• 

DEPARTMENT OF THE INTERIOR 

FRANKLIN K. LANE, Secretary 

UNITED STATES GEOLOGICAL SURVEY 

GEORGE OTIS SMITH, Director 

PROFESSIONAL PAPER 120-D 

THE STRUCTURE AND STRATIGRAPHY OF GRAVINA 

AND REVILLAGIGEDO ISLANDS, ALASKA · 

BY 

THEODORE CHAPIN 

Published August 22, 1918 

Shorter contributions to general geology, 1918 

(Pages 83-100) 

WASHINGTON 

GOVERNMENT P'.lUNTING OFFICE 

1018 
•• 

• 

DEPARTMENT OF THE INTERIOR 

FRANKLIN K. LANE, Secretary 

UNITED STATES GEOLOGICAL SURVEY. 

~EORGE OTIS SMITH, Director 

Professional Paper.J20-D 

, THE STRUCTURE AND STRATIGRAPHY. OF GRAVINA 

·AND REVILLAGIGEDO. ISLANDS, ALASKA 

BY 

THEODORE CHAPIN 

Published August 22, 1918 

Shorter contributions to general geology, 1918 

(Pages 83-100) 

WASHINGTON 

GOVERNMENT PRINTING OFFI'OE 

1918· 

\ 

\ 
.CONTENTS. 

Page. 

Introduction .. _ ...... -.- ... --- ....... -.------~--- 83 Triassic rocks-Continued. 

Scope of paper _____ : _______________________ _ . 83 Origin_ . _ . : ______________ . _ . _ ... ______ : _____ _ 

Location. __ -·-----._-----.---:----------------- 83 Age and correlation. __________ . _____________ _ 

Surface features. ____________________ - ______ : .. 84 Upper Triassic or Jurassic rqck~-. _. ____ . ___ . ______ . 

Physiographic provinces. __________ . __ ~ __ .. 84 Distribution .. ____ .... _ .......... _._·-.- .. __ ._ .. Gravina Island ____________________ ~ ____ _ 85 Character and stratigraphic relations. __ . ______ . 

Revillagigedo Island _________ . __ ....... ' .. 85 Structure. _ ~ _ . : __ .. __ . _______ . ___ " _ _ _ _ _ _ _ _ _ _ . 

Summary of formations and structure ___ ......... . 85 Origin._. _____ . ________ ._. ____ _' ______________ _ 

Devonian rocks .... ·_ ..... ______________ ......... . 87 Age and correlation._~ ________ . _____________ _ 

Vallenar formation. ____ :? _________ •••••••••••• · 87 Upper Jurassic or Lower Cretaceous rocks. ________ _ 

Distribution and character ___ ._ ......... . 87 ·Distribution and character .. ____ .. ___ · _______ _ 

Structure and stratigraphy _____ ......... . 87 Stratigraphic relations.~ ___ -.~ ____ . ___________ _ 

Age and correlation.·._. ________ .. _______ _ 88 Structure_ . _____ ~. __ .. __ . __________________ _ 

Carboniferous rocks .. _. ______________ ............ . 88 Origin ______________ .- ~ ______ . ______________ . __ , 

Beds included. _________ . _________________ · __ _ 88 Age and correlation._. _________ . _______ : ____ _ 

Dist~ibution and character _______ ........... . 88 Tertiary rocks. _______________ ._ .. __ . __ .. ________ . 

Structure and stratigraphic relations. _ ....... . 88 Quaternary rocks .... ____ . ________ .. __ .. ________ _ 

Age and correlation. _____________ ._ .. _______ _ 89 · Glacial deposits.~ ______ .. __ . _ . ____ .. __ . ______ . 

Triassic rocks ............ _ _. __ : ______ .. -. -. -.... . 89 Distribution and character ___ . _ : .. _ ~ ____ _ 

~e-ds includecL _. ______ . _ . __ ... ____ : ________ _ 89 Age and correlation. _________ . __ .. ______ : 

Distribution and character _____________ .. ____ . 90 Marine gravels .. _ . __ : _ . _ . ____ .. ____________ _ 

Structure __ - - _ - - - - - - - - -- - - - - - - __ - - - - - - - - - - - - 91 Intrusive rocks .. ______________ . ______ _. ___ : ______ _ 

Stratigraphic rela:tions. _______________ ~ ___ : __ _ 91 

ILLUSTRATIONS .. 

·PLATE VIII. A, Wave-cut ·bench, Dall Bay, 

Gravina Island, Alaska; B, Conglomerate, Blank Inlet, Gravina 

Island; C, Slate and graywacke, 

Blank . Inlet, Gravina Island; 

D, Anticline on Vallenar Bay, 

Gravina Island ________ · ___ .. · ___ . __ _ 

II 

Page. 

F:rGuRE 10. Map of southeastern Alaska showing 

position of Gravina and Revillagi.: 

· gedo islands ............. _______ . _. 

11. Geologic sketch map of Gravina Island 

and a part of Revillagigedo Island, 

Alaska. _____ ..... _., ... --- ..... '-- .. 

88. 12. Structure section across Gravina and 

Revillagigedo islahcls, Alaska. ___ . 

Page. 

91 

92 

95 

95 

95 

96 

96 

96 

07 

97 

98 

9.8 

98 

98 

99 

99 

99 

99 

99 

99 

99 

Page. 

83 

84. 

87 

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THE STRUCTURE AND STRATIGRAPHY OF GRAVINA AND REVILLAGIGEDO 

ISLANDS, ALASKA. 

By. THEODORE CHAPIN. 

INTRODUCTION. 

SCOPE OF PAPER. 

. This paper is intended to supplement a 

report by Philip S. Smith1 on the same region. 

The finding on Gravina Island of 

Triassic fossils, which previously 

had been reported from but one 

other . locality · in southeastern 

Alaska, made his paper of par-

. ticular interest. Since the publication of . Smith's report ·the 

writer has had the opportunity to 

make further studies and to gather 

additional data on the structure 

LOCATION. 

The area under discussion is included within 

.the Ketchikan district, which lies in the southern part of southe~stern Alaska, the panhandle 

and stratigraphy of the rocks of 

Gravina Island and adjacent re- sa ~~----l---~~-,;!~~~~ ~C). gions. The discovery of fossils of ,.... 

Jurassic or Cretaceous ~ge has ~, 

made possible smne changes in the ...-. ~ 

geologic mapping and furnished 

the basis for· the interpretations 

which nre here presented,,not as C\ 

fi.nal conclusio~1s but as areasonab le s6r------L-\'\-----~~~~~Z--H~~~-n 

working hypothesis for future field ~ 

work. The structure and stratig- "t: 

raphy of . the rocks of Gravina 

Island are more clearly~nderstood 

by a consideration of their relatimi 

tO n..larger area including a part o~......__._, ........... so_._'--'--_._1_._00 _____ 2~00 MILES 

· o~ Revillagigedo Island (fig~ 10) ... s"'C=====~k====~-...:.....+,;=4d2:::~===~~· ~rhis paper is a preliminary re- 138 134° 

port on an area in the southern FIGURE 10.-Map of southeastern Alaska showing position of Gravina and nevillagigedo 

part of the. Ketchikan district. . . islands. , 

A 1nore cmnplete report op. the whole district, strip of Alaska extending along the seaboard 

including a description of the mineral-resources, ~rom Mount Fairweather to Dixon Entrance. 

is now in preparation.. The accompanying Southeastern Alaska is composed of a mainland 

geologic sketch map (fig. 11) is drawn from the area and a fringe of islands known as Alexan-· 

field notes· of P. S. Smith and the writer. ' der Archipelago. The Ketchikan district lies 

t Notes on t;,ho geology· of Gi'avina Island, Alaska: U.S. Geol. Survey 

I'rof. P,aper 95, pp. 97-105, 1915. · · 

wholly within the Pacific Mountain system, a· 

geographic pr~vince which, . as defined by 

83 

\ 
84 SHORTER CONTRIBUTIONS TO GENERAL . GEOI-'OGY, 1918. 

Brooks, 1 includes a broad zone of ranges forming a concave •· belt parallel to the southern 

coast of Alaska. · 

SURFACE FEATUREs.· 

PHYSIQGRAPHIC l?ROVIN.CES. 

Physi6graphically the Ketchikan district 

falls naturally into three main provinces. One 

comprises essentially the mainland area and 

The dominating feature of the .mainland 

province is the Coas~ Range, a high mountain 

mass with elevations of 7,000 to .8,000 feet. 

Bro·oks 2 describes it as follows: 

The Coast Range extends froin near the boundary of 

Washington northward through British Columbia into 

southeastern Alaska, where it lies partiy in Alaska and 

partly.in Canadian territory. Following the coast line 

for nearly 900 miles it passes inland behind the St. Elias 

LEGEND BED.DED ROCKS. 

Prpbably Triassic slates 

ard phyllites wit~ ir:ttrUSIVe quartz d1onte r 

REVILLAGIGEDO 

ISLAND 

=. 

~ }~! lta:.JIConglomerate,slate, :~~~~ 

131'40' 

ANNETTE 

IS LAND 

an~cke JrH" ~ :::l<(:::l Gceen[ffi slate ~, · 

Slate,sandstone,lime- ~ 

stone,and phyllite _g~ 

Limestone 

• ~~ 1-~ 0 ~g. 

Con[][[]]glomerate }~~ 

011)0 

Crysta!line schi~t ~~~ 

Va~l~:i;~~~r.~~on }t (Conglomerate,l/me- 0 

stone, and shale) 

INTRUSIVE. ROCKS 

- . 

Quartz diorite 

and granite 

0 

Strike and dip 

5E:::3:=:::E:::::::3=:::E~Oc=:=:=:=:=:=:=:j5:=:=:=:=:=:=::==310 MILES 

FIGURE H.-Geologic sketch map of Gravina Island and a part of Revillagigedo Island, Alaska. 

adjacent islands. It includes the Qoast Range 

al}d may be denominated the Coast Range or 

.mainland ptovince. The eastern or Prince of 

Wales Province comprises Prince of-Wales Island and the islands to the west. Between 

these two mountainous areas lies · an intermont.a:iJ.e lowland province including Clarencec 

Strait, Revillagigedo Channel, and Behm Canal 

andlow-lying areas alorig their shores. · 

1 Brooks, A. H., Preliminary report on the Ketchili:an mining district, 

Alaska: U. S. Gcol. Survey Prof. Paper 1, p. 14, 1902. 

Range near the head of Lynn Canal. Thence it can be 

traced northward, decreasing in altitude anCl gradually 

losing definition until it finally "merges with the interior 

plateau near Lake Kluane, in longitude 130° 30'. This 

range has no well-defined crest line but is rather a complex of irregular mountain masses occupying a coastal 

strip between the Pacific Ocean and the Central Plateau 

region. 

The Prince of Wales physiographic province 

is dominated by a . complex group .of short, 

2 Drooks, A. H., The geography and geology of Alaska: U. S. Geol. 

Survey Prof. Paper 45, p. 28, 1906. 
GRAVINA AND REVILLAGIGEDO ISLANDSJ ALASK~. 85 

· rugged ranges reaching elevations of about 

4,000 feet. In the northern part of the Alexander Archipelago these are· not sharply dif-

·ferentiated from ·the Coast .Range and were 

regarded by Bi.·~o]~s as a southeastern extension 

of the .St. Elias· Range. 1 In the Ketchikan 

district, however, as has been pointed out, the 

Prince of vV ales mountainous region is quite 

distinct from the Coast Range, from which it is 

separQ.ted by the lowland province. It is· correlated with the Vancouver Range, named by 

Dawson 2 and consisting of "an irregular mountain system which in its submerged parts 

constitutes Vancouver Island and the Queen 

Charlotte Islands." · 

'!"'he lowland province as here defined, comprises the submarine area_ covered by the waters 

of Clarence Strait, Revillagigedo Channel, and 

Nichols P~ssage, and the bordering lowlands. 

This lowland is evidently a subrnerged portion 

of the.Pacific coast downfold, 3 a series of basins 

forming a more or less continuous depression 

extending from the Gulf of California to 

British Columbia and Al,aska. ·This lowland 

province would thus be correlated with Puget 

Sound and with the lowlands covered by the 

sound separating Queen Charlotte and Vancouver islands in British Columbia from the 

mainland. The area under discussion lies 

within and along the border of this lowland 

province, an nrea covered largely by Mesozoic 

sedime.hts which apparently occupy an overturned synclinal fold. 

GH.A YIN A ISLAND. 

Grayina Island lies between Clarence Strait 

and . Revillagigedo ChanJlel Q.nd is separated 

from Revillagigedo Island by Tong ass Narrows, 

a narrow, shallow channOl a quarter of a mile 

to 2 miles wide and in places less than 100 feet 

deep. The island is roughly triangular. it 

has rather straight shore lines on two ~oasts, 

but the southeast cqast is very irregular in 

shape, being broken by two deep bays, Blank 

and Bostwick inlets, and by n1inor indentations. The longest dimension of the island is 

21 miles, and its rnaximum width is 9 -miles. 

It has an area of about 100 square miles. 

1 Brooks, A. H., Tho geography o.nd geology of Alaska: U. S. Geol. 

Survey Prof. Paper 45, p. 29, 1906. 

s Dawson, G. M., Hoy. Soc. Canada Proc. and Trans., vol. 8, sec. 4, 

p. 4,1890. 

8 Willis, Bailey, U.S. Geol. Survey .Gaol. Atlas, Tacoma folio (No. 54), 

p. 1, 1899. 

Gravina Island lies alorig the border line 

between the Coast Range and lowland provinces. Two low ranges of hills, the Oa]ifornia 

Range and Dall Ridge, extend across the entire 

island fro~ northwest to southeast. Dall 

Ridge reaches a maximum elevation of about 

2,000 'feet,· and California Range of over 2,500 

feet. Between these two ranges of hills, extending from V allenar Bay to Bostwick Inlet, 

i.s a low depression only a few feet above sea 

lev.el: Another depression extends from Blank 

Inlet to a point opposite Ward Oove. It lies 

between California Range and a low range that 

borders Tongass Narrows from Gravina Point 

northward for about 4 miles. 

A strikii1g parallelism exists between the 

ranges of hills, the intervening valleys, and the 

large bays-Vallenar, Bostwick, and Blank 

inlets. All have a dominant northwesterly 

direction, approximately parallel to the structure of the bedrock, which has evidently been 

the. controlling factor in the topographic 

development. 

REV~LLAGIGEDO ISLAND. 

Revillagigedo Island, with the exception of a 

fringe of lowland along Tong ass Narrows, belongs to the mainland province. It is a large 

island of 1,120 square miles separated from the 

mainland by Behm Canal, a semicircular fiord 

that borders the northern part of the island. 

The ii1terior of the island is high and rugged, 

its peaks reaching elevations. ·of 4,000 feet. 

Three long fiords-Thorne Arm, Carroll Inlet, 

and George Inlet-break the southern coast 

into irregular-shaped· peninsulas. 

SUMMARY OF ·FORMATIONS AND STRUCTURE. 

. The rocks of Gravina Island and tliat portion of Revillagigedo. Island included in this . 

discussion are essentially Mesozoic formations 

that rest unconformably upon Carboi1iferous 

and Devonian rocks (fig. 11). The :Mesozoic 

formations include three distinct terranes, ali 

of which are fossiliferous. The lower and 

upper terranes, which are dominantly sedimentary·, are respectively Triassic and either 

Upper Jurassic or Lower Cretaceous .. The 

terrane of intermediate stratigraphic position, 

composed of volcanic rocks· and intercalated 

sediments, contains fossils of Upper Jurassic or 

Lower Cretaceous age but may include also 

Tr~assic rocks. Tertiary lava :flows occ~ on 
86 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

the southern part of Gravina Island but are not 

widely distributed; The rriost recent formations are marine glacial deposits that lie above 

the preBen t high tide: 

Island Triassic conglomerate, limestone, and 

black slate are overlain by v:olcanic rocks and 

intercalated sediments of Triassic or Jurassic 

age, which, in turn, are overlain by the slate 

and graywacke of Blank Inlet, determined to 

be Jurassic or Cretaceous.~ Succeeding the 

Jurassic or Cretaceous sediments are volcanic 

rocks, with black slates and tuffs. I-Iere the 

section is interrupted by a boss of granite, 

beyond which are black slates and phyllites, 

limestone, and schistose conglomerate .. · Beyond the conglomerate occur crystalline schists. 

and massive semicrystalline ·Jimeston,e-bearin.g 

fossils regarded as certainly Paleozoic and 

probably Carboniferous. This anomalous posiThe intrusive rocks of the Ketchikan region 

comprise for the most part the clioritic and 

granitic. rocks of the Coast Range .. They are 

not, however, oon:fined to the Coast Range but 

occur in large bodies on the islands. The age 

of these intrusives is regarded a3 late Ju~assic 

and Cretaceous. 

The following table shows the correlation of 

the Mesozoic formations of Gravinaisland with 

those of Graham and Vancouver islands as 

described by ·canadian geologists: 

Correlation table of Mesozoic formations of Gravina, ,Graham, and Vancouver i~lands. 

Graham Island.a Vancouver Island.b 

Age. Gravina Island. 

• Formation. Lithologic charact~r. Formation . Lithologic character. 

Upper Creta.,. Queen Char- Fossiliferous shales Nanaimo se- Conglomerates, sand- ceous. lotte series. and conglomerates, ries. stones, and shales, 

with coal. with coal. 

Lower Creta- Conglomerate, sand-· \ 

ceo us. stone, and gray- I wacke. 

Juras- Batholithic Batholithic l)"pper Granodiorites, Quartz diorite, etc. intrusives, Quartz diorite, etc. and dike in- quartz 

sic. etc. trusives. diorites, etc. 

Sicker series Andesitic flows and 

Middle Juras- Volcanic flows· and Yakoun for- Agglomerates, .tuff~, (Jurassic or tuffs; schistose, slaty, 

sic. tuffs with interca- mation. and flows. t u ff a c e o u s, and 

lated sediments. Triassic). quartzose sediments. : Volcanic" breccias. 

Lower Juras- Dal'k-coloreci, fi n .e- grained, thinly sic. laminated fossilif- erous argillites, Sutton forma- .Crystalline limeMaude for- grading upward into tion, Van- stones, metamorphic 

Conglomerate, · lime- mation. . tuffs and agglomer- couver :vol- andesites, and pyroates. P o s s i b I y canics. clastic rocks. Upper Triassic. stone, sandstone, detrital conglomer- and slate. ates at base. I 

a Mackenzie, J.D., Canada Geol. Survey Mem. 88, 1916. b Clapp, C. H., Canada Gcol. Survey Mem. 13, 1912. 

Across the area including, Gravina Island tior~ of the Paleozoic and Mesozoic rocks is 

and the southern part· of Revillagigedo Island probably due to an overturned position ·of one 

the beds present a dominant northwest strike limb of the syncline. 

and, except locally, dip northeast, giving the ' From the slate and graywacke area of Blank 

appearance of simple monoclinal structure Inlet the general sequence of rocks is the same 

over a wide area. (See fig. 12.) These beds, toward the northeast as l.t is toward the .. southhowever, are believed to represe~t overturned west, and it is believed that the similar rocks 

and truncated isoclinal folds and thus give the are in fact the san;te beds exposed on two limbs 

impression of a much greater·thickness of sedi- of an overturned fold, the trough of which is 

ments than really exists. The ·evidence for exposed on Blank Inlet. The volcanic rocks 

this conclusion lie~ in the following observed on each side of the Blank Inlet area of slate and 

facts~. On the . southwest coast of Gravina graywacke are si.ri:rilar and. may. be correlated, 
GRAVINA AND REVILLAGIGEDO IS:{:.ANDS, ALASKA. 87 

as there is little reason to doubt their identity. 

The slates · and phyllites of Revillagigedo 

Island are much more schistose than the black 

slates and sandstones of Gravin·a Island, with 

which they are correlated, but this is evidently 

due to the contact metamorphism caused by 

the adjoining intrusive granite. The other 

sediments of Revillagigedo Island are also more 

metmnorphosed than their supposed equivalents on Gravina; Island. The limestone ·is 

crystalline and the conglomerate is schj.stose, 

but these differences in metamorphism are also 

easily accounted for, not alone by the small 

body of granite shown in the sketch but also 

by the immense batholith of the Coast Range. 

Along this eastern limb of the syncline is 

a greatly disturbed belt which apparently 

continues the entire length· of southeastern 

Alaska along the bdrde~ of the intrusives of the 

Coast Range. ·In this disturbed ·bel~ the foldsw .. 

TRIASSIC 

(in part or wholly UpperTriassic) 

·S· 0 

bedded with thin limestones, calcareous shales, 

argillites, and sandstones containing Devonian 

fossils. 

STRUCTURE AND STRATIGRAPHY. 

. The structure and stratigraphic relations o~ 

the Devonian beds are not clear. The beds 

are closely folded and deformed, and the relations are obscured by movement along the contacts. From South V allenar Point eastward 

the beds trend dominantly northwest and dip· 

northeast at varying angles. From the present 

position of the beds the base of the formation 

appears to be the conglomerate, which apparently rests upon the Mesozoic volcanic beds of 

the west coast of the island. This anomalous 

condition indicates an overturned position of 

the beds. · 

. On the west coast of V allenar Bay, about a 

mile east of South Vallenar Point, is the· crest 

NE •. 

·, TRIASSIC CARBONIFEROUS 

~i~ part or wholly Upper Triassic) (probably late Carbon·1ferous) 

5 10 Miles 

J!'JGURE 12.-Stmcture section across Grav~a and Revillagigedo islands, Alaska. 

. ing is much more close, and the beds are more 

'tightly compressed and, if correctly interpreted, 

·are in part overturned. · The structural form is 

not. a simp 1~ syncline and might more correctly 

be termed a synclinorium, for one prominent 

anticline and a number of minor folds occur, 

the details of which are brought out; more fully 

.in the di~cussion of the different formation~. 

DEVONIAN ROCKS. 

VALLENAR FORMATION. 

DISTRIBUTION AND CHARACTER. 

The oldest rocks recognized by the writer 

. are the Devonian beds exposed on· the west 

coast of Vallenar Bay, on Gravina Island. 

They consis~ of conglomerate, limestone, calcareo':ls shales, sandstone, and argillites. The 

conglomerate is composed of cobbles of schist, 

quartz, and limestone embedded in a schistose 

matrix. The conglomerate is in p~t interof a very sharply folded anticlirle (Pl. VIII, D), 

which has involved the calcareous shales· and 

argillites. It is believed that these are the 

lowest of the Devonian beds and that the lime-. 

stones and conglomerates to ·the. west are 

stratigraphically higher in the section and 

occupy the overturned limb of a steeply pitching ~nticline that is overturned · toward the 

west. It may be that the schistose conglomerate containing an abundance of limes.tone. 

cobbles is a meinber of the Mesozoic greenstone 

and intercalated sedimentary beds. The contact with the Mesozoic beds at the head of 

V allenar Bay is covered by. gravels. This 

anticline along whiGh· the Devonian beds ·are 

brought to the sti.rface on V allenar Bay is presumably the same fold that extends along Dall 

Ridge to Seal Cove and forms a minor fold in 

the broad synclinoriu:rp. involving all the rocks 

between the west coast o{ Gravina Island and 

George Arm. · . · · 
SHORTER CONTRIB.UTIONS TO GENERAL GEOLOGY, lins·. 

AGE AND CORRELATION. 

The Devonian rocks of V allenar Bay were 

first described as the V allenar ''series'' by 

Brooks/ who states: . 

The Vallenar series [is] so named from its type exposures 

on Vallenar Bay, at the northern end of Gravina Island. 

The series. is composed essentially of limestones and cal-

. careous. schists ·with some black slates. 

Fossils were collected at this locality by 

Brooks in 1901 and determin.ed by Charles 

Schuchert as Devonian. More collections were 

made in 1905 ai;ld 1:906 by the Wrights and 

E.·M. _Kindle 2 who report: 

One mile west of Vallenar Bay,· Gravinf1 Isla1ui.-The material from this _locality, while generally insufficient for 

specific determination, is much better than that obtained 

last year (1905) and leaves ·no doubt as to the Devonian age 

of the beds west of Vallenar Bay. Several specimens of 

. Atrypa· reticularis are present. This· fixes the horizon as 

not later than Carboniferous, while the associatio.n of Ch_onetes·cf; manitobensis, Spirijer sp., 'Proetus sp., and Cyclonemasp. indicate a horizon of Devonian age, probably middle Devonian. · 

Some poorly preserved material collected by 

th_e writer in 1915 was determined by T. W~ 

Stanton as probably Devonian. 

The beds of V allenar Bay were correlated by 

Broo•ks with rocks on Long Island. 3 . Middle 

Devonian rocks of similar character occur on 

. Clover B~y, on the east coast, and Hunter Bay 

and Klakas Inlet, on the west coast of Prince of 

Wales Island and on Hot~ pur Island. 

CARBONIFEROUS ROCKS. 

BEDS INCLUDED: 

The terrri "Ketchikan series" was applied by 

Brooks 4 to argillaceous and carbonaceous roqks 

. with some lim.estone bed~ and fin~ly foliated 

slates and phyllites that occur on 'Tongass N arrows, near Ketchikan, on Revillagigedo Island, 

and on Cleveland Peninsula. Brooks considered these rocks to be in part Carboniferous and 

in part Mesozoic and suggested that probably 

the limestones are Carbqniferou~ and the slates 

are Triassic. 

The Wrights 5 state: 

In view of the comparatively weak paleontologic and 

stratigraphic .evidence bearing on the precise age of these 

argillites and crystalline schists, it is possible that their 

period of deposition extended even beyond the Carboni£· 

erous period into the Triassic, ·especially when the exten· 

sive development of Triassic strata to the south in British 

Columbia, as described by G. M. Dawson, is taken into 

consideration. The evidence thus far gathered, however, 

indicates Carboniferous age for the greater portion of these 

crystalline schists· and argillites. 

Brooks noted that the limestones overlie the 

argillites (slate and greenstone) and thus appear 

to be younger.· This confu~ion was due to their 

overturned position, the oJder beds actually 

overlying the younger ones. · 

The assignment of the rocks mapped by 

Brooks as the '' Ketchikan series'' to both 

Oarboniferous and Triassic is in harmony with 

the writer's interpretations. The part . of 

Brooks's "Ketchikan series" that stratigraphically underlies the conglomerate and · massi~e 

limestone which in this report are correlated 

·with the Upper Triassic· rocks of ·Gravina 

Island is _here regarded as Carboniferous. 

DISTRIBUTION AND CHARACTER. 

The rocks herein assigned. to the Carboniferous are essenti"ally crystalline schists and 

limestone. In areal distribution they occupy 

a broad belt along the border of the Coast 

Range, extending from the mouth of Portland 

Canal northwestward for an unknown distance. 

In the area under discussion they are restricted 

to ·a small area· on Revillagigedo Island nea~ 

the head of George Arm. The schists make up 

the greater part of the series. They are evidently the result of contact metamorphism 

induc~d by the intrusive rocks. An adequate 

description of them would require a detailed 

petrographic study of the rocks in the field 

and an intensive !fiicroscopic study, neither of 

which has been made by the writer. ·The limestone constitutes a relatively small part of the 

ser~es. It occurs in. beds 100 feet in maximum 

thickness intercalated with the crystalline 

s.chists. · It is dominantly white and crystalline. 

STRUCTURE . AND STRATIGRAPHIC RELATIONS. 

The Carboniferous rocks are closely folded. 

·and contorted. They6ccupyagreatlydisturbed 

belt along the western border of the. Coast 

Range and form the. overturned limb of a 

synclinoriuin in which are involved all the 1 Brooks, A. H., Preliminary report on the Ketchikan mining district, . 

Alaska: u.s. Geol. survey Prof. Paper 1, p. 42, 1902. formations east of Clarence Strait. The other 2 Wright, F. E., and c. w., .The Ketchikan and Wrangell mining dis- limb of the syncline comes to the surface on 

tricts, Alaska: U. S. Geol. Survey Bull. 347, p. 50, 1908. 

8 Brooks, A. H., op. cit., p. 43. the western coast of Gravina Island. • Brooks, A. H., Preliminary report on the Ketchikan mining district, The base of the Carbqniferous schists and 

Alaska: U.S. Geol: Stirvey Prof. Paper 1, p. 44, 1902. l . ~Wright, F. E. and c. w., op. cit., p. 56, imestones IS not exposed. .They apparently 
,., -.. ..... ,.. ~ • ... .,. 

U. S. GEOLOGICAL SURVEY PROFESSIO~AL PAPER 120 PLATE VIII 

A. WAVE-CUT BENCH, DALL BAY, GRAVINA ISLAND, ALASKA. B. CONGLOMERATE, BLANK INLET, GRAVINA ISLAND. 

C. SLATE AND GRAYWACKE, BLANK INLET, GRAVINA ISLAND. D. ANTICLINE ON VALLENAR BAY, GRAVINA ISLAND. 
·, 

>, 

• 

G:L'tAVIN A AND REVILLAGIGEDQ. ISLANDS, ALASI\:A, 

overli.e conglo:r;norate and limestone. beds whoso 

age is unknown· l;mt which are regarQ.ed as 

Upper Triassic. The strata at this locality, 

however, are beijeved to be overturned so that. 

t}le normal sequence of tl;l.e beds is reversed. 

(See fig. 12.) 

AGE AND CORRELATION. 

Fossi.ls were collected 'from the limestone 

.near Coon Is~and at the head of George Arm 

by Mr. Brooks; who states that the many 

c:r;inoid stems suggest th.at at least the limestones belong to the .upper Paleozoic. C. W. 

Wright also collected fossils from George Arm, 

on which E. M. IGndle reports as' follows: 1 

This fauna is known to be at but one other point in Alaska 

-Porcupine· mining district-where some of t.l,le . same. 

1:1pecies were obtained last year. This fauna also is entirely dissimilar from those of the Mississippi Valley and 

Appalachian region and finds nearer affinities in those of 

·western United States and of eastern ·Europe (Russia),. 

These schists and- limestones are therefore 

tentatively regarded as late Carboniferous. 

Their relation to the Devonian beds of V allenar 

Bay is not evident, as the two are nowhere 

known to be in contact. If th&se s.chists· and 

limestones are correctly correlated with the 

]ate Carbonife:I:ous it would appear that· there is 

lacking in this region a very thick section of 

Carboniferous rocks which are widely developed 

on the west coast of Prince of Wales Island. 

No. 966. Point on southeast ·side of George Inlet 10 miles h 

from its head, R~villagigedo Island.-The fossils from this T ese include Mississippian limestone and 

point comprise only crinoid stems, which, however, are so cherts,. which are probably the equivalent of 

abundant as to make up a large portion of the rock. Cri- the Lisburne limestone, and Pennsylvanian 

noid stems can not ordinarily be used for correlation. limestones· that include ~1oscovian and GscheHowever, their great al;mndance here seems 'to justify a lian faunas. 

surmise that they very probably represent a Carboniferous TRIASSIC ROCKS. , horizon. Crinoids in such abundance as is shown by these 

specimens occur in very few places, if anywhere, outside 

of Carboniferous horizons. In the absence of other eviBEDS INCLUDED. 

dence the horizon represented may be referred t.entatively The term "Gravina series'' was first. applied 

to the Carboniferous. , ·by Brooks 3 to massive conglomerate overlain 

Other collections made by P. S. Smith in by black slates or shales occurring on the south 

1913 contained nothing of determinative value end of Gravina Island. He included in his 

but were considered as probably Paleozoic. mapping of this "series" the somewhat similar 

No collections were made· by the writer, but massive conglomerates and associated black 

the crinoid-bearing beds were observed at a slates and arenaceous sediments of. the west 

number of places. coast, Bostwick Inlet, and Blank Inlet and 

There is little doubt of the Carboniferous age suggested its probable extension to other 

of at least one of the fossiliferous beds in this places. As a result of recent investigations, 

series of schists and limestones, but their exact however, the sedimentary rocks. of Gravina 

age and stratigraphic position are much in Island originally defined as th~ "Gravina 

doubt. Lithologically, except in amount of series" are now known to include two sediInetamorphi$m; they resemble the slate and mentary formations-one of Triassic age and 

limestone of the Porcupine district. The one of JurassiC or Cretaceous age. These two 

slates and limestone beds of the Porcupine 'dis- sedimentary formations are associated with 

trict are not fos~iliferous, but from Closely volcani,c rocks of intermediate stratigraphic 

associated massive limestone beds fossils were position. Brooks first regarded the "Gravina 

collected which G. H. Girty first doubtfully . series" as Cretaceous and correlated it with 

referred to the lower Carboniferous but later the Queen Charlotte series of Vancouver and 

determined as upper Carboniferous. Regard- Queen Charlotte islands, but later he stated 

ing a similar fauna from Admiralty Island, that its identity with. the "Vancouver series" 

Girty 2 says: (Triassic) seemed equally probable.4 His corTho fauna. from point a.t southeast entrance to Pybus 

Bay, Admiralty Island, which also seems to occur at po~t 

on divide between Chapin Bay and Herring Bay, Admiralty Island, is without do':lbt upper Carboniferous in age 

and probably can be designated Permo-Carboniferous. 

1 Wright, F. E. and C. W., op. cit., p. 55. 9 U.S. Geol. Survey Bull. 287, p. 1431 1906. 

S3449°-l8-2 

relations with both TriassiG and Cretaceous :~ 

formations, made without any fossils, proved 

substantially correct . 

a Brooks, A. H., Preliminary report on tho Ketchikan mining district, 

Alaska: ·U.S. Geol. Survey Prof. Paper 1, p. 45, 1902. 

4 Brooks, A. H., Tho geography and geology of Alaska: U. S. Gool. 

Survey Prof. Paper 45, p. 226, 1906 
90 SHORTER CONTRiBUTIONS TO GENERAL GEOLOGY, 1918. 

DISTRIBUTION AND CHARACTER. of fossiliferous massive .limestone. ·There ·has 

The Upper Triassic rocks on· Gravina Island been some movement along. this contact;,:.~o comprise a gradationally conformable series that the relations are confused, but the limeof conglomerates, lime~tones, sandstones, and stone is apparently above the conglomerate 

slates. They are most typically exposed on and sandstone. On Thompson Cove .the conthe southwest coast of the island, where they formable relations of the limestone to the conoccupy a narrow belt that. extends from a small glomerate, sandstone, and slate are ~oree~ident, cove. opposite :ball Bay northwestward for although here also there has been some faulting. 

about 7 miles. Unfossiliferous rocks, presum- In the upper part of the conglomerate the 

a?ly of the same formation, to judge from litho- beds are thinner and the material is much finer 

logic similarity and structural relations, occupy grained and contains more sandstone, ·slate, 

a narrow belt that extends· across George Arm an~ thin beds of limestone, which are conformand Carroll Illlet.. ably . overlain by the massive limestone. 

On Gravina Island the Upper Triassic rocks The limestone varies in appearance from 

.fall naturally into· three main divisions-one place to place. On Thompson Cove and Open 

composed essentiaily of conglomerate, one Bay it is a soft gray fossiliferous rock, corals 

. essentially of limestone, and one of interbedded being especially abundant. On Threemile 

black slate, sandstone, conglomerate, and Cove it is ~ore closely folded; is consid~rably limestone. These thre·e conformable terraiws silicified, and weathers out · brick-red. The . 

are shown by fossils to be probably Upper limestone here is less fossiliferous, and the fosTriassic. Overlying the Upper. Triassic sedi- sils ~re poorly preserved. The greater an1ount 

n:tents, with apparent conformity, are volcanic of deformation of the limestones on Threemile · 

agglomerates and intercalated sedimentary· Cove is probably the result of contact .metabeds which are, at least in part, of Upper moq)bis,m induced by the intrusive rocks of 

Jurassic or lower Cretaceous age and possibly Dall Head. 

-in part of Upper Triassic age. The massive limestone is conformab_iy overThe exposed basal member of the series on lain. by a great thickness of blacl,< slate with 

Gravina Island is a co~rse conglomerate that intercalated beds _of conglomerate, sandstone, 

:extends along the southwest coast of the island and limestone. These beds crop out alo~g 

from. Conglomerate Point northwestward to the coast of Gravina Island for .a distance of 

Open Bay · al?-<:f occupies thi-ee narrow strips about 3 mil~s north of the limestone area· and 

whose continuity is broken by Fivemile Cove, extend to the high hills of Dall Ridge nortP, of 

Thompson· Cove, and Threemile Cove. The Dall Head. Similar rocks are exposed on 

conglomerate is a heavily- bedded, massive rock. Bostwick Inlet and Seal Cove and extend north-

. The boulders are essentially of angular coarse- westwar.d in a belt from. 2 to 3 miles wide to 

grained- granite resembling ·the granite of. An- V allenar Bay a~d North V allenar Point. 

nette Island, and the matrix is quartz-feldspar The dominant rocks of this series are blacksand presumably derived from the same source clay slates having a pronounced ci~avage. Inas the boulders. Intercalated with the coarse tercalated with the slate are thin beds of quartz 

conglomerate are thin beds of sandstone,. which sandstone, quartzite, and conglomeratic sand-:-

are composed of . the same material a~ the stone. The limestone .beds are not numerous 

~atrix of the conglo_merate, and gradational but are __ usually fossiliferous.. The interbedded 

~eds of grits. Thin beds of fossiliferous lime-- limestone layers· a;re about 20 to 30 feet in 

sto~e and black slate with pronounced cleavage thickness but appear to be lenticular. · 

occur sparingly. The congbmerate and inter- The black slate and ~ssociated sediments 

calated sandstone beds are· strongly indurated are closely folded, especially on, Threemile 

and break with pl'ominent fractures across the Cove, where the beds are thrown into sharp 

boulders. ~ontorted folds. The beds on V allenar Bay 

-On a small cove. south of Threemile Cove 1 are much more regular and show none of the 

the conglomerate is finer grained toward the close folding. 

top and passes upward into grits, sandstone, On Revillagigedo Island assoCiated conglomand slate which ar~ overlain by a large block erate, limestone, schistose slates, and phyllites 

1 Threemile Cove h~ l:lPPe'lr!lu in the literature as "cove 3 miles are correlated with the sedirp.entary series of 

north of Dall Head.'' · · · Gravina Island on the basis of similarit:f of 

• 
•• 

GRAVINA AND REVI.LLAGIGEDO ISLANDS, ALASKA •• 91 

lithology. ancl.sequence and .stratigraphic and STRUCTURE. 

structural relations;.· The beds are.exposed on The Triassic· conglomerate, li~estone; ·and . 

George Arm, .Carroll: Inlet,. and. Thorne Arm. ·black slate of the west coast of Gravina Island, 

The lithologic·· similarity. of these rocks to the although closely·· folded, maintain a gen~ral 

.Triassic rocks of Gravina Island is marked, ·strike'of ~bout N. ·70°·W .. The beds dip northnotwithstanding their .more schistose· ·nature, east and occupy ·one limb of the syn,clinoril.un, 

which is due to their proximity' tq the. large ·which· dips beneath the volcanic rocks and 

masses of intrusive rocks. The conglomerate.is comes to the surface in Bostwick Inlet and 

very schistose, and the cobbles are mashed. ancl presumably also on Rev.illagigedo Island near 

deformed. The limestone is white and crystal- the :head of George Arm, where the beds are 

line, .and the slates are in p~t schistose and · invol~ed in the overturned'. limb of" the ·synmight more correctly. be -termed phyllites and' . clinoriilln. . . . . 

. schists. Similar slates, phyllites," and schists · · STRATIGRAPHlC RELATIONS •. occur on the north end. of. To :rig ass Narrows. ' 

These rocks show different degrees of meta- ·.~::The :·base of the Upper Triassic series of Oramorphism. The most extreme. phases occur. vina ''Island is not visible. The lowest part 

along ··the· borders· of the·· plutonic bodies of· .exposed· occurs at- ·Thr~emile a~d Thompson 

Revillagigedo Island, and the an1oun~ of meta- coves, where the actual base of the conglomerate 

.l:Jl~rphism varies more oi; J~ss direc~ly with its is concealed by the .water. At Vallenar Bay 

nearness to. the igneous rock. ... On Tongass the Triassic rocks overlie Devonian sediments 

Narrows the· slates show little metamorphism that com:e to the surface ·along the axis 9f ·an 

except cleavage and·· the development of a anticline. Th~ contac~ . is . covered. by ·the 

little secondary :niica. As the igneous rocks Quaternary deposits. of Vallenar, Bay. The 

are approached the metap1orphism becomes presence· of Devo~ian pebb.les _iri the conglommore noticeable. On . Ward Cove the slates erate~ of Threem1le Cove 1ndwates an underhave a .very pronounced schistosity and contain lying rock of that age, and on George Arm·the 

considerable secondary mica and hornblende. supposedly Triassic rocks rest upon CarhonifThe most prominent type is a dark-gray ro~k erous rocks. A marked unconformity at the 

with acicular hornblende crystals a centimeter base of the Upper Triassic rocks is·thus evident. 

in length, white n1ica, quartz, and colorless On Gravina and ·R.evillagigedo islands the 

g~:trnet in large euhedral crystals. Pyrite is Triassic rocks are in places conformably· overlocally abundant. Another' type· is ·a dark- lain by volcanic ·beds· of Upper ·Triassic or 

green schist compo'sed es'sentiaJly of hor~bh~nde Jurassic ~ge and in places unconformably by· 

and quartz .. Garnet_ is an abundant .. accessory. slate ·and graywacke ·of Jurassic or Cretaceous 

It occu~·s in euhedral crystals .partly· altered age. The conglomerate 'and sandstone ·of Dall 

to cnlcite nnd quartz. Epidote; .. chloritie, and Head are invaded by dioritic rocks, presumably 

pyrite also occur. of Jl.lrassic ·or· Cretaceous age,' and are· uncon:..: 

No fossils lutvq been fou ~d i!-i .t4e~e rocks, forniably overlain by Tertiary· lavas.:· 

but on account of their stratigraphic -position ORIGIN~ .. , . , . \'. ·~ . . . 

between the Carboniferous rocks of Revillagi- Th u · T · ,. · · k th d t f I . . . . e· pper. . r1assw roc s are e pro uc. s o 

gedo . sla:nd an.~ the supposedly ... T.r~assw or I normal deposition· and are evidently of marine 

Jurassw volcaniC· rocks of .. T0ngass·.:Narrows. .· ·n . h n b th f t · II .. . • • • • • •• . • I or1g1 , ... as s ow . y e auna. presen Ill .a 

and Gravina Island, ·they appE}a;F' -to··o. ccupv th m mbers Th . · b 1· d t · t • • • • u. . e . e • · ey are . e 1eve o represen 

about ti1e same· honzon ~s the Tnas~~c slates . ·an unbroken period of deposition, but the 

and sandsto~es ~ ?~av~na Island~ · The se:.. . abrupt change from· conglomerate to massive 

fu~:nc? of tlus ~enes · ;s als~ t~,e ~all_l.~ as the ·limestone' and black-.slate with numerous inter~ 

.nas~w successwn ° .s~du~e~.tary. beds of :·calations · of ·limestone, conglomerate,. and 

Gr~vma Island-conglomerat?, ; l.Imestone, . sandstone: indicates .that the rocks were de.., 

arg1llaceo~s and ~renaceous sediments,.···· From , posited. und~r r:onstantly ch,anging conditio],)S.· 

the present .relatwn of the beds the sequence ;rh~· massive .conglo~erate was. evidently.: .a 

on the two Islands appear~ to .be reversed, but basal forma.tion. and was laid down in. comthis is due to the overturned position of the· parati-;~ly shallow w~t~r, :rie~r the .. shor~'li~e 

section Oil"Re:villagigedo Island. and derived its materi.al ~ro~. 'the p~od~~ts. ·~f ' ,,;· •••• • '•.· '' ••• 0. • •• 

• 
92 SHORTER CONTRIBUTIONS T0. G~NERAL GEOLOGY, 1918. 

land waste. The. organic· nature of the lime- ing from the uppermost part of the Carbonstone. is clearly proved by the great abundance iferous or from the Mesozoic. These beds were 

of organic remains, especially corals, which it thus doubtfully referred to the Mesozoic for 

contains .. The sc4is~s and phytlites were some years,. but recent collections. have shown 

. originally slates and. sandstones,, probably. of them t_o be :without much doubt Triassic and 

marine. origin, but have been chang.~d to their· probably Upper Triassic. 

present condi,tion by the contact influence of Collections inade by Martin and Overbeck 

the intrusive rocks. ·from ·the- lin1estone beds of Threemile Cove 

AGE AND coRRELATION. occurring beneath the black slate were deter-

. · mined by T. W. Stanton as follows: 

The .age ·of these· rocks is shown by a larg~ 8830 (G. C. M. No.2). Gravina Island, south arm of "cove 

·n.~ber of fossil collections, made from all 3 miles·north of Dall Head." Massive limestone outmembers of the series, to be Triassic, probably crop near anchorage behind wooded island. . 

Upper·Triassi~. .Undetermined corals of Mesozoic type, two or three 

Fossils collected b.y G. C. Ma.rtin and R. :M. genera represented. · Ostrea? sp .. Overbeck from the massive conglomerate at_ . Pseudomelanla? sp., internal cast.. . 

the exposed· base of this series were de-termine-d .Arcestes?? sp., fragment; may not even be an amb~ T. W. Stanton as. follows: monite. 

8831 (G.. C. M. No. 3). Gravina Island 1;1ear 8830. Large Probably Triassic. 

limest()ne nodule in conglomerate. The wh0le nodule 8832 (G. C. M. No. 4). Grav~na Island, near south foreland 

is a nautiloid with deeply lobed suture, possibly refer- on arm of_" cove 3 miles north of Dall Head, " near conable to Cosmonautilus. On the back is a Rhyncho- · tact with conglomerate. 

nella (?). · Probably Triassic. · Undetermined coral f~agment. 

8833 (G. C. M. No. 5). Gravina Island, south arm of cove Pentacnnus sp., segment of column. 

· 3 miles nor:th of Dall Head. Near zo:Qe of nodular Proba~ly Triassic . 

. . masses of limestone in conglomerate. This lot appears 8834 (G. C. M. No'. 6). -Gravina Island, north arm of "cove 

. 3 miles north of Dall Head. " Massive limestone in to be Devqnian. reef west of cabin. frob ably about _100, feet below- 8704 . 

. .' .Mr. Mar£i:q.'·s opinion, with which the writer Corals, probably several genera. 

agrees, is that the Triassic conglomerate res.ts Cassianella sp. 

unconformably upon Devonian rocks, and the Myophoria?? sp. 

Devonian fossils from Threemile Cove ·were ob- Natica .sp. 

t_ ained either from boulderS in the con~lomerate Muichiso:hia? sp .. ...., . Triassic. 

or from unrecognized Devonian rocks occurring 8835 (G. C. M. No. 7). Gravina Island, near 8834, from 

in complex structural relations with the Triassic thin-bedded limestone interbedded with shal~ about 20 

beds. The beds. on Threemile and Thompson or 30 feet below 8834. 

coves are. faulted and also closely folded and. Corals; several genera represented. 

contorted.,, thus being thrown into. complex Spiriferimi? sp. Myophoria? sp, structural relations.. From· the wording of the Natica sp .. 

locality descriptio:q. . given ·above it would Turritella? sp. 

appear quite possible that the fossils were col- Pseudomelania? sp. 

lected from inCluded masses of limestone in the Tracliycera.c;? sp., small fragment. Triassic. · conglomerate. In 1916 A. H. Brooks and the 

writer· collected fossils from ·narrow bands of The follo,wing collections made. by the wr-iter 

limestone definitely interbedded with the con- in 1915 from the limestone overlying the mas- . sive conglomerate were ~etermined by Mr, glomer~te. J. B. Reeside reports. on these· a~ Stanton: 

follows·: 9531 (15 ACh 111). West coast of Gravina Island.. Cove 

9899 (16 ACh 136). Threemile Cove, Gravina Island. 2} miles nqrth qf Dall Head. 

·_Echinoid spine, possibly of Cidari8. Age undetermined. E~hinoid spines. · 

Fossils were first collected from Threemile' Undetermined corals . 

. Cove in 19.06 by the Wrights} rhe exact lo-- g~;~~es~? sp. 

· cality i_s uncertain, but the ma.trix of the fos- p t ec .en sp. 

siliferous rocks is described as calcareous schist, · N atica? sp. 

which is evide~tly -the silic~ous limestone. Turritella? sp. . 

Tl f d db G H a· · · . _ The fossils in this lot are poorly preserved and . · le a una was regar e · Y · · · Irty as· com... mostly fragment~ry. They ar_e apparently Meso1 Wright, F~·:E: and·c. w·.,- op. cit., p. 52.. zoic and may be·Triassic. . 

•• 
'. -

GRAVINA AND REVILLAGIGEDO ISI.ANDS, ALASKA. 93 

15 (ACh 121). Northeast shore of Thompson Cove, Gravina '9532. (15 ACh 113). Threemile Cove, Gravina Island, 300 

Island. yards southeast of north end of cove and about 700 yards 

Corals, genus undetermined. north of 15 ACh 112. Fossils from black slate and limePentacrinus sp. stone. 

Echinoid spine. The black slate shows fragmentary specimens of 

Ostrea? sp. Halobia c£. H. superba Mojsisovics, of Upper.TriasSolemya? sp. ·sic age, while the limestone shows imperfect speciTurritella? sp. Jl!eils which seem to be referable to Aucella and 

Spiriferina? sp. hence is probably of Upper Jurassic age. 

Probably Triassic. The limestone and black slate from which 9534 (15 ACh 122). Northeast shore of Thompson Cove, 

Gravina Island. the fossils regarded as Upper ·Triassic and 

This lot includes undetermined ·corals, Ostrea?,, Upper Jurassic were obtained are apparently 

· 'l'urrite~la?~ and undetermined· gastropods. Prob- interbedded. This anomalous occurre;n.ce may 

. ablyTnassic. . . be. due to very closely infolded Jurassic 

9535(15ACh123). Northshoreof.ThompsonCove,GraVlna k b t "t b bl h Island. roc s, u 1 appears ·more pro a_ e t at 

Pecten? sp. the doubtful ~urassic forms are not co~rectly 

Gryphaea? sp. identified. 

U

Hal

1

obia. s~. · d. 't . 9533 (15 ACh 115). Thompson Cove, Gravina Island. nc etmm1ne ammoru e. H 1 b' 1 t · f lt bl k T · · a o 1a s a es 1n au oc . nas~c. Cidaris? sp., imprint of surface. 

9536 (15 ACh 129). Fivemile Cove, Gravina Islap.d. , Halobia cf. H. superba Mojsisovics. 

Limestone apparently overlying massive conglomerate. . '.frias.sic. 

Corals, several undetermined genera. 

Bryozoa? 

Pecten sp. 

Purpurina?? sp. . 

Undetermined slender gastropod. 

Probably Triassic. 

9537 (15 ACh 130). Fivemile Cove, Gravina Island. 

Corals, several genera. 

Bryozoa? 

Myophoria sp. 

Purpurina ?? sp. 

Turbo? sp. 

Triassic. · 

9538 (15 ACh 132). Fi~emile Cove, Gravina: Island, immediately north of 9537. 

. Gryphaea sp. 

Probably _Triassic. 

Two lots of fossils collected by P. S. Smith 

fron1 the black slate of Threemile Cove (8704). 

and also from Fivemile Cove (8705) were subInitted to :Mr. Stanton, who reported as follows: 

The only fossil species recognized in these two lots is a 

Halobia, which is closely related to if not identical with 

H. super~a Mojsisovics, an Upper Triassic species. The· 

:focN8 from which these collections came are therefore referred to t>ohe Triassic. · . 

The writer n1ade the following collections in 

1915 frmn the black slate and interbedded 

lhnestone and sandstone beds of Threemile 

Cove, Thompson Cove, and Dall Ridge. Stanton reports: · 

15 ACh 112~ Threemile Cove, Gravina Island, 300 yards 

north 'of 15 ACh 111. 

Spiriferina? sp. 

Halobia? sp. 

Probably Triassic. 

i~- ACh 124. Dall 'Ridge, 1 miles north of Dan Head, 

.Gravina Island. 

Echinoid spines, etc. 

Probably Triassic. 

The following collection was. identified by 

J. B. Reeside: 

9900 (16 ACh 137). Threemile Cove, Gravina Island. 

Cerithium? sp., fragment showing part of tw:o 

. whorls. 

Isastraea cf. I. profunda Reuss. 

Thamnastraea cf. T. rectila.mellosa Winkl. 

Spongiomorpha? sp. 

Hydrozoan, undetermined. 

Natica? sp . 

These forms are apparently of the Upper Triassic 

coral fauna referred by J. P. Smith to the Noric. · 

. The conglmnerate s~ndstones near the crest 

of Dall Ridge contain Devonian fossils, but as· 

these are fragmentary and occur in pebbles 

they indicate nothing more than the postDev<;>nian age of the roc_ks. . Kirk reports: 

l5 ACh 126. Dall Ridge, 4 miles north of Dall Head. 

Fragmental fossils apparently occurring as pebbles in conglomerate. ·At least one of the fossils is 

Devonian. It is a coral, probn.biy Alveolites. 

Black slates and sandstone.s exposed on the ' 

west coast of Bostwick Iillet .are correlated, on 

structural grounds a.nd lithologic similarity, 

with the upper part of the Triassic rocks on 

the west coast of the island, but fossils collected 

by different men have served to confuse the 

relations. The fossils listed below were collected in 1914 by-Martin and Overbeck from 
94 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

the west shore of Bostwick Inlet and identified 

by T. W. Stanton: 

wick Inlet, ~hich is unknown elsewhere in 

Alaska, may mark a Lower Triassic horizon, 

8836 (G. C. M. No. 8). Gravina Island, Bostwick Inlet, also unknown in Alaska. He· states: · · 

west' shore near entrance. From angular nodules in a As Mr. Girty points out, this fauna is entirely· unlike 

(.;, brecciated (?) nodular limestone. any other from Alaska. It therefore affords little aid in 

Terebratula sp. solving the stratigraphic position of the beds on Gravina 

Spiriferina? sp. Island. Certain significant deductions, however, suggest 

Pecten sp. · themselves. In general in southeastern Alaska the known 

Plicatula? sp. Carboniferous· rocks are limestones and not lithologically 

Cassianella sp. similar to the rocks of Gravina Island. The rocks here 

Myophoria sp. discussed are much less dynamically metamorphof3ed and 

Myophoria or Trigonia sp. are less intensely deformed than the known Devonian or 

Nacula sp. . Carboniferous rocks of the region. Their relation to the 

Astarte? sp. andesitic agglomerates and flows fits in better with an 

Arcestes? sp.. assumed· Mesozoic age tJ:tan with a Paleozoic age. The 

Triassic. Thisassemblagesuggeststhefaunaofthe almost uninterrupted succession ofbeds,from a basal(?), 

lower ·part of the Modin formation in California, conglomerate to shales containing unquestioned Triassic 

which was tentatively assigned to the Jurassic. · fossils on the southwestern coast suggests that the doubt-

. Near this same locality P. S. Smith 1 also col- fulJy determined genera from that loca;lity, provisionally 

lected fossil~ on which Girty reports as follows: assigned to the Carboniferous, justify the suggestion p1ade 

Three lots Hom Bostwick Inlet, Gravina Island, contain by Mr · Girty at that time that they may mark a Triassic 

horizon. No Lower Triassic is known in Alaska. Is it not . round crinoid stems (13AS170) and a ~ingle compressed possible that this fauna from Bostwick Inlet, which is 

pelecypod (13AS169), suggesting the genus Posidono'f!l,ya. unknown elsewhere in Alaska, fits into this as yet unfilled 

The third lot (13A~l71) contains a pelecypod fauna interesting and varied but entirely new to me. As these fossils gap? 

at best show only the shapes and some of them the sculp- Its reference to the Lower Triassic, however, 

ture, andaspele~ypodsofsimilarexternalappearancemay seems improbable, for the fauna is in the upper 

beiongto widely different genera, the identifications mad<;J part of the formation, above the position of 

here are offered with doubt. Though similar doubt tacitly · 

surrounds many pelecypod identifications in faunal lists the HaZobia-bearing slates (determined as 

and elsewhere, I have expressed it in this case by the use Upper Triassic) and immediately below the 

of question marks because the age of the whole fauna i~ Upper Jurassic or. Lower Cretaceous volcanic 

involved in such uncertainty. In lot 171 five types are rocks. Its reference to the Upper Triassic or 

represented more or less abundantly and by specimens the Lower or Middle Jurassic would be mor~ more or less good. These are Glossites? cf. G. lingualis, 

Schizodus? cf. s. appressus, Paracyclas? cf. P. ellipticus, in keeping with the observed relations. 

Crenipecten? cf. C. crenulatus, Elymella? cf. E. nuculoides. The term "Vancouver series" was first apBesides these, however, there are a good many indeter- plied by Dawson 2 to. the entire mass of volcanic 

minate forms, some of which suggest the genera Leda, materials and interbedded limestones, flaggy 

Chonetes??, Pseudomonotis??, Aviculipecten, and Nucula. argillites, and quart~ites that unconformably 

The. age of the fauna is quite uncertain. you declined underlie the Cretaceous rocks of vancouver 

for the time being to admit it into the Triassic, and nothing. 

resemblingithasthusfarbeen broughtinfrom theAlaskan Islands. At that time, on the evidence. of 

Carboniferous. J'he generic and specific resemblances intercalated fossiliferous beds, this series of 

suggested above might indicate a Devonian fauna, but rocks was regarded as largely Triassic, although 

neither .. has any Devonian fau,na related to this been h 'b'li f · · 1 d·- b th J · obtained from.Alaska .. ThR,genus Chonetes, if definitely t e possl 1 ty 0 Its· ·Inc u mg 0 ura<sslC 

present, would at least lim.lt the geologic age to the upper .and Carboniferous rocks was recognized. The 

Paleozoic, but the specimen is so imperfect that even "Vancouver series" was later subdivided by 

the identification as a brachiopod is doubtful. Clapp,S who has shown that it is in part JurasThus the age of this, the best fauna in the collection, sic and possibly also Carboniferous. The Tri:-

must for the present remain undesignated. It would.be assic rock~ of Gravina Island are probably the 

'highly rash to attempt any definite age determination or conclusion for the other lots, GOntaining, as they do, for the· equivalent ·of parts of the "Vancouver series" 

most part, only crinoid stems (even that being doubtful of'Vancouver and Queen Charlotte islands. 

in some c~ses), though they may tentatively be placed in Triassic rocks are widespread throughout 

the Paleozoic. _Alaska and occur at many places in southe.astRegarding 'this report Smith suggests that ern Alaska. Fossils of Upper Triassic age 

this doubtfully determined fauna from Bost-

.1 Notes on the geology of Gravina Island, Alaska: Prof. Paper 95,p. 

103, 1915. ' 

~Dawson, G . .M:., Canada Geol. Survey Ann. Rept. 1886, p. lOB. 

a Clapp, C. H., Southern Vancouver Islapd: Canada Geol. Survey 

Mem.'I3, p. 37, 1912. 
GHA VIN A AND HEVILLAGIGEDO ISLANDS_; ALASKA. 95 

hnve been found pn· Admiralty, Kupreanof, and an upper series of mixed water-laid tuffs 

and Screen islands. and black slates and limestone, with porphyMartin 1 states that the .abundant corals in ritic basic rocks of :similar composition, evisoine of these Triassic rock:; of Gravina Island dently partly intrusive i:md partly explosiv_e. 

suggest that they 1nay represent the lower No shm:p line separates the two p~rts, which 

Noric coral fauna of Iliamna Lake.· I-Ie con- the· writer has. not differentiated on the geosiders that the Triassic limestones of south- logic map. The black slates pass into green 

eastern Alaska, which in general are charac- fissile tuffs, and· these into more massive varieterizod by faunas contal.ning Iialobia cf. Ii. ties,, so that as a rule· a sharp line of co:n,tact 

superba Mojsisovics, correspond at least· ap- can not be drawn 'between them. By more 

proximately in position to the Chitistone llme- detailed. work the lower purely volcanic matestone of the Copper River valley. · rial might be separated from the upper mixed 

sedimentary and igneous material. The limeUPPER TRIASSIC OR JURASSIC ROCKS. stone bodies are too small to be shown on a 

Tho· rocks designated greenstone and slate ·map. 

on· tho map are made· up of an interbedded The base of the volcanic rocks is evidently 

series of altered tuffs, flows, and black slates, the coarse breccia that overlies the Upp~r Triwith some intrusives·, occurring on Gravina assic sediments of the west coast of Gravina 

and Revillagigedo isl~nds. The greenstones. Island. This breccia occupies the southern . 

and slates overlie the Upper Triassic rockcs with part of Dall Ridge and portions of California 

apparent conformity. ·on the evidence of a Ridge. It· is a fine-grained green rock with 

few fossils found in the intercalated sediments large angular fragments set in a dense matrix 

and on st:ructural grounds and analogy with of the same composition. 

rocks oi known age .th.ese rocks are regarded as Tho breccia passes upward into finerUpper Triassic ~r Jurassic. grained tuffs, in part at least water-laid material with which are interc~lated black slates 

DISTRIBUTION. and thin beds of crystalline limestone. The 

rrho greenstone and slate occupy· three par- black slates are typical clay slates having a 

allel belts on Gravina Island. One extends ·well-developed cleavage, with bedding planes 

f:roni South Vallenar Point to Seal Cove;· discernible in places. The slates are plainly 

n.nother fr01n the east shore of Bostwick Inlet conformable with the green tuffs. 

to Vallenar Point, and the· third along Tongascs The black slates and green bedded ,tuffs are 

Narrows from Old Indian Town to Gravina best developed along the north shore of TonPoint. Northeast of Tong ass Narrows similar gass Narrows from Ketchikan ·to Mountain 

rocks extend from Radio station to I-Ierring Point. The· black slates are in places conBay. siderably metamorphosed and schistose and 

CHARAC'l'El't AND STl'tA'l'IGRAPHIC RELATIONS. might more correctly be called phyllites. The); 

are closely interbedded with and grade into 

The rocks consist essentially of volcanic fissile green tuffs, both of which are interbedded 

breccias and tuffs with interbedded .black with more massive tuffs. The green tuffs are 

slates and water-laid fine-grained tuffaceous also schistose and in places· completely recryssediinents. Certain green schistose tuffs ap- tallized. T4e greater part of the town of 

pnrently interbedded with the more massive .Ketchikan stands on rocks of this character, 

.tuffs n.nd breccias 1nay be older infolded rocks, and excavations for street building show both 

and son1e graywackes and slates may be tl~e blocky and fissile types. The most schisyou:nge:r infolded rocks. tose types are con1pletely recrystallized and 

The detn.iled sequence of the volcanic rocks consist of secondary quartz, feldspar, calcite, 

of Gravina Island has not been worked . out. €pidote, and chloritic material. . The massive 

In general they may be divided into two ·types consist essentially of secondary epidote, 

parts-a lower series of purely igneous material, 'hornblende, chlori~e, sericite, and· calcite, with 

mainly ·coarse pyro.clastic ·rocks and breccias pyroxene and plagioclase feldspar crystals 

1 Martin, G. d., 'l'riassic rocks of Alaska: Goo!. Soc.Amorica Bull., vol. 

27, p. 700, 1916. . 

almost entirely replaced by secondary min-. 

orals. 
96 SHORTER CONTRIBUTIONS TO GENERAL QEOLOGY, 1918. 

The schistose green tuffs referred · to by 

Brooks 1 as greenstone schist and by Smith 2 as 

the green schist riear Ketchikan, were regarded 

by both as Paleozoic: According to the present 

interpretatjon of the structure of Gravina and 

Revillagigedo islands these tuffs stratigraphically overlie the massive igneous rocks· carrying Jurassic fossils and occupy a position near 

the top of the formatipn. Their schistose nature is believed to be due to contact meta.: 

morphism induced by the intrusive masses of 

_Revillagigedo Island, as indicate-d by the increase of schistosity toward th~ igneous rocks 

on the northeast. 

The base of the green~tone and slate series 

is evidently the coarse breccia found along the 

west coast of Gravina Island. It overlies the 

Upper Triassic· sediments with apparent conformity.. As previously brought out, the 

breccia grades upward into a more dominantly 

sedimentary series of green tuffs· and. black 

slates which are . unconformably overlain by 

conglomerate, slate, and grayw-acke of Upper 

Jurassic or lower Cretaceous age. · 

STRUCTURE. 

On Gravina Isl~nd the volcanic rocks of 

Dall Ridge occupy the trough of an overturned syncline, and the other two .areas-one 

extending from Vallenar Point to Bostwick 

Inlet and the other on Tong ass Narrows.:_ 

form the two limbs ·of another overturned 

syncline, the trough of which contains the 

Upper Jurassic or Lower Cretaceous sedimentary rocks of Blank Inlet. The volcanic rocks 

on the northeast shore of Tong ass Narrows are 

also believed to be overturned· and to overlie 

stratigraphically the phyllites of Ward Cove 

and George Arm and the Carboniferous schist 

to the northwest, which seem to overlie the 

volcanic rocks. 

·oRIGIN. 

'rhe volcanic breccias at the base of the series 

are. evidently the product of volcanic explosions. The perfect stratification of 'much of 

this igneous material ·and the conformable 

intercalations of slate and limestone show that 

at least portions of it were deposited or collected in water. The pillow-like structure of 

some of the flows that occur with the frag1 Brooks, A. H., Preliminary report on the Ketchikan mining district, 

Alaska: U. S. Ge:ll. Survey Prof. Paper I, p. 48, 1902. 

2 Smith, P. S., op cit., p. 100. 

mental rocks indicates that they also were 

submarine. 

. AGE AND CORRELATION.· 

·The age determination of the greenstone 

and slates rests upon rather slender paleontologic foundation. The collections from Bostwick Inlet were submitted to Mr. Stantcn, 

who reports as follows: 

9528 (15 AQh 104). Bostwick Inlet, Gravina Island .. 

9529 (15 ACh 105). Bostwick Inlet, Gravina Island. 

Aucella sp. 

Probably Upper Jurassic; possibly Lower Cretaceous. 

9530 (15 ACh 106). Bostwick Inlet, Gravina Island. 

Pecten? sp .. 

Not sufficient for determining age but probably 

from same formation as 9528 and 9529. 

These fossils were collected from the west 

coast of Bostwick "rnlet about half a mile 

south of the localities from which the Triassic 

and- doubtful collections were made. The 

fossils occur in sedimentary beds appareu"tly 

interbedded with the volcanic breccias and 

overlying the Triassic sediments, but the 

relati?ns are somewhat obscured by the gravel 

covering. 

Some correlations with rocks of known age 

are suggested. The greenstone and slate series 

extends more or less continuously along the 

mainland and adjacent islands of sou the astern 

Alaska. In the Juneau region the recent 

detailed work of Spencer and Eakin shows a 

section similar _to that exposed along Tongass 

Narrows and adjacent islands, and Brooks, 

who has studied both sections, believes that the 

black slate and. greenstones of Ketchikan· 

should be correlated with the andesi tic flows 

and black slates of Juneau, which carry 

Triassic fossils. The :finding of Upper Triassic 

fossils by Spencer and Eakin 3 in the slate and 

greenstone series of Juneau suggests to the 

writer t.he possibility that the rocks of .Tongass 

Narrows are of similar age. Smith 4 notes the 

similarity between the volcanic rocks of Gravina Island and the augite melaphyres of the 

region north of Juneau, referred by Knop{5 

to the _Upper Jurassic or Lower Cretaceous, 

and. it is not at all 1:1nlikely that the upper 

portwn of the slate and greenstone series of 

the Ketchikan district may be the equivalent 

3 U. S. Geol. Survey Thirty-eighth Ann. Rnpt., p~ 90, 1917. 

• Smith, P. S., op. cit., p. 164. 

. 5 Knopf, Adolph, The Eagle River region, southeastern Alaska: 

U.S. Geol. Survey Bull. 502, p. 1~, 1912. 
• 

•• 

GRAVINA AND REVILLAGIGEDO ISLANDS, ALASKA. '97 

·of the volcanic rocks associated with the stone contains igneous material. The Maude 

Berners formation. The slate and greenstone formation is regarded . on fossi( evidence as 

of the Ketchikan district may thus be corre- Upper Triassic and Lower Jurassic. 

lated with both the Triassic and Jurassic rocks The conformably overlying Yakoun formaof the Juneau region. tion consists of. water-laid agglomerates, and ' 

Rocks of. greater value .for purposes of tuffs, conglomerates, lavas, and possibly intrucorrelation occur on Vancouver and Queen s1ves. Fossils contained in the intercalated 

Charlotte islands. The ''Vancouver series," sediments indicate a Jurassic age, suggesting 

frrst defined by Dawson as Triassic, was later correlation with the Tuxedni sandstone. (Midsubdivided by Clapp 1 on the basis of distribu- dle Jurassic). 

tion, lithology, and structure into th.e Nitinat The correlati@n of the lower p·art of the valformation, Vancouver. volcanics, Sutton for- · canic rocks of Gravina Island with the Vanmation, Sicker series, and Metchosin volcanics. couver volcanics and tP,e upper part of th.e 

l-Ie uses the term "Van~ouver group" in Maude formation is suggested. · The upper part· 

place of "Vanc'Ouyer series" as used by Daw- of the volcanic rocks of Gravina Island may 

son~ The Nitinat· formation is made up of be the ·equivalent, at ·least in part, of the Sicker. 

highly altered calcareous sediments and is serie~ and the Yakoun formation. The age 

regarded as Jurassic or Triassic ( ~). The of the Gravina Island volcanic ·rocks is by no 

:vancouver volcanics are composed largely of means certain. · It is not at all unlikely that 

volcanic materials with interbedded sediments. they contain both Triassic and Jurassic rocks 

On the evidence of fossils collected from the and possibly some infolded Paleozoic rocks 

intercalated limestone beds ·of the Sutton for- also. · · · 

mation the Vancouver volcanics are regarded . · by Clapp as lowermost Jurassic. The Jurassic UPPER JURASSIC OR LOWER CRETACEOUS ROCKS. 

age of DISTRIBUTION AND CHARACTER. these rocks, however,. is not accepted 

by Martin,2 who considers that the fossil 

evidence is not conclusive and that the Sutton. A formation consisting· essentially of int~rlimestone fauna may with equal confidence be bedded conglomerate, graywacke, and black 

called Upper Triassic. The Sicker series is slate exposed along a wedge-shaped strip 

made up of meta_morphic, chiefly schistose extending from Blank Inlet to the northeast 

sedimentary and volcanic rocks, with intrusive coast of Gravina Island (see Pl. VIII,·B and 0) 

porphyrites. The stratigra.phic .relation of this is determined on fossil evidence to be -Upper 

"series" to the "Vancouver group" of which Jurassic ·or Lower Cretaceous, and similar 

it forms a part is not certain, but it is believed rocks just northwest of Ward Cove are correto occur above the Vancouver volcanics. lated with this formation .. 

On Graham Island, one of the Queen Char- The conglomerate does no~ a~pear to be the 

lotte group, the ''Vancouver group'' \vas sub- basal member ~ut ~ccurs at Int~r;als "throughdivided by MacKenzie a into the Maude forma- ou~ the formatiOn In~erbedded w1th the graytion, referred to the Lower Jurassic and possi- wa~ke and slate. !tIS composed of rather fine 

bly in part to the Triassic, and the Yakoun J grained pe?ble~ With a few bould~rs ~s much 

formation, of Middle Jurassic age. The :Maude as a foot In diameter. The matnx IS essenformation is composed of. a lower part, made tially a graywacke but contains a considerable 

up of banded slaty and flaggy argillites, in amount of igneous rock fragments. The 

places carbonaceous, ~nd an uppel· part of pebbles and cobbles are mostly igneous and 

feldspathic sandstones. Partly crystalline bi- resemble the volcanic rocks of Gravina Island. 

tuminous massive limestones are provisionally Peb~les of dioritic rocks resembling the pluclassed with the Maude formation. Near the tonic rbcks of. the Coast Range also occur. 

top of the formation, ·where it begins to grade No members of this formation are known. to 

into the overlying Yakoun formation, the sand- be of igneous origin, although both matrix 

1 Clapp, c. H., op cit., p. 37. and cobbles of some of _the conglomerates 11 Martin, G. c., op. cit., P· 709. contain so great an amount of igneous material 

s.MacKenzio, J. D., Geology of Graham Island, B. C.: Canada Geol. 

survew Mem. 88, p. 39, 1916. tha~ they resemble agglomerates. 
98 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

The conglomerate passes by increasing fine-. 

ness and ·decrease of igneous material into 

. typic.al-looking graywacke with angular slate 

fragments. · Microscopic examination of the 

graywacke shows it to consist essentially of 

quart~ and feldsp~r grains with angular fragfossil collection from which only one form was 

identified. Mr. ,Stanton reports: 

9527 (15 ACh 99). Blank Inlet, 1! miles north of Blank 

Point, Gravina Island. 

Belemnites sp. 

Jurassic or Lower Cretaceous. 

ments of slate and decomposed particles of Smith 1 correlated the conglomerates and 

dioritic rocks, hornblende, pyroxene, and ·other sediments on the west coast of Gravina 

epidote. Flakes of biotite may be. secondary, Island with these somewhat similar ones on 

but no marked schistosity has developed. Blank · Inlet. However, besides the fossil 

The slates are very 'fissile biack clay slates· evidence which now shows that the two are 

and are closely interbedded ~ith the _gray- distinct, there are very marked differences. in 

wacke members. The slates have a marked the lithology. The conglomerates of Threemilecleavage. (See Pl.. VIII, 0.) Cove are composed essentially of a peculiar 

STRATIGRAPHIC RELATIONS. 

These Upper Jurassic or Lower Cretaceous 

beds overlie the volcanic rocks of Gravina Island 

described above and regarded as Tri,assic or 

Jurassic, but in most places the relations are 

rather obscqre. · On the no~theast shore of 

. Bostwick Inlet near the entrance the slates 

and graywacke overlie. the er<~ded -surface of 

the volcanic rocks with marked unconformity. 

On the northeast shore of Gravina Island con-

. glomerates· and graywacke overlie the volcanic 

rocks without apparent angular unconformity. 

The conglomerate and graywacke of Ward 

Cove are noticeably unconformable upon the 

black phyllite that has been _correlated. with 

the U :pper Triassic rocks. 

STRUCTURE. 

The conglomerate, slate, and graywacke beds 

of Blank Inl<:t occupy the trough of an overturned syncline in which are also inv-olved the 

volcanicroc~ of Gravina Island, which actually 

o·verlie them on the east shore of Blank Inlet. 

· type of igneous rock that was not noted on 

Blank Inlet, where the conglomerates contain 

a great amount of greenstones and dioritic 

rocks similar to the ·jntrusives of the Coast 

Range, types neither of which was observed 

in the· conglomerate of the west coast. The 

seqimentary beds .of both Bostwick Inlet and 

the west coast contain beds ·of siliceous sandstone and quartzite but no appreciable ·amount 

of graywacke. On Blank Inlet the reverse is 

true; the arenaceous beds are dominantly 

graywacke, and pure quartzose beds are lacking. Structural differences are also very apparent. The conglomerate of Blank Inlet is 

considerably . sheared; the roc~s of the west 

coast are massive. This might appear to. indicate a greater age for the rocks of Blank Inlet 

but the greater amou.llt of deformation is no 

doubt due to the difference in composition· of 

the rocks, the soft agglomeratic material being 

much more easily affec-ted than the granite. 

It is a noticeable fact that th~ few diorite 

boulders in the conglomerate of. Blank Inlet 

have withstood the deformation. 

ORIGIN. The rocks of· the Queen Charlotte series, 

These Upper Jurassic or Lower Cretaceous with which these beds of conglomerate, slate, 

conglomerates, graywack~s, and slates are the and graywacke on- Gravina Island were at first 

products of normal deposition in a shallow sea. correlated, are now determined as Upper OreThe constantly changing character of the de- taceous 2 and are thus younger than any 

posits indicates corresponding changes of con- known rocks of Gravina Island. The conglomditions cluring their deposition. The materials . erate, slate, and graywacke of Blank Inlet 

of the rocks were apparently derived in part are possibly the equivalent of the Berners 

from the volcanic rocks of Gravina Island near formation of Eagle River, the conglomerate, 

by and in part from the· su:ITounding regions. graywacke, and slate of Admiralty I~land and 

other places in southeastern Alaska, and t.he 

AGE AND CORRELATION. Valdez groll:p of Prince William Sound. 

The age determination of the slates and 

graywackes of Blank Inlet rests on a single 1 Smith, P. S., op. cit., p. 104. 

2 MacKenzie; J. D., op. cit., p. 65. 
• 

I 

~RAVI~ A AND REVILLAGIGEDO IS;LANDS, ALASKA. 99 

TERTIARY ROCKS. . I i~ ~h~ elevated wave-cut be~ches found in·this 

The only rocks of Tertiary age within the VICinity. That part of Gravina Jsland south of 

area consist of rhyolite, which occurs on the Dall Head, the bold pror~10ntory th~t mar~s 

peaks of Dall Head and ~xte:q.ds from Thomp- the south .end of the h~ghlands o~ the Isl~nd_, Is, 

son Cove nearly to Seal Cove. It is a light- a flat plmn {Pl. VIII, A). This and similar 

gray dense rock composed essentially of quar.tz features on Annette Island and ·at other places 

and feldspar. It· overlies unconformably the near by are regarded .as wave-cut benches.: 

granite and greenstones of Dall Head and has S~aller features of the same character are lownot shm·ed in the mineralization of the under- lying, flat be~ches .along .the c<;>ast, some a few 

lying rocks. It was regarded by th0 Wrights feet above·.high tid~ and others only partly. 

an :l Smith as Tertiary. above. water .. The higher benches are .forested 

c · and dissected by erosion, but others near the 

QUATERNARY ROCKS. water level are bare of vegetation and apparGLACIAL DEPOSITS. . ently have been only recently uplifted, and it 

DISTRlBUTION AND CHARACTER. 

The most recent sediments are the. glacial 

deposits. Rocks of this character are not 

abundant but have been found at one place on 

Gravina Island. About half a inile ·northwest 

of the cabin at the head of Dall Bay is a small 

exposure of glacial material about 80 feet 

above sea level. The outcrop is concealed on 

all sides by vegetation. The exposed base 

contains about 2 feet of glacial till and blue 

clay and is overlain by stratified beds of glacial 

gravel about 5 or 6 feet thl.ck. Fossils occur 

in both the blue cln.y and the stratified gravel. 

AGJ~ AND CORRELATION. 

A s1nall collection of fossils from the glacial 

deposits was sub1nitted to W. H. Dall, who 

reports: 

is not at all unlikely that this emergence has 

continued to the present time. As the benches 

are developed only locally, however, it is pro b-

. able that th~ uplift was of a differential charac:. 

ter and .that some parts of the region are sink- · 

ing below sea level and others are rising, owing 

to the warping nature of the movement. 

MARINE GRAVELS.' 

During the period in. which. the fossiliferous 

deposits:of Dall Bay stood at sea levei the low 

depression. that .connects_ Vallenar Bay and 

Bostwick Inlet was evidently in large part 

_flooded if not whol'y covered. It is thus safe 

to assume that the gravels that mantle the 

floor · of this '' through'' valley are largely 

marine, although partly reworked by later 

stream action. The valley floor is covered by 

vegetation which hides the deposits. 

INTRUSIVE ROCKS. 

The intrusive rocks of Dall Jiead were not 

examined in detail by the writer. On the west 

opposite Juneau the sa.me formation extends to an altitude coast of Gravina Island they are apparently 

of 200 feet. The species c9ntained in Mr. Chapin's col- dioritic in composition and along the contact 

lection a.re as follows: 

I have examined the material from Dali Bay, Gra~ina Island. The material is a "boulder clay" which disintegrates in water, and when it is strained off the fossils remain in a clean condition. Near the Treadwell mine 

Cylichnella n. sp. 

'Bela violacea. Ivlighels. 

Colus sp., fragment .. 

Cardium (Cerastoderma) ci.Iiatum Fabricius. 

Axiuopsis viridis Dall. · 

Macoma calcarea Gmelin. 

Saxicava arctica Linne. 

Mya intermedia Dall. 

Balanus sp., fragment. 

The assembly naturally indicates the colder 

. climate associated with the. period of glaciation. 

These fossiliferous deposits occur about 80 

feet above high tide, a position that indicat~s 

ttn uplift of the land since the glacial epoch. 

Corroborative proof of a recent uplift is found 

with the sediments· are gneissoid. This structure is regarded as purely cataclastic; the 

- gneissoid effect is comn1only noted in the diorites at other place;; in the Ketchikan district 

and is .not regarded as evidence of an older 

gneiss. on· the extreme southern point of 

Gravina Island, however, Smith noted gneisses 

which he regards as Paleozoic, intruded by 

igne~:ms rocks. He states: 1 

At this place they consist of highiy metamo~hic gneisse~ 

and amphibolites which have been intruded. by igneous 

rocks. Undoubtedly the older rocks have been metamorphosed locally by the younger 'intrusives, but apparently . 

1 Smith, P. S., op. cit., p. 97. 
100 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

they were considerably metamorphosed dynamically before 

they were intruded. No definit~ determination of the age 

of the gneisses and amphibolites has been made. Judged 

by their physical character they seem to be much older 

than any of the known late Paleozoic rocks. Although 

their aspect may be due in large ~easure to the contact 

effects produced in proximity to the large intrusive masses, 

the impression gained in the field, and not dispelled by 

later investigations in the office, is that those rocks are 

probably at least as old as the middle or earlier part of the 

Paleozoic era. 

The relation.of the old gneisses and amphibolites to the 

other sedimentary rocks of the region is indefinite, because 

they occupy only a small area at the extreme end of the 

island and are surrounded on all sides by sea or by a large 

area of youngerigneous rock. No other rocks on Gravina 

Island are correlated with these rocks near Dall Head. 

· The great abundance of coarse angula,r 

blocks of gran.ite occurring in the Upper Triassic conglomerate suggests a near-by land mass 

of this rock. The gneisses noted above may 

be a remnant of this old land mass, but if so 

the granite must be locally metamorphosed, 

for the cobbles of the conglomerate are massive 

· and have suffered little metamorphism. As 

previously stated, the material of the conglomerate is lithologically similar to tlie granite of 

Annette Island and is supposedly from a stock 

of similar composition. 

The intrusives of Dall Bay and Seal Co~e ar-:. 

granites and associated pegmatites or alaskites. 

These are composed· essentially of quartz and 

orthoclase with a subordinate amount of mus:.-

covite and secondary sericite. They range iri 

texture from medium-grained holocrystalline 

rocks to fine-grained. aphanitic and porphyritic 

phases. They are evidently the equivalent of 

similar rocks occurring at several places on 

Prince of Wales· Island and other localities. 

Wherever the . relations were observed the 

granite has been found to be later than the 

quartz diorite and associated intrusives of the 

Coast Range, and is probably Cretaceous. The 

granite and quartz diorite of Gravina Island 

have not been separated on the map; 

The intrusive -rocks of Revillagigedo Island 

are quartz diorites and associated types and 

are to be correlated with ·the intrusives of the 

Coast Range. They occur . as batholithic 

masses cutting diagonally' across the sedimentary formations. · They are regarded as Middle 

or Upper Jurassic, as the conglomerates of 

Upper Jurassic or Cretaceous · age contain 

many boulders of similar rock, which is lacking 

in the earlier conglomerates. 

0 .• 

• 

•