This is the Lime Kiln Point lighthouse, on the west side of San Juan Island in the San Juan Islands of Washington. The dark rock consists of pillow basalt that was erupted onto the floor of the ocean. The light-gray rock is limestone that originated as small reefs or accumulations of lime, which formed on the ocean floor during the time the basalts were not erupting. Together, these rocks compose the Deadman Bay Volcanics. Fusulinid fossils in the limestone indicate that the rocks formed in the Late Permian or Early Triassic period.

Most of the rocks that comprise the San Juan Islands are considered parts of accreted terranes. The rocks and their fossils speak of exotic origins, bits and pieces of the ocean floor and island arcs that came from far away. The geologic structures visible in the islands illustrate how the terranes were thrust and stacked into this corner of the west coast of North America.

Lopez Structural Complex

This close-up shows some outcrop-scale features that formed in a thrust fault zone within the earth's crust. The gray rock is sandstone and the black rock around it is shale. The white veins are made of quartz. The sandstone and shale were originally stacked in parallel, horizontal, flat beds. However, the beds of sandstone and shale were caught up in a thrust fault deep within the crust. The gray beds of sandstone and black beds of shale were deformed, disrupted, and interleaved with each other. This was ductile deformation, like two putties with different stiffness being deformed into new shapes without breaking or fracturing. Select the image to see a larger view. Use your browser's back button to return to this page.

The stress of the fault motion has sheared the sandstone beds into discontinuous gray "blobs" surrounded by black shale. The shale has developed metamorphic cleavage and the minerals in the rocks, identified definitively under a microscope, indicate that both the shale and sandstone underwent low-temperature, high-pressure metamorphism. A likely scenario is that the clay and sand beds were turbidites deposited on the seafloor which then got stuffed into a trench at a subduction zone. Within subduction zones the metamorphic environment is one of high pressure and low temperature.

It was probably not until after the thrust faulting was completed that the cracks filled with quartz developed. The quartz-filled veins mark brittle fractures which in some cases appear to be microfaults, where the rocks broke and shifted in opposite directions on each side of the crack. The principle of cross-cutting relationships allows us to deduce that the brittle fracturing, microfaulting, and growth of quartz veins happened after the ductile deformation of the sand and shale layers.

Shear Zone

This picture shows a shear zone. It is in one of the major thrust fault zones on San Juan Island. The white-colored rock is ribbon chert, layers of chert that accumulated on the ocean floor. Most of the dark rock is pillow basalt, from lava that erupted onto the ocean floor. The ribbon chert has been sheared and separated into pieces, and the sheared basalt has flowed, as a plastic solid, around the ribbon chert. All this happened without melting, in a fault zone at some depth in the crust. Select the image to see a larger view. Use your browser's back button to return to this page.

Basalt Pillow (in use)

This person's head rests on a pillow of the Deadman Bay basalt near Lime Kiln Point on the west side of San Juan Island. The rock hardened upon eruption through a crack in the ocean floor, probably over 200 million years ago (Late Permian or Early Triassic period). Now it has accreted to North America as one of the terranes of the San Juan Islands. Select the image to see a larger view. Use your browser's back button to return to this page.

Tethyan Intra-Pillow Limestone

This limestone of light gray color fits between basalt pillows of the Deadman Bay Volcanics. The limestone has bedding in it that shows it formed in place, accumulating between the pillows. This contrasts with some nearby limestone that is broken in pieces and mixed with broken basalt pillows, all now hardened together into a single rock mass, suggesting underwater debris flows. Select the image to see a larger view. Use your browser's back button to return to this page.

The limestone contains Tethyan fusulinids. These are fossils of primitive, one-celled plankton that flourished in tropical oceans in or near the Tethys Sea, on the eastern side of the supercontinent Pangaea, during Late Permian time. Apparently, since they formed, the Deadman Bay Volcanics have worked their way across an ocean and up to high latitudes from the tropics before coming to rest as part of the San Juan Islands. Such an exotic origin is consistent with reconstructions of how plates have moved in the Pacific Ocean basin since the Paleozoic.

Glossary terms that appear on this page: pillow basalt; limestone; plastic; fusulinid; accreted terrane; sandstone; shale; thrust fault; chert; Tethys Sea; Pangaea