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Atlantic Geology

Late Carboniferous Tectonostratigraphy in the Avalon Terrane of Southern New Brunswick R. Damian Nance

Volume 22, Number 3, December 1986 Article abstract Development of the basement-involved, fold-thrust belt in coastal southern New URI: https://id.erudit.org/iderudit/ageo22_3art07 Brunswick is attributed to dextral transpression associated with regional. CarbonlCerous strike-slip displacements. In the vicinity of Saint John, See table of contents deformation strongly influenced Westphalian sedimentation in the penecontemporaneous Lancaster and Balls Lake Formations and reflects sustained NW-SE shortening that coincides with a major compressive bend in Publisher(s) the E-W, Cobequid-Cbedabucto fault system which records significant, right-lateral Pennsylvanian displacement. Atlantic Geoscience Society Purple conglomerates and lithic wackes of the Balls Lake Formation record syntectonic. NW-progradation of an alluvial fan in response to the uplift of a ISSN source to the SB and display facies depicting proximal to ndd-fan. mid-fan, and distal settings. Locally interfingering and laterally equivalent, grey lithic 0843-5561 (print) arenites of the Westpbalian Lancaster Formation are the product of major, NE- 1718-7885 (digital) flowing, meandering streams partially influenced by the prograding distal fan. Initial deformation (Dj) produced NW-directed, basement-involved thrusts that Explore this journal structurally invert regional stratigraphy. Associated lower greenschlst facies metamor-phism accompanied development of a widespread, SB-dipping fabric (Sj). variably expressed as a slaty cleavage, protomylonitic solution cleavage and Cite this article orthomylonitic foliation. The fabric locally bears a strong but variably oriented mineral lineation (L^) and is axial. planar to NW-vergent, isoclinal ndcrofolds Nance, R. D. (1986). Late Carboniferous Tectonostratigraphy in the Avalon and regional overturned structures (Fi) that plunge gently NE and SW. Renewed Terrane of Southern New Brunswick. Atlantic Geology, 22(3), 308–326. thrusting (D2) and back-thrusting (D3) produced conjugate fold sets coaxial with Fj that verge both NW (F2) and SB (F3). Associated axial planar crenulation cleavages (S2 and S3) overprint S\ and dip SB and NW respectively. On-strike variations in deformational style and timing, coupled with the presence of steeply dipping, en echelon zones of Intense deformation, suggest the fold-thrust belt is segmented by right-stepping, convergent wrench faults synthetic to the deeply listric, Cobequid-Cbedabucto system. These faults may shallow into thrusts to form positive flower structures, and locally terminate in thrusts associated with anomalous NW-SE trends in D^ and D2. Thrust uplift in response to transpression on locally downward-steepening, en echelon faults subparallel to the Fundy shore Is proposed to account for the source area of the syntectonic Balls Lake fan. Further displacement and telescoping resulted in structural inversion of regional stratigraphy and deposition of the Balls Lake and Lancaster Formations in advance of an overriding allochthon to the south.

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Vol. 22 December, 1986 No. 3

Late Carboniferous Tectonostratlgraphy In the Avalon Terrane of Southern New Brunswick

R. Damian Nance Department of Geological Sciences Ohio University, Athens, Ohio 45701

Development of the basement-involved, fold-thrust belt in coastal southern New Bruns­ wick is attributed to dextral transpression associated with regional. Carboniferous strike-slip displacements. In the vicinity of Saint John, deformation strongly influenced Westphalian sedimentation in the penecontemporaneous Lancaster and Balls Lake Formations and reflects sustained NW-SE shortening that coincides with a major compressive bend in the E-W, Cobequid-Chedabucto fault system which records significant, right-lateral Pennsylvanian displacement.

Purple conglomerates and lithic wackes of the Balls Lake Formation record syntectonic, NW-progradation of an alluvial fan in response to the uplift of a source to the SB a n d display facies depicting proximal to mid-fan, mid-fan, and distal settings. Locally interfingering and laterally equivalent, grey lithic arenites of the Westphalian Lancaster Formation are the product of major, NE- flowing, meandering streams partial­ ly influenced by the prograding distal fan.

Initial deformation (Dj) produced NW-directed, basement-involved thrusts that struc­ turally invert regional stratigraphy. Associated lower greenschist facies metamor­ phism accompanied development of a widespread, SE-dipping fabric (Si), variably expressed as a slaty cleavage, protomylonitic solution cleavage and orthomylonitic foliation. The fabric locally bears a strong but variably oriented mineral lineation (Li) and is axial planar to NW-vergent, isoclinal microfolds and regional overturned structures (Fi) that plunge gently NE and SW. Renewed thrusting (D2 ) and back- thrusting (D3 ) produced conjugate fold sets coaxial with Fi that verge both NW (F2 ) and SE (F3 ). Associated axial planar crenulation cleavages (S2 and S3 ) overprint § 1 and dip SE and NW respectively.

On-strike variations In deformational style and timing, coupled with the presence of steeply dipping, en echelon zones of Intense deformation, suggest the fold-thrust belt is segmented by right-stepping, convergent wrench faults synthetic to the deeply listric, Cobequid-Chedabucto system. These faults may shallow into thrusts to form positive flower structures, and locally terminate in thrusts associated with anomalous NW-SE trends in D^ and Dy. Thrust uplift in response to transpression on locally downward-steepening, en echelon faults subparallel to the Fundy shore is proposed to account for the source area of the syntectonic Balls Lake fan. Further displacement and telescoping resulted in structural inversion of regional stratigraphy and deposi­ tion of the Balls Lake and Lancaster Formations in advance of an overriding allochthon to the south.

On impute le dAveloppement d'une zone orogAnique chevauchante solidaire du socle au littoral du Nouveau-Brunswick meridional A une transpression dextre accompagnAe de dAcrochements rAgionaux carbonifAres. PrAs de Saint John, la deformation, qui a fortement marque la sAdi mentation westphalienne dans les formations syncbrones de Lancaster et de Balls Lake, provient d'un raccourcissement soutenu NW-SE ayant coincide avec un important coude en compression dans le systAme de failles E-W de Cobequid-Chedabucto. Ce dernier reprAsente un coulissage A droite de grande valeur d'age pennsylvanien.

Les poudingues pourpres et les wackes lithiques de la Formation de Balls Lake soulignent la progradation syntectonique NW d'un cdne de dejection et traduisent la

MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 22, 308-326 (1986) Received April 9. 1986 0711-1150/86/030308~19$3.85/0 Revision Accepted August 25. 1986 309 Nance

proximity de reliefs dmergds au SE; les facias tdmoignent de regimes proximaux k milieux de cone, de milieux de cone, et distaux. Les ardnites lithiques grises de la Formation de Lancaster, du Westphallen, en sont iEquivalent lateral et s'y interdigitent par endroits. Elies sont, pour leur part, le produit de grands cours d'eau k mdandres coulant vers le NE sous 1 'Influence partielle du cone distal progradant.

Des nappes de socle de direction NW Inversent structuralement la stratlgraphie rdgionale et tdmolgnent d'une deformation tnitiale (Dj). Le ddveloppement largement rdpandu d'tme fahrique (S^), pentde SE et prenant l'aspect variable d'un clivage ardoisier, d'un clivage sous solution protomylonltique et d'une foliation orthomylonitique, fut accompagnd d'un mdtamorphisme dans le facids schistes verts infdrieur. Localement, la fahrique acquiert une lindation (Lj) forte mais variable, soulignde par 1 ' orientation de mindraux dana le plan axial de microplis isoclinaux k vergence NW et de structures rdgionales retoumdes (F5 ) qui plongent gentiment vers le NE et le SW. S'y associent des clivages de crdnulation de plan axial (S2 et S3 ), pentds respectivement SE et NW, qui se superposent sur Sj.

Des variations longitudinales dans le style et l'dpoque de la ddformation, coupldes k la prdsence de zones intensdment ddformdes, en dchelon et fortement pentdes, suggdrent que la zone orogdnlque chevauchante est scindde par des ddcrochements vertlcaux ddcalds k droite, qui convergent et sont, eux-memes, synthdtiques du systdme fortement Ustrique de Cobequid-Chedabucto. Ces failles se couchent vers la surface pour devenir des chevauchements qui engendrent des structures en fleur positives; elles s'estompent par endroits en des chevauchements associds aux orientations anormales NW­ SB dans et D 2 . On explique la source du cone syntectcnique de Balls Lake par la surrection des paquets chevauchants sous le jeu en transpression de failles en dchelon subparalldles k la cote de Fundy et dont les pendages se redressent localement vers le bas. Un ddplacement et un tdlescopage suppldmentaires rdsultdrent en une inversion structurale de la stratlgraphie rdgionale et en la ddposition des formations de Balls Lake et de Lancaster en avant du terrain charrid plus au Sud. ^ le journal]

INTRODUCTION tectonics is less clear as the region has been interpreted as a major over­ The southeastern margin of the thrust terrain in which Carboniferous Avalon terrane in southern New Bruns­ sedimentary rocks formed entirely al­ wick forms a narrow, basement-involved, lochthonous members of the overriding fold-thrust belt that trends east- plate (Rast et al. 1978). The tectono- northeast and broadly parallels the stratigraphy of the fold-thrust belt northern shore of the Bay of Fundy and its relationship to regional (Fig. 1). Variously termed the strike-slip faulting have consequently "Maritime Disturbance" (Poole 1967), remained uncertain. the "Fundy Cataclastic Zone" However, Plint and van de Poll (Ruitenberg et al. 1973) and the (1982, 1984) and Currie and Nance "Variscan Front" (Rast and Grant 1973), (1983) have recently shown that late the thrust belt is the product of Car­ Carboniferous sedimentary rocks within boniferous compressive deformation in the fold-thrust belt are, in part, contrast with the regional strike-slip autochthonous and have proposed they faulting that is the dominant late record syntectonic alluvial fan and Paleozoic structural style in Maritime fluvial sedimentation, developed in Canada. Carboniferous deformation in response to the uplift and late Carbon­ much of the Maritimes reflects mainly iferous emplacement of thrust sheets. dextral movements on major wrench Mosher and Rast (1984) have further structures (Keppie 1982; Rast 1984), suggested that development of the over­ and resulted in the development of thrust terrain reflects termination of pull-apart basins recorded in penecon- the Cobequid-Chedabucto fault system of temporaneous sedimentation (Bradley Nova Scotia, on which some 165 km of 1982; Yeo and Ruixing 1986). However, dextral displacement was accommodated in southern New Brunswick the relation­ in the mid-Pennsylvanian and Permian ship of sedimentation to Carboniferous (Keppie 1982). 310 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY

Fig. 1. Geologic sketch map of late Carboniferous fold-thrust belt in southern New Brunswick (modified after Hayes and Howell 1937; Alcock 1938; Rast et al. 1978; Wardle 1978; Ruitenberg et al. 1979; Plint and van de Poll 1982,1984; Currie and Nance 1983; Currie 1984, 1986; McCutcheon 1984; McCutcheon and Ruitenberg 1984; Mosher and Rast 1984; Parker 1984; Ruitenberg 1984; Nance and Warner 1986; and others). Biotite (Bio), chloritoid (Ctd) and garnet (Gam) isograds from Mosher and Rast (1984).

A detailed history of late Carbon­ tionally subdivided into two distinct iferous deposition and deformation is packages (Fig. 1). Late Devonian to recorded in Pennsylvanian sedimentary Mississippian redbeds of the rocks within the fold-thrust belt. Kennebecasis Formation pre-date, and Southeast of Saint John (Fig. 1), these are largely unaffected by, late Carbon­ straddle a major structural front along iferous deformation and record local, which mildly deformed and essentially graben-fill alluvial sedimentation de­ autochthonous units to the north are veloped in response to normal movements overthrust by their polydeformed strat­ on major, through-going faults. In igraphic equivalents to the south. The contrast, clastic rocks of the Pennsyl­ depositional record preserved within vanian Lancaster and Balls Lake Forma­ the autochthon (Caudill and Nance tions, and the lithologically correla­ 1986), coupled with the structural tive Boss Point and Tynemouth Creek history recorded in the allochthon Formations to the northeast (Plint and (Nance and Warner 1986), support a van de Poll 1982, 1984), were deposited syntectonic, alluvial fan to fluvial penecontemporaneously with the develop­ setting that developed in response to ment of the fold-thrust belt and are tectonic uplift along the present Fundy locally intensely deformed. shore during a Westphalian phase of Within the mildly deformed autoch­ transpression on the offshore Cobequid- thon (Caudill and Nance 1986) southeast Chedabucto fault system (Nance 1986). of Saint John (Fig. 2), the Balls Lake Formation comprises greyish purple STRATIGRAPHY AND GEOLOGIC SETTING lithic wackes containing subrounded pebbles and thin conglomeratic hori­ Carboniferous sedimentary rocks in zons; similarly coloured, moderately the vicinity of Saint John are tradi­ sorted, bimodal and polymict orthocon- 311 Nance

SECTION LOCATIONS

q u a t e r n a r y

t r ia s s ic

LANCASTER OVERBANK CARB. F M . 1------

UPPER B ALLS MID-FAN LAKE FM. MID-FAN

DISTAL

HADRYNIAN COLDBROOK Hc GROUP

Fig. 2. Simplified geological map of autochthonous Lancaster and Balls Lake Formations southeast of Saint John illustrating facies distribution and field location of synoptic stratigraphic sections shown in Figure 4 (after Caudill and Nance 1986). 312 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY glomerates; red to greyish red, unsort­ fan facies. Stratigraphically and/or ed, polymict and polymodal paraconglom- structurally overlying grey to tan, erates; and occasional red siltstones. cross-bedded lithic and quartz are­ The formation attains a maximum esti­ nites, thin pebble conglomerates, and mated thickness of 200 m but thins locally plant-bearing black shales are northwestward to a few metres near assigned to the Lancaster Formation. Saint John (Currie and Nance 1983). In At Ploughshare Rock (Fig. 3), the contrast, the Lancaster Formation, Lancaster Formation is itself over­ which attains a maximum thickness in thrust by strongly deformed, clastic excess of 400 m, consists of grey to and volcanogenic sedimentary rocks that greyish brown, lithic arenites with are tectonically overlain by metavol- occasional thin conglomeratic pebble canic rocks. The sedimentary unit lags; common grey-green siltstones; and comprises green calcareous siltstones, grey to dark grey shales. Poorly pre­ purple sandstones and green to pink, served plant fragments within the volcanogenic chlorite-calcite schists. shales have been locally assigned to The predominantly volcanic succession the Westphalian B (Stopes 1914) and includes grey-green volcanogenic sand­ Westphalian C (N. Rast, pers. comm. stones and siltstones, epidotized in­ 1985) and probably span the interval termediate to basic volcanics, frag­ Westphalian A-C. The Lancaster Form­ mental pyroclastics and occasional ation conformably overlies and, local­ green laharic conglomerates. Although ly, may underlie the Balls Lake Forma­ clearly older than the Balls Lake tion while vertical and lateral transi­ Formation, to which they contribute tions from typical Balls Lake litholo­ clasts, the age of these units is un­ gies to grey-green units similar to certain . The volcanic succession has those of the Lancaster Formation sug­ traditionally been assigned to the West gest lateral equivalence of the two Beach Formation of presumed Carbonifer­ units in some areas. An early West­ ous age (Hayes and Howell 1937; Alcock phalian age for the unfossiliferous 1938), whereas the sedimentary unit has Balls Lake Formation therefore seems been assigned to both the West Beach probable. Locally, both formations (Hayes and Howell 1937) and Balls Lake rest unconformably on Eocambrian feld- Formations (Alcock 1937; Ruitenberg et spathic sandstones, volcanogenic con­ al. 1979; Parker 1984). However, both glomerates, felsic tuffs and basalt units most closely resemble the Eocam­ flows that overlie the late Precambrian brian succession and late Hadrynian Coldhrook Group (Currie 1984; Tanoli e£ Coldbrook Group (Currie and Nance 1983; al. 1985). Ruitenberg 1984) which unconformably Further south, late Carboniferous underlie the Balls Lake and Lancaster deformation in parautochthonous units Formations further north (Currie 1984; of the Balls Lake and Lancaster Forma­ Tanoli et al. 1985). tions progressively intensifies towards Extending northeastward from Cape Cape Spencer (Fig. 3) where they are Spencer (Fig. 3) are a series of ortho- overthrust by allochthonous volcano­ mylonitic granitoid and quartz dioritic sedimentary units and the Cape Spencer klippen that occupy the structurally granite (Nance and Warner 1986). Var­ highest position within the allochthon iably deformed, purple to green poly­ and collectively constitute the Cape mict conglomerates, lithic wackes, Spencer granite. A late Hadrynian age siltstones and shales, traditionally for this granite, which is separated assigned to the Balls Lake Formation, from the tectonically underlying closely resemble the mid­ volcano-sedimentary assemblage by a fan facies of the autochthon des­ prominent mylonitic thrust surface, is cribed by Caudill and Nance (1986) and supported by textural and compositional are associated with pale green calcar­ similarities to Hadrynian plutons north eous siltstones similar to their distal of the fold-thrust belt (Wardle 1978; 313 Nance

km

SUBAREA 1 /SUBAREA 2

Carboniferous Ca | Aplite/Silicification n=27 C| Lancaster Formation D1 thrust Cbl Balls Lake Formation D2 thrust Age uncertain F2 axial trace West Beach Formation (s: sediments, v: volcanics) F3 axial trace THadrynian Contact metamorphic assemblage I Hg I Cape Spencer granite Line of section

Fig. 3. Structural sketch map of parautochthonous and allochthonous units and lineation orien­ tations on the eastern shore of Mispec Bay (after Nance and Warner 1986).

Currie et a/. 1981). deformation as both aplites and their The Cape Spencer granite and sedi­ possible contact metamorphic or metaso­ mentary rocks of the volcano­ ma tic assemblages cross-cut the princi­ sedimentary unit and the Balls Lake pal (Si) deformational fabric of their Formation are locally invaded by small, host rocks, but are themselves folded sill-like masses of ?aplite and minor by later (D* and D*) stages of deforma­ felsic dikes that appear to be asso­ tion (Warner 1985). ciated with retrogressed chlorite- ?andalusite spots which also occur SEDIMENTOLOGY AND DEPOSITIONAL SETTING within the Lancaster Formation. Devel­ opment of these enigmatic bodies is Balls Lake Formation associated with intense wall-rock sili- cification and is demonstrably syntec- North of Mispec Bay (Fig. 2) mild­ tonic with respect to Carboniferous ly deformed and essentially autochthon­ 314 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY ous units of the Balls Lake Formation approached (Section 3; Fig. 4), bed have been interpreted to be the product thicknesses further decrease, the pro­ of a northwestward-prograding alluvial portion of siltstone and shale in­ fan (Caudill and Nance 1986). More creases, and the succession becomes proximal portions of the fan (Section dominated by laterally extensive, grey- 1; Fig. 4) display braided stream depo­ green sandstones interbedded with sits interbedded with subaerial deb­ finer-grained overbank deposits. The ris flows. Braided sequences comprise tabular and low-relief, erosively based erosively based, channel-fill deposits sandstones are attributed to channel in which clast-supported pebble deposition within a small meandering conglomerates fine upwards into coarse, stream network draining the fan toe. large-scale, trough-cross-bedded sand­ Here stream-bank stability and flood- stones. In contrast, debris flows form plain development was fostered by the unusually coarse, unsorted and matrix- baffling effect of plant growth and supported, polymict conglomerates of resulted in more sinuous channels than broadly tabular geometry. They are those typical of braided river systems ungraded, devoid of current strati­ (Caudill and Nance 1986). fication, and exhibit textural inver­ Paleocurrent directions within the sion whereby contained pebbles display Balls Lake Formation, based on foreset a significantly greater degree of inclination of trough cross-beds, dis­ rounding than larger cobbles and play a radial dispersal pattern typical boulder-sized clasts. These larger of alluvial fans (Bull 1972) and indi­ clasts are predominantly calcareous, cate northwestward paleoflow consistent cross-bedded sandstones lithologically with a source area to the southeast similar to interbeds of fluvial origin (Caudill and Nance 1986). and suggest that the debris flows orig­ inated on the contemporary fan surface. Lancaster Formation Despite the lack of current stratifica­ tion, these intraclasts further display The Lancaster Formation is largely a strong cleavage-parallel alignment the product of a major, northeasterly that is absent in pebbles of older flowing, meandering stream system (van lithologies. As features indicative of de Poll 1970) that drained the basin intraclast rotation are absent, align­ into which the Balls Lake fan pro- ment of the flattened clasts occurred graded. However, in the vicinity of prior to conglomerate lithification and the fan toe, overbank sediments of the hence provides evidence that Carbon­ Lancaster Formation are influenced by iferous deformation was broadly synde- the distal stream network of the Balls positional (Caudill and Nance 1986). Lake fan. Adjacent to the fan (Section In the mid-fan region (Section 2; 4; Fig. 4), small meandering channel- Fig. 4), bed thickness decreases, de­ fill deposits, comprising pebble con­ bris flows are absent, and the succes­ glomerates with shale rip-ups that fine sion is dominated by braided fluvial upwards to parallel-laminated and deposits similar to, but thinner than, trough cross-bedded sandstones, are those of more proximal settings. Flu­ incised into floodplain shales contain­ vial conglomerates interbedded with ing mud cracks and Westphalian plant thin fining-upward sequences display fragments. However, paleocurrent di­ horizontal to very low angle stratifi­ rections showing a somewhat greater cation and are associated with peri­ dispersion than those of the Balls Lake pheral sandstone wedges resembling mod­ Formation are predominantly westward e m bar deposits (Rust 1972). Lateral­ for these channel deposits (Caudill and ly extensive and parallel-laminated Nance 1986) suggesting that they are sandstones recording sheet-flood depo­ the product of small meandering streams sition predominate down section. that drained the Balls Lake fan onto As distal reaches of the fan are the floodplain of the Lancaster fluvial 315 Nance

Fig. 4. Correlation of synoptic stratigraphic sections for the Balls Lake and Lancaster Formations showing vertical facies distribution suggesting fan progradation. See Figure 2 for section locations (after Caudill and Nance 1986). 316 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY system. Distinctly finer, parallel- terms of a simple alluvial fan model laminated, trough and planar cross- (Fig. 5) in which braided streams of bedded sandstones toward the top of the the mid fan are associated with debris section are attributed to stream depo­ flows derived from steeper portions of sition on an encroaching Lancaster the proximal fan and give way distally floodplain at progressively greater to rapidly migrating streams. These distances from the receding fan toe. distal streams eventually meander across the floodplain of the Lancaster Depositional Model fluvial system where they locally in­ fluence sedimentation that was pri­ Caudill and Nance (1986) have marily controlled by flooding of major, interpreted the Balls Lake Formation in northeast-flowing streams. Fluctua-

US Proximal fan Balls Lake Mid-fan Formation Distal fan

Fig. 5. Schematic alluvial fan model illustrating depositional setting and spatial distribution of Lancaster and Balls Lake Formation facies (modified after Caudill and Nance 1986). 317 Nance tions of the rate of sedimentation In In this area, three major, coaxial the respective depositlonal systems fabric-forming events affect both the produced an Interfingering of the Balls parautochthonous Balls Lake and Lake and Lancaster Formations as the Lancaster Formations and the struc­ fan prograded northwestward. Within turally overlying allochthonous units. the Balls Lake Formation, the encroach­ In the following discussion, these are ment of mid-fan facies into areas in­ designated Di to Da, although their itially and finally occupied by the regional development is likely to have distal fan (Fig. 4) suggests a been diachronous and, in part, conju­ progradation-retreat response of the gate. fan to changing local relief and ero­ sion rates within a southeasterly Di Structures source area and is, hence, consistent with the paleocurrent data. Fan pro­ The earliest phase of Carboniferous gradation requires marked relief to the deformation (Di) records the northwest- southeast while intraclast alignment directed thrusting that emplaced the within debris flow conglomerates of the Cape Spencer allochthons on surfaces Balls Lake Formation suggest sediment­ now represented by the mylonitic basal ation was syndeformational. The model contacts of the Cape Spencer granite is therefore consistent with develop­ and both units of the underlying ment of the fan ahead of an actively volcano-sedimentary section. Asso­ advancing allochthon as proposed by ciated lower greenschist (chlorite Currie and Nance (1983). zone) metamorphism accompanied the de­ A closely analogous depositlonal velopment of a pervasive, broadly model has been presented by Plint and southeast-dipping but variably expres­ van de Poll (1984) for Pennsylvanian sed planar fabric (Si) that locally sediments at the northeastern end of contains a strong but variably oriented the fold-thrust belt (Fig. 1). Here, extensional lineation (Li). Southeast Westphalian redbeds of the Tynemouth of Saint John, Si is axial planar to Creek Formation are interpreted as the rare, isoclinal microfolds (Fi) that product of alluvial fans which, in plunge gently northeast and southwest. response to the tectonic uplift and To the west of the city (Fig. 1), thrust emplacement of a source area to however, Si is associated with the southeast, prograded northwestward northwest-verging overturned structures into a basin previously drained by a of regional extent (Rast et a/. 1978; major northeasterly flowing fluvial Parker 1984; Nance 1986). system (Boss Point Formation). Further Within the parautochthonous Balls thrusting subsequently caused both Lake and Lancaster Formations of sub­ formations to be tectonically overrid­ zone 1 (Fig. 3), Si ranges from a well­ den and was followed by early Mesozoic spaced, protomylonitic solution cleav­ extensional reactivation associated age in competent conglomerates and with opening of the offshore Fundy sandstones, to a closely spaced, Basin during initial rifting of the chlorite-muscovite slaty cleavage in present Atlantic Ocean (Nadon and finer lithologies. Elongate sediment­ Middleton 1984). ary porphyroclasts, mica beards and conglomerate pebbles locally define an STRUCTURAL GEOMETRY AND DEFORMATIONAL Li lineation that parallels Fi axes and HISTORY plunges gently northeast and southwest (Fig. 3). The lineation has been at­ The tectonic development of the tributed to both tectonic and sediment­ Balls Lake fan (Nance and Warner 1986) ary influences (Bradley 1984), the is recorded southeast of Mispec Bay latter being supported by its perpen­ (Fig. 3) where the regional strati­ dicular orientation with respect to graphy has been structurally inverted. paleoflow within the host Balls Lake 318 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY and Lancaster Formations such that (Warner 1985) with predominantly south- alignment of elongate pebbles may part­ southeast-plunging axes of maximum ly reflect depositional processes elongation (X). However, observed (Caudill and Nance 1986). Based on the strain in subzone 2 is complex and shape of conglomerate pebbles, the XY probably reflects the temporal and plane of strain in subzone 1 lies with­ spatial overlap of various thrust- in a broadly flat-lying S» cleavage related processes such as shearing, while the principal axis (X) parallels flattening and pressure solution. Li and Fi. Qualitative strain esti­ Shearing during initial northwestward mates (Warner 1985) plot in both the thrusting produced east-southeast- constrictional and flattening fields plunging prolate ellipsoids, and was and show orientations relative to minor followed by a component of flattening structures that suggest flattening that became increasingly important as during thrust emplacement coupled with thrust imbrication progressed. In addi­ extension parallel to contemporary fold tion, deformation involved significant axes. mass transfer, particularly in the Cape In the allochthonous units of Spencer granite where the conversion of subzone 2 (Fig. 3), deformation is a quartzo-feldspathic protolith to a markedly more intense. Si is commonly muscovite-rich mylonite requires net mylonitic and reflects substantial duc­ loss of silica, part of which is re­ tile shearing, thrust-related flatten­ presented by ubiquitous Si-parallel ing and post-tectonic annealing in quartz veins that locally achieve addition to pressure solution. Li, thicknesses of over 5 metres (Alcock defined by mylonitic quartz ribbons, 1938). elongate porphyroclasts and mica beards, is variable in orientation D* and Ds Structures (Fig. 3) with an east-southeast- plunging maxima taken to indicate the Subsequent Carboniferous deforma­ stretching direction during thrust tion (D* and Ds) was broadly coaxial transport. In klippen of the Cape with Di and produced trains of asym­ Spencer granite, protoraylonitic fabrics metric folds that verge both northwest preserving primary granitic textures (Fs) and, more commonly, southeast progressively give way to orthoraylon- (Fs). Associated axial planar crenula- ites and ultramylonitic muscovite tion cleavages range from barely per­ schists within discrete shear zones and ceptible to intense fabrics dipping towards basal thrust contacts. Isolated southeast and northwest respectively. sigmoidal porphyroclasts indicate top- The resultant fold belt (Fig. 6) is to-northwest shear according to the superimposed on the Di thrust terrain criteria of Simpson and Schmid (1983). at Cape Spencer, is geometrically con­ In the tectonically underlying vol­ jugate in profile, and is quite narrow cano-sedimentary section, Si is variab­ since correlative fold closures rapidly ly expressed as a closely spaced slaty become less frequent and more gentle cleavage, protomylonitic solution north of Mispec Bay (Nance 1985). As­ cleavage and orthomylonitic foliation. sociated D* and D* thrusts are marked In the latter, sigmoidal, solution- by an intensification of their respect­ modified detrital porphyroclasts again ive cleavages and folding but do not indicate top-to-northwest shear and may appear to juxtapose significantly dif­ be associated with optically continuous ferent units and are likely to be of quartz ribbons produced through crys­ modest displacement relative to those tallographic slip, and core-and-mantle of Di. F* folds, distinguished only by structures reflecting pronounced, post- their northwest vergence, plunge gently tectonic annealing. Qualitative strain northeast and southwest and are largely estimates again plot in both the con­ confined to the northern portion of strictional and flattening fields subzone 1. Coaxial but southeast- 319 Nance

Fig. 6 . Schematic cross section, Mispec Bay to Cape Spencer. See Figure 3 for section location (after Nance and Warner 1986). verging Fa folds are rare in areas of rics within the Balls Lake Formation, Fa folding but become more abundant to coupled with its lateral equivalence to the southeast. As both fold sets show part of the Lancaster Formation similar structural style and texturally (Caudill and Nance 1986), constrains identical crenulation cleavages, they the onset of deformation to the early probably represent a conjugate Westphalian. Its duration, however, is folding/backfolding response to renewed less certain. Folding clearly affects or continued northwest-southeast com­ Lancaster sedimentary rocks as young as pression that earlier produced Di. Westphalian C (N. Rast, pers. coram. Asymmetric, conjugate kink sets with 1985) and may have intermittently con­ gently northwest and southeast plunging tinued to the Permian (Rast 1983). axes locally overprint D« and D* and Deformed redbeds of the Lepreau Forma­ are equally dispersed across the entire tion, assigned to the Triassic on the area (Nance and Warner 1986). basis of reptile tracks (Sarjeant and Stringer 1978), might have provided an DISCUSSION AND INTERPRETATION upper time limit for the event as they possess a southeast-dipping fracture Major phases of Carboniferous de­ cleavage and are folded about formation recorded in Westphalian sedi­ northeast- southwest axes (Stringer and mentary rocks southeast of Saint John Lajtai 1979). However, Lepreau Forma­ reflect sustained northwest-southeast tion siltstones have subsequently compression that was initially respon­ yielded Mississippian (late Visean) sible for structural inversion of the spores so that the age of the formation regional stratigraphy during basement- is currently uncertain (Stringer and involved, northwest-directed thrusting, Burke 1985). Triassic redbeds else­ and was later accommodated by conjugate where are undeformed. folding and backthrusting (Nance and Carboniferous sedimentation for Warner 1986). The event strongly in­ much of Maritime Canada has been suc­ fluenced deposition of the Balls Lake cessfully modeled in terms of pull- Formation as shown by facies relation­ apart basin development adjacent to ships, pebble lithologies that match major strike-slip faults that record allochthonous units to the southeast, significant syndepositional displace­ and northwest vectors of paleoflow that ments (Bradley 1982; Yeo and Ruixing are perpendicular to fold axes (Caudill 1986) . Although this is potentially and Nance 1986). The Balls Lake Forma­ consistent with the calc-alkaline as­ tion therefore provides both a deforma- semblage of the enigmatic West Beach tional and depositional record of the Formation (Strong et al. 1979), assum­ event as suggested by Currie and Nance ing that the latter is of Carboniferous (1983). Syndepositional tectonic fab­ age, this is clearly inconsistent with 320 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY

the demonstrably Carboniferous Balls its deformational style, orientation Lake and Lancaster Formations where and intensity. Regional northwest- sedimentation accompanied strong com­ verging Di structures (Rast et al. pressive deformation. Furthermore, the 1978; Parker 1984) and post-tectonic spatial coincidence of the fold-thrust greenschist facies metamorphism that belt with a major convergent bend in culminates in the occurrence of garnet the Cobequid-Chedabucto fault system at Lomeville (Mosher and Rast 1984) (Fig. 7), which is known to record southwest of Saint John (Fig. 1), are significant, right-lateral Westphalian absent southeast of the city where movement (Keppie 1982), stongly sug­ megascopic Di folds are unknown and gests that shortening in southern New metamorphism is syntectonic and lies Brunswick reflects late Carboniferous entirely within the chlorite zone transpression at the western terminus (Nance and Warner 1986). Furthermore of the Cobequid system where the vector major allochthonous zones within the of strike slip is compressionally ob­ fold-thrust belt are not laterally lique to the fault trace. As illus­ persistent but rather show en echelon trated by the regional strain ellipse trends. Thus the intensely deformed (insert, Fig. 7), dextral transpression zone of the Lomeville peninsula tinder these conditions would be accom­ (Currie 1984) does not extend east of panied by a northwest-southeast compon­ Saint John but instead appears to side­ ent of compression capable of producing step to that of Cape Spencer where it fold-thrust orientations closely terminates to the northeast and may matching those of southern New again side-step seawards (Fig. 1). Brunswick. Similarly, the highly deformed zone Support for transpression as the west of Lepreau also terminates to the tinder lying mechanism for deformation in northeast and may side-step to that of southern New Brunswick is also evident Lomeville. Such closely spaced, on- within the fold-thrust belt itself. In strike discontinuities in deformational the absence of megascopic Fi structures geometry, timing and intensity are not southeast of Saint John, the widespread readily resolved within a simple over- development of flattened conglomerate thrust system but rather suggest dif­ pebbles and a pervasive, subhorizontal, fering degrees of convergence on separ­ bedding-parallel Si cleavage within the ate , right-stepping wrench faults. parautochthon of subzone 1 (Fig. 3) These faults segment the fold-thrust suggest that the granitic and volcano­ belt (Fig. 1), are synthetic with re­ sedimentary allochthon, presently pre­ spect to the offshore Cobequid- served on moderate to steeply dipping Chedabucto system, and shallow into thrusts at Cape Spencer, was formerly thrusts to form positive flower struc­ more extensive. Hence the implied tures. Evidence that this may be the geometry of thrusting, in addition to case can be found at Musquash Harbour being basement-involved, is downward- (Fig. 8) where the thrust termination steepening to the south. Furthermore, of one such dextral shear would provide while raylonitic porphyroclasts within an explanation for otherwise anomalous, the allochthon of subzone 2 (Fig. 3) northwest-southeast fold trends at the consistently yield up-dip, top-to- western end of the Lomeville belt northwest shear sense, stretching lin- (Parker 1984; Nance 1986). eations taken to indicate transport Carboniferous deformation on direction predominantly plunge east- Western Head (Fig. 8) mimics that des­ southeast and are, hence, right- cribed by Rast et al. (1978) with major laterally oblique to their associated, recumbent Di folds cut by two sets of southeast-dipping thrust surfaces. younger cleavages. However, whereas A transpressional origin for the the third deformational phase (Da) fold-thrust belt would also account for corresponds in style and orientation the regional, on-strike variations in with the Da backthrusting event south- 321 Nance

Fig. 7. Geologic sketch map and interpretive cross sections of Western Head (see Figure 8 for location). MARITIME SEDIMENTS AND ATLANTIC GEOLOGY

Fig. 8. Interpretation of anomalous cross-folds at the western end of the Lomeville belt as thrust termination of dextral shear buttressed against a major granitoid and detached on carbonates of the Green Head Group. 323 Nance east of Saint John, the earlier two formation of the Balls Lake and phases (Di and D*), although broadly Lancaster Formations southeast of Saint coaxial and coplanar, are perpendicular John in which thrusting is attributed to their equivalent folds southeast of to transpression rather than simple Saint John and trend northwest-south- convergence (Fig. 10). In character east. In this area Di, which records with convergent wrench settings the regional early thrusting event, (Harding and Lowell 1979), individual occurred in response to southwest- upward-splaying faults shallow outwards directed transport and produced major, into near-surface thrusts to define a southwest-vergent overturned structures positive flower structure over a syn­ (Fi) and a pervasive, bedding- thetic, convergent wrench fault subpar­ subparallel Si cleavage (Fig. 8). allel to the present Fundy shore. Up­ Renewed or continued thrusting (Di) lift and structural inversion over the produced coaxial folds (F*) overturned wrench fault provides the southeasterly to the southwest and a pervasive S* source area for the Ball Lake alluvial cleavage that is inclined at a shallow fan which then fed the meandering angle to Si. Ds, however, records stream system of the Lancaster Forma­ regional backthrusting with the devel­ tion further north. However, with opment of southeast-vergent, overturned continued displacement on the wrench folds (Fs) and a strong, northwest­ system, thrusting progressively over­ dipping, axial planar crenulation. The rode the Balls Lake and Lancaster Form­ distribution of the anomalous ations and produced the area's present northwest-southeast folds at the structural geometry in which floodplain western end of the Lomeville belt facies of the Lancaster Formation and (Fig. 9) suggests that dextral movement fan facies of the Balls Lake Formation on synthetic wrench faults was here are overthrust, in turn, by their de­ buttressed against a major, ?Hadrynian formed equivalents, units of the granitoid and terminated in southwest- ?Eocambrian volcano-sedimentary assem­ directed thrusting detached on carbon­ blage and, finally, by the Cape Spencer ates of the ?Helikian Green Head Group granite. (Nance 1986). On a regional scale, a model in­ Nance and Warner (1986) have volving the development of positive therefore proposed a tectonic model for flower structures over en echelon, late Carboniferous deposition and de­ convergent wrench faults synthetic to a

Fig. 9. Simplified structural map of the Bay of Fundy illustrating the coincidence of thrusting in the Saint John region with a major compressive bend in the Cobequid-Chedabucto fault system (after Nance and Warner 1986). Composite strain ellipse (Harding 1974) indicates predicted fold-thrust orientations during dextral shear (Saint John structures modified after Mosher and Rast 1984). 324 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY deeply llstric, Cobequid-Chedabucto matched to rocks of the New Brunswick system (Fig. 11) might also account for Avalon terrane, those of the Tynemouth significant differences in source area Creek Formation contain possible con­ indicated by the conglomerate pebbles tributions from both the Meguma Terrane of individual fan complexes. In con­ and the, then closer, Cobequid terrane trast to the Balls Lake fan where con­ (Plint and van de Poll 1982). The on- glomerate pebbles can be largely strike segmentation of individual fan

structure during regional dextral transpression (after Nance and Warner 1986).

Fig. 11. Regional development of positive flower structures over en echelon. convergent wrench faults synthetic to the deeply listric Cobequid-Chedabucto fault system as an explanation for provenance variation within Westphalian alluvial fan complexes. 325 Nance basins suggested by these variations in John. New Brunswick. In Current Research, Part provenance, while counter to that ex­ A, Geological Survey of Canada, Paper 83-1A, pected for simple overthrust systems, pp. 29-36. CURRIE, K. L.. NANCE, R. D., PAJARI, G. E.. Jr., is the predicted pattern of convergent and PICXERILL, R. K. 1981. Some aspects of the wrench settings. pre-Carboniferous geology of Saint John, New Brunswick. In Current Research, Part A, Geo­ ACKNOWLEDGEMENTS logical Survey of Canada, Paper 81-1A, pp. 23- 30. HARDING, T. P. 1974. Petroleum traps associated Support for this project has been with wrench faults. American Association of provided by the Ohio Univeristy Re­ Petroleum Geologists Bulletin, 58, pp. 1290- search Council (OURC 9710) and Baker 1304. HARDING. T. P . , and LOWELL, J. D. 1979. Structur­ Fund (85-5) and is gratefully al styles, their plate tectonic habitats, and acknowledged. For their assistance, hydrocarbon traps in petroleum provinces. Amer­ encouragement and stimulating field ican Association of Petroleum Geologists Bulle­ tin, 63, pp. 1016-1058. discussions, the author is particularly HAYES. A. 0., and HOWELL, B. G. 1937. Geology of indebted to S.M. Barr, M.R. Caudill, Saint John, New Brunswick. Geological Society K.L. Currie, L.R. Fyffe, R.H. Grant, of America, Special Paper 5, 146 p. S.R. McCutcheon, J.B. Murphy, I.M. KEPPIE, J. D. 1982. The Minas Geofracture. In Patel, R.K. Pickerill, N. Rast, A.A. Major structural zones and faults of the North­ ern Appalachians. Edited by P. St-Julien and J. Ruitenberg, J.A.R. Stirling, P. Beland. Geological Association of Canada, Stringer, S.K. Tanoli, T.C. Teng, H.W. Special Paper 24, pp. 263-280. van de Poll, J.B. Warner, S.E. Watters McCUTCHEON, S. R. 1984. Geology of the gold and P.F. Williams. Ken Currie and bearing rocks in the Lomeville-Lepreau area. New Brunswick Department of Natural Resources, Sheila Watters are further acknowledged Mineral Resources Division, Ninth Annual Review for their constructive reviews of the of Activities, Information Circular 84-2, pp. final manuscript. 2-6. ____ McCUTCHEON. S. R . . and RUITENBERG, A. A. 1984. Geology of Annidale-Nerepis area, southern New Brunswick. New Brunswick Department of Natural ALCOCK. F. J. 1938. Geology of the Saint John Resources. Geological Surveys Branch, Plates region. New Brunswick. Geological Survey of 84-1 and 84-4. Canada. Bulletin 210, 65 p. MOSHER, S . , and RAST. N. 1984. The deformation BRADLEY, D. C. 1982. Subsidence in Late Paleozoic and metamorphism of Carboniferous rocks in basins in the Northern Appalachians. Tectonics, Maritime Canada and New England. In Variscan 1. pp. 107-123. tectonics of the North Atlantic region. Edited BRADLEY, L. M. 1984. Structural analysis of de­ by D. H. W. Hutton and D. J. Sanderson. Geolog­ formed Carboniferous strata, Mispec Beach, ical Society (London), Special Publication 14, southern New Brunswick. Unpublished M.S. pp. 233-244. thesis, State University of New York at Albany, NADON, G. C., and MIDDLETON, G. V. 1984. Tectonic 134 p. control of Triassic sedimentation in southern BULL, B. B. 1972. Recognition of alluvial-fan New Brunswick: Local and regional implications. deposits in the stratigraphic record. Society Geology, 12, pp. 619-622. of Economic Paleontologists and Mineralogists, NANCE, R. D. 1985. Alleghenian deformation in the Special Publication 19, pp. 290-303. Mispec Group, Saint John Harbour, New Bruns­ CAUDILL. M. R . . and NANCE. R. D. 1986. Variscan wick. In Current Research, Part A, Geological tectonostratigraphy of the Mispec Group, Survey of Canada, Paper 85-1A, pp. 7-13. southern New Brunswick: stratigraphy and depo- ______1986. Evidence of a transpressional origin sitional setting. In Current Research, Part A, for the Variscan fold-thrust belt in southern Geological Survey of Canada, Paper 86-1A, pp. New Brunswick. Geological Society of America, 343-350. Abstracts with Programs, 18, (1), p. 57. CURRIE, K. L. 1984. A reconsideration of some NANCE, R. D . , and WARNER, J. B. 1986. Variscan geological relations near Saint John, New tectonostratigraphy of the Mispec Group, south­ Brunswick. In Current Research, Part A, Geolog­ ern New Brunswick: structural geometry and ical Survey of Canada, Paper 84-1A, pp. 193- deformational history. In Current Research, 201. Part A, Geological Survey of Canada, Paper 86- ______1986. The boundaries of the Avalon tecton- 1A, pp. 351-358. ostratlgraphic zone. Musquash Harbour-Loch Alva PARKER, J. S. D. 1984. Geological relationships region, southern New Brunswick. In Current of basement to cover. Musquash Harbour to Black Research, Part A, Geological Survey of Canada, River, Saint John area, southern New Brunswick. Paper 86-1A. pp. 333-341. Unpublished M.Sc. thesis, University of New CURRIE, K. L . , and NANCE, R. D. 1983. A reconsid­ Brunswick, 326 p. eration of the Carboniferous rocks of Saint PLINT, A. G.. and VAN DE POLL, H. W. 1982. Allu­ 326 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY

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