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Journal of the Geological Society , London , Vol. 165 , 2008, pp. 307318. Printed in Great Britain.

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Sedimentation of the Phyllopod Bed within the Cambrian Burgess Shale Formationof British Columbia

S. E . GABBOTT 1, J . ZALASIEWICZ 1 & D. COLLINS 21 Department of Geology, University of Leicester, Leicester LE1 7RH, UK (e-mail: sg21@le.ac.uk)

226 Belvedere Blvd, Toronto, Ontario, M8X1K1, Canada

Abstract: We provide the most detailed sedimentological log to date through the Phyllopod Bed of the mid-Cambrian Burgess Shale Formation of British Columbia, based on millimetre-scale logging of a suite of thinsections. The sedimentary facies is dominated by alternations of homogeneous mudstone and a coarser-grained, laminated, variably sandy and shelly mudstone that is locally micronodular. Most boundaries betweenthese two lithologies are gradational, and discrete ning-upwards turbidite units were rarely recognized. Sucha pattern is interpreted to indicate rapid sedimentation of up to decimetre-thick units at this location from pulsatory, quasi-continuous density currents consistent with earlier proposals of exceptional preservationthrough rapid burial; the density currents responsible were probably largely akin to mud-rich slurries, helpingexplain the transport and entombment of the fossils. The homogeneous mudstone units are characterized bynumerous distinctive lenses of pyrite framboids or subeuhedral crystals, previously interpreted as smallripples. Their 3D shape, however, suggests an origin as subspherical early diagenetic aggregates; their presentmorphology is consistent with the high levels of compaction inferred from the preservation of fossils.

The Burgess Shale is arguably the most celebrated fossil-bearingunit in the world. Since its discovery by Walcott in 1909 and particularly since the detailed re-descriptions of the faunas in the1980s it has served as a key example of both the preservation of soft-bodied faunas and the diversity of life soon after theCambrian evolutionary explosion. Non-mineralized BurgessShale fossil remains are principally composed of kerogenized lms (Buttereld 1990), sometimes with associated aluminosili-cates that according to Orr et al . (1998) replicated decay-pronetissues prior to decomposition.

The sedimentology of the Burgess Shale has received rela-tively little attention, despite the obvious signicance that thisaspect has for the processes that lead to exceptional preservation.Studies to date have interpreted the deposits as turbidites (e.g.Piper 1972), and these have been used to invoke rapid burial(obrution) of the fossils as a preservational mechanism (e.g.Piper 1972; Whittington 1975; Conway Morris 1986; Allison &Brett 1995). However, as turbidites are among the most commonof sedimentary facies worldwide, whereas Lagerstatten are bydenition rare, other factors to explain the exceptional preserva-tion have been suggested. More recently, these have included hypotheses to explain why Burgess Shale-type preservation (i.e.kerogenized organic remains in a siliciclastic sediment; seeButtereld 1990) has not been reported as a major taphonomic pathway after the Cambrian. One suggestion relies on a pre- ponderance of reactive and/or swelling clays in the sediments of the Cambrian Gondwanan continental margins; these clays wereheld responsible for prohibiting bacterial degradation of arthro- pod cuticle and other reasonably recalcitrant organic tissues(Buttereld 1995). Others (e.g. Allison & Briggs 1991, 1993;Orr et al . 2003) have suggested that after the Cambrian theincrease in the amount and complexity of bioturbation eliminated the deep-water low-oxygen taphonomic window, where charac-teristically Burgess Shale-type preservation is found. Gaines et al . (2005) accounted for Burgess Shale-type preservation in theMiddle Cambrian Wheeler Formation of Utah through a combi-nation of inuences that reduced sediment permeability, and thus

oxidant ux, to the extent that microbial decomposition wasseverely restrained. As well as restricted bioturbation owing tonear-bottom water anoxia, low permeability was thought to beeffected through deocculated clays, early precipitation of pore-occluding carbonate cements and an absence of coarse grainssuch as silts, microfossils, bioclasts and faecal pellets (Gaines et al . 2005).

Fig. 1. Schematic representation of the stratigraphic relationships between the platform deposits of the Cathedral Limestone and StephenFormations and the basinal deposits of the Burgess Shale Formation (datacompiled from Fletcher & Collins 1998). The Burgess Shale Formationcontains 10 members including the Walcott Quarry Shale Member, whichcomprises the Greater Phyllopod Bed and the interval studied herein, thePhyllopod Bed. WLM, Wash Limestone Member; GB, Ginger Bed;GML, Great Marrella layer; GEL, Great Eldonia layer.

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To shed more light on the processes involved in the depositionof the Burgess Shale we have sampled a large proportion of thePhyllopod Bed from one vertical section at the southern side of Walcotts Quarry. The sedimentary facies were logged from thinsections at a millimetre scale. The Phyllopod Bed is the best-

known and most fossiliferous unit of the Walcott Quarry Member (Burgess Shale Formation). The results of our analyses providesignicant constraints on the depositional processes involved,and allow insights into the mechanisms of fossil preservation atthis locality.

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Previous sedimentological studies

The Phyllopod Bed has been described as comprising sharp- based units of calcareous siltstone grading up via interlaminationinto mudstone by Piper (1972), who interpreted these units to be

turbidites, where the calcareous siltstone was derived from thenearby Cathedral Formation reef; the muddy deposits and fossilswere interpreted as eroded by the turbidity currents fromintermediate depths. Piper (1972) described a typical turbiditeunit as comprising, at its base, a lower calcareous siltstone with

Fig. 2. Graphic log alongside compositeimage of the thin sections of the Phyllopod Bed. Heights are in centimetres starting at0 cm, which is the base of the Phyllopod Bed just above the Ginger Bed. Whitechevrons on the thin-section image indicate positions where lateral movement along alamina was necessary to capture thecomplete image; it should be noted that inthese cases there is no stratigraphic break.Grey arrows show positions of sharp boundaries. Stratigraphic gap represents aninterval where no rock was collected or aninterval where the rock splintered so thatthin sections could not be made. Sliver missing indicates position where amaximum of 23 mm is missing as theshale splintered into small pieces unsuitablefor thin sectioning. The left-hand side of the graphic log indicates the presence of adiagenetic carbonate cement and the right-hand side of the graphic log indicatesfeatures such as pyrite lenses, shellfragments and bedding structures.

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distinctly irregular laminae (mudstone laminae being absent),overlain by alternating laminae of mudstone and well-packed calcareous siltstone, with a decrease upwards in the thickness,coarseness and packing of the siltstones, and an increase in the

thickness of mudstone laminae; upwards in each unit, mudstonelaminae were described as alternating with carbon-rich laminaeincluding some calcareous silt-sized clasts, this grading into thehighest part of the unit, comprising mudstone with a small

Fig. 3. Photographs to demonstrate the nature of facies seen in the Phyllopod Bed. Heights provided indicate height above the Ginger Bed for the base of each photograph shown. ( a ) The homogeneous mudstone facies from the Great Marrella layer. White arrows and the black arrow indicate pyrite lensesand a oating quartz grain, respectively. Height 1 cm; scale bar represents 2 mm. ( b ) Coarser poorly sorted facies: pale mottled layers represent varyingdegrees of diagenetic carbonate cement. Height 82.2 cm; scale bar represents 2 mm. ( c) Image showing the different textural fabrics (representing differentdegrees of cementation) within the coarser poorly sorted facies. Height 85.2 cm; scale bar represents 2 mm. ( d ) BSE image showing a coarser poorlysorted layer with carbonate cement in the centre and its gradational boundaries with the adjacent homogeneous mudstone. Height 86 cm; scale bar represents 500 m.

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proportion of carbonate and no visible laminae. Piper (1972)suggested that many of the unlaminated mudstone units towardsthe upper portion of the Phyllopod Bed were hemipelagic. Our interpretation of the basic sedimentological pattern (see below)differs substantially from this.

Allison & Brett (1995) have also reported details of deposits atWalcotts Quarry. They described intercalations comprising mas-sive thicker beds and nely laminated thinner beds. The laminaewere commonly seen to ne upwards, to have erosive bases with

small microscours, 23 mm across, and some lamina basescontained detrital quartz, carbonate fragments, and occasionalrip-up clasts. They identied small ripples, some formed from pyrite framboids (Allison & Brett 1995, g. 2c, p. 1080), and reported that in deposits from Walcotts Quarry pyrite is almostinvariably evenly distributed, with rare clustering, except for thin pyrite patinas associated with worm gut traces. Allison & Brett(1995) proposed that organisms preserved in the Phyllopod Bed were engulfed in high-density mudsilt ows; an interpretationconsistent with our conclusions. Gostlin & Miall (2005) reported calcisiltite layers intercalated with massive, sharp-based clay-richmudstones from the Greater Phyllopod Bed. They stated that the presence of massive beds and high clay contents were incon-sistent with deposition of the Burgess Shale via turbidity currents

and uidized mudows, and suggested that deposition of muddysediments and fauna occurred through settling after storm-gener-ated back-currents swept sediment into the basin.

Sampling and methods

The section targeted was the whole of Walcotts original Phyllopod Bed (Walcott 1912), which is just over 2 m thick at this site. The Phyllopod Bed begins immediately above the informally designated Ginger Bed, whichis an ochreous, pyritic arenaceous bed, and extends to the top separation plane of Walcotts Quarry (Fletcher & Collins 1998). The Phyllopod Bed constitutes part of the Walcott Quarry Member of the Burgess ShaleFormation (see Fletcher & Collins 1998); it is the most prolic of fossiliferous beds within this formation (Whittington 1985; Conway Morris1986), containing the classic Burgess Shale fauna, and was the principalfocus of previous studies by Piper (1972) and Allison & Brett (1995). Thelatter also studied material from Raymonds Quarry, which lies c. 35 mhigher up-section (Allison & Brett 1995). The Phyllopod Bed wasdiscovered by Walcott in 1909, and was further excavated by theGeological Survey of Canada (19661967). Subsequent excavations (from1993 to 2000) led by one of us (D.C.) extended 5 m down from Walcottsoriginal quarry oor (and coincident base of the Phyllopod Bed) to the topof the Wash Limestone. Thus the Greater Phyllopod Bed is a stratigraphicinterval extending from the top of the Wash Limestone to the top of thePhyllopod Bed and is about 7 m thick (Fig. 1).

The Phyllopod Bed has been logged in considerable detail in the eld by Fletcher & Collins (1998), who recognized signicant lateral variationin bed thicknesses. One of us (S.G.) joined the Royal Ontario Museumeld crew and collected samples from the Walcotts Quarry Member,

including a sequence of samples from the Phyllopod Bed. The log of Fletcher & Collins (1998, g. 4, p. 419) provided a framework in whichthe Ginger Bed comprises a marker bed that lies directly below thePhyllopod Bed, and heights are measured from 0 cm, which denes the base of the Phyllopod Bed (and the top of the Ginger Bed). The samplescollected from the Phyllopod Bed cover 60% of the total interval; thereare only four signicant gaps between 12 and 42 cm (30 cm missing), 59and 68 cm (9 cm missing), 70 and 78 cm (8 cm missing) and 168 and 186 cm (18 cm missing); here the rock splintered into small pieces ascollection was attempted. Despite this, the material collected representsthe most complete set of samples for detailed lamina-scale sedimentolo-gical analysis of the Burgess Shale yet collected. Two highly fossiliferouslayers were reported by Walcott (1912) and constitute useful marker horizons in the Phyllopod Bed; they are the Great Marrella layer (0

7.4 cm), which clearly identies the base of the section, and the Great Eldonia layer (125130 cm).

A suite of polished thin sections of the entire rock succession collected was prepared. The thin sections were logged at a millimetre scale using aPetroScope 1 to create the log shown in Figure 2. After carbon coating, backscattered electron (BSE) imagery was obtained using a Hitachiscanning electron microscope (S3600N). Elemental analyses of mineralswere determined by energy-dispersive spectrometry using an Oxford Instruments INCA system. Photographs of the thin sections (Figs 35)were obtained by placing the thin sections directly into a Durst M805

enlarger and exposing photosensitive paper.

Sedimentary facies

The sedimentary facies essentially comprises a continuum be-tween two end-members.

(a) The rst end-member is a ne-grained homogeneousmudstone facies (Figs 3a and 4a d) that originally was dom-inantly composed of clay minerals (illitesmectitekaolinite);these were recrystallized to muscovite chloritequartzalbiteduring greenschist-grade metamorphism (Powell 2003). Thisfacies includes a minor component of silt-sized quartz grainsthat, in general, become more numerous as the homogeneousmudstone facies grades into the coarser end-member described

below (Fig. 4c and d); some larger (sand-sized) matrix-supported (oating) quartz grains and shell fragments are also locally present (Figs 3a and 4ad). A feature of this facies is theoccurrence of lenses (up to 1000 m, but more commonly 200 500 m in length) composed of pyrite framboids and more rarely pyrite euhedra (Figs 3a, 4a,c and 6). Intervals over 7 cm thick of essentially massive, ungraded mud occur; these include theGreat Marrella layer (Fig. 2 (07.4 cm) and Fig. 3a) and theGreat Eldonia layer (Fig. 2 (125130 cm)).

(b) The second end-member is a coarser facies characterized by poorly sorted, subordinate quartz silt and sand grains (up to1.5 mm in diameter, but usually 150500 m in diameter) withshelly fossil fragments up to 4 mm in size within a mud matrix(Figs 3bd, 4e and 5a,d). The larger particles, both of quartz and

detrital carbonate, conspicuously oat within a ner mud matrix (Figs 4e and 5a,d). Where present, a locally abundantdiagenetic carbonate component overprints most of the primarylamination (Fig. 3d). Typically, this facies shows a mottled texture with pale carbonate lensoids surrounded by darker mudstone (usually the coarser end-member), but the carbonatelensoids also locally coalesce to give more massive carbonate-cemented layers between which there are bedding-subparallel,wispy, muddy intercalations (e.g. Figs 3b,d and 4e). The mottled textures are, in places, accentuated by stylolitic development (seePowell 2003). In the coarser, poorly sorted facies, pyrite isirregularly disseminated or forms bedding-parallel wisps, and only very rarely occurs as the discrete lenses typical of thehomogeneous mudstone facies.

These end-members, (a) and (b), intergrade in vertical succes-sion in both ning-upwards (e.g. Fig. 2) and coarsening-upwards(e.g. Figs 2 and 5b) units; broadly centimetre-scale trends arecommon (Fig. 2). Coarsening-upwards trends are preponderant(35 being identied, compared with 24 ning-upwards trends)suggesting the frequent incidence of waxing ow events. Clear interfaces that may represent breaks in sedimentation are rare(only eight were identied; see Figs 2 and 4) and, where present,they are commonly relatively ne-based. These might representtime gaps involving cessation of deposition; we cannot constrainthe duration of these gaps other than to note that neither identiable hemipelagic laminae nor bioturbated intervals areassociated with these interfaces. The intergradation between

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coarser and ner mud ranges from centimetre scale to millimetrescale. Higher in the succession (starting at 109 cm), and associated with a general increase in the proportion of coarser material, there are local intervals of inclined millimetre-scalelaminae up to 2 mm thick (Fig. 5c) that may represent ripplecross-lamination. We found no textures that could be clearlyassociated with bioturbation.

Pyrite lensesA characteristic feature of the homogeneous mudstone unitswithin the Burgess Shale are small (200800 m in length) bedding-parallel lenses, each composed of some dozens tohundreds of pyrite framboids or, less commonly, of euhedral tosubhedral crystals (see Figs 3a, 4a and 6). These have been previously observed and interpreted as small-scale ripples of transported pyrite grains (Allison & Brett 1995, g. 2C, p. 1080).

We have observed these on bedding surfaces, where they havea roughly circular distribution (Fig. 6c and f); consequently, their overall geometry is that of ovoids that are highly shortened alongthe vertical axis; this is not consistent with ripple-forms. As far as we could judge, these pyrite lenses showed no spatialassociation with fossils or organic fragments, although patinas of

pyrite are locally associated with both soft tissue and skeletalfragments in the Burgess Shale (e.g. Whittington 1975; ConwayMorris 1985, 1986; Allison & Brett 1995).

The textures seen by BSE imagery are generally consistent witha diagenetic origin of the framboids in the lenses, and the large sizeof most of the framboids (e.g. mean diameter for framboids in the pyrite lens shown in Fig. 6a is 9.7 m) suggests precipitationwithin the sediment rather than within the seawater column (seeWignall & Newton 1998). Powell et al . (2003) suggested that pyriteframboids in the Burgess Shale displayed signicant evidence of recrystallization and accordingly that the statistical analysis of framboid size as a palaeoredox indicator was limited. However, theframboids measured here from the lens in Figure 6a do not showthe recrystallization features reported by Powell et al . (2003), such

as solid spheroidal grains of similar size to framboids, and so weinterpret them as original. The pyritic lenses are strongly bedding- parallel even in mudstone that otherwise appears perfectly homo-geneous, and so growth of the framboids along some pre-existing bedding-parallel fabric can be precluded. Thus, in attempting aninterpretation, both the clustering of the framboids into the lensesand the alignment of the lenses need to be addressed.

Tentatively, we link the clustering of the framboids into thelenses with the rapid sedimentation we infer for the entire unit(see below). Thus, we envisage that the sudden burial of a massof sediment, initially containing a high content of (at least partlyoxygenated) entrapped seawater, might produce, eetingly (be-fore signicant compactional dewatering began), a broad rela-tively permeable zone of redox contrasts (i.e. between sediment

particles and seawater) that led rst to initial random precipita-tion of framboids. Subsequently, we suggest that rapid diffusionof iron and sulphide ions to local centres of precipitation would have taken place, the diffusion paths being driven by concentra-tion gradients produced by the pyrite crystallization process itself (Fig. 7); roughly spherical aggregates of pyrite framboids would result. Subsequent application of the considerable amount of compaction (a minimum compaction ratio for the Burgess Shaleof 8:1 was inferred from fossils by Whittington (1975)) would

result in the transformation of the spherical aggregates of framboids into highly attened ellipses. Thus, their present shapeand alignment, in this interpretation, reects compaction directly,and bedding only indirectly.

Interpretation and discussion

The pattern of lamination observed does not accord with the previously published sedimentological description of WalcottsPhyllopod Bed as a succession of rhythmic couplets of a simpleturbidite model where each couplet represents sedimentationfrom a discrete turbidity current (Piper 1972). Rather, we haveobserved a considerably less ordered pattern that shows asuccession of gradations between relatively coarser and relatively

ner sediment with reverse graded intervals slightly morecommon than normally graded intervals. Reverse grading and oating outsized clasts and bioclasts in an otherwise ungraded mud (facies (a) described above) seems also not consistent withthe model proposed by Gostlin & Miall (2005) of settling of material from the water column after storms.

Likewise, the facies pattern we have observed seems notconsistent with deposition either by hemipelagic processes or from more or less continuous sea-oor currents (i.e. as contour-ites). Modern hemipelagites are mostly intensely bioturbated and so do not provide a good comparison. Better comparison is madewith hemipelagites described from early Palaeozoic basins; for example, those from the central Welsh basin (Cave 1979; Davieset al . 1997). Here, hemipelagites that accumulated on an

essentially anoxic sea oor, as were prevalent in those times,show a clearly laminated structure with organic-rich (pelagic)laminae alternating with clastic laminae deposited from nephe-loid plumes. At intervals when the sea oor was oxygenated thislamina structure was visibly disrupted by bioturbation. None of the deposits we describe resemble this widespread early Palaeo-zoic facies. Similarly, although our idealized log (Fig. 8) super-cially resembles the idealized contourite of Stow et al . (2002: p. 18, g. 10) the gradational boundaries of the latter areachieved through pervasive bioturbation, a phenomenon that wehave not observed in our material from the Phyllopod Bed.

The pattern observed, with only eight boundaries where asignicant break in deposition may be inferred in the material wehave studied (over 60% of the Phyllopod Bed), is more consistent

Fig. 4. Sharp boundaries in the Phyllopod Bed. Heights provided indicate height above the Ginger Bed for the base of each photograph shown. ( a )Homogeneous mudstone facies with three conspicuous pyrite lenses (white arrows) and a shell fragment (black arrow) sharply overlying less well-sorted,coarser mudstone facies; this is the sharpest, most distinct boundary within the sampled Phyllopod Bed. Height 57.7 cm; scale bar represents 2 mm. ( b )Relatively sharp boundary lying just above the large carbonate clast (note large bright quartz clast just to right of this) towards bottom of image, betweenner (below) and coarser (above) mudstone layers. Upper half of image shows gradational contacts between ner and coarser layers, both of whichcontain oating clasts (e.g. shell fragment labelled with white arrow). Height 7.4 cm just above the Great Marrella layer; scale bar represents 2 mm.(c) Rapidly gradational boundaries between coarser (with abundant silt and ne sand) and ner mudstone layers. (Note pyrite lenses (white arrows) inner mudstone layer and large detrital quartz clast (white) in the coarser layer.) Height 68.5 cm; scale bar represents 2 mm. ( d ) BSE image of moderatelysharp boundary between coarser mudstone with a little carbonate cement (below) and ner mudstone layers. Poorly developed pyrite lens (white arrow)and oating quartz clast (black arrow). Height 79 cm; scale bar represents 500 m. (e) Image showing the variable nature of boundaries between layersof different grain size from sharp (white arrow) and irregular gradations (bottom and top parts of image). (Note the oating shell debris.) Height143.8 cm; scale bar represents 2 mm.

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with pulsatory deposition from waxing and waning semi-contin-uous density currents producing successive packets of rapidlyaccumulated sediment (see Best et al . 2005). Some of these unitswould have been at least decimetres thick prior to compaction;for example, the Great Marrella Bed (here 7 cm thick post-compaction).

In terms of the Bouma model the single intervals most closelycompare with units T d (the millimetre-scale coarse ne inter-gradations) and T e (the homogeneous mudstones) with frequentgradations between these. Locally, near the top of the sectionstudied, there are thin intervals consistent with the rippled T cunit. However, such comparisons with the Bouma model (or with

Fig. 5. Gradational boundaries in the Phyllopod Bed. Heights provided indicate height above the Ginger Bed for the base of each photograph shown. ( a )Typical section through the upper part of the Phyllopod Bed showing rapid gradational boundaries between units of different grain size and sorting, and variable expression of the superimposed micronodular, carbonate cement fabric in the coarser layers from discrete bedding-parallel laminae to irregular mottling. Scattered pyrite lenses in ner layers (white arrows) and oating quartz clasts and shell fragments throughout. Height 154 cm; scale bar represents 2 mm. ( b ) Typical gradational coarsening-upwards trend in generally homogeneous silty mudstone. Height 47 cm; scale bar represents 1 mm.(c) Overall ning-up succession with lenticular and wispy lamination (indistinct ripple forms?) in lower part of image. Height 109 cm; scale bar represents2 mm. (d ) Lower half of image shows silty mudstone with large oatingshell fragment (white arrow); this facies gives way upwards to alternating layersdiffering in grain size and with variable degrees of micronodular, carbonate cement (pale). Height 165.3 cm; scale bar represents 2 mm.

Fig. 6. BSE images of pyrite lenses. ( a ) Cross-section through a pyrite lens in the Great Marrella layer composed of framboids of varying sizes. Scale bar represents 100 m. (b ) Close-up of pyrite lens shown in ( a ): the framboids are moderately disordered but show no evidence of overgrowth or alteration;microcrystals are euhedral to subhedral. Scale bar represents 50 m. (c) Bedding-parallel image to show a pyrite lens in the Great Marrella layer. (Notethat the framboids have a roughly circular distribution parallel to bedding.) Scale bar represents 100 m. (d ) Cross-section through a pyrite lens where the pyrite crystals are more closely packed than framboids forming the lens in ( a ) and (b ). Height c. 89.5 cm; scale bar represents 100 m. (e) Close-up of the pyrite lens shown in ( d ), showing pyrite crystals ranging from anhedral to euhedral (octahedral and cubic) in habit. Scale bar represents 50 m. (f )Bedding-parallel image to show a pyrite lens in the Great Eldonia layer. (Note the circular distribution of the octahedral pyrite crystals.) Scale bar represents 30 m. (g) Framboids in pyrite lens from the Great Eldonia layer where the microcrystals are fairly disordered: these are moderately tightly packed in the largest framboid but are loosely packed in the other framboids; microcrystals are cubo-octahedral. Scale bar represents 10 m.

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the ner subdivisions of the T d and Te units proposed by Piper (1978) and Stow (1979)) are probably an oversimplication, for,characteristically in the Phyllopod Bed, over-sized quartz grainsand shell fragments commonly oat within ner-grained sedi-ment accompanying the macrofaunal animal remains. This

indicates that the density current possessed, at least over thedistance between the source of the large clasts and the aggradingsurface (see Branney & Kokelaar 2002), sufcient competence totransport such material. This in turn implies deposition not fromdilute turbidity currents (by sedimentation of mud particles fromsuspension), but from denser, mud-rich suspensions that, at leastintermittently, were perhaps akin to slurries, or slurry-ows(i.e. ows transitional between turbidity currents and debrisows: Lowe & Guy 2000; Lowe et al . 2003) (see also Mulder &

Alexander 2001; Amy et al . 2006). Given the frequency of bothreverse and normal grading in the Phyllopod Bed, we prefer interpretations involving progressive aggradation of the waxingand waning of such ows rather than models involving en-massefreezing of high-density, non-turbulent currents (e.g. McCave &Jones 1988).

For comparison, in mud-dominated turbidite deposits, such asthose of the Welsh Basin (Davies et al . 1997), that portion of theturbidity currents responsible for the deposition of the Bouma Dand E intervals was typically insufciently competent to transportand entomb graptolite rhabdosomes (which would mostly have been hydrodynamically lighter than most of the Burgess animals,and thus more easily transported). Graptolites in these rocks aretypically found as current-sorted accumulations in rippled Bou-

ma C intervals (Davies et al . 1997; Zalasiewicz 2001) of turbidite units, whereas overlying Bouma E mud layers arerelatively well-sorted, ne-grained and contain neither trans- ported graptolite remains nor outsized mineral grains (Fig. 8).

This variation on the standard turbidite model, as regards thePhyllopod Bed, may well be a factor in the exceptional fossil preservation observed within this unit (see below). The best preservation of non-mineralized fossils occurs in intervals of theBurgess Shale that are within the ner-grained, more homoge-neous units within our classication; for example, the Great Marrella layer and the Great Eldonia layer. Our interpretationof the depositional process is consistent with the concept of rapid burial (at least within the interval that we have studied) outlined by earlier workers (e.g. Whittington 1975, 1980; Conway Morris1986). In addition, our interpretation of these units as having been deposited from relatively dense slurries (in which turbu-lence may have been damped), rather than as typical Bouma Eunits, is consistent with the size of the animals or carcasses beingtransported (Fig. 8) and the lateral variability in bed thicknessesrecognized by Fletcher & Collins (1998).

Furthermore, the location of these units within a broader interval (the Phyllopod Bed), which shows signs of having beenrapidly accumulated overall, strengthens the argument that rapid burial is a key factor in exceptional preservation at this locality.If the Phyllopod Bed was indeed deposited effectively as a smallnumber of units, from pulsed, dense, mud-rich slurries, and if thecompaction factor of . 85% deduced from the entombed fossils(Whittington 1975) and the geometry of the pyrite lenses (seeabove) is broadly correct, then one may envisage a geologicallyinstantaneous (and thus catastrophic) accumulation of deci-metre- to metre-scale thicknesses of sediment. The entombed animals would be prevented from oating away as they accumu-lated decay-generated gases and many would also thus beinstantly taken below the highly bacterially active surface layersof sediment.

This scenario contrasts sharply with typical centimetre- or decimetre-scale turbidites that, again drawing analogy with theWelsh Basin, were separated by decadal or centennial intervalsduring which slow hemipelagic sedimentation and sea-oor chemical and biological activity took place (see Fig. 8; Cave1979; Davies et al . 1997). All of the laminated intervals of the

Fig. 8. Schematic logs comparing the distribution of clastic particles(including fossils) between a typical early Palaeozoic turbidite facies,such as the early Silurian strata of central Wales, ( a ) and the Phyllopod Bed (b ). It should be noted that the mudstone layers in the Phyllopod Bed contain outsized clasts (including fossils), whereas these are absentfrom the turbidite mudstone layers in the Welsh example depicted.Hemipelagite facies have not been recognized by us in the Phyllopod Bed, but are an integral and distinctive component of the Welshturbidites, either as laminated organic-rich layers laid down in anoxicsea-oor conditions (AHP) or penecontemporaneously oxidized and burrowed layers (OHP), if laid down on an oxygenated sea oor.

Fig. 7. Schematic illustration showing one possible model for theformation of the lenses comprising pyrite framboids, and more rarelyeuhedral to anhedral crystals. ( a ) Highly uncompacted sediment layer as

mudseawater mixture following deposition from density currents; initialrandomly distributed precipitation of pyrite framboids. ( b ) Creation of Fe2 and sulphide ion species concentration or diffusion gradientsfavouring further pyrite nucleation in localized, roughly sphericalvolumes of sediment. ( c) Compaction of the sediment by . 85% to create pyrite framboid clusters as attened ovoids (pancake shapes).

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Phyllopod Bed have a texture generally consistent with tractionallamination. We observed no interval that appeared to representhemipelagic deposition, as is typical, for instance, of the deep-water graptolite shales of the Early Palaeozoic (e.g. compareArmstrong & Coe 1997; Davies et al . 1997, plate 10AC, p. 65).Similarly, we have seen no evidence of microbial mats asreported by Powell et al . (2003) and Caron & Jackson (2006),although these come mostly from below the Phyllopod Bed. Wecannot preclude the existence of brief pauses to allow the growth

of microbial mats, or to allow brief intervals of colonization(sporadic burrows have been noted in this interval; Fletcher &Collins 1998), but we saw no evidence of these associated withthe sharp interfaces recognized in our material.

How do our data reect upon the status of the Burgess Shalefossils as census life assemblages (i.e. a fossil assemblagecomposed of species belonging to a single community and preserved in the environment in which they lived), deathassemblages (i.e. a transported fossil assemblage), or time-averaged assemblages (i.e. an accumulation of fossil species,over a period of time)? The Great Marrella layer and the Great Eldonia layer are striking examples of the homogeneous mud-stone facies, with evidence of subtle waxing and waning ow,and a lack of interfaces, that we deduce represents material

transported as substantial gravity-driven units akin to slurryows. Hence, the animals entirely enclosed in these layers weresubjected to some transport and were not buried in situ . It is perhaps possible that some animals were able, if buried in situ ,to migrate up through the newly deposited layer. However, wesee no evidence of, for instance, escape traces, nor of fossilsassociated (as colonizers) with the few sedimentary interfacesthat we have recognized. Moreover, the fauna in the Great Marrella layer and Great Eldonia layer includes taxa (e.g.Scenella , Selkirkia and algae: Caron & Jackson 2006) unlikely to be capable of moving through tens-of-centimetres thickness of suddenly deposited sediment.

However, we cannot constrain the distance of transport, whichmay have been minimal, nor the relative coherence of theassemblages as communities. Caron & Jackson (2006) havedemonstrated that single beds in the Great Phyllopod Bed (including the Great Marrella layer and Great Eldonia layer)contain articulated organisms, interpreted as census assemblagesand in situ dissociated and completely dissociated organisms,interpreted as time-averaged assemblages. Their analyses of more than 50 000 specimens indicated that, although manyorganisms were moved, they were not transported out of their community habitat, and hence the often cited concept of a pre-slide environment (where the animals lived) and a post-slideenvironment (where they ended up) was not valid (Caron &Jackson 2006). Some degree of transport is consistent withreported occurrences of numerous organisms preserved atvarious angles with respect to bedding, the existence of sediment between appendages and the preferred (current-aligned) orientations of Selkirkia tubes (Conway Morris 1986).However, we cannot preclude that some of the fossils in thePhyllopod Bed (although not those in the Great Marrella layer and Great Eldonia layer) may have been buried in situ ; this issuggested by the presence of trilobite and other arthropod moults in the Great Phyllopod Bed (D.C., personal observation;Caron & Jackson 2006).

The rate of deposition we infer, together with the absence of recognizable hemipelagic deposits, suggests that it is difcultto constrain the oxygenation state of the Phyllopod Bed (seeCave 1979; Davies et al . 1997). The recognition of sporadic burrowed intervals more broadly within the Burgess Shales

(e.g. Allison & Brett 1995; Powell et al . 2003; Caron &Jackson 2006) suggests a sea oor that was at least intermit-tently oxic, and the presence or absence of burrows has beenused to infer changes in basin oxicity (Caron & Jackson2006). Our analysis suggests that the perceived absence of burrows may here be due to inhibition of colonization throughrapid sediment accumulation, where bioturbators were unableto keep pace with sediment inux, as much as by sea-oor and/or sediment anoxia.

The pervasive occurrences of the distinctive pyrite lenses and our inference of a diagenetic origin for them (see above) suggestsa distinctive geochemical environment of early diagenesis in theBurgess Shale perhaps linked with the distinctive mode of deposition we infer. The originally uncompacted state of thedeposits inferred from our results and from fossils that are found at various angles to bedding (locally very high angles despitesubsequent compaction) suggests that the preservation model of Gaines et al . (2005), for the Wheeler Formation Lagerstatte of Utah, cannot be used to explain fossil preservation in the BurgessShale. Those workers showed that a number of factors resulted insediments with much reduced permeability, a situation further compounded by early, ubiquitous pore-occluding, carbonatecementation. Gaines et al . (2005) suggested that preservation of

kerogenized carbon lms in the Wheeler Formation sedimentswas the result of low permeability that lowered oxidant uxsufciently to restrict microbial decay. However, rapid sedimen-tation from mud-rich density currents would have produced relatively high initial levels of porosity and permeability in theBurgess Shale. This is suggested both by estimates of thecompactional attening of enclosed fossils (Whittington 1975)and pyrite framboid aggregates (see above) and by the high porosities of modern muddy sediments immediately after deposi-tion (commonly up to 80% by volume: Singer & Muller 1983, p. 188). Subsequent burial would squeeze out pore uid, but probably not on the short time scale necessary for effectiveoperation of a low-permeability preservational mechanism. Inaddition, unlike the Wheeler Formation, the Burgess Shale of thePhyllopod Bed does contain skeletal bioclasts and larger detritalgrains that would have served to increase primary porosity. Our observations thus suggest burial rate rather than porosity as amajor factor.

High levels of compaction may have produced signicantdiagenetic ssility in the Burgess Shale. This, with heating to250280 8C (I. Harding, pers. comm., in Buttereld 1995) upon burial to 10 km (Powell 2003), could have produced the reported S1 bedding-parallel cleavage (Powell 2003) by mimetic recrys-tallization on the ssility. As these rocks were not isoclinallyfolded prior to the Mesozoic Laramide Orogeny, and as over- burden (lithostatic) pressure itself does not produce a directionalfabric, this might represent a better explanation for the cleavagereported from these rocks.

Our observations and inferences help provide an explanationfor the exceptional preservation in the Burgess Shale, but maynot offer much insight into the wider problem of the concentra-tion of Lagerstatten around continental margin and shelf-basinenvironments in the Cambrian (Allison & Briggs 1993). The perceived high relative abundance of Cambrian Lagerstatten has been linked to physico-chemical conditions, such as specic claychemistries (Buttereld 1995) and factors reducing permeability(Gaines et al . 2005), or to a post-Cambrian increase in theamount and complexity of bioturbation, which effectively elimi-nated the deep-water slope-basin taphonomic setting (Allison &Briggs 1991, 1993, 1994; Orr et al . 2003; but see Aronson 1992;Pickerill 1994). Whatever the solution to this general problem,

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detailed sedimentological analysis can shed substantial light uponlocal instances of exceptional preservation.

The eldwork for this project could not have been done without theagreement and support of the Canadian Parks Service. S.G. in particular thanks eldwork crew members J. B. Caron, D. Garc a-Bellido, K.Gostlin and M. Myers. Financial help was provided by a University of Leicester Research Support Grant to S.G. and a Royal Society ResearchEquipment Grant to S.G. and J.Z. L. Barber drafted Figure 2. We thank P. Allison and L. Amy for their incisive and helpful reviews, T. Fletcher for valuable comments on an earlier draft, and M. Branney, J. Macquaker,A. Page and J. Schieber for useful discussions.

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Received 6 February 2007; revised typescript accepted 14 June 2007.Scientic editing by Howard Falcon-Lang

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