Sunday, December 6, 2020

Rosario Fault on Crane Island north shore

During a work venture in the San Juan Islands I got a nice look at the bedrock along the north shore of Crane Island. Crane is a rocky island between the southwest end of Orcas Island and the northwest shore of Shaw Island. The ferry between Orcas and Friday Harbor passes along the south shore of Crane, but on this venture I was in a small boat and we ventured along the north shore. 

Crane Island is mapped as Constitution Formation on both the Bellingham 1:100,000 map (Lapen (2000) and the Roche Harbor 1:100,000 map (Logan, 2003). And both maps denote the Rosario Thrust Fault immediately to the north of Crane Island 

Portion of Bellingham Quad geology map (Lapen, 2000)
KJmc = Constitution Formation, JTRmcto = Orcas Chert, pPscg = Garrison Schist, pDit = Turtleback Complex, Qgd = glacial drift. Heavy black lines with saw teeth are thrust faults with teeth on upper plate.

Portion of Roche Harbor Quad geology map (Logan, 2003)

The Rosario Thrust is where the Constitution Formation has been faulted up over the Orcas Chert. The fault zone is a zone of highly sheared rock that in places include slivers and slices of other formations caught up in the fault zone such as the Garrison Formation denoted by Lapen (2000) on Shaw Island south of Crane Island. 

My observations of the fault zone exposed along the base of the steep north slope shoreline on the north side of Crane were consistent with other areas where the Rosario Trust Fault is well exposed along the shore near Rosario on Orcas Island (hence the faults name) and along the southwest shore of San Juan Island. The shoreline areas typically offer the best exposures of the fault zone.     

Highly sheared rock within the Rosario Thrust along the north shore of Crane Island

Brecciated and sheared bedrock at water line

Massive block of Constitution Formation above shear zone

On the uplands of Crane Island, the bedrock is generally massive metamorphosed sandstone of the Constitution Formation.  

Typical massive silica-rich metamorphosed sandstone

Orcas Chert is primarily composed of ribbon chert, alternating layers of silica rich 1 to 2 inches thick with thin layers of shale. The chert is derived from the accumulation of silica diatoms that accumulated on the ocean floor. The shale was derived from fine dust and silt that also reached the ocean floor. The layering developed after deposition when the ooze of silica rich and silty sediment was compacted. The age of the Orcas Chert is between 180 and 280 million years old. The Orcas Chert is part of terrain consisting of the related Deadman Bay Volcanics, which are ocean floor pillow basalts that are somewhat older than the Orcas Chert and likely formed the basement that the silica rich sediment that formed the chert unit was deposited on. The original depositional setting was an ocean floor area far from North America. The formation was moved to and accreted to the edge of North America via plate tectonics. During accretion the Orcas Chert was metamorphosed at high pressures.

The Constitution Formation was also originally an ocean floor assemblage; however, this unit is on the order of 130 million years old. The Constitution Formation consists predominantly of metamorphosed fine sandstone derived from a volcanic arc. There is some chert and basalt in the unit as well. This formation was also accreted to the edge of North America. A lack of older aged sediment, North America sediment, suggests the sediment may have been derived from an ocean island arc. 

The Rosario Thrust Fault and the juxtaposition of the Orcas Chert and Constitution Formation took place after accretion -- note that the younger Constitution is thrust over the older Orcas Chert.    

The map pattern shows that Crane Island is klippen of Constitution Formation on the underlying Orcas Chert. Parts of the Constitution Formation have been stripped away leaving the Constitution Formation as an 'island' on the Orcas Chert and as well as the rock formation that makes up most of Crane Island. 


1 comment:

dave said...

The San Juan Islands/Cascade foothills geology
as a hugely fascinating study in accreted terranes
and anyone interested in hearing a bit more
might wanna go here -
nickzentner dot com/the-nick-zentner-geology-podcast/
Number 54 addresses the subject
but they're all great listening.