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Journal of Volcanology and Geotherma

Eruption processes and facies architecture of the Orion Central kimberlitevolcanic complex, Fort à la Corne, Saskatchewan; kimberlite mass flow

deposits in a sedimentary basin

A. Pittari a,⁎, R.A.F. Cas a, N. Lefebvre b, J. Robey c, S. Kurszlaukis b, K. Webb b,1

a School of Geosciences, PO Box 28E, Monash University, Victoria, 3800, Australiab De Beers Canada Inc., Ste 400-65 Overlea Boulevard, Toronto, Ontaria, Canada M4H 1P1

c RSA Geological Services/De Beers Group Services (Pty) Ltd. Wesselton Village, PO Box 47, Kimberley, 8300, South Africa

Accepted 11 December 2007Available online 10 January 2008

Abstract

The Fort à la Corne diamondiferous kimberlite field consists of at least 70 bodies of volcaniclastic kimberlite, hosted within a contemporaneousnon-volcanic sedimentary succession. This study of the three-dimensional stratigraphy and facies architecture across the Orion Central kimberlitevolcanic complex highlights the variations in upper and extra-vent processes.

The sedimentary succession consists of continental to marginal marine quartz sandstones and mudstones, overlain by marginal to deep marinedark mudstone and muddy sandstones and siltstones. Relatively thin conformable volcaniclastic kimberlite packages are interbedded throughoutthe host rock stratigraphy. Extremely thick (up to at least 211 m thick) discordant to concordant, volcaniclastic packages/series, infill at least threeelongate northwest-trending craters (145A, 145B and 219 craters), and contain laterally equivalent conformable extra-crater deposits bound bymarine mudstones, indicative of a prevailing dominantly marine environment. The volcaniclastic deposits within the 145A and 145B craters,respectively, are separated by a considerable hiatus, whereas the deposit infilling the 219 crater was formed around the same time as 145B craterdeposit.

Multiple depositional units of massive to stratified, olivine-rich sand- to pebble-sized volcaniclastic facies infill craters and were emplaced bymegaturbidite pulses fed by crystal-rich eruption fountains, which interacted with the crater relief. Stacked, normally graded, thick to very thickbedded matrix-supported olivine-rich facies characterized by brief depositional breaks between some beds represent syn- to post-eruptive turbiditepulses associated with the early eruptive event in the 145A crater. Thin layers of light grey kimberlitic mudstone underlie, or occur near the base ofand above the main volcaniclastic packages associated with the 145B and 219 eruptions. Crater-infilling volcaniclastic deposits were laterreworked by storm induced currents into thin to medium graded, moderately sorted, fine to coarse olivine-rich sandstone with intercalateddiscontinuous muddy laminae. Laminated to cross-laminated olivine-rich silt- to sand-sized volcaniclastic facies constitutes a small volumevolcaniclastic turbidite, which marks the last preserved kimberlite emplacement event at Orion Central.© 2008 Elsevier B.V. All rights reserved.

Keywords: kimberlite eruption; volcaniclastic; Fort à la Corne; megaturbidite; sedimentary basin

⁎ Corresponding author. Present address: Department of Earth and OceanSciences, University of Waikato, Private Bag 3105, Hamilton, 3240, NewZealand. Tel.: +64 7 838 4466x8252; fax: +64 7 856 0115.

E-mail address: apittari@waikato.ac.nz (A. Pittari).1 Present address: Mineral Services Canada Inc, 205-930 Harbourside Drive,

North Vancouver, BC, Canada V7P 3S7.

0377-0273/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.jvolgeores.2007.12.019

1. Introduction

Fragmental kimberlite deposits in diatremes provide a uniqueinsight into in-vent volcanic processes, however, in most cases,the upper and external parts of kimberlite diatremes have beenremoved by erosion. Few examples of upper diatreme/crater andextra-crater kimberlite deposits are known (Canadian Prairies,e.g. Fort à la Corne, Scott Smith et al., 1994; Leahy, 1997; Leckieet al., 1997; Field and Scott Smith, 1999; Berryman et al., 2004;

153A. Pittari et al. / Journal of Volcanology and Geothermal Research 174 (2008) 152–170

Zonneveld et al., 2004; Igwisi Hills, Tanzania, Dawson, 1971;Reid et al., 1975; Mbuji-Maya, Zaire, Demaiffe et al., 1991;Mwadui, Tanzania, Edwards and Howkins, 1966; Stiefenhoferand Farrow, 2004; Orapa, Botswana, Field et al., 1997; Venetia,South Africa, Kurslaukis and Barnett, 2003; Catoca, NEAngola,Rotman et al., 2003; Tokapal, India, Mainkar et al., 2004; Yu-bileinaya, Siberia, Kurszlaukis et al., 2006), and of these fewerhave been assessed using a modern volcanological approach.

The Cretaceous Fort à la Corne kimberlite field, Saskatch-ewan, Canada (Fig. 1a), has provided a unique perspective onkimberlite volcanism (e.g. Scott Smith et al., 1994; Leahy, 1997;Leckie et al., 1997; Field and Scott Smith, 1999; Berryman et al.,2004; Zonneveld et al., 2004; Kjarsgaard et al., 2006; Harveyet al., 2006). At least 70 shallow dipping volcaniclastickimberlite bodies aligned along a northwest trend occur withinan ∼200 m thick Lower Cretaceous non-volcanic siliciclasticsuccession near the northeastern margin of the North AmericanInterior Platform. Locally, this succession consists of the oldercontinental, marginal marine and shallow marine MannvilleGroup (Cantuar and Pense formations) and the overlying

Fig. 1. (a) Location map of the Fort à la Corne kimberlite field within Saskatchewan, Cbodies at Fort à la Corne highlighting the Orion Central volcanic complex. (b) Regio1997; Zonneveld et al., 2004; Kjarsgaard et al., 2006).

marginal marine to offshore Lower Colorado Group (e.g. Leckieet al., 1997; Zonneveld et al., 2002, 2006a; Fig. 1b). Numerousadditional kimberlite volcaniclastic packages are interbeddedthroughout the Mannville and Lower Colorado groups. Radio-metric ages from a limited number of kimberlite bodies rangingfrom 94 to 104 Ma, along with stratigraphic constraints (Rb–Srphlogopite: Lehnert-Thiel et al., 1992; Scott Smith et al., 1994.U–Pb perovskite: Leckie et al., 1997; Heaman and Kjarsgaard,2002; Zonneveld et al., 2004), indicate volcanic activityoccurred over a period of more than 10 million years, underchanging palaeoenvironmental conditions.

An ∼400 m thick Cambrian to Devonian mixed carbonate-siliciclastic-evaporite succession occurs below the MannvilleGroup. Precambrian crystalline basement related to the Palaeopro-terozoic Glennie Domain, which is exposed∼175 km to the north,and underlying Archaean Sask Craton, occurs at about 700 mbelow the Fort à la Corne region (e.g. Collerson et al., 1989; Lucaset al., 1993; Leckie et al., 1997; Davis et al., 1998; Zonneveld et al.,2004; Fig. 1b). Pleistocene glacial sediments (∼100 m thick)unconformably overlie the Cretaceous siliciclastic succession.

anada, relative to the exposed Precambrian Shield, and distribution of kimberlitenal stratigraphic units of east-central Saskatchewan (modified from Leckie et al.,

154 A. Pittari et al. / Journal of Volcanology and Geothermal Research 174 (2008) 152–170

Fort à la Corne kimberlite bodies were originally interpretedas either craters infilled with pyroclastic deposits (Scott Smithet al., 1994; Field and Scott Smith, 1999; Berryman et al., 2004)or as positive-relief tephra cones with horizontal or conicaltapering bases above their feeder vents (Leckie et al., 1997).Recent models from the Star and 140/141 (now the Orion Southcluster) kimberlites have interpreted these bodies to be tephracones which were constructed over several eruptions abovean accommodation space which formed as a result of sub-sidence along ring faults and half-grabens around feeder ventsand/or scouring by base surges (Zonneveld et al., 2004;Kjarsgaard et al., 2005, 2006; Harvey et al., 2006; Zonneveldet al., 2006a,b).

Subaerial pyroclastic fallout, and marine reworking andresedimentation have been interpreted to be the main kimberliteemplacement processes at Fort à la Corne (Scott Smith et al., 1994;Leahy, 1997; Leckie et al., 1997; Nixon and Leahy, 1997;Berryman et al., 2004; Zonneveld et al., 2004, 2006a,b).Subaqueous mass flows (debris flows and turbidites) wereoriginally recognised by V. Lorenz (internal unpublished BeBeers internal report, 1992) and were proposed again as a

Fig. 2. (a) Location map of 2005–06 drill holes logged in this study (closed circles) anAdditional drill holes examined in less detail are also shown (grey text, open circles). (bdrill hole (i.e.Main 219 or 145 kimberlite package, or Lower Series where present; truelarge kimberlite bodies, and interpreted to be a complex of at least three craters (219, 1

significant resedimentation process at the Star and 140/141kimberlites (Zonneveld et al., 2004, 2006a,b; Harvey et al.,2006; Kjarsgaard et al., 2006), although minor pyroclastic flow orsurge deposits were also suggested for some depositional units.Kjarsgaard et al. (2006) suggested that some eruptive events at the140/141 kimberlite were subaqueous. However, no studies at Fortà la Corne have assessed in detail the internal facies characteristicsand architecture of kimberlite deposits to understand the style andspatial distribution of physical emplacement processes.

Recent grid drilling (2005–2006) over the Orion Centralvolcanic complex has allowed for a detailed three-dimensionalfacies analysis of this kimberlite volcanic complex through corelogging and correlation. The facies characteristics and archi-tecture provide insight into the types and spatial variation ofphysical volcanological processes within single kimberlitebodies and throughout the basinal sequence.

2. Methodology, nomenclature and limitations

Graphic stratigraphic logs were constructed from 28 (2005–2006 drilling seasons) vertical drill cores (∼3.6 km core),

d the outline of the geophysical anomaly of the Orion Central volcanic complex.) Contour map of the depths to the base of the major interval of kimberlite in eachdepths shown inmetres below surface) representing the basal surface beneath the45A, 145B craters). Cross-section lines for Figs. 4–7 (see below) are also shown.

155A. Pittari et al. / Journal of Volcanology and Geothermal Research 174 (2008) 152–170

retrieved from intervals from near the base of the glacialsediments to variable depths (159 to 324 m below surface)within the Lower Colorado and Mannville Groups, andgenerally ∼200 m apart (locally ∼100 m) in a grid across theOrion Central complex (Fig. 2a). The surface altitude (∼448 mabove sea level) varies by less than 2 m, and all depths quotedhere are true drill hole depths (i.e. metres below surface). Hostrock lithological units and kimberlite packages, their contactrelationships and kimberlite facies characteristics were docu-mented on the stratigraphic logs, and further assessed bymicroscopic thin section petrography. An additional 5 drill coreswere logged in less detail to identify unit thicknesses and hostrock-kimberlite contacts. A facies, as used here, representsa rock interval with a definable set of original depositionalcharacteristics, which are distinct from adjacent facies (Moore,1949). Rock alteration textures and components are notincluded as essential facies descriptors.

Average grainsize is described using the Wentworth sedi-mentary grainsize classification scheme, and both grainsizeand grain percentages are based on visual estimates from drillcore or thin section. Kimberlite volcaniclastic facies at Fort à laCorne have the potential to be syn-eruptive pyroclastic, syn-eruptive modified pyroclastic (e.g. pyroclastic flow enteringthe sea), syn- to post-eruptive rapidly resedimented (e.g. lahar,turbidite), or post-eruptive gradually reworked in origin. Hence,the use of Wentworth grainsize terms (e.g. sand, pebble) inthe descriptive stage is intended here to avoid implying anyinitial genetic connotations to the kimberlite volcaniclasticfacies. A genetic facies name, which may involve a conversionto volcanological grainsize terms (e.g. ash, lapilli), if consideredpyroclastic in origin, is later applied when the environmentaland process origins of each facies is discussed.

The maximum lithic clast size (ML) within a facies interval isdetermined here as the average of the lengths of the maximummeasurable diameters as seen in drill core of the five largest clasts(Cas and Wright, 1987). ML is actually a minimum value asidentification of the true long axis of some large clasts is limitedby the width of the drill core (i.e. 6.4 cm). Bedding thickness iscategorised according to Ingram (1954) as: laminae (b1 cm),very thin (1–3 cm), thin (3–10 cm), medium (10–30 cm), thick(30–100 cm), and very thick (1–10 m), with the addition here ofextremely thick (N10 m).

A kimberlite volcaniclastic package is defined here as acontinuous interval of one or multiple stacked depositional units,with recognisable gradational or subtle sharp (i.e. defined by agrainsize change, not a “knife-sharp” disconformity) internalunit contacts. If an interval of multiple stacked depositional unitscontains internal contacts or thin horizons, which represent asignificant hiatus in kimberlite deposition (e.g. host rocklithologies; a regolith profile; bioturbation; in situ bioclastichorizons; reworked/resedimented mixed kimberlite-host rockzones; disconformities), or if the internal continuity of the in-terval is uncertain, it is identified as a series.

Post-emplacement alteration masks primary kimberliteconstituents and depositional textures at Orion Central. Acomplete assessment of alteration is beyond the scope of thispaper, but it is essential here to note some of the more pertinent

effects of alteration observed in this study and the limitations ithas on assessing primary textures. One common, but ambiguouskimberlite facies constituent are “clast-like”, fine-grainedcrystal-rich domains of olivine (∼20%, b0.5 mm-sized) andsmaller spinels and perovskite (∼15%, b0.01 mm-sized), in afiner-grained optically unresolvable dark serpentine-carbonatematrix. These domains may contain one or more larger anhedralolivine crystals or form partial rims of variable thickness aroundsingle olivine crystals. Some domains may be “juvenile lapilli”(cf. Scott Smith et al., 1994; Berryman et al., 2004; Zonneveldet al., 2004), however, their boundaries are often irregular andscalloped, and commonly grade into the surrounding alteredmatrix, suggesting that, if they were pyroclasts, their deposi-tional morphology has been later modified. Alternatively, fine-grained crystal-rich domains may represent pseudoclasts of aonce continuous matrix, and this is especially evident wherethey form a diffusely interconnected network.

Another ambiguous facies constituent at Orion Central is thefine-grained, crystal-poor, optically unresolvable, serpentine-carbonate alteration matrix between grains and fine-grainedcrystal-rich domains. This matrix (and potentially the matrix ofsome fine crystal-rich domains) may have replaced a volcani-clastic matrix, although no definitive primary textures (e.g.matrix grains, glass shards) are preserved, except for localisedrelict patches of spinels. The fine-grained alteration matrix isoften locally overprinted by irregular domains of crystallinecarbonate and/or amorphous serpentine.

The abovementioned alteration effects inhibit the assessmentof the proportions of different grain types, and the character-istics and proportions of their supporting medium. Majorkimberlite facies are therefore described with emphasis onthe definite primary grain types (crystals, lithic clasts) anddepositional structures. The terms wackestone (matrix-sup-ported, N10% grains), packstone (dominantly clast-supported,but with interstitial depositional matrix) and grainstone (clast-supported, lacks interstitial depositional matrix) of Dunham(1962) are used here to describe the textural configuration offramework grains, with respect to the proportion of clasticmatrix, which may otherwise be masked by alteration.

3. Host rock–kimberlite geometrical relationships

The Orion Central volcanic complex was originally definedas two partially connected geophysical anomalies (Body 219,1.1×0.95 km and Body 145, 1.0×0.66 km; Fig. 2a). Drill coreslogged and correlated in this study have revealed a laterallycomplex succession of interbedded siliciclastic sediments(Mannville and Lower Colorado groups) and relatively thinstratabound kimberlite packages. Host rock siliciclastic lithol-ogies and their inferred depositional environments are sum-marised in Table 1. Contained within this succession there arethree relatively larger volume discordant kimberlite bodiescentred at different locations within the volcanic complex. Themain interval of kimberlite in each drill hole generallyconstitutes one, sometimes two, of these bodies.

By correlating the base of the main interval of kimberlite ineach drill hole it is apparent that the three large discordant

Table 1Local lithologies of the siliciclastic succession and interpreted depositional environments

Group/formation Lithology Environment

Lower Colorado/− Predominantly dark mudstone; lower zone with interbedded glauconiticsandstone beds; one or two b3 m thick bioturbated, muddy quartz siltstoneto medium sandstone beds at higher stratigraphic levels (ie 148 to 169 mbelow the surface, locally 133 m).

Predominantly deep marine; early proximal offshoreto offshore transition 1, and occasional later periodsof shallower/marginal marine conditions

Mannville/Pense b8 m thick, discontinuous; interbedded bioturbated, laminated quartzsiltstone-fine sandstone and black mudstone; and laminated black siltymudstone.

Lower shoreface, intertidal to subtidal andtransitional offshore marine 1

Mannvillle/Cantuar Interbedded quartz sandstone, grey-brown mudstone to very finesandstone, dark organic-rich mudstone to sandy mudstone, occasionalcoal horizons; rare laminated black mudstone and sandstone horizonswith high abundance-low diversity mainly small trace fossil assemblagesand cross laminae.

Fluvial to palludal, locally lacustrine and estuarine 1

1 cf. Christopher (2003), Zonneveld et al. (2004).

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kimberlite bodies are seated within elongate, northwest-alignedcraters (145A, A45B and 219 craters), and with minimumnegative reliefs of 130 to 170 m (Fig. 2b; justification for whythese are craters is provided in Section 6.1). Although there is alack of drill holes at the periphery, at least the 219 crater isclosed at its northwestern end. These geometries are furthersupported by proprietary geophysical models. Two singlecontinuous kimberlite packages, Main 219 and Main 145packages, infill the 219 and 145B craters respectively, althoughthere is insufficient drill hole coverage to establish a strati-graphic relationship between the two packages. The 145A crateris also infilled by a continuous kimberlite package, whichmay be related to a succession of thinner, shallower kimberlitepackages to the southeast, both of which occur stratigraphicallybelow the Main 145 package, and are grouped together as theLower Series. Several relatively thin but regionally extensivekimberlite packages overlie the Main 219 and 145 packages(Upper Series).

Unconformable Pleistocene glacial sediments generally over-lie the main interval of kimberlite in each drill hole, however,a conformable contact between the Main 219 package or theoverlying Upper Series and Lower Colorado dark mudstone ispreserved in the west.

4. Major kimberlite volcaniclastic facies

Given the limitations due to alteration effects (see Section 2),the major definite primary grain types within kimberlite facies atOrion Central are: anhedral and angular crystals (typicallyN0.5 mm diameter) and relatively finer-grained (typicallyb1 mm diameter) euhedral crystals of serpentine- and/orcarbonate-altered olivine (∼20–50%); minor garnet, ilmeniteand phlogopite (b2%); and lithic clasts (generally b5%). Themajor lithic clast types are: (a) Lower Colorado Group darkmudstone and rare Mannville Group carbonaceous siltstonesand mudstones; (b) Palaeozoic carbonate clasts; and (c) olivine-rich kimberlite clasts (autoliths), which could have been co-herent cognate clasts from the erupting or pre-existing magma,intraclasts entrained from earlier volcaniclastic deposits, or amixture of both types. Deeper crustal-derived granitic orgneissic clasts, and mantle-derived xenoliths are also present

in minor proportions. Carbonaceous plant fragments occur insome facies and were derived either from surficial vegetation atthe time of eruption or carbonaceous horizons in the MannvilleGroup. The key characteristics of the major kimberlite facies arediscussed below.

4.1. Facies A

Themost voluminous facies type of the Orion Central complexconsists of multiple depositional units of massive to stratified,garnet-ilmenite-bearing, olivine-rich sand- to granule/pebble-sized volcaniclastic rocks, commonly displaying a subhorizontalto oblique elongate grain alignment fabric (Fig. 3a,b). Frameworkgrain distributions vary from a wackestone to packstone depo-sitional texture.

This facies is subdivided into four gradational subfaciestypes, described as:

(a) massive, relatively lithic-rich, coarse sand- to pebble-sized olivine-rich (A1, Fig. 3a);

(b) diffuse- to well-, medium to very thickly bedded, variablylithic-bearing, coarse sand- to granule-sized olivine-rich(A2);

(c) laminated to medium bedded, lithic-poor, coarse sand-granule-sized to very fine-fine sand-sized olivine-rich(A3, Fig. 3c); and

(d) massive, lithic-poor, medium to very coarse sand-sizedolivine-rich subfacies (A4).

Individual depositional units of subfacies A1, up to 96 mthick, may have finer-grained, reverse-graded, massive tostratified basal zones (b60 cm thick) and lower concentrationzones of pebble- to boulder-sized, intra-basinal-derived lithicclasts. Less then 5% pebble- to boulder-sized lithic clasts(ML: N2 cm, commonly 5–11 cm, up to 20 cm), of all lithotypes,occur throughout this subfacies, or within multiple thick to verythick gradational concentration zones. Dark mudstone clastsare sometimes irregular or deformed, and commonly containembedded olivine grains within their outer margins. Somecarbonate clasts contain radial surface cracks. Olivine, garnetand ilmenite grains are commonly sand- to granule-sized, but

Fig. 3. Photographs of drill core (core width=6.3 cm, up from right to left) and microphotographs of the major facies types. (a) Massive, olivine-rich sand- to pebblygranule-sized volcaniclastic subfacies A1, with dark mudstone (mst) and carbonate (carb) lithic clasts and showing an elongate clast alignment fabric (145-014,∼154.4 to 154.6 m), and (b) the same subfacies showing a predominantly wackestone distribution of crystal grains and complex domainal alteration variations withinthe interstitial matrix (219-014, 277.2 m, PPL). (c) Laminated to medium bedded, olivine-rich sand- to granule-sized volcaniclastic subfacies A3, highlighting a 25 cmthick normal graded bed with a massive basal zone and laminated upper zone (219-017,∼130.3 to 132.3 m). (d) Textural variations at different levels in various 0.7 to1.2 m thick normal graded beds of facies B (145-015, PPL), showing the poorly sorted, very coarse-grained basal zone (i, 143.1 m), medium grained, moderately sortedmid-zone (ii, 142.5 m), and finer-grained upper zone with an increased proportion of phlogopite flakes and larger mudstone lithic clasts (mslt) (iii, 149.0 m)(e) Olivine-rich sandstone facies C showing clast- to matrix-supported olivine grains; minor phlogopite (phlog), bioclasts (biocl) and lithic fragments; anddiscontinuous dark mud matrix-rich domains-the lighter interclast matrix at the top and bottom is serpentine (219-011, 105.4 m, PPL). (f) Laminated to very thin-bedded, sand-sized volcaniclastic facies D showing a blebby alteration texture possibly preserving a coarser- (right) and finer- (left) grainsize primary texture, andserpentine alteration nodules (spnd) (219-007, 125.5 m, PPL).

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may be up to 3 to 4 cm in diameter. Subfacies A4 is also massive,but is generally finer-grained than A1 and lacks lithic clasts.

Subfacies A2 generally contains multiple massive and normalgraded beds, with gradational boundaries, and relatively thinnerand finer-grained pebble- to cobble-sized (ML: commonly 2–3.5 cm) polymictic lithic concentration zones. Relatively thinnerlithic-poor beds with sharper bedding contacts, distinguishes

subfacies A3 from A2. Individual beds are normal graded, withmassive lower zones and laminated to cross-laminated upperzones (Fig. 3c). Groups of beds commonly occur in very thickfining upward cycles. Bedding orientation varies from subhor-izontal to steeply dipping (15–40°).

Mudstone lithic granule and pebble laminae/thin beds occurin all four subfacies, but most commonly in the finer-grained

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stratified subfacies. Carbonaceous plant fragments are rare butalso more common in finer-grained stratified subfacies. Over-steepened (up to 65°) and folded beds, and localised microfaultsoccur locally.

4.2. Facies B

This facies consists of multiple, amalgamated, normalgraded, thick to very thick (0.2–8.5 m), matrix-supported(wackestone) olivine-rich depositional units with massive,poorly sorted coarse sand- to granule-sized (olivine up to 3 to5mm, occasionally 10mm) lower zones, and laminated to cross-laminated, moderately sorted medium to very fine sand-sizedupper zones, often enriched in larger phlogopite flakes (Fig. 3d).The tops of each bed are characterised by discontinuous,convoluted dark muddy laminae, sometimes with rare tracefossils (wide burrows), and/or concentration zones of deformedmudstone clasts (b1 to 13 cm diameter). Thicker (0.7 to 4.2 m)intervals of mixed irregular-deformed dark mudstone breccia,muddy stringers and altered kimberlite occur between somebeds. Bedding contacts are generally sharp with occasionalflame structures and load casts. Dark mudstone and kimberlitelithic intraclasts often occur in the lower zone of each bed, anddispersed plant fragments are common throughout. Rare pebble-sized carbonate and mantle-derived clasts occur in some beds.Mudstone lithic granule to pebble laminae are common from themid to upper parts of each bed. Oversteepened and folded beds,and localised microfaults occur locally.

4.3. Facies C

This facies consists of very thin to medium graded beds ofmoderately sorted, wackestone to grainstone-textured, olivine-rich (∼40–50%) fine to coarse sandstone and intercalateddiscontinuous greenish grey muddy laminae (Fig. 3e). Laminaeand very thin beds with abundant (up to ∼20%) thin intactshell fragments, often concentrated within mud-enricheddomains, characterise this facies. In coarse-grained beds,olivine grainsize is typically bimodal (average visual grainsizeestimate: ∼0.5 and ∼1.5 mm; maximum grainsize: 2 to3.5 mm). Additional lithic clasts (b5% whole rock) includedark mudstone, and minor dark mica rich altered basement/mantle-derived clasts. Minor garnet and phlogopite flakes arealso present (b1% whole rock).

4.4. Facies D

This facies is pervasively altered and characterised bylaminated to very thin-bedded fine- to coarse sand-sized andsilt to fine sand-sized granular material (Fig. 3f). Coarser-grainedbeds consists of indistinguishable pseudomorphed crystalsand alteration blebs (∼40%, average visually estimated size:∼0.1–0.3 mm) and minor phlogopite flakes within an opticallyunresolvable serpentine-carbonate matrix. Localised deposi-tional structures include pinch and swell laminae, cross laminae,mudstone lithic granule to pebble laminae, and a bedding parallelphlogopite alignment fabric. The basal zone of this facies may be

reverse-graded or sometimes massive and relatively coarser-grained. The bulk of the thickness may be normal graded andindividual beds may be grouped into medium to thick finingupward cycles. Minor irregular beds and microfaults have alsobeen observed. Serpentine alteration nodules are commonlylocalised along bedding planes, a distinctive secondary featurewithin this facies (Fig. 3f).

4.5. Facies E

This facies consists of massive to laminated mudstone, 0.1 to5 m thick, with a light grey colour that distinguishes it from darkLower Colorado Group mudstone. N95% of the whole rock ofthis facies consists of optically unresolvable mud-sizedserpentine-rich material, b5% silt to very fine sand-sizedaltered olivine, opaque mineral(s) and single biotite/phlogopiteflakes. Dispersed relatively coarser-grained olivine and garnetgrains are sometimes present. A variation of this facies consistof discontinuous laminae, and minor cross laminae, or very thinbeds of olivine (+phlogopite, rare garnet) silt to sand, which isoften bioturbated or locally deformed.

5. Facies associations and facies architecture

The stratigraphic succession of the Orion Central volcaniccomplex is divided into two broad facies associationscharacterised by (a) intervals consisting mostly of host rocklithologies, with relatively thin interbedded stratabound kim-berlite packages, and (b) intervals mostly of discordant toconcordant kimberlite packages/series with minor intercalatedhost rock or mixed kimberlite-host rock lithologies. Figs. 4–7highlight the three-dimensional stratigraphy of the successionand the internal facies architecture of kimberlite packages/seriesthrough representative stratigraphic log correlations.

5.1. Relatively thin, stratabound kimberlite volcaniclasticpackages, interbedded within host rock lithologies

Multiple stratabound kimberlite volcaniclastic packages areinterbedded within relatively thicker intervals of Mannville andLower Colorado Group lithologies (Figs. 4–6). They generallyhave conformable sharp, planar to irregular, subhorizontal upperand lower contacts, although some are brecciated, oversteepenedand/or show mixing between kimberlite and the host rock (e.g.145–008, 219.0 m; 219-009, 248.0 m and 243.0 m; Figs. 4, 5).Many stratabound packages are pervasively altered, leavinglittle textural detail to identify volcanic facies characteristics.However, most packages with reasonable textural preservation,consist of one or two, depositional units of subfacies A1 or A4.The following stratabound packages were identified in thestudied drill holes, which unless specified (i.e. see g and hbelow) are of subfacies A1 or A4, or pervasively altered:

(a) up to four, commonly b2 m thick (locally up to 5 to 12 mthick), intra-Cantuar packages per drill hole, but not yetcorrelated between drill holes (e.g. 219-005, 006, −009,−015 and 145-008, Figs. 4–6);

Fig. 4. Representative stratigraphic logs of drill core (depth below the surface, m, versus average grainsize, cm) showing the positions of kimberlite intervals (verticalthick grey line), siliciclastic units, Lower Series, Main 145 package, Upper Series and the major facies types, across the 145A crater.

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(b) a 10 cm thick, package at the Cantuar-Pense boundaryin one drill hole only (219-016, not shown in section,Fig. 2);

(c) a 0.8 to 2.6 m thick, intra-Pense package, tentativelycorrelative across four drill holes (219-012, Fig. 6; also219-008, −013, −016, not shown in section, Fig. 2), and

160 A. Pittari et al. / Journal of Volcanology and Geothermal Research 174 (2008) 152–170

an additional 0.4 m thick intra-Pense bed occurring at ahigher stratigraphic level in one drill hole only (219-016,not shown in section, Fig. 2);

(d) a 2.7 m thick package at the Cantuar–Lower Coloradoboundary (i.e. where the Pense is absent, 219-006,Fig. 6);

(e) a 0.6 m thick package at the Pense–Lower Coloradoboundary (219-016, not shown in section, Fig. 2);

Fig. 5. Representative stratigraphic logs of drill core (as in Fig. 4) showing the positfacies types, along the axis of the 219 crater.

(f) one (e.g. 219-006, Fig. 6), locally three (219-013 only, notshown in section, Fig. 2), 0.1 to 23.4 m thick packages,that occur within the lower part of the Lower ColoradoGroup (i.e. below the zone of muddy quartz siltstone–sandstone beds, see Table 1);

(g) a 2 to 5.8 m thick intra-Lower Colorado package, observedin two drill holes but not yet correlated, that occurs above thezone of muddy quartz siltstone–sandstone beds, and which,

ions of kimberlite intervals, siliciclastic units, Main 219 package and the major

Fig. 6. Representative stratigraphic logs of drill core (as in Fig. 4) showing the positions of kimberlite intervals, siliciclastic units, Main 219 package, Upper Series andthe major facies types, perpendicular to the axis of the 219 crater.

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in one drill hole, is characterised as dark brown phlogopite-rich (20–30%) mudstone (e.g. 219-017, Fig. 5); and

(h) three 0.1 to 3 m thick intra-Lower Colorado packages athigher stratigraphic levels, in one drill hole only (219-015,Fig. 4) consisting ofmassive or normal graded, wackestone-textured, mudstone lithic- (10–20%) and quartz-bearing,olivine-phlogopite-rich (20–25%) pebbly-sandy mudstone.

5.2. Relatively thick, discordant to concordant kimberlitevolcaniclastic packages, with little or no intervals of host rocklithologies

This facies association incorporates the three large volcani-clastic kimberlite bodies of the Orion Central complex (i.e.Lower Series, Main 145 package, Main 219 package), whichoccur as extremely thick (up to 211 m) discordant crater-filldeposits which thin rapidly to adjacent crater rims where theybecome concordant with the host rock. Individual packages/series consist of multiple depositional units and displaycomplex internal spatial facies variations. Related thinnerintercalated mixed siliciclastic–volcaniclastic deposits between

the Lower Series and Main 145 package, and the volcaniclasticUpper Series are also included in this facies association.

5.2.1. Lower SeriesWithin the 145A crater the Lower Series consists of a

continuous kimberlite volcaniclastic package, up to 130.6 mthick, with a sharp base above the Cantuar Formation (219-015,145-007, -008, Fig. 4) and is correlated across three adjacent drillholes based on broad similarities in the overall verticalstratigraphic architecture. The thickest interval (145-007,145.0 to 275.6 m, Fig. 4) consists of: (a) a lower relativelyfine-grained zone of subfacies A4 overlain by subfacies A3(43.6 m thick), which grades upward to (c) a coarser-grainedzone comprised of two depositional units of subfacies A1(46 and 40.5 m thick), separated by a 10 cm thick bed ofsubfacies A4. The basal part of the upper zone is characterised byan abundance of large (20 to 50 cm, up to 3.5 m) lithic clasts ofdark mudstone and Cantuar lithologies. The top grades into amixed light grey and dark mudstone lithic breccia, whichtentatively correlates to mixed mudstone–kimberlite zonesabove the Lower Series in other drill holes (e.g. 145-008, Fig. 4).

Fig. 7. Representative stratigraphic logs of drill core (as in Fig. 4) showing the positions of kimberlite intervals, siliciclastic units, Lower Series, Main 145 package,Upper Series and the major facies types, perpendicular to the axis of 145B crater.

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The Lower Series thins upslope 200 m to the south (145-008,151.5 to 214.9 m, Fig. 4), where it consists of a lower zone ofsubfacies A3, and a relatively thicker upper zone of subfacies A1and A2. 200 m upslope to the north (219-015, 120.8-191.8 m,Fig. 4), only the massive upper zone is present (subfacies A1).

Along the margins and to the southeast of the 145A crater,the Lower Series is thinner (7.7 to 50.7 m thick) and, at itshighest stratigraphic level, sharply overlies Lower Coloradomudstone within the zone above a muddy quartz sandstone bed(see Table 1; e.g. 145-010, −013, Figs. 4, 7). Locally, the basal

contact may grade into an underlying mixed zone of mudstone,mudstone lithic breccia and altered kimberlite. These cratermargin deposits consist predominantly of bedded facies Bdeposits, although the upper one or two beds are often relativelythicker (5.7 to 15.7 m) and characterised by subfacies A1, A2and A4 (e.g. 145-010, 168.1 to 161.8 m, Fig. 4).

Correlations of the Lower Series thin crater margin-equivalentdeposits to the thick Lower Series deposit infilling the 145A crateris poorly constrained. However, in both cases the Lower Series isstratigraphically older than theMain 145 package, and the eruptive

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time break between the two (i.e. mixed kimberlite-mudstone zone,see below) is more significant than any eruption pauses, if any,between internal depositional units within the Lower Series.Furthermore, the Lower Series in drill hole 145-010 (Fig. 4) has thetypical facies architecture of the crater margin deposits in other drillholes (e.g. 145-013, Fig. 7), but also occurs at a similar stratigraphicposition to the crater-fill deposits in an adjacent drill hole (145-008,Fig. 4). Hence, for these reasons both the Lower Series crater-filland crater margin deposits are considered penecontemporaneous.

5.2.2. Mixed mudstone–kimberlite zones above the Lower SeriesLower Colorado mudstone conformably overlies the Lower

Series in two drill holes (219-015, 145-007, Fig. 4), and, inanother drill hole (145-013, Fig. 7), the Main 145 packagesharply overlies and has scoured the top of the Lower Series.However, in all other studied drill holes where the Main 145package and the Lower Series were intersected (e.g. 145-008,−010, Fig. 4), there exists an intermediary zone (4.8 to 30.3 mthick) of intercalated thin to very thick pervasively alteredkimberlite horizons and dark Lower Colorado mudstone ofsimilar thickness. Kimberlite–mudstone contacts are generallysharp, and zones may vary between relatively undisturbedintervals, convoluted, oversteepened (up to 25 to 75°) beddedintervals, and mixed, deformed mudstone lithic-rich kimberliticbreccia zones, with muddy laminae. Additional intercalated bedsinclude: graded massive to laminated/cross-laminated, mud-stone lithic-bearing silty mudstone, (145-010, 137.0–143.0 m,Fig. 4); bioturbated sandymudstone (145-008, 149.1 to 150.0 m,Fig. 4); and deformed light grey facies E mudstone (145-008,145.7 to 149.1 m, Fig. 4). A mixed dark and light grey mudstonelithic breccia occurs at a similar depth in an adjacent drill hole(145-007, 144.0-145.0 m, Fig. 4) and grades up into a matrix-supported, dark mudstone lithic pebble- to cobble breccia.

5.2.3. Main 219 and 145 kimberlite volcaniclastic packagesThe Main 219 and 145 South packages infill the 219 crater

and 145B crater, respectively, and consist of multiple internaldepositional units, which have not yet been uniquely correlatedbetween drill holes. Within their respective craters, both pack-ages sharply overlie undisturbed Cantuar lithologies (Figs. 5, 7)and thicken to the southeast (N211 m thick, Main 219 package,219-014, Fig. 5; N176 m thick, Main 145 package, 145-014,Fig. 7), beyond the limit of drill holes. On the crater rims, the twopackages are significantly thinner (e.g. 219-006, −012 and−017, Figs. 5, 6; 145-012, −013, Fig. 7) and sharply overlieundisturbed Lower Colorado mudstone (or locally the LowerSeries in the case of the Main 145 package, e.g. 145-013, Fig. 7)although load casts or mixed substrate mudstone and alteredkimberlite contacts may occur occasionally (e.g. 219-006, 133.6to 153.9 m, Fig. 6). The highest preserved stratigraphic positionfor the basal contact for both packages is within Lower Coloradomudstone above the zone of muddy quartz sandstone bed(s) (seeTable 1; Figs. 5–7). The unconformity with Pleistocene glacialsediments commonly marks the top of both packages, howeverin some drill holes towards the periphery, a conformable uppercontact with the Upper Series and/or Lower Colorado mudstoneis preserved (e.g. 219-012, Fig. 6; 145-013, Fig. 7).

TheMain 219 and 145 packages consist almost entirely of faciesAwhich varies laterally and vertically between subfacies A1 to A4.Thick deposits within the centres of craters consist of subfacies A1,and much thinner intervals of subfacies A2 and A4 (e.g. 219-005,−009, −014, Fig. 5; 145-014, Fig. 7). On the crater rims thinnerdeposits of subfacies A2 andA3 are dominant (e.g. 219-006,−012,−017, Figs. 5, 6; 145-010, −012, −013, Figs. 4, 7). Also on craterrims, light grey facies E mudstone (up to 5 m thick) may underlie,or occur within the basal zone of the Main 219 package.

Although both packages occur at a similar stratigraphic leveland exhibit similar facies characteristics and architecture, theircontinuity through the centre of the Orion Central complex isnot constrained within the studied drill hole configuration.

5.2.4. Upper SeriesTwo less voluminous kimberlite volcaniclastic deposits occur

stratigraphically above the Main 219 and 145 packages. On thewestern side of the Orion Central complex, a deposit consistingpredominantly of Facies C (9.5 to 19.7 m thick) sharply overliestheMain 219 package (e.g. 219-012, Fig. 6). A zone (up to 3.5 mthick) of variably bioturbated, facies E mudstone with silty- tosandy laminae, occurs at the base or within the lower part(e.g. 219-012, 119.5 to 123.0 m, Fig. 6) of this deposit. Towardsthe centre of Orion Central this facies gradationally overliesLower Colorado mudstone or the Lower Series.

Additional deposits of light grey facies E mudstone (0.4 to1.5 m thick), sharply overlie the Main 219 or 145 packages, inthe south (e.g. 145-013, Fig. 7) and central areas of the OrionCentral complex. The uppermost kimberlite volcaniclasticpackage (2.5 to 13.7 m thick) consists entirely of facies Dand sharply overlies facies E (e.g. 145-013, Fig. 7) or C, (e.g.219-012, Fig. 6), and sometimes with basal load casts.

6. Discussion

Approximately 27% of all sedimentary rocks are consideredto be volcaniclastic (Fisher and Schmincke, 1984; Orton, 1996)and form a significant component in many sedimentary basins.Buried relict stratovolcanoes, calderas and maars withinsedimentary basins have been imaged by 3D seismic or gravitymethods (e.g. Taranaki Basin, NZ, Corinne et al., 1993; OuterMoray Firth Basin, UK, Stewart, 1999; Northern South YellowSea Basin, Lee et al., 2006). The major kimberlite bodies at theOrion Central volcanic complex also have distinct high reliefgeometries against the undeformed strata of the host rockbasinal sequence. The three-dimensional facies architectureacross the volcaniclastic deposits, resolved by recent drill corecoverage, enables a reconstruction of the spatial variations inemplacement processes across the buried crater relief.

6.1. Evidence for crater complexes

The high negative relief surface geometries beneath the largekimberlite bodies could have formed by (a) excavation ofelongate or coalesced volcanic explosion crater(s) (e.g. ScottSmith et al., 1994) or fissure vents; (b) through epiclasticerosional processes; or (c) syn- to post-eruptive subsidence only

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(Kjarsgaard et al., 2006), or combinedwith erosion by base surges(Kjarsgaard et al., 2005; Harvey et al., 2006; Zonneveld et al.,2006b). There is no evidence for epiclastic erosional currents,such as fluvial lag deposits overlying the floor of the basalsurfaces, or for the conditions favourable to extremely deepepiclastic incision of the basinal stratigraphy, such as majortectonic uplift. If these basal surfaces are closed at both ends, assuggested by drill core correlations and proprietary geophysicaldata, a major through-flow erosional current is inconceivable.

If the negative relief surface geometries were the manifestationsolely of syn- or post-eruptive graben-like subsidence, then thesubstrate to the deep central parts of the main bodies would be ofthe same lithology as the upper/outer parts (i.e. Lower ColoradoGroup). However, the presence of Cantuar substrate lithologies inthe central parts is indicative of a cross-cutting relationship, ratherthan a downthrown block. Syneruptive graben-like subsidence ofat least 130 to 170 m is also inconceivable due to a lack ofsubsurface accommodation space, unless a substantial near-surfacekimberlite magma reservoir is invoked, for which there is noevidence. Smaller syn-eruptive, or post-eruptive tectonic displace-mentsmay have partly contributed to the negative relief. Variationsin the depth to the top of the Cantuar Formation (189 to 199 m,locally 206 m), for example, may indicate displacements of lessthan 20m, although these variations could equally be explained asan irregular erosional palaeosurface. Nevertheless, there is nodefinitive evidence yet to infer faulting as the main contributingcause for the formation of the negative basal surface relief.

An explosive volcanic excavation mechanism to explain theorigin of the basal surface geometries is consistent with the cross-cutting relationship between the host rock stratigraphy and theinfilling volcaniclastic deposits. Crater excavation processes at Fortà la Corne have previously been likened to those of phreatomag-maticmaars which flared from palaeoaquifer(s) associatedwith theMannville Group (Scott Smith et al., 1994; Field and Scott Smith,1999). Observations which either support or reject phreatomag-matic activity, or which provide clues on the relative influence ofmagma-water interaction versus magma degassing has not beenobserved in the Orion Central deposits, and hence the mechanismfor explosive crater excavation remains to be solved.

Correlation of the basal surfaces of kimberlite bodies at OrionCentral may be more complex than proposed here and that, ratherthan defining a negative relief, kimberlite depositional unitswithin the main packages could be conformable and interfingerwith the siliciclastic succession, correlating laterally with thinnerintra-Cantuar, -Pense and Lower Colorado beds, and thus overalldefining a multi-eruptive tephra cone (e.g. Leckie et al., 1997;Zonneveld et al., 2004). Apart from inferring unsubstantiatedcomplex kimberlite-substrate boundaries between drill holes, thisinterpretation is not consistentwith the lack of internal time breakswithin the main kimberlite packages at Orion Central. Thepreservation potential of thick volcaniclastic kimberlite depositswould also be reduced if not protected within a negative reliefaccommodation space. Furthermore, a multi-eruptive tephra coneof uniform composition and lacking internal supporting lava units(cf. stratovolcanoes), spanning the Mannville to Lower Coloradogroups would require an eruptive history in the order of severalmillion to over 10 million years, including two major interforma-

tional unconformities. Hence, the Orion Central complex has notbeen considered here to be a polygenetic tephra cone.

The crater-fill volcaniclastic packages have a low abundanceof Lower Colorado and Mannville lithic clasts, the total volumeof which does not equal the expected volume of materialexcavated to form the crater complexes. However, the crater-fillpackages represent material erupted after the initial excavationevent. It is likely that the initial explosive excavation event wasintense enough to fragment this country rock material to fineparticles, which are difficult to detect in the siliciclasticsuccession, and disperse it well beyond the crater margins andthe area currently covered by drill holes (cf. Scott Smith et al.,1994; Field and Scott Smith, 1999).

It is apparent that at least three craters were excavated at theOrion Central volcanic complex, and two of these may haveformed around the same time (i.e. 219 and 145B craters). Multi-vent monogenetic volcanic complexes are observed in othervolcanic systems. The Quaternary basaltic Red Rock VolcanicComplex, southeastern Australia, consists of at least 40 individualeruption points (overlapping scoria cones andmaars) accumulatedover a single eruptive event (Cas et al., 1993). In recent times, the1963–67 eruption of Surtsey, was a monogenetic eruption lasting3.5 years, but involvedmultiple eruption points (e.g. Thorarinssonet al., 1964; Kokelaar and Durant, 1983; Moore, 1985).

6.2. Contemporaneous sedimentary environments and timingof eruptions

Kimberlite packages are interbedded throughout the Mann-ville and Lower Colorado Groups representing an overall changein the prevailing sedimentary environments (cf. Zonneveld et al.,2004, 2006b; Kjarsgaard et al., 2006). Intra-Cantuar kimberlitepackages were emplaced within a continental to marginal marinesetting, although the locations and settings of the source vents arenot known, whereas later kimberlite packages were emplaced inpredominantly marine environments (e.g. Christopher, 2003;Zonneveld et al., 2004). Furthermore, where the major kimberlitepackages/series (i.e. Lower Series, Main 219 and 145 packages,Upper Series) become stratabound on the crater rims, they arebound above and below by Lower Colorado mudstones, which isconsistent with their emplacement within a marine environment.

Crater walls comprised of unconsolidated Lower Coloradomudstone would be highly unstable in a marine environment andsuch material would easily slump and become resedimented(e.g. debris flow) on the deep crater floors. However, withincraters the main packages lie directly above undisturbed CantuarFormation lithologies, rather than on resedimented crater-wall-derived deposits. It is plausible that the craters were not open forvery long and were infilled with volcaniclastic deposits shortlyafter they were excavated, during the same eruptive event.

6.3. Process origins of major kimberlite volcaniclastic facies

6.3.1. Facies A: crater-fill to extra-crater, massive to stratified,olivine-rich tuff to lapilli-tuff megaturbidite

Massive to stratified olivine-rich sand- to pebble-sized volcani-clastic facies (faciesA) constitutes the bulk of theMain 219 and 145

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packages and the Lower Series in the 145A crater. In addition, themassive, relatively coarser-grained subfacies A1 constitutes manyof the earlier intra-Cantuar, Pense and Lower Colorado volcani-clastic packages, although they are generally thinner and strata-bound. The following characteristics of this facies, at least withinthe Main 219 and 145 volcaniclastic packages, support itsemplacement by a series of syn-eruptive granular mass flows(Iverson, 1997; Druitt, 1998; Roche et al., 2002) (i.e. gas-supportedpyroclastic flow or its water-supported equivalent, amegaturbidite).

(a) Several metres to tens of metres thick of continuousmassive facies represent sustained deposition underextremely high sedimentation rates.

(b) Lateral transitions from thick massive (valley-ponded)facies to thinner stratified (veneer) facies and theirdependence on the crater relief is a common feature ofhigh energy mass flow deposits, and represent lateralchanges in particle concentration and flow regime (Druitt,1992; Branney and Kokelaar, 2002; Pittari et al., 2006).

(c) The elongate clast alignment fabric is indicative of shearforce in granular flow previously invoked for debris flows,turbidites and pyroclastic density currents (e.g. Hughesand Druitt, 1998; Major, 1998; Sakai et al., 2002). Thisfabric is stronger towards the bedded subfacies A3 at thecrater margins, representing increased frictional grain–grain interactions (Major, 1998). Alternatively, such afabric could arise from shearing during post-depositionalcompaction and slumping (e.g. Tobisch, 1984; Patersonand Tobisch, 1993). In this case, the degree of grainalignment would be heterogenous and enhanced alongshear zones, which is not consistent with the observeduniform dispersed distribution of aligned elongate grains.

(d) Poor sorting indicates grain transport under high particleconcentrations (Lowe, 1982; Druitt, 1995).

The above characteristics are inconsistent with a pyroclasticfallout origin involving individual particle size and densitysorting (e.g. Cas andWright, 1987), as suggested for other Fort àla Corne bodies (cf. Scott Smith et al., 1994; Leahy, 1997; Leckieet al., 1997; Berryman et al., 2004; Zonneveld et al., 2004).

Despite the limitations in documenting primary textures (e.g.quantifying the proportion of ‘juvenile clasts’), this facies is stillcrystal-rich (∼20–50% olivine). Even if all fine-grained crystal-rich domains were originally pyroclasts, the proportion of grainsto matrix would still be high, although largely still matrix-supported. Voluminous crystal-rich volcaniclastic deposits inother volcanic systems have been interpreted to be syn-eruptivesubmarine mass flow deposits (e.g. Merrions Tuff, southeasternAustralia, Cas, 1979, 1983). High crystal contents could be due toan inherent crystal-rich magma, which is applicable to kimberlitemagmas (e.g. Mitchell, 1986); or crystal enrichment and fineselutriation in the primary eruption column and pyroclastic flows;or in secondary eruption columns and trailing ash cloudsassociated with pyroclastic flows entering a large water bodyand transforming into a volcaniclastic megaturbidite (Cas, 1983).

High crystal/grain abundances within a gas-charged, matrix-supported, granular pyroclastic flow could result in high frictional

grain stresses, limiting flow mobility and its ability to createpronounced facies variations over the topographic relief. Awater-charged matrix would support larger grains more effectively, thusimproving flow mobility over topographic relief. Quench-fractured surfaces on some carbonate lithic clasts and deformedmudstone clasts are also consistent with cold water-hot clastinteraction. Even with the effect of a water-charged matrix, flowmobility was still relatively low as indicated by the construction ofa mound above, at least, the 219 crater.

Pebble- to boulder-sized lithic clasts are concentrated withinmassive, relatively coarse-grained subfacies A1 infilling cratersand is a typical feature of topographically-controlled pyroclasticflows (Bryan et al., 1998; Branney and Kokelaar, 2002; Pittariet al., 2006). Larger clasts acted as bedload within a highparticle concentration water-charged crystal, and possible mud-sized, granular mixture. Gradational variations in the content oflarge lithic clasts or discrete lithic concentration zones reflectfluctuations in flow energy or variations in clast expulsion fromthe feeder conduit and sedimentation into a progressivelyaggrading deposit (e.g. Branney and Kokelaar, 1992, 2002;Pittari et al., 2006). Finer-grained, massive subfacies A4 reflecta lower flow energy and/or depletion in lithic clast expulsion,but still maintaining a sustained concentrated current.

Stratified deposits on crater rims (subfacies A2 and A3) arecomparable to ignimbrite veneer deposits (Walker et al., 1981;Wilson, 1985) or overbank deposits from channelised turbiditycurrents (e.g. Walker, 1984a), representing decreased particleconcentrations, increased turbulence and pulsatory depositaggradation from the outer parts of the density current (Druitt,1992; Branney and Kokelaar, 2002; Pittari et al., 2006). Unlikemost subaerial stratified ignimbrite deposits, which show simplegrainsize fluctuations, individual beds within these subfacieshave the characteristic normal grainsize grading and internalgrading in depositional structures of Bouma turbidite beds(Divisions A to C, Bouma, 1962). This is a reflection of bettersorting efficiency and transition across a wider flow regimespectrum associated with subaqueous mass flow pulses. V.Lorenz (internal unpublished Be Beers internal report, 1992)originally reported volcaniclastic turbidite deposits at Fort à laCorne, although at the time there were not enough drill holes togain an appreciation of the three-dimensional facies architecture.

This facies is interpreted as a crater-fill to extra-crater, massiveto stratified, olivine-rich tuff to lapilli-tuff megaturbidite, and isapplicable to the Main 219 and 145 volcaniclastic packages andthe part of Lower Series infilling the 145A crater complex, all ofwhich consist predominantly of this facies. Caution must be takenwhen applying this genetic name to the massive variants of thisfacies associated with relatively thinner stratabound intra-Cantuarpackages, in which, although a mass flow origin is likely, theirsubaqueous origin is poorly constrained.

6.3.2. Facies B: normally-graded, thick to very thickly bedded,matrix-supported, olivine-rich, tuffaceous sandstone turbiditesequence

Normal grading and grading of depositional structuresassociated with individual beds of this facies is comparable totheBouma-like beds of the laminated tomediumbedded subfacies

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A3, and deposition from turbidity currents can also be inferredhere. This is further supported by flame structures and load cast atthe base of beds, and “rip-up” intraclasts within the basal zone ofbeds. Facies B differs from subfacies A3 by the occurrence ofconvoluted, sometimes bioturbated, dark muddy laminae, con-centrations of deformed mudstone clasts, and in some cases,relatively thicker mixed and brecciated mudstone kimberlite zoneat the top of most beds. Wispy, convoluted silty laminae andbioturbation are common in muddy turbidites (Hesse, 1975; Stow,1979; Hill, 1984). In facies B, the discontinous laminae are finer-grained (mud) relative to the enclosing olivine-rich silt to sand,and were probably formed by current shear or loading ofunconsolidated saturated mudstone intraclasts. Discontinuousmudstone lithic granule to pebble laminae (trails) are also theresult of internal flow shear (e.g. Bryan et al., 1998).

Improved grainsize and density (hydraulic) sorting upwardthrough normal graded beds is likely an effect of an aqueoussupporting medium (cf. gas-supported). Zones of large-scalesoft sediment deformation resulted from post-depositionalslumping of a water-saturated deposit.

This facies constitutes a bedded sequence forming the LowerSeries along the margins of the 145 crater complex to thesoutheast of the depocentre for the Lower Series (i.e. 145Acrater).Mud-enriched tops onmany of the beds suggest that briefhiatuses occurred between each turbidite pulse. This facies couldrepresent resedimented volcaniclastic turbidite pulses whichwere shed off the main Lower Series volcaniclastic pile eitherduring or shortly after its emplacement (cf. Zonneveld et al.,2004, 2006a). It is therefore classified genetically as a normallygraded, thick to very thickly bedded, matrix-supported, olivine-rich, tuffaceous sandstone turbidite sequence.

6.3.3. Facies C: shallow marine, storm-reworked, stratified,olivine-rich tuffaceous sandstone

Very thin to medium bedded, bioclastic olivine-rich sandstone(facies C) contains characteristics of a reworked and/or resedimen-ted volcaniclastic deposit comparable to storm-influence sandstonebeds or “tempestites” (e.g. increased degree of rounding of olivinegrains, moderate sorting, bioclastic horizons, graded bedding,intercalated muddy laminae) (Walker, 1984b; Brenchley, 1985)which are known in volcaniclastic successions (e.g. Di Marco andLowe, 1989; Kano, 1991; Fritz and Howells, 1991). The presenceof thin (i.e. fragile) intact shells preferentially concentrated in mud-rich horizons indicate intermittent periods of calmer conditions.

This facies directly overlies the primary pyroclastic facies-dominated Main 219 package. Furthermore, the preservedgeometry of the top of the Main 219 package suggests that itformed a mound N25 m in relief centred above the northerncrater and the preserved storm-reworked/resedimented faciesoccurs downslope to the west of this mound. Hence,volcaniclastic debris constituting this facies may have beenshed from the Main 219 package mound.

6.3.4. Facies D: extra-crater, olivine-rich coarse tuff/tuffaceoussandstone turbidite

The laminated to cross-laminated olivine-rich silt- to sand-sized volcaniclastic package (facies D) occurs above the storm-

reworked package and does not infill a significant crater withinthe vicinity of the Orion Central complex. Pinch-and-swelllaminae and cross laminae suggest relatively dilute, high energytractional processes (e.g. pyroclastic surge, storm currents,turbidity current). Given the significant thickness (2 to 11 m) ofthis facies with no evidence for an internal time break, thisfacies must represent a single depositional event. Furthermore, arelatively coarser-grained massive basal zone in one drill hole,suggests a relatively thick partial Bouma bed at least here. Thisfacies can be classified genetically as an extra-crater, olivine-rich coarse tuff (if syn-eruptive) or tuffaceous sandstone(if post-eruptive) turbidite.

6.3.5. Facies E: light grey mudstoneThe origin of light grey mudstone remains ambiguous, and a

genetic facies name has not been applied. The presence ofkimberlitic constituents (e.g. biotite/phlogopite, olivine, ser-pentine alteration minerals) and stratigraphic association withother kimberlitic facies, especially the Main North and Southpackages suggest a closer relationship to local kimberlitedeposits, rather than background marine sedimentation. Thisfacies could either represent the fine ash fallout from an eruptioncolumn, the elutriated ash cloud deposit from a mass flow, or akimberlitic mudflow.

Zones of light grey mudstone containing silty-sandyhorizons and localised current ripples are associated with thestorm-reworked olivine-rich sandstone (facies C) and suggeststhat at least in some places, the light grey mudstone has beenreworked.

6.4. Eruption columns

The extremely thick deposits of crater-fill to extra-crater,massive to stratified, olivine-rich tuff to lapilli-tuff megaturbi-dite are interpreted to have been fed from pyroclastic eruptioncolumns sourced from vents at or near the deepest parts of thecraters. Eruption column(s) would have been established almostimmediately after the initial crater excavation and clearingphase of the same eruption. Given that these deposits wereemplaced in a marine environment, and that the vent(s) werelocated within a negative relief it follows that the eruptioncolumn(s) were also, at least in their lower parts, subaqueous. Inaddition, the eruption column(s) were likely to have been verydense as they were laden with a high concentration of crystalpyroclasts. Both the high column density and the ambientsubaqueous environment would have acted against the upwardbuoyancy of eruption column(s) causing their collapse almostimmediately upon their formation, and subsequent generation ofvolcaniclastic megaturbidity currents.

The eruptive history of the turbidity current-generatingeruption fountain may have been complex. Accumulation ofmaterial into the crater and on the crater rim probably beganaway from the vent site whilst the fountain was still active, andthen infilled the vent at the final stage of the eruption. Asustained eruption fountain may have continued during thedepositional/crater-infill stage or multiple eruptive pulses couldhave led to the periodic occurrence of collapsed fountains. The

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subaqueous environment may have allowed a relatively highvolume of erupted material to be suspended within the watercolumn (cf. subaerial environments) and caused relativelyslower sedimentation rates back into and outside craters. Overthe course of a single eruption volcaniclastic material infilledcraters to the brim (cf. Surtsey, Moore, 1985), and spilled overthe edges creating low relief volcaniclastic mounds, as a result ofthe low dispersal capability of the crystal-rich megaturbidites.

Fig. 8. Schematic representation of the volcano-sedimentary history of the Orion Cpackages, including mass flow deposits, separated by relatively long hiatuses, from uthe Mannville Group and subsequent marginal marine to marine sedimentation oftemporal change in the local environment (i.e. not spatially contemporaneous) at Orioby megaturbidity currents from a collapsed eruption column (Lower Series, cf. ‘d’Multiple volcaniclastic turbidite pulses (X) punctuated by short depositional pauses wpile. (c) Period of slumping, resedimentation and hemipelagic marine sedimentationboth around the same time, but after a significant hiatus subsequent to the 145A erupinfilling of these craters (Main 219 and 145 packages) by sedimentation from volcaneruptive reworking of the Main 219 (and possibly 145) package by storm currents, a(Upper Series).

6.5. Volcano-sedimentary history

The history of volcanism, and spatial and temporal variationsin volcanic processes at the Orion Central volcanic complex isreconstructed from the stratigraphy and internal facies archi-tecture of kimberlite volcaniclastic packages (Fig. 8). Long-lived periods of continental to marginal marine sedimentationand erosion (Mannville Group, Christopher, 2003), and later

entral kimberlite cluster. (a) Emplacement of multiple extra-vent volcaniclasticnknown source locations, during continental to marginal marine sedimention ofthe Lower Colorado Group. (NB. the diagram, from right to left, represents an Central. (b) Explosive excavation of the 145A crater, and syn-eruptive infillingand ‘e’ below), contemporaneous with a later stage of Lower Colorado Group.ere directed at least to the southeast from the main Lower Series volcaniclasticabove the Lower Series. (d) Explosive excavation of the 219 and 145B craters,tion, and dispersal of excavated debris beyond the crater zones. (e) Syn-eruptiveiclastic turbidity currents derived from a collapsed eruption column(s). (f) Post-nd later small volume volcaniclastic turbidite from an unknown source location

168 A. Pittari et al. / Journal of Volcanology and Geothermal Research 174 (2008) 152–170

marine sedimentation of the Lower Colorado Group, wereperiodically punctuated by emplacement of conformablekimberlite volcaniclastic packages (Fig. 8a), a common featurethroughout Fort à la Corne (e.g. Scott Smith et al., 1994; Leckieet al., 1997; Zonneveld et al., 2004). Many of those at OrionCentral were volcaniclastic mass flow deposits. Whether theywere pyroclastic or resedimented in origin is unknown, nor isthe distance and direction from their source localities, althoughtheir conformable contacts imply that they are extra-craterdeposits. Other volcaniclastic emplacement processes (e.g.pyroclastic fallout, epiclastic reworked deposits), have not yetbeen identified within the stratabound, interbedded kimberlite-host rock facies association probably because of either their lowpreservation potential or that they are represented by some ofthe pervasively altered packages.

The relatively thick, disconcordant to concordant, kimberlitevolcaniclastic package facies association is confined stratigra-phically to the middle of the Lower Colorado Formation andrepresents the proximal-to-vent pyroclastic, resedimented andreworked deposits associated with at least two major subaqu-eous eruptive episodes (Fig. 8b–e). Each eruption wascharacterised by an initial explosive deep crater excavationevent (cf. Scott Smith et al., 1994; Field and Scott Smith, 1999;Berryman et al., 2004), which opened a feeder vent(s) andgenerated a dense crystal-rich eruption column (Fig. 8d). Theeruption columns probably collapsed under their own weightalmost immediately after formation generating multiple pulsesand lobes of volcaniclastic turbidity currents (Fig. 8e). Newlyformed craters acted as small confined basins capturing theturbidity currents, deflecting and reflecting them off the craterwalls (cf. basin confined siliciclastic turbidity currents, Picker-ing and Hiscott, 1985) and accumulating a thick massive todiffusely stratified volcaniclastic deposit. Relatively thinnerstratified deposits accumulated from turbidity currents whichsurpassed the upper and outer rims of the crater.

The first eruption excavated and infilled the N130 m deep145A crater (Lower Series, Fig. 8b). Volcaniclastic turbiditycurrents, punctuated by short periods of quiescence were alsodispersed to the southeast around the same time or shortly after theeruption. A significant hiatus in volcanic activity followed inwhich an up to 30m thick deposit of interbedded slump and debrisflow deposits (mixed, deformed and brecciated kimberlite-mudstone), and in situ marine mudstone was emplaced (Fig. 8c).

Following the hiatus, the northwest-aligned 219 and 145Bcraters were excavated and infilled to form volcaniclasticmounds in the southeast (N140 m deep, Main 145 package,cross-cutting the earlier Lower Series deposit), and in the north(N170 m deep, Main 219 package) of the Orion Centralcomplex (Fig. 8d–e). The relative timing of these two craterexcavation events is poorly constrained, however, given theirsimilar stratigraphic position and facies characteristics, theycould be of the same eruption or two similar penecontempora-neous eruptions. Together, these are the most voluminous andwidely dispersed volcaniclastic packages in the area.

Soon after emplacement the Main 219 (and possibly 145)package was reworked and deposited by storm induced currentsat least in the western and central parts of the study area (Fig. 8f;

cf. Zonneveld et al., 2004; Kjarsgaard et al., 2006). Emplace-ment of a small volume, but widespread, volcaniclasticturbidite, from which its source location is unknown, representsthe last preserved kimberlite emplacement event of the OrionCentral volcanic complex (Fig. 8f).

7. Conclusions

The Orion Central volcanic complex records a long period ofcontinental to marine siliciclastic sedimentation punctuated bydiscrete extra-vent kimberlite volcaniclastic emplacementevents, and culminating in at least two eruptions centred withinthe area. The basic concept of an initial crater excavation blast(Scott Smith et al., 1994; Field and Scott Smith, 1999; Berrymanet al., 2004; Kjarsgaard et al., 2005; cf. Harvey et al., 2006) foreach eruption is maintained here although craters were rapidlyinfilled by deposits from later stages of the same eruption. Themain kimberlite volcaniclastic packages were emplaced in amarine environment (cf. Kjarsgaard et al., 2006). Volcaniclasticmegaturbidity currents fed from dense, collapsed eruptioncolumns, were captured into the newly opened craters andformed the main infilling deposit. Major lateral facies variationsfrom coarse-grained, massive deposits within crater depocentresto thinner, finer-grained, stratified deposits on the crater margins,reflect variations in granular flow processes within volcaniclas-tic turbidity currents which interacted with the substantialnegative relief. Newly emplaced eruption packages weresusceptible to post-depositional slumping and mixing orreworking by subaqueous currents (e.g. storm currents; seealso Zonneveld et al., 2004; Kjarsgaard et al., 2006).

It is anticipated that the range in volcaniclastic depositcharacteristics and processes at the Orion Central complexrepresent only a subset of those expected for the entire Fort à laCorne kimberlite field. However, this study highlights adominant process which must be considered elsewhere at Fortà la Corne and in other kimberlite fields where crater-fill andextra-crater deposits are recognised. Understanding the internaltransportational and depositional processes associated withkimberlite deposit packages, arises from careful documentationof facies characteristics and their spatial distribution. Finally, themasking effect of alteration on primary depositional texturesshould be acknowledged and any emplacement study must befounded only on the limited preserved deposit characteristics.

Acknowledgements

This study is part of a postdoctoral program funded by DeBeers Canada, who are acknowledged along with their formerjoint venture partners (Shore Gold/Kensington Resources,Cameco and UEM) for access to facilities and drill core. Thecurrent Fort à la Corne Shore Gold-Newmont joint-venture isthanked for permission to publish this work, and the usefulcomments from S. Harvey and P. Du Plessis (Shore Gold) weregreatly appreciated. We are grateful for the fruitful discussionswith B. Jellicoe (whilst at Kensington, now at Great WesternDiamonds), S. Harvey (Shore Gold), B. Scott Smith (Scott SmithPetrology), J-P. Zonneveld (Geol. Surv. Canada) and C. Hetman

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(Mineral Resources) and for the logistical support provided byDe Beers staff at the Saskatoon warehouse led by T. French.

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