Monday, July 16, 2012

Basalt Injections Near Brewster

The Waterville Plateau area in central Washington is full of geologic highlights. I have posted on few: moment-in-deep-time-back-to-back, giant-ripples, field-of-erratics, washington-states-other-mounds, glacial-erratics-near-grand-coulee and mini-badland-southeast-of-bridgeport. There are plenty of other sites associated with the Columbia River Basalts, the continental ice margin and the Missoula Floods. 

On a recent trip I stopped along the Big Bend (one of several) east of Brewster on the Columbia River to admire the glacial terraces on the opposite side of the river.

Terraces along lower Okanogan River and Columbia River

The terraces developed along the ice margins of the Okanogan ice lob approximately 13,000 years ago. The level terraces are excellent apple orchard sites if water can be delivered to the terrace. The Columbia at this point opens up into a rather broad valley compared to narrow canyon reaches both up stream and down stream. The valley is filled with water backed up from Wells Dam downstream.

As nice as the view across the river is at this location, there is a spectacular road cut on the opposite side of the road.

The outcrop is mostly Cretaceous to Jurassic granodiorite that has been partially very sheared from faulting with numerous injections of basalt magma. The granodiorite is referred to as the Summit-Frazer Complex is a southern extension of the Okanogan Block - a fairly coherent band of grantic rocks that extends well to the north into British Columbia. This is one of the southernmost exposures of this accreted terrane that was added to the North American margin. Basalt dikes cutting through granitic rocks is not unusual, but in this case the dikes have ends with the injections frozen in place.

Thin sliver of basalt within highly sheared granitic rock

Another thin sliver of basalt within shear zone

Close up of basaltic injection

Unsheared granodiortite country rock

Mini columnar jointing along the basalt dike chill margin

I really do not know much about these rocks. The granitic county rock is much older - on the order of 120 million years. The shearing and basalt injections may be associated with extension in the area in the Eocene roughly 45 million years ago. Complex rocks with a well exposed location.

Location map. A very wide pull out is located along the river adjacent to the outcrop.

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