August 28, 2022

GSOC Strawberry Mountain Field Trip

August 28, 2022/ Carole Miles

Recap of the GSOC Strawberry Mountain Faults Field Trip, August 5-7, 2022

An example of a USGS seismic hazard map. This one maps the relative earthquake hazards in the U.S.

This is a LiDAR image of the Strawberry Mountain Fault.

Field Trip led by: Andrew Dunning, PSU Geology Masters student
Co-facilitated by: Denny Chamberlin and Carole Miles
story text by Carole Miles
photos by Carole Miles, Denny Chamberlin, and Kate Ely

The Strawberry Mountain fault was discovered in 2019 by DOGAMI’s Ian Madin upon reviewing new LiDAR maps of the area. Andrew Dunning, looking for a project for his master’s thesis at Portland State University, realized that a lack of high-quality seismic data in this area of Oregon may have erroneously led to an inaccurate assessment of the seismic risk in Eastern Oregon on the USGS National Seismic Hazard Map. He has been working on gathering data related to timing and offset of the Strawberry Mountain fault for the past two years. He was excited to take GSOC members into the field to show us what he has learned.

We met at the historic Cant Ranch in the John Day Fossil Beds National Monument to organize, then moved on to our first stop — the Mascall Overlook for a view north to Picture Gorge (Sheep Rock is the point between the notch).

Next, we stopped along Hwy 26 for a rare, although somewhat unimpressive, exposure of the John Day fault which controls the morphology of the John Day valley. The fault line is evident where the red rock abuts the black rock.

We stopped at the Kam-Wah-Chung State Park to see one of the oldest buildings in Oregon and the oldest Chinese-American business in the Northwest, built in 1865. It served the Chinese residents who came to work mining gold in Eastern Oregon during that era. It was also a good spot to see the numerous landslides and all the major rock units of the area. Landslides are common due to the meandering John Day River which undercuts and over-steepens the soft rocks and clays of the John Day, Mascall, and Rattlesnake formations. The cap rock of Rattlesnake tuff and the low-angle dip of the formations creates perfect conditions for landsliding.

We ended Day 1 with a stop at the foot of the Strawberry Mountains to look at glacially-carved valleys and alluvial fans.

Day 2 was spent hiking the Onion Creek trail to the location of Andrew’s research study area along the Strawberry fault and helping with auger core sampling and trench digging in search of the fault line.

Starting the 1.5 mile hike at about 5,000 feet elevation…

…and working our way up to about 6,500 feet elevation. We observed the natural re-growth of trees and native plants since a large wildfire in 2002.

Including lots of Mariposa lilies (and bee friends).

We learned how to make an auger core…

Those with strong backs helped to dig a trench in search of the fault line.

And analyze the soil layers found at various depths (including a 10 cm layer of Mt. Mazama ash!).

Andrew jumped in to do some of the detail work, still unsure that we actually found it.

Another 1.25 miles up the trail took us to a beautiful overlook of the John Day valley to north.

And a nice look at the Strawberry Mountain volcanic layers to the south.

On Day 3, we enjoyed the easier 1.4 mile hike up to Strawberry Lake, a glacially-carved lake that has been dammed by a massive landslide at the north end. There is no outflow to this lake — it flows underground through the loose landslide debris and, about 30 days later, comes out of the ground as springs close to the Strawberry campground.

This outcrop at the northwest corner of the lake is where the landslide came from. You can also see the beige layer of Dinner Creek Tuff sandwiched between layers of red scoria.