Deformation of Upper Carboniferous coal measures in the Sydney Basin: evidence for late Alleghanian tectonism in Atlantic Canada

Deformation of Upper Carboniferous coalmeasures in the Sydney Basin: evidence for lateAlleghanian tectonism in Atlantic Canada

M.R. Gibling, W. Langenberg, W.D. Kalkreuth, J.W.F. Waldron, R. Courtney,J. Paul, and A.M. Grist

Abstract: Upper Carboniferous (Stephanian) coal measures of the Sydney Basin are deformed by reverse faults and foldswith dips of up to 50° on the limbs. These structures represent compressional or transpressional events (Donkin Episode)of unknown sequence and timing. Offshore swath-bathymetry data indicate that northwest-striking reverse faults are foldedby prominent northeast-trending folds. Vitrinite reflectance patterns of oriented coal samples from the fold limbs are uniaxialnegative, with minimum reflectance axes (Rmin) near-normal to bedding and fanning across fold axes. This pattern impliesthat the bulk of coalification took place through burial compaction without directed horizontal stress, prior to the deformationthat produced the folds. Slight divergence between Rmin axes and poles to bedding may reflect slight tilting, associatedwith differential subsidence or early reverse faulting, before or during coalification, or may lie within analytical error. Onemoderately biaxial sample suggests that some deformation accompanied coalification locally. Available apatite fission-trackdata for the Sydney Basin indicate that cooling of the coal measures to below 100 ± 20°C was underway by the Middleto Late Triassic or earlier. Because cooling implies that coalification was largely complete, the fission-track results arecompatible with latest Paleozoic or early Mesozoic deformation. We infer that the Donkin Episode represents late Alleghanian(Permian?) compression within the Appalachian Orogen. If so, the Sydney structures are manifestations of a little-knowntectonic episode in Atlantic Canada. However, Mesozoic–Cenozoic deformation cannot be ruled out.

Résumé: Les séries houillères du bassin de Sydney (Carbonifère supérieur - Stéphanien) sont déformées par des faillesinverses et des plis déversés dont les flancs atteignent des pendages de 50º. Ces structures représentent des événementsde compression ou de transpression (épisode Donkin) dont la séquence et la synchronisation sont inconnues. Les données debathymétrie par secteurs au large indiquent que les failles inverses à direction nord-ouest sont pliées par des plis proéminentsà tendance nord-est. Les patrons du facteur de réflexion de la vitrinite provenant d’échantillons de charbon orientés prélevés surles flancs des plis sont uniaxes négatifs, avec des axes de réflectance minimum (Rmin) presque perpendiculaires au litage et sedispersant en travers des axes de plis. Ce patron implique que la plus grande part de la carbonisation a eu lieu par compactiond’enfouissement sans contrainte horizontale directe, avant la déformation qui a produit les plis. La légère divergence entre lesaxes Rmin et les pôles du litage pourrait refléter un léger basculement associé à une subsidence différentielle ou à des faillesinverses précoces, avant ou durant la carbonisation, ou encore cette divergence pourrait être à l’intérieur de la fourchette del’erreur analytique. Notre échantillon modérément biaxe suggère qu’une certaine déformation ait localement accompagné lacarbonisation. Pour le bassin de Sydney, les données disponibles de datation par traces de fission sur l’apatite indiquentque le refroidissement des séries houillères à des températures inférieures à 100 ± 20 ºC était déjà en cours au Triasmoyen à tardif ou plus tôt. Puisque le refroidissement implique que la carbonisation était presque terminée, les résultatsde datation par traces de fission concordent avec la déformation au Paléozoïque terminal ou au Mésozoïque précoce.Nous concluons que l’épisode Donkin représente une compression à l’Alléghanien tardif (Permien ?) à l’intérieur del’orogène des Appalaches. Si tel est le cas, les structures de Sydney sont des évidences d’un épisode tectonique peuconnu au Canada Atlantique. Cependant, une déformation au Mésozoïque–Cénozoïque ne peut être rejetée d’emblée.

[Traduit par la Rédaction] Gibling et al. 93

Can. J. Earth Sci.39: 79–93 (2002) DOI: 10.1139/E01-062 © 2002 NRC Canada

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Received 8 January 2001. Accepted 31 July 2001. Published on the NRC Research Press Web site at http://cjes.nrc.ca onJanuary 30, 2002.

Paper handled by Associate Editor M. Bustin.

M.R. Gibling. 1 Department of Earth Sciences, Dalhousie University, Halifax, NS B3H 3J5, Canada.W. Langenberg. Alberta Geological Survey, 4th Floor, 4999-98 Avenue, Edmonton, AB T6B 2X3, Canada.W.D. Kalkreuth. Coal and Organic Petrology Laboratories, Instituto de Geociencias, UFRGS, Av. Bento Goncalves 9500, 91501–970Porto Alegre, RS, Brazil.J.W.F. Waldron. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E2, Canada.R. Courtney. Geological Survey of Canada (Atlantic), P.O. Box 1006, Dartmouth, NS B2Y 4A2, Canada.J. Paul. Bahnhofstrasse 3, 15938 Golssen, Germany.A.M. Grist. Department of Earth Sciences, Dalhousie University, Halifax, NS B3H 3J5, Canada.

1Corresponding author (e-mail: [email protected]).

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Introduction

The Sydney Basin is a large upper Paleozoic depocentrein Atlantic Canada, part of the regional Maritimes Basin,which covers large onshore and subsea areas (Fig. 1, inset).The Sydney Basin fill is exposed onshore in Cape Breton Islandand extends offshore to Newfoundland and the Burin Platform.It contains the main economic coals of eastern Canadawithin the Upper Carboniferous Sydney Mines Formation,part of the Morien Group (Fig. 2). The coals extend widelyunder the adjacent Atlantic Ocean, where they are worked insubsea mines and have been identified in seismic lines andoffshore wells (Pascucci et al. 2000).

Over much of the basin, Sydney Mines Formation stratadip gently and faults are rare, giving a superficial impressionthat the beds are little deformed. However, prominent foldsare present, locally with dips of 50° on the limbs (Fig. 3),and low-angle reverse faults have been documented (Haites1951; Hacquebard 1983). Deformation is also evident inswath-bathymetry images (Courtney 1996) and seismic profilesoffshore (Pascucci et al. 2000). Haites (1951) suggested thatthe strata were deformed during the Late Carboniferous orPermian by comparison with other parts of the AppalachianOrogen, although no direct evidence was provided.Hacquebard and Donaldson (1970) used coal-rank variationto infer that the peats or low-rank coals that were precursorsof the present bituminous coals were tilted prior tocoalification, implying an early (Late Carboniferous?) phaseof deformation.

The present study is the first attempt to tackle comprehen-sively the deformational history of the coal measures. Afterreviewing previous structural studies, we report new structuralinformation from swath-bathymetry images of an area ofnear-100% sea-floor outcrop, which provides evidence fortwo deformational events. We also report vitrinite reflectancepatterns in 10 oriented coal blocks from folded SydneyMines strata. Vitrinite reflectance in coals is anisotropic atelevated rank levels (approx. >0.60% Ro), and the degree ofanisotropy and the orientation of reflectance axes with respectto bedding, fold axes, and faults can provide informationabout the tectonic regime during coalification, when thevitrinite structure was generated (Hower and Davis 1981;Levine and Davis 1989a). These data sets allow us to inferthe timing of deformational events with respect to coalificationand burial history. Although no absolute means of datingcoal-measures deformation is apparent, available modellingresults for fission tracks in apatite from the coal measureshelp to constrain the timing of tectonothermal events.

Geology of the Sydney Basin

StratigraphyThe late Paleozoic history of the Sydney Basin has recently

been outlined by Pascucci et al. (2000), and a brief summaryis provided here (see Fig. 2). The Lower Carboniferoussuccession comprises three rock groups. The Horton Groupconsists mainly of alluvial conglomerate and sandstone, andits thickness may exceed 3 km. The Windsor Group consistsof marine and continental evaporite, carbonate, and siliciclasticrocks, up to 1 km thick. The Mabou Group comprises about100 m of lacustrine siliciclastic rocks, sulphate evaporite,and limestone. Pascucci et al. (2000) identified three

half-grabens in the southeastern part of the basin, andinferred that they were filled mainly with Horton Groupstrata. The depocentres lie on the southern sides of northeast-oriented fault zones (the North Sydney, Florence, andDominion Fault Zones; Fig. 1). The Windsor and Mabougroups overlap the bounding faults to cover adjacent basementrocks, suggesting that a phase of thermal relaxation followedactive extension (Fig. 2).

The Upper Carboniferous Morien Group rests with angularunconformity on Lower Carboniferous rocks (Fig. 2). Tectonicactivity reflected in the unconformity was inferred to correlatebroadly with the onset of the Alleghanian Orogeny in theU.S. Appalachians by Gibling (1995). The Morien Group is1500–1800 m thick and comprises three formations of lateWestphalian B to Stephanian age. The basal South BarFormation is at least 860 m thick and is a sandstone-dominated, braided-fluvial unit with minor coal. The overlyingSydney Mines Formation is ~1000 m thick in the Glace BaySyncline and consists of sandstone, grey and red mudstone,coals, dark fossiliferous limestone, and calcrete deposited inalluvial to restricted marine settings (Gibling and Bird 1994).The formation contains numerous economic coals up to 4.3 mthick, four of which were sampled for the present study(McRury, Emery, Phalen, Harbour: Fig. 3). In the southeasternpart of the basin, the Waddens Cove Formation comprisesred alluvial deposits with minor coal. Red mudstone andsandstone of the Pictou Group overlie the coal measures underthe nearshore part of the basin and may be ~1000 m thickbased on dip extrapolation. They are undated, but are probablyStephanian to perhaps Early Permian in age (R.C. Boehner,personal communication, 1999): lithologically similar LowerPermian strata are present on Prince Edward Island and underthe Gulf of St. Lawrence (inset in Fig. 1; van de Poll et al.1995).

Fission-track analysis indicates that the Sydney Basin wasbeing exhumed by the Late Triassic (Grist et al. 1995; Gristand Gibling 1999); this exhumation was probably linked toflank uplift adjacent to Atlantic extensional basins. Severalkilometres of latest Paleozoic to early Mesozoic strata mayhave overlain the lowermost Pictou Group prior to exhumationand erosion (Grist et al. 1995). The Orpheus Graben south ofthe Sydney Basin (inset to Fig. 1) contains a thick Late Triassicto Cenozoic succession, associated with Atlantic rifting andsubsequent passive-margin development. Olsen et al. (2000)suggested that the basal Honeycomb Point Formation of theFundy Basin fill in New Brunswick is Permian in age, basedon paleomagnetic data and lithostratigraphic comparisonwith the Moroccan margin; these authors noted that, if thisformation is truly part of the extensional basin fill and not afortuitously preserved outlier, rifting may have commencedas early as the Permian. Lower Cretaceous rocks overlie theUpper Paleozoic section on the Burin Platform (MacLeanand Wade 1992) and the eastern offshore Sydney Basin (Fig. 2),and faulted Lower Cretaceous strata are present locally incentral Cape Breton Island, west of the Sydney Basin(Dickie 1987). Volcanic rocks dredged from Scatarie Ridge(Fig. 1) yielded a 127 ± 15 Ma (Early Cretaceous) date bythe K–Ar method (Delabio et al. 1979), and Cretaceousigneous rocks are also present in the Orpheus Graben (Jansaet al. 1993). Tectonic activity associated with the AvalonUplift affected the Grand Banks from the Late Jurassic to

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Gibling et al. 81

Fig. 1. Generalised geology of the Sydney Basin onshore in Cape Breton Island and in the adjacent nearshore area. F, fault; FZ, faultzone. Three minor faults are: MBF, MacAskills Brook Fault; HF, Homeville Fault; WCF, Waddens Cove Fault. The location of twooffshore wells (F-24 and P-05) is shown. Large box encloses outcrop study area of Figs. 3 and 6. Small box encloses area of swath-bathymetry survey shown in Fig. 4. Fault zones offshore were identified from seismic lines and named by Pascucci et al. (2000). Insetmap shows position of study area within Atlantic Canada; dashed lines enclose areas underlain by upper Paleozoic strata of the re-gional Maritimes Basin, principally the Gulf of St. Lawrence Basin, Sydney Basin and Burin Platform; Modified from Boehner andGiles (1986) and Pascucci et al. (2000). N.B., New Brunswick; N.S., Nova Scotia; NFLD., Newfoundland; P.E.I., Prince Edward Is-land; Que., Quebec; L., Lower; U., Upper.

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Fig. 2. Upper Paleozoic to Mesozoic stratigraphy and inferred history of the Sydney Basin. Cretaceous strata are known in the easternoffshore part of the basin and locally onland in Cape Breton Island, but rocks of the Morien and Pictou groups are at surface overmost of the basinal area. Numbers indicate four phases of stratal accumulation. Formations and ages from Boehner and Giles (1986),White and Barr (1998) and Grant and McAlpine (1990). Time scales derived from Hess and Lippolt (1986), Okulitch (1999), andTucker et al. (1998). Cen., Cenomanian; Eifel., Eifelian; Fras., Frasnian; Givet., Givetian; Steph; Stephanian; Westph., Westphalian.Fm., Formation.

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the Late Cretaceous (Grant and McAlpine 1990), and manyupper Paleozoic faults were reactivated during the Mesozoic(MacLean and Wade 1992). A phase of Mesozoic basininversion and reverse faulting in the Fundy area was provi-sionally dated as Early Jurassic or later by Withjack et al.(1995).

StructurePre-Morien strata onshore in the Sydney Basin are cut by

northeast-striking faults (Fig. 1). Mapped stratal patterns showup to 18 km of stratal offset, predominantly in a dextralsense, but are equally consistent with dip-slip faulting, withdownthrow predominantly to the southeast, or with somecombination of dip-slip and strike-slip motion. Most major,mapped faults cannot be traced across the basal Morienunconformity. This implies that they were active during orbefore the mid Carboniferous, linked to early Alleghaniandeformation, although some were probably reactivated locally(Pascucci et al. 2000).

Within coal mines in the Sydney Mines Formation, Haites(1951, 1952) documented numerous reverse (thrust) faultswith low-angle (~20°) fault planes, northwest strikes, andthrows of a few metres to ~120 m. The faults commonlyincrease inthrow and displacement with depth. Normalfaults are present but rare. A large, southeast-directed thrustwas mapped in Morien Bay by Hacquebard (1983), and bothnorthwest- and northeast-trending faults were identified inswath-bathymetry images over the Cape Perce Anticline byCourtney (1996). The present margins of the Sydney Basinonshore are marked by the Mountain and Bateston faults,which offset the Morien Group, as do three prominent faultsin the southeastern part of the basin (Fig. 1).

Morien Group strata onshore dip gently, typically at 5–15°and exhibit prominent asymmetric folds (Robb 1876; Hayesand Bell 1923). Along the Bridgeport and Cape PerceAnticlines (Fig. 3), the Morien strata dip steeply (up to 50°).Haites (1951) noted prominent cleat (joint) sets in the coalsalong directions of 065° and 335° and suggested that they

developed contemporaneously with folding. DeWolfe (1906),Hayes and Bell (1923), and Gray and Gray (1941) suggesteda link between the folds and uplifted basement blocks in thesouthwest part of the basin, and Bell (1938) noted that thebasement uplifts continue northeastward into anticlines atthe level of the Morien Group. More recent mapping confirmsthat the fold axes strike subparallel to faults in the underlyingstrata (compare Figs. 1 and 3).

Offshore, Pascucci et al. (2000) used seismic lines toidentify a linear, northeast-trending antiform along the lineof the North Sydney Fault Zone (Fig. 1), associated withfaults that appear to diverge upwards and show reverseseparation. The faults appear to cut the Morien and Pictougroups, constraining the deformation to Stephanian – EarlyPermian or later. The local coincidence of the antiform witha Lower Carboniferous fault system suggests that deformationinverted the adjacent half-graben and reactivated the faultzone. The antiform and other antiforms associated with theDonkin and Dominion fault zones (Fig. 1) were interpretedby Pascucci et al. (2000) as positive flower structures inducedby a component of convergence normal to the wrench fault(see Harding 1985). Pascucci et al. (2000) referred to thesewidespread deformation events of compressive or transpressivestyle as the “Donkin Episode.”

Sedimentary evidence indicates that some synclines betweenthe prominent anticlines onshore were paleotopographic lowsand sites of preferential sediment accumulation during Moriendeposition. In the Glace Bay Syncline (Fig. 3), the SydneyMines Formation is especially thick and contains a highproportion of poorly drained facies (grey mudstone andlimestone) and relatively thick coals (Bell 1938; Gray andGray 1941; Hacquebard 1983; Gibling and Bird 1994;Tandon and Gibling 1997; Tibert and Gibling 1999). However,relief during deposition was modest as the major coal seamscover both anticlinal and synclinal areas. Offshore towardsthe northeast, the base-Morien unconformity becomesprogressivelydeeper and shows considerable relief.

Hacquebard and Donaldson (1970), Hacquebard (1983,

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Gibling et al. 83

Fig. 3. Simplified geological map of part of the Sydney Basin onshore (located in Fig. 1). Sample sites record locations of orientedcoal samples. Modified from Boehner and Giles (1986).

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1984), and Hacquebard and Cameron (1989) noted that coalrank shows little change with stratigraphic level over about1150 m of section in coastal areas along Mira Bay and GreatBras d’Or (Fig. 1), where dips are northeasterly at 5–10°.However, coal rank was found to increase with depth in onshoreand offshore boreholes, and the rank of the Harbour Seamincreases downdip in mines under the Atlantic Ocean, withvitrinite reflectance increasing from 0.7 to 1.1% Ro over ahorizontal distance of about 10 km (Hacquebard 1983, fig. 10).Hacquebard and co-workers used these rank observations toinfer that tilting of strata (“warping”) to near-present attitudestook place prior to coalification. Dip estimation based onfigs. 20 and 22 of Hacquebard and Cameron (1989) suggeststhat these authors envisaged tilting to < 2°. The northeastwardrank increase in the Harbour Seam, with highest rank in theGlace Bay and Donkin areas (Fig. 3), was explained byincreased depth of burial. A full analysis of coal-rankvariation in the basin will be presented in a subsequent paper.

New structural data from swath bathymetryand mine plans

Swath-bathymetry images were collected in the offshorearea immediately north of New Waterford using methodsoutlined in Appendix 1. The sea floor in the surveyed areaslopes gently seaward at about half a degree, and the surveyreveals bedrock exposure over much of the area in the formof ridges and furrows (Fig. 4) that reflect differential erosionof the Sydney Mines Formation. Based on comparison withwave-cut platform exposures, the ridges represent induratedsandstone beds, whereas the furrows represent softer shales.This interpretation is confirmed by the presence, in severalinferred sandstone units, of channelized bases and inclinedstratification, both typical features of fluvial channel sandbodies. The remarkably extensive bedrock exposure allowsstructural features to be identified with considerable certainty.

The bedrock exposure is cut by north-trending gullies,generally 1–4 m deep and up to 100 m wide. Some containunconsolidated Recent sediment and probably represent valleyseroded during Quaternary relative sea-level lowstands. Farthernorth, smooth sea floor is inferred to represent recentlydeposited sediment that conceals the bedrock.

In the eastern part of the area, a broad antiformal fold in thebedrock corresponds to the Bridgeport anticline. Small gullies(< 20 m wide) mark sinistral apparent offsets, up to ~30 m,in the trace of bedding, and are interpreted to representfaults. Most faults strike roughly northwest, and can betraced for 500 m to 3 km across the sea floor. Bedding offsetis typically at a maximum approximately midway along afault trace, declining to zero at the ends. Some linear featuresthat otherwise resemble faults show no detectable beddingoffset and are either faults with very small throw or differentiallyeroded joints. Faults that dip gently northeast were observedin former mine workings in the area, and are present in cliffexposures near New Waterford (S. Forgeron, personalcommunication, 1999).

Attempts to construct contours on fault surfaces from pointsof known elevation on the fault trace indicate that the faultsare non-planar and dip generally northeast, with reverseseparations of 0–10 m; however, the sea-floor data give noclear indication of three-dimensional fault shape. One fault

appears to cross the antiform in the eastern part of the areaand is located on-strike with a fault recorded in nearby mineworkings in the Phalen, Hub, and Harbour seams. A structurecontour map of the fault (Fig. 5) based on information frommining operations shows that the positions of the threesubsurface intersections and the sea-floor fault are consistentwith a single, gently curved surface that dips northeast at17–22°. Several observations strongly suggest that faultingpredated folding: (i) the curvature of the fault coincides ingeneral location and sense with the folds in bedding; (ii ) thestructure contours on the fault surface are more stronglycurved than the bedding–fault intersection lines; (iii ) theinterlimb angles of folds in bedding, 10–15°, are comparablewith the orientation difference between different parts of thefault.

Hence, we distinguish two generations of structures thatwere responsible for shortening of strata in the Sydney MinesFormation in the New Waterford area. An earlier episodegenerated localized, northwest-striking and southwest-vergentthrust faults. Subsequent northwest–southeast directed shorteningproduced broad folds that become tighter to the southeast,some of which are cored by older, reactivated faults andbasement uplifts. Previous structural analysis has not establishedthe relationship between faults and folds in the coal measures.We provisionally refer both events to the Donkin Episode ofPascucci et al. (2000). However, in view of the limited resolutionpossible from the seismic profiles, the faults inferred byPascucci et al. (2000) are probably large-scale structures,and it is possible that the suite of relatively small-scale faultsrecognized here constitute an earlier, unrelated episode ofdeformation.

A swath-bathymetry image over the northern limb of theCape Perce Anticline near Donkin (Courtney 1996) imagedthe anticline and associated northeast- and northwest-strikingfaults, the latter with ~50 m of apparently sinistral offset.Although no detailed structural analysis was reported, theseobservations suggest that structural features similar to thoseat New Waterford are widely present in the basin.

Vitrinite anisotropy

ApproachVitrinite is a carbon-enriched organic maceral derived mainly

from woody precursors and is the major organic constituentof most coals. The reflectance (in percent) of vitrinite in oilis the most commonly used optical index for evaluation ofcoal rank and level of thermal maturity of sedimentary strata.Vitrinite, which has reflectance in oil above 0.6%, usuallyshows anisotropy in polarized light, indicating that vitrinitereflectance has directional properties. The three-dimensionalreflectance pattern can be represented by an ellipsoid(termed here an “indicatrix” in comparison with the geometricfigure that represents the refractive indices of a crystal), theradius of which in any given direction is proportional to thereflectance in that direction. The indicatrix can be describedby the magnitudes and orientations of its three principalreflectance axes (Kilby 1988; Langenberg and Kalkreuth1991a, 1991b).

In undeformed sedimentary basins, the minimum reflectionaxis (Rmin) of uniaxial negative ellipsoids is orientednear-perpendicular to bedding. This orientation indicates

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85Fig. 4. Swath-bathymetric results offshore from New Waterford (lower left). Area enclosed by box covers crest of Bridgeport Anticline and is shown in Fig. 1.Bathymetric detail is enhanced by shadowing with light source from northeast.

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that the vitrinite structure was acquired through coalificationduring sedimentary burial, when compression is assumed tohave been essentially vertical (Levine and Davis 1989a).Reflectance axes should fan across fold axes if the coal-bearingstrata were deformed after coalification (Levine and Davis1984). Coals with biaxial reflectance patterns are reportedfrom strongly folded or faulted strata, suggesting that tectonicstresses are responsible for the biaxial anisotropy (Cook etal. 1972; Levine and Davis 1984, 1989a, 1989b; Salih andLisle 1988; Langenberg and Kalkreuth 1991a, 1991b;Langenberg et al. 1998; Taylor et al. 1998). In biaxial negative

coals, Rint is closer in magnitude to Rmax than to Rmin, whereasin biaxial positive coals, Rint is closer in magnitude to Rmin.Biaxial positive patterns indicate stronger deformation (Levineand Davis 1989a). For coals with biaxial reflectance patterns,axial directions are commonly subparallel to fold axes andother structural lineaments (Levine and Davis 1989a;Langenberg and Kalkreuth 1991a).

Thermal experiments on coals have shown that anisotropydevelops during coalification (Goodarzi and Murchison1972; Oberlin and Terriere 1975; Rouzaud and Oberlin1983), although the precise mechanism remains uncertain

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Fig. 5. Structure contour map of Phalen seam and fault, based on swath bathymetry data (Fig. 4) and intersections with coal seams re-corded during surveys of mining operations in the Phalen, Harbour and Hub seams. 1 foot = 0.3048 m.

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(see Levine and Davis 1984). Levine and Davis (1984)noted that early formed organic fabrics identified by theirreflectance patterns can be partially overprinted by later fabricsduring progressive coalification; however, structural eventsthat postdate coalification are not recorded in reflectancefabrics, although they may deform, brecciate, or physicallyrotate coal.

Methods and resultsTen blocks, oriented with respect to local dip and strike,

were collected from the McRury, Emery, Phalen and Harbourcoals. In the laboratory, the oriented samples were cut alongthree perpendicular planes: (1) parallel to the surface withthe measured orientation, (2) perpendicular to this orientedplane and parallel to its dip direction (dip plane), and

(3) perpendicular to the oriented plane and parallel to itsstrike direction (strike plane). These surfaces were ground,polished, and the vitrinite reflectance was measured accordingto methods described by Langenberg and Kalkreuth (1991a,1991b) and Taylor et al. (1998). Magnitudes and orientationsof reflectance axes were determined by mathematically fittingan ellipsoid to the reflectance measurements in six directionsfrom the three perpendicular surfaces, following methodologydeveloped by Kelker and Langenberg (1997). A statistical ttest based on the standard deviation of the mean was used todecide if the difference between the magnitudes of the principalreflectance axes is significant for classification of the ellipsoidsas uniaxial or biaxial. If the difference between maximumand intermediate axis is insignificant, they are considereduniaxial, even if the magnitudes are not exactly the same.

The shapes of the reflectance ellipsoids for the sampleswere investigated using an axial-ratio plot (Levine and Davis1989), which shows the ratio ofA = Rmax/Rint as the verticalaxis andB = Rint/Rmin as the horizontal axis (Fig. 7). Uniaxialpositive coals plot along the vertical axis and uniaxial negativecoals plot along the horizontal axis. The slope of a lineconnecting any point on the diagram with the origin (1,1) is

k = (A – 1) / (B – 1)

This parameter k is used to describe the reflectance ellipsoids,where values between 0 and 1 represent uni- or biaxial“negative” coals and values between 1 and infinity representuni- or biaxial “positive” coals.

Irregularities on bedding surfaces and the use of a standardcompass can cause errors in the estimation of beddingorientation(Charlesworth et al. 1976, p. 55). Additionally,the “local” bed orientation may not reflect the “regional”orientation of the burial surface to which vitrinite structurehas accommodated, and this factor introduces an unknownerror — possibly considerably greater than the measurementerror — in correlating vitrinite structure with bedding. Inview of these uncertainties, we use an arbitrary cut-off angleof 15° between Rmin and the pole to bedding: deviations greaterthan this are considered worthy of special interpretation.

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Fig. 6. Orientation of vitrinite anisotropy, shown on lower hemisphere, equal-area stereonets with respect to bedding, for sample loca-tions in the Sydney Mines Formation (located in Fig. 1).

Fig. 7. Axial-ratio plot of principal reflectances of 10 orientedblocks from the Sydney Mines Formation.

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Table 1 shows the magnitudes and orientations of theprincipal reflectance axes for samples in three structuralareas: Donkin and Long Beach, on both limbs of the CapePerce Anticline (5 samples); Lingan, Glace Bay, and VictoriaMines, on both limbs of the Bridgeport Anticline (4 samples);and Sydney Mines, close to the axis of the Sydney HarbourSyncline (1 sample). Figure 6 shows the magnitudes andorientations of the axes plotted on stereonets, with present-daybedding and the pole to bedding for each. The samples wereclassified as uniaxial or biaxial with the aid of an axial-ratioplot and calculation of the parameterk (Fig. 7; Table 1).

All samples show uniaxial negative reflectance patterns.Sample 10 (McRury Seam at Donkin,k = 0.76) and sample 6(Harbour Seam in the Donkin Tunnel section,k = 0.36) showa biaxial tendency (Fig. 7). However, statistical analysis indicatesthat the difference between Rmax and Rint is not significantfor sample 6 and is only close to significant for sample 10(the difference must be greater than two standard deviationsto reject the t test: Kelker and Langenberg 1997).

For the Cape Perce suite, the angle between Rmin and thepole to bedding ranges from 3° to 12° (average 7.4°), showingthat Rmin is near-perpendicular to bedding in beds that presentlydip up to 50°. For the Bridgeport Anticline suite, the angleranges from 2° to 15° (average 8.7°) in beds that dip up to 40°.For the Sydney Mines sample, Rmin has an angle of 16° withthe pole to bedding, in beds that dip 15°. The average angulardifference for all ten samples is 8.8°. Only two samples haveangles of 15° or more: sample 11 (Glace Bay) at 15°, andsample 19 (Sydney Mines) at 16°. The stereonet plots (Fig. 6)show that the orientation of Rmax and Rint shows no consistentrelationship with the strike and dip of bedding, nor withMorien fold axes.

Relative timing of structural events andcoalification

The predominant uniaxial negative reflectance patterns, aswell as the lack of correlation between reflectance axes andfold axes, indicate that the bulk of coalification took placeduring burial-related compaction, while the beds were flat-lyingand without significant directed horizontal stress. The closeaccord between poles to bedding and Rmin axes in steeply

dipping beds, with fanning of Rmin axes across folds, indicatesthat coalification was essentially complete before deformationassociated with the Donkin Episode generated steep dips onthe limbs of the Cape Perce and Bridgeport anticlines. Thisis clearly seen at Cape Perce where the average divergenceof Rmin from poles to bedding is < 9° where the beds dip atup to 50°.

Coals with > 15° difference between poles to bedding andRmin axes could have been slightly tilted before or during themain phase of coalification, as suggested by Hacquebard andDonaldson (1970). Deviations in other samples are consideredto lie within the uncertainty of the method. Although Rminaxes from samples on the southeast limb of the Cape PerceAnticline differ from poles to bedding (Fig. 6), the axes arevariably shallower or steeper than the poles to bedding; amore consistent relationship might have been expected ifearly tilting were responsible for the deviations. Slight tiltingis consistent with differential subsidence during MorienGroup deposition, which is supported by geological (facies)evidence. It is also consistent with minor tilting during earlystages of the Donkin Episode, perhaps associated withsouthwest-vergent thrust faults, such as those documentednear New Waterford. Unfortunately, we have not been ableto obtain oriented coal samples from folds associated withthese early faults: fault-related folds were recorded by Haites(1952) only in mine workings that are now inaccessible. Weare thus unable to comment on the timing of coalificationrelative to this phase of deformation.

The biaxial negative anisotropy recorded in the McRurySeam at Donkin suggests that a component of coalificationat this locality was associated with horizontal compression.This may imply that the Donkin Episode commenced locallywhile coalification was still underway, although the biaxialpattern is not strongly developed.

In a survey of coalification in North American coal fields,Damberger (1991) noted that coalification predated or ac-companied deformation in most studies and cited the SydneyBasin as the only well documented example in North Americaof coalification postdating deformation. Our results indicatethat, although some early tilting cannot be ruled out, thebulk of deformation at Sydney postdated coalification. Acomponent of postdeformational coalification related to tectonicburial is well documented in the central and southern Alberta

© 2002 NRC Canada


Magnitudes Orientations

ID Coal Seam Rmax Rint Rmin SD Rmax Rint Rmin k

1 Phalen 0.85 0.83 0.73 0.03 237/28 115/45 346/25 0.172 Emery 0.82 0.81 0.71 0.03 240/16 138/37 349/49 0.093 McRury 0.85 0.84 0.70 0.03 143/28 237/09 344/61 0.096 Harbour 1.00 0.96 0.85 0.03 278/02 008/07 175/82 0.36

10 McRury 0.90 0.85 0.79 0.03 315/08 047/08 180/79 0.7611 Emery 0.86 0.85 0.75 0.02 277/09 008/07 137/79 0.1314 Phalen 0.93 0.92 0.80 0.02 078/08 347/05 223/81 0.0715 Emery 0.91 0.89 0.73 0.02 000/19 095/14 219/66 0.1116 Emery 0.82 0.80 0.69 0.02 270/24 014/28 147/52 0.1319 Emery 0.75 0.75 0.66 0.02 066/11 331/26 176/62 0.03

Note: Samples with ID 1–10 are from the Cape Perce Anticline, 11–16 are from the Bridgeport Anticline, and 19 is from the Sydney Mines area (seeFigs. 3, 6 for location). ID, sample number; SD, average standard deviation of the mean reflectances in six directions of the blocks.

Table 1. Magnitudes in percent vitrinite reflectance, orientations and parametersk of principal reflectances of oriented coal blocks.

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Foothills (Hacquebard and Cameron 1989; Langenberg et al.1998).

Absolute timing of events

Deformation of the Sydney Mines Formation can be datedas Stephanian (the age of the youngest biostratigraphicallydated strata) or later — possibly Early Permian or later if thePictou Group offshore Sydney extends into the Early Permian.The oldest Mesozoic strata that overlie the Sydney BasinPaleozoic succession are Early Cretaceous in age (Fig. 2)and appear relatively flat-lying in seismic profiles on theBurin Platform (MacLean and Wade 1992; Pascucci et al.1999). These data suggest that deformation of the Paleozoicstrata was substantially complete by the Early Cretaceous.Although no absolute means of dating the coal-measuresdeformation is apparent, fission-track modelling results fordetrital apatite from the Sydney Basin (Grist et al. 1995;Grist and Gibling 1999) help to constrain the timing ofcoalification and thus of the deformational episode that postdatescoalification; the results are summarized here.

Apatite fission-track modelling constrains the time atwhich the strata began to cool below about 100 ± 20°C, thetemperature below which fission tracks are retained in theapatite structure (Wagner and van den Haute 1992). Althoughorganic maturation reflects both temperature and time (durationof heating), it is most effective at near-maximal burialtemperatures(Lopatin 1971; Hood et al. 1975; Bostick et al.1979; Waples 1980). Thus, to a first approximation, the onsetof cooling implies the end of effective coalification. Burialtemperatures must have been great enough to fully anneal allinherited fission tracks — approximately equivalent to vitrinitereflectance values of 0.7–0.9% Ro (Arne and Zentilli 1994).

Grist and Gibling (1999) modelled fission track-lengthdistributions in samples from the offshore F-24 and P-05wells (Fig. 1). Two samples from sandstone layers in thelower Sydney Mines Formation (Westphalian D) in the F-24well had previously been studied by Grist et al. (1995) usingan early version of the modelling programme. Vitrinitereflectance values from the sampled interval range from ~1.0–1.2% Ro — sufficiently high to ensure that inherited fissiontracks in the apatite grains had been fully annealed. Althoughthe fission-track samples and oriented coal blocks wereobtained from different areas, the two areas appear to haveexperienced a similar deformational history.

Based on the track-length distribution and using a modelof progressive cooling following burial, the onset of trackretention is predicted to have occurred at 206 ± 12 Ma and193 ± 12 Ma for the Sydney Mines Formation samples. Ifthe confidence brackets of the samples are considered, thesampled interval was cooling by ~180–220 Ma — Late Triassicto Early Jurassic, based on the Okulitch (1999) time scale. Asecond model allows for an Early Cretaceous heating eventin view of the Lower Cretaceous strata that cover parts ofthe offshore Sydney Basin and the presence of volcanicrocks nearby. Results for this model yield older dates fortrack retention — 215 ± 13 Ma and 202 ± 11 Ma, respectively.Thus, cooling could have commenced as early as ~190–230 Ma(Middle–Upper Triassic). Arne et al. (1990) inferred a regionalLate Cretaceous to early Tertiary heating phase from fission-

track analysis of Carboniferous basins in mainland NovaScotia.

This evidence for the onset of cooling implies thatcoalification at Sydney was largely complete by the Middleto Upper Triassic. This is a “minimal age estimate”: detritalapatites from younger strata (upper Sydney Mines Formationand Pictou Group) than those sampled here might have begunto retain fission tracks before deeper, hotter strata were cooled.The cooling probably reflects exhumation accompanyingregional rifting and extensional basin development (Ravenhurstet al. 1990), which may have commenced as early as thePermian (Olsen et al. 2000). Although not definitive, thefission-track results are compatible with a relatively earlydeformational event.

Correlation with regional tectonic events

The Alleghanian Orogeny in the U.S. Appalachians andthe Variscan Orogeny of Europe involved major tectonic andthermal events through the Late Carboniferous to Permian(Hatcher et al. 1989; Matte 1991). In the northern to centralU.S. Appalachians, deformation, magmatism, and metamor-phism have been dated as Permian (Dallmeyer 1982;Wintsch and Sutter 1986; West and Lux 1993). Permiandextral shear has been inferred in New England and otherparts of the Appalachians, probably due to a change inconvergence direction during the final stages of Pangeanassembly at ~270–290 Ma (Getty and Gromet 1988; Sacksand Secor 1990; Goldstein 1994; Valentino et al. 1994).Faure et al. (1996) inferred a Permian tectonic phase in Quebecand northern New Brunswick from structural evidence.Elsewhere in Atlantic Canada, a lamprophyric sill on PrinceEdward Island is dated at 247 Ma, close to the Permian–Triassic boundary (Greenough and Fryer 1991), and Permianintrusions and thermal events have been recorded in south-western Nova Scotia (Zentilli and Reynolds 1985; Dallmeyerand Keppie 1987; Pe-Piper and Loncarevic 1989). On thecontinental shelf off western Ireland (formerly contiguouswith Atlantic Canada), Upper Permian strata rest unconformablyon Upper Carboniferous and basal Permian strata (Robesonet al. 1988). Permo-Carboniferous tectonic and volcanic eventswere widespread in Iberia and elsewhere in Europe andnorthwest Africa (Doblas et al. 1998; Arche and Lopez-Gomez1999).

This analysis confirms the importance of Permiandeformational events in areas close to the Sydney Basin. APermian age for the Donkin Episode is compatible with thecompressional or transpressional style of deformation of theSydney coal measures. Additionally, the presence of a biaxialvitrinite reflectance signature in one coal sample may indicatethat deformation was partly coincident with burial compaction.A latest Paleozoic date is in accord with later Permianextensional basin formation, and can be supported from thefission-track data. The significance of reverse faulting priorto folding, indicated from swath-bathymetry analysis, iscurrently unclear.

In view of evidence for an Early Jurassic or youngercompressional phase in the Fundy Basin (Withjack et al.1995), we cannot rule out a Mesozoic age for the DonkinEpisode. However, Mesozoic compressional events have not

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been widely documented in Atlantic Canada, and are unlikelyto have produced profound effects far inboard from theextensional basins.

Some sandstones in the mining area are gas-charged. Becausethe Donkin Episode postdated coalification (organic maturation),hydrocarbon reservoirs in deformed areas could have beenpartially destroyed through fault-induced leakage of fluids.However, structural traps formed during this episode mighthave retained hydrocarbons generated during a subsequentCretaceous thermal event. The faults also affect mine plansand locally deliver water and gas (Haites 1951).

Conclusions

The Upper Carboniferous coal measures of the SydneyBasin are strongly deformed locally. Analysis of swath-bathymetry images shows that northwest-striking reversefaults are postdated by prominent, northeast-trending linearantiforms with reverse faults, interpreted as positive flowerstructures. The deformation represents one or moretranspressional or compressional events (Donkin Episode) ofunknown timing.

Analysis of vitrinite reflectance anisotropy in coals onfold limbs shows that coalification took place while the stratawere essentially flat-lying and undergoing burial compactionwithout directed horizontal stress. On the steep limbs of theantiforms, where strata dip up to 50°, Rmin axes of thevitrinite indicatrix are near-perpendicular to bedding and fanacross the fold axial surfaces, indicating that tilting tookplace after coalification was complete. A slight divergencebetween Rmin axes and poles to bedding may reflect earlytilting linked to differential subsidence over faulted basementblocks and (or) minor movement on early reverse faults;however, this divergence may lie within the error of themethod.

The youngest Sydney Mines strata are Stephanian in age,implying a Stephanian or younger deformational event.Although no absolute dates constrain the timing of deformation,fission-track modelling suggests that stratal cooling wasunderway by the Middle to Upper Triassic, possibly muchearlier. This implies that the bulk of coalification, which ismost effective at near-maximal burial temperatures, wascomplete by the early Mesozoic, allowing for a relativelyearly deformational event. From comparison with other partsof the Appalachian Orogen, we infer that the Donkin Episoderepresents compressional or transpressional events during thelatest, Permian stage of Pangean assembly — the final stagesof the Alleghanian Orogeny. If so, the Sydney structuresrepresent a little-known tectonic episode in Atlantic Canada.Mesozoic–Cenozoic deformation is geologically improbablebut cannot be ruled out.

Some hydrocarbons generated and trapped duringcoalification of the Sydney coal measures could have beendispersed along faults created during the subsequent DonkinEpisode. However, structural traps formed during this episodemight have retained hydrocarbons generated during a subsequentCretaceous thermal event.

Acknowledgments

We thank S. Forgeron and the Cape Breton Development

Corporation for facilitating access to data on faults in theNew Waterford area, Vincenzo Pascucci and John Wade forvaluable discussion, and Andrew Henry for drafting. Weappreciate the thoughtful comments of journal reviewersS.M. Barr and L.P. Stasiuk. Use of coal-petrographicequipment at Geological Survey of Canada Calgary and theassistance of technical staff is gratefully acknowledged. M.Gibling gratefully acknowledges funds provided by a ResearchGrant and a Strategic Grant of the Natural Sciences andEngineering Research Council of Canada.

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Appendix 1. Methods used in obtainingswath-bathymetry images

Swath-bathymetry images were collected in the offshorearea immediately north of New Waterford using a SimradEM3000 multibeam bathymetry system. The output from theSimrad system was recorded on a Sun Sparc 20 workstationin the raw datagram format using the Simrad program Merlin.The integrated sounding data (depth, roll, pitch, heave, position)were recorded as separate files for each survey line. Thesedata were merged with tidal information using the UniversalSystem’s software package HIPS on an HP workstation. Thesoftware was used to visually inspect and clean all therecorded data types interactively on the workstation screen.A correction was applied to take into account variations inthe sound velocity profile, used to calculate the first-order

refraction correction routines in the sounder, which producedmismatches (up to 1 m) between depths recorded in theouter parts of adjacent swaths. After cleaning, merging withtidal information, and releveling, bathymetric data weregridded for imaging using a simple binning algorithm thatwas applied on a line-by-line basis. In this method, the positionsof soundings were first projected into the Cape BretonDevelopment Corporation grid coordinate system used forrecording mining operations. These data were then projectedonto a 4250 × 7200 pixel grid, covering the entire surveyarea with a horizontal resolution of 2 m and shaded to displaybathymetric detail (Fig. 4). Some artefacts remain: a linearWNW–SSE striping represents the residual effect of refractionat the edges of swaths. Nonetheless, geological features ofthe sea floor are clear.

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Documents

Deformation of Upper Carboniferous coal measures in the Sydney Basin: evidence for late Alleghanian tectonism in Atlantic Canada