The Geology of the John Day basin includes scraps of ancient islands and coral reefs, remnants of 45-million-year-old volcanoes, and the southern-most exposures of great lava flows that covered the Columbia basin.
The rugged landscape of the Clarno Volcanics south of Fossil, the colorful, ash-laden volcanic layers of the John Day Formation between Dayville and Spray, and the dark basalt flows that cap most of the landscape all tell part of Oregon’s story.
Structures: Folding and faulting effect the rocks of the John Day basin.
Faulting has sculpted much of the landscape, and major regional faults include the John Day Fault which follows the south side of the John Day Valley from near Prairie City to Dayville. Near Monument, the Hamilton Fault has uplifted Long Creek Mountain about 1,000 feet. North of Dayville, the Middle Mountain fault has tilted Columbia River basalts and other formations. Small faults are abundant, including a highly visible fault that cuts through the face of Sheep Rock.
Eight major formations compose most of the landscape here. From youngest to oldest, they are:
Mazama Ash – 7,700 years: White chalky-appearing material, usually found in lenses where it was washed by rainfall and streams. Erupted from Mount Mazama in the cataclysmic formation of Crater Lake 7,700 years ago.
Rattlesnake Formation – 6-8 million years: Gravels mostly found filling the John Day Valley east of Picture Gorge. These gravels likely mark a time of uplift of the Strawberry and Aldrich Mountains along the John Day Fault.
The Rattlesnake Formation includes the Rattlesnake Ignimbrite – 7.2 million years: A layer of hot ash, now welded into an almost solid rock, blasted out of a huge, flat volcanic vent near Burns. The Rattlesnake Ignimbrite is the flat, almost horizontal layer that caps Picture Gorge.
Mascall Formation – 14-16 million years: Fine-grained ash and lakes beds, as well as river sediments deposited on top of the Columbia River basalts. The Mascall Formation is about 600 feet thick near Picture Gorge and the Mascall ranch. andscape consisted of several broad basins with lakes and meandering streams that formed atop the last of the basalt flows. These deposits were subsequently covered by successive falls of ash from volcanoes to the west and from the much closer Strawberry volcanics to the east. Alternating between the tuffs – consolidated volcanic ash – are layers of ancient soils and stream deposits that provide evidence of a dynamic floodplain. Many of the vertebrate fossils from the Mascall are found in close association with a prominent layer, the 15 million-year-old “Mascall Tuff.” The deposits of the Mascall strata began when the flows of lava, known as the Picture Gorge Basalts, ceased.
Although dramatic fluctuations in the global climate and regional volcanic activity continued, there were enough phases of moderate climate with ample rainfall and fertile soil to allow the growth of lush grasses and mixed hardwood forests. This savanna-like landscape was characterized by broad, level floodplains with scattered lakes.
Columbia River Basalt (Picture Gorge Basalt) – 16 million years: Hot, dark, and very fluid lava flow that covered most of the Columbia basin between 16.5 and 15 million years ago. These lava flows form the caprock and dark outcrops between Picture Gorge and Service Creek, as well as the canyon of Rock Creek along Hwy 26. . Most of this basalt erupted from vents close to Monument. Today, some of these long, linear vents are visible as “dikes” along the John Day River near Kimberly. The basalts here are known as the “Picture Gorge” basalts for their excellent exposure there.. They are closely related, and erupted about the same time as the voluminous basalt flows that cover the Columbia Basin, line Hells Canyon’s upper rims, form the walls of the Columbia River Gorge, and are found on the coast at Haystack Rock, Saddle Mountain Yaquina Head and other coastal promontories.
John Day Group – 18-39 million years: The colorful rocks of the John Day Group include the red and tan beds of the Painted Hills, the blue beds of Turtle Cove, and the soft, cream-colored rocks of Haystack valley and Crazy Woman Rock. These formations are mostly ancient soils with intermingled lakebeds and river deposits. Their colors reflect they types of clays, amount of moisture, and amount of oxidized iron present—and hence are good indications of ancient climates. Until 1996, these diverse rocks were called the John Day Formation. However, based upon their differences, and broad lateral extent, each separate layer has been recognized as a different formation.
Haystack Formation – 18-25 million years: Light-colored soils and stream gravels from a time of cool, temperate grasslands. Beds are largely composed of volcanic ash erupted from Cascade volcanoes.
Kimberly Formation – 25-28 million years: Light colored ash-rich clay and volcanic tuff beds exposed well near Kimberly. Similar in appearance to the Haystack Formation.
Turtle Cove Formation – 28-30 million years: Green claystones, lake beds and ash/tuffs. Color caused by the presence of two minerals: celadonite and clinoptilolite. Both of these greenish minerals developed in cool, temperate environments with relatively low rainfall—about 20-24 inches annually. These rocks, distinct in color from the underlying reds and tans of the Big Basin Member, reflect a dryer, cooler climate that developed globally 30 million years ago.
Big Basin Formation and Bridge Creek Beds (32-35 million years): The distinctive red and tan layers of the Painted Hills represent the Big Basin Formation.
Clarno Formation– 37-54 million years: Best exposed along the John Day River between Twickenham and Clarno, at the John Day Fossil Beds Clarno Unit, and on the Pine Creek Ranch, the Clarno Formation of rocks are the remnants of Eocene volcanoes. The climate of the Eocene was sub-tropical, with rainfall of as much as 100 inches per year. Clarno volcanic rocks are similar to those of the modern Cascades—and the volcanoes were likely as large. The most common rocks types are andesites and lahars or volcanic debris flows but basalts and dacites are also part of the Clarno package. A thick lake-bed series of sedimentary rocks about 45 million years old is exposed in roadcuts on U.S. Highway 26 about 5 miles east of Ochoco Summit.
Goose Rock Conglomerate – 80-100 million years: Generally considered a deposit of a river system—probably near the mouth, the Goose Rock Conglomerate
Gable Creek Conglomerate: Found west of Mitchell and near downtown Mitchell, the Gable Creek Conglomerate is Cretaceous in age—about 100 to 120 million years. It represents environment probably just offshore from Oregon’s first beach. The cobbles in the rock were eroded from mountainous terrain not far from the beach—perhaps the precursors of today’s Bitterroot Mountains and Blue Mountains.
Hudspeth Shale: Finer-grained, and just slightly younger than the Gable Creek Conglomerate rocks, the Hudspeth Formation is a thick pile of marine sediments, deposited in a deepening sea, as sea level rose during the Cretaceous. It is well exposed in roadcuts along Highway 26 from the Painted Hills Road westward about 6 miles.
Baker Terrane—225 million years: The oldest rocks here are known as the Baker Terrane. These formations were added to North America about 100 million years ago. They include a few outcrops of coarse gray marble and a flaky rock known as schist (blueschist) exposed near Mitchell as well as serpentine found just north of the Foree Basin in the John Day Fossil Beds.