Saturday, July 1, 2017

Nanaimo Group Extension Formation on Orcas Island

I got a little glimpse of one small portion of the Late Cretaceous Nanaimo Group on a recent visit to Orcas Island. 

The Nanaimo Group is a group of sedimentary formations exposed along the west side of Vancouver Island and in the islands between the BC mainland and Vancouver Island. Geologic maps indicate there is a bit of Nanaimo Group underling the northernmost portions of the San Juan Islands and along the north part of Orcas Island.  

This group of rocks have been receiving some recent attention by those trying to piece together the western North American margin story. Brown (2012) has interpreted a few scattered outcrops of slightly metamorphosed conglomerates (northwest corner of Blakely and Upright head at the ferry landing on Lopez) as being older Nanaimo Group, suggesting the earliest Nanaimo sediments were incorporated into the final assemblage of the San Juan thrust stack and thus adding a time constraint on when the tectonic terranes that make up the San Juans were stacked up. Mathews and others (2017) and Mahoney, (2016) have attempted to work out some of the source material via dating of detrital zircons in the Nanaimo sediments which have narrowed but not fully resolved the potential sources of some of the sediment. Difficult and complex tectonics that requires piecing a lot of scattered and detailed information together to develop an interpretation. 

My very limited look at the Nanaimo was a chance to see just one tiny bit of the story theses rocks may hold. The part of the Nanaimo Group I was looking at is mapped as the Extension Formation dated at approximately 84 million years. My first view was on a bedrock bald with very thin soils and vegetation.   

As my field work typically involves slopes, I came across a cliff of the bedrock within the forest on my traverse. The slope and cliff were north facing and within the forest so the light was dim despite it being a sunny day.

The Extension Formation is primarily a conglomerate. At this site the conglomerate is primarily cobbles and boulders. Due to the light and steepness of the slope, I ended up with some blurry images, but did get few illustrative shots of the various cobbles and clasts.

Granitic boulder

Andesite boulder

metamorphosed sandstone?

Granitic pebbles

Vein quartz clast?

The short story is that this exposure of the Extension Formation is a clast supported pebble to cobble conglomerate to very coarse sandstone that was deposited in a high energy environment. The size of the granitic cobbles to boulders suggest fairly close proximal to a granitic source material. The Extension Formation has been interpreted to include high-energy deposition in deeper marine submarine canyon and fan facies in northern areas of Nanaimo Group, and shallow marine to coastal to braided fluvial depositional environments in the Nanaimo area where coal is present. This rather wide range of depositional environments seems a bit odd as a single formation, but my grasp of the Nanaimo Formations is limited.  

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