Geology of the Okanogan Highlands 2012 - Stop 8

Stop 8: Granite and Breccia

The parking pull-out is on the Bodie Mountain fault, the main fault along the east side of the Toroda graben. At the parking area, the fault is covered by eroded material. The outcrops of solid rock about 200 feet east of here are granite that was fractured and broken into smaller pieces due to movement along the Bodie Mountain fault.

There are veins of epidote and chlorite, green minerals, in the fractured granite, along with some quartz veins. The quartz veins themselves are partly fractured and incorporated into the veins of broken rock and green minerals. As with Stop 2, these are characteristics of faulting in the shallow crust, where rock is brittle and tends to break into angular pieces.

Just 10s of feet to the west of the parking area, the outcrop is a type of rock called a sedimentary breccia (pronounced brech-ee-uh). Sedimentary breccia is a type of sedimentary rock consisting of angular, sharp-cornered fragments that were eroded from rock nearby and carried by water or landslides, or both, to the base of a slope. Presumably, the slope in this case was the Bodie Mountain fault when it was actively uplifting during the Eocene epoch, around 50 million years ago. Look at the fragments of rock in the breccia and see if you can spot pieces of granite that were eroded from the other side of the fault.

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