Depositional Environments of 

Coal-Bearing and Associated 

Formations of Cretaceous Age 

in the National Petroleum 

Reserve in Alaska 

By HENRY W. ROEHLER 

U.S. GEOLOGICAL SURVEY BULLETIN 1575 
DEPARTMENT OF THE INTERIOR 

DONALD PAUl HODEl, Secretary 

U.S. GEOlOGICAl SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1987 

For sale by the Branch of Distribution 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Roehler, Henry W. 

Depositional environments of coal-bearing and associated formations 

of Cretaceous Age in the National Petroleum Reserve in Alaska. 

(U.S. Geological Survey bulletin ; 1575) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1575 

1. Geology, Stratigraphic-Cretaceous. 2. Geology-Alaska-National 

Petroleum Reserve. 3. Coal-Geology-Alaska-National Petroleum 

Reserve. I. Title. II. Series. 

QE75.B9 no. 1575 557.3 s 8~344 

[QE685] [551.7'7'09798] 
CONTENTS 

Abstract 1 

Introduction 1 

Cretaceous depositional setting 2 

Depositional environments 3 

Alluvial plain 6 

Delta plain 6 

Delta front 7 

Prodelta and marine 9 

Paleogeography 9 

Early stages of deposition (middle Albian) 9 

Middle stages of deposition (late Albian) 9 

Late stages of deposition (Campanian) 11 

Coal-forming environment in NPRA 11 

Peat-forming analogs in the Mississippi River delta 13 

Summary and conclusions 13 

References cited 15 

FIGURES 

1. Index map of NPRA 2 

2. Paleogeographic map of the interior Cretaceous seaway of North 

America 3 

3. Generalized west-east cross section of NPRA showing formation 

nomenclature and depositional environments 4 

4. Photograph of a distributary mouth bar in the Thktu Formation on the 

Nanushuk River at Arc Mountain 4 

5. Block diagrams of coal-forming depositional environments in NPRA 5 

6. Photograph of a distributary channel sandstone in the Killik Tongue of 

the Chandler Formation on the Colville River 6 

7. Cross-sectional model for deltas in NPRA 7 

8. Generalized columnar sections of three types of delta-front and lower 

delta-plain environments in NPRA 8 

9. Aerial photograph of the Kukpowruk Formation in the Utukok River 

Region 8 

10. Photo-panorama of an upper delta plain depositional environment in the 

Killik Tongue of the Chandler Formation on the Colville River 9 

11-14. Photographs of: 

11. Crossbedded distributary channel sandstone in the Killik Tongue 

of the Chandler Formation on the Colville River 10 

12. A splay sandstone in the Killik Tongue of the Chandler Formation 

on the Colville River 10 

13. A distributary mouth bar sandstone in the Grandstand Formation 

on the Nanushuk River 10 

14. A distributary mouth bar sandstone in the Thktu Formation on 

the Colville River 10 

15. Paleogeography of NPRA and adjacent areas in middle Albian time 11 

16. Paleogeography of NPRA and adjacent areas in late Albian time 12 

17. Paleogeography of NPRA and adjacent areas in Campanian time 14 

Contents Ill 

Depositional Environments of Coal-Bearing and 

Associated Formations of Cretaceous Age in the 

National Petroleum Reserve in Alaska 

By Henry W. Roehler 

ABSTRACT 

Coal-bearing rocks in the National Petroleum Reserve in 

Alaska (NPRA) are more then 1,500 m thick and are present 

in the Cretaceous Corwin, Chandler, and Prince Creek Formations. These formations were deposited mainly as parts of a large 

bird-foot delta called the Corwin delta and in smaller, laterally 

adjacent d~ltas. These deltas were fluvially dominated. They 

prograded in digitate fashion in a northeast direction from the 

vicinity of the ancestral Brooks Range into the northern opening of the interior Cretaceous seaway of North America. The 

shorelines, or delta front, of the Corwin delta are composed 

of sandstone and shale that make up all or parts of the 

Kukpowruk, Tuktu, Grandstand, Ninuluk, Seabee, and Schrader 

Bluff Formations. Prodelta and marine sandstone, siltstone, and 

shale are present in the upper part of the Torok Formation and 

in parts of the Seabee and Schrader Bluff Formations. 

The Cretaceous formations investigated in NPRA comprise 

orderly seaward progressions of alluvial-plain, delta-plain, deltafront, prodelta, and marine depositional environments. Each 

of these depositional environments is distinguished by its 

geographic location, stratigraphic position, lithologic composition, and sedimentary structures. Most of the coal beds were 

deposited as organic debris that accumulated in freshwater 

swamps that occupied interdistributary bays in the delta-plain 

environment. Interbedded with the coal are gray and brown 

carbonaceous shale of salt marsh origin; gray, troughcrossbedded sandstone of distributary channel origin, gray, 

parallel-bedded, splay sandstone of stream overbank origin; and 

gray shale of brackishwater, bay-fill origin. The depositional 

environments of the Cretaceous deltas in NPRA are analogous 

to those in the Holocene Mississippi River delta. 

INTRODUCTION 

The National Petroleum Reserve in Alaska (NPRA) 

comprises 95,000 km2 of the North Slope of Alaska. 

NPRA is bounded on the south by the northern part of 

the Brooks Range, on the east by the Colville River, on 

the north by the Arctic Ocean, and on the west by a line 

drawn along 162 ° W. longitude between the Arctic Ocean 

and the Brooks Range (fig. 1). The physiography of 

NPRA includes part of the Brooks Range, part of the Arctic Foothills of the Brooks Range, and part of the Arctic 

Coastal Plain. The structure consists of thrust faults along 

the north margins of the Brooks Range, linear folds in 

the Arctic Foothills, and low-angle, mostly seaward dipping beds across the Arctic Coastal Plain. NPRA is 

located 225 km north of the Arctic Circle, and is primarily 

a treeless tundra habitat, which is underlain by permafrost. Major drainages flow northward to the Arctic 

Ocean and include the Colville River and its tributaries, 

which drain the southern and eastern parts of NPRA, and 

the Kokolik, Utukok, Kuk, Meade, and lkpicpuk Rivers 

and their tributaries, which drain the western and northern parts of NPRA. A detailed analysis of the physical 

environment is included in ''An Environmental Evaluation of Potential Petroleum Development on the National 

Petroleum Reserve in Alaska:' a paper prepared by the 

U.S. Geological Survey under Section 105(b) of the Naval 

Petroleum Reserves Production Act of 1976 (December 

15, 1979). 

The U.S. Geological Survey has investigated NPRA 

as part of a land use study that was authorized by the 

Secretary of Interior under provisions of U.S. Public Law 

99-258 (1976). A major objective of this land-use study 

has been to assess mineral resources, including coal. The 

depositional environments of coal-bearing rocks in NPRA 

have been investigated in an attempt to predict coal-bed 

occurrence, geometry, and composition. 

The importance of coal exploration in NPRA has 

been underscored by the large resources that have been 

reported there in recent years. Martin and Callahan (1978) 

calculated hypothetical resources of about 1.9 trillion tons 

of bituminous coal and about 1.4 trillion tons of subbituminous coal for NPRA. Most of the coal occurs in 

widely spaced beds less than 3 m thick. More than 95 percent of the coal occurs at depths of less than 900 m, and 

35 percent, or about 1.2 trillion occurs at depths of less 

than 300 m. For comparison, the entire coal resources of 

the United States as of January 1, 1974, were estimated 

by Averitt (1975) to be 3.599 trillion tons. 

Coal investigations in NPRA are difficult because 

of the inaccessibility of outcrops, poor exposures, and 

Introduction 1 
50 100 KILOMETERS 

Figure 1. Index map of N PRA. 

environmental factors. Outcrops are accessible only by 

light aircraft, by boats on major drainages, or on foot. 

There are no permanent roads in NPRA except for those 

in and around oil fields and villages, and no vehicles are 

allowed on the tundra during summer months because 

of environmental restrictions. Less than 5 percent of the 

Cretaceous coal-bearing rocks in NPRA area exposed. 

The exposed sections are generally restricted to cutbanks 

along major drainages and to occasional sandstones that 

cap rubble-strewn ridges between drainages. Coalbeds 

rarely crop out on the tundra between drainages, although 

their presence there may be indicated by coal fragments 

mixed with other rock fragments on slumped hillsides, 

or by coal fragments in excavated material at the mouths 

of rodent burrows. The coal-beds that are the source of 

these fragments are seldom locatable in place, however, 

because permafrost and tundra vegetation make trenching 

impossible. The weather in NPRA is unpredictable. Rain 

and fog occur frequently, and even during the summer 

months light snow is possible. 

2 National Petroleum Reserve, Alaska 

CRETACEOUS DEPOSITIONAL SETTING 

The Cretaceous rocks investigated in NPRA include 

the Torok, Kukpowruk, Corwin, Tuktu, Grandstand, 

Chandler, Ninuluk, Prince Creek, Seabee, and Schrader 

Bluff Formations. These rocks were deposited near the 

northern opening of the interior Cretaceous seaway (fig. 2), 

which transected central North America from the vicinity 

of the Holocene Beaufort Sea north of Alaska and 

Canada southward to the Gulf of Mexico (Ryer, 1981). 

Along the western margins of the seaway in northern 

Alaska was an orogenic belt (the ancestral Brooks Range) 

from which sediments were carried northeastward by 

streams to form clastic wedges of nonmarine conglomerate, sandstone, siltstone, shale, and coal, hundreds 

of meters in total thickness. These clastic wedges project 

seaward into finer textured marine mudstone and shale 

(Bird and Andrews, 1979). The subaerial parts of the 

clastic wedges consist of sediments deposited in alluvial 

plains adjacent to the orogenic belt, which graded laterally 
Figure 2. Paleogeographic map of the interior 

Cretaceous seaway of North America in Turonian time. 

Arrow points to the location of the study area. (From 

Ryer, 1981; modified from Williams and Stelck, 1975). 

seaward into deltas. The deltas had bird-foot configurations during early stages of development (Ahlbrandt, 

1979), but during later stages they coalesced to form a 

broad west-northwest trending coastal plain. A west-east 

correlation of stratigraphic units and depositional 

environments across NPRA is illustrated on figure 3. 

The Cretaceous rocks in NPRA are associated with 

a large delta system called the Corwin delta (Ahlbrandt 

and others, 1979), with parts of smaller deltas situated 

lateral to the Corwin delta (Huffman and others, in press), 

or with marine shelves, slopes, and basins (Molenaar, 

1983). The deltas were fluvially dominated, constructional, and prograding, except for two interruptions by 

marine transgressions across the northeast part of NPRA 

during the deposition of the Seabee and Schrader Bluff 

Formations. Beach deposits, which would indicate reworking of the marine shorelines of the deltas by waves, 

longshore currents, or tides have not been identified by 

the author in outcrops, but they have been reported in 

the Grandstand and Kukpowruk Formations in drill holes 

(Ahlbrandt and others, 1979; Fox, 1979; Huffman and 

others, in press). Nearshore sandstone bars have been 

observed locally by the author within marine shale of the 

Torok Formation in the southwestern part of NPRA, and 

most of the delta shorelines in the eastern part of NPRA 

are of distributary mouth bar origin (fig. 4). 

The bird-foot deltas prograded in digitate fashion 

onto shallow, very muddy marine shelves, 75-150 km wide, 

located along the western margins of the interior seaway 

(Molenaar, 1981). The pattern of bird-foot delta deposition envisioned by the author for NPRA is illustrated by 

block diagrams on figure 5. Most of the stream courses 

across the deltas appear to have been straight to slightly 

sinuous, based upon channel profiles and sedimentary 

structures. The channels are usually concave and symmetrical (fig. 6). Sedimentary structures in the channel 

sandstones are dominantly trough crossbeds, although 

parallel and planar crossbeds associated with point bars 

and channel bars are present locally. 

Crevasse splay sandstone occurs interbedded with 

other rocks of deltaic origin (fig. 6). The splay sandstone 

was deposited along the edges of interdistributary bays 

in response to stream overflow during periods of flood. 

The floods probably resulted from either heavy seasonal 

rains or from occasional torrential rains in nearby mountains. From crevasses, or breaches in riverbanks, the splay 

sands fanned outward into adjacent bays and flood basins. 

The sands prograded, and this resulted in upward coarsening and lateral fining away from the crevasses. At its distal 

edges the splay sandstone generally feathers out into siltstone and shale. Sedimentary structures in the splay sandstone indicate that the flow of water across the splays was 

partly confined to a network of small distributary channels characterized by trough crossbeds and partly unconfined to sheetlike or laminar flow across the vegetated 

splay surfaces characterized by thin, subparallel layers of 

current ripples. 

The size and shape of the salt- and brackish-water 

interdistributary bays in the lower parts of the deltas can 

be inferred from limited outcrop and well data. The bays 

were presumably large, roughly triangular, and partly or 

totally open to the sea. Infilling of the bays progressed 

seaward, and large swamps and marshes replaced the open 

waters of the bays as the deltas prograded. The sediments 

and organic debris that filled the bays resulted in the 

deposition of randomly layered sandstone, mudstone, carbonaceous shale, shale, and coal beds, which characterize 

the NPRA deltas. 

DEPOSITIONAL ENVIRONMENTS 

A descending and laterally seaward progression of 

Cretaceous depositional environments in NPRA includes 

alluvial plain, delta plain, delta front, prodelta, and 

marine. These depositional environments have modern 

and ancient analogs that have been described by Wright 

and Coleman (1973), Broussard (1975), Ferm and Horne 

Depositional Environments 3 
t:=_j 

~ 

Alluvial 

plain 

EXPLANATION 

~ D 

Delta front Marine 

and prodelta shelf 

~ Marine transgression 

-L Tuff 

[' METERS 

50 KILOMETERS L.._ __ ___j 

APPROXIMATE SCALE 

A' 

Campanian 

Santonian 

Coniacian 

Turonian 

Cenomanian 

Albian 

Torok Formation 

Figure 3. Generalized west-east cross section of NPRA showing formation nomenclature and depositional environments. 

(1979), and others. A model for the delta plain, delta front, 

and prodelta environments in NPRA has been constructed 

in cross section on figure 7. 

The alluvial-plain, upper part of the delta-plain, and 

prodelta depositional environments have fairly uniform 

lithofacies across NPRA. The lower part of the delta-plain 

and the delta-front depositional environments, on the 

other hand, vary from area to area and from formation 

to formation across NPRA. The three basic types of these 

environments are illustrated in columnar sections on figure 

8. In the western part of NPRA, the delta-front environment (l)rpe A) is situated in the Kukpowruk Formation 

and is composed mostly of stacked distributary channel 

sandstone with some interbedded distributary mouth bar 

sandstone and marine shale (figs. 8 and 9). The lower 

delta-plain environment is situated in the Corwin Formation in the western part of NPRA and is a thick sequence 

of mostly bay-fill shale with a few beds of distributary 

channel sandstone and splay sandstone, and rare thin beds 

of carbonaceous shale and coal. The l)rpe A environments 

4 National Petroleum Reserve, Alaska 

Figure 4. Exposures of the Tuktu Formation on the east side 

of the Nanushuk River at Arc Mountain 105 km southeast of 

Umiat. 1, marine shale; 2, coarsening upward distributary mouth 

bar. 
FOREST SWAMP 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

N 

EXPLANATION 

k;:(:.)~)J Sandstone 

t====.j Mudstone 

[~ Carbonaceous shale 

~Coal 

Figure 5. Block diagrams of coal-forming depositional environments interpreted for Cretaceous rocks in NPRA, North 

Slope of Alaska. 

Depositional Environments 5 
Figure 6. Lens-shaped outcrop near the center of the 

photograph is a distributary channel sandstone in delta-plain 

deposits of the Killik Tongue of the Chandler Formation. South 

bank of the Colville River at Killik Bend 85 km southwest of 

Umiat. Light-colored, parallel beds surrounding the channel 

sandstone are crevasse splay sandstones. 

in the western part of NPRA probably reflect deposition 

in distal parts of the Corwin delta in large, closely spaced 

distributary channels, between which were large, open, 

brackish-water bays. In the eastern part of NPRA, deltafront environments (Types B and C) are present in the 

Tuktu, Grandstand, Ninuluk, Seabee, and Schrader Bluff 

Formations, where the lithofacies consists primarily of 

distributary mouth bar sandstone and interbedded marine 

shale. The lower delta plain in the Chandler and Prince 

Creek Formations, which overlie the delta-front environments in the eastern part of NPRA, are either (1) Type 

B environments, consisting of interbedded distributary 

channel sandstone, splay sandstone, thin carbonaceous 

shale and coal, and some distributary mouth bar sandstone; or (2) Type C environments, consisting of thick 

beds of laterally persistent distributary mouth bar and 

splay sandstone and interbedded bay-fill mudstone and 

fairly thick coal, with rare distributary channel sandstone 

(fig. 8). The Type B and C environments in the eastern 

part of NPRA probably reflect deposition in distal parts 

of the Corwin delta in and between widely spaced 

distributary channels and in largely filled-in, swampy, 

interdistributary bays and flood basins. 

Alluvial Plain 

The Prince Creek Formation is of alluvial plain 

origin where it is exposed along the lower Utukok and 

Kokolik Rivers in the western part of NPRA. The formation there is composed of more than 80 percent fine6 National Petroleum Reserve, Alaska 

grained to very coarse grained, trough-crossbedded 

sandstone that contains lenses of pebble- to cobble-size 

quartzite conglomerate (Chapman and Sable, 1960). 

These sandstones are gray, salt-and-pepper-type, carbonaceous, and locally ferruginous; they occur in lenticular, fining-upward beds up to 15 m thick that have 

scoured bases. Pebble- to cobble-size coal fragments are 

common. Interbedded with the sandstones are thin beds 

of coal, gray shale, and gray to green bentonite. The 

coalbeds are generally less than 1.3 m thick, commonly 

bony, and laterally discontinuous. A similar bed of 

alluvial-plain sandstone containing conglomerate lenses 

is present about 50 m above the base of the Kogosukruk 

Tongue of the Prince Creek Formation on the west bank 

of the Colville River 30 km northeast of Umiat. The 

alluvial-plain environment reflects deposition by large and 

small, broadly meandering, and anastomosing streams, 

which were interspersed with small interchannel mudfilled flood basins. In places, vegetation accumulated 

along the margins of the flood basins to form peats containing impurities derived from the influx of detritus during floods. 

Delta Plain 

Delta-plain environments in NPRA are present in 

the Corwin, Chandler, and part of the Prince Creek Formations. The delta-plain environments in these formations are divided into upper and lower delta-plain subenvironments, primarily by differences in sedimentary 

lithofacies (Roehler and Stricker, 1979), and to a lesser 

extent on the basis of floral zonations (Scott and Smiley, 

1979). The recognition and separation of the delta-plain 

subenvironments are important because thick, 

economically important beds of coal occur most frequently in the upper delta plain. 

The upper delta-plain subenvironment is separated 

from the lower delta-plain subenvironment in modern 

deltas by water salinity changes (fresh to brackish) and 

accompanying vegetation changes that take place across 

the deltas (Coleman, 1976). They are separated in ancient 

deltas by distinctly differing patterns of sedimentation 

(Fisher and McGowen, 1969). The upper and lower deltaplain subenvironments over most of NPRA are similar 

to those described and illustrated in a delta model prepared by Ferm (1976). The upper delta plain is characterized by numerous, thick beds of sandstone with interbeds 

of carbonaceous shale and coal, and sparse, thin beds of 

mudstone and noncarbonaceous shale. One of the best 

exposures of an upper delta plain in NPRA is found at 

Killik Bend on the Colville River, 85 km southwest of 

Umiat (fig. 10). The lower delta-plain lithofacies are 

variable, as discussed above, but usually consist of very 

thick beds of mudstone and noncarbonaceous shale; 

sparse, thin beds of sandstone and carbonaceous shale; 
UPPER DELTA 

PLAIN LOWER DELTA 

PLAIN 

Distributary channel 

sandstone 

DELTA FRONT 

Interior seaway 

PRODELTA 

Sandstone, 

siltstone, 

and shale 

Not to scale 

Figure 7. Cross-sectional model for fluvially dominated Cretaceous deltas in NPRA, North Slope of Alaska. Delta-plain and 

prodelta thicknesses are exaggerated. 

and rare, thin beds of coal. A typical lateral lithofacies 

change from the upper to lower delta plains in NPRA is 

illustrated on the cross-sectional model, figure 7. As suggested on figure 7, the change is gradational, and the 

boundary defining the change is hence arbitrarily located. 

Investigations of plant megafossils and microfossils 

by Scott and Smiley (1979) have resulted in the division 

of Cretaceous rocks in NPRA into eight floral zones and 

21 floral subzones. Each zone and subzone has characteristic genera and species. The zonations are based 

primarily upon the distribution of the megafossils, but 

the accuracy of the zone boundaries is supported by spore 

and pollen data. The lower five floral zones subdivide the 

Torok, Kukpowruk, and Corwin Formations and their 

lateral equivalents, and the upper three zones subdivide 

the Prince Creek Formation and its lateral equivalents. 

The variations in floral zones and subzones are interpreted 

as resulting mostly from regional paleoclimatic changes, 

but changes in depositional environments may also be 

important contributing factors. 

The distributary channel and crevasse splay sandstones of the upper and lower delta plain subenvironments 

weather mostly to tan, hard, resistant escarpments between valleys formed in softer, drab-gray shale and coal. 

The distributary sandstone is broadly lenticular, and occurs in fining upward, trough crossbeds (fig. 11) that have 

scoured bases. Pebble lenses, carbonized wood fragments, 

and ironstone concretions are fairly common, particularly 

in the lower parts of the channel sandstone. The dimensions of this sandstone are variable, but it may be as much 

as 20 m thick and hundreds of meters wide. The splay sandstone coarsens upward and is silty and lenticular. It commonly occurs as very thin, parallel, current-rippled beds 

(fig. 12). Gray nodular limestone concretions and thin 

limestone lenses are found in the distal parts of some splays 

near the places where they wedge out into bay-fill shale. 

Carbonaceous shale beds in the delta-plain sections 

are gray and contain black, carbonized stem and leaf 

fragments and in soine places laminae of coal. Thick 

laminae and beds of coal are generally composed of alternating bright and dull bands of vitrain, durain, and fusain 

up to a few millimeters thick. Nearly perpendicular coal 

cleats trend about N. 10° W. and N. 85 ° E. (Roehler and 

Stricker, 1979). 

Interdistributary bay deposits consist mainly of gray 

shale, but many contain laminae and thin lenses of gray 

siltstone and slightly carbonaceous shale. The bay shale 

is similar in color and texture to marine shale, but it is 

distinguishable by its stratigraphic positions above deltafront sandstone. 

Delta Front 

The Kukpowruk, Tuktu, Grandstand, and Ninuluk 

Formations and the Ayiyak Member of the Seabee 

Depositional Environments 7 
ALLUVIAL 

PLAIN 

UPPER 

DELTA 

PLAIN 

LOWER 

DELTA 

PLAIN 

DELTA 

FRONT 

PRODELTA 

MARINE 

SHELF 

TYPE A 

Not to scale 

EXPLANATION 

Distributary channel sandstone 

~~~ Splay sandstone-siltstone 

tE~Hj Distributary mouth bar 

·· - sandstone-siltstone !I Shale 

i}lJ Carbonaceous shale 

~Coal 

1=-=1 Dolomite concretions 

Figure 8. Columnar sections illustrating the three basic types 

of delta-front and lower delta-plain depositional environments 

in NPRA, North Slope of Alaska. 

Formation were deposited by distributary streams at the 

seaward edges (delta front) of the Corwin delta. These 

deposits consist of irregularly interbedded sandstone, 

siltstone, and shale. The sandstones in the delta front are 

of two types, based upon their mode of deposition and 

8 National Petroleum Reserve, Alaska 

Figure 9. Aerial view of the Kukpowruk Formation looking 

east in the Utukok River region in the southwestern part of 

NPRA. The light-colored bands are mostly superposed, lenticular, distributary channel sandstones. 

geometry. They are either trough-crossbedded 

distributary channel deposits that fine upward and have 

sharp, scoured bases, or they are parallel-bedded 

distributary mouth bar deposits (figs. 13 and 14) that 

coarsen upward and have transitional bases. The sandstones are all graywackes. They are composed of 

subangular, poorly sorted, and well-cemented grains composed 30-70 percent of tan, white, and gray quartzite and 

quartz; 25-65 percent of dark-gray and black chert; and 

about 5 percent of feldspar, muscovite, biotite, siliceous 

or white clay cement, and miscellaneous varicolored rock 

fragments. The overall color of the sandstones varies 

from medium to dark gray. The sandstones that compose 

the distributary mouth bars are characteristically dark 

gray, very fine to fine grained and contain large amounts 

of interstitial clay and mud. The distributary channel 

sandstones are characteristically medium gray, fine to 

coarse grained, quartzose, and cleaner than the 

distributary mouth bar sandstones. Differences in composition, sedimentary structures, stratigraphic position, 

and weathering make it easy to distinguish distributary 

mouth bar and distributary channel sandstones on 

outcrops. 

The delta-front sandstones and siltstones in NPRA 

appear to have been deposited by the simple extension 

of distributary channels into shallow, quiet, nearshore 

marine waters. Where the streams entered the sea, their 

flow velocities and sediment transporting capabilities 

decreased and deposition occurred. In the receiving basin 

near the mouth of a distributary stream, sediment infilling was most rapid across subjacent shallow submarine 

depressions. A stream would discharge its sediments into 

one of these depressions to form a distributary mouth 

bar. After the depression was filled, the stream would 
Figure 10. Exposures of an upper delta-plain subenvironment in the Killik Tongue of the Chandler Formation at Killik Bend 

on the Colville River, 85 km southwest of Umiat. Height of the bluff is approximately 50 m. 1, shale and carbonaceous shale; 

2, distributary channel sandstone; 3, splay sandstone; and 4, coal. 

alter its course to fill another nearby depression. Because 

of this pattern of sediment infilling and channel switching, distributary channel sandstones often occur 

interbedded with or in juxtaposition to distributary mouth 

bar sandstones along delta-front outcrops. The 

distributary channel and distributary mouth bar sandstones of the delta front apparently were not affected by 

tidal currents, although at times they may have been 

affected by north-northwest trending longshore currents 

(Molenaar, 1981; Huffman and others, in press). 

Prodelta and Marine 

Prodelta and marine deposits are present in the 

upper part of the Torok Formation where it intertongues 

with the Kukpowruk, Tuktu and Grandstand Formations, 

in parts of the Seabee Formation, and in parts of the 

Schrader Bluff Formation (fig. 3). These deposits erode 

to form low hills with smooth slopes that are occasionally 

interrupted by small ledges. The exposures are generally 

obscured by tundra vegetation, by the affects of permafrost, and by slumping. The prodelta and marine deposits 

are mostly shale with some interbedded dark gray, very 

fine to fine-grained sandstone and siltstone in thin, coarsening-upward sequences that were deposited under shallow marine conditions on the shelves seaward of the 

deltas. 

PALEOGEOGRAPHY 

Early Stages of Deposition (Middle Albian) 

The first of the Cretaceous deltas to appear in 

NPRA was the Corwin delta (fig. 3). It prograded northeastward from the southwest corner of NPRA onto the 

shallow shelf of the interior Cretaceous seaway (fig. 15). 

The lower part of the Corwin delta near the southwest 

corner of NPRA consists of 200-550 m of prodelta and 

delta-front sandstone, siltstone, and shale of the 

Kukpowruk Formation and hundreds of meters of sandstone, shale, carbonaceous shale, and coal of the overlying delta plain comprising the Corwin Formation. 

The provenance for the early Corwin delta sediments was probably the ancestral Brooks Range southwest 

of NPRA. Chapman and Sable (1960) found a perceptible change in the amount and the direction of the intertonguing and replacement of the Kukpowruk and Corwin Formations by the underlying marine Torok 

Formation across the southwest part of NPRA. They 

observed that the intertonguing of these units took place 

in aN. 32 ° E. direction along a rising replacement gradient of about 11 m/km. A southwest source direction 

for the early delta sediments is further indicated by 

numerous northeast-trending paleocurrent directions 

measured in nonmarine stream channels in the Corwin 

Formation (Bird and Andrews, 1979) and by northeasttrending foreset dips in marine slope deposits in the Torok 

Formation, which are correlatable by seismic and drillhole data (Molenaar, 1981). 

Middle Stages of Deposition (Late Albian) 

The Corwin delta prograded northeastward from 

middle Albian to the end of Albian time across a horizontal distance of more than 400 km and through a vertical 

stratigraphic interval of more than 1,800 m (Molenaar, 

1981). The progradation lasted several million years, and 

during this period of time the delta coalesced along its 

eastern margins with smaller deltas that were building 

Paleogeography 9 
Figure 11. Crossbedded sandstone in a distributary channel 

exposed near the base of the Killik Tongue of the Chandler Formation on the east bank of the Killik River, 25 km southeast 

of the confluence with the Colville River. 

Figure 12. Coarsening upward splay sandstone in a delta-plain 

environment in the Killik Tongue of the Chandler Formation 

exposed at Killik Bend on the Colville River, 85 km southwest 

of Umiat. 

northeastward from the shores of the interior Cretaceous 

seaway. A separate southern provenance in the ancestral 

Brooks Range is indicated for the sediments in one of 

these deltas (called the Umiat delta by some authors) that 

was located mostly east of NPRA. The stratigraphic position and geographic boundaries of this delta are poorly 

defined, but it probably includes parts of the Killik Tongue 

of the Chandler Formation and part of the Grandstand 

Formation. The sandstones in this delta are coarser 

grained and more conglomeratic, but contain less rock 

fragments and more quartz, igneous, and metamorphic 

10 National Petroleum Reserve, Alaska 

Figure 13. Thin, parallel-bedded distributary mouth bar sandstone in the Grandstand Formation on the west bank of the 

Nanushuk River, 65 km southeast of Umiat. 

Figure 14. Thin, parallel-bedded distributary mouth bar sandstone exposed in a delta-front depositional environment in the 

T u ktu Formation on the north bank of the Colvi lie River, 13 km 

east of the confluence with the Etivuluk River, 165 km southwest 

of Umiat. 

grains and pebbles than do the sandstones deposited in 

the Corwin delta in the western part of NPRA (BartschWinkler, 1979). The presence of delta sandstones derived 

from multiple sediment sources is not unique; in the late 

Quaternary Niger Delta, three independent sediment 

sources with characteristic mineral suites have been identified (Allen, 1965). 

The shoreline of the interior Cretaceous seaway 

trended northwest during late Albian time. The Corwin 

delta and the deltas to the east occupied most of the 

central part of NPRA, and a broad alluvial plain was 
100 200 KILOMETERS 

Figure 15. Paleogeography of NPRA and adjacent areas in middle Albian time. 

present landward of the deltas along the northern margin 

of the Brooks Range. Figure 16 interprets the 

paleogeography at the close of deposition of the Killik 

Tongue of the Chandler Formation. 

Late Stages of Deposition (Campanian) 

The next and last period of Cretaceous deposition 

in NPRA that includes coal-bearing strata is represented 

by about 500 m of mudstone, siltstone, shale, bentonite, 

and coal that crop out along the Kogosukruk and lower 

Colville Rivers at the eastern edge of NPRA. These rocks 

comprise the Kogosukruk Tongue of the Prince Creek Formation (Gryc and others, 1951). The Kogosukruk Tongue 

intertongues with and is laterally replaced eastward by the 

upper part of the Schrader Bluff Formation which has 

been dated as Campanian (Whittington, 1956). 

The Campanian shoreline of the interior Cretaceous 

seaway trended northwest across the northeast part of 

NPRA, as shown on figure 17. Tertiary and Quaternary 

erosion has removed all of the Kogosukruk Tongue from 

the central and southern parts of NPRA, and the restoration of the paleogeography of those areas shown on figure 

17 is consequently based upon speculation. 

COAL-FORMING ENVIRONMENT IN NPRA 

Prerequisites for peat and coal formation normally 

include the following physical, chemical, and biological 

conditions: (1) a climate that produces abundant vegetation; (2) water-saturated soils or water-covered surfaces 

that allow for peat preservation without adverse weathering, oxidation, or biogenic degradation; (3) subsidence to 

accommodate peat accumulation and maturation; and (4) 

Coal-Forming Environment 11 
100 200 KILOMETERS 

Figure 16. Paleogeography of NPRA and adjacent areas in late Albian time. 

deep burial to coalify the peat. These conditions were present during the Cretaceous in NPRA. 

The Cretaceous climate in NPRA ranged from subtropical in early Albian time to warm-temperate during 

latest Albian through Turonian time (Scott and Smiley, 

1979). Precise temperature and precipitation ranges are 

unknown, but they favored the luxuriant growth of vegetation including conifers, dicots, cycads, ginkgos, and ferns, 

from which thick peat accumulations were possible. 

The coal deposits in NPRA apparently resulted 

from accumulations of plant remains (peat) in forest 

swamps within the interdistributary areas of delta plains 

in a pattern of development similar to that described by 

Frazier and Osanik (1969) for the Mississippi River. As 

distributary streams prograded across the shallow marine 

shelves at the seaward edges of the deltas, the interdistributary areas evolved into large, brackish-water bays 

that converged landward but were open to the sea. The 

12 National Petroleum Reserve, Alaska 

margins and landward parts of these open-water bays 

rapidly filled with sand, mud, and silt as a result of overbank flooding of the distributaries. Resulting splay and 

levee deposits provided a stable platform for freshwater 

vegetation. As the infilling of the bays continued, the bays 

eventually were abandoned, and their landward parts were 

occupied by freshwater swamps. Plant debris accumulated 

in these swamps near or below water level, and extensive 

beds of peat developed. 

The Cretaceous deltas in NPRA subsided during 

long periods of sedimentation. This is apparent from the 

superpositioning of numerous coal-beds within stratigraphic intervals of delta-plain origin that are as much 

as 1,500 m thick. Because the peat beds from which the 

coal formed were deposited at or near sea level, the deltaplain areas obviously had to subside to accommodate this 

thick interval of rocks. Deep burial and deformation are 

suggested for the beds in the southern part of NPRA 
across the folded foothills of the Brooks Range, where 

the rank of the coal is bituminous (Martin and Callahan, 

1978). Northward across NPRA, the rocks are largely 

undisturbed, the depths of burial decrease, and the rank 

of the coal decreases to subbituminous. 

The amount of compaction or compression of peat 

during the coalification process varies with the composition and rank of the coal (Stach and others, 1982). Coal 

of forest swamp origin and bituminous and subbituminous ranks normally has peat-to-coal compaction 

ratios from 5:1 to 10:1. A few coal beds in NPRA reach 

thicknesses of as much as 4 m, and the peats responsible 

for these beds thus ranged in thickness from 20 to 40 m. 

PEAT-FORMING ANALOGS 

IN THE MISSISSIPPI DELTA 

Peat deposits of the Mississippi Delta have been 

investigated extensively by the Louisiana Geological 

Survey since 1981. During these investigations, Kosters 

(1983) identified several peat-forming depositional settings. The two most important are lenticular peat beds 

that form in subsiding interdistributary basins and 

blanket peat beds that form across the top of slowly subsiding abandoned delta lobes. Of lesser importance are 

peat beds deposited as abandoned channel fill or as 

organic detrital material. One of the peat deposits studied, 

identified as the Barataria Prospect, is presently forming in a subsiding interdistributary basin similar to those 

postulated for the Cretaceous NPRA deltas. The 

Barataria Prospect (Kosters, 1983) occupies an area of 

more than 85 km2 in Jefferson and Lafourche Parishes 

southeast of Lake Salvador. The peat beds in the prospect are as much as 4 m thick, but in most places they 

are horizontally and vertically discontinuous because of 

the influx of clastic detritus. The detritus consists of silt 

and clay that forms wedges and lenses within the peat. 

The detritus is derived from the periodic overbank 

flooding of Bayou Barataria, a distributary located along 

the eastern edge of the prospect. The Barataria peat bed 

is composed of organic material derived mostly from 

brackish-water marsh plants including Spartina and other 

grasses. The Barataria peat bed will produce a thin 

coalbed with numerous splits and partings and a high ash 

content. 

Frazier and Osanik ( 1969) reported peat accumulations as much as 6 m thick between natural levees along 

the Teche and Lafourche Bayous in the Mississippi Delta. 

These peat beds are derived from forest swamp vegetation, mostly cypress and gum, that accumulated in broad, 

subsiding inland flood basins during progradationalaggradational river phases. Analyses of these peat beds 

indicated that they contain less than 1 percent sulfur and 

about 15 percent ash. They may have closer affinities to 

the NPRA peats than do those of the Barataria Prospect 

because of their forest swamp origin and lower sediment 

influx. 

Several swamps are present in the upper parts of 

the delta plain on the east side of the Mississippi River 

northeast of New Orleans. These swamps contain peat 

beds, but they are deteriorating due to the incursion of 

salt water from the Gulf of Mexico associated with delta 

lobe subsidence. Saxena (1979) stated that the swamp 

trees common to these areas, including cypress, red 

maple, black willow, tupelo gum, and cottonwood thrive 

in freshwater, but they cannot tolerate water of even 

moderate salinity. As the freshwater swamps die off, they 

are replaced by salt-tolerant marsh vegetation and 

mangrove. Similar salinity and vegetation changes are 

postulated for NPRA following the marine transgressions 

that took place during deposition of the Seabee and 

Schrader Bluff Formations (fig. 3). 

SUMMARY AND CONCLUSIONS 

The National Petroleum Reserve in Alaska contains 

as much as 3.3 trillion tons of bituminous and subbituminous coal of Cretaceous age in beds mostly less 

than 3m thick and at depths of less than 900 m. The coal 

beds are situated in low- to moderately dipping, partly 

folded and faulted strata in a stratigraphic interval more 

than 1,500 m thick. 

The Cretaceous rocks investigated in NPRA were 

deposited as a thick wedge of clastic sediments that projected northeastward from the ancestral Brooks Range 

into the northern opening of the North American interior 

Cretaceous seaway. They comprise an orderly seaward 

progression of alluvial-plain, delta-plain, delta-front, prodelta, and marine depositional environments. Each of the 

depositional environments is distinguished by its geographic location, stratigraphic position, lithologic composition, and sedimentary structures. 

Alluvial-plain depositional environments were 

situated between the ancestral Brooks Range and deltas 

that were building seaward along the coastline of the 

interior Cretaceous seaway. The alluvial-plain environment was characterized by large and small broadly 

meandering and anastomosing streams that were occasionally interspersed with small interchannel swamps and 

mud-filled flood basins. Rocks deposited in the environment are restricted to upper parts of the Prince Creek 

Formation where they consist of trough-crossbedded, 

conglomeratic sandstone, and some thin interbedded 

shale and coal. 

The coal-bearing rocks in NPRA were deposited in 

delta-plain depositional environments in the Corwin, 

Chandler, and Prince Creek Formations. They are parts 

of a large delta complex called the Corwin delta and parts 

of smaller, laterally adjacent deltas. The Corwin and 

adjacent deltas were fluvially dominated, constructional, 

Summary and Conclusions 13 
16t 

.... ·. 

70° 

69° 

680 ~~--"9// f/\L, "?~~ff;v("'~~...,..:..._ 

100 200 KILOMETERS 

Figure 17. Paleogeography of NPRA and adjacent areas in Campanian time. 

and prograding, except for two interruptions by marine 

transgressions during deposition of the Seabee and 

Schrader Bluff Formations. The delta-plain depositional 

environment consisted of bifurcating distributary streams 

between which were broad areas of freshwater swamps and 

open, brackish-water interdistributary bays. Rocks 

deposited in the delta-plain environment are mostly randomly interbedded sandstone, shale, and coal. The sandstones are either lenticular, trough-crossbedded 

distributary channel deposits, or parallel-bedded, currentrippled, crevasse splay deposits. The shales are partly carbonaceous and of reed swamp origin and partly noncarbonaceous and of bay-fill origin. The coalbeds formed 

from peats that accumulated on the floor of forest 

swamps that occupied abandoned parts of interdistributary bays. 

Delta-front environments are present in the Kukpowruk, Tuktu, Grandstand, Ninuluk, Seabee, and Schrader 

Bluff Formations. The environment was characterized 

14 National Petroleum Reserve, Alaska 

by distributary streams that prograded into the shallow 

waters of marine shelves at the seaward edges of the 

Cretaceous deltas in NPRA. Between the distributary 

streams were large, brackish-water, and salt-water bays 

that were open to the sea. Rocks deposited in the environment are mostly interbedded sandstone and shale. The 

sandstones are partly trough-crossbedded distributary 

channel deposits that fine upward and partly parallelbedded, distributary mouth bar deposits that coarsen 

upward. The sands in these deposits were not reworked 

by tides and longshore currents. The shales are seldom 

carbonaceous. 

The prodelta and marine depositional environments 

were located along the shallow marine shelves seaward 

of the NPRA deltas. Rocks deposited in the prodelta 

environment are mainly shale with some interbedded 

coarsening-upward and seaward-fining beds of sandstone 

and siltstone. The marine environment consists mostly 

of shale that was deposited in deep waters at distances 
off the shores of the interior Cretaceous seaway. The prodelta and marine environments in NPRA comprise the 

upper part of the Torok Formation and parts of the 

Seabee and Schrader Bluff Formations. 

The coal-bearing and associated rocks in NPRA 

weather gray, tan, or brown on outcrops. The sandstones 

are all graywackes and are composed mostly of quartzite, 

quartz, and chert grains with minor feldspar, muscovite, 

biotite, and rock fragment grains. The cementing materials 

are silica and white clay. The sandstones are usually very 

fine to medium grained, poorly sorted, subangular, well 

cemented, and often contain large amounts of interstitial 

clay and mud. 

The paleoclimate in NPRA during the Cretaceous 

Period was warm-temperate to subtropical and supported 

abundant vegetation from which thick peat accumulations 

were possible. The delta environments in NPRA are 

analogous to the Holocene Mississippi River delta. The 

peats in both places accumulated in freshwater swamps 

in forested habitats at or near sea level. The resulting coal 

usually has high heating value and low sulfur and ash 

content. 

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16 National Petroleum Reserve, Alaska 

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·, U.S. GOVERNMENT PRINTING OFFICE: 1987-591-047/66,013 REGION NO.8