Depositional Framework and Regional Correlation 

of Pre-Carboniferous Metacarbonate Rocks of the 

Snowden Mountain Area, Central Brooks Range,

Northern Alaska

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1545
Cover: Massive carbonate rocks chiefly of the Mathews River unit, Chandalar D-6 quadrangle, central 

Brooks Range, northern Alaska. The image is a digital enhancement of the photograph (fig. 4) 

on page 7.

Design and layout by Sunne Rinkus, 1994.
Depositional Framework and Regional Correlation 

of Pre-Carboniferous Metacarbonate Rocks of the 

Snowden Mountain Area, Central Brooks Range, 

Northern Alaska

By J.A. Dumoulin and Anita G. Harris

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1545

Lithofacies, conodont biostratigraphy and biofacies, 

and depositional environments of pre-Carboniferous 

metacarbonate rocks, correlation with other sequences 

across northern Alaska, and paleogeographic and 

paleotectonic implications

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center, Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

Published in the Eastern Region, Reston, Va. 

Manuscript approved for publication August 17, 1993.

Library of Congress Cataloging in Publication Data

Dumoulin, Julie A.

Depositional framework and regional correlation of pre-Carboniferous metacarbonate

rocks of the Snowden Mountain area, central Brooks Range, northern Alaska / by J.A.

Dumoulin and Anita G. Harris.

p. cm.—(U.S. Geological Survey professional paper ; 1545) 

"Lithofacies, conodont biostratigraphy and biofacies, and depositional environments of

pre-Carboniferous metacarbonate rocks, correlation with other sequences across

northern Alaska, and paleogeographic and paleotectonic implications." 

Includes bibliographical references. 

Supt. of Docs. no. : I 19.16:1545 

1. Rocks, Carbonate—Alaska—Snowden Mountain Region. 2. Conodonts—

Alaska—Snowden Mountain Region. 3. Stratigraphic correlation. I. Harris, Anita

G. II. Title. III. Series. 

QE471.15.C3D86 1994

552'.58-dc20 93-21361

CIP
CONTENTS

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

Acknowledgments .............................................................................. 1

Introduction...................................................................................... 2

Geologic Setting ................................................................................ 2

Previous Work .................................................................................. 3

Stratigraphic Nomenclature ................................................................... 3

Methods.......................................................................................... 6

Metacarbonate Succession—Cambrian (and older?) to Middle Devonian

Metasedimentary Rocks .............................................................. 6

Cambrian Rocks .......................................................................... 6

Lithologies........................................................................... 7

Age and Biofacies .................................................................. 7

Distribution.......................................................................... 8

Depositional Environment......................................................... 8

Middle Ordovician Rocks................................................................. 8

Lithologies........................................................................... 10

Age and Biofacies .................................................................. 14

Distribution.......................................................................... 14

Depositional Environment......................................................... 14

Upper Ordovician and Silurian Rocks ................................................ 16

Lithologies........................................................................... 17

Age and Biofacies .................................................................. 20

Distribution.......................................................................... 21

Depositional Environment......................................................... 21

Lower and (or) Middle Devonian Rocks ............................................. 22

Metacarbonate Rocks of Uncertain Age .............................................. 23

Wiehl Mountain Area.............................................................. 23

Dillon Mountain Area.............................................................. 24

Snowden Mountain Massif........................................................ 25

Table Mountain Area .............................................................. 25

Summary of the Metacarbonate Succession in the Snowden Mountain

Area................................................................................ 26

Devonian Metaclastic Rocks.................................................................. 28

Beaucoup Formation ..................................................................... 28

Carbonate Lithologies.............................................................. 28

Age and Biofacies .................................................................. 30

Distribution.......................................................................... 30

Depositional Environment......................................................... 30

Hunt Fork Shale .......................................................................... 31

Carbonate Lithologies.............................................................. 31

Age and Biofacies.................................................................. 31

Distribution.......................................................................... 31

Depositional Environment......................................................... 32

Nutirwik Creek Unit ..................................................................... 33

Carbonate Lithologies.............................................................. 33

Age and Biofacies .................................................................. 34

Distribution.......................................................................... 34

Depositional Environment......................................................... 34

Summary of Devonian Metaclastic Rocks in the Snowden Mountain

Area................................................................................ 34

m
IV CONTENTS

Regional Relationships......................................................................... 35

Pre-Carboniferous Metacarbonate Successions ...................................... 35

Eastern Baird Mountains Metacarbonate Succession ......................... 37

Proterozoic and (or) Cambrian Rocks..................................... 37

Lower and Middle Ordovician Rocks..................................... 39

Upper Ordovician to Devonian(?) Rocks................................. 40

Comparison of the Snowden Mountain and Eastern Baird Mountains

Metacarbonate Successions .............................................. 40

Other Pre-Carboniferous (Meta)sedimentary Successions ................... 42

Western Baird Mountains ................................................... 42

York Mountains............................................................... 44

Shublik and Sadlerochit Mountains........................................ 47

Doonerak Window............................................................ 47

Devonian Siliciclastic Units............................................................. 48

Devonian Siliciclastic Units in the Baird Mountains ......................... 48

Nakolik River Unit........................................................... 48

Unit ftqs ...................................................................... 48

Hunt Fork Shale .............................................................. 49

Comparison of Snowden Mountain and Baird Mountains Devonian

Siliciclastic Units .......................................................... 49

Discussion.................................................................................. 50

Comparative Microfacies Analysis .............................................. 51

Paleobiogeography.................................................................. 53

Conclusions...................................................................................... 57

References Cited................................................................................ 57

Appendix......................................................................................... 62

PLATES

[Plates follow appendix]

1. Ordovician conodonts from the Snowden Creek unit.

2. Late Ordovician and Silurian conodonts from the Mathews River unit.

3. Latest Givetian and Frasnian conodonts from the Beaucoup Formation, Hunt Fork Shale, and Nutirwik Creek 

unit.

FIGURES

1. Index map showing location of Snowden Mountain study area, northern Alaska ................................. 2

2. Map showing generalized distribution of geologic units in the Snowden Mountain area ........................ 4

3. Index map showing geographic names and location of measured sections and lithologic and fossil

collections ....................................................................................................................... 5

4. Photograph showing massive metacarbonate rocks of the Mathews River unit..................................... 7

5. Diagram showing lithologic and paleontologic symbols used in this report......................................... 8

6. Columnar section showing metacarbonate succession in the Snowden Mountain area ............................ 9

7. Photographs of the Snowden Mountain unit and overlying strata..................................................... 10

8. Columnar section showing fossil distribution in the metacarbonate succession in the Snowden Mountain

area............................................................................................................................... 11

9. Photographs of sedimentary features of the Snowden Creek unit..................................................... 12

10. Photomicrographs of clasts in calcareous turbidite, Snowden Creek unit............................................ 14
CONTENTS V

11-15. Photographs showing—

11. Sedimentary features of the Mathews River unit................................................................... 17

12. Sedimentary features and megafossils of the Mathews River unit.............................................. 19

13. Fossils and sedimentary features of the Mathews River unit..................................................... 20

14. Aligned sticklike organic forms in Emsian or Eifelian metalimestone ......................................... 23

15. Sedimentary features of metacarbonate rocks of uncertain age.................................................. 24

16. Chart showing hypothetical stratigraphic position of units of uncertain age within the metacarbonate

succession in the Snowden Mountain area ................................................................................ 27

17. Photographs showing features of the Beaucoup Formation............................................................. 29

18. Photographs showing sedimentary features of the Hunt Fork Shale .................................................. 32

19. Schematic columnar sections showing lithofacies, conodonts, and age relations of the Beaucoup

Formation, Hunt Fork Shale, and Nutirwik Creek unit................................................................. 36

20. Index map of northern Alaska showing distribution of pre-Carboniferous (meta)carbonate rocks .............. 37

21. Columnar section showing lithofacies and fossil distribution in the metacarbonate succession in the

eastern Baird Mountains ...................................................................................................... 38

22. Photographs showing sedimentary features of meatacarbonate succession in the eastern Baird Mountains ... 39

23. Diagram showing correlation of pre-Carboniferous rocks in the Snowden Mountain area and

eastern Baird Mountains...................................................................................................... 41

24-26. Columnar sections showing—

24. Lithofacies and conodont distribution, metacarbonate succession in the western Baird Mountains....... 43

25. Lithofacies and conodont distribution, carbonate succession in the York Mountains........................ 45

26. Lithofacies and fossil distribution, carbonate succession in the Shublik and Sadlerochit

Mountains................................................................................................................. 46

27. Photographs showing sedimentary features of the Nakolik River unit, western Baird Mountains............... 49

28. Chart showing correlation and depositional environments of (meta)carbonate successions in the York,

Baird, Snowden, and Shublik and Sadlerochit Mountains.............................................................. 52

29. Diagram showing schematic reconstruction of pre-Carboniferous carbonate successions in northern

Alaska............................................................................................................................ 54

30. Diagram showing global plate tectonic reconstruction for part of Late Ordovician time ......................... 57

TABLES

1. Names of fossils shown in figures 8, 19, 21, and 24-26............................................................... 15

2. Paleobiogeographic affinities of faunas from Cambrian and Ordovician (meta)carbonate successions in

northern Alaska ................................................................................................................ 55

METRIC CONVERSION FACTORS

For readers who wish to convert from the metric system of units to 

the inch-pound system, the conversion factors are listed below.

Multiply By To obtain

Length

micrometer (|xm) 0.0039 inch

millimeter (mm) 0.039 inch

centimeter (cm) 0.394 inch

meter (m) 3.281 foot

kilometer (km) 0.621 mile

Depositional Framework and Regional Correlation 

of Pre-Carboniferous Metacarbonate Rocks of the 

Snowden Mountain Area, Central Brooks Range,

Northern Alaska

By J.A. Dumoulin and Anita G. Harris

ABSTRACT

A succession of chiefly metacarbonate rocks in the 

Snowden Mountain area of the central Brooks Range 

includes strata of Cambrian (and older?) to Devonian age, 

structurally intercalated with dominantly siliciclastic rocks 

of Devonian age. Both carbonate and siliciclastic rocks are 

deformed and metamorphosed to greenschist facies, but 

locally preserved primary textures and environmentally 

diagnostic conodont assemblages allow interpretation of 

depositional environments. Rocks of the Snowden Mountain area are the first pre-Carboniferous metacarbonate 

succession from the central Brooks Range for which 

detailed sedimentologic and biofacies data are available.

Massive metacarbonate rocks at Dillon Mountain may 

be Cambrian and (or) older in age, contain coated grains 

and stromatolites, and formed in a peritidal environment. 

Phyllite and lesser sandy metalimestone (Snowden Mountain unit) yield early Middle Cambrian trilobites with 

Siberian biogeographic affinities and were deposited on an 

outer shelf or slope. Carbonaceous phyllite, metachert, and 

metalimestone (Snowden Creek unit) produce cosmopolitan, cool-water conodonts of Middle Ordovician (late Arenigian to early Caradocian?) age. These strata accumulated 

chiefly in a slope to basinal setting, but the vertical 

sequence indicates a shallowing-upward depositional 

regime. Middle Ordovician rocks are succeeded by Upper 

Ordovician and Silurian dolostone and metalimestone 

(Mathews River unit) that contain conodonts of Edenian and 

Maysvillian, Richmondian, Llandoverian and early Wenlockian, and Wenlockian and Ludlovian ages. Late Ordovician conodonts include Siberian and North American 

faunal elements; Silurian conodonts are chiefly cosmopolitan. The Mathews River unit is characterized by 

shallowing-upward peritidal cycles and was deposited in a 

range of warm, shallow-water environments; it is locally 

overlain by Lower and (or) Middle Devonian (Emsian and

(or) Eifelian) metalimestone bearing two-hole crinoid 

columnals. Thrust slices of metacarbonate rocks in the 

Wiehl Mountain area, the Snowden Mountain massif, and 

the Table Mountain area retain some primary features 

similar to those of the Mathews River unit but are less well 

dated and may be of Ordovician to Devonian age.

The dominantly metacarbonate succession is structurally juxtaposed with metasiliciclastic rocks of the Beaucoup 

Formation, Nutirwik Creek unit, and Hunt Fork Shale. 

These units contain layers and lenses of metalimestone that 

yield latest Middle and early Late Devonian (latest Givetian 

and Frasnian) conodonts. Thicker limy layers formed as 

biohermal buildups and ooid or bioclastic shoals; thinner 

limy layers are tempestites or turbidites derived from shelf 

or platform carbonate deposits.

Metasedimentary rocks of the Snowden Mountain area 

have some faunal and lithologic similarities to coeval strata 

in the Baird Mountains (western Brooks Range), the Shublik and Sadlerochit Mountains (eastern Brooks Range), and 

the York Mountains (Seward Peninsula). The chiefly 

metacarbonate succession correlates particularly well with 

Ordovician and Silurian strata in the eastern Baird Mountains; Devonian siliciclastic units correspond to Middle and 

Upper Devonian rocks in the Baird Mountains. Available 

biogeographic and lithologic data from northern Alaska are 

best explained by postulating that the pre-Carboniferous 

carbonate successions accumulated on a single continental 

margin or platform that had faunal exchange with both 

Siberia and North America, rather than on a series of 

discrete platforms juxtaposed by later tectonic events.

ACKNOWLEDGMENTS

We thank our colleagues on the Brooks Range segment 

of the Trans-Alaska Crustal Transect program for logistical 

support and useful geologic discussions, particularly Alison
PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

200 KILOMETERS

I

141° W.

Figure 1. Location of Snowden Mountain study area in Chandalar, Philip Smith Mountains, and Wiseman quadrangles 

(1:250,000), northern Alaska.

B. Till. We appreciate the careful technical reviews provided by Godfrey S. Nowlan, Geological Survey of Canada, and James G. Clough, Division of Geological & 

Geophysical Surveys, State of Alaska.

INTRODUCTION

This paper describes pre-Carboniferous metacarbonate 

rocks that crop out along the Dalton Highway through the 

central Brooks Range (fig. 1). In this area, bedrock consists 

largely of intercalated thrust sheets of calcareous and 

siliceous metasedimentary rocks. Our study concentrates on 

a succession of chiefly carbonate rocks, which are Cambrian (and older?) to Devonian in age, and on subordinate 

limy layers within three Devonian siliciclastic units.

The focus of this study is the Snowden Mountain area, 

about 300 km south of Prudhoe Bay. The area investigated 

in detail extends from south of Sukakpak Mountain north to 

Atigun Pass and includes parts of the Chandalar, Philip 

Smith Mountains, and Wiseman l:250,000-scale quadrangles (figs. 1-3).

The pre-Carboniferous stratigraphy of northern Alaska 

is only beginning to be unraveled. Bedrock geology is 

dominated by a Jurassic to Cretaceous erogenic belt, which 

extends more than 1,000 km from the Seward Peninsula 

through the Brooks Range. Outcrops of pre-Carboniferous 

metacarbonate rocks occur throughout this belt, but their 

origin and tectonic significance are uncertain. Did these

rocks once constitute a single carbonate platform, like that 

formed by the chiefly Carboniferous Lisburne Group, 

which has since been tectonically dismembered? Or are they 

parts of several discrete platforms, derived from disparate 

continental margins, that have been juxtaposed by later 

tectonic events?

Rocks of the Snowden Mountain area are the first 

pre-Carboniferous carbonate succession from the central 

Brooks Range for which detailed sedimentologic and biofacies data are available, and these data permit comparison 

with better known lower and middle Paleozoic rocks in the 

western and eastern Brooks Range and Seward Peninsula. 

Detailed lithologic and faunal similarities between strata in 

these areas suggest that, like the Lisburne Group, older 

carbonate rocks display relative stratigraphic continuity 

across northern Alaska.

GEOLOGIC SETTING

During the Jurassic to Cretaceous Brooks Range orogeny, rocks of the central Brooks Range were complexly 

folded, thrust faulted, and underwent syntectonic regional 

metamorphism; contrasting structural levels are now 

exposed from south to north, the direction of tectonic 

transport (Moore and others, 1991). Several schemes have 

been proposed by various authors to describe these contrasting levels; bedrock has been divided into belts, terranes and 

subterranes, and domains and allochthons. These diverse
STRATIGRAPHIC NOMENCLATURE

approaches all recognize the existence of distinct linear 

zones within the orogen that are characterized by rocks of 

contrasting metamorphic grade and structural style.

All of the metacarbonate succession and most of the 

siliciclastic rocks described in this paper occur within a 

region variously designated as the central belt (Till and 

others, 1988; Till and Moore, 1991), the Hammond subterrane of the Arctic Alaska terrane (Moore and others, in 

press), or the Skajit allochthon (Oldow and others, 1987). 

Central belt rocks are markedly less deformed and metamorphosed than the pelitic and calcareous rocks of the 

schist belt (Coldfoot subterrane) to the south; schist belt 

rocks are penetratively deformed and have undergone highpressure metamorphism (Till and Moore, 1991). However, 

central belt rocks are more deformed and metamorphosed 

than rocks of the Bndicott Mountains allochthon (Endicott 

Mountains subterrane) to the north, and the stratigraphy of 

central belt rocks is less well understood (Oldow and others, 

1987).

The precise location and nature of boundaries between 

the schist belt, central belt, and Endicott Mountains allochthon are controversial (Oldow and others, 1991; Moore and 

others, 1991; Till and Moore, 1991), and the northernmost 

thrust sheets of Devonian siliciclastic rocks discussed in this 

paper are considered part of the Endicott Mountains allochthon by some authors. These rocks are included here 

because their lithofacies, conodont faunas, and levels of 

thermal alteration are similar to those of Devonian siliciclastic rocks within the central belt. All the rocks described 

in this report have been dismembered by thrust faults and 

metamorphosed to greenschist facies; they contain conodonts that have color alteration indices (CAIs) of 5 or 

higher, indicating that the host rocks reached at least 300°C 

(Epstein and others, 1977). However, microfossils, megafossils, and primary sedimentary textures are locally well 

preserved; these features allow reconstruction of original 

stratigraphic relationships and depositional environments 

and provide a basis for regional correlation.

PREVIOUS WORK

The geology of much of the central Brooks Range is 

still relatively poorly known, and pre-Carboniferous carbonate rocks in this area have received little attention. 

Reconnaissance geologic mapping and resource assessment 

of this part of the Brooks Range was initiated by Mertie 

(1925, 1929); later, more detailed geologic maps of the 

Chandalar, Wiseman, and Philip Smith Mountains quadrangles were published by Brosge and Reiser (1964, 1971), 

Brosge and others (1979), and Dillon and others (1986, 

1988). Oldow and others (1987) presented a generalized 

geologic map of the central Brooks Range as well as 

balanced cross sections along two north-south transects, one 

of which bisects the' Snowden Mountain area. All of these

maps include brief lithologic descriptions of the metacarbonate rocks described in this report.

Brosge and others (1962) described the general Paleozoic sequence in the eastern half of the Brooks Range and 

assigned most metacarbonate rocks in the Snowden Mountain area to the Skajit Limestone of Middle(?) and Late 

Devonian age. Palmer and others (1984) first recognized 

rocks of early Paleozoic (Cambrian) age in the Snowden 

Mountain area, and Dillon and others (1987a, 1988) documented the occurrence of rocks of Ordovician and Silurian(?) age in this region. The latter two reports summarize 

lithologic and age data that were available from the Snowden Mountain area prior to 1986 but contain little specific 

biostratigraphic or sedimentologic information. The present 

study combines detailed petrologic data, results of more 

than 60 productive conodont collections made during the 

1989 and 1990 field seasons, and a reassessment of all 

previous fossil collections available from the study area in 

order to characterize the sedimentology, stratigraphy, and 

regional relationships of pre-Carboniferous metacarbonate 

rocks in the Snowden Mountain region.

STRATIGRAPHIC NOMENCLATURE

Previous workers assigned most metacarbonate rocks 

in the Snowden Mountain area to the loosely defined Skajit 

Limestone and considered these rocks to be of Devonian 

age on the basis of rare, poorly preserved megafossils 

(Brosge and others, 1962). Metasedimentary, chiefly siliciclastic rocks (for example, units Dsg, Dl, Dls, and Ds of 

Brosge and Reiser, 1964) 1 spatially associated with these 

massive carbonate bodies were thought to stratigraphically 

overlie the Skajit Limestone and were also interpreted as 

Devonian in age. Later publications (Brosge and others, 

1979, and Dillon and others, 1988) referred some finegrained metaclastic rocks in the Wiseman and Philip Smith 

Mountains quadrangles to the Upper Devonian Hunt Fork 

Shale. Dutro and others (1979) proposed a new Upper 

Devonian stratigraphic unit, the Beaucoup Formation, to 

encompass intercalated clastic and carbonate rocks in the 

Philip Smith Mountains, Chandalar, and Arctic quadrangles; this unit was considered to unconformably overlie the 

Skajit Limestone and conformably underlie the Hunt Fork 

Shale. Rocks previously assigned to unnamed units, such as 

units Dl, Dls, and Ds of Brosge and Reiser (1964) in the 

Chandalar quadrangle, were reassigned, in part, to the 

Beaucoup Formation.

In the middle 1980's, conodont and megafossil collections made by field parties of the Alaska State Division of 

Geological & Geophysical Surveys in the Wiseman and

^sg, Devonian siltstone and grit; Dl, Devonian limestone; Dls, 

Devonian calcareous and noncalcareous phyllite, grit, and silty limestone; 

Ds, Devonian slate.
PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

150°00' 149°30'

681J 00' —

EXPLANATION 

[MAP UNITS DISCUSSED IN THIS REPORT]

DEVONIAN METACLASTIC ROCKS 

Dh Hunt Fork Shale (Devonian)

Dn Nutirwik Creek unit (Devonian) 

Db Beaucoup Formation (Devonian)

PRE-CARBONIFEROUS METACARBONATE 

SUCCESSION

SOm Mathews River unit (Silurian and 

Ordovician)

Os Snowden Creek unit (Ordovician)

ftt Metacarbonate rocks of Table 

Mountain area (Paleozoic)

ftw Metacarbonate rocks of Wiehl 

Mountain (Paleozoic)

fts Metacarbonate rocks of Snowden 

massif (Paleozoic)-lncludes 

Snowden Mountain unit

Metacarbonate rocks of Dillon 

Mountain area (Paleozoic and 

(or) Proterozoic)

OTHER MAP UNITS

PMl Lisburne Group (Pennsylvanian and 

Mississippian)--lncludes Lower Mississippian Kekiktuk Conglomerate 

and Kayak Shale

MDk Kanayut Conglomerate (Mississippian? and Devonian)

ft mil Metasedimentary rocks, undivided 

(Paleozoic)

Metasedimentary and metavolcanic 

rocks in Mount Doonerak window 

(lower Paleozoic)

_ _ 9 Contact-Dashed where inferred; 

queried where uncertain

Figure 2. Generalized distribution of geologic units in the Snowden Mountain area, based on preliminary compilations (1992) by personnel 

of the Trans-Alaska Crustal Transect Program; a geologic map of this area was not available for our use. Most contacts of geologic units 

are thrust faults, and many units are internally imbricated. Map units may include areas underlain by rocks of other units too small to show 

at the scale of this map.

Chandalar quadrangles revealed that at least some of the 

massive carbonate bodies included in the Skajit Limestone, 

and some of the associated metaclastic rocks assigned to the 

Beaucoup Formation or to unnamed Devonian map units by 

previous workers, are of Silurian and older age. Dillon and 

others (1987a, 1988) proposed a new stratigraphy to encompass these findings. They defined a sequence of unnamed 

lower Paleozoic map units, including several units of

Cambrian(?) and Cambrian(?) to Ordovician(?) age, two 

Middle Ordovician units (Om, Obpm),2 and an Upper 

Ordovician and Silurian unit (unit SOm2 ; designated as unit

2Om, Ordovician marble; Obpm, Ordovician black phyllite and 

metalimestone; SOm, Ordovician to Silurian marble.
STRATIGRAPHIC NOMENCLATURE

/// / Dillon

'//'-, / Mountain

Wiehl 

AMount

Figure 3. Geographic names and location of measured sections and lithologic and fossil 

collections in the Snowden Mountain area. See figure 2 for distribution and identification of 

geologic units; see appendix 1 for geographic coordinates and key faunal components and 

lithologies for numbered localities.

OSm by Dillon and others, 1987a, 1988). These workers 

retained the Skajit Limestone, Beaucoup Formation, and 

Hunt Fork Shale, considered all three units to be of 

Devonian age, but restricted their geographic extent within 

the northern Wiseman-Chandalar area. In addition, they 

proposed a series of new Devonian units, such as the rocks

of Whiteface Mountain, which were distinguished on the 

basis of different proportions of metasedimentary versus 

metavolcanic rocks.

Our work indicates that most of the massive carbonate 

rocks previously assigned to the Skajit Limestone in the 

Snowden Mountain region are of pre-Devonian age. How-
PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

ever, at least some massive carbonate rocks in this area are 

Devonian in age, and other strata are Silurian or Devonian. 

Reliable lithologic criteria that allow discrimination of 

pre-Devonian from Devonian carbonate rocks have not yet 

been established. Contacts between the massive carbonate 

rocks and the associated metaclastic rocks are generally 

faults, but, locally, contacts appear to be gradational and 

stratigraphic. The various Devonian metaclastic rock units, 

as presently defined, are also difficult to discriminate in the 

field (for example, Hunt Fork Shale versus Beaucoup 

Formation). Our present state of knowledge is sufficient to 

indicate problems with previously established stratigraphic 

nomenclature but is not yet adequate to support the definition of a series of new formal unit names. Hence, in this 

paper, established stratigraphic names are used where they 

are consistent with our new lithologic and faunal data. 

Elsewhere, map-unit designations or informal names 

employed during recent mapping are used, and the relationship of these names to the older stratigraphic nomenclature 

is indicated.

METHODS

The metacarbonate rocks described in this report 

were observed and sampled in a series of traverses and 

partial measured sections east and west of the Dalton 

Highway; locations of measured sections and key lithologic 

and fossil collections are shown in figure 3 and described in 

appendix 1.

Petrographic descriptions are based on field studies of 

lithology and sedimentary structures as well as on examination of about 50 polished slabs and 400 thin sections. 

Sections were measured using Jacob's staff, Brunton compass, and tape; identification of calcite and dolomite was 

made on selected samples using the Alizarin Red-S and 

potassium ferricyanide staining technique of Dickson 

(1966). Carbonate rocks in which original texture is not 

obscured by metamorphism, deformation, or diagenesis are 

classified following Dunham (1962); when descriptive 

modifiers are employed, they are listed in order of increasing abundance. Metalimestone is used to indicate rocks that 

are partially recrystallized but retain some relict primary 

textures; marble is used to describe rocks that are totally 

recrystallized. Biostratigraphy relies largely on conodont 

faunas from our measured sections and spot samples, but 

data from previous megafossil collections (particularly corals and trilobites) are included where appropriate. Interpretations of depositional environments are based on depositional models for carbonate rocks outlined in Wilson 

(1975); the environmental implications of conodont assemblages (for example, Sweet and Bergstrom, 1971; Clark, 

1984) are also used to constrain our interpretations.

METACARBONATE SUCCESSIONCAMBRIAN (AND OLDER?) TO MIDDLE 

DEVONIAN METASEDIMENTARY 

ROCKS

Bedrock in the Snowden Mountain area consists 

mostly of large masses of metacarbonate rocks associated 

with subordinate thinner intervals of carbonaceous phyllite 

(fig. 4). Deformation and metamorphism have obscured the 

original depositional patterns of these rocks, but compilation of a number of partial, but overlapping, measured 

sections allows reconstruction of a generalized stratigraphy 

(figs. 5, 6). The oldest dated strata are of early Middle 

Cambrian age; these rocks are fault bounded, and their 

stratigraphic relationship to the other units discussed here is 

uncertain. Rocks of Late Cambrian and Early Ordovician 

age have not been recognized in the Snowden Mountain 

area, but Middle Ordovician strata are widely distributed 

and comprise a variety of carbonaceous, chiefly finegrained lithofacies laid down in a shallowing-upward depositional regime. These rocks are structurally intercalated 

with, and locally grade up into, massive metacarbonate 

rocks. Many exposures of this lithology are of Late Ordovician and Silurian age, but some are Devonian, and other 

metacarbonate masses in the study area may be of Cambrian 

or older age. The reconstructed stratigraphic column shown 

in figure 6 is undoubtedly incomplete; it does not include 

lithologic units in the Snowden Mountain area (such as units 

ftmu and 1ftmd in fig. 2) that are of unknown or uncertain 

age. However, it portrays our present understanding of the 

relative stratigraphic position, age range, and minimum 

thickness of well-dated lithologic units known to be preLate Devonian in age in the Snowden Mountain area. A 

more speculative column including units of less well constrained age is presented and discussed below (see "Summary of the Metacarbonate Succession in the Snowden 

Mountain Area," p. 26).

CAMBRIAN ROCKS

The oldest well-dated rocks in the study area are a thin 

interval of phyllite and subordinate metalimestone, about 3 

km in lateral extent, that discontinuously underlies massive 

marble on the north side of Snowden Mountain (fig. 3, loc. 

20). These rocks constitute unit Cl (Cambrian marble) of 

Dillon and others (1988) and are here referred to as the 

Snowden Mountain unit. Ten to twenty meters of continuous section are relatively well exposed directly below the 

massive marble along a small north-facing ridge; additional 

phyllitic strata discontinuously exposed on the steep northern side of this ridge also belong to this unit but are 

inaccessible and were not examined during the course of 

this study. The entire phyllite and metalimestone interval is 

no more than a few hundred meters thick.
METACARBONATE SUCCESSION-CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS 7

Figure 4. Massive metacarbonate rocks chiefly of the Mathews River unit, Chandalar D-6 quadrangle.

The Snowden Mountain unit overlies a thick section of 

metasedimentary rocks, including calcschist and metaconglomerate; the nature of the contact between these units is 

uncertain but was thought to be a normal stratigraphic 

contact by Dillon and others (1988) (see further discussion 

below). The upper contact of the Snowden Mountain unit 

has been interpreted as an unconformity (Dillon and others, 

1987a, 1988), but where observed during the course of this 

study it appears to have undergone at least some faulting. 

The basal meter of strata directly overlying the phyllite and 

metalimestone interval consists of dominantly clastsupported, calcareous conglomerate (fig. 7 A), which grades 

upward into massive, cliff-forming gray marble. Clasts are 

1 mm to 15 cm in diameter, subrounded to angular, and 

generally well aligned. Most clasts consist of medium-gray, 

finely crystalline marble; rare pebbles and cobbles of tan, 

fine-grained, dolostone(?) and white, coarse-crystalline 

marble also occur. This conglomeratic layer could represent 

a basal lag developed along an unconformity surface, but it 

(and the immediately overlying gray marble) is locally 

strongly sheared, brecciated, and iron stained, thus indicating at least some deformation along the upper contact of the 

Snowden Mountain unit.

LITHOLOGIES

The dominant lithology in the Snowden Mountain unit 

is noncalcareous, gray to silver phyllite. Thin layers and 

lenses of slightly recrystallized, sandy metalimestone make 

up 5 to 10 percent of the section and are thicker and more 

abundant in the upper part of the section. The top 2 m of 

strata are orange-weathering, dark-gray, bioclastic wackestone to packstone, with phyllitic partings and layers spaced 

4 to 10 cm apart. The wacke-packstone consists of locally 

abundant whole and fragmentary trilobites and phosphatic 

brachiopods, as well as echinoderm and other fossil debris, 

peloids, and a few to 10 percent angular, fine sand- to 

silt-sized quartz grains in a matrix of lime mud; thin seams 

of dolomite euhedra occur locally (fig. IE).

AGE AND BIOFACIES

Limy layers in the uppermost part of the Snowden 

Mountain unit contain trilobites of early Middle Cambrian 

age, including many specimens of Kounamkites cf. K. 

frequens Chernysheva, less common specimens of Chon-
PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

dranomocare cf. C. speciosum M. Romanenko, and fragments of indeterminate olenellids (Palmer and others, 1984; 

Dillon and others, 1988) (fig. 8). The fauna has strong 

Siberian biogeographic affinities and is typical of openshelf facies (Palmer and others, 1984).

DISTRIBUTION

Dillon and others (1987a, p. 337) suggested that strata 

"thought to be stratigraphically below and on strike with 

those at the trilobite locality" within the Chandalar D-6 

quadrangle are also of Cambrian age. These rocks consist of 

units Ccq, Ccs, and parts of OCc, OCvc, and OCvp 

(Dillon and others, 1988)3 and include interlayered tan- to 

orange-weathering, calcareous schist and quartzite, sandstone and granule conglomerate, sandy marble, and gray, 

green, and purple phyllite. Dillon and Reifenstuhl (in press) 

extended this stratigraphy south of the Snowden Mountain 

area on the basis of lithologic correlation and considered 

additional, widely distributed strata in the Chandalar C-5 

and C-6 quadrangles to be of Cambrian age. Thus far, no 

fossils have been found in any of these rocks to confirm a 

Cambrian age; 10 samples taken for microfossils from these 

units are barren, and a single productive sample from the 

Chandalar C-6 quadrangle contains a conodont fragment of 

post-Cambrian age. In addition, at least some of the units 

assigned a Cambrian age by Dillon and others (1988) have 

strong lithologic similarities to rocks of known Devonian 

age, and contacts considered to be stratigraphic by Dillon 

and others (1988) have been reinterpreted as faults by later 

workers (T.E. Moore, U.S. Geological Survey, written 

commun., 1991). In this paper, only the Snowden Mountain unit (unit €1 of Dillon and others, 1988) is considered 

to be of Cambrian age; it has been recognized in only one 

locality, along the north side of Snowden Mountain (fig. 3, 

loc. 20).

DEPOSITIONAL ENVIRONMENT

Sedimentary features of the Snowden Mountain unit 

suggest deposition in an outer shelf or slope setting. The 

fine grain size and lack of current structures characteristic of 

most of the section imply deposition in a quiet setting, 

below fair-weather wave base. Lenses of quartzose lime 

wackestone and packstone intercalated in the phyllite probably represent storm deposits, and the increased abundance 

of such lenses in the upper part of the section suggests a

EXPLANATION FOR STRATIGRAPHIC COLUMNS 

[Figures 6, 8,16,19, 21,23-26, and 29]

3Ccq, Cambrian chlorite quartz schist; Ccs, Cambrian calc-schist; 

OCc, Cambrian(?) to Ordovician(?) calcareous clastic rocks; OCvc, 

Cambrian(?) to Ordovician(?) volcanic conglomerate; OCvp, Cambrian(?) 

to Ordovician(?) interlayered volcanic rocks and phyllite.

ROCK TYPES

Metasandstone 

or quartzite

Silty shale or 

silty phyllite

Shale or 

phyllite

Limestone or 

metalimestone

Argillaceous limestone 

or metalimestone

Dolomitic limestone or 

metalimestone

Dolostone

Marble

Chert or metachert

Tuff

Metabasite

Mafic dike

SEDIMENTARY STRUCTURES 

£ Bioturbation 

Cross lamination

^ Fenestral fabric and (or)

~ cryptalgal lamination

\\ Vertical fenestral fabric

FOSSILS AND GRAIN TYPES

s Bioclast

O Coated grain

»* Intraclast

°°° Peloid

<=> Brachiopod

y Bryozoan

$ Crinoid ossicle

&& Colonial coral

© Dasycladacean alga

^£9 Solitary coral

>fiV Stromatolite

S Stromatoporoid

Boundary— 

C, conformable; 

U, unconformable; 

F, fault;

F/U, fault and (or) 

unconformity

Figure 5. Lithologic and paleontologic symbols used in this 

report.

shallowing-upward depositional regime. The trilobitebrachiopod-echinoderm fauna contained within the limy 

lenses is characteristic of normal marine conditions and 

moderate water depths (Palmer and others, 1984).

MIDDLE ORDOVICIAN ROCKS

Middle Ordovician strata are widely distributed in the 

Snowden Mountain area and consist primarily of phyllite, 

metachert, metalimestone, and marble (fig. 6). These rocks 

were originally included in units Dsk (Skajit Limestone) 

and Dl (limestone) by Brosge and Reiser (1964) and were 

considered by them to be of Devonian age. Dillon and 

others (1987a, 1988) recognized the presence of Middle 

Ordovician conodonts in these strata and assigned them to 

their units Obpm (black carbonaceous phyllite and crinoidal 

metalimestone) and Om (gray to black crinoidal marble 

bodies of mappable extent included within unit Obpm).
METACARBONATE SUCCESSION-CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS

SNOWDEN MOUNTAIN AREA

METERS

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Figure 6. Generalized composite stratigraphic column for metacarbonate succession in the 

Snowden Mountain area, based on a number of overlapping, partial measured sections with good 

biostratigraphic control. Thicknesses are minima, particularly for Upper Ordovician and Silurian 

rocks. European series names are used for intervals containing dominantly cosmopolitan faunas, 

whereas North American series and (or) stage names are used for intervals containing chiefly North 

American Midcontinent Province faunas (NAMP). See figure 5 for explanation of symbols.
10 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

Figure 7. A, Carbonate-clast conglomerate directly overlying 

Snowden Mountain unit; layer could be a lag developed along an 

unconformity surface or a fault breccia (fig. 3, loc. 20) (pencil, 14 

cm long). B, Photomicrograph of bioclastic peloidal packstone, 

Snowden Mountain unit (fig. 3, loc. 20); dark ovoids are peloids, 

and other grains are bioclasts, including echinoderm and trilobite 

fragments.

The basic unit concept established by Dillon and others 

(1987a, 1988) is used here, and these rocks are referred to 

as the Snowden Creek unit. However, our map distribution 

of the Snowden Creek unit (fig. 2) differs somewhat from 

that of unit Obpm (Dillon and others, 1987a, 1988), and the 

unit as defined here includes a slightly broader array of rock 

types. In addition, our interpretation of the stratigraphic 

context of these rocks differs from that of Dillon and others 

(1987a, 1988). These authors stated that unit Obpm unconformably(?) overlies calcareous metasedimentary rocks, 

which they believed to be of Cambrian age, and is in turn 

overlain by "Skajit Limestone of Devonian age" (Dillon and 

others, 1987a, p. 337). Our studies indicate that the basal 

contact of most exposures of the Snowden Creek unit is a 

fault, and a stratigraphic relationship to rocks of definite 

Cambrian age has not been established. The upper contact 

of this unit is also generally a fault, but locally (for

example, fig. 3, loc. 11) a gradational and apparently 

conformable contact with Upper Ordovician and Silurian 

massive metacarbonate rocks of the Mathews River unit is 

observed.

The Snowden Creek unit is at most a few hundred 

meters thick. Exposed sections range from about 10 to 100 

m thick and are typically strongly folded; some sections 

may have been structurally thickened. A generalized composite stratigraphy (fig. 6) has been pieced together from a 

number of overlapping partial measured sections, particularly those at localities 22, 24, 28, 29, and 36 (fig. 3).

LITHOLOGIES

Carbonaceous, fine-grained siliciclastic and calcareous 

rocks characterize the Snowden Creek unit and are interlayered on a scale of a few centimeters to tens of meters. 

Calcareous material dominates some sections but makes up 

less than 5 percent of others (Dillon and others, 1987a) (fig. 

9A); these differences reflect both temporal and spatial 

fluctuations in carbonate input. Elongate lenses of lightcolored calcite marble (equivalent to unit Om of Dillon and 

others, 1987a, 1988) occur locally within the Snowden 

Creek unit.

Siliciclastic lithologies in the Snowden Creek unit 

include phyllite, metachert, and rare metasandstone. 

Phyllite-rich intervals are poorly exposed and crop out best 

in streamcuts such as the headwaters of Snowden Creek 

(fig. 3, loc. 24). More commonly, phyllite forms subordinate layers a few millimeters to a few meters thick within 

sections of metachert or metalimestone. Phyllite is black to 

silvery gray, generally carbonaceous, and locally calcareous; foliation planes are spaced a few millimeters to 3 cm 

apart.

Reddish-brown- to brown-weathering, gray to black 

metachert occurs in intervals of 2 to 30 m; it weathers into 

irregular slabs, 0.5 to 5 cm thick, which may contain 

millimeter- to centimeter-scale laminations of mica, pyrite, 

or carbonaceous material. In some outcrops, this smallscale compositional layering is so pronounced that the rocks 

are rhythmically color-banded (for example, fig. 3, loc. 

29). Local lenses, 15 to 30 cm thick, of brown-weathering, 

black dolostone occur throughout the metachert intervals. 

Both metachert and dolostone are fine grained; crystal size 

is 20 to 100 |xm in metachert and 20 to 50 |xm in dolostone. 

Both lithologies contain spheroids and ovoids, 50 to 400 

|xm (mostly 100-200 jjim) in diameter, that are probable 

radiolarian ghosts. In the metachert layers, these ghosts are 

made of quartz crystals that are slightly coarser than those in 

the surrounding matrix. Ghosts in the dolomitic lenses 

consist of crystalline calcite or dolomite; concentrations of 

carbonaceous material preserve details of the original test in 

some specimens. Radiolarian ghosts generally are rare (a 

few percent) and disseminated but reach abundances of 5 to
METACARBONATE SUCCESSION -CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS 11

METERSs

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LOWER SILURIAN(?) 

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SNOWDEN MOUNTAIN AREA

EXPLANATION

Number of samples that 

yielded a species 

association

• 1

• 3

• 5

69

Figure 8. Generalized composite stratigraphic column showing 

fossil distribution in the metacarbonate succession in the Snowden 

Mountain area; selected fossils (chiefly conodonts) of biostratigraphic, paleogeographic, and (or) paleoecologic significance are 

shown at appropriate points adjacent to the lithologic column. 

Thicknesses are minima, particularly for Upper Ordovician and

Redeposited Late

Cambrian and (or) Early

Ordovician conodonts

Silurian rocks. European series names are used for intervals 

containing dominantly cosmopolitan faunas, whereas North American series and (or) stage names are used for intervals containing 

chiefly North American Midcontinent Province (NAMP) faunas. 

See figure 5 and table 1 for explanation of symbols and numerical 

identification of fossils, respectively.

25 percent in some samples and may be concentrated into 

irregular laminae. Some metachert layers contain small 

knots of chloritoid.

Metasandstone is less abundant in the Snowden Creek 

unit than the finer grained siliciclastic lithologies described 

above but may form 20 to 30 percent of some outcrops. It 

occurs intercalated with phyllite in graded, poorly sorted 

layers, 2 to 25 cm thick, which weather pink, greenish gray,

or brownish gray. Most layers are semischistose, and some 

are slightly calcareous; grain size ranges from very fine to 

very coarse. Clasts are rounded to subangular and consist of 

15 to 30 percent quartz, lesser amounts of plagioclase, and 

abundant lithic grains in a recrystallized matrix of mica, 

chlorite, and calcite. Lithic clasts are primarily sedimentary 

(chert, siltstone, dolostone) and metamorphic (schist, phyllite). Heavy minerals include tourmaline and garnet.
12 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

Figure 9. Sedimentary features of the Snowden Creek unit. A, Section consisting chiefly of noncalcareous phyllite and lesser 

metasandstone (fig. 3, loc. 22). B, Section corsisting chiefly of calcareous turbidites (fig. 3, loc. 28); arrow marks turbidite shown in C 

(hammer, 40 cm long).

Limy layers are found throughout the Snowden Creek 

unit, but, as noted above, the kind and amount of calcareous 

material vary strikingly from section to section. The major 

carbonate rock types are thinly layered, carbonaceous 

metalimestone and massive, light-colored calcite marble.

Carbonaceous metalimestone is most abundant in the 

lower part of the Snowden Creek unit. It occurs as continuous sequences, 10 to 100 m thick, that dominate some 

sections and as subordinate layers, a few millimeters to 1 m 

thick, in sections made mostly of phyllite and (or) metachert. In both settings it forms couplets, 0.5 to 10 cm thick, 

defined by a contrast in color, grain size, and (or) noncarbonate components (fig. 9B-D). The finer grained upper 

part of the couplet is dark gray to black, weathers brown or 

black, and may contain abundant mica and (or) carbonaceous material. The lower part of the couplet is generally 

thicker, lighter in color, and may contain abundant quartz. 

Most contacts between the two parts of a single couplet are 

gradational, whereas those between adjacent couplets are

sharp. In the least deformed and recrystallized outcrops of 

carbonaceous metalimestone, details of original bedform 

and composition are preserved. Most couplets are even to 

slightly undulatory, laterally continuous, and parallel laminated; some form lenses, 0.5 to 1. 5 m long, with scoured 

bases.

The amount of noncarbonate material in the couplets 

varies greatly from outcrop to outcrop. Some couplets 

contain few noncarbonate grains and consist mostly of 

calcareous bioclasts and (or) calcareous lithiclasts and lime 

mud. At locality 28 (fig. 3), at least 40 m of relatively pure 

metalimestone overlies several meters of black metachert. 

Carbonate couplets here consist of skeletal packstone grading up to lime mudstone and are separated by 1 cm or less 

of calcareous phyllite; bioclasts are almost exclusively 

echinoderm debris (crinoid columnals) (fig. 9B-D). The 

basal centimeter of a typical 8-cm-thick couplet at this 

outcrop contains columnals that average 2 mm in diameter, 

as well as 5 to 10 percent rounded grains of quartz silt and
METACARBONATE SUCCESSION—CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS 13

I____1 CM |

Figure 9.—Continued.

C, Sawed slab of turbidite showing well-developed graded bedding. D, Photomicrograph of lower part of turbidite in C; rock 

consists primarily of crinoid ossicles, C, in a matrix of finecrystalline calcite and minor amounts of quartz silt.

fine sand, in a matrix of fine-crystalline (200 jam) calcite. 

The upper part of the couplet contains rare columnals that 

average 0.5 mm in diameter, and less than 1 percent quartz 

silt, in a matrix of microcrystalline (20-40 jjtm) calcite. At 

other outcrops (for example, fig. 3, loc. 19), couplets 

contain 20 to 30 percent rounded to angular carbonate lithic 

clasts (fig. 10A, B) in addition to echinoderm debris. These 

clasts include lime mudstone, peloidal wackestone and 

grainstone, bioclastic packstone and grainstone, and finegrained dolostone with disseminated radiolarian ghosts. The 

largest clasts are as much as 1.5 cm in diameter but in 

general are coarse to very coarse sand sized.

Other metalimestone intervals include abundant noncarbonate material. At locality 30 (fig. 3), just 3 km 

southeast of locality 28, 100 m of quite impure metalimestone overlies 5 m of black metachert. The coarse lower 

parts of couplets at this site contain 20 to 80 percent quartz, 

as well as calcite and subordinate white mica. Elsewhere, 

rounded phosphatic grains and phosphatic skeletal fragments make up as much as 25 percent of the coarse fraction 

of some metalimestone couplets.

Distinctive, light-colored carbonate bodies (unit Om 

of Dillon and others, 1987a, 1988) occur locally within

the Snowden Creek unit. These bodies are typically elongate lenses, 10 to 100 m thick and a few hundred meters 

to several kilometers long, which are white to light gray and 

weather white to beige. Most are massive, fine- to mediumcrystalline, calcite marble in which original texture has 

been obliterated. Locally, however, original fabric can be 

discerned and consists of peloidal-skeletal packstone or 

grainstone. These white marble bodies are most common in 

the upper part of the section.

At several outcrops, carbonaceous metalimestone 

grades upward through an interval of a few tens of meters 

into massive white marble. At locality 28 (fig. 3), 2 m of 

black metachert with minor thin layers of black metalimestone is overlain by about 40 m of metalimestone couplets. 

At the base of the metalimestone sequence, couplets are 

dark gray to black, 3 to 5 cm thick, and typically separated 

by about a centimeter of calcareous phyllite. A few meters 

higher, couplets are medium gray, 5 to 10 cm thick, and 

separated by at most a few millimeters of phyllite. In the 

uppermost part of the sequence, light-gray, flaggy metalimestone grades up into white, sugary, fine crystalline 

marble. Changes in microtexture occur throughout this 

sequence as well. The lowest metalimestone layers (those
14 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

Figure 10. A and B, Photomicrographs of clasts in calcareous turbidite, Snowden Creek unit (fig. 3, loc. 19). C, crinoid ossicle; M, 

carbonaceous mudstone clast; P, peloidal grainstone clast.

intercalated with the metachert) are strongly carbonaceous 

and contain radiolarian ghosts. The metalimestone couplets 

consist primarily of echinoderm packstone, micrite, and 

minor detrital quartz and become less carbonaceous and 

more quartzose upward. The marble consists of 5 percent 

crinoid columnals in a matrix of anhedral calcite.

AGE AND BIOFACIES

Conodonts have been obtained from 22 samples, 

which represent 16 localities and several lithologies in the 

Snowden Creek unit (fig. 3 and app. 1, Iocs. 18, 19, 22, 24, 

25, 28, 30-32, 35, 36), and provide relatively good age 

control for these strata. Dolostone lenses in metachert, thin 

metalimestone couplets intercalated with phyllite, and thick 

sequences of pure and impure metalimestone have all 

yielded conodonts (fig. 8; table 1; app. 1). Key species, 

including Periodon flabellum, Tripodus laevis, Pygodus 

serra, Py. anserinus, Prattognathus rutriformis, and Eoplacognathus elongatus transitional to Polyplacognathus 

sp. (fig. 8; pi. 1, figs. 1, 2, 5, 6, 9-12, 17, 18), indicate an 

age of late Arenigian to at least Llandeilian and probably to 

early Caradocian (early to middle Middle Ordovician) for 

this unit (USGS collns. 10728-CO, 9913-CO, 9911-CO, 

10851-CO); the highest beds (USGS colln. 9909-CO) 

contain Periodon aculeatus (pi. 1, fig. 8) and are thus no 

younger than early Caradocian (middle Middle Ordovician). The most precisely dated parts of the unit represent 

intervals within the serra Zone (USGS colln. 10728-CO) 

and Baltoniodus variabilis Subzone to lowermost B. gerdae 

Subzone of the Amorphognathus tvaerensis Zone (USGS 

colln. 10851-CO) of late Llanvirnian and latest Llandeilian

age to earliest Caradocian age, respectively. The oldest 

beds thus far sampled are very late Arenigian in age (USGS 

collns. 10826-CO and 10827-CO); these strata also contain 

redeposited Late Cambrian and (or) Early Ordovician conodonts (fig. 8; pi. 1, figs. 21-24). Conodonts from the 

Snowden Creek unit are chiefly cosmopolitan, deep- and 

(or) cool-water species of the protopanderodid-periodontid 

biofacies.

DISTRIBUTION

The Snowden Creek unit is exposed primarily in 

several west- to southwest-trending linear outcrop belts. 

The southernmost belt lies south of Sukakpak and Wiehl 

Mountains; age control is provided by a single sample (fig. 

3, loc. 36). The central belt is the best dated and most 

studied; it consists of at least three discrete fault slices and 

can be traced from south of Snowden Mountain west across 

the Dalton Highway to Grotto Mountain. Two smaller 

exposures occur north and northeast of Snowden Mountain 

(fig. 3, Iocs. 11 and 18-19).

DEPOSITIONAL ENVIRONMENT

Sedimentary features and conodont biofacies of the 

Snowden Creek unit indicate that deposition took place 

chiefly in a slope to basinal setting, and the vertical 

sequence of lithologies suggests a shallowing-upward depositional regime. The carbonaceous siliceous lithology is 

interpreted as metachert because of its lithologic association 

and the presence of radiolarian ghosts and fine-scale lami-
METACARBONATE SUCCESSION-CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS 15

Table 1. Names of fossils shown in figures 8, 19, 21, and 24-26.

No. Conodonts No. Conodonts

1 Ancyrodella gigas

2a Ancyrodella lobata

2b Ancyrodella nodosa

3 Ancyrodella sp. indet.

4 Ancyrognathus cf. A. coeni

5 Ancyrognathus aff. A. triangularis

6 Aphelognathus aff. A. divergens

1 Aspidognathus cf. A. sp. B of Mannik (1983)

8 Astropentagnathus irregularis

9 Aulacognathus buttatus

10 Belodina sp.

11 Chosonodina rigbyi

12 Clavohamulus densus

13 Clavohamulus n. sp.

14 Cordylodus angulatus

15 '''Cordylodus" horridus

16 Cordylodus intermedius

17 Cordylodus proavus

18 Culumbodinal cf. C. occidentalis

19 Dapsilodusl similaris

20 Diaphorodus delicatus

21 Distomodusl dubius

22 Drepanodus arcuatus

23 Eoplacognathus elongatus transitional to Polyplacognathus sp.

24 Erraticodon balticus

25 Fahraeusodus marathonensis

26 Fryxellodontusl n. sp.

27 Histiodella n. sp.

28 Icriodellal sp. indet.

29 Icriodus symmetricus

30 Icriodus taimyricus

31 Icriodus sp. indet.

32 Juanognathus variabilis

33 Jumudontus gananda

34 Klapperina ovalis

35 Kockelella amsdeni

36 Kockelella sp. indet.

37 Macerodus n. sp.

38 Mesotaxis falsiovalis

39 Oepikodus communis

40 Oistodus lecheguillensis

41 Oistodus multicorrugatus

42 Oneotodus costatus

43 Oulodusl n. sp.

44 Oulodus sp.

45 Ozarkodina cf. O. cadiaensis

46 Ozarkodina confluens

47 Ozarkodina ex.ca.vata

48 Ozarkodina aff. O. oldhamensis

49 Ozarkodina remscheidensis remscheidensis

50 Ozarkodina n. sp. cf. O. remscheidensis

51 Ozarkodina n. sp.

52 Palmatolepis plana

53 Palmatolepis proversa

54 Palmatolepis sp. indet.

55 Paltodus subaequalis

56 Panderodus sp.

57 Pandorinellina exigua philipi

58 Pandorinellina expansa

59 Pandorinellina insita

60 Paracordylodus gracilis

61 Paroistodus originalis

62 Paroistodus proteus

63 Paroistodus cf. P. proteus

64 Pelekysgnathus dubius

65 Pelekysgnathus sp. indet.

66 Periodon aculeatus

67 Periodon flabellum

68 Phakelodus tenuis

69 Phragmodus n. sp. (= PA. n. sp. of Barnes, 1974)

70 Plectodinal tunguskaensis

71 Plectodinal cf. P/.? dolboricus

72 Plegagnathusl repens

73 Polonodus sp.

74 Polygnathus evidens

75 Polygnathus gronbergi

76 Polygnathus inversus

77 Polygnathus linguiformis

78 Polygnathus pacificus

79 Polygnathus planarius

80 Polygnathus samueli

81 Polygnathus webbi

82 Polygnathus of the Po. xy/i« group

83 Polygnathus sp. indet.

84 Prattognathus rutriformis

85 Prioniodus elegans

86 Proconodontus muelleri

87 Protopanderodus graeai

88 Protopanderodus insculptus

89 Protopanderodus liripipus

90 Protopanderodus varicostatus

91 Protoprioniodus aranda

92 Pseudobelodina adentata

93 Pseudobelodina vulgaris vulgaris

94 Pterospathodus aff. P. cadiaensis

95 Pygodus anserinus

96 Pygodus serra

97 Pygodus sp. indet.

98 Rossodus manitouensis

99 Rossodus n. sp.

100 Rossodus sp.

101 Scolopodus bolites

102 Scolopodus floweri

103 Scolopodus leei

104 Spinodus ramosus

105 Stereoconus corrugatus

106 Teridontus nakamurai

107 Variabiloconus bassleri

108 Walliserodus australis

109 Walliserodus ethingtoni

OTHER FOSSILS

110 Chancelloria sp. (spongelike forms)

111 Acrotretid brachiopods

112 Tcherskidium n. sp. of Blodgett and others (1988) (brachiopod)

113 Pelagiella sp. (primitive mollusk)

114 Homagnostus sp. (agnostid trilobitomorph)

115 Hystricurusl sainsburyi (trilobite)

116 Kounamkites cf. K. frequens (trilobite)

117 Plethometopus armatus (trilobite)

118 Two-hole crinoid columnal
16 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

nation. Metachert and phyllite dominate the lower part of 

the unit and indicate a relatively quiet, starved basin setting; 

the highly carbonaceous nature of these rocks and the lack 

of an indigenous benthic fauna suggest that the basin was 

poorly oxygenated. Rare graded metasandstone layers are 

probable turbidites.

Metalimestone layers occur throughout the Snowden 

Creek unit but are thicker and more abundant in its upper 

part. Sedimentologic and faunal evidence demonstrate that 

most metalimestone layers are also probable turbidites, 

which were deposited against a "background" accumulation 

of fine-grained siliceous material (now metachert) and clay 

(now phyllite). Most calcareous layers are distinctly graded 

and form partial Bouma (1962) sequences (typically ABDE 

or BDE); the lack of a well-developed C division is 

common in carbonate turbidites (Scholle, 1971; Dumoulin, 

1992).

Composition of the carbonate turbidites suggests their 

derivation from a variety of intrabasinal and extrabasinal 

sources and some changes in provenance with time. Lithic 

clasts include grains indicative of a deep-water origin, such 

as black phyllite and dolostone with radiolarian ghosts, and 

lithologies typical of a shallow-water carbonate-platform 

source, such as peloidal mudstone and bioclastic grainstone. Some clasts are rounded and irregular; these were 

probably still soft when deposited and thus derived from 

contemporaneous, intrabasinal strata. Other clasts are angular and contain calcite veins; these were fully lithified when 

eroded and infer an extrabasinal origin. Faunal evidence 

(discussed below) demonstrates that older, extrabasinal 

rocks have been eroded, reworked, and included in these 

turbidites. Calcareous extrabasinal sources appear to have 

been most important in the early history of the Snowden 

Creek unit; samples from lower in the unit contain more 

carbonate lithic clasts, whereas those in the upper part of the 

unit consist mostly of skeletal (echinoderm) debris and 

contain more quartz.

Paleoenvironmental interpretation of the elongate 

white marble bodies in the Snowden Creek unit is problematic because most retain little primary texture. Where 

original fabric can be discerned, it is typical of shallowwater sedimentary environments (peloidal-skeletal packstone or grainstone), and there are no structures, such as 

graded bedding, indicative of redeposition. Previous studies 

of metacarbonate rocks in northern Alaska (Dumoulin and 

Harris, 1987a, 1992) have demonstrated that shallow-water 

accumulations of skeletal grainstone are particularly susceptible to recrystallization and obliteration of primary texture. 

The marble bodies are much less carbonaceous than the rest 

of the Snowden Creek unit, are generally in gradational 

contact with intervals of darker metalimestone (turbidites), 

and occur predominantly in the uppermost part of the unit. 

Most of the marble bodies probably represent carbonate 

banks or shoals formed in situ on local highs that developed 

within the basin. Their concentration in the upper part of the

unit suggests a shallo wing-up ward depositional regime. 

Small, shallow-water carbonate buildups, which pass laterally and vertically into basinal bituminous pelagic sediments, have been described from the Cretaceous of Mexico 

and the Middle East (Jenkyns, 1980; Wilson, 1975).

Conodont biofacies analysis supports the environmental interpretations outlined above. Conodonts from the 

Snowden Creek unit represent the protopanderodidperiodontid biofacies, which is characteristic of a cool- and 

(or) deep-water depositional environment. Assemblages 

from dolostone lenses in metachert (for example, fig. 3, 

loc. 31) include complete and well-preserved fragile elements; such elements could not have survived post-mortem 

transport and were probably deposited by simple settling 

from within the water column to the basin floor. Most 

conodonts from the Snowden Creek unit, however, come 

from carbonate turbidites; these assemblages are hydraulically sorted, and some include elements of anomalous age 

or biofacies introduced by redeposition. The coarser grained 

parts of these turbidites yield mostly large, robust protopanderodids (pi. 1, figs. 3, 4), whereas finer grained layers 

produce more delicate periodontids (pi. 1, figs. 8, 15). 

Several early Middle Ordovician faunas collected northeast 

of Snowden Mountain (fig. 3, loc. 18) include reworked 

conodonts of Late Cambrian and (or) Early Ordovician age, 

such as Phakelodus tennis, Cordylodus spp., Rossodus 

manitouensis, and "Scolopodus" gracilis (fig. 8; pi. 1, figs. 

21-24). In addition, although conodonts of cool-water 

biofacies dominate the Snowden Creek unit, some samples 

include a few forms typical of warmer, shallower water, 

such as belodinids (fig. 3, loc. 35), plectodinids, and 

Prattognathus rutriformis (fig. 3, loc. 22; pi. 1, fig. 6), that 

were transported seaward into a deeper water setting. All 

samples from the elongate marble bodies are barren.

Thus, Middle Ordovician strata in the Snowden Mountain area were deposited primarily in a poorly oxygenated, 

off-platform setting that received minor fine-grained siliciclastic detritus, as well as pulses of allodapic calcareous 

material derived from contemporaneous and older carbonate 

deposits rimming the basin. Carbonate input varied across 

the basin, but, in general, increased through time. Eventually, water depths shallow enough for in situ accumulation 

of calcareous shoals were achieved locally.

UPPER ORDOVICIAN AND SILURIAN ROCKS

The carbonaceous Middle Ordovician strata discussed 

above are structurally intercalated with, and locally grade 

up into, massive metacarbonate rocks (fig. 11A) assigned to 

the Skajit Limestone by previous workers (for example, 

Brosge and Reiser, 1964). Dillon and others (1987a, 1988) 

reported the presence of Late Ordovician and Silurian(?) 

microfossils and megafossils from several thrust slices 

of "Skajit Limestone" north of Snowden Mountain and
METACARBONATE SUCCESSION—CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS 17

Figure 11. Sedimentary features of the Mathews River unit. A, Typical cliff-forming outcrops of massive dolostone and metalimestone 

located in the Chandalar D-6 quadrangle. B, Dark-weathering, fossiliferous, burrowed metalimestone overlain by light-weathering, thinly 

laminated dolostone (lens cap, 6 cm in diameter) (fig. 3, loc. 12). Bioturbated metalimestone beds produce more conodonts than other 

lithologies in this unit.

reassigned these rocks to their unit SOm. This unit was 

described as consisting of "massive gray marble and orange 

dolomite with local replacement bodies of black chert" 

(Dillon and others, 1988). Other massive carbonate outcrops in the Snowden Mountain area were still considered to 

represent the Skajit Limestone of Devonian age, however, 

which was described by Dillon and others (1988) as 

consisting mostly of "massive gray marble and dolomite," 

and subordinate carbonate conglomerate and minor pelitic, 

quartzose, and volcanic layers. Thus, discrimination of 

these two units on lithologic grounds is difficult.

Our studies confirm the Late Ordovician and Silurian 

age of unit SOm but also identify Late Ordovician and 

Silurian fossils in strata both north and south of Snowden 

Mountain that had been referred to the Skajit Limestone 

(Dsk) by Dillon and others (1987a, 1988). In addition, our 

collections produced fossils of definite and possible Devonian age from some occurrences of the Skajit Limestone (as 

mapped by Dillon and others, 1988). Various lines of 

evidence (discussed below) suggest that other outcrops of 

the Skajit Limestone may be pre-Late Ordovician in age. 

Massive carbonate bodies in the Snowden Mountain area 

probably represent dismembered pieces of a long-lived 

carbonate platform; individual thrust slices differ in age and 

the degree to which primary fabric has been preserved. In 

this paper, distinctive dolomitic metacarbonate rocks north 

and south of Snowden Mountain, including unit SOm and 

parts of unit Dsk of Dillon and others (1987a, 1988), are 

referred to as the Mathews River unit. All rocks assigned to 

this unit have yielded some fossils of Late Ordovician and 

(or) Silurian age. Massive metacarbonate rocks in which the 

distinctive lithologic features of the Mathews River unit 

have not been found are shown in figure 2 as ftBd, fts, 

ftw, or Pzi. Some of these strata may be facies equivalents 

of the Mathews River unit or parts of the Mathews River 

unit that have lost their distinctive lithologic features

through metamorphism and (or) deformation. Other strata 

may be older or younger than the Mathews River unit. 

These rocks are discussed below as "Metacarbonate Rocks 

of Uncertain Age."

Most sections of the Mathews River unit are fault 

bounded as well as internally faulted and folded. A composite of several partial measured sections (fig. 3, Iocs. 6, 

12, and 16) shows the unit to be at least 150 m thick. 

Stratigraphic relationships with other units may be discerned in the northern outcrop belt. At locality 11 (fig. 3), 

rocks of the Mathews River unit gradationally and apparently conformably overlie rocks of the Snowden Creek unit. 

At locality 6 (fig. 3), a thin layer of Devonian carbonate 

rocks overlies the Mathews River unit; the contact appears 

to be an unconformity but could be a fault. Carbonate rocks 

lithologically similar to these Devonian strata, but which 

have not yet been dated, overlie the Mathews River unit at 

several other localities.

LITHOLOGIES

Massive cliffs of orange and gray dolostone and 

metalimestone and lesser amounts of marble make up the 

Mathews River unit (fig. 11 A). Some dolostone forms 

irregular masses that crosscut bedding, but most occurs as 

even, laterally continuous beds, which alternate with darker 

weathering metalimestone to produce the decimeter- to 

meter-scale color-banding characteristic of this unit (fig. 

1LB). Black chert constitutes 1 to 10 percent of some 

outcrops; it replaces skeletal material and forms irregular 

bands and stringers a few centimeters thick. Much of this 

unit is notably fetid, particularly those sections dominated 

by dolostone. Where original textures are best preserved, 

the Mathews River unit consists of four main lithologies, 

interbedded on a scale of a few centimeters to tens of
18 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

meters. These lithologies are bioturbated metalimestone, 

parallel-laminated metalimestone, fossiliferous dolostone, 

and algal-laminated dolostone.

The first lithology is gray- to brown-weathering, 

medium-gray to black, locally dolomitic metalimestone, 

which forms slightly undulatory to nodular beds, 2 to 50 cm 

(mostly 5-15 cm) thick, separated by orange-weathering 

argillaceous partings (fig. 12A). Most beds show megascopic and microscopic evidence of bioturbation, ranging 

from an irregularly mottled fabric to a network of discrete 

burrows (fig. 12fi). Mottled fabric typically consists of 

irregular ovoids and cylinders, one to a few centimeters 

across, in a darker matrix. Discrete burrows are light 

colored, a few millimeters in diameter, and subvertical; 

most have a Chondrites- or Trichichnus-like form. Both 

mottles and burrows are less dolomitic, less carbonaceous, 

and coarser grained than the surrounding material. Within a 

single bed, mottled fabric generally grades upward into an 

array of discrete burrows, which in turn decrease upward in 

size and abundance.

Bioturbated metalimestone is mostly skeletal wackestone and lesser packstone and mudstone, which may 

alternate on a scale of centimeters or decimeters. Matrix 

consists of micrite, locally slightly recrystallized (crystals 

2-14 |xm). Skeletal material constitutes 15 to 25 percent 

of most samples but reaches 60 to 80 percent and shows 

overly close packing in some millimeter- to centimeterthick layers. Fossils in wackestone and mudstone intervals 

are mostly whole skeletons; shells are articulated and many 

are coated with and (or) partially replaced by pyrite. 

Bioclasts in packstone layers are commonly 3 mm or less in 

diameter, broken, abraded, and (or) disarticulated. Shelter 

porosity, filled with sparry calcite cement, and biogenic 

geopetal fabric (fig. 12C) are locally well developed in this 

lithology.

A relatively diverse fauna characterizes the bioturbated 

metalimestone intervals. Corals are locally abundant and 

include Catenipora sp. aff. C. rubra (T.E. Bolton, Geological Survey of Canada, written commun., 1992) (fig. 

12D), halysitids, favositids (fig. 12E, F), and syringoporids (R.B. Blodgett and W.A. Oliver, Jr., U.S. Geological 

Survey, written commun., 1991). Most corals occur as 

isolated specimens, and biohermal concentrations were not 

observed. Other fossils include echinoderm debris, brachiopods (including pentamerids), mollusks, and bryozoans.

A second distinctive lithology is tan- to grayweathering, gray metalimestone in even, parallel-laminated 

beds 2 to 5 cm thick; it forms subordinate intervals as much 

as 30 cm thick within intervals of bioturbated metalimestone. These beds consist of packstone and grainstone made 

up of silt- to fine-sand-sized skeletal material, peloids, and 

a few percent detrital quartz. Bioclasts are mostly broken, 

abraded, and not specifically identifiable. Laminae are 

made of concentrations of peloids alternating with bioclasts; 

peloidal layers are generally somewhat dolomitic.

Orange- to light-brown-weathering, dark-gray to 

black, locally calcitic dolostone in even, massive beds 20 

cm to 3 m (typically 50 cm to 1 m) thick constitutes the third 

lithology. Most samples consist largely of euhedral to 

subhedral dolomite crystals 40 to 100 |xm in size. Relict 

microtextures indicate that these rocks were sparsely bioclastic lime wackestone and mudstone prior to dolomitization (fig. 13A); a few to 20 percent silt- to sand-sized 

bioclasts are disseminated in the fine-grained matrix. In 

some samples, skeletal material has not been dolomitized; 

elsewhere, bioclast outlines are preserved as concentrations 

of organic material that crosscut dolomite crystal boundaries. Identifiable skeletal material in these beds consists 

mostly of ostracodes, including Leperditia sp. (Dillon and 

others, 1988), gastropods, dasycladacean algae (fig. 135), 

and rare halysitid coral fragments. Locally, bioclasts have 

been bored or partly to completely micritized. A few to 10 

percent disseminated peloids and oncoids occur in some 

samples. Peloids are ellipsoidal and average 0.2 mm in 

diameter; oncoids are oval, irregularly laminated, range 

from 0.5 to 2.0 mm in size, and are commonly partially 

pyritized.

The fourth lithology is beige-, pink-, orange-, or 

tan-weathering, light- to medium-gray, dolostone to dolomitic limestone; it forms even to slightly irregular beds 2 cm 

to 2 m (mostly 10^4-0 cm) thick. Many samples are finely 

laminated (fig. 13C) and contain fenestral fabric. Laminae 

are 0.2 to 1.0 mm thick, crinkled and irregular, form 

small-scale hummocks with as much as 0.5 cm of relief, 

and are interpreted as cryptalgal in origin. Fenestrae vary 

from horizontal, laminar forms to more irregular shapes; 

most are 2 to 3 mm in diameter. Sheet cracks, 0.5 to 2 mm 

by 1 to 2 cm, occur locally. Both fenestrae and sheet cracks 

are filled with relatively coarse, sparry calcite or dolomite.

Most of this lithology consists of finely crystalline 

(25-80 jjum) dolomite, but some intervals are undolomitized 

or partially dolomitized and display excellently preserved 

original microtexture. Fenestrae in these rocks are surrounded by lime mud or fine-grained peloids; the peloids 

have locally coalesced into a flocculent, clotty mass (structure grumeleuse) in which individual grain outlines are 

difficult to discern (for example, Bathurst, 1976). The 

peloids are made of fine-grained calcite (crystals 2-20 JJUIL),

Figure 12. Sedimentary features and megafossils of the 

Mathews River unit. A, Nodular-bedded, bioturbated metalimestone overlying parallel-laminated metalimestone (fig. 3, 

loc. 10) (hammer, 40 cm long). B, Mottled fabric produced by 

bioturbation (fig. 3, loc. 12) (pen tip, 3 cm long). C, Shelter 

porosity in bioclastic wackestone (fig. 3, loc. 6); gastropod 

shell (arrow) is filled with a thin layer of lime mudstone (dark) 

overlain by sparry calcite cement (light). D-F, Slabs of colonial 

corals in bioturbated metalimestone: D, Catenipora sp. aff. C. 

rubra (primitive halysitid coral) (fig. 3, loc. 6); E and F, 

Favositoid corals (fig. 3, Iocs. 12 and 16, respectively).
>-/>v

XL"-v' "B;-H

. ^% >M»V . Jrf ' ;-^V*v
20 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

Figure 13. Fossils and sedimentary features of the Mathews River unit. A, Photomicrograph of typical texture of fossiliferous dolostone; 

bioclasts float in matrix of peloidal carbonate mud (fig. 3, loc. 12). B, Photomicrograph of dasycladacean algae in fossiliferous dolostone 

(fig. 3, loc. 13). C, Cryptalgal lamination in dolostone (fig. 3, loc. 8) (entire pencil, 14 cm long). D, Photomicrograph of peloidal, P, 

grainstone containing subordinate bioclasts, B (fig. 3, loc. 34).

have maximum diameters of 100 to 160 jxm, and are 

uniformly ellipsoidal.

Laminae in this lithology are produced by alternations 

of lime micrite and fine-crystalline dolomite or by subtle 

variations in dolomite color or crystal size. Some samples 

contain a few percent disseminated bioclastic debris. In 

addition, relatively coarse grained peloidal and (or) bioclastic grainstone and packstone form local layers a few 

millimeters to several centimeters thick. Peloids in these 

layers are irregular to oval and are 0.1 to 0.8 mm in 

diameter (fig. 13D). Recognizable bioclasts include ostracodes and dasycladacean algae.

AGE AND BIOFACIES

The age of the Mathews River unit is constrained 

largely by conodonts, obtained from 26 samples at 17 

localities (fig. 3; app. 1, Iocs. 6, 8-10, 12, 13, 16, 17, 34);

5 coral faunas from 4 localities provide additional age 

control. All four lithologies described above yield conodonts, but burrowed metalimestone is the most productive 

rock type. Conodont collections from all parts of this unit 

are dominated by the long-ranging genus Panderodus, 

which merely indicates a Middle Ordovician through Middle Devonian age. It is the sole constituent of five samples 

but elsewhere occurs with species that allow more precise 

age assignments. The oldest conodont samples (fig. 3, Iocs. 

6, 8-10, 13; app. 1) contain Phragmodus n. sp. (fig. 8; pi. 

2, figs. 20-24; =Phragmodus n. sp. of Barnes, 1974) and 

fewer Plectodinal cf. P.? dolboricus and are of Edenian to 

Maysvillian (early Late Ordovician) age. Two samples 

yield slightly younger faunas. One collection (fig. 3, loc. 

12) consists exclusively of abundant, broken specimens of 

Aphelognathus aff. A. diver gens and is Richmondian in age 

(fig. 8; pi. 2, fig. 16). A sample of peloidal dolostone (fig. 

3, loc. 9) yielded Ozarkodina aff. O. oldhamensis and 

oulodontids (fig. 8; pi. 2, figs. 7-14) and is of very latest
METACARBONATE SUCCESSION-CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS 21

Ordovician to Early Silurian age. The youngest conodont 

collections are Silurian in age; both Llandoverian to early 

Wenlockian (fig. 8; pi. 2, figs. 1-4) (fig. 3, Iocs. 16,17) and 

Wenlockian to Ludlovian (pi. 2, figs. 5, 6) (USGS collns. 

12069-SD, 12070-SD) faunas were found. Silurian samples contain rare pterospathodids, icriodellid fragments, 

oulodids, and ozarkodinids (including O. excavata and O. 

cf. O. cadiaensis), as well as ramiform and coniform 

elements of Kockelella and Pelekysgnathus, respectively 

(pi. 2, figs. 1-6). Coral collections from the Mathews River 

unit include Catenipora sp. aff. C. rubra of probable early 

Late Ordovician age (T.E. Bolton, written commun., 

1992), Tetradium(>) sp. and halysitids of Late(?) Ordovician age, and Mesofavositesl sp. of probable Late Ordovician or Silurian age (Dillon and others, 1988).

Conodonts from the Mathews River unit represent 

warm, shallow-water biofacies. Late Ordovician collections 

are dominated by provincial forms, but Edenian to Maysvillian faunas have slightly different biogeographic affinities than Richmondian faunas. Edenian to Maysvillian 

collections consist mostly of Siberian-northern North American province elements (Phragmodus n. sp. and Plectodincf! 

cf. P.? dolboricus). The single Richmondian sample is a 

monospecific collection of a western North American form, 

Aphelognathus aff. A. diver gens.

DISTRIBUTION

The Mathews River unit crops out in several thrust 

sheets north, east, and southeast of Snowden Mountain. 

These sheets are intercalated with Devonian siliciclastic 

rocks, and, in the south, with metasedimentary rocks of 

uncertain age (fig. 2).

Conodont collections demonstrate that some sections 

of the Mathews River unit have been tectonically thickened, 

and other sections contain fault slices of younger units. At 

locality 9 (fig. 3), peloidal dolostone of latest Ordovician or 

Early Silurian age underlies crinoidal marble of early Late 

Ordovician age; sedimentologic criteria indicate that these 

strata are not overturned. At locality 12 (fig. 3), peloidal 

and algal-laminated dolostone of Richmondian age underlies at least 20 m of Frasnian metalimestone (Nutirwik 

Creek unit), which in turn underlies metalimestone and 

dolostone of Middle and Late Ordovician age.

DEPOSITIONAL ENVIRONMENT

The Mathews River unit was deposited in normal 

marine to slightly restricted, moderate to very shallow water 

environments. Sedimentologic and faunal evidence indicate 

that the major lithologies described above formed in specific settings on the middle to inner shelf. Bioturbated 

metalimestone and parallel-laminated metalimestone were

deposited in open-marine conditions at moderate to shallow 

water depths. Fossiliferous dolostone accumulated in a 

somewhat restricted, shallower water environment. Algallaminated dolostone formed in the most restricted and 

shallowest water setting.

Bioturbated metalimestone and subordinate beds of 

parallel-laminated metalimestone formed primarily below 

fair-weather wave base. These lithologies have features 

such as variable bed thickness, nodular bed form, abundant 

carbonate mud, and argillaceous partings that are characteristic (Wilson and Jordan, 1983) of middle shelf deposits. 

They contain a stenohaline macrofauna of corals, bryozoans, echinoderms, and brachiopods, indicating that open 

circulation and normal marine salinity prevailed during 

deposition. Bioturbated metalimestone is mostly "whole 

fossils wackestone" (usage of Wilson, 1975); the lime mud 

matrix and intact, unabraded skeletal material in these beds 

indicate deposition in relatively quiet water lacking "currents of removal" (Dunham, 1962). Mud-poor beds of 

bioclastic packstone and parallel-laminated metalimestone 

(containing broken and abraded skeletal debris, peloids, and 

quartz) most likely accumulated during storms and (or) in 

local shoals.

Fossiliferous dolostone formed in a shallower, more 

restricted environment than the rocks just described, probably on the inner shelf. The abundance of lime mud (now 

dolomitized) indicates a quiet depositional setting like that 

inferred for the bioturbated metalimestone, but the macrofauna includes few forms characteristic of normal marine 

conditions and is dominated by biota tolerant of restricted 

circulation (Wilson, 1975) such as ostracodes, gastropods, 

and dasycladacean algae. Some bioclasts are micritized, a 

process that most commonly occurs in the photic zone as a 

result of boring by endolithic algae (Bathurst, 1976). Local 

occurrence of oncoids and peloids in this lithology further 

supports the interpretation of a shallow-water, somewhat 

hypersaline depositional setting. Oncoids are biogenic 

encrustations, typically formed on soft substrates in 

moderate-energy environments (Wilson, 1975; Fliigel, 

1982). Oncoids disseminated in a micritic matrix, as well as 

pelleted mudstone and wackestone, typically occur in protected, somewhat restricted environments such as marine 

shelf lagoons (Wilson, 1975).

Sedimentary structures and fauna of the algallaminated mudstone lithology indicate deposition in shallow 

subtidal to supratidal environments on the inner shelf 

(Wilson, 1975; Shinn, 1983; James, 1984). The association 

of algal mats, fenestral fabric, and sheet cracks is best 

developed and most commonly preserved in intertidal to 

supratidal settings (James, 1984). Modern and ancient 

shallow subtidal sediments consist largely of gray (reduced) 

pelleted muds that have been thoroughly bioturbated and 

lack primary sedimentary structures; storms introduce such 

sediments onto tidal flats, where they form layers a few 

millimeters to several centimeters thick that alternate with
22 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

algal laminite (Shinn, 1983). Most peloids in the Mathews 

River unit are similar in size and shape to modern fecal 

pellets, but some grains are larger and (or) quite irregular 

and may be skeletal particles that are completely micritized 

(Bathurst, 1976). The sparse biota of the algal-laminated 

dolostone is consistent with deposition in an inner shelf 

setting. Megafossils are limited to ostracodes and algae, 

forms that are tolerant of restricted circulation and shallow 

water depths (Wilson, 1975).

Interpretations of the depositional environments of the 

Mathews River unit suggested by conodont biofacies agree 

with those outlined above that are based on sedimentary 

features. Collections from bioturbated metalimestone and 

fossiliferous dolostone contain variably abundant Panderodus associated with Phragmodus n. sp., Plectodinal cf. P.I 

dolboricus, and belodinids in Ordovician strata and with 

pterospathodids, icriodellid fragments, oulodids, ozarkodinids, and ramiform and coniform elements of Kockelella 

and Pelekysgnathus in Silurian strata. These assemblages 

indicate deposition in warm, relatively shallow water settings. Samples of clast-rich skeletal packstone and parallellaminated metalimestone also produce Phragmodus n. sp. 

but consist chiefly of abraded, incomplete conodont fragments typical of high-energy depositional environments. 

Algal-laminated dolostone yields monospecific faunas of 

Pander odus sp. or Aphelognathus aff. A. diver gens that are 

characteristic of warm, shallow-water, intermittently 

restricted environments.

Sedimentologic and paleontologic evidence indicates 

that fossiliferous dolostone and algal-laminated dolostone 

were deposited in shallow to very shallow water with 

locally restricted circulation, and this depositional setting 

probably accounts for the pervasive dolomitization of these 

lithologies. Dolomite in the algal-laminated lithology may 

be detrital and (or) authigenic; both types of dolomite are 

common on modern tidal flats (Shinn, 1983). Subtidal 

sediments in the Mathews River unit may have been 

dolomitized by reflux of Mg-rich surficial brines produced 

in nearby supratidal environments; this mechanism has been 

invoked by many authors to explain dolomitization of a 

variety of modern and ancient sediments.

The four major lithologies described and interpreted 

above occur throughout the Mathews River unit, but there is 

some spatial and temporal variation in their abundance. For 

example, bioturbated metalimestone is most abundant in the 

lower Upper Ordovician part of the unit, whereas fossiliferous dolostone and algal-laminated dolostone predominate 

in uppermost Ordovician and Silurian strata. Sedimentary 

features and conodont biofacies suggest that the shallowest, 

most restricted conditions prevailed during Richmondian 

time.

The Mathews River unit also contains small-scale 

lithologic cycles in both Ordovician and Silurian strata. A 

typical cycle is 50 cm to 5 m thick and consists of 20 cm 

to a few meters of burrowed, fossiliferous metalimestone

overlain by a similar thickness of massive, sparsely fossiliferous dolomitic wackestone. The wackestone grades up 

into a thinner interval of algal-laminated, peloidal dolostone 

with fenestral fabric, which is in turn overlain by another 

bed of fossiliferous metalimestone. The cycles appear to 

have formed under shallowing-upward (regressive) conditions because transitions from bioturbated metalimestone to 

dolomitic wackestone, and from dolomitic wackestone to 

cryptalgal dolostone, are generally gradational, whereas 

those from cryptalgal dolostone to fossiliferous metalimestone are commonly abrupt.

LOWER AND (OR) MIDDLE DEVONIAN ROCKS

Rocks of confirmed Early or Middle Devonian age are 

rare in the study area, but Emsian and (or) Eifelian marble 

and metalimestone crop out 4.5 km east of Nutirwik Creek 

(fig. 3, loc. 6). At this locality, a small lens, 10 to 15 m 

thick, of Lower and (or) Middle Devonian strata unconformably overlies, or is faulted above, carbonate rocks of 

the Mathews River unit. These Devonian rocks are strongly 

deformed, lineated, and flattened, but relict sedimentary 

textures are locally preserved.

Devonian rocks at locality 6 consist of yellowweathering, cliff-forming marble with layers 50 cm to 2 m 

thick of dark-gray to black metalimestone and rare pink- to 

orange-weathering, medium-gray dolostone. Relict textures 

preserved in metalimestone include peloidal-bioclastic 

packstone and wackestone; bioclasts are chiefly crinoid 

columnals and coral debris. The most distinctive relict 

texture occurs near the base of the section and consists of 

packstone rich in aligned sticklike forms that do not branch 

and are probably amphiporid stromatoporoids (fig. 14). 

Individual sticks are 1 to 4 mm in diameter, as much as 3 

cm in length, and contain an internal network of 0.2-mm 

pores; details of original wall structure have been destroyed 

by recrystallization. Similar packstone, rich in sticklike 

forms and crinoids, structurally (and stratigraphically?) 

overlies the Mathews River unit at several other sites (fig. 

3, Iocs. 7,8, and 14) and may be correlative with packstone 

at locality 6. Fossils and sedimentary structures in all these 

strata indicate a shallow-water depositional environment.

Two-hole crinoid columnals in the rocks at locality 6 

restrict their age to the Emsian and (or) Eifelian (late Early 

and (or) early Middle Devonian) (R.B. Blodgett, oral 

commun., 1990). These rocks are more likely of Emsian 

than Eifelian age. Our experience in northern Alaska 

indicates that two-hole crinoid columnals most often occur 

in rocks of Emsian age and are unlikely to occur in rocks 

younger than early Eifelian in age. Conodonts from locality 6 (USGS colln. 12064-SD) merely indicate a Silurian 

through Middle Devonian age. Samples taken for conodonts 

from the other localities noted above yielded only Panderodus sp. (Middle Ordovician to Middle Devonian) or were 

barren.
METACARBONATE SUCCESSION-CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS 23

B

cm

Figure 14. Aligned sticklike organic forms (probable amphiporid stromatoporoids) in Emsian or Eifelian metalimestone (fig. 3, loc. 6). A, Slab. B, View of 

thin section in reflected light.

METACARBONATE ROCKS OF UNCERTAIN AGE

Massive metacarbonate rocks that lack the lithologic 

characteristics of the Mathews River unit crop out throughout the study area; they occur in fault slices in the vicinity, 

from south to north, of Wiehl, Dillon, Snowden, and Table 

Mountains (Ffew, FfeEd, Ffes, and Ffet, fig. 2). These rocks are 

undated or contain fossils indicative of a relatively broad 

age range; some may be correlative with the Mathews River 

unit, but others may be older or younger. Some of these 

rocks possess distinctive sedimentary features, but most 

have retained little primary fabric. Lithologic and age 

information for these rocks is summarized by geographic 

area below.

WIEHL MOUNTAIN AREA

Metacarbonate rocks at Wiehl Mountain are lithologically similar to the Mathews River unit and yield fossils 

permissive, but not diagnostic, of correlation with the 

Mathews River unit. But the Wiehl Mountain area metacarbonate rocks are locally intercalated with noncarbonate 

lithologies not seen in the Mathews River unit and so are 

discussed separately here.

Metacarbonate rocks make up the main massif of 

Wiehl Mountain and continue east at least 10 km. They are 

in thrust contact with calcschist to the east, other metacarbonate rocks to the northwest, and the Snowden Creek unit 

and other metasedimentary rocks to the south. Marble is the 

major lithology; it is white to dark gray, white to orange to 

gray weathering, fine to medium crystalline and forms 

prominent cliffs. Local color lamination on a millimeter to 

centimeter scale reflects variation in purity and crystal size; 

darker laminae contain more mica and carbonaceous material and are finer grained. Layers of pink- to orangeweathering, light- to dark-gray dolostone a few centimeters 

to several meters thick form recessive zones or saddles.

Relict sedimentary textures are preserved in some 

metalimestone and dolostone layers. Lime packstone and 

wackestone containing peloids and (or) bioclasts occur at 

several localities on the west and north sides of Wiehl 

Mountain; identifiable skeletal debris in these rocks consists 

of echinoderm and brachiopod fragments. Color mottling, 

probably produced by bioturbation, characterizes some 

dolostones. Other dolomitic layers preserve cryptalgal lamination and fenestral fabric. These sedimentary structures 

are like those found in the Mathews River unit and indicate 

a shallow subtidal to peritidal depositional environment.

Several types of metaigneous rocks are intercalated 

with the massive metacarbonate rocks at Wiehl Mountain. 

Brosge and Reiser (1964) noted four elongate bodies of 

albite-epidote-chlorite-muscovite schist (Dgs) on the east 

side of the mountain. Dillon and others (1987b) reported 

bimodal volcanic rocks in this area; their "Llama Creek" 

sequence includes felsic flows, tuffs, and breccias, as well 

as mafic greenschists. These authors obtained a U-Pb 

isotopic age of 396 Ma (middle Early Devonian, based on 

Bally and Palmer, 1989; Harland and others, 1990) from a 

felsic flow on Wiehl Mountain and suggested that mafic and 

felsic rocks in this region are coeval.

We observed both felsic and mafic metavolcanic rocks 

intercalated with the metacarbonate rocks at Wiehl Mountain (fig. 15A). Rubble of gray metarhyolite, containing 

phenocrysts of potassium feldspar and plagioclase, was 

found at one locality. Elsewhere, reddish-brown to green 

altered tuff forms centimeter- to meter-thick layers in 

marble; these layers consist mostly of fine-grained, felty 

masses of mica, clay, and carbonate that retain relict shard 

textures. Larger bodies (to 40 m thick) of green to greenishgray, andesitic to basaltic metavolcanic rocks are locally 

schistose but elsewhere preserve diabasic textures and 

chilled margins.
24 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

Figure 15. Sedimentary features of metacarbonate rocks of uncertain age. A, Marble, M, intercalated with, B, basaltic metavolcanic 

rocks, northeast side of Wiehl Mountain. B and C, Slabs showing coated grains and intraclasts with cryptalgal laminae, I, in dolomitic 

metalimestone, metacarbonate rocks of Dillon Mountain area (fig. 3, loc. 37).

Conodonts indicate an age of Middle Ordovician to 

Middle Devonian for the metacarbonate rocks at Wiehl 

Mountain. Black peloidal dolostone on the southwest side 

of the mountain produced Panderodus sp. and a fragment of 

Ozarkodinal sp. indet., denoting a Late Ordovician to 

Middle Devonian age and a normal-marine, shallow-water 

depositional environment (USGS colln. 11961-SD). A 

sample of dolomitic marble taken 1.5 km to the northeast 

yielded only Panderodus sp., indicating a Middle Ordovician to Middle Devonian age (field no. 89AD14A). Two 

other samples from the Wiehl Mountain metacarbonate 

rocks were barren.

DILLON MOUNTAIN AREA

Metacarbonate rocks that crop out in the vicinity of 

Dillon Mountain have certain lithologic features that distinguish them from all other rocks in the study area. They 

occur in a northeast-trending fault slice that extends at least 

15 km from Sukakpak Mountain east to the Mathews River 

and that is imbricated with metacarbonate rocks to the south

and east and calcschist to the north and west. Stratigraphic 

thickness of this sequence appears to be several hundred 

meters, but the section may have been tectonically thickened. The rocks are strongly deformed; isoclinal folds are 

obvious in outcrop.

Major lithologies are marble, dolomitic marble, and 

dolostone, with rare, meter-thick layers of orange- or 

green-weathering, calcareous or chloritoid-bearing, quartzose metasedimentary rocks. Most marble is light-gray 

weathering, white, medium crystalline, and relatively pure; 

it forms massive cliffs cut by irregular, flaggy foliation 

planes. Some intervals are distinctly color-banded on a 

scale of a few millimeters to several meters; the banding is 

produced by layers of dark-gray micaceous or dolomitic 

marble, or of orange or gray dolostone. Relict primary 

features are rare in marble and consist mainly of local 

parallel to low-angle cross lamination defined by quartzrich laminae a few millimeters to 2 cm thick. However, 

sedimentary textures are preserved in many dolomitic layers; these layers consist of grainstone to wackestone made 

up of a variety of coated grains (fig. 15B, C).
METACARBONATE SUCCESSION-CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS 25

Coated grains occur in layers 0.5 to 5 cm thick and 

consist both of ooids and oncoids (usage of Fliigel, 1982). 

Ooids in these rocks are relatively homogeneous in size 

(average diameter 1.0 mm) and shape (spherical to ellipsoidal) and contain smooth, multiple, concentric laminae. 

Oncoids vary in size (0.5-3 mm), are more irregular in 

form, characterized by uneven laminae, and include some 

composite grains with multiple nuclei. Both sorts of coated 

grain generally occur in grain support, but some layers 

contain 20 percent or fewer grains floating in a finely 

crystalline matrix. Most of the coated grains in these rocks 

consist of polycrystalline dolomite in a calcite matrix; 

locally, both grains and matrix are dolomite. Concentric 

lamination within these dolomite masses is preserved as a 

palimpsest texture that crosscuts individual dolomite crystals. Coated-grain-bearing layers are most abundant in the 

lower third of the Dillon Mountain section (for example, 

fig. 3, loc. 37). A few layers in this interval contain crinkly, 

hummocky lamination of probable cryptalgal (stromatolitic) 

origin; cryptalgal laminite also occurs as elongate intraclasts 

in some coated-grain-rich layers (fig. 15B, C).

Preserved sedimentary features indicate a peritidal 

depositional environment for the Dillon Mountain area 

metacarbonate rocks. Ooids form through inorganic chemical precipitation in warm, wave-agitated saline or hypersaline waters; oncoids are produced by biogenic encrustation, typically by algae or cyanobacteria (Wilson, 1975; 

Fliigel, 1982). Modern ooids and oncoids form in warm, 

shallow shelf areas of moderate to high wave and current 

activity; oolitic sands generally occur in waters less than 10 

m deep (Choquette, 1978). As noted above, algal mats are 

most common in intertidal to supratidal settings (James, 

1984), particularly in Paleozoic or younger rocks.

Other than oncoids and cryptalgal laminae, no organic 

structures have been found to constrain the age of the Dillon 

Mountain area metacarbonate rocks. Several samples were 

taken for conodonts, but all were barren. Similar carbonate 

sequences containing abundant coated grains (ooids and 

oncoids) and stromatolites but lacking other organic 

remains occur in the western Brooks Range (unnamed units 

described by Dumoulin, 1988) and in the northeastern 

Brooks Range (Katakturuk Dolomite; Clough, 1989; 

Clough and others, 1990). These rocks are thought to be of 

Late Proterozoic and (or) earliest Paleozoic age in the 

western Brooks Range (Dumoulin, 1988) and of Proterozoic age in the northeastern Brooks Range (Blodgett and 

others, 1986).

SNOWDEN MOUNTAIN MASSIF

Massive metacarbonate rocks make up the main massif 

of Snowden Mountain and are in probable fault contact with 

Cambrian rocks (Snowden Mountain unit) to the northwest,

Upper Ordovician and Silurian metacarbonate rocks 

(Mathews River unit) to the northeast, Middle Ordovician 

phyllite and metalimestone (Snowden Creek unit) to the 

southwest, and metacarbonate rocks of uncertain age (Dillon Mountain area metacarbonate rocks) to the south (fig. 

2). Little sedimentologic or paleontologic information has 

been recovered from the Snowden Mountain massif metacarbonate rocks, at least in part because the steep and 

rugged terrain has limited helicopter access. No lithologic 

or biostratigraphic data yet obtained permit definitive correlation of these rocks with any of the metacarbonate units 

described above; available fossil collections indicate that 

strata of several ages are present.

Most of the Snowden Mountain massif metacarbonate 

rocks examined are light-gray-weathering, white to lightgray, fine- to medium-crystalline marble exposed in sheer 

cliff faces. Some marble is color laminated and contains 

rare echinoderm debris. Subordinate intervals of pink- to 

beige-weathering, medium- to dark-gray dolostone preserve 

relict sedimentary features, including sheet cracks and 

fenestral fabric.

No megafossil-based ages are reported from massive 

metacarbonate rocks making up the Snowden Mountain 

massif, and samples taken for conodonts have been mostly 

unproductive. Dillon and others (1988) reported three 

collections from the southern, central, and northern parts of 

the massif, as well as two barren samples. The southernmost collection was made along Snowden Creek in massive 

metacarbonate rock 7 m from the contact with black phyllite 

of the Snowden Creek unit; it yielded a meager fauna of 

probable late Early through early Middle Ordovician (middle Arenigian through Llanvirnian) age (USGS colln. 

9905-CO; fig. 3, loc. 24; loc. 13 of Dillon and others, 

1988). A sample of marble about 2 km to the northwest, 

however, produced a single deformed conodont of Silurian 

through Mississippian morphotype (fig. 3, loc. 23; loc. 16 

of Dillon and others, 1988). The northernmost collection 

came from massive dolostone 5 m above the contact with 

Cambrian rocks and consists of a single fragment of 

Ordovician through Triassic age (fig. 3, loc. 21; loc. 20 of 

Dillon and others, 1988). Three samples taken from the 

massif during the present study were barren.

TABLE MOUNTAIN AREA

Massive metacarbonate rocks in the Table Mountain 

area have some lithologic similarities to the Mathews River 

unit but include little or no dolostone and have produced no 

fossils diagnostic of a Silurian or older age. These rocks 

crop out from the Hammond River northeast to Table 

Mountain (fig. 3) and make up several thrust sheets imbricated with Devonian siliciclastic rocks (fig. 2).

Table Mountain area metacarbonate rocks are mostly 

cliff-forming, light- to medium-gray marble and lesser
26 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

medium- to dark-gray metalimestone; relict sedimentary 

textures are rare and consist of skeletal-peloidal wackestone 

and packstone. Recognizable bioclasts are chiefly echinoderm columnals but include brachiopods, ostracodes, and 

stromatoporoid and bryozoan fragments. Peloids range 

from 30 to 200 jxm in diameter and are rounded to ellipsoidal. 

Few fossils have been found to constrain the age of 

these rocks. Megafossils are poorly preserved; collections 

from two localities east and southeast of Table Mountain are 

of Silurian or Devonian age (Brosge and Reiser, 1964, their 

Iocs. 9 and 10). Conodonts are scarce and nondiagnostic. 

Of seven samples taken during our study, two produced 

fragments indicative of an Ordovician through Permian or 

Triassic age (field localities 89TM290A and 90TM468C, 

respectively), and five were barren. Dillon and others (1988) 

reported three barren samples from these rocks (their Iocs. 

45, 49, and 62).

SUMMARY OF THE METACARBONATE

SUCCESSION IN THE SNOWDEN MOUNTAIN

AREA

The massive metacarbonate rocks and subordinate 

associated lithologies described above represent a preCarboniferous stratigraphic succession that has been metamorphosed, deformed, and dismembered by thrust faults. 

This succession encompasses distinct lithologic units of 

relatively well-constrained ages, as well as other units of 

uncertain age. Figures 6 and 8 summarize stratigraphic 

relationships of well-dated units in the Snowden Mountain 

area; figure 16 attempts to integrate units of uncertain age 

within this framework and presents several alternative 

stratigraphic hypotheses.

Well-dated units in the Snowden Mountain area metacarbonate succession are the Snowden Mountain unit 

(Middle Cambrian), the Snowden Creek unit (Middle 

Ordovician), the Mathews River unit (Upper Ordovician 

through Silurian), and unnamed Lower and (or) Middle 

Devonian metalimestone. Most contacts between these 

units are faults, but some contacts between the Snowden 

Creek and Mathews River units may be depositional, and 

the contact between the Mathews River unit and overlying 

Lower and (or) Middle Devonian metalimestone may be an 

unconformity.

Units of uncertain age but that, by virtue of spatial 

association and lithologic correlation, also appear to be part 

of the Snowden Mountain area metacarbonate succession 

are the metacarbonate rocks of the Wiehl Mountain area, 

the Dillon Mountain area, the Snowden Mountain massif, 

and the Table Mountain area. All of these units (with the 

possible exception of Snowden Mountain massif strata) are 

fault bounded. All consist of massive metacarbonate 

rocks generally similar to, but not as dolomitic as, those 

that make up the Mathews River unit, and all contain

some organic remains. Stratigraphic integration of these 

units within the Snowden Mountain area metacarbonate 

succession is attempted in figure 16 and is constrained by 

fossils, lithologic considerations, and local and regional 

relationships.

The Dillon Mountain area metacarbonate rocks have 

produced no organic remains other than algal structures 

(stromatolites, oncolites). Such forms are known from 

strata of Archean to Holocene age but are most common in 

Proterozoic rocks (Krumbein, 1983). Both algal lamination 

and coated grains are found in Paleozoic strata in the 

Snowden Mountain area; stromatolites occur in tidal-flat 

facies of the Upper Ordovician through Silurian Mathews 

River unit, and coated grains (ooids) make up one bed in the 

Upper Devonian Hunt Fork Shale (discussed below). However, these occurrences also include other fossil debris; 

algal laminites in the Mathews River unit contain local 

ostracodes and are interlayered with coral-bearing metalimestone, and ooid grainstone in the Hunt Fork Shale 

contains brachiopod and echinoderm fragments in addition 

to coated grains. Strata elsewhere in northern Alaska that 

contain only algal forms and no other fossils, such as the 

Katakturuk Dolomite in the northeastern Brooks Range and 

unnamed rocks in the western Brooks Range, are thought, 

on the basis of regional relationships and isotopic data, to be 

of Proterozoic age in the northeastern Brooks Range 

(Blodgett and others, 1986; Clough and others, 1990) and 

Late Proterozoic age and (or) Early Cambrian age in the 

western Brooks Range (Dumoulin, 1988; Till, in press).

We propose that the Dillon Mountain area metacarbonate rocks correlate with other stromatolitic, coated-grainbearing units in the Brooks Range and infer an age of 

Proterozoic and (or) Early Cambrian for them. If this 

inference is correct, the Dillon Mountain area metacarbonate rocks predate the Middle Cambrian Snowden Mountain 

unit and constitute the basal part of the Snowden Mountain 

area metacarbonate succession. The Snowden Mountain 

unit has so far been recognized in only one area, more than 

10 km north of the northernmost outcrops of the Dillon 

Mountain area metacarbonates, so the original stratigraphic 

relationship between these two units cannot directly be 

assessed. Did strata of the Snowden Mountain unit originally accumulate on the Dillon Mountain area metacarbonate rocks? Or did spatially associated rocks of unknown 

age, such as the calcschist exposed north and west of Dillon 

Mountain (included in unit ftmu, fig. 2), originally overlie 

the metacarbonate rocks and underlie the Snowden Mountain unit? Further mapping and structural analyses are 

needed to resolve this problem.

Metacarbonate rocks of the Snowden Mountain massif 

retain little relict texture and have yielded few fossils. 

Meager conodont collections of (from north to south) 

Ordovician through Triassic, Silurian through Mississippian, and late Early through early Middle Ordovician age 

are known (fig. 3, Iocs. 21, 23, and north end of 24). The
METACARBONATE SUCCESSION-CAMBRIAN TO MIDDLE DEVONIAN METASEDIMENTARY ROCKS 27

SNOWDEN MOUNTAIN

METERS

250 —

200 —

150 —

50 — 

0

SYSTEM/ 

SERIES

DEVONIAN

SILURIAN

ORDOVICIAN

UPPER

MIDDLE

CAMBRIAN

SERIES/SUBSERIES/ 

STAGE/SUBSTAGE

EIFELIAN AND (OR) EMSIAN

LUDLOVIAN AND WENLOCKIAN

WENLOCKIAN AND 

LLANDOVERIAN

RICHMONDIAN 

(INCLUDES MINOR 

LOWER SILURIAN(?) 

STRATA)

MAYSVILLIAN 

AND EDENIAN

LOWER 

CARADOCIAN(?) 

AND LLANDEILIAN

LLANVIRNIAN

UPPER 

ARENIGIAN

LOWER MIDDLE

LOWER CAMBRIAN 

AND (OR) 

PROTEROZOIC

STRATIGRAPHIC 

UNIT

1- 

•z.

MATHEWS IRIVER 

Iz

D

WDEN] 

SNOWDEN CREE* NTAIN 

NrTJ

i8"

W5

METACARBONATE 

SUCCESSION

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WIEHL

MOUNTAIN

AREA

TABLE

MOUNTAIN

AREA

Figure 16. Hypothetical stratigraphic position of units of uncertain age within the metacarbonate succession in the Snowden Mountain 

area. Parts of columns with symbols indicate preferred correlation. Thicknesses are minima, particularly for Upper Ordovician and 

Silurian rocks. European series names are used for intervals containing dominantly cosmopolitan faunas, whereas North American series 

and (or) stage names are used for intervals containing chiefly North American province faunas. See figure 5 for explanation of symbols.

Snowden Mountain massif metacarbonate rocks structurally 

overlie Middle Cambrian rocks of the Snowden Mountain 

unit to the north and structurally underlie Middle Ordovician rocks of the Snowden Creek unit to the south, but the 

exact nature of these contacts is disputed. Dillon and others 

(1988) included the Snowden Mountain massif metacarbonate rocks in their Skajit Formation, which they considered 

to be of Devonian age, and interpreted the northern contact 

as an unconformity and the southern contact as a fault. 

However, T.E. Moore (written commun., 1991) suggested 

that the southern contact may be depositional, and we 

observed some evidence (discussed above) that the northern 

contact is a fault.

The stratigraphic position of the metacarbonate rocks 

of the Snowden Mountain massif is thus uncertain, and it is 

possible that rocks of several ages are included within this 

unit. Two hypotheses are most compatible with the available data. In the first, the Snowden Mountain massif 

metacarbonate rocks depositionally overlie Middle Cambrian rocks, depositionally underlie Middle Ordovician 

strata, young from north to south, and are of Early to 

earliest Middle Ordovician age. In this interpretation, the 

lower age of the unit is limited by the Ordovician-Triassic 

conodont fragment obtained just above the contact with 

Middle Cambrian rocks, the upper age is constrained by the 

late Early through early Middle Ordovician age of the fauna
28 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

collected just below the contact with Middle Ordovician 

rocks, and the Silurian-Mississippian collection must be 

attributed to a fault sliver of younger rocks.

A second possibility is that the Snowden Mountain 

massif metacarbonate rocks are a facies equivalent of the 

Mathews River unit and are of Late Ordovician through 

Silurian age; they could also be Devonian in age, as 

suggested by Dillon and others (1988). In this interpretation, the northern contact could be a fault or an unconformity, but the southern contact must be a fault. This hypothesis 

is compatible with two of the three fossil collections from 

these strata; conodonts of late Early through early Middle 

Ordovician age from locality 24 (fig. 3) must be reworked 

or produced by a sliver of the Snowden Creek unit.

The metacarbonate rocks in the Wiehl Mountain and 

Table Mountain areas have some lithologic similarities to 

the Mathews River unit and contain rare fossils of Ordovician to Devonian and Silurian to Devonian age. These data 

are compatible with, but not diagnostic of, correlation with 

the Mathews River unit and assignment of a Late Ordovician and Silurian age. Available evidence also permits the 

interpretation that these rocks are, at least in part, of 

Devonian age, and they could be entirely younger than the 

Mathews River unit. Metaigneous rocks like those intercalated with the Wiehl Mountain area metacarbonate rocks 

have not been noted in the Mathews River unit or in any of 

the other metacarbonate units discussed above. The Table 

Mountain area metacarbonate rocks occur north of all other 

metacarbonate units and are intimately associated with 

Devonian siliciclastic rocks, particularly those of the Nutirwik Creek unit.

Thus, the Snowden Mountain area metacarbonate succession includes rocks of Middle Cambrian, Middle Ordovician, Late Ordovician, Silurian, and Early and (or) 

Middle Devonian ages; less well dated parts of the succession may be Proterozoic and (or) Early Cambrian and Early 

Ordovician in age. Shallow-water shelf or platform deposition occurred during Proterozoic and (or) Early Cambrian, 

Early Ordovician(?), Late Ordovician and Silurian, and 

Early and (or) Middle Devonian time; outer-shelf to basinal 

environments prevailed during Middle Cambrian and Middle Ordovician time. Faunal and lithologic evidence indicate that the deepest water (most basinal) settings existed 

during the early Middle Ordovician, whereas the shallowest, most restricted depositional regimes existed in the latest 

Ordovician. Fossils with specific biogeographic affinities 

found in this succession include Middle Cambrian trilobites 

and early Late Ordovician conodonts with Siberian affinities, an early Late Ordovician cateniporid coral with Canadian Arctic affinities, and latest Ordovician conodonts with 

western North American affinities. Fossils of Middle Ordovician, Silurian, and Devonian ages (mainly conodonts) are 

chiefly cosmopolitan.

DEVONIAN METACLASTIC ROCKS

The pre-Carboniferous Paleozoic metacarbonate rocks 

described above are structurally juxtaposed with Devonian 

metaclastic rocks throughout the Snowden Mountain area 

(fig. 11 A). These metaclastic rocks consist of the Hunt Fork 

Shale (Chapman and others, 1964), the Beaucoup Formation (Dutro and others, 1979; Dillon and others, 1988), and 

the Nutirwik Creek unit (Aleinikoff and others, 1993).

All three units consist chiefly of fine-grained siliciclastic rocks with subordinate layers and lenses of metalimestone and include distinctive subordinate lithologies. The 

units are further distinguished by characteristic carbonate 

lithofacies and conodont biofacies. Conodonts obtained 

from limy layers demonstrate that the three metaclastic units 

are at least in part correlative and in part of Frasnian age.

Our study concentrated on metalimestone intervals 

within the metaclastic units and was less detailed than our 

work on the metacarbonate sequence. The descriptions 

below are based on reconnaissance outcrop observations 

and petrologic and paleontologic analyses of spot samples; 

sections were not measured in the metaclastic units.

BEAUCOUP FORMATION

In the Snowden Mountain area, the Beaucoup Formation consists chiefly of gray slate and phyllite with lesser 

amounts of quartz-muscovite schist and local mafic intrusions. Limy layers are relatively abundant and constitute 5 

to 30 percent of most sections; they are more abundant in 

outcrops in the southern part of the study area. Some 

metalimestone bodies are tens of meters thick and as much 

as 6 km long (Dillon and others, 1988), but most are smaller 

(2-30 m thick, a few tens of meters long) and distinctly lens 

shaped.

CARBONATE LITHOLOGIES

The most southerly exposures of the Beaucoup Formation in the study area, in the vicinity of the sharp bend in the 

Hammond River (figs. 2 and 3), are strongly recrystallized 

and retain little relict texture. Carbonate layers in these 

rocks consist of fine- to medium-crystalline marble with 

minor amounts of detrital quartz and rare brachiopod 

fragments.

Elsewhere in the Snowden Mountain area, original 

textures of the Beaucoup Formation are better preserved, 

and carbonate bodies consist of two main types. The first is 

mostly gray-weathering, dark-gray to black, fine-grained 

lime mudstone, wackestone, and lesser packstone, in 

mound-shaped lenses 3 to 30 m thick (fig. 175). These 

rocks are generally massive but contain irregularly spaced 

gray to orange phyllitic partings and local mottled zones 

produced by bioturbation. Most bodies are texturally and 

compositionally heterogeneous and tend to be darker, finer
DEVONIAN METACLASTIC ROCKS 29

Figure 17. Features of the Beaucoup Formation. A, Beaucoup Formation, including phyllite, gabbro, and Frasnian metalimestone, 

overlies massive metacarbonate rocks (photograph by T.E. Moore). B, Thick, mound-shaped layer of fossiliferous metalimestone (fig. 

3, loc. 21); a sample from this locality (USGS colln. 11966-SD) produced conodonts of latest Givetian or early Frasnian age (pi. 3, figs. 

3, 4). C, Deformed corals in metalimestone (field loc. 89ATi39).

grained, richer in clay, and less recrystallized toward the 

periphery.

The predominant lithology is slightly recrystallized 

lime mudstone. Most samples contain 2 to 10 percent 

bioclasts and peloids disseminated in a matrix of calcite 

crystals 8 to 25 |xm in diameter. Bioclasts, locally bored 

and micritized, include echinoderm columnals and spines, 

ostracodes, and bryozoans. Peloids are ellipsoidal, 40 to 

160 (Jim in diameter, and organic rich. Some samples are 

couplets, a few centimeters thick, of relatively pure lime

mudstone grading up into finer grained, phyllitic lime 

mudstone.

Bioclastic-peloidal wackestone and packstone form 

zones a few centimeters to several meters thick within these 

muddy mounds. Bioclasts here are chiefly corals, stromatoporoids, and fewer brachiopods and echinoderms. Fossils 

are not uniformly distributed; some patches are rich in 

corals or stromatoporoids, for example, whereas others 

contain mostly brachiopods. Corals are tabulate forms and 

solitary and colonial rugosans (fig. Y1C). Coral genera
30 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

include Alveolitesl sp., Cladoporal sp., Macgeea sp., 

Spongophylluntf sp., and Thamnopora sp.; stromatoporoids 

include Amphipora sp. and massive forms (Dillon and 

others, 1988). Articulated brachiopod shells and articulated 

crinoid columnals occur locally and indicate relatively quiet 

water conditions.

The second type of carbonate body consists of buff- to 

orange-weathering, gray lime packstone to grainstone in 

relatively elongate lenses 2 to 10 m thick. Some intervals 

appear thoroughly bioturbated, and discrete Chondritestype burrows occur locally. Clasts, mostly skeletal material 

and peloids, range from very fine to coarse sand sized; 

individual samples are fairly well sorted. Bioclasts are 

generally broken and abraded, and many are micritized. 

Identifiable biotic grains consist of crinoid columnals and 

fewer brachiopod fragments, foraminifers, and algal oncolites. Peloids in these bodies are more irregular in size and 

shape than those in the muddy mounds; they range from 50 

to 700 jxm in diameter, and at least some may be micritized 

skeletal grains. Most samples also contain 1 to 25 percent 

disseminated, angular, quartz silt and sand.

Euhedral rhombs of dolomite, 30 to 50 (JLITI in diameter, occur locally in all of the calcareous lithologies 

described above. They are most common in mudstones and 

grain-poor wackestones and constitute 5 to 30 percent of 

some samples. Silicification of skeletal material is rare but 

was noted at a few outcrops.

AGE AND BIOFACIES

Conodonts indicate that the Beaucoup Formation in the 

study area is of latest Givetian and Frasnian (latest Middle 

and early Late Devonian) age. Megafossils in these rocks 

are chiefly poorly preserved corals, stromatoporoids, brachiopods, and mollusks of Middle through early Late Devonian age (Brosge and Reiser, 1964, Iocs. 2 and 3; Brosge 

and others, 1979, Iocs. 136 and 143; Dillon and others, 

1988, Iocs. 2, 3, 11, 28, 51, 60, 61). Conodonts have been 

obtained from 19 collections at 17 localities (fig. 3, Iocs. 3, 

15, 26, 27, 33). Some conodont collections merely indicate 

a broad Middle to Late Devonian age, but others are more 

diagnostic. Four collections are latest Givetian to early 

Frasnian in age (fig. 3, Iocs. 15, 26, 27, 33), one represents 

an interval within the early Frasnian (upper part of transitans Zone into lower part of Upper hassi Zone; USGS colln. 

12078-SD, about 30 km east of the study area, listed in 

app. 1 and not shown on fig. 3), and another is long-ranging 

within the Frasnian (Upper hassi Zone to linguiformis Zone; 

fig. 3, loc. 3). Map distribution of these diagnostic samples 

suggests that younger parts of the Beaucoup Formation are 

exposed to the north. Conodonts from the Beaucoup Formation mostly represent a polygnathid-icriodid biofacies,

which typifies normal-marine, relatively shallow-water, 

and locally high-energy depositional environments.

DISTRIBUTION

The Beaucoup Formation occurs in large, thrust sheets 

throughout the Snowden Mountain area (fig. 2). It is 

tectonically interleaved with the Hunt Fork Shale and 

Nutirwik Creek unit in the north and with the Mathews 

River unit, the Snowden Creek unit, and metasedimentary 

rocks of uncertain age to the south.

DEPOSITIONAL ENVIRONMENT

Siliciclastic strata predominate throughout the Beaucoup Formation, indicating that conditions suitable for 

carbonate accumulation were only locally or occasionally 

achieved. Lithologic and faunal evidence demonstrates that 

limy layers in this unit accumulated in situ as shallow-water 

bioherms and bioclastic shoals. Turbidity strongly inhibits 

primary carbonate production (for example, Wilson, 1975); 

thus, limy strata formed only in times and (or) places of 

reduced siliciclastic influx.

The two types of carbonate bodies described above 

formed in slightly different settings. We interpret 

mudstone-wackestone "mounds," locally rich in corals, 

stromatoporoids, and other fossils, as biohermal buildups; 

the abundance of mud and the presence of articulated 

brachiopods and crinoid columnals suggest deposition in 

quiet water below fair-weather wave base. The fauna is 

predominantly stenohaline and denotes open circulation and 

normal-marine salinity. Peloids in these rocks resemble 

modern fecal pellets. Thinner layers of packstone and 

grainstone accumulated in shallower and (or) higher energy 

settings, as demonstrated by abraded, size-sorted grains and 

rarity of mud. Some layers were probably deposited in 

carbonate sand shoals; others may be grain-rich lags derived 

from bioherms through current winnowing. Peloids in these 

rocks are most likely micritized grains; micritization is most 

common in the shallow photic zone where it is mediated by 

algae (Bathurst, 1976).

Different lithologies in the Beaucoup Formation produce distinct conodont assemblages. Samples from muddy 

biohermal buildups produce few conodonts; those that occur 

represent chiefly the polygnathid-icriodid biofacies, with or 

without pandorinellinids (pi. 3, figs. 1—4). More diverse 

and abundant faunas are found in bioclastic packstonegrainstone layers. Polygnathids dominate these collections 

but occur with ancyrodellids, Mesotaxis, and rare icriodontids and Klapperina (pi. 3, figs. 5-11). It appears that 

Pandorinellina, Icriodus, and polygnathids lived over the 

biohermal buildups, whereas Ancyrodella, Mesotaxis, and
DEVONIAN METACLASTIC ROCKS 31

many polygnathids lived over and peripheral to grain-rich 

bioclastic shoals and aprons surrounding the buildups.

HUNT FORK SHALE

The Hunt Fork Shale consists of as much as 1,000 m of 

dark-gray to black shale, slate, and phyllite with lesser 

amounts of fine- to medium-grained sandstone, siltstone, 

calcareous sandstone, and ferruginous fossiliferous limestone (Brosge and others, 1979; Nilsen, 1981; Moore and 

Nilsen, 1984) (fig. ISA). The unit was defined by Chapman 

and others (1964) from exposures in the Killik River 

quadrangle, 150 km west of Snowden Mountain but has 

since been recognized throughout the Brooks Range. It is 

the basal formation of the Endicott Group (Tailleur and 

others, 1967) and grades upward into marginal marine and 

nonmarine clastic rocks (Noatak Sandstone and Kanayut 

Conglomerate). In the Philip Smith Mountains quadrangle, 

the Hunt Fork Shale is at least 700 m thick and comprises 

quartzite, calcareous sandstone, limestone, shale, and 

wacke members (Brosge and others, 1979). The limestone 

member consists of lenticular beds, 1 to 15 m thick, that 

occur mostly in a persistent zone as much as 50 m thick 

about 200 to 300 m below the top of the formation. In 

addition, minor thin limy beds occur in other members. 

Most calcareous layers examined for this study are part of 

the shale member of Brosge and others (1979).

CARBONATE LITHOLOGIES

In the Snowden Mountain area, the Hunt Fork Shale 

contains fewer, thinner limy layers than does the Beaucoup 

Formation. Calcareous beds make up 5 percent or less of 

most sections and are generally 5 cm to 1.5 m thick; rare 

carbonate intervals as much as 100 m thick occur at some 

outcrops. Many limy layers in the Hunt Fork Shale contain 

a mixture of calcareous and siliciclastic grains. In the 

descriptions below, calcarenite describes rocks that include 

more than 10 percent noncarbonate detrital clasts.

Thinner calcareous beds in the Hunt Fork Shale are 

graded (fig. 185), locally bioturbated, and consist mostly of 

orange-weathering, medium-gray to black bioclastic grainstone, lesser packstone, and calcarenite composed of fossil 

debris (5-70 percent), carbonate lithic clasts (1-30 percent), and siliciclastic detritus (1-80 percent) (fig. 18C, 

D). Most samples are poorly sorted and contain coarse 

bioclasts (a few millimeters to several centimeters in diameter) in a matrix of fine to very fine carbonate and 

siliciclastic sand cemented by sparry calcite or (rarely) 

silica. Muddy rip-up clasts are locally abundant, and many 

bioclasts are filled and (or) coated with mud. Fossils include 

corals (solitary and colonial rugosans), brachiopods, echinoderms, bryozoans, gastropods, ostracodes, foraminifers, 

ichthyoliths, and hydraulically sorted, reworked conodonts

of mixed biofacies (pi. 3, figs. 12-15). Carbonate clasts are 

primarily lime mudstone and peloidal packstone; most are 

rounded to irregular and of probable intrabasinal origin, but 

some are angular, contain calcite veins truncated at the clast 

margin, and are most likely extrabasinal. Noncarbonate 

grains are angular to subangular and consist chiefly of 

monocrystalline quartz, less abundant chert, and minor 

amounts of plagioclase feldspar, phyllite, white mica, 

chlorite, phosphate, and opaques.

Dark-gray, medium-bedded oolitic grains tone (fig. 

18£) forms a 30-cm-thick bed in the Hunt Fork Shale 2 km 

south of Atigun Pass (fig. 3, loc. 1). Ooids average 0.5 mm 

in diameter, display both concentric and radial lamination, 

and occur in a matrix of sparry calcite cement; many have 

quartz silt nuclei. Other grains make up less than 5 percent 

of this bed and consist of brachiopod and echinoderm 

debris, quartz sand, and intraclasts of oolitic packstone.

Rare, thicker (2-100 m) carbonate intervals in the 

Hunt Fork Shale consist of nodular bedded to massive, 

bioclastic-peloidal lime wackestone and mudstone. These 

rocks contain corals, stromatoporoids, bryozoans, and gastropods dispersed in a matrix of peloids and slightly 

dolomitic lime mud.

AGE AND BIOFACIES

Megafossils indicate a Frasnian and Famennian (early 

and late Late Devonian) age for the Hunt Fork Shale in the 

Philip Smith Mountains quadrangle (Brosge and others, 

1979). Frasnian corals, brachiopods, pelecypods, mollusks, 

and plants have been obtained from the limestone, calcareous sandstone, and shale members of the formation, but 

the uppermost wacke member yields Famennian brachiopods and pelecypods.

In the Snowden Mountain area, conodont assemblages 

from limy layers in the Hunt Fork Shale are dominated by 

polygnathids and suggest a middle to late Frasnian age (pi. 

3, figs. 12-15). Nine samples from eight localities yielded 

conodonts (fig. 3, Iocs. 1,2). Collections are more abundant than those from the Beaucoup Formation, and polygnathids are more diverse; they are chiefly Polygnathus 

pacificus (pi. 3, fig. 13), P. aequalis, P. evidens (pi. 3, fig. 

12), and P. samueli (pi. 3, fig. 14). Some of these species 

have previously been reported only from late Frasnian rocks 

of the Hay River area, southwestern Northwest Territories, 

Canada (Klapper and Lane, 1985), and southeast Alaska 

(Savage, 1992). Most collections represent a post-mortem 

biofacies mixture derived from normal-marine, relatively 

shallow to moderately deep shelf environments.

DISTRIBUTION

The Hunt Fork Shale crops out only in the northernmost part of the study area in the vicinity of Atigun Pass
32 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

1cm

Figure 18. Sedimentary features of the Hunt Fork Shale. A, 

Typical outcrop of phyllite containing subordinate layers of 

metasiltstone and metasandstone (hammer, 40 cm long). B, View 

of thin section (reflected light) showing graded metalimestone bed; 

coarse grains at base of sample are coral fragments (field loc. 

90TM527). C, Photomicrograph of thin calcareous bed containing 

abundant detrital quartz grains, as well as, M, mudstone clasts and 

skeletal debris (fig. 3, loc. 1). D, Photomicrograph of thin 

calcareous bed made chiefly of carbonate bioclasts such as, C, 

crinoid ossicles (field loc. 90TM570). E, Photomicrograph of 

oolitic grainstone (fig. 3, loc. 1).

(fig. 2). It forms several large fault sheets thrust above the 

Beaucoup Formation to the south and the Kanayut Conglomerate to the north.

DEPOSIHONAL ENVIRONMENT

The Hunt Fork Shale, like the Beaucoup Formation, 

accumulated in a marine regime dominated by siliciclastic influx. But Hunt Fork limy layers are thinner and 

rarer than those in the Beaucoup Formation and indicate a 

depositional environment even more inimical to in situ 

carbonate production.

Previous workers observed that the Hunt Fork Shale 

grades upward from deep- to shallow-marine facies (for

example, Nilsen, 1981; Moore and Nilsen, 1984). Most of 

the unit accumulated in a low-energy, relatively deep 

(below fair-weather wave base) depositional environment 

such as a prodelta slope, but the uppermost wacke member 

records delta progradation across a shallower (outer shelf?) 

setting (Moore and Nilsen, 1984). The lower part of the 

Hunt Fork Shale contains thin, graded beds of siliciclastic 

sandstone and siltstone that increase in abundance upward. 

Some of these beds contain partial Bouma sequences and 

are probably turbidites, whereas others may have formed 

through vertical settling from storm-generated overflows 

(Nilsen, 1981). Sandstone bodies in the upper part of the 

Hunt Fork represent shoals, channel-mouth bars, and submerged linear ridges (Nilsen, 1981).
DEVONIAN METACLASTIC ROCKS 33

Figure 18.—Continued.

Sedimentologic and faunal evidence indicate that 

most limestone layers of the Hunt Fork Shale in the study 

area were not generated in place but were redeposited by 

storm waves and (or) turbidity currents. A tempestite origin 

may best explain many of the thin calcareous beds. They do 

not display sedimentary structures characteristic of Bouma 

sequences, such as parallel and cross lamination, but do 

possess typical storm deposit features (Fliigel, 1982), 

including grading, shale rip-up clasts, a variety of carbonate 

lithoclasts, and a mix of fossils derived from various 

shallow-water biofacies. The condition and composition of 

conodont collections from these layers support the interpretation of redeposition. Assemblages consist chiefly of 

robust elements, and most specimens are broken and 

abraded. Platform elements are usually incomplete, and 

very few ramiform elements are recognizable to morphotype, indicating post-mortem, relatively high energy 

hydraulic transport.

A few carbonate layers in the Hunt Fork Shale appear 

to have formed in place in relatively shallow water. Oolitic 

grainstone most likely accumulated in local high-energy 

shoals, and rare thick layers of bioclastic-peloidal wackestone and mudstone probably represent muddy biohermal 

buildups like those in the Beaucoup Formation.

The scarcity of limestone in the Hunt Fork Shale 

relative to the Beaucoup Formation could reflect several 

factors. The Hunt Fork may have been deposited in deeper 

and (or) colder water; in situ carbonate production is highest 

in warm, shallow seas (Wilson, 1975; Fliigel, 1982). 

Alternately, or additionally, conditions during Hunt Fork

deposition may have been too turbid to allow establishment 

of much neritic carbonate production; siliciclastic input may 

have been greater and more continuous than during accumulation of the Beaucoup Formation.

NUTIRWIK CREEK UNIT

The Nutirwik Creek unit consists of purple and green 

phyllite with subordinate volcaniclastic metasandstone, 

pebble conglomerate, and felsic igneous rocks (Aleinikoff 

and others, 1993). Some purple and green phyllite sections 

contain minor calcareous black phyllite and metalimestone, 

which we here provisionally include within the Nutirwik 

Creek unit. These limy layers appear to be in the upper part 

of the unit (T.E. Moore, written commun., 1991). Rocks 

presently assigned to the Nutirwik Creek unit were considered part of the Beaucoup Formation and (or) the rocks of 

Whiteface Mountain (informal unit) by previous authors 

(Brosge and others, 1979; Dillon and others, 1988). Most 

exposures of the Nutirwik Creek unit occur in the Table 

Mountain area, where the rocks are intercalated with 

massive metacarbonate rocks (ftt) (fig. 2).

CARBONATE LITHOLOGIES

Limestone is even less common in the Nutirwik Creek 

unit than in the other Devonian siliciclastic units discussed 

above; it occurs intercalated with recessive intervals of
34 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

slightly calcareous black phyllite and makes up at most a 

few percent of the overall section. Most calcareous beds are 

3 to 20 cm thick and consist of finely laminated, carbonaceous, dark-gray to black, medium- to dark-grayweathering lime mudstone to wackestone. Bioclasts are 

chiefly pelagic fossils such as tentaculitids and calcitized 

radiolarians, as well as, less commonly, echinoderm debris. 

Other silt- and sand-sized grains in these predominantly 

fine-grained rocks include peloids and minor amounts of 

detrital quartz.

A few limy beds consist of redeposited, grain-rich 

packstone like that found in the Hunt Fork Shale. These 

beds are thicker (as much as 1 m), weather orange, and 

contain a mix of bioclasts, calcareous lithic clasts, and 

minor amounts of lime mud. Fossils include coral, bryozoan, and echinoderm fragments; carbonate clasts consist of 

lime mudstone or skeletal wackestone.

AGE AND BIOFACIES

Limestone layers included in the Nutirwik Creek unit 

yield megafossils and conodonts of Middle to early Late 

Devonian age. Megafossils (Dillon and others, 1988, Iocs. 

47, 49, and 50) include solitary and colonial rugose corals, 

tabulate corals (Cladoporcf! sp.), and stromatoporoids 

(Amphipora'l sp.) of early Late Devonian and Middle to 

early Late Devonian age and brachiopods (Mucrospirifer 

sp.) of Middle(?) Devonian age.

Four samples from the Nutirwik Creek unit yielded 

conodonts; the most diagnostic collections are of middle 

Frasnian (early Late Devonian) age (fig. 3, Iocs. 4, 5, 12; 

pi. 3, figs. 16-21). Skeletal packstone (fig. 3, loc. 5) 

produced abundant and diverse conodonts indicative of the 

upper part of the Lower hassi Zone. Bioclastic limestone 

(fig. 3, loc. 4) yielded conodonts of the Upper hassi Zone 

to Lower rhenana Zone. These assemblages, unlike those 

obtained from the Beaucoup Formation and the Hunt Fork 

Shale, are not dominated by polygnathids. Instead they 

commonly contain ancyrodellids and palmatolepids, as well 

as lesser numbers of Ancyrognathus spp., icriodids, and 

polygnathids. Most of these conodonts represent the 

polygnathid-ancyrodellid-palmatolepid biofacies and are 

characteristic of an outer shelf or slope environment.

Two metafelsite samples from the Nutirwik Creek unit 

produced Early Devonian zircon ages (393 ± 2 and about 

385-390 Ma) (Aleinikoff and others, 1993). These ages, 

together with the paleontologic data, suggest that the 

Nutirwik Creek unit was formed by a long-lived depositional regime that existed from at least Early to Late 

Devonian time. Alternatively, the unit could represent a 

heterogeneous assemblage of fault slices of various Devonian ages. Thus far, limestone layers we include in the 

Nutirwik Creek unit have yielded chiefly Late Devonian 

(Frasnian) fossils.

DISTRIBUTION

The Nutirwik Creek unit crops out in the north-central 

part of the study area (fig. 2), where it forms several large 

thrust sheets intercalated with massive metacarbonate 

rocks. Metacarbonate rocks and the Nutirwik Creek unit are 

also intercalated on a smaller scale. At locality 12 (fig. 3), 

a 20-m-thick, fault-bounded layer of the Nutirwik Creek 

unit is structurally overlain and underlain by Upper Ordovician massive metacarbonate rocks of the Mathews River 

unit. The Nutirwik Creek unit at this locality consists of 

black phyllite with fewer carbonaceous lime wackestone 

beds that yield Frasnian conodonts (fig. 3, loc. 12).

DEPOSITIONAL ENVIRONMENT

Lithologic and faunal evidence suggest that calcareous 

layers in the Nutirwik Creek unit accumulated in a more 

offshore and (or) deeper water environment than that 

postulated for limestone in the Beaucoup Formation or the 

Hunt Fork Shale. Most of the Nutirwik Creek limy beds are 

fine-grained, laminated, carbonaceous, and contain rare, 

chiefly pelagic fossils. These beds are interpreted as distal 

turbidites, derived from bioherms or other carbonate deposits on an adjacent platform or shelf, and deposited in quiet 

water at some distance from the original source. Less 

abundant, thicker layers of skeletal packstone probably 

reflect the passage of rare, high-magnitude hydraulic 

events, such as unusually large turbidity currents and (or) 

storm waves. In situ shallow-water carbonate deposits, such 

as the muddy bioherms and ooid grainstone that occur in the 

Beaucoup Formation and Hunt Fork Shale, have not been 

observed in the Nutirwik Creek unit.

Conodont biofacies analysis supports the conclusions 

outlined above. Conodont assemblages from the Nutirwik 

Creek unit show evidence of hydraulic breakage and sorting 

and include a mix of shallow and deeper water species. 

However, most conodonts obtained from this unit are 

characteristic of an outer shelf or slope environment 

(polygnathid-ancyrodellid-palmatolepid biofacies). Thus, 

Nutirwik Creek conodont assemblages formed through 

post-mortem transport of moderate- to shallow-water cosmopolitan species into a deeper water setting.

SUMMARY OF DEVONIAN METACLASTIC 

ROCKS IN THE SNOWDEN MOUNTAIN AREA

Three metaclastic units of Devonian age are recognized in the Snowden Mountain area: the Beaucoup Formation, the Hunt Fork Shale, and the Nutirwik Creek unit. 

Each of these units consists chiefly of fine-grained siliciclastic rocks and includes subordinate amounts of metalimestone. However, the units contain distinctive subordi-
REGIONAL RELATIONSHIPS 35

nate lithologies; mafic intrusions are apparently restricted to 

the Beaucoup Formation, quartzose metasandstone beds are 

most abundant in the Hunt Fork Shale, and the association 

of purple and green phyllite, volcaniclastic sandstone, and 

felsic igneous rocks distinguishes the Nutirwik Creek unit.

Previous authors (for example, Dutro and others, 

1979) proposed that rocks here included in the Beaucoup 

Formation and Nutirwik Creek unit depositionally overlie 

massive metacarbonate rocks of Devonian and older age 

(Snowden Mountain area metacarbonate succession of this 

paper) and are themselves overlain by the Hunt Fork Shale. 

Other workers suggested that most boundaries between 

these clastic units and the Snowden Mountain area metacarbonate succession are faults. The Beaucoup Formation 

and Nutirwik Creek unit are widely distributed throughout 

the study area and are closely associated with rocks of the 

Snowden Mountain area metacarbonate succession, but the 

Hunt Fork Shale occurs only in the northernmost part of the 

map area and is nowhere in contact with the metacarbonate 

succession (fig. 2).

Our work indicates that characteristic carbonate lithologies and conodont biofacies distinguish the three metaclastic units, but all the units are in part correlative and in part 

of Frasnian age (fig. 19). Metalimestone is most abundant 

in the Beaucoup Formation and accumulated mainly in 

relatively shallow water (shelf or platform environment) as 

in situ muddy bioherms and bioclastic shoals. Calcareous 

material is less abundant in the Hunt Fork Shale and is 

chiefly redeposited. Most limy layers in this unit are 

tempestites or turbidites; such deposits can form in a variety 

of settings, but tempestites are most common in inner to 

outer shelf environments. In situ bioclastic-ooid shoals and 

biohermal buildups are also found in the Hunt Fork Shale 

and suggest midshelf or shallower conditions. Metalimestone is least abundant in the Nutirwik Creek unit and 

occurs mostly as fine-grained, distal turbidites. Lithologic 

and faunal evidence infer a more offshore and (or) deeper 

water environment (outer shelf or slope) for this unit.

Available fossil data suggest that limy layers in the 

Beaucoup Formation are in part older than limy layers in the 

other two metaclastic units, whereas limy layers in the Hunt 

Fork Shale are in part younger, but all three units include 

some strata of middle(?) Frasnian age. The oldest welldated beds in the Beaucoup Formation yield latest Middle to 

earliest Late Devonian (latest Givetian to earliest Frasnian) 

conodonts, but other intervals are early Frasnian and middle 

to early late Frasnian in age. The most diagnostic conodont 

collections from the Nutirwik Creek unit are middle Frasnian in age, but Middle(?) Devonian brachiopods are also 

reported from this unit. Limy layers in the Hunt Fork Shale 

contain conodonts of middle to late Frasnian age, but the 

upper (wacke) member of this unit yields Famennian 

brachiopods. Some of the conodont species found in the 

Hunt Fork Shale have previously been noted only in

northern Canada (Northwest Territories) and southeast 

Alaska; specific biogeographic affinities have not been 

suggested for other faunal elements of the metaclastic units.

REGIONAL RELATIONSHIPS

Pre-Carboniferous (meta)carbonate4 rocks are widely 

distributed across northern Alaska (fig. 20), but the lithofacies, biostratigraphy, and interrelationships of these 

sequences are only beginning to be deciphered. In many 

areas, such as the Arctic quadrangle in the eastern Brooks 

Range and the Survey Pass and Ambler River quadrangles 

in the west, pre-Carboniferous metacarbonate successions 

are known to occur (Dumoulin and Harris, 1988 and unpub. 

data), but they have not yet been closely studied. However, 

stratigraphic successions that correlate at least in part with 

rocks in the Snowden Mountain area have been described in 

relative detail from several localities: the Doonerak window 

just west of the study area (Dutro and others, 1976, 1984a, 

1984b; Repetski and others, 1987; Julian, 1989), the Baird 

Mountains in the western Brooks Range (Dumoulin and 

Harris, 1987a, 1987b; Dumoulin, 1988; Karl and others, 

1989; Till, in press), the Shublik and Sadlerochit Mountains 

in the eastern Brooks Range (Blodgett and others, 1986, 

1988, 1992a), and the York Mountains on the Seward 

Peninsula (Sainsbury, 1969, 1972; Till and Dumoulin, in 

press).

Of the sections known in detail, metacarbonate rocks 

of the Snowden Mountain area are lithologically and faunally most similar to strata of the Baird Mountains, 400 km 

to the west (fig. 20). The Proterozoic(?) to Devonian 

metacarbonate succession of the Snowden Mountain area 

correlates best with rocks in the eastern Baird Mountains, 

whereas the Devonian siliciclastic units correlate with rocks 

exposed in the eastern and western Baird Mountains.

PRE-CARBONIFEROUS METACARBONATE 

SUCCESSIONS

The overall progression of lithofacies, depositional 

environments, conodont faunas, and biofacies in the eastern 

Baird Mountains succession is strikingly similar to that in 

the Snowden Mountain area. In the sections that follow, 

metacarbonate rock units in the eastern Baird Mountains are 

briefly described and compared with correlative units in the 

Snowden Mountain area. Other pre-Carboniferous sedimentary successions in northern Alaska are then summarized 

and compared in turn with Snowden Mountain area rocks.

4(Meta)carbonate is used for successions of pre-Carboniferous rocks 

that include metamorphosed and nonmetamorphosed carbonate rocks.
HUNT FORK SHALE NUTIRWIK C

BEAUCOUP FORMATION

59

Figure 19. Schematic sections (based on field observations, conodont biostratigraphy, and published descriptions), selected conodonts (relative abun

and age relations of the Beaucoup Formation, Hunt Fork Shale, and Nutirwik Creek unit. Only the calcareous part of the Nutirwik Creek unit is s

explanation of symbols and numerical identification of fossils, respectively.
REGIONAL RELATIONSHIPS 37

Figure 20. Map of northern Alaska showing general distribution of pre-Carboniferous (meta)carbonate rocks (gray pattern) and 

quadrangle and geographic names referred to in text.

EASTERN BAIRD MOUNTAINS METACARBONATE 

SUCCESSION

Carbonate rocks in the eastern Baird Mountains, like 

those in the Snowden Mountain area, have been deformed, 

metamorphosed (to blueschist and greenschist facies), and 

yield conodonts with CAIs of 5 or higher (Dumoulin and 

Harris, 1987a). In the eastern Baird Mountains (Mt. Angayukaqsraq area), dolostone and marble of Proterozoic and 

(or) Cambrian age contain stromatolites and coated grains 

and are overlain by fossiliferous Middle and Upper Cambrian carbonate rocks. These strata are succeeded by a 

condensed Lower and lower Middle Ordovician section 

of carbonaceous phyllite, rare radiolarian chert, and carbonate turbidites, which shallows upward into Upper Ordovician to Devonian(?) dolostone and metalimestone deposited 

in warm, locally restricted marine environments. The 

lithologic and biostratigraphic summary of the eastern 

Baird Mountains metacarbonate succession given below is 

taken chiefly from Dumoulin and Harris (1987a, b), 

Dumoulin (1988), Harris and others (1988), Karl and others 

(1989), and Till (in press) and includes some of our 

previously unpublished data.

PROTEROZOIC AND (OR) CAMBRIAN ROCKS

The basal unit in the eastern Baird Mountains metacarbonate succession is at least 100 m thick and consists of 

cliff-forming, orange- to light-gray-weathering, dark- to

light-gray dolostone, metalimestone, and marble with subordinate layers of quartz-rich metasedimentary rocks and 

metabasite (unit Cftvc of Till, in press; unit ftBcb of Karl 

and others, 1989)5 (fig. 21). Original sedimentary textures 

are locally well preserved; major carbonate lithologies 

include stromatolitic boundstone and cross-laminated grainstone composed of ooids, oncoids, composite grains, and 

rare pisoids. Stromatolitic morphologies range from tabular 

sheets to club-shaped mounds as much as 15 cm high. 

Regional relationships suggest that these rocks are of 

Proterozoic and (or) Cambrian age; the assemblage of 

sedimentary features indicates an intertidal to shallow 

subtidal depositional environment.

The massive dolostone unit appears to grade upward 

and laterally into a section at least 90 m thick of orange- and 

gray-weathering, variously impure metalimestone, marble, 

and dolostone, and lesser gray to green phyllite and calcareous and chloritic schists (unit CBic of Till, in press; 

subunit one of unit OCc of Karl and others, 19895). 

Prominent lithologies include quartzose and (or) chloritic 

calcarenite and packstone and grainstone containing

5CBvc, Proterozoic and (or) Cambrian dolostone, conglomerate, and 

mafic volcanic rocks; CBic, Proterozoic and (or) Cambrian impure marble 

and phyllite; Cc, Cambrian dolostone and metalimestone; Ope, Ordovician phyllite, metalimestone, dolostone, and marble; DOc, Devonian(?) to 

Upper Ordovician marble and dolostone unit of Till (in press). ftBcb, 

Paleozoic and (or) Proterozoic carbonate rocks and metabasite; OCc, 

Ordovician and Cambrian carbonate rocks; DOc, Devonian(?) to Ordovician carbonate rocks units of Karl and others (1989).
METERS 

550 —

500 — 

450 — 

400 —

350 — 

300 —

150 — 

100 — 

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SYSTEM

O

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SILURIAN

ORDOVICIAN

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AND UPPER 

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LUDLOVIAN 

AND (OR) 

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| UPPER

| MIDDLE ^

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MIDDLE

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EASTERN BAIRD 

MOUNTAINS

EXPLANATION

Number of samples that 

yielded a species 

association

• 1

• 3

• 5

68 110 113 114

Figure 21. Generalized composite stratigraphic column and distribution of 

fossils in the metacarbonate succession in the eastern Baird Mountains; 

selected fossils (chiefly conodonts) of biostratigraphic, paleogeographic, 

and (or) paleoecologic significance are shown at appropriate points adjacent 

to the lithologic column. Rocks of Early Ordovician and early Llanvirnian 

age contain an abundant graptolite succession (see Carter and Tailleur, 1984; 

other data from Dumoulin, 1988; Dumoulin and Harris, 1987a, b, and 

unpub. data; Karl and others, 1989; Till, in press). Thicknesses are minima. 

European series names are used for intervals containing dominantly cosmopolitan faunas, whereas North American series and (or) stage names are used 

for intervals containing chiefly North American province faunas. Letter 

symbols are map units of Till (in press): CBvc, Proterozoic and (or) 

Cambrian dolostone, conglomerate, and mafic volcanic rocks; CBic, Proterozoic and (or) Cambrian impure marble and phyllite; Cc, Cambrian 

dolostone and metalimestone; Ope, Ordovician phyllite, metalimestone, 

dolostone, and marble; DOc, Devonian(?) to Upper Ordovician marble and 

dolostone. See figure 5 and table 1 for explanation of symbols and 

identification of fossils, respectively. Vertical black bar indicates samples 

too closely spaced to show separately.
REGIONAL RELATIONSHIPS 39

Figure 22. Sedimentary features of metacarbonate succession in the eastern Baird Mountains. A, Middle Ordovician carbonate turbidites 

(Ope, Ordovician phyllite and carbonate of Till, in press; lens cap, 6 cm in diameter). B, Upper Ordovician shallowing-upward cycles 

of dark-weathering bioturbated pack-grainstone and light-weathering cryptalgal laminite (lower part of DOc unit of Till, in press).

locally abundant coated grains. Some layers are graded, 

contain parallel and cross lamination, and are interpreted as 

turbidites. No fossils have been found in this unit, but map 

relations suggest that it is a deeper water facies equivalent 

of the upper part of unit CBvc and the lower part of unit Cc 

(see below and footnote 5).

Rocks of definite Cambrian age overlie both units 

•CBvc and CBic and make up a section at least 60 m thick 

(unit Cc of Till, in press; subunit two of unit OCc of Karl 

and others, 1989). The basal part of this section is massive 

marble; it grades up into thin-bedded, platy metalimestone 

and then into thin couplets of bioturbated metalimestone 

and laminated dolostone interpreted as shallowing-upward 

peritidal deposits. Protoconodonts, chancellorid sclerites, 

hyolithids, and phosphatized steinkerns of monoplacophoran mollusks indicate a maximum age of Early (but not 

earliest) Cambrian for the lower part of this unit; acrotretid 

brachiopods and agnostid arthropods demonstrate Middle 

(probably late Middle) and Late Cambrian ages, respectively, for the middle and upper parts of the unit (fig. 21). 

Fossils and sedimentary features infer that most of the

section was deposited in a shallow subtidal to supratidal 

environment; the fauna is cosmopolitan and suggests no 

specific paleogeographic affinities.

LOWER AND MIDDLE ORDOVICIAN ROCKS

A condensed Lower and lower Middle Ordovician 

section at least 50 to 100 m thick overlies rocks of Middle 

and Late Cambrian age in the eastern Baird Mountains (unit 

Ope of Till, in press; subunits three and four of unit OCc of 

Karl and others, 1989; see footnote 5). Carbonaceous 

phyllite with rare layers of radiolarian-bearing metachert 

and fine-grained metalimestone makes up the basal part of 

the sequence. Calcareous layers increase in thickness and 

abundance upward. Thin graded beds of metalimestone, 

separated by phyllitic partings (fig. 22A), pass up into 

thicker bedded, bioturbated, bioclastic packstone and grainstone. The unit contains an excellent Arenigian to Llanvirnian graptolite succession (Carter and Tailleur, 1984) and 

cosmopolitan, chiefly cool-water conodonts of Llanvirnian,
40 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

latest Llanvirnian to Llandeilian, and late Llandeilian to 

earliest Caradocian(?) age (fig. 21). Lithologic andbiostratigraphic criteria indicate a shallowing-upward depositional 

regime; basinal phyllite grades up into shelf-margin carbonate turbidites and then into mid- to inner-shelf bioclastic 

grainstones. The turbidites contain a mix of warm- and 

cool-water conodont species.

UPPER ORDOVICIAN TO DEVONIAN(?) ROCKS

Lower and Middle Ordovician strata in the eastern 

Baird Mountains are succeeded by more than 200 m of 

Upper Ordovician, Silurian, and Devonian(?) dolostone and 

less abundant metalimestone (unit DOc of Karl and others, 

1989 and Till, in press; see footnote 5). The lower part of 

this section forms steep cliff exposures; the upper strata 

crop out as rubble-covered hills. The lower interval is 

distinctively color banded on a scale of one-half to several 

meters; the bands represent cyclic alternations of dark-gray 

to black, bioturbated pack-grainstone and light-gray cryptalgal laminite with fenestral fabric (fig. TIE). Similar 

lithologies occur in the upper part of the section, along with 

layers of bioclastic packstone and wackestone that contain 

corals, stromatoporoids, and pentamerid brachiopods.

Conodonts and brachiopods (Tcherskidium n. sp.) 

constrain the age of this unit. The cliff-forming lower part 

of the section is of Late Ordovician age; the basal beds 

contain Protopanderodus insculptus and the Siberiannorthern North American conodont Plectodinal cf. P.I 

dolboricus and may be as old as middle Maysvillian, but 

most strata yield chiefly Aphelognathus diver gens, a western North American Midcontinent Province (WNAMP) 

conodont, and are Richmondian (latest Ordovician). The 

rubble-forming upper strata are mostly Llandoverian to 

Ludlovian in age (Early and Late Silurian); the uppermost 

beds are no younger than Early Devonian and could be as 

old as latest Silurian. Fossils and sedimentary structures 

indicate a warm, shallow-water depositional environment 

for these rocks; the shallowest, most restricted conditions 

prevailed during Richmondian time.

COMPARISON OF THE SNOWDEN MOUNTAIN AND

EASTERN BAIRD MOUNTAINS METACARBONATE

SUCCESSIONS

The Snowden Mountain metacarbonate succession 

(used hereafter for the Snowden Mountain area metacarbonate succession) has many lithologic and faunal similarities 

to the succession in the eastern Baird Mountains. Ordovician and Silurian strata in the two areas correlate particularly well, but points of correspondence exist throughout 

both metacarbonate successions (fig. 23).

Both the Snowden Mountain and the eastern Baird 

Mountains metacarbonate successions include strata of 

Cambrian and possible Proterozoic age. Massive metacarbonate rocks of the Dillon Mountain area are lithologically 

similar to and may correlate with unit CBvc (fig. 23) in the 

eastern Baird Mountains. The major carbonate lithologies in 

both units are crossbedded, coated-grain grainstone and 

algal (stromatolitic) boundstone; both units contain layers of 

quartzose metasedimentary rocks and were deposited in 

peritidal environments. Stromatolites are more abundant 

and display a wider range of morphologies in the eastern 

Baird Mountains strata, but this difference may reflect a 

preservational bias; stromatolites are best preserved in the 

dolomitic parts of unit CBvc, and dolostone is relatively 

uncommon in the Dillon Mountain area. Unit CBvc also 

differs from the metacarbonate rocks in the Dillon Mountain 

area because it contains layers of metabasite and is spatially 

associated with rocks of known Cambrian age.

Correspondence between other pre-Ordovician units in 

the two areas is less precise. The Snowden Mountain unit 

may correlate with the upper part of unit CBic or, less 

likely, with the lowermost part of unit Cc. Lithologically, 

the Snowden Mountain unit is more like unit -CBic. Both 

units include gray phyllite, impure metalimestone, and 

quartzose metalimestone, and both accumulated in an outer 

shelf or slope setting. The well-dated uppermost part of the 

Snowden Mountain unit is of early Middle Cambrian age, 

older than part of unit Cc, and the shallow-water depositional environment inferred for unit Cc is at odds with the 

setting proposed for the Snowden Mountain unit. No rocks 

have yet been found in the Snowden Mountain area that 

correlate with the Upper Cambrian part of unit Cc. Proof 

that strata of this age once existed in this region, however, 

is provided by the presence of redeposited Late Cambrian 

conodonts in basal parts of the Snowden Creek unit.

Cambrian faunas in the Snowden Mountain area may 

have been more provincial than those in the eastern Baird 

Mountains; Snowden Mountain unit trilobites have Siberian 

biogeographic affinities, whereas the biota of unit Cc is 

cosmopolitan. Such paleobiogeographic interpretations 

remain preliminary, however, because the Cambrian fauna 

thus far recovered from the Snowden Mountain area is 

much more limited than that found in the eastern Baird 

Mountains, and strictly comparable forms with welldefined Cambrian provinciality (for example, trilobites) 

have not been obtained from the eastern Baird Mountains.

Ordovician strata in the Snowden Mountain area and 

the eastern Baird Mountains correlate well. Lithofacies and 

biofacies of the Snowden Creek unit correspond closely 

with those of unit Ope. Lower parts of both sections consist 

of carbonaceous phyllite with layers of radiolarian metachert and allodapic metalimestone; higher strata are predominantly calcareous and comprise dark, thin-bedded 

turbidites grading up into lighter colored, thicker bedded 

bioclastic pack-grainstone. Both units accumulated in
REGIONAL RELATIONSHIPS 41

tA<HIN BAIHU MUUIN 1 AIINS "^ ——

METERS 

550—

500— 

450 — 

400 —

350 —

300 —

250 —

150 — 

100 — 

50 — 

n

SYSTEM

SILURIAN

ORDOVICIAN

CAMBRIAN

SERIES/STAGE/ 

SUBSTAGE

C 

C 

LOWERMOST C 

DEVONIAN(?) 

AND UPPER C 

SILURIAN

LUDLOVIAN 

C

LUDLOVIAN 

AND (OR) c 

WENLOCKIAN 

C

C 

LLANDOVERIAN

OC

LU 

CL0.

MIDDLE

C

C 

RICHMONDIAN C

C 

C

MAYSVILLIANI?) Q

PPER AND 

MIDDLE

- — -c

C

LLANDEILIAN 

C 

C 

LLANVIRNIAN

G.C 

LOWER G 

UPPER C,S,T

MIDDLE S 

LOWERf?)

CAMBRIAN 

AND (OR) 

PROTEROZOIC

n

o

&

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——————————— 4UU MVI ———————————————

-" ^^ F

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————————————————————————— F/UF 

F

Figure 23. Correlation of pre-Ci 

and eastern Baird Mountains. Lette 

C, conodonts; G, graptolites; T, 1 

fossils. Correlation lines are solid 

mate. Letter symbols in stratigraph 

units of Till (in press). Thicknesse 

intervals containing dominantly co 

and (or) stage names are used 

Midcontinent Province faunas. Set

r""""w"""

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WUtIN MUUIN 1 ANN AtttA

STRATIGRAPHIC 

UNIT

MATHEWS RIVRUNIT

fSNO 

SNOWDEN CREEKUNIT MOL

L_u

_J 

METACARBONATE ROCKSWDEN MOUNTAIN OFDILLON NTAI 

NIT AREA ~i

SERIES/SUBSERIES/ 

STAGE/SUBSTAGE

S EIFELIAN AND 

(OR) EMSIAN

^ LUDLOVIAN AND 

WENLOCKIAN

C \ LLANDOVERIAN /

C RICHMONDIAN 

(INCLUDES MINOR 

LOWER SILURIAN(?) 

STRATA)

C MAYSVILLIAN 

AND 

C,S EDENIAN

C 

LOWER 

C CARADOCIAN(?) 

AND

_ LLANDEILIAN 

u

C

C 

C 

LLANVIRNIAN

C UPPER 

ARENIGIAN

SYSTEM/ 

SERIES

DEVONIAN

SILURIAN

UPPER

MIDDLE

ORDOVICIAN

T LOWER MIDDLE CAMBRIAN

LOWER CAMBRIAN 

AND (OR) 

PROTEROZOIC

METERS 

— 400

— 350 

— 300 

-250 

- 200

— 150

— 100

— 50 

n

arboniferous rocks in the Snowden Mountain area 

,rs indicate the position of precisely dated samples: 

rilobites and trilobitomorphs; and S, other shelly 

where well controlled and dashed where approxiic column for the eastern Baird Mountains are map 

s are minima. European series names are used for 

smopolitan faunas, whereas North American series 

br intervals containing chiefly North American 

; figure 5 for explanation of symbols.

shallowing-upward depositional regimes that evolved from 

poorly oxygenated basins to shallow subtidal carbonate 

shoals, and turbidites in both units contain hydraulically 

sorted conodont assemblages, which include a mixture of 

warm- and cool-water species. Conodonts represent chiefly 

cosmopolitan forms of the protopanderodid-periodontid 

biofacies and indicate that deposition of the two sections

was essentially coeval; the lowest exposed beds of unit Ope 

are slightly older than the oldest dated strata in the Snowden 

Creek unit.

Middle Ordovician strata display subtle compositional 

differences between the eastern Baird Mountains and the 

Snowden Mountain area that appear to reflect local variations in depositional environment and turbidite provenance.
42 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

The lower part of the Snowden Creek unit is more siliceous 

and less phyllitic than the lower part of unit Ope. Graptolites have not been found in the Snowden Creek area, but 

this may be an artifact of preservation—cleavage must 

parallel bedding in phyllitic rocks in order for graptolites to 

be observed. Siliciclastic turbidites are rare in both sections 

but slightly more common in the Snowden Creek unit, and 

Snowden Creek calcareous turbidites are in general more 

impure than those in unit Ope. Rare reworked conodonts of 

anomalous (older) age occur in Snowden Creek turbidites 

but have not yet been found in any turbidites of unit Ope. 

Upper Ordovician and Silurian strata in the Snowden 

Mountain area and eastern Baird Mountains also correlate 

well. The Mathews River unit is lithologically and faunally 

similar to unit DOc. Both are massive, dolomitic, and 

characterized by meter-scale color bands; the bands reflect 

shallowing-upward peritidal cycles of bioturbated, bioclastic and (or) peloidal carbonate deposits overlain by cryptalgal laminite. Both units contain similar mega- and microfaunas and were deposited in warm, shallow-water 

depositional environments that were shallowest and most 

restricted during Richmondian time. Some Upper Ordovician strata in both areas produce conodonts characteristic of 

Siberian-northern North American faunas, whereas latest 

Ordovician assemblages are dominated by WNAMP faunas, and Silurian faunas are predominantly cosmopolitan. 

Devonian rocks are rare in the Snowden Mountain metacarbonate succession and rare or absent in the eastern Baird 

Mountains succession.

OTHER PRE-CARBONIFEROUS (META)SEDIMENTARY 

SUCCESSIONS

Several other (meta)sedimentary successions in northern Alaska are at least in part coeval with the Snowden 

Mountain area strata and have been described in sufficient 

detail for sedimentologic and biostratigraphic comparisons 

to be made. (Meta)carbonate successions in the western 

Baird Mountains, the York Mountains, and the Shublik and 

Sadlerochit Mountains have similarities to the metacarbonate succession in the Snowden Mountain area but differ in 

some particulars. Most notably, these successions contain a 

relatively thick sequence of Lower Ordovician platform 

carbonate rocks. Lower Paleozoic metasedimentary rocks 

are also exposed in the Doonerak window; these strata are 

chiefly siliciclastic and volcaniclastic but include minor 

amounts of carbonate that may have been derived from the 

Snowden Mountain metacarbonate succession (Julian, 

1989).

WESTERN BAIRD MOUNTAINS

Description.— Pre-Carboniferous metacarbonate rocks 

are widespread in the western Baird Mountains; these rocks

constitute the Baird Group. In this paper, we restrict the 

Baird Group to metacarbonate rocks of Ordovician through 

Middle Devonian age in the western Baird Mountains; we 

do not include partly coeval metacarbonate rocks of the 

Maiyumerak Mountains and eastern Baird Mountains or 

impure carbonate rocks of Middle and Upper Devonian age 

(for example, Eli Limestone). Like the metacarbonate 

succession in the eastern Baird Mountains, these strata have 

been metamorphosed to greenschist and blueschist facies 

and contain conodonts with CAIs of 5 or higher. But overall 

stratigraphic successions in the two areas differ in detail 

(Dumoulin and Harris, 1987a, b, and unpub. data; Harris 

and others, 1988; Karl and others, 1989).

Rocks of pre-Ordovician age have not been recognized 

in the western Baird Mountains; the metacarbonate succession begins with a relatively thick (at least 400 m) interval 

of Lower and Middle Ordovician carbonate strata deposited 

chiefly in restricted to normal marine, very shallow to 

deeper water platform environments (fig. 24). These rocks 

comprise two distinct, roughly coeval facies: dolostone with 

abundant fenestral fabric and bioturbated to laminated, 

argillaceous metacarbonate rocks. Conodonts indicate that 

these strata were deposited during early Early to early 

Middle Ordovician time (Rossodus manitouensis Zone to 

Histiodella altifrons Zone of the NAMP conodont zonation); the bulk of both facies accumulated during early 

Fauna D time. Conodont faunas include chiefly WNAMP 

and some Siberian-Alaskan (SAP) province endemics. One 

sample of argillaceous metalimestone contains reworked 

conodonts of Late Cambrian age, in addition to Early 

Ordovician conodonts (fig. 24). A thin layer of dark, 

fine-grained metalimestone occurs in the lower part of 

the argillaceous metacarbonate section, and similar layers 

overlie the section. These layers contain cool-water conodonts of middle Early and early Middle Ordovician age, 

respectively; that is, slightly older than the oldest dated 

strata of unit Ope in the eastern Baird Mountains, and 

equivalent in age to the middle beds of unit Ope.

Younger Ordovician rocks appear to be rare in the 

western Baird Mountains and are of uncertain thickness. 

Middle Middle Ordovician dolostone contains Siberian 

province conodonts indicative of a very restricted, warm, 

shallow-water, innermost platform environment. A single 

locality of definitively Upper Ordovician marble contains a

Figure 24. Generalized composite stratigraphic section and 

selected conodonts of the metacarbonate succession in the 

western Baird Mountains (data from Dumoulin and Harris, 

1987a, b, and unpub. data; Karl and others, 1989). Thicknesses are minima. European series names are used for 

intervals containing dominantly cosmopolitan faunas, whereas 

North American series and (or) stage names are used for 

intervals containing chiefly North American province faunas. 

See figure 5 and table 1 for explanation of symbols and 

identification of fossils, respectively.
REGIONAL RELATIONSHIPS 43

METERS 

600 H

550 -

500 -

WESTERN BAIRD 

MOUNTAINS

400 -

350 -

300

200 -

150 -

100 -

50-

Redeposited

Late Cambrian

species

37 26

40 102

14 98
44 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

cosmopolitan conodont fauna that includes some North 

American forms; these strata are equivalent in age to the 

lower part of the Mathews River unit in the Snowden 

Mountain area and the lowest part of unit DOc in the eastern 

Baird Mountains and appear to have been deposited in a 

middle to outer shelf setting.

Silurian strata are lithologically and biostratigraphically similar to those in the eastern Baird Mountains but are 

more restricted in age (Llandoverian rocks have not been 

recognized) and areal distribution; the section may be 

several hundred meters thick. Lower and Middle Devonian 

(Emsian and Eifelian) carbonate strata, rare or absent in the 

Snowden Mountain area and eastern Baird Mountains, are 

widely distributed and were deposited in a variety of shelf 

and platformal environments. These rocks contain chiefly 

shallow-water, cosmopolitan conodonts, as well as representatives of a more provincial form, Icriodus taimyricus, a 

Siberian-northern North American endemic. The section is 

at least 60 m (and probably more than 100 m) thick and 

contains locally abundant amphiporid stromatoporoids, corals, bryozoans, gastropods, and brachiopods.

Correlation. — The metacarbonate succession in the 

western Baird Mountains has some points of correspondence with the metacarbonate succession in the Snowden 

Mountain area but has other notable differences. Cambrian 

conodonts reworked into Lower Ordovician carbonate 

deposits testify that pre-Ordovician sediments once existed 

in the western Baird Mountains, but no strata similar to the 

Dillon Mountain area metacarbonate rocks, or the Cambrian 

Snowden Mountain unit, have been found in the western 

Bairds. On the other hand, the thick, widely distributed 

Lower Ordovician section in the western Baird Mountains 

has no established counterpart in the Snowden Mountain 

area. No lithologic equivalent of the basinal Middle Ordovician Snowden Creek unit is known in the western Bairds, 

although brief episodes of cooler and (or) deeper water 

produced a few thin dark carbonate layers. Upper Ordovician and Silurian strata are lithologically similar in the two 

regions, but rocks of definite Early Silurian age are not 

known in the western Bairds. Lower and Middle Devonian 

metacarbonate rocks are widespread in the western Bairds 

but appear to be rare in the Snowden Mountain area.

YORK MOUNTAINS

Description. — The (meta)carbonate succession in the 

western Baird Mountains is similar in many ways to that in 

the York Mountains on the western Seward Peninsula (fig. 

20). The York Mountains carbonate succession consists of 

more than 1 km of Ordovician, Silurian, and Devonian(?) 

limestone, argillaceous limestone, and dolostone deposited 

in predominantly shallow water environments (Sainsbury, 

1969, 1972; Till and Dumoulin, in press; Dumoulin and 

Harris, unpub. data; Harris and others, 1988; Vandervoort,

1985). It is largely unmetamorphosed; most conodonts have 

CAIs of 2 to 4.

Ordovician rocks predominate in the York Mountains 

carbonate succession (fig. 25). The oldest dated rocks are 

Early Ordovician and comprise two lithofacies, units Oal 

and Ol6 of Sainsbury (1969). Unit Oal consists of at least 

350 m of dolomitic, locally argillaceous, limestone. Unit Ol 

is primarily micritic limestone and is at least 450 m thick. 

Both units are characterized by 8- to 15-m-thick, 

shallowing-upward cycles (Vandervoort, 1985) deposited in 

a range of subtidal to supratidal environments; the overall 

section accumulated in a deepening-upward regime. Units 

Oal and Ol produce chiefly tropical, cosmopolitan conodonts of early Arenigian (low Fauna D) age and yield some 

WNAMP and some SAP elements. The uppermost beds of 

unit Ol contain early middle Arenigian, cooler and (or) 

deeper water conodonts of the Baltoscandic Province, and 

trilobites very similar to forms described from Novaya 

Zemlya, northern Siberia (Ormiston and Ross, 1976).

Unit Ol is overlain, apparently conformably, by 7 to 30 

m of black, graptolitic shale with minor interbedded black 

limestone, which grades upward into flaggy, thin-bedded 

limestone with shale partings (unit Olsh6 of Sainsbury, 

1969). The lower limestone beds contain calcified radiolarians; higher beds are graded and allodapic. Unit Olsh is at 

least 300 m thick; it yields cool-water, pandemic conodonts 

of latest Arenigian and earliest Llanvirnian age and accumulated in a shallowing-upward regime. A fault-bounded 

section of dark limestone (unit Odl6 of Sainsbury, 1969) 

yields cephalopods of late Llanvirnian to early Llandeilian 

age and probably accumulated in middle to outer shelf 

conditions.

Upper Ordovician and Silurian strata (unit SOdl6 of 

Sainsbury, 1972) are at least 340 m thick in the York 

Mountains. Upper Ordovician beds are slightly dolomitic, 

fossiliferous limestone; rugose corals (Bighornid) indicate a 

possibly Richmondian age. Conodonts from these strata 

include pandemic forms, WNAMP elements, and some 

SAP elements (Plectodinal tunguskaensis) and are characteristic of a warm, relatively shallow-water biofacies. These 

rocks also contain the trilobite Monorakos, a form known 

previously only from the Siberian and Kolyma platforms 

(Ormiston and Ross, 1976), and brachiopods and gastropods with Siberian affinities (Potter, 1984; Blodgett and 

others, 1992b). Silurian rocks are chiefly dolostone, contain 

5- to 8-m-thick shallowing-upward cycles, and are of 

Ludlovian age. The youngest rocks known in the York 

Mountains carbonate succession are of probable late Ludlovian to Early Devonian age (Allan Pedder, Geological 

Survey of Canada, oral commun., 1988).

6Oal, Ordovician argillaceous limestone and limestone; Ol, Ordovician limestone and argillaceous limestone; Olsh, Ordovician limestone and 

shale; Odl, dark limestone; SOdl, Silurian and Ordovician limestone and 

dolomitic limestone.
REGIONAL RELATIONSHIPS 45

METERS 

1050-

750- 

600 - 

450- 

300-

150-

0

SYSTEM/ 

SERIES

SERIES/STAGE/ 

SUBSTAGE (FAUNA)

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GAMACHIAN TO 

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NO YOUNGER 

THAN LLANDEILIAN

LOWERMOST 

LLANVIRNIAN 

AND 

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Figure 25. Generalized composite stratigraphic section 

and selected conodonts of the carbonate succession in the 

York Mountains (data from Sainsbury, 1969, 1972; Till and 

Dumoulin, in press; Dumoulin and Harris, unpub. data). 

Letter symbols represent map units of Sainsbury (1969, 

1972): Oal, Ordovician argillaceous limestone and limestone; Ol, Ordovician limestone and argillaceous limestone; 

Olsh, Ordovician limestone and shale; Odl, dark limestone; 

SOdl, Silurian and Ordovician limestone and dolomitic

limestone. All thicknesses are minima, based on partial 

sections measured by the authors; Sainsbury (1969) reports 

greater thicknesses for most of these units, but his estimates 

probably include structural repetitions. European series 

names are used for intervals containing dominantly cosmopolitan faunas, whereas North American series and (or) stage 

names are used for intervals containing chiefly North American province faunas. See figure 5 and table 1 for explanation of symbols and identification of fossils, respectively.
46

METERS 

3900-

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SHUBLIK AND SADLEROCHIT 

MOUNTAINS

112

93

Figure 26. Generalized composite stratigraphic section and fossil distribution in the carbonate succession in 

the Shublik and Sadlerochit Mountains (data from 

Blodgett and others, 1986, 1988, 1992a; Robinson and 

others, 1989; J.G. Clough, 1989, and written commun., 

1992; and A.G. Harris, unpub. data). See figure 5 and 

table 1 for explanation of symbols and identification of 

fossils, respectively.
REGIONAL RELATIONSHIPS 47

Correlation.— Differences between the York Mountains and Snowden Mountain (meta)carbonate successions 

are most marked in pre-Middle Ordovician rocks. No 

definitively Cambrian or older strata are known in the 

Yorks, but Lower Ordovician rocks are thick and widely 

distributed. Middle Ordovician and younger rocks in the 

two areas correspond better. The lower part of unit Olsh 

correlates lithologically and faunally with part of the Snowden Creek unit, which suggests that basins existed in both 

regions during early Middle Ordovician time. The Snowden 

Mountain area basin received more siliciclastic input and 

persisted longer than its counterpart in the York Mountains. 

Upper Ordovician and Silurian strata are also similar in the 

two areas, but Richmondian rocks in the York Mountains 

formed in slightly more normal marine and deeper water 

than coeval Snowden Mountain area strata. Definitively 

Devonian carbonate rocks are rare or absent in the York 

Mountains and appear to be uncommon in the Snowden 

Mountain metacarbonate succession.

SHUBLIK AND SADLEROCHIT MOUNTAINS

Description.—A carbonate platform succession somewhat similar to that in the western Baird and York Mountains occurs in the Shublik and Sadlerochit Mountains in the 

northeastern Brooks Range (Clough, 1989; Blodgett and 

others, 1986, 1988, 1992a; Clough and others, 1988, 

1990). These rocks are not metamorphosed and yield 

conodonts with a CAI of 4. The succession comprises the 

Katakturuk Dolomite (at least 2,500 m thick), the Nanook 

Limestone (about 1,200 m thick), and the Mount Copleston 

Limestone (71 m thick) (fig. 26).

The Shublik-Sadlerochit Mountains carbonate succession (as used herein) is distinguished by a thick sequence of 

pre-Ordovician carbonate rocks. The Katakturuk Dolomite 

consists of a variety of carbonate lithologies deposited in 

shallow subtidal to supratidal settings; it includes abundant 

stromatolites and coated grains and is considered to be of 

Proterozoic age. The overlying Nanook Limestone also 

contains a variety of shallow-water carbonate lithofacies; 

the lower two-thirds of the formation have yielded no body 

fossils (although the basal part of the unit contains burrows) 

and is thought to be chiefly Proterozoic or Early and 

Middle(?) Cambrian in age. These unfossiliferous strata are 

overlain by at least 160 m of Upper Cambrian peloidal 

pack-grainstone, which yields trilobites with North American paleobiogeographic affinities.

The upper part of the Shublik-Sadlerochit Mountains 

carbonate succession is Ordovician (upper one-third of the 

Nanook Limestone) and Devonian (Mount Copleston Limestone) in age. Lower Ordovician strata are as much as 300 

m thick and consist chiefly of peloidal pack-grainstone. 

These rocks yield trilobites with affinities to the Bathyurid 

Province, which occupied low-paleolatitude sites in North

America, Greenland, northeastern Russia, and Kazakhstan, 

and locally abundant Clavohamulus densus, a NAMP conodont that indicates a warm, partly restricted, very shallow 

water depositional environment. Clavohamulus densus is 

diagnostic of the Rossodus manitouensis Zone of early 

Early Ordovician age. An interval of Middle and (or) Late 

Ordovician age, as much as 120 m thick, lies between 

definitive Lower and Upper Ordovician strata. The uppermost 44 m of the Nanook Limestone are definitively Late 

Ordovician in age, consist of interbedded dolostone and 

lime mudstone, and yield a relatively diverse gastropod 

fauna, as well as other mollusks and ostracodes and the 

pentamerid brachiopod Tcherskidium sp., which has Siberian biogeographic affinities. Upper Ordovician strata produce conodont faunas similar to those from the eastern 

Baird Mountains, including the Siberian-northern North 

American element Phragmodus n. sp. but having a greater 

diversity of WNAMP species such as Aphelognathus aff. A. 

diver gens, Culumbodina occidentalis, and Pseudobelodina 

vulgaris vulgaris. Locally, in the Shublik Mountains, the 

Mount Copleston Limestone of Emsian (late Early Devonian) age disconformably overlies the Nanook Limestone. 

This Devonian unit is mostly lime mudstone with local 

banks of pentamerid brachiopods and lagoonal deposits rich 

in amphiporid stromatoporids; other fossils include twohole crinoid columnals and conodonts of the gronbergi to 

serotinus Zones, inclusive.

Correlation. —The carbonate succession in the Shublik 

and Sadlerochit Mountains differs considerably from that in 

the Snowden Mountain area. The Shublik-Sadlerochit preOrdovician section is much thicker than known or suspected 

coeval rocks in the Snowden Mountain area; well-dated 

Cambrian strata are younger than the Middle Cambrian 

Snowden Mountain unit and include faunas with North 

American, not Siberian, biogeographic affinities. The relatively thick, Lower Ordovician part of the Nanook Limestone has no dated equivalents in the Snowden Mountain 

area, and the basinal, Middle Ordovician Snowden Creek 

unit has no lithologic counterpart in the Shublik-Sadlerochit 

carbonate succession. Upper Ordovician strata in the upper 

part of the Nanook correspond lithologically and faunally 

with coeval rocks in the Snowden Mountain metacarbonate 

succession, but Silurian strata are missing in the Shublik 

and Sadlerochit Mountains. The Emsian Mount Copleston 

Limestone is lithologically similar to, and may correlate 

with, Emsian and (or) Eifelian metalimestone in the Snowden Mountain area. Both units contain the distinctive 

two-hole crinoid columnals.

DOONERAK WINDOW

Description.—Lower Paleozoic rocks, informally 

named the Apoon assemblage (Julian, 1986), are exposed in 

a structural high near Mount Doonerak, about 40 km 

northwest of Snowden Mountain (fig. 20). The assemblage
48 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

consists of black slate and argillite, green and gray phyllite, 

volcaniclastic conglomerate, mafic tuff and pyroclastic 

breccia, massive basalt, and minor marble; it is severely 

deformed and metamorphosed to lowest greenschist facies 

(Julian, 1986, 1989). Structural complexity precludes determination of stratigraphic thickness; structural thickness of 

the assemblage exceeds 3,000 m (Julian, 1989).

The age of the Apoon assemblage is poorly constrained 

but appears to be early Paleozoic. A few fossils have been 

recovered from metasedimentary rocks in the western part 

of the Doonerak window. North of Frigid Crags (about 15 

km southwest of Mount Doonerak), dark-gray phyllite 

contains Early Silurian (Llandoverian) graptolites and Silurian conodonts, and siliceous volcaniclastic rocks produced 

Ordovician, probably Middle Ordovician, conodonts 

(Repetski and others, 1987). In the hills south of Wolf 

Creek (about 30 km southwest of Mount Doonerak), sandy 

metalimestone contains Middle (probably early Middle) 

Cambrian trilobites, as well as protoconodonts, hyolithids, 

and acrotretid brachiopods; the fauna has Siberian biogeographic affinities (Dutro and others, 1984a, 1984b). Two 

marble bodies in this area produced protoconodonts of 

Middle Cambrian to Early Ordovician age (Dutro and 

others, 1984a). Mafic dikes and sills in the Apoon assemblage have been dated by K-Ar and Ar-Ar techniques and 

yield ages of about 470 and 350 Ma (Dutro and others, 

1976); these values fall within the early Middle Ordovician 

and late Early Mississippian, respectively (based on Harland and others, 1990; Bally and Palmer, 1989).

The Apoon assemblage is interpreted as a subductionrelated magmatic arc complex, deposited in a back-arc 

basin (Julian, 1989). An undated marble body in the eastern 

Doonerak area locally preserves a limestone breccia texture, 

contains shale-rip-up clasts, and is thought to be a carbonate 

debris-flow deposit (Julian, 1989). Cambrian metacarbonate layers from the Wolf Creek area contain calcareous 

lithiclasts (Dumoulin, unpub. data) and could also have 

been redeposited.

Correlation.—Julian (1989) suggested that limy 

debris-flow deposits within the Apoon assemblage were 

derived from a carbonate platform in the Snowden 

Mountain area. However, no lithologic or fauna! data have 

yet been found to confirm this suggestion because dated 

metacarbonate layers in the Apoon assemblage are older 

(Middle Cambrian to Early Ordovician) than the main 

period of platform carbonate deposition at Snowden Mountain (Late Ordovician and Silurian).

DEVONIAN SILICICLASTIC UNITS

Metasedimentary rocks that are at least in part correlative with Devonian siliciclastic units in the Snowden 

Mountain area occur in the western (and possibly, in the 

eastern) Baird Mountains. The Baird Mountains units are

briefly described below (description based on data in Karl 

and others, 1989) and then compared to the Snowden 

Mountain units.

DEVONIAN SILICICLASTIC UNITS IN THE BAIRD 

MOUNTAINS

NAKOLIK RIVER UNIT

The Nakolik River unit is an informal name proposed 

by Karl and others (1989) for metasedimentary rocks that 

crop out in the western Baird Mountains. Two subunits are 

distinguished, units Dnl and Dnu. 7 Unit Dnl consists of 

metalimestone and marble intercalated with subordinate 

(20-40 percent) quartzose metaclastic rocks (fig. 21 A). 

Metalimestone intervals are a few to several tens of meters 

thick, contain locally abundant corals (fig. 27B), stromatoporoids, and brachiopods, and formed as biohermal buildups on a shelf or platform subject to periodic influx of 

siliciclastic material. Other limy layers consist of quartzose 

calcarenite deposited by storms or in high-energy shoals. 

Unit Dnl yields rare conodonts of latest Givetian and 

Frasnian age that represent the polygnathid-icriodid biofacies. Unit Dnu consists primarily of green, gray, and 

maroon phyllite, as well as lesser amounts of metalimestone, quartzose metasandstone, and metaconglomerate; it 

is laterally gradational to, and locally gradationally overlies, unit Dnl. Metalimestone layers are similar to those in 

unit Dnl but are generally less than 10 m thick. Conodonts 

obtained from these layers are of Frasnian age; polygnathids 

dominate the assemblages, but minor ancyrodellids and 

palmatolepids occur and suggest an outer shelf depositional 

environment.

The Nakolik River unit is gradationally overlain by the 

Hunt Fork Shale; the base of the unit is not exposed. The 

unit is transitional in time and space between the Lower and 

Middle Devonian carbonate platform sequence of the upper 

part of the Baird Group to the south and the dominantly 

clastic, mostly Upper Devonian Endicott Group to the 

north.

UNIT ftqs

Unit ftqs8 of Karl and others (1989) crops out in the 

northeastern Baird Mountains and consists of quartz-pebble 

metaconglomerate, quartzose, locally calcareous metasandstone, and green, maroon, and black (carbonaceous) phyllite. Coarse-grained metaclastic rocks are primarily quartzose but include some volcanic rock fragments, and the unit

7Dnl, Devonian limestone of Nakolik River; Dnu, Devonian phyllite, 

carbonate, and clastic rocks of Nakolik River, undivided.

8ftqs, Paleozoic quartz conglomerate, sandstone, and siliceous 

phyllite.
REGIONAL RELATIONSHIPS 49

-Metasandstone

-Metasiltstone 

and phyllite

/Metalimestone and marble

containing corals 

and stromatoporoids

Figure 27. Sedimentary features of the Nakolik River unit, 

western Baird Mountains. A, Outcrops of the Nakolik River unit. 

B, Colonial coral in fossiliferous metalimestone.

is "intimately associated" (Karl and others, 1989, p. 33) 

with siliceous volcanic rocks (metarhyelite). No fossils 

have been recovered from unit ftqs, but the unit is thought 

to be Middle and Late Devonian in age on the basis of 

regional relationships (Karl and others, 1989). Unit ftqs 

overlies the Devonian(?) and older eastern Baird Mountains 

metacarbonate succession described above. In most places 

the contact is a fault, but a depositional contact has been 

reported from one locality (Karl and others, 1989). The unit 

is overlain by gray phyllite assigned to the Hunt Fork Shale; 

this contact is covered and may be structural and (or) 

stratigraphic.

HUNT FORK SHALE

Rocks referred to the Hunt Fork Shale by Karl and 

others (1989) crop out in both the eastern and western Baird 

Mountains. These strata have been metamorphosed to lower 

greenschist facies and consist of gray to green or black 

phyllite and subordinate siliceous or calcareous metasiltstone and metasandstone. Metasandstone layers are graded, 

as much as 1 m thick, and interpreted as turbidites; some 

contain transported fossil debris. In both its eastern and 

western outcrop areas, the Hunt Fork Shale is intruded by 

massive mafic sills and dikes; at one western locality, the 

unit contains a 30-m-thick section of pillowed(?) mafic 

flows. No fossils have been found in this unit in the eastern 

Baird Mountains, but western outcrops yield brachiopods 

and mollusks of late Frasnian or early Famennian age. Karl 

and others (1989) suggest a prodelta, outer shelf or slope 

depositional setting.

COMPARISON OF SNOWDEN MOUNTAIN AND BAIRD 

MOUNTAINS DEVONIAN SILICICLASTIC UNITS

Like the Snowden Mountain area, the Baird Mountains 

contain several siliciclastic units of Devonian age that are 

structurally intercalated with older metacarbonate rocks. 

The lithology, fauna, age, and general depositional setting 

of the Nakolik River unit are similar to those of the 

Beaucoup Formation and Nutirwik Creek units in the 

Snowden Mountain area. Unit Dnl and the Beaucoup 

Formation contain fine- and coarse-grained siliciclastic 

rocks, biohermal buildups, redeposited calcarenite layers, 

and conodonts of the polygnathid-icriodid biofacies. Unit 

Dnl is more calcareous than the Beaucoup Formation and 

appears to be, in part, older; the oldest faunas known from 

unit Dnl are Eifelian or early Givetian in age, whereas 

well-dated samples from the Beaucoup Formation are no 

older than latest Givetian. Unit Dnu and the Nutirwik Creek 

unit consist primarily of purple and green phyllite but 

include some conglomerate and minor metalimestone; limy 

layers produce Frasnian conodonts, including some forms
50 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

typical of deeper water (outer shelf or slope) settings. Unit 

ftqs is also lithologically similar to much of the Nutirwik 

Creek unit; both include pebble conglomerate, maroon, 

green, and black phyllite, and volcanic material. Limestone 

lenses have not been noted in unit ftqs, however, and its 

age is poorly constrained. Unit ftqs, the Nakolik River unit, 

and the Nutirwik Creek unit are spatially associated with, 

and may have been deposited on, older metacarbonate 

successions.

Age, lithology, and depositional setting of the Hunt 

Fork Shale are similar in both the Baird Mountains and 

Snowden Mountain area, although the unit appears to 

contain less limestone and is consequently less well dated in 

the Baird Mountains. The Hunt Fork Shale is thought to 

depositionally overlie the Nakolik River unit in the western 

Baird Mountains (Karl and others, 1989), but all contacts 

between the Hunt Fork Shale and the Beaucoup Formation 

and Nutirwik Creek units in the Snowden Mountain area 

appear to be faults. Finally, mafic igneous rocks intrude the 

Beaucoup Formation in the Snowden Mountain area but are 

intercalated with and intrude the Hunt Fork Shale in the 

Baird Mountains.

DISCUSSION

The present configuration of pre-Carboniferous rocks 

in northern Alaska is a result of Mesozoic tectonism; the 

original configuration of these rocks is unknown. Paleogeographic reconstruction of northern Alaska remains contentious because fundamental tectonic questions are unresolved. In particular, the cause and effects of Devonian 

orogeny in the Brooks Range, and the position of northern 

Alaska prior to Mesozoic opening of the Canada basin, are 

poorly understood (Nelson and others, 1993; Lawver and 

Scotese, 1990).

Most workers interpret pre-Carboniferous carbonate 

strata of northern Alaska as a platform or continental margin 

assemblage formed on continental crust, but the Paleozoic 

position of this crust is poorly constrained. Proposed 

Paleozoic configurations include northern Alaska as a 

northwest-protruding extension of the North American 

craton (Churkin and others, 1984, 1985) or as an isthmus 

connection between North America and Siberia (Churkin 

and Trexler, 1981; Rowley and others, 1985). Other 

authors have hypothesized that northern Alaska and the 

Chukotka region of northeastern Siberia constituted a discrete continental block with a Paleozoic kinematic history 

distinct from that of North America (for example, Sweeney, 

1982).

Alternatively, pre-Carboniferous strata now found in 

northern Alaska may have accumulated along several disparate continental margins and then juxtaposed by later 

tectonic events. Grantz and others (1991) suggested that the 

Brooks Range is underlain by Siberian and North American

crustal fragments assembled during early Paleozoic convergence. These authors interpret rocks with "Siberian" faunas, such as pre-Devonian strata in the Snowden Mountain 

area, as Siberian crustal fragments and metavolcanic rocks 

in the Doonerak window as remnants of the arc formed 

during this convergent event.

Various techniques can be used to test the models for 

origin of pre-Carboniferous carbonate strata in northern 

Alaska discussed above. Approaches include (1) establishing displacement histories of carbonate successions through 

paleomagnetic studies, (2) identifying structures and (or) 

rock associations (such as ophiolite assemblages or volcanic 

arc deposits) that mark suture zones between carbonate 

successions, (3) using microfacies analyses to assess the 

degree of lithologic and biotic similarity between individual 

(meta)carbonate successions, and (4) comparing paleobiogeographic affinities of faunas in distinct (meta)carbonate 

successions. Reliable primary magnetic components have 

not been obtained from Paleozoic rocks of the Brooks 

Range or the Seward Peninsula (Plumley and Tailleur, 

1987; Plumley and Reusing, 1984), but the other three 

methods have been used to investigate the origin of preCarboniferous strata of northern Alaska.

Identifying boundaries between Paleozoic crustal fragments is hindered by the masking effects of younger 

tectonic events on older structures and the difficulty of 

recognizing and precisely dating ancient suture assemblages. Metavolcanic rocks of early Paleozoic age in the 

Doonerak window, and of Early and Late Cambrian age in 

the northeastern Brooks Range, have been interpreted as 

possible remnants of oceanic crust trapped during a preCarboniferous continental collision (Moore, 1987; Grantz 

and others, 1991) (fig. 20). These rocks yield ambiguous 

geochemical signatures, however, and could also have 

formed along the North American continental margin 

(Moore, 1987).

Deep-water carbonate rocks are another lithology that 

might occur along suture zones between continental carbonate terranes, but these strata could also accumulate in 

intracratonic basins. Slope and basinal carbonate rocks of 

Cambrian to Devonian(?) age occur on the northeastern and 

southeastern Seward Peninsula (Dumoulin and Till, 1985; 

Till and others, 1986; Ryherd and Paris, 1987; Dumoulin 

and Harris, unpub. data), and carbonate turbidites of 

Silurian and (or) Devonian age crop out in the Ambler River 

and Wiseman quadrangles (Dumoulin and Harris, 1988 and 

unpub. data) (fig. 20). Nothing is known, however, about 

the type of basement (oceanic or continental) on which 

these sequences were deposited.

Zones where disparate continental fragments may have 

been juxtaposed are marked by suture assemblages, but 

distinguishing the fragments requires detailed stratigraphic 

comparisons. Comparative lithofacies analysis and paleobiogeography have been used in the Appalachians 

(Williams and Hatcher, 1983) and the North American
REGIONAL RELATIONSHIPS 51

Cordillera (Monger and Ross, 1984) to recognize "exotic" 

Paleozoic terranes thought to have been displaced great 

distances before accretion to the North American continental margin. Application of such techniques to the preCarboniferous strata of northern Alaska has been limited, 

however, because relatively detailed paleontologic and 

sedimentologic data have only recently been obtained from 

much of this region. These data are summarized below, and 

their implications for the Paleozoic history of northern 

Alaska are briefly explored.

COMPARATIVE MICROFACIES ANALYSIS

One approach that could help identify "exotic" continental fragments in northern Alaska is comparative analysis 

of microfacies in the pre-Carboniferous (meta)carbonate 

successions. Carbonate rocks deposited along the same 

margin or platform should show similar successions of 

lithofacies, biofacies, and depositional environments. 

Coeval rocks formed on separate continents, in contrast, 

could differ greatly, because they might have different 

sediment sources, paleoclimates, and (or) subsidence histories. Distinction between these end-member cases is not 

necessarily straightforward, however. Intracratonic basins 

can develop within a single continental block and give rise 

to a variety of dissimilar but coeval facies. Alternatively, 

global changes such as eustatic shifts could produce uniform effects on disparate continental margins.

Similarities and differences between the (meta)carbonate successions of northern Alaska are summarized below 

and in figure 28. The metacarbonate succession in the 

Snowden Mountain area correlates best, lithologically and 

faunally, with the metacarbonate succession in the eastern 

Baird Mountains, but also has similarities with (meta)carbonate successions in the western Baird, York, and Shublik 

and Sadlerochit Mountains. Differences between these 

(meta)carbonate successions could reflect origins in several 

discrete carbonate platforms but also could be explained 

through variation in subsidence rates, erosion, and clastic 

input along a single continental margin.

Strata of Proterozoic and Cambrian age are thickest in 

the Shublik and Sadlerochit Mountains and have not been 

observed in the York and western Baird Mountains. Relatively deep-water facies (deposited in outer shelf and (or) 

slope settings) of Middle Cambrian and older(?) age are 

known in the eastern Baird and Snowden Mountain metacarbonate successions and may also occur in Doonerak 

window rocks. Relatively shallow-water facies of Late 

Cambrian age are recognized in the eastern Baird and 

Shublik-Sadlerochit (meta)carbonate successions.

Lower Ordovician platform carbonate rocks are widely 

distributed in the York, western Baird, and Shublik and 

Sadlerochit Mountains. The western sections are thicker 

and differentiated into at least two distinct lithofacies; the

Shublik-Sadlerochit section appears relatively uniform. 

Carbonate platform rocks of Early Ordovician age have not 

been identified in the eastern Baird Mountains or Snowden 

Mountain area; instead, basinal facies (carbonaceous phyllite, siltstone, and metachert) appear to have accumulated 

during this time.

Middle Ordovician strata in the eastern Baird and 

Snowden Mountain metacarbonate successions consist 

chiefly of deep-water slope to basinal facies; both sections 

shallow upward and contain abundant fine-grained siliciclastic detritus. The Middle Ordovician section in the York 

Mountains is similar but contains less siliciclastic material 

and a larger proportion of somewhat shallower (outer to 

middle shelf) facies. Other Middle Ordovician sections are 

dominated by platform carbonate rocks; deep-water facies 

of Middle Ordovician age appear to be a minor part of the 

western Baird metacarbonate succession and have not been 

reported from the Shublik and Sadlerochit Mountains. 

Global sea level was high during the Llanvirnian; relatively 

deep-water facies characterize many Llanvirnian sections 

worldwide.

Carbonate strata of Late Ordovician through Middle 

Devonian age are generally similar across northern Alaska; 

variations appear to reflect differential erosion during Early 

Silurian and (or) post-Early Devonian time. Upper Ordovician strata were deposited in a shallowing-upward shelf or 

platform setting; this pattern has been documented in all 

successions with the exception of the western Baird Mountains, where Upper Ordovician strata appear to be scarce. 

Lower and Upper Silurian strata occur in the York, eastern 

and western Baird, and Snowden Mountain (meta)carbonate 

successions. No Silurian rocks have been found in the 

Shublik and Sadlerochit Mountains.

Meter-scale shallowing-upward cycles have been 

noted in Upper Ordovician strata in the eastern Baird and 

Snowden Mountain metacarbonate successions and in Silurian strata in the York, western Baird, eastern Baird, and 

Snowden Mountain (meta)carbonate successions. Carbonate facies and depositional patterns in Ordovician and 

Silurian strata in northern Alaska correlate well with published eustatic curves for the early Paleozoic. Global sea 

level was high at the base of the Ashgillian and then fell 

sharply throughout the remainder of the Late Ordovician 

(Ross and Ross, 1988). The Silurian eustatic curve is 

marked by a number of short-term rises and falls (Ross and 

Ross, 1988) so small-scale depositional cycles are not 

surprising in strata of this age.

Lower and Middle Devonian carbonate rocks appear to 

be rare in the Snowden Mountain area, are rare or absent in 

the eastern Baird and York Mountains, and are only locally 

present in the Shublik and Sadlerochit Mountains. The 

thickest and most widely distributed section of Lower and 

Middle Devonian carbonate rocks occurs in the western 

Baird Mountains. Global sea level was relatively low during
EONOTHEM/ 

SYSTEM/ 

SERIES

z 

<

z 

O

LU 

Q

SILURIAN

Z 

0

o 

o 

rr 

O

z 

<

rr 

m

2 

< 

o

UPPER

MIDDLE

LOWER

UPPER

LOWER

UPPER

MIDDLE 

LOWER

UPPER

MIDDLE

LOWER

PROTEROZOIC

YORK MOUNTAINS

?

1

1

M sodl:

HH Dolostone and 

HH limestone 

^ (>340m) 

11 

? 

^SOdl

1

1 

gggg. Odl: Dark limestone; 

fH| middle shelf (>1 00 n

Hi Olsh: Shale and lime 

H turbidites; shallows 

TM (>300 m) 

sss® ^-. Ol : deepening-t

|XS8$£ "NS.^ • 'A* I*

^& n*l- ^ micntichm t£^^£ vy«i. ^* /

SSsjS: **s^ [>

mi Argillaceous -< 

im limestone and dolo- 

| stone (>350 m)

M

outeri) 

stone 

jpward

pward 

estone mi 

300 m)^ 

^^

^*-> ^^

"N^

WESTERN BAIRD 

MOUNTAINS

HH Dolostone and 

lii metalimestone 

iH (>60 m)

Z&&& 

^^ r^ I , j

mg Dolostone and 

m^ metalimestone 

HH (>100 m)

&§&! 

SwS^

? Metalimestone; 

HH outer and middle 

^ shelf(?) (uncertain 

« thickness)

s<x«x Dolostone (uncertain ^_ 

*** thickness) Darkmeta| ime. • 

stone (uncertainBH 

thickness)

&&£ ^vw

^ Dolostone with fenestral 

|H fabric (>300 m) ^- ^§(|

• ^-""Dark/ B

^ ^ -^ metalimestone ^^ 

TM^*^ (uncertain thickness) S* f*^

Argillaceous meta- / 

carbonate (>300 m) ^|

(Reworked Late Cambrian ' | 

conodonts in argillaceous 

metacarbonate rocks)

EXPLANATION

Shallow-water deposits (innermiddle shelf or platform)

Deep-water deposits (outer 

shelf, slope, or basin)

EASTERN BAIRD 

MOUNTAINS

? .

I

,J

i^i DOC

Shallowing-upward cycles 

of packstone-grainstone 

HH (some bioclastic) and 

IHI cryptalgal laminite 

^ (>200 m)

•,

? '

• Ope: 

^M Carbonaceous phyllite, 

^1 metachert, and carbon- 

^1 ate turbidites. Shallows 

^1 upward (50-100 m)

^^ "Cc: 

^^ Massive marble, platy 

metalimestone, metalime- 

^^ stone couplets (>60 m) n 

^^ ^^

HI! --• ^* H 

^ ^- ^ H

? ^ ^ •

^- -CEic: • 

^* Phyllite and ^1 

\ metalimestone ^H 

?. . \ turbidites (>90 m) ^1

jjH ^^ •

^ -CEvc: X \ 1 

Dolostone with \ ^H 

^^ stromatolites x ^* 

^^ and coated grains ' 

^ (>100m)

SNOWDEN MOUNTAI

AREA

• 

Metalimestone (>1 0 m)

2ym 

^^

^^^ 

^^

HH Mathews River unit: 

Shallowing-upward cycles 

of packstone-grainstone 

(some bioclastic) and 

j^ cryptalgal laminite 

(>150m) 

^ 

^ 

^^ 

^^^^

•• Snowden Creek unit: 

^H Carbonaceous phyllite, 

^H chert, carbonate turbidit

^H Shallows upward; slope 

^H basin (> 100m)

?i 

l

I (Reworked Late Cambria

I and Early Ordovician co

' donts in Snowden Creek 

| unit)

I

^_ Snowden Mountain unit: 

^1 Phyllite, sandy metalime

11 stone; storm deposits; o

shelf or slope (>20 m)

Dillon Mountain area 

^^ metacarbonate rocks: 

Stromatolites and coate

^ grains (>1 00m)

^^ 

•
REGIONAL RELATIONSHIPS 53

the Early Devonian but began to rise in the Eifelian (Ross 

and Ross, 1988).

Carbonate-siliciclastic rocks of Middle and Late Devonian age occur in the Snowden Mountain area and western 

Baird Mountains; carbonate content is higher in the Baird 

Mountains units. Possibly correlative sequences that contain little carbonate material crop out in the eastern Baird 

Mountains. No strata definitively of this age are known in 

the York or Shublik and Sadlerochit Mountains.

Thus, carbonate platform development started as early 

as the latest Proterozoic in at least some parts of northern 

Alaska and had ended by the Early or Middle Devonian in 

all of the (meta)carbonate successions discussed above. 

Clastic influx and (or) Devonian orogeny may have caused 

the demise of early Paleozoic carbonate platforms in northern Alaska. Successions of mixed carbonate and siliciclastic 

material (with carbonate generally subordinate) of Middle 

and Late Devonian age are widely distributed across northern Alaska. A depositional relationship between older 

carbonate platform successions and younger carbonateclastic units has been postulated by some workers but has 

not been proven in most areas. The best case for such a 

relationship can be made in the western Baird Mountains, 

where a continuous Devonian succession of Emsian, Eifelian, Givetian, Frasnian, and possibly Famennian age is 

present. The Emsian to Eifelian cessation of carbonate 

platform deposition in most of northern Alaska appears to 

roughly coincide (about 390 Ma; Aleinikoff and others, 

1993) or slightly predate (370 ± 30 Ma; Nelson and others, 

1993; Dillon and others, 1987b) intrusion of granitic plutons. These ages fall within the Early Devonian and 

Frasnian, respectively (based on Harland and others, 1990; 

Bally and Palmer, 1989).

Lithologic data from the pre-Carboniferous (meta)carbonate successions of northern Alaska could be explained 

through origin on three discrete carbonate platforms. The 

Snowden Mountain and eastern Baird metacarbonate successions are distinguished from other northern Alaska 

(meta)carbonate successions by well-developed basinal 

assemblages of chiefly Middle Ordovician age and the 

apparent absence of Lower Ordovician shallow-water carbonate rocks. The York and western Baird (meta)carbonate 

successions are characterized by a thick section of Lower 

Ordovician platformal carbonates and by relatively minor 

amounts of Middle Ordovician deeper water facies. The 

Shublik-Sadlerochit carbonate succession is similar to the 

York and western Baird (meta)carbonate successions but

Figure 28. Correlation and depositional environments of 

(meta)carbonate successions in the York, western and eastern 

Baird, Snowden, and Shublik and Sadlerochit Mountains. All 

rocks shown in the western Baird Mountains column are part of 

the Baird Group. Number in parentheses indicates thickness of 

unit.

includes a thick section of Proterozoic and Cambrian strata 

not known in the west.

An alternate explanation of the available data is that 

the pre-Carboniferous carbonate successions described 

above formed along a single continental margin that was 

near, and shared some faunal elements with, both the North 

American and Siberian continents (fig. 29). During Early 

and Middle Ordovician time, an intracratonic basin developed along part of the north Alaskan margin (eastern Baird 

and Snowden Mountain metacarbonate successions) while 

carbonate platform deposition continued elsewhere (York, 

western Baird, and Shublik-Sadlerochit (meta)carbonate 

successions). By Late Ordovician time, shallow-water carbonate deposition had spread again across the north Alaska 

margin, and these conditions persisted, with some interruptions, through at least the Early Devonian.

PALEOBIOGEOGRAPHY

Paleobiogeographic analysis is another technique that 

can shed light on the Paleozoic configuration of northern 

Alaska. "Siberian" and "North American" biogeographic 

affinities have been reported for various fossils from the 

pre-Carboniferous (meta)carbonate successions described 

above. The biogeography of Cambrian trilobites has 

attracted the most attention (Palmer and others, 1984; Dutro 

and others, 1984a, b; Grantz and others, 1991), but provinciality has also been noted in Ordovician trilobites, conodonts, brachiopods, gastropods, and corals. Silurian and 

Devonian faunas in northern Alaska appear relatively 

cosmopolitan.

The biogeographic affinities of Cambrian and Ordovician faunas from northern Alaska are summarized in table 

2. Late Cambrian trilobites in the Shublik-Sadlerochit 

carbonate succession have North American affinities 

(Blodgett and others, 1986); Early Cambrian trilobites 

found in limy beds in the Marsh Fork volcanic rocks 

(informal unit of Moore, 1987), 50 km south of the Shublik 

Mountains, also have North American affinities (Dutro and 

others, 1972). Middle Cambrian rocks in the Doonerak 

window and at Snowden Mountain yield trilobites with 

Siberian affinities (Palmer and others, 1984). Lower Ordovician strata in the Shublik-Sadlerochit carbonate succession produce trilobites with affinities to the Bathyurid 

Province, which occupied low-paleolatitude sites in North 

America, northeastern Russia, Kazakhstan, and Greenland 

(Blodgett and others, 1986). Early and Late Ordovician 

trilobites from the York Mountains carbonate succession 

have Siberian affinities (Ormiston and Ross, 1976). Late 

Ordovician brachiopods and gastropods with Siberian affinities occur in the York, eastern Baird, and ShublikSadlerochit (meta)carbonate successions (Blodgett and others, 1988; Potter, 1984; Blodgett and others, 1992b). Some 

corals from the York Mountains (Oliver and others, 1975;
54 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

WEST

YORK 

MOUNTAINS

EAST

BAIRD MOUNTAINS 

WESTERN EASTERN

SNOWDEN

MOUNTAIN

AREA

SHUBLIKAND

SADLEROCHIT

MOUNTAINS

MIDDLE DEVONIAN 

TO LATE 

ORDOVICIAN

MIDDLE AND EARLY 

ORDOVICIAN

ym^^m^mmmL

CAMBRIAN AND 

LATE PROTEROZOIC

EXPLANATION

Shallow-water shelf or platform 

carbonate deposits (Cambrian 

and Late Proterozoic)

Shallow-water shelf or platform 

carbonate deposits (Middle 

and Early Ordovician)

Figure 29. Schematic reconstruction of pre-Carboniferous carbonate successions in northern Alaska. During Late Proterozoic 

and Cambrian time, chiefly shallow-water carbonate facies accumulated in the eastern Baird Mountains, Snowden Mountain area, 

and Shublik and Sadlerochit Mountains; rocks of this age have not 

been found in the York Mountains or western Baird Mountains. 

During Early and Middle Ordovician time, chiefly shallow-water

Deep-water, outer shelf, slope, 

or basin deposits (Middle and 

Early Ordovician)

Shallow-water shelf or platform 

carbonate deposits (Middle 

Devonian to Late Ordovician)

carbonate facies were deposited in the York Mountains and 

western Baird Mountains, basinal facies accumulated in the 

eastern Baird Mountains and Snowden Mountain area, and only 

shallow-water carbonate facies formed in the Shublik-Sadlerochit 

Mountains. During Late Ordovician to Middle Devonian time, 

neritic carbonate deposits accumulated across northern Alaska. 

See figure 5 for explanation of symbols.

R.J. Ellas, University of Manitoba, written commun., 

1986) and Snowden Mountain area (T.E. Bolton, written 

commun., 1992) have North American affinities.

Provinciality has also been noted in Ordovician conodont faunas from northern Alaska; species typical of 

WNAMP and SAP have been identified. Lower Ordovician 

strata in the York and western Baird (meta)carbonate successions yield chiefly tropical cosmopolitan (for 

example, Acanthodus lineatus, Variabiloconus bassleri, 

Protopanderodus leei, Rossodus floweri, Scolopodus 

bolites, and "5." gracilis ) and pandemic species (Drepanodus arcuatus, Drepanoistodus forceps, Paltodus subaequalis, and Paroistodus parallelus) together with fewer 

WNAMP (Clavohamulus n. sp., Rossodus n. sp., and

Histiodella n. sp.) and SAP (Fryxellodontusl = Acodinal 

bifida of Abaimova, 1975) elements. Lowermost Ordovician beds in the Shublik and Sadlerochit Mountains yield 

NAMP conodonts (Clavohamulus densus}. Middle Ordovician rocks in all of the northern Alaskan (meta)carbonate 

successions contain chiefly cosmopolitan forms, with the 

exception of some intervals in the western Baird metacarbonate succession; lowermost Middle Ordovician strata in 

this area contain NAMP elements and middle Middle 

Ordovician beds are dominated by SAP species (mostly 

acanthocodinids and stereoconids) (fig. 24). Upper Ordovician strata reveal a relatively complex pattern of conodont 

faunal affinities. Early Late Ordovician collections in 

the eastern Baird, Snowden, and Shublik-Sadlerochit
REGIONAL RELATIONSHIPS 55

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56 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

(meta)carbonate successions are dominated by Siberiannorthern North American forms, whereas latest Ordovician 

faunas in these areas contain WNAMP conodonts (predominantly aphelognathids). Upper Ordovician strata in the 

York Mountains yield WNAMP and SAP conodonts; 

the single known locality of Upper Ordovician rocks 

in the western Baird Mountains produced cosmopolitan 

conodonts.

Specific elements of the paleobiogeographic pattern 

outlined above have been used to infer the presence of 

"exotic" continental fragments in northern Alaska. The 

occurrence of Cambrian trilobite faunas with Siberian 

affinities in the Doonerak-Snowden Mountain area, and 

with North American affinities in the Shublik and Sadlerochit Mountains, has been cited as evidence that the Brooks 

Range is underlain by "Siberian" and "North American" 

crustal fragments (Grantz and others, 1991). The Siberian 

affinities of Ordovician trilobites in the York Mountains led 

Ormiston and Ross (1976) to conclude that the Seward 

Peninsula was part of a Siberia-Kolyma continent during the 

early Paleozoic and was not connected to the rest of Alaska 

and North America until the Mesozoic.

Long-distance translation of tectonic blocks is not the 

only mechanism that can produce "anomalous" faunal 

distributions, however. Modern biogeographic patterns are 

controlled by many variables, including global circulation, 

continental configuration, and larval ecology. Position of 

the paleoequator relative to northern Alaska during the 

Paleozoic is one factor that may have affected faunal 

distributions. The biogeography of the present-day Great 

Barrier Reef demonstrates the importance of continental 

configuration. The reef occurs along the eastern margin of 

the Indian-Australian plate and is oriented roughly perpendicular to the equator; it is more than 2,000 km long and 

extends through 15° of latitude. Distinct biotic zones occur 

within the reef as a direct result of pronounced latitudinal 

variations in temperature and salinity (Davies and others, 

1987). Recent paleogeographic reconstructions (for example, Scotese, 1986) have positioned northern Alaska at a 

moderate angle to the paleoequator during the early Paleozoic, so biotic differentiation in pre-Carboniferous strata 

may reflect, at least in part, latitudinally controlled ecologic 

variation.

Larval ecology also influences biogeography. Many 

modern marine organisms produce pelagic larvae that are 

able to survive long-distance transport by ocean currents; 

such dispersal results in pantropic species distributions. 

Studies of comparative larval morphology suggest that 

some ancient species also had teleplanic larvae (Smith and 

others, 1990). The occurrence of "Tethyan" species in 

Paleozoic faunas of the North American Cordillera may 

reflect pantropic dispersal of certain species rather than 

large-scale tectonic displacements (Newton, 1988). Such 

dispersal mechanisms also may have affected faunal distributions in northern Alaska.

The most precise biogeographic studies compare fossils of the same age and depositional environment and 

consider all elements of a given fauna. The "Siberian" 

trilobites in the central Brooks Range are of Middle Cambrian age, whereas the "North American" trilobites in the 

northeastern Brooks Range are Early and Late Cambrian in 

age. Trilobites of exactly the same age have not been found 

in the two areas, and conclusions based on noncoeval 

species may be misleading. Middle Cambrian trilobites of 

"Siberian" aspect have recently been reported from rocks in 

southwestern Alaska that are considered part of the North 

American continental margin (Babcock and Blodgett, 

1992). The Middle Cambrian may have been a time when 

faunal exchange between Alaska and Siberia was particularly pronounced. Ordovician conodont faunas also show 

temporal variations in biogeographic affinities; in most 

northern Alaskan (meta)carbonate successions, "Siberian" 

faunal elements in lower Upper Ordovician strata give way 

to "North American" faunal elements in upper Upper 

Ordovician rocks. Analogous temporal variations in the 

proportion of "Tethyan" species occur in Paleozoic and 

Mesozoic Cordilleran faunas and are attributed to climaterelated pulses of larval distribution (Newton, 1988).

It is also important to consider the biogeographic 

implications of as many fossil groups as possible within a 

given fauna. Potter (1984) studied brachiopods, corals, and 

trilobites in Upper Ordovician strata of the York Mountains 

and suggested that the biogeographic affinities of the entire 

fauna indicated faunal exchange between the York Mountains area, the North American continent, Chukotka, the 

Siberian platform, and Kazakhstan. The distribution of a 

variety of Late Ordovician brachiopod and gastropod species led Blodgett and others (1992b) to argue that, in early 

Paleozoic time, Arctic Alaska, the Seward Peninsula, and 

Chukotka formed a single tectonic block with faunal ties to 

the Kolyma region.

In our view, the available biogeographic data from 

northern Alaska are best explained by postulating that 

pre-Carboniferous carbonate successions accumulated on a 

single continental margin or platform that had faunal 

exchange with both Siberia and North America. If northern 

Alaska represented a collage of distinct crustal fragments, 

one would expect to find "Siberian" faunas throughout one 

carbonate succession and "North American" faunas 

throughout another. This pattern is not observed. Rather, 

both "Siberian" and "North American" faunal affinities 

occur at different times and in different fossil groups within 

all of the (meta)carbonate successions discussed above. 

"Siberian" influences are particularly noteworthy in Middle 

Cambrian and early Late Ordovician time. This interpretation agrees well with the Phanerozoic plate tectonic reconstructions of Scotese (1986) and Scotese and McKerrow 

(1990) (fig. 30). They treat northern Alaska-Chukotka as a 

discrete continental block that lay between Siberia and 

North America throughout the early Paleozoic. Their recon-
REFERENCES CITED 57

Late Ordovician (Ashgillian)

Figure 30. Global plate tectonic reconstruction for part of Late Ordovician (Ashgillian) time, showing relative proximity of northern 

Alaska, Siberia, and North America (modified from Scotese and McKerrow, 1990).

structions indicate particular proximity between Siberia and 

Alaska during Middle Cambrian and Late Ordovician time.

CONCLUSIONS

Pre-Carboniferous rocks of the Snowden Mountain 

area consist of a dominantly metacarbonate succession of 

Proterozoic(?) through Early or Middle Devonian age that 

has been structurally dismembered and intercalated with 

predominantly metaclastic rocks of chiefly early Late Devonian age.

Ordovician through Silurian rocks constitute the most 

precisely dated and best studied part of this metacarbonate 

succession. Middle Ordovician strata are mainly carbonaceous phyllite, metachert, and allodapic metalimestone 

deposited in a slope or basinal environment; their vertical 

sequence indicates a shallowing-upward depositional 

regime. These strata are succeeded by Upper Ordovician 

and Silurian metacarbonate rocks that were deposited in 

warm, shallow-water environments.

Devonian metaclastic units contain subordinate metalimestone that yields late Givetian(?) and Frasnian conodonts. Thicker limy layers formed as in situ shallow-water 

buildups; thinner layers are mostly calcarenite redeposited 

by storms and (or) turbidity currents.

The metacarbonate succession in the Snowden Mountain area correlates best, lithologically and biostratigraphically, with metacarbonate rocks in the eastern Baird Mountains and also has similarities with (meta)carbonate

successions on the Seward Peninsula and in the western and 

eastern Brooks Range. Detailed comparisons of the lithology, biofacies, and biogeographic affinities of preCarboniferous carbonate successions across northern 

Alaska suggest that these rocks represent a single carbonate 

platform or continental margin dismembered by later tectonic events, rather than a collage of "exotic terranes."

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anticlinorium, central Brooks Range, Alaska, in Hamilton, 

T.D., and Galloway, J.P., eds., Geologic studies in Alaska 

by the U.S. Geological Survey during 1986: U.S. Geological 

Survey Circular 998, p. 40-42.

Robinson, M.S., Decker, John, Clough, J.G., Reifenstuhl, R.R., 

Bakke, Arne, Dillon, J.T., Combellick, R.A., and Rawlinson, S.E., 1989, Geology of the Sadlerochit and Shublik 

Mountains, Arctic National Wildlife Refuge, northeastern 

Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigations 100, scale 1:63,360, 1 sheet.

Ross, C.A., and Ross, J.R.P., 1988, Late Paleozoic transgressiveregressive deposition, in Wilgus, C.K., Hastings, B.S., 

Posamentier, H., Van Wagoner, J., Ross, C.A., and Kendall, C.G. St.C., eds., Sea-level changes—An integrated 

approach: Society of Economic Paleontologists and Mineralogists Special Publication No. 42, p. 227-247.

Rowley, D.B., Lottes, A.L., and Ziegler, A.M., 1985, North 

America-Greenland-Eurasia relative motions—Implications 

for circum-Arctic tectonic reconstructions [abs.]: American 

Association of Petroleum Geologists Bulletin, v. 69, p. 303.

Ryherd, T.J., and Paris, C.E., 1987, Ordovician through Silurian 

carbonate base-of-slope apron sequence, northern Seward 

Peninsula, Alaska [abs.], in Tailleur, I.L., and Weimer, 

Paul, eds., Alaskan North Slope geology: Bakersfield, California, Pacific Section, Society of Economic Paleontologists 

and Mineralogists, book 50, p. 347-348.

Sainsbury, C.L., 1969, Geology and ore deposits of the central 

York Mountains, Seward Peninsula, Alaska: U.S. Geological 

Survey Bulletin 1287, 101 p.
REFERENCES CITED 61

————1972, Geologic map of the Teller quadrangle, western 

Seward Peninsula, Alaska: U.S. Geological Survey Miscellaneous Investigations Map 1-685, 4 p., scale 1:250,000.

Savage, N.M., 1992, Late Devonian (Frasnian and Famennian) 

conodonts from the Wadleigh Limestone, southeastern 

Alaska: Journal of Paleontology, v. 66, p. 277-292.

Scholle, P.A., 1971, Sedimentology of fine-grained deep-water 

carbonate turbidites, Monte Antola Flysch (Upper Cretaceous), Northern Apennines, Italy: Geological Society of 

America Bulletin, v. 82, p. 629-658.

Scotese, C.R., 1986, Phanerozoic reconstructions—A new look at 

the assembly of Asia: University of Texas Institute for 

Geophysics Technical Report No. 66, 54 p.

Scotese, C.R., and McKerrow, W.S., 1990, Revised world maps 

and introduction, in McKerrow, W.S., and Scotese, C.R., 

eds., Palaeozoic palaeogeography and biogeography: Geological Society of London Memoir No. 12, p. 1-21.

Shinn, E.A., 1983, Tidal flat, in Scholle, P.A., Bebout, D.G., 

and Moore, C.H., eds., Carbonate depositional environments: American Association of Petroleum Geologists Memoir 33, p. 171-210.

Smith, P.L., Westermann, G.E.G., Stanley, G.D., Yancey, T.E., 

and Newton, C.R., 1990, Paleobiogeography of the ancient 

Pacific: Science, v. 249, p. 680-683.

Sweeney, J.F., 1982, Mid-Paleozoic travels of Arctic-Alaska: 

Nature, v. 298, p. 647-649.

Sweet, W.C., and Bergstrom, S.M., eds., 1971, Symposium on 

conodont biostratigraphy: Geological Society of America 

Memoir 127, 499 p.

Tailleur, I.L., Brosge, W.P., and Reiser, H.N., 1967, Palinspastic analysis of Devonian rocks in northwestern Alaska, in 

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13, no. 1.

Till, A.B., and Dumoulin, J.A., in press, Seward Peninsula: The 

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of Alaska: Geological Society of America Decade of North 

American Geology.

Till, A.B., Dumoulin, J.A., Gamble, B.M., Kaufman, D.S., and 

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Till, A.B., and Moore, T.E., 1991, Tectonic relations of the schist 

belt, southern Brooks Range, Alaska [abs.]: American Geophysical Union 1991 Fall Meeting: American Geophysical 

Union Program and Abstracts, p. 295-296.

Till, A.B., Schmidt, J.M., and Nelson, S.W., 1988, Thrust 

involvement of metamorphic rocks, southwestern Brooks 

Range, Alaska: Geology, v. 16, p. 930-933.

Vandervoort, D.J., 1985, Stratigraphy, paleoenvironment, and 

diagenesis of the Lower Ordovician York Mountain carbonates, Seward Peninsula, Alaska: Baton Rouge, Louisiana 

State University, M.S. thesis, 141 p.

Williams, Harold, and Hatcher, R.D., Jr., 1983, Appalachian 

suspect terranes, in Hatcher, R.D., Jr., Williams, Harold, 

and Zietz, Isodore, eds., Contributions to the tectonics and 

geophysics of mountain chains: Geological Society of America, Memoir 158, p. 33-53.

Wilson, J.L., 1975, Carbonate facies in geologic history: New 

York, Springer-Verlag, 470 p.

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P.A., Bebout, D.G., and Moore, C.H., eds., Carbonate 

depositional environments: American Association of Petroleum Geologists Memoir 33, p. 297-344.
62 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

Appendix 1. Locality register for key faunal components and lithologic features.

[Named taxa are conodonts and were identified by A.G. Harris, unless otherwise noted. See figures 2 and 3 for geologic and geographic location]

Map no. 

(map unit)

KDh)

(Dh)

2(Dh)

3(Db)

4(Dn)

5(Dn)

6 (SOm)

(SOm)

(Devonian limestone outcrop 

too small to 

show on map)

7 (SOm and 

Devonian? limestone)

Field no. 1 

(USGS colln. no.)

90ABd37A 

(12067-SD)

90ABd37B

90ABd34 

(12065-SD)

90TM529A 

(12082-SD)

84DN199 

(11083-SD)

90TM453B 

(12076-SD)

90AD20G 

(10828-CO)

90AD20H

90ABd31

(12064-SD)

89ATi75

Latitude N./ 

Longitude W.

68°06.97149°32.5'

68°06.97149°32.5'

68°06.87149°32.2'

68°04.1'/149°12.3'

68°00.1'/149°34.5'

67°59.2'/149°21.7'

67°54.87149°31.4'

67°54.87149°31.4'

67°54.87149°31.4'

67°53.47149°32.7'

Key faunal components and lithologies

Skeletal wacke-packstone 

Polygnathus evidens (pi. 3, fig. 12) 

Polygnathus of the Po. xylus group

Oolitic grainstone interbedded with 

90ABd37A.

Quartzose skeletal grainstone 

Polygnathus pacificus (pi. 3, fig. 13) 

Polygnathus aff. Po. planarius 

Polygnathus samueli (pi. 3, fig. 14)

Skeletal grainstone 

Ancyrodella nodosa (pi. 3, fig. 8) 

Polygnathus aff. Po. pacificus

Crinoidal marble 

Ancyrodella gigas 

Ancyrodella lobata (pi. 3, fig. 19) 

Ancyrognathus aff. A. triangularis 

(pi. 3, fig. 21). 

Icriodus symmetricus (pi. 3, fig. 1)

Intraclast-skeletal packstone 

Ancyrodella sp. 

Ancyrognathus cf. A. coeni (pi. 3, fig. 

20). 

Icriodus sp. 

Palmatolepis plana (pi. 3, fig. 16) 

Palmatolepis proversa (pi. 3, figs. 17, 

18).

Peloidal-skeletal packstone 

"Belodina" spp. 

Pander odus spp. 

Phragmodus n. sp. (= Ph. new species 

of Barnes, 1974). 

Plectodinal cf. PL? dolboricus

Coralline wackestone 

Coral: Catenipora sp. aff. C. rubra2

Skeletal pack-wackestone (containing 

probable amphiporid stromatoporids) 

overlying 90AD20H. 

Panderodus sp. 

Ozarkodina of Silurian-Middle Devonian morphotype. 

Echinoderms: two-hole crinoid ossicles3

Bioturbated dolomitic metalimestone 

(SOm) overlain by skeletal wackestone containing probable amphiporid 

stromatoporoids; wackestone is lithologically similar to 90ABd3 1 and 

may be Devonian.

Fossil age

early Late Devonian (Frasnian)

early Late Devonian (Frasnian, 

probably late Frasnian).

early Late Devonian (middle to late 

Frasnian; Upper hassi Zone to 

linguiformis Zone).

early Late Devonian (middle 

Frasnian; Upper hassi Zone to 

Lower rhenana Zone).

early Late Devonian (middle 

Frasnian; upper part of Lower 

hassi Zone).

early Late Ordovician (Edenian to 

Maysvillian).

probably early Late Ordovician

late Early to early Middle 

Devonian (Emsian to Eifelian).

CAI

5

5

5-5.5

5.5

5

5

'Samples collected by R.B. Blodgett (ABd), J.A. Dumoulin (AD), A.G. Harris (7-84), T.E. Moore (TM), George Plafker (APr), and A.B. Till (ATi), 

all U.S. Geological Survey; J.T. Dillon (DN) and D.N. Solie (DNS), State of Alaska Division of Geological & Geophysical Surveys. 

Identified by T.E. Bolton, Geological Survey of Canada. 

Identified by R.B. Blodgett, U.S. Geological Survey.
APPENDIX 63

Appendix 1.—Continued

Map no. 

(map unit)

8 (SOm)

(Devonian? 

limestone)

9 (SOm)

(SOm)

10 (SOm)

11 (Os, SOm)

12 (SOm)

(Dn)

13 (SOm)

14 (SOm and 

Devonian? 

limestone)

15 (Db)

Field no. 1 

(USGS colln. no.)

84DNS110 

(10583-CO) 

89AD39

89AD39D

84DNS109A 

(10582-CO)

89AD42 

(11964-SD)

89AD41A 

(10725-CO)

89AD40

90AD21AA 

(10831-CO)

90AD21E 

(12071-SD)

89APrl70 

(10727-CO)

89AD43 

90ABd28

89ATi71A 

(11969-SD)

Latitude N./ 

Longitude W.

67°52.7'/149°41.9'

67°52.67149°42.3'

67°52.7'/149°41.4'

67°52.87149°41.6'

67°52.5'/149°41.3'

67°52.6'/149°42.3'

67°52.27149°35.7'

67°52.2'/149°35.7'

67°51.3'/149°46.5'

67°51.17149°49.2'

67°50.87149°32.1'

Key faunal components and lithologies

Fossiliferous dolostone 

"Belodina" sp. 

Phragmodus n. sp. (= Ph. new species 

of Barnes, 1974).

Skeletal wackestone containing probable amphiporid stromatoporoids overlies unit SOm; wackestone is lithologically similar to 90ABd31 and 

may be Devonian.

Crinoidal marble 

Culumbodinal cf. C. occidentalis (pi. 

2, fig. 19). 

Pander odus sp. 

Phragmodus n. sp. (= Ph. new species 

of Barnes, 1974) (pi. 2, fig. 22). 

Pseudobelodina dispansa 

Pseudobelodina inclinata transitional to 

Ps. v. vulgaris (pi. 2, figs. 17, 18)

Peloidal dolostone, structurally underlies 84DNS109A. 

Icriodellal sp. indet. 

Ozarkodina aff. O. oldhamensis (pi. 2, 

figs. 7-10). 

Oulodusl n. sp. (pi. 2, figs. 11-14) 

Panderodus sp.

Parallel-laminated metalimestone 

Phragmodus n. sp. (= Ph. new species 

of Barnes, 1974) (pi. 2, figs. 23, 

24).

Units Os and SOm in apparent depositional contact at this locality.

Algal-laminated dolostone 

Aphelognathus aff. A. diver gens (pi. 2, 

fig. 16).

Sooty metalimestone in fault contact 

with 90AD21AA. 

Polygnathus aff. Po. alatus 

Polygnathus aff. Po. planarius

Fossiliferous dolostone 

Panderodus sp. (pi. 2, fig. 15) 

Phragmodus n. sp. (= Ph. new species 

of Barnes, 1974) (pi. 2, figs. 20, 

21).

Fossiliferous dolostone (SOm) overlain 

by skeletal wackestone containing 

probable amphiporid stromatoporoids; wackestone is lithologically 

similar to 90ABd31 and may be 

Devonian.

Lime mudstone with sparse bioclasts 

Icriodus sp. indet. 

Pandorinellinal cf. P. insita 

Polygnathus spp. indet.

Fossil age

early Late Ordovician (Edenian to 

Maysvillian).

early Late Ordovician (Edenian to 

Maysvillian).

very latest Ordovician to Early 

Silurian; very probably Early 

Silurian (Llandoverian).

early Late Ordovician (Edenian to

Maysvillian).

late Late Ordovician (Richmondian)

early Late Devonian (Frasnian)

early Late Ordovician (Edenian to 

Maysvillian).

latest Middle to earliest Late 

Devonian (latest Givetian to early 

Frasnian).

CAI

5.5

5.5

5-5.5

5

5

5

5

5
64 PRE-CARBOMFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

Appendix 1.—Continued

Map no. 

(map unit)

16 (SOm)

17 (SOm)

18 (Os)

(Os)

19 (Os)

20 (Cambrian

Snowden

Mountain unit)

Field no. 1 

(USGS colln. no.)

89AD36E

(11963-SD)

89ATi74A

(11970-SD)

90AD25A

(10826-CO)

90AD25B

(10827-CO)

90TM448

(1085 1-CO)

83DNS300

89AD32

Latitude N./ 

Longitude W.

67°49.5'/149°39.1'

67°49.67149°36.5'

67°49.1'/149°34.8'

67°49.1'/149°34.8'

67°49.5'/149°32.8'

67°48.17149°43.9'

Key faunal components and lithologies

Coralline metalimestone

Icriodellal sp. indet. (pi. 2, fig. 4)

Oulodus! sp. indet.

Ozarkodina cf. O. cadiaensis (pi. 2,

fig. 3).

Pander odus sp.

Pterospathodus aff. P. cadiaensis (pi.

2, figs. 1, 2).

Bioturbated, fossiliferous dolostone

Oulodusl sp. indet.

Pander odus sp.

Pterospathodus! n. sp.

Calcareous turbidite

Drepanodus arcuatus (pi. 1, fig. 20)

Periodon flabellum (pi. 1, fig. 17)

Protopanderodus aff. P. graeai (pi. 1,

fig. 19).

Tripodus laevis

Redeposited Late Cambrian and (or)

Early Ordovician conodonts:

Cordylodus aff. C. proavus

Cordylodus cf. C. intermedius

Eoconodontus notchpeakensis

Phakelodus tenuis (pi. 1, fig. 22)

Teridontus nakamurai

Variabiloconus bassleri

Calcareous turbidite

Drepanodus arcuatus

Paroistodus sp.

Tripodus laevis (pi. 1, fig. 18)

Redeposited Late Cambrian and (or)

Early Ordovician conodonts:

Cordylodus sp. indet. (pi. 1, fig. 21)

Eoconodontus notchpeakensis

Furnishina sp.

"Oistodus" triangularis

Phakelodus tenuis

Rossodus manitouensis (pi. 1, fig.

23)

"Scolopodus" gracilis (pi. 1, fig. 24)

Teridontus nakamurai

Calcareous turbidite

Eoplacognathus elongatus transitional

to Polyplacognathus sp. (pi. 1, figs.

1,2).

Erraticodon balticus (pi. 1, figs. 13,

14).

Protopanderodus varicostatus (pi. 1 ,

fig. 3).

Pygodus anserinus (pi. 1, fig. 9)

Spinodus ramosus

Skeletal wacke/packstone

Trilobites:

Chondranomocare cf. C. speciosum

Kounamkites cf . K. frequens4

Fossil age

Early Silurian (Llandoverian to early

Wenlockian).

Early Silurian (Llandoverian to early

Wenlockian).

very earliest Middle Ordovician (latest

Arenigian) containing redeposited

conodonts of Late Cambrian and (or)

Early Ordovician age.

very earliest Middle Ordovician (latest

Arenigian) with redeposited conodonts of Late Cambrian and (or)

Early Ordovician age.

middle Middle Ordovician (latest

Llandeilian to earliest Caradocian;

Baltoniodus variabilis Subzone to

Lower Baltoniodus gerdae Subzone).

early Middle Cambrian

CAI

5

5

5

5

5

Identified by A.R. Palmer (in Palmer and others, 1984).
APPENDIX 65

Appendix 1.—Continued

Map no. 

(map unit)

21 (fts)

22 (Os)

23 (fts)

24 (Os)

(Os)

(fts)

25 (Os)

26 (Db)

27 (Db)

28 (Os)

29 (Os)

Field no. 1 

(USGS colln. no.)

7-27-84C

7-3G-84A 

(9911-CO) 

89AD24

84DN249

7-27-84H 

(9908-CO)

7-27-84G 

(9909-CO)

7-27-84J 

(9905-CO)

7-30-84E 

(9913-CO) 

89AD28

84DN190 

(11082-SD)

89APrl46 

(11966-SD)

89AD29Z 

(10724-CO)

89AD27

Latitude N./ 

Longitude W.

67°48.07149°44.0'

67°47.07149°44.5'

67°47.47149°41.5'

67°47.1'/149°38.5'

67°47.1'/149°38.5'

67°47.17149°38.5'

67°45.67149°37.0'

67°45.67149°51.7'

67°44.77149°54.3'

67°44.87149°41.8'

67°44.57149°38.7'

Key faunal components and lithologies

Massive dolostone 

Indeterminate conodont fragment

Carbonaceous metalimestone 

Pander odus sp. 

Periodon aculeatus 

Prattognathus rutriformis (pi. 1, fig. 6) 

Protopanderodus varicostatus 

Pygodus sp. indet. 

Spinodus ramosus (pi. 1, fig. 7) 

Section also contains phyllite, dolostone with radiolarian ghosts, and 

metasandstone.

Marble 

Ozarkodinid of Silurian-Mississippian 

morphotype.

Carbonaceous phyllitic metalimestone 

Panderodus(1) sp. 

Periodon aculeatus (pi. 1, fig. 8) 

Protopanderodus varicostatus (pi. 1 , 

fig. 4).

Carbonaceous metalimestone, about 20 

m above 7-27-84H. 

Panderodus sp. indet. 

Periodon sp. indet. 

Protopanderodus varicostatus

Massive metalimestone, about 42 m 

below 7-27-84H. 

Coniform element of probable middle 

Arenigian-Llanvirnian age.

Carbonaceous metalimestone 

Periodon aculeatus 

Protopanderodus varicostatus 

Pygodus anserinus (pi. 1, fig. 5)

Platy gray marble 

Icriodus sp. indet. 

Pandorinellina insita (pi. 3, fig. 2) 

Polygnathus xylus xylus 

Polygnathus cf. Po. linguiformis

Lime mudstone with sparse bioclasts 

Icriodus sp. of Middle to Late Devonian morphotype. 

Pandorinellina aff. P. insita (pi. 3, 

figs. 3, 4). 

Polygnathus spp. of late Middle to 

early Late Devonian morphotype.

Graded crinoidal grainstone (calcareous 

turbidite). 

"Belodina" spp. 

Dapsilodusl similaris 

Protopanderodus aff. P. varicostatus 

Scalpellodusl sp. 

Section also contains metachert with 

radiolarian ghosts, carbonaceous metalimestone, and massive marble.

Section contains metachert with radiolarian ghosts, impure carbonate turbidites, and phyllite.

Fossil age

Ordovician to Triassic

middle Middle Ordovician (latest Llanvirnian to earliest Llandeilian; upper 

Pygodus serra Zone to lower Pygodus anserinus Zone).

Silurian to Mississippian

middle Middle Ordovician (Llanvirnian 

to Llandeilian).

middle Middle Ordovician (Llanvirnian 

to Llandeilian).

probably late Early through early Middle Ordovician (middle Arenigian to 

Llanvirnian).

middle Middle Ordovician (latest Llanvirnian to earliest Caradocian; Pygodus anserinus Zone to lower Baltoniodus gerdae Subzone).

latest Middle-earliest Late Devonian 

(very latest Givetian to earliest 

Frasnian).

latest Middle to earliest Late Devonian 

(very latest Givetian to early 

Frasnian).

middle Middle Ordovician (Llanvirnian 

to Llandeilian).

CAI

6

5

5.5

5-5.5

5

5.5

5

5-5.5

5

5
66 PRE-CARBONIFEROUS METACARBONATE ROCKS, NORTHERN ALASKA

Appendix 1.—Continued

Map no. 

(map unit)

30 (Os)

31 (Os)

32 (Os)

33 (Db)

34 (SOm)

35 (Os)

36 (Os)

37 (ftBd)

Not shown on 

fig. 3; sample 

taken about 30 

km east of the 

study area (Db)

Not shown on 

fig. 3; sample 

from east of 

map area (Dh)

Field no. 1 

(USGS colln. no.)

89TM274B 

(10729-CO) 

89AD30

90AD26B 

(10834-CO)

89APrl44 

(10726-CO)

89TM240B 

(11971-SD)

90AD2A 

(12069-SD)

89ATil8A 

(10728-CO)

89AD20A

(10722-CO)

89 ADS 1

90TM476A 

(12078-SD)

90TM574A 

(12084-SD)

Latitude N./ 

Longitude W.

67°43.9'/149°38.r

67°42.87149°47.7'

67°42.3'/149°52.0'

67°40.47149°58.0'

67°39.5'/149°23.7'

67°34.9'/150°08.2'

67°33.9'/149°41.2'

67°41.0'/149°33.0'

67°59.0'/148°33.7'

68°13.7'/148°58.5'

Key faunal components and lithologies

Impure calcareous turbidites 

Periodon aculeatus 

Protopanderodus aff. P. n. sp. A of 

McCracken (1989).

Dolostone with radiolarian ghosts (lens 

in metachert). 

Dapsilodus! similaris 

Pander odus sp. 

Periodon aculeatus 

Spinodus ramosus

Calcareous turbidite 

Pygodus! sp. indet. 

New genus, new species of Potter 

(1975) (pi. 1, fig. 16).

Lime mudstone with sparse bioclasts 

Icriodus sp. indet. 

Pandorinellina sp. indet. 

Polygnathus sp. of late Middle to early 

Late Devonian morphotype.

Bioturbated skeletal wackestone 

Kockelella sp. indet. (pi. 2, fig. 5) 

Panderodus sp. 

Pelekysgnathus sp. indet. (pi. 2, fig. 6) 

Section also contains peloidal 

grainstone.

Dolostone with radiolarian ghosts 

Belodina sp. indet. 

Dapsilodus! similaris 

Panderodus sp. 

Periodon aculeatus (pi. 1, fig. 15) 

Protopanderodus varicostatus 

Pygodus serra (pi. 1, figs. 10-12) 

Spinodus ramosus

Carbonaceous metalimestone couplets 

(calcareous turbidites). 

Juanognathus! sp. indet. or Rossodus! 

sp. indet. 

Periodon! sp. indet.

Dolostone with coated grains

Skeletal-peloidal wacke-packstone 

Ancyrodella gigas (pi. 3, fig. 7) 

Icriodus sp. 

Klapperina ovalis (pi. 3, fig. 10) 

Mesotaxis falsiovalis (pi. 3, figs. 5, 6) 

Polygnathus aequalis (pi. 3, fig. 11) 

Polygnathus webbi (pi. 3, fig. 9).

Icriodus sp. 

Palmatolepis plana (pi. 3, fig. 15) 

Polygnathus aff. Po. pacificus

Fossil age

early Middle Ordovician (Pygodus 

serra Zone possibly into Pygodus 

anserinus Zone).

early through middle Middle 

Ordovician (Llanvimian to early 

Caradocian).

late Early to earliest Middle Ordovician 

(late Arenigian).

latest Middle to early Late Devonian 

(latest Givetian to early Frasnian).

late Early to early Late Silurian (Wenlockian to Ludlovian).

early Middle Ordovician (late Llanvirnian; Pygodus serra Zone).

Ordovician, possibly Early to earliest 

Middle Ordovician (probably 

Arenigian to Caradocian; possibly 

Arenigian).

early Late Devonian (early to middle 

Frasnian; upper transitans Zone into 

lower Upper hassi Zone).

early Late Devonian (middle to late 

Frasnian; uppermost Lower hassi 

Zone to Upper rhenana Zone).

CAI

5

5

5

5

5-5.5

5

5

5

5
PLATES 1-3
PLATE 1

Ordovician conodonts from the Snowden Creek unit

Figures 1, 2. Eoplacognathus elongatus (Bergstrom) transitional to Polyplacognathus sp., upper views 

of stelliplanate elements, X40, USGS colln. 10851-CO (fig. 3, loc. 19), USNM 475602, 

03. The stelliplanate element of this species is similar to Polyplacognathus ringerikensis 

Hamar from Norway; the Alaskan form, however, appears to have a better developed 

secondary lobe on the posterior processes. Process geometry of the Alaskan form is nearly 

identical to that of Polyplacognathus ramosus, a younger species, but P. ramosus has 

prominently crenulate margins, whereas the Alaskan form has relatively smooth margins. 

No pastiplanate elements were found in the only collection in which this transition form 

occurs. 

3, 4. Protopanderodus varicostatus (Sweet and Bergstrom)

3. Inner lateral view, X30, USGS colln. 10851-CO (fig. 3, loc. 19), USNM 475604.

4. Outer lateral view, X45, USGS colln. 9908-CO (fig. 3, loc. 24), USNM 413584

(= fig. 3L of Dillon and others, 1987a). 

5,9. Pygodus anserinus Lament and Lindstrom, upper views of P elements.

5. X100, USGS colln. 9913-CO (fig. 3, loc. 25), USNM 413583 (= fig. 3K of Dillon

and others, 1987a). 

9. X60, USGS colln. 10851-CO (fig. 3, loc. 19), USNM 475605.

6. Prattognathus rutriformis (Sweet and Bergstrom), upper view of stelliplanate element, 

X60, USGS colln. 9911-CO (fig. 3, loc. 22), USNM 413580 (= fig. 3G of Dillon and 

others, 1987a).

7. Spinodus ramosus (Hadding), lateral view of Sa? element, X65, USGS colln. 9911-CO

(fig. 3, loc. 22), USNM 413579 (= fig. 3F of Dillon and others, 1987a). 

8, 15. Periodon aculeatus Hadding, inner and outer lateral views of Sc and M elements, X105 

and X50, USGS colln. 9908-CO (fig. 3, loc. 24), USNM 413576 (= fig. 3C of Dillon 

and others, 1987a) and USGS colln. 10728-CO (fig. 3, loc. 35), USNM 475606. 

10-12. Pygodus serra (Hadding), upper views of two P elements and lateral view of Sa

element, X40, USGS colln. 10728-CO (fig. 3, loc. 35), USNM 475607^75609. 

13, 14. Erraticodon balticus Dzik, outer and inner lateral views of M and Sc elements, X50, 

USGS colln. 10851-CO (fig. 3, loc. 19), USNM 475610, 475611.

16. New genus, new species (= Persian slipper-shaped elements of Potter, 1975), posterior? 

view, X60, USGS colln. 10726-CO (fig. 3, loc. 32), USNM 475612.

17. Periodon flabellum (Lindstrom), inner lateral view of Sb element, xlOO, USGS colln.

10826-CO (fig. 3, loc. 18), USNM 475613.

18. Tripodus laevis Bradshaw, oblique posterior view of Sd element, X90, USGS colln.

10827-CO (fig. 3, loc. 18), USNM 475614.

19. Protopanderodus aff. P. graeai (Hamar), lateral view, X30, USGS colln. 10826-CO 

(fig. 3, loc. 18), USNM 475615.

20. Drepanodus arcuatus Pander, inner lateral view, X60, USGS colln. 10826-CO (fig. 3,

loc. 18), USNM 475616.

21-24. Redeposited Late Cambrian and (or) Early Ordovician conodonts, fig. 21, X90, figs. 

22-24, X80; figs. 21, 23, and 24 from USGS colln. 10827-CO, fig. 22 from USGS 

colln. 10826-CO (fig. 3, loc. 18).

21. Cordylodus sp. indet., lateral view of Sc element, USNM 475617.

22. Phakelodus tenuis (Miiller), lateral view, USNM 475618.

23. Rossodus manitouensis Repetski and Ethington, inner lateral view, USNM 475619.

24. "Scolopodus" gracilis Ethington and Clark, oblique posterior view, USNM 475620.
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1545 PLATE 1

ORDOVICIAN CONODONTS FROM THE SNOWDEN CREEK UNIT
PLATE 2

Late Ordovician and Silurian conodonts from the Mathews River unit

Figures 1, 2. Pterospathodus aff. P. cadiaensis Bischoff, lateral and upper views of Pa and Pb 

elements, X80, USGS colln. 11963-SD (fig. 3, loc. 16), USNM 475621, 475622.

3. Ozarkodina cf. O. cadiaensis Bischoff, oblique inner lateral view of Pa element, X80, 

USGS colln. 11963-SD (fig. 3, loc. 16), USNM 475623.

4. Icriodellal sp. indet., upper view of Pa element fragment, X40, USGS colln. 11963-SD 

(fig. 3, loc. 16), USNM 475624.

5. Kockelella sp. indet., inner lateral view of M element, X60, USGS colln. 12069-SD (fig. 

3, loc. 34), USNM 475625.

6. Pelekysgnathus sp. indet., posterior view of coniform element, X60, USGS colln.

12069-SD (fig. 3, loc. 34), USNM 475626. 

7-10. Ozarkodina aff. O. oldhamensis (Rexroad), lateral views of Pa, Pb, M, and Sb elements,

x 120, USGS colln. 11964-SD (fig. 3, loc. 9), USNM 475627-475630. 

11-14. Oulodusl n. sp., lateral views of Pa?, Pbx , Pb2?, and M elements, X60, USGS colln. 

11964-SD (fig. 3, loc. 9), USNM 475631-475634.

15. Panderodus sp., fused cluster of at least 10 elements, X50, USGS colln. 10727-CO (fig. 

3, loc. 13), USNM 475635.

16. Aphelognathus aff. A. divergens Sweet, inner lateral view of Sb element, X55, USGS

colln. 10831-CO (fig. 3, loc. 12), USNM 484395.

17, 18. Pseudobelodina inclinata (Branson & Mehl)? transitional to Ps. vulgaris vulgaris Sweet, 

lateral views, X50, USGS colln. 10582-CO (fig. 3, loc. 9), USNM 475636, 475637. 

These specimens have more denticles than Ps. v. vulgaris but fewer than Ps. inclinata 

and probably represent a new species. 

19. Culumbodina! cf. C. occidentalis Sweet, outer lateral view, X90, USGS colln.

10582-CO (fig. 3, loc. 9), USNM 475638. 

20-24. Phragmodus n. sp. (= Phragmodus n. sp. of Barnes, 1974), x60.

20, 21. Anterior and lateral views of P and M elements, USGS colln. 10727-CO (fig. 3,

loc. 13), USNM 475639, 475640. 

22. Inner lateral view of Sb element, USGS colln. 10582-CO (fig. 3, loc. 9),

USNM 475641.

23, 24. Inner lateral views of Sb and Sc elements, USGS colln. 10725-CO (fig. 3, loc. 

10), USNM 475642, 475643.
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1545 PLATE 2

LATE ORDOVICIAN AND SILURIAN CONODONTS FROM THE MATHEWS RIVER UNIT
PLATE 3

Latest Givetian and Frasnian conodonts from the Beaucoup Formation, Hunt Fork Shale, and

Nutirwik Creek unit 

[All specimens are Pa elements]

Figure 1. Icriodus symmetricus Branson and Mehl, upper view, X30, USGS colln. 11083-SD (fig.

3, loc. 4), USNM 475644. 

2. Pandorinellina insita (Stauffer), outer lateral view, X60, USGS colln. 11082-SD (fig. 3,

loc. 26), USNM 475645. 

3, 4. Pandorinellina aff. P. insita (Stauffer), outer lateral views, X60, USGS colln. 11966-SD

(fig. 3, loc. 27), USNM 475646, 475647.

5, 6. Mesotaxis falsiovalis Sandberg, Ziegler, and Bultynck, upper and lower views of Pa 

elements, X50, USGS colln. 12078-SD (sample listed near end of app. 1), USNM 

475648, 475649. 

7, 9-11. From USGS colln. 12078-SD (sample listed near end of app. 1).

7. Ancyrodella gigas Youngquist, upper view, X30, USNM 475650.

9. Polygnathus webbi Stauffer, upper view, x30, USNM 475652.

10. Klapperina ovalis (Ziegler and Klapper), lower view, X40, USNM 475653.

11. Polygnathus aequalis Klapper and Lane, upper view, X30, USNM 475654. 

8. Ancyrodella nodosa Ulrich and Bassler, upper view, X30, USGS colln. 12082-SD (fig. 

3, loc. 3), USNM 475651.

12. Polygnathus evidens Klapper and Lane, upper view, X30, USGS colln. 12067-SD (fig. 

3, loc. 1), USNM 475655.

13. Polygnathus pacificus Savage and Funai, upper view, X30, USGS colln. 12065-SD (fig. 

3, loc. 2), USNM 475656.

14. Polygnathus samueli Klapper and Lane, upper view, X30, USGS colln. 12065-SD (fig.

3, loc. 2), USNM 475657. 

15, 16. Palmatolepis plana Ziegler and Sandberg, upper views, x30, USGS colln. 12084-SD

(sample listed at end of app. 1), 12076^SD (fig. 3, loc. 5), USNM 475658, 475659. 

17, 18. Palmatolepis proversa Ziegler, upper views, X30, USGS colln. 12076-SD (fig. 3, loc.

5), USNM 475660, 475661.

19. Ancyrodella lobata Branson and Mehl, upper view, X30, USGS colln. 11083-SD (fig. 3, 

loc. 4), USNM 475662.

20. Ancyrognathus cf. A. coeni Klapper, upper view, x30, USGS colln. 12076-SD (fig. 3, 

loc. 5), USNM 475663.

21. Ancyrognathus aff. A. triangularis Youngquist, upper view, X30, USGS colln. 

11083-SD (fig. 3, loc. 4), USNM 475664.
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1545 PLATE 3

LATEST GIVETIAN AND FRASNIAN CONODONTS FROM THE BEAUCOUP FORMATION, 

HUNT FORK SHALE, AND NUTIRWIK CREEK UNIT