Members of the CwM team recently traveled out to east-central Oregon for field work, stopping along the way to view some of the major geologic formations of the western Blue Mountains. Their trip took them through the towns of Fossil, Spray, Kimberly, Dayville, and Mitchell. In the process, they drove across 45 million years of geologic history from the Clarno Formation to the Rattlesnake Formation.
Driving south from Fossil, the team encountered several crumbling outcrops of the Clarno Formation, which consists of pale andesite and rhyolite lava flows, ash-fall deposits, and volcanic breccias. The volcanic activity that created the Clarno Formation occurred between about 55 to 40 million years ago, which predates the activation of the Cascade volcanoes. Several debris flow deposits within the Clarno preserve the seeds and leaves of palm and avocado trees, suggesting a tropical climate during this period.
The famous Painted Hills are located near Mitchell, due south of Fossil. The hills are comprised of alternating layers of bright red, yellow, gray, and black paleosols (ancient soil) that were formed in a large floodplain between about 38 and 32 million years ago. The tropical climate represented in the late Clarno Formation continued into this period but began to shift. Wet and warm periods created bright red layers of soil, rich in iron and aluminum. Yellow soils, where metals were leached out, represent shifts to cooler and drier climate. Though not visible in the image above, both the red and yellow soil layers are often dotted with 2-4 foot-wide black patches of organic rich soil. These patched formed where dense vegetation was clustered throughout the floodplain.
On the east side of Highway 19 between Kimberley and the John Day Highway, the pinnacle of Sheep Rock can be clearly seen for several miles. The red, greenish-gray, and pink layers that form the base of Sheep Rock belong to the John Day Formation. The red claystone layer in the foreground is the Big Basin Member, the lowest unit of the John Day Formation, which was formed 40 to 32 million years ago (same time as the Painted Hills). Moving upward on Sheep Rock you next reach the greenish-gray siltstone of the Turtle Cove Member (32 to 26 million years). Cutting across the middle of Sheep Rock is the dark brown Picture Gorge Ignimbrite, which has been precisely dated to 28.8 million years old. Finally, the upper Kimberly Member of the John Day Formation appears as a pale pink claystone that was formed 26 to 24 million years ago and is capped by the Picture Gorge Basalts that hold up the pinnacle.
The Picture Gorge Basalt Flows are a member of the much larger Columbia River Basalt Group that formed about 16 to 14 million years ago. The Picture Gorge flows originated from a series of fissures that were formed due to tectonic pressures from the subducting Farallon Plate. A total of 61 separate eruptions occurred roughly every 15,000 years, eventually producing the 1,300 foot-thick Picture Gorge sequence covering about 2,500 square miles. The John Day River has cut through many of these flows in Picture Gorge along Highway 26 and Highway 19.
The Picture Gorge basalt flows were not the only volcanic activity during this time. Starting at the same time and continuing after the flood basalts ceased, nearby volcanoes erupted and deposited hundreds of feet of ash across the region. The bright white Mascall Formation, seen here in an outcrop near Dayville, was formed between 16 and 12 million years ago from andesite and rhyolite ash. Much of the ash was deposited in large, shallow lakes or was reworked by meandering streams. The resulting volcanic tuff is very light weight and has a chalky consistency. For several million years after the Mascall Formation was deposited, the region experienced uplift and significant erosion that erased about 4 million years-worth of rock. Capping the Mascall Formation across the region is a layer of mixed cobbles, gravels, and silt that belong to the Rattlesnake Formation. This conglomerate was deposited about 8 to 5 million years ago by energetic streams flowing from the recently uplifted mountains nearby.
Sources:
Bestland, E.A., Hammond, P.E., Blackwell, D.L.S, Kays, M.A., Retallack, G.J., & Stimac, J. (2002). Geologic Framework of the Clarno Unit, John Day Fossil Beds National Monument, Central Oregon. State of Oregon Department of Geology and Mineral Industries. Open-file Report O-02-03.
National Park Service. (2013). John Day Fossil Bed National Monument, Sheep Rock Geology. U.S. Department of the Interior.
Schlicker, H.G. & Brooks, H.C. (1975). Engineering Geology of the John Day Area, Grant County, Oregon. State of Oregon Department of Geology and Mineral Industries. Portland, Oregon.
Thayer, T.P. & Brown, C.E. (1966). Geologic Map of the Canyon City Quadrangle, Northeast Oregon. United States Geological Survey.
Thayer, T.P. (1972). Geologic Setting of the John Day Country, Grant County, Oregon. United States Geological Survey.