Peat accumulation on a drowned coastal braidplain: the Mullins Coal (Upper Carboniferous), Sydney Basin, Nova Scotia

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Sedimentary Geology 128 (1999) 23–38

Peat accumulation on a drowned coastal braidplain: the Mullins Coal (Upper Carboniferous), Sydney Basin, Nova Scotia Neil E. Tibert Ł , Martin R. Gibling Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, B3H 3J5, Canada Received 29 May 1998; accepted 20 April 1999

Abstract The Mullins Coal lies within the braided-fluvial South Bar Formation. The lateral extent (15 km) and thickness (2 m) of the coal suggest that controls for peat accumulation were allogenic. Marine inundation of the distal braidplain, as indicated by the high-sulphur content of the coal and the occurrence of agglutinated foraminifera in associated shales, caused ponding of freshwater near maximum marine transgression, with accumulation of thick paralic peat across a stable platform of sandy fluvial sediments. Highstand parasequences with thin capping coals were subsequently incised beneath a sequence boundary as the braidplain readvanced. The Mullins Coal thins and splits in association with muddy bayfill deposits eastward towards the Glace Bay Syncline, a long-lived palaeotopographic element related to fault-bounded basement blocks. Although relative sea-level change controlled the stratigraphic position of the precursor peats, differential tectonic subsidence and=or compaction modified their extent, thickness and quality.  1999 Elsevier Science B.V. All rights reserved. Keywords: Carboniferous; coal; sequence stratigraphy; fluvial; Canada

1. Introduction Thick and extensive peats (coal-precursor) are infrequently associated with low-sinuosity river deposits. Rare examples include the modern peat-forming wetlands (mires) that coexist with low-sinuosity rivers in Bavaria (Diessel, 1992, p. 374) and Permian low-sulphur coals that intercalate with low-sinuosity fluvial channel and wetland deposits in Australia (Martini and Johnson, 1987; Fielding et al., 1993). In general, extensive peat accumulation requires a near exclusion of all detrital sediment from Ł Corresponding

author. Present address: Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA; E-mail: [email protected]

the depositional site, a condition that is rarely sustained for substantial periods of time on the alluvial braidplain. We therefore ask the question: How do ‘windows’ favourable for peat accumulation (Nemec, 1992) open in fluvial systems characterised by active sediment transport? Such windows may represent times when channels occur some distance from the depositional site, so that autogenic effects such as channel avulsion may serve to explain alternations between coal and braided-river deposits. Alternatively, windows favourable for peat formation on an active alluvial plain may arise from allogenic mechanisms, such as tectonism and eustasy. Tectonic and eustatic models developed for coalforming environments are common. For example, Haszeldine and Anderton (1980) suggested that pe-

0037-0738/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 5 9 - 7

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riodic tectonic uplift in the source area allowed braided rivers to spread over a low-energy coastal plain with peat deposits in the Upper Carboniferous of northern England. Titheridge (1993) examined coals and braided-river deposits in the Eocene of New Zealand, and suggested that the stratigraphic architecture reflected syn-depositional faulting and tilting of half-graben blocks. Nemec (1992) documented Lower Cretaceous braidplain deposits in Svalbard, and inferred that some coals formed in deltaic and coastal settings when relative highstands of sea level inundated the braidplain. More recently, the Holocene Canterbury braidplains of New Zealand contain coastal peats that formed in response to glacio-eustatic sea-level fluctuations (Brown and Weeber, 1992). Numerous other studies have noted the association of coals with braided-fluvial channel bodies, typically a few tens of metres thick, and have inferred relative lowstands of sea-level for the channel bodies and transgressive or highstand conditions for the coals. These studies include Gustason (1989), Shanley et al. (1992) and Yoshida et al. (1996) in the Cretaceous of western North America, and Greb and Chesnut (1996) and Hampson et al. (1997) in the Carboniferous of the United Kingdom and Ireland. These authors have suggested both eustatic and tectonic controls on stratal architecture. The Upper Carboniferous South Bar Formation in the Sydney Basin of Nova Scotia is a 1 km thick braided-fluvial deposit that is very much thicker than the channel-body deposits referred to above. Thin coals and mudstones are interspersed with thick sandstones and conglomerates, and were initially interpreted in autogenic terms as peats formed on local braid-bar tops and=or in alluvial backswamps (Rust and Gibling, 1990). However, the Mullins Coal is unusually thick (>2 m) and extensive (15 km), and a detailed study of cores and outcrops suggests that the precursor peat formed in response to marine inundation of the South Bar braidplain. Changes in thickness and quality of the coal across fault-bounded basement blocks indicate that differential subsidence and=or compactional effects exerted a second-order control. Our results indicate that coals within thick and apparently proximal fluvial deposits can originate through relative changes in base level driven by an allogenic control.

2. South Bar Formation and regional stratigraphy The Sydney Basin, 350 by 150 km in present extent, contains the South Bar Formation (Fig. 1) of Westphalian C=D age (Boehner and Giles, 1986). As basal strata of the Morien Group, the South Bar Formation unconformably overlies Lower Carboniferous rocks where the stratigraphic thickness exceeds 1 km at the type section near Sydney. The formation shows local variation in thickness, especially in the northwestern part of the Sydney Basin where the strata occupy two prominent synclinal troughs (Fig. 1). The formation consists predominantly of sandstone, pebbly sandstone and conglomerate, with mudrocks and coals forming <10% of the aggregate thickness (Rust and Gibling, 1990). The overlying Sydney Mines Formation is a meandering fluvial to restricted-marine unit of Westphalian D to Stephanian age that consists of cyclic deposits of sandstone, mudstone, limestone and economic coal. The cyclothems are thought to record high-frequency eustatic events associated with Gondwanan glaciation (Gibling and Bird, 1994). At more easterly localities, the Waddens Cove Formation is laterally equivalent to the upper part of the South Bar Formation and consists of sinuous channel deposits incised into well-indurated, siliceous palaeosols. A series of northeasterly-trending faults cut Lower Carboniferous rocks beneath the Morien Group but appear to be sealed off beneath the unconformable base of the Morien Group (Fig. 1). The faults define a series of structural blocks, cored with pre-Carboniferous basement rocks, that include the Boisdale, Coxheath and East Bay Blocks, separated by the George River and Coxheath Faults (Fig. 1). Offshore to the northwest, the Boisdale Block is a strongly positive feature that delineates structural sub-basins (Hacquebard, 1983). Some of the faults are imaged in offshore seismic profiles (V. Pascucci, pers. commun., 1998) and may have originated as bounding faults to extensional basins that developed during the Early Carboniferous following Acadian deformation. However, the faults offset the Lower Carboniferous strata and most reflect some degree of mid-Carboniferous reactivation during the Maritimes Disturbance (Poole, 1967) that corresponds to the onset of the Alleghanian Orogeny in the U.S. Appalachians (Gibling, 1995).

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Fig. 1. Outcrop area of the Morien Group in the Sydney Basin, Nova Scotia, Canada. White areas onshore indicate underlying Precambrian to Lower Carboniferous bedrock. Anticlinal and synclinal axes are shown for folds that affect the Morien Group. Note that several major faults which cut the underlying Lower Carboniferous strata cannot be traced into the Morien Group. Box indicates the area shown in Fig. 2. Inset map shows location of the map area in northeastern Nova Scotia. Modified from Boehner and Giles (1986).

Gentle post-depositional folds trend northeast where the tectonic dips are typically less than 30º (Boehner and Giles, 1986). Key structural features of the Morien Group include the Bridgeport Anticline and the adjacent Glace Bay and Sydney Harbour Synclines (Figs. 1 and 2). The New Waterford Anticline is weakly developed in the northern part of the area, but is more prominent offshore. The Morien Group is thicker in the synclinal areas. Lithofacies analyses of the Sydney Mines Formation strata indicate that the Glace Bay Syncline formed a persistent, syndepositional topographic low (Gibling and Rust, 1990; Tandon and Gibling, 1997). The persistence of these trends throughout deposition of the Morien Group suggests continued minor displacement of the underlying faults, although compactional draping over stabilised blocks is also possible. South Bar palaeoflow was unidirectional and northeasterly, generally sub-parallel to structural lineaments in the underlying bedrock, but with greater variability in the lowermost strata (Gibling et al., 1992). Rust and Gibling (1990) divided the South Bar

Formation into three facies assemblages that form a continuous fining-upward succession. They concluded that the basal, conglomeratic South Bar strata were laid down in proximal braided rivers partially confined within large bedrock valleys cut into underlying formations. The valleys were subsequently filled with alluvium, and the upper, sandier strata were laid down in braided rivers that traversed an unconfined alluvial plain. Prior to the present study, marine fossils were not identified in the South Bar Formation. This made it difficult to determine if the braided rivers debouched directly into the ocean or passed distally into high-sinuosity channels. Robb (1876) documented numerous coals within what is now termed the South Bar Formation. Apart from the Mullins Coal, all were less than 1 m thick and could be traced for a few kilometres. The detrital strata contain numerous blocks of reworked coal (originally peat mats), indicating that channel erosion destroyed some original peats. In contrast, the Mullins Coal is >2 m thick and is known to extend approximately 15 km along depositional strike from

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Fig. 2. Study area of the Mullins Coal (see Fig. 1). Small black dots indicate drill hole locations.

North Sydney to southeast of River Ryan (Fig. 2). Apparently, the coal seam is displaced by faults in western localities (Haites, 1952) and pinches out at more easterly localities. The primary coal seam lies approximately 430 m above the base of the measured type section and approximately 310 m below the South Bar=Sydney Mines Formation contact (Rust and Gibling, 1990). Dolby (1989) identified Westphalian C=D palynomorphs above the upper South Bar=Sydney Mines Formation contact. The database for this study were obtained from cores drilled by the Nova Scotia Department of Mines and Energy in 1978 and since stored at their storage facility at Stellarton. Facies descriptions and centimetre-scale bedding measurements came from holes NW1 to NW16 (Fig. 2). Gamma-ray and neutron logs are available for holes NW1 to NW 7. In addition, we examined an exposure at the type section south of Victoria Mines (‘Coastal Section’ in Fig. 2) where most of the coal has been mined

out. Visual logs from the C-series holes drilled by the Cape Breton Development Corporation east of Sydney Harbour and other holes near North Sydney were consulted to establish the geometry of the coal. Petrographic analyses of the Mullins Coal were incorporated from Hacquebard and Donaldson (1969). The Mullins Coal Interval (MCI) is used to denote the stratal interval of approximately 50 m above and below the coal.

3. Micropalaeontology Microfossils were isolated from shale samples above and below the Mullins Coal (locations discussed below), and are discussed prior to facies analysis because of their palaeoenvironmental significance. The samples were processed and analysed using the methods outlined by Wightman et al. (1994). Table 1 provides a list of the agglutinated forami-

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Table 1 Number of protozoan specimens extracted from shale samples, Mullins Coal Interval (see Fig. 4 for sample locations) Sample: Strat. level: Total number:

NW2.1 55.47 48

NW2.2 52.12 11

NW2.6 21.94 15

NW2.7 17.37 40

NW4.1a 59.3 5

NW7.2b 30.02 9

NW7.43 20.42 46

NW15.03 27 48

Trochammina Ammobaculites Ammotium Arcellaceans

28 20

8 3

5

16 24

5

7

23 20 3

22 23 3

5 5

2

Table 2 Agglutinated foraminifera and arcellaceans characteristic of modern marginal-marine environments Environment

Estuary

Lower salt marsh

Upper salt marsh

Freshwater marsh

Tidal range Salinity

Sub=inter-tidal 1–35 ppt

0–70 cm above MSL 15–32 ppt

70–110 above MSL 1–25 ppt

Not applicable 0 ppt

Microfossils

Agglutinated

Agglutinated

Agglutinated

Arcellaceans

Modern Protists

Miliammina fusca Ammotium salsum Ammobaculites Eggerella advena Reophax

Ammotium salsum Miliammina fusca Ammobaculites

Trochammina inflata T. macrescans Tipotroncha comprimata Haplophragmoides

Centropyxis constricta Nebela collaris C. aculeata Difflugia

The foraminiferal compositions are characteristic for most temperate latitudes, and tidal range and salinity data are generalised. Data derived from Scott and Medioli (1980), Medioli et al. (1990), Scott et al. (1991) and Wightman et al. (1994).

niferal genera found in the core. Trochammina and Ammobaculites dominate most samples where Ammotium and freshwater arcellaceans comprise only minor constituents. Similar biotic assemblages were identified in shale samples from the overlying Sydney Mines Formation (Wightman et al., 1994). Scott et al. (1991) used agglutinated foraminifera and arcellaceans to establish distinct intertidal marsh zones in Holocene peats from the Mississippi Delta. Their results indicate that Trochammina is most common in the high marshes whereas Ammobaculites characterises the low marshes and estuaries (Table 2). Although modern representatives of the morphotypes identified in this study are considered cosmopolitan marsh species, Ammobaculites and Ammotium are common in brackish lagoons and estuaries (Ellison, 1972; Buzas, 1974). An upper marsh setting is favoured for the samples associated with the Mullins Coal in view of the abundance of Trochammina, and the scarcity of Ammotium and arcellaceans (Table 2). It is important to note, however, that Ammobaculites and Ammotium associated with thicker shales and

fluvial sediments in the MCI may indicate estuarine=lagoonal phases. The Trochammina and Ammobaculites association, common in Cretaceous rocks of the United States and Canada, are generally considered to represent a brackish-water, marginal-marine environment (Eicher, 1965; Wall, 1976; McNeil and Caldwell, 1981; Leckie and Tibert, 1997).

4. Facies assemblages We recognise ten facies (Table 3) and four facies assemblages from the MCI (Table 4). Fig. 3 shows a representative segment for each facies assemblage identified from the outcrop and core logs. 4.1. Fluvial channel facies assemblage 4.1.1. Description This assemblage forms the bulk of the South Bar Formation strata. Sandstones that are massive (Sm), trough cross-bedded (St) and cross-laminated (Sr)

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Table 3 Facies analyses of the Mullins Coal Interval, South Bar Formation (facies codes modified from Miall, 1978) Facies

Sedimentary features

Interpretation

Gmi, intraformational conglomerate

Up to 0.85 m thick; mudclasts 1–5 cm in diameter; organic detritus

Bank-derived clasts as lags on channel bases and scours

Gt, extraformational conglomerate

Up to 0.47 m thick; rounded granules along erosional bases of trough cross-sets

Lagged bases of 3D dunes within channels

Sm, massive sandstone

Up to 6.5 m thick; grey, medium-coarse sand; structureless to weakly stratified with diffuse organic bands

Dewatered dunes within channels; bioturbated sands in floodplain splays

St, cross-bedded sandstone

Up to 5 m thick; trough cross-sets <1 m thick in grey medium sand; load casts, organic partings, rare burrows

3D dunes within channels and standing-water bodies

Sr, cross-laminated sandstone

Up to 1.1 m thick; 2 cm cross-laminated sets, locally climbing, in grey fine sand to silt; convolute laminae, flaser and lenticular bedding (mud interlayers), rare burrows

3D ripples within channels, standing-water bodies and floodplain splays

Fl, laminated siltstone

Up to 3.7 m thick; red=grey silt with clay partings; planar laminated, with ripple cross-laminae, convolute laminae, load casts, organic partings and root traces

Traction=suspension deposition in standing-water bodies, periodically very shallow

Fm, massive mudstone

Up to 2 m thick; grey, poorly stratified silt to clay; root traces, slickensides

Hydromorphic palaeosol (gleysol?)

Fmr, mottled mudstone

Up to 0.85 m thick; poorly stratified with grey=red mottles, root traces, slickensides, carbonate and ferruginous globules

Well-drained palaeosol (calcic vertisol?)

C, coal

Up to 2.13 m thick; banded bituminous coal, pyrite commonly visible; a few centimetres to 30 cm in bayfills

Peat-forming wetland (mire); histosols

Ch, carbonaceous shale

Up to 1 m thick; mixed organic detritus and clay, laminated; associated with facies Sr and Fm

Clastic wetland with shallow standing water

Table 4 Facies assemblages of the Mullins Coal Interval, South Bar Formation Assemblage

Sedimentary features

Depositional setting

Fluvial channel

Cosets of trough cross-bedded and massive sandstone (St, Sm) up to 5 m thick, separated by prominent erosional surfaces. Local fining-up units: Gt=Gmi–St=Sm–Sr–Fm=Fl. Foraminifera in one shaly interbed.

Distal braided rivers with channel and bar deposits; local tidal influence

Bayfill

Coarsening-up units, up to 3 m thick, of Fl–Sr, St and Sm, capped by hydromorphic palaeosols (Fm) and=or carbonaceous shale (Ch). Up to 4 units are stacked in packages up to 9 m thick. Foraminifera in four samples.

Bayhead delta and=or crevasse-splay deposits with local thin peat; brackish-water influence prominent

Mire

Mullins Coal, up to 2.13 m thick where it overlies fluvial channel assemblage to the west; grades laterally into impure coal (Ch) where it overlies bayfill assemblage to the east. Rare thin coal associated with bayfills. Coals overlie rooted palaeosols (Fm).

Blanket coastal (paralic) peat

Well-drained floodplain

Red=grey units of interstratified Fmr, Fm and Fl with sandstone beds (Sr), up to 3 m thick. Intercalated with the bayfill assemblage.

Floodplain with palaeosols indicative of relatively prolonged exposure and crevasse splays

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Fig. 3. Representative portions of the coastal section (A) and core logs (B to E) to illustrate the facies and facies assemblages of the Mullins Coal Interval. Drill holes are NW1 (B), NW5 (C, E) and NW13 (D). Facies codes are shown in Table 3.

comprise most of the assemblage (89% in aggregate thickness). Thin conglomerate (Gt, Gmi) occupies the erosional surfaces and trough bases. Massive mudstones that contain root traces (Fm) comprise less than 8% of the strata. Trough cross-sets at the coastal outcrop are 1 m thick and form 5 m thick stacked cosets that are separated by prominent erosional surfaces (Fig. 3A,B). Some units display a fining-upward trend. Basal to middle units contain pebble-granule beds (Gt) that grade into mediumgrained cross-bedded sandstone (St and Sm). The uppermost beds are often fine-grained, cross-laminated sandstone (Sr) that terminate with grey pedoturbated mudstone (Fm). Vascular plant fragments are ubiquitous in the sandstones.

Channel deposits are thick and multi-storied in most cores. Towards River Ryan, however, channel bodies as thick as 5 m intercalate with locally isolated finer-grained strata. These deposits show a fining-upward trend where large-scale cross-beds and conglomeratic layers are progressively replaced by generally thin packages (<1 m) of siltstone and mottled claystone. A shale sample obtained from a predominantly cross-stratified sandstone succession (approximately 15 m below the Mullins Coal) yielded a relatively rich foraminiferal assemblage of Ammobaculites sp., Trochammina sp., and Ammotium sp. (sample NW15.03, Table 1).

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4.1.2. Interpretation The abundance of trough cross-bedded sandstone arranged as stacked, erosively based units with poorly developed fining-upward trends and a low mudstone to sandstone ratio indicates that deposition occurred in a braided-fluvial setting (Cant and Walker, 1978; Rust and Gibling, 1990). Abundant fossilised plant detritus indicates that deposition occurred adjacent to a vegetated land surface. As noted by Rust and Gibling (1990), the abundance of trough cross-bedding and scarcity of planar cross-bedding suggests that the South Bar fluvial systems were relatively deep braided rivers, rather than forming shallow networks on an extensive braidplain. The presence of agglutinated foraminifera in the shales suggests that the braided river debouched directly into the ocean. However, sedimentological evidence for either tidal or estuarine conditions within the assemblage have yet to be identified. Isolated channel bodies may represent distributary channels associated with bayfills (see below). 4.2. Bayfill facies assemblage 4.2.1. Description Strata assigned to this assemblage consist of one or more coarsening-up units that comprise laminated siltstone (Fl), trough cross-bedded sandstone (St), and cross-laminated sandstone (Sr) (Fig. 3C). The basal fine-grained strata contain wavy and planar laminae and ripple cross-laminae separated by clayrich partings with macerated plant fossils. They pass upwards into cross-laminated sandstone with flaser and lenticular bedding, convolute structures, load casts and rare burrows. Some units are capped by grey mudstones a few decimetres thick, with carbonaceous root traces and slickensides (Fm), that are locally overlain by coal (C) up to 30 cm thick and carbonaceous shale (Ch) up to 1 m thick. The units range from 3–9 m in thickness where they typically comprise three or four stacked parasequences. Of interest is a massive 2 m thick sandstone bed that occurs within a thick, laminated siltstone succession (core NW7). This sandstone unit contains subangular mudstone pebbles (10–15 cm in diameter) and abundant woody-plant detritus and is in sharp contact with the associated bayfill siltstones. Microfossils identified in samples of laminated

siltstone collected 1–10 m below the Mullins Coal include Ammobaculites sp. and Trochammina sp. (samples NW2.1 and 2.2, NW7.2b: Table 1). Sparse palaeoflow data (see below) from ripple cross-laminated sandstone indicates a northeasterly flow that agrees with those measurements taken for the fluvial channel deposits. 4.2.2. Interpretation The coarsening-upward nature of the units and the evidence for subaqueous conditions below a capping palaeosol, indicates that sediment progressively filled standing bodies of water. Sediment was provided both from traction and suspension and the ripple cross-lamination appears to be entirely of a current origin. The compacted thickness of the units (3–9 m) may provide an indication of minimum water depth, provided that water depth remained constant during deposition. The presence of agglutinated foraminifera associated with flaser bedding and sparse burrows indicate restricted marine conditions within the bays. Sedimentological analogues for this assemblage can be found in the deposits of shallow interdistributary basins that contain crevasse splays and small deltas (Coleman and Prior, 1980; Tye and Coleman, 1989), tidal estuaries, backbarrier lagoons, and fluvio-lacustrine systems (Staub and Cohen, 1979; Fielding, 1984; Breyer, 1987; Nemec, 1992). The presence of agglutinated foraminiferal assemblages indicates a marine connection, either by tidal channels or open bay mouths (Coleman and Prior, 1980; Breyer, 1987), and the assemblage is given the general name of ‘bayfill’. The abundance of current ripple lamination, with palaeoflow orientations similar to those measures for the fluvial channel assemblage, suggests that the water bodies were filled by fluvial sediment, probably in the form of crevasse splays and bayhead deltas. The abundance of load casts and convolute stratification indicates that the surface sediments were soft and water-saturated. The grey mudstones that cap some units are immature hydromorphic palaeosols that reflect waterlogged conditions, which we classify as gleysols using the system of Mack et al. (1993). The thin coals and carbonaceous shales may represent flooding surfaces that formed as the progradational bodies were abandoned and drowned. The discrete, thick sandstone bed em-

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placed within fine-grained strata resembles some washover sand sheets described from back-barrier settings by Staub and Cohen (1979) and Reinson (1992). Alternatively, the mud intraclasts within the sandstone bed resemble crevasse splay deposits that form during large, seasonal, flooding events as observed on the Mississippi Delta (Coleman and Prior, 1980).

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the associated bayfills. The splits of impure coal indicate periodic flooding and increased clastic supply to the mire (Kosters et al., 1987), and their presence, coupled with the relatively high ash content, suggest that the mires were planar in form and supplied by groundwater (rheotrophic). The predominantly reed moor to forest moor, limno-telmatic setting for the Mullins Coal, implies that the mire was poorly drained.

4.3. Mire facies assemblage 4.4. Well-drained floodplain assemblage 4.3.1. Description This assemblage constitutes the Mullins Coal Seam and associated fine-grained seat earth including coal (C), carbonaceous shale (Ch), and massive mudstone (Fm) (Fig. 3D). The Mullins Coal is of high volatile bituminous A rank with an ash content averaging 11% and a sulphur content of 5.93% (six analyses: Hacquebard and Donaldson, 1969). The aggregate thickness of coal in the cores ranges from 0.55 to 2.13 m. Units of carbonaceous shale intercalated with the coal range from 0.39 to 5.06 m thick and are composed of admixtures of organic material (including abundant megascopic vascular plant material) and dark, fine-grained sediment; they are interstratified locally with thin mudstones (Fl and Fm). As noted above, thin coals and carbonaceous shales are associated with the Bayfill Facies Assemblage. Hacquebard and Donaldson (1969) studied the petrography of the Mullins Coal at two localities, one near Victoria Mines and the second near the axis of the Bridgeport Anticline near River Ryan (Fig. 2). They identified three dull intervals within the seam, correlatable between the two localities, which are thought to represent limno-telmatic (reed moor to forest moor) conditions. Brighter coal, attributed to telmatic conditions, forms the bulk of the seam. In contrast, duller intervals indicate forest moor conditions prevalent near River Ryan and reed moor conditions near Victoria Mines. 4.3.2. Interpretation The Mullins Coal formed in extensive coastal mire. The high sulphur values match those expected for modern peats subject to marine influence (Bustin et al., 1983; Given et al., 1983), an interpretation further supported by the restricted-marine biota of

4.4.1. Description This facies assemblage, identified only in core, contains cross-laminated sandstone (Sr), laminated siltstone (Fl), mottled mudstone (Fmr), and massive grey mudstone (Fm) (Fig. 3E). Two occurrences of the assemblage were noted, each <10 m thick where they intercalate with strata of the Bayfill Facies Assemblage. A typical succession has a red=grey mudstone that is in sharp contact with overlying grey mudstone which is in turn overlain with cross-laminated siltstone or sandstone (Fig. 3E). The mudstone (Fmr) has red=grey mottles, slickensides on fractures, carbonate globules and some carbonaceous root traces. In thin section, the mudstone comprises silty clay with a well-developed lattisepic fabric (Brewer, 1976). Oriented ferruginous clay line fractures within the clay, and calcite rosettes (aggregates of radial calcite crystals) are present. Hematite concretions contain inclusions of clay, silt and carbonate minerals. 4.4.2. Interpretation The mottled mudstones with their roots, slickensides, oriented clay fabrics, hematite and carbonate globules are interpreted as palaeosols that formed in well-drained settings with generally low groundwater levels. Fractures bordered by oriented clay are interpreted as cutans, indicative of clay illuviation within soils. We classify the palaeosols as calcic vertisols using the system of Mack et al. (1993). Similar red palaeosols occur in the Sydney Mines Formation (Tandon and Gibling, 1997). We interpret the associated sandstones as alluvial splay deposits. The scarcity and thinness of the stratal bodies indicate that well-drained floodplain deposits were relatively rare during deposition of the MCI.

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5. Geometry of the Mullins Coal Interval The MCI extends from North Sydney to southeast of River Ryan, a distance of approximately 15 km (Fig. 2). Fig. 4 shows the relative distribution of the facies assemblages along the crop of the Mullins Coal. Palaeocurrent data from the coastal outcrop at Victoria Mines indicate a northerly palaeoflow, in accordance with measurements taken from upper and lower strata of the South Bar Formation (Gibling et al., 1992). The line of section illustrated in Fig. 4 roughly corresponds to depositional strike. The fluvial channel assemblage dominates at westerly localities on the western limb of the Bridgeport Anticline (Fig. 4). At eastern localities, fluvial channel sandstones wedge out in the 7–30 m interval above the Mullins Coal. Isolated fluvial bodies

(3–5 m thick) also occur in the vicinity of River Ryan where they contain red intraformational conglomerates (Gmi), suggesting episodic erosion of the well-drained floodplain deposits. The bayfill assemblage dominates near River Ryan on the eastern limb of the Bridgeport Anticline and the western limb of the Glace Bay Syncline (Fig. 4). Individual coarsening-up units above and below the Mullins Coal increase in thickness from approximately 1–2 m (core NW10) to 8–10 m (core NW7) (Fig. 4). The well-drained floodplain deposits form two thin lenses that pinch out to the west at 12 m and 30 m above the Mullins Coal in the River Ryan–Glace Bay Syncline area. These deposits represent periods of subaerial exposure. The Mullins Coal is thickest at Victoria Mines where it closely overlies trough cross-bedded sand-

Fig. 4. Correlation of drill cores that intersect the Mullins Coal between Victoria Mines and the Glace Bay area (see Fig. 2). The coal and impure coal belong to the Mire Assemblage, and red sandstone and mudstone to the Well-Drained Floodplain Assemblage. The location of samples that yielded agglutinated foraminifera is also shown (Table 1); both sample localities for core NW2 represent two closely spaced samples. Note the change of scale in the centre of the diagram, with 5 km distance between the two sets of drill holes. The line of section is roughly parallel to depositional strike.

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stone. A thin hydromorphic palaeosol (grey rooted mudstone or ‘seat earth’), less than 2 m thick, separates the coal from the channel deposits. This relationship can be traced laterally to the east from the coastal outcrop to core NW1 near River Ryan. The peat probably developed on a relatively noncompactible platform of sand. Across this same area, a continuous layer of bayfill strata, a few metres thick, overlies the Mullins Coal. Southeast of River Ryan, the coal splits and is gradually replaced by a carbonaceous shale wedge (Fig. 4). North of our cross-section (Fig. 4), the C-series drill cores (Fig. 2) show a gradual thinning of the Mullins Coal Seam to approximately 0.9 m in thickness. The facies relationships are not entirely resolved from the stratigraphic profile illustrated in Fig. 4. Above the Mullins Coal, the fluvial channel assemblage could interdigitate with strata of the bayfill and well-drained floodplain assemblages towards the Glace Bay Syncline, or it could occupy one or more valleys cut through these strata. The thin occurrences of well-drained floodplain deposits may thicken towards the Glace Bay Syncline, where tongues of the predominantly red sediments of the Waddens Cove Formation occupy the same stratigraphic position (Fig. 2). In the North Sydney area (Fig. 2), a fault appears to truncate the Mullins Coal (Haites, 1952). Old mine data suggest that the Mullins Coal and associated bayfill mudstones become progressively thinner towards the west where they are gradually replaced by thick fluvial-channel deposits. A palaeogeographic reconstruction for the study area at the commencement of Mullins peat accumulation is shown in Fig. 5. Braided rivers are depicted as flowing northward across an alluvial plain from bedrock valleys cut into upland areas to the southwest. Through time, channel avulsion created a sheet-like fluvial sandstone body with a relatively even surface. The Glace Bay area formed a topographic low where a restricted-marine embayment was periodically inundated with river-derived sediment. High freshwater discharge and a rising water table associated with relative sea-level rise, were sufficient to maintain substantial peat growth in the low-salinity restricted bay. The precursor wetlands of the Mullins Coal are shown bordering the landward margin of the bay and as local backswamps on the alluvial surface. These swamps extended across the

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Fig. 5. Palaeogeographic reconstruction of the study area at the start of Mullins peat accumulation. Note that the presence of restricted-marine biota in the shales implies a brackish water body, but that the nature of the protecting barrier is uncertain.

infilled bay surface when rates of peat accumulation kept pace with increasing accommodation.

6. Sedimentological analogues The Mullins Coal represents a disjunct facies relationship, in which braided-river sedimentation was interrupted by marine-influenced ponding in the topographic lows. At this time, rates of peat growth matched the rate of relative base-level rise and peat accumulated across the alluvial surface. Rust and Gibling (1990) did not identify coeval meanderingfluvial facies within the South Bar Formation and we infer that the South Bar braided rivers ran directly into the ocean, as in the Canterbury Plains (Brown and Weeber, 1992). This inference is supported by the occurrence of foraminifera within shales of the Fluvial Channel Assemblage. The scenario depicted in Fig. 5 suggests that, in some respects, the Snuggedy Swamp of South Carolina (Staub and Cohen, 1979) forms an analogue to the Mullins Coal Interval. Snuggedy Swamp is a marsh and tidal channel complex developed behind and between active and remnant barrier-island sands. The extensive wetlands are accumulating thick (up to 4.6 m) layers of freshwater peat and thin (up to 0.3 m) layers of salt-marsh peat. The tidal-flat deposits are up to 8 m thick, and consist of stacked coarsening-up sequences of clay to silt. A thin rooted zone

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or salt-marsh peat caps each sequence. The freshwater peats developed on the inner side of the tidal-flat area and advanced seaward over salt-marsh peats at times when peat accumulation outstripped sea-level rise. The stacked coarsening-up units at Snuggedy Swamp may reflect an episodic, rather than a steady, rise in sea level. Although the Snuggedy Swamp analogue is simplistic in many respects, it provides an explanation for the successive flooding and filling of the coastal bays. Similar coarsening-up bayfill stratal stacking patterns have been documented from the interdistributary bays and lakes found on the Mississippi Delta (Tye and Coleman, 1989). In terms of the coal sulphur content, recent models developed for tropical mires show that thick, low sulphur peats can accumulate adjacent to marine areas and may not provide direct evidence for nonmarine conditions (Cecil et al., 1993). In contrast, high sulphur coals may form, well-removed from direct marine influence (Bustin et al., 1983). We emphasize that the high sulphur values recorded for the Mullins Coal, the lateral extent and geometry of the seam, and the presence of agglutinated foraminifera provide compelling evidence that the peat accumulated adjacent to a system where marine influence was significant. The western margin of the Late Cretaceous epeiric sea provides an ancient analogue where alluvial (including braided fluvial) deposition was repeatedly interrupted by relative base level changes. Peats developed along the western margin of the seaway (Ryer, 1984; Roberts and Kirschbaum, 1995), and the resultant coals of the Dakota and Iron Springs Formations in Utah are thin and have sulphur values as high as 4% (Tibert, Leckie, and Eaton, unpublished data). The Iron Springs Formation includes thin coals that alternate with braidplain deposits, shed from the Sevier orogenic belt to the west (Gustason, 1989). Faunal evidence is scarce with the exception of a few brackish-water vertebrate and invertebrate taxa (Gustason, 1989; Eaton et al., 1997). Agglutinated foraminifera include Ammobaculites sp. and Trochammina sp., and a near monospecific population of the brackish water ostracode Cytheridea sp. dominates the assemblage (Tibert, Leckie, and Eaton, unpublished data). Base-level fluctuations in the foreland basin were controlled by the combined influence of tectonism, eustasy,

and climatic=oceanographic perturbations (e.g., Elder and Kirkland, 1993).

7. Controls on Mullins Coal geometry 7.1. Relative sea level and sequence stratigraphic analysis The facies analysis of the MCI suggests that relative rise of sea-level exerted a first-order control on the accumulation of thick, extensive peat, opening a ‘window’ in an environmental setting unfavourable for extensive peat accumulation (Nemec, 1992). A sequence stratigraphic interpretation for the interval is shown in Fig. 6. We suggest that rising sea level caused ponding of freshwater conducive for peat growth and created the accommodation space necessary for thick peat to accumulate. This model compares favorably with the work of Kosters and Suter (1993) who found that the thickest peats lay landward of a maximum transgressive shoreline underlying the Mississippi Delta. The Mullins Coal is inferred to lie close to a maximum flooding level within the succession. The coal is overlain by laminated, plant-rich shale (facies Fl) which may mark maximum flooding, although similar shales are present at several levels within the MCI. No unusually radioactive shales that might indicate condensation are observed on the gamma-ray logs. The Mullins Coal is relatively thick and is separated from the underlying fluvial sandstones west of the Bridgeport Anticline by a thin, muddy seat earth. Peat accumulation commenced shortly after transgression of the braided-river plain. In contrast, the Mullins Coal rests on restricted-marine bayfill units east of the Bridgeport Anticline. Marine waters evidently covered the latter area prior to the major phase of peat accumulation, and the underlying bayfill deposits are interpreted as a stacked set of retrogradational bayfills within the transgressive systems tract. Coarsening-up units of bayfill strata overlie the Mullins Coal. These parasequences contain thin coals and carbonaceous shales that contain agglutinated foraminifera that provide compelling evidence for marine flooding of the marsh surface. These strata are assigned to the highstand systems tract. A similar dichotomy of basal thick coal and

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Fig. 6. Sequence stratigraphic interpretation for the Mullins Coal Interval, simplified from Fig. 7. MFS D maximum flooding surface; TST and HST D transgressive and highstand systems tract, respectively.

higher thin coals was noted in Sydney Mines Formation cyclothems by Gibling and Bird (1994), and in the Late Carboniferous of Kentucky by Aitken and Flint (1995). We interpret the basal surface of the overlying braided-fluvial sandstones (Fig. 4) as a sequence boundary. As noted above, the fluvial strata may occupy one or more palaeovalleys, but the core database is insufficient to resolve this question. Bohacs and Suter (1997) examined the relationship between rates of peat production and generation of accommodation space. They suggested that the thickest coals in paralic successions accumulate during two stages in the relative base-level cycle: late lowstand to early transgressive, and late transgressive to early highstand. Thinner coals should form during middle transgressive and later highstand stages. For the Mullins interval, the lack of exposure that could elucidate proximal to distal trends precludes a full sequence analysis. However, transgressive strata underlie the Mullins Coal in the Glace Bay Basin (Fig. 6), which suggests that thick peat accumulation was not associated with late lowstand conditions. Additionally, the coal-bearing interval lies within predominantly alluvial strata where there is no indication for open-marine conditions, suggesting that the study area is located in a relatively proximal part of the basin close to the landward limit of transgression. Thus, we believe that the thick Mullins Coal corresponds to the late transgressive to early

highstand stage of Bohacs and Suter (1997), and that the overlying thin coals correspond to later highstand conditions. Although speculative, relative sea-level fluctuation during deposition of MCI may reflect the coeval Gondwanan glaciation (Veevers and Powell, 1987) or basin-scale tectonic effects. The presence of a sequence boundary marked by incision below the fluvial sandstones implies both relative rise and fall of sea-level, providing some support for a glacioeustatic explanation. 7.2. Differential subsidence and compaction Lateral facies changes within the MCI suggest that local tectonic activity and=or compactional effects exerted a second-order control on peat accumulation (Fig. 7). Splits and thickness changes in the coal correspond broadly to the position of northeasttrending faults that bound upstanding blocks of older bedrock (the Coxheath and Boisdale Blocks) and associated anticlines and synclines developed in the Morien Group (Fig. 1). The Mullins Coal has a relatively uniform thickness (2 m) where braided-fluvial sandstones cover the western limb of the Bridgeport Anticline, a basinward extension of the Coxheath Block (Fig. 7). To the west, the coal apparently dies out westward across the Sydney Harbour Syncline, close to the George River Fault (Fig. 7). To the

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Fig. 7. Schematic cross-section through the Mullins Coal area of the Sydney Basin to illustrate the relationship between coal geometry and basinal tectonic elements.

east of the Bridgeport Anticline, the coal splits and thins towards the axis of the Glace Bay Syncline on the East Bay Block, where underlying and overlying bayfill mudstones are much thicker. There is strong coincidence between the hinge-line position of the split, as seen in Fig. 4, and the projected trace of the Coxheath Fault (Fig. 7) which throws down to the east in the River Ryan area (Boehner and Giles, 1986). There is evidence for analogous palaeotopographic effects associated with continued minor tectonism and=or compactional draping within the Morien Group (Gibling and Rust, 1990; Tandon and Gibling, 1997). Peat distribution was apparently influenced by differential compaction of facies associated with the tectonic subsidence of the underlying fault blocks (Fig. 7). The presence of a stable platform composed of non-compactible fluvial sand may have promoted the accumulation of thick, high-quality peat, as noted for other peat- and coal-bearing regions by Tankard (1986), Nemec (1992) and Cecil et al. (1993). In contrast, low-quality peat and carbonaceous mud accumulated in topographically lower areas with more compactible sediment, such as the Glace Bay Syncline. Compactional effects have been invoked to

explain aspects of coal distribution in the ancient record (e.g., Fielding, 1987; Gastaldo et al., 1991).

8. Conclusions The Mullins Coal (Westphalian C) is a thick and extensive, high-sulphur coal within the predominantly braided-fluvial South Bar Formation of the Sydney Coalfield. The coal is associated with a mudstone wedge that yields agglutinated foraminifera, and formed during marine inundation of a distal braided-fluvial plain. The availability of accommodation space and ponding of freshwater near the transgressive maximum allowed thick peat to accumulate over a non-compactible platform underlain by the braided-river sands. The coal thins and splits where the marine wedge thickens eastward into the Glace Bay Syncline, a persistent topographic low. We infer that the stratigraphic position of the Mullins Coal reflects basinwide changes in relative sea-level, whereas the quality and local thickness of the coal was controlled, in part, by differential compaction in localities associated with underlying fault-bounded basement blocks.

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Acknowledgements We thank Kevin Gillis, Jim Langille and Don MacNeil of the Nova Scotia Department of Natural Resources for assistance with core logging and for providing geophysical logs, and Winton Wightman for analysis of microfossils. Funding was provided by a grant to M.R.G. from Natural Sciences and Engineering Research Council of Canada.

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