Geology of part of the northern half of the Bend quadrangle, Jefferson and Deschutes counties, Oregon

Abstract

The mapped area is part of the middle Deschutes Basin of central Oregon, which lies in the northwestern corner of the High Lava Plains physiographic province, between the Cascade and Ochoco Mountains. It is underlain by Cenozoic lavas, tuffs and sediments of continental origin. The oldest rocks in the sequence are pyroxene andesites, which crop out as prominent inliers within the area, and dense, fine-grained olivine basalts, which are exposed along the northeastern boundary of the area at Trail Crossing. They are assigned to the late Eocene - Early Oligocene Clarno Formation. Silicified tuffs of the early to middle (?) Oligocene - early Miocene John Day Formation crop out near Terrebonne, and overlie the Clarno basalts with angular unconformity east of Trail Crossing. The thickness of Clarno and John Day rocks is undetermined. The Deschutes Formation, a heterogeneous sequence of sediments, tuffs and lavas, overlies the older rocks with angular unconformity, and reaches an exposed thickness of 600 feet along the northern boundary of the area. The best exposures occur along the Deschutes and Crooked Rivers, which flow northward through the area in steep-walled canyons. On the basis of stratigraphic position, paleontological evidence and radiometric age dating, the formation is considered to be early Pliocene to early Pleistocene in age, and correlates directly with the Rattlesnake Formation of central Oregon. Members of the Deschutes Formation include prominent deposits of diatomite, and a mudflow breccia which contains enormous boulders of Clarno andesite. Epiclastic sediments include fluvial sands, conglomerates and breccias, and tuffaceous eolian sands. Individual beds are generally discontinuous wedge- or lens-shaped units. Pyroclastic sediments include thin, intercalated beds of fine vitric ash and coarse, cindery ash or lapilli, pumice lapilli beds, vitric airborne tuffs, and volcanic breccia. Individual beds are usually widespread and continuous, with a sheet-like form. Nine ash flow tuffs are recognized in the formation. They form widespread sheets, and often crop out as prominent ledges or benches within the canyons. Most of the tuffs are relatively thin, but some exceed 100 feet in thickness. Index of refraction studies indicate that silica percentages range from 56% to 68f, and the tuffs are probably dacitic in composition. Basalt flows are interbedded with Deschutes sediments, intracanyon to them, and also form extensive plateaus capping the formation. Most of the lavas form thin sheets, but others are tabular and range up to 200 feet in thickness. The plateau-forming flows at Terrebonne, and also the oldest interbedded flow, are characterized by a distinctive diktytaxitic texture and an abundance of olivine phenocrysts. However, most of the basalt flows are relatively sparse in olivine, and are sparsely to abundantly porphyritic. The presence of cinder and lava cones, basaltic intrusives, buried cinders and the Localized nature of individual flows indicate that most of the basalts were erupted from cones and fissures within the basin. The youngest rocks in the sequence consist of diktytaxitic olivine basalt, herein designated the Crooked River basalt. This unit covers the plateau in the southeastern part of the area, and caps prominent benches within canyons in the northern part of the area. It ranges in thickness from less than 10 feet on the plateau to 650 feet in the canyons. It is separated from the Deschutes Formation by a striking erosional unconformity, and is tentatively considered to be late Pleistocene in age. Most of the Deschutes strata are horizontal or gently inclined toward the center of the depositional basin. They were deposited on a surface of variable relief, and the attitudes of individual beds reflect the changing slope of the original surface. Lava-covered plateau surfaces in the southern and central part of the mapped area dip 10 to the north and northeast. A series of parallel normal faults, located in the southwestern part of the mapped area, trend N. 250 - 350 W. The present topography is due to fluvial erosion of the volcanic terrain.