A Plunging Syncline in the Salish Sea

Sucia Island with Patos Island in the center distance. Canadian islands and Vancouver Island are in the distance along with a ship moving through the Boundary Pass along the international border.

I had a nice trip over to Orcas Island earlier this week. From Bellingham it is typically less expensive for me to fly the 15 minutes to Eastsound versus an hour drive and hour ferry ride with waiting for the ferry added in as well.

I had a great view of Sucia Island and Patos Island, the northernmost islands of the San Juans. Both islands are Washington State parkland. One of the small islands within the horseshoe shape of Sucia is privately owned and has a house on it. From the air, the distinctive shape of Sucia can readily be recognized as either a plunging syncline (a downward fold) or a plunging anticline (an upward fold). In the case of Sucia it is southeast plunging syncline. The younger rocks are in the middle and the older rocks are on the outer sides. The harder sandstone ridges that stand out can be seen bending around the fold giving Sucia a classical plunging fold shape. Preferential erosion has caused the resistant sandstone ridges to stand out. The island along with all the San Juan Islands was buried under several thousand feet of glacial ice as recently as 16,000 years ago and the topography of the islands is primarily glacial related.

Most of the island is interpreted to be Chuckanut Formation (Vance, 1975) an Eocene age formation that covers a large area around and south, east and north of Bellingham. This formation is broadly folded, but with the water and somewhat drier climate the fold at Sucia is very obvious.

Sucia does have a bit of geologic twist. The far limb making up the ridge on the left side of the photo is part of the older Cretaceous Nanaimo Group named for its presence on the east side of Vancouver Island near Nanaimo. The Nanaimo Group on Sucia contains marine fossils a feature not present in the Chuckanut. The other difference is that the clast makeup is different with the exception of some of the basal (earliest deposits) of the formations. Hence, Vance interpreted the sandstones on Sucia as being the more continental Chuckanut Formation. While the units are concordant, there is non conformity (erosion break) between the older Nanaimo and younger Chuckanut. The non conformity unfortunately can not be observed as it lies somewhere between the two ridges of more resistant rock. Nanaimo Group rocks are well exposed along the north shore of Orcas Island as well. But glacial sediments unfortunately cover up the contacts with underlying older rocks.

Bedrock platform beach on the north shore of Orcas Island showing sedimentary layers within the Nanaimo Group.

The Nanaimo Group is interpreted to have been deposited in a basin significantly south of here and transported along with other older rocks northward along north-south strike slip faults along the then margin of North America. The Chuckanut was deposited at a later date with sediments derived from a continental source at a time prior to the uplift of the current North Cascades range. All of the rocks in the area have been folded since the Chuckanut was deposited.

I have made a couple of trips to Sucia via sail boat with my friend and associate Ted Hammer. Lots of interesting geology including a bunch of dead trees in a back water estuary that causes some head scratching about seismic or tsunami impacts. There is also obvious evidence of long human occupation with at least one very large miden.