GEOLOGIC MAP OF THE NORTH HALF OF THE LOWER CROOKED BASIN OF CENTRAL OREGON REVEALS A COMPLEX EOCENE TO PLEISTOCENE VOLCANIC HISTORY

The Geologic Map of the North Half of the Lower Crooked Basin (LCB) encompasses an area of 2338 km² east of the Cascade Range in central Oregon. New geologic mapping in the LCB is the result of a multiyear project undertaken to determine the geologic conditions controlling the distribution of groundwater resources, character of geologic hazards, and location of potential aggregate/mineral resources. The construction of detailed geochemical and geochronological data sets, application of GIS techniques, and use of high-resolution lidar base maps has enabled us to reveal a complex volcanic history for the LCB.

The LCB has been a locus of magmatism for the past 47 million years, including the formation of two large-scale Paleogene rhyolite calderas and eruption of Neogene basaltic lavas from local vents. The earliest magmatism in the LCB is recorded by 46.4 Ma high-MgO alkali-olivine basalts and ca. 44 to 39 Ma intermediate-silicic calc-alkaline volcanic and intrusive rocks correlative with the Clarno Formation. Clarno volcanism in the LCB peaked at 41.8 Ma with the eruption of the tuff of Steins Pillar and formation of the 16 × 11 km Wildcat Mountain caldera. Clarno rocks are succeeded upward in the LCB by a late Eocene-Oligocene bimodal assemblage of tholeiitic mafic lavas and rhyolitic flows, domes, and ash-flow tuffs correlative to the John Day Formation. John Day rocks in the LCB, in part, make up the Crooked River caldera, a large-scale 41 × 27 km, multicyclic volcano-tectonic depression formed between 29.7 and 27.6 Ma. Sedimentary rocks of the early-middle Miocene Simtustus Formation and 15.7 Ma Prineville Basalt overlie John Day and older rocks by angular unconformity, infilling topographic lows developed across the older Crooked River caldera. Younger olivine basalt flows of the 3.3 to 8.8 Ma Deschutes Formation vented within the western LCB were largely emplaced as intracanyon lavas following channels incised into older rock. Intracanyon Deschutes flows record the late Neogene development of a longitudinal, north-flowing ancestral Crooked River that closely approximated the present-day drainage. Pliocene and older rocks are overlain in the western part of the LCB by a cover of Pleistocene basaltic lavas, erupted from nearby Newberry Volcano. Bedrock is locally covered by Quaternary surficial deposits across the LCB.