Geology of the Nanushuk Group 

and Related Rocks, 

North Slope, Alaska 

U.S. GEOLOGICAL SURVEY BULLETIN 1614 

Geology of the Nanushuk Group 

and Related Rocks, 

North Slope, Alaska 

By A. Curtis Huffman, Jr., Editor 

U.S. GEOLOGICAL SURVEY BULLETIN 1614 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1985 

For sale by the Branch of Distribution 

U.S. Geological Survey 

604 South Pickett Street 

Alexandria, VA 22304 

Library of Congress Cataloging in Publication Data 

Geology of the Nanushuk Group and related rocks, North Slope, Alaska. 

(United States Geological Survey bulletin ; 1641) 

v, 129 p. 

Bibliography: p. 124-129 

Supt. of Docs. No.: 119.3:1614 

1. Sandstone-Alaska-North Slope. 2. Geology, Stratigraphic-Cretaceous. I. Huffman, A. C. II. 

Series. 

QE75.B9 no. 1614 557.3s [557.98'7] 84-600055 

[QE471.15.S25] 
CONTENTS 

Introduction to the geology of the Nanushuk Group and related rocks, North Slope, 

Alaska, by A. C. Huffman, Jr. 1 

Cretaceous tectonics, depositional cycles, and the Nanushuk Group, Brooks Range 

and Arctic Slope, Alaska, by C. G. Mull 7 

Subsurface correlations and depositional history of the Nanushuk Group and related 

strata, North Slope, Alaska, by C. M. Molenaar 37 

Depositional and sedimentologic factors affecting the reservoir potential of the Cretaceous Nanushuk Group, central North Slope, Alaska, by A. C. Huffman, 

Jr., T. S. Ahlbrandt, Ira Pasternack, G. D. Stricker, and J. E. Fox 61 

Petrography of sandstones of the Nanushuk Group from four measured sections, central 

North Slope, Alaska, by Susan Bartsch-Winkler 75 

An analysis of the Umiat delta using palynologic and other data, North Slope, Alaska, 

by F. E. May and J.D. Shane 97 

Uranium potential of the Cretaceous Nanushuk Group, North Slope, Alaska, by A. 

C. Huffman, Jr. 121 

FIGURES 

1. General index map of the North Slope 2 

2. Measured sections from Corwin and Umiat deltas 4 

3. Generalized nomenclature and stratigraphic relationships of the Nanushuk 

Group and related rocks, western and central North Slope 6 

4. Stratigraphic relationships of lower Cretaceous deposits from the Brooks 

Range to the Beaufort Sea 8 

5. Diagrammatic cross section from the Brooks Range to the Beaufort Sea 9 

6. Generalized map showing the distribution of the Nanushuk and Colville 

Groups, and the major allochthons and tectonic features of the Brooks 

Range 10 

7. Photograph showing the Nanushuk Group and Torok Formation in the Pitmegea River area 12 

8. Table showing the vertical sequence of major allochthons in the Brooks 

Range 13 

9. Photograph showing the Fortress Mountain Formation at Ekakevik 

Mountain 14 

10. Chart showing global Cretaceous eustatic cycles compared to generalized Arctic Slope Cretaceous stratigraphy 15 

11. Photograph showing the Okpikruak Formation 16 

12. Photograph showing olistostrome, De Long Mountains 17 

13. Chart describing the pebble shale unit and adjacent rock units in Kugrua 

and Inigok test wells 20 

14. Diagrammatic maps of major tectonic elements during Neocomian and Aptian 

times 26 

15. Photograph showing Mount Kelly Graywacke Tongue of the Fortress Mountain Formation 29 

16. Diagrammatic map of major tectonic elements at the end of Aptian time 32 

17. Index map of part of the North Slope 38 

18. Diagram showing stratigraphic cross sections of Nanushuk Group and related 

rocks, National Petroleum Reserve in Alaska 42 

19. Seismic cross section showing the relationship of the Nanushuk Group and 

Torok Formation 52 

20. Seismic cross section showing disturbed zone in the Torok Formation 53 

Contents Ill 
IV Contents 

FIGURES 

21. Isopach map of Nanushuk Group 54 

22. Isopach map of Torok Formation 55 

23. Plot of directional data in the Nanushuk Group and Torok Formation 56 

24. Paleogeographic maps, mid-Nanushuk and latest Nanushuk time 57 

25. Isopach map of seismic velocities in the lower Nanushuk Group 58 

26. Map showing percentage of sandstone, Nanushuk Group 62 

27. Generalized stratigraphic column of Nanushuk Group, central North 

Slope 66 

28. Diagram showing terminology and major depositional environments 67 

29. Plots showing porosity vs. permeability for Nanushuk Group rocks 69 

30. Plots showing distribution of sandstone within the Nanushuk Group by 

facies 70 

31. Paleogeographic maps showing depositional history of Nanushuk Group and 

related rocks 72 

32. Location map of four measured sections 76 

33. Compositional diagrams of Nanushuk Group sandstones 76 

34. Triangular plots of Nanushuk Group sandstones 77 

35-38. Triangular diagrams showing modal analyses plotted according to grain 

size: 

35. Kurupa anticline section 81 

36. Tuktu Bluff section 82 

37. Arc Mountain section 83 

38. Marmot syncline section 84 

39. Diagrams of vectoral analyses showing compositional trends according to 

grain size 85 

40. Histograms of textural characteristics 86 

41-51. Photographs showing: 

41. Intergrain boundaries in sandstone 87 

42. Tightly appressed sandstone showing grain deformation 88 

43. Replacement by calcite cement in sandstone 88 

44. Kaolinite pore-space filling in sandstone 89 

45. Original porosity and minor secondary porosity 90 

46. Chlorite coating on quartz grains in sandstone 90 

47. Quartz overgrowths on sandstone after clay formation 91 

48. Abundant quartz overgrowths on sandstone 92 

49. Secondary porosity features in sandstone (scanning electron microscope) 93 

50. Secondary porosity features in sandstone (photomicrograph) 94 

51. Possible hydrocarbons in Nanushuk Group rocks 95 

52. Index map of Umiat delta area 98 

53. Diagrams showing locations of invertebrate megafossils observed in Nanushuk 

Group rocks 106 

54-62. Relative frequency diagrams of main palynomorph groups: 

54. Knifeblade-1 well 108 

55. Titaluk-1 well 109 

56. Oumalik-1 well 111 

57. East Oumalik well 113 

58. Wolf Creek-2 and -3 wells 114 

59. Square Lake-1 well 116 

60. Umiat-1 well 118 

61. Umiat-11 well 119 

62. Grandstand-I well 120 
TABLES 

1. Microfauna! and palynological data from Lower Cretaceous rocks 22 

2. Megafossil data from Kemik Sandstone Member 24 

3. Location of North Slope test wells shown in figure 17 39 

4. Location of Nanushuk Group measured sections shown in figure 17 40 

5. Resource-related data from sections measured in Nan~shuk Group rocks 64 

6. Compositional data from four sections measured in Nanushuk Group rocks 78 

7. Sedimentologic data from transitional and nonmarine parts of sections measured 

in Nanushuk Group rocks 122 

Contents V 

Introduction to the Geology of the Nanushuk 

Group and Related Rocks, North Slope, Alaska 

By A. Curtis Huffman, Jr. 

Abstract 

The Cretaceous Nanushuk Group has long been 

considered one of the major hydrocarbon plays on the 

North Slope and as such has been the focus of a number 

of investigations. The primary purpose of this four-year 

study was to evaluate the reservoir potential of the Cretaceous Nanushuk Group in and adjacent to the National 

Petroleum Reserve in Alaska (NPRA). This study sought 

to apply modern stratigraphic concepts as well as up-todate geochemical, petrographic, and paleontologic 

methods in evaluating the hydrocarbon, coal, and uranium potential of the Nanushuk and related rocks. Preliminary results were published in Ahlbrandt (1979), 

which concentrated on the western North Slope. This 

report is primarily concerned with the central North 

Slope, although several preliminary regional syntheses 

are presented as well. 

The results of this and previous investigations indicate that the Nanushuk Group was deposited as two 

delta systems onto a shelf of marine shale of the Torok 

Formation, which prograded into the Colville basin generally from west to east. Deposition of the western, or 

Corwin, delta began in early Albian or perhaps late Aptian time, and the delta prograded northeastward from 

the vicinity of the De Long Mountains, Tigara Uplift, and 

Herald Arch. The deltaic and shelf topsets can be traced 

basinward into large foresets and coeval bottomsets of 

the Torok Formation. Petrographic analyses confirm that 

sediment of the Torok Formation is closely related to 

that of the Nanushuk Group in the Corwin delta and 

is quite different from the Nanushuk of the Umiat delta. 

The Umiat delta was an elongate to lobate, riverdominated delta, which prograded northward from the 

vicinity of the Endicott Mountains, beginning in mid-Albian time. Sandstones of the Umiat delta are dominantly 

phyllarenite, as compared with the sedimentary litharenite of the Corwin delta. The differences in sediment 

type between the two depocenters reflect the sequence 

of lithologies exposed in the various thrust slices of the 

Brooks Range. The Endicott Group of the central Brooks 

Range contains large thicknesses of Upper Devonian and 

Mississippian quartzitic conglomerate and phyllitic shale, 

whereas the sequences exposed in the western Brooks 

Range are dominantly shale, limestone, and chert. Because the Umiat delta was not subjected to high temperatures and deep burial as was the Corwin delta, micropaleontological and palynological analyses have proven 

valuable in the interpretation of its depositional environments. 

A detailed model of the Umiat delta and related 

deposits may be constructed through a combination of 

sedimentology, paleontology, and seismology studies. 

This model suggests that a belt of high-energy coastalbarrier deposits trending northwest from the northern 

edge of the Umiat delta may contain reservoir sandstones and stratigraphic traps. 

INTRODUCTION 

Much of the outcrop and subsurface work by the 

U.S. Geological Survey and the U.S. Navy in the 1940's 

and 1950's focused on rocks of the Nanushuk Group because the most significant oil field on the North Slope 

prior to the discovery of the Prudhoe Bay field in 1967 

was the Nanushuk field at Umiat (fig. 1), estimated by 

Espach (1951) to contain as much as 122 million barrels. 

The transfer of jurisdiction of Naval Petroleum Reserve 

No. 4 from the U.S. Department of the Navy to the U.S. 

Department of the Interior by the Naval Petroleum Reserves Production Act of 1976 changed the name to National Petroleum Reserve in Alaska (NPRA) and called 

for a reevaluation of the resource potential of the reserve. 

As a part of this evaluation, the Nanushuk Group 

and related rocks were studied by a team of U.S. Geological Survey scientists from 1977 to 1981. Fieldwork for 

this investigation was begun in the western part of the 

North Slope in 1977, and preliminary results were published in Ahlbrandt ( 1979). The 1978 and 1980 fieldwork 

concentrated on the central and eastern regions, and many 

of the results are presented in this report. Detailed measured sections from the western North Slope are described 

in Huffman and others (198la) and from the central North 

Slope in Huffman and others (1981b). 

Emphasis during this study has been on the interpretation of depositional environments and their relationship 

to one another and to reservoir potential. This regional 

approach was necessitated by several factors: (1) The 

Nanushuk Group was deposited in two large depocenters, 

the Corwin delta of the western North Slope and the 

Umiat delta of the central North Slope, with a region of 

low sandstone content and meager exposures in between. 

(2) No unit above the Torok Formation can be physically 

traced the length of the outcrop belt. (3) Very few data 

Introduction 1 
168° 16fi0 164° 162° 160° 158° ~-~----~--------·-----~-------------------------------------, -------....,....----------·~.-----~------r 

71o) EXPLANATION ' i ! / I j1 3

,

0 , ~ PALEOTRANSPORT DIRECTIONS DETERMINED FROM PLUNGE ' I ! . 

'----(10)4!t ALONG AXIS OF SYMMETRICALLY FILLED NONMARINE 

Percent of CHANNELS OF THE NANUSHUK GROUP~Number of 

reading measurements indicated by number in parentheses; arrow indicates 

vector mean transport direction; percent of readings in each segment ' ' - ! - : 

Tunalik 1 

• 

of rose is to scale shown 

LINE OF MEASURED SECTION 

WELL LOCATION AND NAME 

ANTICLINE-Dashed where approximately located 

SYNCLINE-Dashed where approximately located 

APPROXIMATE OUTLINE OF BASAL PART OF 

NANUSHUK GROUP-Kukpowruk and Tuktu Formations 

OIL FIELD 

GAS FIELD 

~--·--r -·- ----------------t-- 1 ---+ -----r--

isk. 

: 

Figure 1. Index map showing the study area of Nanushuk Group rocks, locations of sections measured during 

1977 and 1978, and locations of wells, western and central North Slope, Alaska. 

have been found with which to precisely date or correlate 

the units. The cross sections in figure 2 illustrate these 

problems. Very few outcrops exist in the 150-km interval 

separating the Carbon Creek and Kurupa anticline measured sections (fig. 1). The widely separated exposures that 

are present in this region are predominantly shale and 

mudstone. 

By concentrating on genetic units, equivalent environments can be compared between the two depocenters 

without regard to time or formal nomenclature. A stratigraphic nomenclature chart of the Nanushuk Group and 

related units (fig. 3) illustrates some of the problems m2 Geology of the Nanushuk Group and Related Rocks, Alaska 

volved in trying to apply formal nomenclature to these 

units on a regional basis. The correlations implied on this 

chart are approximate at best. 

During this study, a total of 25,000 m of section 

at 33 localities (fig. 1) were measured, described, and 

sampled in detail. Samples were collected for petrographic, geochemical, and paleontologic studies. Relationships observed in the surface exposures were combined 

with subsurface and geophysical data to build a depositional model that was tested and perhaps modified by subsequent drilling. Similar methods were used to project reservoir properties into the subsurface. 
154" 

.;__ ----~-----= ~~-

Figure 1. 

Petrographic and palynologic studies were particularly useful in the analysis and interpretation of depositional environments and their reservoir properties in the 

Umiat delta. Projections of trends northward into the subsurface of the central North Slope could be accomplished 

with some confidence based on petrographic and 

palynologic analyses of subsurface material. These projections were combined with geophysical and well-log information to produce a reservoir model. 

The chapters of this report are arranged first regionally and then topically. In the chapter following the Introduction, Mull set the spatial and time framework for 

150° -------;---·--------·~ 

Continued 

Nanushuk Group deposition in a discussion of the tectonics and related deposition during the Early Cretaceous 

in the central and eastern Brooks Range and North Slope. 

Molenaar combined seismic data and well-log information 

to construct a regional correlation and depositional history 

for the Nanushuk Group. Huffman, Ahlbrandt, Pasternack, Stricker, and Fox analyzed depositional environments in surface exposures as well as from subsurface 

data in the central North Slope and proposed a regional 

model for the Nanushuk depositional system. Petrographic 

results from the Nanushuk of the central North Slope, 

reported by Bartsch-Winkler, show distinctive and signifiIntroduction 3 
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EXPLANATION 

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pattern show lithology 

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COVERED INTERVAL 

Figure 2. Measured sections along lines A-A' (Corwin delta) and 8'-8' (Umiat delta) on figure 1. Distance between 

A' and 8' is approximately 150 km. Datum for both sections is the base of the nonmarine facies, commonly marked 

by a conglomeratic fluvial sandstone. 
SYSTEM 

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Figure 3. Stratigraphic nomenclature for the Nanushuk Group and adjacent rock units, western and central 

North Slope, Alaska (modified from Ahlbrandt and others, 1979). The correlations implied are approximate. 

cant differences from those of the western area. May and 

Shane described the depositional environments in part of 

the Umiat delta utilizing dinoflagellate, pollen, and spore 

6 Geology of the Nanushuk Group and Related Rocks, Alaska 

analyses. The final chapter, by Huffman, is a brief discussion of the uranium potential of the Nanushuk Group and 

related rocks throughout the North Slope. 
Cretaceous Tectonics, Depositional Cycles, and the 

Nanushuk Group, Brooks Range and 

Arctic Slope, Alaska 

By C. G. Mull1 

INTRODUCTION 

The Nanushuk Group, conststmg of sedimentary 

rocks of Albian and Cenomanian Age, is widespread on 

the Arctic Slope of Alaska, where it is a hydrocarbon 

exploration objective in the National Petroleum Reserve 

in Alaska (NPRA). A small oil field at Umiat and minor 

gas accumulations elsewhere have been found in NPRA, 

but the hydrocarbon potential seems to be limited by reservoir quality. The study of the Nanushuk Group has had 

as its major objective the understanding of the depositional processes, distribution, and diagenesis of the sandstone bodies that are the potential reservoir beds. The 

Nanushuk Group represents the culmination of deposition 

that started near the beginning of the Cretaceous as a result of the major orogenic event that formed the present 

Brooks Range and its offshore extension beneath the 

Chukchi Sea, the Herald arch. This report summarizes 

the regional tectonic and depositional framework of the 

Early Cretaceous orogenic belt and depositional basin as 

an aid to understanding the character, distribution, and 

deformation of the Nanushuk Group rocks. Other chapters 

in this volume describe the Nanushuk in detail. 

The generalized nomenclature and stratigraphic relationships of sediments of Early Cretaceous and early 

Late Cretaceous (Cenomanian) age of the western and 

central Arctic Slope are summarized on figure 4. Cretaceous stratigraphy and more detailed nomenclature is 

given by Detterman and others (1975) for the eastern 

Brooks Range and by Detterman ( 1956) for the central 

and western Arctic Slope. Generalized geologic maps of 

the Arctic Slope and Brooks Range have been compiled 

by Lathram (1965), Beikman and Lathram (1976), Beikman (1978), and Grybeck and others (1977). 

The discussion of the tectonic evolution of the 

Brooks Range is from data of many workers, including 

Keller and others ( 1961), Detterman and others ( 1963), 

Tailleur and others (1967), Snelson and Tailleur (1968), 

Tailleur (1969), Tailleur and Brosge (1970), Rickwood 

(1970), Martin (1970), Brosge and Tailleur (1970, 1971), 

Tailleur (1973), Detterman (1973), Mull and others 

'Presently with Alaska Division of Geological and Geophysical Surveys. 

(1976), Mull (1977), Mull and Tailleur (1977), Roeder 

and Mull (1978), Mayfield and others (1978), and Mull 

(1979, 1982). It is augmented by concepts and interpretations of the author based on data available through 1982. 

The onset of the Brooks Range orogeny is recorded 

in Early Cretaceous deposits exposed within and adjacent 

to the Endicott and De Long Mountains of the central 

and western Brooks Range. These sediments record the 

abrupt shift in sediment dispersal from a predominantly 

northern source that persisted through the late Paleozoic 

and early Mesozoic, to a major southern and southwestern 

source derived from a Cretaceous orogenic belt. 

Sediments from the orogenic belt were poured into 

a developing foredeep, the asymmetrical Colville basin; 

seismic data indicate that the total thickness of Lower Cretaceous and Cenomanian sediments may be 10,000 m. 

The sequence records the progressive filling of the basin, 

and it can be considered a typical gradational flysch-tomolasse sequence. 

This basin fill is divided into three stratigraphic sequences: a relatively thin lower sequence of stable platform sediments, and two thick overlying sequences of 

flyschoid and molassoid sediments. Although the sequences can be related to specific tectonic events in the 

area of the Brooks Range and Arctic Slope, they also 

seem to coincide generally with the Early Cretaceous 

global eustatic cycles of Vail and others (1977, p. 85). 

The basal sequence of platform sediments is well developed in the subsurface of the Arctic coastal plain and 

is well exposed in the northeastern Brooks Range. It is 

also present at the base of the Cretaceous on some of 

the thrust sheets in the central and western Brooks Range. 

The lower flyschoid sequence is present in outcrop on 

the thrust sheets of the Endicott and De Long Mountains 

and their foothills, in a few areas in the Philip Smith 

Mountains, and in the subsurface of the northern foothills 

province of the Arctic Slope. The upper sequence, which 

contains the Nanushuk Group, is present only as part of 

the autochthonous complex that underlies the northern 

foothills and coastal-plain area of the Arctic Slope. The 

generalized relationship of the various rock units, their 

ages, and their interpreted basin positions are diagrammatically illustrated in figures 4 and 5. 

Cretaceous Tectonics and Depositional Cycles 7 
SOUTHWEST 

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Figure 4. Stratigraphic relationships of Lower Cretaceous deposits across the central Colville basin from 

the Brooks Range to the Beaufort Sea. The Mount Kelly Graywacke Tongue of the Fortress Mountain 

Formation is confined to the western Arctic Slope, and the Kongakut Formation and Bathtub Graywacke 

are recognized only on the eastern Arctic Slope and in the Brooks Range. Dashed lines are time lines. 

The stable platform sequence consists dominantly 

of clay shale containing scattered pebbles and quartz 

grains. Along the northern front of the northeastern 

Brooks Range and in the subsurface of the northern Arctic 

Slope a discontinuous sandstone and (or) conglomerate 

forms the base of the sequence. A thin coquinoid limestone horizon is interbedded with the shales locally on 

the thrust sheets in the central and western Brooks Range 

foothills. 

The lower flysch sequence consists dominantly of 

rhythmically alternating beds of sandstone, siltstone, and 

shale; the more distal equivalents to the north are almost 

entirely shale. It is of Neocomian to Aptian Age. 

The upper sequence, referred to as the Albian and 

Cenomanian sequence, consists of coarse proximal conglomerates on the south that rapidly grade northward into 

basinal turbidites. The basinal deposits are succeeded by 

prodelta muds deposited in advance of a prograding deltaic complex, the Nanushuk Group, which represents the 

molassoid culmination of the depositional cycle that began 

in the Early Cretaceous. Along the margins of the basin 

the upper sequence seems to be separated from the lower 

8 Geology of the Nanushuk Group and Related Rocks, Alaska 

flysch sequence by a disconformity or depositional hiatus, 

but in the axial part of the basin the two sequences may 

be gradational. 

The Nanushuk Group, a wedge of deltaic deposits, 

extends as a nearly unbroken outcrop belt from the Chukchi Sea on the west to the Sagavanirktok River area on 

the central Arctic Slope. It is also present in the subsurface of the Arctic coastal plain and beneath the Chukchi 

Sea. Throughout this area the Nanushuk contains potential 

hydrocarbon-reservoir beds. East of the Itkillik River, due 

to a facies change to basinal deposits, the Nanushuk is 

limited both in outcrop and in the subsurface, and has 

little reservoir potential. East of the Okpilak River, in 

the Arctic National Wildlife Refuge, little is known of 

the Early Cretaceous. In this area the Cretaceous stratigraphy is inferred from offshore seismic data and from extrapolations from the northern Yukon Territory of Canada. 

Across the Arctic Slope, the southern part of the 

Nanushuk's outcrop belt has been detached from underlying strata and folded into a series of long linear anticlines 

and synclines that form a major decollement. To the 

south, the anticlines have been breached through the 
s 

BROOKS RANGE 

A 

Chand ler Lake 

area 

s 

BROOKS RANGE 

Elusive Lake 

area 

COLVILLE BAS I N 

Umiat area 

Nanushuk Group 

COLVILLE BASIN 

~ 

BEAUFORT SILL 

Cape Halkett 

area 

Colville Group 

Autochthonous 

pre-Cretaceous 

rocks 

BEAUFORT SILL 

Prudhoe Bay area 

CANADA 

BASIN 

BEAUFORT 

SEA 

CANADA 

BASIN 

N 

N 

Co ill -~-- ---- ~roup 

BEAUFORT 

SEA 

B 

Autochthonous 

pre-Cretaceous 

rocks 

'-........ ....._ ___ _____ _ 

Figure 5. Diagrammatic cross section of the central (A) and eastern (8) Colville basin from the Brooks Range 

to the Beaufort Sea. Regional decollement cutting Cretaceous strata is not shown. Heavy lines are faults; arrows 

show direction of movement. Dashed lines show foresets in marine sediments. Allochthonous Paleozoic rocks 

consist dominantly of Carboniferous Lisburne Group carbonate rocks, and Upper Devonian Kanayut Conglomerate and Hunt Fork Shale. The Beaufort sill, also known as the Barrow arch, trends from the area of Barrow 

southeast to the Sadlerochit Mountains area of the northeastern Brooks Range. 

Nanushuk, but to the north, fold amplitudes decrease and 

the Nanushuk is unbreached. 

PHYSIOGRAPHIC SETTING OF THE 

NANUSHUK GROUP 

The Nanushuk Group crops out in the northern 

foothills part of the Arctic Foothills Province of northern 

Alaska; it is widespread in the subsurface beneath the 

coastal plain area of NPRA. Its outcrop belt extends 

nearly unbroken for about 650 km from the sea cliffs of 

the Corwin Bluff area on the west to the Sagavanirktok 

River area of the eastern Arctic Slope (fig. 6). Across 

this distance the province is characterized by generally 

east-west-trending ridges and rolling upland terrain to an 

elevation of about 960 m. The typical expression of the 

Nanushuk and underlying strata is illustrated in figure 7. 

The southern limit of the Nanushuk's outcrop belt 

is in many areas marked by a prominent south-facing escarpment rising above somewhat lower terrain in the adjacent southern foothills. The ridges and uplands of the outcrop belt are broken by numerous generally northward 

flowing streams and rivers, many of which rise in the 

Brooks Range. On the west, four small rivers, the Pitmegea, Kukpowruk, Kokolik, and Utukok, cross the outcrop belt of the Nanushuk Group and flow into the Chukchi Sea. These streams are relatively small, but in places 

they and their tributaries are incised more than 500 m 

into the Nanushuk terrain. As a consequence of this inciCretaceous Tectonics and Depositional Cycles 9 
EXPLANATION 

AUTOCHTHONOUS ROCKS 

Colville Group (Cretaceous: Maestrichtian to Cenomanian) 

Nanushuk Group (Cretaceous: Cenomanian to Aptian) 

Fortress Mountain Formation (Cretaceous: Aptian to 

Cenomanian) . Kfmk, Mount Kelly Graywacke Tongue 

Bathtub Graywacke and Kongakut Formation (Cretaceous: 

Aptian to Neocomian) 

Undifferentiated sedimentary rocks (Upper Triassic to Lower 

Mississippian) 

Undifferentiated metamorphic rocks (Middle Devonian to 

Precambrian) 

Granite (Cretaceous to Devonian) 

1 0 Geology of the Nanushuk Group and Related Rocks, Alaska 

ALLOCHTHONOUS ROCKS-Arranged in order of superposition 

Misheguk Mountain and Copter Peak allochthons (Triassic to 

Lower Paleozoic) 

Nuka Ridge allochthon (Permian to Mississippian) 

lpnavik River allochthon (lower Cretaceous to Middle 

Devonian) 

Kelly River allochthon (Lower Cretaceous to Mississippian) 

Endicott Mountains allochthon (Lower Cretaceous to Upper 

Devonian) 

CONTACT -Dashed where inferred or concealed 

• 4 THRUST FAULT -Dashed where inferred or concealed; 

sawteeth on upper plate 

ANTICLINE-Showing direction of plunge 

-9- EXPLORATORY WELL-Dry hole 
Figure 6 (above and facing page). Generalized geologic map showing the approximate distribution of the 

Nanushuk and Colville Groups, major allochthons, and tectonic features of the Brooks Range. The Okpikruak 

Formation, pebble shale unit, and Kemik Sandstone Members of the Kongakut Formation are not shown. 

Anticlinal axes in the Chukchi Sea are from Arthur Grantz and Steven May (written commun., 1982). 

sion, the exposures of the Nanushuk on the western Arctic 

Slope are generally better than elsewhere on the Arctic 

Slope. This western area is frequently refeJTed to as the 

Utukok-Corwin area and, because of its better exposures, 

was the center of the initial outcrop studies in this project. 

East of the Utukok-Corwin area, the Colville River 

rises in the foothills of the De Long Mountains and has 

a generally east and northeasterly course for more than 

360 km to near Umiat, where it turns northward and flows 

toward the Beaufort Sea. For more than 160 km, between 

long 160° W. and 156° W., the upper Colville River flows 

south of the outcropping Nanushuk Group. As a result, 

the major Colville tributaries, which rise in the Brooks 

Range, do not cross the Nanushuk. From near the confluence of the Etivluk River with the Colville and eastward 

to the Umiat area, the Colville River crosses the outcrop 

belt of the Nanushuk. The major tributaries to the Colville 

in this area, the Kurupa, Killik, Chandler, and Anaktuvuk 

Rivers, all of which rise in the high Endicott Mountains 

to the south-are incised into the Nanushuk Group. This 

area, referred to as the Chandler River area, contains fair 

to good exposures along the major streams. 

Cretaceous Tectonics and Depositional Cycles 11 
Figure 7. Nanushuk Group and underlying Torok Formation on the western Arctic Slope. The relatively competent 

Nanushuk is folded into a number of broad, gentle synclines. Note the scattered resistant beds that appear to 

pinch out eastward in the underlying incompetent shale. View northeast across the upper Pitmegea River. Kn, 

Nanushuk Group; Kt, Torok Formation. Short dashed lines, approximately located contacts; long dashed lines, anticline (opposing arrows) and syncline (converging arrows). Syncline is approximately 15 km long. 

North of the Colville River, exposures of the 

Nanushuk Group are generally poor. This area, which encompasses about one-third of the outcrop belt of the 

Nanushuk, is drained by the Kuk, Meade, Topagoruk, 

Ikpikpuk, and Awuna Rivers and their tributaries. These 

rivers all head in a low rolling terrain on the Nanushuk 

about 300-500 m in elevation. The Kuk River trends 

northwestward into the Chukchi Sea; the Meade, Topagoruk, and Ikpikpuk drainages flow northward to the 

Beaufort Sea; and the Awuna flows eastward into the Colville River. These streams have generally low discharge, 

and, as a consequence, they do not incise the Nanushuk 

deeply; this lack of erosion results in generally poor outcrops. In addition, the detailed studies carried out in this 

project suggest that the Nanushuk in the Kuk-Ikpikpuk 

Rivers area may contain interdeltaic deposits (Ahlbrandt 

and others, 1979). The resulting low-sand content of the 

deposits thus may also have caused the small number of 

outcrops in this area. As a result, only limited field studies 

have been carried out in the Kuk-Ikpikpuk Rivers area 

of the Nanushuk outcrop belt. Most of the knowledge of 

the Nanushuk Group north of the foothills is derived from 

subsurface data from exploratory wells and from seismic 

data. 

On the eastern Arctic Slope, east of the Itkillik 

River, outcrops of the Nanushuk and correlative beds are 

confined to several high mesal ike hills, and to a few 

12 Geology of the Nanushuk Group and Related Rocks, Alaska 

stream and river cutbanks. The rolling upland terrain 

characteristic of the western part of the Nanushuk is present only in a limited area in the vicinity of the 

Sagavanirktok and Ivishak Rivers. The absence of the 

Nanushuk on the eastern Arctic Slope is caused by a rapid 

northeastward facies change to basinal shales, combined 

with a northward swing in the trend of the mountain front 

that brings to the SlJrface these nonresistant sediments. 

In addition, in contrast to the western Brooks Range, extensive Pleistocene glaciation in the eastern Brooks Range 

resulted in mantling of extensive areas north of the mountains by morainal and associated outwash deposits. Details 

of the glaciation and glacial deposits are given by Detterman (1953), Detterman and others (1958), and Hamilton 

and Porter (1975). 

EVOLUTION OF THE BROOKS RANGE 

OROGENIC BELT 

Evolution of the orogenic belt forming the Brooks 

Range, herein termed the Brooks Range orogenic belt, 

began during Neocomian time. This belt apparently developed as a result of counterclockwise rotation of a crustal plate composed of northern Alaska and adjacent areas 

(Tailleur, 1969; 1973; Tailleur and Snelson, 1969; 

Rickwood, 1970; Newman and others, 1979). 
Paleomagnetic data and regional reconstructions suggest 

that this plate, the Arctic Alaska plate (Tailleur and 

others, 1976; Newman and others, 1979), moved relatively southward and, at its southern compressional margin, 

was obducted by oceanic crust at a south-dipping subduction zone (Mull, 1977, p. B31; Mull, 1982, p. 44; and 

Roeder and Mull, 1978). The relationships of continental 

and oceanic crust suggest that the orogenic belt developed 

by underthrusting (Tailleur and Snelson, 1969), rather 

than by the more commonly visualized mode of overthrusting . Underthrusting is the process by which slices 

of sediment are progressively detached from a down-going 

slab as it approaches a subduction zone (Seely and others, 

197 4). Following the stage of major crustal compression, 

a period of dominantly vertical uplift resulted in major 

modifications of the earlier structural style. 

Neocomian and Aptian(?) Time 

Tectonics 

Underthrusting in the orogenic belt in the western 

Brooks Range resulted in formation of multiple allochthonous sheets of mafic igneous and sedimentary 

rock; each allochthon is underthrust by rocks that were 

formerly more northerly facies equivalents. Each allochthon is composed of a distinctive suite of formations, 

most of which maintain a relatively uniform character 

within the allochthon but differ markedly from coeval formations in other allochthons (fig. 8). Several major allochthons can be traced for more than 400 km through 

the De Long and western Endicott Mountains. In the central Brooks Range, most of the allochthons that are present in the western Brooks Range have been eroded so 

that in most places only one, the Endicott Mountains (or 

Brooks Range) allochthon, is present. Parts of some of 

the structurally higher allochthons are present in isolated 

areas, and abundant distinctive detritus in later Cretaceous 

deposits attests to their former widespread presence. During the Neocomian, the stack of allochthonous sedimentary and minor igneous rocks may have been as much as 

10 km thick, with an overlying sheet of imbricated mafic 

to ultramafic rock that may also have been as much as 

10 km thick. 

Dating of the Orogeny 

Fauna from orogenic sediments date the onset of 

orogenic activity as Neocomian. At Ekakevik Mountain 

(fig. 9) in the southern NPRA, autochthonous early Albian conglomerates unconformably overlie and truncate 

imbricated thrust sheets containing orogenic sediments of 

Berriasian and Valanginian Age (see mapping by Tailleur 

MISHEGUK MOUNTAIN ALLOCHTHON 

Gabbro 

Ultramafic rocks 

~-------THRUST FAULT-------___, 

COPTER PEAK ALLOCHTHON 

Basalt 

Middle Devonian limestone, thin 

~-------THRUST FAULT--------~ 

NUKA RIDGE ALLOCHTHON 

Mudstone 

Permian and Triassic cherty shale 

Mississippian to Permian limestone, arkose, sandstone 

~-------THRUST FAULT-------~ 

IPNAVIK RIVER ALLOCHTHON 

Cretaceous wacke 

Permian and Triassic chert, shale 

Mississippian chert, shale, limestone, diabase sills 

Middle Devonian limestone, thin 

~-------THRUST FAULT------------------~ 

KELLY RIVER ALLOCHTHON 

(Southern facies) 

Cretaceous wacke 

Permian and Tri9ssic 

chert, shale 1-... 

ENDICOTT MOUNTAINS 

ALLOCHTHON 

(Northern facies) 

Cretaceous wacke 

.Y Permian and Triassic chert, ~ Mississippian crinoidal "~<. shale 

limestone, cherty ~ Mississippian crinoidal 

Mississipian silty J limestone (east), 

limestone I chert (west) 

Upper Devonian quartzite, 

conglomerate 

Upper Devonian shale 

1---------'---THRUST FAULT 

Other sedimentary rocks 

~--------THRUST FAULT-------~ 

ARCTIC SLOPE AUTOCHTHONOUS COMPLEX 

Middle and Upper Triassic Shublik Formation: 

limestone, shale 

Permian and Triassic Sadlerochit Group: sandstone, 

conglomerate, shale 

Mississippian and Pennsylvanian Lisburne Group 

Lower Mississippian Kayak Shale and Kekiktuk 

Conglomerate: shale, conglomerate, sandstone 

Lower Paleozoic metamorphic rocks 

Figure 8. Vertical superposition of the basal autochthonous complex and the major allochthons in the 

Brooks Range. In general, each allochthon, where 

present, occurs in the same structural position relative to adjacent allochthons. Each allochthon represents a southern facies of rocks in the immediately 

underlying allochthon and has been telescoped relatively northward by thrust faulting over the underlying 

allochthon or by southward underthrusting of the underlying rocks. 

Cretaceous Tectonics and Depositional Cycles 13 
Figure 9. The Fortress Mountain Formation at Ekakevik Mountain (T. 10 S., R. 22 W.). A thick, uniformly south 

dipping section of interbedded conglomerate, sandstone, and shale unconformably overlies (line on photograph) 

a thrust-faulted terrane of Mississippian limestone, mafic igneous rocks, the Okpikruak Formation, and other rock 

units. Facies changes over short distances are characteristic of the Fortress Mountain here. View to the east. 

and others, 1966, and Mayfield and others, 1978). Facies 

study of these conglomerates suggests alluvial-fan, fluvial, 

and shallow marine deposition very near an active source 

of coarse detritus (Hunter and Fox, 1976). Similar relationships are seen at Castle Mountain and Fortress 

Mountain on the central Arctic Slope and at Atigun 

syncline, east of Galbraith Lake. The data suggest that 

the major orogenic activity that formed the Brooks Range 

was pre-Albian, beginning during Berriasian time, and 

continuing into the Aptian. 

Depositional Cycle and Nomenclature 

Neocomian rocks are discontinuously exposed along 

the entire north flank of the Brooks Range and in some 

areas within the mountains, particularly in the western 

Brooks Range. Thin Neocomian deposits are present in 

the subsurface of most of the Arctic Slope. The relationship of the various Neocomian rock units and the 

generalized configuration of the Colville basin north of 

14 Geology of the Nanushuk Group and Related Rocks, Alaska 

the central Brooks Range is diagrammatically illustrated 

in figure 5, and by Carter and others ( 1977, fig. 5, p. 

24-25). 

In general, the depositional sequence coincides with 

the worldwide global eustatic cycle of Vail and others 

( 1977, fig. 2, p. 85) that ended during middle Aptian 

time (fig. 10). These cycles are described in the section 

of this chapter on "Depositional cycle and nomenclature," 

in the section on "Albian and Cenomanian time." 

In the De Long Mountains and on the north flank 

of the Endicott Mountains, the Neocomian sequence, consisting dominantly of flysch, is known as the Okpikruak: 

Formation (Patton and Tailleur, 1964, p. 445-449). 

On the gentle northern flank of the Neocomian depositional basin, stable platform sediments coeval with the 

Okpikruak Formation are informally known as the "pebble 

shale unit" (Alaska Geological Society Newsletter, 1978; 

Bird and Andrews, 1979). Although these deposits are 

sometimes referred to the Okpikruak:, the name is inappropriate inasmuch as they bear little resemblance to the 

thick flysch of the type Okpikruak. In outcrop in the 
RELATIVE CHANGES OF SEA LEVEL ARCTIC SLOPE STRATIGRAPHY 

Cl I 0 u a: 0 w a.. a.. w 

c 

(Q 

"2 

0 

UJ c f- Q) 

<( C/) 

....J 

C/) 

::> 

0 

w 

u 

<( 

1-

w 

a: 

u 

>- ....J 

a: 

<( c UJ (Q .E 

0 

u 

0 

Q) 

z 

AGE --RISING 

Maestrichtian 

70 

Axis of 

/

Colville 

Basin 

Kingak 

Shale 

' 

Arctic 

Coast 

N 

Formation 

Of Kongakut Formation 

Figure 10. Generalized global Cretaceous eustatic cycles (modified from Vail and others, 1977, fig. 2) and generalized 

Arctic Slope Cretaceous stratigraphy discussed in this report. The horizontal axis shows relative positions of sea 

level; 1 is the maximum relative highstand during the Late Cretaceous, and 0 is the Tertiary minimum relative 

lowstand. Relative rises of sea level are plotted toward the left; relative drops are plotted toward the right. Cycles 

in patterned area are generalized. Figures 4 and 5 show stratigraphic relationships in more detail. 

northeastern Brooks Range, the beds were informally referred to as the pebble shale member of the Kongakut 

Formation (Detterman and others, 1975, p. 21-25). 

In the western part of the Arctic National Wildlife 

Refuge, the Kemik Sandstone Member of the Kongakut 

Formation (Detterman and others, 197 5, p. 21) underlies 

the pebble shale unit; the Kemik was formerly considered 

a basal member of the Okpikruak Formation (Keller and 

others, 1961, p. 196). In the Prudhoe Bay area, the Put 

River Sandstone of Jamison and others (1981, fig. 13; 

Mcintosh, 1977, figs. 5, 6, 16, 17) occurs in a similar 

stratigraphic position. 

Lithology and Distribution 

The Okpikruak Formation is dominantly a thick 

flysch sequence and is present in allochthonous sheets 

throughout the De Long and Endicott Mountains. The 

flysch consists of rhythmically interbedded shale and 

sandstone (fig. 11), and pebble-to-boulder conglomerate 

in some areas. In addition, a sedimentary chaos or olistostrome is locally present as part of the Okpikruak. The 

olistostrome is composed of a heterogeneous assemblage 

of exotic blocks of sedimentary and mafic igneous rocks. 

Where well exposed, the blocks are clearly encased in 

mudstone, graywacke, and associated channel conglomerates at depositional, not fault, contacts (Mull and others, 

1976). In some places, the blocks exceed tens of meters 

in maximum dimension. A well-exposed outcrop of the 

olistostrome is illustrated in figure 12. 

At the base of the Okpikruak on some of the allochthons in the western and central Brooks Range, a thin 

section of clay shale and interbedded coquinoid limestone 

of the stable platform sequence forms the base of the section; the limestone characteristically weathers reddish 

brown, and the shale is commonly maroon in color. In 

the western De Long Mountains, a thin quartzose sandstone-siltstone unit is also locally present at this horizon. 

Neocomian flysch is not known along the mountain 

front of the northeastern Brooks Range. In contrast to the 

flysch of the Okpikruak Formation, the pebble shale unit, 

the Kemik Sandstone Member, and the Put River Sandstone of the northeastern Brooks Range and subsurface 

Arctic Slope consist of stable platform deposits. 

The pebble shale unit is present in the northeastern 

Brooks Range and in the subsurface throughout the entire 

northern Arctic Slope; its southern limit in the subsurface 

is unknown. It consists of about 100 m of fissile organicrich clay shale containing scattered grains of medium- to 

coarse-grained quartz and a few cobbles and boulders of 

Cretaceous Tectonics and Depositional Cycles 15 
Figure 11. Typical rhythmically interbedded sandstone and shale of the Okpikruak Formation flysch, central and 

western Brooks Range. Sandstone beds range from 10 em to 15 em thick. 

chert, quartz, and quartzite. In outcrop it is exposed discontinuously from the Kemik Creek-Kavik River area 

eastward through the Sadlerochit Mountains; its easternmost known exposure is in the Niguanak River area 

near Barter Island. Except where truncated by an overlying unconformity, the pebble shale unit has been found 

in most of the subsurface of the northern Arctic Slope. 

Locally, the pebble shale unit is underlain by the 

Kemik Sandstone Member of the Kongakut Formation. 

The Kemik consists of as much as 40 m of very fine 

grained quartzose conglomeratic sandstone. Facies study 

suggests a shoreface-to-beach environment of deposition 

and sediment derivation from a northern source area (Mull 

and Kososki, 1977, p. B21, and unpublished data), in 

contrast to the southerly derived turbidite deposits of the 

siltstone member of the Kongakut Formation, which overlies the pebble shale unit and Kemik Sandstone Member 

in the eastern part of the Arctic National Wildlife Refuge. 

The Kemik Sandstone Member crops out in the Sadlerochit Mountains area, and along the mountain front as 

16 Geology of the Nanushuk Group and Related Rocks, Alaska 

far southwest as the Echooka River area north of the 

Philip Smith Mountains. This part of the Brooks Range 

is autochthonous; thus, the Kemik is not involved in the 

allochthonous part of the orogenic belt. It is not present 

in outcrop southwest of the Echooka River, but was found 

in the Atlantic Richfield Susie-1 and Nora Federal-1, 

Mobil Echooka-1, McCulloch Fin Creek-1, Forest 

Kemik-1 , and Amoco Kavik-1 exploratory holes (Tailleur 

and others, 1978) drilled in the foothills belt between the 

Canning River and Sagavanirktok River area. 

The Put River Sandstone of Jamison and others 

(1981; Mcintosh, 1977) is present locally in the eastern 

part of the Prudhoe Bay oil field, where it consists of 

as much as 22 m of conglomeratic sandstone. The uppermost sandstone horizon in the Kuparuk Formation in the 

Kuparuk oil field probably also correlates with the Kemik. 

In NPRA, similar sandstones have been found in the Cape 

Halkett-1, W. T. Foran-1, Walakpa-1, Peard-1, South 

Meade-1, East Teshekpuk-1 , and Ikpikpuk-1 exploratory 

holes. 
Figure 12. Olistostrome on the north side of the western De Long Mountains, North Slope. Exotic blocks of 

Mississippian limestone are encased in conglomeratic 

mudstone and are overlain by poorly sorted boulder 

conglomerate. Round limestone block in center is approximately 5 m high. 

Thickness 

In the western De Long Mountains, Sable and Mangus (1951, p. 11) reported a thickness of 500 m for the 

Okpikruak Formation; Chapman and Sable (1954, p. 7) 

and Mangus and others (1954, p. 9) suggested that its 

total thickness could be as much as 1,500-1,800 m. Near 

Anaktuvuk Pass on the north flank of the Endicott Mountains, the formation is about 650 m thick (Patton and Tailleur, 1964, p. 447). In the Elusive Lake area, both 

Brosge and others ( 1979) and Keller and others ( 1961, 

p. 197) reported more than 600 m of Okpikruak. 

In contrast to these orogenic deposits, the coeval 

stable platform deposits of the northern flank of the basin 

are very thin. Throughout the subsurface of the Arctic 

Slope and in outcrop in the northeastern Brooks Range, 

the pebble shale unit is of remarkably uniform thickness, 

about 100 m. However, in the British Mountains, Detterman and others (1975, p. 21, 25) reported a thickness 

of 160 m. Where present, the underlying basal sandstone 

and (or) conglomerate of the Kemik, Put River, or unnamed beds may reach a thickness of about 40 m but 

in most places are much thinner. 

Stratigraphic Relations 

On the thrust sheets throughout the western and central Brooks Range, Neocomian rocks unconformably overlie condensed deposits of early Middle Jurassic, Triassic, 

and older age; the Jurassic Kingak Shale is not present 

on the allochthons. In many places, flysch of the Okpikruak is at the base of the section. However, on some 

allochthons, the base of the Neocomian contains a thin 

section of clay shale that in some places contains interbedded coquinoid limestone. These beds were apparently deposited in a stable shallow platform setting prior to the 

onset of subsidence and orogenic sedimentation. Similar 

relationships are present at the base of the Cretaceous 

within the mountains of the northeastern Brooks Range. 

On the stable north flank of the basin, a major regional angular unconformity is present at the base of the 

pebble shale unit or at the base of the Kemik, Put River, 

or unnamed sandstone, where present. The contact of the 

pebble shale unit and Kemik is conformable and gradational through a V2- to 1-m interval. In the subsurface 

the basal unconformity truncates rocks as old as pre-Mississippian; in outcrops on the north side of the Sadlerochit 

Mountains in the Arctic National Wildlife Refuge, the 

pebble shale unit and Kemik Sandstone Member unconformably overlie the Permian and Triassic Sadlerochit 

Group. To the south, both in outcrops along the northeast 

Brooks Range front and in the subsurface, the unconformity dies out and the amount of truncation decreases so 

that thick sections of the Jurassic Kingak Shale are widespread beneath the Cretaceous. 

In the Colville basin, regional relationships indicate 

that the orogenic belt and foredeep migrated progressively 

northward (Snelson and Tailleur, 1968), so that earlier 

orogenic deposits were themselves subsequently involved 

in the orogeny and, in places, were in tum cannibalized 

to form later deposits. The Okpikruak and related sediments likely prograded progressively northward over the 

pebble shale unit and related rocks in the center of the 

basin (fig. 5). Continuous deposition into the Albian may 

have occurred in some areas; elsewhere in the basin the 

top of the cycle may be a disconformity or nondepositional hiatus. This subject is discussed at greater length 

in the section of this report describing Albian-Cenomanian 

stratigraphic relations. 

Toward the northern stable flank of the basin, the 

pebble shale unit and related rocks are overlain by rocks 

of the Albian to Cenomanian (Torok and Nanushuk) depositional cycle (fig. 5). The upper contact is sharp and 

Cretaceous Tectonics and Depositional Cycles 17 
represents the abrupt onset of Albian and older orogenic 

sedimentation prograding over the subsiding platform. The 

hiatus represented by this contact is probably relatively 

short in the deeper parts of the basin, but to the north 

may encompass a large part of the Aptian. In the Atigaru1 , South Harrison Bay-1, and West Fish Creek-1 wells 

in the northeastern part of NPRA, an unconformity at the 

base of the Torok shows erosion of the entire pebble shale 

unit. This relationship is illustrated in figure 5 and by 

Molenaar (figs. 19 and 21 , this volume). 

Age 

Based upon mollusk zonation in the central Brooks 

Range, the Okpikruak Formation is considered to be both 

Berriasian (Jones and Grantz, 1964, p. 1468) and Valanginian in age (Brosge and others, 1979, table 1). Sable 

and others (1951), Sable and Mangus (1951), and Mangus 

and others (1954) also reported Berriasian and Valanginian fossils from the Okpikruak of the western Brooks 

Range. 

At the type section of the Kongakut Formation, the 

Kemik Sandstone Member contains the Hauterivian ammonite Simbirskites sp. (Detterman and others, 1975, p. 

25). Elsewhere at three localities in the Sadlerochit Mountains area of the Arctic National Wildlife Refuge, shale 

horizons interbedded with the Kemik contain a microfauna 

identified by M. B. Mickey (Biostratigraphics Inc., written commun., 1981) as Hauterivian to Barremian (late 

Neocomian) (table 1, localities 80AMu7-2, 80AMu16--

17, and 80AMu27-8). In addition, on a tributary of the 

Kavik River, a horizon interpreted as an offshore marine 

equivalent of the Kemik has yielded a microfauna identified as Neocomian by Anderson, Warren, and Associates 

(written commun., 1978) (table 1, loc. 76AMu113-1). 

Pelecypods from several localities in the Kemik of the 

Sadlerochit Mountains area have been identified by J. W. 

Miller and D. L. Jones as Astarte ignekensis Imlay, 

Tracia stelcki McLearn, Entolium utokokense Imlay, 

Panope(?) kissoumi McLearn, and Camptonectes dettermani Imlay of early and (or) middle Albian age (table 

2, locs. 76AMu15-1, 76AMu27-4, 80AMu12, 

80AMu14--10, 80AMu17, 80AMu19-3, and 80AMu22). 

The close association of these pelecypod collections with 

the microfauna! collections suggests that the pelecypod 

fauna may have a longer range than previously recognized. 

Microfauna from the pebble shale unit of the Kongakut Formation in the Sadlerochit Mountains area were 

dated as Hauterivian to Barremian by M. B. Mickey 

(Biostratigraphics, Inc., written commun., 1981) (table 1, 

locs. 80AMu7-4, 80AMu8-4, 80AMu14--12, 80AMu16--

18, 80AMu16--18A, and 80AMu21). 

Detterman and others (197 5, p. 25) also reported 

18 Geology of the Nanushuk Group and Related Rocks, Alaska 

a Neocomian (late Hauterivian and Barremian) age for 

the pebble shale unit in the southern part of the Demarcation Point quadrangle of the eastern Brooks Range. A Jurassic microfauna reported in some collections was apparently reworked from beds eroded at the regional unconformity beneath the Kemik. In the northern Arctic Slope, 

many wells have penetrated the pebble shale horizon; 

many of these wells in NPRA yielded a definitive Neocomian microfauna. Typical of this horizon are the faunas 

from ditch cuttings in Kugrua-1 and Inigok-1 wells drilled 

in NPRA (Anderson, Warren, and Associates, written 

commun., 1979); these data are summarized on figure 13. 

In the De Long Mountains, beds at the base of the 

Cretaceous similar to the pebble shale unit have yielded 

both micropaleontologic and palynological data reported 

by Anderson, Warren, and Associates (written commun., 

1979) to be probably Neocomian (table 1, loc. 

77AMu16). Elsewhere in the De Long and Endicott 

Mountains these beds are also dated by the interbedded 

coquinoid limestone. This limestone, composed of the 

pelecypod Buchia sublaevis Keyserling, is dated as early 

Valanginian (Imlay, 1959, p. 161, 165; Jones and Grantz, 

1964, p. 1463). 

Paleogeography 

The generalized and inferred distribution of the 

major northern Alaska tectonic elements present during 

Neocomian and Aptian time is illustrated in figure 14. 

The boundaries of the elements are approximate and are 

intended to illustrate the basic concepts in the evolution 

of the Early Cretaceous depositional basin in northern 

Alaska. 

To the north is the expanding Canada basin, formed 

by the rotation of the Arctic Alaska plate. Movement of 

this lithospheric plate apparently created an extensional 

(pull-apart or Atlantic style) plate boundary consisting of 

a broad zone of normal faults that are generally 

downdropped to the north. This zone underlies the 

Beaufort Sea continental shelf and slope and forms the 

north edge of the Arctic Alaska plate. The Brooks Range 

orogenic belt is the compressional plate boundary that 

marks the southern edge of the Arctic Alaska plate. 

Throughout the Brooks Range, the distribution and nature 

of the Neocomian facies suggest a roughly east-west trend 

to this orogenic belt. Beneath the Chukchi Sea, the extension of the orogenic belt is inferred to trend to the northwest. However, control is sparse and is inferred in part 

from the trends of the Herald arch (Grantz and others, 

1970; Grantz, Holmes, and Kososki, 1976) and Tigara 

uplift (fig. 6; Payne, 1955). A limited number of measurements of flute casts and groove casts in Neocomian or 

Aptian flysch in the Tigara uplift near Cape Lisburne 

suggest current flow from west-northwest to east-southeast 
(Mull, unpub. field data, 1974). Paleomagnetic data from 

the Tigara uplift (Newman and others, 1979, and unpublished data) do not suggest rotation of this area relative 

to the remainder of the Brooks Range; thus the current 

measurements are suggestive of a western rather than a 

southern source of sediment for this area during Neocomian to Aptian time. 

During the Neocomian, the northern margin of the 

central and western Brooks Range orogenic belt lay far 

south of the present mountain front (fig. 14A). In this 

area, Neocomian orogenic deposits are widespread on allochthonous sheets that were thrust long distances northward prior to Albian deposition. 

Stratigraphic relationships with allochthonous 

Neocomian rocks unconformably overlain by autochthonous Albian rocks, such as at Ekakevik Mountain (figs. 

6 and 9), document a progressive northward migration 

of the orogenic front and foredeep during the Early Cretaceous. By Aptian time (fig. 14B), they are inferred to 

have migrated northward to locations near or north of the 

present western and central Brooks Range mountain front. 

Only in the northeastern Brooks Range does the evidence 

suggest that the orogenic belt and depositional trough lay 

south of the present mountain front. 

Between the Brooks Range on the south and the 

Canada basin on the . north lie the Colville basin and the 

Beaufort sill (fig. 14). The name "Beaufort sill" is here 

applied to the relatively narrow linear stable platform on 

the north side of the Colville basin. This area has generally been known as the Barrow arch (Brosge and Tailleur, 

1969; Rickwood, 1970, Detterman, 1973, Morgridge and 

Smith, 1972) or the Arctic platform (Payne, 1955, Brosge 

and Tailleur, 1970). Tailleur and others (1976) referred 

to it as the Barrow inflection to emphasize the passive 

nature of the subsidence on its flanks; it was not an active 

uplift. The sill can be recognized in the Cretaceous outcrops of the Sadlerochit Mountains area of the northeastern Brooks Range, and can be traced in the subsurface 

northwest to beyond Barrow. It received relatively little 

sedimentation during the Early Cretaceous, and it separates the deep asymmetrical foreland basin on the south 

from the Canada basin to the north. The sill is the remnant 

of the much larger Arctic platform or cratonic area that 

was rifted during southward rotation of the Arctic Alaska 

plate. Its form results from the combination of northward 

onlapping mid-Paleozoic through Jurassic stable platform 

sedimentation, which formed its southern flank, and the 

Early Cretaceous rifting which formed its northern flank. 

The term "Barrow arch" is deemed inappropriate because 

in a structural-geology context, "arch" is defined as "a 

broad open anticlinal fold of a regional scale; it is usually 

a basement doming" (Bates and Jackson, 1980, p. 2). 

"Arch" thus carries an implication of genesis that is not 

supported by the structural and stratigraphic relationships 

in this part of Alaska. 

Albian and Cenomanian Time 

Tectonics 

Regional relationships indicate that the Early Cretaceous stage of intense crustal shortening in the Brooks 

Range was followed by an episode of dominantly vertical 

tectonics (Mull, 1977, p. B28-B31; 1979, p. 9-10), possibly related to remobilization of granitic plutons in the 

southern Brooks Range. This event of predominantly vertical uplift may be the result of isostatic rebound of continental crust deeply depressed beneath oceanic crust at the 

south margin of the Neocomian orogenic belt. It coincides 

with the deposition of the Nanushuk Group. 

The greatest vertical uplift occurred in the Arrigetch 

Peaks-Mount Igikpak area of the Schwatka Mountains in 

the south-central Brooks Range. Here, granite gneiss in 

two plutons is present in an area greater than 972 km2 

(375 mi2), and as high as 2,600 m (8,510 ft), forming 

the highest peaks of the southern Brooks Range (fig. 6; 

mapping by Brosge and Pessel, 1977; Nelson and 

Grybeck, 1980). The amount of uplift appears to decrease 

both to the east and to the west in the Baird Mountains, 

where the Shishakshinovik, Redstone, and Kaluich plutons are much smaller and occur at much lower elevations 

than the Arrigetch-Igikpak pluton (see mapping by Pessel 

and Brosge, 1977; and Mayfield and Tailleur, 1978). Regionally, the core of the range is a west-plunging anticlinorium that exposes autochthonous or parautochthonous 

rocks flanked by allochthonous sheets. These allochthons 

are particularly well defined in the Endicott and De Long 

Mountains north and west of the core of the range. 

Of particular significance is the allochthon that 

forms the Endicott Mountains and extends from Feniak 

Lake on the west to the Philip Smith Mountains on the 

east (fig .. 6). Evidence for this allochthon was given by 

Mull and Tailleur (1977, p. B27-B29), Dutro and others 

(1976), and Armstrong and others (1976). This allochthon 

includes stratified rocks termed the "Endicott sequence" 

(Mull, 1979, p. 11), the "Brooks Range sequence" (Martin, 1970; Mayfield and others, 1978), and the "Foothills 

sequence" (Tailleur and others, 1966). Unfortunately the 

geographic part of the name "Endicott sequence" is 

preempted by the rock-stratigraphic name "Endicott 

Group". Therefore, to avoid confusion with this well-established stratigraphic name, the sequence is here renamed 

the "Endicott Mountains allochthon" for its occurrence in 

that range. The Endicott Mountains allochthon consists 

of a thick section of Upper Devonian quartzose sandstone, 

quartz- and chert-pebble conglomerate, and shale (the 

Kanayut Conglomerate and Hunt Fork Shale of the Endicott Group) overlain by Mississippian cherty limestone 

of the Lisburne Group. Permian and Triassic siliceous 

beds (Siksikpuk and Otuk Formations, Mull and others, 

Cretaceous Tectonics and Depositional Cycles 19 
1'-.J 

= 

~ 

!! 

~ s. 

-:r 

I'D 

z 

Dl 

= c 

Cll 

:r 

c 

:II:"' 

~ 

a 

c 

'0 

Dl 

Q. = 

~ 

[ 

~ ~ 

> i" 

~ 

GAMMA RAY CURVE 

(API units) 

0 

100 

50 

150 

100 

200 

DEPTH 

(Feet) 

6800 

6900 

7000 

7100 

7200 

150 

250 

SONIC CURVE 

INTERVAL TRANSIT TIME 

(Microseconds per foot) 

100 

200 

KUGRUA 1 TEST WELL 

DUAL INDUCTIONLATEROLOG RESISTIVITY 

CURVE 

50 

15011.0 

(Ohm meters) 

10 100 

z 

0 

~ :'i! 

a: 

0 

IL. 

~ 

0 

a: 

0 

1-

1-

z 

:> 

w 

....J 

<( 

I 

C/) 

w 

....J 

a:l 

a:l 

w 

a.. 

WELL DEPTH 

(Feet) 

4,910-6,890 

6,890-7,220 

w' 7,220-7,330 ....J 

<( 

I 

C/) 

~ 

<( 

(!) 

z 

:;;2 

AGE 

Early Cretaceous, 

Aptian 

Early Cretaceous, 

Neocomian 

Latest Jurassic 

to earliest 

Cretaceous. 

FOSSILS 

Pyritized radiolarian fauna: 

Lithocampe spp. 

Spongodiscus spp. 

Cenosphaera spp. 

Dictyomitra spp. 

Gaudryina tailleuri 

G. tappanae 

Glomospirella sp. 

Ammobaculites erectus 

Trochammina squamata 

T. conicominuta 

T. cf istowensis 

Gaudryinella irregularis 

Glomospira subarctica 

Lenticulina sp. 

Pseudobolivina sp. 

Haplaphragmoides duoflatis 

H. coronis 

H. inf/atigrandis 

Conoboides cf. umiatensis 

Praebulimina sp. 

Saracenaria grandstandesis 

Lituotuba gallupi 

Globulina exserta 

Ouinqueloculina sp. 

Gaudryina milleri 

Trochammina instowensis 

T. gryci 

T. sp. 

Haplaphragmoides canui 

Ammobaculites alaskensis 

lnvolutina orbis 

T. cf. canningensis 
0 

tD 

~ 

~ c 

(II 

~ ::I 

;:;· 

(II 

Dl 

::I 

Q. 

c 

tD 

l 

s· 

::I 

!!!.. 

n 

~ 

r 

!::: 

INIGOK 1 TEST WELL 

GAMMA RAY CURVE 

(API units) 

DEPTH 

(Feet) 

SONIC CURVE 

INTERVAL TRANSIT TIME 

(Microseconds per foot) 

DUAL LATEROLOG 

RESISTIVITY 

(Ohm meters) WELL DEPTH I AGE FOSSILS 

30 

130 

80 

180 

130 

230 100 

8900 

9000 

9100 

9200 f-==::::::::;;;:~--,---

9300 

75 50 1001 

z 

0 

~ ::2: 

a: 

0 

u.. 

~ 

0 

a: 

0 

1-

~I 

~I 

il 

~w 

o(...J 

(!)<( 

ZI 

~(/) 

(Feet) 

5,130-8,310 I 

8,310-9,060 

9,060-9,660 

I 

I 

I 

Aptian to Pyritized radiolarian fauna, 

early Albian containing rare to common 

agglutinated and calcareous 

foraminifera 

Cenosphaera spp. 

Spongodiscus spp. 

Dictyomitra spp. 

Stichomitra spp. 

Archaeodictyomitra sp. 

Lithocampe spp. 

Early Cretaceous,! Sparse Lithocampe fauna 

probable Aptian 

Early Cretaceous, I Ammodiscus mackensiensis 

Neocomian Ouinqueloculina sp. 

Glomospirella arctica 

G. subarctica I Ammobaculites reophacoides 

Ouadrimorphina sp. 

Praebulimina sp. I Gaud'Yina ta#leuri 

Bathysiphon scintillate 

Trochammina sp. 

Pseudobolivina sp. 

I Haplophfflgmoldes duoflat/s 

H. coronis 

H. goodenoubhenis 

H. inflatigrandis 

Conorboides cf. umiatensis 

Figure 13. Log character and paleontological data for the pebble shale unit and adjacent rock units in the U.S. Geological Survey Kugrua-1 and lnigok-1 test wells in NPRA. The base of the pebble shale unit is a major regional erosional 

unconformity; the upper contact probably represents a nondepositional hiatus. Paleontology by M. B. Mickey of 

Anderson, Warren and Associates. 
~ 

~ 

" ~ 

~ 

a 

-; 

z 

Dl 

= c 

"' c =-

'11:" 

" a 

c 

~ 

Dl 

Q. = 

~ 

~ 

[ 

~ ~ 

> E" 

f' 

Table 1. Microfauna! and palynological data from lower Cretaceous rocks, North Slope, Alaska 

[Identifications and dating by Anderson, Warren, and Associates, written commun., 1979, and Biostratigraphics, Inc., written commun., 1981. 

C, common; F, frequent; R, rare] 

Locality Stratigraphic 

No. position 

75AMu99-1-- Lower part of 

Nanushuk Group 

(Corwin Formation). 

7 5AMulOOA-- Upper part of 

Torok Formation. 

75AMulOOB-- ------do------

75AMu16-1-- ------do------

75AMu16-2-- ------do------

77AMu17-- Mount Kelly 

Graywacke 

Tongue of 

Fortress 

Mountain 

(pre-Torok 

sandstone). 

Location 

(quadrangles 

scale 1:250 ,000) 

Corwin Bluff, De Long 

Mountains quadrangle, 

western Arctic 

Slope. 

Pitmegea River area, 

De Long Mountains 

quadrangle, western 

Arctic Slope. 

--------do---------

South of Tuktu 

Bluff, Chandler 

Lake quadrangle, 

central Arctic 

Slope. 

South of Tuktu 

Bluff, Chandler 

Lake quadrangle, 

central Arctic 

Slope. 

Eagle Creek, De Long 

Mountains quadrangle, 

western Arctic 

Slope. 

Township and 

range 

NWl/4 

sec. 34 

T. 6 S., 

R. 55 W. 

SW1/4 

sec. 11, 

T. 8 S., 

R. 53 W. 

SWl/4 

sec. 11, 

T. 8 S., 

R. 53 W. 

SE1/4 

sec. 25, 

T. 8 S., 

R. 2 W. 

SW1/4 

sec. 25, 

T. 8 S., 

R. 2 W. 

NE1/4 

sec. 13, 

T. 9 S., 

R. 46 W. 

Fauna and flora 

Undifferentiated bisaccates 

(F). Gardodinium trabeculasum (single), Muderongia 

sp.-5 (R), Odontochitina 

operculata (F), Oligosphaeridium complex (R). 

Undifferentiated bisaccates 

(C). Cyclonephelium 

distinctum (R), Muderongia 

tetracantha (single), 

Odontochitina operculata 

(R), Oligosphaeridium 

complex (F), 0. c. (thickwall) (R), Palaeoperidinium 

cretaceum (R). 

Undifferentiated bisaccates 

(C). Gardodinium trabeculasum (single), Muderongia 

sp.-5 (single), Odontochitina 

operculata (R), Oligosphaeridium complex (F), 0. c. (thickwall) (R), Veryhac~ium sp. (R). 

Undifferentiated bisaccates 

(F), Osmundacidites ~· (R). 

Odontochitina operculata 

(single), Palaeoperidinium 

cretaceum (R), ?Pseudoceratium retusm (single), 

"TMrnanaceae < R) • 

Bathysiphon vitta (F), 

Gaudryina cf. tailleuri (R). 

Cyclonephelium distinctum 

(R), Gardodinium trabeculosum (R), Oligosphaeridium 

complex (F), Palaeoperidinium 

cretaceum (R). 

Age 

Early Cretaceous 

(Aptian and Albian). 

Do. 

Do. 

Early(?) Cretaceous 

(possibly Aptian 

and Albian). 

Probably Early 

Cretaceous (Aptian 

or older). 

Early Cretaceous 

(probably Aptian and 

Albian). 
0 

tl) 

~ 

2 

c 

~ 

f 

;::;· 

~ 

1:11 

::II 

Q. 

c 

tl) 

l 

cs· 

::II 

e!.. 

1""1 

-< n 

~ 

~ 

w 

75AMu28-1-- Shale underlying Fortress 

Mountain Formation. 

75AMu28-3A-- ------do-------

75AMu28-3B-- ------do-------

75AMu29-- ------do-------

75AMu29-- ------do-------

77AMu16-- Beds similar 

to pebble 

shale unit. 

76AMu29-2-- Shale interbedded with 

Kemik sandstone 

Member of 

Kongakut 

Formation. 

80AMu7-4-- Pebble shale, 

2 m above top 

of Kemik Sandstone Member. 

Base of Fortress 

Mountain, Chandler 

Lake quadrangle, 

central Arctic 

Slope. 

--------do----------

--------do----------

--------do----------

--------do----------

Tingmerkpuk Mountain, 

De Long Mountains 

quadrangle, western 

Brooks Range. 

Ignek Creek, Mt. 

Michelson quadrangle, 

eastern Brooks Range. 

--------do---------

SE1/4 

sec. 14, 

T. 10 S., 

R. 5 W. 

-----do-----

-----do-----

SE1/4 

sec. 2, 

T. 10 S. , 

R. 5 W. 

-----do-----

W1/2 

sec. 21, 

T. lOS., 

R. 45 W. 

NW1/4 

sec. 19, 

T. 3 N., 

R. 25 E. 

NW1/4 

sec. 19, 

T. 3 N. 

R. 25 E. 

Arenaceous spp. (F), 

Bathysiphon vitta (R), 

Glomospira ~ (R), 

Glomospirella gaultia 

(R), Spongodiscus sp. (R). 

Arenaceous spp. (R), 

Bathysiphon sp. (R), 

Glomospirella gaultina 

(R), Haplaphragmoides 

~· (R), shell fragments? (C), fecal 

pellets (F). 

Cenosphaera ~· (F), 

Lithocampe ~· (F), 

fecal pellets (F). 

Arenaceous sp. (R), 

Haplaphragmoides ~· 

(R), Saccammina lathrami 

(R), Inoceramus prisms (R). 

Undifferentiated bisaccates 

(C). Aptea polymorpha 

(single), Cyclonephelium 

distinctum (R), Odontochitina 

operculata (R), Oligosphaeridium complex (R), Palaeoperidinium cretaceum (R). 

Ammobaculites reophacoides, 

Gaudryina tailleuri, Haplaphragmoides coronis, Cyclonephelium distinctum, Oligosphaeridium complex, Canningia/ 

Cyclonephalium sp. 

Ammobaculites reophacoides, 

Gaudryina tailleuri, G. milleri?, Haplaphragmoides 

duoflatis. 

Bathysiphon scintillata (F), 

Conorboides cf. umiatensis 

(R), Glomospira corona (R), 

G. subarctica (F), Glomo-

~irella arctica (F), Haplaphragmoides duoflatis (F),~ 

coronis (F), ~ inflatigrandis 

(F). 

Early Cretaceous 

(Aptian or older). 

Do. 

Age not determined. 

Probably Early 

Cretaceous. 

Early Cretaceous 

(Aptian and Albian). 

Early Cretaceous 

(probably Neocomian). 

Early Cretaceous 

(Neocomian). 

Early Cretaceous 

(Hauterivian to 

Barremian). 
~ 

... 

C'l 

~ 

~ s. 

... 

:r 

I'll 

z 

Ill 

:I 

c 

Ill 

:r 

c 

:II:" 

C'l 

a 

c 

't:l 

Ill 

:I 

Q. 

~ 

[ 

I") 

~ 

g; 

> iii 

~ 

Locality 

No. 

76AMu15-1 

(M7419). 

76AMu27-4 

(M74 20). 

80AMu12--

80AMu 14-10--

80AMu17--

80AMu19-3--

80AMu22--

80Amu8-4--

Table 2. Megafossil data from Kemik Sandstone Member of Kongakut Formation, North Slope, Alaska 

[Identifications and ages reported by J. W. Miller and D. L. Jones, written commun., 1981. C, common; F, frequent; R, rare] 

Stratigraphic 

position 

Upper half of 

Kemik Sandstone 

Member. 

Upper part of 

Kemik Sandstone Member. 

------do------

------do------

Top of Kemik 

Sandstone 

Member. 

Float from 

upper part 

of Kemik Sandstone Member. 

Upper half of 

Kemik Sandstone Member. 

Pebble shale 

unit, composite 

sample 10-15 m 

above top of 

Kemik Sandstone Member. 

Location 

(quadrangles 

scale 1:250,000) 

North side of 

Sadlerochit 

Mountains, Mt. 

Michelson 

quadrangle. 

Tributary of Katakturuk River, north 

side of Sadlerochit 

Mountains, Mt. 

Michelson quadrangle. 

Hogback ridge, upper 

Ignek Creek, south 

side of Sadlerochit 

Mountains, Mt. Michelson quadrangle. 

Katakturuk River, 

south side of Sadlerochit Mountains, 

Mt. Michelson 

quadrangle. 

Marsh Creek, north 

side of Sadlerochit 

Mountains, Mt. Michelson quadrangle. 

---------do----------

---------do----------

Ignek Creek, Mt. 

Michelson quadrangle, 

eastern Brooks Range. 

Township and 

range 

N1/ 2 

sec. 19, 

T. 4 N., 

R. 30 E. 

SE1/4 

sec. 6, 

T. 3 N., 

R. 28 E. 

SW1/4 

sec. 27, 

T. 3 N., 

R. 26 E. 

E1/2 

sec. 3, 

T. 2 N., 

R. 27 E. 

SW1/4 

sec. 18, 

T. 4 N., 

R. 30 E. 

SW1/4 

sec. 18, 

T. 4 N., 

R. 30 E. 

SE1/4 

sec. 13, 

T. 4 N., 

R. 29 E. 

W1/ 2 

sec. 27, 

T. 3 N., 

R. 25 E. 

Fauna 

Astarte ignekensis Imlay 

Thracia stelcki McLearn 

Astarte ignekensis Imlay 

Astarte ignekensis Imlay 

Ditrupa cornu Imlay 

Astarte ignekensis Imlay 

Astarte ignekensis Imlay 

Entolium utukokense Imlay 

Panope(?) kissoumi McLearn 

Camptonectes dettermani Imlay 

Astarte ignekensis Imlay 

Ammodiscus mackenziensis 

(R), Arenaceous spp. (F), 

Bathysiphon scintillata 

(R), Gaudryina tailleuri 

(F), Haplaphragmoides 

duoflatis (F), H. coronis 

(R), Trochammin~s~ 

(R). 

Age reported 

Early Cretaceous 

(middle Albian). 

Early Cretaceous 

(early to middle 

Albian). 

Early Cretaceous 

(Albian). 

Early Cretaceous 

(early to middle 

Albian). 

Early Cretaceous 

(Albian). 

Early Cretaceous 

(middle Albian). 

Early Cretaceous 

(early to middle 

Albian). 

Early Cretaceous 

(Hauterivian to 

Barremian). 
n 

I Ill 

f Q. 

ell 

s. 

~ 

"8 

Ill 

:=: 

s· 

::I 

!!.. 

n 

~ 

ff 

N 

loll 

80AMu14-12--

80AMu16-18--

80AMu16-18A 

80AMu21----

80AMu7-2----

80AMu27-8--

76AMull3-1--

Pebble shale 

unit, 3 m 

above top of 

Kemik Sandstone 

Member. 

Pebble shale 

unit, 0.5 m 

above shaly 

siltstone of 

Kemik Sandstone Member. 

Pebble shale 

unit, same 

location as 

above, composite sample 

7-13 m above 

shaly siltstone 

of Kemik Sandstone Member. 

Upper Kataktuvuk 

River, Mt. Michelson 

quadrangle, eastern 

Brooks Range. 

Marsh Creek, 

Mt. Michelson 

quadrangle eastern 

Brooks Range. 

---------do----------

Pebble shale unit, ---------do-----------

uncertain position above Kemik 

Sandstone Member. 

Shale interbedded with 

Kemik Sandstone 

Member, approximately 10 m 

below upper 

sandstone unit 

of Kemik (same 

location as 

7 6AMu 29- 2) • 

Silty mudstone 

interbedded 

with sandstone 

in top of 

Kemik Sandstone 

Member. 

Shales, 

stratigraphically 

equivalent 

to Kemik 

Sandstone. 

Member. 

Ignek Creek, Mt. 

Michelson quadrangle, 

eastern Brooks Range. 

Tributary of Katakturuk River, Mt. 

Michelson quadrangle, 

eastern Brooks Range. 

Kavik River area, Mt. 

Michelson quadrangle, 

eastern Arctic Slope. 

Center of N1/2 

sec. 3, 

T. 2 N., 

R. 27 E. 

SE1/4 

sec. 19, 

T. 4 N., 

R. 30 E. 

-----do-----

NE1/4 

sec. 13, 

T. 4 N., 

R. 29 E. 

NW1/4 

sec. 19, 

T. 3 N., 

R. 25 E. 

NE1/4 

sec. 32 

T. 4 N., 

R. 28 E. 

NE1/4 

sec. 6, 

T. 1 S., 

R. 24 E. 

Arenaceous spp. (large, 

coarse) (F), Bathysiphon 

scintillata (F), Conorboides cf. umiate~ 

(R), Gaudryina tailleuri 

(R), Glomospirella arctica 

(R), Haplophragmoides 

duoflatis (F), H. coronis 

(F), .!!..:_ inflati/Va~), 

Trochammina cf. sablei (R). 

Ammodiscus sp. (very small) 

(R). Gaudryina subcretacea 

(F), G. cf. tailleuri (R), 

Haplo~ragmoides coronis 

(C), Thuramminoides sp. (R). 

Ammobaculites reophacoides 

(R), Conorboides cf. umiatensis (R), Gaudryina __ _ 

tailleuri (F), Haplophragmoides duoflatis (F), H. 

Wl'atigrandis (R). -

Ammodiscus mackenziensis (R), 

~ cf. elongatus (R), Arenaceous spp. (large, coarse) 

(F), Bathysiphon scintillata 

(R), Gaudryina tailleuri (F), 

G. tappanae (F), Glomospira 

subarctica (R), Haplophragmoides duoflatis (F), H. 

coronis (C), Thurammincldes 

sp. (R). 

Arenaceous spp. (C), Bathysiphon scintillata (F), 

Gaudryina subcretacea (R), 

~ tappanae (F), Glomospira 

subarctica (F), Haplophragmoides coronis (C), .!!..:_ goodenoughensis (R), Lituotuba 

gallupi (R), echinoid spines 

(R). 

Ammobaculites reophacoides 

(R), Arenaceous spp. (F), 

Gaudryina tailleuri (R), 

Haplophragrnoides duoflatis 

(C),.!!..:_ coronis (F). 

Ammodiscus mackenziensis, 

Bathysiphon scintillata, 

Gaudryina tailleuri, Glomospira ~. Glomospirella 

arctica, Haplophragmoides 

duoflatis, H. coronis, 

H. inflatiglr.;n~ 

Do. 

Do. 

Do. 

Do. 

Do. 

Probably Early Cretaceous (Neocomian, 

undifferentiated). 

Early Cretaceous 

(Neocomian). 
A 

Present boundary 

of Alaska 

Blew 

B 

Present boundary 

of Alaska 

BROOKS RANGE OROGENIC 

.-l"' 

""' .-J\ "" '\. ~ '( 0 ' ' ~ ~ '-.............._A ----+~ROXIMATE Colutlle g ~~ --~ 0 ~-''!!.!_te ~---:_Qf __ t COLVILLE BA§lli~ --- _ Aptian T-------- CIJ 

Present ----- m~tain mountain 

RANGE OROGENIC 

CANADA 

BAS IN 

CANADA 

BAS IN 

Figure 14. Diagrammatic map of major tectonic elements in what is now northern Alaska, showing inferred 

trends and approximate boundaries of elements. A, Neocomian time; 8, Aptian time. Hachured lines, diagrammatic fault locations, hachures on downthrown sides. Queries indicate inferred locations. 

1982) and a thin, distinctive Early Cretaceous (Valanginian) coquinoid limestone form the top of the sequence. 

The allochthon is as much as several kilometers thick; 

Upper Devonian rocks make up at least one third of the 

allochthon. It is widespread in the Endicott Mountains, 

but crops out only in local fensters in the De Long Mountains. 

26 Geology of the Nanushuk Group and Related Rocks, Alaska 

The mid-Cretaceous uplift of the core of the Brooks 

Range resulted in refaulting and folding of the Endicott 

Mountains allochthon, and development of regional northem dip into the basin north of the range. Although the 

folding and faulting resulting from this uplift appear complex, by comparison with the thrust faults formed by extreme crustal shortening during the Neocomian, they are 
relatively minor in magnitude. Coincident with the uplift 

of the core of the range was the accentuation of the Colville basin, the deep successor basin north of the mountains. 

Grantz and others ( 1970) suggested that faulting 

along the east margin of the Herald arch was younger 

than the folds that warp the Albian beds to the east. However, stratigraphic data (discussed in the section on 

"Paleogeography") suggest that deformation along the 

Herald arch and Lisburne Hills structural trends was probably occurring at the same time as uplift within the Brooks 

Range. 

Dating of Orogeny 

Evidence from uranium-lead and rubidium-strontium 

dating, regional structural-stratigraphic relationships (Mull 

and Tailleur, 1977), and metamorphic relationships (Nelson and Grybeck, 1979) indicates that the plutons were 

emplaced during the Paleozoic (Turner and others, 1979; 

Dillon and others, 1980; Silberman and others, 1979; 

Grybeck and Nelson, 1981). However, potassium-argon 

dating of these plutons has yielded dates ranging from 

88±4 m.y. to 98.8±2.7 m.y. (Senonian to Cenomanian) 

(Pessel and others, 1973; Brosge and Reiser, 1971; Turner 

and others, 1978). Similar relationships have been 

documented in the Romanzof Mountains in the northeastem Brooks Range (Sable, 1977, p. 57). In addition, 

Turner and others (1979) suggested that dates from the 

300-km-long schist belt in the southern Brooks Range indicate a pervasive thermal event that cooled to argon-retention temperatures for micas in mid-Cretaceous time. 

Paleontological data from the sediments resulting 

from this orogeny is summarized in the section on "Age" 

and in table 1. 

Depositional Cycle and Nomenclature 

Rocks of the Albian-to-Cenomanian depositional 

cycle lie unconformably upon older sediments along the 

margins of the Colville basin. Major rock units deposited 

during this cycle are the Nanushuk Group, Torok Formation, Bathtub Graywacke, Fortress Mountain Formation, 

and the Mount Kelly Graywacke Tongue (new name, this 

report) of the Fortress Mountain Formation; their nomenclature and generalized relationships are diagrammatically 

illustrated in figures 4 and 5. 

Most of the Nanushuk Group deposition appears to 

have taken place during the Albian, but paleontological 

data, summarized in the section on "Age" and in table 

1, from the Torok and Fortress Mountain Formations, 

both at the surface and in the subsurface, suggest that 

the depositional cycle began during the Aptian. Although 

Aptian-age sediments are probably present at its base in 

many areas, for convenience this cycle is referred to as 

the Albian-to-Cenomanian cycle. 

This depositional sequence coincides with the 

generalized Aptian-to-Cenomanian eustatic cycle illustrated by Vail and others (1977, p. 85, fig. 2). They indicate the end of one global eustatic cycle during the late 

Aptian, and the beginning of another that continued 

through the Albian into the Cenomanian. Figure 10, modified from Vail and others ( 1977), illustrates the two eustatic cycles, which, in general, coincide with the Neocomian-to-Aptian and Albian-to-Cenomanian depositional 

cycles in northern Alaska. The relative drop in sea levd 

that separates the two eustatic cycles may coincide with 

the stratigraphic break separating the Albian-toCenomanian depositional cycle from older sediments 

along the margins of the Colville basin. 

The relationship of these northern Alaska depositional cycles to the global cycles may be more than coincidence. Rona (1973), Vail and others (1977), Pitman 

(1978), and many other authors have related eustatic sealevel and ocean-volume changes to changes in elevation 

or volume of midocean ridges, which in tum may be related to changes in the rate of sea-floor spreading. A possible relationship between the major Early Cretaceous 

orogenic activity in northern Alaska and plate movement 

in the Arctic has been mentioned by Rickwood (1970), 

Tailleur (1973), and Newman and others (1979), and thi:s 

plate movement is assumed to have resulted from seafloor spreading and rifting in the Canada basin. However, 

it is not known whether ocean-volume changes inferred 

from tectonic activity in the Arctic would be sufficiently 

large to be noticeable when combined with the effects 

of spreading and orogeny elsewhere. 

In northern Alaska, a major regression during dep·-

osition of the Nanushuk Group, Torok Formation, and 

Fortress Mountain Formation at a time of rising sea level 

can be explained as a function of the rate of sediment 

influx from the orogenic belt exceeding the rate of sealevel rise and isostatic subsidence. Pitman (1978) pointed 

out that regression on a passive plate margin may occur 

as a result of a decreasing rate of sea-level rise. Such 

a decrease could be related to a decreasing rate of sea·-

floor spreading in the Arctic, which can, in fact, be in·-

ferred from the evidence of waning orogenic activity dur·-

ing Albian time in northern Alaska. In northern Alaska, 

the relatively close proximity of the passive extensional 

plate margin to the compressional margin and its orogeni<: 

belt undoubtedly complicates the interpretation of the con .. 

trol of eustatic cycles on sedimentation. 

Lithology, Thickness, and Distribution 

Fortress Mountain Formation 

Along the south flank of the Colville basin, the For .. 

tress Mountain Formation crops out discontinuously in a 

narrow, nearly east-west-trending line of isolated mesas 

Cretaceous Tectonics and Depositional Cycles 27 
and buttes (figs. 6, 9). It typically consists of thick, proximal, cobble-to-boulder conglomerate that varies from well 

sorted to chaotic and poorly sorted. The conglomerate is 

interbedded with shale, siltstone, and sandstone. Very 

rapid facies changes are characteristic. At Fortress Mountain, its type locality, and at Castle Mountain, it is more 

than 3,000 m thick (Patton and Tailleur, 1964). In the 

Killik-Etivluk River area, Chapman and others (1964) 

suggested a thickness of about 2,000 m. At other 

localities, such as at Atigun syncline and Ekakevik Mountain, it contains conglomerate of comparable coarseness 

but in thinner sections. Hunter and Fox (1976) interpreted 

the coarse beds at Ekakevik Mountain and Castle Mountain as generally shallow marine to fluvial deposits. Clasts 

consist dominantly of chert, fine-grained mafic igneous 

rocks, quartzite, and minor amounts of limestone, and 

they were apparently derived from erosion of the allochthons that form the Endicott and De Long Mountains. 

The associated sandstone is poorly sorted and very silty, 

and contains argillaceous lithic graywacke. Other prominent exposures along this trend, such as at Pingaluligit 

Mountain near the Killik River, Smith Mountain near the 

Etivluk River, and Liberator Ridge near the Kiligwa 

River, consist dominantly of thick graywacke and only 

minor conglomerate. Elsewhere along the Fortress Mountain trend, exposures are few and are generally confined 

to stream cutbanks; in these places, the formation consists 

almost entirely of thinly interbedded rhythmic turbidites 

of fine-grained sandstone, siltstone, and shale. The intermittent distribution of coarse-grained fluvial to shallowmarine facies that seem to grade laterally to submarine 

fan deposits suggests a series of major depocenters of 

coarse debris built out into the Colville basin. These depocenters probably coincide in general with the location 

of the major outcrops of Fortress Mountain Formation 

mapped north of the Endicott Mountains (fig. 6). 

In the eastern Brooks Range, the Bathtub 

Graywacke (Detterman and others, 1975, p. 25-28) is an 

apparent equivalent of the lower Torok and the Fortress 

Mountain. It consists of about 750 m of thin and rhythmically interbedded graywacke and shale. Graded beds and 

flute casts and load casts are common; the unit represents 

turbidites deposited on a submarine fan. 

For nearly 100 km northward from the outcrop belt 

of the Fortress Mountain Formation, little is known about 

it. Information is available from only two exploratory 

holes, Texaco East Kurupa-1, and West Kurupa-1, that 

apparently reached this stratigraphic level but did not 

completely penetrate the rocks representing the depositional cycle. In these wells, the section penetrated consists 

predominantly of very fine grained, argillaceous, lithic 

graywacke and shale. Although gas shows are common 

in this part of the stratigraphic section, the rapid rate of 

facies change observed in outcrop, the very dirty character 

of the sediments, and the facies distribution indicating 

28 Geology of the Nanushuk Group and Related Rocks, Alaska 

deposition of turbidites into a deep basin, suggest that 

the horizon has very little reservoir potential in the axial 

part of the Colville basin. 

In NPRA, a number of exploratory holes have penetrated the Albian-to-Cenomanian depositional cycle. However, because of poor paleontologic control, the exact correlation of the basal beds of the cycle with the Fortress 

Mountain Formation in outcrop is uncertain. Typically, 

the basal 100 m of the cycle contains a number of silty 

and clayey sandstone beds, in contrast to a predominantly 

shaly section above. This sandy section may be a distal 

basin equivalent of the Fortress Mountain Formation, but 

is here considered to be basal Torok Formation. 

Mount Kelly Graywacke Tongue (new name) of the 

Fortress Mountain Formation 

In the southern foothills belt of the De Long Mountains, a widespread unit of distinctive calcareous, micaceous graywacke is in a stratigraphic position similar to 

the Fortress Mountain Formation. These rocks were assigned to the Fortress Mountain by Chapman and Sable 

(1960, p. 71-73) who noted (p. 71) that "No exact time 

or stratigraphic correlations between these rocks and the 

rocks in the type area of the Fortress Mountain Formation 

are implied." Because the lithology is quite different from 

that of the type Fortress Mountain Formation, this rock 

unit is here named the Mount Kelly Graywacke Tongue 

of the Fortress Mountain Formation for its characteristic 

development on Mount Kelly (sec. 21, T. 11 S., R. 50 

W., Umiat meridian), considered its type locality. It is 

distinguished from the main body of the Fortress Mountain Formation by its calcareous cement and abundant detrital muscovite. The Fortress Mountain Formation is here 

restricted to the sections of massive conglomerate, shale, 

and noncalcareous, nonmicaceous graywacke and associated rhythmic turbidites present generally north of the 

Endicott and eastern De Long Mountains. The generalized 

distribution and the type locality of the Mount Kelly 

Graywacke Tongue are shown on figure 6. Throughout 

much of the western De Long Mountains foothills the 

tongue is exposed on prominent, rather sharp rubblecovered ridges; scattered well-exposed beds are on some 

hillsides and in stream cuts. Nonresistant, poorly exposed, 

gray silty shale and mudstone constitute 30-50 percent 

of the tongue. A typical exposure of Mount Kelly 

Graywacke Tongue is illustrated in figure 15. 

Sandstone in the Mount Kelly Graywacke Tongue 

ranges from very fine to coarse grained and is slightly 

conglomeratic in some places. The sand is composed of 

quartz, chert, lithic rock fragments and abundant muscovite flakes in an extremely argillaceous matrix. In thin 

section, the grains consist of about 20 percent monocrystalline quartz, as much as 10 percent metamorphic quartz, 

20-40 percent lithic fragments, as much as 10 percent 
Figure 15. Complexly folded and thrust-faulted beds of Mount Kelly Graywacke Tongue of the Fortress Mountain 

Formation, in the foothills of the western De Long Mountains, Arctic slope. Resistant ridges in center of view are 

about 1 km apart. View to the southwest. 

chert and the remainder grains of muscovite, plagioclase, 

magnetite, pyrite, pyroxene, and olivine (R. M. Egbert, 

written commun., 1981). Limestone fragments constitute 

30-75 percent of the lithic grains; metamorphic, volcanic, 

and argillaceous sedimentary-rock fragments constitute the 

remainder of the lithic component. Clasts of shale are 

abundant in some beds; abundant carbonaceous material 

is also seen at the top of some beds. Beds range from 

3 to 6 em in thickness and are interbedded with shale, 

silty shale, and siltstone that commonly is not well exposed. Many beds are massive and apparently homogeneous; however, graded and convolute bedding are common, 

and flute, groove, and load casts are abundant. Other beds 

have large- to medium-scale crossbedding; low-amplitude 

current and oscillation ripples are abundant on the tops 

of some beds; climbing ripples have been observed in a 

few places. In general, the range of bedding features is 

suggestive of turbidite deposition, but deposition in rather 

shallow water is also evident. 

No sections of the Mount Kelly Graywacke Tongue 

have been measured; however, it may be as much as several hundred meters in thickness in some areas. In most 

places no top or base of the formation is exposed; it is 

often structurally deformed by folding and apparent thrust 

faulting so that no thickness determination is possible. 

The base of the tongue seems to be an unconformity, although the base is everywhere obscured by covered intervals of varying extent. In various areas, the exposures underlying the covered interval range from Triassic chert, shale and limestone, to Jurassic clay shale, to 

Neocomian clay shale and some coquinoid limestone. The 

top of the tongue appears to be gradational and to interfinger with the overlying Torok Formation. Although exposures are not good, this relationship is best seen on 

aerial photographs of a southwest-plunging anticlinal nose 

in T. 8 S., R. 46 W., between Poko Mountain and Igloo 

Mountain. This area was referred to by Chapman and 

Sable (1960, p. 71, 133, 134, and pl. 8) as the 

Tingmerkpuk "high." 

The Mount Kelly Graywacke Tongue is dated as 

Aptian and Albian (late Early Cretaceous) based upon a 

flora obtained from shale interbedded with massive 

Cretaceous Tectonics and Depositional Cycles 29 
graywacke apparently near the center of the formation as 

exposed at a locality south of Poko Mountain (table 1, 

loc. 77 AMu17). The apparently interfingering relationship 

of the tongue and the Torok Formation also suggests that 

the tongue is part of the Albian and Cenomanian depositional cycle. 

The distinctive micaceous and calcareous character 

of the Mount Kelly Graywacke Tongue suggests a different source terrain than that which was the major source 

of detritus for the remainder of the Fortress Mountain Formation. Clasts in the Fortress Mountain in general suggest 

derivation from the multiple allochthons of sedimentary 

and igneous rock comprising the De Long and Endicott 

Mountains. The abundance of muscovite in the Mount 

Kelly Graywacke Tongue suggests sediment derivation 

from a western extension of the schist belt that forms 

the southern margin of the Brooks Range. This belt is 

composed predominantly of quartz muscovite schist and 

associated calcareous schist. In the central Brooks Range, 

regional structural evidence suggests that the schists were 

deeply buried by the overlying allochthons and were not 

exposed to erosion during the Albian. However, in the 

western Brooks Range, evidence suggests that the schists 

may also have been involved in the allochthons of that 

area. 

Torok Formation 

In NPRA, seismic and subsurface data indicate that 

the Torok Formation is more than 2 km thick and is a 

record of the progressive filling of the Colville trough. 

The Torok consists predominantly of shale that is silty 

and hard in many places. In outcrops in · the southern part 

of its area, it interfingers with the underlying Fortress 

Mountain Formation and the Mount Kelly Graywacke 

Tongue. It appears to represent turbidite deposition in the 

axis of the Colville basin. In many places in the subsurface, the lower part of the Torok consists of more than 

1,000 m of rhythmic turbidite lacking paleobathymetric 

indicators. The nature of the sediments and the general 

absence of calcareous microfauna suggest submarine fan 

deposition below carbonate compensation depth. Upward 

in the section, the Torok may represent prodelta deposits 

in advance of the prograding deltaic deposits of the 

Nanushuk Group. 

The Torok Formation thins to the northeast but is 

present across most of the Arctic Slope, with the exception of the northeastern Arctic Slope and the southern 

foothills belt. Many of the areas in which the Torok is 

absent in the northeastern Arctic Slope are high on the 

flank of the Beaufort sill and are apparently beyond the 

Torok depositional limit. In this area, the Torok progressively onlapped but did not overstep the crest of the sill 

(fig. 58). Elsewhere, in parts of northeastern NPRA and 

the Beaufort Sea, the Torok may have been truncated by 

30 Geology of the Nanushuk Group and Related Rocks, Alaska 

an overlying unconformity. In the southern foothills belt, 

it is absent by Holocene erosion. 

Nanushuk Group 

From the Chukchi Sea to the area of the Colville 

River, the Torok Formation grades upward into deltaic 

deposits of the Nanushuk Group. The Nanushuk grades 

upward from shallow marine to nonmarine deltaic sediments and represents the final filling of the Colville basin 

by a prograding deltaic complex. The relationships of the 

Torok and Nanushuk across NPRA are illustrated in figure 

5, and by Molenaar (figs. 18-22, this volume). 

The Nanushuk Group forms a nearly unbroken outcrop belt from the Chukchi Sea on the west to the 

Sagavanirktok River on the east (fig. 6); this outcrop belt 

constitutes the northern foothills belt of the Brooks Range. 

In many areas a prominent escarpment formed by the resistant, gently north dipping clastic rocks of the Nanushuk 

marks the southern limit of the Nanushuk. Regionally 

these rocks dip gently into the subsurface beneath the Arctic coastal plain and are present throughout most of the 

northern part of NPRA. In NPRA the Nanushuk is absent 

only in the Brooks Range, the southern foothills belt 

(where it was removed by Holocene erosion), in a limited 

area near Barrow (where it is absent due to an overlying 

unconformity), and to the northeast (beyond its depositional limit). Its original southern depositional limit is unknown. 

Inasmuch as details of the Nanushuk of the central 

and western Arctic Slope are discussed by Ahlbrandt and 

others (1979), Bartsch-Winkler (1979), Bird and Andrews 

(1979), Fox (1979), Huffman (1979), and in other papers 

in this volume, similar discussion is not repeated here. 

Nanushuk Group, eastern Arctic Slope 

On the Arctic Slope east of the Sagavanirktok 

River, rocks of the Albian and Cenomanian depositional 

sequence are limited and consist of a markedly different 

facies than the Nanushuk Group to the west. Major exposures are limited to a mesalike syncline east of the Ivishak 

River and to cutbanks along its west bank. In this area 

the sediments are rhythmically interbedded shale and 

sandstone, in many places with well-developed graded 

bedding and sole markings, suggesting turbidite deposition 

in a submarine fan setting. Apparently correlative beds 

in the subsurface also appear to be basinal deposits. 

In the Arctic National Wildlife Refuge east of the 

Canning River, submarine fan deposits possibly correlative with the Nanushuk are present along Arctic Creek, 

between the Sadlerochit and Hulahula Rivers in the Sadlerochit Mountains area (Reiser and others, 1971). East 

of the Hulahula River, Reiser and others (1978) have rec-
ognized rocks that may also be correlative with the 

Nanushuk. However, the area is remote from the typical 

Nanushuk outcrop area, well control is sparse, correlations 

are uncertain, and other major changes in structure and 

stratigraphy are known to exist. Data from the Yukon Territory (Young and others, 1976) suggest that only Albian 

flysch is present in the British Mountains. 

Stratigraphic Relations 

A sharp depositional break marks the top of the Albian and Cenomanian cycle in some areas. In the UmiatChandler Lake area, deltaic sediments of the Nanushuk 

Group are overlain by the dominantly marine Upper Cretaceous deposits of the Colville Group (Brosge and Whittington, 1966, and Detterman and others, 1963). To the 

east, in the Sagavanirktok-Ivishak River area, continuous 

deposition may have prevailed from the Nanushuk into 

the Colville Group. To the west of the Umiat area, a 

Holocene erosion surface marks the top of the Nanushuk. 

The unconformity at the base of the cycle is clearly 

evident in outcrop mapping along the basin margin. At 

localities such as Castle Mountain and Ekakevik Mountain 

(figs. 6, 9) on the central Arctic Slope, proximal conglomerate of the Fortress Mountain Formation overlies a 

major angular unconformity. Under the Arctic coastal 

plain in northern NPRA, well penetrations also reveal a 

sharp break between the distal sediments of the Torok 

Formation and the underlying sediments on the gentle 

north flank of the Colville trough. This contact may be 

a nondepositional hiatus in much of the area, but locally 

it is an erosional unconformity. Typical subsurface log 

character and paleontologic data at this basal contact are 

shown in figure 13. 

In the central Colville basin beneath the foothills 

province, no wells have penetrated the basal contact of 

the Albian depositional sequence, and here the nature of 

its base is unknown. However, if the magnitude of eustatic sea-level lowering during the Aptian was great enough, 

it is possible that a disconformity may separate the deposits of the two cycles throughout the Colville basin. 

Alternatively, continuous deposition throughout the 

Early Cretaceous can be inferred in the axial part of the 

Colville basin. In outcrops between the Sagavanirktok 

River and Elusive Lake, rocks mapped as the Okpikruak 

and Fortress Mountain Formations by Keller and others 

(1961, p. 196--200, pl. 21) and by Brosge and others 

(1979) total at least 2,000 m of interbedded shale, siltstone, graywacke, conglomeratic sandstone, and conglomerate. Keller and others (1961 , p. 196) noted that the Okpikruak is indistinguishable from the overlying Fortress 

Mountain Formation, although the rhythmic alternation 

that characterizes the Okpikruak is lacking in the Fortress 

Mountain. Formation contacts appear to be gradational. 

Within the depositional cycle, the stratigraphic relationships seem to be entirely gradational. The Fortress 

Mountain Formation interfingers both northward and stratigraphically upward with shale of the Torok Formation. 

Similarly, the marine Torok interfingers with the 

overlying Nanushuk Group. 

Age 

Relatively abundant foraminifera and palynomorph 

assemblages from shale in the upper part of the Torok 

Formation and from the Nanushuk Group date these rocks 

as middle Albian to Cenomanian (Sliter, 1979; and May, 

1979). Microfossil data and megafossils (which include 

ammonites) date the Fortress Mountain Formation as early 

Albian (Patton and Tailleur, 1964, p. 456--458). Data for 

the lower part of the depositional cycle (lower Torok and 

Fortress Mountain Formations) are limited; fossils consist 

dominantly of pyritized radiolaria and some agglutinated 

and calcareous foraminifera. Radiolarian fauna from two 

wells in NPRA are tabulated in figure 13; the fauna is 

considered to be Aptian and early Albian (Anderson, Warren, and Associates, written comm~n., 1979). However, 

C. M. Molenaar (written commun., 1980) pointed out that 

in some areas on the northern flank of the basin, strata 

inferred to be Aptian can be traced on seismic records 

up foresets into strata yielding an Albian fauna. Outcrop 

samples from a number of localities, tabulated in table 

l , have also yielded foraminifera and (or) palynomorph 

assemblages considered to be Aptian or Aptian and Albian 

age by Anderson, Warren, and Associates. Although the 

faunas may be time transgressive, some localities contain 

forms restricted to Aptian rocks in the Canadian Arctic 

(M. B. Mickey, Biostratigrapics, Inc., oral commun., 

1981). 

Paleogeography 

The generalized distribution of the major tectonic 

elements at the end of the Albian and Cenomanian depositional cycle is shown in figure 16. In general, the margin 

of the Canada basin and the Brooks Range orogenic front 

appear to have been in positions approximating those in 

the Aptian, reflecting the shift to dominantly vertical uplift in the mountains. However, a major change in the 

form of the Colville basin is evident. During the Neocomian and Aptian, the Colville basin was apparently a 

linear trough parallel to the orogenic front, but by 

Cenomanian time, progradation of the Torok Formation 

and the deltaic Nanushuk Group resulted in a basin restricted to the eastern Arctic Slope. Northeastward and 

eastward progradation is indicated by a combination of 

outcrop, subsurface, and seismic studies (Payne, 1951; 

Chapman and Sable, 1960; Ahlbrandt and others, 1979, 

Cretaceous Tectonics and Depositional Cycles 31 
CANADA 

8 AS IN 

BROOKS RANGE OROGENIC 

Figure 16. Diagrammatic map of major tectonic elements near the end of Albian time in what is now northern 

Alaska, showing inferred trends and approximate boundaries of elements. 

p. 14-25; Bird and Andrews, 1979, p. 37--40, figs. 15, 

18; Mull, 1979, p. 10). In the Barrow area, the deltaic 

wedge prograded over the crest of the Beaufort sill, so 

that detritus from the orogenic belt was, for the first time, 

deposited onto the oceanic crust of the Canada basin. 

However, on the eastern Arctic Slope, the Beaufort sill 

remained high relative to the basin to the south, and in 

the area from Prudhoe Bay to the Sadlerochit Mountains 

the sill received little sedimentation during the Albian-toCenomanian cycle. 

Ahlbrandt and others ( 1979) recognized a deltaic 

complex, the Corwin delta, that dominates the Nanushuk 

of the western Arctic Slope. The progradation of this delta 

suggests a major sediment source to the west or southwest 

of the western Arctic Slope as well as a source of sediment from the Brooks Range to the south. The Tigara 

uplift (Payne, 1955) in the area of Cape Lisburne, and 

its offshore extension beneath the Chukchi Sea, the Herald 

arch (Grantz and others, 1970, 1976), were probably a 

major source of detritus for the Corwin delta. 

In the Brooks Range foothills of the central Arctic 

Slope (Chandler River to Sagavanirktok River area), the 

generalized Albian facies trends suggest northward progradation. Both the Nanushuk Group and the Fortress Mountain Formation have nearly east-west trends of outcrops 

(fig. 6) that appear to be generally comparable in thickness and facies. To the north the Nanushuk becomes thinner and finer grained. Current measurements in fluvial 

sandstones define a lobate deltaic complex, named the 

Umiat delta, which also appears to prograde generally 

northward (Huffman and others, this volume). The east32 Geology of the Nanushuk Group and Related Rocks, Alaska 

west facies trends and the form of the Umiat delta suggest 

sediment derivation from the east-west-trending Endicott 

Mountains. 

Between Shainin Lake and the Sagavanirktok River, 

the Brooks Range front trends slightly to the northeast. 

East of the Sagavanirktok River the front trends about 

N. 40° E. for more than 100 km to form the major northeast salient of the Philip Smith, Franklin, Romanzof, and 

British Mountains of the eastern Brooks Range. Because 

of this salient, the eastern Brooks Range mountain front 

is more than 120 km further north than the mountain front 

at Anaktuvuk Pass. But, east of the Sagavanirktok River, 

the structural trends mapped by Brosge and others (1979) 

continue into the Philip Smith Mountains toward Porcupine Lake on a .trend of about N. 70° E., and are not 

parallel to the mountain front. In addition, Albian facies 

trends intersect the mountain front near the Sagavanirktok 

River. Where the mountain front begins to change to its 

northeasterly trend, the Fortress Mountain and Nanushuk 

facies maintain their generally east-west trend. In the 

Chandler River area of the central Endicott Mountains, 

coarse Fortress Mountain conglomerates are 20 to 25 km 

north of the mountain front, but at Atigun syncline, they 

are at the eastern Endicott Mountain front. Similarly, in 

the Chandler River area, Nanushuk facies are more than 

40 km north of the front, but at Marmot syncline north 

of the eastern Endicott Mountains, similar facies are only 

10 km north of the mountain front. The convergence of 

these trends is illustrated in figure 7. 

In summary, regional structural trends suggest that 

the source of Nanushuk sediment was an upland area, 
probably having a generally east-west trend that changed 

to a N. 70° E. trend in the eastern Brooks Range. The 

data of Huffman (1979), Bartsch-Winkler (1979), and 

Ahlbrandt and others (1979), and in this volume from 

the central Arctic Slope suggest a discrete delta lobe possibly related to one major drainage system. The highlands 

probably persisted eastward, but at some distance south 

of the mountain front that forms the salient of the present 

northeast Brooks Range. The eastern limit of this trend 

is unknown, but the thin and relatively fine grained Albian 

sediments in the Canning-Sadlerochit Rivers area suggest 

that Albian uplands lay some distance south of the present 

mountain front or had much less relief than to the west. 

Provenance 

The nature of the rocks forming the Herald arch 

beneath the Chukchi Sea is unknown. However, the Tigara uplift and the remainder of the Brooks Range have 

been mapped in fair detail. The nature of the allochthonous rocks that were the source terrain for at least part 

of the sediment in the Nanushuk of the western Arctic 

Slope (Mull, 1979, p. 10-12) is summarized below. 

In general, the structurally low allochthons of the 

western Brooks Range are composed of Mississippian to 

Triassic sedimentary rocks, dominantly chert, limestone, 

and shale, some of which is silicified. Lower Cretaceous 

graywackes are present on most of the allochthons. The 

structurally higher allochthons contain more siliceous sediment and abundant mafic igneous rock; the highest allochthon is composed dominantly of gabbroic to ultramafic igneous rock (Roeder and Mull, 1978). The 

characteristics of the allochthons are summarized in figure 

8; their generalized distribution is shown in figure 6. In 

general, the source terrain for the Nanushuk in the western 

Arctic Slope was deficient in stable detritus other than 

chert. This deficiency, and the relative abundance of 

mineralogically unstable detritus, is seen in the nature of 

the Nanushuk sediment deposited in the western or Corwin delta (Ahlbrandt and others, 1979; Huffman, 1979; 

and Bartsch-Winkler, 1979, p. 61-69, and this volume). 

This deltaic complex is characterized by a relatively low 

sand-shale ratio and by relatively clay-rich sands having 

low porosity and permeability. 

In the Endicott Mountains to the east, the lowest 

allochthon, the Endicott Mountains allochthon, differs 

from the allochthons of the De Long Mountains by having 

a thick section of Upper Devonian quartzitic sandstone 

and conglomerate. Although the structurally higher allochthons are presently limited in the Endicott Mountains, 

there is an abundance of igneous clasts of various types 

in the Fortress Mountain Formation (Chapman and others, 

1964, p. 353-356; Patton and Tailleur, 1964, p. 451-

458). These clasts suggest that prior to the Late Cretaceous vertical uplift and erosion of the core of the range, 

allochthons similar to those of the western Brooks Range 

were also widespread in the central Brooks Range. The 

presence of the thick quartz-rich Upper Devonian sediments in the lower part of the source terrain in the Endicott Mountains is seen in the higher sand-to-shale ratio 

and better reservoir quality of the Nanushuk Group in the 

Umiat delta in contrast to the Corwin delta as discussed 

by Ahlbrandt and others (1979, p. 25), Bartsch-Winkler 

(1979; this volume), Fox (1979), and Huffman (1979; this 

volume). 

East of the Sagavanirktok River, the belt of allochthonous rocks trends approximately N. 70° E., to near 

the head of the Sheenjek River. This trend is most obvious 

in areas in which the Upper Devonian Kanayut Conglomerate is in thrust contact over the Carboniferous Lisburne 

Group and younger rocks; it is less obvious in areas in 

which the allochthonous Lisburne is in regional thrust 

contact with the autochthonous part of the Lisburne. 

These thrust-fault relationships were mapped by Brosge 

and Reiser (1965) and Brosge and others (1979). Because 

of the apparent close relationship of the allochthons of 

the Endicott Mountains to the source terrain of the 

Nanushuk, it is likely that the trend of allochthons in the 

Philip Smith Mountains may approximate the trend of the 

sediment source for Albian rocks that have been stripped 

from most of the northeast Brooks Range. 

Late Cretaceous and Tertiary Tectonics 

and Deposition 

The record of Late Cretaceous and Tertiary tectonics 

and deposition in northern Alaska is confined predominantly to the eastern Brooks Range and Arctic Slope. A 

thick Upper Cretaceous and Tertiary sedimentary section 

is present, and evidence suggests that the northern salient 

of the eastern Brooks Range resulted from Late Cretaceous and Tertiary vertical uplift. 

Tectonics and Dating of Orogeny 

Late Cretaceous and younger uplift of the northeastern Brooks Range apparently involved the eastern end of 

the Colville basin, and included both the Lower Cretaceous orogenic deposits of its southern margin and the 

coeval stable platform deposits of its gentle north flank. 

Evidence for vertical uplift of the northeastern Brooks 

Range is: 

1 . Albian facies trends do not parallel the present 

northeastern Brooks Range mountain front east of the 

Sagavanirktok River. North of the Endicott Mountains, 

the trends are approximately parallel with the mountains, 

but near the Sagavanirktok River they converge with the 

Cretaceous Tectonics and Depositional Cycles 33 
mountain front; this convergence of trends has been discussed previously. 

2. The Late Cretaceous stratigraphy suggests Late 

Cretaceous uplift of part of the northeast Brooks Range. 

In the Juniper Creek-Kavik River area, several hundred 

feet of nonmarine sandstone, conglomerate, and interbedded shale has been mapped as the Tertiary Sagavanirktok 

Formation (Keller and others, 1961, p. 208, pl. 21 ); however, micropaleontological evidence indicates a Late Cretaceous age (H. R. Bergquist, in Keller and others, 1961, 

p. 208-209). This faunal evidence and close lithologic 

similarities suggest a more likely correlation with the 

lower part of the Upper Cretaceous Kogosukruk Tongue 

of the Prince Creek Formation of the Colville Group 

mapped by Brosge and Whittington (1966, p. 563-570) 

in the Umiat area. Brosge (1970, p. D6, fig. 1) and Pessel 

and others ( 1978b) also assigned these deposits in the 

Kavik River area to the Late Cretaceous. In the Umiat 

area the Kogosukruk interfingers with the underlying 

marine beds of the Schrader Bluff Formation of the Colville Group (Brosge and Whittington, 1966, p. 570), but 

in the Kavik River area, the base of the beds here considered to be Kogosukruk Formation appear to be an erosional unconformity (Keller and others, 1961, p. 206--

207). In addition, the Upper Cretaceous beds underlying 

the unconformity seem to contain more sandstone and 

conglomerate than coeval beds in the Umiat area. The 

increased thickness of Upper Cretaceous strata in the 

Sagavanirktok-Ivishak River area, indicated on isopach 

mapping by Pessel and others (1978b, 1978d), may also 

suggest Late Cretaceous uplift in the northern salient of 

the eastern Brooks Range. 

3. The Neocomian pebble shale unit, the Kemik 

Sandstone Member of the Kongakut Formation, and the 

Upper Cretaceous Colville Group of nearly identical facies 

as the coeval sediments exposed elsewhere, are widespread in the valley between the Sadlerochit and Shublik 

Mountains and to the east. This distribution indicates that 

at least the Sadlerochit and Shublik Mountains area of 

the northeastern Brooks Range was not uplifted until the 

Tertiary. 

4. The dominant structural style of the frontal 

Brooks Range east of the Sagavanirktok River is of vertical, basement-involved tectonics, in contrast to a style of 

dominantly compressional tectonics in the Endicott Mountains to the west. The Franklin, Romanzof, and British 

Mountains, which form the northern salient of the eastern 

Brooks Range, contain autochthonous Mississippian and 

younger rocks folded into a series of broad anticlinoria 

and synclinoria. Erosional breaching of the axes of the 

anticlinoria exposes extensive areas of metamorphic basement consisting of lower Paleozoic low-grade metasedimentary rocks, metavolcanic rocks, and a pre-Mississippian granitic body (Reiser and others, 1971, 1978; and 

Sable, 1977). In contrast, most of the Endicott and De 

34 Geology of the Nanushuk Group and Related Rocks, Alaska 

Long Mountains in the central and western Brooks Range 

consist of allochthonous Upper Devonian and younger unmetamorphosed rocks that have been telescoped many tens 

of kilometers relatively northward (Mull and Tailleur, 

1977, p. B27-B29). These allochthons contain none of 

the metamorphic rocks of the type exposed in the northeastern Brooks Range. The contrasting structural styles 

are clearly illustrated in regional geologic maps by 

Grybeck and others (1977) and Beikman (1978). In the 

eastern Brooks Range, thrust sheets indicative of major 

crustal shortening are 7 5 km and more south of the mountain front. Although this evidence does not bear directly 

on the timing or sequence of tectonic events, it is indicative of a fundamental tectonic difference between the 

northeastern Brooks Range and the mountains to the west. 

Arctic Slope Decollement 

Much of the Cretaceous section beneath the northern 

foothills belt has been folded into a series of long, generally east-west-trending gentle synclines and sharp anticlines (Lathram, 1965; Beikman and Lathram, 1976). In 

general, fold amplitudes decrease from south to north and 

die out in an area of essentially homoclinal dip beneath 

the Arctic coastal plain. 

In the southern part of the Nanushuk outcrop belt, 

south-dipping high-angle reverse faults or thrust faults cut 

the Nanushuk Group along some of the anticlinal axes. 

Many of the anticlines are breached through the Nanushuk 

into the underlying incompetent shale of the Torok Formation. The shale, which is commonly crumpled and contorted in contrast to the gently dipping overlying 

Nanushuk, generally forms poor exposures. The characteristic expression of these folds is illustrated in figure 

7. To the north where fold amplitudes are less, the 

Nanushuk extends unbroken across the crests of the anticlines. This structural style has long been interpreted as 

detachment of the Nanushuk (Brosge and Tailleur, 1971, 

p. 71), which has been deformed independently from the 

underlying strata. The inferred zone of Torok detachment 

has been confirmed by seismic data that show a relatively 

uniform southward dip of Jurassic and older strata beneath 

the anticlines and synclines of the foothills belt. 

The decollement was probably an indirect result of 

the later stages of uplift and compression in the core of 

the Brooks Range. A minor gravitational component of 

movement of the Nanushuk Group and overlying beds 

northward may have contributed to the decollement. On 

the central and western Arctic Slope, deformation probably began during early Tertiary or latest Cretaceous time 

and waned during the Tertiary. No clear evidence of deformation contemporaneous with either the Nanushuk or 

with later Cretaceous deposition is known. 

On the eastern Arctic Slope the decollement is probably younger than to the west, because, as discussed 
above, the uplift of the northeastern Brooks Range is apparently younger than to the west. In the northern Arctic 

National Wildlife Refuge, the Marsh Creek anticline has 

beds as young as Miocene or Pliocene (Reiser and others, 

1971), dipping as much as 30°. Similarity in structural 

style of this anticline to the foothills anticlines to the west 

suggests that much of the deformation of the Tertiary beds 

in the wildlife refuge may also be a decollement that does 

not involve underlying, more competent beds. Grantz and 

Mull ( 1978) referred to these folds as thrust folds. 

Depositional Cycle 

Regional compilations of geologic mapping of 

northern Alaska (Beikman and Lathram, 1976; Lathram, 

1965) and the results of drilling in NPRA show that postNanushuk deposits in northern Alaska are confined to the 

coastal plain and northern foothills of the central and eastern Arctic Slope. No Upper Cretaceous or Tertiary rocks 

are known west of approximately long 157° W. In the 

Umiat area, Upper Cretaceous deposits are relatively thin; 

but to the east in the Sagavanirktok River area, the Upper 

Cretaceous section is more than 3 km thick (Pessel and 

others, 1978d). Tertiary rocks are even more restricted 

in distribution, and outcrops are found still farther east 

and north. Details of the Upper Cretaceous and Tertiary 

outcrop stratigraphy are given by Brosge and Whittington 

(1966) and Keller and others (1961); generalized discussion and isopach maps including subsurface data are given 

by Pessel and others (1978a, b, c, d) and Brosge and 

Tailleur ( 1970). 

In general, the areal distribution of Upper Cretaceous and Tertiary rocks records a continuation of the 

eastward and northward progradation that occurred in the 

Early Cretaceous. However, in addition, there is evidence 

of a major sediment influx from the northeastern Brooks 

Range; this evidence has been discussed previously. 

Late Tertiary and Holocene erosion has stripped 

some of the Upper Cretaceous and Tertiary deposits. 

However, the distribution of nonmarine facies in both the 

Upper Cretaceous and Tertiary suggests that these deposits 

may not have been deposited very far west and south of 

their present outcrop limits. 

SUMMARY 

The Cretaceous sedimentary rocks of Arctic Alaska 

can be divided into three discrete depositional cycles, here 

referred to as the Neocomian to Aptian, Albian to 

Cenomanian, and Late Cretaceous cycles. These cycles 

coincide very closely with the global eustatic cycles of 

Vail and others (1977). 

The Nanushuk Group of the Arctic Slope is a major 

deltaic complex at the top of the Albian to Cenomanian 

cycle. It is derived from erosion of an orogenic belt in 

the central and western Brooks Range. On the western 

Arctic Slope, a major source of sediment from the west 

is evident; this terrain probably coincided with the Tigara 

uplift and its offshore extension, the Herald arch. Development of the Brooks Range orogenic belt began in 

Neocomian time with the obduction of oceanic crust over 

continental crust at a major compressional plate boundary 

at the south margin of a small continental plate that rotated counterclockwise out of the Canada basin. Major 

northward thrusting in the orogenic belt continued into 

the Aptian. A foredeep north of the orogenic belt, the 

Colville basin, was filled with thick flysch derived from 

erosion of the Brooks Range allochthons. The flysch 

grades upward into progressively shallower water Aptian 

and Albian prodelta shales, which culminate in the molassoid deposits of the Albian and Cenomanian Nanushuk 

Group. From the Albian into the Late Cretaceous, sedimentary rocks filled the basin and progressively overlapped the Beaufort sill, which separates the Colville 

basin from the oceanic Canada basin to the north. 

Erosion of the generally east-west-trending orogenic 

belt in the Brooks Range and filling of the Colville basin 

was accentuated by vertical uplift of the core of the range 

in Albian time. This vertical uplift became the dominant 

tectonic force, possibly due to isostatic rebound of the 

deeply depressed continental crust at the compressional 

plate margin. However, in the area of the eastern Arctic 

Slope, the rate of basin subsidence apparently exceeded 

the rate of sediment influx, so that deltaic deposits prograded only a short distance into the basin. As a result, 

flyschoid sediments correlative with the deltaic complex 

forming the Nanushuk Group to the west are present in 

this area. These basinal sediments are north of the Philip 

Smith Mountains, which are oblique to the trend of the 

Early Cretaceous orogenic belt and foredeep and were uplifted in the Late Cretaceous. 

The reservoir potential of the Nanushuk Group is 

strongly influenced by the nature of its provenance, which 

contained abundant quartzose and quartzitic sediments in 

the central Brooks Range but was deficient in stable detritus to the west. As a result, the best reservoir potential 

is on the central Arctic Slope where these sediments prograded far to the north. In the subsurface to the east, 

where progradation of the delta was inhibited, the 

Nanushuk has no reservoir potential. 

Continuing Late Cretaceous and Tertiary uplift in 

the Brooks Range resulted in a regional decollement on 

the Arctic Slope. The relatively competent Nanushuk 

Group was detached and folded independent of the Jurassic and older rocks. The structural style resulting from 

this decollement consists of broad synclines and sharp 

shale-cored anticlines which die out in amplitude to the 

north. 

In the southern part of the decollement belt, the 

anticlines are breached through the Nanushuk into the unCretaceous Tectonics and Depositional Cycles 35 
derlying Torok Shale, so that there is little potential for 

_hydrocarbons in the Nanushuk. The northern part of the 

decollement belt, where the Nanushuk has not been 

breached, contains structural and mixed structural-stratigraphic hydrocarbon prospects. 

36 Geology of the Nanushuk Group and Related Rocks, Alaska 

To the north of the decollement, hydrocarbon prospects are probably dominantly stratigraphic in combination 

with regional dip. A discussion of the organic geochemistry of the Nanushuk Group and its control on hydrocarbon 

potential was given by Magoon and Claypool (1979). 
Subsurface Correlations and Depositional History of 

the Nanushuk Group and Related Strata, 

North Slope, Alaska 

By C. M. Molenaar 

INTRODUCTION 

The Nanushuk Group of Albian to Cenomanian Age 

(mid-Cretaceous) is present in the subsurface in the westem and central Arctic Slope from the northern foothills 

to the Arctic coast. Except for a narrow band in front 

of the outcrop belt that extends as far east as the 

Sagavanirktok River, most of the Nanushuk in the subsurface extends only a little farther east than the north-trending course of the Colville River. Because most of the 

subsurface Nanushuk occurrence is in the northern twothirds of the National Petroleum Reserve in Alaska 

(NPRA), most of the areal descriptions used here relate 

to NPRA (fig. 17). 

This chapter: ( 1) presents a regional stratigraphic 

synthesis of the Nanushuk Group and related strata by 

means of facies-correlated stratigraphic cross sections, (2) 

presents quantitative data on the basin geometry and basin 

paleobathymetry, and (3) relates the depositional history 

to the development of potential hydrocarbon reservoir 

facies. 

The Torok Formation of Albian Age, which underlies and is largely coeval with the Nanushuk, is included 

in this study in order to better understand the Nanushuk 

depositional system. 

Subsurface information was provided by more than 

60 wells and core holes that penetrate the Nanushuk in 

and adjacent to NPRA. Forty-eight of these wells, as well 

as selected outcrop sections, are shown in figure 17 and 

are listed in tables 3 and 4. Eighteen of the wells have 

been drilled since 1975 and have modem log suites for 

more detailed evaluation. In addition, about 10,000 linekilometers of six-fold common-depth-point seismic data 

acquired since 197 4 cover most of the area in a 10- to 

20-km grid. (The seismic surveys were made in conjunction with the NPRA exploration program, and the data 

are available from the National Oceanic and Atmospheric 

Administration, EDIS/NGSCD, 325 Broadway, Boulder, 

CO 80303). Integrating the seismic data with the well 

data, plus the lack of structural deformation in the area 

north of the foothills, provide an excellent means to study 

the Nanushuk-Torok depositional system. 

Nanushuk outcrops have been studied by many 

workers over the years. Some of the more definitive 

works are Gryc and others (1956); Chapman and Sable 

(1960); Detterman and others (1963); Chapman and others 

(1964); Patton and Tailleur (1964); Brosge and Whittington (1966); Ahlbrandt and others (1979); and Huffman 

and others (this volume). Much of these works were used 

in extrapolating depositional environments and styles to 

the subsurface. Subsurface studies by Collins and Robinson (1967) were also used in the well correlations. However, additional wells and modem seismic data have greatly enhanced the understanding of subsurface relations. 

Bird and Andrews (1979) explained and presented 

data on the subsurface stratigraphic relationships of the 

Nanushuk Group and the Torok Formation; how the alluvial-deltaic and shallow marine shelf deposits of the 

Nanushuk are laterally equivalent to and underlain by 

outer shelf topsets, slope foresets, and basinal bottomsets 

of the Torok; and how the Nanushuk delta prograded generally from west-southwest to east-northeast across the 

subsiding Colville basin. This chapter describes the continuation of these subsurface studies. 

EXPLANATION OF STRATIGRAPHIC 

CROSS SECTIONS 

A network of five stratigraphic cross sections of the 

Nanushuk Group and related strata, correlating most of 

the wells in and adjacent to NPRA, is shown in figure 

18. Seismic data, well data, and surface stratigraphic 

studies have been integrated in the construction of these 

sections. The sections ( 1) show the facies relationships 

and correlations between wells, (2) show how the facies 

are time-transgressive and how they rise stratigraphically 

as the deltaic sequence progrades over a subsiding basin, 

and (3) show zones or trends of thicker sand development. 

Also, these cross sections can be used to (1) correlate 

data from new wells, and (2) make additional observations 

or interpretations. 

The entire Torok Formation and the basal Lower 

Cretaceous transgressive pebble shale unit are included on 

Subsurface Correlations and Depositional History 37 
]10 

69° 

165° 

EXPLANATION 

o 2 Well 

x3 Outcrop section 

--Line of cross section 

162° 159° 156° 153° 150° 

, limit 

L----------' ~,of?. I - ;,q 

~~v~ I' 

-'---.. 

68° 

BROOKS R A N G E 

50 100 150 KILOMETERS 

Figure 17. Index map of part of the North Slope of Alaska showing selected Nanushuk Group control 

points (wells and outcrops), eastern extent of Nanushuk Group, lines of stratigraphic cross sections, and 

boundary of the National Petroleum Reserve in Alaska (NRPA). Refer to tables 3 and 4 for listing of wells 

and outcrop sections. Shaded area is Nanushuk Group outcrop belt. 

the cross sections in order to show the gross stratigraphic 

relationships of these units to the Nanushuk. These units 

are clearly shown by seismic data, which depict the depositional pattern of topset bedding (alluvial-delta plain and 

shallow marine shelf), foreset bedding (basin slope), and 

bottomset bedding (basin floor). (The terms "topset," 

"foreset," and "bottomset" refer to the large geomorphic 

forms of the basin-edge profile and not to parts of an 

individual delta front such as a Gilbert-type delta.) Figures 

19 and 20 are examples of these relationships as displayed 

by seismic data. The relationships of the Nanushuk with 

the overlying Colville Group, where it is present, are also 

shown on the cross sections. 

Correlating between wells on the cross sections was 

greatly aided by seismic data. Where possible, the individual wells were correlated to the nearest seismic line 

by using either synthetic seismograms or transit-time logs. 

Topset reflector beds were then correlated between wells, 

where possible. This method worked fairly well in northeastern NPRA (north and east of the East Topagoruk and 

Inigok wells), the area covered by 197 4 seismic lines. 

However, seismic data are of inferior quality on the 1975, 

38 Geology of the Nanushuk Group and Related Rocks, Alaska 

197 6, and 1977 lines, which cover the other areas. This 

reduction in quality is reportedly because the geophone 

placement along these lines did not optimize the relatively 

shallow reflections from the Nanushuk-Torok interval. 

Additional processing may enhance the shallow data from 

these lines. Despite these lower quality data, however, 

gross correlations can usually be made by projecting a 

weak or nonexistent seismic reflection between more pronounced reflections. Also, the base of the topset reflectors 

(or shelf break) can be identified on these lines and hence 

related to nearby wells. However, in the northern foothills 

area where most of the old U.S. Navy wells were drilled, 

structural complications generally precluded correlating 

those wells to seismic lines. 

The base of the Colville Group is used as a horizontal datum in the eastern part of the area where this unit 

is preserved. This surface was a very low gradient shelf 

or delta plain prior to the transgression of the Colville 

sea except possibly in the southernmost area where clastic 

material derived from the ancestral Brooks Range continued to build an aggradational coastal plain while the 

area to the north was being transgressed. 
Table 3. Location of wells shown in figure 17, and stratigraphic data on Nanushuk Group, North Slope, Alaska 

[?, data not resolved; ±, approximate top of section; +, eroded top of Nanushuk Group, incomplete section] 

Locality 

No. 

(fig. 1) 

l 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

l7 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

44 

45 

46 

47 

48 

49 

Well name 

USN Umiat-1-------------

USN Skull Cliff C.T.-1--

USN Umiat-2-------------

USN Simpson-1-----------

USN South Barrow-1------

USN Fish Creek-1--------

USN Oumalik-1-----------

USN Meade-1-------------

USN East Oumalik-1------

USN Simpson C.T. 31-----

USN East Topagoruk-1----

USN Titaluk-1-----------

USN Gubik-1-------------

USN Topagoruk-1---------

USN Knifeblade-2A-------

USN Kaolak-1------------

USN Square Lake-1-------

USN Grandstand-1--------

USN Wolf Creek-3--------

British Petroleum 

Shale Wall-!. 

Sinclair Little Twist-1-

British Petroleum 

Kuparuk-1. 

British Petroleum 

Itkillik-!. 

McCulloch Colville U-2--

Texaco-Newmont E. 

Kuparuk-1. 

Gulf Colville Delta-----

Pan Am. Aufeis-1--------

USN Cape Halkett-1------

Forest Lupine-1---------

Texaco W. Kurupa-1------

Texaco E. Kurupa-1------

USN East Teshekpuk-1----

USN So. Harrison Bay-1--

USN Atigaru Point-1-----

USN W.T. Foran-1--------

USN South Simpson-1-----

USN West Fish Creek-1---

Texaco Tulugak-1--------

USGS Drew Point-1-------

USGS North Kalikpik-1---

USGS Kugrua-1-----------

USGS South Meade-1------

USGS East Simpson-I-----

USGS Peard-1------------

USGS J.W. Dalton-1------

USGS Inigok-1-----------

USGS Ikpikpuk-1---------

USGS Tunalik-1----------

USGS Seabee-1-----------

Sec. 

34 

23 

3 

32 

28 

15 

30 

19 

13 

36 

12 

23 

20 

25 

2 

25 

2 

32 

2 

34 

11 

15 

10 

19 

30 

5 

13 

33 

9 

16 

6 

19 

13 

22 

11 

26 

26 

3 

8 

31 

18 

25 

14 

34 

25 

20 

5 

Location 

T. 

l N. 

18 N. 

l s. 

19 N. 

23 N. 

11 N. 

6 N. 

8 N. 

5 N. 

19 N. 

14 N. 

l N. 

l N. 

15 N. 

4 s. 

7 N. 

2 N. 

5 s. 

s. 

5 s. 

3 s. 

2 s. 

N. 

s. 

2 s. 13 N. 

3 s. 

16 N. 

4 s. 

6 s. 

7 s. 

14 N. 

12 N. 

14 N. 

l7 N. 

l7 N. 

ll N. 

5 s. 

18 s. 

13 N. 

14 N. 

15 N. 

18 N. 

16 N. 

18 N. 

8 N. 

13 N. 

10 N. 

l s. 

1Fault repetition of 760 ft (232m) has been removed. 

2Base of Nanushuk Group not reached. 

The two outcrop sections (fig. 18D and 18E) were 

correlated to subsurface data by correlating the marine deposits at the base of the Ninuluk Formation, which actuR. 

2 w. 

22 w. l w. 

13 w. 18 w. 

l E. 

16 w. 

22 w. 15 w. 11 w. 

14 w. 11 w. 3 E. 

16 w. 

12 w. 

34 w. 

6 w. 

l E. 

7 w. 

5 E. 

4 w. 

5 E. 

6 E. 

E. 

8 E. 

6 E. 

11 E. 

2 w. 

14 E. 

4 w. 

6 w. 

4 w. 2 E. 

2 E. 

2 w. 

12 w. l w. 

3 E. 

8 w. 

2 w. 

26 w. 19 w. 

10 w. 28 w. 5 w. 

5 w. 

10 w. 36 w. l w. 

Top 

(ft) 

915 

10 

80 

20 

70 

2,890 

20 

45 

35 

220 

90 

40 

3,350 

so 

0 

113 

1,630 

110 

30 

l ,080 

0 

5,365 

Nanushuk Group 

Base Thickness 

(ft) (ft) (m) 

2,840 

450 

1,060 

980 

695 

4,110 

2,770 

3,450 

2,970 

( 2) 

2, 280 

3,500 

4,005 

2,150 

( 2) 

5 '205 

3,940 

l ,070 

3,575 

3,150? 

3,075 

6,330 

1165 

440+ 

980 

960 

625+ 

l, 220 

2,750+ 

3,405+ 

2,935+ 

( 2) 

2,190+ 

3,460 

655 

2, 100+ 

1,805+ 

5 ,092+ 

2,310 

960+ 

3,545+ 

2,070? 

3,075+ 

965 

1355 

134+ 

299 

293+ 

190 

37 2 

838+ 

1 ,038+ 

895+ 

( 2) 

667+ 

1,055 

200 

640+ 

500+ 

1,55 2+ 

704 

293+ 

1,080+ 

631? 

937+ 

294 

-----------------Not present----------------

1,765 2,820 1,055 322 

6,680 6,860 180 55 

-----------------Not present----------------

2,450 3,474 1,025 312 

3,118 4,235 1,117 340 

0 

0 

1,585 

3, 220 

3,470 

3,480 

50± 

2,550 

1, 230 

2,395 

100 

50± 

350 

50± 

2,660 

2, 250 

150 

50± 

280 

1,730 

2,190 

3,100 

4, 220 

4,400 

4,380 

1,915 

3,915 

3, 200 

3,470 

2,330 

2,490 

2,690 

2,470 

4,135 

3,935 

2, 950 

6' 250 

1,300 

1 '730+ 

2,190+ 

1,515 

1,000 

930 

900 

1 ,865+ 

1,365 

1, 970 

1,075 

2, 230+ 

2,440+ 

2,340 

2,420+ 

1,475 

1,685 

2,800 

6' 200+ 

1,020 

527+ 

667+ 

46 2 

305 

283 

274 

568+ 

416 

600 

328 

680+ 

759+ 

713 

738+ 

450 

514 

853 

1,890+ 

311 

ally represents initial deposits of the Colville transgression. The base of the Ninuluk is inferred to rise to the 

south, owing to intertonguing. The Killik River surface 

Subsurface Correlations and Depositional History 39 
Table 4. Location of outcrop sections of Nanushuk Group shown in figure 17, North Slope, Alaska 

[ + , incomplete section] 

Locality Location 

No. Outcrop name T. R. 

Corwin Bluff----- 6-7 s. 53-56 w. 

6-7 s. 53-56 w. 

2 Barabara N.-2 S. 44-45 w. 

syncline. 

3 Kokolik warp 3-4 s. 40-41 w. 

syncline. 

4 Elusive syncline--- 1-3 N. 33 w. 

5 Carbon Creek 2 s. 28 w. 

anticline. 

6 Colville River----- 5 s. 15-17 w. 

7 Kurupa River------- 6 s. 12-14 w. 

8 Knifeblade Ridge--- 3-4 s. 12 w. 

9 Killik River------- 5-6 s. 6-7 w. 

10 Mammoth Creek and 7-8 s. 1-2 w. 

Chandler River. 

Tuktu Bluff------

11 Type Grandstand---- 6 s. 4 E. 

12 Rooftop------------ 7 s. 6-7 E. 

13 Nanushuk River----- 9 s. 6 E. 

14 Marmot syncline---- 8 s. 13 E. 

15 Lupine River------- 4 s. 14 E. 

section shown on the south-to-north cross section across 

eastern NPRA (fig. 18D) is generalized from the measured section of Detterrnan and others (1963, pis. 29, 32, 

and 33). The Tuktu Bluff section shown on the southeastto-northwest cross section across northeastern NPRA (fig. 

18£) is a combination of a section by Detterman and 

others (1963) and one by Huffman and others (1981b). 

The thickness used here for the pre-Ninuluk part of the 

Nanushuk is the average of 1,180 m (3 ,870 ft) measured 

by the earlier workers and 1 ,842 m, plus approximately 

394 m to the base of the Ninuluk (6,040 + 1,300 ft), 

measured by the later workers. This wide discrepancy in 

thickness is probably due to a different choice of dips 

(7-13°) in measuring across a long, flat area along the 

Chandler River. 

The Grandstand-! well, which was located at the 

surface near the top of the main marine part of the section 

(Grandstand Formation), was placed about 365 m (1 ,200 

ft) below the Ninuluk datum as projected by Detterman 

and others (1963, pl. 37) (fig. 22). In areas where the 

top of the Nanushuk is not preserved, the vertical placement of the wells on the cross sections is related to a 

near-horizontal assumed time datum (topset bedding) near 

the base of the Nanushuk between adjacent wells. This 

datum can be either a thick marine shale tongue and shelf 

sandstone bed that may be correlated between wells, or 

a seismic reflector in the topset bedding. The correlations 

40 Geology of the Nanushuk Group and Related Rocks, Alaska 

Nanushuk Group 

thickness 

(m) Source 

4 '7 23+ Chapman and Sable, 1960. 

3,444+ Smiley, 1969b. 

3' 110+ Chapman and Sable, 1960. 

2,091+ Huffman and others, 1981a. 

1,977 Chapman and Sable, 1960. 

786+ Huffman and others, 1981a. 

1 '911+ Chapman and others, 

1964. 

1,83 2+ Huffman and others, 1981b. 

1 '45 2+ Brosge and Whittington, 

1966. 

1 ,361+ Detterman and others, 

1963. 

1,430 Detterman and others, 1963; 

Huffman and others, 1981 b. 

1 ,842+ 

812+ Huffman and others, 1981b. 

195+ Do. 

1 ,070+ Detterman and others, 

1963. 

853+ Huffman and others, 1981 b. 

563+ Do. 

of the lower part of the Nanushuk and upper part of the 

Torok between the South Meade and Topagoruk wells 

shown on the northern coastal plain cross section (fig. 

18B) are the result of this technique. These units are assumed to have been deposited on a nearly horizontal or 

very gently sloping surface at approximately the same 

water depth, and that differential subsidence between 

wells was negligible. This assumption may not hold for 

the entire section because the different rates of subsidence 

between areas may be significant for thicker sections and 

more time. However, this method shows the overall timestratigraphic relationships quite well. 

In general, the horizontal correlation lines on the 

cross sections within the Nanushuk are considered nearhorizontal time lines. The dashed correlation lines shown 

within the Torok correlate from topset to foreset to bottomset beds and are considered to be time lines. 

Because the wells are logged in the inch-pound system of measurement, this system is used for the cross 

sections to facilitate comparisons with other well logs. 

STRATIGRAPHY AND DEPOSITIONAL SETTING 

Nanushuk Group 

The Nanushuk Group is a deltaic sequence of complexly intertonguing marine and nonmarine deposits. It 

ranges in thickness from at least 3,444 m in outcrops at 
Corwin Bluff (Smiley, 1969b) on the far southwest of 

the study area to a pinchout edge in the area of the present 

Colville River delta (fig. 21). The thickest subsurface penetration is about 1,900 m (top eroded) in the Tunalik-1 

well on the west. As indicated by foreset dip directions 

in the Torok Formation on seismic sections, the deltaic 

sequence prograded east-northeast across the subsiding 

Colville basin (fig. 23; Bird and Andrews, 1979). The 

basin was bounded on the north by the passive Barrow 

arch, which separated it from the actual continental margin farther north. The ancestral Brooks Range bounded 

the south side of the Colville basin. 

The term "delta" or "deltaic system" is used in a 

broad sense. The Nanushuk sediments in most of the area 

were supplied by many distributaries, and undoubtedly 

there were many interdeltaic areas. The main or western 

delta complex, which has been called the Corwin delta, 

is considered to be a river-dominated, high-constructional 

delta (Ahlbrandt and others, 1979, p. 17). It had a relatively low-sand and high-mud content in contrast to the 

southern-source Umiat delta of the outcrop belt south of 

Umiat (Huffman and others, this volume). 

The overall prograding regressive sequence was interrupted many times by marine transgressions due in part 

to delta shifting, but more importantly, to episodic pulses 

of basin subsidence . Basinal subsidence (or relative sealevel rise) is necessary to account for stacking of shallow 

marine sequences or for the stratigraphic rising of the section as the sequence pro grades. (Refer to the section on 

"Stratigraphic Rise and Basin Subsidence.") Therefore, 

the Nanushuk is not one simple regressive sequence but 

a composite of intertonguing shallow marine shale, shallow marine sand, and subaerial delta-plain deposits. 

In this chapter, the term "shoreface sandstone" refers to a sandstone body that separates open marine environments from lagoonal, marsh, back-bay, or nonmarine 

environments. These bodies would include barrier islands, 

tidal channels and deltas, strand-plain coastal barriers, and 

distributary-mouth bars. Distributary-mouth bars are probably the most common in the Nanushuk, especially in 

the western part of the area. 

The lower part of the Nanushuk consists of intertonguing marine shoreface or shelf sandstones and neritic 

shales and siltstones. For convenience and uniformity, the 

contact with the underlying Torok Formation in the subsurface is usually placed at the base of the sandy section 

as determined by electric log or gamma ray-transit time 

log response. Because of the gradational and intertonguing 

nature of the contact and because of differing log qualities, the selection of the contact becomes quite arbitrary 

in some wells. An example of this arbitrariness is the 

selection of the contact at about 2,800 ft in the Umiat-1 

well shown on the cross section across the northern foothills (fig. 18A). Actually, there are several thin sandstone 

beds with poor spontaneous-potential (SP) log response 

for more than 1 ,000 ft below. Another example is the 

selection of the contact at about 2,150 ft in the Topagoruk-! well shown on the cross section across the 

northern coastal plain (fig. 18B). There are three or four 

thin sandstone beds in the 1 ,200-ft interval below. One 

must be aware of these variations when evaluating isopach 

or sand-percentage maps. 

Seismically, the base of the Nanushuk occurs within 

the lower part of the top set reflectors. The distance between the base of the topset beds and the base of the 

Nanushuk ranges anywhere from 15 m as in the W. T. 

Foran-1 well, to 335 m, as in the South Simpson-1 well. 

In some places the basal Nanushuk sands extend to the 

shelf break as at Fish Creek-1 and West Fish Creek-1 

wells shown on the cross section across the eastern part 

of NPRA (fig. 18D), but generally the basal sand occurs 

well back from the shelf edge. Figures 19 and 20 show 

the relationship of the base of the Nanushuk to the base 

of the topsets. 

On the cross sections, the Nanushuk Group is divided into a lower, dominantly marine facies (Kukpowruk 

Formation on the west or Grandstand and Tuktu Formations on the east) and an upper dominantly nonmarine 

facies (Corwin Formation on the west or Chandler Formation on the east) to show the lateral relationships. These 

major facies are the basis for the differentiation of most 

of the formations of the group. Bird and Andrews (1979) 

referred to these parts as facies A (predominantly nonmarine) and facies B (predominantly marine). In the subsurface, the differentiation is based on log character, the 

presence or absence of marine fauna, and the presence 

or absence of coal or highly carbonaceous beds. In many 

places the boundary is transitional in a zone of intimate 

intertonguing of the two facies. In these transitional 

zones, the contact is usually placed at the top of the highest well-developed shoreface sandstone or the highest occurrence of marine fauna. In most wells, the selection 

of this contact is highly interpretive, especially in western 

NPRA where the Corwin delta was more river dominated. 

Because of the complex intertonguing of the two 

major facies of the Nanushuk and the shales of the Torok, 

and because of the distance between wells, better than 

gross correlation of the major facies is difficult. Some 

of the correlations of individual sandstone or shale 

tongues are shown only where wells are relatively close 

together. Also, the small scale of the cross sections mitigates against showing these details. 

The generalized time-stratigraphic relationships of 

the formational units of the Nanushuk Group that were 

described by Detterman (1956) from outcrops in the Colville River area southeast of NPRA and by Sable ( 1956) 

from outcrops on the west side of NPRA are portrayed 

on the cross sections. Two of the cross sections are tied 

to outcrop sections south of Umiat where some of these 

units were originally named. The outcrop sections are porSubsurface Correlations and Depositional History 41 
w 

Tunalik Kaolak Meade Oumalik 

2 

3 

4 

5 

6 

7 

8 

9 

10 

No. 1 No. 1 No. 1 No. 1 

~43km------~-----------------107km------------------+---------62km---------+~~~ 

A 

EXPLANATION 

Dominantly nonmarine facies 

Dominantly shallow marine 

sandstone and shale 

-------

1700 ft of Torok removed 

to account for tectonic 

thickening 

Pre-pebble shale 

Figure 18. Stratigraphic cross sections of Nanushuk Group and associated strata across National 

Petroleum Reserve in Alaska and adjacent areas. See figure 17 for location of sections. Horizontal 

distance between wells or sections is indicated. Well depths are numbered at 1 ,000-ft intervals. 

Vertical exaggeration is approximately 58 x. SP, spontaneous potential; R, resistivity; GR, gamma 

ray; TT, transit time. Dashed lines are correlations of miscellaneous beds or reflectors. Dip angle 

42 Geology of the Nanushuk Group and Related Rocks, Alaska 
East E 

Oumalik 

No. 1 

Titaluk 

No.1 

Wolf Creek 

No. 3 

Umiat Umiat McCulloch Gubik B. P. Itkillik 

No. 1 No. 2 Col. U. 2 No. 1 No. 1 

-- ~-----57km------~-----40km----~-----46km------•+1·~ ~·~1-·~~·~1-·-+ --•+l·~-35km~ km km km SP R 

Seabee Formation, Colville Group 

.SP R 

SP R 

\ 

\ 5 

\ \ 

4 

I 6 

6° I I 6.70 

Foresets projected from \ 

seismic line 68E-77 

in syncline to south \ 

\ 

'--

5 

6 

Bottomsets of westerly derived Torok deposited on 

bottomset turbidite facies of southerly derived 

Torok and Fortress Mountain Formations at some 

horizon below 

Bottom sets 

(basinal) 

Base of pebble shale unit from 

well and seismic data arid 

adjusted for tectonic thickening 

of Torok Formation 

.~ '\ 

'\ 

Pre-pebble shale 

SP R 

_____ __§ 

5 

50 ft of fault 

repetition 

removed 

Torok 

Formation 

7 

of foreset beds in respect to topset beds is indicated. Datum is base of Colville Group (Seabee 

Formation) where it is present; otherwise datum generally conforms to topset beds. A, west-east 

across northern foothills; 8, west-east across northern coastal plain; C, west-east across northeastern 

coastal plain; D, south-north across eastern part of NPRA; f, southeast-northwest across northeastern part of NPRA. 

"0 

Q) 

"0 Q) ·:;: 

]:0 

co c Q) :::l 

(/) - ' (J) ~ c e .Q o.., 

f- E 

0 

u.. 

Subsurface Correlations and Depositional History 43 
w 

Tunalik 

No.1 

Peard 

No. 1 

Kugrua 

No. 1 

South 

Meade 

No. 1 

~--------------96km----------------~20km~~--------65km--------~---36km--

2 

0. 

::J 

e 

0 3 ~ 

::J 

..c. 

rJl 

::J 

c 

ro 

z 4 

5 3 

4 

6 \ 

\ 

\ -.:___ 

\\1.5° 

5 

7 5 

----- ~ "' ~ 10 \ 6 

""' ""' 8 """ ""' ,6 

"---.. 0.7° 

---- 9 ----- ~ 

----

10 

11 

B 

Figure 18. Continued 

trayed in a generalized way to show the gross facies relationships. 

The upper contact of the Nanushuk is an erosional 

surface underlying Pleistocene deposits in most of the area 

except for the eastern third of NPRA, where the Seabee 

44 Geology of the Nanushuk Group and Related Rocks, Alaska 

Formation (Cenomanian-Turonian) of the Colville Group 

overlies the Nanushuk (fig. 21). In the latter area, the 

top of the Nanushuk is placed at the base of a transgressive bentonitic shale section, which is usually easily identified on the logs. In some places, however, a basal trans-
E 

East East North Atigaru 

Point 

No. 1 

Gulf Colville 

Delta 

No. 1 

Topagoruk Topagoruk lkpikpuk Teshekpuk Ka likpik 

No. 1 No. 1 No. 1 No. 1 No. 1 

--~~20km~-----41 km----~-------54km------~--22km~25km~~----43km----~ 

Torok 

Formation 

4 

\ 

\ 

\ 

\ 

3 

-o 

Q) 

-o 

:~ -o 4 c 

:J 

vi 

c 

0 

·.p 

co 

E 

0 

u.. 

EXPLANATION 

Dominantly nonmarine facies 

Dominantly shallow marine 

sandstone and shale 

Figure 18. Continued 

gressive sand or sandy zone occurs at the base of the 

Seabee Formation, thereby making the precise boundary 

selection somewhat interpretive; micropaleontologic data 

are helpful in selecting this boundary. 

The Ninuluk Formation of Cenomanian Age occurs 

in the upper part of the Nanushuk Group in the outcrop 

belt in the Chandler River-Killik River area (Detterman, 

1956; Detterman and others, 1963; and Chapman and 

others, 1964). It is a marine unit that intertongues with 

the nonmarine Niakogon Tongue of the Chandler Formation (of the Nanushuk Group) and is as thick as 350 m 

(Detterman and others, 1963, p. 264). As portrayed on 

the cross sections in the southeastern part of the area (fig. 

18£), it is interpreted as representing deposition by the 

southern-source Umiat delta during the time of the initial 

Colville transgression farther north. In other words, the 

Umiat delta was still actively aggrading after the eastnortheasterly prograding delta to the north became inactive 

and was being transgressed by the Colville sea. 

The facies relationships and the seismic data do not 

indicate an unconformity at the base of the Seabee Formation in the subsurface. The Ninuluk and Seabee appear 

to be parts of the same major cycle of transgression. However, in the outcrop belt to the south, earlier workers have 

reported a significant unconformity (Detterman, 1956, p. 

233; Detterman and others, 1963, p. 270; Chapman and 

others, 1964, p. 382). Continuing tectonic activity in the 

ancestral Brooks Range could have resulted in an unconformity at that time along the outcrop belt. Brosge and 

Whittington (1966, p. 527) suggested that warping in the 

folded belt could have started during Nanushuk time. This 

suggestion is supported by the apparent thickening of section within the Ayiyak Mesa syncline 50 km south of 

Umiat, as indicated on an old, poor-quality U.S. Navy 

Subsurface Correlations and Depositional History 45 
seismic line in that area (Detterman and others, 1963, 

pl. 42). 

Torok Formation 

The Torok Formation represents offshore or deeper 

water deposition of shale, siltstone, and minor amounts 

of sandstone. It is largely coeval with the Nanushuk 

Group in the northern half of the area, but to the south 

the lower part thickens and intertongues with or is equivalent to the Fortress Mountain Formation of early Albian 

age (Chapman and others, 1964, p. 357). The lower 

Torok in that area appears to be older than any part of 

the Nanushuk Group. 

The thickness of the Torok is largely controlled by 

the asymmetry of the preserved part of the Colville basin, 

especially to the south, as shown in figure 22. This 

isopach map was constructed by using the widely scattered well data, and where a well did not completely penetrate the Torok, seismic data were used (Miller and others, 

1981). In the anticlinal belt of the northern foothills, the 

Torok has been thickened tectonically by thrust faulting 

and possible diapirism in the cores of the anticlines. An 

attempt was made to adjust for this thickening in the cross 

sections and the isoach map by subtracting the amount 

of structural relief of each anticline from the present 

Torok thickness (fig. 22). 

The Torok interval is represented on seismic lines 

by the lower part of the topset reflectors and the foreset 

and bottomset reflectors. The Torok in the topset beds 

consists of neritic shelf shales and siltstones; the foreset 

beds consist of slope shales and siltstones and minor sandstone beds; and the bottomset beds consist of deep-water 

shales containing common thin turbidites or deep-water 

sandstones and siltstones. 

Well penetrations indicate that these deep-water 

sandstones are generally very fine grained, as would be 

expected inasmuch as they were filtered through the 

Nanushuk delta where the coarser grained sands would 

have been deposited in the fluvial facies. A possible exception to this deltaic filtering would be if there were 

times of lowered sea level when fluvial deltaic facies extended to the shelf break. Wells recently drilled in the 

northern part of NPRA have had numerous gas shows 

from these deep-water sandstones. Torok foreset reflectors 

that can be correlated to wells seem to be either silty 

or sandy zones or thin zones of low-velocity shales. The 

sandy intervals could serve as paths for hydrocarbons 

migrating from source rocks within either the Neocomian 

pebble shale unit or the lower Torok interval to reservoirs 

in the Nanushuk. 

On some seismic lines, reflectors can be traced continuously from topset beds within the Nanushuk to bottomset beds within the Torok deep-water deposits. These 

reflectors, which are represented by short dashed lines on 

46 Geology of the Nanushuk Group and Related Rocks, Alaska 

the cross sections, are time lines and represent the nearoriginal depositional profile of the basin. 

In the northern part of NPRA where the seismic 

data are of good quality, bottomset beds downlap 

(Mitchum, 1977, p. 207) onto or near the basal Cretaceous pebble shale unit of Neocomian Age as shown on 

three of the northern cross sections (fig. 18). Because 

these bottomset beds can be correlated seismically to beds 

of Albian Age, the hiatus or thin interval of Torok between these downlapping beds and the pebble shale unit 

is interpreted to be a deep-water condensed or nondepositional zone representing part of Neocomian, all of Aptian, 

and much of Albian time. The thickness of this interval 

is less than the resolution of the seismic data, that is, 

less than 25 m. Indeed, this condensed section may be 

included in the upper part of the pebble shale unit. This 

starved depositional system is interpreted to have occurred 

as a result of the deeper part of the Colville basin to 

the south receiving all the sediments being derived from 

the south and southwest after deposition of at least most 

of the pebble shale unit. Not until the latest stages of 

the Nanushuk-Torok depositional cycle did the Colville 

basin fill enough for sediments to reach the northern part 

of NPRA. A similar depositional pattern continues to the 

northeast of the Nanushuk depositional limit in younger 

Cretaceous and Tertiary formations. On seismic lines in 

northeastern NPRA, topset, foreset, and bottomset bedding can be seen in the overlying shales of the Colville 

Group (figs. 19 and 20). In the Prudhoe Bay area, bottomset beds of the Colville Group downlap onto or near the 

pebble shale unit (Bird and Andrews, 1979, p. 35). 

In the southern part of NPRA (south of a line connecting the Inigok and Tunalik wells, fig. 17), seismic 

data indicate a southward-thickening wedge of strata that 

onlap the south-dipping pebble shale unit as shown on 

the two cross sections in the southeastern part of NPRA 

(fig. 18D and E). Some of these beds can be traced to 

east-northeasterly dipping foresets on the west, but the 

lower part of this wedge may be deep-water basinal beds 

or distal turbidites of southerly derived Torok and (or) 

Fortress Mountain Formation equivalents. Structural complexities and poor data quality preclude tracing these beds 

to the south. 

In northeastern NPRA, in the area from the Fish 

Creek wells to beyond Atigaru Point, the lower Torok 

interval is disturbed, as indicated by seismic data (fig. 

20). Inasmuch as this interval is laterally adjacent to bottomset beds and underlies fore set and bottomset beds, the 

disturbed interval is thought to have been caused by largescale submarine slumping. In some of this area, the pebble shale unit is missing, at least in part, indicating either 

that it was involved in the slumping, or that it was 

scoured by submarine currents that may have triggered 

the subsequent slumping. The areal association of the 
WSW ENE 

East South East Drew J. W. Cape W. T. 

Topagoruk Topagoruk Simpson Simpson Point Dalton Halkett Foran 

No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 

r--20km--~---30km-----+~18km~~--26km--~~---28km--~~---30km----~-9km_, 

4 

c 

EXPLANATION 

Dominantly nonmarine facies 

Dominantly shallow marine 

sandstone and shale 

SP R SP R 

Figure 18. Continued 

slumping and scouring suggests that they were caused by 

related events. 

Provenances-Their Effect On Depositional 

Patterns 

The provenance for the Nanushuk Group has been 

discussed and summarized by Mull (1979, p. 10-12), 

Bartsch-Winkler (1979, p. 67), and Huffman (1979, p. 

84). Based on the interpretation of regional paleogeography and compositional differences in the Nanushuk 

sandstones, all these workers agree that the western De 

Long Mountains, the adjacent Tigara uplift south of Cape 

Lisburne, and its northwestern offshore extension, the 

Herald arch (fig. 31), was a dominant source area for 

the western or Corwin delta complex, and that the ancestral Brooks Range on the south was a predominant source 

area for the Umiat delta. I am in general agreement with 

these interpretations, but I offer additional comments 

about the relative size or extent of these two major source 

areas. 

Subsurface Correlations and Depositional History 47 
ssw 

Killik River 

surface 

section 

Sinclair 

Little Twist 

No. 1 

Wolf Creek 

No. 3 

Square 

Lake 

No. 1 

lnigok 

No. 1 

~38-43km----~----39km----~--22km~~-------51 km------~~----

Colville Group 

Torok 

Formation 

SP R GR TT 

---\ 

\ 

\ 

\40 

\ 

"' 

6 

--......... --------7 

Bottomsets (basinal) 

.---...--:: 

---- ...... ------------ ---- ---- ---- ---- ---- ---- - ----- ---- ---- ---- ---- ---- ---- -------- ---- __..... 

--------------------:-------------------- ---- -------- -------- ---- ---- __..... ---- ---- ---- -------- ---- ---- ----

Torok and(or) Fortress Mountain 

--------

equivalent~ ----- --------~ 

D 

distal turbidites onlap south-dipping 

Neocomian pebble shale unit. 

Figure 18. 

Except for some coarse units such as the conglomerate member at Corwin Bluffs (Chapman and Sable, 1960), 

which must have been derived from a nearby source such 

as the Tigara uplift, most of the Nanushuk in the western 

area had a relatively low-sand and high-mud content 

(Ahlbrandt and others, 1979, p. 25). This mud-rich delta 

system controlled the east-northeasterly direction of progradation across the Colville basin, as indicated by foreset 

dip directions in the prodeltaic Torok Formation (fig. 23). 

48 Geology of the Nanushuk Group and Related Rocks, Alaska 

Continued 

The huge volume of mud-rich sediments represented by 

this western delta complex suggests that a relatively large 

source area encompassing a large drainage area was present to the southwest, undoubtedly much more extensive 

than the western De Long Mountains, Tigara uplift, and 

Herald arch as they were defined by Mull ( 1979, fig. 

3), and Grantz and Eittreim (1979, fig. 3). Furthermore, 

the Herald fault zone, which bounds the northeast side 

of the Herald arch, is considered, at least in part, a post-
NNE 

West Fish Fish South Atigaru 

Creek Creek Harrison Point 

No. 1 No. 1 Bay No. 1 No. 1 

----53km--------~8km~14km~15km~ 

SP SP TT SP R 

Seabee Formation 

of Colville Group 

---

2 

2 "C 

Q) 

"C 

·:;: 

ii 

Foreset beds c:: 

:l 

_..........in Seabee 

\ Formation ~ "-- 0 

·.;:; 

Cll 

E 

0 

u. 

Q) 

Q) 

.0 

Cll 

Q) 

(/) 

~ 0 

0 

I-

--6 

Submarine 

slumping 

and scouring . 

7 Pebble shale 

unit removed 

EXPLANATION 

Dominantly nonmarine facies 

Dominantly shallow marine 

sandstone and shale 

Figure 18. Continued 

Nanushuk feature because it offsets Nanushuk Group 

strata (Grantz and Eittreim, 1979, p. 26). Therefore, I 

suggest that during Nanushuk time a land mass of larger 

areal extent lay farther west or southwest than the present 

Tigara uplift and Herald arch, probably in the area of 

the present Hope basin farther southwest in the Chukchi 

Sea or beyond. 

The ancestral Brooks Range was undoubtedly a 

prominent highland that contributed sediments both to the 

Colville basin on the north and to the Koyukuk basin on 

the south during Nanushuk time. However, because of 

a limited drainage area and because of more rapid subsidence on the south side of the Colville basin, the southern 

source deltas did not control the progradational pattern 

of the Nanushuk except along the south side of the basin 

in the area of the present outcrop belt. However, the sands 

from this delta probably contributed significantly to the 

east-northeasterly prograding system when the prodelta 

shelf of the western delta had built out adjacent to the 

southern deltas. (Refer to the section on "Hydrocarbon 

Reservoir Sand Development" for additional comments.) 

If it were not for the presence of the easterly prograding 

western delta, the southern-source Nanushuk would not 

have extended too far to the north. This contention is 

supported by observations southeast of Umiat and east of 

the pinchout of the easterly prograding system where welldeveloped southern-source alluvial-deltaic clastics such as 

those at the Marmot syncline outcrop section (the southeasternmost section with the 853 + value, fig. 21) grade 

rapidly into finer clastics of the Torok. The Nanushuk 

at this section, which is more than 850 m thick, 800 m 

of which is nonmarine and contains coarse clastics 

(Huffman and others, 1981b), grades to Torok shales and 

siltstones within 60 km to the north as shown on the east 

in figure 21. Some of the 60 km may have been shortened 

by folding and thrusting, but even so, the gradation is 

abrupt when compared to that of the western delta complex. Inferred paleogeographic maps of a part of northern 

Alaska and adjacent areas at middle and late Nanushuk 

time are shown in figure 24. 

Subsurface Correlations and Depositional History 49 
SE 

Tuktu Bluff 

surface Grandstand 

section No. 1 

Umiat 

No. 1 

lnigok 

No. 1 

f-19-30 km ___,_ ____ 50 km-----l------- 77 km ______ ...,.__ 

a. 

::J 

0 

t5 

.::L 

::J 

.L: 

CJ) 

::J 

c 

<ll 

z 

c 

0 

·.:; 

<ll 

E 

0 

u.. 

.::L 

0 

0 

1-c-

------------- ----- ----- -------- ---- -""1"9'·"'""1""' 

---- --------- ----- ----- ----- --------- - ------------ --------- ---- ---- ---- ----- 00 ---- e;,"-= 

-:-------- &~ Torok and(or) Fortress Mountain equivalent it 'li 

distal turbidites onlap south-dipping 0 ..:> 

Neocomian pebble shale unit. q-q, 

E 

""0 <Qq, 

~ 

~..s0 

~0 

Figure 18. Continued 

STRATIGRAPHIC RISE AND BASIN 

SUBSIDENCE 

In prograding across the Colville basin, the shallow 

marine part of the Nanushuk, that is, the part between 

the basal contact and the marine-nonmarine boundary, becomes younger and shifts upward in the section owing 

to facies changes. Because these shallow marine facies 

are largely confined to a rather narrow range of water 

depths, possibly no more than 50 m and probably much 

less, these upward shifts or rises are then a measure of 

relative sea-level rise (or basin subsidence). The term 

50 Geology of the Nanushuk Group and Related Rocks, Alaska 

"stratigraphic rise" is used to denote these upward stratigraphic shifts of water-depth-related deposits. 

Stratigraphic rises are directly observable on seismic 

lines. The change from top set beds to fore set beds represents the shelf break, a geomorphic feature separating the 

basin shelf from the basin slope. As indicated on seismic 

lines, this break is abrupt, especially in the eastern half 

of NPRA (figs. 19 and 20). There are several interpretations of the origin of shelf breaks, and discussion of them 

is beyond the scope of this report, but most hypotheses 

relate the shelf breaks to water depth. The breaks are assumed herein to be water-depth controlled, possibly deter-
lkpikpuk 

No.1 

South 

Simpson Simpson 

No.1 No.1 

NW 

South Barrow 

No. 1 and 

Arcon Barrow No. 1 

--------69km ----------~-----46km------~21 km~---------64km--------~ 

Seabee Formation, 

Colville Group 

Topsets (shelf) 

Foresets (slope) 

(strike section) 

Bottomsets (basinal) 

-------------

Pebble shale unit 

GR TT 

------

4 

Torok 5 Formation 

6 

7 

Pre-pebble shale 

/Post-Nanushuk erosion 

Seabee Formation 

4 3 

5 4 

6 

t 

Barrow arch 

7 

c:i 

z 

~ 

e 

ro SP R m 

c 

0 

u 

~ 

+ 

c:i 

z 

~ 

e 

ro 

al 

£ 

:::J 

0 

(/) 

EXPLANATION 

Dominantly nonmarine facies 

Dominantly shallow marine 

sandstone and shale 

Figure 18. Continued 

mined by tidal or other marine currents on the shelf. Most 

modem shelf breaks are at an average water depth of 132 

m (Shepard, 1963, p. 257). However, most of these features resulted from Pleistocene low stands of sea level 

(Emery, 1965, p. 1383). Therefore, the Nanushuk-Torok 

shelf breaks must have formed at much shallower depths 

than the 132-m average of present shelf breaks; a range 

of 50 to 75 m is suggested. 

As the Nanushuk progrades and rises to the northeast, the shelf break (or base of the topset bedding) also 

rises upward in the section, as observed on seismic sections. Like the Nanushuk deltaic deposits, the shelf break 

appears to be predominantly a constructional feature in 

which the basin-slope deposits prograded seaward. Figure 

19 shows that between shot points 49 and 29, a distance 

of 8 km, the base of the topsets rises about 180 milliseconds (two-way time) or approximately 260 m (850 

ft). The Nanushuk-Torok contact shows these vertical 

rises of the shelf break, although it would be offset toward 

land. An example of this landward offset is shown by 

the vertical rise of the base of the Nanushuk between the 

Peard-1 and Kugrua-1 wells on the northern coastal plain 

cross section (fig. 18B). The actual rise of the shelf break 

occurs about 8 km east of the Kugrua-1 well as indicated 

by seismic data (seismic line 2fr-76 between shot points 

643 and 653). This location places the rise of the base 

of the Nanushuk about I 0 km from the rise of the shelf 

break as projected across depositional strike. 

The rise of the shelf break would be indicated in 

Nanushuk deposition either by vertically stacked shoreface 

Subsurface Correlations and Depositional History 51 
East Teshekpuk No. 1 

(Projected from 4 km north-northwest) 

Total depth 10,664 ft (3250 m) 

SEISMIC LINE 12-74-G-186 

WSW -<>- ENE 

48 47 46 45 44 43 42 41 40 24 23 

-o 

-1 1-

w 

w 

u.. 

-2 u.. 

0 

(/) 

0 

(/) 

0 

z 

0 

u 1.0 w 

-3 z 

~ 

(/) 

:J 

0 -4 I (/) 1-

~ ~ 

ui -5 u) 

~ 

i= 

>-

~ 1.5 

I 

1-

-6 a.. 

w 

0 

~ -7 ~ w 0 

$: 

1-

Cl( 

-8 m 

:J 

(/) 

-g 

2.0 

-10 

2.5 

Figure 19. Seismic section in northeastern National Petroleum Reserve in Alaska showing relationships 

of topset bedding of Nanushuk Group and uppermost Torok Formation, foreset bedding of Torok, and 

bottomset bedding of lower Torok. Upper dashed line is Colville-Nanushuk contact, lower dashed line 

is Torok-pebble shale contact, and dots represent position of shelf break. Note stratigraphic rise of shelf 

break from west to east. See figure 17 and table 3 for location of East Teshekpuk well (32). Numbers 

across top of figure are shot-point numbers. 

sandstones or, more likely, by minor transgressions and 

neritic shale deposition between prograding Nanushuk 

shoreface or shelf sandstones. The major stratigraphic 

rises may be correlatable basin-wide transgressive events. 

Smaller rises could be caused by the local effect of delta 

shifting, or by basin subsidence caused by isostatic loading of delta lobes. 

In some areas, the shelf break remains at the same 

stratigraphic level, indicating that relative sea level must 

have been constant as the shelf break prograded seaward. 

A relative drop in sea level and a subsequent beveling 

of the top of the foreset beds could also cause this feature. 

As well as being detectable on seismic lines, the shelf 

break can also be noted on dipmeter logs of wells in 

northeastern NPRA by a 3-5° dip increase in the upper 

part of the Torok. 

The total incremental stratigraphic rise of the base 

of the Nanushuk from the Tunalik-1 well on the west 

to the Atigaru Point-1 well on the east, a distance of 

52 Geology of the Nanushuk Group and Related Rocks, Alaska 

350 km, is about 2,100 mas shown on the northern coastal plain cross section (fig. 18B). The average rate of rise 

is about 6 m/km. A significant additional rise takes place 

between the Tunalik-1 well and the Corwin Bluff surface 

section 220 km to the southwest, but correlation from the 

outcrops to the subsurface in that area is too imprecise. 

However, Chapman and Sable (1960, p. 91-95), in an 

excellent photogeologic and surface study of a large part 

of the western outcrop belt between Corwin Bluff and 

the Kaolak well, determined the average rate of facies 

change (or stratigraphic rise) of the base of the Nanushuk 

to be 21 m/km in a N. 32° E. direction. This amounts 

to about 3,000 m of rise in their study area, a distance 

of about 145 km across depositional strike. This rate may 

be excessive, but it does help to explain the change from 

the 3,444 + m-thick Corwin Bluff section to thinner sections in the subsurface to the northeast. Also, the high 

rate of stratigraphic rise may indicate an increased rate 

of subsidence toward the south. 
SEISMIC LINE 15-74-G-186 

WSW 

North Kalikpik No. 1 

Total depth 7395 ft (2254 m) 

-¢- ENE 

49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 

I 

0 -o Iw 

w 

u. 

-1 u. 

0 

(/) 

0 

z 

-2 <t 

(/) 

:::;) 

0 

:I: (/) -3 I0 

z ~ 

0 en u 

w -4 :I: 

(/) Ia.. 

~ w 

-5 0 

u.J <t :::2: w 

f= eft 

>- -6 Q) 

1.5 :::;) 

<t (/) 

~ -7 0 

~ IFigure 20. Seismic section in northeastern National Petroleum Reserve in Alaska showing disturbed zone 

in Torok Formation and discontinuity of the pebble-shale reflector. This disturbed zone and discontinuity 

is thought to be caused by submarine slumping and scouring. Upper dashed line is Colville-Nanushuk 

contact, lower dashed line is Torok-pebble shale contact, and dots represent position of shelf break. 

Note development of slope-foreset beds in lower Colville Group. Apparent structure in pre-pebble shale 

reflectors is due to seismic velocity of disturbed zone being greater than velocity of undisturbed adjacent 

sections. See figure 17 and table 3 for location of North Kalikpik well (40). Numbers across top of figure 

are shot-point numbers. 

Further evidence for the increased rate of subsidence 

to the south is indicated by less and (or) later subsidence 

of the Barrow arch on the north side of the Colville basin. 

The Barrow arch was probably a passive high, but was 

never high enough to be subjected to erosion during 

Nanushuk time, as indicated by the presence of the distinctive pebble shale unit of Neocomian Age over the 

arch. (The absence of pebble shale in the area between 

the Fish Creek and Atigaru Point wells was caused by 

postdepositional submarine slumping or scouring, as previously explained.) The cross sections that extend to the 

north or northeast part of NPRA show the basal Torok 

Formation onlapping the pebble shale unit (fig. 18). This 

onlapping indicates that the northern side of the Colville 

basin subsided at a lower rate and (or) later time . A lower 

rate is indicated by the progessive southward depositional 

thickening of the Torok shown on the two cross sections 

in the southeastern part of NPRA (fig. 18D and E) as 

opposed to the facies-controlled southwestward thickening 

of the Nanushuk (fig. 21). 

An interesting pattern of depositional thinning of the 

Nanushuk in a landward direction (west or southwest) is 

indicated on many of the east-west-trending seismic lines 

in the northern part of NPRA. Several measurements of 

topset-bed convergence were made along line 26-74 from 

the Fish Creek area to the area near the Ikpikpuk-1 well 

and on line 26-76 from the area of the South Meade well 

to the western coast (fig. 17). Topset reflectors traced 

continuously for distances of from 8 to 60 km along these 

lines showed westward convergence ranging from 0. 2 to 

2. 3 cm/km for each meter of thickness. This convergence 

may be explained by either compactional differences or 

actual differences in basement subsidence. Compactional 

differences may be caused by the delta front prograding 

over prodelta shales that were less compacted than adjacent, but older, prodelta shales to the west, thus allowing 

Subsurface Correlations and Depositional History 53 
165° 162° 159° 

EXPLANATION 

o Well 

X Outcrop section 

70° 

69° 

68° 

BROOKS 

156° 153° 

R A N G E 

50 100 150 KILOMETERS 

Isopach 

interval 

150° 

TOROK 

FORMATION 

1070 + 

Figure 21. Isopach map of Nanushuk Group (modified from Bird and Andrews, 1979). Refer to both figure 

17 and tables 3 and 4 for identification of control points. Numbers beside control points indicate thickness 

of Nanushuk Group; a + after number by well indicates minimal thickness owing to top of Nanushuk 

having been eroded. A + after number by an outcrop section indicates an incomplete section of Nanushuk. 

a greater Nanushuk thickness to accumulate. Differences 

in basement subsidence, on the other hand, may be the 

result of deltaic loading in which slightly greater subsidence occurred in the delta-front area. In both explanations the locus of greater subsidence would migrate as 

the delta prograded. 

SHORELINE TRENDS AND BASIN 

CONFIGURATION 

Most of the Corwin delta and, to a lesser extent, 

the Umiat delta were river-dominated systems (Ahlbrandt 

and others, 1979, p. 17), and, as a result, the shorelines 

were probably moderately to extremely digitate, or in 

other words had a bird's-foot pattern. However, the overall shoreline trend of most of the basin throughout 

Nanushuk time was north-northwest or normal to the dip 

of the slope-foreset bedding of the Torok as indicated by 

seismic data and facies patterns from well data (figs. 21 

and 23). East-dipping foresets can be seen on seismic 

lines as far as 8 km south of the Umiat and Wolf Creek 

wells (fig. 17). Immediately to the south of that area, 

54 Geology of the Nanushuk Group and Related Rocks, Alaska 

however, the shoreline trends abruptly change to east-west 

because of the effect of north-prograding deltaic systems 

from the ancestral Brooks Range (fig. 24). This east-west 

orientation is indicated in the outcrop belt south and 

southeast of the Colville River by north-directed fluvial 

channel trends (fig. 23; Huffman and others, this volume) 

and the long linear east-west trend of Tuktu Bluff, which 

is made up of thick, stacked delta-front sands of the Tuktu 

Formation. 

Because the preserved part of the Colville basin was 

asymmetric with the greatest subsidence on the south, the 

north-prograding deltas from the ancestral Brooks Range 

probably had narrow delta or coastal plains and more abrupt facies changes. More deep-water sandstones might 

be expected in the Torok to the north in front of these 

active sediment sources. An example would be the 790-mthick section of sandstone and shale in the Gilead CreekIvishak River area 65 km northeast of the Marmot 

syncline section (Keller and others, 1961 , p. 204) that 

are thought to be deep-water equivalents of the Nanushuk 

(Huffman and others, this volume). The southern parts 

of the two cross sections that connect to outcrop sections 
165° 162° 

71° 

EXPLANATION 

o Well 

X Outcrop section 

70° 

69° 

68° 

BROOKS 

159° 156° 

50 100 

153° 

R A N G E 

150 KILOMETERS 

Isopach 

interval 

150° 

Figure 22. Isopach map of Torok Formation (modified from Bird and Andrews, 1979). Numbers beside wells 

indicate thickness of Torok Formation; thickness values in parentheses have been corrected (approximately) 

for tectonic thickening in thrust-faulted anticlines. Refer to both figure 17 and table 3 for identification of 

control points. 

at Killik River and Tuktu Bluff, respectively (fig. 18D 

and E), show the inferred relationships of the southernsource Umiat delta and the areas to the north. These sections also demonstrate the southward thickening of basin 

fill during Early Cretaceous time. 

In addition to the rapid facies change of the Tuktu 

Formation grading to Torok shale between the Tuktu Bluff 

section and the Grandstand-I well, depositional thickening of the Nanushuk to the south similar to that noted 

in the underlying Torok Formation, probably has occurred. However, this thickening could not be verified 

by the poor-quality old U.S. Navy seismic line in that 

area. 

Foreset bedding is better developed in the east or 

northeast parts of NPRA. In the western part, these features become more obscure and less numerous partly because of poor seismic data in the shallow Nanushuk-Torok 

interval. Some foreset bedding can be seen, however, as 

far west as the Tunalik-1 well and just east of the 

Kaolak-1 well where the foreset beds are at a much lower 

angle than the foresets to the east. 

By measuring the time intervals on the seismic section from top to base of foresets and applying proper velocities from nearby well data, calculations were made 

of the approximate foreset slope angle and the relief of 

the fore sets, which approximate the depth of water (less 

compaction) at the time of deposition. The dip angles of 

the foreset beds are calculated relative to the topset beds, 

which are considered to have been almost flat at the time 

of deposition (probably less than 0.25° dip seaward). The 

calculated foreset dips range from less than 2° on the west 

to 4-6° on the east and northeast part of NPRA. These 

calculations were made on the steeper upper parts of the 

foresets averaged over at least 4 km of horizontal distance. The calculations are from unmigrated seismic data, 

but migrated data would not change the figures significantly. The relief of the foresets from top to base is 45~ 

900 m. Allowing for compaction, the basin slope was 

slightly steeper than the indicated dip, and the water depth 

at the base of the slope was slightly greater than 45~900 

m. Water depths probably increased to the south beyond 

the areas of interpretable seismic data. 

No large canyons cutting the slope or foreset bedding have been observed or reported on any of the seismic 

Subsurface Correlations and Depositional History 55 
153° 150° 

0 CEAN 

EXPLANATION 

o Well 

X Outcrop section 

><A~· x Nanushuk Group 

(2~} outcrop belt 

Cape Lisburne ~~ ~ fo & j( 

(~~-.. 8) ~(1 0) ·X •.• (~.4) ~:~ •... !?) .... ~?.)·· ...... · ····. •• •• (2) • ,· ••• COLVILLE •• •• 

-S~~h~-r~·-··(20)·. limit of R!v.cl? 

l. . --------- . ..--....- ' ~,of?. Nanushuk I - ;,q 

~~u~ ~· 

"'---· BROOKS R A N G E 

50 100 150 KILOMETERS 

Percent of 40 O A 

readmgs l...u..!..J.6S: ( 1 O) Number of 

EXPLANATION 

~ 4. ..... ""' readmgs measurements 

Paleotransport directions determined Average progradation 

from nonmarine channels of the 

Nanushuk Group 

direction of Nanushuk 

(Chapman and Sable, 1960) 

Direction of true dip of 

foreset reflectors in 

Torok Formation 

Dipmeter data from 

sandstone of 

Nanushuk Group 

Figure 23. Plot of directional data from seismic foreset dips in the Torok Formation, dipmeter data from 

Nanushuk Group sandstones, direction of Nanushuk progradation from outcrop (Chapman and Sable, 1960), 

and direction of plunge of symmetrically filled nonmarine channels of the Nanushuk Group as measured 

by Ahlbrandt and others (1979) and Huffman and others (this volume) (modified from Bird and Andrews, 

1979). 

lines (excluding the Turonian-shale-filled Simpson can-

_yon, which was probably cut as a submarine canyon in 

late Nanushuk time). However, there are probably small 

gullies or slumps that may be feeder channels for deepwater sands or turbidites at the base of the foreset or in 

bottomset beds. Many irregularities, in fact, are visible 

on the seismic lines, suggesting slumping and truncation 

of beds (figs. 19 and 20). 

The width of the basin shelf during Nanushuk time 

can be estimated from the cross sections. The distance 

from the marine-nonmarine boundary to the apparent timeequivalent shelf break on the sections, adjusted for depositional strike, ranges from 75 to 150 krn. Undoubtedly, 

the shelf was much narrower along the east-west-trending 

shoreline southeast of NPRA. Seventy-five to 150 krn 

seems reasonable when compared with modem continental 

56 Geology of the Nanushuk Group and Related Rocks, Alaska 

shelves of passive plate margins. For instance, the continental shelf of the Atlantic-type margin ranges in width 

from 30 to more than 300 km (Heezen, 1964, p. 13). 

However, as mentioned previously, the present shelf 

breaks are relict Pleistocene features, and shelves were 

obviously narrower during Pleistocene low stands of sea 

level. 

HYDROCARBON-RESERVOIR SAND 

DEVELOPMENT 

Factors reducing the porosity and permeability of 

Nanushuk sandstones are clay content, alteration of immature mineral grains, compaction, and cementation. These 

factors are all influenced by depositional environments 

and depositional history. The following interpretation and 
200 Kl LO METERS 

Figure 24. Inferred paleogeographic maps of a part of northern Alaska and adjacent areas. A, at midNanushuk time; and B, at latest Nanushuk time (maximum regression). 

line of reasoning regarding depositional history and basin 

shape may be useful in outlining areas of more favorable 

reservoir development. 

The east-northeast delta progradation is thought to 

have been controlled by a large source area to the southwest, by subsidence of the Colville basin, and by the silling effect of the passive Barrow arch. The presence of 

the arch would also explain the low marine energy and 

relative paucity of good marine fauna in much of the 

Nanushuk, especially to the west. It is interpreted that 

Subsurface Correlations and Depositional History 57 
165° 162° 159° 156° 153° 150° 

0 CEAN 

71° /Point Barrow 

V?<s 

V?s ""' EXPLANATION ' \ 

\ 

2.9 0 

3.6°) 3.23 /-,0 ........_ o Well 

\ ( \ / 3. 35 ......... 0 3./ 3 02 

' .._,. ' 

0

3.17 . 3.96 '-. "-. ..._ __ , 0 o · 

X Outcrop section 

' ......... ..._____ " 3.14 '.......... .....__ ......... ...__ '-. 

I 

......... -ll...___ .......... 0 ........ 

....._ --- --0 -..._ ' ....._ 

X 4 .25----~---

........ ..._ --- X I --- ~...._3 9b-+ Cape Lisburne -...:>-~- o - o o

4

·

21 

•X X ~miat 4.05 ~ 

······· ~ ................. .f.,-· ··· ~--··· ·~~~~;~~~ --.~- -··· ··..... . X X ~.42 :' 0X 04.05 , 

Southern ' limit of lv£Jj> ··,· ·? ............ .. . o >.<. X ___ )f../ 

L_ . .........-- · ....---.._,~~Ji'-<1 Nanushuk I Group · ........ r·o;:~··· ... ~. .-" 

68° 

8 R 0 0 K s '{~(/~ I' R A N G E 

"'--._, 

50 100 150 KILOMETERS 

figure 25. Isopach map of average seismic velocities of sandstones in the lower 150 m of the Nanushuk 

Group. Velocities are in kilometers per second. Isopach interval is 0.25 km/s. 

as the arch continued to subside, it reached a level that · 

reduced or eliminated the silling effect on the Colville 

basin, presumably in late Nanushuk time. The subsidence 

of the passive arch resulted in more open marine conditions and increased wave energy, thereby permitting 

thicker shoreface sands to develop. The increased wave 

energy may account for the zone of thicker, better sorted 

sand along the Umiat-Inigok-Simpson shoreline trend 

(fig. 17). Also, an important aspect of this favorable reservoir sandstone trend is the contribution to the deltaic 

system by the sands of the Umiat delta on the south 

(Ahlbrandt and others, 1979). These sands, which are relatively richer in quartz (although still classified as lithic 

sands) than sands of the Corwin delta (Ahlbrandt and 

others, 1979, p. 24), were brought into the basin by northtrending fluvial systems in late Nanushuk time. Longshore 

currents are postulated to have then transported the sands 

to the north-northwest across the broad prodelta shelf of 

the larger northeasterly prograding western delta, depositing the sand either along that shoreline or as offshore 

bars. These events would account for the greater percentage of sandstone in the eastern part of the delta compared 

with that in the western part, which was deposited by 

a sand-poor delta. Another explanation for the higher sand 

58 Geology of the Nanushuk Group and Related Rocks, Alaska 

percentage in the northeastern part of NPRA may be the 

contribution from the western fluvial system late in 

Nanushuk time. Evidence for such a fluvial system is 

lacking because of erosion of that part of the section in 

areas to the west (fig. 18). A third possibility may be 

simply the change from a river-dominated to a wavedominated delta setting related to lowering of the sill (Barrow arch). This change would concentrate sands into 

thicker shoreface sequences. 

Thus, the sandstones occurring along the Umiat-Inigok-Simpson shoreline trend and extending east several 

kilometers beyond the eastern extent of subaerial plain 

deposits (fig. 21) are likely to be in the zone of highest 

marine energy, and hence to be more winnowed, less 

clayey sands. In addition, the northern part of this trend 

is likely to contain mineralogically more mature sands because the labile constituents would have been removed 

by winnowing and abrasion due to longer transport distance or higher wave energy in that area. Partly offsetting 

these favorable factors may be a slight decrease in grain 

size, owing to increased distance from the sediment 

source, especially the Umiat delta. Permeability retention 

is generally reduced in finer grained sandstones. 

A comparative measure of porosity reduction by 
compaction can be correlated with acoustic-velocity trends 

of these rocks. Increases in velocities are caused by compaction (greater depth of burial) and (or) cementation. Interval transit times of all comparable lithologies in the 

Nanushuk Group as determined from transit-time logs 

show a south-southwest increase in velocities. (Data are 

from only the post-1960 wells; transit-time log surveys 

were not available for the old U.S. Navy wells.) The average velocities of sandstones in the lower 150 m of the 

Nanushuk range from about 2.9 km/s (9,500 ft/s) in the 

East Simpson-I and J. W. Dalton-1 wells on the north 

to 4.0 km/s (13,000 ftls) and higher in the Tunalik, Seabee (Umiat), and Sinclair Little Twist (southwest of 

Umiat) wells (fig. 25). Exceptions to the trend and lower 

velocity porous zones do occur in an otherwise higher 

velocity sandstone, as in the Umiat area where porosities 

are as high as 20 percent (Fox and others, 1979, p. 23). 

Higher acoustic velocities for the Nanushuk in the 

southern or southwestern areas suggest that it probably 

had been more deeply buried in that area. Compressional 

tectonic forces could have caused some of this increase, 

but this increase is also apparent in areas of undeformed 

rocks. In general, therefore, reservoir potential of 

Nanushuk sandstones is expected to be lower in the southem and southwestern areas. 

In summary, on the basis of regional trends, the 

most favorable area for thicker, better sorted, and cleaner 

Nanushuk sandstones with better reservoir properties appears to be in northeastern NPRA along a broad belt between the Umiat and Simpson wells. Within this belt, 

better porosity and permeability would be expected in the 

north because of less compaction in that area. 

SUMMARY 

The Nanushuk Group of Albian to Cenomanian Age 

is a deltaic deposit in which a main delta system, the 

Corwin delta, prograded across the Colville basin from 

west-southwest to east-northeast. The provenance was 

probably a large landmass to the southwest in the area 

of the present Chukchi Sea or beyond. The Colville basin 

was bounded on the north by the passive Barrow arch, 

which was covered by the deltaic system late in Nanushuk 

time. The ancestral Brooks Range bounded the south side 

of the asymmetric basin and was a secondary but important source area. The depositional system represented by 

deposits from this southern source has been called the 

Umiat delta. 

Seismic data indicate that the Nanushuk is laterally 

equivalent to and prograded over shelf, slope, and basinal 

deposits represented by the Torok Formation. Measurements of these geomorphic features on seismic sections 

indicate that ( 1) the basin slope angle generally steepened 

from less than 2° on the west to as much as 6° on the 

east, (2) the depth of water in the deep part of the basin 

in the area of study ranged from 450 to 900 m plus the 

amount of compaction, and (3) the prodelta shelf was 75-

150 km wide. The foreset and bottomset beds in the Torok 

downlap onto or near the Neocomian pebble shale unit 

in the northern part of NPRA. The hiatus or very thin 

interval of Torok between the downlapping bottomset beds 

and the pebble shale represents part of Neocomian, all 

of Aptian, and a large part of Albian time. 

The total incremental stratigraphic rise (a measure 

of relative sea-level rise and (or) basin subsidence) of the 

base of the Nanushuk from the Tunalik-1 well on the 

west to the Atigaru Point-1 well on the east, a distance 

of 350 km, is about 2,100 m. However, subsidence was 

not uniform throughout the basin, as indicated by less 

and (or) later subsidence of the Barrow arch. 

Because of the relatively rapid subsidence on the 

south side of the basin, the southern-source deltas probably did not extend too far to the north. However, the 

growth of the Umiat delta just southeast of NPRA coincided with the presence of the prodelta shelf of the larger, 

easterly prograding western delta so that the southerly derived clastics could be distributed along the north-northwest-trending shoreline and shelf, probably by longshore 

currents. It is postulated that during late Nanushuk time, 

wave action and longshore currents increased because of 

the more open marine conditions that prevailed after the 

Barrow arch subsided enough to lose its silling effect on 

the Colville basin. It is further postulated that prior to 

that time marine circulation was somewhat restricted in 

the Colville basin. This restricted circulation would account for the low marine energy and the relative paucity 

of good marine fauna in much of the Nanushuk in its 

western part. 

Thus, the eastern part of NPRA along the alinement 

of the Umiat, Inigok, and Simpson wells has thicker, better sorted, cleaner sandstone units. In addition, the sand 

contribution from the Umiat delta was relatively richer 

in quartz than sands from the Corwin delta, especially 

farther north where labile constituents were more likely 

to have been removed by winnowing and abrasion. Alternate hypotheses for the better sandstone development 

along that trend may be simply that increased wave energy 

could have concentrated sand into thicker units, or that 

a western fluvial system in late Nanushuk time may have 

been responsible for the increase in sand percentage. Evidence for such a fluvial system is lacking because of erosion of that part of the section to the west. 

The northeastern part of NPRA has been less deeply 

buried than areas to the southwest, as indicated by transittime logs. The resulting reduced compaction coupled with 

deposition of better winnowed, although finer grained, 

sandstone beds in that area makes it a more favorable 

area for petroleum reservoir development. 

Subsurface Correlations and Depositional History 59 

Depositional and Sedimentologic Factors Affecting 

the Reservoir Potential of the Cretaceous 

Nanushuk Group, Central North Slope, Alaska 

By A. Curtis Huffman, Jr., Thomas S. Ahlbrandt1 , Ira Pasternack2 , Gary D. Stricker, and 

james E. Fox3 

INTRODUCTION 

The Nanushuk Group of Albian to Cenomanian Age 

is a regressive sequence of marine, transitional, and nonmarine deposits exposed in an outcrop belt 30 to 50 km 

wide and approximately 650 km long in the Arctic Foothills Province, North Slope, Alaska (fig. 1). Field and 

laboratory studies suggest that the Nanushuk Group was 

deposited in deltaic systems throughout most of the length 

of the outcrop belt and that it can be divided into two 

sedimentary provinces separated roughly by the 157° W. 

longitude line. West of long 157° W., deposition was in 

the elongate, river-dominated Corwin delta (Ahlbrandt and 

others, 1979), which prograded to the northeast from the 

vicinity of the De Long Mountains, Tigara uplift, and 

Herald arch (fig. 26). Along the central North Slope, from 

long 157° W. to the Sagavanirktok River, deposition took 

place in the lobate to elongate river-dominated Umiat 

delta (Ahlbrandt and others, 1979), which prograded to 

the north, northwest, and northeast from the vicinity of 

the Endicott Mountains. 

The Corwin delta was studied by a team of U.S. 

Geological Survey geologists during the 1977 field season, and the results were reported in Ahlbrandt and others 

(1979), Ahlbrandt and Huffman (1978), and Huffman and 

others (1981a). Other investigations of the Nanushuk 

Group outcrop of the western North Slope were done by 

Chapman and Sable (1960), Smiley (1966, 1969b), and 

Roehler and Stricker ( 1979). These studies described the 

Nanushuk of the western area as having a high mud content, few stacked sandstone bodies, and no extensive 

delta-front sandstones, and that it prograded and was thus 

time transgressive to the north and east from the Corwin 

Bluff area. Recent work in the Kaolak well on the subsurface extension of this delta has resulted in further 

documentation of the delta's muddy character and the interpretation that, at this location, concentrations of sand 

are in distributary mouth bars (Pasternack, 1981). 

1

Hereford, Colo. 

2

AMOCO Production Company, Denver, Colo. 

3

South Dakota School of Mines, Rapid City. 

The Umiat delta of the central North Slope was 

studied during the 1978 field season (Huffman and 

Ahlbrandt, 1979) using methods identical to those described by Ahlbrandt and others ( 1979). Stratigraphic sections totaling approximately 8,000 m were measured and 

sampled in detail (fig. 1) and are described in Huffman 

and others ( 1981 b). Various aspects of these sections are 

summarized in table 5, in addition to being discussed elsewhere in this volume. Previous studies of the Nanushuk 

outcrop of the central North Slope were reported on by 

Gryc and others (1956), Detterman and others (1963), 

Chapman and others ( 1964), Brosge and Whittington 

( 1966), and Smiley ( 1969a). Fisher and others ( 1969) 

used a deltaic model in discussing the Nanushuk and concluded that it was deposited in a high-constructive delta. 

The present investigation confirms this model and 

suggests a lobate-to-elongate form. 

Along the outcrop belt northeast of the Marmot 

syncline measured section (fig. 1), much of the Nanushuk 

apparently grades laterally into a thick sequence of interbedded mudstone and sandstone having many characteristics of a turbidite deposit. East of the Ivishak River, these 

rocks were mapped as the Ignek Formation by Keller and 

others (1961), but the name was later abandoned (Detterman and others, 197 5). The lower member of the Ignek 

was assigned to the Nanushuk Group and the upper member to the Colville Group by them. No detailed work was 

done in the Ivishak River area as a part of this project; 

however, reconnaissance inspections and sampling at several localities indicate that these deposits represent a deltaic to deep-marine sequence. 

In the subsurface north of the outcrop belt, the 

Umiat delta rapidly loses its identity. The belt of high 

sand content extending from Umiat to the vicinity of the 

Ikpikpuk well (fig. 26) appears to be composed primarily 

of coastal barrier sandstone bodies (a general term used 

here to denote coastal and shallow marine sandstone 

bodies deposited along a regressive strand line). The 

buildup of sandstone in the South Simpson-I well area 

(fig. 1) probably reflects the interaction of several factors 

such as deposition on an extension of the Corwin delta 

and longshore drift from the southeast. 

Depositional and Sedimentologic Factors Affecting Reservoir Potential 61 
168(! 

EXPLANATION 

APPROXIMATE OUTLINE OF BASAL PART OF 

NANUSHUK GROUP 

-10- SANDSTONE PERCENTAGE CONTOUR-Contour 

interval 5 percent 

X 

/4--. 

0 10 25 Percent of reading 

~}d 

TEST WELL 

MEASURED SECTION 

PUBLISHED SECTION 

FLUVIAL PALEOTRANSPORT DIRECTION 

APPROXIMATE AREA OF COASTAL-BARRIER 

SANDSTONE BUILDUP 

50 

Figure 26. Map showing sand-percentage contours, fluvial transport directions, and area of coastal-barrier sandstone buildup, Nanushuk Group, North Slope. 

DEPOSITIONAL ENVIRONMENTS 

South-Central North Slope 

Where examined at the surface in the southern part 

of the central North Slope, the lower part of the Nanushuk 

Group consists of thick delta-front sandstones and siltstones interbedded with and gradationally overlying prodelta mudstone and siltstone of the Torok Formation, as 

shown schematically in figure 27. These delta-front sandstones coarsen upward and are in tum overlain by a thick 

62 Geology of the Nanushuk Group and Related Rocks, Alaska 

transitional sequence of alternating marine and nonmarine 

deposits representing a number of constructive delta cycles 

separated by transgressive marine deposits as defined by 

LeBlanc (1972). Thick nonmarine deposits of the delta 

plain cap the whole progradational sequence. Along the 

northern edge of the outcrop belt and in the subsurface, 

delta-plain deposits are overlain by a transitional sequence 

(Ninuluk Formation and Niakogon Tongue of the Chandler Formation, of the Nanushuk Group) and subsequently 

by the marine deposits of the Upper Cretaceous Colville 

Group. 
Figure 26. Continued 

The following discussion briefly describes the major 

depositional environments (fig. 28) associated with the 

Umiat delta. In ascending stratigraphic order, these environments are: prodelta, distal bar, distributary mouth bar 

and coastal barrier, interdistributary fill, and delta plain. 

Environments associated with the succeeding transgressive 

facies are then briefly discussed. Terminology used in 

these descriptions of depositional environments is modified from Coleman (1976); "delta front" is used as a general term including the distal bar and distributary mouth 

bar environments. 

Prodelta 

Prodelta deposits are well exposed at only two of 

the measured sections, Tuktu Bluff and Marmot Syncline 

(fig. 1). At both locations they are indistinguishable from 

the underlying shelf deposits and are included in the 

Torok Formation. They consist of medium-gray to olivegray laminated shale and mudstone with thin interbedded 

siltstone layers that are iron stained and ripple laminated 

in places. Fossils and trace fossils are rare, possibly due 

to unsatisfactory living conditions produced by high 

Depositional and Sedimentologic Factors Affecting Reservoir Potential 63 
Amount 

Table 5. Synthesis of resource-related data from measured 

[M, marine; T, transitional; NM, nonmarine; ss, sandstone; No., number; 

Total Total 

Measured section Environment of section Base of section Top of section 

measured 

thickness 

(m) 

ss 

thickness 

(m) 

Ss 

(per 

exposed lat N. long W. lat N. long W. cent) 

(percent) 

Carbon Creek------- M,T,NM----- 11 69°13 I 26" 158°45 1 35' 69°14 1 51" 158°44 1 15" 969 20 2 

Section Creek------ M,T-------- 27 69°04 1 33" 156°00' 16" 69°03 1 31" 155°58 I 16" 161 20 12 

Awuna River-------- T, NM------ 65 69°01 1 27" 155°52'51" 69°021 03" 155°5 21 08" 169 30 18 

Kurupa anticline--- M,T,NM----- 36 68°5 21 44" 155°26 1 21" 68°56 1 16" 155°021 23" 1832 413 23 

Killik type-------- M,T-------- 97 68°51 1 07" 153°22 103" 68°52 1 05" 153°23 1 34" 313 237 76 

Tuktu Bluff-------- M,T,NM----- 22 68°43 1 30" 152°15 1 50" 68°46 1 10" 152°06 1 54" 1868 385 21 

Type Grandstand---- M,T-------- 44 68°56 1 47" 151°13'20" 68°55 1 46" 151°13'03" 812 298 37 

Roof Top anticline- M,T-------- 58 68°50 1 37" 150°33 I 31" 68°50 1 30" 150°34 I 49" 195 103 53 

Arc Mountain------- M,T-------- 97 68°40 1 09" 150°36 1 31" 68°36 1 57" 150°33 I 28" 516 205 40 

Marmot syncline---- M,T,NM----- 47 68°43 1 37" 149°01' 11" 68°44 1 34" 149°03 1 56" 1060 271 26 

Lupine River------- M,T-------- 21 69°05 1 41" 148°46 1 48" 69°05' 23" 148°45 1 19" 563 91 16 

sedimentation rates. Grain size increases upward as does 

the thickness and number of siltstone units and the amount 

of included carbonaceous plant debris. The total thickness 

of prodelta sediments was not determined but probably 

is several hundred meters. 

Distal Bar 

The distal bar deposits (fig. 28) are very fine 

grained, poorly sorted sandstones and siltstones as thick 

as 25 m, interbedded with mudstone and shale intervals 

as thick as 90 m. The sandstones and siltstones are dark 

gray to olive gray, as are the mudstones and shales. The 

predominant sedimentary structures are laminations, but 

the sandstones may be rippled or rarely crossbedded. Carbonaceous plant debris is a common constituent of both 

the sandstones and mudstones. Both vertical and horizontal burrows (for example, Cruziana and Planolites) and 

bioturbated beds are common. Molds and casts of 

pelecypods (Pleuromya, Corbicula, and Tellinimerids) are 

scarce but widespread. Although the distal-bar deposits 

could not be traced their full length in outcrop, they extend several kilometers parallel to depositional strike 

(roughly east-west) where exposed on the Tuktu escarpment (fig. 26). In the subsurface 75 km to the north, 

such as in the Umiat and Square Lake wells, there is 

no evidence of thick delta-front sandstones or siltstones. 

Distributary Mouth Bar and 

Coastal Barrier 

The primary sites of clastic sedimentation in the 

southern part of the Umiat delta were distributary mouth 

bars (fig. 28). These fine- to medium-grained sandstone 

64 Geology of the Nanushuk Group and Related Rocks, Alaska 

bodies are 10 to 25 m thick and are generally lenticular 

in cross section, although they may coalesce laterally to 

form sheets of sandstone several tens of kilometers wide. 

Their down-dip extent to the north is unknown because 

of limited outcrop and lack of suitable marker horizons. 

Grain size and porosity increase upward in individual 

bars. 

The distributary mouth bars commonly have lowto moderate-angle trough crossbedding in the lower and 

middle parts and some parallel bedding in the upper part. 

Bases and tops are variable; the basal contact of a bar 

with a distal-bar deposit produced by normal regressive 

succession is typically gradational, whereas the contact 

with fine-grained deposits of the prodelta or interdistributary bay is commonly marked by contorted bedding probably caused by rapid sediment loading and an abrupt 

change in grain size and energy regime (fig. 27). In a 

normal regressive or prograding succession, the bar deposits are overlain by fluvial deposits of the distributary 

channel at a scour surface or by fine-grained interdistributary bay sediments. Locally they may be overlain by prodelta or delta-front mudstone and siltstone at a sharp erosional surface, indicating a transgressive event and the 

beginning of another delta cycle. 

Distributary mouth bars of the Umiat delta contain 

a wide variety of both trace fossils and megafossils. The 

fossil assemblages change with the energy of the environment so that the lower bar assemblage is similar, to that 

of the distal bar, and the trace fossils Cruziana and Planolites are common. The higher energy deposits are characterized by Scolithos, Diplocraterion, Rhizocorallium, 

Gyrochorte, and Helicodromites. Carbonaceous plant 

fragments increase in both size and quantity upward in 

the bars. Both pelecypods (Inoceramus, Corbicula, 

Nucula, Pecten, and Tellinimirids) and scaphopods (Ditrupa) are common in the upper parts as well. Stacked 
sections of Nanushuk Group rocks (1978), North Slope, Alaska 

mD, millidarcy; leaders(--), not observed] 

No. Average Maximum ss Porosity (Eercent) ss ss thickness (m) Average 

beds thickness and environ- Range and 

(m) ment (no. of 

measurements) 

14 1.4 2.4 T 4.3-14.5 8.84 (7) 

8 2.5 8. 2 T 4.6-14.5 7.9 (3) 

17 1.8 9.7 NM 5.5-11.1 7.25 (4) 

143 3.0 33.8 T 2.6-17.7 7.34 (48) 

63 3.8 69.2 T 4.1-17.2 9.53 (10) 

78 4.9 43.6 T 3.4-16.4 7.06 (39) 

48 6.2 32.6 T 2.8-14.9 9.79 (31) 

13 7.9 21.3 M 5.4-10.2 7.82 (6) 

55 3.7 67.1 M 2.7- 8.4 5.22 (18) 

63 4.3 29.5 T 2.3- 8.0 4.69 ( 29) 

57 1.6 9.7 T 3.7- 7.2 5.5 (13) 

regressive beach sequences are present within the Umiat 

delta complex such as at the Kurupa anticline measured 

section and may record periods of increased wave and 

current activity as well as decreased sediment supply or 

periods of more rapid basin subsidence. Net-sand and 

sand-percentage contour maps (fig. 26) as well as electric 

logs of test wells just north of the outcrop belt suggest 

that deposition was in coastal-barrier rather than deltafront sandstone bodies (Molenaar, this volume), also probably due to increased wave and current activity and a 

change in the balance of sediment supply to subsidence. 

lnterdistributary 

Embayments between active delta lobes (interdeltaic 

subembayments of Fisher and McGowen, 1967) or between distributaries on a lobe are filled with deposits of 

organic-rich shale and mudstone; crevasse-splay deposits 

of siltstone and sandstone; and claystone, siltstone, and 

coal deposited in marsh and swamp environments (fig. 

28). Because large amounts of interdistributary bay sediments are fine grained, they are less resistant to erosion 

and are therefore poorly exposed, making accurate thickness and lateral extent measurements difficult to obtain. 

Fine-grained bay-fill deposits consist of laminated 

and rippled, bioturbated, brownish-black to olive-gray carbonaceous shales, mudstones, and siltstones that range in 

thickness from 10m to more than 100m. The areal extent 

of these deposits was not determined. Overlying and interbedded with the fine-grained deposits are crossbedded and 

ripple-laminated, olive-gray to light-gray sandy siltstones 

and fine-grained sandstones deposited in crevasse splays 

and small distributary channels (fig. 27). These sandstones 

are typically 3 to 6 m thick, but may be as much as 

30 m thick, and contain Corbicula and Unio. Marsh deposits of claystone, siltstone, and coal, which may be 

Permeability (mD) Coal beds 

Average Average 

Range and Thickness No. thickness 

(no. of (m) (m) 

measurements) 

0.07-2.2 0.67 (7) 

0.11-6.6 2. 29 (3) 

0.099-0.29 .18 (3) 0.5 3 0.17 

o. 2-374 15.77 (45) 12.75 16 0.79 

0.04-2.9 4.94 (8) 1.5 4 0.38 

0.7-320 23.58 ( 25) 0.15 1 0.15 

0.4-89 15.31 (30) 0.45 3 0.15 

0.13-4.4 1.03 (6) 

0.5-3.9 .44 (18) 1.5 2 0.75 

0.3-13 .71 ( 28) 

0.3-0.36 .19 (13) 

rooted in crevasse-splay sandstones, are typically less than 

1 m and rarely more than 3 m thick. Complete sequences 

of bay-fill environments were seldom observed in the 

Umiat delta primarily because of the paucity of good exposure but also because of periodic interruptions of local 

sedimentation and progradation, resulting in deltaic cycles 

similar to those described by Coleman and Gagliano 

(1964). Three or more cycles involving bay-fill sediments 

are apparent in the transitional part of the section at 

Kurupa anticline and Tuktu Bluff. 

Delta Plain 

Sediments deposited on the delta plain of the Umiat 

delta are predominantly fluvial-channel and overbank deposits and minor fine-grained lacustrine and swamp or 

marsh deposits (fig. 28). In the transitional part of the 

section, these nonmarine deposits are interbedded with interdistributary bay and distributary mouth bar sequences. 

The channel deposits are typically light-gray to 

light-olive-gray, fine- to medium-grained, trough-crossbedded sandstones or conglomeratic sandstones. They 

range in thickness from 1 to 25 m and were deposited 

by both braided and meandering streams. The contact between the transitional and nonmarine parts of the 

Nanushuk is typically marked by high-energy fluvial conglomerate beds (fig. 27) as thick as 14m. These conglomerates are composed predominantly of well-rounded white 

quartz and gray chert clasts as large as 30 em (but more 

commonly 4 to 8 em) and contain large plant fragments. 

Transport-direction measurements on trough and channel 

axes taken in the fluvial deposits (fig. 1) indicate a general 

northerly progradation from the vicinity of the Endicott 

Mountains, as do the sand-percentage contours of figure 

26. 

Poorly exposed intervals of fine-grained overbank 

Depositional and Sedimentologic Factors Affecting Reservoir Potential 65 
TRACE FOSSILS MEGAFOSSILS 

COLVILLE Marine GROUP -----------

Transgressive sandstone 

Swamp Coal Cii Marsh ---------- c 

0 Distributary-mouth bar Plant fragments ·.;:::; 

"iii 

c ------------ Scolithos Inoceramus ro Diplocraterion ~ Prodelta Areniculites 

Planolites -------- Cruziana .,, Transgressive sandstone Bone fragments swan1P _____ Pelecypods- Unio Coal 

Lacustrine 

Meand8rin9-- Plant fragments 

fluvial channel -------

Q) 

c Overbank -~ 

E ---------- c Braided fluvial channel Plant fragments 0 

z ----------

Overbank Teredo borings Logs 

------

Braided fluvial Plant fragments 

channel 

------

Swamp 

Coal 

Marsh 

------

Distributarymouth 

c.. bar :::> 

0 ro -----

cc c Swamp 

~ 0 Coal Plant fragments ·.;:::; 

~ "iii Crevasse Gastropods :::> c I ro splay lnterdistributary Pelecypods (/) ~ :::> bay fill Corbicula z 

<X: 

z Bay fill 

Distributary channel 

Plant fragments 

---------- Scolithos Ammonites ---- Distributary- } Diplocraterion 

Rhizocorallium Scaphopods 

mouth Gyrochorte Ditrupa 

bar Cruziana Pelecypods 

Helicodromites Inoceramus Q) ------- Pecten -~ DistribUtary-mouth 

Planolites 

a; Corbicula 

~ -------- bar Nucula 

Plant fragments 

Distal bar } Cruziana Ammonites 

Planolites Pelecypods 

Corbicula 

Pleuromya 

EXPLANATION Tellinimerids z ~Q Q) Sedimentary structure Lithology of- Prodelta cc<X: .!: 

0~ ro Crossbedding Conglomerate f-CC ~ 0 

LL. Parallel bedding Sandstone 

Ripple marks Siltstone 

Climbing ripples Mudstone 

Contorted bedding Shale, claystone 

Figure 27. Generalized stratigraphic column of the Nanushuk Group, central North 

Slope. 

deposits as much as 150 m thick are composed primarily 

of medium-gray to dark-yellowish-brown siltstones and 

mudstones that are laminated, rippled, and rarely contorted. Burrowing and bioturbation and significant 

66 Geology of the Nanushuk Group and Related Rocks, Alaska 

amounts of carbonaceous plant debris are common. Interbedded with the mudstones and siltstones are thin (10-30 

em) crossbedded and ripple-laminated silty sandstones. 

Marsh and swamp deposits commonly consist of lami-
Figure 28. Schematic diagram showing terminology and major depositional environments discussed in text. 

nated and rippled siltstones and mudstone interbedded 

with claystone, carbonaceous shale, and coal beds. Individual coal beds rarely exceed 2m. 

Marine Transgression 

A major regional transgression in the early part of 

Late Cretaceous time (Detterman and others, 1963) halted 

growth of the Nanushuk deltas. In the outcrop area this 

transgression is characterized by intertonguing marine and 

nonmarine deposits similar in gross aspect to the transitional beds in the regressive phase of Nanushuk deposition. However, there are two major differences: the presence of abundant bentonitic shale and bentonite beds (7-

30 em thick), and the high sandstone content (37 percent 

at Ninuluk Bluff). Where observed at Ninuluk Bluff, several thin (30--60 em) beds of crossbedded, conglomeratic 

sandstone containing both pelecypod and bone fragments 

are interpreted to be transgressive sandstones that record 

several pulses that occurred during the regional transgression. Most of the marine intervals are typical regressive 

distributary mouth bar and coastal-barrier sandstones interbedded with fine-grained prodelta and bay-fill deposits. 

The intertongued nonmarine sediments are predominantly 

fluvial channel sandstones, overbank mudstones, and coal 

beds (30 em to 1 m thick) deposited on the delta plain. 

This sequence of intertonguing sandy marine and coalbearing nonmarine units is overlain by marine bentonitic 

shales and thin sandstones of the Seabee Formation of 

the Upper Cretaceous Colville Group. Detterman and 

others ( 1963) recognized an angular relationship between 

the Nanushuk and Colville Groups in the Chandler River 

region. We were unable to confirm this relationship during the course of this study. 

Northeastern North Slope 

Nanushuk sediments change facies in a short distance along the outcrop belt to the northeast of Marmot 

syncline and apparently grade into a thick sequence of 

mudstone and turbidites that were mapped as the lower 

Depositional and Sedimentologic Factors Affecting Reservoir Potential 67 
member of the Ignek Formation east of the Ivishak River 

by Keller and others ( 1961). The lower member of the 

Ignek (now the Nanushuk) was described as being composed of subgraywacke-type sandstone, siltstone, shale, 

and less commonly conglomerate, and the sandstone as 

being locally crossbedded and ripple marked, and characterized by "curly bedding or hassock structure" (Keller 

and others, 1961, p. 204). 

Reconnaissance examination and sampling of the 

outcrops along the Saviukviayak and Ivishak Rivers and 

Gilead Creek (fig. 26) by Ahlbrandt supports the correlation of at least a part of these deposits with the Nanushuk 

Group based on the occurrence of Nanushuk-like fluvial 

lithologies in the uppermost part. Although some of the 

exposures are structurally complex, the general sequence 

includes a thin nonmarine section overlying thick shallowmarine deposits, which in turn overlie a thick deep-marine 

turbidite sequence. Observed lithologies in the turbidite 

deposits include thick mudstone or silty claystone sequences and very fine grained sandstones. Sedimentary 

structures in sandstone and siltstone beds include parallel 

laminations, current ripples and climbing ripples, sole 

markings, and rare low-angle cross bedding. Parts of 

Bouma cycles, especially the C, D, and E intervals, are 

common. All indications are that the turbidites were 

deposited in deep-marine environments and are approximately time equivalent to the lower deltaic Nanushuk deposits (Keller and others, 1961). These observations help 

to establish the position of the shelf break during 

Nanushuk deposition. 

Farther to the northeast, in the vicinity of the Sadlerochit Mountains and the Jago River (fig. 26), Detterman and others (1975) measured reference sections in 

what were thought to be Nanushuk or equivalent rocks. 

Both their Arctic Creek and Sabbath Creek sections were 

examined and sampled during the present study. Lithologically, there is little resemblance between these sections 

and the typical Nanushuk Group sections other than general environments of deposition. Detailed petrographic and 

paleontologic analyses have not yet been completed, however, and any conclusions would be premature. 

North-Central North Slope 

The Umiat delta as an entity extends a very short 

distance northward into the subsurface. The actual shape 

and northward extent of the delta as well as the effect 

and influence of the Corwin delta are open to various 

interpretations (for example, Molenaar, this volume). The 

belt of relatively high sandstone concentrations extending 

generally northward from the vicinity of Umiat to the Ikpikpuk well and the apparent sandstone buildup in the 

vicinity of the South Simpson-I well (fig. 1) are both 

probably due in part to redistribution of sand from the 

68 Geology of the Nanushuk Group and Related Rocks, Alaska 

Umiat delta as well as to some influence from the Corwin 

delta. 

Petrographic results reported by Bartsch-Winkler 

( I979) indicate that in most of the wells sampled, the 

sandstones in the lower parts of the section bear a strong 

compositional similarity to Corwin delta sediments. 

Higher in the section, however, there is a marked change, 

and the sandstones show stronger affinities to the Umiat 

delta sediment types. Although the data are not yet complete enough to warrant definitive conclusions, most of 

the sampled wells also suggest a degree of mixing of sediment types in some beds as well as possible interbedding 

of the two sediment types. 

Several explanations for the different lithologies 

satisfying both the outcrop and petrographic data are possible. The most probable at this time is that the Umiat 

delta prograded northward onto a shelf built in part by 

sediment from the Corwin delta. In the vicinity of Umiat, 

the Umiat delta was subjected to strong wave and current 

activity that tended to move sediment to the northwest. 

A coastal-barrier-type shoreline was built by sediment 

transported to the coast by Corwin streams and by sediment transported northward by longshore drift from the 

Umiat delta. 

The sand buildup in the vicinity of the South 

Simpson-I well was also probably due to an interaction 

between Corwin deltaic sedimentation and sand redistribution from the Umiat delta. The reason for the actual concentration of sand in this area is unknown, but it may 

have been due at least in part to the effects of the Barrow 

arch and the Meade arch on wave energy and current directions as well as drainage patterns. 

RESERVOIR CHARACTERISTICS 

Both the quantity and quality of petroleum reservoir 

rocks depends a great deal on geographic and stratigraphic 

position in a deltaic system, because the higher energy 

environments generally produce the more porous and 

permeable sandstone deposits. In a river-dominated delta 

such as the lobate to elongate Umiat delta, the highest 

energy environments are the fluvial channels and the distributary mouth bars which are concentrated on the depositional lobes. North of the delta, between Umiat and the 

South Simpson-I well, the highest energy deposits are 

the coastal-barrier sandstones. 

In the sections measured, distributary mouth bars 

and coastal-barrier deposits make up 44 percent of the 

total sandstone. These deposits have an average bed thickness of 6.5 m and considerable lateral extent, whereas 

the fluvial-channel deposits that compose 25 percent of 

the total sandstone have an average bed thickness of 4. 7 

m and limited lateral extent. Porosity values within distributary mouth bars range from 2. 8 to 1 7. 2 percent and 
400 

300 

200 

100 

(/) 80 G 6o a: 40 

~ 30 

j 20 

~ 

~ 1~ 

>-' 6 f--

:::i 4 

co 3 <t: 

UJ 2 ~ 

a: 

~ 1.0 a: 0.8 

<{ 0 .6 

0.4 

0 .3 

0 .2 

0.1 

0.08 

0 .06 

0.04 

0.03 

0.02 

• 

• 

• 

• 

• 

• • 

• 

• 

·~·. 

,

. :· 

~ . 

• 

• . ~ ....... 

.. ..... .... 

POROSITY, IN PERCENT 

A 

• 

• 

• • 

• 

• 

• 

• •• 

• 

• 

• • 

• 

.... 

• • 

.. , • • f ... 

• 

• •• • ~ •" . •. .., • 

. ... 

•• 

• 

• 

5 10 

• 

• 

• 

15 

• MARINE 

.a. NONMARINE 

B 

20 25 30 

POROSITY, IN PERCENT 

35 

Figure 29. Porosity versus permeability plots for outcrop samples from A, Corwin delta (western); 8, Umiat 

delta (eastern). Permeability measurements were generally not made on samples from the Corwin delta 

having less than 8 percent porosity. 

average 7.6 percent, and permeability values range from 

0.02 to 245 mD (millidarcies) and average 6.3 mD. Several 10--15-m-thick intervals of nearly unconsolidated sand 

in the Kurupa anticline and Killik type sections are interpreted as stacked foreshore and upper distributary mouth 

bar deposits. In both of these sections, the unconsolidated 

intervals are near the top of the transitional part of the 

section. Fluvial-channel sandstones average 8.1 percent 

porosity, ranging from 2.5 to 17 .7 percent, and average 

19.2 mD permeability, ranging from 0.03 to 374 mD. 

The interdistributary bays between channels on a 

prograding lobe are filled with mudstone and shale deposits interbedded with crevasse-splay sandstones. Although individual sandstone beds average only 4.5 m 

thick, they are commonly stacked on top of each other 

for aggregate thicknesses of as much as 30 m. The average porosity and permeability of these sandstones are less 

than those of the higher energy deposits but locally were 

found to be as high as 14.2 percent and 64 mD, respectively. Crevasse-splay deposits occur in the interdeltaic 

subembayments as well as on the lobes, although not in 

such thick accumulations. 

Distal-bar deposits, though widespread, are commonly thin and silty, although aggregate thicknesses of 

as much as 25 m are not rare. Porosity and permeability 

of these deposits are generally low, averaging 5. 6 percent 

and 0.06 mD respectively, and most deposits contain 

abundant intergranular silt and clay. No porosity of more 

than 9.4 percent or permeability of more than 0.33 mD 

were found in these sandstones in any of the outcrop samples of the Umiat delta. 

Along the outcrop belt of the Umiat delta, the 

higher energy environments are concentrated in the transitional and upper parts of the marine part of the Nanushuk 

section. This part of the section contains as much as 70 

percent of the total sandstone content of the Nanushuk 

and ranges in thickness from 200--1 ,000 m. In the subsurface the same relationship is apparent in electric logs 

northward to the vicinity of Umiat. From Umiat to the 

vicinity of the South Simpson-I well, sandstones are conDepositional and Sedimentologic Factors Affecting Reservoir Potential 69 
(/) 

a: 

UJ 

fUJ 

::;;: 

~ 

u) 

(/) 

UJ 

z 

~ 

~ 

I 

f-

_J 

<( 

f--

0 

f20 

0 

40 

20 

0 

100 

60 

40 

20 

0 

0 

NONMARINE 

537 m 

83 m sandstone 

1 5 percent sandstone 

1 9 percent total sandstone 

0 0 

TRANSITIONAL 

1044 m 

248 m sandstone 

24 percent sandstone 

58 percent total sandstone 

0 

MARINE 

286m 

1 00 m sandstone 

35 percent sandstone 

23 percent total sandstone 

TOTAL SECTION 

1867 m 

431 m sandstone 

23 percent sandstone 

3 m average sandstone 

0 0 

A 

3 6 9 12 15 18>18 0 

NONMARINE 

(not measured) 

TRANSITIONAL 

576 m 

1 72 m sandstone 

30 percent sandstone 

58 percent total sandstone 

0 8 

0 

MARINE 

209m 

126 m sandstone 

60 percent sandstone 

42 percent total sandstone 

TOTAL SECTION 

785 m 

298 m sandstone 

38 percent sandstone 

6.2 m average sandstone 

B 

3 6 9 12 15 18 >18 0 

NONMARINE 

447 m 

70 m sandstone 

16 percent sandstone 

26 percent total sandstone 

TRANSITIONAL 

345m 

130 m sandstone 

38 percent sandstone 

48 percent total sandstone 

MARINE 

268m 

71 m sandstone 

27 percent sandstone 

26 percent total sandstone 

TOTAL SECTION 

1060 m 

271 m sandstone 

26 percent sandstone 

4.3 m average sandstone 

c 

20 

15 

'5 

20 

15 

10 

f5 z 

UJ 

u 

a: 0 UJ 

a.. 

~ 

>-' 

fCi) 

0 

a: 

0 

a.. 

20 

15 

10 

5 

0 

20 

15 

5 

0 

3 6 9 12 15 18 >18 

INDIVIDUAL SANDSTONE BED THICKNESS IN 1.5 METER (5 FOOT) CLASSES 

Figure 30. Distribution of sandstone within 1.5-m-thickness classes from four sections measured in the Umiat 

delta deposits, central North Slope. A, Kurupa anticline; 8, Grandstand type; C, Marmot syncline; D, Tuktu 

Bluff. Dots, porosity values by thickness class; open circles, sample has a permeability in excess of 10 mD. 

For each facies of the Nanushuk Group present (marine, transitional, and nonmarine) and for the total measured section, values are given for total thickness, net sandstone, percent sandstone in that facies or section, 

percent of sandstone in total section contained in that facies (percent total sandstone), and average bed 

thickness of sandstone in total section (average sandstone). 

70 Geology of the Nanushuk Group and Related Rocks, Alaska 
D 

120 NONMARINE 

488 m 

100 45 m sandstone 

9 percent sandstone 

80 1 2 percent total sandstone 20 

60 0 15 

40 10 

20 5 

0 0 

120 TRANSITIONAL 

588 m 

100 83 m sandstone 

14 percent sandstone 

80 22 percent total sandstone 20 

r.tl 

c:: 60 15 w 

rw 

~ 40 10 

~ . : 

crl 20 r- 5 z r.tl w w u z c:: ~ 0 0 w 

u o.._ 

I ~ rw MARINE >-' 140 z 747 m r- 0 Ui r- 2 57 m sandstone 0 r.tl 120 c:: 0 34 percent sandstone 0 z 66 percent total sandstone o.._ 

<( 

r.tl 100 ...J 

<( 

r0 80 20 r60 0 15 

40 10 

20 5 

0 0 

160 TOTAL SECTION 

1823 m 

140 385 m sandstone 

21 percent sandstone 

120 4 .9 m average sandstone 

100 

80 20 

0 

60 0 15 

40 10 

20 5 

0 0 

0 3 6 9 12 15 18 > 18 

INDIVIDUAL SANDSTONE BED THICKNESS 

IN 1.5 METER (5 FOOT) CLASSES 

Figure 30. Continued 

centrated in the upper part of the marine part of the section in what are apparently coastal-barrier deposits. Porosity and permeability values are high in these sandstones, 

averaging 28 percent and 203 mD, respectively, further 

indicating deposition in high-energy environments. 

The general relationship of porosity and permeability to depositional environment is apparent on a plot of 

porosity versus permeability as shown in figure 29. Because of the low marine energy acting on the Corwin delta 

(fig. 29A), the higher energy deposits and thus generally 

better porosities and permeabilities are found in the fluvial 

sandstones, whereas in the Umiat delta (fig. 29B) with 

its greater marine influence, better porosities and permeabilities occur in the transitional and marine parts of 

the section. These relationships are also demonstrated on 

figure 30, which indicates that the best reservoir development in the Umiat delta is in the transitional part of the 

section on depositional lobes as at the Kurupa, Grandstand, and Marmot measured sections (fig. 1) and in the 

marine part of the section away from the lobes (Tuk:tu 

Bluff). Along the high-sandstone-percentage belt between 

Umiat and the Ikpikpuk well, the same relationship would 

be expected, with the best reservoir capacity in the coastal-barrier sandstone bodies within the marine part of the 

section. 

DEPOSITIONAL HISTORY 

The following interpretation of Nanushuk depositional history is intended as a generalized representation 

only. Although ideas and data are drawn liberally from 

various chapters in this volume and Ahlbrandt ( 1979), 

only a few specific references are included. 

Deposition of the Nanushuk Group began in the 

area that is now the far western and southwestern North 

Slope during early Albian or perhaps late Aptian time 

(Scott and Smiley, 1979). The Nanushuk was deposited 

as a fine-grained, elongate, river-dominated delta system 

onto a broad, shallow shelf of prodelta Torok mud, silt, 

and clay (fig. 31A), which was prograding into the Colville basin from west to east. The sources of both the 

Nanushuk and much of the Torok were probably the De 

Long Mountains, Tigara uplift, and Herald arch 

(Huffman, 1979; Mull, 1979). Both wave and current energies acting on the Corwin delta were apparently low 

due to the elongate nature of the Colville basin, the wide, 

shallow shelf, and possibly the sill effect of the Barrow 

and Meade arches. Because of the low wave activity and 

the general structural stability, the available sand was concentrated in long, narrow distributary mouth bar and fluvial-channel deposits. Much of the delta was apparently 

bay and marsh, resulting in extensive thin coal and crevasse-splay deposits. 

The Corwin delta prograded in several major lobes 

Depositional and Sedimentologic Factors Affecting Reservoir Potential 71 
150° 

Endicott 

160° 150° 

B 

50 100 150 KILOMETERS 

EXPLANATION 

-<:>- TEST WELL 

~ DIRECTION OF LONGSHORE 

~ 

DRIFT AND CURRENTS 

BRAIDED STREAM 

~ MEANDERING STREAM 

-:--?--- POSTULATED NORTHWESTERN BOUNDARY 

OF CORWIN DELTA 

Figure 31. Paleogeographic maps depicting the depositional history of the Nanushuk Group, central North 

Slope. A, Early-mid Albian time; 8, mid-late Albian time; C, late Albian-Cenomanian(?) time; D, Cenomanian 

time (at time of maximum regression). 

72 Geology of the Nanushuk Group and Related Rocks, Alaska 
160° 150° 

c 

71 

70° ----?----

MountainS 

150° 

70° 

Mountains 

50 100 150 KI LOMETERS 

Figure 31. Continued 

to the northeast and east with large deltaic subembayments 

between them (fig. 31B and C). The southern lobe did 

not extend east of long 159° W. as a recognizable entity. 

Between long 159° W. and 157° W., deposition was predominantly interdeltaic, with little sandstone deposition 

but thick delta-plain and coastal-plain deposits (fig. 31B 

and C). This environment existed as far north as the 

Meade-I test well. The northern lobe of the Corwin delta 

Depositional and Sedimentologic Factors Affecting Reservoir Potential 73 
apparently swung to the north of the Meade-! well and 

continued around the northern end of the ancestral Meade 

arch. 

As the Corwin delta prograded northeast and east, 

the shelf and slope migrated generally eastward. During 

mid-Albian time the Umiat delta began building northward 

from the Endicott Mountains onto a shelf constructed of 

material derived from the south and west (fig. 31B). Because of the narrow shelf and the proximity to the open 

end of the Colville basin, wave and current activity was 

greater than that which affected the construction of the 

Corwin delta. The Umiat delta began as an elongate riverdominated system (fig. 31B), but as the effects of waves 

and currents increased, the delta assumed a more lobate 

character (fig. 31C). 

Material supplied to the Umiat delta was both 

coarser and more quartzose than that of the Corwin delta. 

As the delta prograded generally northward, clastics were 

carried northwestward by longshore drift and current activity. These sediments were deposited along a north- to 

northwest-trending strandline that slowly prograded eastward (fig. 31C). Streams from the Corwin delta continued 

to transport and deposit sediment in deltaic environments 

7 4 Geology of the Nanushuk Group and Related Rocks, Alaska 

in the vicinity of the Topagoruk and South Simpson-I 

test wells. Mixing of the Corwin and Umiat sediment 

types occurred throughout much of the marine and transitional parts of Nanushuk deposition from the Umiat delta 

northward. 

Faunal evidence suggests that the Kurupa-Umiat 

lobe of the Umiat delta was initiated prior to the Grandstand-Marmot lobe (F. E. May, oral commun., 1979). 

Initiation of delta construction was probably determined 

more by the presence of a shelf that was prograding from 

west to east than by activity within the source area. The 

presence of coarse conglomerates at the base of and interspersed through the lower nonmarine part of the section 

suggests renewed uplift in the Endicott Mountains accompanied by rapid progradation of the delta northward. 

Delta progradation was followed by a marine transgression in Late Cretaceous time (fig. 31D). Intertonguing 

of marine and nonmarine deposits and thin transgressive 

sandstones record transgressive pulses followed by still 

stands or regression. This transitional phase was followed 

by deposition of the marine Colville Group in what appears to be a normal transgressive relationship. 
Petrography of Sandstones of the Nanushuk Group 

From Four Measured Sections, 

Central North Slope, Alaska 

By Susan Bartsch-Winkler 

INTRODUCTION 

Ahlbrandt (1979) includes a reconnaissance petrographic examination of the Lower and Upper Cretaceous 

Nanushuk Group (Bartsch-Winkler, 1979, p. 61) and a 

detailed examination of the Barabara syncline section in 

the western outcrop belt (Huffman, 1979, p. 77). This 

report, a study of the eastern outcrop belt north of the 

central and east-central Brooks Range, is a continuation 

of the earlier study. Included are descriptions of the 

Kurupa anticline, Tuktu Bluff, Arc Mountain, and Marmot syncline sections, all south and east of the National 

Petroleum Reserve in Alaska (NPRA), and the Colville 

River (fig. 32). Modal analyses were performed on a total 

of 94 thin sections, and many more slides were examined 

for textural details. The stratigraphic sections range in 

depositional setting from fluvial to deltaic to shallow 

marine and show variations in texture, composition, and 

diagenetic alteration. 

DESCRIPTION OF MEASURED SECTIONS 

Stratigraphic sections were measured using a combination of tape and Jacob-staff techniques and described 

in Huffman and others ( 1981 b). The sandstone intervals 

were measured and described on a meter-by-meter basis; 

the shale intervals were described in less detail. Sampling 

for porosity, permeability, and petrographic studies was 

not systematic, but instead concentrated on the coarser 

grained sandstones that would be most favorable as petroleum reservoirs. The thicker, coarser beds were sampled 

at several horizons. All of the sections grade from marine 

at the base to nonmarine at the top (fig. 33). 

Kurupa Anticline Section, Killik River Quadrangle 

The base of the Kurupa anticline section forms 

bluffs at the junction of Heather Creek and the Kurupa 

River north of the Tuktu escarpment. The Kurupa anticline section, the westernmost described here, extends for 

4 km and totals 1,867 m in thickness. About 672 m (36 

percent) of section is exposed, and approximately 413 m 

(22 percent) of the section is sandstone. The basal part 

consists of prodelta mudstone; it is overlain by estuarine 

and intertidal mudstone and siltstone and beach sandstone 

deposits. Braided and meandering stream deposits of sandstone and conglomerate cap the section. 

Tuktu Bluff Section, Chandler Lake Quadrangle 

The Tuktu Bluff section crops out just south of the 

Chandler River as part of the Tuktu escarpment between 

Tuktu Bluff on the west and Gunsight Pass on the east. 

The section is 5 km long, and about 1,823 m of strata 

was measured. Approximately 22 percent of the interval 

is exposed, and 21 percent (385 m) is sandstone. The 

sequence of depositional environment is lower shoreface 

at the base, succeeded by upper shoreface, delta front, 

tidal delta and tidal channels, and river-mouth bar and 

braided stream at the top (fig. 33). 

Arc Mountain Section, Chandler Lake 

Quadrangle 

The Arc Mountain section, north of the Tuktu escarpment, is 3 km long and comprises 516 m of strata. 

About 97 percent of the section is exposed, and 40 percent 

is sandstone (205m). Foreshore and shoreface sandstones, 

interpreted as the base of the section, grade upward into 

prodelta and delta-front sandstones that are capped by fluvial and interdistributary bay-fill deposits (fig. 33). 

Marmot Syncline Section, Phillip Smith 

Mountains Quadrangle 

The Marmot syncline section, the easternmost section studied, is west of the junction of the Sagavanirktok 

River and Accomplishment Creek, is about 2 km long, 

and had a total thickness of 1,060 m. About 496 m (47 

Petrography of Sandstones 75 
164° 160° 156° 152° 148° 

50 100 K1LOMETERS 

Figure 32. Location of the four measured sections 

discussed in this study. 

percent) of the section is exposed, and about 271 m (26 

percent) of the section is sandstone. The lower 270 m 

is interpreted as being deposited in lower shoreface and 

foreshore environments and is succeeded by about 20 m 

of intertidal sedimentary rocks, overlain by 770 m of fluvial (braided and meandering stream) deposits (fig. 33). 

PETROGRAPHY 

The thin sections were stained with sodium colbaltinitrite to aid in identification of potassium feldspar, and 

with alizarin red S to aid in identification of dolomite. 

Owing to the poor quality of the thin sections, especially 

Environment 

COMPOSITION, IN PERCENT Ql 

c 

·;:; 

(Q 

a: 

w 

Cll 

:E 

::::> 

z 

w 

...J 

116 

113 

108 

103 

80 

79 

76 

70 

64 

59 

49 

47 

43 

33 

32 

31 

30 

25 

24 

23 

21 

20 

49U 

49L 

45l 

43U 

39U 

39l 

38U 

38l 

37M 

35l 

30 

25 

23 

21 

20l 

17 

16 

13U 

12 

!1: AH 41U ..:; 41M <( 41l (/) 32 

31 

30 

27 

26 

25 

22 

20 

19 

18 

17 

16 

6 

CH56 54 

53 

52 

46 

45 

44 

43 

38 

37 

35 

34 

32 

31 

30 

29 

28 

26 

25 

24 

20 

18 

16 

14 

O=Om+Op 

ooooooo M"""ll)co""a:un 

Om Op 

oooooo 0 ooooo ~NM ..... ItHC - NMq-llHO 

L=Lv+Lm+Ls 

0 0 0 0 0 N M '<t Ill CO 

76 Geology of the Nanushuk Group and Related Rocks, Alaska 

lm 

OltlOitl Ln--NN 

Ls Lt=L+Op 

0 0 0 0 "'''<t "' co 

Cal'?ite Ql 

Arg-Carb grains c 

0 O&n0 0 ·~ ,._ ltl~~N N'<tCOCX>~ ::iE 

E 

c 

0 

z 
FA 

Volcanic litharenite 

Lv C 

Q 

Ls 

Sedimentary 

litharenite 

,··. 

Measured 

section 

Kurupa Anticline 

Tuktu Bluff 

Arc Mountain 

Marmot syncline 

L 

Lm Cc D 

EXPLANATION 

Average modal 

composition 

• 

0 

Qm 

Sub- SubLithic Feldspathic 

feldsarenite litharenite 

Ac 

Sedimentary 

litharenite 

( 

/ / / 

\ \ . Calc-lithite 

Outer limit of 

modal plots 

\ ----\ __ 

Chert-arenite \'- ------ :\ ......._ __ _ 

Figure 34. Triangular plots showing outer limits of modal plots for each measured section discussed in this 

report and the average modal composition of each. Q, monocrystalline and polycrystalline quartz, quartzite, 

and chert; Qm, monocrystalline quartz; F, feldspar grains; L, volcanic, metamorphic, and sedimentary lithic 

fragments; Lt, total lithic fragments and polycrystalline quartz; Lt, total lithic grains; Lv, volcanic lithic fragments; Lm, metamorphic lithic fragments; Ls, sedimentary lithic fragments; Qp + cht, polycrystalline quartz, 

quartzite, and chert; Ac, argillaceous and carbonaceous grains; Cc, calcareous grains. 

Figure 33 (facing page). Graphs showing environment of depositionand compositions of sandstones as determined 

from modal analyses of samples from four measured sections, Nanushuk Group, North Slope, Alaska. A, Kurupa 

anticline. 8, Tuktu Bluff. C, Arc Mountain. D, Marmot syncline. Q, monocrystalline and polycrystalline quartz, quartzite, and chert; Qm, monocrystalline quartz; Qp, polycrystalline quartz, quartzite, and chert; F, feldspars; l, volcanic, 

metamorphic, and sedimentary lithic fragments; lv, volcanic lithic fragments; lm, metamorphic lithic fragments; 

ls, sedimentary lithic fragments; Lt, total lithic fragments and polycrystalline quartz; Arg-Carb, argillaceous and carbonaceous grains. All samples have less than 30 percent matrix and less than 30 percent cement by volume. Total 

matrix and cement may not exceed 35 percent. 

Petrography of Sandstones 77 
Table 6. Modal analyses of surface samples of sandstone from four measured sections in the Lower and Upper 

Cretaceous Nanushuk Group, central North Slope, Alaska 

[Q, monocrystalline and polycrystalline quartz, quartzite, and chert; Qm, monocrystalline quartz; Qp, polycrystalline quartz, chert, and quartzite; 

F, feldspars; L, volcanic, metamorphic, and sedimentary lithic fragments; Lv, volcanic lithic fragments; Lm, metamorphic lithic fragments; Ls, 

sedimentary lithic fragments; Lt, total lithic fragments and polycrystalline quartz; Ac, argillaceous and carbonaceous grains; Cc, calcareous grains. 

All values in percent; all samples listed have less than 30 percent matrix and less than 30 percent cement, by volume. Total matrix and cement 

do not exceed 35 percent. Modal data not recalculated to 100 percent but is percentage of total point counts for each sample. Grain size classes, 

measured petrographically, are: fs, fine sand; ms, medium sand; cs, coarse sand; vcs, very coarse sand] 

Grain- Grain type 

Sample size 

No. class 

Q Qm Qp F 1 1v 1m 1s 1t Ac Cc Mica Matrix Cement 

Kurupa anticline section 

20 fs---- 22.3 9.3 13.0 3.7 45.0 14.7 12.3 18.0 58.0 5.0 7.0 0.3 4.7 25.6 

21 ms---- 34.3 15.3 19.0 3.7 42.0 15.7 15.0 11.3 61.0 2.7 .3 .o 7.0 7.3 

23 cs---- 29.0 9.0 20.0 3.0 44.7 24.0 9.0 11.7 64.7 7.0 • 7 .o ll.6 3.7 

24 cs---- 29.9 12.0 17.0 2.7 43.3 16.6 14.4 12.3 60.3 7.3 .3 .o 6.3 20.3 

25 fs---- 38.8 27.4 11.4 6.0 22.9 9.0 4.0 9.9 34.3 6.6 2.0 1.3 12.1 20.0 

30 fs---- 31.6 18.3 13.3 11.7 30.6 15.7 8.6 6.3 43.9 3.3 1.0 .3 17.0 6.7 

31 fs---- 34.0 21.7 12.3 12.0 24.9 9.6 8.0 7.3 37.2 2.3 1.7 .o 9.7 11.3 

32 fs---- 28.7 16.0 12.7 6.3 39.3 18.4 10.0 10.9 52.0 5.3 1.3 1.3 8.0 9.9 

33 ms---- 31.3 13.3 18.0 8.0 35.8 14.7 9.7 11.4 53.8 7.7 .o .7 11.0 2.0 

43 fs---- 21.0 7.7 13.3 4.3 43.4 14.7 12.0 16.7 56.7 4.7 7.7 1.0 6.0 28.3 

47 ms---- 18.9 8.3 10.6 3.7 51.4 18.0 13.3 20.1 62.0 5.7 8.7 .3 6.6 22.7 

49 fs---- 32.2 17.9 14.3 5.0 36.4 9.3 17.9 9.2 50.7 4.3 1.3 1.3 21.9 7.2 

59 ms---- 23.4 8.7 14.7 6.7 47.0 14.4 20.0 12.6 61.7 6.0 .3 .3 11.0 8.7 

64 fs---- 32.3 17.3 15.0 3.7 36.0 4.3 24.0 7.7 51.0 3.3 3.7 .3 5.7 25.7 

70 fs---- 32.0 20.0 12.0 2.3 36.4 9.4 20.3 6.7 48.4 2. 7 1.7 .7 24.3 4.9 

74 ms---- 29.6 19.3 10.3 2.3 27 .o 6.7 15.6 4.7 37.3 3.7 .o 1.3 1. 7 39.7 

76 cs---- 38.0 28.7 9.3 2.3 35.0 7.3 20.0 7.7 44.3 5.0 .o .o 16.0 4.7 

79 ms---- 39.6 23.3 16.3 2.3 35.4 4.7 20.7 10.0 51.7 3.3 .7 2. 7 17.0 3.7 

80 fs---- 31.3 19.0 12.2 3.7 28.4 6.7 18.7 3.0 40.7 1.0 .3 15.3 19.3 .7 

103 cs---- 50.6 34.3 16.3 .o 31.6 4.0 18.0 9.6 47.9 3.0 1.3 4.3 11.3 4.3 

108 cs---- 31.7 19.0 12.7 1. 7 29.7 4.3 19.7 5.7 42.4 .7 1.7 14.3 16.0 7.0 

113 fs---- 33.3 16.3 17.0 1. 7 28.7 7.3 18.7 2.7 45.7 1.0 .o 21.7 18.7 1.6 

116 fs---- 38.3 23.0 15.3 3.3 27.7 6.7 15.7 5.3 44.6 1.3 .3 10.3 18.3 1.6 

Average----- 32.2 18.0 14.7 4.4 36.5 11.7 15.0 10.0 51.4 4.3 1.8 3.4 12.4 10.1 

Tuktu Bluff section 

12 fs---- 31.3 19.0 12.3 5.3 26.9 3.7 14.6 8.6 39.2 5.3 0.3 9.3 26.0 3.0 

13U fs---- 29.0 16.0 13.0 2. 7 25.0 3.0 17.4 4.6 38.0 .3 2.0 17.7 19.7 8.3 

16 fs---- 30.0 18.0 12.0 4.3 29.2 4.0 18.6 6.6 41.2 3.3 2.0 17.7 14.6 9.0 

17 ms---- 42.0 23.0 19.0 2. 7 36.6 6.0 20.0 10.6 55.6 1.3 .o 2.1 18.4 5.0 

201 fs---- 34.1 22.7 11.4 6.0 29.7 4.6 18.4 6.7 41.1 2.7 .o 28.7 25.7 3.0 

21 cs---- 39.0 27.3 11.7 1.7 35.6 9.0 16.3 10.3 47.3 1.0 .o 4.7 14.3 6.4 

23 fs---- 30.0 12.7 17.3 2.7 38.0 13.0 19.3 5.7 55.3 1.7 1. 7 5.7 9.7 15.7 

25 ms---- 41.0 24.0 17.0 2.0 41.7 10.3 21.7 9.7 58.7 4.7 .o 3.3 9.3 10.0 

30 fs---- 26.0 15.0 11.0 1.3 33.4 6.0 25.4 2.0 44.4 • 7 .3 8.0 28.0 5.0 

351 fs---- 34.6 15.8 18.8 2.6 33.1 5.3 22.8 5.0 51.9 • 7 .3 14.5 17 .1 3.0 

37M fs---- 43.9 25.3 18.6 3.2 24.9 4.8 17.9 2. 2 43.5 .3 .o 10.9 14.4 7.1 

381 ms---- 51.5 33.1 18.4 .3 33.7 6.8 13.3 13.6 5 2.1 2.0 .o 1.4 15.9 .6 

38U ms---- 58.7 36.7 22.0 1.3 27.6 5.7 12.2 9.7 49.6 2.8 .o 4.1 11.0 1.1 

391 fs---- 51.3 35.2 16.1 3.2 24.3 8.1 10.6 5.6 40.4 1.0 2.3 5.8 13.6 4.3 

39U cs---- 51.2 28.7 22.5 .7 35.4 13.3 12.4 9.7 57.9 2.9 .o 2.0 12.7 2.0 

43U cs---- 59.0 33.0 26.0 1.0 26.0 6.6 12.0 7.4 5 2.0 1.7 .o .3 17.4 .o 

451 fs---- 42.1 28.7 13.4 1.0 28.0 7.6 14.7 5.7 41.4 2. 7 .o 8.3 21.0 1.0 

49U cs---- 50.3 34.3 16.0 2.7 24.6 5.6 13.0 6.0 40.6 1.3 .o 4.3 19.0 1.0 

491 fs---- 53.6 17.3 36.3 .3 36.7 18.3 14.0 4.4 73.0 2. 7 .o .o 2.3 1.3 

Average----- 42.0 24.5 17.5 2.4 31.1 7.5 16.6 7.1 48.6 2.1 .5 7.8 16.3 4.6 

78 Geology of the Nanushuk Group and Related Rocks, Alaska 
Table 6. Continued 

Grain- Grain type 

Sample size 

No. class 

Q Qm Qp F L Lv Lm Ls Lt Ac Cc Mica Matrix Cement 

Arc Mountain section 

6 fs---- 38.7 25.7 13.0 3.0 17.4 2.0 8.4 7.0 30.4 4.7 0.3 14.7 25.3 3.3 

16 fs---- 36.9 21.0 15.9 1.9 21.3 6.1 ll.1 4.1 37.2 .o .o 18.2 19.7 2. 2 

17 fs---- 39.4 26.0 13.4 2.3 23.3 5.0 12.0 6.3 36.7 1.7 .3 10.0 23.0 1.6 

18 vcs---- 6 2. 9 42.7 20.2 .o 31.5 10.9 9.3 ll.3 51.7 9.6 .o .3 5.3 1.0 

19 ms---- 62.6 42.3 20.3 1.0 26.0 7.0 8.6 10.4 46.3 5.7 .o 1.3 10.0 1.6 

20 ms---- 63.3 43.3 20.0 .3 29.3 6.3 12.7 10.3 49.3 5.0 .o 2. 7 7.3 .6 

22 fs---- 38.3 23.3 15.0 3.7 18.7 6.0 10.0 2.7 33.7 1. 7 .o 16.0 18.7 4.7 

25 ms---- 55.1 40.7 14.4 2.3 24.8 6.7 12.4 5.7 39.2 1.7 .o 4.3 16.4 .6 

26 ms---- 56.3 31.3 25.0 2.3 26.6 3.6 16.0 7.0 51.6 1.0 .o 7.3 12.0 .3 

27 cs---- 62.3 45.0 17.3 .7 28.7 8.7 8.0 12.0 46.0 7.3 .o .o 2. 7 2.0 

30 ms---- 34.3 19.7 14.6 2.0 26.7 8.0 14.0 4.7 41.3 .7 .o 9.0 22.0 .7 

31 cs---- 61.6 42.3 19.3 .7 25.2 ll.3 7.6 6.3 44.5 1.3 .o .7 6.3 4.4 

32 ms---- 64.7 47.3 17.4 .7 21.1 6.7 8.4 6.0 38.5 1.3 .o .7 10.7 .6 

411 ms---- 59.9 41.2 18.7 1.0 28.6 6.6 ll. 7 10.3 47.3 1.7 .o 2.0 9.0 1.0 

41M cs---- 58.7 32.3 26.4 .3 28.4 4.3 14.7 9.4 54.8 4.7 .o 3.3 12.3 1.6 

41U cs---- 58.6 33.0 25.6 .3 29.3 .3 16.0 10.0 54.9 1.3 .7 3.3 12.7 .3 

Average----- 53.4 34.8 18.5 1.4 25.4 6.4 ll.3 7.7 44.0 3.1 .1 5.9 13.3 1.7 

Marmot syncline section 

14 fs---- 51.4 31.0 20.4 1.7 31.3 7.3 17.4 6.6 51.7 1.3 .o 6.0 13.6 2.3 

16 cs---- 52.9 30.3 22.6 3.3 32.0 6.0 23.0 3.0 55.6 1.0 .o 6.3 10.0 2. 7 

18 fs---- 48.0 26.7 21.3 2.0 29.3 6.3 18.7 4.3 50.6 1.3 .o 7.0 13.0 3.0 

20 cs---- 52.3 23.3 29.0 .o 45.4 9.0 26.7 9.7 74.4 3.7 .o 1.3 10.3 .3 

24 vcs---- 59.3 25.3 34.0 .o 33.3 15.0 8.0 10.3 67.3 5.3 .o .o 2.7 1.4 

25 cs---- 58.3 41.0 17.3 .o 24.4 6.4 16.3 ll. 7 41.7 2.0 .o .7 8.3 2.1 

26 ms---- 62.7 37.0 25.7 .o 22.1 4.4 ll.O 6.7 47.8 2.0 .o 4.0 9.4 2.0 

28 fs---- 49.0 31.0 18.0 .7 21.7 6.4 10.3 5.0 39.7 .7 .o 3.3 23.7 2.3 

29 ms---- 54.7 27 .o 27.7 .o 39.7 12.0 15.4 12.3 67.4 5.3 .o 2.3 7.4 2.9 

30 cs---- 59.3 41.0 18.3 .3 27.0 4.7 15.3 7.0 45.3 1.0 .o 6.0 12.3 1.0 

31 vcs---- 59.0 35.3 23.7 .7 34.4 8.3 18.7 7.4 58.1 3.7 .o 1.0 8.4 .3 

32 ms---- 55.6 27.3 28.3 1.0 30.6 8.0 14.3 8.3 58.9 4.3 .o 4.7 9.0 .2 

34 fs---- 52.4 32.7 19.7 1.7 24.8 5.0 14.4 5.4 44.5 1.7 .o 8.7 13.4 .3 

35 cs---- 58.0 35.7 22.3 .o 24.3 7.0 ll.O 6.3 46.6 2.3 .o 4.7 14.0 .3 

37 cs---- 60.9 21.3 39.6 .3 33.3 15.7 9.3 8.3 7 2.9 7.0 .o .3 5.0 2.6 

38 cs---- 53.3 39.0 14.3 1.3 31.0 13.4 10.3 7.3 45.3 3.7 1.3 .3 8.7 8.3 

43 cs---- 54.0 33.2 20.8 1.5 31. 1 8.8 10.4 ll.O 51.9 7.0 .o .o 14.9 1.8 

44 cs---- 63.0 37.0 26.0 .o 31.4 7.0 12.7 11.7 57.4 5.7 .o .7 6.6 .3 

45 ms---- 62.0 41.7 20.3 .3 23.3 7.4 9.3 6.6 43.6 .3 .o 3.3 ll.3 2.3 

46 fs---- 53.3 35.7 17.6 2.3 30.0 9.7 15.0 5.3 47.6 1.0 .o 4.3 11.0 .6 

52 vcs---- 6 2. 7 32.7 30.0 .o 35.0 9.7 15.0 10.3 65.0 5.0 .o 1.7 3.6 .3 

53 ms---- 67.7 49.7 18.0 1.3 21.0 8.3 6.3 6.4 39.0 .7 .o 1.7 9.0 .6 

54 fs---- 47.3 31.3 16.0 2.3 23.3 7.0 9.0 7.3 39.3 1.3 .o 6.7 21.0 1.7 

56 fs---- 40.4 28.7 ll. 7 2. 7 21.0 12.3 5.7 3.0 3 2. 7 .3 .o 12.7 24.4 2.3 

Average----- 55.7 33.1 22.6 1.0 29.2 8.5 13.5 7.6 51.8 2.8 .1 3.7 ll.3 1.8 

in staining and impregnation, modal data on visible poros- easternmost Arc Mountain and Marmot syncline sections 

ity and potassium feldspar content are considered tenta- have more chert and monocrystalline and polycrystalline 

tive. For modal analyses, 300 grains were counted per quartz grains than do the more micaceous sandstones rich 

thin section, according to the methods described by Dick- in lithic grains to the west, which compose the Kurupa 

inson (1970). anticline and Tuktu Bluff sections. 

Compositional Data Mineralogy and Grain Types 

Compositionally, sandstones of the Nanushuk Quartzose grains include monocrystalline quartz, 

Group from the eastern outcrop belt are classified as polycrystalline quartz, chert, and quartzite. The monolitharenites (Folk, 1968). The sandstones making up the crystalline grains include undulose types as well as those 

Petrography of Sandstones 79 
with straight extinction angles. Quartzite grains are those 

with elongated crystallite morphologies, lacking any mica. 

Polycrystalline quartz grains may have a wide range in 

crystallite dimensions, even in a single grain. For this 

reason, grains with crystallites smaller than 0.03 mm, 

which most workers would classify as chert, were not 

separated from the polycrystalline quartz category in this 

study. Chert grains include only fibrous types, which are 

rarely found in these rocks. 

Feldspar is not a significant component in sandstones of the Nanushuk Group, and where it is present, 

it is significantly altered. Most feldspar grains are blocky 

and show albite and Carlsbad twinning. Perthitic and myrmekitic feldspar types are also present. 

Lithic grains are typically altered by compaction and 

(or) replacement. Sedimentary lithic grain types include 

calcareous, argillaceous or carbonaceous grains, and unfoliated quartz-mica grains. Metamorphic lithic grains include quartzite and foliated quartz-mica grains. Volcanic 

lithic grain types include felsite, microlitic volcanic, and 

microgranular-hypabyssal grains. Trends in lithic grain 

compositions may be questionable due to their generally 

smaller grain size and their apparent susceptibility to alteration and recrystallization. Also, microcrystalline felsite 

grains and quartzite grains having only slightly elongate 

crystallites may be difficult to distinguish from polycrystalline quartz grains. 

More detailed descriptions of grain types in sandstones of the Nanushuk Group may be found in Huffman 

(1979, p. 80-83) and Fox (1979, p. 50-52). 

Modal Distribution 

Compositional data for each of the measured sections are shown graphically in figure 33. No consistent 

vertical compositional trends were found in a comparison 

of all four sections. Tuktu Bluff and Arc Mountain sections are the most comparable in that both show an upwardly decreasing content of feldspar and increasing content of total quartzose grains, polycrystalline quartz 

grains, and total lithic grains. The Kurupa anticline section has a decreasing amount of volcanic lithic grains and 

feldspars and an increasing amount of metamorphic lithic 

grains. In contrast, the plot of the Marmot syncline section indicates a decreasing amount of metamorphic lithic 

grains upward. 

Ternary plots (figs. 34A-D) of the data shown in 

figure 33 show that the sandstones of the Nanushuk Group 

from the eastern outcrop belt are compositionally litharenite (Folk, 1968). On the triangular plot with quartzose 

grains, feldspar grains, and lithic grains (fig. 34A), an 

eastward trend from Kurupa anticline to Marmot syncline 

is clearly shown by increasing quartzose grains and decreasing lithic and feldspar grains. Total percentages of 

quartz, monocrystalline quartz, and polycrystalline quartz 

80 Geology of the Nanushuk Group and Related Rocks, Alaska 

increase eastward by 13, 3, and 11 percent, respectively 

(table 6). The ternary plot having lithic grains as end 

members (fig. 34C) shows the average composition of the 

sandstones to be phyllarenite, although some samples of 

sandstones of the Nanushuk Group may fall into volcanic 

and sedimentary litharenite categories as well. Characteristic lithic types (argillaceous/carbonaceous, calcareous, 

and chertlpolycrystalline quartz) plotted on a ternary diagram (fig. 34D) show an eastward trend of increasing 

chert and polycrystalline quartz grains with a reduction 

in both argillaceous/carbonaceous grains and calcareous 

grains; the sandstones are classified as chert arenites. Percentages of calcareous grains in sandstones decrease rapidly eastward from the Kurupa anticline section. Only trace 

amounts (less than 1 percent) of calcareous grains may 

be found in the Arc Mountain, Marmot syncline, and 

Tuktu Bluff sections (table 6). 

Textural Data 

Maximum and mean grain size was measured under 

the petrographic microscope and estimated in the field 

using the American Stratigraphic Company grain-size 

chart. Roundness was estimated petrographically by comparison with the Krumbein (1941) visual estimator. Rock 

chips were mounted in Lakeside1 cement, coated with 

gold-palladium, and examined using a Cambridge-1801 

scanning electron microscope (SEM) with an attached energy-dispersive X-ray analyzer in order to further disclose 

the relation between grains, pores, and cements. 

Using the petrographic microscope, textural parameters were determined by estimation of sorting and packing 

density. Sorting was estimated by referring to sorting images illustrated by Pettijohn and others, (1972, p. 585). 

Estimated packing density was determined visually using 

these criteria: high, framework grains tightly packed with 

abundant penetration involving squeezing of relatively soft 

lithic fragments between and around more resistant grains 

such as quartz and feldspar; moderate, closely packed 

framework with minor grain interpenetration and little or 

no deformation of lithic fragments; and low, framework 

separated by or floating in matrix (including highly deformed material without recognizable grain boundaries) or 

authigenic cement with no observable grain interpenetration. Type of grain contacts (Taylor, 1950) include (1) 

sutured grains, mutual stylolitic interpenetration of two 

or more grains (which must be carefully distinguished 

from grains of polycrystalline quartz); (2) concavo-convex 

grain contacts; (3) long or straight contacts; (4) point or 

tangential contacts; and (5) floating grains that are not 

in contact with other framework constituents. Most sam1

Use of trade names is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey 
Q 

• 

••• 

• 

F L F 

Ls 

• • 

• • • • ~ • • • 

• • • • •I 

, 

Lv Lm Cc 

Amount 

Symbol Class n (percent) Q F L 

• Fine sand----- 12 52 45 8 47 

• Medium sand--- 6 26 40 6 54 

• Coarse sand--- 5 22 50 3 47 

Total------- 23 100 

Average--------------------- 45 6 49 

Qm 

Ac 

• 

•• 

Qp+ 

Qm Lt Lv Lm Ls cht 

26 67 32 43 25 67 

20 74 30 42 28 65 

28 70 28 47 25 70 

25 70 31 44 26 67 

• 

• 

• 

•• 

Jl>. 

• 

-: . 

• 

• 

• • •• • 

• • • 

• aI 

• 

Lt 

• • • • • • 

• •• • 

Qp + cht 

Ac Cc 

21 13 

27 8 

25 5 

23 10 

Figure 35. Triangular diagrams showing modal analyses plotted according to grain size, and table showing 

average modal composition for each grain-size class, Kurupa anticline section, North Slope, Alaska. n, number 

of samples; Q, quartzose grains (including monocrystalline and polycrystalline quartz, chert, and quartzite); 

F, feldspar grains; L, lithic grains; Qm, monocrystalline quartz; Lt, total lithic grains; Lv, volcanic lithic fragments; Lm, metamorphic lithic fragments; Ls, sedimentary lithic fragments; Qp + cht, polycrystalline quartz, 

quartzite, and chert; Ac, argillaceous and carbonaceous grains; Cc, calcareous grains. 

pies show more than one type of grain contact. Textural 

components and cement plus matrix were determined from 

the modal analysis of each thin section. Matrix (as discussed by Dickinson, 1970) may be high in samples having significant amounts of phyllosilicate coatings and (or) 

squeezed or compacted lithic fragments. Results of porosity and permeability tests are discussed by Huffman and 

others (this volume). 

Estimates of average grain size measured with the 

petrographic microscope show that the sandstones of the 

Petrography of Sandstones 81 
Q 

J 

• 

...... • .. 

F L 

Ls 

• 

• • 

• .. • .... ~ 

• .. • • • .... 

• • 

Lv Lm 

Amount 

Symbol Class n (percent) Q 

• Fine sand----- 11 58 53 

• Medium sand--- 4 21 57 

.... Coarse sand--- 4 21 59 

Total------- T9 TOO 

Average--------------------- 55 

F 

Cc 

F L Qm Lt Lv Lm 

5 43 32 64 21 61 

2 41 34 64 21 48 

40 32 67 35 41 

3 42 32 64 24 54 

Qm 

Ac 

• 

Qp+ 

Ls cht 

19 80 

32 81 

24 89 

23 82 

~ • .& 

··-

Ac 

14 

19 

11 

14 

• 

• 

• 

• 

• • 

• 

Cc 

6 

0 

0 

4 

Lt 

Figure 36. Triangular diagrams showing modal analyses plotted according to grain size, and table showing 

average modal composition for each grain-size class, Tuktu Bluff section, North Slope, Alaska. n, number 

of samples; Q, quartzose grains (including monocrystalline and polycrystalline quartz, chert, and quartzite); 

F, feldspar grains; L, lithic grains; Qm, monocrystallline quartz; Lt, total lithic grains; Lv, volcanic lithic fragments; Lm, metamorphic lithic fragments; Ls, sedimentary lithic fragments; Qp + cht, polycrystalline quartz, 

quartzite, and chert; Ac, argillaceous and carbonaceous grains; Cc, calcareous grains. 

Nanushuk Group sampled are progressively coarser eastward (figs. 35-38). Sorting changes eastward from moderate to well sorted (fig. 40). Grain size determines the 

composition of these sandstones (figs. 35-39) and, in gen82 Geology of the Nanushuk Group and Related Rocks, Alaska 

eral, the coarser sandstones are more quartzose, containing fewer feldspathic, micaceous, and calcareous components. Therefore, the coarsest sandstones to the east probably have the most favorable values of porosity and, 
Q Om 

• 

F L F Lt 

Ls Ac 

... • • ... •• .... ... • • ... • • • 

~ 

• • 

Lv Lm Cc Qp + cht 

Amount Qp+ 

Symbol Class n (percent) Q F L Qm Lt Lv Lm Ls cht Ac Cc 

• Fine sand---- 4 25 63 5 33 39 56 21 51 25 85 14 

• Medium sand--- 7 44 67 2 32 45 54 25 46 30 87 13 0 

... Coarse sand--- 5 31 68 0 32 43 56 27 39 34 78 22 

Total------- T6 TOO 

Average--------------------- 66 2 32 42 55 25 45 30 83 16 

Figure 37. Triangular diagrams showing modal analyses plotted according to grain size, and table showing 

average modal composition for each grain-size class. Arc Mountain section, North Slope, Alaska. n, number 

of samples; Q, quartzose grains (including monocrystalline and polycrystalline quartz, chert and quartzite); 

F, feldspar grains; L, lithic grains; Qm, monocrystalline quartz; Lt, total lithic grains; Lv, volcanic lithic fragments; Lm, metamorphic lithic fragments; Ls, sedimentary lithic fragments; Qp + cht, polycrystalline quartz, 

quartzite, and chert; Ac, argillaceous and carbonaceous grains; Cc, calcareous grains. 

owing to these mechanically and chemically more stable 

quartzose grains, have the least visible alteration by compaction. Lithic grains show no significant compositional 

variation with grain size. Vectoral analyses of grain size 

and compositional trends in these sandstones are shown 

in figure 39. 

The sandstones of the Nanushuk Group have undergone considerable alteration by compaction and are typiPetrography of Sandstones 83 
Q Om 

• 

F L F Lt 

Ls Ac 

A A 

A 6 • 'i· • A A A • ,. . 

• • • A 

Lv Lm Cc Qp + cht 

Amount Qp+ 

Symbol Class n (percent) Q F L Qm Lt Lv Lm Ls cht Ac Cc 

• Fine sand---------- 7 29 64 3 34 41 59 31 49 21 93 7 0 

• Medium sand-------- 5 21 69 31 42 58 30 41 29 90 10 0 

... Coarse sand-------- 9 38 64 35 38 61 27 46 27 83 16 

6 Very coarse sand--- 3 12 64 0 36 33 67 32 40 27 83 17 0 

Total------------ 24 TOO 

Average-------------------------- 65 34 38 61 30 44 26 87 12 0 

Figure 38. Modal analyses plotted on triangular diagrams according to grain size, and table showing average 

modal composition for each grain size class, Marmot syncline section, North Slope, Alaska. n, number of 

samples; Q, quartzose grains (including monocrystalline and polycrystalline quartz, chert and quartzite); F, 

feldspar grains; L, lithic grains; Qm, monocrystalline quartz; Lt, total lithic grains; Lv, volcanic lithic fragments; 

Lm, metamorphic lithic fragments; Ls, sedimentary lithic fragments; Qp + cht, polycrystalline quartz, quartzite, 

and chert; Ac, argillaceous and carbonaceous grains; Cc, calcareous grains. 

cally well compacted. The degree of compaction is a function of grain size, sorting, and composition. Estimates of 

packing densities of the sandstones of the Nanushuk 

84 Geology of the Nanushuk Group and Related Rocks, Alaska 

Group show them to be highly compacted (fig. 40) with 

intergrain boundaries that are sutured, concavo-convex, or 

long and straight (fig. 41). Sandstones having high pack-
Q Om 

4 

1~ 

F L F Lt 

Ls Ac 

Lv Lm Cc 

Figure 39. Vectoral analyses showing compositional trends according to grain size of samples from four measured sections, North Slope, Alaska. 1, Kurupa anticline section. 2, Tuktu Bluff section. 3, Arc Mountain section. 

4, Marmot syncline section. Vectors begin at fine-grained mean and end at coarse-grained mean. Q, quartzose 

grains (including monocrystalline and polycrystalline quartz, chert, and quartzite); F, feldspar grains; L, lithic 

grains; Lt, total lithic grains; Qm, monocrystalline quartz; Lv, volcanic lithic fragments; Ls, sedimentary lithic 

fragments; Lm, metamorphic lithic fragments; Cc, calcareous grains; Ac, argillaceous and carbonaceous grains; 

Qp + cht, polycrystalline quartz, quartzite, and chert. 

ing densities may also contain large percentages of 

pseudomatrix, which is thought to result from less competent (lithic?) grains being squeezed into pore spaces and 

molded around adjacent grains until the original grain is 

indistinguishable (fig. 42). Pseudomatrix may also be included in the low packing density category, because abundant pseudomatrix may result in floating grains or separated grains with no interpenetration along grain boundaries. Rocks of the Tuktu Bluff and Kurupa anticline sections have higher percentages of low packing density, 

probably because pseudomatrix is the predominant form 

of matrix material in finer grained sandstones containing 

a higher lithic grain component or in sandstones with 

abundant mica. 

Alteration by Cementation 

Cementation by carbonate, silica, and various clays 

is a major contributor to reduction and filling of the intergranular pores in Nanushuk rocks. Grain size, thickness 

of the sandstone beds, the amount of burrowing and mixing of sediment layers, and composition of the framework 

grains probably help to determine the amount and type 

Petrography of Sandstones 85 
KURUPA 

ANTICLINE 

SECTION, 

n =53 

TUKTU 

BLUFF 

SECTION, 

n=24 

ARC 

MOUNTAIN 

SECTION, 

n =20 

MARMOT 

SYNCLINE 

SECTION, 

n=28 

DOMINANT GRAIN 

SIZE (ESTIMATED), 

IN PERCENT 

SORTING, 

IN PERCENT 

C> 

C> 

0.. "' "' "' i ~ ~ ~; 

@~ 

~~ 

~LllJ 

GRAIN 

CONTACTS, 

IN PERCENT 

! .f c "' "' C> 1- u::: 

TEXTURAL 

COMPONENTS, 

IN PERCENT 

MATRIX + 

CEMENT, 

IN PERCENT 

PACKING 

DENSITY, 

IN PERCENT 

GRAIN 

ANGULARITY, 

IN PERCENT 

[J5 ~6 EIJ95 

17 9 14 19 65 16 

. 5 4 0 

Figure 40. Histograms of textural characteristics shown in thin sections for measured sections discussed 

in this report, North Slope, Alaska. Generally, the most favorable reservoir characteristics are shown on 

the right side of each histogram. n, number of samples. 

of cement present. Other workers have shown that cementation increases with proximity to shale contacts (Fothergill, 1955; Fuchtbauer, 1974). The cementation is attributed to the process of ion diffusion as connate water is 

released from the shales (Jonas and McBride, 1977). Cements may also originate from internal sandstone sources 

by dissolution and recrystallization of the framework 

grains (such as quartz, lithic, feldspar, and carbonate 

grains) as pressures and temperatures increase in the sandstone during the compaction process. 

The most cemented rocks are found in the westernmost measured section, Kurupa anticline (table 6). Of 

the samples from Kurupa anticline section, 81 percent 

have values of matrix plus cement exceeding 20 percent 

(fig. 40). The amounts of cement and cement plus matrix 

decrease significantly eastward; for example, the Marmot 

syncline samples average only 2 percent cement and between 10 and 15 percent when matrix is added. 

Carbonate cement is a major component in some 

86 Geology of the Nanushuk Group and Related Rocks, Alaska 

of the sandstones of the Nanushuk Group. In some samples, the amount of carbonate cementation is related to 

the abundance of detrital calcareous grains in the sandstones (Bartsch-Winkler, 1979; Huffman, 1979). Outlines 

of original (calcareous?) grains replaced by calcite cement 

may be visible (fig. 43A). Replaced framework grains, 

such as quartz, may appear to be floating in carbonate 

cement (fig. 43B). The carbonate cement apparently replaces clay and siliceous cements in sandstones of the 

Nanushuk Group and commonly appears as the last stage 

of alteration. 

Early diagenetic clay cements, which probably 

formed at intermediate depths of burial (Jonas and 

McBride, 1977), include chlorite, kaolinite, and illite and 

are common in samples of the Nanushuk (Bartsch-Winkler, 1979). The clay is typically authigenic rather than 

detrital (that is, it develops in place rather than having 

been transported and deposited contemporaneously with 

the grains) and forms by alteration and (or) compaction 
Figure 41. lntergrain boundaries in sandstone from the 

Nanushuk Group. A, Displacement of pebble boundary 

and extensive cracking as a result of compaction pressure. 8, Stylolitic interpretation of rounded quartzose 

of framework grains. The clay cement may be oriented 

fibers growing in and around grains and pores (figs. 44, 

45, and 46) or, in some places, may grow secondarily 

after partial dissolution or replacement of detrital grains 

(figs. 44 and 45). Other workers have shown that clay 

cement that coats quartz-grain surfaces may prevent development of quartz overgrowths (Fuchtbauer, 1967; Pitman, 1972; Heald and Larese, 1974) and this relationship 

may apply to Nanushuk sandstones (fig. 46). Clay cements, because of their stacked or platy morphology (such 

as kaolinite), may give the impression of better porosity 

in laboratory tests that measure porosity and permeability 

(fig. 44). According to Sarkisyan (1971), regularly 

stacked plates of kaolinite may retain as much as 20 percent porosity and a permeability of 8.8 millidarcies. 

Silica is mobilized for cementation by compaction 

pressures and temperatures of sandstones; pressure solution of framework grains (especially quartz), clay-mineral 

alteration during burial, kaolinization of feldspar, dissolution of grains in adjacent shale beds, and replacement of 

quartz and silicate grains by carbonate are thought to provide silica for cementation (Jonas and McBride, 1977). 

Siliceous cementation in sandstones of the Nanushuk 

pebbles, probably resulting from high compaction pressures at grain boundaries. Sample 78AAh47l, Tuktu Bluff, 

magnification, 2.5X10, photomicrograph. 

Group may result from a combination of these factors. 

Higher pressures along resistant quartz-grain boundaries 

cause silica to dissolve (seen as sutured, concavo-convex, 

and long or straightgrain boundaries) and to be redeposited as quartz overgrowths (figs. 47 and 48). Typically, 

quartz overgrowths in samples of the Nanushuk occur 

after formation of clays. However, where framework 

grains do not have clay rims, or where siliceous cementation is abundant, overgrowths and areas with significant 

siliceous cement may be sometimes difficult to distinguish 

from the angular detrital grains. The amount of siliceous 

cementation in these sandstones may, therefore, have been 

underestimated. Chalcedony and feldspar, in rare instances, may also be present as cement. 

POROSITY 

Because of unsatisfactory impregnation of original 

pore spaces by blue plastic dye before thin-section preparation and because of abundant grain plucking, conclusions on the type and amount of pores retained by these 

Petrography of Sandstones 87 
Figure 42. Tightly appressed sandstone from the 

Nanushuk Group, showing squeezing and contortion of 

less competent framework grains around more resistant 

grains and into intergranular spaces. Some cracking of 

resistant grains has also taken place. Sample 77 AAh21, 

Tuktu Bluff; magnification, 6.3X10, photomicrograph. 

sandstones and the relation between pores and cements 

are only tentative. Further study is warranted if details 

are to be ascertained. 

Average visible porosity, calculated from questionable point-count data, is not encouraging from a reservoir 

standpoint. Visible porosity values are highest in the 

Tuktu Bluff samples (average 2 percent) and lowest in 

the Arc Mountain samples (average 1 percent). Significant 

values (as much as 16 percent) of visible porosity are 

sporadic. The data suggest that the coarser sandstones 

have a higher incidence of visible porosity and that the 

Figure 43 (at right). Replacement by calcite cement in 

sandstone from the Nanushuk Group. A, Calcite replacement of detrital grains, leaving a clay rim around original 

(calcareous?) grain. Sample 78ACh35, Kurupa anticline; 

magnification, 6.3X10. 8, Framework quartz grain appears to float in calcite cement after partial replacement, 

but relict grain boundary is still visible. Sample 78ACh16, 

Kurupa anticline; magnification, 16x10, photomicrograph. 

88 Geology of the Nanushuk Group and Related Rocks, Alaska 
Figure 44. Kao-linite filts-poTe spaces created by dissolution and (or) replacement of framework grains, in sandstone from the Nanushuk Group. A, Sample 78ACh54, 

Kurupa anticline. 8, Sample 78ACh67, Kurupa anticline. 

Magnification, 16X10, photomicrographs. C, Books of 

hexagonal kaolinite crystals growing in pore space. Sample 78ACh41, Marmot syncline; magnification, x1 ,340, 

SEM. 

porosity is localized. The medium- and coarse-grained 

sandstones have a larger percentage of quartzose grains, 

and original intergranular pore spaces are more likely to 

be preserved in these rocks than in the finer grained sandstones that are composed of less resistant, squeezed 

grains. The grains in the coarser, quartz-rich sandstones 

apparently have more exposure to dissolving fluids that 

move through the rock creating secondary porosity. The 

diagenetic processes that result in grain dissolution are 

complex; two factors thought to contribute to the development of secondary porosity, pressure and temperature, 

were discussed by Jonas and McBride (1977, p. 91-95A). 

Although the porosity values in the Nanushuk 

Group are generally not significant, leaching and corrosion of siliceous framework grains apparently help to 

create secondary porosity in these rocks (fig. 49; BartschWinkler, 1979). Secondary porosity was recognized using 

a petrographic microscope and the criteria listed in Scholle 

( 1979, p. 171). Such features as partial dissolution and 

Petrography of Sandstones 89 
Figure 45. Original porosity is still retained in this sandstone sample from the Nanushuk Group, but some reduction in porosity probably took place during compaction. Quartzose grains were dissolved at grain contacts 

and welded. At some later stage, minor secondary porosity was created by dissolution of quartzose grains 

(arrow). Growth of chlorite in pore spaces completes 

diagenetic cycle. Sample 78ACh32, Kurupa anticline; 

magnification, 6.3X10, photomicrograph. 

corroded grains and cements, inhomogeneity of packing, 

oversize pore spaces, floating grains, and fractured or 

cracked grains were observed (fig. 50). Secondary porosity may subsequently be totally or partly destroyed by 

infilling of diagenetic clays and cements. 

Very small quantities of black to dark-brown, 

slightly translucent material in some samples was suspected to be oil, though no hydrocarbon tests were conFigure 46 (at right). Quartz-grain surfaces are coated 

with chlorite in sandstone from the Nanushuk Group, 

which may inhibit development of quartz overgrowths. 

A, chlorite cement might facilitate retention of primary 

porosity by reducing siliceous cementation (presolution). 

Sample 78ACh17, Kurupa anticline; magnification, X370, 

SEM. B, Quartz overgrowth is surrounded by a halo in 

which no chlorite cement exists. It is uncertain which 

cement is inhibiting growth of the other. Sample 

78ACh32, Kurupa anticline; magnification, x319, SEM. 

90 Geology of the Nanushuk Group and Related Rocks, Alaska 
Figure 47. Quartz overgrowths on sandstone grains 

after clay formation, Nanushuk Group. A, Quartzose 

grains have overgrowths at points of low pressure (pore 

space) adjacent to areas that were probably under high 

pressure (grain contacts). Overgrowths probably develop 

where clay coatings are absent. A polycrystalline quartz 

grain (upper left) has a pod of kaolinite growing on its 

surface. Sample 78ACh23, Kurupa anticline; magnification, 6.3x10, photomicrograph. 8, Framework quartz 

grain with vacuoles (F) is outlined by diagenetic clay. 

Clear areas of quartz-overgrowth development (Q) are 

probably initiated where clay rim is absent. Secondary 

porosity (P) may be created by leaching of siliceous cement; no clay development is taking place in the secondary pore space. Sample 78ACh32, Kurupa anticline; 

magnification, 6.3X10, photomicrograph. C, Chlorite cement and quartz overgrowths clog intergranular pore 

spaces. Sample 78ACh32, Kurupa anticline; magnification, X234, SEM. 

ducted. The hydrocarbons(?) appear to have migrated into 

the sandstone during or after the compaction process (fig. 

51). 

PROVENANCE 

The sandstone compositions of the Nanushuk Group 

from the eastern outcrop belt are high in quartzose and 

metamorphic lithic grains and low in feldspar grains (table 

Petrography of Sandstones 91 
92 Geology of the Nanushuk Group and Related Rocks, Alaska 
Figure 49. Secondary porosity in Nanushuk Group sandstones. A, Scanning electron microscope shows leaching, corrosion, and honeycombing of framework grains. 

Sample 78ACh76, Kurupa anticline; magnification, X143, 

6). The average sandstone contains about 45 percent 

quartzose grains (27 percent monocrystalline quartz 

grains) and about 30 percent lithic grains ( 14 percent 

metamorphic lithic grains). Feldspars, almost exclusively 

plagioclase grains, make up from 1 to 5 percent. According to Dickinson (1970) and Folk (1968), sandstones of 

this composition originate in a tectonically active terrane, 

and this origin is borne out for the Nanushuk sequence 

by geologic mapping in the North Slope and Brooks 

Range regions (Grybeck and others, 1977). Thrust sequences active in the Brooks Range during Cretaceous 

Figure 48 (facing page). Quartzose sandstone from the 

Nanushuk Group, having abundant quartz-overgrowth 

development. Sample 78ACh25, Marmot syncline. A and 

8, iron-rich(?) clay outlines original grain boundaries and 

overgrowths from adjacent quartz grains in optical continuity with parent grain. A, plain light; 8, polarized 

light. Magnification, 2.Sx10. C and D, quartz cement 

migrated from the monocrystalline quartz grain into the 

adjacent polycrystalline quartz grain, which has been 

corroded. C, plain light; 0, polarized light. Magnification, 6.3X10, photomicrographs. 

SEM. B, Irregular surface on quartz-overgrowth crystal 

indicates possible dissolution of siliceous cement. Sample 78ACh17, Kurupa anticline; magnification, x1220, 

SEM. 

time provided a structural and probably a topographic high 

that shed sediments northward into the Colville basin 

(Detterman, 1973; Mull, this volume). In the Brooks 

Range, clastic rocks of the Upper Devonian Kanayut Conglomerate and Hunt Fork Shale, both of the Endicott 

Group, may have been the source of the quartzose component in the sandstones of the Nanushuk. Chloritic phyllite 

and graywacke sequences of the Hunt Fork Shale and related sedimentary rocks probably contributed metamorphic 

detritus to the Nanushuk Group. Shale clasts containing 

sponge spiculites similar to those pictured in Mamet and 

Armstrong (1972, figs. 5C and 5D, p. C134) from the 

Early Mississippian Kayak(?) Shale and also present in 

the Lisburne Group (A. K. Armstrong, oral commun., 

1979) are abundant in sandstones of the Marmot syncline 

section. 

PETROLEUM POTENTIAL 

The results of this study indicate that the sandstones 

of the Nanushuk Group are borderline petroleum reservoirs. Sandstone comprises only 20 percent of the meaPetrography of Sandstones 93 
94 Geology of the Nanushuk Group and Related Rocks, Alaska 

Figure 50. Secondary porosity features in sandstone 

from the Nanushuk Group. A, Apparent partial dissolution of a rounded polycrystalline quartz grain creates 

secondary porosity in a highly compacted sandstone. 

Sample 78ACh72, Kurupa anticline; magnification, 

6.3X10, photomicrograph. 8, Abundant partial dissolution of grains results in elongate and oversize pores and 

floating grains. Sample 78ACh72, Kurupa anticline; magnification, 6.3x10, photomicrograph. C, Embayed grains 

give secondary pores rounded borders. Pores are oversize and may have resulted from complete dissolution 

of a framework grain or from dissolution of cement that 

replaced a framework grain. Sample 78ACh79, Kurupa 

anticline; magnification, 2.Sx10, photomicrograph. 

sured sections, on the average, and most of this is in 

lenticular bodies containing extensive shale interbeds. 

Many sandstones are shaly or muddy because of mixing 

by burrowing organisms. The sandstones, classified as 

phyllarenites or litharenites, are mostly fine to medium 

grained and are moderately to well sorted. Porosity has 

been destroyed by compaction and cementation, and permeability values are expected to be correspondingly low. 

Where visible porosity is present, it is localized and occurs probably as a result of depositional environment of 

the sandstones, which influences the grain size, or of postdepositional processes, which create secondary porosity. 
Figure 51. Possible presence of hydrocarbons(?) in 

Nanushuk Group rocks. Migration apparently occurred 

during or after compaction of the sandstones. A, Possible hydrocarbon(?) migration is facilitated along solution 

seams probably formed during compaction. Sample 

78ACh34, Marmot syncline; magnification, 2.5X10, 

photomicrograph. 8, Possible hydrocarbon(?) migration 

into intergranular pore spaces and into a grain cracked 

by compaction. Dissolution pore (P) unfilled by hydrocarbon indicates that migration may have occurred prior 

to creation of dissolution porosity. Sample 78ACh25, 

Marmot syncline; magnification, 2.5X10, photomicrograph. 

Petrography of Sandstones 95 

An Analysis of the Umiat Delta Using Palynologic 

and Other Data, North Slope, Alaska 

By Fred E. May1 and john D. Shane2 

INTRODUCTION 

Fieldwork on the northern Alaska Nanushuk Group 

(Albian-Cenomanian) in the eastern National Petroleum 

Reserve in Alaska (NPRA) yielded sedimentological evidence of a northward prograding delta herein referred to 

as the Umiat delta (fig. 52). This delta is different from 

the northeastward prograding Corwin delta, which is generally considered to be the result of most of the AlbianCenomanian deposition in the NPRA (Ahlbrandt and 

others, 1979). Evidence of the Umiat delta was observed 

in many detailed measured sections of the Nanushuk, and 

consists of paleotransport-direction measurements and sedimentary structures. Sand-percentage contours and sandisopach maps plotted from data from wells within the area 

(Bird and Andrews, 1979; Huffman and others, this volume) show the general shape of the delta. 

Because palyniferous assemblages occur in wells 

within the area of the proposed Umiat delta, this study 

was undertaken to determine the geologic history of the 

Umiat delta using palynology. These wells are within the 

boundaries of the western part of the delta. Although the 

eastern boundary of the delta is in the area of the Marmot 

syncline, no wells have been drilled there. Outcrop samples in this eastern area, between the Grandstand-! well 

and the Marmot syncline, yielded poor pollen, spore, and 

dinoflagellate-acritarch assemblages and are not reported 

here. 

Palynological counts were made on 164 core samples from the Nanushuk Group from 10 test wells on the 

North Slope of Alaska. Pollen groups counted were gymnosperms, spores, angiosperms, dinoflagellates, and acritarchs; approximately 25,000 grains were counted. These 

count data are presented here as composite relative-frequency graphs, and a preliminary paleoecological interpretation is made for each well. 

Also evaluated were the general characteristics of 

the relative abundances of pollen groups in each well and 

the coal occurrence. This report illustrates and discusses 

the data, but the meaning of the data is left to the reader 

to interpret. Some of the data may yield direct 

1

Presently with Idaho State University, Pocatello. 2 Presently with Exxon Co., U.S.A., Houston, Tex. 

paleoecological information, whereas other data may yield 

information about the hydrodynamics of the delta and associated marine systems responsible for the deposition of 

the sediments. 

Some conclusions are presented as preliminary 

hypotheses. For example, the paleoecological interpretations are based on the assumption that most of the pollen 

and spores observed originated from near the well site. 

This assumption is probably more accurate for the spores 

and angiosperm and nonconiferous gymnosperm pollen 

than for the conifer, dinoflagellate, and acritarch pollen, 

but until detailed work is completed on the distribution 

of individual taxa, we treated all major groups counted 

as having been deposited near their localities of origin. 

There is reason to assume that nearby deposition was possible, and this possibility is discussed. 

Field localities for invertebrate megafossils reported 

by Imlay ( 1961) are noted to show what types of mollusks 

and other animals have been found at particular points 

within the Umiat delta. These data help to determine the 

range of paleoenvironments for the formations at certain 

localities within the delta. Paleobotanical data (Smiley, 

1967, Lowther, 1957) has aided in suggesting the type 

of climate during the time of deposition. 

STUDY AREA 

The proposed Umiat delta is in an area between 

lat 68°50' and 69°50' N. and long 156°00' and 151 °20' 

W. on the North Slope (fig. 52). In general, the area 

lies in the southeast corner of the NPRA, and also slightly 

south and east of it. It is called the "Umiat delta" because 

one of its proposed main central lobes moved over the 

Umiat area. Palynological count data were gathered from 

the following wells: Grandstand-!; Umiat-1 and -11; 

Square Lake-1; Wolf Creek-2and-3; Titaluk-1; Knifeblade-!; Oumalik-1; and East Oumalik-1. Some observations are used from Gubik-I and -2, and Umiat-2. 

GEOLOGIC UNITS 

The Nanushuk Group in this area was deposited by 

the prograding Umiat delta. Formations in the Nanushuk 

are the Grandstand and Tuktu Formations (delta-front deAnalysis of the Umiat Delta 97 
Chukchi Sea Beaufort Sea 

NATIONAL PETROLEUM RESERVE IN ALASKA 

~-----...... 

I 

I 

I 

I 

I 

.J 

50 MILES 

EXPLANATION 

UMIAT DELTA • TEST WELLS 

Figure 52. Index map showing the general Umiat delta area (Huffman and others, this volume) and test-well 

locations, North Slope, Alaska. 

posits of middle to late Albian age), the Killik Tongue 

of the Chandler Formation (delta deposits of probable late 

Albian to early Cenomanian age), and the Ninuluk Formation (mixed delta and marginal-marine facies deposited as 

a result of the Seabee marine transgression into the Umiat 

delta; this unit is early Cenomanian in age). 

All formational units referred to in this report are 

based on the work of Collins (1958, 1959) and Robinson 

(1956, 1958, 1959) of the U.S. Geological Survey. 

METHODS 

All samples were processed by standard palynological maceration procedures; samples were not sieved. 

98 Geology of the Nanushuk Group and Related Rocks, Alaska 

Palynomorphs were mounted in A Y AF1 plastic mounting 

medium on microscope slides. Palynomorphs were 

examined with Nomarski interference contrast. Counts 

were made on representative samples from each core; 

counts of as many as 200 specimens (angiosperms, gymnosperms, spores, and dinoflagellates-acritarchs) were 

made on each sample, depending upon the quality and 

quantity of grains present on a slide. Relative frequencies 

of these palynomorph groups were calculated and plotted 

as composite relative-frequency diagrams for purposes of 

comparing the fossil groups. 

Dominant fern-spore frequencies are considered to 

1

Use of trade names does not imply endorsement by the U.S. Geological Survey. 
represent wetter parts of the delta; dominant gymnosperm 

pollen (primarily bisaccates) are considered to represent 

drier or better drained parts of the delta; dinoflagellate 

and acritarch frequencies are considered to represent 

marine areas, or encroaching marine areas that were likely 

often brackish. 

Coal is considered to represent periods of subaerial 

deposition or exposure. Leaf impressions and carbonaceous material in the strata are generally considered to 

represent subaqueous deposition within the delta or near 

the delta front. The presence of marine dinoflagellates and 

various marine megafossils indicates intervals of marine 

deposition. The presence of brackish-water dinoflagellates 

indicates intervals of brackish-water deposition. 

PHYSIOGRAPHY AND PLANT ECOLOGY OF 

MODERN DELTAS IN WARM, HUMID 

CLIMATES 

Although the specific kinds of vegetation growing 

on modem deltas are different from those of the AlbianCenomanian, it is still reasonable to expect that the physical environments in the deltas have remained much the 

same. It also seems possible that these physical environments supported vegetation that was structurally similar 

to vegetation that grows in similar environments today. 

Modem deltaic environments can be grouped under 

the following headings (from Fosberg, 1964): 

1. Water 

a. Distributaries.-Main branches through which 

the river empties into the sea. These tend to be 

the larger channels in the delta, and are arranged 

fanwise, forking and spreading out, and often 

becoming very wide in their lower reaches. 

b. Delta channels and tidal channels.-Channels 

that originate within the delta itself, draining the 

lakes, ponds, swamps, and marshlands. They 

tend to be closely meandering and intricately 

branched. This pattern is striking, very uniform, 

and immediately recognizable from the air. The 

origin of the pattern is not clear, though it is 

undoubtedly related to the near lack of any slope 

and consequent low cutting or scouring power 

of the tidal currents. The water escaping through 

the delta and tidal channels is primarily from rain 

floods and that which entered at high tides. 

c. Lakes and ponds.-Bodies of still water ranging 

from extensive basins to small ponds and pools. 

Some have outlets; others do not. All have a 

tendency to fill with sediment and plant remains 

and to change gradually to marshes or swamps. 

(1) Interconnecting basins.-These originate as areas of shallow sea cut off by 

sediments deposited, as the delta builds 

2. Wetlands 

outward, as natural levees, sand bars, 

and ridges. At first they may be practically open to the sea, merely deeper 

areas in the shallow water over the extensive silt deposits of the extending 

delta. 

(2) Levee flank depressions.-The sediments that make up deltas normally contain a large component of water and 

much organic matter. Organic matter 

may be especially abundant in the 

swampy strips along the outer slopes of 

natural levees. During periods of low 

water, both decomposition of organic 

matter and loss of water from the sediments by evaporation and gravity flow 

take place at greater than normal rates. 

Compaction and settling, both from the 

above processes and from slow but continuous action of gravity, tend to produce long narrow ponds alined along the 

outer edges of the natural levees. 

(3) Abandoned channels or oxbow 

lakes.-As channels, especially distributaries, fill up with silt, they tend to 

cut through their enclosing natural levees 

and assume new courses, especially 

when floods occur. This commonly 

takes place on the seaward side of a seaward loop of a meander, and the new 

channel, being straighter at first, takes 

most of the water. The old channel is 

soon cut off by newly formed natural 

levees and is left as a lake, often a Cshaped lake called an oxbow. 

a. As lakes, ponds, and channels fill with silt and 

organic debris, they gradually change from areas 

of permanent standing water to land that, although still wet, is firm enough to support emergent vegetation. In the tropics the vegetation of 

such land tends to be woody, trees or shrubs, 

and the ecosystem is called a swamp. Marshes 

and wet savannahs are, however, not completely 

lacking in woody plants. There trees become 

dominant in wet sandy areas, peat accumulates, 

and a peat swamp results. Wet savannahs and 

peat swamps are generally well away from the 

strongly salty end of the salinity gradient, toward 

the head of the delta. They are also confined 

to areas where the water supply is principally 

rain and ground-water seepage rather than 

floods. 

3. Drylands 

a. Delta terraces.--On many deltas, there are eroAnalysis of the Umiat Delta 99 
sional remnants of former delta land surfaces that 

formed during times of higher sea-level stands. 

They are flat or very gently sloping platforms, 

found near the head of or the landward margins 

of the present-day deltas. They are normally 

drier than the rest of the delta. The soil is older 

and more weathered, often severely leached if 

the delta is in a rainy region. In very humid regions the vegetation may be the normal lowland 

rain forest of the region. 

b. Natural levees.-In the distributaries, the water 

in the center of the channel flows faster than 

at its sides. Consequently, more sediment is 

dropped at the sides than in the center, and the 

bottom tends to build up along the sides while 

the center is kept scoured clean, and even 

deepens. As the incipient natural levees thus 

formed are left dry during low water in the dry 

season, the propagules of mangroves and other 

plants gain a foothold. At high water the resulting vegetation, though small and sparse, slows 

the water still more and causes the deposition 

of more silt, building up the levee. Further silt 

deposition takes place during each flood season, 

and further establishment of vegetation occurs 

during each dry season. As the level of the strip 

of land becomes higher, flooding may become 

less frequent. The composition of the vegetation 

changes as dryland conditions become more 

prevalent. Swamp species give way to terrestrial 

plants. A riparian forest is formed, made up of 

trees well adapted to land having a rather high 

permanent water table. 

c. Sand ridges and flats.-Along the seaward margins of deltas, beaches are formed in proportion 

to the roughness of the sea, and sand is piled 

up in ridges. Offshore bars may form and move 

shoreward under normal storm-wave action until 

they are piled against the beach ridges, forming 

a broader ridge or a parallel series of ridges. 

Eventually, narrow sand flats are formed, parallel with the shore. These ridges and flats tend 

to invade and cover marshes or swamps or to 

protect them in some measure from wave erosion, or both. The sand is soon covered by a 

considerable assortment of beach plants: grasses 

with deep rhizomes and roots, various creepers 

that root at the nodes and form mats anchored 

to the sand, and firmly rooted herbs and shrubs 

that tend to form a low shrub vegetation, often 

dense. Trees in the Old World, especially 

Casuarina equisetifolia, tend to grow on these 

ridges, usually starting while there is still bare 

sand and only grass; although slow growing, 

1 00 Geology of the Nanushuk Group and Related Rocks, Alaska 

they gradually overtop the scrub and form a 

characteristic strand forest. 

A COMPARISON OF TEMPERATE AND 

TROPICAL DELTAS 

Because the Umiat delta appears to have formed 

under humid, warm-temperate to subtropical conditions, 

indicated by the palynoflora (see the section on 

"Paleoecology"), comparison of temperate and tropical 

deltas is of interest. Two main differences between temperate and tropical deltas are that tropical deltas are 

flooded to greater depths than are temperate ones, and 

tropical deltas tend to have woodier vegetation. These two 

factors seem to go together, as deep flood waters likely 

reduce photosynthesis for smaller herbaceous vegetation, 

whereas taller woody vegetation would grow better in 

such conditions. Woody vegetation also creates peat bogs, 

which are characteristic of tropical deltas. Temperate deltas have fewer peat bogs; they seem to be replaced by 

freshwater marshlands (Fosberg, 1964, p. 229). 

PALEOECOLOGY 

Paleobotanical information was interpreted by 

Smiley (1967, p. 853) as indicating that the climate of 

northern Alaska during Albian-Cenomanian time was 

warm and humid, and that similar vegetation today grows 

south of lat 30° N. During the present palynological study, 

as many as 65 genera of fern and other spores have been 

observed, suggesting a diverse fern-lycopod-bryophyte 

flora within the wetter parts of the Umiat delta. The approximately 10 genera of bisaccate gymnosperm pollen 

are interpreted as having originated partly from within better drained parts of the delta. Angiosperm pollen make 

up a minor component of the overall palynoflora. Thus, 

the vegetation consisted mainly of ferns, lycopods, some 

bryophytes, and a variety of gymnosperms existing within 

an actively building delta system in a warm, humid environment. No doubt much of the gymnosperm flora existed 

in the uplands. 

Coastal waters are thought to have been temperate, 

not warm, due to the presence of pectens and abundant 

burrowing pelecypods. Pelecypods that lived in and near 

the Umiat delta are ones common to shallow Albian Age 

seas of northwest Europe (Imlay, 1961, p. 16-17; Nicol, 

1955, p. 122). 

The relatively small amount of coal formed within 

the Umiat delta, as compared to the large amount formed 

within the Corwin delta, is likely related to higher energy 

within the shifting Umiat fluvial system and to biogenic 

degradation of organic material. Relative frequencies de-
termined in this study for spores, gymnosperms, angiosperms, and dinoflagellates and acritarchs for 10 test wells 

within the confines of the Umiat delta area suggest that 

adjacent areas of vegetation were nearly always undergoing change. This change is reflected in the alternation of 

dominance and the widely fluctuating frequencies. Although dinoflagellate and acritarch percentages are generally lower than those of gymnosperms and spores, rapid 

increases in percentages of dinoflagellates and acritarchs 

suggest frequent marine inundation of large areas within 

the delta, and such inundations too would be detrimental 

to coal formation, both in the cleaning out of peat deposits 

as well as increasing the salinity in large areas of freshwater that supported coal-producing vegetation. The widespread presence of dinoflagellates and acritarchs in samples examined from these 10 wells suggests that the delta 

was deposited primarily at sea level. Mollusks collected 

within the Umiat delta (discussed later) also indicate a 

wide range of environments, from freshwater to inner 

shelf. 

The lush vegetation suggested by the diverse pollen 

and spore assemblages in the Nanushuk Group is difficult 

to reconcile with a long Arctic night during AlbianCenomanian time. Today, the Arctic winter months are 

dark due to the 23 1/2° angle of inclination of the Earth's 

rotational axis. Other than small wobbles through time, 

the Earth's rotational axis has been considered to have 

remained essentially in its present orientation. Because of 

this, continental plate movements, based on paleopole estimates, have provided one of the main tools for continental-drift and sea-floor-spreading reconstructions. The 

highly diverse vegetation found in the Arctic AlbianCenomanian strata suggests a warm, humid climate somewhat similar to present-day climates at lower latitudes. 

It is difficult to visualize such a climatic setting with a 

long Arctic night. 

If the long Arctic night did not exist during AlbianCenomanian time, then only a few explanations are possible. Either the angular inclination of the Earth's axis has 

changed considerably, or the Arctic-Alaskan plate has 

been much more south due to continental drift. 

The present understanding of plate tectonics 

suggests that North America has rotated counter-clockwise 

to its present position. During Albian-Cenomanian time, 

Alaska appears to have been north of its present position 

(Irving, 1979, p. 684), suggesting that such an Arctic 

night would have been even more of a factor than today. 

Thus, continental drift does not appear to explain the rich 

and diverse flora in this time and location. 

The other explanation of why there was such a diverse flora in the Arctic during Albian-Cenomanian time 

is a change in the inclination of the Earth's rotational 

axis relative to the plane of the ecliptic. Wolfe (1978, 

p. 701-702) provided an interesting discussion of this 

idea, using paleobotanical evidence. His studies of Tertiary Arctic vegetation suggest that the Earth's axis inclined from about 1 oo during the Paleocene to about 5° 

during the middle Eocene, thus nearly negating the Arctic 

night. Although the lower to middle Cretaceous floras of 

the same region are more difficult to equate with modem 

ones than were these reported by Wolfe from the Tertiary, 

a change in axis inclination could still have occurred during the Cretaceous. No new evidence is furnished here, 

but the high numbers of fern, bryophyte, and lycopod 

spore taxa, some 65 genera, combined with cycad pollen 

suggest a flora that would not have survived the yearly 

dark period. 

Another possibility suspected by Smiley (1967, p. 

861) is that the Albian-Cenomanian forests of the Arctic 

had adapted to extended periods of darkness and had survived in spite of it. Smiley's argument revolves around 

deciduous plants, and he suggests that this kind of vegetation would have been dormant during the prolonged 

winter darkness and have not needed daylight. Therefore, 

Smiley thought that no change in axial inclination was 

necessary. He pointed out some possibilities that could 

have helped cause warmer climates in the Arctic, such 

as periods of extreme volcanism. He also mentioned the 

possibility of changes in extraterrestrial influences on the 

Earth. Such factors have been reported by Gribbin (1979), 

who discussed climatic change in terms of fluctuations 

in solar radiation, solar flares, cosmic seasons, the effect 

of passing through the spiral arms of the galaxy or through 

galactic dust lanes, and variations in eccentricity of the 

Earth's orbit. 

It seems reasonable that Alaska was either farther 

south during Albian time, or that the Earth's axial inclination was different, or else that a combination of both factors reduced considerably the effect of the prolonged Arctic darkness. 

Paleofloral Components 

Delta-Front Colonizers 

One consideration in the development of deltaic 

vegetation is whatkind of vegetation constitutes the deltafront colonizers. Typically, in temperate deltas, herbaceous forms colonize new areas, but in tropical areas several species of trees, called mangroves, grow in newly 

built areas due to their viviparous dispersal mechanism 

and their ability to trap sediment and withstand high 

salinities. Mangrove species today are angiosperms, a 

group of plants that did not appear in the fossil record 

until Albian time, but it appears unlikely that angiosperm 

mangroves existed then. The earliest angiosperms on the 

North Slope, such as the few primitive forms in the Umiat 

Analysis of the Umiat Delta 101 
delta, were most likely small herbaceous forms that occupied disturbed areas within the delta. Perhaps, though, 

nonangiospermous Albian mangroves did exist, and may 

be identified through palynological and paleobotanical 

work in ancient deltas. These might be trees that could 

tolerate the high salinity, fluctuating depth from tides, and 

rather high energy that exists in the area where the delta 

meets the sea. (We have no idea what Arctic tidal and 

wave energies would have been. Today, wave energies 

are low due to ice cover). Angiosperms probably did not 

form the colonizing vegetation, as they appear to have 

been a new and minor component during the Albian-early 

Cenomanian in the Arctic areas. The colonizers probably 

would have been gymnosperms (conifers), lycopods, or 

ferns. If they were trees, they could be called mangroves. 

If they were herbaceous, then the seaward margin of the 

delta would likely have appeared similar to some modern 

temperate deltas, lacking mangroves. It is noteworthy that 

robust and complex root systems that characterize mangroves have not been reported among the fossil plant remains of the Albian of the North Slope. This lack of 

record would be unusual, because if mangroves had been 

present they would likely have formed substantial root 

mats with aerial structures acting as silt catchers. 

Any fossil mangrove roots should have been buried 

in fine-grained sediment that would have allowed preservation. A considerable amount of paleobotanical work has 

been done on the North Slope deposits of Albian-early 

Cenomanian age, and many well cores and measured sections have been examined with no apparent observation 

of such root systems. It seems likely then that such mangrove root systems were not present, and that the most 

probable delta-front colonizer was not like present mangroves, at least in its root systems. It seems at present 

that the most likely colonizers would have been euryhaline 

herbaceous plants. These could have been gymnosperms, 

lycopods, or ferns. The gymnosperm pollen observed is 

primarily coniferous. It is difficult to imagine conifers as 

mangroves or as euryhaline colonizers. Except for Cypress, conifers are not known to inhabit modern 

euryhaline environments. Lycopod spores are not nearly 

as diverse as the ferns. Therefore, the most likely deltafront colonizers appear to be ferns. 

Although few modern ferns favor aquatic, brackish 

environments, one genus, Acrostichum, does favor terrestrial brackish-marsh conditions (Copeland, 1947, p. 64). 

In Lake Maracaibo, Venezuela, Acrostichum grows in a 

swampy delta-front region of brackish water in areas of 

dense vegetation that are difficult to walk through 

(Tschudy, 1969, p. 81; Tschudy, oral commun., 1979). 

Similar fern overgrowths could have been successful colonizers in the Albian-Cenomanian Umiat delta because 

tide ranges were likely low, as they are today, and there 

would have been little destruction at the delta front. For 

102 Geology of the Nanushuk Group and Related Rocks, Alaska 

example, a spring tide, measured 130 km northwest of 

Point Barrow in 1962, was only about 18 em. In the 

Chukchi Sea, sea level has been noted to change by only 

about 1 m in 1 week, and in general, tide range in the 

Chukchi Sea is about 20 em (Coachman and Aagard, 

1974, p. 31), due to local wind and (or) atmospheric pressure fields. In such conditions, herbaceous colonizers 

would have had little difficulty in becoming established. 

The main tolerance they would have had to develop was 

to higher salinities, and, therefore, such fern types as the 

modem Acrostichum would be very likely to have been 

colonizers in the Umiat delta. 

Mixed Swamp-Forest Vegetation 

Behind the delta-front colonizers, we postulate a 

mixed swamp-forest vegetation occupying primarily the 

wetlands and water areas described previously. Leveed 

distributaries and delta channels likely meandered through 

these areas. Because these areas probably contributed 

most of the spores and pollen counted in this study, dominances of particular groups, such as fern spores, probably 

represent these areas. It is suggested that as fern, lycopod, 

and bryophyte spore trends increased to dominance in the 

count data for a particular well, that areas near the well 

site were becoming wetter, suggesting development of 

these water and wetland areas. Suspected taxodiaceous 

pollen observed in the wells possibly represent the conifer 

vegetation within such areas. Elatides, a suspected 

taxodiaceous plant megafossil (Lowther, 1957, p. 16), has 

been identified from Oumalik-1 well (Robinson, 1956, 

p. 27). The rare angiosperm pollen observed likely represent the newly appearing angiosperms as they occupied 

disturbed areas, such as along cuts in river banks, 

breached levees, or other areas affected by crevasse 

splays. Otherwise, most of the representative 

palynomorphs probably come from the highly diverse 

ferns and the less diverse lycopods and bryophytes. 

Highland Forest Vegetation 

Behind the delta system, southward in the highlands, which may have been a few tens of kilometers 

away based on the nearest persistent granular or conglomeratic sediments, in the foothills of the ancestral 

Brooks Range, we postulate a primarily coniferous forest. 

This upland area probably yielded pollen such as Pinuspollenites, Vitreisporites, Podocarpus, Araucariacites, 

and possibly Sequoiapollenites. As dry lands developed 

within the delta system, we believe that the conifer vegetation, represented by their pollen types, colonized these 

areas, creating the source for some of the coniferous pollen that influenced the trends in the illustrated counts 

(figs. 54-62) for the gymnosperm group. Most of the fossil wood collected within the boundaries of the adjacent 
Corwin delta system is coniferous, and petrified coniferous tree trunks have been observed there in probable 

growth position (R. A. Scott, oral commun., 1979) in 

the Corwin Formation, which is considered to have been 

environmentally similar to the Umiat delta Killik Tongue 

of the Chandler Formation discussed here. Fossil conifer 

leaves possibly of the fossil genus Elatocladus have been 

observed by May (unpub. data) in great numbers in the 

Killik Tongue of the Chandler Formation on the Anaktuvuk River at the type locality of the Grandstand Formation. Other conifer remains have been observed by us in 

Titaluk-1 well. Robinson (1956, p. 27) reported Elatides 

in the Grandstand of Oumalik-1 well. Collins (1959) reported Cephalotaopsis from Square Lake-1 well. These 

occurrences suggest the presence of conifers within the 

delta system. Dominances of bisaccate conifer pollen have 

been observed in the Grandstand Formation in the westemmost part of the Umiat delta from Titaluk-1 and East 

Oumalik-1 wells. These dominances may reflect the development of better drained land areas near there. These 

dominances are discussed specifically later. 

An argument can be made that these pollen and 

leaves have washed in along distributaries, or that the pollen have been transported by wind or water currents for 

considerable distances. Such interpretations can be readily 

determined by the reader, and we feel that some degree 

of the count data represents this amount of transport. 

Based on a few published criteria, we also think that it 

is possible that most of the palynomorphs counted originated within a few kilometers of the well sites (this is 

even more true of spores). Traverse and Ginzberg (1966, 

p. 428) stated that "it must be pointed out that the great 

bulk of pine pollen, or any other pollen, is dropped within 

a few miles of the tree of origin, unless the air is very 

turbulent." They also indicated that pine-pollen deposition 

is generally greater near the source, and that it decreases 

away from the source. Tschudy (1969, p. 80) stated that 

"experiments on modem pollen rains suggest that probably 

most pollen that occur as fossils have been transported 

relatively short distances before becoming incorporated 

into sedimentary deposits." He further stated that the 

"plants growing within a basin or an area of deposition 

may drop their pollen and spores in situ. The pollen will 

then be moved about only by such currents as may be 

active within the area" (Tschudy, 1969, p. 80-81). An 

example provided by Tschudy shows that fern spores 

mainly of the genus Acrostichum growing near the mouth 

of the Rio Catatumbo at Lake Maracaibo, Venezuela, are 

deposited in greatest percentages directly off the mouth 

of that river. We apply these criteria solely as a preliminary approach to the areas within and immediately in front 

of the Umiat delta, anticipating that some unknown 

amount of the pollen and spores counted originated from 

greater distances. 

Marine and Brackish-Water Palynomorphs 

Dinoflagellates and acritarchs occur in most samples 

examined and counted. Although often in low percentages, their presence indicates that the Umiat delta was 

frequently inundated by seawater, which indicates a low 

delta relief, near sea level. Some assemblages in the Killik 

Tongue of the Chandler Formation and the Ninuluk Formation are unique in having small deflandreoid and 

ceratioid complexes not previously described, which probably represent brackish-water environments (May, 1979). 

Some of these forms illustrated in May ( 1979) are identified as: 

Deflandrea species A through H 

Muderongia species A and B 

Other assemblages in the Grandstand Formation and 

the Killik Tongue of the Chandler Formation appear to 

be more characteristically marine and are represented by 

the following (May 1979; May and Stein, 1979): 

Apteodinium granulatum 

Batioladinium jaegeri 

Callaiosphaeridium assymmetricum 

Chlamydophorella nyei 

Cribroperidinium edwardsi 

Gardodinium elongatum 

Hystrichosphaeridium arundum 

Lithodinia stoveri 

Luxadinium propatulum 

Muderongia asymmetrica 

New Genus A 

0/igosphaeridium albertense 

Ovoidinium verrucosum 

Pseudoceratium expolitum 

Spinidinium vestitum 

The study of these probably brackish versus marine 

associations appears to be extremely useful in paleogeographical reconstructions of the Umiat delta. 

CON DillON OF PLANT MATERIAL PRESERVED 

IN TEST WELLS STUDIED 

Fossil plant remains found during logging of the 

test wells in this study were described, for the most part, 

in general terms by Collins (1958, 1959) and Robinson 

(1956, 1958, and 1959). Plant remains seem to be more 

complete and better preserved in two of the westernmost 

wells, Titaluk-1 and Oumalik-1, where nearly complete 

leaves, plant-leaf coquina, numerous black plant impressions, abundant black plant fragments, Elatides sp., and 

Ginkgo digitata were found. In the remaining wells, rather 

nondescript terms were used by Collins and Robinson to 

describe the plant remains, such as plant impressions, carbonaceous plant fragments, coaly plant remains, and carAnalysis of the Umiat Delta 1 03 
bonaceous partings, which suggests that plant material 

east of the Titaluk-Oumalik area is more degraded. One 

reason for the difference in preservation may be because 

sedimentological data suggest that a delta lobe with natural levees extended from the Titalik area to the Oumalik 

area (fig. 1; Huffman and others, this volume). Natural 

levees are one of the longer lasting features of modern 

deltas because leaf litter protects them from erosion by 

rain. This leaf litter comes from riparian forests that 

develop on the levees as dryland conditions become more 

prevalent (Fosberg, 1964, p. 230). One hypothesis, then, 

is that the apparently better preservation of plant megafossils in the Titaluk-Oumalik area resulted from leaf litter 

on levees that were built out from the Titaluk area to 

the Oumalik area. The apparently poorer condition of fossil-plant remains east of there may have resulted from 

the influence of more destructive marine and (or) fluvial 

systems, and the plant material became quite degraded 

before final deposition. 

One curious aspect of the Umiat delta is its small 

amount of coal as compared to that of the Corwin delta 

system to the west. For example, the Nanushuk Group 

of the Corwin delta system at Kaolak-1 well appears to 

have about 60 m of coal (Ira Pasternack, U.S. Geological 

Survey, oral commun.), whereas Titaluk-1 and all of the 

other Umiat delta test wells appear to have less than 6 

m of coal each. The high diversity in the palynomorphs 

(more than 100 genera), especially the spores, suggests 

a lush vegetation within or near the Umiat delta. Apparently the relative lack of coal resulted from biodegradation 

of plant material within the swamps. Nutrients in the 

water probably were abundant, and biological agents 

could have caused deterioration of the organic material. 

This deterioration may account for the carbonaceous 

shales described from some of the eastern wells. At any 

rate, both fluvial and paludal processes appear to have 

degraded the organic material in the east. 

COAL AND SHORELINES 

The presence or absence of coal in parts of the wells 

studied was helpful in determining the general positions 

of shorelines. The presence of coal indicates periods of 

terrestrial deposition, interpreted here as indicating that 

at times the shoreline was somewhat seaward, or north 

of the wells. Intervals containing marine fossils and lacking coal suggest marine deposition and that at times the 

shoreline lay landward, or to the south of the wells. The 

relative direction of the shoreline from each well site, 

based on coal or marine fossils, is one of the main tools 

used here in the reconstruction through time of the Umiat 

delta. 

1 04 Geology of the Nanushuk Group and Related Rocks, Alaska 

PALEOECOLOGY OF INVERTEBRATE 

MEGAFOSSILS 

Invertebrate megafossils collected from the 

Nanushuk Group within the Umiat delta are listed below 

with associated environments (N. F. Sohl and D. L. 

Jones, U.S. Geological Survey, Washington, D.C., and 

Menlo Park, Calif., respectively, oral commun., 1979. 

See fig. 53 for general localities by formation based on 

Imlay, 1961): 

Grandstand Formation (fig. 53A and B) 

Arctica. Slightly brackish water, sluggish, shallow 

burrower. 

Entolium. Bysally attached pectinoid. Can be ambulatory into bays. Definitely marine, but may extend into lower estuaries. Shallow water. 

Corbula. Brackish water. 

Lingula. Muddy sand and fine-grained sediments. 

Also found in Gulf Coast chalks. Sometimes brackish water but not freshwater. 

Inoceramus. Almost completely marine. Nearly any 

water depth. 

Nucula. Likely deepest part of shelf, fully marine. 

Psilomya. Marine shore to shelf; more common at 

shallow ends. Burrows about 6 in. Bottom dweller. 

Nonturbid. Sluggish. 

Panope. Similar to Psilomya. 

Tancredia. Possibly brackish water, extremely shallow. Hard substrate. 

Thracia. Mainly marine. Thin shelled. Good burrower. Shallow mud and silt. 

Killik Tongue of the Chandler Formation (figs. 53C and 

D) 

fish. 

Entolium. See Entolium above. 

Leptosolen. Razor clam. Deposit feeder. Deep burrower. Can be shallow marine. 

Lingula. See Lingula above. 

Panope. See Panope above. 

P silo my a. See P silomya above. 

Thracia. See Thracia above. 

Veniella. Fully marine. Muddy sand. 

Unio. Freshwater. 

Also reported were fish teeth, fish scales, and starANALYSIS OF PALYNOFLORAS 

The palynological history of each well is described 

below in terms of the floras inferred to have existed in 

the vicinity of each well. As a preliminary approach, we 

assume that most pollen and spores were deposited within 

a few kilometers of their vegetation, but we also acknowl-
edge that some pollen and spores were carried greater distances (for example, bisaccate conifer pollen) (Traverse 

and Ginzberg, 1966). 

Knifeblade-1 Well (Robinson, 1959) (Fig. 54) 

Well information.-Knifeblade-1 well contains 

about 243 m of the Killik Tongue of the Chandler Formation (from surface to about 250-m depth) and about 305 

m of the Grandstand Formation (250 m to 550 m). It 

is not known how much Grandstand Formation lies beneath the bottom of the well, but at nearby Titaluk-1 well 

there is about 488 m of Grandstand Formation, suggesting 

that about 183 m of Killik has been eroded at Knifeblade. 

Sandstone and shale alternate through the Nanushuk, and 

coals are present in the Killik and upper Grandstand Formation. 

General palynology .-Samples are lacking for 

much of the Grandstand Formation between 305 and 518 

m, but were available in the bottom 30 m and the top 

61 m. Adequate samples were available through the Killik. 

Dinoflagellates are numerous in the bottom 30 m 

of the Grandstand Formation (15-36 percent), which indicates marine conditions. Gymnosperms dominate in this 

interval (36 to 40 percent) over dinoflagellates and acritarchs, but alternate in dominance with spores. In the 

upper 61 m of the Grandstand, where samples are again 

available, spores dominate strongly, reaching peaks of 68 

percent, gymnosperms have decreased slightly, having 

peaks of 33 to 40 percent, and dinoflagellates-acritarchs 

have decreased markedly to 3 to 10 percent. 

Percentages remain about the same for each group 

passing from the Grandstand Formation into and through 

the Killik Tongue of the Chandler Formation. However, 

spores increase slightly, gymnosperms decrease slightly, 

and dinoflagellates-acritarchs remain about the same, having very low percentages of 2-9 percent. 

Trends.-Although little data are available for the 

Grandstand Formation, trends exist in the Killik and can 

be inferred in the lower Grandstand. Spores increase from 

the base of the Grandstand up and through the Killik; 

gymnosperms remain about the same in the Grandstand 

but decrease through the Killik, being intermediate between spores and dinoflagellates; and dinoflagellates and 

acritarchs decrease from the lower Grandstand into the 

upper Grandstand where they are greatly dominated by 

both gymnosperms and spores, and they remain about the 

same, in very low percentages, through the Killik. 

Coal.--Coal is not present in the lower two-thirds 

of the Grandstand Formation, but it is fairly common from 

350 m up through the remaining Grandstand and Killik 

Tongue of the Chandler Formation. 

Paleoenvironmental interpretation.-The presence 

of dinoflagellates and acritarchs throughout the well, although generally in small percentages (less than 10 percent), suggests a continual marine influence. The abundance of terrestrial palynomorphs indicates a close proximity to shoreline in the lower half of the Grandstand 

Formation, but the lack of coal and presence of dinoflagellates in the lower Grandstand suggest marine nearshore deposition. Higher in the Grandstand, the repeated 

presence of coal suggests frequent subaerial exposure 

along an oscillating shoreline. The high concentration of 

spores in the upper Grandstand and all of the Killik 

suggests wet conditions within the delta. The continual 

increase in spores and decrease in gymnosperm pollen indicate that conditions were becoming continually wetter. 

Thus, in this area, the adjacent delta was initially dry 

enough to sustain occasionally dominant gymnosperm 

floras (base of Grandstand Formation), but it became wetter, and spore-producing vegetation became dominant. At 

the well locality, subaqueous deposition persisted until 

upper Grandstand time when the first coals formed. After 

this time, an oscillating shoreline controlled deposition, 

allowing coal to form on marginally marine shales and 

siltstones. The continual increase in spores suggests shoaling or continual establishment of the delta in the area, 

and that the general paleoenvironment was wet. 

Titaluk-1 Well (Robinson, 1959) (Fig. 55) 

Well information.-The Titaluk-1 well penetrated 

the Ninuluk Formation, the Killik Tongue of the Chandler 

Formation, and the Grandstand Formation. The Ninuluk 

occurs at the surface, and it is not known how much of 

the upper part is missing. Because more Ninuluk is present here than in any of the other wells, probably most 

of it is present. Samples were available for the lower 122 

m of the Ninuluk. All of the Killik is present (disconformities are not suspected), and several samples were 

available for analysis throughout this unit. The underlying 

Grandstand Formation was also completely penetrated, 

and adequate sampling was done throughout the Grandstand Formation. 

Throughout the Nanushuk Group, sandstones alternate with shales and siltstones typical of a continually 

shifting delta. The Killik has much less sandstone per unit 

of section than does the Grandstand. 

General palynology.-In the lower 152 m of the 

Grandstand Formation, spores and gymnosperm pollen are 

similar in relative frequencies, alternating in dominance. 

Through the next 244 m, gymnosperm pollen dominates 

markedly, reaching peaks of 66 to 75 percent. In the 

upper 122 m of the Grandstand, gymnosperm pollen and 

spores again are similar in numbers, but alternate, the 

Analysis of the Umiat Delta 105 
A 

B 

Inoceramus X 

Panope 

Arctic a 

. 

. 

DELTA 

Knifeblade 1 

DELTA 

Knifeblade 1 

• Wolf Creek 2 and 3 

Corbula Entolium 

Nucula 

\Gubik 1 

•Gubik 2 

Trancredia X 

Arctic a 

Panope 

Entolium 

Thracia 

Thracia 

PanopeX 

Arctic a 

40 KILOMETERS 

Grandstand 1 

Lingula• 

Grandstand 1. 

Arctica 

Panope 

• Gubik 1 

•Gubik 2 

Figure 53. Invertebrate megafossils observed in Nanushuk Group rocks (Imlay, 1961). Shape of delta (heavy 

solid line) based on configuration of sand percentage contours (Huffman and others, this volume) and 

shows composite of features that developed over the lifetime of the delta. A, lower part of Grandstand 

Formation; B, upper part of Grandstand Formation; C, lower part of Killik Tongue of the Chandler Formation; D, upper part of Killik Tongue of the Chandler Formation. 

1 06 Geology of the Nanushuk Group and Related Rocks, Alaska 
Thracia 

Lingula 

Cephalotaxopsis {conifer) 

DELTA 

. 

Knifeblade 1 X Unio xUnio Unio x Pan ope 

Starfish 

69° 

c 

155° 154° 

DELTA 

. 

Knifeblade 1 

D 

Figure 53. 

spores becoming dominant. Spores then dominate in the 

lower 122 m of the Killik, and reach peaks of 68 to 81 

percent. Spores dominate in the remainder of the well. 

• Wolf Creek 2 and 3 

Grandstand 1 

. 

153° 152° 

• Wolf Creek 2 and 3 

Grandstand 1. 

40 KILOMETERS 

Continued 

• Gubik 1 

•Gubik 2 

Panope 

Psilomya 

Entolium x 

151° 

• Gubik 1 

•Gubik 2 

Gymnosperm pollen appears to be dominant 

throughout most of the Grandstand, although alternating 

somewhat with spores at the base and top. In the Killik, 

Analysis of the Umiat Delta 107 
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EXPLANATION 

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Knifeblade-1 well, Umiat delta area, North Slope. 

1 08 Geology of the Nanushuk Group and Related Rocks, Alaska 
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EXPLANATION 

Sample 

Carbonaceous material 

Coal 

Core 

Interval containing 

angiosperm pollen 

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dinoflagellates 

Interval containing 

spores 

Interval containing 

gymnosperm pollen 

Figure 55. Composite relative-frequency diagram of main palynomorph groups, environmental data, and spontaneous-potential log from Titaluk-1 well, Umiat delta area, North Slope. 

Analysis of the Umiat Delta 109 
gymnosperm pollen are subordinate to spores, but 

amounts remain rather uniform to the top of the Ninuluk. 

Dinoflagellates and acritarchs dominate in two samples at the base of the Grandstand, but are almost totally 

subordinate to both gymnosperm pollen and spores 

throughout the remainder of the well. 

Angiosperms are present as a very minor component 

in several samples scattered throughout the well. 

Trends.-Dinoflagellates and acritarchs generally 

decrease in numbers from the base to the top of the 

Grandstand (40-4 percent), but remain relatively uniform 

up through the Killik and Ninuluk, although increasing 

slightly upsection into the Ninuluk. 

Gymnosperm pollen generally increases from the 

base of the Grandstand Formation for about 305 m (25-67 

percent), and then generally decreases to the top of the 

Ninuluk (67-22 percent). 

Spores generally decrease from the base of the 

Grandstand Formation upward for about 305 m. In the 

upper Grandstand, spores increase (20-47 percent) andremain rather uniform upward through the remaining Killik 

and Ninuluk. 

Coal.-One thin coal seam occurs in the lower part 

of the Grandstand Formation. In the middle Killik, coal 

is common in thin layers. In the upper Killik and lower 

Ninuluk, coal is sparse but is present in somewhat thicker 

layers in the middle Ninuluk. It is sparse in the upper 

Ninuluk. 

Paleoenvironmental interpretation.-The continual 

decrease in dinoflagellates and acritarchs upward in the 

Grandstand Formation suggests a lessening in marine conditions throughout that interval. This change suggests that 

deltaic buildup was reducing the marine influence more 

and more. The one thin coal seam in the lower part of 

the Grandstand Formation suggests rare subaerial exposure 

at that time. Subaerial exposure was common in middle 

Killik time. In the lower and upper Grandstand Formation 

gymnosperms (mainly conifers) become markedly dominant over spores, suggesting better drained conditions in 

adjacent deltaic areas. In the uppermost Grandstand, 

spores first begin to dominate over gymnosperm pollen, 

suggesting that deltaic conditions near the Titaluk well 

were becoming wetter. This environment becomes more 

striking in the lower Killik where spores dominate over 

gymnosperms by as much as 60 percent. At 122 m up 

into the Killik, spores and gymnosperms alternate several 

times, but spores dominate upward through the remainder 

of the section, which suggests general wet conditions. 

The rare occurrence of coal in the Grandstand 

suggests that the Titaluk well area was generally submerged. The shoreline appears to have reached the area 

once in early Grandstand time, but not again until middle 

Killik time. 

Lower Killik deposition was much the same as the 

upper Grandstand. Coal is sparse and sporadic. During 

110 Geology of the Nanushuk Group and Related Rocks, Alaska 

middle Killik time here, coal formation was more common, suggesting that the area was emergent more frequently and that the shoreline, therefore, was often to the 

north. 

Coal formed occasionally during middle Ninuluk 

time. At this time the shoreline, though oscillating back 

and forth, was probably mostly to the south, due to the 

more common marine influence. Dinoflagellate occurrence tends to increase from the lower Killik upward 

through the Ninuluk, which suggests the establishment of 

estuarine conditions within the delta, possibly due to the 

Seabee transgression that ultimately covered the delta. 

In general, in most of the Grandstand Formation, 

the pollen, spores, and dinoflagellates-acritarchs suggest 

an initial period of draining when gymnosperm vegetation 

dominated adjacent deltaic areas. This period was followed by wet conditions during latest Grandstand time 

and throughout the remainder of the time represented in 

the well, when spore-producing vegetation dominated. 

The shoreline appears to have remained to the south most 

of the time. 

Oumalik-1 Well (Robinson, 1956) (Fig. 56) 

Well itiformation.-Oumalik 1 well penetrated the 

Grandstand Formation from the surface to a depth of 929 

m. Therefore, no data are available for either the Killik 

or the Ninuluk, which typically overlie the Grandstand. 

Sandstones, shales, and siltstones alternate throughout the 

Grandstand, sandstone being less common in the upper 

half. 

General palynology .-From the base of the Grandstand Formation upward, spores and gymnosperm pollen 

alternate in dominance through about 152- or 183-m intervals, implying slow changes in the wetness and dryness 

of the delta. In the bottom 152 m, spores reach peaks 

of 50 to 56 percent, dominating gymnosperm pollen by 

10 to 25 percent. Through the next 152-m interval, gymnosperms reach a peak of 62 percent, dominating spores 

by as much as 32 percent. Through the next 183 m, 

spores and gymnosperm pollen remain relatively uniform, 

spores dominating gymnosperms by only 1 to 5 percent. 

Through the next 213 m of the Grandstand, gymnosperms 

increase continually from 43 to 77 percent, and spores 

drop continually from 45 to 19 percent. No samples were 

available for the upper 152 m of the well. Angiosperm 

pollen was not observed. 

Dinoflagellates and acritarchs are present in each 

sample examined, but they dominate in only one sample 

from about 137 m above the base of the Grandstand. In 

all other samples they are subordinate to both spores and 

gymnosperm pollen throughout the section. 

Trends.-Dinoflagellates and acritarchs decrease 

generally from the base of the Grandstand to the top. 
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EXPLANATION 

· • Sample 

Carbonaceous material 

==:Coal 

I Core 

-·- Interval containing 

angiosperm pollen 

--Interval containing 

dinoflagellates 

--- Interval containing 

spores 

.... Interval containing 

gymnosperm pollen 

Figure 56. Composite relative-frequency diagram of main palynomorph groups, environmental data, and spontaneous-potential log from Oumalik-1 well, Umiat delta area, North Slope. 

Gymnosperm pollen increase slightly up through the well, 

and spores decrease slightly up through the well. 

Alternation of dominance between gymnosperm pollen and spores is almost rhythmic, changing over each 

152 to 183m. 

Coal.--Coal occurs sparsely through the GrandAnalysis of the Umiat Delta 111 
stand Formation, but is more common in the Grandstand 

here than in any of the other wells examined. 

Paleoenvironmental interpretation.-The general 

decreasing trend in dinoflagellates and acritarchs suggests 

growth of the delta and accompanying restriction of 

marine influences through time. The few occurrences of 

thin coal seams suggest some subaerial exposure and that 

the shoreline was occasionally to the north. However, the 

shoreline was oscillating, as dinoflagellates are also persistent, though in low percentages, most samples having 

less than 18 percent. 

Dominant spores in the lower 152 m suggest that 

adjacent deltaic areas were wet. Drier conditions are 

suggested by dominant gymnosperm pollen in the next 

higher 152m. For 183m above this, fern spores dominate 

the gymnosperms slightly by a few percent. This relationship suggests some equivalence between areas of gymnosperm and spore floras in adjacent deltaic areas. In the 

upper 213 m, gymnosperms increase consistently in dominance, which suggests drying conditions in adjacent deltaic areas where gymnosperm floras became dominant. 

The striking overall feature in the Oumalik well is 

that spores as well as dinoflagellates and acritarchs consistently decrease, and gymnosperms consistently increase 

even though spores and gymnosperms alternate in dominance. This relationship suggests that marine influence 

was continually decreasing and that better drained conditions were becoming more prevalent in the adjacent deltaic areas, or that nearby distributaries were bringing in 

more bisaccate pollen from the highlands. 

East Oumalik-1 Well (Robinson, 1956) (Fig. 57) 

Well information.-East Oumalik-1 well penetrated 

a complete section of Killik, 207 m thick, and Grandstand, 707 m thick. The entire section consists of alternating sandstone, shale, and siltstone. 

General palynology.-Of all the wells examined, 

East Oumalik has the least variation in relative frequencies 

for each group of palynomorphs. Dinoflagellates are present in each sample examined, generally between 5 and 

18 percent, and dominating the other two main groups 

(gymnosperm pollen and spores) only in the uppermost 

Killik in one sample. Spores fluctuate with the dinoflagellates, remaining uniform up through the Grandstand, always dominated by gymnosperm pollen. In two samples, 

at 289 and 518 m in the Grandstand, gymnosperm pollen 

dominate spores by only about 1 percent. In the Killik, 

however, spores dominate gymnosperm pollen by a considerable amount (3-41 percent). The relative frequencies 

of gymnosperm pollen are very uniform up through the 

112 Geology of the Nanushuk Group and Related Rocks, Alaska 

Grandstand, generally between 43 and 55 percent. In the 

Killik, however, gymnosperm pollen shows a rather continual decreasing trend, and is dominated by both dinoflagellates and spores near the top of the tongue. 

Trends .-Relative frequencies are fairly uniform for 

each group throughout the Grandstand Formation. Gymnosperm pollen dominate at least slightly in almost every 

sample examined, spores are slightly subordinate to the 

gymnosperm pollen, and dinoflagellates and acritarchs are 

dominated by both. In the Killik Tongue of the Chandler, 

the trend of gymnosperm pollen amounts is to lower percentages, and these amounts are subordinate to spores 

throughout most of the tongue. The trend of dinoflagellates and acritarchs is to higher percentages, dominating 

both spores and gymnosperms near the top of the tongue. 

No angiosperm pollen were observed in any Grandstand 

or Killik samples. 

Paleoenvironmental interpretation.-The presence 

of dinoflagellates and acritarchs in all samples examined 

suggests continuous marine influence throughout 

Nanushuk time, although the Killik assemblages appear 

more estuarine than those of the Grandstand. The almost 

continuous and uniform dominance of gymnosperm pollen 

throughout the Grandstand Formation suggests that better 

drained conditions existed for a long period in adjacent 

deltaic areas, or that deltaic distributaries were transporting bisaccates to this area. The decrease in numbers of 

gymnosperm pollen in the Killik and the resulting dominance by spores suggests that conditions became wetter 

in adjacent areas. This conclusion is substantiated by the 

increase in numbers of dinoflagellates, which suggests 

brackish water through Killik time. Bodies of standing 

water likely communicated with the sea by increased mixing with saltwater through tidal channels. Swamp conditions appear more prevalent in the basal Killik, as is indicated by increased coal amounts. 

Wolf Creek-2 and -3 Wells (Collins, 1959) 

(Fig. 58) 

Well information.-In the Wolf Creek-3 well, 

palynological samples were available only for the Grandstand Formation. For this reason, palynological data for 

the Killik Tongue of the Chandler Formation were 

gathered from Wolf Creek-2 well and were included with 

the Grandstand data from Wolf Creek-3 as a composite 

section. In Wolf Creek-3, a complete section of Grandstand Formation was penetrated from about 427 m to 841 

m. In Wolf Creek-2, a complete section of Ninuluk Formation was penetrated, but no samples were available. 

Underlying the Ninuluk, a complete section of Killik was 
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EXPLANATION 

Sample 

Carbonaceous material 

Coal 

Core 

Interval containing 

angiosperm pollen 

Interval containing 

dinoflagellates 

Interval containing 

spores 

Interval containing 

gymnosperm pollen 

Figure 57. Composite relative-frequency diagram for main palynomorph groups, environmental data, 

and spontaneous-potential log from East Oumalik-1 well, Umiat delta area, North Slope. 

Analysis of the Umiat Delta 113 
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EXPLANATION 

I 

1\ 

1\ 

Palynomorph groups (percent) 

10 20 30 40 50 60 70 I I I I 

.. .-.: 

: 

( 

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• Sample X Gap between parts of composite section 

Interval containing angiosperm pollen 

Interval containing dinoflagellates 

Interval containing spores 

Carbonaceous material 

Coal 

Core Interval containing gymnosperm pollen 

Figure 58. Composite relative-frequency diagram of main palynomorph groups, environmental data, and spontaneous-potential log from Wolf Creek-2 and -3 wells (composite 

section), Umiat delta area, North Slope. 

penetrated between 198 and 472 m, for which samples 

were available. 

The Grandstand Formation is composed of alternat114 Geology of the Nanushuk Group and Related Rocks, Alaska 

ing sandstone and shale or siltstone. The Killik is primarily shale and siltstone, but thin sandstones are also present. 
General palynology .-Spores tend to dominate 

through both the Grandstand and Killik, even though their 

relative frequencies fluctuate considerably from sample to 

sample, especially within the Grandstand Formation. Rare 

peak abundances of gymnosperm pollen or dinoflagellates 

and acritarchs occur. Whereas dinoflagellates and acritarchs are dominated by gymnosperm pollen at the base 

of the Grandstand, they alternate with the gymnosperm 

throughout the remainder of the Grandstand. In the Killik, 

where spores tend to dominate all groups, the dinoflagellates and acritarchs are subordinate to the gymnosperms, with one exception in the middle of the Killik. 

Angiosperms are present in low percentages (0-4 

percent) and form an almost negligible component, as in 

other wells. 

The most striking feature of the relative frequencies 

from the Wolf Creek wells is the degree of fluctuation 

of each group from sample to sample within the Grandstand Formation; this fluctuation reflects unstable conditions near the well sites. Although hydrodynamic conditions could be involved, such as facies-controlled deposition of palynomorphs, we make a preliminary interpretation here of rapidly changing vegetational environments 

in adjacent deltaic areas. 

Trends.-Dinoflagellates and acritarchs appear in 

low percentages (2-18 percent) at the base of the Grandstand, but they rise to 46 percent in the lower Grandstand 

before decreasing again to 9 percent. This decrease is followed by a relatively consistent rise through the rest of 

the Grandstand to 58 percent. Dinoflagellates and acritarchs then decline abruptly at the top of the Grandstand, 

followed by continuously increasing peaks into the Killik. 

Spores dominate through most of the Nanushuk, but 

it was difficult to determine any particular trends. The 

consistent wide fluctuation in their percentages is striking, 

but peaks or lows do not seem to follow any particular 

pattern. 

Gymnosperm pollen percentages appear to have 

rhythmically fluctuating patterns. At the base of the 

Grandstand, gymnosperm pollen percentages are high 

(two peaks of 65 and 92 percent), then they decrease to 

24 percent, rise to 68 percent at the base of the upper 

Grandstand, then decrease to 8 to 21 percent in the upper 

Grandstand, then increase to 69 percent at the top of the 

Grandstand. In the Killik, gymnosperm pollen generally 

decrease from the 69 percent peak at the top of the Grandstand to 29 percent at the top of the Killik. 

Coal.-Except for one thin coal seam at about 465 

m (uppermost Grandstand), no coal is present throughout 

the Grandstand Formation, although carbonaceous partings and material were frequently recorded in the well 

description. A few thin coal seams appear in the base 

of the Killik at both Wolf Creek-2 and -3 wells and become more abundant through the middle Killik at Wolf 

Creek-2. Coal is present in the upper Ninuluk at both 

Wolf Creek-2 and -3. 

Paleoenvironmental interpretations.-The lack of 

coal in the Grandstand suggests continual subaqueous 

(marine to estuarine) deposition, but the presence of carbonaceous material throughout this interval indicates nearshore deposition. Dinoflagellates and acritarchs have several peaks between 25 and 59 percent in the Grandstand, 

which suggests relatively good marine conditions. The 

general dominance of spores throughout the Grandstand 

suggests wet conditions in adjacent deltaic areas; a few 

gymnosperm peaks suggest that better drained conditions 

existed somewhere nearby. 

In the Killik, dinoflagellate and acritarch percentages are consistently lower than in the Grandstand, 

and the assemblages are assumed to be estuarine. The 

drop in dinoflagellates corresponds to coal development 

at the well site, which suggests shoaling and some subaerial deposition. The dominance of spores and the decreasing gymnosperm pollen in the Killik suggests increasingly 

wet conditions in adjacent deltaic areas. 

The high degree of fluctuation of percentages for 

all groups in the Grandstand is interpreted as suggesting 

frequent changes in wetness in the nearby deltaic areas. 

On rare occasions, adjacent areas may have become dry 

enough to support dominant gymnosperm vegetation 

(primarily conifer), but this appears exceptional. 

Square Lake-1 Well (Collins, 1959) (Fig. 59) 

Well information.-Square Lake-1 well penetrated 

the Nanushuk Group from 576 to 1,215 m. The formations include Chandler-Ninuluk undifferentiated from 576 

to 756 m and Grandstand Formation from 756 to 1,215 

m. Although the well bottomed in the Grandstand Formation, we believe that most of the formation is present in 

the well. 

Although the Killik Tongue of the Chandler and the 

Ninuluk are undifferentiated in the well, we believe that 

most of this interval is probably Killik because the thickness of the unit is only 183 m, which is not unreasonable 

for the Killik in this area. 

The section consists of alternating sandstone, shale 

and siltstone, with shale predominant. 

General palynology.-In the lower 152 m of the 

Grandstand Formation, gymnosperms dominate initially, 

but decrease from 60 to 38 percent, whereas the spore 

palynoflora increases through the same interval from 35 

to 49 percent. The transition interval from gymnosperm 

dominance to spore dominance shows much alternation 

in the dominance of spores and gymnosperms. Although 

dinoflagellates and acritarchs are a minor component, they 

show one peak of 40 percent. Angiosperms are present, 

but they are always less than 3 percent. 

Analysis of the Umiat Delta 115 
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EXPLANATION 

• Sample 

Carbonaceous material 

Coal 

Core 

X Interval containing no palynomorphs 

Interval containing angiosperm pollen 

Interval containing dinoflagellates 

Interval containing spores 

Interval containing gymnosperm pollen 

Figure 59. Composite relative-frequency diagram of main palynomorph groups, environmental 

data, and spontaneous-potential log from Square Lake-1 well, Umiat delta area, North Slope. 

The overlying 213 m of the Grandstand Formation 

lacked samples. At the top of this interval, at 866 m, 

spores dominate markedly (81 percent) over gymnosperms 

116 Geology of the Nanushuk Group and Related Rocks, Alaska 

(13 percent). Dinoflagellates and acritarchs are present in 

low numbers (about 6 percent). Angiosperms are absent. 

From 866 m (upper Grandstand Formation) to the 
top of the Nanushuk Group (576 m), spores generally decrease from 80 to 40 percent, whereas the gymnosperm 

pollen generally increases from 13 to 53 percent. In the 

upper 15 m of the Chandler-Ninuluk undifferentiated sequence, gymnosperms and spores alternate briefly in 

dominance, but near the top of this interval spores dominate (71 percent) over gymnosperm pollen (22 percent). 

Trends .-In a general way, spores and gymnosperm 

pollen vary inversely to each other in the well. As gymnosperms decrease in dominance from the base of the well 

in the lower Grandstand to the upper Grandstand, spores 

increase. From the upper Grandstand to the top of the 

Nanushuk, gymnosperm pollen increase in numbers and 

spores decrease. Spores, however, maintain a slight dominance at the top of the well in what is probably the 

Chandler Formation (Killik Tongue). 

Dinoflagellates and acritarchs are relatively stable 

throughout the well with some peaks of less than 20 percent, and they do not appear to be affected by trends 

in either the spores or the gymnosperms. 

Angiosperms are an insignificant part of the overall 

palynoflora, and trends are difficult to detect. 

Coal.-Two occurrences of coal are indicated in the 

well description. These are thin layers in the upper Chandler-Ninuluk interval. A few carbonaceous partings and 

carbonaceous plant remains were also noted throughout 

the Nanushuk. 

Paleoenvironmental interpretations.-The presence 

of dinoflagellates throughout the Nanushuk Group, although generally in small percentages (less than 18 percent), suggests a continuous marine influence. The dominance of terrestrial palynomorphs suggests a close proximity to shoreline. The general lack of coal suggests general 

subaqueous deposition. General dominance of spores 

suggests that adjacent deltaic areas were wet; the slight 

dominance of gymnosperm pollen at the bottom of the 

well (lower Grandstand), however, suggests somewhat 

better drained conditions nearby at that time, or that deltaic distributaries were draining into the area. 

Umiat-1 Well (Collins, 1958) (Fig. 60) 

Observations on the Umiat-1 well were made only 

in the Ninuluk Formation and Killik Tongue of the Chandler Formation for comparison with the same intervals in 

nearby Umiat-11 well. The unusually high amount of 

shale in Umiat-1 contrasts strongly with the higher concentrations of sand from the same interval at Umiat-11 

and suggests dissimilar environments of deposition, probably an embayment versus a delta lobe. 

Well information.-A complete section of the 

Ninuluk Formation, Killik Tongue of the Chandler Formation, and the Grandstand Formation was penetrated to a 

depth of 613 m. 

General palynology.-Dinoflagellates and acritarchs 

in the Killik Tongue of the Chandler Formation range between 5 and 15 percent, suggesting some marine influence. Specific assemblages suggest estuarine conditions 

as well. Higher in the Ninuluk Formation, two samples 

show a range for dinoflagellates of 3 to 63 percent, probably indicating quiet or brackish-water areas possibly 

caused by the initial effects in the delta of the Seabee 

marine transgression. 

Spores appear to dominate most of the Killik 

Tongue of the Chandler,suggesting relatively wet environments in adjacent deltaic areas. Gymnosperms appear to 

be intermediate between dinoflagellates and spores within 

this interval. In the upper part of the Killik Tongue of 

the Chandler and in the Ninuluk Formation, gymnosperms 

have relative frequencies similar to those of the spores, 

but with a slight dominance. Angiosperms are a minor 

part of the palynoflora, and occur in only two samples. 

Trends.-Spores decrease generally up through the 

section. The limited data make it difficult to interpret 

trends for the gymnosperms and dinoflagellates and acritarchs. 

Coal.-No coal was described in the well log. Carbonaceous partings are common. 

Paleoenvironmental interpretations.-The overall 

lack of coal suggests general subaqueous deposition. Dinoflagellate occurrences suggest marine conditions with 

some brackish water. The presence and persistence of carbonaceous material indicates nearshore shallow water. 

Umiat-11 Well (Collins, 1958) (Fig. 61) 

Well information.-Vmiat-11 well penetrated a 

complete section of the Ninuluk Formation, Killik Tongue 

of the Chandler Formation, and the Grandstand Formation 

to a depth of 93 7 m. The Grandstand is about 198 m 

thick, the Killik about 79 m thick, and the Ninuluk Formation is about 36 m thick. Lithologies are alternating 

sandstone and siltstone or shale. 

General palynology.-Dinoflagellate and acritarch 

relative frequencies are generally higher in Umiat-11 than 

in most of the other wells examined, and range from 17 

to 38 percent in the Grandstand Formation, 13 to 37 percent in the Killik Tongue of the Chandler Formation, and 

24 to 40 percent in the Ninuluk Formation. 

Gymnosperm pollen are generally intermediate in 

abundance between dinoflagellates-acritarchs and spores, 

except in the lower Grandstand and Ninuluk, in which 

they appear to be subordinate to both dinoflagellates-acritarchs and spores. 

Spores dominate throughout the Grandstand interval, alternate in dominance with gymnosperms through 

much of the Killik, and then dominate through most of 

the Ninuluk. 

Analysis of the Umiat Delta 117 
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1200 366 

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EXPLANATION 

• Sample Interval containing angiosperm pollen 

Carbonaceous material Interval containing dinoflagellates 

Core Interval containing spores 

Interval containing gymnosperm pollen 

Figure 60. Composite relative-frequency diagram of main palynomorph groups, and environmental data and 

spontaneous-potential log from Umiat-1 well, Umiat delta area, North Slope. Megafossils reported from the 

underlying Grandstand Formation include: Voldia and Corbula? (1,635 ft, 498 m); Corbula? (1,693 ft, 516 m); 

and Arctica (1 ,712ft, 522 m). 

Angiosperm pollen are a minor component, generally less than about 3 percent, but they are persistent in 

most of the samples examined. 

Trends .-Dinoflagellates-acritarchs decrease from 

38 to 13 percent up through the Grandstand Formation 

and into the lower Killik, but they peak at 36 percent 

in the middle Killik. In the Ninuluk they have two peaks 

of 35 and 40 percent. 

Gymnosperm pollen are somewhat stable in number 

in the Grandstand, but increase through the Killik, and 

then decline through the Ninuluk. 

Spores fluctuate widely through the Grandstand Formation, and have high relative frequencies (35 to 62 percent), then decrease continuously into the middle part of 

the Killik. From the middle Killik to the upper Killik 

the spores generally increase, although they fluctuate in 

numbers, but they drop to 15 percent in the uppermost 

Killik. Through the Ninuluk the spores rise sharply. 

Coal.-There is no coal reported from the Grandstand Formation. One thin seam occurs in the lower 

Ninuluk or uppermost Killik. No coal is reported from 

the remainder of the Ninuluk. Carbonaceous material is 

reported from the Killik and Ninuluk. 

Paleoenvironmental interpretations .-Dinoflagellates and acritarchs range generally between 13 and 25 

percent in the Nanushuk Group, which suggests a rather 

118 Geology of the Nanushuk Group and Related Rocks, Alaska 

continual marine influence. A few peaks of more than 

30 percent were observed. 

The overall dominance of the terrestrial palynoflora 

suggests close proximity to shoreline. The spore dominance in the Grandstand and lower Killik indicates that 

adjacent deltaic areas were wet during that time interval. 

The low relative frequencies of dinoflagellates and acritarchs and high frequencies of gymnosperms in the upper 

Killik suggest that the area was becoming less marine and 

that better drained environments were developing within 

the adjacent delta. The decrease in both gymnosperms and 

spores through the Ninuluk and sharp increase of dinoflagellates and acritarchs suggests that the nearby areas 

were again coming under a marine influence, likely from 

the Seabee transgression. 

Grandstand-1 Well (Robinson, 1958) (Fig. 62) 

Well information.-The Grandstand-! well penetrated the Nanushuk Group from 34 to 326m. Formations 

penetrated include the Killik Tongue of the Chandler Formation from 36 to 64 m and the Grandstand Formation 

from 64 to 226 m. No samples were studied from the 

part considered to be Killik. 

This section consists of alternating sandstone, shale 

and siltstone. Shales appear to dominate. 
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EXPLANATION 

• Sample 

Carbonaceous material 

Coal 

Core 

Interval containing angiosperm pollen 

Interval containing dinoflagellates 

Interval containing spores 

Interval containing gymnosperm pollen 

Figure 61. Composite relative-frequency diagram of main palynomorph groups, environmental data, and 

spontaneous-potential log from Umiat-11 well, Umiat delta area, North Slope. 

General palynology .-Dinoflagellate relative frequencies fluctuate widely throughout the Grandstand Formation (3 to 49 percent). Gymnosperm pollen dominate 

through the lower part of the Grandstand but become subordinate to spores in the middle Grandstand. At the top 

of the Grandstand (67 m), gymnosperms again dominate. 

Spores are intermediate between dinoflagellates-acritarchs 

and gymnosperm pollen in the lower Grandstand, but they 

dominate through the middle Grandstand. In the upper 

Grandstand the spores decline and are finally dominated 

by both gymnosperm pollen and dinoflagellates-acritarchs. 

Angiosperm pollen are present in many samples, but they 

are rare. 

Trends .-Dinoflagellate relative abundances fluctuate widely throughout the Grandstand. Generally, percentages are low, but some high percentages occur as 

short-lived peak abundances. It is difficult to say whether 

dinoflagellates and acritarchs generally increase or decrease up through the section because of the wide fluctuations and the relatively short section. Spores generally appear to increase up through the section into the upper 

Grandstand, but they then decrease through about four 

samples. Gymnosperm pollen decrease generally through 

the lower Grandstand, but then appear to increase to the 

position of the upper sample. Angiosperms are too rare 

to indicate any trends. 

Coals.-Carbonaceous partings generally occur 

throughout the Grandstand Formation. Sparce coal was 

reported in the upper Grandstand-Chandler interval. 

Paleoenvironmental interpretations .-Marine dinoAnalysis of the Umiat Delta 119 
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EXPLANATION 

• Sample 

Carbonaceous material 

Coal 

Core 

Interval containing angiosperm pollen 

Interval containing dinoflagellates 

Interval containing spores 

Interval containing gymnosperm poller. 

Figure 62. Composite relative-frequency diagram of main palynomorph groups, environmental data, and spontaneous-potential log from Grandstand-1 well, Umiat delta area, North Slope. 

flagellates occur throughout the Grandstand Formation, indicating relatively continuous marine influence. Several 

samples have rather low percentages (less than 6 percent), 

but several peaks of more than 30 percent exist, indicating 

that marine conditions fluctuated to a large degree. This 

fluctuation is substantiated by some coal and carbonaceous 

partings in the well. 

Gymnosperm pollen dominate in the lower Grandstand, which suggests better drained conditions than dur120 Geology of the Nanushuk Group and Related Rocks, Alaska 

ing middle Grandstand time when spores dominated. The 

general increase in spores in the middle to upper part of 

the Grandstand suggests a relatively continuous increase 

in wet conditions in adjacent deltaic areas. In the upper 

Grandstand Formation, spores decrease in numbers, 

suggesting that the wetter conditions subsided. That better 

drained conditions developed is suggested somewhat by 

the dominance of gymnosperm pollen in the uppermost 

sample of the Grandstand. 
Uranium Potential of the Cretaceous Nanushuk 

Group, North Slope, Alaska 

By A. Curtis Huffman, Jr. 

INTRODUCTION 

The deltaic Nanushuk Group of Cretaceous age was 

evaluated as a uranium host rock at the same time that 

it was being evaluated as a hydrocarbon reservoir. Although no uranium occurrence has ever been reported in 

the Nanushuk, host-rock environments potentially favorable for sandstone-type deposits have been described by 

many investigators (for example, Chapman and Sable, 

1960; Detterman and others, 1963; Chapman and others, 

1964; Patton and Tailleur, 1964; Brosge and Whittington, 

1966; Roehler and Stricker, 1979). 

Exposures of the Nanushuk Group were examined 

and described in detail at 32 localities (fig. 1; Huffman 

and others, 1981a, b) from Corwin Bluffs on the west 

coast to the Sagavanirktok River in the central North 

Slope (Ahlbrandt and others, 1979). Many more outcrops 

were visited during the course of this study but were described and sampled in less detail. Radioactivity measurements were made at each station and used for relative 

comparisons only; no attempt was made to convert values 

to absolute uranium content. However, approximately 700 

samples of Nanushuk Group and related rocks were 

analyzed for uranium using the delayed-neutron counting 

method. A summary of these analyses is given in table 

7. 

SANDSTONE-TYPE URANIUM DEPOSITS 

Sandstone-type uranium deposits can be loosely divided into peneconcordant deposits and vein deposits 

(Finch, 1967). The peneconcordant types are closely related to lithology and depositional environment, whereas 

vein deposits are controlled by structure and tectonics and 

occur in a wide variety of lithologies. Because the types 

of structures associated with vein deposits (high-angle 

fractures, collapse structures, and so forth) are very rare 

or absent in the Nanushuk Group, and because these types 

of deposits are not dependent on lithology, this discussion 

is concerned primarily with the favorability of the 

Nanushuk as a host for peneconcordant deposits. 

Finch ( 1967) stated that most peneconcordant deposits "are in lenticular sandstone beds that accumulated 

from fresh-water streams," and that "some uranium deposits are in sandstone beds deposited on deltaic coastal 

plains bordering shallow or deep seas." Fischer (1974) 

added that "Sedimentation on a low-lying terrane with a 

high water table, yielding nonoxidizing conditions of 

water-saturated beds, is indicated by the preservation of 

coalified fossil plants, which are present in almost all host 

beds." In addition to these two general characteristics, 

such criteria as attitude of the bedding, sandstonemudstone ratio, percentage of stream deposits, and color 

have been frequently used to determine the favorability 

of a rock unit as a potential host for uranium deposits. 

The possibility of suitable host rocks actually containing uranium deposits depends largely on an accessible 

source of the uranium. This source could either be within 

or adjacent to the host rocks, or connected to the host 

rocks by an aquifer. Sources within the host originate, 

together with the sediment itself, from uranium-rich rocks 

such as tuff, granite, black shale, phosphatic rocks, or 

from pre-existing uranium deposits. Adjacent sources and 

those connected to the host rock by aquifers include overlying or underlying units and the sediment source terrain 

itself. Fluids from intrusive bodies are another possible 

source (Page, 1960). 

This assessment of the uranium potential of the 

Nanushuk Group is based on an evaluation of its suitability as a host rock, and includes factors such as . depositional environments, hydrologic characteristics, texture, 

the presence of reductants, and the presence of possible 

uranium sources. 

SUITABILITY OF THE NANUSHUK GROUP 

AS A HOST ROCK 

The Nanushuk Group contains more than 1,500 m 

of nonmarine and fluvial sediments deposited on a lowlying deltaic plain, as has been documented elsewhere in 

this volume and in previous work. The abundance of carbonized plant debris throughout the Nanushuk has also 

been documented by many workers, most notably by 

Smiley (1966, 1969a, b). Because of the preservation of 

the fossil plant material, the general gray to greenish-gray 

color of a fresh surface, and the widespread occurrence 

of pyrite, it appears that the Nanushuk has never been 

Uranium Potential 121 
Table 7. Sedimentologic data and uranium content from the transitional and nonmarine parts of 

[NA, not applicable; uranium determination by delayed neutron activation; analyses by H. T. Millard, C. McFee, C. M. Ellis, C. A. 

Transitional 

Sample Measured and nonmarine 

No. section facies thickness (m) 

33 Barabara syncline----------- 1, 295 

8 Pitmegea syncline----------- 451 

14 Tupikchak syncline---------- 814 

26 Tupikchak Mountain---------- 808 

18 Kokolik warp---------------- 1,551 

14 Folsom Point---------------- 540 

12 Lookout Ridge--------------- 214 

15 Carbon Creek anticline------ 432 

140 Average Corwin delta-------- NA 

60 Kurupa anticline------------ 1,516 

45 Tuktu Bluff----------------- 1,075 

30 Type Grandstand------------- 576 

20 Arc Mountain---------------- 226 

32 Marmot syncline------------- 792 

187 Average Umiat delta--------- NA 

strongly oxidized and contains abundant material to act 

as a reductant for uranium-bearing solutions. 

Table 7 lists values of several sedimentologic factors for 13 measured sections in the transitional and nonmarine parts of the Nanushuk Group. The most obvious 

conclusion to be drawn from this compilation is that the 

Corwin delta of the western North Slope (Ahlbrandt and 

others, 1979) was a muddy system containing a low percentage of sandstone, and of fluvial-channel sandstone in 

particular. The Umiat delta of the central North Slope 

(Huffman and others, this volume) contains more sandstone but little fluvial-channel sandstone. Low sandstone 

content, and especially the lack of long, continuous, and 

coalesced fluvial-channel sandstone bodies, is thought to 

be unfavorable for uranium mineralization in that the lack 

of such deposits impedes migration of the mineralizing 

fluids that form uranium deposits of minable size. 

Along the southern margin of the North Slope the 

Nanushuk Group is folded into large open folds having 

limbs that dip as steeply as 70°. The folds die out to 

the north, however, and beneath the coastal plain dips 

range from 1 to 5°. Dips less than 5° are thought to be 

favorable for the development of peneconcordant sandstone-type uranium deposits, whereas intense deformation 

is generally considered unfavorable. 

Based on these criteria, the Nanushuk Group of the 

western North Slope would be unsuitable as a potential 

host rock for uranium deposits, and would be only slightly 

122 Geology of the Nanushuk Group and Related Rocks, Alaska 

Fluvial channel Sandstone--

Sandstone sandstone mudstone 

(percent) (percent) ratio 

14.7 6.1 0.17 2 

3.2 1.5 .033 

7.6 6.4 .083 

13.0 8.1 .151 

5.9 3.4 .063 

10.9 2.5 .123 

4.2 2.4 .045 

3.5 .8 .036 

7.9 3.9 .088 

21.8 6.7 .279 

11.9 5.4 .135 

29.7 3.1 .424 

52.7 25.4 1.120 

25.2 21.5 .337 

28.6 12.4 .459 

more suitable in the central region because of the increased sandstone and fluvial-channel sandstone content, 

particularly in the upper part. The nonmarine deposits 

along Sabbath Creek in the eastern North Slope (fig. 26) 

(correlated with the Nanushuk by Detterman and others, 

1975) are also considered marginally suitable because of 

the presence of fluvial-channel sandstones and organic debris. 

POSSIBLE URANIUM SOURCES 

The Nanushuk Group is composed of sediments derived primarily from preexisting sedimentary rocks with 

a variable contribution from mafic to ultramafic rocks, 

metamorphic rocks, and volcanic detritus (Huffman, 1979; 

Mull, 1979; Bartsch-Winkler, 1979, and this volume). 

None of the sedimentary source rocks are known to contain uranium and none were found to be anomalously 

radioactive during this study. The mafic and ultramafic 

rocks in the source terrain are low in uranium (0.13-0.65 

ppm) and fall in the gabbroic to ultramafic range 

documented by Z. E. Peterman (unpub. data, 1963) and 

Rogers and Adams (1969). 

Anomalous radioactivity has been reported from 

phyllitic carbonaceous shales that contain sulfide deposits 

in the southwestern Brooks Range (Curtis and others, 

1979), but it is doubtful that these distant rocks con-
selected measured sections in the Nanushuk Group of the western and central North Slope, Alaska 

Bliss, M. F. Coughlin, R. B. Vaughn, M. N. Schneider, and W. R. Stang, U. S. Geological Survey. Locations of sections shown on figure 1] 

Average Alternations Average No. samples 

sandstone of sandstone Average High uranium + greater than average 

thickness to mudstone uranium uranium 2 standard uranium + 2 standard 

(m) per 3o05 m ( 10 ft) (ppm) (ppm) 

1o86 Oo 24 1o58 3o17 

2.08 .04 1o18 1.6 2 

3.69 o06 1o 12 1o56 

4o05 olO 1o45 1o99 

1.86 o10 1o 20 2o14 

4o54 o07 1o41 2o59 

2o 29 o05 1o14 2o 23 

1.68 o06 2o21 2o8 2 

2o76 o09 1o 41 3o17 

2o80 0 24 1o58 4o19 

5.58 .06 1o 91 2o89 

6 o13 0 15 1o73 3o59 

7o43 o21 1o81 2o94 

4o63 .17 1o73 2o 76 

5o31 o17 1o75 4o19 

tributed significant amounts of sediment to the Nanushuk 

Group, and they are not connected to the Nanushuk by 

an aquifer. 

The uppermost Nanushuk contains thin bentonitic 

beds as does the overlying Colville Group of Late Cretaceous age (Chapman and Sable, 1960; Smiley, 1969b; 

Detterman and others, 1963). The bentonites of the upper 

part of the Nanushuk in the western North Slope are associated with a thick shale section, so it is problematical 

whether any uranium released from these source beds 

could find its way to the host sandstones in sufficient 

quantities to form economic deposits. In the central North 

Slope, the upper part of the Nanushuk is more favorable, 

as it contains both bentonite and suitable host rocks and 

is overlain by the bentonite-rich Seabee Formation of the 

Colville Group. The source of the bentonites is not 

known, but possible sources include the granitic centers 

of east and west-central Alaska, some of which contain 

anomalous concentrations of uranium (Miller, 1970; Miller and Johnson, 1978). 

The nonmarine deposits along Sabbath Creek in the 

eastern North Slope (fig. 26) do not contain bentonite 

beds but are close to the Romanzof Mountains, which 

are composed of granitic plutons reported to contain as 

much as 80 ppm uranium (Brosge and Reiser, 1976). The 

same authors also reported 20 to 30 ppm uranium in the 

Jurassic Kingak Shale and 30 to 80 ppm in phosphatic 

beds of the Triassic Shublik Formation in the same area. 

deviations deviations 

(ppm) 

2o4 2 3 

1o30 2 

1o43 2 

1o63 6 

1o92 1 

2oll 2 

1o 7 2 2 

2o 7 2 1 

1o 91 None 

3o60 2 

2o 74 3 

2o78 1 

2o89 1 

2o38 4 

2o88 None 

SUMMARY 

The Nanushuk Group of the central and eastern 

North Slope contains marginally suitable host rocks for 

peneconcordant sandstone-type uranium deposits. Host 

rocks that were deposited in favorable environments are 

present in the nonmarine and transitional parts of the 

Nanushuk Group throughout the North Slope, but the generally low sandstone content and the limited extent of 

sandstone bodies that do exist severely limit its overall 

favorability, particularly in the west. 

Potential sources of uranium are present in the bentonitic clays of the uppermost Nanushuk and in the bentonitic and tuffaceous beds of the overlying Colville 

Group. Because of the high mudstone content of the upper 

part of the Nanushuk in the west, it is problematical 

whether any uranium released from these source beds 

could accumulate in the host sandstones in sufficient 

quantities to form economic deposits. Suitable host rocks 

exist in the upper part of the Nanushuk Group in close 

proximity to the tuffaceous sediments in the central North 

Slope, but no evidence of mineralization was found during 

this study. 

The easternmost North Slope appears to have somewhat more potential for economic uranium deposits, based 

primarily on the presence of possible sources of uranium 

in the source terrain of the host rocks. This area is currently being evaluated. 

Uranium Potential 123 
REFERENCES CITED 

Ahlbrandt, T. S., ed., 1979, Preliminary geologic, petrologic, 

and paleontologic results of the study of Nanushuk Group 

rocks, North Slope, Alaska: U.S. Geological Survey Circular 794, p. 163. 

Ahlbrandt, T. S., and Huffman, A. C., 1978, Cretaceous 

Nanushuk Group, North Slope, Alaska, in K. M. Johnson, 

ed., The United States Geological Survey in Alaska-Accomplishments during 1977: U.S. Geological Survey Circular 772-B, p. 13-15. 

Ahlbrandt, T. S., Huffman, A. C., Fox, J. E., and Pasternack, 

Ira, 1979, Depositional framework and reservoir-quality 

studies of selected Nanushuk Group outcrops, North Slope, 

Alaska, in T. S. Ahlbrandt, ed., Preliminary geologic, petrologic, and paleontologic results of the study of Nanushuk 

Group rocks, North Slope, Alaska: U.S. Geological Survey 

Circular 794, p. 14-31. 

Alaska Geological Society, 1978, North Slope stratigraphic 

nomenclature: Anchorage, Alaska, Newsletter, May issue, 

p. 1. 

Armstrong, A. K., Mamet, B. L., Brosge, W. P., and Reiser, 

H. N., 1976, Carboniferous section and unconformity at 

Mount Doonerak, Brooks Range, northern Alaska: American Association of Petroleum Geologists Bulletin, v. 60, 

no. 6, p. 962-972. 

Bartsch-Winkler, Susan, 1979, Textural and mineralogical study 

of some surface and subsurface sandstones from the 

Nanushuk Group, western North Slope, Alaska, in T. S. 

Ahlbrandt, ed., Preliminary geologic, petrologic, and 

paleontologic results of the study of Nanushuk Group rocks, 

North Slope, Alaska: U.S. Geological Survey Circular 794, 

p. 61-76. 

Bates, R. L., and Jackson, J. A., eds., 1980, Glossary of geology [2d ed.]: Falls Church, Va., American Geological Institute, 749 p. 

Beikman, H. M., 1978, Preliminary geologic map of Alaska: 

U.S. Geological Survey Map, scale 1:2,500,000. 

Beikman, H. M., and Lathram, E. H., 1976, Preliminary 

geologic map of northern Alaska: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-789, scale 

1:1,000,000, 2 sheets. 

Bird, K. J., and Andrews, Jack, 1979, Subsurface studies of 

the Nanushuk Group, North Slope, Alaska, in T. S. 

Ahlbrandt, ed., Preliminary geologic, petrologic, and 

paleontologic results of the study of Nanushuk Group rocks, 

North Slope, Alaska: U.S. Geological Survey Circular 794, 

p. 32-41. 

Brosge, W. P. , and Pessel, G. H. , 1977, Preliminary reconnaissance geologic map of Survey Pass quadrangle, Alaska: 

U.S. Geological Survey Open-File Report OF-77-27, scale 

1:250,000. 

Brosge, W. P., and Reiser, H. N., 1965, Preliminary geologic 

map of the Arctic quadrangle, Alaska: U.S. Geological Survey Open-file report, scale 1:250,000. 

---1971, Preliminary bedrock geologic map of the Wiseman 

and eastern Survey Pass quadrangles, Alaska: U.S. Geological Survey Open-file report, scale 1:250,000, 2 sheets. 

---1976, Preliminary geologic and mineral resource maps 

(excluding petroleum), Arctic National Wildlife Range, 

124 Geology of the Nanushuk Group and Related Rocks, Alaska 

Alaska: U.S. Geological Survey Open-file report 76-539, 

scale 1:500,000, 4 sheets. 

Brosge, W. P., Reiser, H. N., Dutro, J. T., Jr., and Detterman, 

R. L., 1979, Bedrock geologic map of the Philip Smith 

Mountain quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-879-B, scale 1:250,000, 

2 sheets. 

Brosge, W. P., and Tailleur, I. L., 1969, Isopach maps of upper 

Paleozoic and Mesozoic rocks, northern Alaska: U.S. 

Geological Survey Open-file report, 13 p., maps, scale 

1:2,500,000. 

---1970, Depositional history of northern Alaska, in W. L. 

Adkison and M. M. Brosge, eds., Geological seminar on 

the North Slope of Alaska, Palo Alto, Calif., 1970: Los 

Angeles, Calif., American Association of Petroleum 

Geologists, Pacific Section Meeting, Proceedings, p. DlDl8. 

---1971, Northern Alaska petroleum province, in I. H. 

Cram, ed., Future petroleum provinces of the United 

States-their geology and potential: American Association 

of Petroleum Geologists Memoir 15, p. 68-99. 

Brosge, W. P., and Whittington, C. L., 1966, Geology of the 

Umiat-Maybe Creek region, Alaska, with a section on 

Heavy-mineral studies of the Umiat-Maybe Creek region, 

by R. H. Morris: U.S. Geological Survey Professional Paper 

303-H, p. 501-638. 

Carter, R. D., Mull, C. G., Bird, K. J., and Powers, R. B., 

1977, The petroleum geology and hydrocarbon potential of 

Naval Petroleum Reserve No. 4, North Slope, Alaska: U.S. 

Geological Survey Open-File Report 77-475, 61 p. 

Chapman, R. M., Detterman, R. L., and Mangus, M.D., 1964, 

Geology of the Killik-Etivluk Rivers region, Alaska: U.S. 

Geological Survey Professional Paper 303-F, p. 325-407. 

Chapman, R. M., and Sable, E. G., 1954, Stratigraphy and 

structure of the Kokilik and Kukpowruk Rivers area, 

Alaska: U.S. Geological Survey Open-file report, p. 71. 

---1960, Geology of the Utukok-Corwin region, northwestem Alaska: U.S. Geological Survey Professional Paper 303-

C, p. 47-167. 

Coachman, L. K., and Aagaard, K., 1974, Physical oceanography of Arctic and subarctic seas, in Y. Herman, ed., 

Marine geology and oceanography of the Arctic seas: New 

York, Springer-Verlag, p. 1-72. 

Coleman, J. M., 1976, Deltas-Processes of deposition and 

models for exploration: Champaign, Ill., Continuing Education Publication Co., Inc., p. 102. 

Coleman, J. M., and Gagliano, S. M., 1964, Cyclic sedimentation in the Mississippi River deltaic plain: Gulf Coast Association of Geological Societies Transactions, v. 14, p. 67-

80. 

Collins, F. R., 1958, Test wells, Umiat area, Alaska: U.S. 

Geological Survey Professional Paper 305-B, p. 71-206. 

---1959, Test wells, Square Lake and Wolf Creek areas, 

Alaska: U.S. Geological Survey Professional Paper 305-H, 

p. 423-484. 

Collins, F. R., and Robinson, F. M., 1967, Subsurface stratigraphic, structural, and economic geology, northern Alaska: 

U.S. Geological Survey Open-file report, 252 p. 

Copeland, E. B., 1947, Genera Filicum, the genera of ferns: 

Waltham, Mass., Chronica Bontanica Co., p. 247, 10 pis. 
Curtis, S. M., Rossiter, R., Ellersieck, I. F., Mayfield, C. F., 

and Tailleur, I. L., 1979, Gamma-ray values in the Misheguk Mountain region, northwestern Alaska: U.S. Geological Survey Open-File Report 79-1086, 2 sheets. 

Detterman, R. L., 1953, Northern Alaska (Sagavanirktok-Anaktuvuk region), in T. L. Pewe and others, Multiple glaciation 

in Alaska: U.S. Geological Survey Circular 289, p. 11-13. 

---1956, New and redefined nomenclature of Nanushuk 

group, in George Gryc and others, Mesozoic sequence in 

Colville River region, northern Alaska: American Association of Petroleum Geologists Bulletin, v. 40, no. 2, p. 233-

244. 

---1973, Mesozoic sequence in Arctic Alaska, in M. G. 

Pitcher, ed., Arctic geology: American Association of Petroleum Geologists Memoir 19, p. 37&--395. 

Detterman, R. L., Bickel, R. S., and Gryc, George, 1963, 

Geology of the Chandler River region, Alaska-Exploration 

of Naval Petroleum Reserve No. 4 and adjacent areas, 

northern Alaska, 1944-53-Pt. 3, Aerial geology: U.S. 

Geological Survey Professional Paper 303-E, p. 233-324. 

Detterman, R. L., Bowsher, A. L., and Dutro, J. T., Jr., 1958, 

Glaciation on the Arctic Slope of the Brooks Range, northem Alaska: Arctic, v. 11, no. 1, p. 43-61. 

Detterman, R. L., Reiser, H. N., Brosge, W. P., and Dutro, 

J. T., Jr., 1975, Post-Carboniferous stratigraphy, northeastem Alaska: U.S. Geological Survey Professional Paper 886, 

46 p. 

Dickinson, W. R. , 1970, Interpreting detrital modes of 

graywacke and arkose: Journal of Sedimentary Petrology, 

v. 40, no. 2, p. 695-707. 

Dillon, J. T., Pessel, G. H., Chen, J. H., and Yeach, N. C., 

1980, Middle Paleozoic magmatism and orogenesis in the 

Brooks Range, Alaska: Geology, v. 8, no. 7, p. 338-343. 

Dutro, J. T., Jr., Brosge, W. P., Lanphere, M. A., and Reiser, 

H. N., 1976, Geologic significance of the Doonerak structural high, central Brooks Range, Alaska: American Association of Petroleum Geologists Bulletin, v. 60, no. 6, p. 

952-961. 

Emery, K. 0., 1965, Characteristics of continental shelves and 

slopes: American Association of Petroleum Geologists Bulletin, v. 49, no. 9, p. 1379-1384. 

Espach, R. H., 1951, Recoverable petroleum reserves in the 

Umiat structure, Naval Petroleum Reserve No. 4, Alaska: 

U.S. Bureau of Mines Petroleum and Natural Gas Branch, 

Open-file report, 9 p. 

Finch, W. 1., 1967, Geology of epigenetic uranium deposits 

in sandstone in the United States: U.S. Geological Survey 

Professional Paper 538, p. 121. 

Fischer, R. P., 1974, Exploration guides to new uranium districts and belts: Economic Geology, v. 69, no. 3, p. 362-

376. 

Fisher, W. L., Brown, L. F., Jr., Scott, A. J., and McGowen, 

J. H. , 1969, Delta systems in the exploration for oil and 

gas-a research colloquium, Austin, Texas, 1969, Syllabus 

[1]: Austin, University of Texas, Bureau of Economic Geology, 78 p. 

Fisher, W. L., and McGowen, J. H., 1967, Depositional systems in the Wilcox Group of Texas and their relationship 

to occurrence of oil and gas: Gulf Coast Association of 

Geological Societies Transactions, v. 17, p. 105-125. 

Folk, R. L., 1968, Petrology of sedimentary rocks: Austin, 

Texas, Hemphill's, 170 p. 

Fosberg, F. R., 1964, Vegetation as a geological agent in tropical deltas, in Scientific problems of the humid tropical zone 

deltas and their implications: United Nations Educational, 

Scientific, and Cultural Organization, Proceedings of the 

Dacca Symposium, Paris, p. 227-233. 

Fothergill, C. A., 1955, The cementation of oil reservoir sands 

and its origin: World Petroleum Congress, 4th, Rome, 1955, 

Proceedings, Section 1B, Paper 1, p. 310--314. 

Fox, J. E., 1979, A summary of reservoir characteristics of the 

Nanushuk Group, Umiat test well 11, National Petroleum 

Reserve in Alaska, in T. S. Ahlbrandt, ed., Preliminary 

geologic, petrologic, and paleontologic results of the study 

of the Nanushuk Group rocks, North Slope, Alaska: U.S. 

Geological Survey Circular 794, p. 42-53. 

Fox, J. E., Lambert, P. W., Pitman, J. K., and Wu, C. H., 

1979, A study of reservoir characteristics of the Nanushuk 

and Colville Groups, Umiat test well 11, National Petroleum 

Reserve in Alaska: U.S. Geological Survey Circular 820, 

47 p. 

Fuchtbauer, Hans, 1974, Sediments and sedimentary rocks, 1; 

Part II: New York, Halsted Press, 464 p. 

Grantz, Arthur, and Eittreim, S. L., 1979, Geology and physiography of the continental margin north of Alaska and implications for the origin of the Canada basin: U.S. Geological Survey Open-File Report 79-288, 75 p. 

Grantz, Arthur, Holmes, M. L., and Kososki, B. A., 1976, 

Geologic framework of the Alaskan continental terrace in 

the Chukchi and Beaufort Seas, in Canadian continental 

margins: Canadian Society of Petroleum Geologists Memoir 

4, p. 669-700. 

Grantz, Arthur, and Mull, C. G., 1978, Preliminary analyses 

of the petroleum potential of the Arctic National Wildlife 

Range, Alaska: U.S. Geological Survey Open-File Report 

7~89, 20p. 

Grantz, Arthur, Wolf, S. C., Breslau, Lloyd, Johnson, T. C., 

and Hanna, W. F., 1970, Reconnaissance geology of the 

Chukchi Sea as determined by acoustic and magnetic profiling, in W. L. Adkison and M. M. Brosge, eds., Geological 

seminar on the North Slope of Alaska, Palo Alto, Calif., 

1970: Los Angeles, Calif., American Association of Petroleum Geologists, Pacific Section Meeting, Proceedings, p. 

F1-F28. 

Gribbin, J., 1979, Climatic change: Cambridge, England, Cambridge University Press, 191 p. 

Grybeck, Donald, Beikman, H. M., Brosge, W. P., Tailleur, 

I. L., and Mull, C. G., 1977, Geologic map of the Brooks 

Range, Alaska: U.S. Geological Survey Open-File Report 

77-16&--B, scale 1:1,000,000, 2 sheets. 

Grybeck, Donald, and Nelson, S. W., 1981, Map and interpretation of the structural geology of the Survey Pass quadrangle, 

Brooks Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-117&--B, scale 1:250,000. 

Gryc, George, and others, 1956, Mesozoic sequence in Colville 

River region, northern Alaska: American Association of Petroleum Geologists Bulletin, v. 40, no. 2, p. 209-254. 

Hamilton, T. D., and Porter, S. C., 1975, Itkillik glaciation 

in the Brooks Range, northern Alaska: Quaternary Research, 

v. 5, no. 4,p. 471-497. 

References Cited 125 
Heald, M. T. , and Larese, R. E. , 197 4, Influence of coatings 

on quartz cementation: Journal of Sedimentary Petrology, 

v. 44, no. 4, p. 1269-1274. 

Heezen, B. C., 1974, Atlantic type continental margins, in C. 

A. Burk and C. L. Drake, eds., The geology of continental 

margins: New York, Springer-Verlag, p. 13-24. 

Huffman, A. C., Jr., 1979, Stratigraphy and petrography of a 

measured section on the south limb of Barabara Syncline, 

North Slope, Alaska, in T. S. Ahlbrandt, ed., Preliminary 

geologic, petrologic, and paleontologic results of the study 

of Nanushuk Group rocks, North Slope, Alaska: U.S. 

Geological Survey Circular 794, p. 77-88. 

Huffman, A. C., Jr., and Ahlbrandt, T. S., 1979, The Cretaceous Nanushuk Group of the western and central North 

Slope, Alaska, in K. M. Johnson and J. R. Williams, eds., 

The U.S. Geological Survey in Alaska-Accomplishments 

in 1978: U.S. Geological Survey Circular 804--B, p. 46-50. 

Huffman, A. C., Jr., and others, 1981a, Measured sections of 

the Cretaceous Nanushuk Group undivided, western North 

Slope, Alaska: U.S. Geological Survey Open-File Report 

81-176. 

---1981b, Measured sections of the Cretaceous Nanushuk 

and Colville Groups undivided, central North Slope, Alaska: 

U.S. Geological Survey Open-File Report 81-177. 

Hunter, R. E., and Fox, J. E., 1976, Interpretation of depositional environments in the Fortress Mountain Formation, 

central Arctic Slope, in E. H. Cobb, ed., the United States 

Geological Survey in Alaska-Accomplishments during 

1975: U.S. Geological Survey Circular 733, p. 3~31. 

Imlay, R. W., 1959, Succession and speciation of the pelecypod 

Aucella: U.S. Geological Survey Professional Paper 314--G, 

p. 155-169. 

---1961, Characteristic Lower Cretaceous megafossils from 

northern Alaska: U.S. Geological Survey Professional Paper 

335, 73 p., 20 pls. 

Irving, E., 1979, Paleopoles and paleolatitudes of North 

America and speculations about displaced terrains, in R. A. 

Price, coord., Geodynamics Canada: Canadian Journal of 

Earth Sciences, v. 16, no. 3, pt. 2, p. 669-694. 

Jamison, H. C., Brockett, L. D., and Mcintosh, R. A., 1981, 

Prudhoe Bay-A ten-year perspective, in M. T. Holbouty, 

ed., Giant oil and gas fields of the decade 1968-1978: 

American Association of Petroleum Geologists Memoir 30, 

p. 289-314. 

Jonas, E. C., and McBride, E. F., 1977, Diagenesis of sandstone and shale-Application to exploration for hydrocarbons: American Association of Petroleum Geologists Continuing Education Program, Publication 1 , 120 p. 

Jones, D. L., and Grantz, Arthur, 1964, Stratigraphic and structural significance of Cretaceous fossils from Tiglukpuk Formation, northern Alaska: American Association of Petroleum Geologists Bulletin, v. 48, no. 9, p. 1462-1474. 

Keller, A. S., Morris, R. H., and Detterman, R. L., 1961, 

Geology of the Shaviovik and Sagavanirktok Rivers region, 

Alaska: U.S. Geological Survey Professional Paper 303-D, 

p. 169-222. 

Krumbein, W. C., 1941, Measurement and geological significance of shape and roundness of sedimentary particles: Journal of Sedimentary Petrology, v. 11, no. 2, p. 64--92. 

Lathram, E. H., 1965, Preliminary geologic map of northern 

126 Geology of the Nanushuk Group and Related Rocks, Alaska 

Alaska: U.S. Geological Survey Open-file report, scale 

1:1,000,000, 2 sheets. 

LeBlanc, R. J.., 1972, Geometry of sandstone bodies, in Underground waste management and environment implications: 

American Association of Petroleum Geologists Memoir 18, 

p. 133-190. 

Lowther, J. S. , 1957, A Cretaceous flora from northern Alaska: 

University of Michigan unpublished Ph.D. dissertation, 184 

p, 19 pls. 

Magoon, L. B., and Claypool, G. E., 1979, Hydrocarbon source 

potential of the Nanushuk Group and the Torok Formation, 

a preliminary report, in T. S. Ahlbrandt, ed., Preliminary 

geologic, petrologic, and paleontologic results of the study 

of Nanushuk Group rocks, North Slope, Alaska: U.S. 

Geological Survey Circular 794, p. 54--60. 

Mamet, G. L., and Armstrong, A. K., 1972, Lisburne Group, 

Franklin and Romanzof Mountains, northeastern Alaska, in 

Geological Survey research, 1972: U.S. Geological Survey 

Professional Paper 800---C, p. C127--C144. 

Mangus, M. D., Detterman, R. L., Lachenbruch, M. C., Jr. 

and Lachenbruch, A. H., 1954, Stratigraphy and structure 

of the Etivluk and Kuna Rivers area, Alaska: U.S. Geological Survey Open-file report, 15 p. 

Martin, A. J., 1970, Structure and tectonic history of the westem Brooks Range, De Long Mountains and Lisburne Hills, 

northern Alaska: Geological Society of America Bulletin, 

v. 81, no. 12, p. 3605-3622. 

May, F. E., 1979, Dinoflagellate and acritarch assemblages 

from the Nanushuk Group (Albian-Cenomanian) and the 

Torok Formation (Albian), Umiat test well 11, National Petroleum Reserve in Alaska, northern Alaska, in T. S. 

Ahlbrandt, ed., Preliminary geologic, petrologic, and 

paleontologic results of the study of Nanushuk Group rocks, 

North Slope, Alaska: U.S. Geological Survey Circular 794, 

p. 113-127. 

May, F. E., and Stein, J. A., 1979, Dinoflagellate and acritarch 

assemblages from the Grandstand Formation (middle to 

upper Albian) of the Nanushuk Group, Simpson core test 

25, National Petroleum Reserve in Alaska, northern Alaska, 

in T. S. Ahlbrandt, ed. , Preliminary geologic, petrologic, 

and paleontologic results of the study of Nanushuk Group 

rocks, North Slope, Alaska: U.S. Geological Survey Circular 794, p. 128-146, 5 pis. 

Mayfield, C. F., and Tailleur, I. L., 1978, Bedrock geology 

map of the Ambler River quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-120A, scale 1:250,000. 

Mayfield, C. F., Tailleur, I. L., Mull, C. G., and Sable, E. 

G., 1978, Bedrock geologic map of the south half of National Petroleum Reserve in Alaska: U.S. Geological Survey 

Open-File Report 78-7~B. scale 1:500,000, 2 sheets. 

Mcintosh, R. A., 1977, Prudhoe Bay operating plan, reservoir 

description-geologic: Testimony at conservation hearing on 

Prudhoe Bay Unit operating plan, May 5, 1977, 33 p.; 

transcript on file at Alaska Oil and Gas Conservation Committee, Anchorage. 

Miller, C. C., Pickard, J. E., Taber, E. C., and Rogers, R. 

B., 1981, Summary geophysical report-FY 1979: NPRA 

Summary Report, 18 p. Available from National Oceanographic and Atmospheric Administration (E.D.S.), Mail 

Code D621, Boulder, CO 80303. 
Miller, T. P., 1970, Petrology of the plutonic rocks of west-central Alaska: U.S. Geological Survey Open-File Report 1511, 

132 p. 

Miller, T. P., and Johnson, B. R., 1978, An occurrence of 

parsonite, a secondary uranium mineral, in alaskite of the 

Wheeler Creek pluton, in K. M. Johnson, ed., The United 

States Geological Survey in Alaska-Accomplishments during 1977: U.S. Geological Survey Circular 772-B, p. 42-

44. 

Mitchum, R. M. , Jr. , 1977, Seismic stratigraphy and global 

changes of sea level, Part 11-Glossary of terms used in 

seismic stratigraphy, in C. E. Payton, ed., Seismic stratigraphy-Applications to hydrocarbon exploration: American 

Association of Petroleum Geologists Memoir 26, p. 205-

212. 

Morgridge, D. L., and Smith, W. B., Jr., 1972, Geology and 

discovery of Prudhoe Bay field, eastern Arctic Slope, 

Alaska, in R. E. King, ed., Stratigraphic oil and gas 

fields--classification, exploration, methods, and case histories: American Association of Petroleum Geologists 

Memoir 16, p. 489-501. 

Mull, C. G., 1977, Apparent south vergent folding and possible 

nappes in Schwatka Mountains, in K. M. Blean, ed., The 

United States Geological Survey in Alaska-Accomplishments during 1976: U.S. Geological Survey Circular 751-B, 

p. B29-B31. 

---1979, Nanushuk Group deposition and the late Mesozoic 

structural evolution of the central and western Brooks Range 

and Arctic Slope, in T. S. Ahlbrandt, ed., Preliminary 

geologic, petrologic, and paleontologic results of the study 

of Nanushuk Group rocks, North Slope, Alaska: U.S. 

Geological Survey Circular 794, p. 5-13. 

---1982 [1983], Tectonic evolution and structural style of 

the Brooks Range, Alaska-An illustrated summary, in 

Powers, R. B., ed., Geologic Studies of the Cordilleran 

Thrust Belt: Denver, Colo., Rocky Mountain Association 

of Geologists, V. 1, p. 1-45. 

Mull, C. G., and Kososki, B. A., 1977, Hydrocarbon assessment of the Arctic National Wildlife Range, eastern Arctic 

Slope, Alaska, in K. M. Blean, ed., The United States 

Geological Survey in Alaska-Accomplishments during 

1976: U.S. Geological Survey Circular 751-B, p. B20-B22. 

Mull, C. G., and Tailleur, I. L., 1977, Sadlerochit(?) Group 

in the Schwatka Mountains, south-central Brooks Range, in 

K. M. Blean, ed., The United States Geological Survey 

in Alaska-Accomplishments during 1976: U.S. Geological 

Survey Circular 751-B, p. B27-B29. 

Mull, C. G., Tailleur, I. L., Mayfield, C. F., Ellersieck, Inyo, 

and Curtis, S., 1982, New Upper Paleozoic and Lower Mesozoic stratigraphic units, central and western Brooks Range, 

Alaska: American Association of Petroleum Geologists Bulletin, v. 66, no. 3, p. 348-362. 

Mull, C. G., Tailleur, I. L., Mayfield, C. F., and Pessel, G. 

H. , 197 6, New structural and stratigraphic interpretations, 

central and western Brooks Range and Arctic Slope, in E. 

H. Cobb, ed., The United States Geological Survey in 

Alaska-Accomplishments during 1975: U.S. Geological 

Survey Circular 733, p. 24-26. 

Nelson, S. W., and Grybeck, Donald, 1979, Tectonic significance of metamorphic grade distribution, Survey Pass quadrangle, Alaska, in K. M. Johnson, ed., The United States 

Geological Survey in Alaska-Accomplishments during 

1978: U.S. Geological Survey Circular 804-B, p. B16-B17. 

---1980, Geologic map of the Survey Pass quadrangle, 

Brooks Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map 1176-A, scale 1:250,000, 2 sheets. 

Newman, G. W., Mull, C. G., and Watkins, N. D., 1979, 

Northern Alaska paleomagnetism, plate rotation, and tectonics, in Alexander Sisson, ed., The relationship of plate 

tectonics to Alaskan geology and resources: Alaska Geological Society, 6th symposium, Anchorage 1977, Proceedings, 

p. Cl-C7. 

Nicol, David, 1955, An analysis of the Arctic marine pelecypod 

fauna: Nautilus, v. 68, no. 4, p. 115-122. 

Page, L. R., 1960, The source of uranium in ore deposits, in 

Genetic problems of uranium and thorium deposits: International Geological Congress, 21st, Copenhagen 1960, sec. 

15, pt. 15, p. 149-164. 

Pasternack, Ira, 1981, The depositional factors affecting the reservoir potential of the lower Cretaceous Nanushuk Group, 

northwestern Alaska: Golden, Colorado School of Mines 

M.S. thesis 2384. 

Patton, W. W., Jr., and Tailleur, I. L., 1964, Geology of the 

Killik-Itkillik region, Alaska: U.S. Geological Survey Professional Paper 303--G, p. 409-500. 

Payne, T. G., 1951, Geology of the Arctic Slope of Alaska: 

U.S. Geological Survey Oil and Gas Investigations Map 

OM-126, scale 1:1,000,000, 3 sheets, text. 

---1955, Mesozoic and Cenozoic tectonic elements of 

Alaska: U.S. Geological Survey Miscellaneous Geological 

Investigations Map 1-84, scale 1:5,000,000. 

Pessel, G. H. , and Brosge, W. P. , 1977, Preliminary reconnaissance geologic map of Ambler River quadrangle, Alaska: 

U.S. Geological Survey Open-File Report 77-28, scale 

1:250,000. 

Pessel, G. H., Garland, R. E., Tailleur, I. L., and Eakins, G. 

R. , 1973, Preliminary geologic map of southeastern Ambler 

River and part of Survey Pass quadrangles, Alaska: Alaska 

Division of Geological and Geophysical Surveys Open-File 

Report 28, scale 1:63,360. 

Pessel, G. H., Tailleur, I. L., and Bird, K. J., 1978a, 

Generalized structure map of top of the Colville Group, 

eastern North Slope petroleum province, Alaska: U.S. 

Geological Survey Miscellaneous Field Studies Map MF928-C, scale 1:500,000. 

---1978b, Generalized isopach map of the sandstones within 

the Colville Group, eastern North Slope petroleum province, 

Alaska: U.S. Geological Survey Miscellaneous Field Studies 

Map MF-928-D, scale 1:500,000. 

---1978c, Generalized isopach map of shale in the Colville 

Group, eastern North Slope petroleum province, Alaska: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF-928-E, scale 1:500,000. 

---1978d, Generalized isopach map of the Colville Group, 

eastern North Slope petroleum province, Alaska: U.S. 

Geological Survey Miscellaneous Field Studies Map MF928-F, scale 1:500,000. 

Pettijohn, F. J., Potter, P. E., and Siever, Raymond, 1972, 

Sand and sandstone: New York, Springer-Verlag, 618 p. 

References Cited 127 
Pitman, E. D., 1972, Diagenesis of quartz in sandstones as revealed by scanning electron microscopy: Journal of Sedimentary Petrology, v. 42, no. 3, p. 507-519. 

Pitman, W. C., III, 1978, Relationship between eustacy and 

stratigraphic sequences of passive margins: Geological Society of America Bulletin, v. 89, no. 9, p. 1389-1403. 

Reiser, H. N., Brosge, W. P., Detterman, R. L., and Dutro, 

J. T., Jr., 1978, Geologic map of the Demarcation Point 

quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-526, scale 1:250,000. 

Reiser, H. N., Brosge, W. P., Dutro, J. T., Jr., and Detterman, 

R. L., 1971, Preliminary geologic map, Mount Michelson 

quadrangle, Alaska: U.S. Geological Survey Open-file report, scale 1:200,000, 2 sheets. 

Rickwood, F. K., 1970, The Prudhoe Bay field, in Geological 

seminar on the North Slope of Alaska, Palo Alto, Calif., 

1970: Los Angeles, Calif., American Association of Petroleum Geologists, Pacific Section meeting, Proceedings, p. 

L1-L11. 

Robinson, F. M., 1956, Core tests and test wells, Oumalik area, 

Alaska: U.S. Geological Survey Professional Paper 305-A, 

p. 1-70. 

---1958, Test well, Grandstand area, Alaska: U.S. Geological Survey Professional Paper 305-E, p. 317-339. 

---1959, Test wells, Titaluk and Knifeblade areas, Alaska: 

U.S. Geological Survey Professional Paper 305-G, p. 377-

421. 

Roeder, Dietrich, and Mull, C. G., 1978, Tectonics of Brooks 

Range ophiolites, Alaska: American Association of Petroleum Geologists Bulletin, v. 62, no. 9, p. 1696--1702. 

Roehler, H. W., and Stricker, G. D., 1979, Stratigraphy and 

sedimentation of the Torok, Kukpowruk, and Corwin Formations of Cretaceous age in the Kokolik-Utukok River region, National Petroleum Reserve in Alaska: U.S. Geological Survey Open-File Report 79-995, 80 p. 

Rogers, J. J. W., and Adams, J. A. S., 1969, Uranium, chap. 

92, in K. H. Wedepohl, ed., Handbook of geochemistry: 

Berlin, Springer-Verlag, v. 11-4. 

Rona, P. A., 1973, Relations between rates of sediment accumulation on continental shelves, sea-floor spreading, and eustacy inferred from the central north Atlantic: Geological Society of America Bulletin, v. 84, no. 9, p. 2851-2872. 

Sable, E. G., 1956, New and redefined Cretaceous formations 

in western part of northern Alaska: American Association 

of Petroleum Geologists Bulletin, v. 40, no. 11, p. 2635-

2643. 

---1977, ,Geology of the western Romanzof Mountains, 

Brooks Range, northeastern Alaska: U.S. Geological Survey 

Professional Paper 897, 84 p. 

Sable, E. G., Dutro, J. T., Jr., and Morris, R. H., 1951, Preliminary report on the stratigraphy and structure of the 

Driftwood-Noluk area, Alaska: U.S. Geological Survey 

Open-file report, 28 p. 

Sable, E. G., and Mangus, M. D., 1951, Stratigraphy and structure of the upper Utukok-Kokolik Rivers area, Alaska: U.S. 

Geological Survey Open-file report, 19 p. 

Sarkisyan, S. G., 1971, Application of the scanning electron 

microscope in the investigation of oil and gas reservoir 

rocks: Journal of Sedimentary Petrology, v. 41, no. 1, p. 

289-292. 

128 Geology of the Nanushuk Group and Related Rocks, Alaska 

Scholle, P. A. , 1979, A color illustrated guide to constituents, 

texture, cements, and porosities of sandstones and associated rocks: American Association of Petroleum 

Geologists Memoir 28, 201 p. 

Scott, R. A., and Smiley, C. J., 1979, Some Cretaceous plant 

megafossils and microfossils from the Nanushuk Group, 

northern Alaska-a preliminary report, in T. S. Ahlbrandt, 

ed., Preliminary geologic, petrologic, and paleontologic results of the study of Nanushuk Group rocks, North Slope, 

. Alaska: U.S. Geological Survey Circular 794, p. 89-111. 

Seely, D. R., Vail, P. R., and Walton, G. C., 1974, Trench 

slope model, in C. A. Burk, and C. L. Drake, eds., The 

geology of continental margins: New York, Springer-Verlag, Inc., p. 249-260. 

Shepard, F. P., 1963, Submarine geology [2nd ed.]: New York, 

Harper and Row, 557 p. 

Silberman, M. L., Brookins, D. G., Nelson, S. W., and 

Grybeck, Donald, 1979, Rubidium-strontium and potassium-argon dating of emplacement and metamorphism of the 

Arrigetch Peaks and Mount Igikpak plutons, Survey Pass 

quadrangle, Alaska, in K. M. Johnson and J. R. Williams, 

eds., The United States Geological Survey in Alaska-Accomplishments during 1978: U.S. Geological Survey Circular 804--B, p. B18, B19. 

Sliter, W. V. , 1979, Cretaceous foraminifers from the North 

Slope of Alaska, in T. S. Ahlbrandt, ed., Preliminary 

geologic, petrologic, and paleontologic results of the study 

of Nanushuk Group rocks, North Slope, Alaska: U.S. 

Geological Survey Circular 794, p. 147-157. 

Smiley, C. J., 1966, Cretaceous floras from Kuk River area, 

Alaska-stratigraphic and climatic interpretations: Geological Society of America Bulletin, v. 77, no. 1, p. 1-14. 

---1967, Paleoclimatic interpretations of some Mesozoic 

floral sequences: American Association of Petroleum 

Geologists Bulletin, v. 51, no. 6, p. 849-863. 

---1969a, Cretaceous floras of Chandler-Colville region, 

Alaska-stratigraphy and preliminary floristics: American 

Association of Petroleum Geologists Bulletin, v. 53, no. 

3, p. 482-502. 

---1969b, Floral zones and correlations of Cretaceous Kukpowruk and Corwin Formations, northwestern Alaska: 

American Association of Petroleum Geologists Bulletin, v. 

53, no. 10, p. 2079-2093. 

Snelson, Sigmund, and Tailleur, I. L., 1968, Large scale thrusting and migrating Cretaceous foredeeps in western Brooks 

Range and adjacent regions of northwestern Alaska [abs.]: 

American Association of Petroleum Geologists Bulletin, v. 

52, no. 3, p. 567. 

Tailleur, I. L. , 1969, Rifting speculation on the geology of 

Alaska's North Slope: Oil and Gas Journal,v. 67, no. 39, 

p. 128-130. 

---1973, Probable rift origin of Canada Basin, Arctic 

Ocean: American Association of Petroleum Geologists 

Memoir 19, p. 526--535. 

Tailleur, I. L., and Brosge, W. P., 1970, Tectonic history of 

northern Alaska, in W. L. Adkison and M. M. Brosge, 

eds., Geological seminar on the North Slope of Alaska, Palo 

Alto, California, 1970: Los Angeles, Calif., American Association of Petroleum Geologists, Pacific Section Meeting, 

Proceedings, p. El-El9. 
Tailleur, I. L.. Brosge, W. P., and Reiser, H. N., 1967, 

Palinspastic analysis of Devonian rocks in northwestern 

Alaska, in D. H. Oswald, ed., International symposium on 

the Devonian system, Calgary, 1967: Alberta Society of Petroleum Geologists Proceedings, v. 1, p. 703-716; 1968, 

V. 2, p. 1345-1361. 

Tailleur, I. L., Kent, B. H., and Reiser, H. N., 1966, Outcrop 

geologic maps of the Nuka-Etivluk region, northern Alaska: 

U.S. Geological Survey Open-file report, scale 1:63,360, 

5 sheets. 

[Tailleur, I. L., Mull, C. G., and Pessel, G. H.] 1976, 

Geologist calls Prudhoe one of a kind: Oil and Gas Journal, 

v. 74,no. 44, p. 32. 

Tailleur, I. L., Pessel, G. H., Levorsen, J. A., and Engwicht, 

S. E., 1978, Maps showing land status and well locations 

and tables of well data, eastern North Slope petroleum province, Alaska: U. S. Geological Survey Miscellaneous Field 

Studies Map MF-928A, scale 1:500,000, 5 sheets. 

Tailleur, I. L., and Snelson, Sigmund, 1969, Large-scale thrusting in northwestern Alaska possibly related to rifting of the 

Arctic Ocean [abs.]: Geological Society of America Special 

Paper 121, p. 569. 

Taylor, J. M., 1950, Pore-space reduction in sandstones: American Association of Petroleum Geologists Bulletin, v. 34, 

no. 4, p. 701-716. 

Traverse, Alfred, and Ginsburg, R. N., 1966, Palynology of 

the surface sediments of Great Bahama Bank, as related 

to water movement and sedimentation: Marine Geology, v. 

4, no. 6,p. 417-459. 

Tschudy, R. H., 1969, Relationship of palynomorphs to 

sedimentation, in R. H. Tschudy and R. A. Scott, eds., 

Aspects of palynology: New York, Wiley-Interscience, p. 

79-96. 

Turner, D. L., Forbes, R. B., and Dillon, J. T., 1979, K-Ar 

geochronology of the southwestern Brooks Range, Alaska: 

Canadian Journal of Earth Sciences, v. 16, no. 9, p. 1789-

1804. 

Turner, D. L., Forbes, R. B., and Mayfield, C. F., 1978, K-Ar 

geochronology of the Survey Pass, Ambler River, and eastern Baird Mountains quadrangles, southwestern Brooks 

Range, Alaska: U.S. Geological Survey Open-File Report 

78-254, 41 p. 

Vail, P. R., Mitchum, R. M., Jr., and Thompson, S., III, 1977, 

Seismic stratigraphy and global changes of sea level-Part 

4, Global cycles of relative changes of sea level, in C. E. 

Payton, ed., seismic stratigraphy-applications to hydrocarbon exploration: American Association of Petroleum 

Geologists Memoir 26, p. 83-97. 

Wolfe, J. A., 1978, A paleobotanical interpretation of Tertiary 

climates in the northern hemisphere: American Scientist, v. 

66, p. 694-703. 

Young, F. G., Myhr, D. W., and Yorath, C. J., 1976, Geology 

of the Beaufort-Mackenzie Basin: Geological Society of 

Canada Paper 76--11, 65 p. 

References Cited 129