Lower Cambrian consists of two unconformity-bounded, on1ap-off1ap sequences ..... Stage .... to. ~ z. < a: co. ::t. I. I. Meishucunian. Slage. Meishucunian Stage.
GEOLOGICAL ASSOCIATION OF CANADA MINERALOGICAL ASSOCIATION OF CANADA CANADIAN SOCIETY OF PETROLEUM GEOLOGISTS
ASSOCIATION GEOLOGIOUE DU CANADA ASSOCIATION MINERALOGIQUE DU CANADA SOCIETE CANADIENNE DE GEOLOGUES PErROLlERS
FIELD TRIP GUIDEBOOK
TRlP A3. CAMBRIAN DEPOSITIONAL HISTORY AND STRATIGRAPHY, AVALON-BONAVISTA REGION, SOUTHEASTERN NEWFOUNDLAND
Ed Landing' and Alison P. Benus I,2
I
New York Stale Geological Survey. the State Education Department. Albany. NY 12230.
l
Department of Earth Sciences. Memorial University of Newfoundland. St. John's, Newfoundland, Am 3X5.
May, 1988
III
,
..
III
CONTENTS INTRODUCTION ••••••••••••••••••••••••••••••••••••••••••••••••••• 1
GEOLOGICAL SETTING ••••••••••••••••••••••••••••••••••••••••••••• 5
UPPER PRECAMBRIAN Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Avalonian Oroqeny •••..•••••.•.•••••••••••••••••••••••••••• 7
UPPERMOST PRECAMBRIAN-CAMBRIAN
Depositional Sequences •••••••••••••••••••••••••••••••••••• 10
Lowest Depositional Sequence
Rencontre Formation •••••••••••••••••••••••••••••••••• !1
Chapel Island Formation •••••••••••••••••••••••••••••• !1
Random Formation ••••••••••••••••••••••••••••••••••••• l~
Biota•••••••••••••••••••••••••••••••••••••••••••••••• 12
Higher Lower Cambrian Depositional Sequences
Bonavista Group .••.•••••..•..•••••••••••••••••••••••. 15
Brigus Formation (emended) ••••••••••••••••••••••••••• 20
Biota .......................•.•...... 21
Middle-Upper Cambrian Sequences
Chamberlains Brook Formation ••••••••••••••••••••••••• 22
Manuels River Formation •••••••••••••••••••••••••••••• 23
Elliot Cove Group .•.•.•••••.••..•••..••••••••••.••••• 25
it • • • • • • • • • • • • • • •
AC'KN'O'WL:EDG.MENTS •••••••••••••••••••••••••••••••••••••••••••••••• 26
ITINERARY-DAY 1
Purpose ....•..••.•.•......•...........•....•..........•..• 27
Route ...••.••.••.•...•.......•..•.•••.••..•..•.••..•.•.••• 27
Stop 1--Cambrian Unconformities at Manuels River •••••••••• 28
Stop 2--Linked Lithofacies/Biofacies Shift at the
Chamberlains Brook-Manuels River Transition, Kelligrews
Quarry ...............•..•.•••.••..•.....••..•••••••..•.• 29
Stop 3--Regional Extension and the Feather-Edge of the
Bonavista Group at Duffs •••••••••••••••••••••••••••••••• 30
Stop 4--Long Cove, Unconformities Bounding the Bonavista
Group ...•.•.•.....•.....•.....•..•••.•..•......••••••••• 32
ITINERARY-DAY 2
Purpose .•..•.•••..............•.•••........•..•.•..•.••••. 34
Route •..•.....................••••••••...........••••.•... 35
Stop 5--Sunnyside, Representative Sections of the Petley, West Centre Cove, and Cuslett Formations of the Bonavista Group .•••.•.......•...........••.•••••••••.•.• 35
Stop 6--Monkstown, Chapel Island-Bonavista Unconformity in East Burin............. . •..•••.•....•.....••.••••• 38
ITINERARY-DAY 3
Purpose •...•.••••.........•.••.••..••.•••.•...•••••••••.•• 40
Route .•••......•....•.......••......•.•......••••.••••...• 40
Stop 7--Keels, a Cambrian ·Worm· Reef in the West Centre
Cove Formation ••....•.•..••••.....•..••...•••....••..••• 41
Stop 8--Smith Pointj Unconformity Between the Sub
trilobitic and Trilobite-Bearing Lower Cambrian ••••••••• 43
Stop 9--Rickmans Harbour, Type Section of the Random
Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . 45
Stop 10--Fosters Point, Manuels River-Elliot Cove Inter-
formational Unconformity •••••••••••••••••••••••••••••••• 46
REFERENCES CITED ••••••••••••••••••••••••••••••••••••••••••••••• 47
IV
FIGURES 1 Field trip stops •••••..••••••••••••••••••••••••••••••••• 1
2 Upper Precambrian-Cambrian stratigraphy ••••••••••••••••• 3
3 Uppermost Precambrian and Lower Cambrian correlations ••• 14
4 Lithostratigraphy of the Bonavista Group •••••••••••••••• 16
5 Onlap-offlap cycles of the Bonavista Group •••••••••••••• 19
6 Section at Long Cove, Stop 4 •••••••••••••••••••••••••••• 33
7 West Keels section, Stop 7 •••••••••••••••••••••••••••••• 42
8 Smith Point, Stop 8 ••••.••.•..••••••.••••••••••••••••••• 43
1
INTRODUCTION
The Burin,
Avalon,
and Bonavista Peninsulas
of
southeastern
Newfoundland exhibit completely exposed and structurally noncomplex outcrops of Cambrian platformal siliciclastics and minor limestones (Figure 1). made
with
Because direct lithological comparisons can often be coeval
sequences
America and Britian,
in
other
Avalonian
regions
of North
insights into the stratigraphic sequence and
depositional history of the Cambrian of southeastern Newfoundland provide a basis for an understanding of the depositional tectonics and dynamic stratigraphy of the Avalonian Cambrian.
Figure 1. The Avalon Zone in Newfoundland. Figure shows uppermost Precambrian through Ordovician outliers (in black) and location of field trip stops (S 1 to S 10).
Bonavista Peninsula
St. John's
o,
50 , km
In
the
course
emphasize five
of
this
field
trip,
the
authors
intend
to
fundamentally important aspects of the Cambrian of
the Avalon-Bonavista region: 1) The Cambrian rests everywhere with angular unconformity or nonconformi ty on Upper (Stops 1, (Signal
3,
Precambrian rocks of the Avalonian orogen
9). Continuous deposition from the Upper Precambrian
Hill
Group
and
Precambrian--1owest
equivalents)
Cambrian
is
not
through known
the in
uppermost
southeastern
Newfoundland (Figure 2). 2)
The
uppermost
Precambrian and
lowest,
or sub-trilobitic,
Lower Cambrian consists of two unconformity-bounded, sequences in southeastern Newfoundland (Landing include A)
a
shelf
and
muds
sandstones
sequence sands
including (Chapel
(Random Formation)
Bonavista Group.
red
beds
Island and R)
~
on1ap-off1ap
al., 1988). These
(Rencontre
Formation),
Formation),
and
macrotidal
shales and limestones of the
The Random Formation
(=middle Lower Cambrian)
or
younger units overlie the Avalonian orogen in the Avalon-Bonavista region (Stops 1, 3, 9) while older Early Cambrian units (Rencontre, Chapel Island) are present in the Burin Peninsula. 3)
The
Random
overlying units
Formation
and does
not
is
not
form
a
depositionally
related
time-transgressive,
deposit of the regional Cambrian onlap (fide McCartney,
to
initial
1967~
Greene
and Williams, 1974). The Random is unconformably overlain by younger Cambrian deposits
(Stops
4,
5,
see Figure
3)
and was apparently
never deposited in eastern regions such as Conception Bay (Stops 1, 3). Local absence of the Random in the western Burin Peninsula (Stop 6)
is
apparently
due
to
erosion
following
post-Random
and
pre
Bonavista block faulting (Landing and Benus, 1988). 4)
Post-Random,
Lower Cambrian accumulation is controlled by
differential subsidence along narrow, northerly-trending "troughs." The
thickest
sections
of
the Bonavista Group
(275+ rn)
occur
in
3
SYSTE~_ SOUTHWEST BURIN
...
PB':lIN~A
WESTERN AVALON PENINSULA
not exposed
Elliot Cove Group
EASTERN AVALON PENINSULA
(l)
a. a. :::::> r- (l)
Z is
«
c:OJ
------
Manuels River Fm.
Manuels River Fm.
:2 Chamberlain's Brook Fm. ::E
Chamberlain's Brook Fm.
j
~
~
Brlgus Formation
Brigus FOrmation
« 0
I
Bonavista Group
r-
Q)
;:
0
Random Formation
Random Formation
....I
~ ~ ~ lIIelll. 2
Chapel Island Formation
m;;:n:t
Q1
Rencontra Fm.
1
1I
Crown Hill Fm. Trinny Cove Fm. Maturln Pond Fm.
Q.
ci.
0
:J
I I I I I I I I 1.lJ..l ~ (; iii 0
Z
«
c
):
c:
(J)
>
~
« 0
Marystown Group
III
..a
ci.
0
:J
2
W
c: a.. c: W a.. a..
Big Head Formation
0
-;
OJ
c .2'
Bull Arm Formation
-1
- -1- -1- !-1 .....
-
-1
-
-1
."c J:.
.., 0
Cape Ballard Fm. Cuckold Fm. Farryland Head Fm. Gibbett Head Formation Cappahayden Formation Renews Head Formation Fermeuse Fm. Trepassey Fm.
~~ ~-----Mistaken Point Fm. Q.
Connecting Poinl Group
:::::> Rock Harbour Fm.
1-1
-
-7-
-
-1
Love Cove Group
i-1
-
-1-
-
-1
0
c .2
Q. CD
u
c 0
(;)
Briseal Fm. Drook Fm. Gaskiers Fm. Mall Bay Fm. Harbour Main Gp.
Burin Group
Figure 2. Upper Precambrian to Camhrian correlations, Avalon Zone, southeastern Newfoundland. Upper Precambrian correlations after King (1979) and Strong (1979). As discussed in the text, the Big Head Formation may be a lateral equivalent of part of the Conception Group while the Trepassey Formation may be better grouped in the Conception Group. See Landing and Benus (1988) for definition of the Bonavista Group. Vertical rulings indicate major unconformities. Recently obtained U-Pb dates on zircons from the Wandsworth Gabbro sill (ca. 763 myr) indicate that the Burin Group is significantly older than the "Pan-~frican" ages (ca. 630-585 myr) obtained from the Mary~town and Rock Harbour Groups, the Harbour ~ain volcanics and Holyrood pluton (Krough ~ ~., In press).
4
western Trinity Bay and the Bonavista Peninsula (Stops 5, 8) along the
"Placentia-Bonavista
axis If
(Landing
and Benus,
1988).
"Thin"
Bonavista sequences lie east of this axis, and the feather edge of the Group lies in eastern Conception Bay
(Stops 3,
4). An eastern
shift of the Lower Cambrian depocenter to eastern Trinity Bay took place
during
Brigus
time,
and
thinner
sections
of
the
Brigus
Formation occur on the old "Placentia-Bonavista axis" (Stop 7). 5) Sediment-filled fissures in the uppermost Random and Upper Precambrian
(Stops
3,
5)
suggest that post-Random Lower Cambrian
onlap reflects long-term regional extension. 6)
Limestone
beds
and
manganese
carbonate
horizons
are
particularly important in stratigraphic syntheses; these lithologies represent
the
shallowest
water
conditions
of
shale-dominated
sequences, assist in the recognition of offlap events (see Figure 5) and
serve
as
regionally
extensive
marker
horizons
(Stops
1-8).
Manganese carbonates appear above the uppermost Lower Cambrian (Stop 1)
and define
a
possible
soil
horizon
(Stop 7)
at the regional
Lower-Middle Cambrian unconformity. 7)
A
key
development
in
the
post-Random
Cambrian
is
the
replacement of red- and green-colored sediments by black shales in the middle Niddle Cambrian. This color change, approximately coincident with a volcanic event (Manuels "metabentonite," Stops 1, 10),
reflects
Ordovician) black
development
of
long-term
(through
basin stratification across the Avalon platform.
shale
trilobite
the
basin
limestones
unconformity surface
was
not
(Stop
deep, 10),
and an
abundant
uppermost
Lower This
storm-winnowed Middle
Cambrian
(Stop 10), and sandstones with wave-generated
structures in the lower Upper Cambrian suggest relatively shallow marine environments.
b
5 GEOLOGICAL SETTING Southeastern Newfoundland east of the Hermitage Bay -- Dover (Figur~
fault system
1) has a geological history and stratigraphic
record which are comparable to those of the coastal northeastern United
States,
southern
New
Brunswick,
northern
mainland
Nova
Scotia, Cape Breton Island, Wales and central England (see Rast et al., 1976). This eastern terrane of the Appalachians, or the Avalon Zone, collided with the easterly remnants of the Paleozoic Iapetus Ocean
(Gander
Zone)
during
the
Devonian
Acadian
orogeny
(see
Williams et al., '1974). The
Avalon
dominated
by
Zone
of
southeastern
relatively
Newfoundland
unmetamorphosed,
is
upper
areally
Precambrian
volcanics, siliciclastics, and local mafic and granitoid intrusives.
An important compressive event, frequently designated the "Avalonian orogeny" and equated with the Cadomian orogeny of western Europe, deformed this older sequence. overlying,
non-conformities the
Precambrian
the
onlap of the
siliciclastic-dominated upper Precambrian to Ordovician
marine platform sequence
between
Erosion preceded
1).
Angular unconformities· and
at
many
localities
Precambrian
and
are
upper through
(Figure
present
Ordovician.
The
the
at
the
overlying
Acadian orogeny
contact
uppermost is
probably
responsible for the down-folding and -faulting that preserves the uppermost Precambrian through Ordovician in a
number of outliers
across southeastern Newfoundland (Figure 1). Extensive areas of Late Devonian granitic intrusives that accompanied the Acadian orogeny, and which also intrude Silurian (?) fluvial
clastics
and
local
and Devonian-age subaerial to
limestones,
are
present
along
and
adjacent to the Hermitage Bay -- Dover fault system. The youngest rocks
of
the
Carboniferous
area
occur
granites
and
in
the
Burin
Peninsula
fluorite-bearing
outcrops of Lower Carboniferous
fanglomerate;
and
pegmatites fluvial
include
and
local
sandstones,
o
G
and thin limestones. These intrusives and sediments accompanied the Alleghenian
(e.g.
Hercynian)
orogeny_
Upper
Triassic
to
Lower
Jurassic mafic intrusives and rift-associated sediments that record the opening phase of the modern Atlantic in the Maritime Canadian and
United
States'
Appalachians
are
not
present
in
subaerial
outcrops in southeastern Newfoundland (see O'Brien et al., 1983, for a regional summary of Avalonian geology) • Glacial drift, a dense spruce forest and bogs cover the bedrock in most interior localities. with extensive sea cliffs,
However, a deeply incised coast line fjords,
and shore outcrops as well as
local road and river cuts have allowed southeastern Newfoundland to become
a
standard
of
reference
in
the
interpretation
of
the
geological history of the Avalon Zone.
UPPER PRECAMBRIAN Stratigraphy Among the various tectonic models that have been proposed for the upper Precambrian of the Avalon Zone
(see O'Brien and King,
1982), a geological history involving initial rifting and subsequent compression appears
to be most
appropriate
to
the
stratigraphic
record in the eastern part of the Avalon Peninsula 1986; see Figure 2). ensialic
rift
(King,
This geologic history includes a)
setting
with
bimodal
volcanism
and
1979,
an early associated
subaerial to shallow marine volcaniclastic deposition (Harbour Main Group)
(ca. 1.5+ km thick), b) subsequent deep marine (lower slope
to rise) and
turbiditic sedimentation
King,
volcanism
1979) (ca.
at
a
rapidly
3-5 km thick),
(Conception Group; e.g. williams
subsiding margin with c)
southerly progradation of dark
shales and thin sands of a slope facies km
thick),
sandstones
d) and
southward laterally
diminishing
progradation equivalent
(St. John's Group, ca. 1.6 of
coarsening-upward
fanglomerates
(Signal
shelf Hill
7 7
., Group,
km
3-8
thick),
and
e)
very
late
Precambrian
folding
(Avalonian orogeny). Ava10nian Orogeny The
1
J
effect
Newfoundland
of
is
late
Precambrian deformation
curiously
syntheses even though the
understated
in
in
many
southeastern
recent
"Avalonian orogeny" was
tectonic
first proposed
(Lilly, 1966) in this region. Very divergent opinions on the style of
Avalonian
deformation
and
the
nature
of
the
contact of the
uppermost Precambrian and Cambrian with the upper Precambrian in southeastern Newfoundland are held by various authors. For example, Keppie
(1982,
p.
asserted
125)
a
"general
absence
of
such
deformation in the Avalon Peninsula" and even excluded southeastern Newfoundland from the latest Precambrian terrane collisions which he imagined as leading to the assembly of an Avalon Zone. Alternatively King (1979) interpreted an episode of uplift largely to the north of the modern Avalon Peninsula that supplied upper Precambrian arkosic sediments
(Signal Hill Group)
and a subsequent onlap of Cambrian
sediments (Random Formation) with, seemingly, little evidence of a regionally
extensive
unconformity
O'Brien et ale
(1983)
geographically
limited
latter
authors
extensional
the
base
of
the
Cambrian.
also regarded the Avalonian orogeny as event
believed
tectonism
at
in
that
that
southeastern
the
limited
orogeny the
Newfoundland. was
The
dominated
sub-Cambrian
a
by
Avalonian
\ unconformity to horsts and allowed continuous deposition from the Proterozoic into the Cambrian in grabens to the southeast. which
is
regionally
preferred
herein
extensive,
key
is
that
event
in
the
Avalonian
the
A
orogeny
geological
view is
history
a of
southeastern Newfoundland that 1) precedes the region's transition into a
nearly passive,
with
change
a
from
a
shallow marine platform, generally
southerly-
to
2) a
is associated westerly-facing
ts
8
depositional slope, and 3) is marked by profound unconformity at the base of the uppermost Precambrian and Cambrian. McCartney (1969: see King, 1979, Fig. 1) noted that the Random 1!ormation and higher Lower Cambrian units overlie progressively older Precambrian units on
a
west-to-east transect across the Avalon Peninsula. For example, the Random overlies upper Signal Hill equivalents in eastern Trinity and Placentia Bays while higher Lower Cambrian units overlie units as low as the Holyrood Granite and Harbour Main Group in Conception Bay (see Landing and Benus, Duffs
localities).
1988J
Bacon Cove, Brigus South Point and
A stratigraphic
cut-out
of
approximately
ten
kilometers of the upper Precambrian prior to Lower Cambrian onlap is indicated
by
these
relationships.
Local
angular
unconformities
between nearly vertical upper Precambrian sediments and overlying gently dipping Cambrian units are known at several
localities
in
Conception Bay (McCartney, 1969; Landing and Benus, 1988) and seem to
be
compatible
regime
in
the
with late
a
compressive
Precambrian.
rather
than
Angular
an
extensional
unconformities
and
stratigraphic cut-out at the Avalonian unconformity are well known in the southeastern Trinity Bay region on the basis of divergences in
fold
axes
Cambrian
in
the
upper
(nearly north-south)
cut-out
of
the
upper
Precambrian
(nearly
(Hutchinson,
1953)
Precambrian
below
east-west)
and
and by 1.3 km of
the
Random
Formation
(McCartney, 1957). Older beliefs that the quartzites and siltstones of the Random Formation
are
Musgravetown challenged
by
depositionally
Group
(Jenness,
Fletcher
(1972)
continuous 1963:
on
with
McCartney, the
basis
the
were
first
sections
along
1967)
of
underlying
southeastern Placentia and southwestern St. Mary's Bays. region,
In this
a unique chert and rhyolite pebble conglomerate occurs at
the base of the Random Formation, and a minimal stratigraphic cut
out of 140 m of upper Precambrian is demonstrable across Cape St. Mary's.
Fletcher's
(1972)
proposals
that
the
Random
is
depositionally unrelated to the upper Precambrian and that apparent local conformable Bonavista
relationships
Peninsulas
are
due
of
the Random
to
reworking
in the Avalon and of
eroded
upper
Precambrian sediments have been corroborated by subsequent work in the
southwestern Burin Peninsula.
In
this
region,
a
conformable
sequence of the Rencontre-Chapel Island-Random Formations is present (Landing et al., 1988). The absence of the uppermost Precambrian- lowest Cambrian Rencontre and Chapel Island Formations in the Avalon and Bonavista Peninsulas emphasizes the magnitude of the sub-Random unconformity in these areas of southeastern Newfoundland. Evidence of the Avalonian unconformity in the Burin and eastern Avalon Peninsula regions is less convincing because of unresolved probl~ms
in correlation of uppermost Precambrian and lowest Cambrian
sediments. Probable evidence for the Avalonian unconformity in the southwestern (1978, p.
Burin Peninsula has been reported by Strong et al.
25)
who noted a pronounced angular unconformity between
the "Inlet Group"
(=Chapel Island Formation) and pillow basalts of
the much older Burin Group. Possible evidence for folding associated with
compressive
tectonics
during
the
Avalonian
orogeny
may
be
present in the northeastern part of the Avalon Pentnsula. The "Lilly unconformity" at Knobby Hill to the north of St. John's (see King, 1982)
features
an
angular unconformity ,between
folded Conception
Group strata with conglomerates assigned to the top of the local upper Precambrian (Flat Rock Cove Formation) •
rD UPPERMOST PRECAMBRIAN-CAMBRIAN Depositional Sequences As
discussed above,
the
uppermost
Precambrian
through Lower
Cambrian appears to lie unconformably on the rocks of the Avalonian orogen
in
interval
southeastern (Rencontre
Newfoundland.
and
Chapel
western part of this area
The
Island
oldest
Formations)
(Fortune Bay,
overlie
the
eroded
upper
(see Landing and Benus,
1988).
sense
cumulative
a
deposi tional
slope
in
onlap the
and
latest
occur
this
in
the
(Random through Brigus
Precambrian
localities of
of
Burin Peninsula, eastern
Placentia Bay) and, in general, higher units Formations)
rocks
in
eastern
A generalized easterly
westerly-facing
Precambrian
and
regional
Early Cambrian
differs from the southerly facing paleoslope of the late Precambrian (see
King,
1979).
toward the
If
The
apparent
easterly
or
southerly
direction
interior" of the Avalon Platform during the Cambrian
also seems to be appropriate to other regions of the North American portion of the Avalon Platform. Landing available
evidence
that
suggests
that
(In press) Lower
has summarized
Cambrian onlap
took
place towards present southerly and easterly directions in eastern Massachusetts, Rhode Island and southern New Brunswick. The
uppermost
southeastern
Precambrian
Newfoundland
is
bounded depositional sequences 2).
through
divisible
into
Lower three
Cambrian
in
unconformity
(e.g. Vail et al., 1977; see
Fi~ure
The lowest of these is the Rencontre Formation-Chapel Island
Formation-Random Formation interval. demonstrable Peninsula
between
these
(Landing et al.,
stratigraphic
package
Conformable relationships are
formations 1988)
represents
in
the
southern
Burin
while the lower contact of this the
Avalonian
unconformity.
The
feather edge of this depositional sequence lies between Trinity and
Conception Bays and within St. Mary's Bay at the eastern limit of the Random Formation. The top of the Random Formation is a regional
tl
unconformity surface at all localities with overlying Lower Cambrian units in southeastern Newfoundland The
second
depositional
(see Landing and Benus,
sequence
is
the
1988).
unconformity-bounded
Bonavista Group in the Trinity-Placentia Bay and eastern Conception Bay .and St. Mary's Bay regions
(see Landing and Benus, 1988). The
Bonavista Group is unconformably overlain by the Brigus Formation (emended)
while a condensed manganese nodule bed or manganiferous
shales mark the onset of Middle Cambrian deposition Brook Formation)
following a
Cambrian boundary interval
(Chamberlains
regional hiatus in the Lower-Middle 1962~
(see Hutchinson,
Fletcher,
1972).
Lowest Depositional Sequence Rencontre Formation.--Conglomeratic red beds of the Rencontre Formation unconformably overlie acidic volcanics on the southwestern tip of the Burin Peninsula and mark the onset of latest Precambrian deposition.
These
proximal
alluvial
deposits
are
succeeded
by
fluvial and marginal marine siliciclastic sandstones, siltstones and mudstones. formation
Depositional near
the
environments
transition
into
Formation were probably peritidal
for the
(Myrow,
the
upper
overlying 1987).
part Chapel
of
the
Island
The depositional
history of the Rencontre Formation was complex with sedimentation and accumulation strongly controlled by fault activity
(Smith and
Hiscott, 1984). Chapel Island Formation.--The thickness of the Chapel Island Formation
in
the
southwestern Burin Peninsula
is not known with
precision. It is at least, but probably not much greater than, 1,000 m thick.
The most useful stratigraphic subdivision of the Chapel
Island Formation is Bengston and Fletcher's divided
the
formation
into
five
(1983).
informal members,
These authors numbered
1-5,
based on outcrops at Grand Bank (lower members) and Little Dantzic Cove
(upper
members).
This
same
modification by Crimes and Anderson
scheme
was
also
used
without
(1985). These informal members
12
haqe been more clearly defined by Narbonne et al. (1987). Random Formation.--The Random Formation is a very significant stratigraphic
unit
in
the
Lower
Cambrian
of
southeastern
Newfoundland. The prominent quartz arenites of the Random record an influx of quartz sands within the sub-trilobitic Lower Cambrian of th:i.s
region.
Formation
is
Landing the
(In
local
press)
has
suggested
representative
of
a
that
the
Random
high-energy,
quartz
arenite facies that was regionally extensive on the Avalon Platform. Local names that have been applied to this
lithosome in various
Avalon
Quartzite
Platform regions
Random Formation
include
the
Wrekin
(=Blue Pinion Formation)
(England),
(Newfoundland),
quartzite" in the Little Hollow Formation
"unnamed
(mainland Nova Scotia),
"beds 5-8" of the Morrison River Formation
(Cape Breton Island),
"White Quartzite" of the Glen Falls Quartzite (New Brunswick), North Attleboro Formation
(Massachusetts).
quartzite
is
lithosome
In these regions,
unconformably
overlain
by
this white
sub-trilobitic
Lower Cambrian or higher strata (see Landing and Benus, 1988). The Random Formation conformably overlies Formation in the Fortune Bay area Myrow,
1987).
The
sections
on
(Anderson,
the
the Chapel
Island
1981, Hiscott, 1982;
southwest
part
of
the
Burin
Peninsula indicate a gradual transition from the storm- and waveinfluenced environments of member 5 of the Chapel Island Formation to one in which strong tidal currents vigorously re-worked nearshore sediments.
On
the
Burin Peninsula,
the
upper
two-thirds
of
the
Random Formation consists of trough cross-bedded and herringbone cross-bedded
quartz
arenites
and
minor
shales.
These
represent
deposition under macrotidal conditions in shallow subtidal and lower intertidal environments (Myrow, 1987). Biota.--Trace fossils and skeletalized metazoans appear in the first
depositional
'sequence
above
the
Avalonian
orogen
in
southeastern Newfoundland. Although no fossils are presently known
13 in the Rencontre Formation, late Precambrian-aspect trace fossils of the
Harlaniella podolica Zone appear in member 1 and persist into
the lower 2.4 m of member 2 of the Chapel Island Formation. This low diversity assemblage of sub-horizontal grazing traces
is
rapidly
rep1aced by feeding and deep dwelling burrows of the Phycodes pedum Zone in lower member 2. This second assemblage provides a practical and
correlatable
base
of
the Cambrian
System
(Narbonne
et al.,
1987). All species of the Phycodes pedum Zone persist through the upper
part
of
the
Chapel
Island
Formation
and
into
the
Random
Formation. However, arthropod traces (Rusophycus, Dimorphichnus) and burrows with spreiten appear in the lower part of member 2 and are used
to
define
the
stratigraphically
long
ranging
Random Formation) Rusophycus avalonensis Zone.
(through
Skeletal
the
fossils
occur through much of the Chapel Island Formation although these remains are not preserved in the coarse-grained facies of member 5 or
the
Random
Formation.
Organic-walled
tubes
of
Sabellidites
cambriensis occur in upper member 1 and lower member 2 of the Chapel Island and are locally abundant in the Precambrian-Cambrian boundary interval.
However,
problematical
a
few
metazoan
poorly
ftLadatheca ft
preserved
conchs
cylindrica
of
(Grabau)
the were
recovered near the top of member 2. A somewhat more diverse small shelly fossil assemblage appears in the siliciclastic mudstones of upper member 3 with the association of "L. ft cylindrica, Aldanella attleborensis
(Shaler and Foerste), and Watsonella crosbyi Grabau
appearing in member 3.
This assemblage persists through member 4
where it is accompanied by halkierids, and species of Fomitchella, orthothecids
Tiksitheca, in
occurrence of A. "lower
Aldanella
the
bedded
Cambrotubulus limestones
and
Lapworthella,
Coleoloides
of member
4.
The
and
lowest
attleborensis is used to define the base of the attleborensis
Interval"
in
the
Chapel
Island
Formation while the underlying interval from upper member 2 through
most of member 3 is the "Ladatheca" cylindrica Interval" (Landing, In press; see Figure 3). Neither of these faunal intervals should be regarded as having a practical biostratigraphic significance within the Chapel Island Formation. Indeed early diagenetic dissolution of small shelly fossils in the somewhat lighter green colored shales at the Little Dantzic Cove section seems to be responsible for the loss of almost
all small shelly fossils
cylindrical
(with the exception of ilL. n
from upper member 3 and lower member 4. These remains
are preserved as pyrite casts through the upper 41.25 m of member 3 at Fortune North in darker-colored shales.
The stratigraphic range
of small shelly fossils extends well below their lowest occurrence as reported by Bengtson and Fletcher (1983)
from member 4 of the
Chapel Island Formation (Landing et al., 1988). Figure 3. Lithostratigraphy and proposed correlations of the uppermost Precambrian and Lower Cambrian of southeastern Newfoun~land (modified from Landing, In press). The transit10n from the upper Aldanella attleborensis assemblage to the overlying Camenella baltica assemblage is controlled by regional shoaling and the appearance of bedded limestones of member 2 of the Cuslett Formation. Avalon Platform
.!
Iii en
southeastern Newfoundland leurin Trinity
shelly sequence
1-0.,
Srigus Formation
I
E
to
1'I~1'1-'l' Y '1' fz" on'Vel 'l' ·'VI.Y YlWi ~wer)
Z
Yudoma Formation and older units
Z
u
5~ 6~ 7 L\I/~I
8~
..,..
..It" ..I-
91 \\\"1
.L
.J..
\\\\
.J..
\\\\
..L
Meters
1.0
Jf""
..r-
..a-
..I- Q:::) -'-
-'-
-'-
~
S'"
CD ......
CI:> -'-
/
......
WKBo 114.85
