Sunday, January 3, 2016

Upright Head Conglomerate

With the short winter days and three projects on three different San Juan Islands I had a bit of a lay over wait for the ferry from Lopez Island to San Juan Island.

The ferry landing at Upright Head has plenty of interesting sites on public access land. Hence, one can get in the ferry wait lineup early (not really an issue mid winter on a week day) and spend the time waiting by taking nearby short hikes.

The very first site to check out is the great road cut exposing pebble conglomerate adjacent to the road down to the ferry.

Conglomerates can have a very good story to tell, but despite the great exposure and easy access to this conglomerate deciphering this conglomerate's history has been a challenge. Whetten (1975) recognized that this conglomerate was younger and had a different history than conglomerates on the south part of Lopez. Brown (2012) has incorporated the information extracted from the pebble make up and age dates of zircons in the conglomerate to constrain the final assembly of the San Juan thrust nappe sequence.

Figuring out a story from conglomerates requires knowing a lot about potential source material. Where did the pebbles come from? This particular conglomerate has pebbles that resemble two thrust nappes to the north - Orcas Chert/Deadman volcanics and East Sound Group/Turtleback Complex. Pebbles associated with other thrust nappes are not present suggesting that those formations had not yet been thrust onto the San Juan nappe sequence when this conglomerate was deposited see (Brown, 2012) .

There are lots of other conglomerates in the San Juans. The older conglomerates are more metamorphosed. In this conglomerate the metamorphism is subtle. At the outcrop level there are a few hints. In most places, but not all the bedrock fractures at least partially through the pebbles (see above pictures). But in other places the fractures go around the pebbles suggesting some differential strain. But in the images below note the dissolution fractures in the rock including some that cut straight through pebbles.

While the best exposure is the fresh cut slope in the rock that the road, the same formation can be observed in its more native condition on bedrock outcrops along the shore and shore bluff to the east.

A walk to the shore bluffs is well worth while and includes great views as well as an interesting mix of trees.

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