Stratigraphy and Geologic History of Alaska
EA-I. (George O. Gates)
STRATIGRAPHY AND GEOLOGIC HISTORY OF ALASKA
CONTENTS
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Page
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Pre-Paleozoic Rocks
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1
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Paleozoic Rocks
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4
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Sedimentary
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5
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Cambrian System
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5
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Ordovician System
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5
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Silurian System
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7
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Devonian System
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9
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Carboniferous System
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11
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Igneous
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17
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Volcanic
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17
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Intrusive
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18
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Mesozoic Rocks
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19
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Sedimentary
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19
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Triassic System
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20
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Jurassic System
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22
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Cretaceous System
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24
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Undifferentiated Mesozoic Rocks
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29
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Igneous
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31
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Volcanic
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31
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Intrusive
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34
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Cenozoic Rocks
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38
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Sedimentary
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38
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Tertiary System
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38
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Quaternary System
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41
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Igneous
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41
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Volcanic
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41
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Intrusive
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43
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Geologic History
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44
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Pre-Cambrian
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44
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Paleozoic
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44
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Mesozoic
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45
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Cenozoic
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46
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Bibliography
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48
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EA-I Gates: Stratigraphy and Geologic History of Alaska
MAPS
This manuscript was accompanied by one colored Geologic Map of
Alaska. Because of the high price of reproducing such maps only a few
submitted will be used in Volume I. The selection of the number of these
maps will be determined by the publisher and the choice of those used
should be made in conjunction with a representative of the publisher.
All maps are, therefore, being held at the Stefansson Library until
a selection can be made.
EA-I. (George O. Gates)
STRATIGRAPHY AND GEOLOGIC HISTORY OF ALASKA
The following summary of the stratigraphy and historical geology
of Alaska is essentially a condensation of
Areal Geology of Alaska
, by
Philip S. Smith, published as U.S. Geological Survey
Professional Paper
192
^
,^
^
✓^
in 1939. Most of the statements should be considered as broad generalize–
tions and, therefore, may not be completely applicable to a specific locality
or set of conditions.
PRE-PALEOZOIC ROCKS
Regionally metamorphosed rocks, mainly of sedimentary origin, underlie
rocks of Paleozoic age in southeastern Alaska, east-central Alaska, parts
of the lower Yukon-Kuskokwim region, the Seward Peninsula, the Brooks Range,
and other parts of Alaska. It has not been possible to date these metamorphic
rocks with assurance, as the oldest overlying Paleozoic rocks of known age
are Orodovician, Silurian, or Devonian, except in east-central Alaska where
Middle Cambrian rocks overlie a thick series of the older rocks. These
older rocks in east-central Alaska are known as the Birch Creek Schist and
the Tindir group (somewhat younger), and are shown on the accompanying
^
Fig 1^
geologic map as pre-Cambrian in age. In other parts of Alaska old meta–
^
Fig 1^
morphosed rocks are not distinguished on the map from undifferentiated
Paleozoic or older rocks.
EA-I Gates: Stratigraphy and Geologic History
The Birch Creek schist underlies a large part of the area between the
Yukon River, Tanana River, and the Canadian border, and extends eastward
into Canada. It is also exposed along the north flank of the Alaska Range
in a belt a few miles wide that includes the Kantishna Hills and extends
east to and beyond the Tok River. As now used, the Birch Creek schist
includes all the older pre-Cambrian metamorphic rocks that were originally
of sedimentary origin. It is comprised principally of quartzite, quartzite
schist, quartz-mica
^
schist,^
mica schist, feldspathic and chloritic schists, and a
^
✓^
minor proportion of carbonaceous and calcareous schists and crystalline
limestone. Data on the Birch Creek schist are far too scanty to define its
stratigraphy in detail but, in general, the rocks are more quartzose in the
lower part of the sequence. Calcareous schist and limestone comprise only a
small part of the sequence and are restricted to its upper part. Fol
^
i^
ation
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✓^
is well developed in all of the rock types except massive quartzites.
Superposition of one cleavage on another indicates that the rocks of the Birch
Creek schist have suffered more than one period of regional metamorphism.
Although the structure is regionally complex, major cleavage tends to strike
N. 60° E. The thickness of the Birch Creek schist is not known, but it is
believed to be many thousands of feet. Its age is probably early pre-Cambrian.
North of Eagle, Mertie found that it is overlain by slightly metamorphosed
sediments, the Tindir group, which is turn underlies Middle Cambrian rocks.
The Tindir group of rocks in the area of the Tatonduk River, a tributary
of the Yukon River near the international boundary, is believed to be largely
or wholly pre-Cambrian. Rocks of the Tindir group differ markedly from the
Birth Creek schist in lithology and structure. The Tindir group forms a
sequence, 20,000 to 25,000 feet thick, of sedimentary rocks interbedded with
EA-I Gates: Stratigraphy and Geologic History
basic lavas and intruded by basic dikes and sills. The sediments are
principally dolomite, limestone, shale, slate, and quartzite. The upper
part of the sequence contains a group of redbeds: hematitic dolomite,
quartzite, chert, and tuff. All of these rocks are characterized by a high
content of hematite. In the few samples of tuffs collected by Merite the
maximum content was 27 per cent of ferric oxide (Fe
2
O
3
). Redbeds are also
known in the lower part of the sequence. In the middle and lower parts of
the sequence are amygdaloidal and ellipsoidal basic lavas, associated tuffs,
and basic dikes and sills. The upper part of the group may be Lower Cambrian.
The group may be correlative with the Belt series of British Columbia and
Western United States.
In central Alaska metamorphic rocks crop out in the following areas
^
Fig 1^
shown on the accompanying map as “Paleozoic and Older Rocks”: in the Kaiyuh
Hills, south of Ruby, between Poorman and Lake Minchumina, between Ruby and
Tanana, and between Stevens and Bettles on the Yukon and Koykuk rivers,
respectively. Taking all of these areas as a whole, schist, quartzite,
phyllite, greenstone, and limestone are common rock types. Limestone is
especially common in the areas along the south of the Yukon River. Some of
these metamorphic rocks are believed to be early Paleozoic and may be as young
as Ordovician and Devonian.
Others are certainly pre-Orodovician age. Litho–
logically, some of them bear a strong resemblance to the Birch Creek schist of
the Yukon-Tanana region. Hence, taken as a whole, these rocks may range in
age from pre-Cambrian to mid-Paleozoic.
A variety of regionally metamorphosed rocks in the Nome area, on the
Seward Peninsula, underlie little-deformed fossiliferous Or
o
dovician rocks.
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✓^
The lower age limit is not known but pre-Cambrian rocks may be included.
EA-I Gates: Stratigraphy and Geologic History
On the south flank of the Brooks Range, older sedimentary and igneous
rocks that have been regionally metamorphosed to a variety of schists, form
a belt that extends west from the head of the Chandalar River to the mouth
of the Kobuk River. Generally these rocks are highly micaceous and very
quartzose. They underlie the Skajit limestone of Silurian age. The lower
age limit is not known though there is no evidence to preclude the possi–
bility that it is pre-Cambrian.
The oldest igneous rocks known in Alaska are associated with the Birch
Creek schist. They are basic extrusives and perhaps instrusives that have
been metamorphosed to amphibolites, hornblende schists, and chlorite schists.
The Pelly gneiss, which occurs mainly in the eastern part of the
Yukon-Tanana region, comprises a second group of metamorphosed igneous rocks
associated with the Birch Creek schist. It consists mainly of granitic
rocks, but locally includes darker varieties of monzonitic, dioritic, and
gabbroic character. The age of the Pelly gneiss has not been determined
definitely. It is, of course, younger than the Birch Creek schist and it
is fa
ri
^
ir^
ly certain that it is pre-Paleozoic. Its age relation to the Tindir
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✓^
group is not known.
PALEOZOIC ROCKS
Paleozoic rocks in Alaska are largely limited to southeastern Alaska,
the Alaska Range, central Alaska between the Alaska Range and the Brooks
Range, and Seward Peninsula, and Brooks Range. Rocks of this era are
by far the most extensive of the rocks in southeastern Alaska, the Seward
Peninsula, and the Brooks Range. The Paleozoic sedimentary rocks of Alaska
contrast with the Mesozoic and Tertiary rocks in that limestone forms an
important and, in some areas, dominant part of the section whereas there is
EA-I.
^
G^
ates: Stratigraphy and Geoligic History
relatively little limestone in the younger rocks. Surface volcanic
activity was common in parts of Alaska at different times during the
Paleozoic era. Intrusive activity, however, was limited during this era.
Sedimentary
Cambrian System
. Cambrian rocks are known only in eastern Alaska near
the Canadian boundary north of the Yukon River, although they may be present
but unrecognized in other parts of Alaska. The sequence consists dominantly
of limestone and is of Middle and Upper Cambrian age. It has been divided
into four units, from oldest to youngest: Middle Cambrian beds of limestone,
600 to 800 feet thick; slate and quartzite, about 300 feet thick; limestone,
about 3,000 feet thick; and Upper Cambrian limestone, about 2,000 feet thick.
The Middle Cambrian limestone is mainly massive, white, finely crystal–
line and, in part, silicified. Oölitic limestone and limestone conglomerate
are also present. The Upper Cambrian limestone is mainly massive, white,
and coarse-textured in its lower part and brownish-gray, fine-textured with
prominent bedding plane
z
^
s^
in its upper part.
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✓^
Ordovician System
. Sedimentary rocks of Ordovician age are known in
southeastern Alaska, central Alaska, and the Seward Peninsula. Except in
southeastern Alaska, limestone is the dominant rock.
In southeastern Alaska, Ordovician rocks are exposed in scattered
localities on Prince of Wales Island and nearby islands, and in a nearly
continuous belt along the western coast of Kuiu Island. In all these places
the sequence consists dominantly of greywacke with slate and andesitic
volcanic rocks. Less commonly, conglomerate, grit, limestone, black chert,
and
s
quartzite are present. Fossils, mainly graptolites, indicate a range
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✓^
in age from Lower to Middle Ordovician.
EA-I. Gates: Stratigraphy and Geologic History
Near the Canadian border, north of the Yukon River, Ordovician
rocks overlie Cambrian rocks without structural discordance. In places
these rocks are limestone, very similar to the underlying Cambrian. In
other places they are dark gray to black, thin-bedded, siliceous slates and
cherts.
Black slates and shales of lower Ordovician age are exposed along the
northwest flank of the White Mountains, about 50 miles north of Fairbanks.
These rocks grade downward without stratigraphic break into older metamorphosed
rocks.
Ordovician fossils have been identified in limestones in the Ruby and
McGrath areas. These rocks occupy a large area at the head of the Sulukna
River, a tributary of the Kowitna River. In part they are dolomitic lime–
stones. Highly metamorphosed crystalline limestones are exposed along the
banks of the Yukon, both above and below Ruby. No fossils have been found
but from other data they are believed to be no younger than Ordovician.
Northeast of McGrath, a large body of limestone is exposed in a belt 7 to
8 miles wide and more than 30 miles long. Both thin-bedded and massive
limestones occur, the former predominating. Some calcareous sandstone, shale,
and chert are intercalated with the thin-bedded varieties. The series has
a minimum thickness of 5,000 to 7,000 feet. It contains fossils of Ordovician
and of Silurian age. Data available are not adequate, however, for drawing
the boundary between the two systems.
The most extensive area of Ordovician rocks in Alaska is in the western
part of the Seward Peninsula, where these rocks are exposed over an area of
2,000 square miles. The rocks are dominantly limestones, the Port Clarence
limestone, and total at least 5,000 feet in thickness. In the western part
EA-I. Gates: Stratigraphy and Geologic History
of the area they are little metamorphosed except locally in the vicinity
of granitic intrusions. Eastward these limestones become progressively
more metamorphosed and locally contain thin beds of graphitic and other
schists.
Silurian System
. The distribution and kinds of Silurian rocks are
very similar to the Ordovician. The Silurian rocks are widespread in
southeastern Alaska, central Alaska, and the Seward Peninsula. They are,
however, also widespread farther north along the southern part of the Brooks
Range. Limestones and related calcareous rocks are everywhere the dominant
types except in southeastern Alaska where much of the section consists of
clastic rocks.
Silurian rocks are exposed in three main areas in southeastern Alaska:
Dall and other islands off the west coast of Prince of Wales Island, the
northern part of Prince of Wales Island, and the Glacier Bay areas. These
rocks range in age from early to middle Silurian and possibly later. The
Silurian rocks in southeastern Alaska can be divided into four units which
differ in kind of rock and age. These are, from oldest to youngest: (
1
)
greywacke with black slate and small amounts of conglomerate and lime beds,
thickness undetermined; (
2
) andesite and andesite-porphyry lava flows, con–
glomerate, and some greywacke, tuff, volcanic breccias, and limestone,
thickness about 3,000 feet; (
3
) thick-bedded, dense limestone in which locally
are thick beds of conglomerate, thin-layered limestone, nodular shaly
argillaceous limestone, and sandstone, total thickness about 4,500 feet;
and (
4
) green-gray greywacke with sparse conglomerate beds interbedded with
red, gray-green, and gray greywacke-like sandstones, and small amounts of
shales, totaling at least 5,000 feet thick.
EA-I. Gates: Stratigraphy and Geologic History
Two facies of Silurian rocks have been recognized along the Porcupine
River in east-central Alaska. The lower and thicker facies consists of
buff magnesian limestones, estimated to be 2,500 feet thick. The upper
facies is shale, in its upper part fissile black shale, and interbedded dark
siliceous limestone. Fossils in the magnesian limestone show that it is
middle Silurian about equivalent to the Niagaran of the central United States.
The shales are either late Niagaran or Cayugan in age.
Middle Silurian limestone, probably not less than 3,000 feet thick,
^
Fig. 1^
is exposed in the two belts shown on the geologic map, west and north of
Fairbanks. This limestone is called the Tolovana limestone. It is in
part dolomitic.
As discussed under Ordovician rocks, the limestones in the areas of
Ruby and McGrath are in part Silurian.
The Silurian limestone in the southern part of the Brooks Range is
called the Skajit limestone. It is a prominent lithologic unit of the
range. This formation consists almost exclusively of massive, bluish-gray
limestone that is somewhat silicified. Except in the valley of the Alatna
River where Silurian slate, schist, and thin-laminated limestone overlies
the Skajit limestone without stratigraphic break, the Skajit limestone is
the uppermost unit of the Silurian. The few collections of fossils obtained
from the Skajit limestone indicate an age no older than middle Silurian,
though part of it may be upper Silurian.
Silurian rocks may be present in the Alaska Range in the western part
of the belt of undifferentiated Paleozoic rocks that crosses McKinley Park.
A belt of argillites in part interbedded with greywacke and chert underlies
Devonian rocks and overlies Ordovician rocks. No reliable thickness is known,
EA-I. Gates: Stratigraphy and Geologic History
but the thickness of these rocks is estimated to be about 5,000 feet. The
structure is complex and, in part, the rocks are schistose.
Though it is possible that they are present at other places on the
Seward Peninsula, Silurian rocks have been identified only at White Mountain
a short distance southeast of Council, and near Cape Deceit on the north
coast of the peninsula. At both localities the Silurian rocks are magnesian
limestones several hundred feet thick. They have been assigned to the latter
part of the Silurian age. Both occurrences are isolated by surface materials
and the relation of the limestones to older and younger rocks is not known.
Devonian System
. Devonian rocks are widespread in southeastern Alaska.
Because of the lack of detailed mapping and other factors, it has not been
possible to correlate closely sections exposed at different places. The
general sequence, however, is believed to be about as follows from oldest to
youngest: conglomerate, greywacke, and sandstone with locally interbedded
limestone, total thickness about 2,000 feet; interbedded limestone, slate,
chert, andesitic lava, brecci
s
^
a^
, tuff, and locally conglomerate; andesitic
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✓^
lava, in part with pillow structure, breccia, tuff, and locally rhyolite
lava, total thickness 2,000 feet; gray to green andesitic tuff and greywacke,
locally with fine conglomerate, intercalated limestone, and minor amounts of
andesitic lava and breccia, total thickness 2,400 feet; limestone member,
600 feet thick. Except possibly the upper part of the limestone unit, these
rocks are all assigned to Middle Devonian. Upper Devonian rocks in south–
eastern Alaska consist of basalt, andesite, tuff, limestone, sandstone,
slate, and conglomerate.
The area between the Yukon and Tanana rivers and the international
boundary contains several large tracts of Devonian rocks. The stratigraphic
EA-I. Gates: Stratigraphy and Geologic History
sequence of these rocks can be divided into two parts: a lower part of clay
slate, siliceous slate, chert, and quartzite, with minor amounts of limestone
and conglomerate; and an upper part of volcanic rocks, basaltic flows, and
related tuffs. All of these rocks are believed to be Middle Devonian, ex–
cepting possibly some Upper Devonian in the upper part of the sequence.
The total thickness is estimated to be 10,000 feet.
In the valley of the Porcupine River, Middle Devonian massive light-gray
to blue limestone, 325 feet thick, is overlain by brownish shales at least
several hundred feet thick, believed to be of Upper Devonian age.
Devonian rocks are widespread in the Brooks Range. In the eastern
part of the range, north of Wiseman and in the vicinity of Arctic, these
rocks consist of fine dark-gray quartzite, black flint, calcareous black
shale, and impure limestone, cut by basaltic dikes. The age is not certain
but they are believed to be Upper Devonian. Farther west in the Brooks Range,
the Devonian rocks form a monotonous assemblage of quartzite, sandstone, and
slate with subordinate amounts of conglomerate, coarse grit, and a little
limestone. These rocks have been deformed to the stage where characteristic
fracture cleavage has developed. Close folding has no doubt duplicated parts
of the section. The width of outcrop is sufficient, however, to allow the
estimate that these rocks are at least several thousand feet thick.
Devonian rocks have been recognized in the Alaska Range in places all
the way from the vicinity of McKinley Park to the international boundary.
A unit of Middle Devonian limestone is the most definite as to age. In
places associated with the limestones are clastic rocks of less-certain age,
but believed to be Devonian. These rocks include conglomerate, slate, shale,
greywacke, quartzite, thin-bedded and massive limestone. The thickness of
EA-I. Gates: Stratigraphy and Geologic History
these rocks may be as much as 10,000 feet. In the extreme eastern part of
the range near the international boundary, Middle Devonian rocks consist of
basic lavas, agglomerates, and tuffs associated with considerable black
shale and minor amounts of greywacke. Fossils in this sequence seem com–
parable to ones in southeastern Alaska.
Some fossils of Devonian age have been found in the thick sequence of
limestones northeast of McGrath. These were described under Ordovician
rocks as some of them are of that age.
No rocks on the Seward Peninsula have been definitely identified as
Devonian. At a few localities, however, the fossils contained in limestones
indicate possible Devonian age.
Carboniferous System
. Rocks of the Carboniferous system are widespread
in Alaska. In addition to the rocks known to be of Carboniferous age, it is
likely that rocks formed during this period are present in areas now mapped
as undifferentiated Paleozoic. All of the known marine Carboniferous rocks
belong to either the Mississippian series or the Permian series with one
exception, a massive Pennsylvanian (?) limestone at one locality in south–
eastern Alaska.
Rocks of the Carboniferous system are distributed throughout southeastern
Alaska. The localities where these rocks are known are scattered and too
^
Fig. 1^
small to be shown appropriately on the accompanying geologic map. It is
probable that a considerable part of the belt of metamorphic rocks on the
mainland along the southwest boundary of the Coast Range batholiths is of
Carboniferous age. The main areas of Mississippian rocks are near Klawak
on the west side of Prince of Wales Island, in the northern part of Kuiu
Island, and in the Iyoukeen Peninsula on the east side of Chichagof Island.
EA-I. Gates: Stratigraphy and Geologic History
The beds consist of coarsely crystalline, highly fossiliferous limestone and
black, tin-layered chert overlain by interbedded, dense, gray quartzite and
cherty limestone with sparse sandstone, conglomerate, and limestone.
Pennsylvanian rocks are limited to a massive, white, marine limestone
at least 100 feet thick on the island in Soda Bay on the west coast of
Prince of Wales Island.
Permian rocks in southeastern Alaska can be divided into two units.
The lower unit comprises a variety of rocks: basaltic and andesitic flows,
tuffs, and volcanic breccias, limestones, sandstone, and shale. The upper
unit is more uniform and consists of white limestone and intercalated layers
of white chert. The most representative sequence of Permian rocks is exposed
on the Cornwallis Peninsula at the northern end of Kuiu Island, and on the
Keku Islands. In general, faunas of the Permian rocks of the southeastern
Alaska are more like those from Russian localities than those in the United
States.
In the Copper River region, Carboniferous rocks form two belts that
trend west-northwest. The southern belt, south of the Wrangell Mountains,
follows the Chitina River west across the Copper River to and beyond Klutina
Lake. The northern belt forms the eastern part of the Wrangell Mountains
and extends northwest along the southern flanks of the Nutzotin Mountains
and the Alaska Range to and beyond the Richardson Highway. The rocks com–
prise three units. The lower unit, of Mississippian age, consists of slate
and schist with minor amounts of crystalline limestone, cherts, tuffs, and
basalt flows. The middle unit, of Permian age, is a great thickness of lava
flows, tuffs, and volcanic breccias, interstratified with limestone, shale,
sandstone, and conglomerate. The upper unit is a great thickness of basaltic
EA-I. Gates: Stratigraphy and Geologic History
lava flows called the “Nikolai greenstone.” It is considered to be Permian
in age though part of it may be Triassic.
Although Carboniferous rocks may be present in other parts of interior
Alaska, they are best known in its eastern part, in a belt northwest and
north of Fairbanks, along the Yukon River upstream from Circle, along the
Porcupine River, and at the head of the Sheenjek River in the Brooks Range.
The Carboniferous rocks in the belt northwest and north of Fairbanks
have been divided into three more or less distinct units, from oldest to
youngest: Livengood chert, undifferentiated Mississippian rocks, and
Rampart group. All are Mississippian in age. As the name implies, the
Livengood chert consists dominantly of chert with a few interbeds of lime–
stone and argillaceous rocks; total thickness is probably several thousand
feet. The structure is complex and the sequence is overturned from the south.
The undifferentiated Mississippian rocks are, at least in part, younger than
the Livengood chert and older than the Rampart group. They consist of shale,
sandstone, chert, and some limestone. In some places deformation has been
sufficiently intense to form schist and slate. Limestone is more abundant
in the upper part of the section and, where it is predominant, is considered
a separate overlying unit. The Rampart group consists mainly of ba
l
saltic
^
✓^
flows, tuffs, and breccias, altered to greenstone, with interbedded chert,
shale, slate, and sandstone. The Paleozoic volcanic rocks west and south
of Circle are similar to, and are believed to be correlative with, the Rampart
group. The total thickness of the Rampart group has not been determined but
it may be as much as 10,000 feet.
Near the international boundary, Carboniferous rocks are known along the
Yukon and Nation rivers. The Calico Bluff formation, apparently the oldest
^
,^
EA-I. Gates: Stratigraphy and Geologic History
is believed to be Mississippian in age and to overlie stratigraphically
the Rampart group. It is essentially a sequence of alternating beds of
marine limestone and shale, with some slate. Total thickness is about
1,300 feet. In contrast to most of the known Mississippian rocks in
interior Alaska, rocks of the Calico Bluff formation are highly fossili–
ferous.
Overlying the Calico Bluff formation and best exposed along the Nation
River is a sequence of clay shale, sandstone, and conglomerate called the
Nation River formation. In contrast to most Carboniferous rocks in Alaska,
it is of continental origin and contains bituminous coal in pods and kidneys.
Present evidence suggests that the Nation River formation may be Pennsylvania.
The Tahkandit limestone, the uppermost formation of the Carboniferous
system, consists mainly of limestone, with interbeds of conglomerate,
sandstone, and shale in the lower part. It is Permian and is about 600 feet
thick. The abundant marine fauna these rocks contain are apparently more
closely related to Asiatic than to North American Permian faunas.
Carboniferous rocks along the Porcupine River apparently grade downward
into Devonian rocks. Beds similar to the Calico Bluff formation have been
recognized. On the basis of fossils contained, other parts of the section
are Permian.
Carboniferous rocks are known in the area at the headwaters of the
Sheenjek and the east for, of the Chandalar rivers. At the base of the
sequence is a unit of chert and slate, with associated basic intrusives.
The age is not known definitely, but is Carboniferous or late Devonian.
Conformably overlying this unit is a unit of quartzite, conglomerate, and
slate. The grains and pebbles are predominantly chert and vein quartz, and
EA-I. Gates: Stratigraphy and Geologic History
are set in a highly siliceous matrix. It is believed to be early Mississippian.
At the top of the Carboniferous sequence in this area is a limestone unit as
much as 6,000 feet thick. Fossil evidence is limited but this limestone is
believed to be late Mississippian and correlative with the Lisburne limestone.
In western and southwestern Alaska, Carboniferous rocks are known in
widely spaced areas. These areas are relatively small compared to most of
the areas of Carboniferous rocks heretofore described. As more detailed
field work is accomplished, Carboniferous rocks no doubt will be recognized
^
Fig. 1^
in some of the areas shown on the map as undifferentiated Paleozoic.
At Cape Mountain near Cape Prince of Wales on the Seward Peninsula, at
least 1,500 feet of bluish t
o white limestone has been recognized as of
^
✓^
Carboniferous age. The few fossils collected, though they do not allow
specific determination, appear most similar to fauna of the Lisburne lime–
stone of Mississippian age. The limestone has been strongly deformed and
intruded by granite.
A prominent section of gray limestone is known in the western part of
^
Ref. to map^ the area across the Kuskokwim River from Aniak
shown
as undifferentiated
Paleozoic rocks. Fossils indicate a Permian age.
The area of Carboniferous rocks north of Goodnews Bay comprises dark–
gray limestone with some red and black slates. Overlying is a thick series
of argillite, sandstone, and greywacke, which is locally metamorphosed.
Assignment of these rocks to the Carboniferous era is based on stratigraphic
and li
g
^
t^
hologic evidence. Meager fossil evidence indicates Perm
^
i^
an age.
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✓✓^
Three units have been recognized in the area of Carboniferous rocks
north of Nuyakuk Lake in the eastern part of the Kilbuck Mountains. The
lower unit, probably as much as several thousand feet thick, consists
EA-I. Gates: Stratigraphy and Geologic History
dominantly of chert and chorty grit. Included also are conglomerate,
quartzite, shale, slate, argillite, and limestone. Sheared and semischistose
varieties of all these rocks occur. This unit is definitely pre-Permian and
is believed to be Mississippian. The middle unit, 500 to 1,000 feet thick,
is a yellowish limestone that grades upward through tuffacaous beds into
greenstone. Many well-preserved fossils show definitely that this limestone
is Permian. The upper unit, at least 4,500 feet thick, is mainly basic
lava flows altered to greenstone. At least the lower part of the unit is
Permian; but its upper part may range into the Triassic. It seems fairly
certain that this sequence of greenstone is correlative with the Nikolai
greenstone in the Copper River region. In its lithology, thickness, and
stratigraphic relation to Permian limestone,
^
it^
is very similar to the Ni
c
kolai
^
✓^
greenstone.
Mississippian rocks comprise a belt along the north side of the Brooks
Range that appears to be continuous throughout the length of the range.
Two formations have been recognized. The older, the Noatak formation, con–
sists of a few thousand to several thousands of feet of marine quartzitic
sandstone, chert conglomerate, chert breccia, shale, slate, and limestone.
Overlying is the Lisburne limestone which consists of several thousand feet
of marine beds: coarse bioclastic limestone, dense gray limestone, chert
layers in the upper part of the sequence, and black shale. One bed of
phosphatic rock, in part oölitic, has been recognized. These two formations
are conformable. Underlying the limestone in the Cape Lisburne area on the
west coast are Mississippian black shales and slates that contain several
coal seams.
In the eastern part of the Brooks Range, in the Canning River area,
the Lisburne limestone is confirmably overlain by the Sadlerochit formation,
EA-I. Gates: Stratigraphy and Geologic History
which comprises a few hundred feet of marine beds mainly of fine quartzite
sandstone. Northward, the formation thickens, coarsens, and becomes conglome–
ratic. The Sadlerochit formation is believed to be Permian though it may
include some Pennsylvanian.
Igneous
Volcanic
. Some occurrences of Paleozoic volcanic rocks have been mentioned
in connection with the description of sedimentary rocks. A more complete
separate treatment will be given here.
Early Paleozoic or older metamorphosed volcanic rock, the “Tota
lankia
^
tlanika^
” ^Totlanika^
schist, forms a belt 100 miles long on the north side of the Alaska Range
between the Kantishna and Little Delta rivers. These rocks
^
,^
feldspar schists
^
✓^
and gneisses, were originally rhyolites and rhyolite porphyries with tuffs
and some sedimentary beds. This sequence of rocks is older than Middle
Devonian and is believed to be early Paleozoic though it may be older.
In
the
southeastern Alaska volcanic rocks are recognized in each of the
systems of the Paleozoic era from Ordovician through Carboniferous. These
^
Fig. 1^
rocks are not shown on the accompanying geologic map as the areas of outcrops
are imperfectly known and small. The rocks are mainly andesitic and basaltic
flows and tuffs. Some basalts and rhyolites are also present. Included in
eash system are pillow lavas which indicate the flows were likely, at least
in part, submarine.
Three dominantly volcanic units are recognized in east-central Alaska.
The Fossil Creek volcanic of Ordovician age extend for about 40 miles along
the flanks of the White Mountains north of Fairbanks. The sequence, probably
not less than 2,000 feet thick, consists mainly of basic lavas, tuffs, and
breccias altered to greenstone. The Rampart group extends from Rampart
EA-I. Gates: Stratigraphy and Geologic History
eastward to and beyond Circle. It also consists dominantly of basaltic
lavas, tuffs, and breccias. Some sediments are interbedded. The total
thickness is unknown but may be as much as 5,000 to 10,000 feet. The age
is Carboniferous. The Woodchopper volcanics along the Yukon nearer the
boundary are basalfic lavas and tuffs altered to greenstone and are
Devonian.
Volcanic rocks of late Paleozoic or early Mesozoic age are known on
the south side of the Alaska Range between Denali and the Richardson Highway,
west of Slana, and on the south side of the Wrangell Mountains. In general
the rocks are basaltic flows and tuffs. On the south side of the Wrangell
Mountains, they comprise a sequence of basic
^
lava^
flows, the Nikolai greenstone,
which totals more than 6,500 feet in thickness. Fossils
of
^
in^
interbeds of
^
✓^
sedimentary rocks show that at least the lower part of the sequence is
Permian, and it is believed likely that most or all of the sequence is of
this age though it may range up into the Triassic. So little is known of
the rocks at the other localities mentioned that their age is even less
perfectly known though they may be more or less contemporaneous with the
Nikolai greenstone.
Intrusive
. The intrusion of igneous rocks formed only a small part of
the geologic history of Alaska during the Paleozoic era. These intrusive
rocks are largely basic and ultrabasic. No bodies of granitic intrusive
rocks are known. It may not be true, however, to say that none exist, as
Silurian conglomerates in southeastern Alaska contain boulders of granitic
rock. The source of these boulders has not been recognized. The age of the
intrusive from which they were derived would be early Paleozoic or pre-Paleozoic.
The best-known intrusive of the Paleozoic era form two discontinuous.
EA-I. Gates: Stratigraphy and Geologic History
linear belts in the Yukon-Tanana region. The larger begins near the
Tanana River between the Chena and Salcha rivers and extends easterly to
and probably beyond the international boundary. The second belt extends
northeast and southwest from Livengood. The rocks of these intrusive
are largely ultrabasic and basic in character and are, in large part,
altered to serpentine and greenstone. The time of the intrusions has not
been definitely determined. Available evidence, however, strongly suggests
late Devonian age.
MESOZOIC ROCKS
Sedimentary
Mesozoic rocks are widespread in Alaska and give evidence of marine
inundations of large extent during that era. Sedimentary rocks of Triassic
age are mainly limestone, chert, and some shale. Those of Jurassic and
Cretaceous ages are thick sequences of clastic rocks, shale, argillite,
slate, dark argillaceous sandstone (graywacke), sandstone, and conglomerate,
all mainly marine. Volcanic rocks, flows, and tuffs are present but are
largely restricted to the southern coastal ranges and southeastern Alaska,
and are most common in the lower part of the Mesozoic section.
In southeastern Alaska, Mesozoic rocks occupy two northwest-trending
belts. The eastern belt extends from Gravina Island north of Lynn Canal
and is separated from the Coast Range batholiths by a belt of Paleozoic
rocks except at its northern end, where it is in contact with the batholiths.
The western belt is along the west coasts of Chichagof and Baranof Islands.
The southern coastal ranges, the southwestern part of the Alaska Range, and
the Aleutian Range consist in part of Mesozoic rocks. In interior and
southwestern Alaska, much of the area between the Alaska Range and the Bering
EA-I. Gates: Stratigraphy and Geologic History
Sea and north from Bristol Bay to the Brooks Range is occupied by rocks
of Mesozoic age. In this region, rocks of Mesozoic age probably underlie
large parts of the unmapped areas, and areas concealed by Quaternary
sediments. Rocks of Mesozoic age crop out or underlie Quaternary and
Tertiary sediments over most of the arctic slope north of the Brooks Range.
Triassi
s
^
c^
System
. It is surprising that Lower and Middle Triassic
^
✓^
rocks are everywhere unrecognized or missing in the sedimentary sequence
of Alaska. Alaska was probably emergent throughout this gap in the record.
This time was not marked by orogeny, however, as Triassic rocks everywhere
are structurally conformable with the Paleozoic rocks beneath. Triassic
rocks are largely restricted to northern, southern coastal, and southeastern
Alaska. In southeastern Alaska these rocks are all Upper Triassic. They
have been identified on Gravina Island and at several localities north to
Juneau. In general, they are divisible into two unconformable units: a
lower unit, about 1,600 feet thick, generally of conglomerate, sandstone,
and limestone; and an upper unit, more than 1,400 feet thick, mainly of
andesitic volcanic rocks, including brecci
s^
a^
with limestone matrix and lava
^
✓^
flows which, in part, show pillow structure.
In southern Alaska, Upper Triassic rocks have been recognized in the
belt of Mesozoic rocks on the south side of the Wrangell Mountains. The
sequence is divided into three units, oldest to youngest: Chitistone
,
^
✓^
limestone, Nizina limestone, and the McCarthy formation. The Chitistone
limestone rests, structurally conformable, on the Nikolai greenstone, a
sequence of basic lava flows at least 6,000 feet thick. The earliest of
these flows was extruded in Permian time but the extrusions may have continued
into Triassic. The Upper Triassic sediments grade upward from thick-bedded
EA-I. Gates: Stratigraphy and Geologic History
limestone (Chitistone limestone), into thin-bedded limestone and shale
(Nizina limestone), and into the black shale and argillite of the McCarthy
formation. The Upper Triassic sequence totals about 5,000 feet in thickness.
Elsewhere in southern Alaska, Upper Triassic rocks are known at several
localities in a discontinuous belt from Shelikof Strait north and east to
the south side of the Alaska Range: at Puale Bay (formerly Cold Bay) near
Kanatak, in the area of Kamishak Bay, near Iliamna Lake and Lake Clark, on
the east side of Cook Inlet from Port Graham north to
e
^
a^
nd along the east
^
✓^
shore of Kachemak Bay, in the valley of the Chulitna River, in the Talkeetna
Mountains, and east of Denali on the south side of the Alaska Range. At the
localities in the northern part of this belt, the rocks are mainly limestone,
shale, and argillite. In the southern part of the belt, volcanic tuff and
contorted chert are present as well as limestone.
Upper Triassic limestone is known on the south shore of Lake Nuyakuk
north of Bristol Bay.
Triassic rocks are practically unknown throughout interior Alaska.
Thin-bedded limestone and calcareous shale and oil shale of Upper Triassic
age are known in eastern Alaska near the mouth of the Nation River and on
Trout Creek, north of Eagle. Oil shale collected by a prospector from the
Christian River north of Fort Yukon contained Upper Triassic fossils.
A belt of Upper Triassic rocks extends along the north front of the
Brooks Range from Cape Lisburne east at least as far as the Sadlerochit
River. Though the section differs in detail from place to place, at the
localities studied, bedded chert, limestone, oil shale, and some thin beds
of sandstone are the common types of rock. This sequence may range up to
2,000 feet in thickness. Everywhere it rests with structural conformity on
EA-I. Gates: Stratigraphy and Geologic History
Paleozoic rocks, which, throughout most of the belt, are Mississippian
limestones (Lisburne limestone). A thin sequence of Upper Triassic and
Jurassic shale and fine sandstone was penetrated in a test hole on the east
shore of Dease Inlet, southeast of Barrow. These rocks are beneath a
sequence which consists mainly of Tertiary and Cretaceous sediments about
6,000 feet thick and rest on steeply dipping, slightly schistose argillite
of probable pre-Cambrian age. It is probable that Upper Triassic rocks
underlie much of the arctic slope north of the Brooks Range.
Jurassic System
. Rocks of this period are confined to the country
bordering the Pacific Ocean and to northern Alaska. No rocks of this age
have been recognized in interior Alaska between the Alaska Range and the
Brooks Range. Rocks of probable Jurassic age in the belt bordering the
Pacific Ocean from Kodiak Island to southeastern Alaska are discussed under
the heading “Undifferentiated Mesozoic Rocks” as, in most places, they
cannot with certainty be distinguished from Cretaceous rocks. Jurassic
rocks are best known in the belt along the southeast side of the Alaska
Peninsula from Tuxedni Bay southwest to Chignik Bay. Here is to be seen
probably the most complete sequence of Jurassic rocks in North America.
In the northern part of the Cook Inlet belt, the oldest Jurassic rocks
comprise 1,000 feet or more of porphyry tuffs, and basaltic and andesitic
lavas. These rocks are overlain by the Tuxedni sandstone of Middle Jurassic
age. This marine formation is composed of sandstone and sandy siltstone and
includes conglomerate, grit, and arkose, the coarser types being more common
in the lower part of the sequence. The total thickness is about 8,000 to
9,000 feet though it varies considerably. Several of the oil seeps known in
this part of Alaska are in areas where Tuxedni sandstone crops out. Upward
EA-I. Gates: Stratigraphy and Geologic History
the Tuxedni sandstone grades into black and reddish argillaceous siltstone
in which are some sandy and calcareous beds. This formation is about 2,000
feet thick and is called the Chinitna siltstone. It is overlain by the
Naknek formation which is comprised of sandstone, shale, conglomerate,
arkose, and tuff.
Southwest from Kamishak Bay along the Alaska Peninsula, the over-all
characteristics of the Jurassic sequence is comparable with the section
just described. An exception is at Puale Bay, where the lowest members
are limestone and calcareous sandstone.
Jurassic rocks crop out on the east shore of Cook Inlet south from
Seldovia. These rocks are tuff and volcanic agglomerate interbedded with
thin strata of marine limestone, shale, and sandstone.
The eastern part of the Talkeetna Mountains is occupied by a Jurassic
sequence of fossiliferous marine rocks that are closely correlative with
the Jurassic rocks bordering Cook Inlet. The Talkeetna formation at the
base consists of lava, breccias, and tuff, interbedded with some sandstone
and shale and resembles the Jurassic section near Seldovia. Overlying in
succession are the Tuxedni sandstone, Chinitna siltstone, and Naknek forma–
tion, all closely similar to their Cook Inlet equivalents in lithology
and paleontology.
In northern Alaska, Jurassic rocks, all marine, are known in at least
two places: a thick section of black shale (Kingak shale) in the area of the
Canning River, and a thin section of fine sandstone and shale near the bottom
of the test hole on the east shore of Dease Inlet. It is possible, however,
that the thick sequence of black shale, greywacke, and siltstone overlying
Triassic rocks in the foothill belt north of the Brooks Range may include
EA-I. Gates: Stratigraphy and Geologic History
Jurassic rocks which have not been recognized because of the sparseness
of fossils. At least a considerable part of this sequence, however, is
known to be of Lower Cretaceous age.
Cretaceous System
. In the Cretaceous period the sea spread over a
much
p
^
l^
arger part of Alaska than during Jurassic time. In addition to
^
✓^
covering the sites of Jurassic marine sedimentation in southeastern,
southern, and northern Alaska, the sea occupied at one time or another much
of the rest of Alaska, though at no one time were all parts submerged simul–
taneously. As during the Jurassic period, deposition of clastics was the
dominant type of sedimentation.
Most of the post-Triassic and pre-Tertiary rocks in southeastern
Alaska and southern coastal Alaska are barren of fossils or contain, at
some places, only meager assemblages that furnish inconclusive evidence of
age. For these reasons, rocks of this age in these two areas will be dis–
cussed under the heading “Undifferentiated Mesozoic Rocks.” Only those rocks
whose Cretaceous age has been determined with considerable certainty are
considered here.
In the upper part of the Chitina Valley, north of the Chitina River
and south of the Wrangell Mountains, Cretaceous clastic marine sedimentary
rocks are exposed within an irregular area, fifty miles long and twenty
miles wide. These rocks rest on an irregular erosion surface cut across
folded older rocks. Although the Cretaceous rocks show marked changes
laterally, the general sequence can be described as follows, from bottom
to top. Conglomerate or grit, which in most places fills the depressions
in the old land surface; sandstone and sandy shale, 300 to 500 feet thick;
black shale not less than 3,000 feet thick, fairly homogeneous in its western
EA-I. Gates: Stratigraphy and Geologic History
part but containing much red shale, thin beds of limestone, sandstone,
and conglomerate in its eastern part; conglomerate, sandstone, and sandy
shale, as much as 3,000 feet thick. In many places the beds of this
sequence are abundantly fossiliferous. For the most part the sequence is
Lower Cretaceous. Upper Cretaceous sediments may be present in an area
to the south and west.
In the eastern part of the Talkeetna Mountains, in the headwaters
of the Nelchina River, there are several patches of Lower Cretaceous rocks
that consist of a lower unit of tuff and conglomerate, 100 to 200 feet thick,
and an upper unit of limestone, as much as 400 feet thick (Nelchina limestone).
The two units are conformable and conformably overlie the Jurassic rocks of
this area. Some of the beds of the lower unit contain marine fossil
.
^
s.^
^
✓^
Upper Cretaceous rocks (Matanuska formation) are extensively developed
in the Matanuska Valley above Palmer. They comprise a sequence at least 4,000
feet thick, of which the lower half is mostly shale and the upper half is
interbedded sandstone and shale. The base of this series of beds has not
been observed but it
is probably rests unconformably upon an erosion surface
that cuts across rocks, ranging in age from Upper Jurassic to Lower Cretaceous.
The Cantwell formation of Upper Cretaceous age crops out in two east-west
belts in the Alaska Range. The northern and larger belt occupies much of the
northeastern part of Mount McKinley National Park and extends eastward along
the valley of the Yanert River to the vicinity of Mount Deborah. The southern
belt which is narrower and shorter extends southwest from Cantwell along the
southern boundary of the park. The formation is a series of continental
clastic sediments that show marked lateral variation. The series consists
of interbedded sandstone, grit, shale, and conglomerate, at least 4,000 feet
EA-I. Gates: Stratigraphy and Geologic History
thick. Locally the lower part is a massive basal conglomerate. In a few
places, beds of bituminous coal are present but none of commercial importance
has been found. The Cantwell formation rests unconformably on older Mesozoic
and Paleozoic rocks.
The Cretaceous rocks at Herendeen and Chignik bays on the lower part
of the Alaska Peninsula comprise a fairly complete sequence of the Cretaceous
system. The Staniukovich shale at the base, apparently unconformable on
Upper Jurassic rocks, is dominantly thin-bedded shale with lesser amounts of
sandstone and conglomerate. The thickness is more than 1,000 feet. The
fossils found are marine and indicate Lower Cretaceous age. The Herendeen
limestone, a cross-bedded, arenaceous, marine limestone, also of Lower
Cretaceous age, conformably overlies the Staniukovich shale. At the top of
the Cretaceous system and unconformably overlying the Herendeen limestone is
the Chignik formation, the lower part of which is a series of beds of marine
shale, 200 feet thick. Overlying is a unit, 300 feet thick, consisting of
shale with which are interlayered numerous beds of bituminous coal and some
sandstone. The top unit of Chignik formation is partly marine and consists
of conglomerate, sandstone, and shale. The Chignik formation is Upper
Cretaceous.
West of the 150th meridian, marine Cretaceous rocks form extensive
outcrops and are dominant in the area between the Brooks Range, the Bering
Sea, the Alaska Range, and the Aleutian Range. It seems probable that bedrock
is also dominantly Cretaceous in those parts of this area unexplored or over–
lain by Quaternary sediments. Rocks of this age are less common east of
the 150th meridian and are known mainly along the Yukon River near the
Canadian boundary (Woodchopper district), and along the Canadian boundary
between the Yukon and Porcupine rivers.
EA-I. Gates: Stratigraphy and Geologic History
Throughout this whole area, from the Bering Sea and Bristol Bay to
the Canadian boundary, the Cretaceous section is dominated by two lithologic
facies, a greywacke facies and a shale, argillite, and slate facies. Where
the base has been observed, these rocks rest unconformably on older rocks,
which in most places are Paleozoic. In the Tikchik Mountains, north of
Bristol Bay, they rest unconformably on Upper Triassic rocks. Relative
ages within the Cretaceous are not everywhere clear. However, east of the
152nd meridian, fossils indicate that the Cretaceous rocks in the Rampart
and Woodchopper districts are largely Lower Cretaceous, and of marine origin.
The shale facies, a black shale, predominates but the section contains thin
interbeds of gray to brown sandstone. Overlying these in the Woodchopper
district are coarser clastics, greywacke, sandy shale, and coarse conglomerate
at the top, the age of which are believed to be Upper Cretaceous. These rocks,
however, have not been clearly distinguished from early Tertiary rocks with
which they are structurally conformable.
Though the evidence is by no means definite, west of the 152nd meridian,
the Cretaceous rocks appear to be mainly Upper Cretaceous in age. The coarser
clastics, greywacke, and conglomerate are more prominent and comprise a
large part of the section in contrast to the Lower Cretaceous rocks of the
Rampart and Woodchopper districts. In many places the bottom of the section
is marked by a basal conglomerate. As a whole, the sequence is a monotonous
succession of greywacke and shale. Although abrupt lateral changes in the
lithology are characteristic, greywacke tends to predominate in the lower
part of the sequence, and shale tends to predominate in the upper part.
Parts of the sequence are of marine origin, and other parts that contain
coal beds are nonmarine.
EA-I. Gates: Stratigraphy and Geologic History
Accurate estimates of thickness of Creta
n
^
c^
eous rocks are not available
^
✓^
in the areas considered above. The thickness is at least several thousand
feet, however, and in places may be greater than 10,000 feet.
The largest area in Alaska known to be underlain by Cretaceous rocks
is that north of the Brooks Range. Rocks of this age are known to underlie
almost all of that part of Alaska north of the Brooks Range. They form the
bedrock almost everywhere within the Foothills province bordering the Brooks
Range. Though concealed by Tertiary and Quaternary sediments, seismograph
studies and drill tests show that they extend on northward and underlie
the arctic coastal plain. This assemblage of rocks has a maximum aggregate
thickness of about 25,000 feet in the Foothills province. Northward beneath
the arctic coastal plain, the thickness is 6,000 feet or less. This assemblage
of Cretaceous rocks, consisting almost entirely of clastic sediments is
divided into two groups. In the Foothills province, these two groups are
separated by an angular unconformity. Within this province, discordance
decreases northward and disappears beneath the arctic coastal plain where
the two groups seem to be conformable.
Two lithologic units, both entirely marine, have been recognized in the
older group within its belt of outcrop bordering the north front of the
Brooks Range, roughly the southern third of the belt of Cretaceous rocks
shown on the accompanying areal geologic map of Alaska. The lower unit is
largely dark-greenish graywacke with some black shale and lenses of dark
chert-pebble conglomerate. The upper unit in most of the outcrop belt is a
monotonous succession of thin beds of black argillaceous shale and silty
shale, and thin interbeds of siltstone and fine-grained graywacke. To the
south, however, it grades to graywacke and conglomerate. In its belt of
EA-I. Gates: Stratigraphy and Geologic History
outcrop this older group of Cretaceous sediments is about 12,000 feet
thick. Fossils are rare. Though it may include some Jurassic sediments,
it is believed to be in large part of Lower Cretaceous age.
The younger group of Cretaceous sediments in its belt of outcrop
(approximately the northern two-thirds of the belt of Cretaceous rocks
^
Fig. 1^
north of the Brooks Range shown on the accompanying map) comprises graywacke,
siltstone, shale, sandstone, conglomerate, and some tuff and bentonite.
The proportions of these rock type vary markedly within the column and the
whole section is marked by lateral change in lithology. The sequence is
in part marine and in part nonmarine. Parts of the latter contain numerous
coal beds. The results of drilling indicate that to the north, beneath the
arctic coastal plain, the younger group of Cretaceous sediments is largely
offshore facies of marine black shale. In the belt of outcrop, the younger
group is about 13,000 feet thick; northward, it thins markedly. The greater
part is Upper Cretaceous. The lower part is believed to be Lower Cretaceous.
Undifferentiated Mesozoic Rocks
In southeastern Alaska, the southern coastal ranges, and the Alaska
Range, there are large areas of Mesozoic rocks in which scarcity of fossils,
complexity of structure, or both, have prohibited determination of age
according to period with any degree of certainty. These rocks will be
described here.
Mesozoic rocks in southeastern Alaska, assigned to the Triassic on the
basis of fossils contained, have been described. Fossils are rare in the
rest of the rocks; no fossils have been found over large areas and across
large stratigraphic thicknesses. The fossils that have been found have not
allowed determination of age with certainty. In most places the best statement
EA-I. Gates: Stratigraphy and Geologic History
of age that can be given is that the rocks are either Jurassic or Cretaceous,
and more likely either Upper Jurassic or Lower Cretaceous. At a few locali–
ties it has been possible to make a somewhat more accurate determination of
age on the basis of fossils. Here the rocks are believed to be Lower
Cretaceous. These localities are: Pybus Bay in the southern part of Admiralty
Island, tip of Point Hamilton on northwest coast of Kupreanof Island, Etolin
Island, Blank Inlet on Gravina Island, and Slocum Arm on Chichagof Island.
All of the rocks in the two belts, ranging in age from Jurassic to
Cretaceous, can be divided into two general lithologic facies: (
1
) the older,
a predominantly volcanic facies consisting of schistose greenstone volcanic
rocks, mainly hornblende and rarely augite-porphyry brecci
s
^
a^
, with intercalated
^
✓^
tuff and flows, black slate, and graywacke; (
2
) a predominantly sedimentary
facies consisting of graywacke, black slate or shale, and conglomerate.
At some places thin beds of limestone are intercalated in the sedimentary
facies.
A wide belt of undifferentiated Mesozoic marine rocks is shown on the
^
Fig. 1^
map extending all the way from Kodiak Island, northeast through the Kenai
Mountains, and east through the Chugach Mountains to the headwaters of the
Chitina River. The islands of Prince William Sound are included in this belt.
This belt is composed of a monotonous assemblage of rocks that are strikingly
similar, both from place to place and from bottom to top. The series as a
whole consists of alternating beds of dark slate or argillite and graywacke,
from an inch to a foot or so in thickness. In places the bedding is so fine,
it is varvelike; in other places the beds are several feet thick; and in
some places as much as a hundred feet thick. Conglomerates are present but
form only a small part of the whole sequence. A few lenticular beds of
limestone are known. Locally the rocks are schistose. Strong folding and
EA-I. Gates: Stratigraphy and Geologic History
faulting and lack of key beds or horizons make it almost impossible to
determine stratigraphic relations between areas. As a whole, fossils are
rare in the assemblage; in great bodies of rock, none whatsoever have been
found. The ones found commonly do not allow accurate determination of
age. During the course of work within this belt, evidence has accumulated,
however, that indicates late Mesozoic age for these rocks. In fact, a
considerable part of the assemblage may be Upper Cretaceous in age. If
this is so, a perplexing problem is raised when the rocks in this belt are
compared to the adjacent belt of Mesozoic rocks that includes the Jurassic
rocks along the west coast of Cook Inlet, the Upper Cretaceous rocks of the
Matanuska Valley, the Jurassic and Lower Cretaceous rocks of the eastern
part of the Talkeetna Mountains, and the Triassic, Jurassic(?), and Upper
Cretaceous rocks along the north side of the Chitina Valley. The rocks of
the Kenai-Chugach belt are much more severely folded and show more advanced
metamorphism than rocks of the other belt, even though rock much older may
be present in the latter. This was due in part to the more argillaceous
character of the Kenai-Chugach belt. It seems likely, however, that rocks
of this belt were subjected to greater deforming stresses.
Extending southeast from Slana, Mesozoic marine rocks, several thousand
feet thick, form much of the Nutzotin Mountains. Typically they consist of
alternating thin beds of dark argillite and graywacke. To the northwest,
the beds are less than one inch thick; beds two inches thick are rare. The
beds are commonly in pairs; a sandy bed below grades upward into an argilla–
ceous layer. To the southeast, beds gradually become thicker; the graywacke
beds become coarser and the proportion of graywacke to argillite increases.
Diagnostic fossils tho
o
ugh few in number have been found in this sequence
EA-I. Gates: Stratigraphy and Geologic History
and show that Upper Triassic, Upper Jurassic, and Lower Cretaceous rocks
are represented. No stratigraphic breaks between these systems have
been recognized.
A discontinuous belt composed of a thick sequence of Mesozoic marine
sediments extends from Iliamna Lake northeast along the Alaska Range to
Cantwell. These rocks are dominantly argillite and graywacke, commonly
thin-bedded. They bear a striking resemblance to the rocks of the
Kenai-Chugach belt already described. Fossils are very rare; some are
definitely Jurassic and probably Upper Jurassic, others are definitely
Cretaceous and probably Upper Cretaceous.
Igneous
Volcanic
. Mesozoic volcanic rocks are practically restricted to the
southern coastal regions of Alaska and to southeastern Alaska. Volcanism
was much more widespread and intense in the early part of the era. It
gradually waned and became almost nonexistent in the last period. It has
^
Fig.1^
not been possible everywhere on the geologic map to distinguish volcanic
^
✓^
rocks of this era from sedimentary rocks.
Volcanic rocks belonging to the first two periods of the Mesozoic era
have been recognized in southeastern Alaska. Those of Triassic age are
known on the northern part of Kupreanof Island and the northeastern part
of Kuiu Island. They consist of andesitic lavas and breccia with limestone
matrix. Many flows show pillow structure. Those on Kuiu Island rest on
Permi
e
^
a^
n rocks. Those on Kupreanof Island rest on Upper Triassic sediments.
^
✓^
Near Juneau, the Gastineau volcanic group, about 5,000 feet thick, consists
of members of andesitic lava, calcareous slate, and andesitic tuff. The
slate contains Upper Triassic fossils.
EA-I. Gates: Stratigraphy and Geologic History
Volcanic rocks of doubtful Jurassic age are known in southeastern
Alaska near Ketchikan, on Gravina and Prince of Wales Islands, in the
Wrangell district, and in the Juneau district. In the Ketchikan district,
these rocks are mainly coarse andesitic tuffs which grade upward into
slate and limestone. Andesitic basaltic flows are included in the sequence
on Prince of Wales Island. In the Wrangell district, schistose breccia
with interbedded tuffs and flows are associated with slate and graywacke.
Two units of volcanic rocks in the Juneau district, the Thane group (the
older) and the Douglas Island group, have tentatively been assigned to the
Jurassic system. From the base upward, the Thane group, 5,000 feet thick,
consists of tuff and slate, limestone, and tuff. The Douglas Island group,
15,000 feet thick, consists of flows, tuffs, and agglomerate.
As already explained, the series of altered amygdaloidal basaltic lavas,
the Nikolai greenstone, in the Copper River area may in part be of Mesozoic
age.
^
Fig. 1^
As seen on the geologic map, Mesozoic volcanic rocks are widespread in
a discontinuous belt along the general
^
a^
rea of the Talkeetna Mountains, the
^
✓^
valley of the Susitna River, and Cook Inlet. Though the evidence is not
conclusive, these rocks in the Talkeetna Mountains are mainly Lower Jurassic.
They form a sequence about 1,000 feet thick in which are intercalated some
layers of sedimentary rocks. In the eastern part of the
M
^
m^
ountains the
^
✓^
sequence is dominantly andesitic breccias and tuffs, whereas in the western
part it is mainly basic lava flows. All of these rocks have been more or
less altered to greenstone and have been subjected to some folding and
faulting.
Greenstone tuffs, about 1,000 feet thick, form a belt along the west
EA-I. Gates: Stratigraphy and Geologic History
front of the Chugach Range in the vicinity of Turnagain Arm and Knik Arm.
These rocks lie with angular unconformity on Upper Cretaceous graywackes
and slates but are regarded as older than the early Tertiary rocks in nearby areas.
Mesozoic volcanic rocks are known on the east side of Cook Inlet near
Seldovia, though the areas are too small to show adequately on the geologic
^
Fig. 1^
map. At Seldovia, tuff and volcanic breccia, several hundred feet thick,
are associated with Triassic limestone. A series of Jurassic rocks
^
,^
2,000 to
^
✓^
3,000 feet thick, extends along the shore south from Seldovia to Point Bede.
This series consists of tuffs and volcanic agglomerates interbedded with
marine sedimentary beds.
On the west side of Cook Inlet, volcanic rocks are known at many places
from the Skwentna River on the north to Iliamna Lake on the south. Where
these rocks are intimately associated with sedimentary rocks they have not
^
Fig. 1^
been distinguished from the sedimentary rocks on the geologic map. Two more
or less distinct groups of the rocks have been recognized. The older group,
of questionable Triassic age, consists of greenstones formed by the alteration
of basic volcanic materials. These rocks form a discontinuous belt from the
northeastern part of Iliamna Lake to the head of Lake Clark. The second group
is widely distributed in discontinuous areas from the south shore of Iliamna
Lake to the Skwentna River. The age of these rocks has been tentatively
assigned to the Lower Jurassic. This group consists of tuffs, volcanic breccias,
lava flows, many of which are porphyritic, and locally interbeds of graywacke.
Petrologically the volcanic rocks include andesitic, rhyolite, and basalt.
Greenstones, believed to be Mesozoic, are known in the area of Prince
^
Fig. 1^
William Sound. On the geologic map, these rocks are distinguished from the
sedimentary rocks with which they are associated on Latouche and Knight
EA-I. Gates: Stratigraphy and Geologic History
Islands. In general, these greenstones range from fine-textured ellip–
soidal flows to more massive diabase.
Mesozoic volcanic rocks occupy a belt crossing the eastern end of
the Seward Peninsula. These rocks are mainly hornblende and pyroxenite–
andesite flows. Some brecci
^
a^
s of similar composition are present.
^
✓^
Interbeds of bentonite and volcanic tuff are common in the Upper
Cretaceous series of sedimentary rocks in northern Alaska, north of the
Brooks Range.
^
Fig. 1^
Intrusive
. The intrusive rocks as shown on the geologic map are not
distinguished with respect to age or kinds. They range in composition from
acidic to u
o
^
l^
trabasic. Although for many of the intrusions the evidence is
^
✓^
too meager to allow accurate dating, the over-all weight of evidence indicates
the bulk of the intrusive igneous rocks in Alaska formed in the time between
mid-Jurassic and mid-Cretaceous.
Intrusive rocks are a dominant geologic feature of southeastern Alaska.
They are part of a great composite batholith and its satellitic intrusions
that extent north 1,100 miles from the Fraser River in Briti
t
^
s^
h Columbia.
^
✓^
They form the backbone of the Coast Range. Outlying stocks, dikes, and
batholiths are believed to be of the same general period of intrusion as
the main batholith and genetically related to it.
The part of the Coast Range batholith in southeastern Alaska average
^
s^
^
✓^
50 miles in width and is 350 miles long. Taken as a whole, data on it are
meager, but more abundant for the southern half. The bulk composition
ranges from quartz-diorite to quartz-mon
s
^
z^
onite. From west to east across
^
✓^
the body, the rocks contain an increasing
emount of alkali f
o
^
e^
ldspar and
^
✓^
quartz and a decreasing amount of hornblende, biotite, and calcic feldspar.
EA-I. Gates: Stratigraphy and Geologic History
These changes are not gradual but abrupt between three main belts: (
1
) a
southwest border facies, 5 to 15 miles wide, of quartz-diorite; (
2
) a core,
15 to 25 miles wide, mainly of granodiorite but with quartz-diorite and
quartz-monzonite; (
3
) a northeast border facies of quartz-monzonite. Contacts
are rather abrupt and although there is little brecciation along contacts
between facies, certain features suggest that the batholith is a composite
of interlocking batholiths.
Satellitic intrusions are concentrated in the mainland and island are
east of Clarence Strait between Wrangell and Ket
^
e^
hik
e
^
a^
n, in the eastern part
^
✓^
of Prince of Wales Island, on Baranof and Chichagof Islands, and in the area
of Glacier Bay. Those in the Wrangell-Ket
^
c^
hik
e
^
a^
n area are mainly quartz-diorite
^
✓^
and grandiorite. The ones on Prince of Wales and Chichagof Islands are
mainly hornblende-diorite and some monzodiorite; quartz-diorite and grano–
diorite are also present. Characteristics of the intrusive rock and its
relations to country rock strongly suggest that substantial parts of the
hornblende-diorite bodies were formed by granitization or related processes.
The bodies on Baranof Island are mainly quartz-diorite with some granodiorite.
Intrusions of basic and ultrabasic rocks as stocks and sheetlike bodies
are found in all of the above areas though they constitute only a small part
of the total volume of igneous rock present. Some of the more notable of
these intrusions are: Mount Burnett, northeast of Ketchik
e
^
a^
n, stock of
^
✓^
dunite and pyroxenite with marginal gabbro and hornblend
e
^
ite^
; Salt Chuck,
^
✓
cp. orig. p. 45^
Kasaan Peninsula, synclinal body of copper-and palladium-bearing gabbro and
pyroxenite; Blashke Island, cylindrical mass of dunite surrounded by pyroxenite
and gabbro; R
u
^
e^
d Bluff Bay, east side of Baranof Island, stock of chromite-
^
✓^
bearing dunite and pyroxenite; Mirror Harbor on northwest side of Chichagof
EA-I. Gates: Stratigraphy and Geologic History
Island and Bohemia Basin on Yakobi Island, nickel-bearing noritic and
gabb
o
roic parts of composite stocks; Lituya Bay, thick-layered body of
^
✓^
norite, dunite, troctolite, anorthosite, and bronzitite.
Though the ages of all these intrusions, both acidic and basic,
have not been definitely determined, it seems likely that they range from
Upper Jurassic to Lower Cretaceous.
Bodies of intrusive rock are very common in east-central Alaska between
the Yukon and Tanana rivers. Many more
[] are known than could be shown
^
Fig. 1^
on the geologic map. Most of these rocks are believed to have been intruded
during the Mesozoic era. Granite and quartz-diorite are the most common
rock types. The granites in order of abundance are biotite granite, horn–
blends granite, and hornblende-biotite granite. These different types
^
Fig. 1^
have not been mapped separately. Intermediate types such as granodiorite
and quartz-mozonite have not been found.
Although it is clear that these bodies of granit
e
^
ic^
rock were intruded
^
✓^
after the Paleozoic era and before the Tertiary period, a more precise
determination of their age is difficult to make. Jurassic rocks have not
been found in the area.
Most of the larger intrusives in the Alaska Range are of granitic
compositon. These are post-Upper Triassic and pre-Terti
e
^
a^
ry. Some of them
^
✓^
cut the Cantwell formation, which is Upper Cretaceous, but do not out the
Eocene coal-bearing sediments, and hence are of very late Mesozoic age.
Quartz-diorite is the dominant intrusive rock of the Talkeetna
Mountains. Though available evidence indicates, it is not conclusive that
the batholith forming much of the Talkeetna Mountains was intruded in
mid-Jurassic. Lower Jurassic rocks are cut and Upper Jurassic conglomerates
EA-I. Gates: Stratigraphy and Geologic History
contain boulders similar in composition to the intrusive rock.
^
Fig. 1^
Large batholithic intrusions are shown on the geologic map between
Illiamna Lake and the Skwentna River in the southern Alaska Range. The
prevailin
t
^
g^
rocks are of granitic
i
texture and consist of hornblende granite,
^
✓^
hornblende-granite porphyry, biotite granite, sodic granite, granodiorite,
quartz-diorite, and diorite. All tend to be porphyritic with phenocrysts
of feldspar. Granite facies are more common than diorite facies. Though
some of the intrusions may be as old as Upper Jurassic, most are believed
to be Upper Cretaceous in age.
Smaller intrusive bodies of granitic rocks are common in the Kenai–
Chugach belt of Mesozoic sedimentary rocks. Acidic dikes are common,
especially in the vicinity of Turnagain Arm and at Nuka Bay on the Kenai
Peninsula. Three stocks of ultrabasic rocks are known in this belt:
(
1
) south of Knik Arm, near its head; (
2
) Red Mountain southeast of
Seldovia near the southern end of Kenai Peninsula and (
3
) Claim Point near
Portlock at the southwest end of Kenai Peninsula. These are chromite-bearing
bodies composed mainly of dunite, in small part serpentinized, with bends of
pyroxenite. These dikes and stocks are presumed to be of Mesozoic age.
In the Seward Peninsula, the largest areas of intrusive rocks are in
the Kigluaik, Bendeleben, and Darby Mountains. As well, sporadic occurrences
are known all the way from the hills between the Buckland and Kiwalik rivers,
in the eastern part of the peninsula, to the westernmost tip near Cape Prince
of Wales. The most typical rocks are granites composed of quartz, orthoclase
feldspar, and biotite with some plagioclase. These intrusions range in age
from pre-Cambrian to Mesozoic; the majority are believed to be of Mesozoic age.
Granitic intrusive rocks are known in the Brooks Range. There are reasons
EA-I. Gates: Stratigraphy and Geologic History
to suspect that this type of rock may be more common than now known and
that large granit
e
^
ic^
intrusions may be present in the unexplored parts of
^
✓^
the range. The intrusions known are post-Paleozoic and it is presumed
they are Mesozoic.
CENOZOIC ROCKS
Sedimentary
Tertiary System
. The Tertiary sedimentary rocks of Alaska can be divided
broadly into two groups: one is mainly of marine origin and limited to
scattered localities along the coast of southern Alaska and to the coastal
plain of northern Alaska; the other is of continental origin, in part coal–
bearing, and is found in local areas widely distributed over Alaska.
In southeastern Alaska, early Tertiary sediments are present in several
widely separated localities on Prince of Wales Island, Cleveland Peninsula,
and Kupreanof, Kuiu, and Admiralty Islands. The sediments are mainly sand–
stone and conglomerate of continental origin. At Kootznahoo Inlet on
Admiralty Island, they are coal-bearing. Volcanic rocks, lavas, tuffs, and
breccias overlie the sediments.
The Tertiary sediments in the vicinity of Lituya Bay comprise as much
as 12,000 feet of sandstone, shale, and conglomerate, dominantly marine.
Marine sediments predominate along the coast in the areas of Yakataga
and Katalla. In the Katalla area the sediments of early Tertiary age are
divided into four formations, roughly youngest to oldest: Tokun (about
2,500 feet thick), Kushtaka (about 2,500 feet thick), Stillwater (more than
1,000 feet thick), and Katalla (about 8,600 feet thick). All are marine
except the Kushtaka, which is terrestrial and coal-bearing. The sediments
EA-I. Gates: Stratigraphy and Geologic History
of these formations are shale, sandstone, and conglomerate. Parts of the
Katalla formation are petroliferous.
Localities of early Tertiary sedimentary rocks are numerous within the
lowland area of Cook Inlet and the Susitna River and its tributaries. It
seems likely that much of this lowland is underlain by sediments of early
Tertiary age. In most places, however, bedrocks are concealed beneath
Quaternary deposits. The early Tertiary sediments are terrestrial and
are probably at least a few thousand feet thick. They consist dominantly
of shale, siltstone, and sandstone with some
A
^
a^
rkose and conglomerate.
^
✓^
There are numerous coal beds in the sequence.
On the Kenai Peninsula, the sequence is called the Kenai formation.
In the area of Wishbone Hill, north of Palmer, the sequence consists of
the coal-bearing Chickaloon formation, at least 3,300 feet thick, and the
overlying Eska conglomerate, at least 1,700 feet thick.
The Tertiary coals of Alaska are commonly subbituminous or high-rank
lignite. In places where the beds have suffered considerable deformation,
the coal may be of higher rank. In the Wishbone Hill coal field, the coal
is bituminous; in the Chickaloon area in the upper part of the Matanuska
Valley, and north of Bering Lake in the Katalla area, the coal ranges in
rank from bituminous through coking coal to anthracite.
On the Alaska Peninsula and in southwestern Alaska, the Tertiary rocks,
although predominantly volcanic, comprise some scattered areas of sediments
in part marine and in part nonmarine in origin. Marine sediments, sandstone,
shale, and conglomerate are exposed on the east shore of Nushagak Bay, on the
Alaska Peninsula between Herendeen and Belboa bays, and on Unga Island.
Between Herendeen and Balboa bays they are present as interlayers in coal-bearing
EA-I. Gates: Stratigraphy and Geologic History
terrigenous sediments and are Eocene. Tertiary marine sediments of the
Miocene epoch are locally present along the shores of these two bays and
the shores at the other localities mentioned. Tertiary sediments of terres–
trial origin are known in discontinuous patches along the southeast side of
the Alaska Peninsula all the way from Cape Douglas to and beyond Pavlof Bay.
These are Eocene and, except as noted above, are not associated with marine
deposits. There are numerous beds of lignite in the section. The thickness
of the sequence is variable and reaches a maximum
^
^
of 5,000 feet.
^
✓^
Small areas of early Tertiary rocks, mainly of terrestrial origin,
are known on many of the Aleutian Islands. The Tertiary sedimentary rocks
on Nelson and Nunivak Islands in the Bering Sea are coal-bearing, land-laid
deposits.
In interior Alaska the principal areas of Tertiary sedimentary rocks
are along the foothill belt on the north side of the Alaska Range and along
the Yukon River between Eagle and Circle. Along the Alaska Range these
rocks are divided into two units. The older, in local structural basins,
is a coal-bearing sequence of poorly consolidated terrestrial sands,
^
silts,^
and
^
✓^
clays with minor amounts of gravel. The coal is in numerous beds, some as
much as 40 feet thick, and is lignitic or subbituminous. This unit grades
upward conformably with a sequence called the Nenana gravel that is dominantly
gravel with a small amount of sand interbeds. The ages of these two units
have not been definitely determined. The Nenana gravel is pre-Pleistocene.
Plant fossils in the underlying coal-bearing sequence indicate upper Eocene
age. A fossil fish from the coal-bearing sequence in Healy River indicates
Miocene age.
On the south side of the Yukon River downstream from Eagle, terrestrial
EA-I. Gates: Stratigraphy and Geologic History
sediments of sandstone, shale, and conglomerate occupy a belt about 85 miles
long and as much as 15 miles wide. Beds of lignite are present locally.
On the basis of fossils collected, the sequence is in part Upper Cretaceous
and in part Eocene.
Over most of the arctic coastal plain the sediments are concealed
beneath a thin blanket (not exceeding 250 feet thick) of unconsolidated
fine sand, silt, clay, peat, and gravel. Vertebrate remains, including
mammoth, bison, and horse, are present. Some marine invertebrates are also
present. Some Pliocene may be included though the sequence is largely
Pleistocene.
Quaternary System
. With
^
a^
few exceptions near present shores, all of the
^
✓^
deposits of sedimentary origin of this period are land-laid. As shown on
^
Fig. 1^ the accompanying map, Pleistocene river and wind deposits, and Recent river
deposits are not distinguished. Glacial deposits are largely limited to
the borders of the Brooks Range and to the southern coastal ranges.
The better-known Pleistocene deposits of central Alaska are sands and
gravels as terrace deposits, fillings of old river channels, and fill the
earlier of present-day river channels. Presumably these deposits accumulated
before extensive glaciations. In most places they are buried beneath a layer
of silt called “muck,” as much as 100 feet thick. The origin of the muck
is not everywhere clear, but it appears to have formed mainly by the deposi–
tion of silt by the wind. Glacial outwash plains afford a source for the silt.
Igneous
Volcanic.
Records of volcanic activity in the Cenozoic era are widespread
in Alaska. Although activity in one place or another seems to have been taking
place through most of this era, it was more or less concentrated in Eocene
EA-I. Gates: Stratigraphy and Geologic History
and Quaternary times and is continuing today.
In southeastern Alaska about 1,500 feet of Eocene rhyolitic and ande–
sitic flows and tuffs, and associated sedimentary rocks, crop out on Gravina,
Zarembo, Kupreanof, and Kuiu Islands. A later sequence, also probably Eocene,
of basaltic and andesitic flows is known on Admiralty, Kupreanof, and Kuiu
Islands. Rhyolite, andestie, and basalt on Suemez Island off the west coast
of Prince of Wales Island are believed to the Pliocene in age. Evidence of
Quaternary activity has been recognized at many places in southeastern Alaska.
Mount Edgecumbe on Kruzof Island was reported active in 1796.
In the Copper River region, the Wrangell Mountains consist of a cluster
of volcanoes formed since late Eocene(?). The rocks of these volcanoes —
flows, tuffs, and breccias — are most commonly composed of hypersthenes or
hornblende-andesite. Only Mount Wrangell has been reported active in
historic time.
Mount Spurr is the northernmost of a chain of volcanoes, many of which
are active, which extends southwestward along the crest of the Alaska Peninsula
and on out through the arc of the Aleutian Islands. On the peninsula, activity
began as early as Eocene. To the west in the Aleutian Islands, activity
generally began in late Tertiary. Andesites and dacites are the more common
types of the early Tertiary volcanics; the later ones are more typically
basalts and closely allied rocks.
Cenozoic volcanic rocks are exposed extensively in central Alaska.
Some are early Tertiary in age, such as those exposed northeast of Tanacross
not far from the international boundary, and those in the southwest-trending
belt south of Tanana. These rocks are flows and tuffs primarily of rhyolite,
dacite, and andesite. Basalt is very rare. It is generally true throughout
EA-I. Gates: Stratigraphy and Geologic History
interior Alaska that the older Cenozoic volcanics are more acidic than
later ones, which are typically basalts. Recent basalt flows cover more
than 1,000 square miles near Imuruk Lake in the northern part of Seward
Peninsula.
Tertiary to Recent volcanic rocks have been found on practically all
the islands of the Bering Sea region, south of the Diomede Islands. The
older ones are andesitic and the younger ones basaltic.
Intrusive
. In general, the intrusive igneous rocks in interior Alaska
west of Fairbanks differ petrographically and in age from those east of
Fairbanks. Most of those east of Fairbanks are granites, quartz-diorites,
and closely related types, and are mid-Mesozoic in age; whereas those west
of Fairbanks are typically monozonites and quartz-monzonites and are mainly
Tertiary in age.
Several intrusive of the latter group, too small to be shown appro–
^
Fig. 1^
priately on the accompanying map, are known in the area of Hot Springs and
Rampart. The rocks typically consist of orthoclase and plagioclase
(andesine or oligoclase) in about equal amounts, augite, hornblende-biotite,
and quartz. The belt of intrusives extending south and southwest of Ruby
consists of similar rocks. In the Nushugak district north of Bristol Bay,
some of the intrusives are granites and some are quartz-monzonites.
In most places the evidence does not allow definite determination of
the age of the Tertiary instrusives. Most of them, however, are believed to
have been intruded in mid-Tertiary; some may be early Tertiary.
EA-I. Gates: Stratigraphy and Geologic History
GEOLOGIC HISTORY
Only the more widespread and outstanding events in the geologic history
of Alaska will be summarized here. These are listed in more or less chrono–
logic order.
Pre-Cambrian
1. The oldest known rocks in Alaska comprise the Birch Creek schist.
The first step in their formation was an accumulation of a thick sequence
of arenaceous and argillaceous sediments, in part carbonaceous and calcare–
ous near the top. No fossils, but sediments likely accumulated mainly in
marine waters.
2. Basic lavas extruded when the upper sediments were forming.
3. Intrusion of large bodies of granitic rocks into the sequence of
sediments.
4. Severe deformation and metamorphism of the sediments to quartzites
and schists, the basic igneous rocks to amphibolite and related rocks, and
the granitic rocks to the Pelley gneiss.
5. Elevation of the region, followed by a long period of erosion.
6. Subsidence of parts of the region below the water and deposition
of a sequence of calcareous and clastic sediments, 20,000 to 25,000 feet
thick, called the Tindir group.
7. Sedimentation was interrupted or accompanied by several periods
of volcanism.
Paleozoic
1. There is no evidence for a period of deformation between the
pre-Cambrian and Paleozoic.
2. The sediments that were deposited during the several periods of this
EA-I. Gates: Stratigraphy and Geologic History
era accumulated mainly in seaways that occupied parts of Alaska at different
times. Clastic sediments predominate in southeastern Alaska, whereas in
other parts of Alaska calcareous sediments are much more important and
probably predominate.
3. At the close of the Silurian period, the rocks over all or prac–
tically all of Alaska were deformed by folding and to a greater or less
degree metamorphosed. The whole region was elevated and the folded rocks
were beveled by erosion.
4. At other times during the Paleozoic era, the region was elevated
above the sea and eroded. These elevations, however, were not accompanied
by deformation of the rock except locally.
5. Most of Alaska was a land mass during Pennsylvanian time.
6. Sedimentation was interrupted or accompanied by volcanic activity
numerous times. This was especially true in southeastern Alaska. In parts
of east-central Alaska in the Ordovician, Devonian, and Carboniferous
periods, volcanism occurred on a grand scale. Layer after layer of basic
lava spread over the surface. Beds of ash and breccia thrown from volcanic
vents accumulated with the flows. At several places south of the Alaska
Range, the Paleozoic era closed with outpourings of great quantities of
basaltic lavas (the Ni
c
kolai greenstone).
^
✓^
7. Intrusive activity was largely limited to the intrusion of basic
and ultrabasic bodies in two discontinuous west-trending belts in east–
central Alaska.
Mesozoic
1. At the close of the Paleozoic era, all or practically all of Alaska
was elevated above the sea and subjected to a period of erosion that continued
EA-I. Gates: Stratigraphy and Geologic History
through early and Middle Triassic time. With a few possible exceptions
the oldest known Mesozoic rocks in Alaska are of late Triassic age.
These rocks are structurally conformable on Paleozoic rocks; hence, the
two eras were not separated by a period of deformation.
2. Deposition in marine waters was widespread over Alaska. Excepting
the calcareous and siliceous sediments of Upper Triassic time, the sediments
deposited were dominantly clastics of the graywacke suite.
3. In about mid-Mesozoic or Jurassic time great volumes of granitic
magma were intruded to form batholiths at many places in Alaska, such as
the Coast Range batholith, the batholiths in east-central Alaska, and the
Talkeetna Mountains batholith.
4. Though the intrusions were not as widespread as those of mid-Mesozoic
time, at or soon after the close of the Mesozoic era, granitic magmas were
again intruded into pre-existing rocks. The igneous rocks thus formed out
Upper Cretaceous rocks in the Alaska Range, but do not cut the early
Tertiary coal-bearing rocks.
5. Volcanic activity during the Mesozoic era was at its peak during
the Jurassic period, especially in southeastern and southern Alaska.
6. At about mid-Cretaceous time the land was elevated above the sea,
and over most of Alaska the rocks were folded. A period of erosion followed
before parts of the region again sank beneath the sea to receive Upper
Cretaceous sediments. This is the second period of widespread folding
since the beginning of the Paleozoic era.
Cenozoic
1. Except for local invasions of the sea around the margins of Alaska,
the events of the era took place on an emergent land mass.
EA-I. Gates: Stratigraphy and Geologic History
2. In early Tertiary time, continental types of deposits — in
contrast to marine deposits — formed in numerous areas widely distributed
over the Territory. The deposits are of specific importance because coal
beds formed in nearly all areas of deposition and are now being mined in
two separate basins.
3. Marine deposits of Oligocene age along the south coast contain
petroleum, but the latter may be derived in part or wholly from underlying
deposits of Mesozoic age or older.
4. Sediments of Miocene and Pliocene age have been recognized but
their distribution is not well known.
5. Mid-Tertiary orogeny deformed the areas of early Tertiary sedimenta–
tion into graben and synclines.
6. Granitic masses now exposed on the present erosion surface show that
magma invaded the pre-existing rocks locally in Tertiary time.
7. Volcanic activity was particularly prominent in the Eocene and
Quaternary, and continues today in the Alaska Peninsula and Aleutian Islands.
8. Climatic conditions since the beginning of Tertiary time have been
both milder and more severe than now. Fossil plants of Tertiary age indicate
intervals or areas characterized by tropical and subtropical, temperate, and
boreal floras. In the Quaternary period, conditions permitted the accumulation
of extensive snow and ice fields. The present glaciers of the Coast, St. Elias,
Chugach, and Alaska ranges are much restricted by comparison. However, the
large central or interior region is unglaciated.
9. Movements of the land surface have continued into Quaternary time,
some amounting to thousands of feet and many others to one hundred feet or more.
10. Quaternary deposits of both beach and stream types have been of
specific economic interest because of the local concentrations of placer gold.
EA-I. Gates: Stratigraphy and Geologic History
BIBLIOGRAPHY
1. Atwood, W.W.
Geology and Mineral Resources of Parts of the Alaska Peninsula
.
Wash.,D.C., G.P.O., 1911. U.S. Geol.Surv. Bull. 467.
2. Brown, J.S. “The Nixon Fork country and silver-lead prospects near Ruby,”
Smith, P.S., and others.
Mineral Resources of Alaska…1924
. Wash.,
D.C., G.P.O., 1926, pp.97-150.
Ibid
. 783-D.
3. Buddington. A.F., and Chapin, Theodore.
Geology and Mineral Deposits of
Southeastern Alaska
. Wash.,D.C., G.P.O., 1929.
Ibid
. 800.
4. Capps, S.R.
The Bonnifield Region, Alaska
. Wash.,D.C., G.P.O., 1912.
Ibid
. 630
5. ----. The Chisana-White River District, Alaska. Wash.,D.C., G.P.O., 1916.
Ibid. 630.
6. ----. “The eastern portion of Mount McKinley National Park.” Moffit, F.H.,
“The Kantishna district.” Moffit, F.H. “Mining development in the
Tatlanika and Totatlanika Basins,” Smith, P.S., and others.
Mineral Resources of Alaska…1930
. Wash.,D.C., G.P.O., 1933,
pp.219-345.
Ibid
. 836-D.
7. ----.
Geology of the Alaska Railroad Region
. Wash.,D.C., G.P.O., 1940.
Ibid
. 907.
8. ----. “Kodiak and adjacent islands,” Smith, P.S., and others.
Mineral
Resources of Alaska…1935
. Wash.,D.C., G.P.O., 1937, pp.111-84.
Ibid
. 880-C.
9. ----.
The Southern Alaska Range
. Wash.,D.C., G.P.O., 1935.
Ibid
. 862.
10. ----. “The Toklat-Tonzona River region.” Smith, P.S. “Geologic
investigations in northern Alaska,” Moffit, F.H., and others.
Mineral Resources of Alaska…1925.
Wash.,D.C., G.P.O., 1927,
pp. 73-122.
Ibid
. 792-C.
11. Chapin, Theodore.
The Nelchina-Susitana Region, Alaska
. Wash.,D.C., G.P.O.,
1918.
Ibid
. 688.
12. Coller, A.J., Hess, F.L., Smith, P.S., and Brooks, A.H.
The Gold Placers
of Parts of Seward Peninsula, Alaska. Including the Home, Council
,
Kougarok, Port Clarence, and Goodhope Precincts
. Wash.,D.C., G.P.O.,
1908.
Ibid
. 328.
13. Eakin, H.M.
The Cosna-Nowitna Region, Alaska
. Wash.,D.C., G.P.O.,1918.
Ibid
. 667.
EA-I. Gates: Stratigraphy and Geologic History
14. ----.
The Yukon-Koyukuk Region, Al
^
a^
ska
. Wash.,D.C., G.P.O.,1916.
Ibid
. 631.
15. Harrington, G.L.
The Anvik-Andreafski Region, Alaska (Including the
Marshall District)
. Wash.,D.C., G.P.O.,1918.
Ibid
. 683.
16. Knappen, R.S. “Geology and mineral resources of the Aniskchak district,”
Smith, P.S., and others.
Mineral Resources of Alaska…1926
. Wash.,
D.C., G.P.O.,1929. pp.161-227.
Ibid
. 797-F.
17. Leffingwell, E. de K.
The Canning River Region, Northern Alaska
. Wash.,
D.C., G.P.O., 1919. U.S Geol.Surv.
Prof.Pap
. 109.
18. Maddren, A.G. “Geologic investigations along the Canada-Alaska boundary,”
Brooks, A.H., and others.
Mineral Resources of Alaska…1911
.
Wash.,D.C., G.P.O.,1912. pp.297-314. U.S.Geol.Surv.
Bull
. 520-K.
19. ----.
The Koyukuk-Chandalar Region
, Alaska. Wash.,D.C., G.P.O.,1913.
Ibid
. 532.
20. Martin, G.C.
Geology and Mineral Resources of Controller Bay Region,
Alaska
. Wash.,D.C., G.P.O.,1908.
Ibid
. 335.
21. ----.
The Mesozoic Stratigraphy of Alaska
. Wash.,D.C., G.P.O.,1926.
Ibid
. 776.
22. ----, Johnson, B.L., and Grant, U.S.
Geology and Mineral Resources of Kenai
Peninsula, Alaska
. Wash.,D.C., G.P.O.,1915.
Ibid
. 587.
23. Mather, K.F. “Petroleum on Alaska Peninsula: Mineral resources of the
Kamishak Bay region.” Smith, W.R. “The Cold Bay-Katmai district,”
Martin, G.C. “The outlook for petroleum near Chignik,” Brooks, A.H.,
and others.
Mineral Resources of Alaska…1923
. Wash.,D.C., G.P.O.,
1925. pp.159-213.
Ibid
. 773-D.
24. Mendenhall, W.C.
Geology of the Central Copper River Region, Alaska
.
Wash.,D.C., G.P.O., 1905. U.S.Geol.Surv.
Prof.Pap
. 41.
25. Mettie, Jr., J.B. “The Chandalar-Sheenjek district,” Smith, P.S. and
others.
Mineral Resources of Alaska…1927
. Wash.,D.C., G.P.O.,
1930, pp.87-139. U.S.Geol.Surv.
Bull
. 81
[]
^
0^
-B.
26. ----. “The Kaiyuh Hills,” Smith, P.S., and others.
Mineral Resources
of Alaska…1934
. Wash.,D.C., G.P.O., 1937.
Ibid
. 868-D.
27. ----. “Mineral deposits of the Ruby-Kuskowim region,” Smith, P.S., and
others.
Mineral Resources of Alaska…1933
. Wash.,D.C., G.P.O.,
1936, pp.115-225.
Ibid
. 864-C.
EA-I. Gates: Stratigraphy and Geologic History
28. ----. “Notes on the geography and geology of Lituya Bay,” Smith, P.S.,
and others.
Mineral Resources of Alaska…1930
. Wash.,D.C.,
G.P.O., 1933, pp.117-35.
Ibid
. 836-B.
29. ----.
The Nushagak District, Alaska
. Wash.,D.C., G.P.O., 1938.
Ibid
. 903.
30. ----.
The Yukon-Tenana Region. Alaska
. Wash.,D.C., G.P.O., 1937.
Ibid
. 872.
31. Miller, Don J., and others.
Geologic and Topographic Map and Sections
of the Katalla Area, Alaska
. Wash.,D.C., G.P.O., 1945. Mimeographed.
32. Moffit, F.H.
Geology of the Chitina Valley and Adjacent Area, Alaska
.
Wash.,D.C., G.P.O., 1938. U.S.Geol.Surv.
Bull
. 894.
33. ----. “Geology of the Gerstle River district, Alaska, with a report
on the Black Rapids Glacier,” Smith, P.S., and others.
Mineral
Resources of Alaska…1939
. Wash.,D.C., G.P.O., 1942, pp.107-60.
Ibid
. 926-B.
34. ----.
Geology of the Slana-Tok District, Alaska
. Wash.,D.C., G.P.O., 1938.
Ibid
. 904.
35. ----. “Geology of the upper
[]
Tetling River district, Alaska,” Smith P.S.,
and others.
Mineral Resources of Alaska…1938
. Wash.,D.C., G.P.O.,
1941, pp.115-57.
Ibid
. 917-B.
36. ----.
Headwater Regions of Gulkana and Susitna Rivers, Alaska, with
accounts of the Valdez Creek and Chistochina Placer Districts
.
Wash.,D.C., G.P.O., 1912.
Ibid
. 498.
37. ----.
The Iniskin-Chinitna Peninsula and the Snug Harbor District, Alaska
.
Wash.,D.C., G.P.O., 1927.
Ibid
. 789.
38. ----. “The Slana district, upper Copper River region,” Smith P.S.,
and others.
Mineral Resources of Alaska…1929
. Wash.,D.C., G.P.O.,
[] 1932, pp.111-24.
Ibid
. 824-B.
39. ----. “The Suslota Pass district, upper Copper River region, Alaska,”
Smith, P.S., and others.
Mineral Resources of Alaska…1931
.
Wash.,D.C., G.P.O., 1933, pp.137-62.
Ibid
. 844-C.
40. ----. “Upper Copper and Tanana Rivers,” Smith, P.S., and others.
Mineral Resources of Alaska…1934
. Wash.,D.C., G.P.O., 1936,
pp.135-43.
Ibid
. 868-C.
41. ----, and Knopf, Adolph.
Mineral Resources of the Nabesna-White River
District, Alaska. With a section on the Quarternary, by S.R. Capps
.
Wash.,D.C., G.P.O., 1910.
Ibid
. 417.
EA-I. Gates: Stratigraphy and Geologic History
42. Reed, J.C., and Coasts, R.R.
Geology and Ore Deposits of the Chichagof
Mining District, Alaska
. Wash.,D.C., G.P.O., 1942.
Ibid
. 929.
43. Smith, P.S.
Areal Geology of Alaska
. Wash.,D.C., G.P.O., 1939.
U.S.Geol.Surv.
Prof.Pap
. 192.
44. ----.
The Lake Clark-Central Kuskokwim Region, Alaska
. Wash.,D.C.,
G.P.O., 1917. U.S.Geol.Surv.
Bull
. 655.
45. ----, and Eakin, H.M.
A Geologic Reconnaissance in Southeastern Seward
Peninsula and the Norton Bay-Nulato Region, Alaska
. Wash.,D.C.,
G.P.O., 1911.
Ibid
. 449.
46. ----, and Mertie, Jr., J.B.
Geology and Mineral Resources of Northwestern
Alaska
. Wash.,D.C., G.P.O., 1930.
Ibid
. 815.
47. Smith, W.R., and Baker, A.A. “The Cold Bay-Chignik district, Alaska,”
Brooks, A.H., and others.
Mineral Resource of Alaska…1922
.
Wash.,D.C., G.P.O., 1924. pp.151-218.
Ibid
. 755-D.
48. Steidtmann, Edward, and Cathcart, S.H.
Geology of the York Tin Deposits
,
Alaska
. Wash.,D.C., G.P.O., 1922.
Ibid
. 733.
49. U.S. Geological Survey.
Alaskan Volcano Investigations, Report No.2.
Progress of Investigations in 1946
. Wash.,D.C., G.P.O., (n.d.)
Limited edition.
50. U.S. Geological Survey.
Geology of the Iniskin Peninsula, Alaska
.
Wash.,D.C., G.P.O., 1949.
Oil and Gas Investigations. Preliminary
[]
Map
95.
George O. Gates