Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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146 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 GEOLOGY IN THE STUDY AREA The sedimentary rocks within the study area are domi- nantly turbidites of Mutti and Ricci-Lucci (1978) facies B and C, with less common facies A and D. The map pattern of lithofacies is roughly symmetric about a northeast- southwest-trending axis that extends from the south end of Wells Bay to the east end of Kadin Lake (S. Nelson, unpublished mapping, 1992). Bedding thicknesses range from about 3 cm to about 3 m. Conglomerates (facies A) up to about 300 m thick are present in the northwest (fig. 2). There are also conglomerates adjacent to the Columbia Red Metals massive sulfide occurrence (CRM, fig. 2), but are too small to be shown on our map. Volcaniclastic sedimentary rocks, broken pillow breccia, rare pillow lava, and limestone was found at two small localities (fig. 2), one about 3 krn southeast of Miners Lake and the other west of Terentiev Lake and north of Long Bay. There are four felsic to intermediate plutons within the study area, informally named: the Cedar Bay, Granite Cove, Terentiev Lake (which has an apophysis along the Columbia Glacier), and Miners Bay plutons (fig. 2). The first three of these are texturally homogenous, fine- grained, equigranular granite. The Cedar Bay pluton has some tonalite. The other pluton, the Miners Bay, has two compositional phases, with the main (western) part that consists chiefly of gabbro and diorite, and the eastern "tail" is granite. This pluton is the only one in the study area that has been radiometrically dated, with K-Ar ages on biotite and hornblende of 32 and 38 Ma, respectively (Nelson and others, 1985). Nelson and others (1985) correlated the Cedar Bay, Terentiev Lake, and Granite Cove plutons with the other Paleocene to Eocene plutons in Prince William Sound based upon petrography. However, recent major-, minor-, and trace-element geochemical analyses (S. Nelson, unpublished data) indicate that these plutons are distinct from the Paleocene to Eocene plutons and are most similar to the -35-Ma plutons in the western part of Prince William Sound. We thus consider all the plutons in the study area to be in that group. Dikes of several compositions are present in the study area. There are pegmatite and autobrecciated granitic dikes (discussed below) associated with the Granite Cove pluton. Felsite dikes with a fined-grained sugary texture are present throughout the study area south of the Contact fault (fig. 2). Two dikes on the peninsula between Unakwik Inlet and Wells Bay contain phenocrysts of centi- meter-size K-feldspar and quartz in a fine-grained matrix. Dikes with this distinct texture and composition are only found in the vicinity of Unakwik Inlet (Nelson and others, 1985). Because of the spatial association of all the dikes with the plutons, they are probably related and the same age. Metamorphism in the study area is generally to the laumontite to mid-greenschist facies (white mica 2 chlorite -+ epidote * incipient biotite) in lithic graywackes (Goldfarb and others, 1986). Macroscopic metamorphic minerals were not observed outside of contact aureoles, and slatey cleavage was not commonly observed in the field or in thin section. When present, it was a weak pressure-solution cleavage, like that documented in correlative rocks to the southwest (Gibbons, 1988) (fig. 1). Mapping by Nelson and others (1985) indicates that a slatey cleavage was observed to the north of the study area. The contact aureoles of the plutons are upper greenschist to amphibolite facies in pelitic rocks (biotite + muscovite + quartz + staurolite k andalusite * cordierite + garnet), and are generally no more than 0.5 km wide. The aureoles are up to 2 km wide on the north side of the Terentiev Lake pluton and 2.5 km wide along the southwest side of the Cedar Bay pluton, which suggests that the plutons may extend beneath these outcrops. Mapping around Terentiev Lake demonstrates that the erosion level coincides with the top of the Terentiev Lake pluton. The contact between the igneous rocks and the overlying sedimentary rocks is subhorizontal, and is spectacularly ex- posed in outcrops recently revealed by a 100-m drop of lake level within the last 10 years due to retreat of the Columbia Glacier. STRUCTURAL GEOLOGY The early structural history of the rocks in the study area involved minor soft-sediment deformation and the de- velopment of web structure. Fine-grained clastic dikes up to 2 cm wide and 5-6 cm long intrude mudstone and fine- grained siltstone at one locality, and similar soft-sediment features were observed outside of the study area. Web structure (Cowan, 1982; Byrne, 1984) was not observed in the field but was seen in thin sections, and has also been reported in correlative rocks to the southwest by Bol and Gibbons (1992; Gibbons, 1988; see fig. 1). We have divided the study area into 11 structural domains for analysis (fig. 3). We define each domain by roughly consistent strike of bedding, and domain boundaries are defined by discordances in bedding (figs. 3, 4). Therefore, each domain should be characterized by cylin- drical folds with the trend of fold axes in each domain discordant to those in adjacent domains. In one case, fold axes be traced between domains (B and C) because these domains were divided on the basis of the overall change in the strike of bedding between the west end of domain B and the east end of domain C, despite no drastic change in strike at the B-C boundary (fig. 3). We also separated domain A from those to the south by the Miners Bay-Kadin Lake splay of the Contact fault, in order to examine the differences between the domains on either side of the fault. Several other domain boundaries can be demonstrated to be faults. Domain F is bounded by a high-angle fault on its
STRUCTURAL EVOLUTION OF THE CHUGACH-PRINCE WILLIAM TERRANE 147 southwestern margin and inferred faults on its other mar- gins. A northeast-southwest-striking fault corresponds with the boundary between domain D and domains F and H. A fault with a similar orientation roughly corresponds with the domain I-J boundary. FOLDS Folding in the study area is indicated by rocks dipping in opposite directions, but having the same strike and fac- ing direction. The wavelength of these folds based upon our mapping (fig. 2) is commonly 1.5 to 2.5 krn, but may range from 1 to 4.5 krn. The amplitude of the folds is not known, although it is almost certainly greater than their wavelength. Because the wavelength is so large, fold hinges were not generally observed. However, between Miners Lake and Wells Bay in domain C there are gentle undulations of bedding with a wavelength of about 100 m that are probably at the crest of an anticline (M-folds). We generally do not have enough data to trace fold axes later- ally more than 2 to 3 km. In domain C, however, con- glomerate marker beds allow a fold to be traced for 12 km. Stereograms of poles to bedding in each domain help to define the geometry of the folds, and we determined a best-fit fold axis for each domain, if enough data had been collected to define a girdle. Four of the eleven domains did not have sufficient data (N r 20) to clearly discriminate fold axes. Although the trends of the best-fit fold axes are variable between domains, these fold axes are subhorizontal, except in domain F, which has a different structural style and is discussed separately below. Outside of domain F, 12 mesoscale fold axes were measured, and in most cases these fold axes do not coincide with the best-fit fold axes determined from the girdle of poles to bedding. In correlative rocks to the southwest Bol and Gibbons (1992) (fig. 1) found that mesoscale fold hinges were ori- ented similarly to the best-fit fold axis determined from the Figure 3. Form-line map of interpretive strike of bedding (solid lines) and structural domains, A to K (dotted lines). Data for domains A and B also taken from unpublished mapping in Anchorage A-2 and Seward D-2 quadrangles north and west of study area by S.W. Nelson, M. Miller, J. Dumoulin, G. Winkler, S. Karl, and J. Case.
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Geologic Studies in Alaska by the U
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CONTENTS Introduction Cynthia Dusel
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CONTENTS CONTRIBUTORS TO THIS BULLE
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GEOLOGIC STUDIES IN ALASKA BY THE U
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LATE HOLOCENE LONGITUDINAL AND PARA
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DEEP-WATER LITHOFACIES AND CONODONT
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Table 1. Locality register for key
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Table 1. Locality register for key
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LITHOFACIES AND CONODONTS OF CARBON
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LITHOFACES AND CONODONTS OF CARBONI
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Table 1. Conodont faunules and lith
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DEPOSITIONAL SEQUENCES IN ATIGUN SY
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U-Pb AGES OF ZIRCON, MONAZITE, AND
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APPEAL FOR NONPROLIFERATION OF ESCA
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FAVORABLE AREAS FOR METALLIC MINERA
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GOLD AND CINNABAR IN HEAVY-MINERAL
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GOLD AND CINNABAR IN HEAVY-MINERAL
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Table 3. Summary of geochemical sig
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GEOCHEMICAL CHARACTER OF UPPER PALE
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RECONNAISSANCE GEOCHEMISTRY OF BASA
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OSTRACODE ASSEMBLAGES FROM MODERN B
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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US. GEOLOGICAL SURVEY REPORTS ON AL
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U.S. GEOLOGICAL SURVEY REPORTS ON A
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U.S. GEOLOGICAL SURVEY REPOR TS ON
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REPORTS ABOUT ALASKA IN NON-USGS PU
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REPORTS ABOUT ALASKA IN NON-USGS PU
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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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