Geology of the Delarof and
Westernmost Andreanof Islands
Aleutian Islands, Alaska
By GEORGE D. F'RMER end H. FRANK BARNETT
INVESTIGATIONS O F A L A S K A N VOLCANOES
Prepared in coofirration with the
Departments of the Army,Nawy,
and Air Force
UN1TE.D STATES G O V E R N M E N T PRINTENG OFFICE, W A S H I N G T O N
1
1959
UNITED STATES DEPARTMENT OF TffE INTERIOR
FRED A, SEATON, Smrettny
GEOLOGICAL SURVEY
Thomen B. Nolan, Mrectw
Rawer, George M i t t , 1926
GBolom of the Delnrof and Westernmost Andreanof Islands, Aleutian Islands, Alaska, by Gear D. Fraser and
H. Frank Barnett. JF7ashington, U.S. &vt. Print. Off.,
1959.
v, 211-248 p. Ww, maw (4 fold. cal. in pocket) profile, tablea,
25 cm. (U.8. QmIoglcal Survey. Bulletin 1028-1. Investigations
of Alaekan volcanws)
"Prepared in mperatlon with the D e m e n t s d the Army, Navy,
and Air Force?'
Blbliograpag : p. 2444H.
1. (feology-Aleutian Island& 2. Rocks, Igneous.
I. Barnett,
Henry Frttnk, 1921joint author. u TltIe: I3ela.d and westernmmt Andreanof Idmds, Aleutian Man&, Alaska.
(SerIea: C . 8 .
Qeologlcal Bumey. Bulletin 10-I.
Scrim : V.& ~eologlcal Survey. Investigattona of Alnskan volcanoes)
557.984
The U. S. Geological S m y , in response to the October 1945.
request of the War Department (now Department of the Army),
made rr reconnsriassxrce during 1946-64 of volcanic activity in the
Aleutian Islands-Alaska Penimula area. Rwdi% of the first
year's research, fieId, and laboratory work were hastily wembled
as mo administrative reports to the War Department. Some of
the early findings, as recorded by Robert R,Coats, were published
in Bulletin 974-B (1950), "Volcanic Activity in the Aleutian Arc,"
and in Bulletin 989-A (1961), "Geology of Euldir Island, Ahutian
IsIands, Alaska!'
Unpublished results of the early work and all data gathered
in later studies are being published u %parate c p t e r r t of
Bulletin 1028.
The investigations of 1946 were financed principally by the
Military Intelligence Division of the Office, Chief of Engineers,
U.S. Army. From 1947 until 1955 the Departments of the Army,
Navy, and Air Force jointly furnished financial and logistie
wistrtnce.
m
CONTENTS
Lo~ation,,,------------------------,-.--,----~-------------~Climate and ~ W t E o,,,----------n
--,
,- ,- --------------*,
- --
-----
k 1 0 ~,.,,,---y
*
* ---------..,----* * * * - * ..------------------.
hydrncks-------,,,,,.***-------------*-----------*-*---*
- -- --- --
-
- --
-
Amatfgnak laland,, , - --- - - - - -- - - - - Ulak Idand,,,----,,,,--------------------~--------------Central Delamf Islands and southern parts of Tanaga and Ramp
+
-I
-I
I
- - - - - - - ------ - - - - - - - - - - - --- --- ------ - -
Xwlands-- - - -,-, , ---
Early eMeld wlcanw -,--,,
,
,
,
- --- --- - - - - - -- - - Recent volcanic cones,,- ---,
.,,,,-------,-,---- - --,,----Surficial deposits-----------,,,,,---,-----,----megrained i n t m i v e rocks-. .-,-,,-,,.,,,-,----,---,---- ----Coamgmined intrusive rocks,,- - -,,,,.,- - - -- - - - * - - - - - ,-
_,-
- - I
-- - -- -,,-
References ~ i k d - - - - - - - - ~ - - - - - . - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - , - - - - , -
ILLUSTRATIONS
PLATE27. Gmneralhd geoldc map of Delamf and weeternmost
Andreanof Islands.
28. Geolouie map of westeft) peninmls of Kanaga Ialand.
Ram
29. Geologic map of Kavdga Island.
80. Geologic map of Ogliuga and Skagul Idands.
31. Garetoi Volcano from Skagul Island - - ,,- --- ---- -- --- , Facing 212
a2. Kavalga Idaad, looking west-Facing 213
41. Index map of Aleutian Islands
213
236
46. Detsiledaubmarine topography of central Delamf Islands atea
46. Surfaces of planation and drainage map, central Ranaga
---,--.,----,,,,,,,,------,--.,,,---------------
I ~ l a n d - - - - - - - - - , . . - - - - ~ - - - - - - - - - - - - ~ , ~239
~47. Seleckd bpographic proAIea showing orientations of planar
242
aurfam of five islends of Delarof groap,,
,
,
,
- ,,,----,
.-.
TABLES
T m
1. Fosrdls from Kansga and Tanaga Ialands- - - - - - - - - - ,,,..-,
2. Chemical composition of roeka from D e l d and areshrnmoirt
Andreanof I a l a n d e - - - - - - - - - - , . . - . - - - + - - - - - - - - - , . . , , , , -
w
218
222
fn~~&~-.-~---~---*-----------~,-------------*--------.--~--.
~praphy-,--,---rr--------------c--------------It------.--.-c---
hcation,,,----------------------------------------------.--Climate and v ~ h t i o n ------,,,-.------------- ---------------
,,--.
Layared rocks------------------.-----------* - - - - - - - ---------Amatdgnak Idand,, -------- --,------------------,
------Ulak Idand---------------------------------~------------Cenh.aI D e b f Ialanh and muthem parts of Tamp and Xanaga
-
- - - -- - - - - - - - - ------- - - - -- -- - -- - - -- - -
Islands-,,
,
,
- - - ,- - - -,
,
,
,
,
- - - - -- --- -Early shield v o l c a m -,----------,
-.------- -,---------,Recent volcanic cones--- - - - - ,,
--- - - - Surficid deposits- - - -- -- - - - - - - ,,- - -,
,
,
- ,.- .,- - - + - - --a - - - nne-grained intrusive rucks-. .-- - - - , -,
,
- - .,,- , - - - - - Coam+graid intrusive rocks-----,
- -----.
---- ,I
--
r--
a
+-
,-
--+-+-------
Submarine topography,- ---- ---- ------------- -------------ph@ogr~phy--.-~.-.~------------,---------,---,-------------~---logic
history,. ,-, ,,,,.-- -- - - - - - - -- - - - - - -- Referencescited---,----------+------------------------------------
.
-
-
ILLUSTRATIONS
mrn 27. Generalized geologic map of Delarof and weaternmat
28.
29.
SO.
31.
32.
44,
45.
45.
47.
Andreanot Islands.
Geologic map of western peninmula of Hmga Ialand.
Geologic map of Kavalga Inland.
Geologic map of Ogliuga and Skagul Idanda
Gareloi VoIcano from Sbgul Idand - - ,,----,----.,-,Facing 212
Kavalga bland, looking west-, - ,------,-------,,-.,.
Facing 213
Index map of Aleutian Islands ,-,,-------,---- -- - ,,- -,, 213
DebiIedaubmarine topography oZ central Delamt hLands area- 236
Surfacea of planation and drainage map, central Kanaga
Island--------------,--..,.--.---,-2----------------.239
Selected topographic profiles ehowing orientations of planar
surfaces of five islanch of Delarof group- ----------,,--,,
242
TABLES
1
-
1. Fodm from lhnaga and Tanaga Islands,, ,-, ,,- - - - - - - - - - - 218
R Chemical composition of m k a from Delmf and w&enrrnost
Andreanof Islands -,--,,---,.---,.-,,,--,,,----------222
v
INVESnGATEONS OF A M U N VOLCANOES
GEOLOGY OF TWE DELAROF AND WESTERNMOST
ANDREANOF ISLANDS, ALEUTIAN ISLANDS, ALASKA
By
GEORGE D. FRAsER and
H.FRANK BmmTT
Th8 layered m k s of the Delarof and westernmoat Andreanof I h d n
are divisible Into dve unlta, generaIlp younger from south to north. Unit
1, the rockn of Amatignak IsIand, ia composed of sltered, well-bedded
Macmua m k a including thick, finely banded, p e n 6 ~ ~ 1 3 I b e m p a ~ ad~ e~ t r ~ l ~
formed argillib. These rocks apparently grade northward into unit 2, the
altered pillow lava3 and pyroclastic depwita of Ulak Island which are interpreted as a near-~oamfacies of unit 1. The age of these twa d t a is
unknown but preamablp 31 Tertiary. Unit 3 is a composite sequence o f
almost unaltered ahallow marine and mbaerial wleanic r& cmpoeed
Thin
of tuf-breccia, lava, pillow lava, and mrbrdinate sedimentary r&
units extends in a belt acrasa the central Delarof Idands and the sonthem
part^ of Tanage and Kanaga Islands. Upper Tertiary f w f l a are found in
the roc3ts on Tanaga and Kanaga; the rocks of the central D e h o f s are
comlsted with these because of lithologic ~imilarities. The rocks in unit
3 may be partly qaivalent, extreme faciea of the Amstignak
OF
they may be younger. T h e mtact is not exposed. Unit 4 is composed of
Pleistmene precaldere lam that fonna fragments of old sbidd volcamw
near the presently active cones. This lava, younger than most of unit 3,
Etpp~terI* intertongum in places with the upper part of the older sequence.
Unit 6 comprises the composite cones of presently active Gareloi, Tanaga,
and &ma@ Volcanoes. All five bedrock units are andesitic to basaltic in
composition.
Unita 1, 2, and 3 are cut by many dlkea, dike mrrns, and sills of andeb
ikic of basaltic cornpodtion. One limburgite Intrusive mass and one Eight
rhpdacite dike represent divergent compodtiona.
The older nnib are alao cut by coaree-grained tlills, dikes, and plnhna,
mostly of granodioritic cornpo~ition, 31ak Island is part of a granodiorlte
pluton cut by aplite and andedte diben; none of the invaded rock b mposed
and the age is unknown.
None of the r& have been m'onally mhmorphoeed: mem l o w e e
foliated meks, auch as slate and phyllite, are absent.
Submarine contour8 reveal a major trench and ridge abuctare for t h o
Alsutian arc Islands are actnally peah of a great arcuata mountain rIn this area the range rises 25,000 feet from a foxdeep on the Pacific ~ I d e
and 13,000 f e e t from a deep on the north. The main ridge in apparently
an arch botlnded on the uouth bp; a norkhaipping thrust soae and cnr the
nwth by a high-angle fault mne, downthrown to the north. Volcanoes are
located dong the inferred fault an the north. A grmp of transverse mbmarine and rmbaerial linear elements nuggesh further that the area bas
been segmented by high-angle faulting. An A l d a n island arc wan probably
formed in this general area as early as Miocene time, but the p m e n t
voleanoss end the ieland loeationa and Plhapas were formed much labr,
probably In P l e i w n e time.
Several stage8 of Iate Tertiary or Quaternary wave planation nnd mbaequmE differential tilting e
b
a the ~late hiatory of the era; sporadic
central-type volcanism at ita north edge began in early Pleiistocene and has
continued to the present time. Widespread deposition d Recent ash, and
hi~torjeernptiana from Gamloi, Tanaga, and Kanaga Volmnoes, are the
most recent volcanic evenh Glaciem were active on Tanaga and Amatignak
during the Pleistocene, and ice patches are now present nesr dormant east
Tanaga volcanoes and on the high ssEopm of Garelor Volcano.
INTRODUCTION
Nost of the western Aleutian Idands here described were
mapped in reconnaissance (fig. 44) during the fieId mason of
1952 as pad of a ~eneralstudy of t h e geology of the Aleutian
Islands. The Geological Survey's motorahip Eider transported
the field parties during the frequent changes of base camps. Tho~e
who participated in the field study were: R. E. Wilcox (geologistineharge), H. F. Barnett, B. H. Bieler, G. D. Fraser, E. H.
Meitzner, W.H. Nelson, R. A, Robie, and G, L. Snyder.
Two-man traverses were made on foot from base campa establiehed along the coasts. Coverage is summarized a8 follows:
Kana~a(south of latitude 51°54') 7 camps, 34 traverses; Tanaga
(south and southeast coasts) 4 amps, 8 traverses; Amatignak
(east coast) and Uak, 3 travemes each; Kavalgn, 2 traverges;
Unalaa, Ilak, Skagul, Ogliuga, 1 traverae each. Traverses ranged
from 4 Ito 12 miles, and averaged about 7.
Contour maps prepared in 1943 by the Corps of Engineera,
U. S"Army, at rr scale of 1:26,000 were available for Kanaga
IaIand. The Delarof IaIandn were mapped on Coast and Geodetic
Survey topographic manumript sheets, scsle 1:20,000. For
southern Tanaga the only maps available were form-line Coast
and Geodetic Survey topographic hydrographic charts (Nos.
9145, 9146), scale, 1:40,000. High-altitude aeriaI photographs
cover most of the area.
Data included here for northern Kanaga are based upon the
monnaiasances of Robert R, Costs, who also touched briefly
at aome of the Delarof Islands m d part of Tanaga (Coab, 1950,
1952, 1956b, e ; written comrnunieation, 1964).
W't~c-plnnegl -1rr1xcc of Sk.sgn1 Islxmrl conrmsle w i r h \ r ~ l r : ~ n i c c o a c 1% milnortl~wwr. S o l s *mall m o u n h In l o r r ~ r o u ~ ~ dS .k a p u I a : d
O ~ l i u ~(not
a
rhnrn) nre uniqoe Among !kc llelsrol lulnnds in Illat the ltqeal rc.1 clitl i, ntrl wcll ~ l r r r l ~ , ~ r r e l\ir
.
Frlrrc Plloto .%ir
Plcolt~crirphic and C h ~ n i n : Service (If ( T S ! . 1-niinrl S t r l ~ r 4ir Fnrcr.
GEOLOGICAL SrRVEY
r'lifl, pnnly corrrrd clnrm beach. a n d prcl-nl day rock bcucll ( t i d a l O u t ) . S n ~ a l l! I I P , , I T I1111
~ ~ Irppcr IIITILLI.I.
moy hare originnrcd nr b o u l d r n and irrcg~llariticc nn xhvc-cut sur1nr.r: cnrnparc b i t h featvr..s nn prcscnt H.SI.C.I.LII ~ o r f n c c . .\I, Z'ureo
I'hoto Air T ' l ~ n f n P a p l i i ~.inJ
~ C)rzrling Semicir (31 \Tli), United Slnfrr \ir Forcr.
X o l n ~ - a v e ~ p l a n rrlrl p * r 8 u i r f a c ~ . rr>
The 10 islands dkussed Fie along the Aleutian Islands BFC
between longitude 177"039nd 179'09' W.,and between latitude
61°13' and 51°56'30" N. (fig. 44). The two large islands of
Kanaga and Tanaga have areas of about 100 and 160 square
miles, respectively, and are the westernmost of the Andreanof
group. Four of the eight Delarof Idanda, Skagul, Ogtiuga,
Kavalffa, and Unalga, constitute a narrow projection which extends 35 miles west of the larger islands and is separated from
them by Tanaga Pass. Ulak and Amatignak form a spur trending
southweatward from this westerly projection and from the general
curvilinear trend of the Aleutian chain. El& and Gareloi are,
respectively, 5 and 15 miles south mnd north of the line between
Skagul and Unalga. The total area of the scattered Delarof
group is about 65 square miles.
Etmm 4 I l - I n k
map of the AIeutInn IslQnds, A l ~ k s .a h o w l n ~lacatlon
wenternmost Andmnnof Idanda.
m
T
of the Delnmf w d
E AM73 VEGETATION
Weather in the Aleutian Islanda varies locally and h related
to the inbadion of ocean currents, moving air masaes, and
island topography. Winds, clouda, rain, and fog generally prevail
the year around. JAW heavy fogs often limit visibility to 100
feet for days at a titime, particularly in the summar, and traverses
mud often be completed solely by compasa and aneroid barometer
observations, Annual precipitation ia about 40150 inchee.
4R85QMQ-2
In the western Andreanof s summer winds wiginate mostly from
the aoutheast, south, and southwest. Average velocities are about
25 miles per hour; during sudden atoms velocities increase to
70 miles per hour. From October to March prevailing winds
are from the north and west, Excessively strong, gusty downslope winds, sometimes exceeding 100 miles per hour, result from
the rapid spilling over of air masses dammed against the windward sides of mountains or ridges.
The North Pacific drift from the southwest warms the cold
waters brought south from the Eering Sea. Winter temperatures
eeldom fall as low as 0°F. The mean temperature of the warmest
month (usually August) is about 65" ; the daily temperature
rangers in Bummer from 46' to 75OF. Total annual snowfall,
falling mostly from December through March, averages 70 inches,
but little mcumulates except at higher mountain altitudes,
Vegetation is abundant as a consequence of the temperate humid
marine environment. Mosses, grasses, fungi, and a wide variety
of flowering phnta (many with edible parb) blanket the sIopea
t o a thickness of 2 or 3 feet. The only trees are a few stunted
recumbent willow^.
Except at gullies and cliffs the plant cover ia unbroken on the
flat or gently rolling uplands below 1,600 feet altitude. The irregular, hummocky, springy surface rnakea walking slow-less
than 2 miles per hour is an average rate. Rock outcrops in the
- interiors of all the flat islands are rare and widely separated.
Geologic investigations in wch islands were largely confined to
the sewliff and rock-bench zone around the island perimeters.
The landforms among the western Andreanof blands vary
greatly. Kanaga and Tanaga IsIands exhibit compound landforms.
Lofty volcanic heights at the north give way southward and we&ward to poorly drained lowlands surrounded by sea cliffs. The
highest, least eroded, and most recently active volcanic area
a r e at the extreme north of both islands. Like aeveral of the
larger Aleutian Islands, Kana- has a narrow low-lying westward
dension.
Gareloi fsland is rr conical volcano which rises abruptly from
the ocean floor a few thousand feet below sea leveI to a summit
5,160 feet in altitude (pl. 31). Two emall glaciers hang on the
s h p , slightly gullied slopea. The shoreline is precipitous along
most of itEi lengkh. An eruption in 1929 opened a aeries of venter ;
it spilled dark lava from a rift zone ~ o u t h of~ the
t wmmit vent
and spread pyroclaatic materiala over the southern face of the
volcano. This recently active, elevated maas is north of lower
landa, paralleling the arrangement on Kanaga and Tanaga.
Amatignak Island is a dissected glaciated upland marked by
cirques, sharp arEtes, troughlike valleys, and small intermontane
depressions. The island has 1,875 feet of relief. No constmctiond
volcanic landforms were seen.
The other islands of the Delarof group are low, flabtopped
featureless platforms bounded mostly by sheer cliffs and sea-level
rock benches; they resemble t h e southern parts of Ransga and
Tanaga Islands. Small ponds commonly dot the poorly drained
surfaces, Ulak has a somewhat higher relief than the low
Delarofs.
The flat Dekrof IaIands are dotted with small conical mounda
(pls. 31,32) generally only a few feet high and too small to ahow
on a topographic map. A few larger mounds are visible as dosed
contours (pl. 30) ;dense vegetation, possibly the result of fertilization by birds, gives some credence to the idea that the mounds
are '"bird perches," Some of the structures may have begun aa
irregularities on the wave-phned surface (PI. 32).
QEOLOQY
LAYERED ROOKS
AMATIQXAX IB-
CkQmctw.-Sedimentary rocks composed of volcanic detritus
make up moat of the bedrock on Amatignak Island (pl. 27). The
rocla are of dark-gray, greenish-gray, olive-green, bIuiah-gray,
and light-brown tuffaceous argillite and siltstone, tuffaceous sandatone, and tuff aceous breccia. Greenish coIars are most common.
Grain sizes less than one-half inch predominate, but volcanic
bombs are visible in a few beds, and altered volcanic glass i~
seen in thin mtion; accordingly many of these rocks have been
classified as volcanic wacke (Williams, Turner, and Gilberk, 1954).
Generally the rocks are well indurated, chloritized, pyritized, and
silicified. Cryptocrystalline quartz veinleta are common.
A common rock of the island is a finely laminated argiIli@
banded green and gray. The rock is brittle, breaks with a con-
choidal fracture, and often shows structures recording penecontemporaneous deformation. Similar argillite has been found on
Ulak Island and elsewhere in the Aleutian Islands in association
with altered pillow Iavaa and marine pyroclastic deposits.
Microscopic examination of specimens in the sand- and siIL
size range confirmed their origin a tuffaceous dirnents. One
210
INVESTIGATIONS OF ALASKAN VOLCAHOEB
m i m e n had fragments (< 0.04 mm) of &reem chlorttie material,
oligwlaae-andesine, and augite cemented by carbonate. Another,
with zeolitic ( ?) cement had partidea 1< 0.2 m) of chloritized
volcanic glass containing feldspar latha and other crystals, and
cr~ratalfragmenh of unaltered angite and albitized plagioclase
(< 0.09 mm). Most of these rocks are of andesitic or basaltic
composition.
Many andmitic or basaltic dikea and small d l s intrude the
sedimentary rocks along the eastern and southern coast. Small
dioritic or disbaaic sills are also found; a large quartz diorite
ill forma Knob Point on the e a d coast. Shipboard observations
and study of aerial photographs suggest large areas of massive
rock, perhaps plutonic, in the northwest quadrant of the island.
Age.-No precise age can be assigned te the rocks of Amatimak
Island. Parts of some formations on Attu, Agattu, Adak, and
Unalaska dating from late Paleozoic to early Miocene have similar
lithology. The two oldest stratigraphic unib in the Rat Islands,
generally coareer grained than the rmks of Amatignak, have been
altered and intruded by quartz diorite, Fossils from the Rat
Islands indicate an Eocene(?) to Miocene, age for the younger
aequence intruded by quartz diorite, but the older unit ie not
fossiliferoaa. Mesozoic fossils have been found only on Atka
Island; Paleozoic fossils have been found in m e srnaIl area on
Adak (Coats, 1956a). Tertiary fossils have been found on Attu,
Agattu, and Unalaska in rock8 resembling those of Amatignak.
On Amatignak, ~tructuresreaernbling worm tubea were found in
two outcropts; one unidentifiable foraminifer, probably post-Cm
bceoua, was found in thin section. The rocks of Amatignak are
pro~isionallyassigned to the Tertiary and are probably Miocene
or older.
The rocks on Amatignak have been tentativeIy wmelated with
those on nearby Ulak Island, and both are probably at leaat aa
old aa those on t h e central Delarof Islands which have not been
regionally altered or invaded by plutonic rocks (p. 217).
The rmka of Ulak Island resemble those of Amatignak. They
are greener and more altered than those of the central Delarof~.
The layered aequence is composed of altered palagomite. breccia
and tuff, basaltic pillow lavas, and tuffamua wdimentary rock,
Those smka are intruded by sndeaite and hrralt dike~land sills.
In contrast with the well-bedded sedimentary mtion on Amatignak, the rocks of Uak are dominantly igneous and pyrocIastic,
Pillow laveas, abundant on Ulak were not obgerved on Amatignak.
The rocks of UIak are chloritized, but the pillow lavae and tuff
are apparently not spilitic.
A 2-foot thick layer like the distinctively banded zargi1Lit.e
found on dmatignak lies between altered tuff-breccia layers on
the west c a d of Ulak. This sugwts a correlation of the r&
of the two islands, and possible facia change from fine-grained,
wdl-bedded sedimentary rock8 on Amatigndc to more dominantly
volcanic rocks on Ulak.
Bedding on the south half of Ulak Island dips toward Amatignak, and beds on northeast Amatignak dip steeply east, suggesting
either a syncline beneath the straits between Amatignak and
Ulak or drag on an interisland fault (pl. 21). If there ia a
fault between Amstignak and Ulak, and if the relative altitudes
indicate relative movement, then the apparent facies change may
be better explained as a change upward in the aection toward
Ulak. The rocks on UEak are believed to be about the same age
wa the rocka on Amatignak.
CIPNTRaL PglrdROF I W A M D B AXD
B
O
EANAGA IBLBNDS
PA.UTB OF TAHAGA A N D
Chmcter-Unaltered pyroc-lastic rock8 and lava make up the
bulk of the other islands of the area (pla. 27-30). Tuff-breccia
and tuff are interlayered with black, gray, or brown andaitic
and basaltic flows, pillow lava, and flow breccia. Tuff-breccia
is mbst common, lava flows are abundant, and tuff is somewhat
less common. Sedimentary rocks are least abundant, except on
southeastern Tanaga. Plates 28, 29, and 30 show proportions of
the rock types in selected areaa.
Volcanic centera have not been found. The relative scarcity
of reworked volcanic material and the abundance of coarse debris
~uggeatthat the volcanic centers were nearby, Fo~sils(table 1)
indicate that m n y of the beds are of shallow marine origin and
some of them are subaerial.
The composition of these rocb, as revealed by field mapping,
brief petrographic study, and four chemical analysea seems to be
consistently basaltic and andesitic, with no representatives of
more si;iliceoua types of lava. Table 2 presents chemical analyses
of nine samplee from Kanaga, Tanaga, and the Delarof Islands.
The fird four analyses are of rocks considered in this section. The
main differences between roeks of the sequence apparently are
textural and structural rather than chemical. AII sections studied
under the microscope fall in the basaltic: and andesitic groups
TAB- 1.-PoaBila j r m Kanaga and Tantlgo Zslande
h3
t-r
[1dantj5@tlon of losrlls: P. 8. MaeNelL m e g a f a oca 1-9, 12); Ruth Todd, PaamlaiIm flw. 2 6 , 7 , 12): P. B. ClOu f i b 0 &a 16); 0. W.Owke, e&kd&
B. B.%sr&oom, pollan and spore not. W; R. W. Brown, mssi1 wood (loo.&)
~
C
1. On W. side o[ Kmaga Bay 0.6
of b y .
mile N.ofmoulb
Barnarb
Idemtlfioatlon
Y
Lnta 'Pert-.
PO*
P~leeypods:
c w r a sp. (lqf‘3)
abb *mane
Curdium (Caoaldama)n. dp.7 d.0. (Odldurn Pabrlciru
Clrrl dla:
&nw
2. h'ortb mast of Xaaag~BC long 17PJ3'35".
In bedded gravel dbposll about 60 L above
benab.
rp. (e s-,
stm:lg
Ow. Is);
h l o n an age b d largely on ~ i m
of oyatm.
" h e m e to Recent rpww n
w l
a
ponderom.
~ b b )-,d
Pelwypodn:
Pabm n. q.8. (raemant
CardUa (C&oardfa) sp. sK. C. u h k u n n Dd
*
Lew TerLLnry (Puo- P&en name er
~ n 0 b
loc. 4.
-
Cm%%#
sp. (a large, -ulj
ribbed apocles)
---.--.-.-.-------------.--------------------------.--.-------------.-.Forsminllera:
2 ~ ; y m r M tLP,
EYpAidJclm 3p.
mpAldkUn mdlm
(Pa~kwand Jones)
U&a/lw urhmanl Todd
h r l d d t n a limbam cushman md Eugba~?
sv.
a.
I n vaUa of large stream 2 1 mil= N N W . 01
bead of &s
B&y. C~ILU%%IM ma-
OnldmtLBable molds of g a s t r o w and pelecypoda
g10merat.e..
4, OD W. ooast N. ofcanrtd Lakes 4.5 mllcs SSW.
of R a w s V o W o In geao, loUaceaul, s a d atone layom. Consplcuou~ Psckn m q u h a
here.
Pelbcypda:
Paen n. sp. B bi!. P.dbidw DnU
Braoblo b-:
8&
valr6, e.msnd m. h d e t
'
pllOaure(7)
Anlntsraftl moldofa P
&
bably Identical
~ c *a
m ap. D, ~sb b E & j e c t i a o born
8% o y Ioland nssociated wtch s k g 0
hum be wed to be Plloane. Thh Patea
also m n t w aorthan Adak. B e l o w to
Pcd6n WW
group.
.--------------.---.--------------------------------Poramlnlfam:
-----*-a---.--------.-
~ i d u l hsp.
l
C1Mdidfnn %p.
~i
but ~3 yds 9. o( loe. 4 I~I
bedded, mmlronsolldatad pumleeoua sediment8 lntwbeddtd
w l l h v o ! m l o breccia.
Oaatrowda:
Buctnum 6D.
Notlm up.
P d W Plloonae
AU moUmb ara
far Paden, &,
molL.
ly pr6~erpsdand axapt
and Mw nm I r r W
@J
Psleespoda:
Pcam n. sp. C. b ~ ~ m e n t s
&a&
Pp. an. A. adla Dall
M m a BD.Inlet.
&Iw
cI.S. ~ h & d b L l ~ n 6 4 ~ 1
Pcrlrn a rp. 0 rmmhles a spdmw Mrn
MlddletmIa., OUU of Alaska, tmm M
beUeved to lx PU-.
BeIowa to Pada
.-.
o&'
nnd sp. Indet. but bevln name mlrrosoulp
turs u r-cm
from ~oc.
%
HtmUh(vr1alarluocsa (Omell4
numrotphlna spp.
?'
onimellat sn.
Nonlon labradnrfcum
Damn)
Glmllar to buna on Arnchltkn B. (Cushrnnn
sod Todd 1@47) but lseklng lanktonlc9.
Tha ~nnab
PorarnWfera w e %pmlted
~
In
&allow water potmbly not lass than b
fathoms nor bare than 100 more llkely
toward sh~~llowerend of thk ranp. The
Isuno la ehmctorlstlc of cnld wnter 8lmlle.1
tn those known ~n
Alaska, northern b,
and Oreonland Ln Recent was.
~ l p h l d h o ahbk#VW
l
6whmm and Todd
jrloldtror, Cusbmm
FIp%Ula d i m (Prrker and Jones)
Flawrim mardm& (MontRcul?
nD.
UdpnCM el. cuahmanl Todd
Awulogcrinn u r n Todd
B L Y C/.Jgld!
~ ~ [Cushman)?
Rponlde~eolumbimsb (Cuahmnn)
CbLaldullv~ocdtjorn~Cushman a d Etughea
llrnbda Cushmna and ncc~bcs
PuUnla a a L b t w g l R. E.nnd K.0.Stmart
CiHcidca lahoiulus (Wnllror and Jacabl
6.
-1
In \tme%me brecck trapmen= from v a l ~ i c Palec
se1I-l
bedS dong the beach 1.7 miles N. 01
A. Eashmd~s
Cepc chonu.
Macomn sp.
Sea fmtn?&u
at end d tabla
wh&)
Myo sp.
tang83 born
ear&
PUofocle to
f
m
Tan-
7.
1.-Poseila
from Kariage and Tanaga I~lnnds-Continued
b sand )mu B eonslmaerale 0.9 mile N. at Pel PC&%:
Cape Chunu.
%,i(deot!~abk tragmant
Clrrl dla:
&donLl~shle hn-nt
......................................................
.-..-...,.-..-.....--Fmmlnlfera:
Uwoldfna sp.
~!Cponddrelsp.
GlMcldu sp.
Cll,uldh SF.
SP.
8 In masslvc, p.oorly mnsolMetBd nnbedded tug
2.2 m l l NE.
~ of Western R.
Pelecw~ch:
A& fhgmeol
Late Tertky
~ u c l t $onttillnmla Conrnd
Ludm d W
to Recent.
a mugw barn mid-TwUarg
Macom~,Iragment
n.
1. masalve, poorly coosolldsted unberlded cull Oss~m~ods:
Nalka sp.
NE. shorn o t M o m Bey (bay B. of Wuv~cm
Pelecypodn:
Prqmcat oflarge shcl),unldent LBnble
J't.).
10.
On
west shorn
of N. Ran-
01 Knnnga V o l w o .
1 8 rnllea S6 W. Algne:
Oenasslrnllarto LUhophqKam m y be new genus
I I . soulb shbrc at wlnl (Tnmp Polnb) on 8.
mar& of El06 SprIngs Bay. From bedded
tu7 h
m
m
h layer (304t thlckncss mposed)
dlpplng gcotly wthwnrd.
'
12. Lst 6Im41'2%"
N. long 177°W90 W.,In b W
ft lens ol tuI?aeeo;s C O t l u l W Eoaglomcrato ow,
andoslte flow.
Late
~~ or
Plekmcme
b samnatmtlgapblasequ~~~~b
loa 4md 6.
or sobsenus
P e 1 e :
stuicnlrofldanrS Llmaeus
Oatropodn:
ACM
n. BP AIL unwrmaodg lqe, heavy
ape&
Pelwypmb:
Odhm n. SP?
Lgte Torllary to
Remt
Late Tertlar
pmdbly hfioaens
La e henry agatcrs am mom &araeterfatIe of
&hen6 than later de Its. Appannc
prodambane of uo-E
brms atmost
preolud& s very bite Tertlnry or poab'PerLI.
ary we.
A heavy amcavo o ster hav- small nrdlal
rlbo rssemb
tiass of some spedes ol
Spondplws or%nnh.
*ef%
t %?;
an-
PMgbdantm. Mold wit& r dsoprnmcbMU on thn
abG. -- - -- ,.
~1"adaf
sml tmeslmfkbtbatof
but the valve b mar8
6g;pa&md
cbrf=:
Luaa. o m bsrvlly BnQd #-dm p"W of
urn. w.7
Ibb B. rarl.wamupU& er( e h
Pachard omloned that Dull's awdmen
reslly dsme from TartlarJ & k u s e lt
msambka so closely a P l e h t o m e M d W t
sbeelaa barn the P d c C o m t . Tbers la na
iroubt t h a t the %wentg &en was en-c
in tbe -us
&merate.
-----......--.----------------*-------------------..
~ommtnlfm:
EtpnidlclI'o wdkn dakwMd Jonea)
Ranulbaap.
Cul(cidc4 sp.
la.
W.eosst Cape Basmlk
on bscb.
paplaru)e,
The wrlters fmd no fleld evlamee tor ass
Cape Bssrnlb rocks to a sequrnm
mnOg oldei than mckr In lorn. 12 anp14.
Bemum of genezh
menw augga~ted
~ss species of
dabin may be -%ere
~ i n l k f aIn MMioDene Mottramp formntl~ngl
trom b l a s Bahtno1dsa: (Speranpoldlll)
'
Two spwlrneng probably same sped&?. M a y
be Maropncr&ra or L U l a , but la& dieg-
ayd
nmtlc fu8811~~
C W M .
Late PUocm4 a
Plektmne
M: L a g 177°W10" W lat 51%"06' N kam % l l e n ( N o . o f ~ ) :
, cokgbmratonnd t w T ' ~ e ? jr h h in wbbbd
p e t debrls., Slump block burn tas cUlG
7% sedlmmts w m ap arently de
nt to abdow, probRbEBdIn a
b&
Bd
water.
w m exknslve dder
in the & o n . spruce and Rt 10a l mslderabla dIsuno8. and Umltad fme corer Jn
bebly
the area . d.varled bmbaceous&ra
e S t d ahma martdm of the sed.f%uw
z&%
k%a
(tnLerpl&d)
bftsm
Cllmetlc mndltlcms wem probably gomewhae
mddw than l b o found
~
today in same sma
Erlcaeaaa. 3
Them are no trees Ln area M a g .
I
I
..
16. Long lWS5'46" W.. let 6 l W 1 4 " N . IJItuf- .&fu sp.
lecssus aendstanuand cowlomerate. Fmmmtr
or krwll wood.
- .
1
3
I
..
a)
..
I
.
.
.
.
, . .,
.
.
Kmsy localltias h m acsle 1% 000 U 8 Army C o w of E n g h m maps 1%
N. sp. tor Paim dP~lgnntB4by t e t t e r a ' ~ ' ~B, dther let$em h sarles lor n.'s on 0th- A l e o w
-ga
w w Liom U. 8. ~ o a s b e n dd ~ i s ucm p h n s 8146 a d a 1 4
,
,
I
...
PcukMe#~do.-.
I)ao&).
. ...
I
1
.
.
..
.
. -
.
.
-
-
-
.
Tars 2.-Chsmied
aomcpo6itwn of mcka jrom ihe Dsbroj and locslsrnmosl Andranof
B. I.
Tomeail
LoaaLCp,arme. .ndooomTena d s
m
-
~-MOW
um.b
H ~ S E ~
OKIWB
amnwm m . g s a s g a
a.
P
A-
mddte,
d m
~
flnw
Wp$k.
4wwdl
4
ew-m
SFS-M
mm
Be0 ..............................---.......
8 78
4.62
t.m3 0 ..,.-
TOtnl-..
.............-................
................................
~ m p d mdmity.
lBw
LBN
...................................
............................-......
.....................................
........................................
&O- ......................................
50+......................................
TI& .......................................
oob........................................
P 01........................................
d
..........................................
F- .........................................
8.. -........................................
-0- ............... .
......................
MgO -...
OaO
NC~O..
&rO
a re
P70
aw
b79
449
LOB
8.66
.d8
aw
.78
P
..70b0
.
..a7BD
.m
.
.m
.12
Z2
.a
.00
-13
.01
.40
.
.oh
.a
.b0
.la
.M
BBBd
99.98
am
m. a
-01
Auddnr
besal$ h w
a m
uol......................................
Irdk ......................................
Labredorit6.
a t e
.
.
......--...-..
8101-......................
'
~
Aglts
.odd&
62go97
twn-irn
a.76
a m
n~
489
2U
8 87
.do
lt. 01
6.00
mu
4M
879
.U
.in
a. n
.ell
.a9
i-01
m
-01
(IB
.w
8.w
.
-143
lOa M
1 M lo
.Ob
I O M~
89.86
laM
ILI
Ln
1.87
abd
a lo
.-29
RMn-ldl
nn
14n
..81u
.80
L 40
1. M
898
8. 24
4%
..49
8.78
OB
.-46
Od
.a
.cn
.U
.ae
.a
.(w
c
ire
..a2
.W
-04
-00
*WLR
W78
BQBB
.m
a81
.a
78
Pn
QQBb
PB.n
-17
.01
a
3
-ad
w. n
am
.---.------.
L E ~
8.66
1P I
ria
..a,
.a
.82
WOP
~b 77
-08
.18
M
.u
.a
m78
!La
dg~ln-167
17.88
.m
..a
a
..a5
.01
a66
~ W - M
.OR
lQL OB
.01
-
-
=
l.40
.61
I7
-74
-8)
-
-
Pherlblc rooks
P28
L2B
2 01
,'Kl
62
-68
84
.la
L 16
6 06
an
188
.P
..maJ
17.16
4M
b.2~
. . . .
,
a
Dark
ApUte
rhwdmlta I l h M t e , Labredmltd Ornnbdlorite, Uslet 61
cUkn
plutan
0 8
ddb3.d
plotcm
440
lase
..-.
-
4aw
................................. ..-.-..-....................... --.-
h h d e a au pamsat BaO, not +own.
O
r
m
Iabndc
.26
.oa
.
16
.M
.m
.[W
.a3
.a
.a
.01
.OB
.16
,'
.u
.10
............................
whether the rmk is pyrocl&ic or lava, and the sedimentam rocla
appear to have been derived from similar rocks. Significant
variations within t h e basdtic and andesitic groups are indicated
by a wide variation in type and frequency of mafic phenocrysts,
and Et seems probable that s few of the lighter colored rocks,
in particular the pyroclastic deposits, would fall in the dscite or
rhyodacite groups of the chemical classification of Rittmanlr
(1952). No rocks as light colored as the rhyodacite dike (specimen 52Sn-100) were found in the layered sequence, and at Feast
90 percent of the sequence is probably more basic than dacite.
The xwt names {andesite, baaaIt) used in this report f o ~
analysed aphanitic rocka are those appearing in the chemical
claasifimtion of Rittmann. In the text the full Rittmann name
ia given parenthetically for each analysed rock. Pigeonite appears
in the calculated Rittmann name for apeclrnens 62-N-64, 52W-64,and 62-W-70,but no pigeonite was found in the modes,
Moreover, pigeonitea, as used by Rittmann (1952, p. 78) are
""mixed cryatala of diopside and clinohypersthene" not further
defined by optic: angle and may, therefore, include augite and
other clinopyroxenes as defined by others.
S w i m e n 6 2 4 - 6 4 ia a dark, greenish-gray auglb andesih
(Rittrnann: pigeonite andssite). Phenocrysh of augite, magnetite, and zoned (oscillatory) calcic plagiodaae are set in a groundmass of plagioclase microfihs (#An,,) and opaque oxides. The
larger pl agioclase phenocryste are often poikilitic and usually
contain aeveral zones of dusty inclusions.
Specimen 52-W-64(Rittmann : pigeonita andedte) is a lightgray porphyritic augite andesite. The phenocryste (<2 mm)
are progressively zoned calcic plagioclase, augite, magnetite, and
one ghost crystal of hornblende ("opacite"). The groundmass
contains microphenocrysts of plagioclase, clinopyroxene, and
opaque oxide in a cryptocrystEtIline matrix.
Specimen 62-W-70 is dark-gray labradorite-augite andesite
(Rittmsnn: pigeonite labradorite andesite) with nurneraua black,
subangular ta rounded cryptocrystalline inclusions (0.1-1.3 cm3
of unknown eompositian, but poasibly altered basaltic glass or
tuff. The inclusions are opaque on the borders and are crowded
with opaque dust throughout; small crystals of plagicsclase and
clinopyroxene have begun to form within the inclusions. The
igneous matrix for these inclusions is markedly porphyritic with
zoned, poikilitic cr~rstaIsof calcic plagiocIase (wide dusty zones
and areas within the cr~ratals),augite, and corroded divine in
a groundmass of labradorite microlites and opaque oxides. There
is a marked flow structure in the groundmass.
S p i m e n 6%F&94
O W b a m : andmine baa3t) is a bfack,
wry fine grained rock About I percent condab of scattered
ghenwrysts of plasioclaae, clinopgroxene, and pseudamorphs a f k
olivine. The groundmasa haa a strong flow structure and cansigh
of mimlites of d c i c sndesine, cfinopyroxene, and opaque oxides.
Other rocks a2 this group tire generally similar. Augitic
pyroxene, hgpersthene, oxyhornblende, and olivine are common
mafic phenmrysts, Phenocrystic plsgimlase is wuaUy bbradorite
or more calcic and strongly zoned. Andeaiba with both cliaopyroxene and orthopyroxene are perhaps the most common rocks,
but many fall close to the andesite-bdt boundary a d are diflcult to classify without chemical mlyaes.
Throughout the islanda aof bleached and ironata1ned
Wka, beleved t o m u l t from fumarolic wtidb, sre common;
one large, poorly defined area contains abundant epidote &ringers
which probably indicate a diiTerent type of alteration. mis area
extends from ICltnaga Bay to Cape Chlanak and northward toward
Naga Point on the east coast of Kanaga Island. An andenitit
dike swam and at lea& one ~modioriticdike invade the aolcanie
complex in this area and may be related to the epidotimtion.
Plates 28, 29, and 30 show typical a r d distributions of the
rocks. Outcrops are confined largely to coastal areas; many
major Uthologic changes occur across and along etrikes. Bedding
s t t i t u d ~vary greatly and in thick, coarse p m l a s t i c deposits
or rnaasive tuff layers are often impo~ibleto obtain; 1-1
unconformities &st in many outmops. Key beds have not been
found; consequently, mapping of h u h r e e r and m a r a t i n g even
large are= into broad map units Js not poasible in reconnaismnce.
Much more field and laboratory detail is necasary before an
adequate generalized map of the area can be made.
Age-Fossils were found on KBnaga and Tanaga Ishnda. lRocEts
of the central Delarof Islands are g-rouped with the fossiliferaus
rocks h u s e of lithologic similaritiee and geographic proximity.
Table 1 is a summary of the paleontologic evidence far dating
rocks on Kanaga and Tanaga. The rocks are believed to be laxgel9
Miocene and Fiimene but may include Borne Pleistocene.
Similar paleontologic evidence for & older than Miocene L
lacking. The epidotized r&e between Ibnaga Bay and Nags
Point mwrnbk Finger Bay volcanics known h be, in part,
Paleozoic (Coate, 1956a) ;but upper TertIar~rfossil8 weze found
in the altered a m on Kanags (table 1, lm. 1). A m a 1 contact
with older m k s has not been found. The upper contact with lava
of the precaldem shield ~olcanoesis apparently unmnfomabie
in Rome areas and gradational elsewhere (p. 225).
Q W Q F AND WJ?&TERNMOST ANDREhNOF ISLAND8
-I'T
a-l
225
'POW~OEa
The. radially dipping lava flow8 of partially destroyed early
ahield wlcanm on northern Ranaga and Tanaga IsEanda have
been mapped separately on plate 27, although the contact with
mka farther south is probably transitional in part. Coats (1956b)
hw dmcritied thoae m k s for northern Kamga and mapped them
in more detail than i ~ shown
l
here. In one area on the northwest
coast of Kanaga Idand, lava flows of ancient Mount Kanaton
(Coata, 195633, p. 721, which predate caldera subsidence, overfie
the fosdliferous beds (locs. 4,6, and 10 in table 1) with apparent
angular discordance. The time intern1 belmee~deposition of
these contrasting rock types may be very short;. Because the
fassiliferans sedimentary racks are characterized by variable
initial dipa and local unconf ormities, an apparent unconformitg
here may lack significance. Lava indintingwishable from that of
the nhield volcano is intercalated farther south with p p l a s t i c
debris and adiment typical of southern Xanaga, A break in
topogmphie slope separates ancient Mount Kanaton from planed
southern Krtnage; this critical area is covered by a thick ~ h ,
aoil and tundra mantle. On the north& coast of finaga Island,
6.6 rniles muth of Kanaga Volcano, lava flowa of ancient Mount
ICanatonC?) are interlayered with p g r ~ ~ l a s t ideposits,
c
and a
madational contact ia suggested.
The mrmponding contact an Tanaga Island w h m the t o p
graphic break ia even less well developed than on Kanaga, waa
not vi~ited.
The upper mtmt on bdb Emaga and Tanaga is bawd largely
Qn h p o m p h y and laation of uadimted volcanoas within caldera
remnants.
A g c a t s i (1966b, p. 78) has signed a late Tertiary and
H e i s m e age t.5 the lava of ancient Mount Kanatan and msoc i a M precaldera voIcanic centera on Kanaga Island. H e baa
pmtufaEed a late Pleistocene date for caldera formation. Corresponding rocks on Tanaga Island are assumed to be about
the game age.
-
V
O
~ CO
O
-
Gareloi, Tanaga, and Kernaga Volcanoes, which have relatively
u n d i m composite cones with records of hiatoric activib
(Coats, 1950), and dormant remnants of slightly older volcanoee
on Tanaga Island are of Recent age. Relations of Recent volcanw
with the early shieId volcanoes on Tanaga have not been detmdned. The rocks composing the three active conea have initial
dip8 aa deep aa 35" and reat unconformably on older rmts in
Kanaga and Tanaga. The younger cones are built within calderas
formed in older volcanw.
Gareloi and Kanaga Volcanoes and parh of northern Tanaga
have been studied by Coats (1950, 1952, 1966b, c; 1954, written
wmmunication). The rocks of Kanaga Volcano and ancestral
Mount Kanabn are basaltic or andmitic and their lime and alkali
parameters show the province to be caIc-alWne (Coats, 1952,
p. 485). Basaltic and (or) mdesitic rocks occur on Gareloi
Volcano, and presumably exist on Tanagrr Volcano. The sbikingly
similar chemical composition of these rwks and those farther
south indicates that thia sector of the Aleatian a r c has been
dominated by baaaltiic and anderritic rocks since Miocene time
and perhaps longer.
The material which forms the present surfaces of t h e conical
volcanic mountain8 ia largely postglacial. That glaciation was
followed by volcanic eruptions i s evident on northera Tanaga
bland where an older glaciated cone is capped by lava extruded
from a reactivated volcanic vent
Depotsite, of volcanic mh and aoil covered by heavy tundra
vegetation (wfthout permafrost) mantle the islanda inland from
the sea diffs. LocslIy the s ~ l han northern Kanaga, which contains thin layem of pumiceous IapilEi and reddish-brown eoil zones,
reachea a maximum thickness of 20 feet and sometimes forms
dunes. The ash blanket is continuous on the low, flat islands
of Unalga, Havalga, Ogliugs, and Skagul, but quite thin and discontinuoan on Ulak and Amatignak. Quaternary volcanoes nearby
were probably the aource. Because of abundant loose, freah
purniemus lapilli in the deposits of the central Delarofa, the
ash layer there may be younger than the stratified sequences
with intercalated soil layers on Kanaga and Tanaga Llsnds. A
thin dark-gray soil is usually formed on the surface of the a&.
Pea* deposits have fomed in some boggy areas,
On several islands, notabIp Ogliuga, Skagtrl, and Kanaga, extensive sand dunes are stabilized by vegetation (pls. 28, SO).
Stream-transported alluvium, mostly fine pyroclastic material,
locally filIa small basins and valIeys. Deltas form at protected
bay heads.
Discontinuous gravel, cobble, and boulder b a h e a fringe all
the coves and bays of the islanda, commonly with a wave-cut
h c h offshore. Abandoned beach deposita at higher level are
sometimes preserved. Coats ( 1 9 5 6 ~p.~ 92) reports that gouthem
Tanaga is mantled by marine gravels resting on a wavwut pIab
form, and he also reports that similar boulder gravels on Ogliuga
are overlain by coarse volcanic cinders.
Glacial debris was recognized in cirques m d cola at higher altitudes on Amatignak and on northern Tanaga Island, Ulak Island
presents a scoured appearance, but no morainal material was
found.
Coah (1956b, pl. 16) has mapped an andesitic ash layer of
caldera age on northern Kanaga Island. This layer, conspicuous
as coarse and fine pyroclastic debris on the surface of Kanabn
Ridge, is buried hneath more extensive layers of younger ash
south of ancient Mount Knnaton. On the south bank of a large
lake 6.4 miles south of Kanaga Volcano the older ash is beneath
about 20 feet of the finer Recent ash as a decomposed zone of
coarse pumice one foot thick. Neither the younger ash nor f i e
buried ash are mapped in this report.
Mimy shallow intrusive rocks, usually rtndesitic or basaltic,
either porphyritic or aphyric, cut the bedrock units nonth of
Recent volcanic cone8 and older shield volcanoes. Nearly vertical
dikes are usual, either as single units or as dike swarms,
The m o ~con~~picuous
t
dike #warn is in the area on southeastern
h g a Island extending 2% milea north of Cage Chlanak. The
dikea are andesitic and contain needle hornblende. Most trend
northwegt, but aeveral at Cape CRIanak trend wed.
Basalt.-Many basalt dikes, most trending northwest, are on
tbe north coast of Kanaga Island about 5 miles northwest of
Kmsga Bay. Several domelike masses of basalt, as much as 400
yards in greateat dimension, are visible along the coast in the
same area. One basalt mass, with a radius of 150 to 200 yarda,
in crosa section is steep sided, bulbous, and concentrically divided
into layers about 10 feet thick. Each concentric layer has a
separate set of radial columnar joints.
These rockg contain conspicuous, sporadicalIy large inclusiam
of dark gabbroic rocks rich in anorthite, olivine, clinopyroxene,
and chrornite. Many of the inclusione have been stretched Into
vague schlieren. One of the inclusions covers a ~ u r f a c earea of 400
aquare feet or more. Less abundant angular inclusions sf tuffaceoua(?) giltatone, apparently derived from the sedimentary and
pyroclastie rocks, are also found.
The basalt is characterized by a strong fluidal structure of calcb
phgimlase rnicrolites. A great variety of dark minerals--olivine,
orthopyroxene, clinopyroxene, and 03~yhornblende-may appear
228
~ B T I G A T I O N BOF ALASKAN
VOLCANOES
in the same trlide. Some of
the rnac minerals are probably xenofrom gabbroic hclusiona. Elongate vesicles filled with mcondary minerals are present in eeveral outcrops.
These intrusions may be one of the many lmes of volcanic
domea usually found in and near voicanic vent8 (WiIlians, 1932),
bnt mare probably they are bulbous developments in shallow sills.
Evidence of doming was found at one contact with overlying tuffbreccia; basalt dikes aa much sa 60 feet wide also invade the tuffbreccia. Although the sill-like character of the Kaaaga ~tructwea
is not &in,
the joint patterna do suggest bbular mama (either
sills or flows) and this origin seema more likely than plugs or
domea within vents.
Limbwgite-A half mile inland from the coastal area which
ehewa domed sills ( T } and dikes, a fragmental rock with abundant
olivinerich inclusions aa much as a foot in diameter forms an is+
hted outcrop about 10 by 60 feet, The olivine inclusions, both
rounded and tabular, are mixed with lw abundant, very angular
inclusions of tuffaceoua lime~tonein a Iimburgite matrix, Ross
and othem (1954, p. 701) examined some of these jaclasione and
reported that chromium diopside, enshtik, and c h m i u m spinel
are associated with dominant olivine in the ultrabasic inclusions.
Possibly this outcrop is a petrographically extreme development
of the incIusion-rich, domelike structures just described. The
abundance, size, and angulariw of the inclueriona and their =miation with gedimentary inclusions suggest a shallow nearby aource
(cf. Ross and others, 1964, p. 693).
A7Edesite.--On southwestern Kana= (pl. 28) thick, emmely
porphyritic eills of andesite comprise the sea cliff for long disc
tances. One of these sills haa vertical columns 10 feet across and
at lea& 100 feet high. Topography on the headland 1 mile southeast of Western Point sugge* s thickness greater than 300 feet
Top and bottom contacts were not o b s e d , but nearly vertical
end contacts al; t w o locations and one exposure of contact breccia
suggests either postemplacement faulting or forceful transgres-
c-
sive intrusion.
These rocks are characterizad by ragged phenmryab of biotite
and (or) oxyhornblende with '~pacite'~rima.
The rocks abo contain abundant phenocrysta of zoned plagioclme (IabradoritPoligoclase) with or without dusty zonee. Progressive, oscillatory, and
reversed zoning occur, Clinopyraxene and orthopyroxene, magnetite, and a few rounded quartz grains are also present. The rmka
are probably aiIiceoua andesite, but chemicer1 analyses would be required to classify them exactly.
Spee;Imen 52W-$8(Rittmann: labradorite dacih) is frOlll a
r ~ h b g m ycolumnar sill 2.6 miles northwest of C a p Tusik on
southern Httnaga. This sill is finer grained and megarnopically less
porphyritic than the sille on southwest Kanagk Phenocry~b,
mostly lws than X miIlirneter in diameter, make up 26 percent of
the rock as foIlows: abundant zoned labradorite-bJrtownite with
thin rims of oligoclase-andesine and, commonly, oscillatory zoning; Iesa abundant oligmlase-andesine crystals eurrounded by a
zone crowded with dark inclusions, then a rim of labradoriteoligoclam with strong oscillatory zoning (included here are maw
phenmry~tsof labradorite with center parts shewing patchy extinction ) ; clinopyroxene snd orthopyroxene, both rare ( <0.6
nm); abundant subhedral opaque oxidea ( ~ 0 . 2 mm) ; rare
sphene ; ghost hornblende (<2 mm) now composed of fine pyroxene, opaque oxide, and feldspar; rounded quartz relic8 ( ~ 0 . 6
mm) surrounded by rims of granular ~Einopyroxenein a brown,
areakgy anistropic m a w . The strongly fluidal groundrnma is composed of: abundant lath- and blmky-ahaped crystale of zuned
oligodaa~ndesine ( 4 . 2 mm) ; many stubby crystals of cline
pyroxene (<0.16 mm) ; many blocky c~ystala and grains of
opaque oxide (<0.07 mm) ; irregular graina and masses of carbonate, mme within pyroxene crystals and Borne within plagioclase
cryatala ;abundant interstitial iaotropk, material having a low index of refraction and a fracture pattern suggestive of rrisbbalite
or opal.
Both the wesand southern Hanaga sills are complex rocks.
Feldspars of several types ahowing internal zones crowded with
inclusions and dilatory or reversed compositional zones,
rounded qua* relics, and hornblende gh&
all guggest hybridization or at leaat a very complex cooling history.
RhyodQcitee-Specimen 62-Sn-100 (Rittmann : rhyodacite) is
from a very light-colored podlike body about 200 yards wide, expaged 4.8 miles west of the mouth of Ranam Bay on the muth
coast ~f Kanaga Island. This was the anJy light rhyodacite found
in the Delarof-Ksnaga area. Quartz, oligoclase, and biotite are
pment in the white b light-green glassy matrix InEJ1.476)
which makes up about 85 percent of the rock.
Specimen 62-Sn-37[Rithnann :dark rhyodacite) ia of quite different composition (table 2) and would be classified microscop
ically as vitrophydc labradorite dacite. This specimen is a darks
a
y r k from t h e chill border of a ~ m a I djke
l
on northwest Kanag& Its texture is porphyritic (,35 percent phwmrysts) with
a hyd-felty ground mass and strong flow structure, Phenocrystic
pIagioclase (<4 mm) ia owmionally uniform, but u~uallyshows
either progressive or distinct oscilhtury zoning. Composition
ranges from Anrme, with most of the determinations about An,,.
A few of the crystds contain du&y mnes of glassy inclusions, Augite (often as an overgrowth on hypersthene), hypersthene,
opaque oxidea, and rare ghost hornblende with a rim of granular
pyroxene are aIso present sa phenoerysts ; ma11 grains of c r i a b
balitel?) are less conspicuous, and these are d1 8et in a brown
glass (rt between 1.606 and 1.510) containing numerous plagioclase (Anrp4s) miwolites. Potash, preaent in the analysis, is not
expressed mineralogically and must be contained, along with occult quartz, in the low-index glass.
Summ.um and age.-Most of the aphanitic intru~iomstudied
microscopfcally have a porphyritic texture with phenocrysta of
zoned calcic plagioclase (rarely, more sodic types) and one or
more of the following dark minerals : augitic pyroxene, arthopyroxene, oxyhornblende, olivine, and (rarely) biotite. The cornpositions and textures present in the dikes and dlL are more variable than those found in the flaws of the intruded sequence, but
most of the small intrusive masses are andesite or barsdt,
No single period of intrusion can be assumed. Dikes found in
the chloritized rock on Ulak laland are themselves altered; however, fresh-appearing andesite dikes cut the granodiorite plutan
that constitutes Ilak &land, and most of the dikes in the Naga
PoinbKanaga Bay altered area are fresh. If alteration is associated with subjacent plutonic invaaion, diking probably both preceded and followed that event. Diking almost certainly accompanied extrusive volcaniarn throughout the Tertiary.
One dike which cuts t h e fossiliferous sequence on northweat
Kanaga (specimen 52-Sn-37) stops at the contaet with overlying
Ranatan lava. The dike raises the lava flows slightly but does not
cut them,suggesting that it failed to penetrate the massive flows ;
thus aome of the dikes may be Quaternary.
Coarse-grained intmive rocks m found in several areas. A
thick sill of quartz diorite makes up the outer end of Knob Point
(PI. 23) on Amatignak Island. Other sills of diabase or diarite and
perhaps larger plutons exist on that ialand.
Xlak Island i~scomposed of medium- to coame-grained gmnodiorite (specimen 52-Sn-167) and quartz diorite cut by numemua
small aplite dikes (specimen 52-Sn-181)and a few andeaite dikw
10 to 200 feet wide. The largest andesite dike has a contact with '
the m o d i o r i t e indicating that both rocks may have been fluid at
the same time. There are many rounded inclu~ionaof coarse- and
fine-grained material in the gtsnodiorih, but c o n k i s with the
host rock are not exposed.
A granodiorite dike m u r s on Kana- Isbnd beltween Cape
Chlanak and Nags Point. Possibly the alteFation (p. 224) and
coarse dikes, together with the andesite dike swarm (p. 227), indicate a plutonic mass relatively close to the surf= in that a m .
ImEmhts and boulders.-Inclusions ranging En compsitiw
from dunite to granite are found in the shallow intrusions and
Aom on Kntnaga. Large rounded boulders of gabbroic to granitic
composition and also boulders of schist and gneiss were found in
association with indasiona of aimilar rmk in the lava cropping
out along the nhore of an inland lake 400 feet above aea level. Near
the s o u t h w t tip of Skagul boulders of diorite, marsre-grained
granite gneiss, and fine-grained granite occur. Boulders of t ~ ~ s e grained rmka, possibly of local derivation from silh h o r n to
exid on the island, were also found on Amatignak
Some of these boulders undoubtedly are weathered-out incln#ions; othm may be ice-rafted from known plutonic areas. One
bou1der on krnatignak was locked in the roots of a driftw~odh.
As the= are no trees in the Aleutians, this boulder may have come
fmm s great distance, probably from the west. Zarge areas of
metamorphic rock are not necessary to account for these boulder8
of gneirrei and achiet. On Unalaska Island the foliated w k a are
confined to aheared roof pendant8 and narrow border zones on
k g e plntom. No large areas of regionalIy metamorphosed rocks
of even alate grade are known in the Aleutian lalands, and there
is no evidence for an oldland of metmnorphic and igneoue m h
However, induaions in late Tertiary aphanitea indicate that plutonic m & of
~ diverse composition are present beneath the islandy
in mme areas.
Age.-All dated pIutona in the Aleutian Blands u e Tertiasg,
The flak plabn and other coars~grainedrmka in the DelarofKainaga me8 cannot b dated, nor is it c e h i n that they are all
the game age. At least two episodes of comegrained in-ion
are possible. The inclusions in lava of late Tertiarg age on
mags indicate older rocks, and the dike there permits inference
of intrusion after late Tertiary. However, it cannot be ebted d a .
nihly that two episdm are required ;apophyses from one inkuc d d cut Mme Upper Tertiarg rocks, and the same i n k u s i a ~
could aupply inclusions for alightly younger upper Tertiarg ha
00-
A middle Tertiary e p i ~ d eof intrueion, in the gouthem parts
of several ialands or groups of islands, followed by general uplift,
erosion, and late Tertiary, central-type voleanicrm along the north
edge of the chain has been suggested for other areas (Gatea, F r s
ser, and Snyder, 1954). Possibly the Ilak pluton i8 one of several
roughly synchronous inkvsione belonging to this episode. The
coarse-grained sills on Amtignak may be connected with magma
chamber8 that supplied andesitic Iava at different times daring the
Tertiary, and may or may not be genetically related to the manodiorite on Ilak,
mvc!Pom
The Aleutian Islands may be considered aa &a
of a nabmerged mountain range rising 25,000 feet from the floor of the
Aleutian Trench and 13,000 feet from the deep on the concave
side. Major configurations are apparent from regional ahdies of
submarine contours (Murray, 1945). The Aleutian arc ia ~ingle
in ita western part and therefore is probably simpler and younger
than double arcs in the churn-Pacific belt ('Urnbgmve, 1947, p.
185).
T ~ m tines-Most
d
of the area is covered by the sea and many
~tructuresmuat be presented rta interpretations (pl. 27) from
fragmentary subaerial and aubmlrrine data (p1s. 2731). Structures interpreted by trend lines on plate 27 are of three types: (1)
linear subaerial drainage basina, (2) straight shorelines, and (3)
submarine scarps, troughs, and slopes. Moat of these alinemente
probably reflect faults whose precise locations are not known,
Some of the linear features may reflect folda. Individual trend
lines offer no positive proof of dxuctural control. Together the
trend linea farm a geometric pattern which would be difficult to
obtain by nonatructural methods. We believe that the submarine
topography ia controlled by longitudinal, perpendicular transverse, and obIique transverrre fractures, which apparently are
geometrically and genetically related to the arc aa a whole.
MAJOR ISTRUCTlfRES
In general the main ridge north of the Aleutian Trench prob
ably is a complex arch bounded on both sides by faults. One thrust
zone dipping north from its trace on the north aide of the Aleutian
Trench i s indicated by seimic evidence and by comparison with
better known arcs where the eeimic data furnish compelling evidence for such a fault (Benioff, 1949 ; 1956).
A gecond major fault bounds the arch on the north. Alinement
of volcanoes aIong the north edge of the islands has been noted by
all workers in the area. Gates and Gibaon (1956) and Snyder
DELhROF AND WESTERNMOST ANDREANOF ISLANDS
233
(1967)have demonstxatd by detailed submarine antouring that
this alinement, in some apeas, is coincident with a high-angle,
normal(?) fault downthrown on the north side. A 1,200-footsubmarine scarp is evident west of the Delarofs between Little Sitkin
and Semisopochnoi VoIwnw. A similar fault or fault zone ie
probably buried beneath coalescing volcanic piles in the DeIarofAndreanof area
This latter fault appears t o have been offset by transverse
faults. The Delaraf sector shows wutherIy displacement relative
to the Rat and Andreanof sectors, ao that Gareloi Volcano lies
aouth of lines connecting volcanoes on either side. Amchitka Pass,
west of the Delarofs, and Tanaga Pam probably were formed by
large transverse faults.
The Amchitka Pass area is unique for the Aleutian chain. Amatignak Ialand, east of the pass, is farther #souththan any other
island of the chain and is bounded on the northwest by a 6,000-foot
depression (pl. 27). Amchitka Island, west of the paas, and the
Southern Delarof Islands appear to be dragged south with respect
to adjacent parts of the chain. West of Amchitka Pass the trend
of the arc is northwest rather than we& Most significant is the
346-mile arcuate ridge, Bowers Rank (Murray, 1945), that curves
north and west of Amchitka Pass. The Amchitka Pasa area, therefore, ie the gite of an intersection of t w o large mountain ranges,
but an analysis of the structures is not possible with available
data.
The trend af the main arch ia shown on plate 27. IB crat passes
north of the central Delarofs and south of Tanaga and Kmaga.
UnfauIted segments of the south flank of the arch show a smooth
slope more gentle than that to the north, where longitudinal faulting modified by volcanic deposition gives a different profile. How
much of the movement on this arch is bending and bow much is
discontinuous movement on high-angle longitudinal faults similar
to the north boundary fault is unknown. Abundant high-angle
dikes on most of the islands may indicate a tensional environment
(Anderson, 1951,p. 3). Dynamic metamorphism and tight folding
are absent, possibly confirming the hypothesis of tension near the
crest of the main arch. The entire arch may be ta drag feature on
the main underlying thrust zone, or it may be the anticlinal lip of
a *togene formed by plastic deformation (cf. Beniofl, 1966;
Vening Meinesz, 1956).
Folds-In addition to the main arch, dip reversals indicate sevemf srnlrIIer folds (pi. 27). Data are fragmentary, and 1-1
changes in dip resulting from conditions of deposition and faulb
ing make all of these folds queationable.
Probable northeaahtrending folda on Raand Tanaga (and
also on Adak Island 10 miles east of Kana-) contrast with posaible northwesbtrending folds on Amatignak Island. Unuauallp
deep beds on northeast Kavalga are probably dragged on a etrong
north-trending fault zone (pla. 27 and 29).
The large and poorly exposed area of central Eanaga shows
many beds trending north and northeast; many have dips too
steep or persistent to be attributed t o chaotic deposition. A dome
at Cape Tusik and northerly folds or faults elsewhere may explain
some of these anomalous attitudes.
Fawltk-The major longitudinal and transverse faulta be1iewd
to exist beneath the sea have been mentioned. Probably some of
the faulta that cut the islands have not been found becauae of the
lack of lithologic marker beda, erasure of scarps by wave planation, and cover by ash and tundra. Faults that have been recognized in the field a r e mostly high-angle shear zones, eilicifled and
stained with iron oxidea. North- and northwest-trending fautts of
this type are evident on Kavalga (pl. 2 9 ) .
A small reverse fault on the south shore of Kanam Island (pl.
27) shows apparent vertical displacement of 20 feet, A steadng
fissure pasaing through Kanaga Volcano atrikes N, 66"-TO" W.
On western Kanaga large andesite porphyry sills commonly end
in eteep side contacts that are probably faalta (pl. 28). Small
faults are shown an Ulak and Tanaga (pl, 27).
A possible north-trending fault between Uak and Amatign&
is suggested by the different attitudes of the beda on the two islands, and by the steepening of the beds on Amatignak. Relative
altitudes af the t w o islands and the dragged beds suggeat that the
west side is upthrown (PI. 27).
Catdems-Coats
(1950, p. 43) found fncomplete calderas on
both Kanaga and Tanaga Islanda, the northern sections presumably destroyed by faulting or marine erosion. These structures
are approximately coincident with the east and muth margins of
coastal areas of volcanic rocks of Quaternary age ~ h o w non
plate 27 for Ranaga and Tanaga Ialands. Possibly the north r i m
of these structurea were never completely formed. Arcuate normal
faults, downthrown on the north and roughly alined with a larger
east-weat struchra? trend, rnBy explain the calderalike rims better
than the theory of partly eroded conventional calderas, (Williams,
1941, p. 242 ;van Eemmelen, 19!54, p. 88-92.)
SOXIMARZNB TOPOGB.AFXY
Submarine contours in most of the area from longitude 180"
to Adak Strait, and including northern Tanaga IsIand and
Amatignak, are whown on plate 27, The map was prepared from
U. S. Coast and Geodetic Survey boat sheets at several larger
scales. The contour interval is 300 feet. The mag ~ e a l eand the
density and precision of location of aonndings give the contour
lines the status of form lines. Figure 45 is a detailed map of the
bottom topography around the central Delarof Islands.
Many lsea valleye, ridges, scarps and paasea show strong alinements miles in length. These alinements commonly trend in one
of four directions :west, almost parallel with the terrestrial Aleutian Ridge line; obliquely northwest; obliquely northeast; or
roughly north at right angles t o the ridge. The linear character of
ridgea, valleys, and garpa, and the presence of blmk1ike seahighe
suggest that much of the bottom topography reflects geologic
structure.
The islands of UnaIga, Kavalga, Ogliuga, and Skagnl emerge
from a shallow submarine platform orientated jpneralIy emtrwest
alow the Aleutian arc (pl, 27 and pl. 31). The platform extends
west of tthe Delarofs across Amchitka Pass; east of this group of
islands it appears to be offset to the north and presumably is coincident with the flat southern parts of Tanaga and Kanaga Islands.
The crest of the Aleutian Ridge between the 180th meridian
and Skagul Island is bounded on the south by a steep slope at
least 1,500 feet high. At the foot of the slopea just south of Kavalga and west of longitude 179°50' W. the bottom flattens to
form a ahallow trough with seaknolls alined along its outer edge.
The knolls are fiat topped with a steep northern margin and a more
gentle aouthern slope. Probably the trough wrta once continuous
along the Delaraf Islands and across Amchitka Pass, but it is now
disrupted north of Amatignak by a 2,700-foot slope which drops
into a large depression whose maximum depth is 6,000 feet. The
depression is a triangula~area between the Arnatignak-Ulak Islands bIock, a aeahigh 16 miles northwest of Amatignak, and the
crest of the Aleutian Ridge. The greatest depths are at the southeast corner of the triangle.
The east-we&, or longitudinal, trend is dominant along the eentral Ddarofs and west to longitude 180°, but it is only generally
evident east of Tanaga Pass as the boundary of tthe emergent
Aleutian Ridge. The submarine topography north of the islands is
only partly contoured, but additional evidence of U. S. Coast and
Geodetic Survey hydrographic charts and analogy with the Kiskaeab3t
P l a m 46.-DetnUed
aabmarlne t o ~ o g r a ~ hofy ocptral D d w f Idu& area
Amchitka ares {Gibson and Nichols, 1963, pl. 1) indicates the
slopes are steeper than those mth of the blmb.
Tanaga Pam rrnd the pass east of Kavalga Wand (fig. 45) have
features with strong northwesterly alinemenh IN.35' W. to N.
60° WJ.The errst side of Ulak Island is the west edge of a pardlel-sided, flabbottomed trench trending northwest; the pass between Kavalga and Unalga apparently is rm extension of this
trend offset to the east. Other features with evident northwesterly
trenda intersect the seahigh 16 miles northwest of Amatigxlak and
bound it on the northeast; they can be seen on central Ulak and
southeast Tanaga as linear subaerial features. The north margin
of' Tanaga Bay trends northwest, in line with the linear mne on
the island.
The most obvious northwesterly alinementa of bottom topography are in the DeIarof IsIands-Amchitka Pass area ; such aIinements are largely absent east of Tanaga Pass.
A zone of c b e l y spaced northeasterly trends (N,26' E. to N.
60" E.) is apparent south of flak and Tanaga Islands, and may be
the main structural control on Kanagrl Pass. Thk trend also outlines the south margin of the Amatignak-Ulak Islands block, Submarine valleys and seaknollla oriented northeast are evident in
Amchitka Pass to a lesser extent.
Northerly trends are also evident and two north-south scarps
are particularly noticeable : one is south of Kanaga bland, in line
with Ad& Strait; the second ia in Amchitka Paw, near the 180th
meridian and intersects the Aleutian Ridge crest at the base of a
s b P wesbfscing dope. Amchitka Seavalley (Gibmn and Nichols,
1953, PI. 1) also trends north. Skagul Pass (pls. 27, 31) is an example of this trend, and many smaller alinemenh have been contoured,
The muthwest peninsula of Ulak Ialand is triangular, This
shape is probably the result of the intersection of northwest-,
nosthe&, and north-trending structural breaks. In general, the
orienbtion of long straight coastlines and interisland passes
throughout the area shown on plate 27 ier closely coincident with
the four main directions of submarine alinementa (p. 235), with
the directions of subaerial trends, and with the strike of dikes.
K w g a I d i m d - L m d f o m on Qnaga Island illustxate the
concept of constructive voleaxli~mcoupled with continual erosion.
Zlhe ~ymmetrical,complete cane of Kanaga Volcano at the north
end of the island is the most recent constructional landform.
Steam issues from a vent near the summit, and flow8 of blocky
lava in 1906 (Coats, 1947, p. 93) indicate preeent day volcanic
activity. Continuing construction of the cone has inhibited diasectian by atreama. The shoreline is concentric around the voIcano
and is scarcely eroded by the ma.
The arcuate calderalike scarp of finaton Ridge south and e a t
of Kanaga Volcano confine8 the younger deposita within a halfbowl depression (Coah, 1956b, p. 76). Many branching streams
flow southward from the scarp lip. They are underfrt in broad
valleys which radiate from Kanaton Ridge, rand are separated by
almost continuous sharp interfluves of high relief, The valleys
were developed upon the slopea of the andent Kanaton Vobano,
C I S recession on the west ahore has carved bluffs at least 400 feet
high into the rocks of the old volcano and the underlying sedimentary rocks.
Planed surfaces on Kamga I8lamd.-About 3 miles aouth of the
lip of anaton Ridge, the atream join rr transversle drainage direction in the flatlands below 500 feet in altitude. This northwesb
wntheast drainage is the first expression of the older, wave-planed
surfaces of the southern and western parts of Kanags.
Five major levels of planation are inferred from the topographic
profiles (fig. 46). Each level is continuous and bounded by distinct breaks in slope many feet high. Abundant ponda and l a b
on the several levels emphmize their flatness. The irregularly
dashed lines on the map, which indicate the levels, were drawn
from drainage patterns and slope changes shown on topographic
mapa and aerial photographs.
The broad central area of the island ia topped by the highest
planar surf ace, level 1, which haa an altitude of more than 500 feet
on the northwest and 400 feet on the ~outheaat.(On the mag, Ievel
1 includes a minor aurface about 50 feet below the uppermost fiatlands.) A steep scarp 100 to 150 feet high separabs level d from
bvel 2, rat an altitude of 260 to 280 feet on the northwe~tand
slightly more than 200 feet on the southeast side of the island.
Level 2 is easily seen at the narrowed neck separating the northern volcanic highlands from the rest of the island. The high@&
flat gonded area west of the central area, along the narrow prolongation of the idand, is correlated with level 2 ;presumably Ievel
2 aha can be correlated with the higher altitudes at the extreme
west end of Kanaga, north and west of Cape Chunu. Level 3,
mostly about 220 feet in altitude, is weU defined near the west edge
of the central area ;it makea up much of the slender we~ternextension of the island and may be present lmdIy along the steep
burlaces ot danatlon: boundulM
rpprmlmtely locatad; 2 Is aldar Man 3
- --
Fmmm 46.--Sd&xs of plmatlon md drahage map.
central Kamgn Island.
Andreanof group, Aleutian Ialanda, Alaska
coast of the central part of the island on the northwe&. Level 4
ie at altitudes from 100 to 120 feet and is b a t preserved on, the
western elowtion of the idand ; it it^ definable, however, near the
w a t coast of the narrow neck area and as small remnants elsewhere. Level 5 (not shown in fig. 46) includes the narrow benchea
along the present ahoreline. Sheer clifis commonIy separate level
6 from higher levela. Minor terraces which are suggested by the
topographic map conffgurations and are locally apparent in the
field, particularly in the levels below level 2, are not discussed here.
The planed surfaces are almost continuous in the broad centraI
region and western elongation of h n a g a Island (fig. 461, but
a l o n ~the eaat c o a t north of Naga Point only levels X and 2 are
present. Level 2 merges with the south slopes of Kanaton Ridge,
The continuity of the eurfmes inland from Cape Chlanak, southeastern Kanaga, is less apparent because of extensive stream er+
sion parallel with a broad northwest-trending dike swarm.
The drainage divide for most of Kanaga Island, excluding the
volcanic highlands, is clam to the north shore (fig. 46). Streams
flowing northward into the Bering Sea from the central rej$on
drop as much aa 600 feet per mile down a mries of cliffs and
scarpn ; those flowing southward into the Pacific Ocean are not so
steep. The asymmetry is as marked fa~therwest, but altitudea
are lower. The tendency of the drainage to take southerly courses
aeroes most of the central region is commonly thwarted by structural controls, mostly bedding and diking.
Preeumably, the water age and higher altitudea of the central
ares are responsible for the close coincidence of drainage direction with four or five known structural trends. On level 1 the
stream flow gouthwest or northeast, roughly parallel with the
length of the island. Streama below level 1 commonly follow the
other structural trends. Southeast drainage is dominant along
the east coaat. The two deepIy incised valleys leading inta Ranaga
Bay from the north reflect w north-trending structural control visible in Kanaga Bay and extend in^ north along the weat margin of
northern Kanaga Island (pl, 27). The parallel southward drainage along the island's western elongation may be simply downslope or may follow northerly strike8 in the area.
Sea-level benches now forming on the south aide ai Kanags are
notably broader and more continuous than those on the north. This
difference in width is consistent with the concept of a pIanation
platform of marine origin tilted by upfaulting at the north. The
fault w m p , subject to direct wave action, would rmde with much
of its steepness preserved; the newly inundated platform areas
a the ialand, on the other hand, would faws water-1-1
and
subaerial weathering and the consequent development of a wider
bench (Hills, 1949). Predominant southerly winda probably also
contribute to the formation of broader benches along the south
m
coast.
3
Physiographic evidence and geometric construction aug&
mutheastward tilting of the entire Kanaga Island block, excepting the volcanoes. A slope of at least 150 feet in 5.4 milea is in&csted across centrat Kanaga. This dope is opposite to the prevalent dips,
Tenagcl IsEarad-The physiography of Tanaga Island i~ similar
ta that of Kanaga. The northernmost park of the island is, like
Kanaga, a mountainous area that has been active volcanically during and since the Pleistocene, The volcanoes are alined east-west,
and include relatively undissechd, ashovered cones and dissected
shield volcanoes, gome heavily glaciated. The gIaciated peak nearest G u ~ t yBay has crevassed ice around the summit depression, and
at leaet four recent cones to the east and aouth have poured lava
and ash upon radial valleys and ridges. Tanaga Volcano and ita
twin to the west are located within an arcuate ~kructure,p o d b l y
a caldera. The volcano at the northwest tip of the island appears
also t~ have been built up within s similar structure.
Ponded areas and abandoned beach ridges resemble those on
Hanaga, and imply a related emergence. Coats (1966c, p, 92)
states that mast of south Tanaga is a w a v ~ u platform
t
mantled
by marine gravel. Correlation with the planed surfaces of Kanaga
Island has not been 'attempted,
The Delarof Islands,-The low, nearly Rat D d m f Islands (pls.
31, 32) appear t o be blocks planed smooth by waves and later
faulted. Profiles drawn transverse to the trend of main topographic breaks on five islands (fig. 47) show the apparently unrelated orientations of subaerial planar ~urfaces.
Bedding traces are well exposed on wsrve-planed surface8
around most of these isIands. On Ulak, where the ash cover is
thin, a west-northwest bedding trend i~ reflected inland by dinement of ponds. This higher aurface probably resulted from wave
planation, possibly followed by glacial scouring.
Glacial erosion has etched a north-south backbone through the
south half of Amatignak Island. The ridge crests trend northnortheaat acroas the north half. A high area near the northweatern coast is underlain by more massive rocks. No continuoulr
sarfacea of planation are evident around the island. Either the
emergeme of Arnatimak was later than the benching observed on
nearby islanda, or glaciation removed all trace of wave-cut planes.
SW.
NE.
K AVALG A,
.
w.
E.
W.
e.
UNALGA
OGLlUGA
S
B O Y i =1)1
UL AK
h
mawus 47.--Belected bpogmpbtc p m h rhaaln~ortentatbw o f pimu ncufnwa d five ManldPndr of tbe D e l d -D.
D W O F AND WESTERNMOBT ANDRFANOF 18
-
243
GEOLOGIC HXlsTORY
The pr+iocene history of this area ie unknown, but t w o posaibler interpretations are sugge~tedby available data: (I) Rocks
in the Amrrtignak-Uhk area were deposited in early Tertiary in a
fairly stable geosynclinal trough or on its Ranks and were invaded
and altered by granodiorite plutons. The area waa uplifted and
eroded during or after the time of intrusion. Later, in Miocene
time, marine and subaerial volcanism began in the central Delarof -southern h a g a area. Thew young Tertiary chaotic deposit&
contrast markedly with the well-bedded older rocka farther south.
(2) Another possibility i a that rocka in the Amatignak-UIak area
are actually the well-bedded, deeper water fades of Miocene deposits farther north (which are nearer their source) and the alteration of the southern faciea is a reault, not of greater age, but
either of proximity to late Tertiary plutons or of diff went diagenetic conditions within the geosyncline.
The known geologic history, then, begins in Miocene time with
the deposition of poorly bedded volcanic xocks. Easbwmt normal
faults probably localized a chain of volcanoes near the latitude of
the central Delarof~,and this zone apparentIy moved northward
to ib present location. Depth contours of the present volcano belt
on the north edge of the islands reveal many volcanic cones.
These, and the volcanoes projecting above the sea, form a series
of coalming volcanic pilea. Conditions in Tertiary time may have
been simiIar to those of today; but ahifting volcanic centera, concurrent and subsequent erosion of volcanic landforms, and d e p
aition of Recent volcanic ash have all obscured the record. No centers have been found for the Tertiary deposits; and large areas
of mame pymclastic rocks, not obviously related to volcanic centers, wggeat episodes of generalized eruption (much of it submarine) during which mudflows and flood eruptions m y have
been important.
Many local unconfarmitiea record unstable conditions during
late Tertiary time and no persistent lithologic units were d e p m
ited. Lava flows, pillow lavas, and poorly sorted tuff-breccia are
the dominant rocks; many dikes and, locally, large ~shaliowsills
invaded this sequence. Some of the sills were uncovwed by eroaion
and large colnmlzrtr talus block8 from them were incorporated in
later eedimentary and pyroelwtic beds. Relief featurea produced
by faulting, erosion, and deposition were masked by subsequent
floods of lava and coarae pyroclastic rocks. Pocketa of well-sorted
tuff, some of which contain fossils, filled deprensiom on southwe6
ern Kanrrga
244
LNYESTIGATIONS OF h L h 8 W VOLCANOES
During the entire period of volcanism recorded here, andaitic
and bmaltic rocks were dominant, with only sporadic and minor
confributions of more acid rocks. Shallow-water marine life left
a fragmentary record, and foregted land areas existed where today there are no trees,
Near the beginning of Pleisbene time broad ehield volcanoes
formed near the aites of present volmoes. Them volcanoes were
partly destroyed by caldera-forming precesses, and Recent voIcanoes of sharply conical farm grew in the old, incomplete a l deras. At almost the same time another volcano, Gareloi, formed
on the ocean floor. This late volcanism waa all confined to the
northern ends of the islande.
Wave planation carved a series of platforms on the late Tertiary volcanic rocks during the Pleistocene; Anatignak Island,
becausre it has not been planed, may have been uplifted in Pleistocene time. Glaciers mudifred Amatignak and the higher volcanic
areas, and may have overridden She flat areas on other islands,
particularly Ulak.
REFERENCEB CITED
Anderaon, E. M., 1951, The dynamics of faulting, 2d eB. r e v i d , Edinbnra,
Oliver and Boyd, 206 p.
Bemmelen, R. W. van, 1954, Mountain bnildhg, The Hague, Msrtinnn
Nijhoff, 1TI p.
Benioff, W., 1049, Seismic widenee for the fault origin of oceanic deeps:
Geol.
America Bull., v. 60, p. 1837-1856.
1956, Seismic evidence for crustal atructam and k t m i c activity:,
Geol. Soc. America Special Paper 62, p. 61-73.
h t n , R. R., 1960, Volcanic actfvitg in the Aleutian am: U. S. Geol. Survey
Bull, 974-B,p. 96-49.
1061, Gedogy of Bnldir Island, Aleatian Islands, Al&:
U. B. Geol.
SQBall. 989-A, p. 1-26.
-1952,
Magmatic differentiation in Tertiary and Quaternary. wrlermic
rocks from Adak and Ranaga I~lands,Aleutian Islands, Alagka: Geol.
Soc, America Ball., v. 63, p. 486-514.
-t966a,
G e o l o g ~of
~ northern Adak Island, Alaska: W. S. Geol. Survey
Bull. 1028-C, p. 45-67.
1966b, Geology of northern Kanaga Island, Alaska: U. 8. GeoI. SUWW
Bull. 1028-D,p. 69-81.
1 9 6 6 ~Reconnttissanoe
~
geobgg of mme western Aleutian Ialands,
Alaska: U. S. Geol. Survey Bull. 102&E, p. 83-100.
Coshtnan, J. A,,and Todd, Ruth, 1941, A foraminifera1 fauna from Am~hitka
Ialand, Alaska: Cufihman Lab. Foram. Research Contr., v. 28, pt. 8, no.
&.
297, p. 80-72.
Gates. O., Fraser, G.
D., and Snyder, G. L,,1954, Preliminary rep& on thegeologg of the Aleutian Eslands: Sdenee, v. 119, p. 446.
Gates, b., and Gibwn, W., 1956, Interpretation of the configuration of the
Aleutian Ridge: C;eol. Soe. America Bull., v. 67, p. 127-148.
DEWROF AND WESTERNMOST ANDREANOF IBtANDS
245
W, d Nichols, H.,1958, Configuration of t h Aleutian Ridge: Gml.
Soe. b e r i c s Bull., 7. 64, p. 1173-1188.
Hnla, E. S., 1949, Shore platforma: Geol. Mag., v. 86, no. 8, p. 187-165.
Mnrrag, H. W.,1945, Profiles of the Alutlaa Trench: GeoL Soc. America
Bull., v. 66, p. 761-782.
Rittmann, A., 3952, Nomenclahre of volcanic mlts: Bulletin VoTcan01ogiq~8,
1'Aamciation de Volcanolopie. Shrie 11, tome XII, p. 75-102.
Ram C. 8., Foster, M. D, and M g e r ~ A.
, T., 1954, Origin of dunitas and of
olidnerich inclusions in basaltic rocks: Am. Mineralogist, v. 89, p. 698131.
Snyder, G. L,1967, Oceen flmr structures, northeastern Rat Islands, blaslm:
U. S. Geol. S u m BnU. 1028-6,p. 161-167.
Umbgrove, J. ET. F, 1947, The pulse of the earth, The Hague, MartinNuboff, 368 p.
Venting Meinem, FP.
A., 1965, Plastic bnckling of the earth's crust: tba origin
of geosyncIinea: &I.
Soc. America Spee. Paper 62, p. 319-930.
WWiarns, Howel, 1932, The hiabry and character of volcanic domes: Calif.
Univ, Dept, Geol. Sci. Bull., v. E l , no, 5, p. 61-186.
-1941,
Calderas and their origin: California Univ., Depk Geol. Set. BulL,
v. 25, no. 6, p. 239-346.
Williams, Howel, Turner, F. J., and Gfibwt, C. Y,, 1964, Petrography: S m
Francism, W . H.Freeman, 408 p.
G,-
INDEX
Pant
ts...........................
AC~OW.
Idak Island. Finger Bas v o l d e ...........
~
af a s l
...
~..... n$
~
Trim
...................................... a&9-234
.......... 224
a12 FQlh
rW F d b . K m w and T m m IuhL
Ad& Btralt, submerlne topc%spby ........ 2.36, 271
@Eta Island TertIary M
a .................
218
....,...........+...+.... .
Neutlm Ridge....
235
Aleutlm T m c h
232
Allurinm .
mf
AmatIrlsnd, m
o
m
I ................. nM1&
.........................
...................................
.............................. 241. 244
.......... %I-m
landforms................................
21h
EIaCktlUU
mtmdons and W d m L -....
....................
submarim 6qm%mphy
Terttq l&ls ..................*.*......
................
.............
2S
21 9
Amchitka island. lrnrn....
m
Amchitka P m stmctnre
submdne to-pbp
235
~ m c b f t k 6eavalIey
s
n7
hndesite...
2l6.
ZB-E& EKb281, XJ(
Andreanof M m ds. c b w f d m l ~ 1 k l o oor
234.230
rocks, Cable....................... m Kmngn nay. dlkm
atructudc m M
240
AuortMts
227
&Wits
2l3211 Kanml8lmd n n d d t k ..................... 223
.
........
--...,...........
..........................
ma,
....................................
.......-..................+.......
~ 8 t depoai
1
ca. fcrms................. rn
219
Atks Istand.Mesosole fosfila .................
.
..................
A Mu Island T e r t I a ~
iosslls
hudte..-.............+...........
21R
.......W2XI
.....................-.
........................
gndcsltlc ash ........ .
..........,....,.
a m ......................................
.
...........
Pn
213
mldm
...................... a 4
c h e m l d m m W h al ah. tab
....al
ara
.
Idmtlflcd
.............................. nlcm
B&t am.................. ....-........ m-m
Bowem Amk. a@Ylcsnce ....................
2U
B r o w . R . W..l m U w d Meafffied
bg .... 218-221
lnyer~drooks...........................
2l7-W
C d m W .....................................
234
phyuloaraphp..................+.-.- .237-241
caw ~ b ~ m d&l, b ......................... m
planed aurfnma......................... B.3-241
Chedcal cornpaltlm al mb,tahls......... X22
rhyodscltc
229
Chromlte.....................................
321
alns
22+21B
Ollmete .................................... 213-214
volmuIc ash d e d t s ................... S H - a n
c l b o p p m..........*..................m.228
Kaoegn Paae, gubmarlna togographg.---.--Z3f
Cloud. P. E., elme identW bg ........... 21R-221
Kanwn Volcano................ 2a&Z2% =.83f, 2%
B m b m . E S., pohnaend
up^^
bp
....-+
...-.+
..................
......
Cook& C . W., echlnolda Identified by
2lR-22l
Kannron R i d v .............................
l i s v a l ~ aIalmd. folds
lwatiun ..................................
XU
......................... 231
213
p h y................ S 2 3 T
auh.nsrlne m
v o l m l c as\ depmlt......................
Knob Pofnt, nil! ..............................
L a M m l t a ~ u ~ l mdwita
t6
...................
Zta
Landforms...........+..................-.
214-216
tam AOWB .........................
aaa, m,
281, US
hged r o c h
a15-rn
225
XA4 LlmbnruIFe...................................
.... 213
217 LwatIon 01 ares..........................
...................................... m-m
.........................
Flows ........................................
F d b
Flnper Rag volcanlca
2%
230
..............................
247
Psre
P.F
..........
-...--......
.............
...........................
m*mlmadcddaa
---...---.------
ap7
MapNcdl, P. a,, me&mIh Idenby--. 21&Xa BhW Fm.mbmariue Wrmnhpb~
M w a k fosalb...............................
218 BtrncWm, p l @ c
ZS%m
MlcrolIte
224 Gurflcial daposfta
-23
MLneralagp, dehl1tJm o( mma
,.----,
5-23
2M
Mloccne imatigns........................ 214224
ctdcd
-darn tsble-**-**
Mloc~negmloglc hlatorp...................... 2'43
f
u
t
e
Mwut K~~
lam
..-------..---..
aaq ns
folds
m
Imlla
PI&fEn,iS
N ~ gP
s h t , &HI
m
a
3
0
~l~L8tlrn
m
Oalhpa k b d . o h e m i d oomtdtltnt d mkq
l a n d l m l.
314
rsbk-+---+----------------------- Z22
layered mb
2t7-261
md md mlmniu d e ~ m i ~
=am
~oraz~on
2x3
~uatw
211
phym~nphy..
ai
a b m d n e -hp
pa& Tanwn P w . fwmd b y f s u l k . .
233
O l l ~..
h...................................
PR
ablbmarlnc topography-237
Mop---------------------+------Tnnnge Volcaao
%226
Orybamb6enBa
mu Tertlnor formatiom
2%. B12# 1.231
..................................
.....................................
.....--...............................
...............................
..........................
.................................
..............................
...........................
-..--....--.
..................................
...................... ....
.-.-.---.
..............
m,
--..-*.-...-...+
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
..........
.,-.-.---.-----............
....................
Tarllnry fpsella ..............................
219
wmmlu lormatma
a16 m ,rRr'rnryEBoloR1a h'ntorp-phpslmphgBr-Na Todd, Huth, FotamInllera ldQatIilcd by.-.. 916221
P W w l a m ............................ 84217,243 ..
l o~l~t181,R.J., snuyllaa by ...................
Xi3
PIeMxem l m t l o m........................ 2U Topwnphy, wbmarlaa
.................... 285-P7
PleWoctrne w l @ c h b m...................
Trnnd Ilnta.................................
Pllocene formatlma..-..--..-..-----..-------.
hhhhhhhhhhh
,..--.-..-----..*----+-
' -.------*--**--*--
---------------*------+*-+---
---
s13-¶4*838
-,*..*....**..--.-.....---....-.-.
p2a
VO~CSU~C
M'I &&b-. .
-,.--.......-,,.,,
%X&'
Volcmoea, early ahiald
225
BkRgU1 hlslld, boaldsl .
......................
w w l c h~~
- - ----- - ---244
hadd m
BUh
...................................Zl4%Ma
.
---...................................
Ch*mjd -mlt'm
1ocsUon
dmh*
219
"-------+---+-----------------
..-.-.._-_-,+--------
*++*
*
w n k -.-,,-.-.----------------~
2
%
Psre
P.F
..........
-...--......
.............
...........................
m*mlmadcddaa
---...---.------
ap7
MapNcdl, P. a,, me&mIh Idenby--. 21&Xa BhW Fm.mbmariue Wrmnhpb~
M w a k fosalb...............................
218 BtrncWm, p l @ c
ZS%m
MlcrolIte
224 Gurflcial daposfta
-23
MLneralagp, dehl1tJm o( mma
,.----,
5-23
2M
Mloccne imatigns........................ 214224
ctdcd
-darn tsble-**-**
Mloc~negmloglc hlatorp...................... 2'43
f
u
t
e
Mwut K~~
lam
..-------..---..
aaq ns
folds
m
Imlla
PI&fEn,iS
N ~ gP
s h t , &HI
m
a
3
0
~l~L8tlrn
m
Oalhpa k b d . o h e m i d oomtdtltnt d mkq
l a n d l m l.
314
rsbk-+---+----------------------- Z22
layered mb
2t7-261
md md mlmniu d e ~ m i ~
=am
~oraz~on
2x3
~uatw
211
phym~nphy..
ai
a b m d n e -hp
pa& Tanwn P w . fwmd b y f s u l k . .
233
O l l ~..
h...................................
PR
ablbmarlnc topography-237
Mop---------------------+------Tnnnge Volcaao
%226
Orybamb6enBa
mu Tertlnor formatiom
2%. B12# 1.231
..................................
.....................................
.....--...............................
...............................
..........................
.................................
..............................
...........................
-..--....--.
..................................
...................... ....
.-.-.---.
..............
m,
--..-*.-...-...+
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
..........
.,-.-.---.-----............
....................
Tarllnry fpsella ..............................
219
wmmlu lormatma
a16 m ,rRr'rnryEBoloR1a h'ntorp-phpslmphgBr-Na Todd, Huth, FotamInllera ldQatIilcd by.-.. 916221
P W w l a m ............................ 84217,243 ..
l o~l~t181,R.J., snuyllaa by ...................
Xi3
PIeMxem l m t l o m........................ 2U Topwnphy, wbmarlaa
.................... 285-P7
PleWoctrne w l @ c h b m...................
Trnnd Ilnta.................................
Pllocene formatlma..-..--..-..-----..-------.
hhhhhhhhhhh
,..--.-..-----..*----+-
' -.------*--**--*--
---------------*------+*-+---
---
s13-¶4*838
-,*..*....**..--.-.....---....-.-.
p2a
VO~CSU~C
M'I &&b-. .
-,.--.......-,,.,,
%X&'
Volcmoea, early ahiald
225
BkRgU1 hlslld, boaldsl .
......................
w w l c h~~
- - ----- - ---244
hadd m
BUh
...................................Zl4%Ma
.
---...................................
Ch*mjd -mlt'm
1ocsUon
dmh*
219
"-------+---+-----------------
..-.-.._-_-,+--------
*++*
*
w n k -.-,,-.-.----------------~
2
%