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University of Washington Publications in Geology
The Geology of the San Juan Islands |
DESCRIPTIVE GEOLOGY
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SEDIMENTARY ROCKS
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DEVONO-MISSISSIPPIAN SYSTEMS
ORCAS GROUP
Principal Features. The Orcas group3 is the oldest one exposed on the San Juan Islands, and the outcrops are all included within a belt 22 miles long and 10 miles wide. The underlying strata are covered by the waters of the sea. The Orcas group is overlain with approximate conformity by the tuffaceous graywackes of the Leech River group. The rooks of the Orcas group are complexly folded and contorted, while fracture zones and faults of un known magnitude further complicate the structure. Up to the present time it has not been possible to distinguish the Devonian formations from those belonging to the Mississippian.
3McLellan, R. D., The Devonian Orcas Group of Washington: Amer. Jour Sci., 5 Ser., vol. 8, pp. 217-222, 1924.
Lithology and Structure. The Orcas group was laid down by normal processes of marine sedimentation in fairly deep water and at a considerable distance from the shore. Periodic conditions produced thin strata of fine grained and perhaps semi-colloidal silica sands alternating with thin strata of mud. The sediments have been highly metamorphosed and contorted as a result of folding and igneous intrusions.
The outstanding lithological component of the Orcas group is a light bluish-gray chert. The chert is so badly fractured and the joint planes so intricately recemented by white quartz that it is impossible to obtain a thin section that will not show several quartz veinlets within its area. The veinlets are usually parallel for the bedding plane and in places they are so abundant that the rock passes into a cherty quartzite.
The chert strata range from half an inch to three inches in thickness with an average of slightly more than an inch. Each band of chert is separated from the next by layers of argillite. The thickness of the argillite bands is generally slightly less than that of the chert layers and they vary in color from light brownish-gray to dark brown. Near the uppermost part of the Orcas group the rhythmical alternation of thin strata of chert and argillite is somewhat broken. Occasional strata of argillite here reach a thickness of several feet, while above and below them the regular alternation of thin strata persists. By differential erosion, especially along the coast, the argillite layers have been largely removed, while the cherty layers stand out in relief and present a surface and structure resembling that of shredded wheat. The argillite has a tendency to pass into the chert, although in places it is highly schistose.
Thin sections of the chert show numerous small round "spots" filled with quartz, and these are occasionally present in the interbedded argillites. The size of the "spots" is approximately that of the radiolarians seen in the cherts of the Franciscan formation of California, and since radiolarian cherts have also been found in northern California4 in formations of similar age and lithology, it is assumed that the quartz-filled "spots" were originally radiolarian tests which have since recrystallized. A recent examination of selected samples of the chert has revealed the fact that the "spots" are due to microorganisms which were probably radiolarians. These microorganisms occur in such a poor state of preservation that little can be specifically learned concerning them.
4Diller, J. S., Mineral Resources of Southwestern Oregon: U. S. Geol. Survey, Bull. 546, pp. 15-16, 1914.
Numerous lens-shaped beds of limestone occur interbedded with the cherty quartzite. These are not confined to any one horizon, but occur scattered throughout the whole group. However a fairly persistent series of lenses occur at a horizon near the lowest part of the group exposed, and another one near the top of the group. The limestone is always recrystallized and recemented with calcite. In the vicinity of igneous dikes, the limestones are usually silicified and irregular masses of flint are found. Limestone beds occurring in the vicinity of aplite and pegmatite dikes are sometimes replaced by contact minerals such as tremolite and wollastonite. Still other limestones contain a considerable quantity of carbonaceous matter, and in such cases the limestone is black in color and the joint planes are often covered with glistening films of graphite. The limestones of the Orcas group contain remarkably low percentages of magnesium, and this fact is all the more remarkable when it is considered that they are frequently intruded by dikes of igneous materials containing high percentages of magnesium.
A series of cherty and tuffaceous graywackes having a thickness exceeding a hundred feet appear to occupy a definite horizon somewhat higher than the middle of the Orcas group. It is possible that this horizon marks the dividing-line between the Devonian and the Mississippian.
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| Figure 1. Table of Formations. (click on image for an enlargement in a new window) |
ORCAS ISLAND
It is possible that the conglomerate exposed along the shore near the foot of Orcas Knob represents the base of the Orcas group. The coarse greenish sandstone that forms its matrix greatly predominates over the enclosed boulders which consist chiefly of altered andesite, quartzite, and granite. Its thickness is at least 35 feet. The conglomerate is overlain by 25 to 50 feet of thin alternating beds of quartzite and argillite and these are followed by a fairly persistent limestone bed about 30 feet thick. This limestone bed contains Devonian fossils and the surrounding rocks are somewhat less metomorphosed than those occurring in other parts of the region.
The limestone is overlain by thin-bedded quartzite and agillite, the former being cherty only at intervals. Like the true cherts which occur farther up in the group, this material contains the quartz-filled "spots." It is noticeable that some of the chert strata of the Orcas group contain a much larger number of the quartz-filled "spots" than others. Some of the cherts are, free or nearly free from the quartz-filled "spots."
The succeeding strata are so broken up by igneous intrusions of several ages, that scattered remnants of the chert are all that remain. These remnants usually retain the same general strike and dip as that of the unintruded portions, i. e., the strike is northeast and the dip southeast. On the western half of Orcas Island the igneous intrusions take the form of a wide belt or zone running parallel to the strike of the chert. To the southeast of this igneous belt, which is shown on the accompanying geological map as the Turtleback Complex, the cherts are again almost free from igneous intrusions. If a straight line be drawn northeastward from Reef Island to the village of West Sound and continued on towards East Sound, it would mark the southern or upper boundary of the igneous belt.
In the southern part of Orcas Island the major trend of the Orcas group is modified by the presence of several minor folds which trend north and south or slightly northwest and southeast. The southern part of East Sound follows a minor anticlinal fold, while the structure of the region to the south of Mount Woolard is complicated by faulting.
The cherty rocks of the Orcas group appear along the west and northwest sides of the Mount Constitution Range. Here they trend northeasterly and dip towards the southeast at a moderate angle. These rocks make up the upper part of the group on Orcas Island and they contain several large limestone lenses which appear to belong to the same horizon as those occurring to the north of Mount Woolard.
VICTIM ISLAND
The cherty rocks occurring on Victim Island trend about N 60° W and dip towards the southwest. The same attitude is assumed by the rocks along the shore of Orcas Island to the west of Victim Island. The cherts on this island are cut by several igneous dikes or sills which follow the general trend of the sedimentary rocks.
DOUBLE ISLAND
The Orcas cherts occurring on Double Island have the same general trend and dip as those on Victim Island. The peninsula between West Sound and Deer Harbor is located on the axis of a minor synclinal fold which plunges towards the southward. The cherty rocks occurring on Double Island and on Victim Island are a part of the eastern limb of this fold.
BELL ISLAND
Bell Island is located on the axis of the synclinal fold which embraces the peninsula to the northward. The cherty rocks are here so broken up that they are practically structureless.
CRANE ISLAND
The western limb of the synclinal fold mentioned above embraces the rocks of the Orcas group occurring on Crane Island. In general these rocks are badly broken and dislocated, but they commonly trend northeasterly and dip towards the southeast. Near the southwest corner of Crane Island there is a limestone lens which is apparently a continuation of the limestone ledges occurring on Cliff Island.
CLIFF ISLAND
The cherty rocks belonging to the Orcas group which occur on Cliff Island, trend northeasterly parallel to the long direction of the island. They dip steeply towards the southeast. A series of large limestone lenses occur along the northwest shore of the island. Intrusions of the Eagle Cliff porphyrite have destroyed a large part of the cherty rocks on Cliff Island.
SHAW ISLAND
On Shaw Island the rocks of the Orcas group are so badly dislocated and broken up that each isolated outcrop may show a structure which does not conform to that of its neighbors. In general the rocks occurring along the southwest shore trend northwesterly and dip towards the northeast at moderate angles. Along the Wasp Passage the rocks trend northeasterly and dip toward the southeast. Rocks belonging to the Leech River group have been folded and faulted in with those of the Orcas group at many localities on Shaw Island. The rocks of the Orcas group occurring on Shaw Island contain number of limestone lenses, but none of these are large.
JONES ISLAND
On Jones Island the rocks belonging to the Orcas group are confined to the remnants of chert and limestone which have not been destroyed by the igneous rocks which make up the greater part of the island.
A limestone stratum with a maximum thickness of about 20 feet, outcrops along the east shore of the island and trends northwesterly. At the head of the large harbor which penetrates the north end of Jones Island, this limestone stratum turns abruptly and continues in a westerly direction until it reaches the western shore of the island.
SAN JUAN ISLAND
San Juan Island is located on the axis of a large synclinal fold which embraces practically all of the Paleozoic rocks occurring in the map-area. The fold plunges towards the southeast and as a consequence the outcrops of the rocks belonging to the Orcas group on San Juan Island take the general shape of a horseshoe which forms the northern shores of the island.
The contact between the cherts of the Orcas group and the overlying tuffaceous and somewhat cherty graywackes belonging to the Leech River group, may be seen along the northeast shore of San Juan Island, about a mile to the east of Sportsmans Lake. The rocks of the Orcas group occur at the northwest end of the island, to the northwest of San Juan Range. They also include Mount Dallas Range. The most southern outcrop of the Orcas cherts occurs at the southeast side of False Bay.
Along the southwest margin of San Juan Island the cherty rocks trend approximately parallel with the shore-line and dip towards the northeast. Along the northeast margin of the island the cherty rooks dip towards the southwest. The structure at the northwest end of the island has been complicated by minor folding and by faulting.
The cherty rocks belonging to the Orcas group on San Juan Island, contain numerous lenses of limestone. The largest of these is located at Roche Harbor. The cherty rocks have been intruded by scattered dikes and sills of igneous rocks which were not sufficiently abundant to destroy the structure of the cherts.
HENRY ISLAND
Henry Island is located on the axis of the plunging synclinal fold which embraces the Paleozoic rocks on San Juan Island. The two parallel elevated ridges which make up the island trend parallel with the strike of the Orcas cherts. In the vicinity of McCracken Point the cherts trend about N 75° W and dip steeply towards the southwest. The same trend and dip are seen on Battleship Island and on Pearl Island. Several limestone lenses, some of them quite large, are located on Henry Island.
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| PLATE XIII. The view across the islands as seen from the southwest slopes of the Turtleback Range. (Photograph by J. A. McCormick). |
O'NEAL ISLAND
O'Neal Island is composed largely of rocks belonging to the Orcas group. These rocks seem to have been dislocated from those occurring along the nearby shores of San Juan Island, for they trend about N 45° E and have a dip which is nearly vertical. A limestone stratum having a maximum thickness of about 20 feet crosses the southern part of the island.
Age and Correlation. In the Orcas Lime Quarry, near the foot of Orcas Knob, Sections 30 and 31, T 37 N, R 2 W, the writer discovered some fossil brachiopods on the weathered surface of the limestone ledge as it outcrops along the face of the cliff. These fossils occur to a depth of only one inch and beneath this the limestone is entirely recrystallized and shows no trace of fossil remains. Apparently this fossil-bearing ledge has been preserved in some unknown manner while the surrounding limestones have been completely recrystallized.
These brachiopods have been examined by Dr. Charles Schuchert, who determined them to be a variety of Atrypa reticularis Linneaus. He says in a letter to the writer;—
"All of them are Atrypa reticularis, one of the commonest of all Paleozoic forms. This species ranges throughout the Silurian and Devonian but your specimens look to me like the Euro-Asiatic variety seen in the late Middle and Upper Devonian throughout the west, from the Pacific to Iowa. The spirals show in a number of specimens, and these prove that their apexes turn to the center of the dorsal shells. Very few genera do this, so there is no doubt you have Atrypa reticularis, and, I think, a late Devonian form."
The Orcas group is to be correlated with the lower part of the Cache Creek series of British Columbia. The Cache Creek series was first described by Dawson5 who found in it the Pennsylvanian fossil Fusulina. He also found fossils in the lower formations, however, which ranged back into the Devonian, and he remarks that "The lower portions of the Cache Creek formation may be older than the Carboniferous period."
5Dawson, G. M., Report on Area of the Kamloops Sheet, British Columbia: Geol. Survey Canada, Ann. Rept., new ser. 7, pp. 39B-49B, 1896.
In the Bridge River district, west of Lillooet, British Columbia, McCann6, 6a describes the Bridge River series, which he considers to be of Pennsylvanian Permian age, but his description applies equally well to the rocks of the Orcas group.
6McCann., W.S., Geology and Mineral Deposits of the Bridge River Map-Area, British Columbia.
6aIbid., Memoir 130, pp. 23-26, 1922.
The Orcas group may also be partly equivalent to the Hozomeen series7 which occurs in the Skagit and Hozomeen ranges; near the forty-ninth parallel.
7Smith, G. O., and Calkins, F. C., A Geological Reconaissance Across the Cascade Range near the Forty-nineth Parallel: U. S. Geol. Survey, Bull. 235, pp. 22-23, 1904.
Deposits of chert, similar to that of the Orcas group, are known to occur in the Olympic Mountains of Washington, but as yet no detailed geological work has been done in this area.
The chert deposits in Northern California, described by Diller,8 are similar in lithology, and in addition, they contain identical fossils. Radiolarian remains are still preserved in these cherts.
8Diller, J. S., Mineral Resources of Southwestern Oregon: U. S. Geol. Survey, Bull. 546, pp. 15-16, 1914.
While the cherts of the Franciscan formation of California9, 10are tentatively referred to the Triassic or Jurassic periods, their age is not definitely known, and it may prove that they too belong to the late Devonian and Mississippian periods.
9Lawson, A. C., U. S. Geol. Survey, Geol. Atlas, San Francisco folio (no. 193), pp. 34-57, 1915.
10Davis, E. F., The Radiolarian Cherts of the Franciscan Group: Univ. Calif. Publ. Bull. Dept. Geol., vol. 11, pp. 353-408, 1918.
The limestone ledges exposed on Orcas Island, to the south of Mount Woolard, in Sec. 2, T 36 N, R 2 W, are structurally near the top of the chert group. These limestones contain the Carboniferous coral Lithostrotion.
The argillites and graywackes of the Leech River group in this area contain fragments of the "spotted" chert, and also include Fusulina-bearing limestones, hence it is probable that the Lithostrotion-bearing limestones are of Mississippian age.
The Orcas group, therefore, had its beginning in the Middle or Upper Devonian, and the deposition of its sediments continued on into the Lower Carboniferous.
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