Geological Survey Circular 838 Guides to Some Volcanic Terrances in Washington, Idaho, Oregon, and Northern California

Paul T. Robinson, Department of Esrth Sciences, University of California, Riverside, California 9252)
and
Gerald F. Brem, Department of Earth Sciences, California State University, Fullerton, California 92634

INTRODUCTION

The John Day Formation of north-central Oregon is a widespread, largely pyroclastic unit lying between the Eocene Clarno Formstion and the Miocene Columbia River Basalt (Figure )). The bulk of the unit consists of andesitic to dacitic tuffaceous claystone and air-fall tuff derived from vents west of the present-day outcrops. The age of the formation based on vertebrate fossils and K-Ar dating ranges from approximately 16 to 18 m.y. B. P. Woodburne and Robinson, 1977). Hence, the formation is believed to provide a well exposed depositonal record of middle Tertiary Cascade volcanism not available elsewhere.

Detailed studies of the formation have been carried out by a number of workers in the last 20 years. Studies in the vicinity of the type area, near Picture Gorge, have been carried out by Fisher (1966a, 1966b, 1967, and )968), Fisher and Rensberger (1972), Fisher and Wilcox, (1960) and Hay (1962a, 1962b, 1963). Outcrops in and west of the Blue Mountains have been studied by Peck (1964), Robinson (1975), Robinson and Stensland (1979), Swanson and Robinson (1968), and Woodburne and Robinson (1977). Reconnaissance geologic maps have been completed for most of the unit (Swanson, 1969; Robinson, 1975). Detailed chemical and mineralogical studies of the ash-flow and air-fall tuffs are in progress by the authors.