Geology of the Okanogan Highlands 2011 - Stop 4

Continue along Chesaw Road for 1.4 miles until you see a pull-out on the right. The fourth stop provides a dramatic outcrop of accreted terrane, with metamorphic rock that is still somewhat layered.

Explained Dawes, "This was sedimentary rock; it has been slightly metamorphosed. It got buried, squeezed, and especially heated enough to partly recrystallize and grow some new minerals, which show up as shiny mica flakes on the flat surfaces."

"This is not high enough grade metamorphism to have melted itself; it probably got intruded by one of these other, bigger bodies of magma nearby... probably forcing injection into it along the cracks somehow. You can still see the original sedimentary bedding, and it may even be somewhat of an... ocean floor mud and sand sequence. This is from one of the accreted terranes..."

The diverse rock types at this outcrop warrant a closer look.  Dawes continued, "Being careful of the traffic, see if you can see beds in here where it changes from more fine-grained, dark rock, to lighter colored rock, and so on. Those may be original sedimentary beds. During metamorphism, or at some stage maybe even from the Bonaparte Mountain pluton or Buckhorn Mountain pluton, there were some little intrusions that sent some fingers into part of this body of rock. Those are the lighter colored sections [seen on the geologic map in the handout]. 

Dawes helped the group understand the history of this rock, found in the outcrop. "The black part is igneous rock that intruded and crystallized, but it is not your standard granite composition... it's a black granite. Some molten, metal rich rock got into some metamorphic rock along cracks, and crystallized into this dark, igneous rock. There's a bit of standard granite in there too. The rest might be the original metamorphic rock that got caught up in the intrusion." 

Look for Schist, with larger mica flakes...

...and fine layers. "Schist is a German word for layered metamorphic rock with large enough mica flakes to see all over the surface... The big shiny mica flakes were not original sedimentary; they grew when the rock was recrystallized during metamorphism. You can see form the layering that these were originally sedimentary rock from the floor of the ocean, probably near one of these island arcs... and the whole mass got shoved in and the beds have been tilted and metamorphosed, etc., since then. This is Permian in age, 250-300 million years old."

The hydrochloric acid test confirms that marble is also present here.

Pegmatite with white mica (coarse-grained granite with white mica). 

Another marble sample.