The Backpit seam, Sydney Mines Formation, Nova Scotia: A record of peat accumulation and drowning in a Westphalian coastal mire

Palaeogeography, Palaeoclimatology, Palaeoecology, 106 (1994): 223-239

223

Elsevier Science B.V., Amsterdam

The Backpit seam, Sydney Mines Formation, Nova Scotia: A record of peat accumulation and drowning in a Westphalian coastal mire J u d i t h C. W h i t e a, M a r t i n R . G i b l i n g a a n d W o l f g a n g D . K a l k r e u t h b

Department of Earth Sciences, Dalhousie University, Halifax, N.S. B3H 3J5, Canada b Institute of Sedimentary and Petroleum Geology, 3303-33rd St., N. W. Calgary, Alta. T2L 2A7, Canada a

(Received October 3, 1992; revised and accepted March 10, 1993)

ABSTRACT White, J.C., Gibling, M.R. and Kalkreuth, W.D., 1994. The Backpit seam, Sydney Mines Formation, Nova Scotia: A record of peat accumulation and drowning in a Westphalian coastal mire. Palaeogeogr., Palaeoclimatol., Palaeoecol., 106: 223-239. Extensive Westphalian D to Stephanian coal seams divide the Sydney Mines Formation of Cape Breton Island into largescale repetitive sedimentary packages. The Backpit seam, one of the most continuous seams within this formation, was studied in detail to establish compositional trends and relate these to paleomire development within an overall transgressive setting. The seam is of high-volatile B to A bituminous rank and ranges in thickness from 0.6 to 1.5 m onshore. Agglutinated foraminifera occur in strata directly below and above the seam, indicating a coastal setting for the mire. Ash and sulphur contents average 15.3 + 6 and 5.2 + 2%, respectively. Sulphur, predominantly in the form of pyrite, increases near the roof of the seam, consistent with a brackish influence in the roof strata. The planar Backpit mire was subjected to widespread, periodic flooding, marked by thin dull to coaly shale lithotype intervals. Some intervals can be correlated across the onshore portion of the basin for more than 45 km and this distribution suggests regional controls on their formation. Seam lithology changes frequently in vertical section and banded lithotypes predominate. Vitrinite macerals and vitrinite-rich microlithotypes are abundant and thin discrete fusain bands, the remains of ancient fires, are also common. Coal facies patterns record a series of wetting upward pulses in the upper portion of the seam that culminated in the drowning and termination of the mire. A broad, relatively shallow embayment subsequently formed that supported a fresh- to brackish water fauna.

Introduction The Late Carboniferous Sydney Mines F o r m a t i o n , o f the S y d n e y Basin, N o v a Scotia, has long been the m a j o r source o f eastern C a n a d a ' s e c o n o m i c coal. T h e B a c k p i t s e a m is one o f the m o s t laterally extensive seams in the o n s h o r e p o r tion o f the S y d n e y M i n e s F o r m a t i o n a n d is e x p o s e d in a series o f cliff sections, f r o m n o r t h w e s t to southeast, for m o r e t h a n 45 k m parallel to d e p o s i tional strike (Fig. 1). H o l o c e n e c o a s t a l peats, which serve as m o d e r n a n a l o g u e s for m a n y C a r b o n i f e r o u s coals, c o m m o n l y have a c c u m u l a t e d , with o n l y m i n o r interr u p t i o n , for several t h o u s a n d years, d u r i n g which

time m a j o r climatic a n d sea-level changes have t a k e n place. C o a s t a l peats in s o u t h w e s t e r n F l o r i d a are presently being transgressed due to a rise in sea level o f a p p r o x i m a t e l y 3 m d u r i n g the last 4000 yr, which has resulted in the d e p o s i t i o n o f m a r i n e a n d b r a c k i s h w a t e r s w a m p sediments over freshw a t e r s w a m p deposits (Scholl, 1969). A t S n u g g e d y S w a m p , S o u t h C a r o l i n a , freshwater peats f o r m e r l y e x p a n d e d laterally as the rate o f p e a t a c c u m u l a t i o n was greater t h a n that o f sea-level rise; however, recent sea-level rise has o u t p a c e d p e a t a c c u m u l a tion, so t h a t in m a n y l o c a t i o n s salt m a r s h a n d l a g o o n a l d e p o s i t s overlie the freshwater p e a t s ( S t a u b a n d C o h e n , 1979; G a y e s et al., in press). Thus, t o d a y ' s coal seams are p o t e n t i a l i n d i c a t o r s

0031-0182/94/$07.00 © 1994 - - Elsevier Science B.V. All rights reserved. SSDI 0031-0182(93)E0093-9

224

J.C. WHITE ET AL.

ATLANTIC OCEAN N.B

"~1"

~,PEPERCE iN SYNCLINE

km

Fig. 1. The onshore distribution of the Morien Group, including the Sydney Mines Formation, within the Sydney Basin. Letters represent coastal outcrops of the Phalen-Backpit Interval as follows (from west to east): A = Bras d'Or; B = Sydney Mines; C = Victoria Mines; D = New Waterford; E = Glace Bay West; G = Donkin West; H = Donkin East; J = Longbeach. Symbols represent core locations as follows (from west to east): • = NC-87-1; • = SMS-91-29B; x = C-137; * = C-136. Inset shows the location of the study area; N.B. = New Brunswick, P.E.I. = Prince Edward Island.

of environmental fluctuations, including base level, that occurred during the peat-forming stage. Numerous workers have used lithotype, microlithotype, maceral and mineral analyses of coals to infer conditions and processes active during peat accumulation (Hacquebard and Donaldson, 1969; Cameron, 1978; Marchioni, 1980; Diessel, 1986; Kalkreuth and Leckie, 1989; Lamberson et al., 1991). These types of analysis are applied to intact oriented blocks of the Backpit seam, to interpret environmental changes in the original mire and assess relative sea-level variation within an overall transgressive setting. Geological setting for the Backpit seam The fault-bounded Sydney Basin is mostly submarine, and occupies at least 36,000km 2

(Hacquebard, 1983). It is a sub-basin of the Maritimes Basin that covered much of Atlantic Canada during the late Paleozoic. Rocks of this study occur within the coal-bearing Sydney Mines Formation (Fig. 1), and are of Westphalian D age (Barss and Hacquebard, 1967; Zodrow and McCandlish, 1978). The relatively simple structural style of the formation suggests accumulation during a stable phase in the basin's history. The basin gradually subsided, providing accommodation space for thick sequences of fluviolacustrine to marginal marine deposits (Rust et al., 1987). Gentle, seaward-plunging folds mimic the dominant northeast to easterly structural trends of the region (Fig. 1). Dips are generally shallow but locally can be as high as 45 °. Dominant paleoflow direction for the Sydney Mines Formation parallels these structural trends (Gibling et al., 1992).

BACKPITSEAM, SYDNEY MINES FORMATION,NOVA SCOTIA: PEAT ACCUMULATIONAND DROWNING

Summary of methods The Backpit seam was sampled in detail at five coastal sections across the onshore portion of the Sydney Basin (A, B, C, E and G, Fig. 1). Detailed observations were also made on adjacent lithologies. Bulk coal channel samples, representing sequential seam intervals, were collected and analysed for ash and sulphur content, following standard procedures. Oriented coal blocks, covering the total seam thickness, were also collected at each site for detailed lithotype and microscopic analysis. The blocks were mounted in resin and polished perpendicular to bedding. Lithotype logs were constructed for each section using a modified Australian classification scheme (Table 1) and a minimum band width of 5 mm (Diessel, 1965). Each lithotype log was examined for repetitive cycles; brightening and dulling upward cycles are characterized by the increase-decrease in lithotype brightness in at least three consecutive lithotype intervals. Oscillatory patterns are characterized by an interval of alternating lithotypes. The volume percent distribution of lithotypes for each seam section was calculated based on the aggregate thickness of each lithotype. TABLE 1

Lithotype classificationscheme (after Diessel, 1965) Lithotype

Description

Bright (BR)

Subvitreous to vitreous lustre, or conchoidal fracture, < 10% dull

Bright banded (BB)

Bright coal with some thin dull bands, 10-40% dull

Banded (B)

Bright and dull coal bands in equal proportion, 40-60% dull

Dull banded (DB)

Dull coal with some thin bright bands, 10-40% bright

Dull (D)

Matt lustre, uneven fracture, < 10% bright

Fibrous coal (F) (fusain)

Satin lustre, friable

Note: M i n i m u m thickness for each lithotype is 0.5 cm with the exception of fusain layers (F), pyrite bands (PY) and coaly shales (CS); lithotype abbreviations (in brackets) used in tables and figures throughout the text.

225

Detailed maceral, group maceral and microlithotype composition of the Glace Bay West section (E) was determined on polished blocks using a manual point counting technique and a stepping distance of 1 mm. Microscopic data were averaged by lithotype and by seam interval. Seam intervals were selected based on the occurrence of laterally extensive dull coal to coaly shale layers. Both maceral (Diessel, 1982) and microlithotype (Hacquebard and Donaldson, 1969) facies diagrams were used to assess coal facies. Random vitrinite reflectance measurements were also recorded on several samples from each seam section sampled.

Results The Backpit seam and associated strata

The Backpit seam is of high-volatile B to A bituminous rank. The mean vitrinite reflectance (random) of selected Backpit seam samples ranges from 0.69 to 0.76% and increases slightly from west to east (White, 1992). The seam ranges in thickness from 0.6 to 1.5 m onshore, and is thickest in the Sydney Mines and Glace Bay sub-basins (Fig. 2). This thickness distribution may reflect a topographic influence on seam development. Hacquebard and Donaldson (1969) suggested that most Sydney Basin seams were initiated in the Glace Bay Syncline and then spread across slight topographic highs as the mire developed. The seam thins considerably (locally < 20 cm) in offshore wells and methane test holes drilled in mining districts within 10 km of the coast. This distribution suggests an elongate form for the original mire, parallel to depositional strike. In contrast to other Sydney Basin seams, the Backpit seam does not split near the basin margins in the west and east, suggesting that the mire was not subjected to major fluvial incursions. The Backpit seam contains several thin, dull to coaly shale intervals (III, V, VII, IX, Fig. 2) that can be widely correlated. At most locations, a light grey, kaolinitic mudstone with abundant carbonaceous root traces forms an underclay to the seam. Coiled agglutinated foraminifera are common in this interval

226

8O cm

J.C. W H I T E ET AL.

NC-87-1

SMS-9129B

50

B

roof na

50

coal na

IV

_V. coal na

100

II I 150

Sydney Mines sub-basin

Glace Bay sub-basin

[";'~-~" I m u a s i o n e na

not a n a l y s e d

Fig. 2. Distribution of the Backpit seam and Backpit roof unit, and lateral correlation of facies within the study area. Datum is the top of the Backpit seam. Sectionlocations shown in Fig. 1. x = sideritebands

(W.G. Wightman, pers. comm., 1992). The Backpit seam is overlain by an extensive, organic-rich limestone to platy black shale sequence (the Backpit roof unit), 30-75 cm thick, which contains a fresh- to brackish water assemblage of sharks, fish, ostracods, bivalves, serpulids and agglutinated foraminifera (White, 1992). A fuller account of the interseam sedimentary facies successions can be found in White (1992). Mineral matter

The vertical and lateral distribution of ash and sulphur in the Backpit seam is illustrated in Fig. 3. The coal samples in this study contain, on average, 5.2_+2% sulphur by weight. Average sulphur, by seam section, ranges from 1.93 to 7.25%. Ash content averages 15.3_+6% and ranges from 5.89 to 21.41% by weight, per seam section studied. Both ash and sulphur tend to increase upward within a seam section, and commonly exhibit the highest values adjacent to the roof strata. Hacquebard and Donaldson (1969) quoted average

values of 11.53% ash and 6.21% sulphur from the Backpit seam in the Sydney Mines mining district. A large portion of the sulphur in the Sydney Basin coals is pyritic (Hacquebard and Avery, 1982; Birk et al., 1986). Microscopic observations suggest that the Backpit seam contains abundant syngenetic (e.g. crystals, framboids, clusters, globules) and some epigenetic (e.g. veinlets, cleat-filling) pyrite. Pyrite is concentrated above the clay parting (Interval III) and near the top of the seam in the Glace Bay West seam section. Secondary sulphates and iron stains were noted locally on weathered cleat surfaces at outcrop locations. Detrital clays constitute the major siliciclastic component observed in the Backpit coal matrix. Peats that yield economic coal seams are usually well removed from clastic input except for rare flooding events (McCabe, 1984). The Backpit seam records one such event in the lower to middle portion of the seam. This unit (Interval III, Fig. 2) can be traced for more than 45 km across the onshore portion of the basin and varies from coaly

227

BACKPIT SEAM. SYDNEY MINES FORMATION, NOVA SCOTIA: PEAT ACCUMULATION AND DROWNING

West cm

0

,,+1 ,2oo

o3+ ,8o..

,7oo ,2,

76.26 o.681~72.2 °

Roof

-

50

--136

'-na

7.25

FIP-O-r--r ----II~-,-~q 89"91 1.93I 5.89 20

10

*

0

50

I

• 20

na

100

53720

10

0

5'0

• 10

i 0

, 50

• 100

i 100

mI~----~72"21 20 10 0 50 100 2'0

10

0

50

100

East

2.78 0 ~

RI

84.54

2.08 0.42

84.62

-

+915 8 weighted d average I 1 _ 1 % sulphur

na 3

' 20

~ 10

" 0

20 50

10

0

50

na

100

2'o lo FI 20

L . _ 1 % ash

100

4 0

50

,

not analysed

100

8.58 10

0

50

100

Fig. 3. Sulphur and ash distribution in the Backpit seam and associated strata. Section locations shown in Fig. 2.

shale (locally > 60% ash) to a dull banded lithotype. Other minor sediment influxes are recorded in the numerous thin clay, inertodetriniteand liptodetrinite-rich detrital layers recognised microscopically.

Lithotypes Stach et al. (1982, p. 171) defined "coal lithotypes" as "the different macroscopically recognizable bands of coal seams". Lithotype logs (Fig. 4) illustrate the vertical megascopic variability of the Backpit seam. The volume percent distribution of lithotypes for each seam section is illustrated in Fig. 5. Total seam lithotype proportions are comparable across the basin suggesting that relatively similar conditions prevailed during the accumulation of the peat. Banded lithotypes comprise at least 40% of each seam section and bright lithotypes are present in minor amounts in the west (sections A and B). The Victoria Mines (C) seam section contains the greatest concentration of

megascopic pyrite and coaly shales increase in proportion eastward. Three patterns of lithotype succession were observed in the Backpit seam (Fig. 4): (1) brightening upward (average 10 cm thick), (2) dulling upward (average 12 cm thick) and (3) oscillatory (variable thickness). Oscillatory patterns are especially common between banded and bright banded lithotype intervals. Brightening upward cycles predominate in the lower to middle portion of the seam, whereas dulling upward cycles predominate in the upper portion of the seam (Fig. 4). The Backpit mire terminated during a dulling upward cycle that culminated in the deposition of the Backpit roof unit. Numerous fusain layers occur in all seam sections studied. These layers are generally lenticular, thin 0 - 2 mm) and represent 3-4% of the total seam thickness. Bedding-plane surfaces of the Backpit seam reveal randomly oriented fusain patches, suggesting that they may have drifted to the depositional site. These fusains exhibit charac-

228

J.C. WHITE ET AL.

~C

West

East

E G

B

A

©,.

15km

'°::~

\

4km

cm

9km

12

\ >

8O _

NN

12 __

I

1

/ \ / / nfii~giii I mR

I

-.

/

LITHOTYPE

'~

Coal Mudstone

©

/

BR - Bright

LITHOLOGY

Shale ncDos

BB B DB D CS

- Bright Banded - Banded - Dull Banded - Dull - Coaly Shale

R

- Rock

" " :" u

Limestone

|m

V~ \

dulling upward trend

. example of oscillatory trend

/ brightening upward trend

Fusain Layer Pyrite Layer

i

Interval III

Fig. 4. Lithotype logs for the Backpit seam: A = Bras d'Or; B = Sydney Mines; C = Victoria Mines; E = Glace Bay West; G = Donkin West. The top of the seam is the datum.

East

West A o

2~ I

BRBB- ~'] B-

B ~o I

o

~p

C ~o I

oI

E

2~ I

~o

oi

G

~5 I

0.4%

l

-7 ]

DB-D.?

1

-~ _1

I

7 I

I ?l J

Fpycs

-]

[-1

Fig. 5. Proportions of lithotypes, pyrite and coaly shale (vol.%) in sections through the Backpit seam. Abbreviations as follows: bright; B B = bright banded; B = banded; D B = dull banded; D = dull; F = fusain; P Y = pyrite; C S = coaly shale. Seam outcrop locations in Fig. 1.

BR=

229

BACKPIT SEAM, SYDNEY MINES FORMATION, NOVA SCOTIA: PEAT ACCUMULATION AND DROWNING

teristics of pyrofusain, the products of wildfires within a mire (Stach et al., 1982; A.C. Scott, 1989).

E 0

50

Macerals and microlithotypes

100

I

np

Numerous workers have shown a relationship between microscopic (macerals and microlithotypes) and macroscpoic (lithotype) composition of coal (e.g. Diessel, 1965; Cameron, 1978; Marchioni, 1980). This section integrates the lithotype divisions of the Backpit seam with detailed coal compositional data collected using standard coal petrographic techniques on the Glace Bay West (E) seam section. Group maceral composition

The average group maceral composition of bright versus dull lithotypes is distinct (Fig. 6). Vitrinite is predominant in the bright and banded lithotypes and liptinite and inertinite increase gradually, at the expense of vitrinite, in the dull banded and dull lithotypes. The group maceral composition of the Backpit seam is variable over a short vertical distance. This variability is reflected in the lithotype compositional profile of the Glace Bay West seam section analysed (Fig. 7). Vitrinite is the main maceral group and seam average vitrinite proportion is generally greater than 70% (excluding coaly shale intervals). Brightening upward cycles show an upward decrease in liptinite plus clays and/or inertinite and a concomitant increase in vitrinite. Dulling upward cycles exhibit the opposite trend. Slight compositional variations in maceral components are observed in the oscillatory sections.

Z

B

J

F Py, CS

Legend [-----I

Liptinite Clays

[

] Vitrinite Inertinite Pyrite

Fig. 6. Average group maceral composition, by lithotype, for the Glace Bay West seam section (E, Figs. 1 and 2). Lithotype abbreviations in caption Fig. 5. np = not present; na= not analysed.

Detailed maceral composition

Vitrinite is the most abundant maceral in the Backpit seam. Telocollinite tends to be enriched in the bright banded and locally in banded lithotypes, and is slightly more abundant in the lower portion of the seam, whereas desmocollinite is predominant in the upper portion of the seam. Vitrodetrinite is intimately mixed with clays in coaly shales and some dull lithotypes and is common in the transitional interval at the base and roof of the seam. This association suggests transport and reworking of plant fragments prior to deposition.

Corpocollinite occurs in minor amounts throughout the seam. These rounded to oval cell fillings occur in situ within tellocollinite or as displaced fragments within desmocollinite. Sporinite is the major liptinite component and includes microspores, megaspores and sporangia. Rare intact sporangia, with visible spores, occur in intervals I and II (Fig. 2, below the parting). Liptodetrinite tends to be concentrated in the duller lithotypes, commonly associated with inertinite fragments and clays, supporting a detrital

230

J.C. WHITE ET AL.

LEGEND

Liptinite Clays [~

Vitrinite Total Inertinite Fusain Layer Pyrite Layer III

Coal Interval Seam Average (excluding coaly shale intervals)

0

50

100

0

Seam Average* f [ ]

50

100

~/~!

Fig. 7. Group maceralcompositionprofiles,by lithotypeand seam interval for the Glace Bay Westseam section.

origin for these dull layers. Cutinite (leaf cuticle) occurs rarely in lithotypes above and below the parting (Interval III). Alginite is also rare. Distinctive oval to flattened resinite bodies occur in certain layers in this Backpit seam section. They exhibit high relief and very low reflectance under white light, and high fluorescence under blue light. Common oxidation rims suggest transport. The bodies are concentrated in dull layers above the coaly shale parting (Interval III). Some resinite bodies are vesicular and the cavities are filled with pyrite. Hacquebard (1952) referred to these bodies as "squat bulky spores" but later described them as "peculiar resin bodies" (Hacquebard and Donaldson, 1969). Secretion sclerotia are commonly associated with fusain layers and probably represent a fusinitized equivalent of the resin bodies described above. Draping of other macerals around these bodies

suggests that they were in a fusinitized form prior to compaction. Lyons et al. (1982) and Thompson et al. (1983) attributed similar bodies in Appalachian Carboniferous coals to fusinitized resin rodlets and inferred that these bodies were once resin ducts from medullosan seed ferns. They suggested the term resino-sclerotinite to differentiate these macerals from sclerotinite of fungal origin. Misra et al. (1990) suggested that resinosclerotinite could provide a correlation tool for Indian Permian coals. Fusinite is common typically as discrete layers (Fig. 4). Microscopically, these layers consist of well-preserved, moderate- to high-reflecting cell wall structures, commonly filled with pyrite or other mineral matter. Inertodetrinite is common in the duller lithotypes and the curved, shard-like shape of these fragments suggest that they represent the brittle remains of charred cells, probably

BACKPIT SEAM. SYDNEY MINES FORMATION, NOVA SCOTIA: PEAT ACCUMULATION AND DROWNING

transported by wind and/or water and incorporated into the peat. Semifusinite is common in some layers and represents the partial or incomplete charring or oxidation of woody tissues. Cell structure is visible but cells are generally deformed or flattened in a non-brittle manner. Macrinite and semimacrinite fragments are commonly associated with clays and liptodetrinites, suggesting transport. Groundmass macrinite was not observed in this study. Micrinite is common as lenses parallel to bedding and is often associated with liptinite macerals.

Microlithotype composition Average microlithotype composition for each lithotype (Fig. 8) generally correlates with lithotype brightness. Vitrinite-rich microlithotypes predominate and tend to decrease from bright to dull lithotypes. Inertinite- and liptinite-rich microlithotypes are present in minor amounts in the bright banded, banded and coaly shale lithotypes. Their content increases in the dull banded and dull lithotypes. Fusain contains a large proportion of inertite, as would be expected. Mineral-matter-rich microlithotypes occur in minor amounts in the brighter lithotypes; their proportion increases substantially in the dull to coaly shale lithotypes.

Coalfacies analysis Microlithotype-derived coalfacies Hacquebard and Donaldson (1969) introduced a coal facies classification based on the microlithotype composition of intervals from Sydney Basin seams. They integrated mire groundwater level as documented by Osvald (1937) with paleoenvironmental studies of Tertiary brown coals (Teichmfiller, 1950; Teichmiiller and Thompson, 1958) and proposed four main swamp environments as outlined in Table 2. The average microlithotype content of each lithotype and each seam interval was plotted on a four component facies diagram (Fig. 9a and b, respectively). Microlithotype groups (A, B, C and D, Fig. 9) were modified slightly to reflect the additional categories counted in this study. According to this classification scheme, the telmatic forest moor, represented primarily by bright

231

banded to banded lithotypes, is the most prevalent environment in the Backpit seam. One bright banded lithotype plots anomalously in the limnotelmatic forest moor environment. It occurs near the top of the seam and contains abundant finely disseminated mineral matter. Dull to dull banded lithotypes plot in distinct fields in Fig. 9a and represent either telmatic reed moor or limnotelmatic reed to forest moor environments. A small cluster of dull banded lithotypes plot in the telmatic forest moor region. These lithotypes are relatively rich in inertite and vitrinertite, thus accounting for their dull appearance. Open moor environments are represented by dull to coaly shale lithotypes. Fusain layers contain abundant inertite and plot in a cluster in the forest-terrestrial-moor environment (Fig. 9a). The reader is reminded of the problems associated with using fusain layers as environmental indicators and they are plotted on this diagram for consistency only.

Maceral-derived coalfacies Diessel (1982, 1986) devised a coal facies classification based on the ratios of what he considered to be environmentally diagnostic macerals as outlined in Table 2. Diessel (1986) used this classification to characterize Permian Australian coal facies from a variety of established depositional settings, ranging from back barrier to piedmont plain. Australian coals plotted in well-defined compositional zones on Diessel's facies diagram and suggested environmental controls on average seam composition. Facies variations in the Glace Bay West section are here described using this classification scheme. Bright banded to banded lithotypes are concentrated in Diessel's marsh to fen environments with minor wet forest swamp contributions (Fig. 10a). Most of the dull to dull banded lithotypes are associated with a wetter limnic environment. Fusain layers have high TPI values and correspondingly low GI values and plot in the dry forest swamp environment in this classification scheme. Maceral ratios for seam intervals tend to cluster in the fen to limnic zone (Fig. 10b), very close to the TPI = 1 guideline. Most lithotypes in the Glace Bay West seam section have a high G1 (> 1.0, Fig. 10a) suggesting

232

J.C. WHITE ET AL. 50

A. Vitrinite-rich m i c r o l i t h o t y p e s

A. 1. 2. 3. 4.

40-

V V+L V+I V>I+L

B. 5. 6. 7. 8. 9.

C. 10. 11. 12. 13. 14.

I>V+L L>V+I I+L L I

Car+Ar Can+An Csi+Si Cpy+Py Coth+Oth

30-

20-

c

8

o

E= 20

t1

234= 13B

O.

34L B

i1 2 3 4 I 13B

i1 2 3 4 ~

~1 2 3 4 ,

6

~1 2 3 4 =

#

cs 76.7%

B. Inertinite- a n d liptinite-rich m i c r o l i t h o t y p e s

10

o

6

30

8 9= E3B

~5 6 7 8 9j 13

FHq q5 6 7

9~

~

13B

6 7

b

91

9,

~5 6

L

~s

IQ,

62.4%

C. M i n e r a l m a t t e r - r i c h m i c r o l i t h o t y p e s

20

10 ¸

i10

13 B'B

,

=10 1213 j I~

tlO

1213 D'B

i

tlO

1213 I3

I

t

13 ~ F

d0111213 C'S

Fig. 8. Average microlithotype composition for each lithotype, Glace Bay West seam section. Microlithotype abbreviations as follows: V= vitrite; V + L = clarite; V + I = vitrinertite; V > I + L = duroclarite; I > V + L = clarodurite; L > V + l = vitrinertoliptite; I + L = durite; L = liptite; I = inertite; C a r + A r = carbargillite+ argillite; Can+An= carbankerite+ankerite; Csi+Si= carbosilicate + silicate; Cpy + Py = carbopyrite + pyrite; Coth + Oth = other minerals and mineral mixtures.

that relatively high water, low Eh conditions prevailed and enabled the preservation of large quantities of vitrinite. TPI values are mainly less than 1.0, and indicate that much of the vitrinite is unstructured desmocollinite. Anaerobic decomposition is postulated as the mechanism to account for the loss of cell structure without the production of large volumes of oxidised macerals.

Discussion

Paleogeographicsetting Paralic basins form in coastal areas and are influenced to some degree by relative sea-level fluctuation. Paralic peat-forming environments include back-barrier, delta (lower, middle and

BACKPIT SEAM. SYDNEY MINES FORMATION. NOVA SCOTIA: PEAT ACCUMULATION AND DROWNING

TABLE 2 Coal facies definitions Hacquebard and Donaldson (1969)

Forest-terrestrial-moor (FtM)

Terrestrial zone, above high water mark; abundant inertite and inertinite-rich vitrinertite

Forest moor (FM)

Telmatic to limno-telmatic zone; transitional between terrestrial and open moor environment; abundant vitrite, and vitrinite-rich clarite and vitrinertite

Reed moor (RM)

Telmatic to limno-telmatic zone; transitional between terrestrial and open moor environment; abundant liptite, liptinite-rich clarite, duroclarite and vitrinertoliptite

Open moor (OM)

Limnic zone; subaquatic; abundant clarodurite, liptinite-rich durite, carbominerite and mineral matter

Diessel (1986)

Tissue Preservation Index1

Compares the portion of those macerals in which tissue structure is preserved with those without structure; indicates the abundance of arborescent vegetation and/or the effects of oxidation

Gelification lndex 2

Compares partially and completely gelified macerals with those that are not gelified; indicates the degree of wetness within the mire

1Tissue Preservation Index ( T P I ) = telinite + telocollinite+ semifusinite+ fusinite desmocollinite+ macrinite + inertodetrinite 2Gelification Index (61)= vitrinite + macrinite semifusinite+ fusinite+ inertodetrinite

upper delta plains) and coastal and interdelta plains (Bustin et al., 1983). Hacquebard et al. (1967) interpreted the Sydney Basin as a paralic basin based on the relatively uniform seam thickness and variable vertical petrographic composition. Outcrop configuration precludes study of proximal distal trends and marine fossils were not then known. They postulated a

233

floodplain environment for peat accumulation, distal from marine influence. The recent discovery of foraminifera above and below major seams in the basin (Wightman et al., 1992) confirms a paralic, restricted marine setting for these coals. By comparison with foraminiferal assemblages in modern coastal settings (D.B. Scott et al., 1991), high marsh and low marsh-estuarine conditions prevailed, respectively, below and above the Backpit seam. High sulphur values at the top of the Backpit seam may also be related to marine or brackish conditions as marine-influenced coals tend to be enriched in sulphur, especially adjacent to the r o o f strata (Williams and Keith, 1963; Casagrande et al., 1977). Some sulphur may also have originated from dissolution of Windsor G r o u p evaporites in the source area (Bell, 1928; Gibling et al., 1989). We propose a broad, low-lying coastal plain, protected from most clastic input, for the accumulation of the Backpit peat. Regional facies patterns are too poorly known to make a more detailed classification. The Backpit seam thickens in the Sydney Mines and Glace Bay sub-basins possibly due to slight differential subsidence of a series of NE-trending pre-Carboniferous basement blocks as suggested by Gibling et al. (1987) and Gibling and Rust (1990). A series of dulling upward trends in the upper portion of the seam indicate progressive drowning of the mire. This is consistent with peat accumulation during a relative sea-level rise (transgressive phase), a situation inferred for many coal seams in the Carboniferous Ruhr coalfield (Stach et al., 1982) and some modern coastal peats (Scholl, 1969; Staub and Cohen, 1979; Gayes et al., in press). Backpit mire evolution

McCabe (1984) reported peat accumulation rates of 0.1-2.3 m m / y r for recent mires. Estimates of peat:bituminous coal compaction ratios range from 1.4:1 to 30:1, with a median value of 7:1 (Ryer and Langer, 1980). Assuming a peat accumulation rate of 1 m m / y r and a peat:coal compaction ratio of 10:1 (McCabe, 1984; Gibling and Bird, in press), the Backpit mire (up to 1.5 m thick) would have existed for up to 15,000 yr (i.e. 1 cm coal per

J.C. WHITEET AL.

234 a. TERRESTRIAL FtM B

b. TERRESTRIAL FtM

8

O ~

OBB Z~B []DB ID

3 © Forest Terrestrial

TELMATIC 7 " ~,Z

Forest ~ Moor IJ /

FM ?..--~

eCS n= 3 ©F n = 24 • seam n = 8

Forest Terrestrial

intervals

Reed Moor

"~ "~

--

TELMATIC

Forest • V l Moor I •

•

Reed Moor

T~'LMATIC

..:- : - RM FM

.:' Reed

.NO

"~-1;

Forest

L.NO

n= 7 n=11 n=lO n= 2

TELMATIC

Open

.. ~'~M

TELMATIC

I

AV

Ree(/ L'MN°-

oor

Moor

TELMATIC

A - Liptite + Clarite-L + Duroclarite + Vitrinertoliptite

/

B - Inertite + Vitrinertite-I

\ ~

/

C - Vitrite + Clarite-V + Vitrinertite-V

D

OM

D-

LIMNIC

Samples with >20 % dull components ( D ) are plotted on the lower triangle

D OM LIMNIC

Clarodurite + Durite + Carbominerite + Mineral Matter

Fig. 9. Microlithotype group composition of the Backpit seam, Glace Bay West; plotted on a modified facies diagram of Hacquebard and Donaldson (1969). (a) Lithotypes. (b) Seam intervals.

a.lO0

LIMNO~TELMATIC

TELMATIC

OaBBB [] DB ID

.1

MARSH

¢04

10

0/ ~ 441

[]

DO

GI

~

0 [][] 4DD []

o WET FOREST SWAMP

e4

~EN D

b. 100 nn-- 17 r n = 102 n

oF

• cs

ni, n i

• seam

n

intervals

LIMN( TELMATIC

TELMATIC

MARSH

10 " GI

:~

slY FEN I1.',1 VIII=HX ,VI

WET FOREST SWAMP

0

1.0

1.0 DRY FOREST SWAMP o

0.1

0

i

&V

o

TERRESTRIAL

1.0

i

210

20

i

31.0

0.1

0

I

TPI

decreases

Tree Density

1.0

DRY FOREST SWAMP

TERRESTRIAL = =

210

310

TPI increases

)

< decreases

Tree Density

increases

Fig. 10. Facies characterization of the Backpit seam, Glace Bay West; plotted on a facies diagram of Diessel (1986). (a) Lithotypes. (b) Seam intervals.

BACKPIT SEAM, SYDNEY MINES FORMATION, NOVA SCOTIA: PEAT ACCUMULATION AND DROWNING

100 yr). This method provides at best, a broad estimate as to the time period for peat accumulation without substantial clastic intervention. Brightening and dulling upward lithotype successions within the seam vary considerably in thickness; average thickness values (10 and 12cm) represent approximately 1000 and 1200 yr, respectively, using this method. Although lithotype successions are the result of a variety of interrelated factors, the brightening upward cycles probably represent relatively stable periods during which the mire gradually built up and progressed from very wet (dull to coaly shale) to moderately wet (banded, bright banded) conditions. Peat accumulation slightly exceeded baselevel rise. These stable periods were too short for the mire surface to build substantially above the water table, as there is no evidence for subaerial exposure of the mire (e.g. "dry" dull layers of Diessel, 1982). These brightening upward cycles are commonly abruptly punctuated by a dull to dull banded layer, probably induced by local flooding. Some of these flood deposits can be traced for up to 45 km and may be allogenic in origin. Dull lithotypes in the Backpit seam probably accumulated in a wet environment as inferred from the presence of abundant detrital clays, inertodetrinite and liptodetrinite and the absence of groundmass macrinite. Dulling upward cycles in the Backpit seam probably represent gradual rise in base level such that moderately wet (banded to bright banded) evolved to high water conditions (dull to coaly shale). Base-level rise exceeded peat accumulation in this situation. Dull lithotypes are locally succeeded by brighter lithotypes. Esterle and Ferm (1986) inferred that similar patterns recorded the response of vegetation to the increase in available nutrients due to the influx of water whereas Shibaoka and Smyth (1975) attributed it to the ability of arborescent vegetation to establish in relatively deep water. The Backpit seam may represent an upward change from a proximal to distal situation within the mire perhaps from tens of kilometres inland (lower portion, dominated by brightening upward cycles) to a position close to the coast (mid to upper portion, dominated by dulling upward

235

cycles). Agglutinated foraminifera occur in the underclay and in the roof strata and suggest that the mire formed during a minor regression or stillstand within an overall transgressive phase. Woodroffe et al. (1985) described similar peatforming episodes more than 100 km up the South Alligator River, Australia, during a Holocene transgression. The Backpit mire eventually was drowned and an extensive organic-rich limestone to shale unit caps the seam and represents deposition in a broad shallow embayment. Hoffmann (1933, in Teichmiiller, 1989, p. 60) described similar dulling upward facies trends from the Carboniferous Ruhr Coalfield where a progression from vitrinite-rich to liptinite-rich macerals and clays was interpreted as a development from drier (forests) to wetter (marshes, open-water) environments. Mastalerz and Wilks (1992) also described similar trends from the Intrasudetic Basin, Poland. Rheotrophic conditions probably prevailed during the history of the Backpit mire as suggested by the high ash content and the occurrence of a clay-rich parting near the centre. High detrital clay content in the upper portion of the seam accounts for the megascopic dulling upward trend and precludes the development of the mire significantly above the water table. Abundant syngenetic pyrite throughout the Backpit seam suggests that sulphate- and ferrous-enriched groundwater circulated freely within the mire and that the pH was high enough to permit the activity of sulphatereducing bacteria. Cecil et al. (1985) suggested that most late Pennsylvanian mires in the Appalachian region developed a planar geometry due to the switch to a "seasonal tropical" climate: rainfall was no longer consistent enough throughout the year to support mire growth above the water table. Evidence that suggests a seasonal climate in the Sydney Mines Formation includes gilgai structures in paleosols, deep desiccation cracks, and upright trees and in situ rhizoconcretions in channel bodies (White, 1992; Gibling and Bird, in press).

Vertical coal facies interpretation Microlithotype- (Hacquebard and Donaldson, 1969) and maceral-derived (Diessel, 1986) coal facies analyses are based on the assumption that

236

variations in coal facies represent changes in the depositional environment of the mire. These facies variations are dependent on changes in groundwater level, vegetation type, peat accumulation rates and degree of organic matter decomposition in the mire. The bright banded to banded lithotypes in the Backpit seam contain abundant vitrinite and plot in Hacquebard and Donaldson's forest moor environment. Since unstructured desmocollinite predominates (low vPI), these lithotypes plot in marsh to fen environments with minor wet forest swamp contributions on Diessel's diagram. This may not represent a conflict in environmental setting between the two facies classifications if low vP1 values are interpreted to be the result of anaerobic microbial attack in a wet forest environment. This would account for an abundance of vitrinite (forest environment) and subsequent loss of vitrinite structure (biochemical gelification). Dull banded to dull lithotypes commonly plot in limno-telmatic to limnic (reed moor) environments in both facies diagrams. Fusain layers are indicative of oxidation in both schemes and plot in the terrestrial-forest-moor (Hacquebard and Donaldson) or the dry forest swamp environment (Diessel). Fusain in the Backpit seam probably originated from mire fires and the use of this component to interpret environmental setting is dubious. Wildfire occurred regularly from at least the Late Devonian onward (Cope and Chaloner, 1985). These fires could potentially occur in any environment, thus limiting the use of fusain as a sensitive paleoenvironmental indicator (A.C. Scott, 1989; Calder et al., 1991). The allochthonous nature of many of these fusain-rich layers also supports this conclusion. Since evidence for desiccation of the peat is lacking in the Backpit mire (e.g. layers of massive macrinite), crown fires are envisioned as the most probable mechanism for the production of fusain layers. The abundance of fine inertodetrinite shards in some layers supports transport by wind and/or water. According to the microlithotype facies interpretation (Fig. 9b), accumulation of the Backpit peat began in a telmatic forest moor environment (Interval I). Three large-scale facies trends are apparent during the history of the seam. A "wet-

J.C. WHITE ET AL.

ting" upward trend, from forest (I), to reed (II), to open moor (III) facies characterizes the lower portion of the seam at Glace Bay. This was followed by a "drying" upward trend, from open moor (III), to reed (IV), to forest moor (VI) environments. An extensive fire event is recorded in Interval V and high spore content associated with inertinite suggests a water-lain origin for this interval. A gradual rise in base level in a forested setting is recorded from intervals VI to IX (wetting upward trend). Interval VII is not present in the Glace Bay West section; deposition was probably limited to the Sydney Mines sub-basin. Eventually water level rose to a point where peat accumulation was no longer possible and a large embayment covered the Backpit mire (White, 1992). According to the maceral facies interpretation (Fig. 10b), the Backpit mire began as a fen (intervals I and II) in a limno-telmatic environment. Interval III does not plot on this diagram due to negligible structured inertinite (GI= oo), however, open-water conditions existed with an influx of clastic sediments (limnic environment). The fen was re-established in Interval IV and drier forest conditions are indicated for Interval V. The upper portion of the seam, intervals VI-IX plot in a limnic zone similar to Australian coals deposited in a transgressive back-barrier environment (Diessel, 1986). Despite the differences in environmental interpretation, both facies classifications indicate a predominantly wetting upward trend for the Backpit mire. This is consistent with a mire developed during a transgressive phase, on a low-lying coastal plain. Conclusions

The Backpit seam is a banded bituminous coal that contains considerable mineral matter in the form of detrital clays and syngenetic pyrite. It was deposited within a broadly transgressive sedimentary package, but records evolution from regressive to transgressive conditions. The discovery of agglutinated foraminifera, both below and above the Backpit seam indicates a coastal setting for the mire. Other supporting evidence includes the elongate form of the mire,

BACKPITSEAM,SYDNEYMINESFORMATION,NOVASCOTIA:PEATACCUMULATIONAND DROWNING

parallel to depositional strike, the recognition of cyclothem patterns in associated strata that reflect fluctuations in relative sea level, and the high sulphur content of the coal, especially near the roof strata. Coal facies trends also support the periodic regional drowning of the mire with the occurrence of laterally traceable dull to coaly shale layers and the predominance of multiple dulling upward trends in the upper portion of the seam. The seam dulls near the top due to detrital clay content, and drowning terminated peat accumulation. A rheotrophic, planar Backpit mire is inferred from the abundance of detrital mineral matter (flooding) and high syngenetic pyrite content (moderate to high pH) in the coal. Dull to dull banded lithotypes contain abundant detrital macerals (inertodetrinite, liptodetrinite) and/or minerals, indicative of subaquatic deposition and there is no evidence for "dry" dull intervals (groundmass macrinite), characteristic of mire desiccation. Lithotypes in the Backpit seam are petrographically distinct and are useful for seam correlation. Brightening upward cycles predominate in the lower portion of the seam whereas dulling upward cycles characterize the upper seam section. This reflects the transition from forest moor to open moor environments or from fen to limnic (transgressive back-barrier) environments and records a shift of the mire closer to the coastline.

Acknowledgements Coal quality analysis was completed by the Coal Laboratory, Cape Breton Development Corporation, Sydney, Nova Scotia. Mai Nguyen is thanked for field assistance and drafting some figures. Access to cores was provided by Nova Scotia Department of Natural Resources and the Cape Breton Development Corporation. Winton Wightman provided valuable information on foraminiferal assemblage distributions. Financial support was provided by a Natural Science and Engineering Research Council of Canada (NSERC) postgraduate scholarship and a Texaco Research Scholarship to J. White and a NSERC operating grant to M.R. Gibling. Thanks also go

237

to reviewers M. Mastalerz and T.A. Moore whose suggestions greatly improved the text.

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BACKPITSEAM.SYDNEY MINES FORMATION.NOVASCOTIA:PEATACCUMULATIONAND DROWNING Nova Scotia. Thesis. Dalhousie Univ., Halifax, N.S, 287 pp. (unpublished). Wightman, W.G., Scott, D.B. and Gibling, M.R., 1992. Upper Pennsylvanian agglutinated foraminifers from the Cape Breton Coalfield, Nova Scotia: Their use in the determination of brackish-marine depositional environments. Abstr. Geol. Assoc. Can., Mineral. Assoc. Can., Joint Annu. Meet., Wolfville, N.S., p. AllT. Williams, E.G. and Keith, M.L., 1963. Relationship between

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sulphur in coals and the occurrence of marine roof beds. Econ. Geol., 58: 720-729. Woodroffe, C.D., Thorn, B.G. and Chappell, J., 1985. Development of widespread mangrove swamps in midHolocene times in northern Australia. Nature, 317 (24): 711 713. Zodrow, E.L. and McCandlish, K., 1978. Distribution of Linopteris obliqua in the Sydney coalfield of Cape Breton, Nova Scotia. Palaeontographica, 16: 17-22.