcic. The chemical variation diagrams
and AFM plot of samples, however, follow talc-alkaline trends and compare favorably with rocks considered to be talc-alkaline by others. Most of the thesis area lies on the southern flank of the east-northeast trending Sutton Mountain Syncline. The northeast trending Mitchell Anticline is found east
of the project area and the Lower Clarno rocks follow its' trend. The easterly trending strike-slip Mitchell Fault cuts the northern part of the area. Strata of the area show five kilometers of right-lateral movement along the fault. The angular unconformity separating the formations is found in the eastern part of the area and
has been displaced by the Mitchell Fault. The occurrence of andesitic talc-alkaline volcanism near consuming plate margins is well documented and it is possible that the Clarno rocks may be the surface manifestations of subduction. It would be well to remember, however, that "Not all talc-alkaline rocks of the interior of the western United States are related to subduction" (Robyn, 1977).
