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Open-File Report 99-362

Preliminary Geologic Map of the Chugach National Forest Special Study Area, Alaska

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 1990, both the U.S. Geological Survey and U.S. Bureau of Mines were contacted by the Chugach National Forest (CNF) for the purpose of providing mineral resource information for the CNF Master Plan during the planning period fiscal years 1991-1994. This information is to address the terms and requirements of the 1986 Settlement Agreement and to provide mineral and geologic information useful to the CNF for making land-use decisions.

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 Chugach National Forest, located in the Kenai-Chugach Mountains physiographic province of Alaska (Wahraftig, 1965) is the second largest national forest in the United States and is about 9,000 square miles in area. This area encompasses scenic Prince William Sound, the largest embayment along the coast of Alaska between Cook Inlet and Cape Spencer on the Alaskan panhandle to the southeast.

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&deg 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 previous geologic mapping efforts by federal agencies in the Chugach National Forest were reconnaissance-scale efforts produced for regional geologic and resource studies. One of the earliest geologic map compilations covering the area was done by Moffit (1954). Nelson and others (1985) published a geologic map of the entire Chugach National Forest.

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.



The geology of the National Forest is dominated by two major lithologic units, the Valdez Group (Late Cretaceous) and the Orca Group (Paleocene and Eocene) (Schrader, 1900). The Valdez Group is part of a 2,200-km-long by 100-km-wide belt of Mesozoic accretionary complex rocks called the Chugach terrane (Jones and others, 1987). This terrane extends along the Alaska coastal margin from Baranof Island in southeastern Alaska to Sanak Island in southwestern Alaska. The Orca Group is part of an accretionary complex of Paleogene age called the Prince William terrane that extends across Prince William Sound westward through the Kodiak Island area, underlying much of the continental shelf to the west (Plafker, 1969, 1971; Tysdal and Case, 1979). Both groups consist largely of graywacke, siltstone and shale; the finer grained rocks commonly display a slaty fabric. Petrographic study of the clast composition from the graywackes has suggested that the sediments represent the progressive unroofing of a volcanic arc to its plutonic core (Dumoulin, 1987). A recent petrographic study Phillips (1996) concluded the composition does not vary with turbidite facies units. Framework modes of Orca Group sandstones from the Special Study Area range from 19 to 53 percent quartz grains, 25 to 70 percent feldspar grains, and 5 to 74 percent total lithic fragments (Phillips, 1996). The compositional variations reflect differences in the proportions of lithic fragments, derived from volcanic cover of the arc, versus quartz and feldspar, derived from deeper roots of the arc. Additional isotopic and petrographic studies of the sedimentary rocks from the two groups indicate the major source of these clasts was likely the Coast Mountains provenance in Canada (Farmer and others, 1993) and that a secondary input may have come from the Wrangellia and Peninsular terranes (Phillips, 1996).

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.

Tertiary Plutonic Rocks

Plutonic rocks in the study area were probably intruded around 39 Ma. The plutons are compositionally, lithologically, and petrographically similar to other dated 32-38 Ma plutons in Prince William Sound (Tysdal and Case, 1979; Nelson and others, 1985), and the Miners Bay pluton has been dated by K/Ar on biotite at 32.2±1.6 Ma and 38.4±1.9 Ma (Nelson and others, 1985). We obtained new 40Ar/39Ar analyses on samples of biotite, plagioclase, K-feldspar, and microcline from the major plutons, which lie between 59.1 and 25.4 Ma (see Table 2). We do not have 40Ar/39Ar release spectra to assess individual analyses. We note that three of the biotite ages from the Miners Bay pluton lie between 38.6±0.6 Ma and ~ 41 Ma, which is consistent with the K-Ar ages, but this suggests the 32.2 Ma K-Ar date suffered some argon loss. However, one 40Ar/39Ar isochron age on the main diorite phase of the Miners Bay pluton was 59.1±0.1 Ma (Table 2). This unusually old age may indicate the diorite is part of the older Sanak-Baranof suite of intrusive rocks in Prince William Sound or that the isochron age includes gas with excess argon or recoil effects. The fact that an isochron age, versus a plateau age, was calculated indicates the argon systematics were not straightforward. Other new dates on intrusions are on K-feldspar or microcline, which are known to have low argon closure temperatures (<200 &degC; McDougall and Harrison, 1988). All these dates are younger than K-Ar ages on the 32-38 Ma granitoids and thus we conclude these ages reflect uplift and final cooling of these intrusions. This is exemplified by the 25.4±0.1 Ma plateau on K-feldspar from granite of the Miners Bay pluton-this is significantly younger than the 38-41 Ma 40Ar/39Ar dates on biotite from the intrusion.

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.


After incorporation of the sediments into the accretionary complex the sediments were folded into kilometer-scale tight folds. The Special Study Area is at the hinge of the orocline of southern Alaska (Grantz, 1966) where the regional strike of bedding changes by about 90&deg . Adjacent structural domains in the study area defined by the trend of bedding may have up to a 90&deg difference in strike of bedding in adjacent domains. This change reflects a structural pattern developed during oroclinal bending similar to the style of bending a bar (Haeussler and Nelson, 1993). Because the plutons are locally but not pervasively involved in faulting related to oroclinal bending, Haeussler and Nelson (1993) suggested that the bending occurred during and soon after intrusion of the plutons.

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.


Barker, Fred, Farmer, G.L., Ayuso, R.A., Plafker, George, and Lull, J.S., 1992, The 50 Ma granodiorite of the eastern Gulf of Alaska: melting in an accretionary prism in the forearc: Journal of Geophysical Research, v. 97, p. 6757-6778.

Bradley, Dwight C., Haeussler, Peter J., and Kusky, Timothy M., 1993, Timing of early Tertiary ridge subduction in southern Alaska: in Till, A., and Dusel-Bacon, C., eds., Geologic studies in Alaska by the U.S. Geological Survey, 1992, U.S. Geological Survey Bulletin 2068, p. 163-177.

Bol, A.J. and Gibbons, Helen, 1992, Tectonic implications of out-of-sequence faults in an accretionary prism, Prince William Sound, Alaska: Tectonics, v.1, p. 1288-1300.

Bouma, A.H., 1962, Sedimentology of some flysch deposits: Elsevier, Amsterdam, 166 p.

Connelly, William, 1978, Uyak complex, Kodiak Islands, Alaska: a Cretaceous subduction complex: Geological Society of America Bulletin, v. 89, p. 755-769.

Crowe, D.E., Nelson, S.W., Brown, P.E., Shanks III, W.C., and Valley, J.W., 1992, Geology and geochemistry of volcanogenic massive sulfide deposits and related igneous rocks, Prince William Sound, south-central Alaska: Economic Geology, v. 87, p. 1722-1746.

Dumoulin, J.A., 1987, Sandstone composition of the Valdez and Orca Groups, Prince William Sound, Alaska: U.S. Geological Survey Bulletin 1774, 37 p.

Farmer, C.L., Ayuso, Robert, Plafker, George, 1993, A Coast Mountains provenance for the Valdez and Orca Groups, southern Alaska, based on Nd, Sr, and Pb isotopic evidence: Earth and Planetary Science Letters, v. 116, p. 9-21.

Grantz, Arthur, 1966, Strike-slip faults in Alaska: U.S. Geological Survey Open-File Report 267, 82 p.

Jones, D.L., Siberling, N.J., Coney, P.J., and Monger, J.W.H., 1987, Lithotectonic terrane map of Alaska (west of the 141st meridian): U.S. Geological Survey Miscellaneous Field Studies Map MF 1847-A.

Haeussler, Peter J., Bradley, Dwight C., Goldfarb, Richard J., Snee, Lawrence W., and Taylor, Cliff, 1995, Link between ridge subduction and gold mineralization in southern Alaska: Geology, v. 23, no. 11, p. 995-998.

Haeussler, P.J., and Nelson, S.W., 1993, Structural evolution of the Chugach-Prince William terrane at the hinge of the orocline in Prince William Sound and implications for ore deposits, in Dusel-Bacon, Cynthia, and Till, A.B., eds., Geologic Studies in Alaska by the U.S. Geological Survey, 1992: U.S. Geological Survey Bulletin 2068, p. 130-142.

Hudson, Travis, Plafker, George and Peterman, Z., 1979, Paleogene anatexis along the Gulf of Alaska margin: Geology, v. 7, p. 573-577.

Lanphere, M.A., 1966, Potassium-argon ages of Tertiary plutons in the Prince William Sound region, Alaska in Geological Survey Research 1966: U.S. Geological Survey Professional Paper 550-D, p. 195-198.

McDougall, Ian, and Harrison, T. Mark, 1988, Geochronology and thermochronology by the 40Ar/39Ar method, Oxford Monographs on Geology and Geophysics, 9, 212 p.

Moffit, F.H., 1954, Geology of the Prince Sound region, Alaska: U.S. Geological Survey Bulletin 989-E, p. 225-310.

Mutti, E. and Ricci Lucchi, F., 1978, Turbidites of the northern Apennines: introduction to facies analysis: American Geological Institute, Reprint Series 3, p. 127-166.

Nelson, S.W., Dumoulin, J. and Miller, M.L., 1985, Geologic map of the Chugach National Forest, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1645-B, 16 p., 1 pl., scale 1:250,000.

Nelson, S. W., Miller, M. L., and Dumoulin, J. A., 1989, Resurrection Peninsula ophiolite in Guide to the geology of Resurrection Bay, eastern Kenai Fjords area, Alaska, Guidebook, edited by S. W. Nelson, and T. W. Hamilton, Geological Society of Alaska, Anchorage, p. 10-20.

Nelson, S.W., Miller, M.L., Goldfarb, R.J., Snee, L.W., Sherman, G.E., Roe, C.H., and Balen, M.D., 1994, Mineral resource assessment of the Chugach National Forest Special Study Area in northern Prince William Sound, Alaska: U.S. Geological Survey Open-File Report 94-272, 20 p.

Nelson, S.W., and Nelson, M.S., 1993, Geochemistry of ophiolitic rocks of Knight Island, Prince William Sound, Alaska: in Dusel-Bacon, Cynthia, and Till, A.B., eds., Geologic Studies in Alaska by the U.S. Geological Survey, 1992: U.S. Geological Survey Bulletin 2068, p. 130-142.

Phillips, Patti J., 1996, Sandstone composition and provenance of the Orca Group, Chugach National Forest study area, south-central Alaska: in Moore, Thomas E. and, Dumoulin, Julie A. eds., Geologic studies in Alaska by the U.S. Geological Survey, 1994, U. S. Geological Survey Bulletin, B 2152, p. 157-167.

Plafker, George, 1969, Tectonics of the March 27, 1964, Alaska Earthquake: U.S. Geological Survey Professional Paper 543-I, p. 1-74.

----1971, Possible future petroleum resources of Pacific-margin Tertiary basin, Alaska in Future petroleum provinces of North America: American Association of Petroleum Geologists, Memoir 15 p. 120-135.

Plafker, George, and Campbell R.B., 1979, The Border Ranges fault in the Saint Elias Mountainsin Johnson, K.M., and Williams, J.L., eds., Geologic Studies in Alaska by the U.S. Geological Survey, 1978: U.S. Geological Survey Circular 804-B, p. 102-104.

Plafker, George, Jones, D.L., and Pessagno, E.A., Jr., 1977, A Cretaceous accretionary flysch and melange terrane along the Gulf of Alaska marginin Blean, K.M., ed., The United States Geological Survey in Alaska: Accomplishments During 1976: U.S. Geological Survey Circular 751-B, p. B41-B43.

Plafker, George, and Lanphere, M.A., 1974, Radiometrically dated plutons cutting the Orca Groupin Carter, Claire, ed., United States Geological Survey Alaska Progream, 1974: U.S. Geological Survey Circular 700, p. 53.

Roe, C.H. and Balen, M.D., 1994, U.S. Bureau of Mines mineral investigations in the Unakwik Inlet area, Chugach National Forest, Alaska: U.S. Bureau of Mines Open-File Report 50-94.

Schrader, F.C., 1900, A reconnaissance of a part of Prince William Sound and the Copper River district, Alaska, in 1898: U.S. Geological 20th Anniversary Report, pt. 7, p. 341-423.

Tysdal, R.G., and Case, J.E., 1979, Geologic Map of the Seward and Blying Sound quadrangles, southern Alaska: U.S. Geological Survey Miscellaneous Investigation Series Map I-1150, scale 1:250,000.

Wahrhaftig, Clyde, 1965, Physiographic divisions of Alaska, U.S. Geological Survey Professional Paper 482, 52 p.

Winkler, G.R., 1973, Geologic map of the Cordova A-7, A-8, B-6, B-7, and B-8 quadrangles, Hinchinbrook Island, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-531, 1 sheet, scale 1:63,360.

-----1992, Geologic and summary geochronology of the Anchorage 1&deg x 3&deg quadrangle, southern Alaska: U.S. Geological Survey Miscellaneous Investigations Series, Map I-1984.

Winkler, G.R., and Plafker, George, 1981, Geologic map and cross sections of the Cordova and Middleton Island quadrangles, southern Alaska: U.S. Geological Survey Open-File Report 81-1164, 24 p.

-----1993, Geologic map of the Cordova and Middleton Island quadrangles, southern Alaska: U.S. Geological Survey, Miscellaneous Investigation Series, Map I-1984.

Winkler, G.R., and Tysdal, R.G., 1977, Conglomerate in flysch of the Orca Group, Prince William Sound, southern Alaskain Blean, K.M., ed., U.S. Geological Survey in Alaska: Accomplishments During 1976: U.S. Geological Survey Circular 751-B, p. B43-B44.

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