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

LOWER CRETACEOUS SYSTEM OR SERIES

SPIEDEN FORMATION

Principal Features. Isolated outcrops of conglomerate, sandstone, and shale, called here the Spieden formation, occur in the northwest portion of the map-area. The outcrops of this formation are apparently confined to Spieden Island, Sentinel Island, and Sentinel Rock. The latter two islands are composed chiefly of conglomerate, while Spieden Island is also composed of conglomerate except along its northern margin. The rocks of the Spieden formation are evidently separated from those of the Nanaimo series to the north ward by a normal fault trending in an east and west direction. Spieden Island is located on the upthrow side of this fault. Apparently there is another normal fault which follows the bed of the channel to the south of Spieden Island. In this case Spieden Island is located on the down-throw side of the fault.

Lithology and Structure. The Spieden formation is composed of conglomerate, breccia, sandstone, shale, and argillaceous limestone. Within the map-area the conglomerates make up about 85 per cent of the rocks. They contain fragments up to a foot in diameter although they are usually not larger than one inch. The fragments are composed of andesite, diorite, granodiorite porphyry, milky quartz, jasper, graywacke, chert, argillite, and limestone. Boulders of granodiorite occur very sparingly and these are generally fine grained. A calcareous sandstone forms the matrix in most cases, although in some beds a ferruginous matrix prevails. The conglomerates on Sentinel Island are identical in lithology with those on Spieden Island.

SPIEDEN ISLAND

Along the northern margin of Spieden Island there is a group of thin bedded and somewhat carbonaceous shales. These continue at least as far as low tide, but their thickness is unknown. Although they are soft and unmetamorphosed they have been badly contorted by folding. They grade upward into sandy shales and sandstones, with an occasional bed of argillaceous limestone. These beds are usually olive-gray in color, and the individual strata average six inches in thickness. Throughout a thickness of about 35 feet these beds are richly fossiliferous.

The fossiliferous layers are overlain by fine-grained conglomerate and breccia. The arenaceous or calcareous matrix of the conglomerate greatly predominates over the boulders. Here and there, thin and well stratified sandstones occur throughout the conglomerate beds, but the sandstones pinch out along the strike and do not form definite horizons. The conglomerates have a thickness exceeding 2,000 feet.

The conglomerate beds of the Spieden formation have an average strike of N 70°-75° W, and they dip to the southward at angles of 45-60 degrees. Along the north shore, at the foot of Spieden Bluff, the fossiliferous shales and sandstones strike N 65° W, and dip 65° SW.

The conglomerates of the Spieden formation differ from those of the Nanaimo series, in the scarcity of boulders of granodiorite and other batholithic rocks. The late Jurassic batholiths apparently had not been deroofed in this region by upper Knoxville time.

SENTINEL ISLAND

Sentinel Island is composed of conglomerates identical in lithology with those occurring on Spieden Island. The alternating calcareous and ferruginous matrix of the conglomerates is peculiar to the Spieden formation. At the north edge of Sentinel Island the beds strike N 80° W and dip 45° SW. At the south edge of the island the beds strike N 82° W and dip 55° S.W. The thickness of the strata outcropping on Sentinel Island is about 800 feet.

The outcrops occurring on Sentinel Rock are the equivalent of some of the strata on Sentinel Island.

Age and Correlation. Along the north shore of Spieden Island there is a belt of fossil-bearing sandstone and shale. The fauna was examined by Dr. T. W. Stanton, who determined its age to be lower Cretaceous and equivalent to the upper part of the Knoxville formation of California.

Aucella crassicollis Keyserling is by far the most abundant fossil and it makes up fully ninety-five per-cent of the fauna. The identical fossils are found on the Nooksak River, north of Mount Baker. One of these fossil beds is located in a road-cut about two miles east of the village of Glacier, in section 5, T 39 N, R 7 E.

The fossils collected on Spieden Island include the following;—

The Spieden formation is to be correlated with the Pasayten formation which outcrops in the Hozomeen Range and other localities in north central Washington.¹⁶

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¹⁶Smith, G. O., and Calkins, F. C., A Geological Reconnaissance Across the Cascade Range near the Forty-ninth Parallel: U.S. Geol. Survey Bull. 235, pp. 28-30, 1904.

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DESCRIPTION OF NEW SPECIES

Order Ammonoidea
Family SILESITIDAE

GENUS HOLCODISCUS UHLIG
HOLCODISCUS?? STANTONI n. sp.

Plate XII. Figs. 3, 4, and 5.

Shell compressed, convex, and narrowly umbilicated; the umbilicus somewhat rounded with steep inner wall, occupying about one-fifth of the entire diameter; volutions closely involute, the inner ones being almost covered by those which succeed them; aperture higher than wide, sub-elliptical, but deeply emarginate by the encroachment of the preceding volution.

Surface marked with numerous slightly elevated, flexuous, transverse ribs, which are always somewhat narrower than the shallow concave spaces between them; the ribs are always bifurcate, and usually one or both branches are again divided so that near their summits they are generally trifurcate or tetrafurcate. There are approximately 25 major or parent ribs to each volution. The major ribs, though less strongly developed, continue to the inner wall of the umbilicus.

Sutural line not well seen, there being only two immature specimens collected that show a sutural line.

Dimensions.

The largest specimen known to the writer, and preserved only as a cast, measured about 125 mm. in diameter. The largest specimen actually collected has the dimensions: maximum diameter, 65 mm.; maximum thickness, 17 mm.; diameter of the umbilicus, 12 mm.

Locality. The type specimen was found on the north shore of Spieden Island at the foot of Spieden Bluff. The paratype was found at the same locality.

The same species is found in a road-cut along the south bank of the Nooksak River, about two miles east of the village of Glacier, in section 5, T 39 N, R 7 E.

Disposal of Type. University of Washington Paleontological Collection.

H. stantoni differs from Holcodiscus cumshewaensis, which species it most closely resembles, in the size of the umbilicus. H. cumshewaensis, in the type specimen at least, is much more loosely coiled than H. stantoni.

Named in honor of Dr. T. W. Stanton of the U. S. National Museum at Washington.

Family PHYLLOCERATIDAE

GENUS PHYLLOCERAS SUESS
PHYLLOCERAS SPIEDENENSIS n. sp.

Plate XII. Figs. 1 and 2.

Shell smooth and moderately inflated; the umbilical margin rounded and indistinctly defined; volutions increase rapidly in size, are closely convolute, the inner ones being completely covered by each succeeding volution; aperture higher than wide, nearly circular in the type specimen, but more elliptical in some of the other specimens collected; aperture, emarginate because of the encroachment of the preceding volutions, is somewhat pointed in the direction of the umbilicus, the specimen being very thin at this point.

Sutural line.

Dimensions.

The thickness at the axis of the umbilicus approaches zero.

Locality. The type specimen was obtained on the north shore of Spieden Island at the foot of Spieden Bluff.

Disposal of Type. University of Washington Paleontological Collection.

P. spiedenensis, in the type specimen, is more inflated than is normal in the genus Phylloceras. In some of the specimens this is reduced more nearly to the normal inflation for the genus. The type specimen does not seem to be distorted, for several other specimens were collected possessing a similar shape.

PELECYPODA
Superfamily ANATINACEA Dall

GENUS PLEUROMYA AGASSIZ
PLEUROMYA THOR n. sp.

Plate XII. Figs. 8 and 9.

Shell compressed, being most convex near the anterior margin; valves closed in front but slightly open behind; anterior end short and sloping nearly in a line from the beaks to the ventral margin; posterior end much longer, and somewhat pointed at its junction with the ventral margin; cardinal margin excavated and gently curved; umbones large, broad, and prominent; beaks small, anterior, and curved forward and downward; umbonal ridges nearly obsolete.

Surface marked by irregular, moderately fine concentric striations. Hinge teeth and muscular impressions unknown.

Dimensions.

Locality. The type specimen was collected on Spieden Island, at the foot of Spieden Bluff.

Disposal of Type. University of Washington Paleontological Collection.

P. thor is distinguished from the typical Pleuromya subcompressus Meek, by the fact that its beaks are less prominent, its surface markings finer and more irregular, and its posterior end is more pointed.

PLEUROMYA TYPA n. sp.

Plate XII. Fig. 7

Shell moderately convex, rounded in outline, the height being but little less than the length; umbones broad and somewhat flattened; beaks small, elevated, slightly anterior, and curved forward and downward; valves closed in front, but apparently open behind; anterior end sloping rapidly to the ventral margin; posterior end rounded, though somewhat longer than the anterior end; anterior umbonal ridges well developed; posterior umbonal ridges obsolete; ventral margin convex and strongly curved.

Surface marked by deep, irregularly disposed, concentric striations. Hinge teeth and muscular impressions unknown.

Dimensions.

Locality. Collected on Spieden Island at the foot of Spieden Bluff. Poorly preserved casts were seen at the Nooksak River locality, two miles east of the village of Glacier.

Disposal of Type. University of Washington Paleontological Collection.

The ventral margin of P. typa is much more strongly curved than that of any other Pleuromya known to the writer.

Superfamily PECTINACEA Reeve

GENUS LIMA BRUG
LIMA SPIEDENENSIS n. sp.

Plate XII. Fig. 6.

Shell small, moderately convex, obliquely subovate, posterior side produced below; beaks sharply incurved; ears small; surface markings consisting of twelve narrow, radiating ribs; spaces between the ribs smooth and much wider than the ribs themselves; characters of the interiors of the valves unknown.

Dimensions. A single fragmental specimen has the following approximate dimensions,—length, 12+mm.; height, 15+mm.; thickness, 7 mm.

Locality. Collected on Spieden Island, at the foot of Spieden Bluff.

Disposal of Type. University of Washington Paleontological Collection.

L. spiedenensis differs from Lima suciensis in the fact that it contains no minor radiating ribs, and by the lack of concentric lines of growth.