Geology Term Project

Geology Term Project

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Mysterious Swakane Terrane

The topic of my term project is study of the Swakane Terrane. My field site is specifically located along Highway 97, which runs along the west side of the Columbia River about past 3 miles north of Wenatchee. While the terrane itself stretches from Wenatchee north to Chelan and west towards Peshastin, the best perceptible locations are along Highway 97. While travel up Swakane Canyon Road, about a mile past Rocky Reach Dam, provided interesting observations, I chose my site location about a mile preceding Rocky Reach Dam, along a left-hand turnout that runs for several hundred feet inside the guardrail. The site is located before the last curve in the road before the dam, and the entry to the turn-out is furthest south from the Dam reference point.

See map here

Photo 1

Overview of site

My site presented generous tangible examples of Swakane characteristics I had been researching previously. The Swakane terrane is an old formation of metamorphic rock with a striking uniformity and a mysterious past. The rock is either believed to have been originally sandstone or volcanic. Metamorphic change has made distinguishing which difficult. But regardless of content, the formation is historic due to its age. Through testing of the zircon crystals - which were the original sand grains if the rock was sandstone, or crystals formed during volcanism if the rock was volcanic the date of origin appears to be around 1,600 million years ago or older Precambrian. The age and location combination of this formation is distinct, as all other rocks of that age are much further east on our continent because much of the Pacific Northwest did not exist in its present state then. This points to the Swakane Terrane having been an accreted terrane, with evidence of the semi-overlying Chelan Mountains terrane pointing to the occurrence as possible. The terrane was possibly accreted between the older Precambrian age of origin and the Cretaceous Period when its metamorphic event occurred as part of the Metamorphic Core Domain around 90 million years ago. Another hypothesis ventures that the base rock is really Mesozoic in date and contains large amounts of Proterozoic zircon. The same accretion and metamorphic event timeline could still apply with this model.

Photo 2

Example of slaty cleavage

The major rock types of the Swakane Terrane are the original substance rock and Swakane Biotite Gniess - biotite meaning containing mica and gneiss being a regionally, high temperature metamorphisized rock with striped foliation made up of different minerals. Substances contained in that foliation are marble, hornblende biotie gneiss, amphibolite, and other gneiss with marked contents.

Photo 3

Example of foliation (white horizontal stripes across reddish rock in center of photo).

The characteristics of my site showed large amounts of the uniform base metamorphic rock. The rock gives the entire outcroppings along Highway 97 their shape with slaty cleavage, schistose foliation, and gneissic foliation. Bands of the Swakane Biotite Gniess can be seen as white or light colored bands appearing as foliation intrusions sporadically along the exposed surfaces.

Photo 4

Close up sample example of foliation

Photo 5

Close up rock sample example - possibly marble?

Photo 6

Close up example of base rock.

This site is a good example of an accreted terrane, the mysterious past old formations can contain, and the characteristics behind commonly viewed formations.

Stratigraphy

Bibliography:

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by Amanda Thomas