Reconnaissance Geology of Amlia Island, Aleutian Islands, Alaska

Reconnaissance geology of Amlia Island, Aleutian Islands, Alaska

HUGH McLEAN JAMES R. HEIN } U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 TRACY L. VALUER)

174°

ABSTRACT

Amlia Island, located in the central part of the Aleutian Island Atka island chain, exhibits excellent shoreline exposures of Paleogene sedimentary and volcanic rocks, which record the earliest observable history of the volcanic arc. Volcaniclastic sedimentary rocks deposited by debris flow, turbidite, and grain-flow processes interfinger with volcanic flows and breccia. Pillowed flows as well as massive columnar flows ranging in composition from basalt to rhyolite record interfingering episodes of submarine and subaerial deposition. The occurrence of Reticulofenestra reticulata and Dictyococ- lOL cites scrippsae in laminated siltstone and sandstone beds indicates Haystack that the rocks on Amlia are late middle Eocene to early Oligocene Rock in age. Whole-rock radiometric ages of 32 and 39 m.y. from igneous rocks that interfinger with the sedimentary rocks corroborate the fossil ages and are correlative in age with the Hidden Bay pluton on 52' Adak Island. Some of the rocks on Amlia Island are only slightly altered and contain authigenic zeolite phases; other rocks are highly altered and contain prehnite, pumpellyite, and laumontite. Areal distribution of 180° 160° highly altered rocks may reflect proximity to shallow-seated plu- u.s.s tons, such as those that crop out on nearby Atka Island, which probably underlie much of Amlia. The island is structurally simple, with dips rarely exceeding 10° or 15°. High-angle faults with small displacement are common, however, and may reflect emplacement of shallow-seated intrusive rocks. Wave-cut terraces around much of the island appear to be in equilibrium with present sea level and suggest recent tectonic stability.

INTRODUCTION Amlia Island is about 72 km long in an east-west direction, Amlia Island, located in the central part of the Aleutian Island about 8 km wide, and has a rugged shoreline marked by high cliffs chain, is the easternmost island of the Andreanof Island group and numerous bays. The topography of the island is characterized (Fig. 1). It lies just 2 km east of Atka Island; the two islands are by a central east-west-trending ridge with peaks ranging from 500 separated by the narrow and often turbulent waters of Amlia Pass to 600 m above sea level. Vegetation consists of a mat of tundra and (Fig. 1). grass; no trees grow on the island.

Supplementary data in the form of six tables for this article may be secured free of charge by requesting Supplementary Data 83-12 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 94, p. 1020-1027, 5 figs., August 1983.

1020

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Amlia is an ideal place in which to map the older rocks of the contains a reconnaissance geologic map with photos of Atka and central Aleutians because there are no active volcanoes, nor any Amlia Islands. Pliocene or Pleistocene volcanic terrane. The nearest active volca- STRATIGRAPHY AND PALEONTOLOGY noes are on Atka Island, which lies to the northwest, and Seguam Island, which lies to the northeast. Prior to our work in 1979, Amlia The principal rock types on Amlia Island are volcanic sand- Island had not been mapped geologically; however, reconnaissance stone and siltstone, volcanic conglomerate, and volcanic breccia geologic studies on Atka Island in 1977 (Hein and McLean, 1980a) that interfinger with abundant pillow lavas and massive flows of indicated that the western end of Amlia consisted mainly of Ter- basalt, andesite, dacite, and rhyolite (Fig. 2). Plutonic rocks are tiary sedimentary rocks. restricted to two small gabbro plugs(?) on the north side of the In the summer of 1979, we spent nine days mapping the geol- island and a tonalite sill complex on the south side. Stratigraphic ogy of Amlia Island. The island was circumnavigated using small relations between the sedimentary sequences and the volcanic flow boats powered by outboard motors, a method that permitted close sequences are complex, and rapid changes in facies complicate inspection of sea-cliff and nearshore outcrops. Frequent landings detailed correlations.

173'

Sandstone, siltstone, and massive conglomerate

Volcanic rocks with massive and columnar flows

Volcanic rocks with pillow flows

\o Strike and dip of beds Figure 1. Index map showing location of Aleutian Islands and Amlia Island, and generalized geologic map of Amlia Island Apparent strike and dip of beds and eastern tip of Atka Island, Aleutian Islands, Alaska. Anticline

Syncline

Fault

Triangulation station

were made to collect samples for studies of lithology, chemistry, The west end of the island, from Hungry Bay to Amlia Pass and paleontology. Outcrops in the interior of the island are sparse (Fig. 1), is mainly massive volcanic sandstone and volcanic breccia because of extensive vegetative cover; however, two short traverses with minor pillow lavas and dikes. The eastern end, similarly, is into the hills around Hungry Bay permitted measurement of strati- massive beds of volcanic sandstone and tuff interbedded with vol- graphic sections of volcanic and sedimentary rock. canic flows. The central part of the island along both north and This paper describes the geology, stratigraphy, structure, age south shores is mainly volcanic flows, both pillowed and massive, relations, sandstone petrology, and alteration of Amlia Island interlayered with subordinate volcanic breccia and thin-bedded based on our reconnaissance mapping. Igneous petrology and sandstone and siltstone. Columnar flows appear to predominate chemistry are summarized herein and discussed in more detail by along the mountain ridges above elevations of 300 ip (Figs. 2 and 3). T. L. Vallier and others (unpub. data). Hein and others (1981) Several megafossils, including a fossil fish tentatively identified

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E X PL ANATION

Volcanic flows, V. E. = 2.5 x nTrrTTl massive and columnar 2 3 MILES . I _L Pillowed flows "I 2.5 5 KILOMETERS

Sandstone

Volcanic and Figure 2. Generalized cross section of structural and stratigraph- • c • c. sedimentary breccia ie relations through the central part of Amlia Island. See Figure 1 for Fault-Arrows indicate location. direction of relative movement

as a member of the Jack family Carangidae (Fig. 4A), fragments of metric ages of Amlia Island are correlative with ages reported for mollusks, and a specimen of alga1 are not age diagnostic. the Hidden Bay pluton on Adak Island (Citron and others, 1980). Calcareous, nannofossils (coccoliths), however, from seven localities contained age-diagnostic species (Table I).2 A single specimen of SEDIMENTARY ROCKS Coccolithus eopelagicus in one sample indicates a Paleogene age; the occurrence of Reticulofenestra reticulata (Gardner and Smith) The sedimentary sequence at the east and west ends of the is diagnostic for a late middle Eocene to early Oligocene age. A island is gently folded, with dips averaging about 10°. Bed forms sample from the southwest shore, located about 2.5 km north of range from massive, channelized, pebble- and cobble-bearing, very Haystock Rock, also contains R. reticulata and R. umbilica as well. coarse-grained sandstone deposited by debris flows to very thin- A sample from the northeast part of the island from a locality about bedded and laminated, fine-grained sandstone and siltstone depos- 8 km west of triangulation station Brat contains one fragment of ited by turbidity currents (Figs. 4B and 4C). Rapid stratigraphic Dictyococcites scrippsae (Bukry and Percival) that indicates a mid- changes in grain size and bed thickness are common, and inter- dle Eocene to Oligocene age. bedded pyroclastic breccia and tuff attest to pulses of volcanism Three samples analyzed for palynomorphs were found to be contemporaneous with marine sedimentation. nearly barren; however, another sample contained rare palyno- Massive sandstone beds exposed at the eastern tip of the island morphs including Picea, Pinus Haploxylon, and Triplanosporites- (Fig. 4D) resemble the inner submarine fan facies B and C of Mutti Lygodium?. and Ricci-Lucchi (1978). Turbidite sandstones usually consist of

We believe that the volcanic rocks that interfinger with the Bouma Tb and Tc intervals; beds rarely contain the graded Ta inter- sedimentary rocks are coeval and equivalent in age. Two minimum val. Tracks, trails, and soft sediment deformation are common fea- K-Ar ages from slightly altered volcanic rocks are late early Oligo- tures in thin-bedded turbidite sequences. Thin-bedded and lam- cene; this age approximates the early Oligocene nannofossil ages inated sequences, whicl for the most part are composed of tuff (Table 2).3 A pyroxene andesite from a columnar flow located in a (altered to secondary minerals), probably are the result of reworked small bay 6 km west of Sviechnikof Harbor is 32.0 ± 1.0 m.y.; a pyroclastic debris or water-lain volcanic ash. rhyolite dike in a cove 2 km west of triangulation station Brat is Massive, coarse-grained, pebbly sandstone at the west end of 32.7 ± 1.0 m.y. old. A fragment of gabbroic float from the east shore the island locally contains low-angle trough cross-bedding that dips of inner Hungry Bay is 39.8 ± 1.2 m.y., which corresponds to a late to the west and may indicate grain flow produced by tidal currents. middle Eocene age (Lowrie and Alvarez, 1981). The 32-m.y. radio- Imbricated clasts in debris flows near Hungry Bay show southwest- erly flow.

'A specimen of alga(?) having a radial form, about 10 cm in diameter, Sandstone Petrography that resembles the so-called Annularia stellata on Adak Island was found in beds of laminated silty sandstone along the east shore of Hungry Bay. See Forty sandstone and sandy siltstone samples were examined discussion of Annularia stellata in Scholl and others (1970, p. 3586). petrographically and by X-ray diffraction to determine the compo- Tables 1 through 6 may be secured free of charge by requesting Sup- plementary Data 83-12 from the GSA Documents Secretary. sition and abundances of framework grains and cement and to 3See footnote 2. determine the nature and extent of diagenesis.

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LITHOLOGY — E DESCRIPTION I — H 400- Top of section iirrrrrTrHTn;

350 Porphyritic basaltic andesite flows, most are 12 80 massive. Rare beds of volcanic breccia and volcanic sandstone. Forms cliffs.

300- Interlayered basaltic andesite flows, volcanic II 30 breccia, and volcanic sandstone. Some flows may be pillowed. Deeply weathered.

A A Â-.'û'-'û .A A Pebbly volcanic breccia and volcanic sandstone. ufi ifnti fi fi fi I ftu 10 35 Some crossbedding. Rare thin basaltic andesite O 250- a*¿v.-¿^aVV-' flows. a. a-.A •. A .• a. A • a'/fl'' M M 1 M 1 III / IIMil l l/lllllllll Massive andesite(?) flows, columnar jointing common, light-brown, aphanitic. llllllllll 1 llllll Coarse volcanic breccia, dark-brown, interbedded with rare thin volcanic sandstone beds. - 200 Porphyritic basaltic andesite flow. Pebbly volcanic breccia and volcanic sandstone, interbedded. Attitude U. 70° E., 11° S. Some crossbedding.

Interlayered basalt (dominant) and porphyritic basaltic andesite flows, massive, non-pillowed. Some interlayered sandstone beds as much as 1 m thick.

Thinly bedded volcanic sandstone. (See unit 2.) 100 Massive porphyritic basaltic andesite flows. Thinly bedded, pale-brown volcanic sandstone.

Interlayered basaltic andesite and basalt flows. Basaltic andesite is porphyritic, is brownish- black, and weathers pale brown. Basalt is less porphyritic, is black, and weathers pale brown. Interval is covered; volcanic breccia and sandstone probably are interbedded.

Figure 3. Stratigraphie column of upper part of rock sequences on Amlia Island. Measured by altimeter and Brunton traverse on S20-m peak located south of the head of Hungry Bay. Base of section begins at an elevation of 150 m.

Sandstone framework grains are composed mainly of volcanic Accessory grains include opaque minerals, rare recrystallized rock fragments and lesser amounts of plagioclase; clinopyroxene, foraminifers, diatoms, shell fragments, rare epidote, and polycrys- usually augite, is present. Rock fragments include glass shards, talline quartz. We believe that the quartz grains are probably pumiceous vitrophyre, porphyritic vitrophyre, aphyric semiopaque reworked amygdules derived from erosion of volcanic flows. Some groundmass, microporphyritic andesite, and felsic groundmass rocks contain zeolite and rare prehnite grains that may be of detrital consisting of microcrystalline quartz. Framework grains are mod- origin. erately to well sorted and are chiefly angular. Open framework packing and the absence of mechanical grain

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Figure 4. A. Macrofossils are very scarce on Amlia Island; this fossil fish is tentatively identified as a member of the Jack family Carangidae but is not diagnostic of an age. B. Thin-bedded turbidite sandstone overlain by disorganized volcanic conglomerate; a reflection of rapid facies changes within the sedimentary sequence. Outcrop located at southwestern end of island, about 3 km north of Haystack Rock. C. Pebbly sandstone at base grading up into bedded sandstone and siltstone, and capped by a pillowed flow (top center). Man at right for scale. Outcrop located on north shore, 10 km east of Cape Idalug. D. Massive sandstone beds exposed at southeastern tip of Amlia Island resemble the inner submarine fan facies B and C of Mutti and Ricci-Lucchi (1978).

deformation indicate that sand grains probably were cemented at The composition of the sedimentary rocks on Amlia Island shallow burial depths and probably never were deeply buried. Well- reflects erosion of a volcanic terrane and the likelihood that the sorted framework grains cemented by sparry calcite or by clear volcanoes were also active during sedimentation. Lithofacies reflect zeolite attest to high initial porosity and permeability. Analyses of massive movement of sediment into marine environments locally four samples of well-sorted sandstone indicate that although poros- dominated by strong tidal(?) currents and turbidite sedimentation. ities are moderately high, the permeabilities are rather low The assemblage of lithofacies on Amlia Island indicates a paleogeo- (Table 3).4 graphic setting similar to that of the modern Aleutian arc; an Analyses of total organic carbon content and percentage of archipelago with active volcanic centers interspersed with small sed- CaCC>3 in sandstone, siltstone, and mudstone samples are shown in imentary basins located along the crest of the arc massif. Table 4.5 Organic carbon content is uniformly low, less than 1%. Calcite ranges from 1% to 55%; the higher values are from calcare- IGNEOUS ROCKS ous siltstone and sandstone. On Amlia Island, igneous rocks occur as massive and pillowed flows, as dikes, as flow breccias, as sills, and as intrusive plugs. 4'5 See footnote 2. Flows range in composition from basalt to rhyolite. Rock names

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Figure §. A. Volcanic flow rocks exposed along south shore of island. Columnar-jointed volcanic flow interbedded with volcanic breccia. B. Pillow basalt and filled lava tube. C. Pillowed flow of dacite lava exposed along northeast shore of island. Largest pillows have diameters as large as 3 m. Light-colored rims result from prehnite replacement of glassy pillow selvage. D. Elongate bulbous pillow lava.

are based on the classification system of Irvine and Baragar (1971, lowed flows are common on Amlia, in contrast to Atka Island, p. 538), which plots normative color index versus normative plagio- where they are rare. Pillows in basaltic flows include elongate bul- clase composition. This classification is modified by SÍO2 content as bous forms (Fig. 5D), whereas pillows in more silicic flows tend to follows: basalt < 51%, basaltic andesite 51% to 55%, andesite 55% be larger in diameter (as large as 3 m) and more equant (Fig. 5C). to 63%, dacite 63% to 69%, and rhyolite >69% (Table 5).5 Detailed Stratigraphic relations between massive columnar jointed flows and chemistry of igneous rocks in other Aleutian Islands is reported by pillowed flows are complex; in an outcrop 3 km west of Sviechnikof T. L. Vallier and others (unpub. data); details of pétrographie Harbor, a filled lava tube is surrounded by pillows (Fig. 5B). Flows modal analyses are described by Friesen (1982). also interfinger with breccia and fine-grained sedimentary rocks. Outcrops of breccia are massive and are crudely bedded; nearby Field Relations source areas are inferred from the coarseness of breccia clasts. Other igneous rocks include dikes, volcanic plugs, and lava domes, Volcanic-flow rocks composed of basaltic andesite and ande- which have compositional ranges similar to those of flows. Dikes site are the volumetrically predominant rock types on Amlia Island, seem to be oriented variably, but many trend northeast, a direction and they crop out extensively along the central part of the island. that is subparallel to the long axis of the island. Dacite and rhyolite flows are mainly restricted to the northeast Intrusive igneous rocks that have plutonic textures (weakly shore. Flows are both massive and pillowed (Figs. 5A, 5B). Pil- porphyritic, granular) crop out in three places on Amlia Island. A

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25 -m-thick sequence of layered fine-grained pyroxene tonalite crops of secondary minerals are layered. Prehnite is rich in Fe and Ca out on the south shore 14 km east of Cape Misty. The layered units relative to laumontite, which is relatively enriched in Si and A1 are about 4 m thick and persist laterally for several hundred metres. (Table 6).6 The mineralogic association prehnite-quartz-laumontite- Gabbro float observed along the beach on the east shore of pumpellyite, the paucity of trace-element concentration (Table 6), inner Hungry Bay presumably comes from a nearby outcrop that is and the textures suggest that these minerals, in part, precipitated covered with alluvium. The gabbro consists of calcic plagioclase, from hydrothermal solutions into void space and in part replaced clinopyroxene, orthopyroxene, and opaque minerals. A whole-rock glassy volcanic debris or minerals that replaced the glass originally; K-Ar age of 39.8 ± 1.2 m.y. was calculated for this rock. A small the temperatures of formation were probably in the range of about hypabyssal pluton composed of pyroxene gabbro is exposed 1 km 150 to 250 °C at low confining pressures (Kuniyoshi and Liou, south of triangulation station Smog on the north shore of Amlia 1976; Liou, 1971, 1979). Island. The rock is overlain by massive and columnar flows of Smectite is the most common secondary mineral in sedimen- andesite. tary rocks; prehnite, pumpellyite, and epidote are minor compo- nents. Quartz, analcite, heulandite, and laumontite are also Mineralogy and Summary of Chemistry abundant secondary minerals. In volcanic rocks, the Na-zeolites analcite and natrolite are Petrographic work by Friesen (1982) indicates that all igneous most abundant. Chlorite is much greater in volcanic rocks from the rocks on Amlia Island contain plagioclase, clinopyroxene, and east end of the island, whereas smectite predominates in rocks from orthopyroxene as phenocrysts. Primary groundmass minerals the west. Pyroclastic rocks contain much celadonite and secondary include plagioclase, clinopyroxene, orthopyroxene, quartz, K-feld- quartz. Laumontite, heulandite, and chabazite are more abundant spar, opaque ore minerals, olivine (totally replaced by calcite), and in rocks of the west end of the island, and mordenite and analcite apatite. K-feldspar occurs as a groundmass phase only in rhyolite more abundant in eastern rocks. and basaltic andesite. Quartz is absent as a phenocryst phase in From the irregular patterns of rock alteration on Amlia Island, extrusive flow rocks but is common as a disseminated anhedral in addition to the presence of small intrusive bodies, we infer that phase in groundmass. magmatic activity is primarily responsible for producing the The chemical data of T. L. Vallier and others (unpub. data) observed assemblages of secondary minerals. Plutons of middle indicate calc-alkaline and tholeiitic differentiation trends for Amlia Miocene age on nearby Atka Island have produced pervasive Island igneous rocks. Basalts are typical island-arc types and are greenschist-facies alteration in preplutonic stratified rocks (J. R. characterized by relatively flat REE patterns, low K.2O, Cr, Ti, and Hein and others, unpub. data), and we suggest that plutons of Ni contents. The chemical data reveal early fractionation of olivine, similar age may exist beneath Amlia Island. pyroxene, and perhaps chrome spinel, and that plagioclase was not a dominating phase, as also noted by Mahlburg Kay and others STRUCTURE (1982) for other rocks in the Aleutian chain. The large-scale structure of Amlia Island is simple. Strata ALTERATION mainly dip south from 10° to 15°, with many local variations in strike. Dips as steep as 49° are exposed in a north-dipping fault All rocks examined petrographically are altered, ranging from block along the east side of the headland that forms Cape Idalug; incipient to thoroughly propylitic. Significant geographic trends in they are the steepest dips that we observed on Amlia Island (Fig. 2). alteration were not noted except for the general observation that The structure on a local scale is complicated by numerous rocks along the north shore, especially the northeast shore, appear high-angle normal faults with small (less than 4 m) displacements. to be more highly altered and discolored than the rocks exposed Intrusive dikes and sills, which are abundant throughout the island, along the south shore. Friesen (1982) reported that primary miner- further obscure structure on a local scale. als in igneous rocks are replaced by secondary minerals as follows: A broad, open anticline occurs in the predominantly sedimen- clinopyroxene by amphibole, chlorite, smectite(?), pumpellyite(?), tary section between Hungry Bay and Amlia Pass. The fold axis and chlorophaeite; olivine by calcite, smectite, and chlcirophaeite; trends in a northeast direction and appears to plunge gently to the ore phases by hematite and leucoxene; potash feldspar by kaolinite, southwest; the limbs of the fold have dips ranging from 9° to 16° pumpellyite, sericite(?), and zeolites; orthopyroxene by calcite, (Fig. 1). The existence of broad, wave-cut terraces that seem to be in chlorite, hematite, and clay minerals; and plagioclase by epidote, equilibrium with present sea level indicates vertical stability in Hol- kaolinite, prehnite(?), pumpellyite, sericite(?), smectite, and ocene time. zeolites. Large masses, as much as 60 cm, of pale green reniform preh- SUMMARY AND CONCLUSIONS nite with quartz, laumontite, and minor pumpellyite fill interpillow void space in a pillow lava sequence on the east side of Hungry Bay. The geology of Amlia Island provides insight into the devel- Prehnite fills voids and replaces laumontite that originally filled opment of the central Aleutian ridge during late middle Eocene to pore space. The laumontite may have replaced an earlier void-fill zeolite such as heulandite. Laumontite is disseminated through the prehnite, even in the botryoidal masses. In places, these large pods 'See footnote 2.

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early Oligocene time. Interfingering sedimentary and volcanic rocks Calcareous nannofossils were identified by David Bukry, palyno- indicate volcanism contemporaneous with marine deposition. morphs by Norman Fredericksen, and a fossil fish by Bruce Welton Abundant pillow lavas interbedded with massive and columnar- of the Los Angeles County Natural History M useum. Argon analyses jointed lavas suggest that some flows were subaerially erupted, but and age calculations were supervised by M. L. Silberman, who was that much lava flowed into the sea. assisted by L. B. Gray, C. L. Conner, and E. Sims; potassium was A variety of sedimentary processes including debris flows, measured by D. Vivit. X-ray diffraction work was done by L. A. grain flows, and turbidity currents moved volcanogenic material Morgenson, M. J. Wilson, and Sara Monteith, fluorescence by from a volcanic source terrane into shallow marine basins. Sedi- L. F. Espos, and partial chemistry by P. R. Klock. Porosity and ment was locally modified by the winnowing action of strong permeability were measured by Core Laboratories, Inc., Bakers- (tidal?) currents and by the addition of airborne volcanic ejecta. field, California. Sandstones with open framework packing and highly variable patterns of alteration indicate that the sedimentary and volcanic REFERENCES CITED sequence has remained structurally high, and that it has been neither deeply buried nor regionally metamorphosed. The patterns Citron. G. P., Kay. R. W., Mahlburg Kay, S., Snee. C. W„ and Sutler. J. F., 1980, Tectonic significance of early Oligocene plutonism on central Adak Island, central Aleutian Islands. Alaska: Geology, v. 8, of alteration are more closely related to the hydrothermal effects of p. 375-379. shallow-seated plutons and hypabyssal intrusions as observed on Friesen, W. B., 1982, Petrography of igneous rocks from Amlia Island, Aleutian Island Arc, Alaska: U.S. Geological Survey Open-File Report 82-302, 31 p. southwestern Adak Island and on Atka Island (Hein and McLean, Hein, J. R., and McLean. Hugh, 1980a, Paleocene sedimentary and volcanogenic rocks from Adak Island, central Aleutian Islands, Alaska, ifI Shorter contributions to stratigraphy and structural geology, 1979: 1980b, 1980c). U.S. Geological Survey Professional Paper 1126-E, p. EI-E16. - 1980b, Reconnaissance geology of Atka Island, central Aleutian Islands, Alaska [abs.]: Abstracts with The igneous rocks on Amlia range in composition from tholeii- Programs, Cordilleran Section. Geological Society of America, Corvallis, Oregon, p. 110. tic basalt and gabbro through calc-alkaline andesitic basalt to rhyo- 1980c, Paleogene sedimentary and volcanogenic rocks from Adak Island, central Aleutian Islands, Alaska: U.S. Geological Survey Professional Paper 1126E, p, 1-16. lite, and they have chemical characteristics that are typical of Hein. J. R.. McLean. Hugh, and Vallier. T. L., 1981. Reconnaissance geologic map of Atka and Amlia Islands, Alaska: U.S. Geological Survey Open-File Report 81-159. one sheet, scale 1:125.000. ensimatic or oceanic island-arc volcanism. Irvine. T. N., and Baragar, W.R.A., 1971. Guide to the chemical classification of the common volcanic rocks: Broad, open folds and high-angle normal faults indicate that Canadian Journal of Earth Sciences, v. 8. p. 523-548. Kuniyoshi, S., and Liou. J. G., 1976. Burial metamorphism of the Karmutsen volcanic rocks, northeastern the central Aleutian ridge has not been subjected to tectonic com- Vancouver Island. British Columbia: American Journal of Science, v. 276, p. 1096-1119. pression since at least late Paleogene time. Tectonism has been Liou, J. G., 1971, P-T stabilities of laumontite, wairakite, lawsonite, and related minerals in the system CaAl2Si208-Si02-H20: Journal of Petrology, v. 12, pt. 2, p. 379-411. limited to mild extension, probably due to the upward movement of 1979, Zeolite I'acies metamorphism of basaltic rocks from the East Taiwan Ophiolite: American Mineralogist, v. 64. p. 1-14. magma associated with episodes of volcanism and emplacement of Lowrie. William, and Alvarez, Walter. 1981. One hundred million years of geomagnetic polarity history: Geology, v. 9. p. 392-397. plutons. Undeformed wave-cut terraces apparently in equilibrium Mahlburg Kay, S., Kay. R. W„ and Citron, G. P., 1982. Tectonic controls on tholeiitic and calc-alkaline with present sea level may indicate minimal vertical tectonic move- magmatism in the Aleutian arc: Journal of Geophysical Research, v. 87, p. 4051-4072. Mutti, E., and Ricci-Lucchi, F,, 1978, Turbidites of the northern Apennines: Introduction to facies analysis: ment during Holocene time. International Geology Review, v. 20, no. 2, p. 125-166. Scholl, D. W., Greene, H. G., and Marlow. M. S.. 1970. Eocene age of the Adak Paleozoicf?) rocks. Aleutian Islands. Alaska: Geological Society of America Bulletin, v. 81. p. 3583-3592. ACKNOWLEDGMENTS

Field work on Amlia Island was made possible with the per- mission and assistance of the Atka Village Council. Our boats were MANUSCRIPT RECEIVED BY THE SOCIETY MAY 28, 1982 REVISED MANUSCRIPT RECEIVED SEPTEMBER 2. 1982 operated by Nicolas Dirks and Edward Nevzarof of Atka Village. MANUSCRIPT ACCEPTED SEPTEMBER 7, 1982

Printed in U.S.A.

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