Monday, July 1, 2013

More Notes on the Bonnie Lake Steptoe

Precambrian muscovite-biotite-quartz schist on Bonnie Lake Steptoe Island

While the setting of the Bonnie Lake Steptoe is unique and it was a fun adventure just to get to the island (bonnie-lake-precambrian-schist), my previous post did not provide much information on the Precambrian schist that makes up the island. These Precambrian rocks are highly metamorphosed and so isolated any correlation is a bit tentative so there really is not any brilliance I can add as I am not real familiar with the Precambrian rocks of the area and the exposures are sparse.

Waggoner (1990) suggested that the schist is a completely recrystalized unit of the Pritchard Formation based on the likely protolith of unit being a pelitic sedimentary rock. The Pritchard Formation is a lower member of the Belt Supergroup, but is nowhere nearly as metamorphosed as the Bonnie Lake schist. These rocks were deposited along the margin of North America after a rifting event - that is North America was part of a bigger continent and the part attached to where Washington State is today rifted off to become a another continent. After the rifting the new North America may have looked something like this:

Modified from Bentley's modification of Blakely

Waggoner (1990) describes the one other outcrop of this Precambrian schist within the Rosalia 1:100,000 Quadrant. That outcrop is about 3 or 4 miles south of the Bonnie Lake Steptoe Island. Waggoner's description of that outcrop is completely consistent with the Bonnie Lake outcrop. Waggoner described garnets and sillmanite in thin sections (microscopic). I looked for garnets and alluminum silicates (sillimanite being one) but did not see any in hand specimens or outcrops.

I found that that unit was completely recystalized at least at hand specimen level. The foliation plains parallelled bedding and generally had an approximately N10E strike with a 60 degree dip to the west.  However, I did observe a lots of tight folding and rather spectacular folds. I also observed some north plunging lineations on a few outcrops where exposures were just right. Waggoner noted foliation striking to the northwest and dipping gently to the southwest. She also noted distinct southwest and northeast dipping cleavages, but did not describe any folds, lineations or veins.

General trend of strike and dip throughout the island on well defined bedding and foliation

North plunging lineation on a north striking southwest dipping foliation plain

Kink fold with vein or pressure dissolution

Tight folds like this are common

A ponderosa pine finds a niche within a siltite layer

More tight folds shot through with veins of quartz

The inside portion of this fold had broken and slipped

Steeply dipping schist with Grande Rhonde (lower layer) and
Wanapum (upper layer) basalts of the Columbia River Basalt Group.
The schist is perhaps 100 times older than the 15 million years old basalts

No comments: