THE JOHN DAY FORMATION
In central Oregon a distinctive group of rocks between the Eocene Clarno Formation and the Miocene Columbia River Basalt Group is referred to as the John Day Formation. It exists in three distinct regions: (1) south of the Ochoco Mountains, (2) west of the Blue Mountains Uplift, and (3) east of the Blue Mountains Uplift.
Focus here is on the east region, located northeast of Mitchell, which includes the Painted Hills Unit of the John Day Fossil Beds National Monument,
well-known for colorful scenery and beautiful photographic opportunities, especially during clear-day early-mornings.
Colorful John Day beds and very interesting exposures also occur east of Mitchell along the John Day River.
The formation consists chiefly of silicic air-fall and ash-flow tuffs and it has been a subject of study since first exploration by Thomas Condon in 1869.
Subsequent exploration has established four members: (1) the Big Basin Member is dominated by red oxidized tuffs and contains traces of a welded ash-flow sheet known as unit “A” from the western region.
Next above is (2) the Turtle Cove Member which is dominantly green to greenish gray and contains a widespread ash-flow sheet known as the “Picture Gorge Ignimbrite”,
overlain by (3) the Kimberly Member of light gray to buff colored bedded tuffs, and
(4) the Haystack Valley Member containing light-gray lacustrine tuffs with conglomerates.
Members 2 and 3 have produced the well-preserved fossils of plants and animals that have become recognized as one of the world’s best records of mid-Tertiary evolution. The Painted Hills unit has yielded leaf fossils from ginkgo trees and fronds of metasequoia while the Kimberly unit along the John Day River has been famous for demonstrating evolution of hoofed animals such as oreodons and horses.
The Picture Gorge Ignimbrite contains two distinct cooling units with fragments of pumice in a matrix of fused glass shards.
This welded tuff has been estimated to have covered nearly 2000 square miles, and it’s age is 28.7 (Ar/Ar) Ma.
The John Day formation ash beds accumulated for about 15 million years, based on well-documented radiometric ages from 37.4 to 22.7 Ma. This interval corresponds to the time of pronounced activity in the Western Cascade volcanic belt. Pyrogenic minerals in the John Day air-fall tuffs (pyroxenes, calcic plagioclase) match those of Cascade rocks while the crystals common to John Day ignimbrites (sanidine, quartz) are found in intrusive bodies east of the Cascades. It has been proposed and generally accepted that John Day tuffs of all three regional members represent ash-falls carried eastward by prevailing winds from Western Cascade eruptions.
Investigation of the John Day ash reveals separate layers that were deposited in beds of inches to a few feet thick and then were subjected to long-continued alteration, weathering, and development of paleosols. In some localities the accumulated layers were eroded and then covered by more ash-fall. The abundant fossil records suggest an environment of open woodlands with well-developed stream systems. The general impression of lengthy periods of quiescence briefly interrupted by widespread distribution of volcanic ash implies that Western Cascade eruptions were infrequent but extremely violent.
Many dikes and lava flows of basalt that occur near Mitchell correspond to those of the western John Day region. Their chemical analyses reveal high content of FeO and TiO2 with crystals of ilmenite visible in the rocks. A lava flow of this type of basalt can be seen east of and adjacent to the highway between Mitchell and Painted Hills.
Another more extensive example of this basalt occurs along Highway 207 in the Donnelly Basin south of the John Day River. Also, there are 17.8 aggregate-miles of basaltic dike segments trending NW-SE through the Mitchell area.
Their age is 34.1 Ma and they are of the same high Fe-Ti composition. These dikes are easy to recognize by their distinctive red, oxidized color and prominent, resistant margins. A large body of Lawson Mountain intrusive dacite, 8.7 miles east of Mitchell, does not appear to be related to any John Day air-fall or air-flow tuff but is of late John Day age, 22 Ma.
John Day tuffaceous beds and Picture Gorge Ignimbrite both dip 60 degrees westward at a location named “The Blue Banks” (actually green) 15 miles north of Mitchell. The overlying Picture Gorge basalts are horizontal.
This represents an unusual structural disturbance of John Day rocks. I have traced this upturned, resistant ignimbrite unit several miles south from blue banks past Girds Creek to the lower south slopes of Sutton Mountain without finding a trace of a fault system, landslide blocks, or any satisfactory explanation for this feature. There are many as yet unsolved problems revealed in the field geology of the Mitchell region.