Sedimentation in a subaqueous arc back-arc …directory.umm.ac.id/Data Elmu/jurnal/P/Precambrian...Precambrian Research 101 (2000) 111–134 Sedimentation in a subaqueous arc:back-arc

Precambrian Research 101 (2000) 111–134

Sedimentation in a subaqueous arc/back-arc setting: theBobby Cove Formation, Snooks Arms Group,

Newfoundland

Pierre A. Cousineau a,*, Jean H. Bedard b,1

a Sciences de la Terre, Uni6ersite du Quebec a Chicoutimi, Chicoutimi Que., Canada G7H 2B1b Commission Geologique du Canada, Centre Geoscientifique de Quebec, C.P. 7500 Sainte-Foy Que., Canada G1V 4C7

Abstract

The Bobby Cove Formation of the Snooks Arm Group, north-central Newfoundland, displays cycles of lava- andsediment-dominated successions. The formation, located between pillow basalts and black graptolite-bearing mud-stones, was deposited in a subaqueous setting. Volcaniclastic rocks were produced by calc-alkaline volcanism thateither occurred during quiescence of, or was synchronous with, periods of tholeiitic volcanism. Variation in rockchemistry supports a change in tectonic setting from an expanding fore-arc oceanic crust to a progressively moremature magmatic arc/back-arc system. Combined chemical, sedimentological and detailed petrographic studies defineseveral types of volcaniclastic sedimentary rocks of pyroclastic, epiclastic, and mixed origin. The volcaniclastic rockswere divided into three groups based of clast components and textures: (1) a monogenic facies with minor mudstoneinterbeds, composed of basaltic to basaltic andesite clasts, is interpreted to have originated from subaqueousvertically-directed, pyroclastic eruptions; (2) a polygenic facies with abundant mudstone interbeds composed ofrounded fragments and a lack of vitric vesicular mafic fragments, was probably derived from chemical weathering andmechanical erosion of volcanic rocks (i.e. epiclastic volcanolithic sandstone); and (3) a second polygenic facies withminor mudstone interbeds is characterized by diverse clast compositions and textures, and is interpreted asremobilized deposits from previously deposited, fresh and/or altered, pyroclastic debris. © 2000 Elsevier Science B.V.All rights reserved.

Keywords: Basaltic subaqueous eruption; Subaqueous mass flows; Tuff turbidite; Lapilli tuff; Tuffaceous conglomerate; Breccia;Back-arc basin; Newfoundland

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1. Introduction

Intense arc-related volcanism, high sea-level,and abundant volcanogenic mineralization char-

acterize both the Ordovician and the Archean(e.g. Tiley, 1993). These conditions result in envi-ronments where subaqueous deposits of pyroclas-tic debris are likely to form. Ordoviciansubaqueous pyroclastic deposits have been de-scribed previously (Francis and Howells, 1973;Kokelaar et al., 1984; Fritz and Howells, 1991)

* Corresponding author.1 Geological survey of Canada contribution 1999050.

0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 0301 -9268 (99 )00097 -2

P.A. Cousineau, J.H. Bedard / Precambrian Research 101 (2000) 111–134112

and are easier to interpret than Archean deposits,because actualistic plate tectonic models can beapplied with fewer assumptions, and because fos-sils provide better constraints on the paleogeo-graphic context (i.e. water depth, salinity, etc.).Nevertheless, there are considerable similaritiesbetween Ordovician and Archean volcaniclasticdeposits. Plants and animals had not yet colo-nized the land in the Ordovician, so that erosionrates were more similar to the Archean than tothe modern period. Recent work implies that sub-duction of pelagic sediments plays an importantrole in the geochemistry of arc-related volcanicrocks (e.g. Davidson, 1987; White and Dupre,1989; Brenan et al., 1995; Elliott et al., 1997;Turner et al., 1997; Plank and Langmuir, 1998;Bedard, 1999). During the Ordovician, the deep-sea floor had begun to be covered by pelagicbiogenic siliceous ooze (radiolarians), whereas or-ganisms producing present-day carbonate oozehad not yet evolved. Thus, the similarity of Or-dovician and Archean sedimentation patternscould lead to a similarity in the geochemistry ofarc-related rocks.

This study examines a volcaniclastic-rich for-mation of Ordovician age from north-centralNewfoundland. Distinguishing between primarydeposits of pyroclastic origin, and secondary de-posits of epiclastic origin is difficult, but crucial,for the paleogeographic reconstruction of ancientdeposits. The distinction between primary andsecondary deposits may be subtle because bothtend to be emplaced by mass-flows with highconcentrations of debris and limited turbulence(Fisher and Schmincke, 1984; Cas and Wright,1987; Stix, 1991; McPhie et al., 1993). Freshlydeposited pyroclastic debris is readily remobilizedby rain, wind and ice in subaerial environments,and by density currents of various origin inmarine environments.

The distinction between primary and secondarydeposits in subaqueous settings is generally basedon petrography and sedimentary structures(Walker and Croasdale, 1972; Dimroth and Ya-magishi, 1987; Stix, 1991). Both provide informa-tion on the nature of constituents, on the mode offragmentation, on mechanisms of transport, andabout sedimentation processes. It is possible to

identify the composition and origin of individualpyroclasts, and clast populations are preserved inboth primary and rapidly resedimented deposits.However, the brittle nature and metastable miner-alogy of pyroclasts do not favor their preservationin deposits resulting from epiclastic weatheringand erosion of previously sedimented pyroclasticdeposits. Our understanding of subaqueous erup-tions and their deposits is currently limited, as aresult of imperfections in theoretical modeling,and a paucity of well documented examples. Acombined geochemical and stratigraphic approachusing facies analysis is employed to distinguisheruption-related facies and secondary resedi-mented deposits.

The Bobby Cove Formation is part of a se-quence of alternating flow-dominated and sedi-ment-dominated successions. Basaltic volcanism ischaracterized by the accumulation of thick (400–500 m) sequences of pillow lavas and sheet flows(Bedard et al., 1999b). These basalt-dominatedevents alternate with periods of relative quies-cence and erosion marked by tilting of faultblocks, erosion, and/or deposition of thick epi-clastic and debris flow deposits (Bedard et al.,1999c; Kessler and Bedard, this volume). Thereare also periodic influxes of pyroclastic and epi-clastic debris and lavas from a nearby calc-alka-line volcano (Bedard et al., 1999b). Calc-alkalinevolcanism may have occurred during basaltic qui-escence, leading to alternating sequences ofbasaltic flows and calc-alkaline deposits; or tomixed sequences of calc-alkaline tuffs and vol-canogenic epiclastic deposits derived from erosionof the tholeiitic flows (Bedard et al., 1999a). Alter-natively, calc-alkaline activity may have occurredsynchronously with basaltic volcanism, leading tointerfingering between tholeiitic basalts and calc-alkaline tuffs. The more explosive style of calc-al-kaline sequences is probably related to differencesin lava chemistry and volatile content (Gill, 1981;Arculus, 1994).

2. Geological and tectonic setting

The Bobby Cove Formation is part of theSnooks Arm Group in the Notre Dame Bay area

P.A. Cousineau, J.H. Bedard / Precambrian Research 101 (2000) 111–134 113

of Newfoundland (Fig. 1; Hibbard, 1983; Bedardet al., 1999b). Rocks of this group overlie theobducted oceanic crust of the Middle OrdovicianBetts Cove Ophiolite sub-conformably, whichprobably formed in a marginal basin adjacent tothe Laurentian margin (Tremblay et al., 1997). Theoceanic crust at Betts Cove is dominated by rocksof boninitic affinity (Coish et al., 1982; Bedard etal., 1998, 1999b; Bedard, 1999), and so formed ina supra-subduction zone environment, either inresponse to arc splitting, or to extreme fore-arcextension leading to seafloor-spreading (Stern andBloomer, 1992; Tremblay et al., 1997; Bedard et al.,1998). Recent mapping has revealed the presencethroughout the Snooks Arm Group of numeroushigh- angle cross-cutting normal faults (Bedard etal., 1999a,b). Rapid changes in thickness and facieson either sides of these faults, an up-section de-

crease in the magnitude of lithological offsets alongindividual faults, and the local presence of basaltwithin fault breccias and talus breccias, togethersuggest synvolcanic and synsedimentary faults.Extension was synchronous with eruption andsedimentation. Basaltic eruptions in the SnooksArm Group were subaqueous, as indicated by thedominance of pillow lavas and pillow breccias.Interbedded black mudstones locally contain grap-tolites (Snelgrove, 1931; Williams, 1992) and indi-cate deep-water. The Snooks Arm Group consistsof three repeated cycles of basaltic lavas andvolcanogenic sediments whose geochemistry docu-ment basin changes of tectonic regime (Fig. 2). Therepeated alternation of these distinct volcanic suitescollectively imply a long-lived, extensional, subsid-ing back-arc setting with a simultaneous develop-ment of nearby, mature arc volcanoes.

Fig. 1. Geological map of part of the Betts Cove ophiolite and its cover rocks, showing the overall stratigraphy of the Snooks ArmGroup. Adapted from Bedard et al. (1999b).


The lower Snooks Arm Group (Mount Misery,Scrape Point and Bobby Cove Formations) formsthe lowermost lava-sedimentary cycle. The MountMisery Formation is dominated by basaltic pil-lowed and sheet flows of arc tholeiite composi-tion, geochemically transitional between theboninitic lavas of the ophiolite and the evolved,undepleted, ovelying tholeiites. This reflects atransition from the arc-dominated system of theophiolite to a more mature back-arc system (Be-dard et al., 1999b). The Scrape Point Formationconsists of interfingering volcanic and sedimen-tary rocks. The flows have only a weak arc signa-ture and are interpreted to represent matureback-arc basin basalts. The sedimentary rocks arevery similar in mineralogy, texture and geochemi-cal signature to those of the overlying BobbyCove Formation and are interpreted as precursorsor distal equivalents of the conformably overlyingBobby Cove Formation. They show a dominantcalc-alkaline affinity and, thus, the interfingeringof tholeiitic and calc-alkaline rocks suggests stabi-lization and maturation of an arc system syn-chronous with back-arc spreading.

The Bobby Cove Formation is the first map-pable unit of formational rank dominated bysedimentary rocks, and was emplaced during ahiatus in tholeiitic volcanism. The formation isdominated by a lower member that is about 400m thick and contains a diverse suite of mafic tofelsic calc-alkaline volcaniclastic rocks whosechemistry implies that they represent a mature arcvolcano. Analyses of flows, dikes, and of individ-ual clasts suggest that there are two magmaticlineages: a High-Ti and a Low-Ti series (Fig. 2;Table 1). The High-Ti series is essentially indistin-guishable from underlying (Mt. Misery) tholeiites,suggesting a genetic link with these magmas. TheLow-Ti series exhibits a characteristic calc-alka-line trend of depletion of FeO and TiO2, andSiO2-enrichment; a trend that cannot result fromanhydrous crystallization of these magmas. Felsicvolcaniclastic rocks become proportionally moresignificant upwards. The felsic rocks show consid-erable scatter, but most fall along the extension ofthe Low-Ti series trend.

The lower member of the Bobby Cove Forma-tion records the principal calc-alkaline eruptive

Fig. 2. Variation diagrams illustrating the range of magmaticand sedimentary compositions in the Snooks Arm Group.Field designated ‘SP, VB, RH’ encloses lavas from the ScrapePoint, Venam’s Bight and Round Harbour Formations, aswell as subvolcanic feeder sills. Calc-alkaline/Tholeiitic dis-criminant from (Miyashiro, 1973). Curves labeled ‘B’ and ‘SP’show pattern of low-pressure anhydrous fractional crystalliza-tion (40%) of Bobby Cove and Scrape Point lavas (respec-tively) calculated with the program of Weaver and Langmuir(1980). Data from Bedard et al. (1999a) and references listedtherein.

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Table 1Average analyses (normalized to 100%) from different lithofaciesa

Unit facies BCLM BCUM

Clast Clast Clast Mafic tuff Mafic tuff Mafic tuff Dacite class Dacites Felsic tuffHigh-Ti lavas Low-Ti lavas Dyke

SiO2 53.750.3 47.3 47.8 51.5 62.9 61.5 73.452.1 49.6 48.3 55.60.80 0.52 0.88 0.95 0.31 0.650.85 0.301.28TiO2 0.881.140.86

15.717.2 14.4 315.0 14.0 16.7 16.4 17.1 12.615.9 15.7 14.0Al2O3

Fe2O3* 9.3812.5 8.98 10.2 9.64 4.60 6.03 3.4110.0 125 10.2 6.550.17 0.14 0.22 0.19 0.07 0.100.11 0.090.19 0.15MnO 0.21 0.17

4.905.51 7.62 10.0 8.27 6.30 1.26 3.07 1.157.40 5.02 9.96MgO5.92 6.38 6.11 6.58 2.04 5.39CaO 4.188.64 9.00 11.3 11.0 9.495.25 0.27 3.83 4.96 6.22 3.992.12 1.493.95Na2O 1.881.903.55

2.080.26 0.36 1.81 3.12 0.72 3.38 1.78 1.430.91 0.08 0.92K2O0.10P2O5 0.060.20 Bdl 0.07 0.10 0.16 Bdl0.08 0.27 0.10 0.112.68 10.3 3.34 2.87 1.01 2.433.21 2.113.87 2.76LOI 3.44 3.50

100.7100.0 100.4 100.8 97.78 100.4 98.17 100.0 100.1100.0 100.4 100.9Total

78 361 266 418 389Cr 11099 33166 107 398 8446 162 47 32 20 5526 Bdl21Ni 691569

261327 260 269 260 266 52 89 33288 440 373V100 74 87 52 12Cu 1475 1776 88 64 53

57 74 99 69 38 7261 5890 77Zn 116 662235 34 30 41 45 4 10 1340 38 55Sc

30 50 24 27 Bdl 16Co Bdl40 30 66 62 304.8 49 70 10.4 37 4131 315.7Rb 13615

12737 89 96 343 134 335 219 38893 B50 B50BaSr 111169 147 121 203 82 285 440176 373 121 149

1.3 B3 1.4 2.8 3.6 10.4B3 4.3B3 B3Nb 2.1 3.0B0.30.18 0.08 B0.3 0.11 0.16 0.26 0.50 –0.10 B0.3 B0.3Ta

53 46 38 36 108 149Zr 14966 56 55 58 5917 12 16 20 13 2523 2827Y 242119

1.471.65 1.72 1.02 1.31 1.22 2.91 3.30 4.101.31 1.12 1.43Hf5.66 3.26 4.86 5.71 11.1 18.4La 13.14.20 4.27 5.58 3.97 4.24

13.8 7.45 13.5 14.2 26.0 41.410.3 31.612.4 9.45Ce 11.5 10.4–– 1.84 – 1.94 1.86 3.11 – 3.99– – –Pr

8.33 B5 8.45 8.29 12.5 18.4Nd 16.58.17 7.41 7.13 7.22 7.332.40 1.16 2.37 2.42 2.83 3.952.37 4.032.69Sm 2.192.412.46

0.801.01 0.73 0.49 0.86 0.80 0.81 1.13 0.890.89 1.02 0.80Eu– – 2.99 2.80 2.09 2.83 3.74Gd 3.68 2.59 2.63 – –0.48 0.220 0.52 0.52 0.39 0.430.59 0.690.61Tb 0.540.450.50

–4.52 – – 3.35 3.31 2.19 2.04 4.323.20 3.02 –Dy0.63 B0.5 0.65 0.67 0.45 0.32Ho 0.910.96 0.64 B0.5 0.59 0.65– – 1.98 2.08 1.48 0.98– 3.062.93Er –1.982.02

0.3080.41 0.302 B0.2 0.293 0.282 0.22 0.13 0.460.29 B0.2 0.268Tm1.83 1.23 1.76 1.74 1.49 0.87 3.17Yb 2.39 1.97 1.86 2.52 2.200.29 0.166 0.270 0.247 0.25 0.140.33 0.500.29 0.32Lu 0.37 0.28

–14.0 – – 6.9 3.74 4.56 8.2 156.0 4.62 –Pb1.37 1.10 1.08 1.22 3.01 5.15 4.37Tb 0.66 0.91 0.88 0.94 0.880.56 0.69 0.45 0.51 1.36 2.360.55 1.790.59U 0.31 0.46 B05

Cs 0.690.079 1.26 3.47 0.49 1.86 1.53 0.610.60 B0.5 B0.5 0.70

a ‘BCLM’ and ‘BCUM’, Bobby Cove lower and upper members, respectively.


episode in the Snooks Arm Group. Sharply con-trasting patterns of geochemical evolution (Fig.2), and distinct geochemical signatures imply thatthe Bobby Cove magmas were not cogenetic withthe undepleted back-arc basin tholeiites (ScrapePoint or Venam’s Bight formations), though theymay be cogenetic with the arc tholeiites (Mt.Misery Formation). Consequently, the SnooksArm Group appears to record two distinct mag-matic suites. There probably were two distinctvolcanic plumbing systems fed by two differenttypes of basalt.

The upper member of the Bobby Cove Forma-tion is about 200 m thick and consists principallyof volcanogenic turbidites, with rare rhyolitic tuffs(Bedard et al., 1999b). The contact between theupper and lower members is presumed to beconformable, although the contact is commonlyoccupied by thick (up to 100 m) tholeiitic ferro-gabbro sills comagmatic with the tholeiitic rocksof the overlying cycle (Bedard et al., 1999a,b).Presumably the magmas exploited the mechanicalweakness represented by this contact. The turbid-ites themselves have bulk compositions reflectingderivation from calc-alkaline rocks similar tothose of the lower Bobby Cove (Table 1; Fig. 2),suggesting erosion and redeposition of unconsoli-dated, possibly originally subaerial tuffs. Notwith-standing, the absence of interbedded tholeiiticbasalt implies that deposition of the Bobby CoveFormation took place during a period of quies-cence in the back-arc tholeiitic basaltic system.

The entire upper Snooks Arm Group consistsof alternating or simultaneous deposition of: (1)back-arc basin tholeiites; (2) arc tholeiites andcalc-alkaline basalts to rhyolites; and (3) the ero-sion products of these two volcanic sequencesduring periods of volcanic quiescence. The dis-tinction between the first and second volcano-sed-imentary cycles lies mainly in the nature ofvolcaniclastic rocks found at their summits. Thevolcaniclastic rocks capping the first cycle (theBobby Cove Formation) are a mixture of epiclas-tic and pyroclastic rocks, and in part represent thedirect products of calc-alkaline volcanism. Thosecapping the second cycle (Balsam Bud Cove For-mation) are principally epiclastic rocks emplacedby mass-flow currents following slope failure.

These deposits probably reflect a gradual waningof basaltic volcanic activity and incipient erosionof the basaltic volcanic edifice, though interbed-ded felsic tuffs in the basal part probably repre-sent a brief recrudescence of activity from thecalc-alkaline volcano responsible for the BobbyCove deposits. Clast compositions in the large-scale volcaniclastic mass-flow deposits (debrites;Kessler and Bedard, this volume) also suggest thaterosion was not limited to the freshly eruptedtholeiitic lavas and felsic pyroclastic debris, butalso included deeper parts of the volcanic stratig-raphy. The third cycle is incomplete, and onlyconsists of tholeiites (Round Harbour Formation)essentially identical to some underlying tholeiites(Venam’s Bight Formation), though cross-cuttingfelsic dikes of calc-alkaline affinity similar to theBobby Cove felsic magmas suggest a subsequentpulse of calc-alkaline magmatism.

3. Classification and petrography of pyroclasts

Pyroclast-rich rocks can be classified accordingto schemes proposed by Fisher and Schmincke(1984), whereby classification is based on grain-size: tuff (B2 mm), lapilli tuff (2–64 mm), pyro-clastic breccia (\64 mm). In contrast, rocks withpolygenic clast compositions, lack of vitric andvesicular clasts, and which have rounded clastshapes, are most easily considered and classifiedas epiclastic rocks: mudstone (B0.06 mm), sand-stone (0.06–2 mm), conglomerate (\2 mm). Theterms volcanogenic or volcanolithic may be addedto distinguish them from other non-volcanicrocks. Rocks with intermediate features (i.e. withsome pyroclasts, angular clast shapes, and a largevitric component) are classified according toSchmid’s (1981) classification which employs theepiclastic grain-size scheme, but with the addedterm tuffaceous: tuffaceous mudstone, sandstoneand conglomerate. Deposits with abundant juve-nile vitric and vesicular debris tend to be classifiedas pyroclastic, in comparison to those that con-tain abundant accessory and accidental lithicdebris.

Rocks of the Bobby Cove Formation were af-fected by post-depositional hydrothermal circula-


tion, regional greenschist metamorphism, andsyn- to post-obduction deformation. Chemicalanalyses were performed on relict fresh minerals,and by sampling large clasts (conglomerate, brec-cia, and lapilli in size). In the Betts Cove Complex(which includes the Snooks Arm Group), defor-mation was concentrated in the weak serpentinitesand shales, so that the original sedimentary struc-tures, clast morphologies and internal textures arenearly always preserved in the Bobby Cove rocks,especially in the coarser grained conglomeratesand sandstones (Figs. 3 and 4). These primarystructures and textures show little evidence ofpervasive shearing, or extensive compactional flat-tening. It is common for both delicate scoriafragments with angular shapes, and angular crys-tal fragments in tuffaceous sandstones, to be per-fectly preserved (Fig. 3A, B).

Table 2 summarizes the descriptions and inter-pretations of the various petrographic con-stituents observed in the tuffs of this study. Mostsignificant are three types of mafic fragments.Types 1 and 2 fragments are aphanitic, non-vesic-ular to moderately vesicular, and tend to have ablocky shape. The clasts are finely crystalline orglassy basalt fragments, interpreted as juvenilepyroclasts produced by steam explosions. Type 3fragments are scoriaceous and are also in-terpreted to be juvenile pyroclasts producedby magmatic explosions (Heiken and Wohletz,1985; Kokelaar, 1986). Many of these maficfragments are elongated (Fig. 3B), which canbe interpreted as a diagenetic feature, the result ofpost-alteration compaction as a result oflithostatic pressure. However, many deposits con-tain equant clasts and scoria fragments(Fig. 3A,B; Fig. 4D–F), an observation that ap-pears at odds with pervasive flattening as aresult of compaction. Furthermore, frag-ments with equant shapes may contain elongatedvesicles, some of which show elongation normalto the bedding planes. This appears to indicatethat flattening of vesicles is not as a resultof compaction, but is a primary feature of theclasts.

Crystals of feldspar, pyroxene and hornblendeare abundant in the Bobby Cove deposits. Thesecrystals typically have shapes, sizes, proportions

and compositions similar to those of phenocrystsfound in the dominant lithic and scoriaceous frag-ments in the same beds. The crystals were derivedfrom sources similar to those of the mafic clasts.Some of the crystals are partly to completelysurrounded by thin turbid rinds of altered vesicu-lar glass (Fig. 3A). The extremely angular shapesof the crystals, and the preservation of thin‘glassy’ coatings are consistent with a mechanismby which most of the crystals were separatedfrom their matrices during explosive eruptions. Inaddition, they suggest limited transport by cur-rents.

Constituents of the finer grained material asso-ciated with coarse-grained tuffs cannot be distin-guished optically. Their composition seemsuniform and we infer that the fine-grained tuffsprobably had compositions similar to thoseof the coarser-grained tuffs, which are mainlycomposed of fine glassy mafic debris with ad-mixed free crystals of clinopyroxene, plag-ioclase and hornblende. The finer-grained tuffslocally display spherulitic structures (Fig. 3C).Such spherulites often form during early to latediagenesis of glass in lavas and tuffs (Lofgren,1971).

4. Description and interpretation of facies

The Bobby Cove Formation forms a continu-ous band of volcaniclastic rocks, 16 km long, withan average thickness of about 500 m. The unitappears to thicken westwards (Fig. 1). Subdivisionof rocks into facies is based on (1) petrography;(2) sedimentary textures and structures; and (3)rock-type association. The lower member of theBobby Cove contains numerous facies and sub-fa-cies with complex stratigraphic relationships.Coarser-grained facies (i.e. conglomerate and/orbreccia) are better developed at the base of theformation. Coarse-grained facies with abundantfelsic deposits are located only near the dacite tufffacies, in the central part of the formation. Notransition to a felsic lava dome can be docu-mented in the Bobby Cove Formation, thoughsome of the rhyolites of the Balsam Bud Covemay represent such a feature. The upper member


Fig. 3. (A) Photomicrograph of coarse grained tuff turbidite TT1 bed (plane light, bar scale is 1 mm). Abundant fresh pyroxenes(X; medium light gray) and plagioclase (P; white to light gray) crystals, with type 1 light-colored non-vesicular basalt fragment (1;light gray), type 2 dark-colored basalt fragment (2; black), and type 3 vesicular to pumiceous basalt fragment (3; back with light graydots). Note also coating of type 3 fragments on large pyroxene crystal marked X. (B). Photomicrograph of basaltic lapilli of the tuff— lapilli tuff facies (crossed nicols, bar scale is 1 mm). A large type 3 fragments shows changes of vesicle shape from flattened alongits edge (lower left) to elliptical (white dots, upper left); also note the rounded shapes of vesicles within the fragment (upper leftcorner) between adjacent pyroxene crystals (X). Type 2 fragments (2; lower one with small pyroxene crystals and some vesicles) arepresent, as well as type 1 fragments with the blurred contacts. (C) Devitrification spherulites aligned parallel to lamination in theupper part of a tuff turbidite bed; larger spherulites are developed in the finer-grained upper part of a TT1 bed. Scale in centimeters.(D) Tuff turbidite bed (TT1) with massive base overlain by diffuse parallel laminations. (E) Upper part of a TT1 bed with aprotruding clast of basalt overlain by sets of TT3 beds. Lowermost TT3 bed begins by a fine grain sand division overlain by amudstone division whose upper part is brecciated, scoured, and filled by angular to rounded mud pebbles, some imbricated. (F)Close-up of mud-rich TT3 beds that show parallel to undulating laminations and low angle scouring. Width is 10 cm.

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Table 2Petrographic constituents of tuff turbidite and related facies

Vesicle Size and texture Nature OrigineLapilli and % Phenocrystcoarse ash

LithicFinaly crystalline or glassy basalt Steam explo-NoCPXType 1 20–40 B0.25 mm, indistinguishable; 0.25–0.50

sion; thermalmm, blockyB10 FracturingCPX, PL B15%; smallType 2 \0.50 mm, irregular

Scoraceous basaltB10 Magmatic ex-CPX, PL \70% IrregularType 3plosion

Crystal5–40 MagmaticClinopyrone Idiomorphic to broken; B15.0 mm

Idiomorphic to broken; B2.0 mmPlagioclase 5–20Broken; B5.0 mmAmphibole 0–10Indistinguishable Cannot be de-Devitrified and recrystallyzed basaltic10–15Fine ash

fine ash termined


of the Bobby Cove Formation only contains onefacies. Vertical or lateral changes in the ratio ofconstituent rock-types are too insignificant tomerit subdivision of the upper member into subfa-

cies. Small, representative stratigraphic columnsof the various facies described are presented witha map of the central part of the formation whereall facies crop-out (Figs. 5–8 and 10).

Fig. 4. (A) Lapilli tuff breccia (LTB) facies with beds of lapilli and blocks, and tuff interbeds. Note lateral changes of bed thickness,and amalgamation of beds. Hammer for scale. (B) Close-up of lapilli tuff breccia facies. Lapilli and blocks are mostly sub-angularto sub-rounded. Note faint clast imbrication (orientation of two pens). (C) Amoeboid blocks with a (glass?) rim indicative of plasticbehavior during emplacement in the lapilli tuff-breccia facies. (D) Tightly packed basalt blocks in a LTB. Note sub-rounded blockwith concentric laminations, which may represent an ejecta-type bomb (?). (E) Tuffaceous conglomerate facies with clast supportedangular fragments of different composition. (F) Angular dacite clasts in a tuffaceous conglomerate. Note wedge shape of beds thatinterfinger with tuffaceous conglomerate rich in basalt clasts. Hammer for scale.


Fig. 5. Geological sketch-map of the central part of the Bobby Cove Formation, showing the geographic extent of the principalmembers and facies.

4.1. Tuff turbidite (TT) facies of the lowermember

The TT facies constitutes about 50% of theformation. This facies can be further subdividedinto three sub-facies that are intimately interbeddedwith one another. The TT facies occurs at allstratigraphic levels and interfingers with all otherfacies present within the lower member of theBobby Cove Formation (Fig. 5). TT beds have amonogenic composition with abundant mafic crys-tals and clasts interpreted to be pyroclastic inorigin. This allows these rocks to be classified aseither vitric or crystal tuffs (Schmid, 1981).

The most widespread TT subfacies is TT1 (Fig.6), which consists predominantly of beds a fewcentimeters to several decimeters thick composed ofmedium to very coarse sand-sized debris. The TT2subfacies is composed of thicker-bedded (up to afew meters) and coarser-grained (composed ofcoarse sand- to granule-sized) debris. Both TT1 andTT2 beds (Fig. 3D) exhibit a massive base or crudenormal grading that passes up to crude and/or towell-defined parallel laminations at the tops. Dune-

or ripple-sized cross-laminations are rare and later-ally discontinuous in TT1 beds, whereas TT2 havebases with either clast- or matrix-supported pebble-size clasts. Typically, the massive division of TT2beds is overlain by diffuse millimeter-thick laminae.These laminae are subparallel to one another, butlaterally discontinuous on the scale of a few ten’sof centimeters. Debris within the laminae alternatesbetween medium sand to granule in size. Somelaminae also have trains of isolated or groupedsub-angular pebble-size clasts. Locally, large frag-ments protrude from the top of the beds (Fig. 3E).Pebble compositions are similar to those in the tuffbreccia with some intraclast pebbles from TT1 andTT3 beds. TT3 beds are thinner (0.5–10 cm) andare composed of very fine- to medium-sand sizeddebris and mud. TT3 beds commonly show anincomplete set with a lower, medium-grained, nor-mally graded division, overlain by finer-grained,parallel to low angle cross-laminated sands, fol-lowed by a laminated, muddier upper division. Theupper division is laterally discontinuous and com-monly fills, or is cut by low-amplitude archedsurfaces (Fig. 3F). Other common sedimentary


Fig. 6. Stratigraphic column for a portion of the lower member of the Bobby Cove Formation, showing beds of the tuff turbiditefacies.


structures in the tuff turbidite facies includeslumps as well as numerous syn-sedimentary nor-mal faults, indicating that deposition was syn-ex-tensional. Load casts and flame structures arelocally present at the contacts between overlyingcoarser-grained and underlying finer-grained beds.Well-bedded TT3 beds may pass within a fewmeters into a brecciated layer with angular torounded mud pebbles, and then to a train of mudpebbles and granules within a thick bedded tuffturbidite (Fig. 3E).

TT1 beds are similar to those associated withdecelerating flows containing a high concentrationof sand-size debris, such as massive sandstones inproximal turbidites (Walker, 1978). Similarly,TT3 beds are interpreted to have formed as classicto distal turbidites by more dilute turbulent flows.Thicker TT2 beds were also emplaced by flowswith a high concentration of debris. High particleconcentrations can damp turbulence within flowsand lead to en-masse freezing, thus producingmassive divisions (Lowe, 1988). Alternating dif-fuse laminations of different coarse grained-sizedebris (sand to granule), as well as trains ofisolated pebbles, are not solely produced by trac-tion in low density turbulent flows (i.e. Tb). Thelaminae are thin and do not show well developedstructures typical of traction carpets. However,large oversized clasts or trains of clasts, foundeither within the laminae or at the top of beds,imply mechanisms such as limited differential set-tling, dispersive pressure, and/or kinetic sieving(Lowe, 1980, 1988; Postma et al., 1988; Smith andLowe, 1991; Kneller and Branney, 1995; Iverson,1997). Whatever mechanism was dominant, thethickness of the diffuse lamination division re-quires rapid vertical aggradation with a high par

Fig. 7.

Fig. 7. Stratigraphic column for a portion of the lower mem-ber of the Bobby Cove Formation, showing contacts from thelapilli tuff-breccia facies to the tuffaceous conglomerate facies.Note channel feature (dotted line; center) and tuff turbiditeinterbeds in lapilli tuff — breccia. Upper contact of lapilli tuff— breccia with tuffaceous conglomerate is gradual. Bothfacies show similar structures, and distinction between them isbased on composition. Lower contact between tuffaceous con-glomerate and lapilli tuff — breccia is, however, sharp andboth beds have markedly different texture and composition.Pie diagrams of clast counts show a higher proportion ofaccessory fragments in the tuffaceous conglomerate facies.


ticle concentration. These flows probably hadhigher concentrations of particles and higher fall-out rates than those that produced the TT1 beds.

Sedimentation from high- to low-density turbu-lent flows probably best accounts for the sedimen-tary textures and structures of the Bobby Covepyroclast-rich TT facies. In marine turbidite mod-els, coarse-grained beds are episodic events thatalternate with background sedimentation, oftenpelagic mudstones. Pelagic non-volcanic mud-stones are absent in the lower member of the BobbyCove Formation. Furthermore, had pyroclastsrested for some time before resedimentation theywould likely have been altered to some degree, andhave been more prone to destruction during resed-imentation. One would not expect delicate glassyrims or fragile scoria to be preserved. Most of thefeatures found in the Bobby Cove tuff turbiditesimply rapid, repeated, and sustained high sedimen-tation rates from pyroclast-rich flows, and suggestsyn-eruptive deposition.

4.2. Breccia and conglomerate facies

Coarse-grained facies in the Bobby Cove Forma-tion include a lapilli tuff-breccia (LTB), a tuffa-ceous conglomerate (TC), and a subordinatebasaltic tuff-lapilli tuff (TL). Individual breccia andconglomerate beds are several meters to decametersthick, and can form compound sequences of tuffbreccias and tuffaceous conglomerates up to 120 mthick. Individual beds can be traced a few hundredmeters laterally. Clast compositions weredetermined both petrographically and geochemi-cally.

The lapilli tuff-breccia (LTB) contains cobble- toblock-sized clasts (Fig. 4A) that differ fromone another by the proportion of vesiclesand phenocrysts (Fig. 7). Decimeter-thick,crude- to well-defined bedding is accentuated bycentimeter-thick tuff interbeds or by reverse, re-verse-to-normal and normal grading of fragments.The beds are matrix- and clast-supported andshow variation in structures or textures upsection. Lapilli tuff-breccia beds (LTB) may passvertically or laterally into the TC or TT facies. Atone locality, LTB and TC beds abut a thin basalticflow that is compositionally almost indis-

Fig. 8. Stratigraphic column of part of the mudstone —tuffaceous sandstone facies. Transitional character stands outby comparing it with Figs. 3 and 9.


tinguishable from the clasts in the LTB (Table 1).Locally, the lapilli tuff-breccia is cut by small,weakly chilled dikes with irregular contacts, sug-gestive of emplacement in soft sediment. Thesedikes are compositionally similar to those of theclasts and interbedded flows (Table 1).

The LTB facies is characterized by a monogeniccomposition with fragments that are possibly ju-venile pyroclasts. Fragments in the LTB have asub-equant, angular shape, commonly with avesicular, plagioclase+clinopyroxene+horn-blende-porphyritic core, and a partial rim of finer-grained, weakly-phyric, weakly-vesicular material.The finer-grained rims are inferred to have re-sulted from chilling of magma in water. Smallfragments of detached rim material constitute asub-population of clasts, giving a false impressionof a bimodal clast population. Because the irregu-larly outlined fragments do not mold each other(Fig. 4B), their shape is almost certainly not aresult of late diagenetic or tectonic processes, butprobably records the evolving shape of hot pyro-clasts as they cool and fragment during transportand deposition. Some LTB beds contain highproportions of elliptical or amoeba-shaped clastscharacterized by complete fine-grained rims (Fig.4C). Such shapes are similar to fragments inhyaloclastite breccias and seamounts (Cousineauand Dimroth, 1982; Dolozi and Ayres, 1991). TheLTB facies is not associated with pillowed ormassive flows, nor emplaced hyaloclastite brec-cias, but rather pyroclast-rich mass flows. Clastshapes are interpreted as products of subaqueousfire-fountains (Kokelaar, 1986), with fragmenta-tion produced during subsequent mass-flowemplacement.

Sedimentary structures in the LTB reflect rapidsedimentation from a succession of flows, or asuccession of pulses within a flow. The monogenicclast population and sedimentary structures implyrapid, repeated, and sustained high sedimentationrates of pyroclast-rich flows, which would be bestachieved if sedimentation was concurrent witheruption. Gravity and fluidization flow transfor-mations could then explain in part the develop-ment of matrix-poor deposits as well as thepresence of sandy interbeds by elutriation of finergrained debris from within the flow (Fisher, 1983;Huppert et al., 1986).

The basaltic tuff-lapilli tuff facies (TL) is lessabundant than the LTB facies. TL beds are lessthan a few meters thick, and are composed oflapilli- and tuff-sized deposits with poorly definedupward thinning and fining sequences. Individualbeds are mostly massive and poorly sorted,though sorting improves up section with develop-ment of normal grading and parallel laminations.Compared with the LTB facies, TL beds contain agreater amount of matrix and scoriaceous frag-ments. These features support an origin from amagmatic eruption, with sedimentation from sub-aqueously-erupted pyroclastic mass flows (Fiskeand Matsuda, 1964; Stix, 1991; Cousineau, 1994).

The tuffaceous conglomerate (TC) facies (Fig.4D, Fig. 7) differs from the LTB and TL facies inthe following ways: (1) clasts are polygenic (pres-ence of tuff turbidite, intermediate volcanic, dac-ite, mafic cumulates, mudstone fragments); (2) aclast-supported texture is common; (3) clasts aretypically more angular or show a greater diversityin degree of roundness (attributed to mechanicalabrasion); and (4) beds are generally thicker.There is a reduction in the diversity of mafic clasttypes in the sand-sized matrix, and a greaterdifficulty in identifying their morphology. Localderivation of clasts is probable because beds withabundant dacite fragments are located below andnext to a dacite tuff (Fig. 5), and some large clastshave very high aspect ratios. Beds of the TC wereemplaced by high density mass-flows, however,the compositions and textures indicate that theTC facies represents resedimentation of debris,which is only partly composed of juvenile pyro-clasts. Schmid’s classification scheme can be ap-plied in this case. TC beds could result from slopefailure of a volcanic apron and/or of a dacitedome. High variability in clast composition andlack of background pelagic sedimentation implythat: (1) sedimentation occurred repeatedly and atbrief intervals, possibly triggered by syn-volcanictremors; and (2) that more than one stratigraphiclevel, or more than one source was sampled.

4.3. Mudstone-tuffaceous sandstone (MTS) facies

The mudstone-tuffaceous sandstone (MTS) fa-cies represents less than 5% of the lower member


of the Bobby Cove Formation and is transitionalto the upper member. This facies consists of cen-timeter- to decimeter-thick beds of sand-sized andmud-sized clasts (Fig. 8). Local beds of conglom-erate, siliceous mudstone, and felsic crystal tuffsare also present. Less polygenic beds of sand-sizeddeposits are similar to those of the tuff turbiditefacies. More polygenic beds are like those of thevolcanolithic facies of the upper member. Thereare fewer scoriaceous fragments, but abundantaltered quartz- and feldspar-phyric rhyolitic todacitic fragments. Consequently, the MTS de-posits are classified as tuffaceous sandstones andconglomerates. Sedimentary structures are similar

to those found in sandy proximal turbidites (Fig.4F); with structures such as normal grading over-lain by parallel laminations and then mudstone(Table 2). The MTS facies could have formed byepiclastic processes in environments where turbid-ity currents are prominent, such as deep-sea fans.The violet color of the mudstones (partial oxida-tion of iron) and presence of siliceous mudstones,both suggest that these rocks formed near a vol-cano, where hydrothermal activity might havebeen operative.

4.4. Other facies of the lower member

A single thick (�150 m) lenticular body ofdacite tuff caps the basal member near the EastPond outlet. The rock contains 15% fragments ofbasalt, diabase, and feldspathic hornblendite (1–2cm in size). These are set in an entirely recrystal-lized matrix of quartzo-feldspathic compositionwith 40–80%, 1–3 mm-size, euhedral crystals ofhighly sericitized plagioclase, hornblende, mag-netite and quartz. The virtual absence of visiblesedimentary structures and intense recrystalliza-tion limits interpretation of depositional pro-cesses. The presence of this facies at the top of anotherwise mafic tuffaceous succession and its as-sociation with TC beds rich in clasts of similarcomposition suggests a shift towards more felsicvolcanism.

Basaltic to andesitic, massive to pillowed lavaflows compose only a few percent of the BobbyCove Formation. The lavas are petrographicallyand geochemically (Fig. 2, Table 2; Bedard, 1999)almost indistinguishable from the dominant por-phyritic basalt/andesite clasts of the TC and LTBfacies.

4.5. Volcanolithic turbidites (VL) in the uppermember of the Bobby Co6e Formation

Rocks in the upper member of the Bobby CoveFormation (Fig. 9A) exhibit sedimentary struc-tures typical of distal turbidites (i.e. Tabce, Tbce,Tce), with occasional thick to very thick, moremassive beds (3–4 m) of coarser-grained material.There are no consistent upward fining or coarsen-ing cycles (Fig. 10). The composition of the

Fig. 9. (A) Volcanolithic turbidite facies with successive bedsshowing typical Bouma divisions. (B) Photomicrograph ofvolcanolithic sandstone (plane light, bar scale is 1 mm) withvolcanic rock fragments of different compositions: felsic vol-canic (F), mafic volcanic (M), quartz-feldspar granitoid (G)and sedimentary (dark gray). Volcanic quartz grains (Q) arepresent while pyroxene crystals are absent. Note roundedaspect of fragments and grains.


Fig. 10. Stratigraphic column of part of the volcanolithicturbidite facies. Note increased mudstone: sandstone ratio andsedimentary structures typical of classic turbidite successions.

coarser-grained sedimentary rocks throughout thevolcanolithic (VL) facies is uniform. Mafic vol-canic rock fragments, feldspar grains, felsic vol-canic rocks, and volcanic quartz grains are allabundant (Fig. 9B). Mudstone beds are moreabundant in the VL than in the MTS facies. TheVL mudstones are typically dark gray, rather thanviolet, greenish or yellowish as in the lower mem-ber. Similarly, sandstones in the VL facies aremedium gray, rather than greenish as in the TTand MTS facies. Grains in VL sandstones are alsobetter rounded than in the lower member. Theabundance of rounded grains of different compo-sition supports an epiclastic origin. Successions ofsedimentary structures (Tabcde, Tcde, Tae) andmudstone/sandstone ratios (about 1:1) are typicalof classic turbidite deposits, and imply similarsedimentation processes and depositionalenvironments.

5. Evolution of the sedimentary basin

Problems relative to the interpretation of theBobby Cove Formation are similar to those ofmost ancient volcaniclastic rocks. Most importantare: (1) the source volcano is not exposed; and (2)rocks are metamorphosed, folded and faulted.Emplacement models for these rocks may be con-strained by several factors. The overall tectonicand paleogeographic setting is provided by thegeochemistry of these rocks and of those from thebounding formations, and by the fossils containedin bounding formations. These data confirm andsupplement information gathered by petrographyand facies analysis.

Petrographic analysis of the various facies inthe Bobby Cove Formation allows distinction ofthree groups on the basis of clast diversity andtexture. Monogenic facies (TT, LTB, LT) arecomposed nearly exclusively of basaltic to basalticandesite clasts. Clast morphologies in TT, LT,and possibly LTB facies are reminiscent of thosereported from several other mafic volcanogenicdeposits (Kokelaar, 1986; Ross, 1986; Cas et al.,1989; Dolozi and Ayres, 1991; Doucet et al.,1994) which were also interpreted as havingformed by subaqueous pyroclastic eruptions of


phreatomagmatic origin. The polygenic VL facies,with its rounded fragments and lack of vitricvesicular mafic fragments, forms a second groupwhich was probably derived through chemicalweathering and mechanical erosion, as are manyother epiclastic volcanolithic sandstones (e.g.Archer, 1984). The polygenic TC and MTS faciesconstitute the third group, which is characterizedby diverse clast compositions and textures thatvary from bed to bed. They are interpreted tohave been derived from previously deposited,fresh and/or altered pyroclastic deposits with vari-able addition of accessory and accidentalfragments.

Rocks of the Bobby Cove Formation were em-placed by mass-flow sedimentation. The absenceof structures suggestive of terrestrial, nearshore orshallow-water sedimentation favors a deep-waterbasin. No fossils have been reported in this for-mation, though graptolites in bounding forma-tions suggest a deep water environment.Sandstone beds of the VL and MTS facies regu-larly grade into, and alternate with mudstonebeds. This could reflect alternating periods ofmass-flow, triggered by episodic events, and moreregular pelagic sedimentation. Mudstone in-terbeds are absent in the coarser-grained LT,LTB, and TC facies. Yellowish mudstone bedsrepresent less than 5% of the TT facies and theirclose association with laminated medium- to veryfine-grained tuffs favors sedimentation from asimilar volcanic source rather than from pelagicsuspension. Absence of mudstone interbeds in setsof successive near-identical beds of high densitymass flow deposits can be interpreted as reflectingsustained sedimentation from rapidly succeedingflows, or from pulses within a single flow (Hup-pert et al., 1986; Iverson, 1997; Nemec et al.,1998). Mudstone-poor, pyroclast-rich facies couldhave been deposited from mass flows fed directlyby subaqueous pyroclastic eruptions. Mudstone-poor facies with a more polygenic clast composi-tion could result from repeated,earthquake-generated slumping of volcaniclasticdebris originally deposited closer to the vent.

Pillow lavas and pillow breccias occur in allformations of the Snooks Arm Group, indicatingthat eruptions were persistently subaqueous.

However, depth of eruption cannot be ascertainedsolely by studying the Bobby Cove Formation.Clasts in various facies of the formation are vesic-ular. The presence of abundant vesicles in flowsand pyroclasts has previously been used to infereruption at shallow depth (Jones, 1969; Williamsand McBride, 1979; Lackschewitz et al., 1994),but vesicular to highly vesicular basalts appear tobe a common component of deep-water sub-marine arc volcanoes (Burnham, 1983; Dudas,1983; Gill et al., 1990; Wright, 1996; Fiske et al.,1998). The massive influx of coarse pyroclasticdebris marking the base of the Bobby Cove For-mation records an intense, explosive mafic vol-canic event from a calc-alkaline volcano (Fig. 12).The complex lateral and vertical distribution offacies and sub-facies in the lower member ofBobby Cove Formation, together with the wide-spread evidence for synsedimentation extensionalfaulting and slumping, support models of rapiddeposition within a basin characterized by a fault-controlled topography.

Several models can be proposed to explain themafic pyroclast-rich facies of the Bobby CoveFormation. Some models could have operatedconcurrently or subsequently during a single erup-tion or an eruptive cycle. Vertically-directed,phreato-magmatic eruption fed by continuousmagma supply best explains the mafic pyroclast-rich facies of the Bobby Cove Formation. Usingthis model, steam cupolas and fire-fountainsformed above the vent during paroxysmal erup-tions (Kokelaar, 1986). Expansion of the buoyantplume (umbrella) is limited and the plume istransformed into a cloud with a high particleconcentration. Large ejected blocks and bombsthat cross the steam cupola could retain a plasticshape and develop a chilled margin (Kokelaar,1986; Mueller and White, 1992). Most ejecta,however, would be comminuted to finer-graineddebris (Kokelaar, 1986; Lackschewitz et al., 1994)then fall back and accumulate near the vent.Collapse of the plume generates a mass-flowwhich is fed nearly continuously by the ongoingeruption. Pulses in the eruption could lead torepeated collapse events generating closely-spacedsets of similar flows. The pyroclast-rich facies ofthe Bobby Cove Formation (TT, LTB, LT) could


Fig. 11. Depositional model for the facies of the lower Bobby Cove member. (A) The lapilli tuff-breccia and basalt tuff — lapillituff facies with their abundance of juvenile fragments probably resulted directly from a phreatomagmatic subaqueous eruption (rightside of diagram). Fragments generated during collapse of fire fountains and/or of magma bubbles were rapidly entrained in asubaqueous mass flow that moved downslope. A resedimentation model (left side of diagram) is inferred for the tuffaceousconglomerate, following post-eruption mass wasting from the flanks of the volcano. If resedimentation is nearly concurrent witheruption, this model could also explain deposition of the lapilli tuff — breccia facies. (B) The tuff turbidite facies could result froma primary subaqueous eruption (right side of diagram) by a process similar to A, but this would imply greater production of ash-sizeparticles. Also present are vertical gravity flows issued from the ash-laden eruptive plume, which move downslope as they reach theflanks of the volcano. Both mass flow processes could have operated at the same time. Again, this facies could result from masswasting (left side of diagram), that occurred soon after (if not contemporaneously with) the eruption.


have resulted from such eruptions and could rep-resent beds sedimented at some distance withinthe basin, down-slope from the vent (Fig. 11).More powerful, vertically-directed eruptions arepresumed to produced pumice and glass-rich de-posits, typically with a massive base overlain by adoubly-graded succession of tuff turbidite beds(Stix, 1991). Such deposits are not present in theBobby Cove Formation, though such a successionis weakly developed in the TL facies, which con-tains the most vesicle-rich pyroclasts.

Another possible model is one in which debrisis fed rapidly to the eruptive plume (umbrella) byclosely-spaced eruptions and explosions. As theplume breaches the water surface, debris accumu-lates and forms a cloud that spreads laterally.Gravity instabilities develop along the lowerboundary of this debris-laden cloud that subse-quently generate vertical gravity currents. Debrisis thus delivered to the ocean bottom much fasterthan by individual particle settling (Carey, 1997;Fiske et al., 1998). These currents probably con-tinued to flow down the slopes of the volcanicedifice as they reached the ocean bottom. This

mechanism could also generate the succession ofmass-flows that formed the TT facies (Fig. 11).

Debris that accumulates near the vent of avolcano following a pyroclastic eruption will lieon a steep surface, near the angle of repose. Suchareas are prone to earthquakes generated by vol-canic tremors and explosions that can rapidlyremobilize freshly deposited debris and resedimentit into deeper parts of a basin, without substan-tially modifying the general composition of theoriginal deposit. In subaqueous environments,freshly erupted pyroclastic debris may rest in shal-low water, where it is susceptible to reworking bytidal currents and waves (Kokelaar, 1986). Evenwater currents generated by the pyroclastic erup-tion itself may affect both recently formed de-posits and those concurrent with the eruption. Allof these scenarios could have produced mass-flows responsible for the pyroclast-rich facies ofthe Bobby Cove Formation.

Near vent deposits from previous eruptions canalso be affected by these processes. Debris fromvarious styles of eruptions could be incorporatedinto the remobilized mass, increasing the propor-

Fig. 12. Idealized cross-section through the Bobby Cove Formation showing infilling history of the basin. The lower member wasemplaced on a faulted basement topography covered by numerous small tuff and scoria cones. The uppermost part of the lowermember is marked by a change from basaltic explosive volcanism to felsic explosive volcanism with increasing periods of quiescenceand possible hydrothermal activity. Volcanolithic turbidites abruptly appear in the basin, reflecting quiescence and erosion of a largestratovolcano, perhaps coupled with a shift to a more distal or more diverse source area.


tion of accidental and accessory fragments. Thetime elapsed between successive eruptions mayallow seafloor weathering of debris, reducing theamount of juvenile vitric material. Volcanictremors are at a maximum during eruptions, thusresedimentation of near vent deposits from previ-ous eruptions is probably most frequent duringeruptions. This would lead to the formation ofinterbedded deposits of monogenic, pyroclast-richfacies with polygenic, pyroclast-poor facies, andwith facies of intermediate composition. This isthe most probable scenario to explain the TCfacies and its relationship to the bounding pyro-clast-rich facies.

The transition from the lower to the uppermember of the Bobby Cove Formation appears tocoincide with a shift towards more felsic volcan-ism. Frequency of volcanic activity was rapidlyreduced, allowing for enhanced weathering ofnear-vent deposits. Earthquake-triggered resedi-mentation of previously erupted deposits may oc-cur even if there is no eruption occurring. Thearea affected by seismic tremors is very wide, andcould induce landslides from surrounding, possi-bly older and/or inactive volcanoes. With waningof volcanic activity in the area, deeper, moreheterogeneous portions of the volcanic edificewould be exposed by erosion. Coarse-grainedclastic sedimentation in the basin would becomemore polygenic, and background pelagic sedi-ments would have the time to settle and formmudstone interbeds. Hydrothermal activity maypersist for some time. This scenario could explainthe transitional nature of the MTS facies. As thevolcano became dormant, sedimentation in thebasin became dominated by epiclastic deposits ofthe VL facies. This is further supported by thewidespread distribution of this facies in the uppermember of the Bobby Cove.

The period of quiescence ended by the massiveoutpouring of tholeiitic basalt (Venam’s BightFormation), signaling the beginning of a newcycle. However, if the rhyodacites and rhyolites ofthe second volcanic cycle (Balsam Bud Cove For-mation) are considered to represent evolvedequivalents of the Bobby Cove magmas, then thisimplies that the calc-alkaline volcano probablycontinued to fractionate as a closed system. This

further implies that little or no basalt was beingfed into the calc-alkaline volcano’s magma cham-ber, even though this was a period when there wasvoluminous tholeiitic volcanism (Venam’s Bight).This dichotomy of behaviour between the twosystems (calc-alkaline and tholeiitic) supports thenotion that the calc-alkaline and tholeiitic suitesof the Snooks Arm Group had distinct plumbingsystems and were not fed by the same parentalmagmas. A possible modern analogue for theBetts Cove-Snooks Arm Group is the southernMariana arc, where arc tholeiites and dacite of theactive arc erupt through an older boninitic base-ment which is being split by an active back-arcspreading center (the Mariana Trough) whicherupts back-arc basin basalts (Fryer et al., 1998).

6. Conclusions

The Snooks Arm Group consists of alternatingvolcanic flow- and sediment-rich cycles. TheBobby Cove Formation represents one of thesediment-rich phases, with a lower member thatformed largely by mafic pyroclastic activity. Theupper member reflects epiclastic sedimentation re-lated to weathering and erosion of a mature arc.The Snooks Arm Group conformably overlies thesupra-subduction zone Betts Cove ophiolite. Theshift towards a more explosive style of eruptionrelates to changes in lava chemistry from boninite(Betts Cove ophiolite), to arc tholeiite (Mt. Mis-ery Formation, basal Snooks Arm Group), toback-arc basalts (Scrape Point Formation), andfinally to calc-alkaline basalts and andesites(Bobby Cove Formation). The change in chem-istry also relates to a change in tectonic settingfrom an expanding fore-arc oceanic crust to aprogressively more mature magmatic arc/back-arcsystem. The abundance of pillowed flows andgraptolite-bearing mudstones in bounding forma-tions imply a persistent deep-water environment.

All facies in the Bobby Cove Formation resultfrom mass-flow sedimentation, bu can be subdi-vided into: (1) monogenic pyroclast-rich, andpolygenic pyroclast-poor facies; and (2) mud-stone-rich, and mudstone-poor facies. Lack ofexposure of more proximal and more distal facies


of the Bobby Cove Formation, especially the crit-ical near-vent facies, limits discussion of the exactmechanism that led to the formation of the vari-ous facies. Pyroclast-rich and mudstone-poormass-flow deposits (tuff-turbidite, lapilli tuff-brec-cia and lapilli tuff facies) are intimately interfin-gered and were probably deposited from a rapidsuccession of high density flows with limited tur-bulence, and less commonly from lower density,more turbulent flows. They were possibly em-placed from subaqueous Surtseyen-type eruptions.What cannot be ascertained is whether the flowsissued: (1) directly from pyroclastic flows or pyro-clastic falls, modified or not by flow transforma-tion; or (2) from remobilization of pyroclast-richdeposits formed from contemporaneous or near-contemporaneous eruptions. The tuffaceous con-glomerate facies contains no mudstone interbeds,and interfingers with the pyroclast-rich facies. It isgrouped with the pyroclast-poor facies because ofits more polygenic, often locally variable, clastcompositions and is interpreted to have resultedfrom rapid sedimentation contemporaneous withvolcanism, but the debris was probably derivedfrom previously deposited, near-vent volcaniclas-tic deposits. Resedimentation may have been trig-gered by earthquakes associated with eruptions.

The uppermost facies of the lower member stillcontain a few pyroclasts, as well as the character-istic greenish color of the coarser facies of thelower member, but also contain maroon to purplemudstones, as well as sandy deposits resemblingturbidites. The transition from lower to uppermembers of the Bobby Cove Formation is charac-terized by a progressive shift toward facies de-posited from flows resembling classic turbidites, inconjunction with greater proportions of mudstoneinterbeds and more polygenic clast populations.The upper member is composed of only one fa-cies, which is the most heterolithic in compositionand the most mudstone-rich of all the BobbyCove facies. The abundance of mudstone in-terbeds reflects extensive periods of quiescence,allowing for background pelagic sedimentation tooccur. The more polygenic composition of thispyroclast-poor facies relates possibly to a greaterdiversity of source terranes as the calc-alkalinematerial is eroded away. Thus, in the Bobby Cove

Formation, a combination of heterolithic compo-sition and mudstone interbeds favors an epiclasticmechanism of deposition, while monogenic, pyro-clast-rich compositions and the paucity of mud-stone interbeds favor a more primary (pyroclastic)origin of deposits.

Acknowledgements

The authors wish to acknowledge permissionfor access by Richmont Mines and the residentsof Snooks Arm. Logistical assistance from RayWimbleton and Kirsten Oravec was greatly appre-ciated. Financial support was from NSERC(P.A.C.) and from the Geological Survey ofCanada (J.B.). Giff Kessler (Marathon Oil) isthanked for great evenings of discussion duringfield work. We also wish to acknowledge theefforts of Tomasz Dec, who first suggested apyroclastic origin for the lower Bobby Cove. Thismanuscript was significantly improved by the dili-gent review work of Gary Smith, Cathy Busby,and of editorial work by E.H. Chown and WulfMueller. The fine is by Claude Dallaire.

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