Early Tertiary Deformation in North-Central Oregon

Abstract

Structural and stratigraphic evidence from the Oligo-Miocene John Day Formation in north-central Oregon indicates that the regional structural trends were formed during early Tertiary time prior to deposition of the John Day Formation, and that a period of deformation began late in John Day time and continued into Picture Gorge Basalt time. The Monument Dike Swarm is at right angles to the tightest structures formed in late John Day time. The short time span involved for this period of deformation, the outpouring of basalt, and the establishment of the right-angle relation of dike swarm to fold trend suggests that the three events are related. The Blue Mountain anticline, which extends from the vicinity of Prineville to the region of the Wallowa Mountains in northeastern Oregon, has had a long-time controlling influence on structural patterns of the region, perhaps since mid-Paleozoic time, and appears to be the result of a major deep-seated earth feature, the nature of which is not known.