Historical Geology: Field Trip to the John Day Basin, May 2007

Cassie Strickland, Instructor

The 2007 Historical Geology class traveled to the John Day Basin of east-central Oregon to explore the incredible Cenozoic history exposed there. The John Day basin is famous for its nearly complete Cenozoic record, which chronicles the evolution of mammals in western North America, and the gradual shift from a tropical to semi-arid climate in eastern Oregon. Much of the trip was conducted within the boundaries of John Day Fossil Beds National Monument (JODA).

DAY ONE

We descended down into the basin through flood basalts of the Columbia River Basalt Group (CRBGs) and into the town of Monument. The Monument Dike Swarm (1) is the source for the Picture Gorge basalts, a localized series of flows which are only found in the region. Most basalt flows in the John Day Basin are Picture Gorge basalts (2).

Near Hamilton, we saw our first outcrop of Cenozoic sedimentary rocks, namely the contact between Picture Gorge basalts and the Upper John Day Formation (Haystack Valley Member) (3). The tuffs and tuffaceous claystones (4)of the Oligocene John Day Formation are rich in plant and animal fossils.

We entered the Sheep Rock Unit of JODA. The first stop was in the Foree area, where the distinctive blue-green tuffaceous claystones of the Turtle Cove member of the John Day Fm. are well-exposed (5). Looking westward across highway 19, the Picture Gorge basalts cap the entire John Day formation (6-see picture at top of page).

After walking the trails, we stopped at the Thomas Condon Paleontology Center (7). Group Photo in front of Sheep Rock, for which the park unit is named, left to right: Irma Bissonnette, Karen Roberts, Warren Clark, Lindsey McCallum, Michael Bermudez, Samuel (Samo) Wise, Lisa Cooper, Daniel Escobar and Lester Smith (8). After leaving the center, we passed through Picture Gorge, for which the CRBG basalts are named. The Gorge itself is named for the numerous Indian pictographs on the basalt walls (9).

We hunted for fossils in a lacustrine member of the Mascall Formation (10). The rock here is completely composed of diatoms. Students learned that banging rocks & getting dirty is fun, but better if you actually remember to bring a hammer (11,12). We camped up in the mountains bordering the east side of the basin, at Magone (pronounced Magoon) Lake. Lester couldn’t resist bringing one more rock back for bashing (13).

DAY TWO

On the second day, student prospected for fossils in Cretaceous shales of the Hudspeth formation. This locale is known for ammonites, but we only found sea urchins. Irma found an incredible specimen (14)..) After that we headed over to the second portion of JODA, the Painted Hills unit (15- Karen Roberts in foreground). The Painted Hills themselves are John Day formation, but largely unfossiliferous. The multi-colored claystones of the Painted Hills received their brilliant hues during diagenesis. Each different color represents a different combination of elements that impart their distinct color to the layer. Notice Sam’s bare feet (16). Punishment for his cruelty to other students: hiking at the Painted Hills, sans shoes.

After a bit of geology @ 55 m.p.h. enroute from Mitchell to Fossil, we arrived at our final stop, Wheeler High School in Fossil, OR. Students collected in John Day formation (Bridge Creek flora) (17- Lisa Cooper, right side.) Several leaves were found, including metasequoia, and Michael Bermudez even found a metasequoia cone (18). Our scheduled stop at the Clarno Fm. was cancelled, and this concluded the trip. This trip will be held again on May 3^rd & 4^th, Spring Quarter, 2008. If interested, sign up for Historical Geology.

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