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    Stratigraphy and Geologic History of Alaska

    Encyclopedia Arctica Volume 1: Geology and Allied Subjects

    Unpaginated      |      Vol_I-0230                                                                                                                  
    EA-I. (George O. Gates)




    Pre-Paleozoic Rocks 1
    Paleozoic Rocks 4
    Sedimentary 5
    Cambrian System 5
    Ordovician System 5
    Silurian System 7
    Devonian System 9
    Carboniferous System 11
    Igneous 17
    Volcanic 17
    Intrusive 18
    Mesozoic Rocks 19
    Sedimentary 19
    Triassic System 20
    Jurassic System 22
    Cretaceous System 24
    Undifferentiated Mesozoic Rocks 29
    Igneous 31
    Volcanic 31
    Intrusive 34
    Cenozoic Rocks 38
    Sedimentary 38
    Tertiary System 38
    Quaternary System 41
    Igneous 41
    Volcanic 41
    Intrusive 43
    Geologic History 44
    Pre-Cambrian 44
    Paleozoic 44
    Mesozoic 45
    Cenozoic 46
    Bibliography 48

    Unpaginated      |      Vol_I-0231                                                                                                                  
    EA-I Gates: Stratigraphy and Geologic History of Alaska



            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.

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    EA-I. (George O. Gates)



            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.



            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.

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            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

    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

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    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.

    The lower age limit is not known but pre-Cambrian rocks may be included.

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    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

    group is not known.



            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

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    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.



            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.

            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

    in age from Lower to Middle Ordovician.

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    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


            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


            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

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    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


            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.

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    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,

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    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

    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

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    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

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    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.

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    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


            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

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    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 ,

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    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–


            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

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    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. ✓✓

            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

    016      |      Vol_I-0247                                                                                                                  
    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


            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,

    017      |      Vol_I-0248                                                                                                                  
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    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.



            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 tlanikaTotlanika

    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

    each 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

    018      |      Vol_I-0249                                                                                                                  
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    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


            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.

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    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 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

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    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

    021      |      Vol_I-0252                                                                                                                  
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    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

    022      |      Vol_I-0253                                                                                                                  
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    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

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    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

    024      |      Vol_I-0255                                                                                                                  
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    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

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    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

    026      |      Vol_I-0257                                                                                                                  
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    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


            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.

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            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.

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            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

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    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

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    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


            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

    030      |      Vol_I-0262                                                                                                                  
    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

    031      |      Vol_I-0263                                                                                                                  
    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.



            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.

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    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


            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


            Greenstone tuffs, about 1,000 feet thick, form a belt along the west

    033      |      Vol_I-0265                                                                                                                  
    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

    034      |      Vol_I-0266                                                                                                                  
    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 amount of alkali f o e ldspar and

    quartz and a decreasing amount of hornblende, biotite, and calcic feldspar.

    035      |      Vol_I-0267                                                                                                                  
    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

    036      |      Vol_I-0268                                                                                                                  
    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

    037      |      Vol_I-0269                                                                                                                  
    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

    038      |      Vol_I-0270                                                                                                                  
    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.





            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

    039      |      Vol_I-0271                                                                                                                  
    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

    040      |      Vol_I-0272                                                                                                                  
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    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


            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

    041      |      Vol_I-0273                                                                                                                  
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    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


            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.



            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

    042      |      Vol_I-0274                                                                                                                  
    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

    043      |      Vol_I-0275                                                                                                                  
    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


            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.

    044      |      Vol_I-0276                                                                                                                  
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            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.



            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


            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.



            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

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    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.



            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

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    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.



            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.

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    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.

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    EA-I. Gates: Stratigraphy and Geologic History


    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.

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    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,

    . 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.

    050      |      Vol_I-0282                                                                                                                  
    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.

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    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

    . 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

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