Orcas Chert on the Southwest Coast of San Juan Island

Orcas Chert on the west side of San Juan Island

Closeup of Orcas Chert

I always enjoy seeing the Orcas Chert. Orcas Chert of coarse is found on Orcas Island with extensive outcrops along the shores and steep slopes of East Sound and West Sound on Orcas. The formation also crops out along the the headlands on the west side of Shaw Island and makes up significant portions of the shoreline of the east, northwest and southwest sides of San Juan Island. 

Orcas Chert is predominantly ribbon chert and has a great mangled look and with twisted geometry and unexpected surprises within the highly sheared rock. Chert is formed by the accumulation of silica bearing skeletons of organisms such as radiolaria, siliceous sponges and diatoms on the sea floor (see HERE for a view of a diatom deposit in eastern Washington). The silica bearing organisms sink to the bottom of the sea and accumulate enough to create silica rich layers. Chert formation typically requires low sediment input and thus are indicative of sites being far from land or at the least far from areas where much erosion is taking place. They are generally though of as being in deeper water as well or otherwise calcium carbonate bearing shells or corral would predominate. So the presence of chert gives a hint as to the source environment - ocean floor possibly deep water or far from an eroding land mass. 

The Orcas Chert consists primarily of ribbon chert consisting of alternating chert layers with thin layers of shale. The shale consisting of metamorposed mud stone. Orcas Chert also includes lesser amounts of pillow basalts (lava that erupted under water), volcanic tuff and limestone. The limestone is interesting - more on that later. 

The Orcas Chert is part of a suite of rocks belonging to the Northwest Cascades System (NWCS). The NWCS is not a simple assemblage and taking a walk along the the Orcas Chert exposed on the west side of San Juan Island is a good reminder.  Lappen (2000) assembled the Geologic Map of the Bellingham 1:100,000 Quadrangle that includes much of the San Juan Islands. The accompanying report provides only a brief description of the geologic setting but I think it sums up the NWCS rather well as "This structural system is a thrust stack of mainly oceanic lithologic packages (terranes) of varying age, structure and metamorphic history." I would emphasize "varying" as an understatement. When I get asked about these rocks or other assemblages of metamorphic rocks in the San Juans or Northwest Cascades I often say these rocks have had a long hard life. 

The west side of San Juan Island parallels one of the many thrust faults that juxtapose formations within the NWCS. This fault juxtaposes the Orcas Chert with the Constitution Formation to the east. But it is within the Orcas Chert that one can get some sense of how complicated the NWCS is.    

Highly sheared Orcas Chert with fragments of green and possible blue schist

While the NWCS can be described as a melange (French for mix) belt, the Orcas Chert could be described as a melange within the melange. Within the outcrops along the west coast of San Juan Island there are number of highly deformed zones with fragments of other formations embedded withing the sheared Orcas Chert. Its a great place to exercises your ability to determine shear direction and metamorphic fabrics. I take some comfort in that sense of shear has been variously interpreted in these rocks. One thing is clear - these rocks have gone through multiple tectonic events during there journey from the quiet ocean floor sediment basin to being accreted onto the North American Continent and then further faulting and crunching.

Orcas Chert with fragments of limestone

Fossil radiolari in the less mangled chert indicate that the chert is Jurassic to Triassic in age. However, fossil fusulinds within the limestone within the Orcas Chert are Permian. More evidence of the hard life these rocks have had since deposition on the ocean floor. The west coast of San Juan Island is a rare place to see continuous rock outcrops in the low lands of western Washington. Not an unpleasant place to contemplate complex metamorphic and tectonic questions. And there are plenty of other landscape features on this coast as well.  

Southwest coast of San Juan Island