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DEPARTMENT OF THE INTERIOR
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138° 50° 45! 40! 135° 35! Interior—Geological Survey, Reston, Va —1989
Base from U S$ Geological Survey, Geology by D.A. Brew, 1981-86; D.E.
Sitka (A-5) and Sitka (A-6), 1951 z — 4 Me Champion, 1985-86; M.A, Lanphere,
Universal Transverse Mercator Projection 3 3000 3000, 000. 0x ca 1985-86; and J.R. Riehle, 1981-86
$ ee =
3 eee ° : : ° Manuscript approved for publication,
= CONTOUR INTERVAL 100 FEET April 6, 1988
NATIONAL GEODETIC VERTICAL DATUM OF 1929
APPROXIMATE MEAN al
DECLINATION, 1989 AREA OF MAP
DISCUSSION identified, owing both to glacial erosion and to the absence of cone construc-
. —_ - tion by the low-volume, fluid lava flows. Probable older vents are marked by
The Mount Edgecumbe volcanic field on southern Kruzof Island was infor- three hills: (1) 1 km west of Mud Bay at the north end of the volcanic field,
mally named by Berg and Hinckley (1963). The first petrographic descriptions (2) approximately midway along a line between Crater Ridge crater and Port
and map of the volcanic deposits were provided by Brew and others (1969) Krestof, and (3) immediately north of the mouth of Freds Creek. Other, pos-

whose report was based on reconnaissance sampling, primarily of coastal expo-
sures, and photointerpretation. Two dissertations on the Mount Edgecumbe
volcanic rocks were subsequently completed (Myers, 1979; Myers and Marsh,
1981; Kosco, 1981a, b); both focused on the major- and minor-element and iso-
topic compositions of the lava flows. The present study was undertaken with
the following objectives: (1) to prepare a bedrock geologic map (this report), (2)
to study the pyroclastic deposits that mantle much of southern Kruzof Island,
including the lava flows (to be reported elsewhere), and (3) to determine, where
feasible, the ages of the various deposits that comprise the Mount Edgecumbe
volcanic field (this report).

The bedrock of southern Kruzof Island is obscured to an exceptional de-
gree by rain-forest vegetation and a thick mantle of postglacial pyroclastic depos-
its, The pyroclastic deposits are mapped separately only where they are unusual-
ly thick near source vents (units Qmet, Qmea, Qsca, and Qscb) and (or) where
the underlying bedrock is extensively covered (unit Qerp). Outcrops, located
from a low-flying helicopter, were found mainly in creek beds, where typically
10 to 100 m2 of rock are exposed. Contacts, which were rarely seen, were
drawn between outcrops of different lithologic types and along photointerpreted
flow boundaries where possible, The degree of confidence in such contacts is
least where exposures are far apart and where the morphology of flows is most
obscured by pyroclastic deposits; the area of least confidence is indicated on the
map by the diagonal pattern. Because of the reliance on photointerpretation,
the present map is conceptually similar to the map presented by Brew and others
(1969). The main difference is that in seeking all outcrops in the interior of the
island, we have added nearly 200 stations (fig. 1) of which about 150 have yield-
ed bedrock information, We have 85 new, major-element analyses and 150 new
thin sections that we incorporate with all previously published petrographic and
chemical data in this report.

Our rock classifications are based wholly on chemical composition (table
1). Our classification scheme is slightly modified from Le Bas and others (1986);
namely, we have shifted the upper and lower boundaries of the basaltic andesite
field from 57 percent to 56 percent silica and from 52 percent to 51 percent sil-
ica, respectively. The new field boundaries coincide better with natural breaks
in the distribution of the silica content of our samples (fig. 2). According to the
field boundaries of Irvine and Baragar (1971), both calc-alkaline and marginally
tholeiitic compositions are present (fig. 3).

Potassium-argon ages were determined for 17 whole-rock samples from 15
localities in the Mount Edgecumbe volcanic field. Potassium oxide measure-
ments were made by flame photometry following lithium metaborate fusion and
dissolution (Ingamells, 1970). Argon (Ar) analyses were made by isotope dilu-
tion using equipment and techniques described by Dalrymple and Lanphere
(1969). Potassium-argon ages and analytical data are given in table 2. Duplicate
‘Ar measurements were made on 13 samples, and a weighted mean age was cal-
culated for these samples where individual ages were weighted by the inverse of
their variances. Calculated ages for most individual measurements have large
uncertainties because of low percentages of radiogenic 40Ar, typically less than
3 percent, and because of the young age of the lava flows.

Source vents of the nonglaciated lava flows and pyroclastic deposits obvi-
ously define a northeast-trending, linear zone that we infer to be the surface ex-
pression of a regional fracture. Sources of the older deposits are less readily

sible older vents include the two small outcrops of the andesite lava flows and
domes(?) (unit Qaf), the outcrop of older plagioclase basalt flows (unit Qbpo)
at the north end of the volcanic field, the isolated exposure of olivine basalt
flow (unit Qbo) 1 km north of Neva Bay, and the several outcrops of the ande-
site lava flows of St. Lazaria Islands (unit Qsla) off the south shore of Kruzof
Island. Clearly most of the volume of the Mount Edgecumbe magmas was erup-
ted from vents located along the main fracture; nearly all of the magmas in the
field with more than 56 percent silica were erupted from such vents

REFERENCES CITED

Berg, H.C., and Hinckley, D.W., 1963, Reconnaissance geology of northern Ba-
ranof Island, Alaska: U.S. Geological Survey Bulletin 1141-0. 24 p.

Brew, D.A., Muffler, L.J.P., and Loney, R.A., 1969, Reconnaissance geology of
the Mount Edgecumbe volcanic field, Kruzof Island, southeastern Alaska
U.S. Geological Survey Professional Paper 650-D, p. D1-D18.

Dalrymple, G.B., and Lanphere, M.A., 1969, Potassium dating: San Francisco,
W.H. Freeman and Company, 258 p.

Ingamelis, C.O., 1970, Lithium metaborate flux in silicate analysis: Analytical
Chemica Acta, v. 52, no, 2, p. 323-334.

Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classification of
the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, no.
5, p. 523-548.

Johnson, B.R., and Karl, $.M., 1982, Reconnaissance geologic map of the west-
ern Chichagof and Yakobi Islands Wilderness Study Area, southeastern
Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1476-
A, scale 1:125,000.

Kosco, D.G., 198 1a, Part I, The Mt. Edgecumbe volcanic field, Alaska: An exam-
ple of tholeiitic and ‘calc-alkaline volcanism; Part Il, Characteristics of an-
desitic to dacitic volcanism at Katmai National Park, Alaska: Berkeley,
University of California, Ph.D, dissertation, 249 p.

——~,.1981b, The Mt, Edgecumbe volcanic field, Alaska: An example of tholei-
itic and cale-alkaline volcanism: Journal of Geology, v. 89, no. 4, p. 459-
477.

Le Bas, M.J., Le Maitre, R.W., Streckeisen, A., and Zanettin, B., 1986, A chemi-
cal classification of volcanic rocks based on the total alkali-silica diagram:
Journal of Petrology, v. 27, no. 3, p.745-750.

Loney, R.A., Brew, D.A., Muffler, L.J.P., and Pomeroy, J.S., 1975, Reconnais-
sance geology of Chichagof, Baranof, and Kruzof Islands, southeastern
Alaska: U.S. Geological Survey Professional Paper 792, 105 p.

Myers, J.D., 1979, Geology and petrology of the Edgecumbe volcanic field,
southeastern Alaska: Transform fault volcanism and magma mixing:
Baltimore, Maryland, The Johns Hopkins University, Ph.D. dissertation,
205 p.

Myers, J.D., and Marsh, B.D., 1981, Geology and petrogenesis of the Edgecumbe
volcanic field, SE Alaska: The interaction of basalt and sialic crust: Con-
tributions to Mineralogy and Petrology, v. 77, no. 3, p. 272-287.

136045" 136935"
T T

57°10"

SHELIKOF BAY

87°05’ 4

4596

SOUND

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87°00'F 4 KILOMETERS

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Figure 1. — Locations of stations examined on southern Kruzof Island
(some locations represent more than one station). Numbers refer
to samples listed in table 1 and (or) stations in table 2. Geologic
contacts between map units shown.

Qs

Qerp

Qmet

CORRELATION OF MAP UNITS *

UNCONSOLIDATED METAMORPHIC AND.
DEPOSITS PLUTONIC ROCKS

VOLCANIC ROCKS AND DEPOSITS
n

| coo t

<~———————Deposits and rocks in glaciated area > —~<— Deposits in nonglaciated area ————

Qs

Qsma

Postglacial

Unconformity
49435 ka

137
189:
297:

313414 ka

438118 ka

+ HOLOCENE

Qerp

Qmet

coe ka

+42 ka
+51 ka
£27 ka

674111,
56643,

611

Unconformity

1 Ar ages shown in thousands of years B. P. are from table 2.

DESCRIPTION OF MAP UNITS

UNCONSOLIDATED DEPOSITS

Surficial deposits (Holocene) — Consists primarily of alluvial, colluvial, and tidal-
flat deposits. Poorly sorted deposits of blocks and sand derived by mass
movement from adjacent slopes in the crater of Mount Edgecumbe and
downslope from Crater Ridge domes. Moderately sorted sandy gravel and
boulders in stream deposits; moderately to well-sorted sand and (or) gravel
in beach deposits; silt and sand in tidal-flat deposits

METAMORPHIC AND PLUTONIC ROCKS

Prevolcanic bedrock (Tertiary and Cretaceous) — Consists of the Sitka Gray-
wacke (Cretaceous) and Kruzof Island pluton (early Tertiary). The Sitka
Graywacke comprises low, glacially scoured knobs of steeply dipping, dark-
gray argillite and interbedded medium-gray graywacke; moderate degree of
contact metamorphism imparts slaty cleavage to argillite. The Kruzof Is-
land pluton consists of light-colored outcrops of one- and two-mica gran-
ite, granodiorite, and adamellite. (Ages and modified descriptions: Sitka
Graywacke, Johnson and Karl, 1982; Kruzof Island pluton, Loney and
others, 1975)

VOLCANIC ROCKS AND DEPOSITS ,

Dacite lava flows of Crater Ridge crater (Holocene) — Dark brownish gray, vesic-
ular to diktytaxitic, plagioclase-phyric. Mainly massive but has local vague
flow foliation. Forms hummocky topography (flow tops?). Subseriate
porphyritic; foliated microcrystalline groundmass. Phenocrysts are plagio-
clase, 10-12 percent; orthopyroxene, 1-2 percent; clinopyroxene, 1-2 per-
cent; opaque minerals, 0.1-0.3 percent. None of the air-fall units described
below overlie lava flows of this unit

Pryoclastic-flow deposits of Crater Ridge (Pleistocene) — Pumiceous lapilli are
low-silica rhyolite. Maximum thickness is 980m on south wall of vent at
Crater Ridge crater, where at least five major depositional units, some
having internal stratification, are recognized. Near margins, unit consists
of only one or two depositional units and is only 2-3 m-thick. Deposits
comprise poorly sorted silt, sand, rounded pwmiceous lapilli, and angular
blocks of dark-gray vitrophyre that are pieces of older Crater Ridge domes
incorporated in explosive eruptions, Lapilli and blocks each range from
10 to60 percent by volume. Thermal demagnetization experiments indicate
that some beds were deposited at temperatures above 100 °C (Duane
Champion, written commun., 1987), implying an origin as hot pyroclastic
flows; however, unit includes some rhyolite air-fall beds. Mapped margins
are approximate and gradational and are based partly on photointerpreta-
tion. Unit overlies the dacite air-fall tuffs of Mount Edgecumbe (unit
Qmet)

Dacite air-fall tuffs of Mount Edgecumbe (Pleistocene) — Pumiceous lapilli and
ash, Lapilli are >75 percent glass by volume and are yellow, pale orange,
or pale brown when dry. Deposit is compacted and agglutinated into
dense gray and red bands of vitrophyre on rim of Mount Edgecumbe,
where deposit is >50 m thick. Contact with andesite lava flows and
breccias (unit Qmec) near base of Mount Edgecumbe cone is approximate
and placed mainly at break in slope. Unit ranges from 0.5 to 15 m in
thickness beyond this contact; for example, unit is 15 m thick at Shelikof
Bay along major distribution axis of fallout, where unit is covered by
pyroclastic-flow deposits of Crater Ridge (unit Qcrp). Unit is absent from
steep slopes on north and west flanks of Mount Edgecumbe owing to ero-
sion during and after deposition. Phenocrysts are plagioclase» orthopyrox-
ene=clinopyroxene> opaque minerals>apatite

Basaltic andesite lava flow of Shell Mountain (Pleistocene) — Postglacial, heavily
vegetated, and poorly exposed. Three samples are all porphyritic; total
phenocrysts <15 percent (plagioclase) orthopyroxene=clinopyroxene? oli-
vine) in very fine grained, microvesicular groundmass. Blocks of probable
flow carapace overlie unmapped air-fall beds of andesite, basaltic andesite,
and basalt lapilli (units Qsca and Qscb)

Andesite air-fall tuffs of Mount Edgecumbe (Pleistocene) — Red-brown and dark-
gray, scoriaceous lapilli, breccia, and foliated vitrophyre inferred to be
compacted and agglutinated air-fall deposits. Intruded(?) by andesite
dome of Mount Edgecumbe (unit Qmed) but probably nearly contempor-
aneous with dome, Phenocrysts are plagioclase, 15-20 percent; ortho-
pyroxene, 2-3 percent; clinopyroxene, 1-2 percent; opaque minerals,
0.1-0.2 percent. Glassy inclusions common in zones in plagioclase grains;
plutonic inclusions ranging from <1 to 730 cm across also common

Andesite dome of Mount Edgecumbe (Pleistocene) — Porphyritic, holocrystal-
line, dark-gray to brown lava having near-vertical foliation and rubbly ap-
pearance in outcrop. Locally vesicular. Phenocrysts are plagioclase, 10-
15 percent; clinopyroxene, 3-5 percent; orthopyroxene, 2-4 percent;
opaque minerals 0.1-0.3 percent. Grains mainly subhedral but include
some that are rounded; glassy inclusions common in plagioclase grains.
Groundmass grains are 0.05-0.1 mm across

Andesite air-fall tuffs (Pleistocene) — Postglacial. Uinit consists of dark- to med-
jum-gray and grayish-brown, blocky, scoriaceous andesite lapilli, As map-
ped, unit consists of beds>50 m thick that comprise scoria cones at vents.
Unmapped outcrops of unit <SO m thick locally overlie basalt and basalt-
ic andesite air-fall tuffs (unit Qscb) which were erupted from other, adja-
cent vents, Mapped margins of unit arbitrarily placed at base of cones,
although deposits range from 10 cm to SO m in thickness beyond map-
ped contact

Basalt and basaltic andesite air-fall tuffs (Pleistocene) — Postglacial. Unit differs
from the andesite air-fall tuffs (unit Qsca) only in consisting essentially
of mixed orange (basalt) and gray (basaltic andesite) scoriaceous lapilli

Low-silica rhyolite domes of Crater Ridge (Pleistocene) — Dark gray, commonly
fractured, having scattered vesicles and (or) near-vertical foliation. Spar-
sely porphyritic; pilotaxitic, microcrystalline groundmass. Phenocrysts are
plagioclase, 3-10 percent; clinopyroxene, 0-1 percent; orthopyroxene,
0-1 percent; rare opaque minerals. Andesite lapilli that may be unmapped
outcrops <50 m thick of andesite air-fall tuffs (unit Qsca) and thick pryo-
clastic-flow deposits of Crater Ridge (unit Qerp) overlie unit

Younger plagioclase basalt flows (Pleistocene) — Postglacial(?). Exposed mainly
in intertidal zone near mouth of Freds Creek southward for 3 km, where
flows engulf glacial erratics now comprising pebble-boulder beach deposits,
Morphologically fresh, dark-gray flows having 1 to 3 m of relief on their
upper surfaces, pressure ridges that range from 10 to 20 m across, ropy
structures, small lava lakes, and columnar jointing ranging from 10 to 30
cm in diameter. Vesicular; vesicles, locally as much as 35 percent of rock,
define bands, Individual to clustered plagioclase laths prominent. Glom-
eroporphyritic; holocrystalline to vitrophyric; groundmass grains <0.2 mm.
Phenocrysts are plagioclase, 25-35 percent; olivine, 1-3 percent; clinopyr-
oxene, 0-1 percent. Overlain(?) by unmapped basalt and basaltic andesite
air-fall beds <50 m thick of unit Qscb

' Classification of rock types based on chemical composition (table 1); age of
units based on potassium-argon age determinations (table 2).

+74 ka

—1—

—
aos,

20———

410455 a)

3647 ka
}- QUATERNARY
‘PLEISTOCENE

95426 ka

TERTIARY

AND.
_| CRETACEOUS

Andesite lava flows and breccias of Mount Edgecumbe cone (Pleistocene) —
Medium gray or grayish brown, locally vesicular, commonly having near-
horizontal platy foliation. Subseriate porphyritic or glomeroporphyritic;
trachytic or, less commonly, hyalopilitic groundmass with rare brown
glass. Phenocrysts are plagioclase, 20-35 percent; clinopyroxene, 3-6 per-
cent; orthopyroxene, 3-6 percent; trace of opaque minerals. Plagioclase
grains commonly have zones of glassy inclusions. Lava flows high on
flanks of cone probably include younger rootless flows formed of fused
air-fall deposits (unit Qmea). Overlain by rhyolite pyroclastic-flow de-
posits of Crater Ridge (unit Qcrp). However, basalt and basaltic andesite
air-fall tuffs (unit Qscb) <50 m thick have nowhere been found overlying
this unit, although andesite and dacite air-fall tuffs (units Qmet and Qmea)
are so thick that older air-fall deposits could easily have escaped detection

Andesite lava flows of Crater Ridge vent (Pleistocene) — Glaciated(?). Gray or
grayish brown, having near-horizontal platy foliation, Phenocrysts are
plagioclase, 15-20 percent; clinopyroxene, 2-3 percent; orthopyroxene.
1-2 percent; trace of opaque minerals. Pilotaxitic groundmass; glassy in-
clusions common in subhedral plagioclase grains. Although major-element
compositions are similar to those of andesite lava flows of Mount Edge-
cumbe cone (unit Qmec; see fig. 4), lava flows in this unit are inferred
from photointerpretation to have clearly originated at Crater Ridge, Only
known stratigraphic relations involving unit are with the overlying pyro-
clastic-flow deposits of Crater Ridge (unit Qcrp) and with the underlying
older plagioclase basalt flows (unit Qbpo)

High-silica andesite lava flows of Cape Edgecumbe (Pleistocene) — Sparsely por-
phyritic, thinly plated, dark-gray flows forming seacliffs as much as 60 m
high. Local red blocky zones midway up some seacliffs are probable flow
boundaries, implying closely succeeding flows each 15-30 m thick. Phen-
ocrysts are <2 percent (plagioclase >orthopyroxene=clinopyroxene) in
hyalopilitic to pilotaxitic groundmass. Flow foliation pervasive and highly
variable in orientation. Unit clearly overlies high-silica basaltic andesite
lava flows (unit Qbah) and, on basis of photointerpretation, probably over-
lies basaltic andesite lava flows (unit Qbal)

Andesite lava flows of St. Lazaria Islands (Pleistocene) — Dark-gray lava perva-
sively jointed to equant blocks ranging from 10 to 30 cm across. Seriate
porphyritic to hyalopilitic; also includes rare brown glass. Phenocrysts are
plagioclase, 10-20 percent (larger grains anhedral, smaller subhedral); clin-
opyroxene, 2-4 percent; orthopyroxene, 0-1 percent; olivine, 0-1 percent;
submicroscopic opaque dust. Domes(?) at vents implied by concave-down,
radiating pattern of columnar joints in seacliff exposures

Basaltic andesite lava flows (Pleistocene) — Thinly plated, dark gray, nearly
aphyric. Locally vesicular. Porphyritic; trachytic microcrystalline ground-
mass. Phenocrysts are plagioclase, 8-10 percent; clinopyroxene, 0-1 per-
cent; olivine, 0-1 percent; opaque minerals, 0-2 percent. Unit crops out
mostly on southwest shore of Kruzof Island, but true extent is uncertain
because mapped contacts with the underlying older plagioclase basalt lava
flows (unit Qbpo), overlying lava flows of Mount Edgecumbe cone (unit
Qmec), and the underlying(?) high-silica basaltic andesite lava flows (unit
Qbah) are largely photointerpreted

Olivine basalt flows, breccias, and aquagene tuffs (Pleistocene) — Unit crops out
mainly north of Freds Creek, where locally overlain by thin deposits of
glacial till; as mapped, also includes small outcrops of prevolcanic bedrock
(unit pv) One small outcrop of olivine basalt is found on west shore of
Kruzof Island, 2 km north of Neva Bay. A tuff containing lapilli of vesic-
ular hyalopilitic basalt crops out 1 km south-west of head of Port Krestof
on east coast of Kruzof Island; tuff contains marine macrofossils and ben-
thic foraminifers that imply a Pleistocene age and water depth of 50-100
m (William Sliter, written commun., 1985). Lava-flow textures range be-
tween (1) medium gray, equigranular, vesicular to diktytaxitic, fine grain-
ed, diabasic, containing grains ranging from 0.3 to 1.0 mm across of pla-
gioclase (50-60 percent), olivine (15-25 percent), subophitic clinopyrox-
ene (15-25 percent), and trace of opaque minerals and (2) dark gray,
vesicular, porphyritic to hyalopilitic, containing 50-100 percent mineral
grains of plagioclase ») clinopyroxene=olivine. Such wide textural varia-
tion may reflect varying depths of exposure of multiple flows following
glacial erosion. Marginally tholeiitic by chemical criteria (fig. 3)

Older plagioclase basalt flows (Pleistocene) — Highly vesicular, massive, dark
gray. Holocrystalline, porphyritic. Phenocrysts are glomeroporphyritic
plagioclase, 5-35 percent; olivine, 3-6 percent; clinopyroxene, 0.5-1 per-
cent. Morphologically fresh flows having pressure ridges and columnar
joints like those of the younger plagioclase basalt flows (unit Qbpy), but
differing in that (1) groundmass grains of this unit (0.1-0.3 mm) are lar-
ger than those of the younger unit (<0.2 mm or glassy), (2) abundance of
plagioclase phenocrysts ranges more widely in samples of this unit (5-35
percent) than in samples of the younger unit (25-35 percent), and (3)
olivine abundance exceeds 3 percent in most samples of this unit but is
less than 3 percent in all samples of the younger unit

High-silica basaltic andesite lava flows (Pleistocene) — Dark gray; local bands of
vesicles. Flow foliation near-vertical to concave-down semicircles that
range from 10 to 20 m across in seacliff exposures. Minor rubbly, red-
gray zones are presumably flow tops. Porphyritic and holocrystalline;
trachytic groundmass. Phenocrysts are plagioclase, 10-15 percent; clin-
opyroxene, 0-3 percent; olivine 0-2 percent; orthopyroxene, 0-1 percent;
opaque minerals, 2-4 percent. Plastically deformed xenolith of partially
melted plutonic rock, which has glassy rims on mineral grains, found in
beach exposure on south shore of Kruzof Island 2 km west of Shoals Point

Andesite lava flows and domes(?) (Pleistocene) — Poorly exposed, widely scat-
tered small outcrops. Dense, hard, dark gray; well-developed banding and
foliation. Subseriate porphyritic; pilotaxitic groundmass. Phenocrysts are
plagioclase, 2-13 percent; clinopyroxene, 0.5-2 percent; orthopyroxene,
0.3-1 percent; opaque minerals, 1-2 percent; rare olivine(?). Some clino-
pyroxene and orthopyroxene grains have reaction rims; small plutonic in-
clusions common. Marginally tholeiitic by Si02(FeO+.9Fe20)/MgO and
(FeO+.9Fe203)-Mg0-(Na0+K 20) relations, but calc-alkaline by Al2Og-
normative An relations (fig. 3)

Low-silica basaltic andesite lava flows (Pleistocene) — Dark gray, thinly plated,
locally coarsely vesicular. Columnar joints 0.5 to 1 m in diameter. Por-
phyritic, holocrystalline; very fine grained groundmass. Phenocrysts are.
plagioclase, 2-15 percent; clinopyroxene (pigeonite?), 2-4 percent; olivine,
1-3 percent; opaque minerals 3-5 percent

Thick pyroclastic deposits — Bedrock exposures and morphologic expression of
lava flows are obscured by exceptionally thick pyroclastic deposits

Contact — Queried where gradational and (or) photointerpreted

- Photointerpreted morphologic boundary of lava flow

Direction of lava flowage as inferred by photointerpretation
Linear ridges inferred to be lateral moraines — Tick marks on side of glacier
Isopachs of aggregate thickness of air-fall pyroclastic deposits — Contours in

meters. Deposits comprise numerous beds, basaltic andesite at base,
overlain by andesite, dacite, and low-silica rhyolite at top

NagO + K20 CONTENT, IN WEIGHT PERCENT

MISCELLANEOUS INVESTIGATIONS SERIES
MAP I-1983

Table 1, — Major-element analyses of samples representative of the volcanic map units, Mount Edgecumbe volcanic field, Alaska

[Analysis in weight percent. Analysts: A.J. Bartel, E, Brandt, P. Klock, S. Macpherson, R.V. Mendes, S, Neil, K. Stewart,

4. Taggart, JS. Wahlberg, U.S. Geological Survey,

Methods: X-ray fluorescence for all species except FeO, which i b , not determined)
‘Map unit Qerd Qerp ‘Omet Qmea | med | Asma Qsca ‘Qscb, orange ‘Oscb, gray Oar _ ‘mec
Major [ Sampie no 3 5 6 7 at 9 w | on 12 13 14 15 16 ” 18 19 20
clement [Fietdno | 828)145 82Aj147 |81RjO6on B1AjOBAc [B2Rj124 B1RjO7Se | 89R}284 620440] 81j044 | BERIO13 | B1RIO7DE B3RI2E0e |B1RjO73» BIR|2EOc [81RIO73a BIR}2B0c |BSRIOOI a5Le015a|B1RjO64» B4Rj0329
SiOz 630 «632s SCSSCATSC(ié«iSSSCSCOHSCTA CASE )BN aa aBSC*DsCST—Ss“(iOH TO BO. «8D
Al, 6 165 AHO HS GON 1B BSG HB 79
Fes0a 114-119-096 087158 1.82 370 nay 1503.30.98 7427893640608] 105 1.5169
FeO 393 394 «292226274284 284 475.18 6003.89 885 3071.99 8494992372142
‘Mao 257 268 052053094.) 3B 3G aay 410238373 kw? 339 B.D 5.08 osa 0813.28
C20 529 535 210 «213-310 325 B73 BC728 BTA SAT 753781 279072272138 S97
Noz0 399 396 4390-425 4.47 43GB 4280 3D 403,378 2a) 2H 388A? 5393396819
K20 135 1330208198165. 1.719081 oa. 0B 098019) 27G 2G NB COND
TiO 06s 066 «040 «039004073 O74 ek R207 taka
F205 020 018 009 «0.11 «0200028 Oza) 04G 036037 0a). 0.35 1.090289
Mn0. 0.08 008 © 006. «= 00500800702) 17 30.2 0BS OBO 005- 08-009 09
H,0* nd ond. ond, tnd, nd. 0.39 ond, (0.48). nd nd. nd, nd, nd, (0.47) 0.23, nd
H20- nd ond ond nd, nd 09nd, TT nd. nd. nd, nd, (0005) 007 nd. nd.
02 nd ond ond nd nnd, =< 05nd, «SS (004) nd, nd ond nnd, O12). nd
Loi oss 071 441 4163883531 ond, «0638S ZB 037.81 B71. HB OY 128 78
TOTAL 95 998 931 99.3 100.1 985 100.3 1000-995 100.1 «99.9 «100.0 100.2 gas 100.004 B10
Map unit Gera ‘Gbpv Ocee Osle ‘bah Oba Oat ‘Qbpo Obes bo
Major [Sample no} 21 2 2 | 26 25 2% 28 29 0 a 2 re 3a 36 36) a7
element [ Field no. | 84R]0138| 84000232 84Rj001a }85Le014 4000130] 85Le022a BGAIONG |85Le008 B5LO15a |B4RjO14e 84000379 B5DH010a _BARjO12 | 8400199 B5Le004a | 6100725 | B40002Ga_B4Rj011
SiO, 572 #06 = 495) GIB CGI HZ_BTO BABS. 5BHS1877— HU2_— MBB HO. BG AGS
AlzOa 79 «1621607 BY BT 79 NBT BHSWB QHD
Fe203 184 20323384245 122d 23318760 386198 2022.23.82 20 8B 270
FeO 574 806 «= 80S. 4G8~ 401 G07 god 4324352095 MGCBCHS 701
MgO 28a «696 = 6992.21 23012320 402,57 G77 269 353 «676 = 692 9.34
20 721 «112d 49 5.458770 889836 893g S48 A112 106
Nag 409-333-336 472 4535736069338 368357396 = 380 32183 244
k,0 oss 02020216 .1B_——8B 99a 101093018005 0.15
TiO2 107.601.6599 O87 094.0298 .08 G95 AST 0.97
P20 038 ©0026. «=o 040 A310 32 038703635 100.2520 034 0.12
Mino 012 017 otB 00ND ZANT ONE ONE ON. 0.16
H,0* nd. nd, nd, (0.11) nd, (0.53) (0.62) (0.25) (0.64) nd, nd, (0.04) nd. nd, (0.14) 081 nd.
H20 nd. nd nd (01) md. (0.12) (0.13) (0.21) (0.15) n.d, = (008), nd. Sd, ~SKON) O10 nd.
02 nd, ng. nd (0.06) nd. (0.03) (0.06) 10.12) (009) nd, nd 02) nd. nd, (0.08) <.05 nd,
Lor 058 085 = 8108567095 = 097089084 = 07008800106, 261
TOTAL 998 1005 10051006 +1000» 1005999 100.6 += 100.3. 100.1. ©1002» 100.4-—«100.3.S 100.6. +1009 += 10001008 +1000

1 From Brew ancl others (1969),
2.01, loss on ignition: value i corrected for oxidation of FeO to Fe,0, and comprises only volatiles such as H0*, HO , CO, F,Cl, S03. Because H20*, H20TM , and COs
are included in LOI, values of volatiles are in parentheses where LOI is also reported.

9 T T

Lava flows

Pyroclastic deposits

SILICA CONTENT, IN WEIGHT PERCENT

8h aot

7h 4
6 4
BE 4
ak 4
3h 4
2 L 14 ! L L ! L 1

4 48 «+51 54 57 6 63 66 69 72 75

Figure 2. — Classification of lava flows and pyroclastic deposits of Mount Edge-
cumbe volcanic field; classification scheme slightly modified from Le Bas
Oxide values normalized to 100 percent on volatile-

‘SiOp CONTENT, IN WEIGHT PERCENT

and others (1986).
free basis.

| L

. 2 3
(FeO +9 Fep03)/MgO CONTENT, IN WEIGHT PERCENT

4 5

EXPLANATION
+ Lava flow
* Pyroclastic
deposit

iG 7 3

‘Ai203 CONTENT, IN WEIGHT PERCENT.

Table 2. — Potassium-argon ages and analytical data for volcanic rocks from
Mount Edgecumbe volcanic field, Alaska

[Age determinations from whole-rock samples]

‘on no. 1 2 Argon Calculated | Weighted mean
= ro" | Map unit | KO (wt. x) sample age | calculated age
Ar ggimol/or *10") | Arzag(%) | (10% yr) 3 | (10% yn 4
9 ‘med 1.030 4 0,007 0.1430 on 95415
(85ALe018) 0.9692 3.37 65 +30 2413
9 Qmed 0.770 +.0.003 0.7668 2.69 69 £23
(86ALe014) 0.4886 151 44 +25 58417
W Qerr 2.468 + 0.005 0.6349 3.07 1et7
(86ALe013¢) 0.5104 3.34 at7 1625
18 Qerr 2.483 £ 0.009 1.002 2.10 2848
(85AL0015)
2 cra 0.950 # 0,005 0.6700 0.60 49 + 35
(85ALe001)
28 Qbah 0.632 + 0,002 3.951 1.39 435 + 62
(85ALe008) 2.942 ww 3232116 410455
30 Qbal 0.537 + 0,002 0.7387 0.95 96 + 46
(85ALe006) 0.7280, 1.25 94431 95 +26
32 Gat 0.999 # 0.003 6271 276 438 £18
(86ALe012b)
37 Qbo 0.144 +0.007 1.034 0.63 4994 158
(85AL013) 1.315 1.64 634157 5674111
39 Qbpy 0.223 0.005 0.1832 0.42 57 £90
(85ALe010) 0.1046 0.29 33487 45 + 63
40 Qmec 0.821 + 0.005 0.4102 1.04 3548
{5ALe003) 0.4944 1.02 42418 3647
a Qcea 1.188 + 0.006 1.567 4aa7 9246
{85ALe009) 1.281 497 7549 8745
42 Qbpo 0.378 + 0.002 1.781 5.42 327416
(85ARj005a) 1.465 5.14 269 +29 ai3t14
43 Qbo 0.160 + 0,002 0.3187 1.79 138 + 60
(85ALe012) 0.3132 1.63 136+ 60 137442
44 Qbo 0.232 0,002 1.839 5.09 51454
(85ALe020) 1.982 3.68 593472 566443
44 ‘Qbo 0.239 + 0.002 1.071 252 311430
(85ALe021) 0.9196 3.59 267 £44 297427
45 Qbo 0.208 + 0.002 0.3997 1.15 134458
{85ALe005) 0.9117 1.50 305 484 199 +51
46 ‘Qbo 0.217 £0,005 1.823 7.02 583 486
(85ALe011) 2.172 3.50 eoat1ag 611474

1 Refers to numbers on station location map (ig. 1)
2 Mean and standard deviation of four measurements.

4.96210 Myr, #°K/K = 1.167%10"4mol/mol, Errors are estimates of the standard deviation of

For two analyses, weighting is by inverse of variance of individual measurements; uncertainty assigned is weighted standard

deviation

150}—

cai

atalio®

Tholeitie

Cate

saline

FeO + 9 FeO

Qseb (orange)

NORMATIVE An, IN MOLECULAR PERCENT

1NagO + K20

Mao

700

Figure 3. — Classification of lava flows and pyroclastic deposits of Mount Edgecumbe volcanic field as calc-alkaline or thol-

GEOLOGIC MAP OF THE MOUNT EDGECUMBE VOLCANIC FIELD, KRUZOF ISLAND, SOUTHEASTERN ALASKA

By

James R. Riehle, David A. Brew, and Marvin A. Lanphere

1989

eiitic. Samples normalized to 100 percent on volatile-free basis. Field boundaries from Irvine and Baragar (1971). See
“Description of Map Units” for identification of map-unit symbols.

OXIDE CONTENT, IN WEIGHT PERCENT

13.5 4

12.0}-

10.5}- a}

9.0 4

75

‘Abpy, Abpo +

6.0}

45 ‘Omed, Omec, Qera 4

3.0F sed (orange)
Omet

ere
Qcrr

bo: 8)
10.5

9.0

6.0[- Asch (orange)

75

SILICA CONTENT, IN WEIGHT PERCENT

Figure 4, — Silica content versus selected oxide content showing grouping by

map units of Mount Edgecumbe volcanic field. Dot, lava flow; triangle,
pyroclastic deposit. Samples normalized to 100 percent on volatile-free
basis. See “Description of Map Units” for identification of map-unit sym-
bols.

Any use of trade names in this publication is for
description purposes only and does not imply
endorsement by the U.S Geological Survey

For sale by U.S. Geological Survey, Alaska Distribution Section, New Federal Bldg.,
Box 12, 101 Twelfth Avenue, Fairbanks, AK 99701, and U.S. Geological Survey,
Map Distribution, Box 25286, Federal Center, Denver, CO 80225