DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY

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138° 50" 738°35" Tnterior—Geological Survey, Reston, Va —1989
Base from U $ Geological Survey, Geology by D.A. Brew, 1981-86; D.E.
Sitka (A-S) and Sitka (A~6), 1951 z a sme Champion, 1985-86; M.A. Lanphere,
Universal Transverse Mercator Projection = 3000 c 0 = 1985-86; and J.R. Riehle, 1981-86
§ SSS ee
x ——— Manuscript approved for publication,
= CONTOUR INTERVAL 100 FEET April 6, 1988
NATIONAL GEODETIC VERTICAL DATUM OF 1929
"APPROXIMATE MEAN 7 :
DECLINATION, 1989 BER OE ae
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, 198 1a, 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 and

field from 57 percent to 56 percent silica and from $2 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 40 Ar, 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(2) (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.

Ingamells, 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, seale 1:125,000.

Kosco, D.G., 198 1a, Part 1, The Mt. Edgecumbe volcanic field, Alaska: An exam-
ple of tholeiitic and cale-alkaline volcanism; Part II, 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 calc-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"
T

87°10

SHELIKOF BAY

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‘a 2 4 KILOMETERS

i S 1

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.

CORRELATION OF MAP UNITS *

UNCONSOLIDATED METAMORPHIC AND

DEPOSITS PLUTONIC ROCKS

VOLCANIC ROCKS AND DEPOSITS
L

ro r

~<—_——eposits and rocks in glaciated area —————>—

Qs

Postglacial

—~— Deposits in nonglaciated area——~—

f+ HOLOCENE

1645 kal
2848 kal

MISCELLANEOUS INVESTIGATIONS SERIES
MAP I- 1988

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, J. Taggart, J S. Wahlberg, U.S. Geological Survey.
‘Methods: X-ray fluorescence for all species except FeO, which is by titration. .d., mot determined)

Unconformity
49435 ka

137442 ka
189451 ka
297427 ka
313414 ka

438118 ka

5674111,
566243,
611474 ka

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 7

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 pumiceous 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 t060 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)

Qerp

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 Qerp). 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

— Qmet

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 >30 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-

ium-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 <50 m thick loca lly 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

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

410455 ka

36+
45463 ka

95426 ka

QUATERNARY
PLEISTOCENE

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 Qerp). 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 Qerp) 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)

[ieee | 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+.9Fe203)/MgO and
(FeO+.9Fe203)-Mg0-(Na20+K 20) relations, but calc-alkaline by AlgOg-
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

A2+~X Linear ridges inferred to be lateral moraines — Tick marks on side of glacier

20:

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
©

50 CONTENT, IN WEIGHT PERCENT

‘Map unit Ocrd Gap ‘Omer Gmed | Osmo sca ‘seb, orange sch, gray Oar ‘mec

Major [Sample no 3 4 5 6 7 st 9 10 u 12 13 4 15 16 7 18 19 20

element | Fiekdna | 82Ri145 82/147 |81RjO6On B1RIOBAc |82Rj124 B1RjO7Sa| 83Rj284 620446] 81Rj044 | BERIONZ | 81RIO7Oe BIRj2BOe |B1RIO73e B3A/2B0c |81RjO73a _GIRj28Oe |B5RIOOI BSLeO15a]B81RI0G42 B4RjO32

S102 0 632 6okCTSCAL)SC(“‘éimkkSSCCSOOCTA SAS OSGT) SG} aa2SCBZZCSTSsCOSSTO BDC

Ags 6 165 «154 150 SB)? S76 DBT 1B DBS GMS ASB

F203 14119096097 1.59 37 «12 141150330198 712759 326 405 08S 1051.89

FeO 393 394 «6252 «2260-274 «284A 7B 60D 399 585 307, 199 5a 4992572 AS

Mgo 257 268 052 «053094 «= 1.01348 Gay 4102387343395. 505883313 3.28

C20 539 535 «2100213 310 325 G73 GB 728 BTA SAT) = 7537.61 6279 907,227 213 8977

Nez0 399 396 439 «425447 436413, 42380 36D 403374242 AT 483 493398 413

K20 135° «4133°~«209s1.991.66 1.77 091081 oat 276 236) 9B ON

TiO2 066 066 «040 «0.39 006473074 «=e 98120107 rh rh nS 2

F205 020 018 009 0.11 020 019 028 03 029 032 046 035 037 04 033 035 0.10 009 028 029

MnO 008 008 006 «= 0.05 006-007, tS 012 ots 1732 BSB 0056008

tot nee ws nd ond. nd. 039 ond, (0.48). nd. ns. ad, ad. nd 047) 023 ond nd,

420" nd ond ond nd. nd ond nd 0.09 ond 11) nd. ad. nd. ad nd, (005) 0.07, ng,

0: nd ond ond. nd nd. ond nd. <05 nd. (004), nd, ad nd. ad nd (0.12) 2nd. nd,

Loi 056 071 «441 416 388 353 st ond SSS 326103761138 079° 144 = 08802112874

TOTAL 95 998 991 993 100.1 985 100.3 1000 995 1001 999 1000 1002 996 1001 1004 994 98 997 1000
Map unit_|Ocra ‘Gbpy Qeea Osta ‘bat Oat ‘Qbpo ‘bas ‘Qbo

Major [Sample no] 21 2 23 2 25 26 2 28 2 30 a 2 3 35 361 a 28

element | Field no. | 84Rj013s| 84D0023s B4RjO01a |B5Le014 84013 |B5Le022a BGR|OIG | B5Le008s B5DL015a | BAR|OI4s B4Db037a|B50L010e B4R|OI2 | 84000192 _B5LeO04e | 610725 | B4D026a_BARIOIIb.

SiOz 872486 495 6181 (GZ DABS 835 531 577-862 498 50.1 513486 465

Alz05 179 162 160 0167, 68 798 we 74 187 «179 167168 17.3182 wr 5917.3

Fe203 184 2.03 233 «(188 0246022042331 5 160 «356 198 202223182 20 188.278

FeO 574 8.06 e0s 468 «401 6.07 gs 543 G24 G35 5.20 695 «696 6886S 61 102 7.01

MgO 284 = 6.96 699 221 «230412320 402457 46.477 269 353 676 692 65 713-934

C20 72 2 na 5495468077240 889 836 BS 654 698A. 102 106 = 106

Na20 409 333 3.36 472 465 357 3.60 3.69 3.38 3.68 357 415 396 3.30 321 33 314 244

K20 086 = 020 021 11601130888. 1090.48.49 101 093° 0.18 ONG 5022S

TiO. 1.07 1.60 166 099 097 094 102 098 wn 1061.16 1460235438. aw 168 (097

P205 036 = 0.26 030 040 040 031 032 035 037 036 = 036 041 040 025 020 034 017 012

Mino 0120.17 018 6010 © «0.10.22 st 015 «OSS OGG

H,0* nd, nd nd. (0.11) A. (053) (0.62) (0.25) (O64). ad, (004) nd. ad. 014 = 051d. nd.

#20 ad. nd, ad. (001) nd. (0.12) (0.13) 0.21) 0.15). ng. (0.08) nd. ng, co) 010. nd

COs ng. nd, nd. (0.06) nd, (003) (0.06) (0.12) (009) nd. ad. «02nd nd 0.04) <05 nd. nd

Lolz 058 = 08s. og: 055 067 «095 «=o? 08998 070 © 098 070 «106 «= 075? nd 102-261

TOTAL 998 1005 1005 1006 += 1000 1005 999 ©1006 = 100.3 «© 001 10021004 ©=— 1003. 1006 ©1009-1000 008 ©~—1000

1 From Brew and others (1969),

2LOI, loss on ignition: value is corrected for oxidation of FeO to Fe,0, and comprises only volatiles such as Hz0*, HzO , CO, F,Cl, S02. Because H2O*, H20TM , and CO;

are included in LOI, values of volatiles ae in parentheses where LOI is also reported,

9 T T T T T T T T T

Lava flows

4b 4
co 4
2 i 4 i 1 it 1 1 1 1

45 48 51 54 57 60 63 66 69 72 75

SILICA CONTENT, IN WEIGHT PERCENT

Figure 2. — Classification of lava flows and pyroclastic deposits of Mount Edge-
cumbe volcanic field; classification scheme slightly modified from Le Bas
and others (1986). Oxide values normalized to 100 percent on volatile-
free basis.

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

[Age determinations from whole-rock samples}

er ‘Argon Catculated | Weighted mean
Station 9." | wap unit | 0 wt)” sample age_| calculated age
. Ar aqimol/gr x10) | Ar ag) | (0% ye 3 | (oy 4
9 Omed 1.030 + 0.007 0.1430 on 95215
(85ALe018) 0.9692 3.37 65 +30 21413
9 Omed 0.770 + 0.003 0.7668 269 69423
(B6ALe014) 0.4886 151 44225 se+17
wv Qerr (2.468 + 0.005 0.6349 3.07 1wt7
(86ALe013c) 0.5104 3.34 447 16t5
18 Oc 2.483 40,009 1.002 210 228
(8SALe015)
21 cre 04.950 + 0.005 0.6700 0.60 494.95
(85ALe001)
28 bah 0.632 + 0.002 3.951 1.39 435 + 62
(85ALe008) 2.942 way R3t116 410+ 55
20 bal 0.537 £0,002 0.7387 0.95 96446
(85A.Le006) 0.7290 1.25 94431 5 +26
32 Oat 0.999 + 0.003 6271 276 438218
(86ALe012b)
37 Qbo 0.144 + 0.007 1.034 063 499+ 158
(85ALe013) 1.315 1.64 634+ 157 se7 +011
39 Qbpy 0.223 + 0.005, 0.1832 0.42 57290
(B5ALe010) 0.1046 029 33187 45163
40 Omec 0.821 + 0.005 0.4102 1.04 ts
(85ALe003) 0.4944 1.02 42218 3647
al Qcea 1.188 + 0.006 1.567 417 9246
(85ALe009) 1.281 437 t9 8745
42 Qbpo 0.378 + 0.002 1.781 5.42 7416
(85ARj005a) 1.465 5.14 269 +29 313214
43 Qbo 0.160 + 0.002 0.3187 1.79 138 + 60
{85ALe012) 0.3132 1.63 1364 60 137442
44 Qbo 0.232 + 0.002 1.839 5.09 551454
(85A.Le020) 1.982 3.68 593472 566443
44 ‘Qbo 0.239 + 0.002 1.071 252 311230
(85ALe021) 0.9196 359 2674 44 297427
45 ‘Qbo 0.208 + 0.002 0.3997 wis 134258
(B5ALe005) 0917 1.50 305 +84 189751
46 Qbo 0.217 40.005 1,823 7.02 583486
(BSALe011) 2172 350 694+ 148 e1t74

1 Refers to numbers on station location map (fig. 1)
Mean and standard deviation of four measurements
3 c= 0.581=10" yr, Ag= 4.962*10- yr, “K/K = 1.16710" ¢mol/mol. Errors are estimates of the standard deviation of
analytical precision.
4 Fortwo analyses, weighting is by inverse of variance of individual measurements; uncertainty assigned is weighted standard

deviation,

200}—

‘Alz03 CONTENT, IN WEIGHT PERCENT

150}

| i 1 1 1 | 1

1

awe

L

oT £3 2 ¢ ® 6 78 2

(FeO + 9 Fez03)/Mg0 CONTENT, IN WEIGHT PERCENT

FeO + 9 FeQ03

Tholeitic
EXPLANATION
+ Lava flow
* Pyroclastic
deposit

sb (oransed
7

#

NORMATIVE An, IN MOLECULAR PERCENT

oo

Nag0 +K20

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

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.

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

By

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

1989

120--

9.0}-

75

x

45h

30)

seb (orange)
Oscb

(gray)

Fey03

135,

10.5

sol

60-

OXIDE CONTENT, IN WEIGHT PERCENT

45h

3.0

seb (orange) Oscb (gray)

Oat

‘Omed, Qmec, Qcra

Qcea

Ocrd

© oo
gs”

9.0

sob

Qscb (orange)

n 4 1

1

Omer

T

C20

T

48 3T 34

37

60 63

SILICA CONTENT, IN WEIGHT PERCENT

75

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