The Coal-Bearing 

Group in the Nenana 

Coal Field, Alaska

By CLYDE WAHRHAFTIG, JACK A. WOLFE, ESTELLA B. LEOPOLD, and 

MARVIN A. LANPHERE

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1274-D

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington, D.C. 20402 - Price 25 cents (paper cover)
CONTENTS

	Page

Abstract _______________ _ Dl

Introduction _____ _ 1

Coal-bearing group _ _____ __ 4

Healy Creek Formation _________________________ 7

Sanctuary Formation ____ ___ _ 12

Suntrana Formation _________________ 16

Lignite Creek Formation __________________ _____ 19

Grubstake Formation ___________________________ 22

Significance of the radiometric dates ___________ ___ 27

References cited _________________________________ 28

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map of the Healy D-2, D-3, D-4, 

and D-5 quadrangles and the Fairbanks A-2,A-3,A-4, 

and A-5 quadrangles ______________________ D2

2. Geologic map of sees. 23 and 24 and parts of sees. 13, 14,

25, and 26, T. 12 S., R. 7 W., Healy D-4 quadrangle ___ 6

3. Stratigraphic column of the type sections of the formations of the coal-bearing group at Suntrana _______ 6

4. Geologic map of part of T. 9 S., R. 6 W., Fairbanks A-4

quadrangle ____________________________ 10

5. Map of the Nenana coal field and environs, central Alaska 

Range, showing the major coal basins and northern 

limits of several formations __________________ 14

6. Geologic map of part of the valley of Tatlanika Creek,

Fairbanks A-3 quadrangle __________________ 23

TABLES

Page

TABLE 1. Indices of refraction of volcanic glass from the Grubstake

Formation ____________________________ D25 

2. Potassium-argon ages and analytical data _________ 25

m

CONTRIBUTIONS TO STRATIGRAPHY

THE COAL-BEARING GROUP IN THE 

NENANA COAL FIELD, ALASKA

By CLYDE WAHRHAFTIG, JACK A. WOLFE, 

ESTELLA B. LEOPOLD, and MARVIN A. LANPHERE

Abstract

The coal-bearing group of rocks of the Nenana coal field in the central 

part of the Alaska Range is formally subdivided into five new formations, in 

ascending order: the Healy Creek, Sanctuary, Suntrana, Lignite Creek, and 

Grubstake Formations. Fossil leaves and pollen indicate that the coal-bearing 

group, which is wholly continental, ranges in age from the late Oligocene 

through the late Miocene. Volcanic glass from the Grubstake Formation has 

an apparent age of 8.1 million years, based on potassium-argon analyses.

INTRODUCTION

The Nenana coal field in the northern foothills of the Alaska 

Range is a group of synclinal basins containing coal-bearing rocks 

of Tertiary age. The coal field was probably discovered about 1898, 

but the first geologic observations were by A. H. Brooks and 

L. M. Prindle in 1902 and 1906 (Brooks, in Collier, 1903, p. 46; 

Prindle, 1907; Brooks, 1911, p. 95-103). Brooks and Prindle 

correlated the coal-bearing beds with the Kenai Formation. 

Capps (1912, p. 26-29), who outlined the coal basins about as 

we know them today, first called the coal-bearing rocks simply 

the "coal-bearing beds of early Tertiary (Eocene?) age," and 

this usage was followed by Martin (1919) in his detailed study 

of the central part of the coal field. By 1919, however, Capps 

(1919, p. 44-51) was referring to these rocks as the Tertiary 

coal-bearing formation, and this usage has been in general practice to the present day (Capps, 1924, p. 103; 1932, p. 272; 1940, 

p.'HS; Wahrhaftig and others, 1951, where the rocks are called 

the coal-bearing formation (Tertiary); Wahrhaftig, 1958, p. 9; 

Pewe and others, 1966, where the rocks are referred to informally as the coal-bearing formation).

Dl
D2 CONTRIBUTIONS TO STRATIGRAPHY

64

149°30' 149°00' 148°30'

63°45'

i Lignite Creek Formation

FIGURE 1. Generalized geologic map of the Healy D-2, D-3, D-4, and D-5 

quadrangles and the Fairbanks A-2, A-3, A-4, and A-5 quadrangles, showing distribution of Tertiary formations and localities mentioned in text.
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D3

_148°30' 148°00' 147°30'

Contact

Dashed where approximate, 

queried where uncertain

Cantwell Formation

Fault

Dashed where covered 

by alluvium 

i I D3469

Measured section with 

U.S. Geological Survey 

plant microfossil localities

XD1535-1

U.S. Geological Survey 

plant microfossil locality

FIGURE 1. Continued.
D4 CONTRIBUTIONS TO STRATIGRAPHY

In this paper the coal-bearing formation is informally designated a group and is divided into five formations; four of the 

five formations are assigned to provincial floral stages recognized 

in the Cook Inlet region (Wolfe and others, 1966). This stratigraphic revision is the outgrowth of about 15 seasons of 

geologic mapping of the Nenana coal field and surrounding areas, 

between the years 1943 and 1958 (Wahrhaftig and others, 1951; 

Wahrhaftig, 1951; Wahrhaftig and Hickcox, 1955; Wahrhaftig 

and Birman, 1954: Wahrhaftig, 1958). The areal results of this 

mapping appear as geologic quadrangle maps of eight 15-minute 

quadrangles (Wahrhaftig, 1968a 1968h). The areal distribution of the, /Tertiary formations is shown in figure 1, which is 

generalized from the eight geologic quadrangle maps. The age 

assignments are based on studies of plant megafossils and palynological materials collected throughout the history of the study 

of the coal field, but chiefly by Wolfe and Leopold in July 1963. 

A large quantity of volcanic ash from a bed in the Grubstake 

Formation, collected at that time, was dated by Lanphere with 

the results reported herein. The sections on physical stratigraphy 

are by Wahrhaftig. The biostratigraphy and age assignments are 

by Wolfe and Leopold.

COAL-BEARING GROUP

The informal term coal-bearing formation is here redesignated 

the coal-bearing group and is divided into five formations. The 

group is represented by the section continuously exposed in the 

Ei/2 sec. 23 and SW^4 sec. 24, T. 12 S., R. 7 W., Fairbanks 

base line and meridian, Healy D-4 quadrangle, along the north 

bank of Healy Creek from a basal contact on the Birch Creek 

Schist at the Usibelli tipple westward to the mouth of Suntrana 

Creek and up Suntrana Creek for 2,500 feet where, at the forks 

of the creek, the coal-bearing group is overlain apparently conformably by the Nenana Gravel (figs. 1, 2). Four of the five formations into which the group is here divided have their type 

sections in this exposure, and a reference section for the fifth 

exists here (fig. 3).

Other places within the Nenana coal field that have nearly 

complete exposures of the group include the exposures on Lignite 

Creek in sees. 25 and 35, T. 10 S., R. 7 W. (in Healy D-4 

quadrangle), about 6 miles above its mouth; the banks of
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA

148°50'

DB

I ^-^-^\ -i- \--IS ^--. r^ ! 

Base from U.S. Geological Survey, 1950 4000 FEET

CONTOUR INTERVAL 500 FEET 

DATUM IS MEAN SEA LEVEL

EXPLANATION

Lignite Creek Formation

Contact

Dashed where approximately located; 

doited where concealed

Strike and dip of beds

Birch Creek Schist

A B

Location of representative section of the coal-bearing group 

and type sections of the Healy 

Creek, Sanctuary, Suntrana, 

and Lignite Creek Formations

Letters indicate location of columnar sections shown on figure 3

FIGURE 2. Geologic map of sees. 23 and 24 and parts of sees. 13, 14, 25, and 

26, T. 12 S., R. 7 W., Healy D-4 quadrangle, showing the Tertiary coalbearing group and type localities for the Healy Creek, Sanctuary, Suntrana, and Lignite Creek Formations.

322-557 O - 69 - 2
D6 CONTRIBUTIONS TO STRATIGRAPHY

Nena 

Gravi

i 

na J

61 if

Reference 

section of 

Grubstake 

Formation

1

m

2

cS 

£1 

^' A

Coal bed No. 6

Coal bed No. 6

c.2 

Coal bed No. 5 cE

m

Coal bed No. 4 

-7622, 7634

Coal bed No. 3

Coal bed No. 2 ° |

Coal bed No. 1

G coal bed

Coal bed No. 2 

Coal bed No. 1 

G coal bed

F coal bed

E coal bed

D coal bed/fBurned and

\ covered 

C coal bed

B coal bed

A coal bed

" 9926

F coal bed

OOT

B

FEET 

r 200

- 100

: 0

FIGURE 3. Stratigraphic column of the type sections of the formations of 

the coal-bearing group at Suntrana, showing locations of plant megafossil 

localities and microfossil localities. Section A, measured along Suntrana 

Creek from coal bed No. 6 to the base of the Nenana Gravel; section B, 

measured along Suntrana Creek from near the railroad bridge to coal bed 

No. 6; section C, measured on the north bank of Healy Creek from the 

Usibelli tipple to top of the badland exposure east of the Suntrana 

mine. 1, coal (showing bone or clay parting); 2, bony coal; 3, bone; 4, 

claystone and shale; 5, siltstone; 6, sandstone, in part crossbedded; 7, pebbles and conglomerate; 8, schist (unconformity at top).
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D7

Tatlanika Creek and its headwaters, Moose Creek and Sheep 

Creek, from about 1,500 feet south of their junction northward 

to the mouth of Grubstake Creek (fig. 6); and the headwaters of 

Coal Creek, a tributary of the Wood River, in southwestern Fairbanks A-2 quadrangle and northwestern Healy D-2 quadrangle 

(fig. 1).

HEALY CREEK FORMATION

The lowest formation within the coal-bearing group is here 

named the Healy Creek Formation from exposures along Healy 

Creek in the Healy D-4 quadrangle. The Healy Creek Formation 

includes all strata of the coal-bearing group below the top of 

the F coal bed of the Suntrana coal mine. Its type section, in 

the SW14 sec. 24, T. 12 S., R. 7 W., Fairbanks base line and 

meridian (figs. 2, and 3, section C), is the exposure on the northeast wall of the canyon of Healy Creek at Suntrana, from its 

basal contact with the unconformably underlying Birch Creek 

Schist a few feet south of the Usibelli tipple, northward for 

1,000 feet to the top of the F coal bed, where it is overlain 

conformably by the brown-weathering shale of the Sanctuary 

Formation. The Healy Creek Formation includes all the lower 

member of the coal-bearing formation of Wahrhaftig, Hickcox, 

and Freedman (1951, p. 147-152) below the brown shale.

At the east end of the Healy Creek syncline, about 6i/2 miles 

east of the type section, the Healy Creek Formation includes beds 

that are probably much older than those of the type section. 

This is about the thickest well-exposed section of the Healy Creek 

in the Nenana coal field and is therefore designated a reference 

section. It includes beds on the south bank of Healy Creek between 

the basal unconformity at the east edge of sec. 12, T. 12 S., R. 

6 W., and the top of the Moose coal bed, the stratigraphic equivalent of the F coal bed at Suntrana, about 2,000 feet west of the 

basal unconfomrity along the creek bank. (See Wahrhaftig and 

others, 1951, pi. 19, section M.)

The Healy Creek Formation is probably the most widely distributed of the formations that constitute the coal-bearing group, 

but its distribution is patchy (fig. 1). As the lower member of 

the coal-bearing formation, it was recognized from the Sushana 

River on the west (Wahrhaftig, 1951, p. 174-175) to the Jarvis 

Creek coal field on the east (Wahrhaftig and Hickcox, 1955, p. 359),
D8 CONTRIBUTIONS TO STRATIGRAPHY

a distance of 125 miles. Many if not most of the isolated patches 

of coal-bearing rocks, which are a few hundred yards to a mile 

or two across and are widely scattered throughout the Alaska 

Range and adjacent regions, should probably be correlated with 

the Healy Creek Formation.

The Healy Creek Formation was deposited on an irregular 

surface of considerable relief; in places this surface stood above 

the level of the base of the overlying formations, and in these 

places the Healy Creek Formation was never deposited. Because 

of this irregular basal surface, the thickness of the Healy Creek 

Formation varies markedly over short distances.

The maximum measured thickness in the Nenana coal field 

is 1,150 feet on the south side of the Lignite Creek coal basin 

near the western edge of Healy D-3 quadrangle (fig. 1). It is 

800 feet thick at the east end of the Healy Creek coal basin and 

about 1,000 feet thick in the Mystic Creek coal basin. About 

2,000 feet of beds in the Jarvis Creek coal field have been correlated on the basis of lithology with the Healy Creek Formation (Wahrhaftig and Hickcox, 1955, p. 357-359).

The Healy Creek Formation consists of interbedded poorly consolidated sandstone, conglomerate, claystone, and subbituminous coal. The most characteristic features of this formation 

are the lenticularity of its beds and the tendency for its lithologic components pebbles, sand, clay, and coal to be mixed together in the same bed rather than to be cleanly separated as 

they are in the overlying formations. Thus, many of the coals 

are thin bedded, lenticular, and bony, and thin claystone partings thicken abruptly and grade into coarse sandstone; sandstones, commonly have a clay binder; and claystones with pebbles and angular rock fragments are common, especially near 

the base of the formation.

The sandstone and conglomerate characteristically consist of 

materials from nearby basement rock. Where quartz-sericite 

schist is the basement, the characteristic sandstone is buff to 

cream and consists largely of grains of quartz and fine flakes 

of silvery sericite. The associated conglomerate generally consists wholly or largely of quartz pebbles, presumably derived 

from quartz veins in the schist. Where the basement is feldspathic schist and gneiss, the sandstone tends to be strongly
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D9

arkosic and the associated conglomerate contains an abundance 

of pebbles of the schist and gneiss.

The Healy Creek Formation appears to have gradually buried 

a fairly rugged topography on the north side of the Alaska Range 

and its foothills by filling the valleys first and covering the hills 

later. Current directions, such as crossbedding, indicate a diversity of directions of streamflow during its accumulation. The 

prominent direction of streamflow was northward, which was 

probably the general direction of outflow from the area of the 

Nenana coal field to the sea at that time.

Biostratigraphy and Age

The flora, both megafossil and microfossil, of the Healy Creek 

Formation can be separated into two markedly distinct assemblages: the lower Healy Creek (or Rex Creek) flora and the 

upper Healy Creek flora.

The lower part of the Healy Creek Formation on Rex and California Creeks (fig. 4) contains a distinctive assemblage of fossil 

plants that has not been recognized elsewhere in Alaska. Two of 

the most characteristic species of this assemblage are new: 

Alnus sp. aff. A. carpinoides Lesq. and Spiraea sp. aff. S. hopkinsi Wolfe. This assemblage also contains Engelhardia, which 

is not known in Miocene rocks in Alaska, and Sequoia affinis Lesq., 

which is not known above the Oligocene; S. affinis probably is 

ancestral to the Miocene through Holocene species S. sempervirens 

Endl. Some of the Rex Creek species are known in the Tsadaka 

flora and its age equivalents; particularly noteworthy is the 

occurrence of Alnus evidens (Holl.) Wolfe because this species 

is thought to be ancestral to the early and middle Miocene A. 

cappsi (Holl.) Wolfe (Wolfe, 1966, p. B18).

The Tsadaka flora (and by inference the Tsadaka Formation) 

was placed in the lower (?) Seldovian, which was considered 

to be of latest Oligocene (?) and earliest Miocene age (Wolfe 

and others, 1966, p. A16-A17). At the time the Seldovian Stage 

was erected, it was thought that certain floras, although showing 

a close relationship to the floras of the type section of the Seldovian, were somewhat older, and these probably older floras 

were designated as lower (?) Seldovian floras (Wolfe and others, 

1966, p. A15). At the time, no floras were known from the 

lowest part of the type section of the Seldovian Stage, and it
D10 CONTRIBUTIONS TO STRATIGRAPHY

was thought that lower (?) Seldovian floras might be represented in the type section. Additional collecting and study from

148°40'

64°05

EXPLANATION

Nenana Gravel

Contact

Long dashed where approximately located; short dashed where inferred; 

Queried where position uncertain

Totatlanika Schist

X9928 __D3834 =

U.S. Geological Survey plant Measured section with U.S. 

megafossil locality Geological Survey plant 

microfossil localities

FIGURE 4. Geologic map of part of T. 9 S., R. 6 W., Fairbanks A-4 quadrangle, showing plant fossil localities in the Rex Creek coal basin.
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA Dll

the type section has, however, indicated that the lowest floras 

from the type section of the Seldovian are those that were referred to as "upper(?) Seldovian." Therefore, the "lower(?) 

Seldovian. The Tsadaka flora and other "lower(?) Seldovian" 

eluded from the Seldovian, and the "upper(?) Seldovian" of 

former usage, of early and (or) middle Miocene age (Wolfe 

and others, 1966, p. A17), is now considered tq be the only valid 

Seldovian. The Tsadaka flora and other "lower(?) Seldovian" 

floras are therefore considered to be of pre-Seldovian (preMiocene) age. Because the Rex Creek flora is close stratigraphically to true Seldovian floras, it is probably of late Oligocene 

age. The Tsadaka Formation and flora are probably older than 

the Rex Creek flora and are now considered to be of middle and 

late(?) Oligocene age.

The eight pollen and spore assemblages from the lower part 

of the Healy Creek Formation are perhaps even more distinctive 

than the megafossil assemblages. The lower part of section 

D3551 and the two samples at D3550 (fig. 4) are characterized by the most diverse pollen floras yet found in the coal-bearing group and by several forms not found elsewhere in the group. 

These forms include: Aquilapollenites n. spp. (heteropolar and 

isopolar), Orbiculapollis, Saxifraga, Itea, Pachysandra/Sarcococca, Cuphea-type, Melia/Cedrela, Engelhardia/Alfaroa, and 

Proteacidites globisporus Samoil. The species of Aquilapollenites are abundant in some samples and occur in most. Engelhardia/Alfaroa occurs in one-third of the samples, although it 

is not common in the tallies. All the samples contain a diversity 

of pollen that came from deciduous broad-leaved trees. Pollen 

of Pinaceae is present but is rare compared to that of Taxodiaceae. Of the 40 vascular plant genera identified, 60 percent 

are now exotic to Alaska; this is the highest percentage for 

any pollen assemblage from the coal-bearing group.

The flora from the upper part of the Healy Creek Formation 

(fig. 3, Iocs. 9925, 9926), is an assemblage clearly distinct from 

the older assemblage and is of Seldovian age. Alnus healyensis, 

A. cappsi, Quercus furuhjelmi, Cocculus auriculata, and Cladrastis 

japonica make their first appearances in the Alaska Range Tertiary rocks in the upper part of the Healy Creek Formation. A 

few species, for example Ulmus appendiculata Heer, Betula sp.
D12 CONTRIBUTIONS TO STRATIGRAPHY

aff. B. thor Knowl., and Carya sp. aff. C. simulate, (Knowl.) 

R. W. Br., appear to be ancestral to species that occur in 

early (?) and middle Miocene floras in conterminous United 

States. At least one of these species Betula sp. aff. B. thor 

occurs in floras of late Oligocene(?) and early Miocene age in 

Oregon. Thus, the Seldovian floras such as the upper Healy Creek 

flora are considered to be of early Miocene age.

The eight pollen assemblages from the upper part of the 

Healy Creek Formation contain several forms not found in the 

lower part: Ephedra, Quercus, Nyssa, and Compositae (only one 

grain of a short-spined type). In addition, the upper Healy Creek 

flora lacks the forms listed above that characterize the Rex 

Creek flora; the percentage of genera now exotic to Alaska is 

58, which is slightly lower than that of the assemblage from 

the lower part of the Healy Creek. Pollen of Viburnum, which 

is also found in the Rex Creek flora, has not been found above 

the upper part of the Healy Creek Formation. The extinct genus 

Gynkaletes is apparently restricted to the upper Healy Creek 

flora. The pollen flora, in general, contains a diversity of the 

deciduous broad-leaved trees and is comparable to the pollen 

floras from the type and reference sections of the Seldovian 

Stage. The pollen thus substantiates that the upper part of the 

Healy Creek Formation is within the Seldovian Stage. Therefore, the Healy Creek Formation ranges in age from late(?) 

Oligocene to early Miocene, but in its type locality, is of early 

Miocene age only.

SANCTUARY FORMATION

The Sanctuary Formation is the brown-weathering shale that 

lies between the top of the F coal bed and the base of the 

coarse conglomerate beneath the No. 1 and G coal beds at the 

Suntrana mine, in SW^, sec. 24, and SE^ sec. 23, T. 12 S., 

R. 7 W., the type locality, in the Healy D-4 quadrangle (figs. 

2, 3). The type section is designated as the outcrops east of the 

Suntrana tipple, shown on section C of figure 3. It is the "thick 

bed of yellowish-brown varved silt and clay" at the top of the 

lower member of the coal-bearing formation of Wahrhaftig, 

Hickcox, and Freedman (1951, p. 147). At the type locality the 

Sanctuary Formation is 90-130 feet thick and forms a prominent brown band of rounded landforms extending across the
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D13

north wall of the badland amphitheatre in which the surface 

workings of the Suntrana mine are located. It is named for 

exposures of similar brown-weathering shale on the east bank 

of the Sanctuary River % to 1 mile south of the north boundary 

of Mount McKinley National Park, which are believed to be the 

stratigraphic equivalent of the Sanctuary Formation at its type 

locality (Wahrhaftig, 1951, p. 176).

The Sanctuary Formation occupies an interval 350 feet thick 

between the Moose coal bed and the coal beds beneath the No. 1 

bed at the east end of the Healy Creek coal basin (Wahrhaftig 

and others, 1951, pi. 19, section M). In the Lignite Creek coal 

basin it ranges in thickness from 5 to 250 feet. It is present 

in the Rex Creek coal basin (figs. 1, 4), at the head of Tatlanika Creek, on Coal Creek tributary to the Wood River, on 

Kansas Creek (fig. 1), and from the Sanctuary River southwestward to the divide between the Sushana and the Toklat 

Rivers (Wahrhaftig, 1951). It may be present in exposures on 

the east bank of the Delta River about 2y% miles west of Donnelly Dome in Mount Hayes D-4 quadrangle ( Pewe and Holmes, 

1964; Wahrhaftig and Hickcox, 1955, p, 359-360).

Over most of its area of exposure the Sanctuary Formation 

rests conformably on the Healy Creek Formation. However, to 

the north and northwest it overlaps the Healy Creek Formation 

to rest directly on schist and other basement rocks and is in 

turn cut out by overlap of the Suntrana Formation. The formation thickens southeastward from this line of overlap (shown 

in fig. 5), and its original southern limits are unknown.

The Sanctuary Formation is predominantly a gray shale that 

weathers to a characteristic chocolate brown or yellowish brown. 

In clean exposures, such as the narrow gully floors that score 

its outcrop at Suntrana, the shale is seen to be finely banded 

(possibly varved) with alternating pale-weathering and darkweathering laminae a fraction of an inch to an inch thick. Exposures that exhibit bedding are rare, for the shale breaks down 

quickly on exposure to a mass of flat yellow-brown chips that 

have split along the bedding, and where it is continually saturated with water, it forms masses of brown to gray sticky mud 

and is the locus of persistent large landslides. Its yellow-brown 

color on weathered outcrops and the tendency to break into flat

322-557 O - 69 - 3
D14 CONTRIBUTIONS TO STRATIGRAPHY

 43

s-s * "

i a) <U :*i
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D16

yellow-brown chips make the Sanctuary Formation a useful 

stratigraphic marker in the Nenana coal field.

In the type section at the Suntrana mine the upper half of the 

formation is silty and contains numerous beds of fine sand a 

few inches to a few feet thick. Sand beds are common in the 

Sanctuary Formation elsewhere in the Nenana coal field. Coal 

and bone are also common near the top of the Sanctuary Formation in such places as upper Lignite Creek and Coal Creek 

on the Wood River.

The Sanctuary Formation probably accumulated in a large shallow lake, the north shore of which is the line of overlap of the 

Sanctuary Formation by the Suntrana Formation, shown in 

figure 5. The change from gradual burial of an old land surface, 

with deposition keeping pace with subsidence, which was the 

history of Healy Creek time, to flooding of a large area by a freshwater lake, could only have been brought about by sudden increase in the rate of downwarping, depressing a large area 

below some local base level. The local basin of subsidence indicated by the southward and southeastward thickening of the 

Sanctuary Formation continued to subside during the period 

of accumulation of the overlying formations.

Biostratigraphy and Age

The megafossils from the Sanctuary Formation represent two 

species: Metasequoia glytostroboides Hu and Cheng and Fagus 

antipofi Heer. Thus far, the latter species has not been found in 

Alaska in rocks younger than Seldovian in age.

The microfossil flora, known from six samples from the Sanctuary Formation, is notable for the occurrence of numerous 

aquatic plants: dinoflagellate algae, algal cysts referred to 

Tetraporina, colonial bluegreen algae, Nymphaea, Scirpits, Polygonum, and Myriophyllum.

Pollen of deciduous broad-leaved trees is consistently represented in the Sanctuary pollen floras. Pollen of this element 

represents about 5 percent of the pollen tallied and is somewhat 

more common than in the upper part of the Healy Creek Formation. One sample has pollen of Pinus as a dominant, but in 

the other samples pollen of Taxodiaceae is more common than 

Pinaceae. Of the 33 identified vascular genera, 52 percent are
D16 CONTRIBUTIONS TO STRATIGRAPHY

now exotic to Alaska. This percentage in conjunction with the 

consistent representation of the deciduous broad-leaved tree element indicates that the Sanctuary Formation should be placed 

in the Seldovian Stage. Thus the Sanctuary is assigned to the 

early or middle Miocene or both.

SUNTRANA FORMATION

The Suntrana Formation is that part of the coal-bearing group 

between the top of the conformably underlying Sanctuary Formation and the top of the No. 6 coal bed which underlies, with 

apparent conformity, the Lignite Creek Formation at the Suntrana mine. It is equivalent to the middle member of the 

coal-bearing formation of Wahrhaftig, Hickcox, and Freedman 

(1951). It is named for its exposures on the walls of Suntrana 

Creek, the type locality, in SE14 sec. 23, T._12 S., R. 7 W., Healy 

D-4 quadrangle (fig. 2 and fig. 3, section 5). All the numbered 

coal beds of the Suntrana mine are in the Suntrana Formation, 

and it contains the bulk of the coal reserves of the Nenana coal 

field, including nearly all the thick and persistent coal beds.

The Suntrana Formation is a many-times-repeated sequence 

with coarse pebbly sandstone grading upward from the base 

through medium- and fine-grained sandstone to clay and coal at 

the top. Generally there is only one thick coal bed at the top of 

each unit, but in some places two or three thin coal beds are 

interbedded with the claystone. In a typical sequence the sandstone at the base is 20-100 feet thick, the clay and silt unit is 

2-15 feet thick, and the coal is 2-40 feet thick. On the average there are eight repetitions of this sequence in the formation, but in the northern part of the Lignite Creek coal basin 

there are as few as five or six, and in the sections at the head 

of Lignite Creek and at the east end of the Healy Creek coal 

basin there are as many as 10 or 12. The formation as a whole 

averages about 70 percent sandstone (including pebble beds) 

and 15 percent each coal and clay.

The sandstone is poorly consolidated, generally clean, well 

sorted, and crossbedded. It is chalk white to very light buff, 

although it may be stained by iron oxide to orange or red for 

a foot or two above each coal bed. In the field this type of sandstone was called salt-and-pepper sandstone because under a hand 

lens it is seen to consist predominantly of white grains with 

a scattering of black- to dark-gray grains of chert. This saltand-pepper appearance is quite different from that of the more
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D17

colorful sandstone of the overlying Lignite Creek Formation. 

Petrographic studies by M. C. Blake (written commun., 1959) 

showed that quartz makes up 70-75 percent of the sandstone; 

orthoclase, 5-10 percent; plagioclase, 1-5 percent; chert and 

rock fragments, 5-10 percent; and heavy minerals, chiefly of 

low-grade metamorphic suites, 6V2 percent.

The pebbles, which average J4 to 1 inch in diameter, consist 

of more than 65 percent quartz, chert, quartzite, argillite, and 

jasper resistant rock types and of less than 35 percent nonresistant rocks such as granitic rocks, gabbro, greenstone, graywacke, or volcanic rocks. Below the No. 5 coal bed the proportion of resistant rock types in the pebble population is about 

80 percent.

The coal beds, which are generally 10-60 feet thick, can be 

traced laterally for long distances and appear to thicken regularly to a single maximum for each bed, located either in the 

eastern part of the Healy Creek coal basin or in the southern 

part of the Lignite Creek coal basin. The coal is black with a 

dark-brown streak, is comparatively low in ash, and characteristically breaks into small equidimensional blocks.

The Suntrana Formation appears to have accumulated in a 

subsiding basin whose axis lay at the south edge or to the south 

of the present coal field. Current directions indicated by the 

persistent crossbedding of the sandstone are predominantly southward in the Healy Creek and Lignite Creek coal basins and are 

westward in the Tatlanika Creek and Wood River coal basins. 

The source area of the clastic components probably lay in the 

southern Yukon-Tanana Upland, including the area of the Livengood Chert (Mertie, 1937, p. 105-111).

The lateral continuity of the coal beds and of the intervening 

sandstones indicates that at any one time conditions throughout 

the coal basin were fairly uniform, but that they alternated 

between periods when the entire plain was a coal-forming swamp 

and periods when vigorous streams from the north were depositing sheets of gravel and sand over the plain.

The Suntrana Formation is widely exposed in the Healy Creek 

and Lignite Creek coal basins and may also occur in the center 

of the Rex Creek coal basin. It is exposed at the head of Tatlanika Creek and in the large badland at the head of Coal Creek
D18 CONTRIBUTIONS TO STRATIGRAPHY

on the Wood River. It is apparently absent from the western 

part of the Nenana coal field (Wahrhaftig, 1951).

The Suntrana Formation pinches out northward along a line 

shown in figure 5. Southward from this line, the formation thickens at a rate of 50-100 feet per mile to maximum measured 

thicknesses of 1,000 feet on Coal Creek on the Wood River and of 

1,290 feet on Coal Creek on Healy Creek. The individual coal 

beds and sandstone units of the Suntrana Formation also thicken 

regularly southward, but some of these reach their maximum 

thickness in the Lignite Creek coal basin.

Biostratigraphy and Age

The megafossil flora of the Suntrana Formation is not well 

known, primarily because the lack of induration of the shales 

makes collecting entire leaves difficult. Species of Salix and 

Populus are typically the most abundant leaves at each locality. 

Leaves of broad-leaved tree genera now exotic to Alaska are, 

however, present at most localities. The Suntrana leaf floras 

differ from those of the upper part of the Healy Creek Formation by the presence of "Ficus" overbecki Holl. (this should be 

transferred to Populus) and Ulmus californica Lesq. The latter 

species is probably descended from U. appendiculata Heer, which 

occurs in the upper part of the Healy Creek. U. californica is 

known in the conterminous United States from floras of early (?) 

Miocene and younger age. Alangium aequifolium (Geopp.) 

Kryst., however, is known in Japan in floras of middle Miocene 

and older age. The Suntrana flora also contains Alnus cappsi and 

A. fairi, which are not known to range into the late Miocene 

in either Alaska or the conterminous United States. A. cappsi is 

particularly significant because it is thought to be ancestral to 

the late Miocene A. corylina Knowl. and Cocker. The presence 

in the Suntrana flora (fig. 3, loc. 7622) of Quercus (Leucobalanus) also indicates a pre-Homerian age; Quercus is not known 

to occur in Alaska in beds younger than the Seldovian.

The pollen floras (known from 26 samples) contain an abundance of four-pored Alnus and Betula grains; in one sample these 

compose 75 percent of the pollen tallied. The deciduous broadleaved tree element is considerably reduced in consistency of 

occurrence and generally represents about 3 percent of the pollen 

tallied. The percentage of now exotic genera, however, is the
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D19

same as for the Sanctuary Formation (52 percent). Both Fagus 

and Acer have their highest local occurrence in the Suntrana.

The pollen and megaf ossil floras thus indicate that the Suntrana 

Formation should be placed in the Seldovian Stage. Because of 

the overlap in ranges of some of the megafossil species, the 

Suntrana Formation is probably of middle Miocene age.

LIGNITE CREEK FORMATION

The Lignite Creek Formation is the sequence of interbedded 

buff sandstone, greenish-gray claystone, and thin coal beds that 

lie between the top of the No. 6 coal bed of the Suntrana mine 

and the base of the prominent greenish-gray shale at the top of 

the coal-bearing formation. It includes sandstone beds in the 

northern part of the Nenana coal field thought to be contemporaneous with this sequence. The type section is in sec. 23, T. 

12 S., R. 7 W., Healy D-4 quadrangle, along Suntrana Creek 

from the top of the No. 6 coal bed northward for about 1,000 feet 

to the top of the sandstone outcrop between the first forks of 

the creek and totals about 540' feet in thickness (figs. 2, 3). It 

makes up approximately the lower five-sixths of the upper member of the coal-bearing formation of Wahrhaftig, Hickcox, and 

Freedman (1951).

The Lignite Creek Formation has two facies: a coal-bearing 

facies, exposed in Healy Creek and Lignite Creek coal basins and 

in the southern part of the Tatlanika Creek and Wood River 

coal basins; and a noncoal-bearing facies in the northern and 

western parts of the coal field. Their common boundary is shown 

approximately in figure 5. The noncoal-bearing facies was mapped 

by Capps (1940, pi. 3) as the Nenana Gravel.

The coal-bearing facies is about 500 feet thick in the northwestern part of the Lignite Creek coal basin and thickens southeastward to about 750 feet thick at the east end of the Healy 

Creek coal basin. It is about 620 feet thick in the Tatlanika 

Creek coal basin and about 800 feet thick in the Wood River 

coal basin. Like the Suntrana Formation, it is made up of a 

repeated sequence of pebbly sandstone, claystone, and coal. In 

contrast to the Suntrana Formation, the top of each sequence in 

the Lignite Creek Formation has several thin coal beds interbedded with claystone. Five sequences are present at the type 

locality. The coal-bearing facies averages about 5 percent coal, 

30 percent siltstone and claystone (with interbedded fine-grained
D20 CONTRIBUTIONS TO STRATIGRAPHY

sandstone), and 65 percent sandstone, pebble beds, and conglomerate.

Sandstone units in the coal-bearing facies are as much as 160 

feet thick. Some are laterally persistent, but others grade laterally into claystone or lens out altogether. The sandstone is generally poorly consolidated, clean, crossbedded, well sorted, and 

permeable. It is buff and under the hand lens is seen to contain 

an abundance of variously colored grains. This contrast with the 

salt-and-pepper sandstone of the Suntrana Formation is one of 

the most readily recognizable distinctions between the two formations. Petrographic study by M. C. Blake (written commun., 

1959) showed the sandstone to consist of 65-70 percent quartz, 

10-15 percent plagioclase, 5-10 percent orthoclase, 10 percent 

chert and rock fragments, and 10 percent heavy minerals. Etched 

pigeonite grains, an abundance of fresh ferromagnesian minerals, and predominance of plagioclase over orthoclase distinguish 

the sandstone of the Lignite Creek Formation petrographically 

from that of the Suntrana Formation.

Pebble layers and conglomerate beds make up 20-50 percent 

of the lower parts of some of the sandstone units, and conglomerate beds are as much as 20 feet thick. Average pebble size is 

1-2 inches and the maximum size at a given locality is 6-8 

inches. Rocks that are relatively unresistant to weathering such 

as granite, diorite, gabbro, greenstone, basalt, andesite, rhyolite, 

schist, conglomerate, and graywacke make up 35-60 percent 

of the pebble population, including most of the cobbles and boulders ; resistant rock types quartz, chert, quartzite, argillite, and 

jasper make up the remainder. Individual counts of 75-150 

pebbles were repeated many times in each measured section, and 

the abrupt change in pebble population at the top of the No. 6 

bed was the principal means for distinguishing the Lignite Creek 

from the Suntrana Formation.

Coal beds in the Lignite Creek Formation are thin and discontinuous and are difficult to correlate from outcrop to outcrop. 

The coal breaks up on weathering into masses of long narrow 

flakes parallel to the bedding. Broken edges of these flakes show 

tightly compressed annual growth rings, and the flakes are believed to be highly compressed branches and twigs of the coalforming forest. This character of the coai contrasts strongly 

with the blocky fracture of the coals of the Suntrana Formation.
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D21

Only in the top few feet of the No. 5 and No. 6 coal beds is flaky 

weathering coal like that of the Lignite Creek Formation found.

The noncoal-bearing facies interfingers with the coal-bearing 

facies along the banks of Tatlanika Creek for a few miles north 

of Hearst Creek. Along the east and north sides of the Tatlanika 

Creek basin both facies are overlain by the Grubstake Formation, but the Grubstake Formation pinches out along a line extending roughly from the mouth of Buzzard Creek to the Canyon of 

Tatlanika Creek (fig. 5), and the Lignite Creek Formation and 

Nenana Gravel are in contact northwest and west of this line. 

The two formations are poorly consolidated and have few exposures, and their contact in this area (fig. 1) is drawn largely 

on a change in color from buff in the Lignite Creek Formation 

to dark brown or dark gray in the Nenana Gravel and on a 

corresponding change in pebble population. In the upland between 

Tatlanika Creek and the Totatlanika River, the noncoal-bearing 

facies is about 250 feet thick, increasing to 1,000-1,300 feet 

along the north edge of the foothills. Between Moose Creek and 

Eva Creek in southwest Fairbanks A-4 quadrangle (fig. 1), it 

is 70-180 feet thick, and near Slate Creek it is about 300 feet 

thick. In all these areas bedding in the Lignite Creek Formation 

and Nenana Gravel is almost parallel. In the area between Sunday Creek and Dry Creek in the southwest part of Healy D-5 

quadrangle (fig. 1), angular unconformities appear to separate 

the Nenana Gravel from the Lignite Creek Formation and the 

Lignite Creek from the Healy Creek Formation.

Crossbedding in the sandstone of the Lignite Creek Formation 

indicates current directions to the west and southwest in the noncoal-bearing facies and in the coal-bearing facies of the Tatlanika Creek coal basin. Elsewhere current directions are persistently to the south. A northward increase in grain size and 

pinching out of coal and clay indicate a source of clastic components to the north.

Biostratigraphy and Age

The flora of the Lignite Creek Formation is known only from 

six microfossil samples. In these samples, Pinus, Alrnis, or 

Betula are the dominants. The deciduous broad-leaved tree element is considerably reduced, both in regard to abundance (less
D22 CONTRIBUTIONS TO STRATIGRAPHY

than 2 percent of the pollen tallied) and diversity of types. In 

addition, the percentage of genera now exotic to Alaska is 44, 

which is considerably less than in the underlying formations. The 

Lignite Creek, however, contains the highest occurrence of Nyssa, 

which has its highest occurrence in the Cook Inlet basin in the 

Seldovian. The Lignite Creek Formation is probably of late Seldovian and hence middle Miocene age.

GRUBSTAKE FORMATION

The Grubstake Formation includes interbedded dark claystone, 

dark sandstone, and fine conglomerate between the uppermost buff 

sandstone beds of the Lignite Creek Formation and the coarse 

conglomerate of the Nenana Gravel. The type locality, in Fairbanks A-3 quadrangle, consists of the exposures along the east 

bank of Tatlanika Creek from the mouth of Grubstake Creek to 

a point about 1 mile south where it is underlain by the buff 

sandstone of the Lignite Creek Formation (fig. 6). It is between 

600 and 1,000 feet thick at this locality, rests with apparent 

conformity on the Lignite Creek Formation, and is overlain by 

the Nenana Gravel. The flat-lying beds of the Nenana Gravel 

apparently rest unconformably across the upturned edges of the 

coal-bearing group at the heads of Grubstake and Buzzard Creeks, 

but elsewhere in the Tatlanika Creek basin the beds are parallel 

to the bedding of the underlying formations.

The 90 feet of greenish-gray shale at the top of the section of 

the coal-bearing group on Suntrana Creek, in the W1/^ sec. 23, 

T. 12 S., R. 7 W., Healy D-4 quadrangle, is designated as a 

reference section (fig. 3).

The Grubstake Formation appears to underlie three separate 

areas in the Nenana coal field. The largest and thickest body 

extends from the western part of the Tatlanika Creek coal 

basin to the Wood River and appears to underlie much of the 

plateau of Nenana Gravel around Bonnifield and Gold King Creeks 

(fig. 1). Maximum thicknesses of this body are about 1,500 feet 

north of Coal Creek and 1,000 feet in the basin of Grubstake 

Creek, and the formation appears to thicken abruptly (in places 

by as much as 500 feet per mile) from its line of pinchout 

(shown in fig. 5). The second body lies on lower Buzzard Creek 

in the southwest part of Fairbanks A-3 quadrangle and reaches
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D23

a maximum thickness of 500 feet. The third body is the unit 

of greenish-gray shale on top of the Lignite Creek Formation in

EXPLANATION

64°00'

0 5000 10,000 FEET

i i i i i i i i i i i

0 Vz 1 MILE

Contact

Long dashed where approximately located; short dashed where inferred; 

dotted where concealed; queried 

where position uncertain

Inclined Horizontal 

Strike and dip of beds

V 9930 

D3497

Plant fossil locality

FIGURE 6. Geologic map of part of the valley of Tatlanika Creek, Fairbanks 

A-3 quadrangle, showing the type locality for the Grubstake Formation, the 

section on upper Tatlanika Creek, and the site of USGS paleobotanical locality 9930.
D24 CONTRIBUTIONS TO STRATIGRAPHY

the Lignite Creek and Healy Creek coal basins; it thickens southward from a line of pinchout where the Lignite Creek-Nenana 

Gravel contact crosses Elsie Creek in southwestern Fairbanks 

A-4 quadrangle (fig. 1) to about 250 feet at the east end of the 

Healy Creek coal basin.

In the Buzzard Creek and Tatlanika Creek-Wood River bodies, the claystone of the Grubstake Formation is dark gray, poorly 

consolidated, silty, and massive. Locally, it is thin bedded or 

varved and greenish gray. It contains numerous coaly partings 

and thin beds of coal and bone. The coal is usually about an inch 

thick; the thickest bed is 1 foot. Interbedded with this claystone 

are beds of dark-gray, poorly consolidated sandstone 3-40 feet 

thick, with abundant grains of black chert, dark rock fragments, 

and dark minerals. This sandstone weathers brownish red and 

is stained orange on joints and cracks. Conglomerate layers with 

pebbles J/i to 1/2 inch in diameter (maximum l 1/^ inches) are also 

present. Pebbles are milky quartz and dull-black chert, both apparently reworked from the conglomerate of the Cantwell Formation 17-35 miles to the south.

Two thick beds of fine white vitric ash are exposed in the 

lower part of the Grubstake Formation at USGS paleobotany 

locality 9930 on the east bank of Tatlanika Creek between the 

mouths of Roosevelt and Hearst Creeks (fig. 6). The lower ash 

is about 24 feet thick in the exposure, with small-scale crossbedding, and the upper ash is about 13 feet thick. White outcrops seen in the bluffs from a helicopter for about 2 miles south 

of the fossil locality are thought to be of the ash. No white outcrops were spotted in a helicopter traverse of Grubstake Creek, 

nor was any ash seen in a ground traverse of the large gully 

exposure in the Grubstake Formation at the head of Coal Creek, 

but the base of the formation is not exposed in the gully.

The ash consists of more than 99 percent glass in lunelike 

shards and contains less than 1 percent crystals of chlorite, 

muscovite, quartz, plagioclase, and sanidine. The refractive index 

of the glass is given in table 1. Eruption of the lower ash apparently buried a forest growing at the site, and erect coalified 

trunks, rooted in place, rise 15-20 feet into the ash. USGS paleobotany locality 9930 is the buried litter of this forest.
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D25

TABLE 1. Indices of refraction of volcanic glass from the 

Grubstake Formation

[Measurements by G. D. Eberlein (written commun., Nov. 1967)]

Sample Locality Index of refraction

( ((p 25°C)

63Awg 32 USGS paleobotany loc. 9930, east bank of Tat- 1.499±0.001 

lanika Creek, Fairbanks A-3 quadrangle. 

(See fig. 6.) ____________________

________Hard layer with fine white markings perpen- 1.499+0.001 

dicular to bedding, 30 ft above base of Grubstake Formation, forks of Suntrana Creek, 

Healy D-4 quadrangle. (See figs. 2, 3.) ____

It was not possible to obtain pure separates of the minerals 

because of their rarity and small size. The biotite separate 

( 150 + 200 mesh fraction) contains about 90 percent biotite 

and 10 percent chlorite. The muscovite separate ( 150-f-270 

mesh fraction) only contains about 50 percent muscovite; plagioclase, quartz, sanidine, and glass make up the remaining 50 

percent. Potassium-argon analyses of the biotite and muscovite 

concentrates gave ages of 57.3 and 54.4 million years, respectively. A determination on the glass itself gave an age of 8.1 

million years. This should be regarded as a minimum age for 

the ash. The analytical data for these determinations are given 

in table 2. A discussion of the implications of these dates follows 

the fossil data.

TABLE 2. Potassium-argon ages and analytical data

[Field No. 63Awg 32. Potassium analyses by H. C. Whitehead and L. B. Schlocker; argon 

analyses by M. A. Lanphere. Decay constants for K4°: XC =0.585X10 1° yeai '; XB= 

4.72X10 10 yeai i. Atomic abundance of K40=1.19X 10--*]

Material

Biotite

Muscovite _

Glass

K2O analyses 

(pei'cent)

2.82, 2.85

__ 3.52, 3.60

_ 3.98, 3.98

Average K2O 

(percent)

2.84

3.56

3.98

Ar«

(10 '0 rad 

moles/g)

2.439

2.902

.6936

Al-40

rad

Ar40 

total

0.33

.36

.30

(millions of 

years )

57.3+2.3

54.4+2.2

8.1+ .4

The Grubstake Formation in the Healy Creek and Lignite Creek 

coal basins is greenish-gray thin-bedded shale and claystone. At 

its base is a thin bony coal bed. The upper part of the shale has 

several sand and silt beds and commonly grades into dark sandstone at the top. At about one-fourth to one-third of its thickness 

above the base is a zone of thin chalky weathering layers with 

closely spaced lines of parting perpendicular to the bedding. Mi-
D26 CONTRIBUTIONS TO STRATIGRAPHY

croscopic examination of these layers showed they contained 

abundant glass shards with the same refractive index as the ash 

on Tatlanika Creek. (See table 1.) The presence of this ash in 

both areas establishes the correlation of the shale on Healy and 

Lignite Creeks with the type Grubstake Formation.

The Grubstake Formation appears to have accumulated mainly 

in a lacustrine environment in two relatively small basins of 

rapid subsidence a larger basin including the exposures on 

Tatlanika Creek, Wood River, Healy Creek, and Lignite Creek, 

and a much smaller basin on Buzzard Creek. Its abrupt thinning 

and its absence from structural highs such as the dome north of 

Buzzard Creek (fig. 1) suggest that it accumulated during a 

period of tectonism. Crossbedding and pebble lithology suggest 

a southerly source. It probably represents the period of transition from south-flowing drainage during accumulation of the 

Suntrana and Lignite Creek Formations to north-flowing drainage during deposition of the Nenana Gravel. Although lithologically it is somewhat more akin to the Nenana Gravel than to 

the underlying formations, it has been mapped in the past as 

part of the coal-bearing formation. In areas of poor exposure 

where mapping depends heavily on topographic expression, the 

only contact to be marked topographically is the GrubstakeNenana contact at the sharp conclave angle at the base of the 

steep bluffs of the coarse permeable Nenana Gravel.

Biostratigraphy and Age

The megafossil flora of the Grubstake Formation is known 

only from the tuff bed on Tatlanika Creek, which is in the basal 

part of the formation. At this locality, the forest has been preserved in place, and around individual stumps organs of a particular species presumably from the tree of which only a stump 

remains are abundant. The dominants of this vegetation were 

Betula papyrifera Michx. and Pinus monticola Dougl.; other 

genera of Pinaceae and Salix are also common.

The occurrence of the extant Betula papyrifera indicates that 

the Grubstake can be no older than late Miocene (Homerian), 

because a probably ancestral species, B. thor, is present in the 

Seldovian. Elsewhere in western North America, B. papyrifera 

is unknown in rocks older than late Miocene. The species of 

Pinaceae of the Grubstake flora are all apparently extant species
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D27

that are known to range into the Miocene, and they are thus of 

little value in placing an upper age limit on the Grubstake Formation. One fragmentary leaflet, however, appears to represent the 

typical Homerian species Spiraea hopkinsi. The available megafossil evidence thus indicates that the basal part of the Grubstake 

Formation is of late Miocene age. Lacking any data on the upper 

age limit of the Grubstake, the late Miocene age assignment for 

the entire unit should be considered provisional.

The microfossil floras of the Grubstake (known from 11 samples) are problematic in that some of the pollen may have been 

redeposited from the older formations of the coal-bearing group. 

As in the megafossil flora, Pinaceae and Betulaceae dominate the 

microfossil floras. Only one sample contains pollen of several 

broad-leaved exotic genera; this sample has the only occurrence 

in the Grubstake of Liquidambar, Juglans, /tea-type, and Aquilapollenites and also contains genera that are rare in other Grubstake samples such as Carya, Pterocarya, Ulmus, and Ostrya/ 

Carpinus. We think it is highly probable that this sample contains significant amounts of redeposited pollen and should, therefore, be considered an unreliable indicator of the Grubstake flora 

and vegetation. Similarly, another sample contains Quercus, 

Carya, Pterocarya, Ulmus-type, and Tilia. In the remaining nine 

samples, the only broad-leaved exotic genera are Ostrya /Carpinus, 

Diervilla, Carya, Pterocarya, and Tilia, and no more than two 

or three of these occur in any one sample. The Grubstake samples are, therefore, comparable to the samples frpm the type 

section of the Homerian Stage in which the frequency of genera 

of broad-leaved exotics is low (Wolfe and others, 1966), and 

Betulaceae and Pinaceae are dominant.

SIGNIFICANCE OF THE RADIOMETRIC DATES

The apparent age of 8.1 million years on the glass from the 

Grubstake Formation is not incompatible with a late Miocene 

age for the flora from the ash. The Miocene-Pliocene boundary 

is currently placed at about 10 million years (Evernden and 

others, 1964, p. 147, 164, 167). The argon retention characteristics of glass are known to be poor, and leakage of argon sufficient 

to lower the apparent age of the glass by 2 million years is not 

unreasonable.
D28 CONTRIBUTIONS TO STRATIGRAPHY

The dates of 57.3 million years for the biotite and 54.4 million 

years for the muscovite concentrate are in good agreement with 

each other Iput are inconsistent with the late Miocene age of the 

flora. The biotite and muscovite dates suggest an early Tertiary 

age for these minerals (Evernden and others, 1964). The ash 

is remarkably pure and contains no recognizable epiclastic contaminants. The ash fall appears to have stripped the trees and 

left the trunks standing as now coalified fossils. Shards and 

crystals of the ash have been found at the same stratigraphic position in the Grubstake Formation as much as 25 miles away.

There seems to be little likelihood that the ash could be redeposited from an earlier eruption, as this would have required 

transport of the ash across several miles of poorly consolidated 

Tertiary rocks and probable contamination during transport. In 

addition, no ash of the appropriate stratigraphic position is 

known from the vicinity of the Nenana coal field. Thus, it seems 

likely that the biotite and muscovite are contaminants from the 

vent from which the ash was erupted.

Dalrymple (1964) has shown that xenoliths in a basalt flow 

were not completely degassed of their accumulated radiogenic 

argon. In a basalt flow xenoliths probably would be heated to 

above 1,000°C and held at a temperature of several hundred degrees at least several days. In an eruption of the sort that deposited the ash of the Grubstake Formation, individual crystals and 

glass shards would cool rapidly. The character of the buried vegetation uncharred and merely coalified shows that the ash was 

cool when it was deposited. This reasoning combined with the 

concordant ages of the two xenocrystic minerals suggests that 

they have lost little if any radiogenic argon during incorporation 

in the ash. The minerals probably were derived from crystalline 

rocks of the Alaska Range whose ages are poorly known at present.

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Brooks, A. H., 1911, The Mount McKinley region, Alaska, with descriptions 

of the igneous rocks and of the Bonnifield and Kantishna districts, by 

L. M. Prindle: U.S. Geol. Survey Prof. Paper 70, 234 p.

Capps, S. R., 1912, The Bonnifield region, Alaska: U.S. Geol. Survey Bull. 

501, 64 p.

 1919, The Kantishna region, Alaska: U.S. Geol. Survey Bull. 687, 118 p.

 1924, Geology and mineral resources of the region traversed by the 

Alaska Railroad, in Brooks, A. H., and others, Mineral resources of 

Alaska, report on progress of investigations in 1922: U.S. Geol. Survey 

Bull. 755, p. 73-150.
COAL-BEARING GROUP, NENANA COAL FIELD, ALASKA D29

Capps, S. R., 1932, The eastern portion of Mount McKinley National Park, in 

Smith, P. S., and others, Mineral resources of Alaska, report on progress 

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 1940, Geology of the Alaska Railroad region: U.S. Geol. Survey Bull. 

907, 201 p.

Collier, A. J., 1903, The coal resources of the Yukon, Alaska: U.S. Geol. Survey Bull. 218, 71 p.

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America: Am. Jour. Sci., v. 262, no. 2, p. 145-198.

Martin, G. C., 1919, The Nenana coal field, Alaska: US. Geol. Survey Bull. 

664, 54 p.

Mertie, J. B., Jr., 1937, The Yukon-Tanana region, Alaska: U.S. Geol. Survey 

Bull. 872, 276 p.

Pewe, T. L., and Holmes, G. W., 1964, Geology of the Mount Hayes D-4 

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of the Fairbanks quadrangle, Alaska: U.S. Geol. Survey Misc. Geol. Inv. 

Map 1-455, scale 1:250,000.

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and others, Report on progress of investigations of mineral resources of 

Alaska in 1906: U.S. Geol. Survey Bull. 314, p. 205-226.

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the Nenana coal field, Alaska, in Barnes, F. F., and others, Coal investigations in south-central Alaska, 1944-46: U.S. Geol. Survey Bull. 963-E, 

p. 169-186.

 1958, Quaternary geology of the Nenana River valley and adjacent 

parts of the Alaska Range: U.S. Geol. Survey Prof. Paper 293-A, 

p. 1-68.

 1968a, Geologic map of the Healy D-2 quadrangle, Alaska: U.S. Geol. 

Survey Geol. Quad. Map GQ-804. (In press.)

 1968b, Geologic map of the Healy D-3 quadrangle, Alaska: U.S. Geol. 

Survey Geol. Quad. Map GQ-805. (In press.)

 1968c, Geologic map of the Healy D-4 quadrangle, Alaska: U.S. Geol. 

Survey Geol. Quad. Map GQ-806. (In press.)

 1968d, Geologic map of the Healy D-5 quadrangle, Alaska: U.S. Geol. 

Survey Geol. Quad. Map GQ-807. (In press.),

 1968e, Geologic map of the Fairbanks A-2 quadrangle, Alaska: U.S. 

Geol. Survey Geol. Quad. Map GQ-808. (In press.)

 1968f, Geologic map of the Fairbanks A-3 quadrangle, Alaska :U.S. 

Geol. Survey Geol. Quad. Map GQ-809. (In press.)

 1968g, Geologic map of the Fairbanks A-4 quadrangle, Alaska: U.S. 

Geol. Survey Geol. Quad. Map GQ-810. (In press.) 

-1968h, Geologic map of the Fairbanks A 5 quadrangle, Alaska: U.S.

Geol. Survey Geol. Quad. Map GQ-811. (In press.) 

Wahrhaftig, Clyde, and Birman, J. H., 1954, Stripping-coal deposits on lower 

Lignite Creek, Nenana coal field, Alaska: U.S. Geol. Survey Circ. 310, 

lip.
D30 CONTRIBUTIONS TO STRATIGRAPHY

Wahrhaftig, Clyde, and Hickcox, C. A., 1955, Geology and coal deposits,

Jarvis Creek coal field, Alaska: U.S. Geol. Survey Bull. 989-G, p. 353-

367. 

Wahrhaftig, Clyde, Hickcox, C. A., and Freedman, Jacob, 1951, Coal deposits

on Healy and Lignite Creeks, Nenana coal field, Alaska, in Barnes, F; . F.

and others, Coal investigations in south-central Alaska, 1944-46: U.S.

Geol. Survey Bull. 963-E, p. 141-165. 

Wolfe, J. A., 1966, Tertiary plants from the Cook Inlet region, Alaska: U.S.

Geol. Survey Prof. Paper 398-B, p. B1-B32. 

Wolfe, J. A., Hopkins, D. M., and Leopold, E. B., 1966, Tertiary stratigraphy

and paleobotany of the Cook Inlet region, Alaska: U.S. Geol. Survey

Prof. Paper 398-A, p. A1-A29.

U. S. GOVERNMENT PRINTING OFFICE : 1969 O - 322-557