Open-File Report 99-362
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey (USGS) editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
In early 1992 an Interagency Agreement between the U.S. Geological Survey, the U.S. Bureau of Mines and the Chugach National Forest was signed. In this agreement the U.S. Geological Survey is to provide a report which estimates the undiscovered mineral endowments of the 'special' study area and to identify the potential for mineral discovery and development. The U.S. Bureau of Mines was to prepare a report updating the discovered mineral endowment of the Special Study Area. These reports are now published (Roe and Balen, 1994; Nelson and others, 1994). This geologic map is a component of the U.S. Geological Survey contribution to the overall project.
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The Special Study Area is located in the northern part of Prince William Sound (Fig. 1 in PDF file linked below). The Special Study Area is about 400 mi2 in area and is bounded by Unakwik Inlet on the west and Columbia Glacier and Columbia Bay on the east. The southern part of the area includes Glacier Island and the northern boundary is approximately latitude 61° 07' N. The coastline of the area is indented by fjords that were produced by south-flowing glaciers. The lower slopes are densely vegetated with stands of spruce, cedar, hemlock, alder, and devils club. Timber line is located at about 1500' elevation. Relief in the area ranges from sea level to 4,800' at "X Mountain" (T11N, R12W, Sec. 3, 4).
Most geologic mapping under the U.S. Geological Survey's Alaska Mineral Resource Assessment Program (AMRAP) covered 1:250,000-scale quadrangle maps. Quadrangle maps covering the Chugach National Forest include: Tysdal and Case (1979) for the Seward and Blying Sound quadrangles, Winkler and others (1981) for the Valdez quadrangle, Winkler (1992) for the Anchorage quadrangle, Winkler and Plafker (1993) for the Cordova and Middleton Island quadrangle, Winkler (1973) published a 1:63,360-scale geologic map covering Hinchinbrook Island in the Cordova quadrangle.
Both groups also contain thick sections of mafic volcanic rocks (Nelson and others, 1985). In the Orca Group the mafic rocks comprise an ophiolite section that contains, from base to top, ultramafic rocks, gabbro, sheeted dikes, and pillow basalt (Crowe and others, 1992; Nelson and others, 1985; and Nelson and Nelson, 1993). The volcanic section in the Valdez Group also contains thick pillow basalt, lesser sheeted dikes, gabbro, and ultramafic rocks. These units are spatially associated with each other. Both volcanic sections host massive sulfide deposits that were worked primarily for copper (Crowe and others, 1992). These volcanic sections can be distinguished geochemically because those in the Valdez Group are more alkaline than those in the Orca Group (Crowe and others, 1992).
The contact between the Orca and Valdez Groups is designated as the Contact fault (Winkler and Plafker, 1981). In eastern Prince William Sound the location of the Contact fault is based on the change in structural trend in the two groups. Between the Copper River and Port Fidalgo the regional strike of the Orca Group is northeast. The Valdez Group in this area exhibits an east-west regional strike. In western Prince William Sound the regional strike of the two groups is parallel. This coupled with the close lithologic similarities of the two groups, makes location of the contact problematic in western Prince William Sound (Bol and Gibbons, 1992).
Plutonic rocks in the forest were emplaced during two main intrusive episodes. The earliest intrusive episode has been dated by potassium-argon methods as 50 to 53 Ma (Plafker and Lanphere, 1974; Nelson and others, 1985) and at around 53-54 Ma by 40Ar/39Ar (Haeussler and others, 1995). Rocks of this age are found in eastern Prince William Sound and to the west of Prince William Sound. These plutons have been assigned to the Sanak-Baranof plutonic belt (Hudson and others, 1979) are thought to have formed from large melt fractions of the graywacke host (Barker and others, 1992) during subduction of a spreading center beneath the accretionary complex (Bradley and others, 1993 and references therein). The younger plutonic episode has been dated by both potassium-argon and 40Ar/39Ar methods (Lanphere, 1966; Nelson and others, 1985; this study). Potassium-argon dates of hornblende and biotite for the younger intrusive episode in Prince William Sound lie between 32.2±1.6 Ma and 38.4±1.9 Ma (Nelson and others, 1995).
Rocks of both the Orca and Valdez Groups are found in the Special Study Area. The Valdez Group is represented by low-grade metamorphosed turbidites found north of the Contact fault. Rocks of the Orca Group are found to the south of the Contact fault and consist of both turbidites and mafic volcanic rocks.
Geologic mapping at 1:63,360-scale of both groups has focused on the distribution of the depositional facies (Mutti and Ricci Lucchi, 1978) in turbidites both for structural interpretation and controls for mineralization (Haeussler and Nelson, 1993). Two volcanic rock-associations are found in the study area. One association is the volcanic rocks found on Glacier Island that consist of pillow basalt and sheeted dikes typical of an ophiolite association (Crowe and others, 1992). The ophiolite of Glacier Island is part of a 100 km-long belt of ophiolite that extends from Elrington Island in the south, north through Glacier Island and east to Ellamar (Crowe and others, 1992). These ophiolitic rocks are inferred to have formed around 57 Ma, which is the radiometrically determined age of the Resurrection Peninsula ophiolite near Seward (Nelson and others, 1989). We obtained a 40Ar/39Ar plateau on plagioclase from the Glacier Island sequence in the study area of 37.6±0.6 Ma. We interpret this age as being reset during intrusion of the 32-38 Ma plutons.
The second association of volcanic rocks consists of volcaniclastic rocks and pelagic limestone. These rocks are found in two fault-bounded areas within the Orca Group turbidites. Volcanic mudstone with broken pillow breccia, purple or green calcareous shale, and thin beds of gray, green, and purple limestone are characteristic of the unit. These rocks are unusual because they contain fossils older than the enclosing Orca Group and older than the Valdez Group that lies to the north of the Contact fault. These rocks are younger than the McHugh Complex of (Clark, 1973), are age-correlative with the 'Cape Current terrane' of Connelly (1978), and thus they are probably correlative, even though structurally they appear dissimilar. These rocks also contain mid-Eocene fossils, which may be explained by tectonic mixing of sediments of dissimilar ages.
The other plutons in the study area, which have similar chemistry (Table 3) and petrography, include the Terentiev Lake, Granite Cove and Cedar Bay plutons. Petrographically these plutons have modal compositions that overlap the modal compositions of the two dated suites (Fig. 2), however, their chemistry is distinctly more enriched in alkalis and silica than any of the dated plutons (Fig. 3) from other locations in western Prince William Sound (Nelson and others, 1985).
Strontium and neodymium isotopic data (Table 4) indicate the ~38-41 Ma granites in Prince William Sound and in the study area were derived from melting of relatively evolved rocks. This hypothesis is consistent with their origin being anatectic melts of the greywackes. The Knight Island ophiolite was derived from a relatively primitive source region. The Miners Bay gabbro has a modeled initial Sr and Nd ratios intermediate between the ophiolite and the granitoids.
The major structural break in the area is the Contact fault. This fault separates the Valdez Group sedimentary rocks from the Orca Group rocks to the south. A crude minimum displacement along the fault can be estimated by the apparent offset of the conglomerate unit to the south of Miners Lake. The closest conglomerate on the north side of the Miners Bay-Kadin Lake splay is located to the east of the of the Miners Bay pluton. The apparent left-lateral offset of the Miners Bay pluton may not be real since the eastern sliver is coarse grained granite and no granite is found along the south side of the Miners Bay pluton.
Numerous faults and lineaments cross the study area (Haeussler and Nelson, 1993). Most faults are parallel with a north-south or NNW-SSE orientation. Displacements are minor since no significant offset of geologic contacts were observed. One exception is the fault that cuts the north end of the Cedar Bay pluton. Although this fault does not offset the pluton-sedimentary rock contact it does structurally enclose the undivided Late Cretaceous volcanic and sedimentary rock unit within the Orca Group. This structural relation may indicate a significant, pre-Cedar Bay pluton, extensional(?) fault. Strike-slip displacements of several kilometers are permissible on some faults that juxtapose different sedimentary facies if they are older than the plutons.
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