The Knight Island Group

excerts of

Chapter 4 Part VIII
Geology of Prince William Sound


(Photographs and illustrations omitted)


Knight Island is one of the most geologically interesting areas in Prince William Sound. The Knight Island group seems to represent a slice of ocean crust ("an ophiolite") that originated at an oceanic spreading center perhaps about 57 million years ago. These pillow basalts and underlying sheeted dikes were later transferred across a subduction zone (i.e. obducted) onto the crust of the North American continent. Unlike most ophiolites, the Knight Island sequence lacks an overlying layer of pelagic sedimentary rock (chert) created by a continuous rain of the silica-rich bodies of deep water microorganisms. Secondly, layered gabbro underlying the sheeted dikes have not been exposed. However, small gabbro intrusions and inclusions of gabbro and peridotite occur in several locations indicating that these rocks may underlie the exposed greenstones. Gravity and magnetic measurements suggest the presence of mafic and ultramafic material beneath the surface. A six-mile thick layer of dense ultramafic material is necessary to explain the high gravity measurements. Finally, the pillow basalts near the Bay of Isles on the southern end of the island are interbedded with ashy layers of typical Orca turbidites. The lack of deep-water sediments and the interbedding of the volcanic rocks with turbidites suggest that these rocks originated at a spreading center located near a continental margin.

The Knight Island Ophiolite, then, may represent a chunk of the Kula-Farallon Ridge which was interacting with the Border Ranges Trench as the rocks of the Chugach/Prince William Terrane were being carried northward aboard the Kula Plate sometime during the early Eocene.

Turbidity flows cascading down into the trench buried the sporadic extrusions of lava oozing from the spreading center creating interlayered turbidites and pillow basalts. The Ophiolite, thus, was probably incorporated into an accretionary prism which was later offscraped onto the North American Plate. During uplift (probably during the Miocene - about 15 million years ago) the Knight Island rocks were bowed up into a broad anticline and regionally metamorphosed into green-schists (greenstones). The western limb of the anticline facing Knight Island Passage dips gently westward exposing a three mile thick layer of pillow basalts whose tops face westward. The eastern flank of the island consists of a steeper dipping but thinner layer of pillow basalts whose tops face eastward. Thrust upward between these opposed pillows in the central core of the island are the sheeted dikes which once acted as underlying volcanic feeder vents to the extruding pillows. . . .


86. Point Eleanor, near the navigation marker, has a good outcrop of pillow basalts observable from a boat.


87. Lower Passage is a good place to observe geological features typical of the Knight Island group. From the protected Disk Island anchorage, one can explore the island's southern shore. This is an excellent location to observe typical pillow basalt features including pillow breccia, multilobed pillows, radial cooling cracks and vessicles. The cove about 1/2 mile south of the entrance to the anchorage contains a 10 foot adit near its head on the east shore. On the east side of the eastern entrance point to this cove, one can find some intensely malachite stained pillows. The easternmost point of the island displays the characteristic iron-stained pillows which mark most mineralized areas of the Knight Island group.

The east arm of Louis Bay to the south of Disk Island preserves traces of the area's mining history. Two prospects have been dug into the west shore near the head; near here, the remains of a miner's cabin marks the trail head to the main prospects at 550 ft. and 900 ft. above the bay. The pilings on the southeastern shore of the bay mark the location of a sawmill and steam plant dating from 1908.


88. Drier Bay hosts a number of mines and prospects and provides access to geological features characteristic of the rugged central section of Knight Island. The greenstones along the east and southeast shores of the bay contain xenoliths of gabbro and peridotite. Look for light greenish gray to dark inclusions in the massive greenstones.

An old mine tunnel in iron-stained, mineralized rock can be explored at the south end of Cathead Bay while several tunnels penetrate the cliffs between Mallard and Barnes Cove. At Barnes Cove old mining wreckage can be seen on the beach presumably supporting the claims of the Copper Coins Group 1/2 mile southwest of the cove at the 518 ft. lever or the Hercules prospect 1/2 mile southeast of the cove between the 300ft and 1000 ft. levels.

Another mine is located at the 700 ft. level 1/2 mile southwest of the head of Northeast Cove. The mountains at the head of Northeast Cove are a good place to observe sheeted dikes. A zone of sheared rock separating the pillow basalts from the zone of sheeted dikes (the Port Audrey Shear Zone) extends through Northeast Cove to the head of Mummy Bay. One mile south of the head of the cove in this sheer zone is a body of dense peridotite 650-350 ft. in diameter - a possible clue to what may lie below.

The two most successful mines in Drier Bay were above Port Audrey: the Jonsey prospect and the Nellie prospect. The Jonsey prospect is located in a mineralized shear zone containing pyrrhotite and chalcopyrite 1000 feet up on the ridge 1/2 mile east of the lagoon. Cables from the tramline to his mine can be found on the shores of the lagoon. The mine shipped only a small quantity of ore. The Nellie group is in a steel-walled ravine 3/4 miles east of Port Audrey and also contains pyrrhotite and chalcopyrite. . . .


90. Squire Island Anchorage is a good place to observe the interbedding of greenstones and turbidites. At its head in the eastern bight, one can see lavas intermingling with sandstones and slates. In the western bight, pillows intrude thick, tuffaceous layers of sediments. . .

(Chapter VIII. pp. 193-196, selections).