Sentinel Hill Core Test 1: Facies Descriptions 

and Stratigraphic Reinterpretations of the 

Prince Creek and Schrader Bluff Formations, 

North Slope, Alaska

Professional Paper 1747

U.S. Department of the Interior

U.S. Geological Survey

&IG

"OTTOM

4UFFACEOUSBED
Sentinel Hill Core Test 1: Facies Descriptions 

and Stratigraphic Reinterpretations of the 

Prince Creek and Schrader Bluff Formations, 

North Slope, Alaska

By Romeo M. Flores, Gary D. Stricker, Paul L. Decker, and Mark D. Myers

Professional Paper 1747

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Flores, R.M., Stricker, G.D., Decker, P.L., and Myers, M.D., 2007, Sentinel Hill Core Test 1—facies descriptions and 

stratigraphic reinterpretations of the Prince Creek and Schrader Bluff Formations, North Slope, Alaska: U.S. Geological 

Survey Professional Paper 1747, 31 p.

Library of Congress Cataloging-in-Publication Data

Sentinel Hill core test 1 : facies descriptions and stratigraphic reinterpretations of the Prince Creek and Schrader Bluff Formations, North 

Slope, Alaska / by Romeo M. Flores ... [et al.].

p. cm. -- (Survey professional paper ; 1747)

Includes bibliographical references and index.

ISBN 978-1-4113-2022-2 (alk. paper)

1. Stratigraphic correlation--Alaska--North Slope Region. 2. Facies (Geology)--Alaska--North Slope Region. 3. Alluvium--Alaska--North 

Slope Region. 4. Sequence stratigraphy. 5. Geology--Alaska--North Slope Region. I. Flores, Romeo M. 

QE652.55.N68S46 2007

557.98’7--dc22

 2007046718

ISBN 978-1-4113-2022-2
Contents

Abstract ..........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Previous Lithostratigraphic Descriptions .................................................................................................2

Prince Creek Formation .......................................................................................................................9

Schrader Bluff Formation ...................................................................................................................9

New Biostratigraphic Age Constraints .....................................................................................................9

Sequence Stratigraphy and Depositional Setting..................................................................................11

Facies Descriptions and Interpretations .................................................................................................12

Prince Creek Facies............................................................................................................................12

Descriptions ...............................................................................................................................12

Coarser Grained Facies ..................................................................................................12

Finer Grained Facies .......................................................................................................15

Organic-Rich Facies ........................................................................................................17

Interpretations ...........................................................................................................................17

Schrader Bluff Facies ........................................................................................................................18

Descriptions ...............................................................................................................................18

Coarser Grained Facies ..................................................................................................18

Finer Grained Facies .......................................................................................................24

Interpretations ...........................................................................................................................24

Acknowledgments ......................................................................................................................................25

Summary .......................................................................................................................................................25

References Cited .........................................................................................................................................25

Figures

1. Map showing locations of National Petroleum Reserve in Alaska and study area where 

Sentinel Hill Core Test 1 is located.............................................................................................1

2. Map showing drill site for Sentinel Hill Core Test 1 at the base of the bluffs on the 

west bank of the Colville River, Alaska, in relation to seismically defined subsurface 

structure and surficial fold axes.................................................................................................2

3. Stratigraphic section penetrated in the Sentinel Hill Core Test 1 showing detailed 

lithologies and comparison between correlations used in the present study and those 

used by Robinson and Collins (1959)..........................................................................................3

4. Stratigraphic section penetrated in the Sentinel Hill Core Test 1 showing facies types, 

facies succession, lithology, contacts, and ages of the Prince Creek and Schrader 

Bluff Formations ....................................................................................................................... 4–8

5. Lenticular bedding and burrows in silty sandstone of the Prince Creek Formation.......12

iii
6. Mottled and root-marked silty sandstone facies type of the Prince Creek Formation...13

7. Alternating red and gray variegated, mottled mudstone of the Prince Creek 

Formation......................................................................................................................................13

8. Trough crossbeds in Type 1 sandstone of the Prince Creek Formation............................14

9. Mud drapes, root marks, and syneresis or mud cracks (all indicated by arrows) in the 

upper part of Type 1 sandstone of the Prince Creek Formation..........................................14

10. Subparallel laminations in Type 2 sandstone of the Prince Creek Formation ..................14

11. Rooted (indicated by arrows) and burrowed upper part of Types 1 and 2 sandstones 

of the Prince Creek Formation...................................................................................................15

12. Ripple laminations in Type 3 sandstone facies of the Prince Creek Formation................15

13. Matrix- and grain-supported sandstone conglomerate in Type 1 sandstone facies of the 

Prince Creek Formation..............................................................................................................16

14. Interbedded tuffaceous (or bentonitic) mudstone, siltstone, and carbonaceous 

mudstone of the finer grained facies of the Prince Creek Formation.................................16

15. Interbedded coal, carbonaceous shale, and tuffaceous beds of the organic-rich 

facies of the Prince Creek Formation.......................................................................................17

16. Bidirectional crossbeds in Type I sandstone facies of the Schrader Bluff Formation....18

17. Pebble lag conglomerate in the Schrader Bluff Formation..................................................19

18. Hummocky cross stratification in Type II sandstone facies in the Schrader Bluff 

Formation ......................................................................................................................................19

19. Probable Bergaueria trace fossil (indicated by arrow) in Type II sandstone of the 

Schrader Bluff Formation...........................................................................................................19

20. Probable Conichnus trace fossil (indicated by arrow) in Type II sandstone of the 

Schrader Bluff Formation...........................................................................................................20

21. Phycosiphon incertum trace fossil (indicated by arrows) in Type II sandstone of the 

Schrader Bluff Formation...........................................................................................................20

22. Trace fossil Schaubcylindrinus (indicated by arrows) in Type II sandstone of the 

Schrader Bluff Formation...........................................................................................................21

23. Trace fossil Teichichnus (indicated by arrows) in Type II sandstone of the Schrader 

Bluff Formation.............................................................................................................................21

24. Trace fossil Zoophycus (indicated by arrows) in Type II sandstone of the Schrader 

Bluff Formation.............................................................................................................................22

25. Trace fossil Thalassinoides (indicated by arrows) in Type II sandstone of the 

Schrader Bluff Formation...........................................................................................................22

26. Articulated pelecypod body fossil in Type II sandstone of the Schrader Bluff 

Formation ......................................................................................................................................23

27. Slumped structures in Type III sandstone of the Schrader Bluff Formation.....................23

28. Siltstone and mudstone in the finer grained facies of the Schrader Bluff Formation .....24

iv
Table

1. Palynological ages of samples collected from the Sentinel Hill Core Test 1 well, North 

Slope of Alaska...........................................................................................................................10

v

By Romeo M. Flores,1

 Gary D. Stricker,1 Paul L. Decker,2 and Mark D. Myers1

1

 U.S. Geological Survey.

2

 Alaska Division of Oil and Gas.

Sentinel Hill Core Test 1: Facies Descriptions and Stratigraphic 

Reinterpretations of the Prince Creek and Schrader Bluff Formations, 

North Slope, Alaska

Abstract

The Sentinel Hill Core Test 1 well penetrated an 

intertonguing sequence of (1) the marine Schrader Bluff 

Formation in the depth intervals 950–1,180 ft and 690–751 

ft, which consists of shoreface and offshore deposits that 

accumulated along a storm-dominated, barred shoreline; 

and (2) the nonmarine Prince Creek Formation in the depth 

intervals 751–950 ft and surface to 690 ft, which consists 

of fluvial channel, crevasse splay, backswamp, and ash fall 

deposits. The strata range in age from early Campanian to 

early Maastrichtian.

An erosional contact at a depth of 690 ft at the base of 

the upper unit of the Prince Creek Formation is interpreted 

as a major regional sequence boundary, and the overlying 

conglomeratic fluvial channel deposits are interpreted to have 

accumulated in a paleovalley. In its more proximal reaches 

along the Colville River, channels of this paleovalley cut 

down 75 ft into the lowermost 

Prince Creek Formation 

and the uppermost Schrader 

Bluff Formation. Farther 

offshore, the equivalent 

surface to the aforementioned 

paleovalley appears to be a 

subtle discontinuity between 

middle and lower Schrader 

Bluff Formation shelfal marine 

strata. Still farther offshore, the 

equivalent paleovalley surface is 

interpreted as a marine masswasting surface that locally cuts 

through the lowermost Schrader 

Bluff Formation and into the 

underlying Seabee Formation.

Introduction

The Sentinel Hill Core Test 1 well is one of many holes 

drilled by the U.S. Navy between 1944 and 1953 to evaluate 

petroleum potential in the Naval Petroleum Reserve No. 4 

(now the National Petroleum Reserve in Alaska [NPRA]) on 

the Alaskan North Slope (Gryc, 1988) (fig. 1). This core hole 

was drilled primarily to ascertain the nature of the shallow 

subsurface formations on the Sentinel Hill anticline at the east 

edge of the NPRA. Drilled in 1947 to a total depth of 1,180 

ft, the Sentinel Hill Core Test 1 well was positioned approximately 12 mi east and more than 1,000 ft structurally below 

the crest of the anticline, on an eastward-plunging subsurface 

structural nose (fig. 2); the well was subsequently abandoned 

as a dry hole. The well site is on a gravel bar on the west 

side of the Colville River at the base of 350-ft bluffs of the 

coal-bearing Upper Cretaceous Prince Creek Formation near 

the axis of a small surface syncline. The present study is in 

164° W

68°N

70°N

152°W

C h u k c h i S e a 

Beaufort Sea

0 50 100 Miles

Foothills

Study area 

Colville

River

Province

Fish Creek

Kuparuk River

Sagavanirktok River

National Petroleum Reserve in Alaska

(formerly Naval Petroleum 

Reserve No. 4)

Shivugak Bluff

 

 

 

 

 

EXPLANATION

Kuparuk River Unit

Milne Point Unit

Prudhoe Bay Unit

Figure 1. Map showing locations of National Petroleum Reserve in Alaska (formerly Naval 

Petroleum Reserve No. 4) and study area where Sentinel Hill Core Test 1 is located (see fig. 2).
2 Sentinel Hill Core Test 1

Figure 2. Map showing drill site for Sentinel Hill Core Test 1 at the base of the bluffs on the west bank of the Colville River, Alaska, 

in relation to seismically defined subsurface structure and surficial fold axes. Note that this core test was drilled more than 1,000 ft 

structurally below the crest of the anticline on an east-plunging subsurface nose. Letters A–E indicate the flexures recognized during 

field studies in 1945 (Robinson and Collins, 1959). Base map from 1:63,360-scale Umiat C-3 quadrangle, Alaska, 1955.

conjunction with recent outcrop studies conducted along the 

lower Colville River (Flores and others, 2007). Both studies 

describe and interpret the shallow subsurface stratigraphic 

relations differently than they were presented by Robinson 

and Collins (1959), because the present study and Flores and 

others (2007) are based on modern sequence stratigraphic 

principles and the revised stratigraphic nomenclature of Mull 

and others (2003).

Previous Lithostratigraphic 

Descriptions

The Sentinel Hill Core Test 1 well was continuously 

cored below a depth of 100 ft, a method that provided 

excellent stratigraphic information on the intertonguing nature 

of the Upper Cretaceous Schrader Bluff and Prince Creek 

Formations. Robinson and Collins (1959) identified the 

rock units as interbedded Kogosukruk Tongue of the Prince 

Creek Formation (nonmarine) and Sentinel Hill Member of 

the Schrader Bluff Formation (marine), both of which are 

parts of the Upper Cretaceous Colville Group (fig. 3), in the 

following ascending order: (1) Sentinel Hill Member of the 

Schrader Bluff Formation, thickness 231 ft; (2) Kogosukruk 

Tongue of the Prince Creek Formation, thickness 109 ft; (3) an 

upper Sentinel Hill Member, thickness 371 ft; and (4) an upper 

Kogosukruk Tongue, thickness 460 ft (fig. 3). These rock 

units were differentiated mainly on correlations with outcrops 

and on the presence of marine macrofossils and microfossils. 

The nomenclature for these and many other stratigraphic units 

of the Brookian sequence has been substantially revised (Mull 

and others, 2003) such that the names Kogosukruk Tongue and 

the Sentinel Hill Member, as well as the Colville Group, are 

now abandoned. Both this study and that by Flores and others 

(2007) continue to recognize the fundamental intertonguing 

nature of the Prince Creek and Schrader Bluff Formations in 

the Sentinel Hill Core Test 1, as described in Robinson and 

Collins (1959), but the placement and genetic significance of 

the intertonguing contact are reinterpreted as shown in figure 4 

(see fig. 3 in Flores and others, 2007).

151°50'0"W 151°35'0"W

69°40'0"N

0 2 4 Miles

Dry hole

EXPLANATION

Seismic structure contour lines, approximately 

located, queried where inferred. Contour interval 

100 ft; datum is mean sea level

Surficial anticline,

approximately located 

Surficial syncline, 

approximately located

R.1W. R1E. R.1E. R2E. R.2E. R.3E.

T.4N. 

T.3N.

T.3N. 

T.2N. 31

35

2

6 1

36 36 31 

31 36

1 6

36 31

36 31

36 31

1 6

4,500

A

B

D

E

Sentinel Hill

 Core Test 1

Colville

4,000

?

C

River

4,000 ?

5,000
Previous Lithostratigraphic Descriptions 3

Sentinel Hill Core Test 1

(This study) (Robinson and 

Collins, 1959)

1,180 

469

100

840

949

Kogosukruk Tongue

Prince Creek Formation

Group Colville

Sentinel Hill Member

Schrader Bluff Formation

Sentinel Hill Member

Schrader Bluff Formation

Kogosukruk Tongue

Prince Creek Formation

EXPLANATION

Conglomerate sandstone

Conglomerate

Sandstone

Siltstone

Silty shale and 

shaly siltstone

Coal

Tuffaceous

Gap in core

Bentonite

Ironstone concretions

Prince Creek Formation Prince Creek Formation

Mudstone

Variegated paleosoil

x x x x x x x 

x x x x x x x 

x x x x x x x x 

x x x x x 

x x x x x 

x x x x x x x x 

x x x x x x 

x x x x x x 

x x x x x 

x x x x x 

X X X X 

100

200

300

400

500

600

700

800

900

1,000

1,100

1,180

Schrader 

Bluff 

Formation

Schrader Bluff Formation

Feet Feet

x x x x x x x x x x x x 

x x x x x x 

x x x x x x x x x 

x x x x x x x x x 

x x x x x x x x x 

x x x x x x x x x 

x x x x x x x x x 

X X X X X X 

X X X 

X X 

X X X X X X 

X X X 

X X X 

Figure 3. Stratigraphic section penetrated in the Sentinel Hill Core Test 1 showing detailed lithologies and comparison between 

correlations used in the present study and those used by Robinson and Collins (1959). For lithologic explanation, see figure 4.
4 Sentinel Hill Core Test 1

Figure 4. Stratigraphic section penetrated in the Sentinel Hill Core Test 1 showing facies types, facies succession, lithology, 

contacts, and ages of the Prince Creek and Schrader Bluff Formations. Locations of samples for palynomorph analyses are also 

shown. Numbers alongside columns indicate measured depth, in feet, below kelly bushing datum.

x x x x x x x 

x x x x x x x 

x x x x x x x x 

x x x x x 

x x x x x 

x x x x x x x x 

B

B

0

10

20

30

40

50

60

70

80

90

100

100

110

120

130

140

150

160

170

180

190

200

200

210

220

230

240

250

260

270

280

290

300

Type 2

sandstone

Type 2

sandstone

Type 3

sandstone

Type 3

 sandstone

Silty

sandstone

Silty

sandstone

Silty

sandstone

P (102.6)

Early Maastrichtian

Feet Feet Feet
Previous Lithostratigraphic Descriptions 5

x x x x x x 

x x x x x x 

B

x x x x x V

x x x x x 

X X X X 

300

310

320

330

340

350

360

370

380

390

400

400

410

420

430

440

450

460

470

480

490

500

500

510

520

530

540

550

560

570

580

590

600

Type 1 

sandstone

Type 2

sandstone

Type 3

sandstone

Silty

sandstone

Silty

sandstone

Silty

sandstone

P (364.3) 

Late CampanianLate Campanian

Late Maastrichtian Late Campanian

Feet Feet Feet
6 Sentinel Hill Core Test 1

x x x x x x x x x x x x 

HCS

HCS

HCS

x x x x x x 

x x x x x x x x x 

x x x x x x x x x 

x x x x x x x x x 

x x x x x x x x x 

x x x x x x x x x 

X X X X X X 

X X X 

X X 

X X X X X X 

Ravinement and flooding

 surface lag

Sequence boundary

600

610

620

630

640

650

660

670

680

690

700

700

710

720

730

740

750

760

770

780

790

800

800

810

820

830

840

850

860

870

880

890

900

Tonstein

Coaly stringers

Type 1 

sandstone

Type 2

sandstone

P (608.3)

P (615.5)

P (699.6)

P (781.2)

P (832.6)

Schrader Bluff

Formation Prince Creek Formation

Prince Creek Formation Schrader Bluff Formation

Feet Feet Feet
Previous Lithostratigraphic Descriptions 

7

X X X 

X X X 

HCS 

900

910

920

930

940

950

960

970

980

990

1,000

 1,000

1,010

1,020

1,030

1,040

1,050

1,060

1,070

1,080

1,090

1,100

1,100

1,110 

1,120

1,130

1,140

1,150 

1,160

1,170

1,180

Partly lost: 

interbedded mudstone, 

coal, and carbonaceous 

shale (rooted) 

Partly lost: 

mudstone, coal 

withtonstein, 

and carbonaceous 

shale (rooted) 

Partly lost: 

mudstone and

tuffaceous 

Teredo-bored wood

Partly lost: 

mudstone and

tuffaceous 

Type III

sandstone

Type II 

sandstone

 

Type I

sandstone

P (900.9) P (941.3) 

P (1142.5) 

Sequence boundary

(approximately, 

inferred)

Schrader Bluff Formation Prince Creek Formation

Early CampanianEarly Campanian

Late Campanian Early Campanian

Feet Feet Feet
8 Sentinel Hill Core Test 1

Sandstone

Mudstone (clay) 

Carbonaceous shale

Coal

Clinker gravel

Siltstone

EXPLANATION

Conglomerate

Silty sandstone 

Carbonaceous mudstone

Tuffaceous

Covered or lost

Coal and tuff

Coal and carbonaceous shale

Coal and carbonaceous mudstone

Dinosaur bones

Mottles

Hummocky cross stratification 

V Tuffaceous

Convolution

Ripple laminations

Bidirectional crossbeds

Trough crossbeds

Root marks

Ironstone

Plant leaves

Slump

Lenticular lenses

Wispy lenses

Pelecypod shells

Variegated color (reds)

X X X X 

Burrows

Ophiomorpha burrows

Cross laminations

Subparallel laminations

Planar crossbeds

B Bentonitic

Mudstone

Siltstone

Very fine sandstone

Fine sandstone

Medium sandstone

Coarse sandstone

Conglomerate sandstone

Grain size

Silty sandstone 

Pollen sample depth (feet)

(see table 1)

P (102) 
New Biostratigraphic Age Constraints 9

Prince Creek Formation

Robinson and Collins (1959) assigned two intervals of 

the core test hole to the Prince Creek Formation: an upper unit 

at depth interval 0–469 ft and a lower unit at depth interval 

840–949 ft (fig. 3). They described these intervals as mostly 

sandstone, siltstone, and bentonitic, silty, carbonaceous, and 

calcareous claystone that is interbedded with, and grades 

into, clay ironstone. Sandstone and siltstone constitute 

only about 25 percent of the section they assigned to the 

Prince Creek Formation. The sandstone ranges from fine 

to medium grained and consists of about 50 percent quartz 

grains and lesser amounts of metamorphic rock fragments, 

chert, mica, and volcanic glass shards. The siltstone is similar 

in composition to the sandstone; both also contain plant 

fragments. Coal and carbonaceous shale constitute about 

10 percent of the Prince Creek Formation strata as described 

by Robinson and Collins (1959); the coal is black and brittle 

and exhibits blocky fracture. Tuff or bentonite and limey 

mudstone are less common rock types, the former being 

mainly disseminated in the claystone but also occurring in 

white to yellowish gray units interbedded with other rock 

types. The limey mudstone is dense and sideritic and exhibits 

conchoidal fractures.

Schrader Bluff Formation

Robinson and Collins (1959) assigned two intervals of 

the core test hole to the Schrader Bluff Formation: an upper 

unit at depth interval 469–840 ft and a lower unit at depth 

interval 949–1,180 ft (fig. 3). They reported these units as 

being mostly sandstone, siltstone, clay shale, and claystone 

that is less calcareous and sideritic than strata in the Prince 

Creek Formation. Sandstone and siltstone constitute as much 

as 45 percent of the total rock thickness, with sandstone units 

reaching thicknesses of 100 ft. The sandstone is silty to 

medium grained and is composed of as much as 85 percent 

quartz with minor mica and rock fragments. The siltstone is 

composed of similar framework grains but is more calcareous 

and locally bentonitic. Overall, bentonite is as common in 

the Schrader Bluff Formation as it is in the Prince Creek 

Formation. Marine pelecypods and foraminifera are common 

fossils. The common presence of foraminifera microfossils 

in the Schrader Bluff Formation permitted identification 

of related rock units as zones of Eoeponidella strombodes

(Tappan, 1951).

New Biostratigraphic Age Constraints

A total of 12 core samples from Sentinel Hill Core 

Test 1 were submitted for new biostratigraphic analysis 

by consultants of the irf group, inc. These consisted of 10 

samples of organic-rich mudstone and carbonaceous shale 

that were examined for organic-walled palynomorphs from 

the depth interval 102.6–1,142.5 ft and 2 samples of probable 

marine mudstones that were examined for foraminifera from 

depths of 699.6-ft and 1,142.5 ft. The stratigraphic position 

of the core samples is shown in figure 4, and the assemblages 

and point counts for each genus are summarized in table 

1. The results indicate that a majority of the palynomorphs 

are nonmarine along with a few nonmarine algae and fungal 

remains.

Ages of the samples submitted for biostratigraphic 

analysis range from early Campanian (sample at depth 

1,142.5 ft) to early Maastrichtian (sample at depth 364.3 ft) to 

early Campanian (sample at depth 102.6 ft). The uppermost 

sample (at depth 102.6 ft) contains a relatively low-diversity 

terrestrial assemblage with few age-diagnostic taxa. The 

next sample down (at depth 359 ft), however, contains a rich 

and characteristically late Campanian assemblage including, 

in particular, numerous specimens of the important late 

Campanian indicator pollen species Aquilapollenites trialatus. 

Because of the absence of typical late Maastrichtian taxa 

from the uppermost sample, it is interpreted as being early 

Maastrichtian in age. The late Campanian assemblage persists 

throughout most of the rest of the core, down to 941.3 ft, and 

is characteristically nonmarine in nature. Of the two samples 

examined for foraminifera, the uppermost sample (at depth 

699.6 ft) was barren, whereas the lowermost sample (at depth 

1,142.5 ft) yielded 11 distinct types that are consistent with 

the early Campanian age, including the major marker marine 

dinoflagellate cysts of the species Spongodinium delitiense

and Chatangiella ditissima.

These biostratigraphic results, particularly the 

palynomorph analyses, permitted a much improved and clearer 

resolution of the chronostratigraphy of the Sentinel Hill Core 

Test 1 than was reported by Robinson and Collins (1959). 

The new chronostratigraphy indicates that the interval from 

100 to 359 ft, which includes the uppermost Prince Creek 

Formation, is early Maastrichtian in age and that the interval 

from 359 to 941.3 ft, which includes the upper and lower parts 

of the Prince Creek Formation and intervening tongue of the 

Schrader Bluff Formation, is late Campanian in age. The 

lower part of the Schrader Bluff Formation is interpreted as 

early Campanian in age on the basis of marine dinoflagellate 

cysts.
10 Sentinel Hill Core Test 1

1,142.5 104 2 25 30 7 3 2 2 5 2 2 1

Taxon 

Nonmarine, pollen/sporesNonmarine, algaeFungal

EXPLANATION

33 Number of taxa per sample

102.6 33 132 5 9 4 7 1 1 2 3 1 1 1364.3 32 36 29 23 2 17 3 1 29 24 1 1 1 1608.3 42 7 33 62 16 1 14 2 17 1 2 2 1615.5 20 3 145 9 2 5 1 1 6 1 1 2 1 1 1699.6 101 2 4 29 45 4 1 5 1 1 3 1 1 2781.2 23 2 4 27 4 3 22 11 2 1 2 1 1 3 20 1 2 69 1 1832.6 10 6 1 2 2 177 1 1900.9 58 46 6 45 3 2 29 1 2 5 1941.3 27 20 94 2 1 25 1 5

Bisaccategymnospermpollenindet.

Stereisporitesspp.indet.

Deltoidosporaspp.indet.

Osmundaciditeswellmanii

Laevigatosporitesspp.indet.

Retitriletesspp.indet.

Tsugaepollenitessp.indet.,sma

ll

Gleicheniiditessenonicus

cf.Converrucosisporitessp.small,pale,vague

Stereigranisporisregius

cf.Anacolosiditessp.indet.

Betulaceoipollenites/Carpinuspollenitesgroup

Camarozonosporitessp.indet.,small

Selagosporissp.1

Taxodiaceaepolleniteshiatus

Aquilapollenitestrialatus

Mancicorpuspseudosenonicus

Aquilapollenitescf.fusiformis

Erdmannipollisprocumbentiformis

Tricolporopollenitessp.indet

.,small,prolate

Aquilapollenitesspp.-indet.

Echinatisporisspp.indet.

Sigmopolliscarbonis

Linearseptatefungalhypha

Fibulapollisscabratus

Hannisporisscollardensis

Expressipolliscf.accuratus

Verrucosisporitessp.indet.,small

Cibotiumsporajuncta

Lobatiainvolucrata

cf.Divisisporitessp.indet.

Undulatisporitessp.indet.

Ovoiditesarcticus

Cicatricosisporitessp.1

Trilitessp.indet.

Podocarpidite

ssp.indet.

cf.Perinopollenitessp.indet.

Cicatricosisporitessp.2

cf.Loranthacitessp.indet.

Anacolosidites/Expressipollisstriatus

Cranwelliasubtilis

Aquilapollenitesamygdaloides

Ovoiditesparvus

Sequoiapollenites polyformosus

Polycingulatisporites cf. reduncus - vague

1

Prince CreekPrince CreekPrince CreekPrince CreekSchrader BluffPrince CreekPrince CreekPrince CreekPrince CreekSchrader Bluff

Depth,in feet Formation

Table 1. Palynological ages of samples collected from the Sentinel Hill Core Test 1 well, North Slope of Alaska. See vertical stratigraphic section in figure 4 for position of palynology samples.[indet, indeterminant ]
Sequence Stratigraphy and Depositional Setting 11

Sequence Stratigraphy and 

Depositional Setting

The contacts of the Prince Creek and Schrader Bluff 

Formations in the Sentinel Hill Core Test 1 well as originally 

defined by Robinson and Collins (1959) were based primarily 

on the presence or absence of coals and marine beds. 

Although some of the contacts have now been revised (Flores 

and others, 2007), these traditional criteria demonstrate the 

intertonguing of terrestrial and marine deposits that result 

from regressive and transgressive shifts of the paleoshoreline, 

respectively.

In sequence stratigraphic terms, a regressive (seaward) 

retreat of the shoreline occurs during relative sea-level fall 

and results in a regional erosion surface that is recorded as a 

sequence boundary. Transgressive (landward) advance of the 

shoreline occurs during relative sea-level rise and generates a 

transgressive surface of erosion (ravinement surface) that is 

cut by wave energy. The sharp or erosional contacts between 

the Prince Creek and Schrader Bluff Formations in the core 

test include both types of sequence stratigraphic surfaces.

In the Sentinel Hill Core Test 1, the contact juxtaposing 

the upper Prince Creek Formation interval (above) and the 

upper Schrader Bluff Formation interval (below) is at a depth 

of 690 ft and is a clear sequence boundary (Mid-Campanian 

unconformity [MCU] of Decker [2007]; see fig. 4 of Flores 

and others [2007]). This erosional boundary is at the base of a 

conglomeratic, fine- to coarse-grained multiscoured sandstone, 

which contains vertical burrows in the uppermost part. 

Outcrop evidence links this sequence boundary to an incised 

paleovalley that was eroded during a falling sea level and then 

was filled with sediment during the subsequent sea-level rise.

Flores and others (2007) suggest that this sequence 

boundary is present in outcrops along the Colville River Bluffs 

as a widespread, channelized surface with erosional relief 

from 25 to 75 ft. At Shivugak Bluff (fig. 1), the sequence 

boundary forms a paleovalley that scoured out 100 ft or more 

of stratigraphic thickness, thereby incising through lowermost 

Prince Creek strata and into uppermost shoreface sandstones 

of the Schrader Bluff Formation (Flores and others, 2007). In 

the Sentinel Hill Core Test 1, the same sequence boundary 

is incised into a lower shoreface or offshore mudstone. If 

the upper shoreface sandstone below the contact at Shivugak 

Bluff and the lower shoreface to offshore mudstone at Sentinel 

Hill Core Test 1 are parts of the same parasequence, then the 

depth of incision at the sequence boundary would appear to be 

greater at the core hole than at Shivugak Bluff. Alternatively, 

Decker (2007) correlated the Schrader Bluff Formation strata 

beneath the contact at Shivugak Bluff with the lower part of 

the Schrader Bluff Formation in Sentinel Hill Core Test 1 

(depth interval 950–1,180 ft). In that model, erosion on the 

sequence boundary would be greatest at Shivugak Bluff, where 

all strata equivalent to the upper Schrader Bluff Formation 

interval in Sentinel Hill Core Test 1 were erosionally removed 

and where the deepest part of the paleovalley was incised even 

through the lower part of the Prince Creek Formation.

The contact between the upper Schrader Bluff Formation 

interval and the underlying lower Prince Creek Formation 

interval is now placed at the base of the conglomeratic lag 

at 751 ft (fig. 4) (Flores and others, 2007). This contact 

is interpreted as a ravinement surface (also known as a 

transgressive surface of erosion), and the lag conglomerate—

which consists of tuff, coal, and rock fragments—represents 

older terrestrial deposits reworked during the landward 

advance of the paleoshoreline (during sea-level rise). The 

thin nature of the ravinement lag conglomerate suggests rapid 

transgression. The ravinement lag conglomerate is in the 

middle of a hummocky cross-stratified, storm-related upper 

shoreface sandstone. The contact between the lower Prince 

Creek Formation interval and the underlying lower Schrader 

Bluff Formation interval is estimated to be at the 950-ft depth 

(the contact is within an interval of lost core, so the sequence 

boundary predicted at this relatively abrupt facies change 

could not be directly observed).

Recognition of key sequence stratigraphic surfaces 

(two sequence boundaries and one ravinement surface) has 

resulted in a reinterpretation of the positions of the contacts 

between the Prince Creek and Schrader Bluff Formations 

previously identified by Robinson and Collins (1959). In the 

reinterpretation, the following revisions were made: (1) the 

contact (sequence boundary) between the upper Prince Creek 

Formation and upper Schrader Bluff Formation is placed much 

lower (from 469 to 690 ft), resulting in a thicker Prince Creek 

nonmarine section; and (2) the contact (transgressive surface 

of erosion) between the upper Schrader Bluff Formation and 

lower Prince Creek Formation is placed higher (from 747 to 

777 ft), resulting in a thinner Schrader Bluff marine section.

The Schrader Bluff and Prince Creek Formations in the 

Sentinel Hill Core Test 1 represent an overall progradational 

succession of genetically linked, cyclic marine and nonmarine 

depositional environments. The transition grades from 

entirely marine at the bottom to entirely nonmarine at the 

top, but it was interrupted by one significant transgressive 

pulse and punctuated by sequence boundaries. The upper 

Schrader Bluff Formation marine interbed is substantially 

thinner than the Schrader Bluff Formation sections observed in 

outcrops nearby to the south, which represent more proximal 

depositional settings. It is likely that this marine interbed is 

absent from the lower accommodation settings further south 

either because of nondeposition or erosional removal at the 

upper sequence boundary described above. The Schrader 

Bluff Formation rock record preserved in the core test consists 

mainly of lower shoreface environments. The overlying 

Prince Creek Formation was deposited in a variety of settings 

landward of similar shoreface deposits. Initial sediment 

infill in the paleovalley, represented by the conglomeratic 

sandstone in the lowermost part of the upper Prince Creek 

interbed, reflects bedload sedimentation by aggrading rivers 

concomitant with the rise of sea level. Subsequent filling of 
12 Sentinel Hill Core Test 1

the paleovalley occurred in a tidal-estuarine-influenced coastal 

and alluvial environment.

Facies Descriptions and Interpretations

Facies of the Prince Creek and Schrader Bluff Formations 

in the Sentinel Hill Core Test 1 represent coastal and alluvial 

depositional environments. They are best described as they 

are below on the basis of lithology, color, nature of contact, 

grain size, sedimentary structures, megafossils, trace fossils, 

and vertical facies associations of the rocks (fig. 4). The 

descriptions are followed by discussions of interpreted 

depositional environments. The continental Prince Creek 

Formation sandstones (Types 1–3) are differentiated from the 

marine Schrader Bluff Formation sandstones (Types I–III) by 

the use of arabic and roman numerals, respectively.

Prince Creek Facies

Descriptions

The Prince Creek Formation, in the depth intervals 

100–690 ft and 751–950 ft, consists of three general facies 

that are distinguished on the basis of grain size and organic 

content: a coarser grained facies, a finer grained facies, and an 

organic-rich facies.

Coarser Grained Facies

The coarser grained facies, constituting about 60 percent 

of the Prince Creek Formation strata, includes sandstones 

and conglomerates. Silty sandstones are the main rock 

type, constituting 40 percent of the Prince Creek Formation. 

Individual units, as much as 27 ft thick, are typically sharp 

based and rarely erosional based. Internal sedimentary 

structures vary from abundant ripple laminations to commonly 

alternating, fining-upward lenticular, trough, and subparallel 

laminations (depth intervals 400–410 ft and 550–573 ft) (fig. 

5). Trace fossils include Ophiomorpha-like and nondescript 

vertical burrows, and root marks and mottled structures are 

pervasive throughout this facies type (fig. 6). The mottled 

structures are commonly formed by vertically alternating red 

and gray horizons (fig. 7). Slumped structures are common 

(for example, see depth intervals 116–120 ft and 510–520 ft, 

fig. 4).

Sandstones range from very fine to coarse grained with 

gradational, sharp, or erosional bases. Three sandstone facies 

types are recognized on the bases of the nature of their basal 

surfaces, vertical grain size variations, sedimentary structures, 

and facies associations.

The Type 1 sandstone facies, in units as much as 69 ft 

thick, is characterized by vertically stacked multierosional 

surfaces or multiscours, upward fining, and trough-, 

subparallel-, and ripple-bedded deposits (see depth intervals 

324–354 ft and 621–690 ft in fig. 4 and photograph in fig. 

8). The upper part of this sandstone type shows mud drapes, 

syneresis or mud cracks, and root traces (fig. 9). The Type 2 

sandstone facies, as much as 20 ft thick, is characterized by a 

single erosional basal surface; mainly uniform grain size from 

base to top; and trough-, subparallel-, ripple-, and lenticularbedded deposits (see depth intervals 221–235 ft and 441–449 

ft in fig. 4 and photograph in fig. 10). This sandstone facies 

may be stacked, as shown in the depth interval 138–170 ft (see 

fig. 4). Types 1 and 2 sandstone facies are either capped by 

root traces or by vertical burrows (fig. 11). Type 3 sandstone 

facies, as much as 9 ft thick, is characterized by a sharp base 

and mainly ripple-bedded sandstones (see depth intervals 

121–130 ft and 458–463 ft in fig. 4 and photograph in fig. 12).

Figure 5. Lenticular bedding and burrows in silty sandstone of 

the Prince Creek Formation. Photograph is of a core surface.

&IG
Facies Descriptions and Interpretations 13

&IG

&IG

Figure 6. Mottled 

and root-marked silty 

sandstone facies type of 

the Prince Creek Formation. 

Photograph is of slabbed 

surfaces.

Figure 7. Alternating red and gray 

variegated, mottled mudstone of the 

Prince Creek Formation. Left side of 

photograph is of a core surface, and 

right side of photograph is of a slabbed 

surface.
14 Sentinel Hill Core Test 1

Figure 9. Mud drapes, root marks, 

and syneresis or mud cracks (all 

indicated by arrows) in the upper part 

of Type 1 sandstone of the Prince 

Creek Formation. Photograph is of 

slabbed surfaces.

Figure 10. Subparallel laminations in 

Type 2 sandstone of the Prince Creek 

Formation. Photograph is of a core 

surface.

Figure 8. Trough crossbeds in Type 1 sandstone of the 

Prince Creek Formation. Photograph is of a core surface.

&IG

&IG

&IG
Facies Descriptions and Interpretations 15

&IG

Figure 11. Rooted (indicated by arrows) and burrowed 

upper part of Types 1 and 2 sandstones of the Prince 

Creek Formation. Photograph is of a core surface. &IG

Figure 12. Ripple laminations in Type 3 sandstone facies of the Prince 

Creek Formation. Photograph is of slabbed surfaces.

Conglomerate beds, which are 0.5–1.5 ft thick, exhibit erosional 

bases and contain rounded to subrounded pebbles and cobbles of 

sandstone, ironstone, chert, coal spar, mudstone, and carbonaceous 

shale. The fragments are matrix to grain supported, and there is some 

crude imbrication (fig. 13). The conglomerates mainly lie above 

erosional surfaces within the Type 1 sandstone facies (see depth 

intervals 324–355 ft and 621–690 ft in fig. 4).

Finer Grained Facies

Siltstone, mudstone, and tuffaceous beds compose the finer 

grained facies (fig. 14) and constitute 12–40 percent of the Prince 

Creek Formation (fig. 4). The siltstone beds are gray, have 

gradational to sharp contacts, and are as much as 12 ft thick (depth 

interval 769–781 ft, fig. 4). The mudstone beds are light to dark 

gray, have sharp to gradational contacts, and are as much as 32 ft 

thick (depth interval 590–622 ft, fig. 4). The finer grained facies is 

mainly lenticular bedded, burrowed, rooted, and mottled and contains 

abundant plant fragments. Ironstone and variegated layers are 

abundant and commonly interbedded with mudstones (depth intervals 

195–202 ft and 392–398 ft, fig. 4) and silty sandstones (depth 

intervals 510–520 ft and 530–540 ft, fig. 4); these beds are typically 

associated with plant-rich horizons and are the result of oxidation.

The tuff or bentonite beds are a minor part (1 percent) of 

the finer grained facies (fig. 14). They form vitreous (glassy) 

white layers that vary from a few inches to as much as 6 ft thick 

(depth interval 606–612 ft, fig. 4) and contain root traces and plant 

fragments. The tuffaceous beds mostly either are interbedded with 

(depth interval 889–896 ft, fig. 4) or underlie (depth interval 797–799 

ft, fig. 4) coal and carbonaceous shale beds. Tuffaceous beds are 

interbedded with mudstone beds (fig. 14).
16 Sentinel Hill Core Test 1

&IG

Figure 13. Matrix- and grain-supported sandstone 

conglomerate in Type 1 sandstone facies of the Prince 

Creek Formation. Photograph is of core surfaces.

&IG

"OTTOM

4OP

4UFFACEOUSBED

Figure 14. Interbedded tuffaceous 

(or bentonitic) mudstone, siltstone, 

and carbonaceous mudstone of the 

finer grained facies of the Prince 

Creek Formation.
Facies Descriptions and Interpretations 17

&IG

"OTTOM

4OP

Organic-Rich Facies

The organic-rich facies includes coal and carbonaceous 

shale beds (fig. 15), which constitute approximately 2 percent 

of the Prince Creek Formation. Coal is in relatively pure 

seams that are as much as 4 ft thick (depth interval 800–884 ft, 

fig. 4) or in beds as much as 7 ft thick with tuffaceous partings 

that are as much 1.5 ft thick (depth 889–896 ft, fig. 4). The 

coal is composed mainly of bright banded units interbedded 

with dull, nonbanded units. The bright banded units contain 

vitrain macerals (mainly woody plant fragments), and the dull, 

nonbanded units consist of macerals that are mainly degraded 

cellular plant remains.

Carbonaceous shale beds are black sheeted mudstone 

(at depths 211, 252, 267, 312, 374–276, 384, 799, and 886 ft, 

fig. 4) that contain abundant macerated plant leaf and stem 

fragments and coal stringers. These beds 

either are interbedded with or underlie 

the coal beds, are commonly rooted with 

vertical root traces as large as 0.20 inch in 

diameter and 1 ft in length, and typically are 

mottled and variegated (see figs. 6 and 7) 

where there are high concentrations of plant 

fragments.

Interpretations 

The coarser grained facies is interpreted 

as fluvial channel deposits on the basis of 

erosional basal contacts; fining-upward 

profiles; the vertical succession of internal 

sedimentary structures; and multiple 

stacked, internal erosional contacts and beds. 

The Type 1 sandstone facies, distinguished 

by multiple stacked, fining-upward beds—

each of which is underlain by an erosional 

base and contains basal conglomerate 

beds—probably was deposited in bedload 

fluvial channels. The fining-upward 

profiles indicate that this facies infilled 

waning flow fluvial channels. The Type 

1 sandstone facies is typified by the lower 

part of the upper tongue of the Prince Creek 

Formation (depth interval 622–690 ft, fig. 

4), which consists of a thick, fining-upward 

sequence characterized by a succession 

of thinner, fining-upward sandstone beds, 

each of which is underlain by an erosional 

base marked by imbricated, matrix- to 

framework-supported, pebble-cobble 

conglomerates. This sandstone succession 

probably indicates vertical aggradation 

of the fluvial channels. Investigations 

of the Prince Creek Formation in outcrops 

along the Colville River by Flores and others (2007), which 

shows similar vertical facies variations, suggest that Type 1 

sandstones were deposited in sandy braided streams. The 

conglomerates of the Sentinel Hill Core Test 1 are bottom 

channel lag or armor deposits bypassed by sands that 

formed longitudinal dunes and bars. The rippled and smallscale crossbedded sandstones in the uppermost part of the 

succession indicate deposition during waning flow and lateral 

aggradation of the braided streams in chute bars and flood 

plains. The presence of mud drapes, syneresis or mud cracks, 

and root marks suggests point bar accretion during the lateral 

aggradation (Flores and others, 2007).

The Type 2 sandstone facies is interpreted as a linear 

fluvial channel deposit on the basis of the single erosional 

basal surface; largely uniform grain size from base to top; and 

trough-, subparallel-, ripple-, and lenticular-bedding styles. 

Figure 15. Interbedded coal, carbonaceous shale, and tuffaceous beds of the 

organic-rich facies of the Prince Creek Formation.
18 Sentinel Hill Core Test 1

These sandstones probably represent vertically aggrading 

channels that were cut and filled by migrating sand dunes 

and small-scale ripples. Their sharp bases and dominant 

rippled bedding indicate that Type 3 sandstones were probably 

deposited in crevasse channels that aggraded laterally into the 

flood plain. The silty sandstone facies probably represents the 

crevasse splay environment at the mouth of crevasse channels.

The finer grained facies—consisting mainly of rooted, 

mottled, rippled, burrowed siltstone and mudstone—represents 

vertically aggraded flood plain detritus deposited by rivers that 

overtopped channel levees or by crevasse channels cut into the 

levees. The rooted, mottled, and variegated horizons reflect 

diverse paleosols formed during alternating lowering and 

rising of groundwater tables during dry and wet paleoclimatic 

periods (Flores, 2003). The variegated horizons interbedded 

with mudstones that formed in distal flood plains reflect more 

mature paleosols than do the variegated horizons interbedded 

with silty sandstones that formed in well drained, proximal 

flood plains resulting from wet and dry paleoclimatic changes. 

According to Buurman (1975), mottling is characteristic of 

gley soils in which movement of reducing pore waters is 

sluggish. The tuffaceous or bentonitic layers interbedded 

with the fine-grained detritus represent deposition of volcanic 

ash falls. Eruption of volcanic ash must have been intense, 

as indicated by the thick tuffaceous layers. These layers are 

dominant in the flood plain environments rather than in the 

fluvial channels because of the higher preservation potential 

in the vertically aggrading flood plain compared to laterally 

aggrading and erosional fluvial channels.

The organic-rich facies, which consists of coal and 

carbonaceous shale, represents flood plain swamp deposits. 

The woody vitrain bands of the coal beds indicate that the 

swamps were likely vegetated by trees. That the swamps were 

low lying and topogeneous is indicated by the interbedded 

carbonaceous shale composed of mixed organic matter and 

mud. The mud probably resulted from flood deposition into 

the low-lying, flat swamps. Like the detrital flood plain areas, 

the swamps were also episodically covered with airborne 

volcanic ash, which was preserved as interbedded tuffaceous 

layers. The ash falls probably caused intermittent suffocation 

of swamp vegetation.

Schrader Bluff Facies

Descriptions

The Schrader Bluff Formation (depth intervals 690–751 

ft and 950–1,180 ft) consists of about 40 percent silty to 

coarse-grained sandstone and conglomerate beds and about 60 

percent siltstone and mudstone beds. The strata are assigned 

to two principal facies mainly on the basis of grain size—a 

coarser grained facies and a finer grained facies.

Coarser Grained Facies

The coarser grained facies includes sandstone and 

conglomerate beds. Two types of sandstone are recognized 

on the basis of the nature of their basal surfaces (sharp 

to erosional), vertical grain size variations, very fine 

to coarse-grained textures, sedimentary structures, and 

facies associations. Intervals of Type I sandstone beds are 

characterized by uniform grain size throughout, bidirectional 

crossbedding, subparallel laminations, lenticular bedding, 

and burrowing (see depth interval 1,005–1,038 ft in fig. 4 and 

photograph in fig. 16). Framework-supported conglomerates 

with basal erosional contacts are common at or near the base 

and top of Type I sandstone beds, a presence which reflects a 

vertically multiscoured sandstone body in which each scouring 

event is marked by a basal conglomerate. The conglomerates 

consist mainly of subrounded pebbles and cobbles of 

sandstone, coal spar, chert, volcanic, and metamorphic rocks 

(see depth interval 746.2–747.0 ft in fig. 4 and photograph in 

fig. 17).

&IG

Figure 16. Bidirectional crossbeds in Type I sandstone facies of 

the Schrader Bluff Formation. Photograph is of a core surface.
Facies Descriptions and Interpretations 19

Intervals of Type II sandstone are marked by a sharp 

basal surface; largely uniform grain size from base to top; 

and bidirectional, hummocky cross-stratified (fig. 18), and 

lenticular bedding. In the depth interval 1,054–1,100 ft 

(fig. 4), this facies is commonly interbedded with burrowed 

mudstone; also the sandstone is commonly reworked by 

vertical and lateral burrows of the trace fossils Bergaueria, 

Conichnus, Phycosiphon incertum, Schaubcylindrinus,

Teichichnus, Zoophycus, and Thalassinoides (figs. 19–25). 

In addition, the Type II sandstone is locally interbedded 

with erosional-based conglomerates consisting of chert, 

volcanics, and bivalve shell fragments (fig. 26).

Type III sandstones have sharp bases, are commonly 

heavily burrowed to bioturbated with rarely preserved 

trough-like shapes, and exhibit subparallel laminations and 

slumped stratification (see depth interval 963–994 ft in fig. 

4 and photograph in fig. 27).

&IG

&IG

Figure 18. Hummocky cross stratification in Type II sandstone 

facies in the Schrader Bluff Formation. Photograph is of a slabbed 

surface.

&IG

Figure 17. Pebble lag 

conglomerate in the Schrader 

Bluff Formation. On the left and 

right are core surfaces, and in 

the middle is an interior surface 

parallel to bedding plane.

Figure 19. Probable Bergaueria trace fossil (indicated by arrow) 

in Type II sandstone of the Schrader Bluff Formation.
20 Sentinel Hill Core Test 1

Figure 20. Probable Conichnus trace 

fossil (indicated by arrow) in Type 

II sandstone of the Schrader Bluff 

Formation.

Figure 21. Phycosiphon incertum

trace fossil (indicated by arrows) in 

Type II sandstone of the Schrader Bluff 

Formation.

&IG

&IG
Facies Descriptions and Interpretations 21

&IG

&IG

Figure 23. Trace fossil Teichichnus

(indicated by arrows) in Type II 

sandstone of the Schrader Bluff 

Formation.

Figure 22. Trace fossil 

Schaubcylindrinus (indicated by 

arrows) in Type II sandstone of the 

Schrader Bluff Formation.
22 Sentinel Hill Core Test 1

&IG

&IG

Figure 25. Trace fossil Thalassinoides

(indicated by arrows) in Type II 

sandstone of the Schrader Bluff 

Formation.

Figure 24. Trace fossil Zoophycus

(indicated by arrows) in Type II 

sandstone of the Schrader Bluff 

Formation.
Facies Descriptions and Interpretations 23

&IG

&IG

Figure 27. Slumped structures 

in Type III sandstone of the 

Schrader Bluff Formation.

Figure 26. Articulated 

pelecypod body fossil in Type II 

sandstone of the Schrader Bluff 

Formation.
24 Sentinel Hill Core Test 1

Figure 28. Siltstone and mudstone in the finer grained facies 

of the Schrader Bluff Formation. Note burrows (indicated by 

arrows).

Finer Grained Facies 

Beds of siltstone and mudstone (fig. 28) compose 60 

percent of the Schrader Bluff Formation (fig. 4). Siltstone 

beds are gray, have gradational to sharp contacts, and are 

as much as 78 ft thick (depth interval 1,102–1,180 ft in fig. 

4). Mudstone beds are dark gray, have sharp to gradational 

contacts, and are as much as 20 ft thick (see depth interval 

690–710 ft in fig. 4 and photograph in fig. 28). The siltstonemudstone facies is mainly lenticular bedded and contains a 

trace fossil assemblage closely similar to that observed in the 

sandstones. These burrows are filled either with sand from 

overlying beds or by reworked siltstone and mudstone (fig. 

28). The dark-colored mudstone contains finely comminuted 

plant fragments.

Interpretations

The coarser grained and finer grained facies in the 

1,005–1,100-ft depth interval (fig. 4) are interpreted as 

shoreface deposits on the basis of their sharp to erosional 

basal contacts, uniform grain size, vertical succession of 

internal sedimentary structures, and the amalgamation of 

beds across multiple stacked, internal erosional contacts. 

The Type I sandstone facies in the upper part of the sequence 

(depth interval 1,005–1,038 ft in fig. 4) is a composite of 

three depositional cycles, each marked by an erosional base, a 

basal lag conglomerate about 1 ft thick, an upper sandstone of 

uniform grain size, and a highly variable preserved thickness. 

This interval commences with a 2-ft-thick lower package; a 

28-ft-thick middle package dominated by a crossbedded and 

subparallel laminated, sparsely burrowed sandstone; and an 

upper package with trough crossbedded sandstone. These 

amalgamated conglomerate-sandstone cycles are interpreted as 

upper shoreface to foreshore (beach) facies (Reading, 1986). 

The remainder of the succession between 1,038 ft and 1,100 

ft (fig. 4) is composed dominantly of Type II sandstone and 

of lesser amounts of Type III sandstone that are sharp based 

and separated by thin mudstones and siltstones. The Type 

II sandstones are replete with hummocky cross stratification 

and bioturbation and are only partially amalgamated, which 

is consistent with their origin as storm-influenced middle 

to lower shoreface deposits. Between 1,090 ft and 1,100 ft 

(fig. 4), hummocky cross stratification is absent from the 

bioturbated, lenticular, slightly thinner sandstone beds, which 

is consistent with deposition either in lower shoreface to 

offshore transitional settings or during weaker storm events. 

The lenticular bedded, heavily bioturbated, silty sandstones 

common to most of the Type III sandstones represent a 

depositional setting still further offshore.

The sandstones are interbedded with lenticular-bedded, 

bioturbated mudstones and siltstones. The finer grained facies 

either alternates with Type II sandstone facies or underlies 

both Types II and III sandstone facies. The alternating coarser 

and finer grained facies indicate multicycle, coarseningupward sequences of parasequence sets. The lowermost 

parasequence set is in the depth interval 1,054–1,180 ft (fig. 

4) and consists mainly of amalgamated Type II sandstone 

and finer grained facies that are indicative of a progradational 

shoreline. The uppermost parasequence set, depth interval 

1,005–1,054 ft (fig. 4), includes the coarsening-upward 

sequence of finer grained facies overlain by Type III sandstone 

facies, which in turn is overlain by erosional-based Type I 

sandstone facies. The Type I sandstone facies is interpreted 

as the first-stage filling of a paleovalley incised into lower 

shoreface deposits in response to a lowering of sea level.

The coarser grained and finer grained facies of the 

Schrader Bluff Formation, between the depths of 690 ft and 

751 ft (fig. 4), are interpreted as a transgressive succession 

consisting of a basal shoreface sandstone overlain by offshore 

transitional siltstones and mudstones. The sandstone in 

&IG the lower part of this fining-upward unit is interpreted as a 
References Cited 25

shoreface deposit on the basis of sharp to erosional basal 

contacts, uniform grain size, and vertical succession of internal 

sedimentary structures. Siltstones and mudstones dominate 

the upper part, which is interpreted as offshore transitional 

deposits. The thinness of the sequence probably reflects a 

slow rate of sea-level rise (low accommodation potential) and 

minimal sediment supply during transgression. The onset of 

transgression is believed to coincide with the conglomerate at 

the 747-ft depth (fig. 4), which is interbedded with the Type II 

sandstone facies. This conglomeratic deposit is interpreted as 

a ravinement lag reworked from older deposits, the erosional 

base of which marks a transgressive surface of erosion.

Acknowledgments

Financial support for this study was from the Alaska 

Division of Oil and Gas. The paper was reviewed by Ronald 

C. Johnson, Peter D. Warwick, William R. Keefer, and 

Douglas J. Nichols.

Summary

The Sentinel Hill Core Test 1 well penetrates 

intertonguing deposits of the nonmarine Prince Creek 

Formation and the marine Schrader Bluff Formation, which 

range in age from early Campanian to early Maastrichtian. 

Strata in the depth interval 751–1,180 ft (fig. 4), which range 

in age from early to late Campanian, represent a regressive 

succession that began with shoreface units being deposited 

along a storm-dominated, barred shoreline and then was 

succeeded by deposition of fluvial channel, crevasse splay, 

and backswamp deposits. These deposits were overprinted 

by volcanic ash falls, which may have suffocated swamp 

vegetation.

The Schrader Bluff Formation in depth interval 690–751 

ft (fig. 4), which is late Campanian in age, represents a 

relatively condensed transgressive succession of shoreface and 

offshore deposits, which in turn was succeeded by regressive 

fluvial channel, crevasse splay, and backswamp deposits. The 

upper interval of the Prince Creek Formation, extending from 

the surface to the 690-ft depth (fig. 4), is late Campanian to 

early Maastrichtian in age and is mainly composed of fluvial, 

crevasse, and backswamp deposits. The erosional contact 

at the base of the upper intertongue of the Prince Creek 

Formation (depth 690 ft) occurs at the base of the fluvial 

channel deposits and is interpreted as a sequence boundary.

The sequence boundary at the base of the upper tongue 

of the Prince Creek Formation may be a regionally significant 

surface. Decker (2007) tentatively traces this sequence 

boundary from Shivugak Bluff to Sentinel Hill Core Test 1 on 

the Colville River and farther northeast through the subsurface 

toward the Milne Point field on the coast of the Beaufort Sea. 

The character of the boundary changes along the depositional 

dip toward the north. In its more proximal reaches along 

the Colville River, the boundary is an incised, nonmarine 

erosion surface within the Prince Creek and Schrader Bluff 

stratigraphic interval, whereas farther offshore it appears to 

be a subtle discontinuity between middle and lower Schrader 

Bluff Formation shelfal marine sediments. Still farther 

offshore, the boundary is interpreted as a marine mass-wasting 

surface that locally cuts through the lowermost Schrader Bluff 

Formation and into the underlying Seabee Formation.

Correlations of this mid-Campanian sequence boundary 

and other Brookian horizons above and below it (Decker, 

2007) indicate that Schrader Bluff strata at the Sentinel Hill 

Core Test 1 well and in nearby Colville River outcrops are 

probably at least as old as, if not older than, the isolated 

Tabasco sandstone near the bottom of the formation in the 

Kuparuk River Unit. If the Schrader Bluff Formation strata at 

the Sentinel Hill Core Test 1 well are as old as suggested by 

Decker (2007), then the producing Schrader Bluff, West Zak, 

Aurora, and Borealis oil pools in the Milne Point, Kuparuk 

River, and Prudhoe Bay Units of the central North Slope are 

in significantly younger reservoirs than (although possibly 

in a similar facies to) the time-transgressive deposits of the 

Schrader Bluff and Prince Creek Formations at and near the 

Sentinel Hill Core Test 1well.

References Cited

Buurman, P., 1975, Possibilities of palaeogeology: 

Sedimentology, v. 22, p. 289–298.

Decker, P.L., 2007, Brookian stratigraphic correlations, Umiat 

field to Milne Point field, west-central North Slope, Alaska: 

Alaska Division of Geological and Geophysical Surveys 

Preliminary Interpretive Report 2007-2, 19 p., 1 sheet.

Flores, R.M., 2003, Paleocene paleogeographic, paleotectonic, 

and paleoclimatic patterns of the northern Rocky Mountains 

and Great Plains region, in Raynolds, R.G., and Flores, 

R.M., eds., Cenozoic systems of the Rocky Mountain 

region: Denver, Colo., Rocky Mountain Section SEPM 

(Society for Sedimentary Geology), p. 63–106.

Flores, R.M., Myers, M.D., Houseknecht, D.W., Stricker, 

G.D., Brizzolara, D.W., Ryherd, T.J., and Takahashi, K.I., 

2007, Stratigraphy and facies of Upper Cretaceous Schrader 

Bluff and Prince Creek Formations in the Colville River 

Bluffs, North Slope, Alaska: U.S. Geological Survey 

Professional Paper 1748, 52 p.

Gryc, George, ed., 1988, Geology and exploration of the 

National Petroleum Reserve in Alaska, 1774-1982: U.S. 

Geological Survey Professional Paper 1399, 940 p., 58 pls.
26 Sentinel Hill Core Test 1

Mull, G.G., Houseknecht, D.W., and Bird, K.J., 2003, Revised 

Cretaceous and Tertiary stratigraphic nomenclature in the 

Colville Basin, northern Alaska: U.S. Geological Survey 

Professional Paper 1673, 36 p.

Reading, H.G., 1978, Sedimentary environments and facies 

(2d ed.): London, Blackwell Scientific Publications, 615 p.

Robinson, F.M., and Collins, F.R., 1959, Core test, Sentinel 

Hill area and test well, Fish Creek area, Alaska: U.S. 

Geological Survey Professional Paper 305–I, p. 485–521.

Tappan, Helen, 1951, Northern Alaska index foraminifera: 

Cushman Laboratory Foraminifera Research Contribution, 

v. 2, p. 1–8, 1 pl.

Prepared by the USGS Lafayette Publishing Service Center.
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Romeo M. Flores, Gary D. Stricker, Paul L. Decker, and Mark D. Myers—Sentinel Hill Core Test 1—Professional Paper 1747