Probable Permian Age of 

the Rampart Group, 

Central Alaska

By W. P. BROSGE, M. A. LANPHERE, H. N. REISER, and R. M. CHAPMAN

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1294-B

Fossil collections and rock samples 

from a limestone bed within the volcanic 

sequence of the Rampart Group show that 

the group is probably Permian in age
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.G. 20402 - Price 20 cents (paper cover)
CONTENTS

Page

Abstract_ _______________________________________________________ Bl

Introduction, _____________________________________________________ 1

Regional stratigraphy of the Rampart Group and associated rocks____ 2

Rampart Group at the fossil locality_________________________________ 3

Apparent lithologic sequence___________________________________ 8

Fossiliferous limestone.________________________________________ 10

Paleontology._____________________________________________________ 11

Age of intrusive rocks._____________________________________________ 13

Potassium-argon analysis.______________________________________ 16

Conclusion.__ _____________________________________________________ 17

References cited.__________________________________________________ 18

ILLUSTRATIONS

Page 

FIGURE 1. Generalized geologic map of the Rampart region-___________ B5

2. Geologic map of the Yukon River shoreline near Point No

Point-__________________________________ 7

3. Photomicrographs of pelecypod-prism calcarenites___________ 14

4. Photomicrograph of pelecypod prisms in a Permian limestone

in lower part of Gemuk Group_______-________----__--_- 16

TABLES

Page 

TABLE 1. Chemical analyses of volcanic rocks of the Rampart Group and

associated intrusive rocks_____________________________ Bll

2. Potassium-argon data for hornblende from the gabbro at

locality©._____________________________ 17

in

CONTRIBUTIONS TO STRATIGRAPHY

PROBABLE PERMIAN AGE OF THE RAMPART GROUP, 

CENTRAL ALASKA

By W. P. BROSGE, M. A. LANTHERE, H. N. BEISER, 

ANT> E. M. CHAPMAN

ABSTRACT

The Rampart Group, consisting largely of volcanic rocks, was assigned by 

Mertie in 1937 to the Mississippian System because of bryozoans found in a 

limestone bed thought to be in or below the Rampart. Examination of new 

collections as well as the old collections shows that this limestone bed is within 

the volcanic sequence and is more likely to be Permian than Mississippian in 

age. The limestone itself is composed largely of pelecypod prisms and in this 

respect resembles one of the Permian (?) limestone beds in the partly volcanic 

(Carboniferous? to Lower Cretaceous) Gemuk Group of southwestern Alaska. 

Hornblende from gabbro that seems to intrude the Rampart volcanic rocks near 

the fossil locality has a potassium-argon age of 205±6 million years (Triassic). 

Thus, the age of the Rampart Group is considered to be Permian (?).

INTRODUCTION

Volcanic rocks crop out along the margins of the Yukon Flats, and 

magnetic data indicate that volcanic rocks underlie most of the flats 

themselves (Zietz and others, 1960). The volcanic assemblages have 

been known only from reconnaissance mapping, and until recently 

there has been little direct evidence of the age of any of them except 

for the Eampart Group. This group of bedded volcanic and sedimentary rocks was described by Mertie (1937, p. 122) in his regional 

summary of early investigations. He assigned a Mississippian age to 

the Eampart, mainly on the basis of identification of Bryozoa that he 

collected from a limestone bed on the north bank of the Yukon Eiver 

12 miles northeast of Eampart. Eeiser, Lanphere, and Brosge, using 

radiometric methods, recently dated another assemblage of volcanic 

rocks of presumed Paleozoic age on the northeast edge of the Yukon 

Flats as Jurassic (Eeiser and others, 1965). This suggested that the 

Eampart Group might also be Mesozoic rather than Paleozoic in age.

Bl
B2 CONTRIBUTIONS TO STRATIGRAPHY

In 1965, the authors visited the fossil locality in the Eampart Group 

described by Mertie (1937, p. 126) in order to collect more fossils as 

well as samples of the volcanic rocks for radiometric dating. Scanty 

fossil collections were made at the limestone outcrop described by 

Mertie and also from another outcrop of the same bed about a mile 

downstream. Helen Duncan (written commun., 1968) examined the 

two fossil collections and Mertie's original collection and found the 

bryozoans to be Carboniferous or Permian, but probably Permian, 

in age. Examination of thin sections shows that the limestone itself 

is composed mainly of pelecypod prisms like those of the Mesozoic 

Inoceramus. Similar prisms were also found, however, in Permian 

limeskmes of the Gemuk Group and Tahkandit Limestone, southwest 

and east of the area underlain by the Rampart Group. The volcanic 

rocks of the Rampart Group were too altered for radiometric dating, 

but hornblende from a gabbro body that seems to intrude the Rampart 

Group yielded a Triassic age of 205±6 million years. The Permian(?) 

age of the bryozoans, as determined by Duncan, indicates that the age 

of the Rampart Group should be revised to Permian (?). The Triassic 

age of the intrusive rock would be compatible with a Permian (?) age 

for the Rampart Group.

REGIONAL STRATIGRAPHY OF THE RAMPART 

GROUP AND ASSOCIATED ROCKS

The Rampart Group, as redefined by Mertie (1937), consists of 

basaltic to andesitic flows; basaltic, andesitic, and rhyolitic tuffs and 

breccias; and a slightly smaller amount of interbedded chert, shale, 

argillite, and sandstone. Locally, the rocks are calcareous, but the 

only limestone mentioned by Mertie is the limestone bed at the fossil 

locality. Figure 1 shows the distribution of these rocks as mapped by 

Mertie (1937, pi. 1) south of the Yukon River and similar rocks that 

had previously been mapped by Eakin (1916) north of the Yukon. 

Mertie included these northern rocks in the Rampart Group and 

described their good exposures along the Yukon for about 60 miles 

northeastward from the town of Rampart.

No complete section of the Rampart Group is known. It is much 

folded and broken by faults, and no upper limit was specified by 

Mertie. The oldest beds known to overlie it are Tertiary conglomerates; they rest unconformably upon it and contain pebbles of the 

volcanic rocks. The beds thought to underlie it are an assemblage of 

sparsely fossiliferous Mississippian clastic rocks and unfossiliferous 

limestone. These rocks dip north beneath the Rampart Group near 

Rampart. Farther east they dip southward away from the Rampart 

Group in an area where the Mississippian and older rocks are believed 

(Prindle, 1913) to be overturned.
AGE OF RAMPART GROUP, CENTRAL ALASKA B3

The relation of the Eampart Group to the mica schist and granite 

to the north (fig. 1) is uncertain. Eakin, who crossed the northern 

contact of the Eampart Group at two places, suggested (1916, p. 33) 

that in the eastern part of the area the interbedded sedimentary and 

volcanic rocks have been altered by the granite and might grade 

northward into the schist. Photointerpretation suggests that the contact he saw near the west edge of the area may be a high-angle fault.

Diabase dikes and larger bodies of gabbro and augite diorite are 

common within the mapped boundaries of the Rampart Group, especially in the western half of the area shown in figure 1. Although for 

convenience these intrusive rocks were mapped as Rampart Group, 

Mertie excluded them from the Rampart Group as a stratigraphic 

unit. As described by Mertie, the intrusive rocks seem to have the 

same range in mineral composition as the basaltic to andesitic flows, 

and, like them, they contain no olivine but commonly contain a little 

quartz. Like the flows, the intrusive rocks have usually been altered, 

and albite, kaolin, sericite, chlorite, and calcite have been produced. 

However, a few of the intrusive rocks have been much less altered than 

the flows, and Mertie suggested that these might be considerably 

younger than the Rampart Group.

Mertie considered the Rampart Group to be of Mississippian age, 

partly on the basis of its apparent position above the supposedly 

Mississippian clastic rocks but mainly on the identification of three 

genera of Bryozoa from a limestone bed that underlies some of the 

volcanic rocks on the Yukon River (loc. A, fig. 2). Although Mertie 

assigned this limestone bed to the Rampart Group, he suggested that, 

because its stratigraphic position was uncertain, this bed might actually be older than the Rampart Group. He further suggested that, 

because the limestone is conglomeratic, it might be correlative with 

the fossiliferous conglomerate beds in the presumably underlying 

Mississippian clastic rocks. However, the provenance of the Rampart 

limestone is unlike that of the Mississippian conglomerates. Noncarbonate clasts in the limestone are almost entirely volcanic material 

and argillite, whereas the clasts in the Mississippian conglomerate are 

shown by Mertie's description and by examination of hand specimens 

and thin sections of matrix rock from the fossil collections to be mainly 

chert with minor amounts of quartz and black phyllite.

RAMPART GROUP AT THE FOSSIL LOCALITY

The fossiliferous limestone bed that Mertie (1937, p. 126) described 

forms a small cliff opposite Point No Point (loc. A, fig. 2) in the 

eastern part of a bend in the Yukon River, where the Rampart Group 

is exposed for about 4 miles. For a distance of about 10 miles above 

and below this bend, only alluvium and folded Tertiary rocks are
B4 CONTRIBUTIONS TO STRATIGRAPHY
EXPLANATION

Metamorphic and 

granitic rocks 

'Mica schist and granite Clastic rocks and limestone

Contact

May include faults -+- -V

Syncline Anticline

Major fold axes 

Interpreted from aerial photographs

Mississippian fossil locality 

(Mertie, 1937)

FIGTJBE 1. Generalized geologic map of the Rampart region showing area of 

figure 2 and distribution of the Rampart Group and adjacent Mississippian 

rocks. From Mertie (1937) and Eakin (1916). Northwest boundary of Rampart 

Group partly from photointerpretation. w
B6 CONTRIBUTIONS TO STRATIGRAPHY
EXPLANATION

Alluvium and landslide debris

Mafic intrusive rocks

Rampart Group

Pr, interbedded volcanic and 

sedimentary rocks. May include some intrusive rocks of unit 1m

Prl, limestonePrs, mostly sedimentary rocks

a

Contact

Dashed where approximate

Inferred steeply dipping

fault 

Dashed where concealed

Fossil locality Radiometric age sample locality

o6 

Outcrop described in text

50 ffi 

 I t±)

Inclined Horizontal Strike and dip of beds

Strike and dip of beds from 

distant views and aerial photographs

FIGUKE 2. Geologic map of the Yukon River shoreline near Point No Point. Geology away 

from the river inferred from aerial photographs.

W
B8 CONTRIBUTIONS TO STRATIGRAPHY

exposed along the river. Within the bend, the Eampart Group crops 

out at intervals along the shoreline in wooded bluffs that rise about 

800 feet above the river. Landslides from these bluffs, now covered, 

obscure most of the outcrop, and a small point is formed immediately 

upstream from the limestone cliff by a prominent fresh slide that came 

down about 1940. The surface of this slide is composed almost entirely 

of limestone blocks that are derived from the cliff formed by the 

steeply dipping fossiliferous limestone bed, mixed with some red 

argillite from the adjacent beds.

The geologic map (fig. 2) is based on a study of the outcrops along 

the shoreline and on interpretation of aerial photographs of the timbered slopes away from the river. In this area, the Eampart Group dips 

generally north into a large syncline (fig. 1). The rocks are broken by 

many faults too small to be shown on the map and by small folds 

overturned southward. The beds at fossil locality A are part of a 

structurally discordant block in which the beds dip northwest, almost 

normal to the other rocks. This block seems to be bounded by faults 

that are parallel to a system of north-south lineations visible on the 

aerial photographs. A small east-west fault at locality (c), where samples were collected for radiometric age dating, separates the steeply 

dipping intrusive rocks from a small outcrop of flat-lying Tertiary 

rocks in the riverbank, and this fault probably continues westward 

into the Rampart Group.

No section of the Rampart Group can be measured along the river 

because of the small-scale structural complexity and the discontinuous 

outcrops, but a general sequence can be suggested because of the 

northward regional dip. The best exposed part of the Rampart Group 

has been distinguished as a separate member of mostly sedimentary 

rocks in figure 2. The beds below this sedimentary member are not 

exposed along the river. The few beds exposed above it are mainly 

volcanic rocks but include the fossiliferous limestone bed. This bed 

crops out at locality B in the upper part of the apparently continuous 

sequence, as well as at locality A in the discordant block.

APPARENT LITHOLOGIC SEQUENCE

The fossiliferous limestone, judging from its position at locality B, 

is in the upper part of the north-dipping sequence exposed along the 

river. The rocks in this sequence are described below in the order in 

which they crop out going downstream from a point a quarter of a 

mile east of locality B. There is, however, no direct evidence of superposition between the outcrops. The rocks throughout the sequence are 

a similar volcanic-chert-argillite assemblage; even the limestone contains volcanic detritus. Only the carbonaceous wacke of unit 6 seems
AGE OF RAMPART GROUP, CENTRAL ALASKA B9

anomalous in this respect, as the grains in this rock are mostly quartz 

and the only identifiable volcanic detritus is a minor amount of 

plagioclase.

The apparent stratigraphic sequence in the Kampart Group, as 

exposed along the Yukon, is as follows: (Locations of units 1, 3, 6, 

and 10, and Iocs. B and © are shown in fig. 2.)

1. Dark-greenish-gray and grayish-red lithic tuff and breccia consisting of rounded fragments of andesite (?) and of chloritized 

glassy (?) lava that contains amygdules of calcite and quartz. 

Dark-green trachyandesitic crystal tuff of plagioclase, pyroxene, 

and amphibole fragments in a matrix of chlorite and fine mosaic 

quartz (table 1). Interbedded minor amount of siliceous argillite, 

one lens of limestone, and a partly covered zone of spherical 

phosphatic concretions.

Covered interval.

2. Limestone at locality B. Fossiliferous limestone similar to that at 

locality A. Overlain by 25 feet of sheared silty argillite succeeded 

by volcanic breccia beds.

Covered interval.

3. Brown-weathering volcanic breccia with some pebbles and cobbles 

of .green argillite and limestone. Interbedded dark-gray and red 

siliceous argillite with lenses of dark-gray calcareous siltstone. 

Forms upper beds of sedimentary member.

Covered interval.

4. Greenish-gray shale and chert and dark-gray bedded chert interlayed with basalt. 

Fault (not shown in fig. 2).

5. Altered grayish-green vitric tuff(?) consisting of few fragments 

of plagioclase and volcanic quartz in cryptocrystalline chloritic 

matrix.

Fault (?).

6. Medium-dark gray very fine grained micaceous lithic wacke and 

siltstone with layers of carbonaceous pebbles. Interbedded 

micaceous silty shale with few carbonaceous partings and silty 

concretions.

Covered interval.

7. Outcrop of andesitic basalt; mapped as probable mafic intrusive

rocks of Triassic age, but may be part of Rampart Group. 

Covered interval.

8. White and yellow laminated to thick-bedded chert. 

Fault (not shown in fig. 2).

9. Orange-weathering sheared silicified volcanic breccia, partly 

calcareous.
BIO CONTRIBUTIONS TO STRATIGRAPHY

10. Grayish-red and grayish-green mottled siltstone, chloritic very 

fine grained sandstone and altered fine crystalline pyroxene andesite. Brecciated and exposed in landslide. Lowest exposed beds in 

the mainly sedimentary part of the Kampart Group. Outcrops 

downstream are only intrusive rocks and Tertiary conglomerate.

FOSSILIFEROUS LIMESTONE

Only the upper part of the limestone unit is exposed at locality B, 

but at locality A about 100 feet is exposed. The limestone unit consists of tuffaceous limestone in beds 5-10 feet thick interbedded with 

calcareous volcanic granule-to-pebble conglomerate in beds as much 

as 6 feet thick. The limestone grades vertically into the conglomerate, 

and both are composed of pelecypod prisms and calcite fragments, 

albite and andesine crystals, and fragments of green argillite and volcanic rocks, all cemented by carbonate. The limestone in its purest 

phase is pale-yellowish-brown to olive-gray pelecypod prism grainstone. The pelecypod prisms are generally about 0.4^0.8 mm (millimeters) long and 0.03-0.05 mm thick and are alined with their long 

dimension parallel to the bedding. (See fig. 3, left photo.) A few shell 

fragments were found; each was composed of about a dozen prisms 

of equal width alined normal to the shell edge. A transverse section of 

a shell fragment shows that these prisms are regular hexagons of uniform size. Extinction is subparallel (±15°) to the prisms in the shell 

fragments and in most of the individual prisms in the rock.

Bryozoans occur as rare granule-to-pebble-size clasts in the limestone 

at both localities. Small fragments are visible in most of the rock 

samples, but the fossiliferous lens from which Mertie collected several 

large specimens was not found. One specimen large enough to be identified was collected at locality A and another, at locality B. The 

bryozoan fragments are water worn, and in thin section the pelecypod 

prisms are seen to be packed tightly against the abraded surface of the 

bryozoans. No matrix of older limestone was seen on any of the Bryozoa 

clasts.

Although the bryozoans are abraded clastic particles, it seems unlikely that they have been reworked from old rocks into a much later 

shell deposit. Helen Duncan suggests that the bryozoans were transported and deposited penecontemporaneously (J. T. Dutro, Jr., written commun., 1968). The bryozoans are no more damaged by wave 

action than are the completely disaggregated pelecypod shells that 

make up most of the limestone, and the fact that no matrix of preexisting rock was found on the bryozoans argues against their having 

been reworked from older rocks. Moreover, there is no other evidence 

of a nearby source area of pre-Rampart rocks. All clastic particles in
AGE OF RAMPART GROUP, CENTRAL ALASKA Bll

the limestone, as well as all clastic particles in the rest of the Kampart 

Group that were larger than very fine sand, could have been produced 

locally by volcanoes or organisms or could have been derived from 

underlying beds of the Kampart Group itself.

At both localities A and B, the limestone is overlain by argillite 

which, in turn, is succeeded by tuff and breccia. At locality A, this 

argillite is about 50 feet thick, is red, green, and black, and is silty 

and siliceous. The argillite contains lenses of sheared limstone and a 

few beds of crystal tuff composed of sodic plagioclase and secondary 

calcite after augite( ?) in a matrix of glass and felted feldspar microlites. Above the argillite is rubble of andesine-bearing vitric tuff, 

lithic tuff, and volcanic breccia. The vitric tuff has the composition 

of alkali basalt (table 1). The breccia contains fragments of andesite 

and altered amygdaloidal quartzose lava as well as broken crystals 

of andesine, unstrained quartz, and pyroxene.

PALEONTOLOGY

The limestone contains pelecypod prisms, a few Bryozoa, some nondiagnostic fish teeth and fish plates (J. W. Huddle, written commun., 

1965 and 1968), and an unidentifiable foraminifer. Conodonts were 

looked for in the limestone but were not found; samples collected for 

pollen from the carbonaceous wacke lower in the sequence were barren.

TABLE 1. Chemical analyses of volcanic rocks of the Rampart Group and associated

intrusive rocks

[Analyses by rapid-rock method described by Shapiro and Bannock (1962). Analysts- Leonard Shapiro. 

Lowell Artis, S. D. Botts, G. W. Chloe, P. L. D. Elmore, J. L. Glenn, James Kelsey, Hezekiah Smith]

Si02 -- -- - .

AlsOj ... ...................

FejOs---- -------------------

FeO.. ...... ....... ......

MgO... ........................

CaO . Na2O. - - . ...

K2O. .-...... ...... .........

H2O-_ ................... .....

H2 0+...._ ....................

TiO2 ----- -- - -- -.

P2 0s - . . MnO. .........................

CO2 .. .........................

Total.- . .

...... ....... 49.2

..... ..... 12.9

 ... 5.8

..... ...... ..... 9.4

 . .. ... 5.3

 ... 6

. .. .... 3.2

 ..... . 1.6

 .. . .47

................ 2.6

................ 2.1

 ..... .27

................ .16

... . . <.05

................ 99

52

17.4

2.9

5.7

3.2

5.1

4.7

1.9

1.1

3.3

.76

.35

.36

.24

100

49.1

17.3

1.6

6.1

7.2

10.1

2

.52

.66

2.9

.89

.10

.17

<.05

100

41.9

12.7

7.9

11.4

6.7

10.5

1.4

.30

.39

2

3.1

.13

.22

<.05

99

49.2

14.3

1.5

10.3

6.4

10.8

2

.32

.27

2.2

1.8

.20

.21

.20

100

48.1

13.2

2.9

12.4

4.7

8

2.7

.42

.31

2.8

3

.26

.26

.08

100

1. Kampart Group, vitric tuff at loc. A (65ABe97C).

2. Kampart Group, crystal tuff a quarter of a mile east of loc. B (65ABe98).

3. Intrusive rock, andesine gabbro at loc. © (65ABel06CX).

4. Intrusive rock, diabase, radiometrically dated sample at loc. © (65ABel06CZ).

5. Intrusive(?) rock, diabase, 1 mile north of loc. © (65ABel05B).

6. Float, andesine gabbro, same loc. as 5 (65ABel05C).
B12 CONTRIBUTIONS TO STRATIGRAPHY

The byozoans have been identified by Helen Duncan. The specimen 

from locality B (USGS 22401-PC) is a trepostomatous bryozoan aff. 

Dyscritella G-irty that may be a new genus, and the smaller specimen 

from a limestone block in the slide at locality A (USGS 22400-PC) 

is probably of the same species (Helen Duncan, written commun., 

1966). These specimens were originally referred to the Carboniferous 

or Permian (Helen Duncan, written comm., 1966). Helen Duncan (in 

J. T. Dutro, Jr., written commun., 1968) reexamined them and the 

specimens of other genera in Mertie's collection (USGS 5411-PC) and 

reported that the generic identifications published in Mertie's (1937) 

report proved to be erroneous and that "* * * they belong to genera 

commonly found in the Permian, although very rare reports of similar 

forms from the Triassic are on record."

The pelecypod prisms are by far the most abundant fossil material 

in the limestone; they constitute most of the rock in the thin sections 

examined. The genus represented by all this material is unknown because the few visible shell fragments are too small to be identified, but 

the very abundance of the prisms suggests that they might be Mesozoic 

in age. The only similar pelecypod-prism limestone previously reported 

in Alaska is the Nelchina Limestone of Early Cretaceous age (Jones 

and Detterman, 1966), composed mainly of Inoceramus prisms and 

fragments and some volcanic detritus. As in the Rampart Group, identifiable shells are rare compared with the total volume of disaggregated shell material in the rock (D. L. Jones, oral commun., 1968). In 

thin section the pelecypod prisms in the Eampart Group limestone 

(fig. 3, left photo) are similar in shape to those in the Nelchina Limestone (see fig. 3, right photo) and are similarly arranged within the 

shell fragments, but they are generally not as wide. Prisms as wide as 

0.08 mm are common in the Nelchina but are rare in the Rampart.

Known pelecypod prisms in limestone beds, in addition to those of 

Mesozoic age, include prisms of Atomodesma, a guide fossil to the 

Permian in New Zealand, that locally forms a prism limestone bed 

380 feet thick (Waterhouse, 1958; Wood and others, 1956), and prism 

limestones in rocks of probable Permian age in Alaska. Beds of pelecypod prism limestone occur in the Gemuk Group at two localities near 

Goodnews Bay, about 550 miles southwest of Rampart. The Gemuk 

Group is a sedimentary and volcanic sequence of Carboniferous to 

Cretaceous age that contains fossiliferous Permian limestone beds in 

the lower part (Hoare and Coonrad, 1961). One of these fossiliferous 

beds is a very fine grained tuffaceous limestone about 80 feet thick 

(J. M. Hoare, written commun., 1968). In a thin section provided by 

Hoare, this limestone appears to be composed mainly of slender 

pelecypod prisms similar to those in the Rampart Group (fig. 4).
AGE OF RAMPART GROUP, CENTRAL ALASKA B13

At the same locality of the Gemuk Group, this limestone also contains 

productoid and spiriferoid (?) brachiopods of probable Permian age, 

a gastropod of probable Paleozoic age, and large pieces of unidentified 

prismatic pelecypod shells (Hoare and Coonrad, 1961, fossil loc. 16; 

J. S. Williams and E. L. Yochelson, written commun., 1953). In thin 

section, a probable Permian brachiopod valve from the fossil collection is seen to be enclosed in a matrix of closely packed pelecypod 

prisms, so there is no physical evidence that the Permian(?) fossils 

were reworked from a matrix rock older than the prisms.

A second bed of fine-grained pelecypod-prism limestone occurs in 

the undifferentiated rocks of the Gemuk Group on the south shore of 

Goodnews Bay. No identifiable fossils were reported from this bed, 

but a collection made by Coonrad from limestone in the same terrane 

31/2 miles to the east contains fragments of brachiopods of possible 

Permian age (Hoare and Coonrad, 1961, fossil loc. 23; J. S. Williams, 

written commun., 1954) and many fragments of unidentified pelecypod shells composed of parallel prisms 0.03-0.08 mm wide and 1-2 mm 

long.

These straight slender pelecypod prisms in the Gemuk Group have 

been found only in tuffaceous limestones with poorly preserved fossils 

of questionable Permian age, and they have not been found in the 

crinoidal limestone fades from which the better Permian fossil 

collections in the Gemuk were made. However, similar pelecypod 

prisms have been found in a specimen of sandy crinoidal limestone 

provided by E. E. Brabb from the type section of the Permian 

Tahkandit Limestone near Eagle about 250 miles east of Rampart. 

This specimen contains a few individual prisms and many small shell 

fragments composed of parallel prisms about 0.04 mm wide and 1 mm 

long.1 Thus, the pelecypod prisms in the Rampart Group limestone 

are as likely to have been derived from some unidentified Permian 

form as from the Mesozoic Inoceramus.

AGE OF INTRUSIVE ROCKS

Four resistant bodies of basalt and gabbro crop out along the 

Yukon River near the Rampart Group fossil locality (fig. 2). No 

intrusive contact with the Rampart Group is exposed. However, judging from the aerial photographs, these intrusive bodies are on strike 

with the Rampart Group and probably intruded the lower part of the 

Rampart.

1 Pelecypod prisms also occur with productoid brachiopods in three collections (USGS 

15820, 15827, and 15828) from the Permian Echooka Member of the Sadlerochit Formation 

of northern Alaska (J. T. Dutro, Jr., oral commun., 1969).
B14 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 3. Pelecypod-prism calcarenitea Above: Rampart Group, locality B of 

figure 2. Right: Nelchina Limestone, containing Inoceramus prisms. Specimen 

provided by D. L. Jones. Photomicrographs in plane-polarized light.

The body at locality © contains gab'broic zones in the outcrop and 

is thus clearly an intrusive. The other outcrops are fine-grained rocks 

that might be flows in the bedded volcanic sequence; they are thought 

to be intrusive because they are dikelike in shape and because they are 

associated with coarse-grained float that might be locally derived.

Only the northernmost of the four bodies lies between outcrops of 

the Eampart Group, and that body may well be a flow. (See unit 7 

in the sequence of bedded rocks.) The other three bodies lie outside 

the zone in which the Rampart Group crops out along the riverbank, 

and they were seen in contact only with flat-lying beds of Tertiary 

conglomerate, claystone, and lignite. At the easternmost locality, the 

Tertiary beds overlap a deeply weathered greenstone body; at locality 

©, the contact with the Tertiary is a steep fault; elsewhere, the contact 

with the Tertiary is obscure or faulted.
AGE OF RAMPART GROUP, CENTRAL ALASKA B15

Whereas chemical analyses indicate that pyroclastic rocks in the 

Rampart Group are alkaline, the intrusive rocks are modally or 

chemically tholeiitic. The body exposed about 1 mile north of locality 

© is fresh fine-grained tholeiitic diabase with minor hornblende. (See 

table 1.) Boulders of tholeiitic andesine gabbro with a little late hornblende and micropegmatite were found nearby on the beach and may 

be related to the diabase. (See table 1.)

The intrusive body at locality © forms a steep-faced promontory 

about 100 yards wide. It is mainly coarse-grained, light-gray-weathering andesine gabbro composed of fresh poikilitic augite, cloudy 

andesine, chlorite, and a little chloritized hornblende (table 1). The 

gabbro grades into darker and finer grained rocks in the middle of 

the body, and along the northern margin it grades into fine-grained 

tholeiitic augite diabase, which has a crude textural banding that dips 

steeply northeast and is parallel to the major joints. The augite diabase contains about 20 percent green and brown hornblende (table 1), 

and a single sample of this hornblende was used for the potassiumargon determination. Although a small part of the hornblende has 

been altered to chlorite and biotite, most of it is fresh; it surrounds
B16 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 4. Photomicrograph of pelecypod prisms in a Permian limestone in lower 

part of Gemuk Group near Goodnews Lake (Hoare and Oooonrad, 1961, fossil 

loc. 16). Specimen provided by J. M. Hoare. Photomicrograph in planepolarized light.

the labradorite and is intergrown with the augite. The latest minerals 

in the diabase are interstitial quartz and abundant crosscutting 

magnetite.

POTASSIUM-ARGON ANALYSIS

Standard isotope dilution techniques were used for the argon determinations ; the potassium determinations were done by flame photometry using a lithium internal standard. The hornblende used for 

the determinations contained less than 5 percent chlorite. Potassium 

was determined on two splits of the hornblende concentrate, and argon 

was determined on two other splits. The average potassium-oxide 

value was used with each argon determination in turn to calculate an 

apparent age (table 2). The plus-or-minus value for each calculated 

age is the standard deviation of analytical precision that is estimated 

using the method of Cox and Dalrymple (1967).
AGE OF RAMPART GROUP, CENTRAL ALASKA B17

The average age for the hornblende from the gabbro at locality (c) 

is 205 ±6 million years, where the plus-or-minus value is the pooled 

estimate of the standard deviation. This age is Triassic according to 

all recent compilations of the geologic time scale. The age of the 

Triassic-Jurassic boundary has been placed at 180 million years by 

Holmes (1959), at 181 million years by Kulp (1961), and at 190-195 

million years in the Phanerozoic time scale of the Geological Society 

of London (Harland and others, 1964).

TABLE 2. Potassium-argon data for hornblende from the gabbro at locality ©

[Argon analyses and age calculations by J. C. Von Essen andM. A. Lanphere; potassium analyses by L. B. 

Schlocker. Decay constants for K4»: X«=0.585X10-10 year-i; XB=4.72X10-10 year-i. Atomic abundance of 

K«=1.19XHH]

KsO (percent)

0.174 

.175

Ar«d 

(10-w moles per g)

0. 5656 

.5528

A..40 Arrad

ArA

0.41 

.54

Apparent age 

(millions of years)

207 ±7 

203 ±6

0. 174 (avg) 205 ±6 (avg)

CONCLUSION

The fossiliferous limestone bed exposed in the Rampart Group 

along the Yukon Eiver was originally thought to be Mississippian in 

age and was thought possibly to underlie the rest of the Rampart 

Group. Reexamination of the outcrop shows that the limestone bed 

is definitely within the volcanic sequence of the Rampart because it 

contains abundant clasts of volcanic rock and is ov >rlain and apparently underlain by volcanic beds. The bryozoans in old and new collections from this limestone have been reidentified and are now believed 

to be Permian (?) rather than Mississippian in age,. Examination of 

thin sections shows that this limestone is composed largely of pelecypod prisms like those of Inoceramus; similar prisms are equally abundant in the Permian part of the Gemuk Group of southwestern Alaska 

and occur in the Permian Tahkandit Limestone of east-central Alaska.

Gabbro and greenstone intrude the Rampart Group throughout 

much of its area of outcrop. A body of gabbro and diabase that probably intrudes the Rampart Group 4 miles from the fossil locality has a 

radiometric age of 205 million years. The age of this intrusive body 

probably indicates that the Rampart Group volcanic beds exposed 

along this part of the Yukon are no younger than Triassic. It also 

supports the Permian (?) age that is indicated for these volcanic beds 

by the fossil evidence.

The age of the beds that enclose the fossiliferous limestone is therefore revised from Mississippian to Permian(?). As these are the only
B18 CONTRIBUTIONS TO STRATIGRAPHY

dated beds within the Rampart Group, the age of the Rampart is also 

revised to Permian(?). However, the Rampart may prove to contain 

beds younger than Permian, as its upper stratigraphic limit is not 

known, and the presence of Triassic intrusive rocks nearby suggests 

that there may also be Triassic eruptive rocks in the volcanic terrane.

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