Fossil evidence for fault-derived stratigraphic repetition in the northeastern Newfoundland Appalachians

BEN A. VAN DER PLUIJM Department of Geological Sciences, University of Michigan, 1006 C. C. Little Building, Ann Arbor, MI 48109 U.S.A.

KARL E. KARLSTROM Department of Geology, Northern Arizona University, Flagstaff, A2 8601 1, U.S. A.

AND

PAUL F. WILLIAMS Department of Geology, University of New Brunswick, Fredericton, N.B., Canada E3B 5A3

Received September 26, 1986

Revision accepted March 9, 1987

Two types of bedding-parallel faults are common in the Dunnage Zone of eastern Notre Dame Bay. They are (i) early thrusts, which together with bedding were rotated by later folding into steep attitudes; and (ii) postfolding transcurrent faults. Both types of faults occur at all scales and give rise to repetitions in the stratigraphic sequence.

A compilation of the ages of various rock types based on fossil evidence indicates a relatively simple stratigraphy for the Dunnage Zone. This interpretation appears to be at variance with many reported stratigraphic successions, until repetitions by bedding-parallel faulting are taken into account.

Examples are given of sequences in which the repetition of various rock types, dated by means of fossils, is due to bedding- parallel faulting. We believe that the simple lithostratigraphy is a reliable aid for structural interpretations in areas where fossils are scarce or absent.

Deux types de failles parallkles i la stratification apparaissent frkquemment dans la zone de Dunnage de la partie orientale de la baie Notre-Dame. Ce sont ( i ) d'anciens chevauchements, qui avec la stratification ont Ctk bascults en position subvexticale par un kvknement ultkrieur de plissement; et (ii) des dkcrochements postkrieurs B la dkfomation. Les deux types de failles prksentent des dkplacements a toutes les kchelles, et des kpktitions dans la skquence stratigraphique sont apparues.

Une compilation des ages des diffkrentes variktks de roches, fond& sur les fossiles, kv t le que dans la zone de Dunnage la stratigraphie est relativement simple. Cette interpktation d'une stratigraphie simple apparait contraire aux versions proposkes pour plusieurs autres skquences stratigraphiques avant de connaitre les rkpktitions par ces failles parallkles B la stratification.

Des exemples sont dCcrits de skquences dans lesquelles il y a eu kpktition causCe par des failles paralltles i la stratification de diffkrentes roches datkes au moyen de fossiles. Nous croyons qu'une lithostratigraphie simple peut constituer une aide valable dans les interpktations structurales pour les kgions oh les fossiles sont peu abondants ou absents.

[Traduit par la rewe]

Can. J. Earth Sci. 24, 2337-2350 (1987)

Introduction In early efforts to unravel the complex stratigraphy of New-

foundland's northeastern Dunnage Zone (Fig. I), Kay and Williams (1963) and Williams (1964) suggested that the only unambiguous way to reconstruct stratigraphy and help unravel structure is through use of fossil evidence. However, despite intense geologic investigations in the northeastern part of New- foundland over the past two decades, no comprehensive com- pilation of fossil localities has been published in the literature. Instead, data on fossil ages are scattered in at least 38 references (Table 1). One of the principal aims of this paper is to present a compilation of fossil localities in the Dunnage and Gander zones that were published up to 1985. We feel such a compilation will be useful to a wide variety of workers in Appalachian geology both inside and outside Canada. We also wish to draw attention to the fact that structural observations indicate an abundance of bedding-parallel faults, which can only be recognized under special conditions. The result is that every bedding plane has to be considered a possible fault. It follows, therefore, that no sequence of rocks in the area can be assumed to be an unmodified stratigraphic sequence. This con- clusion, combined with an examination of the compiled fossil data, raises some interesting questions regarding tectonic inter- pretations of the Central Mobile Belt (Fig. 1).

Our compilation and tabulation of fossil localities suggest a simple chronostratigraphy and lithostratigraphy for the entire Dunnage Zone, assuming that the more than 200 known fossil localities are a representative sample for Dunnage Zone stra- tigraphy. The idea of a simple regional stratigraphy is not new. Dean (1978) and Kean et al. (1981) proposed similar stratig- raphies for the zone, although they were unable to reconcile the simple stratigraphy with the conventional view of structure in some places within the Dunnage Zone. In view of increasing evidence for bedding-parallel faulting and complex deforma- tion within the zone (Karlstrom et al. 1982; Karlstrom 1985; van der Pluijm 1986), we would like to suggest (i) that a simple regional stratigraphy (modified from Kean et al. 1981) is well enough established that attempts should be made to apply it in areas where fossils are scarce or absent in order to reach a first-order understanding of structural geometry and tectonic history; and (ii) that this stratigraphy suggests that the northeastern Central Mobile Belt can be viewed as a single paleotectonic element that was deformed in early to middle Paleozoic times (see also van der Pluijm 1987). The latter is in contrast to some tectonic interpretations that view this part of the northern Appalachians as a composite of originally sepa- rate terranes (e.g., McKerrow and Cocks 1977; Cume et al. 1980).

Printed in Canada / Imp'iim6 au Canada

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2338 CAN. J . EARTH SCI. VOL. 24, 1987

FIG. 1. Map of Newfoundland showing the location of the Dunnage and Gander zones of the Central Mobile Belt (dotted) and the area covered in this paper. Zone boundaries after Williams (1979) and Kean et al. (1981); see also van der Pluijm (1987).

Regional setting of the Dunnage Zone

The Dunnage Zone of Newfoundland is a zone of mainly marine volcanogenic and volcaniclastic rocks that reaches a width of 150 km along the well-exposed coast of Notre Dame Bay in northeastern Newfoundland (Fig. I). These rocks lie between the Humber Zone to the west, with Grenville-age North American basement overlain by Cambro-Ordovician miogeoclinal deposits, and the Gander and Avalon zones to the east, which are believed to be parts of a continental margin sequence and a Precambrian continental block, respectively (e.g., Blackwood 1982). The Dunnage Zone was thus inter- preted by Williams and Hatcher (1983) and Williams (1984) as an oceanic suspect terrane enclosed between an autochthonous craton to the west and accreted continental terranes to the east. Many workers now view the Dunnage Zone as an island-arc succession built upon oceanic crust (e.g., Dean and Strong 1977; Dean 1978; Williams 1984), andd basalts with mid- ocean-ridge basalt (MORB) or ocean-island affinity are recog- nized locally (Jacobi and Wasowski 1985; Wasowski and Jacobi 1985). There has, however, been controversy over whether the Dunnage Zone represents a single arc or multiple terranes and (or) lithostratigraphic zones (cf. Williams et al. 1972; McKerrow and Cocks 1977). Furthermore, mechanisms and timing of accretion of the Dunnage Zone to North America and tHe Avalon Zone remain incompletely understood (cf. Colman-Sadd 1980; Williams 1980; Blackwood 1982; Karl- strom 1983; Williams and Hatcher 1983; van der Pluijm 1987).

Part of the difficulty in assessing these problems is that we still have only an imperfect understanding of the geometry of the exceedingly complex deformation within and at the margins of the Dunnage Zone. Ongoing structural studies in many areas document the presence of regional-scale thrusts, complex folding, and transcurrent faulting (e.g., Dean and

FIG. 2. Field sketches of small-scale thrusts in turbidites. (a) From Grassy Island, Hamilton Sound. The fault, which has a displacement of approximately 60 cm, is only recognizable where it cuts across bedding. (b ) From Dildo Run. The sequence youngs upwards in the sketch, and the fault, which has a displacement of more than 3 m, can only be recognized by the hanging-wall ramp.

Strong 1977; Nelson 1979, 1981; Thurlow 1981; Karlstrom et al. 1982; Colman-Sadd and Swinden 1984; van der Pluijm 1986). However, the geometry and scale of displacements on many of the faults are poorly constrained, and postulated dis- placements range from kilometres to thousands of kilometres (e.g., McKerrow and Cocks 1977). Thus, at present, structural data are insufficient to prove or disprove many "terrane" interpretations for the Dunnage and adjacent zones.

Biostratigraphic data have potential to delineate different ter- ranes (e.g., Neuman 1984), but evidence for thrusting and complex superimposed folding and faulting in the eastern Notre Dame Bay area and the shortage, if not complete absence, of unmodified stratigraphic sections in the Dunnage Zone necessitate that stratigraphic studies be combined with detailed structural studies. Superposition of lithological units cannot be assumed in reconstructing stratigraphy in many areas of the Dunnage Zone because faults and disrupted folds cause repetition of time-equivalent units within what have been con- sidered simple homoclinal sections (e.g., Karlstrom et al. 1982; Karlstrom 1982, 1985; van der Pluijm 1986).

Disruption of the stratigraphic sequence by faulting

Disruption of stratigraphic sequences by faulting is not an unusual problem, but we wish to stress that it is particularly important in the parts of the Dunnage Zone with which we are familiar. Detailed work is revealing a very high density of faults that are difficult to recognize because they are generally bedding parallel. Thrusts, for example, can be recognized where ramps are exposed, but if they are traced away from the ramp, into a flat, there is commonly no evidence of their exis- tence, even where outcrop is continuous; such thrusts are depicted in Fig. 2. Small thrusts of this type are very common throughout the region, and larger examples have also been observed. Some of the latter are more easily identified because of their association with olistostromes, but where the olistro- stromes are thin they are readily obscured by breaks in outcrop.

Silurian thrusting was followed first by folding, and then by

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VAN DER PLUUM ET AL. 2339

FIG. 3. Transposed bedding from a ductile fault zone intersected by brittle faults. Shale is unomamented, and siltstones and fine-grained sandstones are stippled. The faults are very difficult to trace in the shale but are clearly seen where they cut the coarser grained beds. Note the extreme east-west extension, which appears to be achieved exclusively by faulting. The faults are shown as heavy lines, and two quartz veins are shown as extra heavy lines. The figure was traced from a polished rock slab.

transcurrent faulting. The transcurrent faults vary considerably in strike between approximately 0 and 135" azimuth. Where their strike is markedly different from that of bedding, they are easily recognized, but where they have approximately the same strike as steeply dipping bedding, they tend to exploit the bedding fissility and are, therefore, less obvious. The bedding- parallel transcurrent faults may be ductile or brittle, and ongoing work has shown that on and around New World Island the ductile faults are mostly dextral and the brittle faults both dextral and sinistral. These faults, though generally parallel to bedding, do cut across stratigraphy in ramp-like structures; they can therefore rearrange the stratigraphy in exactly the same way as the thrusts. Since they can be dextral or sinistral, can have a strike clockwise or anticlockwise of the strike of bedding, and affect a stratigraphy that was already repeated by thrusting and folding, they can give rise to any juxtapositions of stratigraphy including "old over young" and "young over old." These faults are very numerous, as can be seen from Fig. 3, which is typical of many parts of the area. The example chosen is one in which the transcurrent faults are inclined to bedding and therefore easily identified. However, abundant bedding-parallel phyllonites and mylonites and displaced dykes and veins suggest that bedding-parallel examples are equally numerous.

In addition to early thrusts and late transcurrent faults, there are zones of tight folding in which all folds occur as single hinges with one limb truncated by a fault (Figs. 4a and 4b). Younging evidence indicates that these folds should each be members of asymmetrical fold pairs, and the way in which they can develop from such pairs is illustrated in Fig. 4c. The same model was proposed by Sander (1911) for the develop- ment of "Umfaltung" or transposed bedding. He pointed out that where bedding-parallel faults develop in folded sequences the faults must die within the fold limb or must cut through the fold hinges. The significance of the structure here is that there are broad zones (e.g., on Farmers Island and Yellow Fox Island) where all of the folds are truncated, indicating that bedding-parallel faults are abundant in the area.

FIG. 4. Field sketches of small folds in turbidites, truncated by brittle-ductile (a) or brittle (b) faults. Note that younging is the same on both sides of the faults, except locally, where the short limbs of the folds are preserved. ( c ) Interpretation of the formation of the struc- tures, depicted in (a) and (b ) , by the development of a bedding-, parallel fault (heavy line with crosses) in the fold limb. - -

It might be thought that the presence of mylonites should render the faults easy to recognize, but many of the faults lack any such expression, and when present the mylonites are easily misidentified in the field. A ' 'thinly laminated limestone' ' on Farmers Island (McKerrow and Cocks 198 I), for example, has recently been shown to be a blastomylonite (C. Antonuk, writ- ten communication, 1986) in which the layering is of deforma- tional origin.

Thus the problem is that there are two generations of bedding-parallel faults, both of which are very numerous. The thrusts can only be identified where they ramp, and many of the transcurrent faults can only be recognized where they cut across bedding. Others can be identified by mylonites or phyl- lonites, but they are generally not easy to recognize in the field. Both generations of faults disrupt the stratigraphy, and it is never safe to assume therefore that a sequence of rocks is a stratigraphic sequence, even though there are no visible faults. .

Fossil dating of stratigraphic units has helped to clarify the stratigraphy and structure, but suitable fossils are not common enough to solve all the structural problems. However, as stated above, existing fossil data indicate a simple stratigraphy throughout the region covered by Figs. 5 and 6 , and if it is assumed to be correct, this simple stratigraphy can be used to solve structural problems.

Working hypothesis-a simple stratigraphy for the Notre Dame Bay area

Figures 5 and 6 show fossil locations from the Notre Dame Bay area and their age. Table 1 lists the lithologies in which the

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lss

Vol

cani

clas

tic a

nd c

alca

reou

s un

its i

n ba

salt;

lat

e A

reni

g: B

altic

66

30 5

4913

15

an

d Sc

oto-

App

alac

hian

pm

vinc

es

28

ga,

ce,

tr,

b Pe

bbly

ss

Cal

care

ous

unit;

lat

e A

reni

g: B

altic

and

Sco

to-A

ppal

achi

an

6633

549

13

15

prov

ince

s 28

tr

, b

tuff

1s

Ass

ocia

ted

with

cal

care

ws

tuff

and

bas

alt

6639

549

18

15

b, t

r, b

r ca

lc t

uff

Lat

e A

reni

g; B

altic

and

Sco

to-A

ppal

achi

an p

rovi

nces

65

78 5

4847

28

gr

sh

N

. gr

acili

s Z

one

to D

. rl

inga

ni Z

one

ages

59

53 5

4758

1603

0 54

5151

61 13

547

59

5 gr

sh

Po

ssib

ly D

. cl

inga

ni Z

one

(mis

iden

tifie

d?)

6763

549

9916

801

5502

9 5

gr

sh

Prob

ably

Clim

acog

rapt

us pe

ltifP

r Z

one

or

C.

wils

oni

Zon

e 68

99 5

455 1

5

gr

sh

D.

clin

gani

Zon

e or

P.

linea

ris

Zon

e 65

61 5

4290

1656

1 54

3041

6574

543

09

5 gr

, co

bl

arg

C

hert

y ar

gilli

te;

only

Car

adoc

ian

grap

toli

tic

shal

e no

rth

of L

ukes

59

25 5

4940

5

Arm

Fau

lt (L

ushs

Big

ht t

enan

e):

sequ

ence

: lim

esto

ne.

31

sand

ston

e. c

hert

y ar

gill

ite,

mar

ine

volc

anic

s a1

1s

B

lock

in

cong

lom

erat

e:

late

Mid

dle

or e

arly

Lat

e O

rdov

icia

n;

6531

548

41

16

Now

egia

n af

fini

ties

gr

bl

arg

M

iddl

e un

it G

ande

r L

ake

Gro

up;

age

unce

rtai

n 69

88 5

4629

36

gr

bl

arg

Pm

babl

e ag

e. l

ate

Mid

dle

to L

ate

Ord

ovic

ian

6866

546

8316

869

5468

8 1 18

36

gr

bl s

h In

terl

ayer

ed

with

Silu

rian

sil

tsto

nes

and

grey

sla

tes

(mid

dle

5923

552

28

36

Are

nig)

; S

nmks

Arm

-

-

Pag

e

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

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w.n

rcre

sear

chpr

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com

by

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IVE

RSI

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OF

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W M

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on

10/0

6/14

For

pers

onal

use

onl

y.

h)

TA

BL

E I (c

ontin

ued)

W

i3 L

oc.

No.

A

ge

Foss

ils"

Roc

k ty

peb

Com

men

ts

Gri

d re

fere

nces

R

ef.'

Pane

co,

cr,

ga,

b

b

Is

Is

shlv

olc

Poss

ibly

ear

ly M

iddl

e O

rdov

icia

n L

ense

s in

tu

ffam

us

basa

lt, a

ppm

x. l

ocal

ity

Shal

c in

che

rty

sedi

men

t in

vol

cani

c as

sem

blag

e; o

ne s

hell;

pr

obab

le a

ge

34

6 36

32

22

4

34

5 36

32

1

map

36

32

36

59

Mid

dle

Onl

ovic

ian

or y

oung

er

No

iden

tific

atio

ns:

Pill

ey's

Isl

and

Fcw

bnd

ies

iden

tifie

d; a

ll pr

obab

ly C

amdo

c

b, c

, cr

, tr

b,

br,

c,

ce,

ga,

etc.

C

ongl

omer

ate

mat

rix

or in

terb

edde

d ar

gilli

te;

pmha

ble

age

Bw

lder

in c

ongl

omer

ate:

old

est

age

No

bodi

es i

dent

ifie

d L

imy

shal

e m

atri

x w

ith l

imes

tone

bou

lder

(co

nglo

mer

ate)

; la

te

Lla

ndov

ery

to e

arly

Wen

lock

: m

atri

c sa

me

age

as b

ould

ers

36

88

33

1755

34

9

F 36

94

z C

3 12

6 36

95

F 2

36

96

x rA

1 m

ap

R

c, b

, et

c.

74

Sll

b, g

r, e

tc.

Wen

lock

to

earl

y L

udlo

w

c, c

r, b

b.

c

Inde

term

inat

e In

terl

ayer

ed f

ossi

life

mus

bed

s: p

ossi

ble

age

calc

ss

36

99

3 !- 36

99

y

1 m

ap

- 17

22

P

-J

18

84

36

97

6 6

31

137

12

12

26

18

12

13

26

26

33

1759

10

76

10

67

36

72

10

38

Is

Is

sh

calc

arg

In s

ilts

tone

; pos

sibl

e ag

e (b

rach

, un

cert

ain)

In

sha

les

and

silts

tone

: te

ntat

ive

age

Con

glom

erat

e m

atri

x an

d ca

lcat

eous

san

dsto

ne;

sugg

este

d ag

e T

rilo

bite

is

from

rcf

ercn

ce 5

, ap

prox

, lo

calit

y: A

reni

g to

Lla

nvir

n;

Eur

opea

n af

fini

ties

L

ense

s in

silt

ston

c; I

ndia

n Is

land

s G

roup

In

vol

cani

cs:

late

Tre

mad

oc t

o ea

rly

Arc

nig;

N

OTE

: m

isid

entif

ied

in r

efer

ence

18

(p.

19

) In

san

dsto

ne;

earl

y M

iddl

e Si

luri

an:

east

sho

re W

hite

Bay

tr,

co

c, b

, cr

Plan

t G

my

and

blac

k sh

ale;

Ear

ly M

issi

ssip

pian

; ea

st s

ho

~

Whi

te B

ay

Is

Islc

ongl

Is

In b

oth

shal

e an

d sa

ndst

one

Bou

lder

s in

con

glom

erat

e (s

ee a

lso

No.

71)

; pro

babl

e ag

e B

lock

in

pebb

ly m

udst

one;

Ash

gill

to L

land

over

y

C c, b

, tr

, et

c.

C

Pods;

Hey

l (1

937.

an

d un

publ

ishe

d):

Hel

wig

(19

67)

cons

ider

ed

age

unlik

ely

Indi

cate

d ag

e A

gglo

mer

ate;

lat

e L

land

over

y or

ear

ly W

enlo

ck

Lat

e L

land

over

y to

ear

ly W

enlo

ck;

Indi

an I

slan

ds G

mup

It1

situ

mar

ine

foss

ils:

earl

y L

land

over

y N

. ~

raci

lis

89

SI-

1 90

S

I-3

91

SI-

3 92

S

I 93

o

v-l

C c, g

a, b

r, c

r C

. cr

SS

volc

Is

lss

cong

l bl

sh

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TY

OF

NE

W M

EX

ICO

on

10/0

6/14

For

pers

onal

use

onl

y.

TA

BL

E 1 (c

oncl

uded

)

Loc

. N

o.

Age

F

ossi

lsa

Roc

k ty

peb

Com

men

ts

Gri

d re

fere

nces

R

ef.'

Pag

e

C b, b

r, t

r gr

b, b

r, t

r, g

a b,

br,

c

gr

c, b

C

O

cr,

OS

, ce

gr

b b, t

r gr

gr

gr

CO

C

volc

Is

ca

lc s

s bl

sh

gw

ss,

arg

sh

SS

Is

Is

bl s

h vo

lc

Is

bl s

h

bl s

h bl

sh

Is

turb

Bou

lden

in

cong

lom

erat

e: p

ossi

ble

age

Are

nig

to L

land

eilo

(br

yozo

ans)

: In

dian

Bay

Big

Pon

d

Sim

ilar

to

No.

19

: po

ssib

ly C

amdo

c N

ew l

ocal

ities

(un

iden

tifi

ed)

Dis

conf

orm

ity:

pss

ihle

lat

e L

land

over

y L

ense

s; l

ate

Lla

nvir

n to

ear

ly L

land

eilo

: NOTE: C

obbs

Arm

st

one

age

(Lla

ndei

lo)

is gi

ven

Len

ses

in p

illow

vol

cani

cs;

tent

ativ

e ag

e

Sli

ver

in S

iluri

an B

otw

ood:

ear

ly t

o la

te C

and

oc

Pmha

bly

Mid

dle

to L

ate

Ord

ovic

ian

Len

s in

vol

cani

c fl

ow;

Mid

dle

to L

ate

Old

ovic

ian

D.

clin

~an

i

P. I

inea

ris

P. l

ineo

ris

or

poss

ibly

Dic

t.llo

grap

rus

com

plan

afus

L

ocal

ly d

eriv

ed c

last

s in

vol

cani

cs (

Buc

hans

Gm

up)

Prob

able

age

34

7 35

17

38

52

8 2

1 34

19

100

1 m

ap

14

map

27

18

31

14

0 43

7 32

15

76

4 5

4

11

7 11

7

21

34

21

34

21

34

29

286

37

1228

"a].

alga

e; b

, bra

chio

ws:

be,

bel

lem

phon

ts; h

i, bi

valv

es; b

r, b

ryoz

oans

; c. c

oral

s; c

e, c

epha

lopo

ds; c

h. c

hitin

ozoa

ns: c

o, c

onod

onts

; cr.

crin

oids

; ga.

gas

trop

ods;

gr, gr

apro

lites

: or.

ort

hids

, os,

ost

ra-

cads

: pc

, pe

lecy

pods

; h,

rhyn

chon

ellid

s: t

r. m

lobi

tes;

noe

h, h

ocho

nem

otid

s.

'arg

, ag

illi

te;

bl,

hlac

k: c

alc,

cal

care

ous:

con

gl, c

ongl

omcr

alc;

gw

. gre

ywac

ke; I

s, l

imes

tone

; mic

. m

icac

eous

; mud

st, m

udst

one;

sh,

sha

le; s

iltst

, silt

ston

e: s

s, s

ands

tone

: tuf

f, tu

ffac

eous

; tur

b, tu

rbl-

di

tes:

vol

c, v

olca

nics

. 'I,

And

enon

and

Will

iam

s (1

970)

: 2.

Ber

gstri

jm e

f al.

(197

4); 3

. Beny a

nd B

ouco

t (19

70):

4,

Bla

ckw

ood

(198

1): 5

, Dea

n (1

978)

; 6.

Dea

n (1

970)

: 7,

Dea

n (1

973)

: S. E

astle

r (19

71);

9. F

arae

us a

nd

Hun

ter (

1981

): 1

0, H

elw

ig (

1967

); 1

1. H

eyl(

19.7

6); 1

2, H

eyl

(193

7);

13, H

ibba

rd a

al.

(197

7):

14,

Hom

e (1

968)

; 15

. Hom

e (1

976)

; 16

, Hom

e an

d Jo

hnso

n (1

970)

; 17

. Jen

ness

119

58);

18, J

enne

ss

(1%

3):

19, K

arls

tmm

(19

85);

20,

Kay

and

Eld

ridg

e (1

968)

; 21.

Kea

n an

d Ja

yasi

nghe

(19

82);

22.

Mac

Lea

n (1

947)

: 23.

McK

emw

and

Cocks (

1977

); 2

4. M

cKem

w a

nd C

ocks

(197

8); 2

5, M

cKer

mw

an

d C

ocks

(19

81);

26.

Nea

le a

nd N

ash

(196

31:

27,

Nel

son

(197

9): 2

R,

Neu

man

(19

70; 2

9, N

owla

n an

d T

hudo

w (

1984

): 3

0, S

toug

e (1

98%

); 3

1, S

toug

e (1

980b

); 3

2, S

tmng

and

Kea

n (1

972)

, 33,

T

wen

hofe

l and

Sch

mck

(19

37);

34,

Will

iam

s (1

962)

: 35,

Will

iam

s (1

963)

. 36.

W~

ll~

ams (197

2); 3

7. W

illia

ms

and

Nob

le (

1986

): 3

8. W

onde

rley

and

Neu

man

(19

84).

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TY

OF

NE

W M

EX

ICO

on

10/0

6/14

For

pers

onal

use

onl

y.

CAN. J . EARTH SCI. VOL. 24. 1987

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TY

OF

NE

W M

EX

ICO

on

10/0

6/14

For

pers

onal

use

onl

y.

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TY

OF

NE

W M

EX

ICO

on

10/0

6/14

For

pers

onal

use

onl

y.

2346 CAN. J. EARTH SCI. VOL. 24, 1987

fossils were found, other pertinent data on the fossil localities, and source references. This compilation is a summary of ages and lithologies reported in the literature up to 1985 (age and lithology were taken from the most recent reference, if refer- ences conflict). Figure 7 plots lithology versus age for these locations and is the basis for our interpretations regarding stratigraphy in the area as a whole. This plot of fossil age versus lithology is an unconventional way to reconstruct stratigraphy, but our belief is that conventional stratigraphic studies in the Dunnage Zone, which use type sections, law of superposition, and correlation charts, can be (and have been) in error because of unrecognized fold and fault repetitions of stratigraphic units (see, e.g., Williams and Noble 1986). The Ordovician and Silurian periods (shown on the left in Fig. 7) are drawn approximately to scale in terms of time; radiometric age calibration is from Gale et al. (1980). Fossil ages are plotted as time intervals in an attempt to express the uncer- tainty in fossil age determinations (see Karlstrom et al. 1983). Figure 7 shows a remarkably consistent trend, which we have expressed in terms of major depositional intervals of various lithologies (black bars), and a simple stratigraphy for the Notre Dame Bay area as discussed below from oldest to youngest rocks.

Ultramafic rocks in the Dunnage Zone occur as part of ophiolite sequences and as isolated zones of ultramafic rocks such as the Gander River Ultrabasic Belt (Jenness 1958; Black- wood 1979), which probably also represent ophiolite (Black- wood 1979, 1982, 1982). Radiometric dating has shown that ultramafic, trondhjemitic, and other ophiolitic rocks from diverse localities give ages in the time span of 463-510 Ma (Fig. 7) (Stukas and Reynolds 1974; Mattinson 1975, 1976; Williams et al. 1976). Recent results by Dunning and Krogh (1985) have reduced this time span to less than 20 Ma (477 -495 Ma). This restricted time range of ophiolitic rocks suggests a relatively limited area for the basin from which these ophiolites originated.

Volcanism other than ophiolites occurred in the Dunnage Zone during two time periods (Fig. 7). Mafic pillow volcanics of island-arc affinity are Early Ordovician, mainly Arenig and Llanvirn, as dated by fossiliferous sandstones and tuffaceous limestones preserved in pods between pillows or as lenses interbedded with the basalts and pillow breccias. Late Ordo- vician to Silurian volcanics are bimodal basalt-rhylolite sequences, which are interpreted as largely subaerial deposits because of a preponderance of pyroclastics and an association with subaerial sandstones (Dean 1978).

Massive limestones in the Dunnage Zone, such as the Cobbs Arm limestone of New World Island and its equivalents, over- lie mafic pillow volcanics and tuffaceous limestones and yield Llandeilian fossils (Fig. 7). Miscellaneous limestones also occur as blocks and clasts in mtlange and conglomerate. The oldest fossil reported from the Dunnage Zone, for example, is a Cambrian trilobite from a limestone block in the Dunnage MClange (Kay and Eldredge 1968), and limestone clasts of probable Silurian age have been found in polymictic con- glomerates (Williams 1972). Thin limestone units also occur interbedded with siltstones in some Silurian successions (Fig. 7). Thus, there is evidence that limestone deposition took place during various time intervals, with a relatively major limestone depositional interval in the Llandeilo. However, all of the limestones are thin and lenticular on a regional scale.

Black shales include graphite-rich slates, black argillites,

chert, and dark, argillitic turbidites. These rocks are pre- dominantly Caradocian in age (Fig. 7), as shown by well- preserved graptolite faunas (Bergstrom et al. 1974). Many workers have suggested that these widespread black shales represent pelagic muds deposited during tectonic quiescence after volcanism ceased in the Dunnage Zone (Dean 1978; Nelson 1979; Kean et al. 1981). There are also Early Ordo- vician black shales and black argillites present in the Dunnage MClange area (Hibbard et al. 1977) and elsewhere.

Field and fossil evidence suggests that Caradocian black shale deposition gave way to turbidite deposition (Dean 1978). Thick and well-developed turbidite successions (with graywacke, slate, and conglomerate interbeds) were first deposited in late Caradocian times and continued into the Llandovery (Fig. 7). The main turbidite deposition became coarser grained with time, and graywacke sequences were gradually overlain by polymictic conglomerate in many localities.

The youngest sedimentary rocks in the northern Dunnage Zone are quartz arenites, which yielded Silurian fossils as young as Ludlow (Fig. 7). These rocks are believed to be sub- aerial and probably represent lateral equivalents of the gray- wackes deposited in a continued shallowing environment during the Silurian.

The striking progression of lithologies with time suggests a simple gross-scale stratigraphy for the Notre Dame Bay area. The general lithological succession is Cambrian and Tremado- cian ophiolites, Tremadocian to Llanvimian submarine vol-

i canics, Llandeilian limestones, Caradocian black shales, I Ashgillian to Llandoverian turbidites, Llandoverian polymictic conglomerates, late Llandoverian to Wenlockian subaerial vol- canic~, and Wenlockian to Ludlovian subaerial sandstones.

I

I

Examples of large-scale stratigraphic repetitions due to bedding-parallel faulting

The proposed stratigraphy (Fig. 7) is very similar to the lithostratigraphy suggested by Kean et al. (1981). A major dif- ference, however, is the position of the Buchans ~ r o u ~ (and probable correlatives) in the sequence. Kean et al. correlated the pillow volcanics of the Buchans, Roberts Arm, Cottrell's cove, and Chanceport groups and considered them Silurian largely based on the observation that they apparently overlie rocks of Late Ordovician age and have contrasting geo- chemistry, volcanic style, and mineralization when compared with pre-Caradocian volcanics. This led to tectonic models postulating the existence of a post-Caradocian volcanic-arc succession (Strong 1977; Kean et al. 1981). The stratigraphy suggested here agrees with the correlation of the four groups but in contrast places them with Early to Middle Ordovician pillow volcanics. This interpretation has been confirmed recently. New Llandeilian fossils from locally derived lime- stone clasts in volcanic breccias (Nowlan and Thurlow 1984) and other age dates (Bostock et al. 1979) indicate that the pillow volcanics are Ordovician in age (see also Nelson 1981; Nelson and Kidd 1979; Arnott et al. 1985). Further, recent zircon UIPb analyses yield ages of 473:: Ma for the Buchans and Roberts Arm groups, respectively (Dunning el al. 1987).

On New World Island the Chanceport Group forms part of a north-younging homoclinal sequence that lies to the north of younger Silurian turbidites. The stratigraphic repetition in this case is due to a late, steeply dipping, approximately east- west-striking fault that overprints the regional folds. The fault

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

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by

UN

IVE

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W M

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on

10/0

6/14

For

pers

onal

use

onl

y.

z

VO

LC

AN

lCS

L

IME

ST

ON

E

BL

AC

K

SH

AL

E

TU

RB

IDIT

E S

C

ON

GL

V

OL

C

SS

5 2

with

Is.

len

ses

and

tuff

. ch

ert,

bl. a

rgil

lite

, ar

gil

liti

c tu

rbid

~te

s gw

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is marked by a narrow break in outcrop, so its movement vec- tor cannot be determined directly. However, it is interpreted here as a dextral transcurrent fault, since adjacent outcropping faults of the same relative age and orientation can be shown to be dextral transcurrent faults by means of kinematic indicators.

Similar repetitions can be demonstrated on Yellow Fox Island in the Bay of Exploits. Here, in a sequence of consis- tently north-younging rocks, dated Caradocian black argillites are overlain by dated Llandeilian limestones and volcanics (McKerrow and Cocks 1981). These in turn are overlain by Sansom-like turbidites, which are overlain by volcanics, neither of which are dated. The lower contacts of both volcanic sequences are marked by approximately east-northeast - west- southwest-trending faults. These faults are narrow and are defined by brittle fractures, thin phyllonites, and carbonate mylonites. A well-developed lineation plunges 10-20" to the west, indicating that the faults are transcurrent. Asymmetrical folds indicate that the displacement at one of the contacts is sinistral; no kinematic indicators have been recognized at the other contact. However, if it is assumed that both faults are sinistral, and since they have a strike that is slightly clockwise (with respect to the acute angle) of the strike of bedding (i.e., bedding trends more east - west), they should both place old on young, as is observed for the lower dated sequence and as is interpreted from the simple stratigraphy proposed here for the upper sequence.

On New World Island there are three repetitions of the stratigraphy from Ordovician volcanics to Silurian conglomer- ates (van der Pluijm 1986). Repetition was recognized by Kay and Williams (1963), who initially considered it a product of thrusting. Later, Kay (1967, 1976), having recognized the abundance of transcurrent faults, interpreted the repetition as a product of transcurrent faulting. The three sequences are sepa- rated by the base of the Cobbs Arm Zone and a movement zone through Little Byrne Cove that Kay (1967) referred to as the Toogood Fault. The base of the Cobbs Arm Zone is marked by a mClange (Jacobi and Schweickert 1976; Arnott 1983) and has been interpreted as being associated with a thrust cutting the depositional interface (Karlstrom et al. 1983; van der Pluijm 1986). Similarly, the Byme Cove movement zone coincides with an olistostrome outcropping in Toogood Arm and Back Cove (Arnott 1983). Both movement zones show similar characteristics and are overprinted by trans- current faults and related folds. We, therefore, also interpret the Byrne Cove movement zone as a thrust.

Conclusions and discussion Our premise is that the structure of northeastern Newfound-

land is complex but the overall lithological succession rela- tively simple on a regional scale. We believe that this lithological succession reflects regional lithological changes through time because the same age progression of various lithologies can be documented in different thrust sheets on New World Island and from widely separated areas within the Dunnage Zone.

In view of the proposed simple stratigraphy, the numerous lithological repetitions observed in the field are interpreted as stratigraphic repetitions due to faulting. The faults are parallel to bedding over much of their areal extent but cut across the stratigraphy by means of short steps (ramps where the faults are thrusts). The faults include early thrusts and late, post- folding, transcurrent faults, and both are very common at all scales in the area studied.

In view of the abundance of the faults, the difficulty in recognizing them, and the consistency of the data presented in Fig. 7, we have greater confidence in the validity of the simple stratigraphy than in the interpretation of complex stratigraphic sequences involving repetition of the various rock types. By rock types in this context we mean the rock types represented in Fig. 7. Certain repetitions are, of course, stratigraphically acceptable. Thus we would not suggest that alternations of tur- bidite and conglomerate are necessarily due to faulting, since the two rock types overlap in age and interfinger laterally. Nor would we attach importance to the repetition of limestone. However, we do suggest that an alternation of turbidites and mafic volcanics most likely represents an original sequence of older volcanics followed by younger turbidites, repeated by bedding-parallel faulting.

Because the proposed regional stratigraphy has potential for simplifying structural and tectonic interpretations, it is neces- sary to closely examine its validity within the Notre Dame Bay area as well as possible extensions of the stratigraphy to other areas of Newfoundland and elsewhere in the northern Appala- chians. Sampling problems, such as the possible presence of major unfossiliferous rock units that would be left out of the proposed generalized stratigraphy, have not been addressed here. Similarly, no attempt has been made to fully explore the numerous questions raised by the proposed lithostratigraphy, such as the compatability of faunal assemblages, faunal prov- inces, sedimentological data, and geochemical data within the major groupings (see, e.g., Neuman 1984). The major aim of this paper, aside from providing a compilation of fossil locali- ties for reference, is to draw attention to the abundance of bedding-parallel faults and to stimulate further testing of the idealized stratigraphy and discussion of its problems and limi- tations.

Our conclusions regarding paleotectonic reconstructions based upon the working hypothesis of a simple regional stratig- raphy (modified from Dean (1978) and Kean et al. (1981)), combined with the observed complex structural relationships, are as follows.

(i) The stratigraphic succession is not the same in each thrust sheet (not all are complete sections), but each can be derived from the regional stratigraphy shown in Fig. 7. This stratig- raphy has been proposed in the literature but not fully utilized for interpretation of map geometries. It should be tested further.

(ii) A regional stratigraphy for the Dunnage Zone implies that the thrust sheets were all derived from a single oceanic terrane, the nature of which is constrained by the originally uniform stratigraphy. Because of the lithologic sequence shown in Fig. 7, we tend to support the view that the Central Mobile Belt largely represents a single telescoped basin, although the width of the basin and internal variations in the basin remain to be determined.

Acknowledgments We thank our numerous colleagues in Newfoundland for

many discussions on various aspects of Notre Dame Bay geol- ogy, which considerably contributed to our understanding of the area. Research was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) grant A7419 and Energy, Mines and Resources Canada (EMR) grants 39, 49, and 71 (P. F. Williams), the Geological Society of America (B. A. van der Pluijm), and the University of New Brunswick. We thank K. L. Cume, Paul Dean, and Baxter

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Kean for thoughtful criticism, which resulted in an improved paper. The staffs a t the universities of New Brunswick and

' Michigan, in particular S h e m Townsend, are thanked for word processing and figure preparation.

I

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