University of Washington Publications in Geology The Geology of the San Juan Islands

In detail the structure of the rocks in the San Juan Island map-area is extremely complicated. A large part of the structural geology of this region will remain long unsolved, for the major fault lines are occupied by deep channels and the islands are often separated so far from each other that the continuity of the structure is broken. The Paleozoic rocks have been subjected to so many periods of folding and faulting, and in addition they have been intruded by igneous rocks of so many periods of igneous activity, that it is indeed remarkable that any continuity can be traced in their structural relations.

The three large islands, Orcas, San Juan, and Lopez, serve as a nucleus to which the structural relationships of the smaller and more isolated islands must be connected. These three islands reveal a structure which in its major aspect is a broad open syncline plunging at a vertical angle of about 35 degrees toward the southeast.

An examination of the topographic maps of the islands fringing the eastern shore of southern Vancouver Island, shows how clearly the rock structure is related to the surface features. The Cretaceous rocks retain an unbroken northwest and southeast trend until the International boundary is reached. Beyond this point the rock structure is broken, and with the exception of the Stuart Island group, the smaller islands show little or no apparent structural relationship to each other. Superficially at least it would appear that Saturna Island and South Pender Island exhibit drag, such as would occur if the San Juan Islands had been thrust to the northward.

The former topography of the San Juan Islands has been greatly modified by glaciation, but the erosion beneath the glaciers was no doubt guided to a considerable extent by valleys and by the fracture zones and fault zones that were already in existence. It is probable that a fault of considerable magnitude occupies each of the major channels.

In an earlier paper³⁷ the writer described a fault extending from False Bay to Doe Bay, as the Kanaka fault. It now appears that although a fracture zone or fault occurs throughout the greater part of this line, it does not cross the map-area as a single fault. Its movement, in different portions of the fault line, did not have the same direction.

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³⁷McLellan, R. D., The Devonian Orcas Group of Washington: Amer. Jour. Sci., vol. 8, p. 220, 1924.

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Judging from the soundings, a very profound fault occurs in Haro Strait, quite close to the shore of San Juan Island. Another fault of considerable magnitude follows San Juan Channel and crosses Lopez Island near the village of Lopez. Whether this fault continues on through Lopez Pass, or whether it is cut off by a north-south fault in Lopez Sound, has not been determined. The south side of the fault apparently moved southeastward with respect to the north side.

It is assumed that a fault or fracture zone follows Harney Channel, and before the formation of the San Juan fault, this zone crossed over in a direct line to Rocky Bay on San Juan Island. If such a fault zone occurs, its horizontal displacement cannot be very large.

The line connecting the villages of East Sound and West Sound is probably the location of a pre-Jurassic fault-zone, because of the numerous igneous intrusions along this line. The strike of the rock formations is parallel to this line.

A fault of small horizontal displacement follows East Sound and divides Orcas Island into two almost equal parts. Whether the fault continues on into Lopez Sound is problematical. From evidence occurring on the north shore of Orcas Island the fault is post-Cretaceous in age, and the eastern side moved southward and upward with respect to the western side.

Along the north shore of Orcas Island, from Point Lawrence to the foot of Buck Mountain, the fossiliferous Leech River sediments strike parallel to the shore-line and dip about 65 degrees to the southward. On the top of Mount Constitution the Leech River sediments strike about N 45° E, and dip towards the south-east. The rocks occurring on Mount Constitution Range are clearly overthrust upon the Leech River sediments which outcrop along the shore to the northward. The fault-line between them is visible at several localities at an elevation of about 1000 feet.

The structure of the region to the southeast of Mount Woolard is almost hopelessly complicated. The rocks of the Orcas and Leech River groups appear to be scrambled together in this district.

A local anticline crosses East Sound at its southern end and all of the rocks to the south of Dolphin are affected by it. The southern part of East Sound is located on the axis of the anticline, but to the north of Rosario and Dolphin there is no evidence of such a fold for the rocks on both sides of the channel trend northeasterly and dip towards the southeast.

The Leech River sediments on Lummi Island appear to be structurally related to those on the north shore of Orcas Island, both having approximately the same strike. The south part of Lummi Island is apparently the up-throw side of a fault that cuts sharply across the island.

In the western portion of the map-area normal faults occur in New Channel, Spieden Channel, and on the south side of the point connecting Davidson Head with San Juan Island. Each of these faults trends in an east-west or slightly northwest direction, and in each case the south side is the up-throw side.

It is evident that the sediments of the Nanaimo series at one time extended over all excepting the eastern margin of the map-area. Erosion has removed these sediments from the more uplifted areas.

The small islands in the northern part of the map-area do not display much similarity in structure. The structure of each of these islands has been treated in the description of the rocks of the Nanaimo series.

Presumably the greatest period of folding and faulting occurred during the late Jurassic. Apparently the Paleozoic rocks were folded into the large plunging syncline at this time, although they probably were folded to some extent at the close of the Paleozoic. If the syncline was formed at the close of the Paleozoic, the Leech River group could not possibly contain any Mesozoic formations for these rocks were all folded at the same time.

The rocks of the Nanaimo series and the Chuckanut formation were evidently folded during the same periods of folding. As no rocks between the Eocene and Pleistocene occur on the San Juan Islands, it is necessary to look to the adjoining districts for evidence as to the periods of folding of the Cretaceous and Eocene rocks.

The first period of marked folding succeeding that of the late Jurassic took place during the late Eocene or early Oligocene. As noticed by Clapp,³⁸ the deformation, though ranging from British Columbia to California, was intense only in isolated localities. From evidence found on southern Vancouver Island, Clapp considers that the deformation was quite certainly of early Oligocene age.

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³⁸Clapp, C. H., Sooke and Duncan Map-Areas, Vancouver Island: Geol. Survey Canada, Mem. 96, pp. 254-255, 1917.

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According to Weaver³⁹ no sharp break separates the Miocene from the Oligocene, but during the middle or latter part of the Miocene an extensive elevation of the sea floor took place.

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³⁹Weaver, C. E., A Preliminary Report on the Tertiary Paleontology of Western Washington: Wash. Geol. Survey Bull. 15, pp. 25, 1912.

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Since the last glacial period the whole region has been uplifted to such an extent that marine fossils have been found in glacial sediments 290 feet above the present sea-level. In the San Juan Island map-area there appears to have been a recent uplift of 15 to 25 feet.