Dynamic palaeoredox and exceptional preservation in the ... et al 2011.pdfthe succession. A diverse skeletonized benthic fauna, including various polymerid trilo-bites, hyolithids,

Dynamic palaeoredox and exceptional preservation in theCambrian Spence Shale of Utah

DANIEL E. GARSON, ROBERT R. GAINES, MARY L. DROSER, W. DAVID LIDDELL AND AARON SAPPEN-

FIELD

Garson, D.E., Gaines, R.R., Droser, M.L., Liddell, W.D. & Sappenfield, A. 2011: Dynamicpalaeoredox and exceptional preservation in the Cambrian Spence Shale of Utah. Le-thaia, DOI: 10.1111 ⁄ j.1502-3931.2011.00266.x

Burgess Shale-type faunas provide a unique glimpse into the diversification of metazoanlife during the Cambrian. Although anoxia has long been thought to be a pre-requisitefor this particular type of soft-bodied preservation, the palaeoenvironmental conditionsthat regulated extraordinary preservation have not been fully constrained. In particular,the necessity of bottom water anoxia, long considered a pre-requisite, has been the sub-ject of recent debate. In this study, we apply a micro-stratigraphical, ichnologicalapproach to determine bottom water oxygen conditions under, which Burgess Shale-typebiotas were preserved in the Middle Cambrian Spence Shale of Utah. Mudstones of theSpence Shale are characterized by fine scale (mm-cm) alternation between laminated andbioturbated intervals, suggesting high-frequency fluctuations in bottom water oxygena-tion. Whilst background oxygen levels were not high enough to support continuousinfaunal activity, brief intervals of improved bottom water oxygen conditions punctuatethe succession. A diverse skeletonized benthic fauna, including various polymerid trilo-bites, hyolithids, brachiopods and ctenocystoids suggests that complex dysoxic benthiccommunity was established during times when bottom water oxygen conditions werepermissive. Burgess Shale-type preservation within the Spence Shale is largely confinedto non-bioturbated horizons, suggesting that benthic anoxia prevailed in intervals, wherethese fossils were preserved. However, some soft-bodied fossils are found within weaklyto moderately bioturbated intervals (Ichnofabric Index 2 and 3). This suggests that Bur-gess Shale-type preservation is strongly favoured by bottom water anoxia, but may notrequire it in all cases. h Anoxia, Burgess Shale, Burgess Shale type-preservation, LangstonFormation, Spence Shale Member, Utah.

Daniel E. Garson [[email protected]], Department of Earth Sciences, University ofCalifornia, Riverside, CA 92521, USA; Robert R. Gaines [[email protected]],Geology Department, Pomona College, 185 E. Sixth St., Claremont, CA 91711, USA; MaryL. Droser [[email protected]], Department of Earth Sciences, University of California,Riverside, CA 92521, USA; W. David Liddel [[email protected]], Department ofGeology, Utah State University, Logan, UT 84322-4505, USA; Aaron Sappenfield[[email protected]], Department of Earth Sciences, University of California,Riverside, CA 92521, USA; manuscript received on 14 September 2010; manuscript acceptedon 04 February 2011.

Burgess Shale-type (BST) biotas are named after theworld famous locality from which they were firstdescribed, but represent a global, if rare, phenomenon.Well-described BST localities are known from NorthAmerica, Europe, Siberia, Asia and Australia and arelargely confined to Series 2 and Series 3 of the Cam-brian (Conway Morris 1989a; Butterfield 1995). Thesedeposits provide a unique window on the early diver-sification of the Metazoa (Conway Morris1989a,1992). BST assemblages are characterized by a com-mon preservational style, in which soft tissues oforganisms were conserved as carbonaceous compres-sions in fine-grained marine sediments (Butterfield1995; Gaines et al. 2008). However, the mechanismscontrolling this taphonomic pathway, referred to asBST preservation, and this pathway’s restriction intime are not fully understood. Insight into the

preservation of BST assemblages may be just asimportant as the exceptional fossils themselves as itspeaks directly to the unique environmental condi-tions that were widespread in the marine realm at thetime.

Anoxia, at least within the sediments, has been con-sidered a necessary pre-requisite for BST preservation(Allison & Brett 1995; Butterfield 1995; Gaines et al.2005). Anoxia may increase the preservation potentialof soft tissues in two ways: by preventing the directscavenging of tissues by animals, and by potentiallyhelping to slow the normal processes of microbialdecay (Allison & Briggs 1991). Oxygen is the mostenergetically favourable oxidant for degradation oforganic matter (Berner 1981) and soft tissues aredegraded quickly under oxic conditions. However,anoxia alone is not sufficient to explain the

DOI 10.1111/j.1502-3931.2011.00266.x � 2011 The Authors, Lethaia � 2011 The Lethaia Foundation

preservation of soft tissues as oxidation of organicmatter by sulphate-reducing bacteria can occur atcomparable rates to oxic degradation (Foree &McCarty 1970; Henrichs & Reeburgh 1987; Allison1988; Lee 1992). Actualistic experiments using shrimpand other non-mineralizing taxa have also shown thatdegradation of soft tissue occurs rapidly under anoxicconditions (Allison 1988; Briggs & Kear 1994).Although anoxia is generally considered as a necessarypre-requisite, anoxia alone is not sufficient to accountfor BST preservation. Some additional mechanism isrequired for the early diagenetic stabilization of labiletissues. This preservational mechanism remains con-troversial (Butterfield 1995; Petrovich 2001; Gaineset al. 2005).

The necessity of anoxic conditions as a pre-requisitefor BST preservation is not universally accepted. Onthe basis of trace metal ratios interpreted as palaeore-dox indicators, Powell et al. (2003) argued for mini-mally oxic conditions during deposition of much ofthe Burgess Shale, including intervals containing BSTpreservation. In addition, Caron & Jackson (2006,2008) have argued for in situ preservation of BurgessShale assemblages; obviously, consistently oxic or dys-oxic bottom water conditions would be required tosustain a benthic fauna. This interpretation was basedon taphonomic evidence suggesting ‘minimal’ trans-port of the Burgess Shale biota (Caron & Jackson2006).

Allison & Brett (1995) reported that bioturbationand BST preservation occur in mutually exclusivehorizons in the Burgess Shale and argued for oscilla-tions between anoxic and dysoxic bottom waters dur-ing its deposition. Anoxic benthic conditions wereinterpreted to have prevailed with periodic and strati-graphically brief (cm) episodes of bottom water oxy-genation that allowed colonization by an in situbenthic fauna.

Evidence from the Lower Cambrian ChengjiangBST deposit of South China suggests a complex palae-oredox history. Dornbos et al. (2005) found that sedi-ments of the Maotianshan shale in the Yu’anshanFormation, which contain BST preservation, arealmost entirely unbioturbated whilst underlying sedi-ments of the Shiyantou Formation, which is not aBST deposit, are consistently moderately bioturbated.However, Zhang et al. (2007) found examples of tracefossils in direct contact with soft-bodied fossils in theYu’anshan Formation, demonstrating that it is possi-ble for BST preservation to occur in close associationwith sediments deposited under bottom water withoxygen high enough to support a benthic fauna, atleast periodically. The orientations of Chengjiang fos-sils indicate transport of most assemblages; only rarelyfossils appear to have been preserved in situ (Zhang &

Hou 2007), which again would have required preser-vation in close temporal and spatial association withat least marginally oxygenated bottom waters.

Gaines & Droser (2005) described three distinctoxygen related microfacies within superficially monot-onous claystones of the Middle Cambrian WheelerFormation, a BST deposit, which occurs in the HouseRange and Drum Mountains of Utah, based on ich-nofabric index (i.i.), a semi-quantitative method todistinguish relative amounts of bioturbation (Droser& Bottjer 1986) and diagnostic body fossil assem-blages. This model was subsequently applied to theMarjum Formation, which also occurs in the HouseRange (Gaines & Droser 2010). These studies demon-strated that nearly all occurrences of BST preservationin both formations occur within laminated intervals,entirely lacking bioturbation (i.i.1), consistent withbenthic anoxia. In addition, this anaerobic microfaciesincludes some skeletal body fossils, dominantly agnos-tid trilobites (Gaines & Droser 2005). A dysoxicmicrofacies was defined by the presence of weaklydeveloped, shallow ichnofabrics (i.i.2 and i.i.3) and itincludes a low-diversity shelly benthic fauna. At themargin of the dysoxic environment, the trilobite Elra-thia kingii occurs in dense (up to 500 individuals ⁄ m2)monospecific assemblages that represent an exaerobicmicrofacies (Savrda & Bottjer 1987) at the transitionbetween laminated (i.i.1) and weakly bioturbated(i.i.2) sediments (Gaines & Droser 2003). These threemicrofacies, recognized at the millimetre scale, may beclosely interbedded (mm-cm scale), and representoxygen levels that alternated between anoxic environ-ments conducive to the preservation and low-oxygenenvironments conducive to the establishment of ben-thic metazoan communities. Applied more broadly,this model suggests that BST preservation occurredprimarily through transport from the living environ-ment to the anoxic preservational trap. Gaines & Dro-ser (2005, 2010) recognized a proximal–distal gradientin composition and articulation of BST assemblages,and also documented examples of in situ preservation,although such occurrences are most rare.

Here, we apply a micro-stratigraphical approach tothe Spence Shale Member of the Langston Formation,a BST deposit of early ‘Middle’ Cambrian age, occur-ring in the lower part of the yet unnamed CambrianSeries 3 in the Wellsville Mountains of Utah (Fig. 1),to determine the palaeoredox context of diverse BSTassemblages known from that deposit (Robison 1991;Briggs et al. 2008). The Spence Shale is similar in pal-aeogeography and depositional environments (Liddellet al. 1997) to the Wheeler Formation, and it containsa similar fossil biota within a fine-grained claystonelithofacies, providing an opportunity for comparisonwith previous findings.

2 Garson et al. LETHAIA 10.1111/j.1502-3931.2011.00266.x

Geological setting

Burgess Shale-type preservation occurs worldwide inLower and Middle Cambrian (Cambrian Series 2–3)strata with older occurrences known from Proterozoicstrata of several palaeocontinents (Butterfield 1995).Middle Cambrian occurrences are largely confined toNorth America and represent the deposition in conti-nental slope or basin settings on what was at the timethe northern margin of Laurentia (Conway Morris1989b; Butterfield 1995). Three Middle CambrianBST deposits occur in Utah: the Spence Shale Memberof the Langston Formation, the Wheeler Formationand the Marjum Formation. During the Middle Cam-brian, the present-day western margin of Laurentiawas a passive margin with three distinct marine faciesbelts (Robison 1976). A near-shore inner detrital beltwas separated from a more distal fine-grained outerdetrital belt by a broad carbonate belt. BST deposits inUtah and elsewhere are characterized by depositionin the outer detrital belt, near the distal margin of thecarbonate belt (Robison 1991).

The Spence Shale is exposed in Northern Utah inthe Wellsville Mountains and in Southern Idaho inthe Bear River Range (Liddell et al. 1997). The SpenceShale is the Middle Member of the Langston Forma-tion, and is dominated by shale with minor intervalsof limestone at most localities. The Upper and LowerMembers of the Langston Formation are both domi-nantly limestone representing deposition within the

carbonate belt. The Spence is within the Peronopsisbonnerensis agnostid trilobite zone and is older thanthe Burgess Shale, Wheeler and Marjum Formations.

The Spence Shale Member is comprised of a seriesof metre-scale shallowing-upward cycles, which repre-sent parasequences (Liddell et al. 1997). Each cycle isexpressed as shale passing upwards into lime mud-stones, grainstones or nodular limestones recordingthe influence of the impinging carbonate platform.An onshore-offshore trend is apparent within thisSpence Shale of the Wellsville Mountains with locali-ties in the southwest lying proximal to a carbonateplatform, and becoming deeper and more distaltowards the northeast (Liddell et al. 1997). The num-ber of metre-scale shallowing-upward cycles expressedand the type of carbonate capping the cycles variesfrom locality to locality based on proximity to the car-bonate platform; sections interpreted to be deeperexpress fewer, thicker, cycles. Miners Hollow repre-sents the most distal deposition within the WellsvilleMountain localities and consists of six metre-scalecycles, the lowest two of which were capped by limemudstone, and all higher cycles capped by nodularlime mudstone and fossiliferous wackestone interbed-ded with shale (Liddell et al. 1997; Fig. 2).

The Spence Shale contains a diversity of biominer-alizing fossils including multiple polymerid trilobites,agnostid trilobites, hyolithids, brachiopods, echino-derms including eocrinoids and ctenocystoids andsponges (Walcott 1908; Resser 1939; Robison & Sprin-kle 1969; Gunther & Gunther 1981; Sprinkle 1985).In addition, the Spence Shale also preserves a numberof non-mineralizing organisms preserved as carbona-ceous compressions, including a diverse array ofarthropods, eldoniids, annelids, priapulids, algae andcyanobacteria (Resser 1939; Robison 1969; ConwayMorris & Robison 1986, 1988; Robison & Wiley 1995;Briggs et al. 2008).

Using trace fossils as a proxy forbottom water oxygenation

A relationship between bottom water oxygenation andextent and depth of bioturbation was first describedby Rhoads & Morse (1971), who developed an oxy-gen-related biofacies model in modern environments.An aerobic biofacies, defined by bottom water dis-solved oxygen concentrations above 1.0 ml ⁄ l, is char-acterized by great depth and extent of bioturbationand a diverse benthic fauna. Dissolved oxygen concen-trations between 1.0 ml ⁄ l and 0.1 ml ⁄ l are consid-ered dysoxic and are characterized by a low-diversitybenthic fauna and reduced depth and extent of

41˚

39˚

38˚

37˚

42˚

40˚

41˚

39˚

38˚

37˚

40˚

112˚ 111˚113˚114˚

112˚ 111˚ 110˚ 109˚113˚114˚

SaltLake City

100 km

Ogden

N

Provo

Utah

15

70

80

Fig. 1. Geographic map of Utah, USA, showing location of theSpence Shale localities in the Wellsville Mountains (star).

LETHAIA 10.1111/j.1502-3931.2011.00266.x Exceptional preservation in the Spence Shale 3

bioturbation. Oxygen concentrations below 0.1 ml ⁄ lare considered anoxic and characterized by a lack ofbioturbation and the absence of a benthic fauna. Sub-sequently, Savrda et al. (1984) developed a similarmodel distinguishing anaerobic from dysaerobic bio-facies based on trace fossils in modern marine sedi-ments that could be applied to ancient sediments.Savrda & Bottjer (1986) developed a model for creat-ing relative dissolved oxygen curves based on size andtype of trace fossil and their cross-cutting relation-ships, allowing further differentiation of oxygen levelswithin the dysaerobic zone. Savrda & Bottjer (1987)defined the exaerobic biofacies, which is characterized

by a monotypic skeletonized fauna appearing in greatabundance at the micro-stratigraphic boundarybetween laminated and weakly bioturbated sediments.This environment, lying at the distal margin of oxy-genated bottom waters promotes the development ofextensive populations of sulphur-oxidizing bacteria,which may provide a food source for metazoansadapted to low-oxygen concentrations (e.g. Savrda &Bottjer 1986; Gaines & Droser 2003).

The ichnofabric index (Droser & Bottjer 1986) pro-vides a semi-quantitative means of assessing the rela-tive extent of bioturbation. Completely undisturbed,laminated sediments are assigned an ichnofabric index(i.i.) of 1, and sediments which have been completelyhomogenized by bioturbation are assigned to i.i.6,with intermediate values representing specific rangesof percentage of cross-sectional area of disturbed lami-nation. Ichnofabric indices have become a standardtool for determining relative bottom water oxygena-tion for palaeoecological studies (e.g. Twitchett &Wignall 1996; Boyer & Droser 2007; Herringshaw &Davies 2008; Kakuwa 2008). Because ichnofabricindices are determined independently of specific tracefossil morphologies, they are particularly useful forCambrian strata in which trace fossil diversity indeep-water environments was lower than in youngerstrata (Orr 2001). Furthermore, the resolution of dataproduced from trace fossil analysis may be much finerin Cambrian sediments; an overall shallower depth ofbioturbation results in better preservation of thinevent beds and higher resolution of detail within theshallow tier (Droser et al. 2002). This increased reso-lution due to shallower burrowing is especially benefi-cial to the interpretation of low-oxygen environmentswith low-sedimentation rates, where even short termoxygenation events have the potential to overprintsedimentary evidence of overall low-oxygen condi-tions.

Materials and methods

Collection of material in the field

Intervals of two cycles at the Miners Hollow locality inthe Wellsville Mountains (Figs 1, 2) were targeted forcontinuous sampling. This locality was chosen as itcontains intervals bearing abundant BST preservationof a diverse, soft-bodied biota (Robison 1991; Briggset al. 2008). Three continuous intervals ranging inthickness from 61 to 361 cm were collected andreturned to the laboratory for micro-stratigraphicanalysis.

At each interval, the section was measured perpen-dicular to observed bedding and marked. Additional

High Creek Limestone Member

Naomi PeakLimestone Member

60

55

50

45

40

35

30

25

20

15

10

5

0

Met

ers

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Cycle 6

Cycle 7

MH6

MH3U

MH3L

Fig. 2. Stratigraphic section of the Spence Shale at the Miners Hol-low locality (after Liddell et al. 1997). Continuously sampled inter-vals studied herein (MH3L, MH3U and MH6) are highlighted.


adjacent material was also marked for fossil collection.The interval was then collected by removal of blocksamples using hand tools and a petrol-powered rocksaw. Additional material taken adjacent to the contin-uous section was split along bedding planes as finelyas possible to identify fossils. Any sample containing apartial or complete body fossil was retained and itsorientation and approximate position within the sec-tion were recorded. Laboratory analysis of fine lami-nations within the fossil-bearing shale allowed for theprecise position of each fossil to be determined andtied into micro-stratigraphic logs.

Two intervals were collected from Cycle 3, whichexceeds 20 m in thickness and comprises nearly halfof the thickness of the Spence Shale (Fig. 2). A 66-cminterval (MH3L) was collected from an exposure ca.3 m above the base of Cycle 3 (contact with Cycle 2covered), which has been interpreted to represent themaximum water depth at this locality (Liddell et al.1997). A second interval, 97.5 cm in thickness, wascollected from 62 cm below the top of Cycle 3(MH3U). This interval was chosen because it containsabundant soft-bodied fossils, including those of meta-zoans, and fresh material is well-exposed in the wall ofa small quarry.

A 458 cm interval representing the majority of theshale portion of Cycle 6 was also collected (MH6).Cycle 6 contains at least two intervals bearing abun-dant soft-bodied metazoans, and fresh material fromthe base of this cycle is well-exposed in a large quarry(Fig. 2). The bottom portion of the interval collectedrepresents the uppermost part of Cycle 5.

Laboratory analysis

After collection, samples were cut perpendicular tobedding, polished and scanned wet or partially sub-merged in water at ‡600 dpi on an Epson desktopscanner. Once scanned, all samples were arranged instratigraphic order and digitally assembled in Photo-shop, and any overlaps between samples removed.A micro-stratigraphic log was drawn digitally usingthe digital images of the continuous sections as a basis.Primary sedimentary structures (bedding), bioturba-tion and authigenic mineral growths were recordedon the logs. i.i. were recorded at the millimetre scalefrom the digitized continuous section.

The approximate position of samples containingbody fossils was recorded in the field. The shale sam-ples containing fossil specimens were cut perpendicu-lar to bedding, as close to the fossils as possible. Cutsurfaces were then polished and scanned in the waydescribed above for the continuous section samples.Sedimentary features from the fossil specimens werematched with those of the continuous section to find

the position of the specimen within the section to thenearest 0.5 mm and marked on the stratigraphicsection. Articulation and attitude were described forbiomineralized body fossils.

Results

The majority (95.7%) of section MH3L is non-biotur-bated (i.i.1); however, a few thin (<2.5 mm) intervalsexhibit in weakly developed ichnofabrics (i.i.2, 3.3%;i.i. 3, 1.0%; Fig. 3). In contrast, the majority of sectionMH3U is weakly bioturbated (Fig. 4). Only 36.6% ofthe thickness of the section is non-bioturbated (i.i.1),with weakly to moderately developed ichnofabricspresent in the rest of the interval (50.1% i.i.2; 13.0%i.i.3, 0.4% i.i.4). The sampled interval of Cycle 6 showssignificant variability in the extent of bioturbation(Fig. 5), with 43.3% of the section assigned to i.i.1,

Fig. 3. Section MH3L showing ii. Scale in cm. Darker intervalsindicate intervals that were darker in appearance after polishingand indicate higher carbonate content. Intervals missing ii weretoo weathered for determination.


20.6% assigned to i.i.2, 28.4% assigned to i.i.3, 4.5%assigned to i.i.4 and 2.9% assigned to i.i.5. The basal0.49 m of section MH6 represents the top part ofCycle 5; several intervals (0–13; 19–21; 45–49 cm) areextensively bioturbated (i.i. ‡ 3). The sampled intervalof Cycle 6 shows significant variability in extent ofbioturbation (Fig. 5), with 43.3% of the sectionassigned to i.i.1, 20.6% to i.i.2, 28.4% to i.i. 3, 4.5%

to i.i.4 and 2.9% to i.i.5. The overall pattern withinCycle 6 is increasing extent of bioturbation upsection,consistent with the upward shallowing and increasingbenthic oxygen content described by Liddell et al.(1997).

Section MH3L is dominated by Morania, a putativecyanobacterium, and agnostid trilobites. The positionsof soft-bodied and skeletonizing fossils within a 10 cminterval from MH3L are shown in Figure 6. Soft-bodied fossils are far more abundant than skeletoniz-ing fossils within this interval and consist almostexclusively of Morania (Fig. 11A). Agnostid trilobitesof the genus Peronopsis are also abundant in this inter-val and commonly occur in the same beds as soft-bodied fossils. The remainder of the skeletonizingfauna consists of polymerid trilobites (mostly pty-choparid trilobites less than 2 cm in length) and bra-chiopods. Almost all fossils occur in non-bioturbatedsediments (i.i.1), however, on a single bedding planeBST preservation of Morania occurs with agnostid tri-lobites and small ptychoparid trilobites associatedwith a thin (1.5 mm) interval of moderate bioturba-tion (i.i.3; Table 1).

Both soft-bodied and skeletonizing fossils are com-mon within section MH3U (Fig. 7), but different taxaoccur in this section. More than half of the examplesof soft-bodied preservation in this interval are repre-sented by the alga Marpolia spissa (Fig. 11B). Twoexamples of unidentified metazoans with preservedgut traces also occur within this section (Fig. 11C).Other specimens of soft-bodied organisms consistedof indeterminate disarticulated or degraded soft-bod-ied material. Despite an overall higher abundance ofskeletonizing fossils, MH3U contains few examples ofagnostid trilobites, with only four specimens identifiedin four different horizons. The remaining skeletoniz-ing fossils represent ptychoparid trilobites in botharticulated and disarticulated states, brachiopodsincluding Acrothele (Fig. 11D), as well as the hyolith-ids Haphlophrentis (Fig. 11D) and Hyolithes, both ofwhich were absent from MH3L. The majority (69%)of occurrences of soft-bodied fossils in this intervaloccur within non-bioturbated sediments (i.i.1) andthe remainder in weakly bioturbated (i.i.2) sediments(Table 1). Sixty three percent of benthic (non-agnos-tid) skeletonizing fossils occur within i.i.2 horizonsand the remaining 27% occur in non-bioturbated(i.i.1) sediments (Table 1).

The occurrences of soft-bodied and skeletonizedfossils within three intervals of section MH6 areshown in Figures 8–10. Soft-bodied fossils are far lesscommon than skeletonized fossils in MH6. Composi-tion of the soft-bodied assemblage in MH6 is similarto that of MH3U and is dominated by Marpolia spissa,(Fig. 11B). The skeletonizing fauna is also more

Fig. 4. Section MH3U showing ii. Scale in cm. Darker intervalsindicate intervals that were darker in appearance after polishingand indicate shale with higher carbonate content. Darkest intervalsrepresent carbonate deposition.


similar to MH3U than MH3L; however, only a singleagnostid trilobite was collected from MH6. Hyolithidsoccur in the lowermost third of the entire cycle. Thectenocystoid, Ctenocystis utahensis occurs at threehorizons within MH6, but was not recovered fromMH3L or MH3U. The three occurrences of this cteno-cystoid occur within i.i.1 horizons and, in one hori-zon, it co-occurs with soft-bodied fossils. BSTpreservation of fossils in these intervals preferentiallyoccurs in i.i.1 sediments (77% of soft-bodied fossiloccurrences n = 13; Table 1), although two specimens(15%) occur in i.i.2 intervals, and a single specimen ofAcinocricus occurs in an interval of moderate biotur-bation (i.i.3). Skeletonizing fossils within these inter-vals preferentially occur in i.i.2 and i.i.3 intervals(82%) with two occurrences identified in i.i.5 intervals

(4%; Table 1). Only 14% of skeletonized fossils inMH6 occur in non-bioturbated (i.i.1) sediments.

Within MH6 between 21 and 41 cm (Fig. 5) is aninterval informally known by collectors as the ‘150¢layer for its approximate position 150 feet above thebase of the Spence Shale section at Miners Hollow.Fossil samples collected from this 20-cm intervalcould not be placed precisely within the intervalbecause weathering has obscured primary fabric.However, this interval is known by collectors to con-tain a higher than average density of soft-bodied fos-sils and has produced specimens of the algae Marpoliaand Yuknessia, arthropods including a canadaspid andMetitosoma paradoxum, and a soft-bodied sponge(Skabelund, personal communication 2008). Thisinterval is characterized by low extent of bioturbation

Fig. 5. Section MH6 showing ii. Scale in cm. Darker intervals indicate intervals that were darker in appearance after polishing and indicateshale with higher carbonate content. Darkest intervals represent carbonate deposition.


Fig. 6. Ichnofabric indices and occurrence of fossils within studied interval of MH3L. Ichnofabric indices are represented by different shades ofgrey according to the key in the column header. The remaining columns from left to right represent soft-bodied occurrences, agnostid trilobites,disarticulated polymerid trilobites, articulated polymerid trilobites, brachiopods, hyolithids and ctenocystoids. Scale in cm from base of MH3L.

Table 1. i.i. of fossil occurrences within all studied intervals.

Fossil occurrences

i.i.

MH3L MH3U MH6 Total

benthicskeletonizingfauna

soft-bodiedfossils


soft-bodiedfossils


soft-bodiedfossils


soft-bodiedfossils

i.i.1 6 24 7 9 7 10 20 43i.i.2 – – 12 4 20 2 32 6i.i.3 1 1 – – 20 1 21 2i.i.4 – – – – – – – –i.i.5 – – – – 2 – 2 –n = 7 25 19 13 49 13 75 51

Fig. 7. Ichnofabric indices and occurrence of fossils within studied interval of MH3U. See figure 6 for explanation of columns. Scale in cmfrom base of MH3U.


relative to the MH6 as a whole and is dominated byi.i.1.

Discussion

The extent of bioturbation increases up-section inboth Cycle 3 (Figs 3, 4) and Cycle 6 (Fig. 5). This, incombination with sedimentological evidence for shal-lowing-upward cycles (Liddell et al. 1997), suggestswater depth as a primary control on bottom wateroxygenation during the period of deposition of theSpence Shale. Fluctuations between laminated andbioturbated intervals occurs on millimetre and centi-metre scales throughout, however, and appear to rep-resent oscillations in bottom water content that weresuperimposed on the larger cyclic pattern. The Spence

Shale Member, like the Wheeler and Marjum Forma-tions (Gaines & Droser 2005, 2010; Brett et al. 2009),was deposited at the edge of a fluctuating oxycline.

Soft-bodied fossil occurrences suggest a broadlysimilar pattern to those of previous studies from otherdeposits in which it was found that BST preservationoccurs preferentially in the absence of bioturbation(Allison & Brett 1995; Dornbos et al. 2005; Gaines &Droser 2005, 2010). In all sections, the majority ofoccurrences of soft-bodied fossils are within the lami-nated sediments (i.i.1), whereas skeletonized fossilsoccur preferentially in weakly moderately bioturbatedintervals. This suggests that anoxic benthic conditionsfavour BST preservation, whereas in situ skeletonizedfossils tend to occur under oxygenated bottom waterswithin Spence Shale and BST deposits in general.Specifically, the oxygen-related biofacies model

Fig. 8. Ichnofabric indices and occurrence of fossils within lowermost studied interval of MH6. See figure 6 for explanation of columns. Scalein cm from base of MH6.


developed for the Wheeler Formation (Gaines & Dro-ser 2005) is largely applicable to the Spence Shale. Amostly anoxic assemblage consisting of transportedsoft-bodied fossils and pelagic organisms, such as ag-nostid trilobites (for deeper water intervals), whichoccur in i.i.1 and a dysaerobic assemblage consistingof benthic skeletonizing fossils, which occur in i.i.2and higher is for the most part consistent with thefindings here.

The soft-bodied fossils of the anoxic zone withinthe Spence Shale can be subdivided into two separateassemblages controlled by depth. This is also in accor-dance with the proximal–distal gradient in soft-bodiedfossil assemblages recognized in the Wheeler and Mar-jum Formations (Gaines & Droser 2005, 2010). Thedeeper water assemblage seen in MH3L is character-ized by abundant Morania. Agnostid trilobites are thesecond most common fossil in this assemblage andalthough some skeletonizing benthic fossils includingbrachiopods and small ptychoparid trilobites arefound within the same bedding planes and adjacentones, they are rare. A more proximal soft-bodiedassemblage represented in MH3U and MH6 is charac-terized by abundant Marpolia spissa. Rare benthicmetazoans and Acinocricus also occur in this moreproximal shallower water anaerobic assemblage.

The Wheeler Formation contains a monospecificexaerobic assemblage of individuals of the trilobite

Elrathia kingii (Gaines & Droser 2003), but no analo-gous assemblage from the Spence Shale can be defini-tively assigned to the exaerobic zone. Occurrences ofthe ctenocystoid Ctenocystis utahensis in laminatedsediments in an interval that also contains soft-bodiedfossils suggests tolerance of very low-oxygen levels,but their great rarity suggests that an opportunisticmode of life was unlikely. Ctenocystoids are thoughtto be benthic, based on functional morphology (Robi-son & Sprinkle 1969) and their preferential occurrencein deep-water laminated sediments has been notedbefore (Parsley & Prokop 2004).

Although this broad pattern is similar to that docu-mented from the Wheeler Formation, a significantminority of soft-bodied fossils within the Spence Shaleoccur in weakly bioturbated sediments. This could besuggestive of a higher benthic oxygen threshold forBST preservation within the Spence Shale; however,some caution must be exercised in reaching this con-clusion, as it is possible for soft-bodied fossils to occurin bioturbated sediments without exposure of soft tis-sues to oxygen after burial. This is possible under twodifferent scenarios, each of which predicts a specificpattern of soft-bodied fossil occurrence relative to thelocation of discrete burrows. In the first hypotheticalcase (Fig. 12A), soft tissues were buried under anoxicbottom water conditions resulting in burial either atthe base of the bed or within it. At some time

Fig. 9. Ichnofabric indices and occurrence of fossils within the second studied interval of MH6. See figure 6 for explanation of ii andcolumns. Scale in cm from base of MH6.


following burial, the return of dysoxic bottom waterconditions promoted burrowing into the sediment,but by this time-soft tissues had already collapsed intotwo dimensions (e.g. Briggs & Kear 1994), and werestabilized by the surrounding sediment. Associationsof this type between trace and soft-bodied fossils havebeen reported from the Chengjiang (Zhang et al.2007), the Kaili Formation (Yang et al. 2009; Linet al. 2010), and the Stephen Formation (Caron et al.2010). In the second hypothetical scenario, dysoxicconditions permitted bioturbation of a bed exposed atthe sea floor, but bottom water conditions subse-quently became anoxic. Transportation and burial ofsoft-bodied fossils resulted in preservation of soft tis-sues at the top of a burrowed interval directly underly-ing a laminated interval.

These two possible scenarios, however, are unlikelyto explain all occurrences of BST preservation withinweakly bioturbated intervals of the Spence Shale. One

occurrence of Acinocricus (Fig. 13) in MH6 is withinan interval, where multiple beds are bioturbated. Thisfossil lies near the top of an interval with dense small(0.5 mm in diameter and smaller) burrows and isdirectly overlain by an interval with somewhat larger(0.5–1 mm diameter) burrows. The multiple bur-rowed horizons suggest that at least this example waslikely deposited under or in very close temporal prox-imity to dysoxic conditions.

Average extent of bioturbation is higher in mud-stone facies of the Spence Shale than the WheelerFormation. The maximum observed extent of biotur-bation in mudstones of the Wheeler Formation is i.i.3(Gaines & Droser 2005), whilst i.i.4 and i.i.5 intervalsare present within the Spence Shale. Relatively fewtrace fossils in the Raymond Quarry and their com-plete absence from the Walcott Quarry of the BurgessShale (Allison & Brett 1995; Gabbott et al. 2008) indi-cate that the Burgess Shale likewise experienced a

Fig. 10. Ichnofabric indices and occurrence of fossils within the uppermost interval of MH6. See figure 6 for explanation of ii and columns.Scale in cm from base of MH6.


much lesser maximum extent of bioturbation thandid the Spence Shale. Chengjiang sediments are alsodominantly non-bioturbated (Dornbos et al. 2005).Higher average extent of bioturbation and multipleinstances of soft-bodied preservation occurring inhorizons that also bear trace fossils suggests a dynamicchemocline and with higher amplitude fluctuationsand periods of greater benthic oxygen availability forSpence Shale than for other BST deposits. Dynamicpalaeoredox conditions may also explain, in part, therarity and low diversity of soft-bodied fossils in theSpence Shale compared with Chengjiang and BurgessShale, as the Spence may represent taphonomic

conditions at the very margins of those, where BSTpreservation was possible, rather than environmentswhere those conditions were maximized for metres ortens of metres of continuous section.

Conclusions

Occurrences of soft-bodied and skeletonizing fossilswithin the Spence Shale fit the broad pattern of twogeneral oxygen related microfacies: an anoxic micro-facies characterized by common occurrences of BSTfossils in association with pelagic agnostid trilobites

Fig. 11. Soft-bodied fossils from the Spence Shale. All scale bars 1 cm. (A) Morania fragmenta with the brachiopod Acrothele.(UCR10829 ⁄ 1). (B) Marpolia spissa (UCR10832 ⁄ 1). (C) Unknown metazoan with gut trace (UCR10831 ⁄ 1). (D) Hyolithid Haphlophrentiswith gut trace, Acrothele and various disarticulated trilobite material. (E) Acinocricus stichus (UCR1083 ⁄ 1).


and a dysoxic microfacies characterized by benthicskeletonizing fossils. The anoxic microfacies can besubdivided into two depth-related biofacies, whichpreserve different types of soft-bodied fossil assem-blages: a shallow water anoxic assemblage, whichaccumulated in close proximity to oxygenated bottomwaters and contains benthic metazoans, and a deep-water anoxic assemblage farther removed from oxy-genated waters. However, compared with the similarWheeler Formation (Gaines & Droser 2005), a signifi-cant proportion (16% n = 51) of BST fossils in theSpence Shale occur in horizons that also contain tracefossils. The Spence Shale represents deposition undermore dynamic benthic redox conditions than theWheeler Formation, with higher maximum benthicoxygen concentrations inferred from significantlygreater maximum extent of bioturbation observed inmudstone facies. Whilst sustained benthic anoxiaappears to have promoted BST preservation, it is nei-ther a sufficient (Allison 1988) nor entirely necessarycondition to account for the extraordinary preserva-tion of BST assemblages worldwide.

Acknowledgements. – We thank Jake Skabelund for access to col-lecting pits and extensive field assistance, Paul Jamison, LucasAllen-Williams, Mike Balint, Stephan Hlohowskyj, Paul Hong,Greg Lawson, Brad Markle and Ryan McKenzie for field assistance,and Nigel Hughes for valuable discussion and comments. Themanuscript was improved considerably by thoughtful reviews byCarlton Brett and one anonymous reviewer. This research was sup-ported in part by National Science Foundation grants (EAR-0518732) to RRG and MLD and (DMR-0618417) to RRG.

References

Allison, P.A. 1988: The Role of anoxia in the decay and miner-alization of proteinaceous macro-fossils. Paleobiology 14, 139–154.

Allison, P.A. & Brett, C.E. 1995: In situ benthos and paleo-oxygen-ation in the Middle Cambrian Burgess Shale, British Columbia,Canada. Geology 23, 1079–1082.

Allison, P.A. & Briggs, D.E.G. 1991: Taphonomy of nonmineral-ized tissues. In Allison P.A., Briggs D.E.G. (eds): Taphonomy.Releasing the Data Locked in the Fossil Record, 25–70. PlenumPress, New York.

Berner, R.A. 1981: Authigenic mineral formation resulting fromorganic matter decomposition in modern sediments. Fortschritteder Mineralogie 59, 117–135.

Boyer, D.L. & Droser, M.L. 2007: Devonian monospecific assem-blages: new insights into the ecology of reduced-oxygen deposi-tional settings. Lethaia 40, 321–333.

Brett, C.E., Allison, P.A., DeSantis, M.K., Liddell, W.D. & Kramer,A. 2009: Sequence stratigraphy, cyclic facies, and lagerstatten inthe Middle Cambrian Wheeler and Marjum Formations, GreatBasin, Utah. Palaeogeography, Palaeoclimatology, Palaeoecology277, 9–33.

Briggs, D.E.G. & Kear, A.J. 1994: Decay and mineralization ofshrimps. Palaios 9, 431–456.

Briggs, D.E.G., Lieberman, B.S., Hendricks, J.R., Halgedahl, S.L. &Jarrard, R.D. 2008: Middle Cambrian arthropods from Utah.Journal of Paleontology 82, 238–254.

Butterfield, N.J. 1995: Secular distribution of Burgess-Shale-typepreservation. Lethaia 28, 1–13.

Caron, J.-B. & Jackson, D.A. 2006: Taphonomy of the greaterphyllopod bed community, Burgess Shale. Palaios 21, 451–465.

Caron, J.-B. & Jackson, D.A. 2008: Paleoecology of the greaterphyllopod bed community, Burgess Shale. Palaeogeography,Palaeoclimatology, Palaeoecology 258, 222–256.

Caron, J.B., Gaines, R.R., Mangano, G.M., Streng, M. & Daley,A.C. 2010: A new Burgess Shale-type assemblage from the ‘thin’Stephen Formation of the Southern Canadian Rockies. Geology38, 811–814.

Conway Morris, S. 1989a: Burgess Shale Faunas and the Cambrianexplosion. Science 246, 339–346.

Conway Morris, S. 1989b: The persistence of Burgess Shale-typefaunas; implications for the evolution of deeper-water faunas.Transactions of the Royal Society of Edinburgh: Earth Sciences 80,271–283.

Conway Morris, S. 1992: The early evolution of life. In BrownG.C., Hawkesworth C.J., Wilson R.C.L. (eds): Understanding theEarth, 436–457. Cambridge University Press, Cambridge.

Conway Morris, S. & Robison, R.A. 1986: Middle Cambrian pria-pulids and other soft-bodied fossils from Utah and Spain. Uni-versity of Kansas Paleontological Contributions 117, 1–22.

Conway Morris, S. & Robison, R.A. 1988: More soft-bodied ani-mals and algae from the Middle Cambrian of Utah and BritishColumbia. University of Kansas Paleontological Contributions122, 23–48.

Dornbos, S.Q., Bottjer, D.J. & Chen, J.-Y. 2005: Paleoecology ofbenthic metazoans in the Early Cambrian Maotianshan Shalebiota and the Middle Cambrian Burgess Shale biota: evidencefor the Cambrian substrate revolution. Palaeogeography, Palaeo-climatology, Palaeoecology 220, 47–67.

Fig. 12. Two possible scenarios for anoxic soft-bodied preservationin which trace fossils are present in association. (A) 1 – Soft-bodiedorganism is transported into or killed by anoxic bottom water con-ditions. 2 – Burial under anoxic conditions 3 – Conditions changeto dysoxic allowing other organisms to burrow down into theanoxic sediments alongside where the soft-bodied organism is pre-served. (B) 1 – Dysoxic bottom water conditions allow for borrow-ing into the sediment. 2 – Bottom water conditions change andsoft-bodied organism is transported into or killed by anoxic bot-tom water conditions. 3 – Burial preserves soft-bodied organismon the same bedding plan from which burrows originate.

Fig. 13. Position within MH6 of soft-bodied Acinocricus specimen,also showing cross-section of associated sediments with multiplelevels of trace fossils above, below and within the same layer as thespecimen. Scale in cm from base of section MH6.


Droser, M.L. & Bottjer, D.J. 1986: A semiquantitative field classifi-cation of ichnofabric. Journal of Sedimentary Research 56, 558–559.

Droser, M.L., Jensen, S. & Gehling, J.G. 2002: Trace fossils and sub-strates of the terminal Proterozoic–Cambrian transition: Impli-cations for the record of early bilaterians and sediment mixing.Proceedings of the National Academy of Sciences of the UnitedStates of America 99, 12572–12576.

Foree, E.G. & McCarty, P.L. 1970: Anaerobic decomposition ofalgae. Environmental Science and Technology 4, 842–849.

Gabbott, S.E., Zalasiewicz, J. & Collins, D. 2008: Sedimentation ofthe Phyllopod Bed within the Cambrian Burgess Shale forma-tion of British Columbia. Journal of the Geological Society ofLondon 165, 307–318.

Gaines, R.R. & Droser, M.L. 2003: Paleoecology of the familiar tri-lobite Elrathia kingii: an early exaerobic zone inhabitant. Geology31, 941–944.

Gaines, R.R. & Droser, M.L. 2005: New approaches to understand-ing the mechanics of Burgess Shale-type deposits: from themicron scale to the global picture. The Sedimentary Record 3,4–8.

Gaines, R.R. & Droser, M.L. 2010: The paleoredox setting of Bur-gess Shale-type deposits. Palaeogeography, Palaeoclimatology,Palaeoecology, 297, 649–661.

Gaines, R.R., Kennedy, M.J. & Droser, M.L. 2005: A new hypothe-sis for organic preservation of Burgess Shale taxa in the MiddleCambrian Wheeler Formation, House Range, Utah. Palaeogeog-raphy, Palaeoclimatology, Palaeoecology 220, 193–205.

Gaines, R.R., Briggs, D.E.G. & Yuanlong, Z. 2008: Cambrian Bur-gess Shale-type deposits share a common mode of fossilization.Geology 36, 755–758.

Gunther, L.F. & Gunther, V.G. 1981: Some Middle Cambrianfossils of Utah. Brigham Young University Geology Studies 28, 1–79.

Henrichs, S.M. & Reeburgh, W.S. 1987: Anaerobic mineralizationof marine sediment organic matter: Rates and the role of anaero-bic processes in the oceanic carbon economy. GeomicrobiologyJournal 5, 191–237.

Herringshaw, L.G. & Davies, N.S. 2008: Bioturbation levels duringthe end-Ordovician extinction event: a case study of shallowmarine strata from the Welsh Basin. Aquatic Biology 2, 279–287.

Kakuwa, Y. 2008: Evaluation of palaeo-oxygenation of the oceanbottom across the Permian–Triassic boundary. Global and Plan-etary Change 63, 40–56.

Lee, C. 1992: Controls on organic carbon preservation: The use ofstratified water bodies to compare intrinsic rates of decomposi-tion in oxic and anoxic systems. Geochimica et CosmochimicaActa 56, 3323–3335.

Liddell, W.D., Wright, S.H. & Brett, C.E. 1997: Sequence stratigra-phy and paleoecology of the Middle Cambrian Spence Shale inNorthern Utah and Southern Idaho. Brigham Young UniversityGeology Studies 42, 59–78.

Lin, J.P., Zhao, Y.L., Rahman, I.A., Xiao, S. & Wang, Y. 2010: Bio-turbation in Burgess Shale- type Lagerstatten – case study oftrace fossil-body fossil associations from the Kaili Biota (Cam-brian Series 3), Guizhou, China. Palaeogeography, Palaeoclima-tology, Palaeoecology 292, 245–256.

Orr, P.J. 2001: Colonization of the deep-marine environment dur-ing the early Phanerozoic: the ichnofaunal record. GeologicalJournal 36, 265–278.

Parsley, R.L. & Prokop, R.J. 2004: Functional morphology and pal-aeoecology of some sessile Middle Cambrian echinoderms fromBarrandian region of Bohemia. Bulletin of Geosciences 79, 147–156.

Petrovich, R. 2001: Mechanisms of fossilization of the soft-bodiedand lightly armored faunas of the Burgess Shale and of someother classical localities. American Journal of Science 301, 683–726.

Powell, W.G., Johnston, P.A. & Collom, C.J. 2003: Geochemicalevidence for oxygenated bottom waters during deposition of fos-siliferous strata of the Burgess Shale Formation. Palaeogeography,Palaeoclimatology, Palaeoecology 201, 249–268.

Resser, C.E. 1939: The Spence Shale and its fauna. SmithsonianMiscellaneous Collection 97, 1–29.

Rhoads, D.C. & Morse, J.W. 1971: Evolutionary and ecologic sig-nificance of oxygen-deficient marine basins. Lethaia 4, 413–428.

Robison, R.A. 1969: Annelids from the Middle Cambrian SpenceShale of Utah. Journal of Paleontology 43, 1169–1173.

Robison, R.A. 1976: Middle Cambrian trilobite biostratigraphy ofthe Great Basin. Brigham Young University Geology Studies 23,93–109.

Robison, R.A. 1991: Middle Cambrian biotic diversity; examplesfrom four Utah Lagerstaetten. In Simonetta A.M., Conway Mor-ris S. (eds): The Early Evolution of Metazoa and the Significanceof Problematic Taxa, 77–98. Cambridge University Press, Cam-bridge.

Robison, R.A. & Sprinkle, J. 1969: Ctenocystoidea: new class ofprimitive echinoderms. Science 166, 1512–1514.

Robison, R.A. & Wiley, E.O. 1995: A new arthropod, Meristosoma:more fallout from the Cambrian explosion. Journal of Paleontol-ogy 69, 447–459.

Savrda, C.E. & Bottjer, D.J. 1986: Trace-fossil model for recon-struction of paleo-oxygenation in bottom waters. Geology 14,3–6.

Savrda, C.E. & Bottjer, D.J. 1987: The exaerobic zone, a new oxy-gen-deficient marine biofacies. Nature 327, 54–56.

Savrda, C.E., Bottjer, D.J. & Gorsline, D.S. 1984: Development of acomprehensive oxygen-deficient marine biofacies model; evi-dence from Santa Monica, San Pedro, and Santa Barbara Basins,California continental Borderland. AAPG Bulletin 68, 1179–1192.

Sprinkle, J. 1985: New Edrioasteroid from the Middle Cambrian ofwestern Utah. The University of Kansas Paleontological Contribu-tions 116, 1–4.

Twitchett, R.J. & Wignall, P.B. 1996: Trace fossils and the after-math of the Permo-Triassic mass extinction: evidence fromnorthern Italy. Palaeogeography, Palaeoclimatology, Palaeoecology124, 137–151.

Walcott, C.D. 1908: Cambrian trilobites. Smithsonian MiscellaneousCollection 53, 13–53.

Yang, Y., Lin, J.P., Zhao, Y.L. & Orr, P.J. 2009: Palaeoecology ofthe trace fossil Gordia and its interaction with nonmineralizingtaxa from the early Middle Cambrian Kaili Biota, Guizhou Prov-ince, South China. Palaeogeography, Palaeoclimatology, Palaeoe-cology 277, 141–148.

Zhang, X.-G. & Hou, X.-G. 2007: Gravitational constraints on theburial of Chengjiang Fossils. Palaios 22, 448–453.

Zhang, X.-G., Bergstrom, J., Bromley, R.G. & Hou, X.-G. 2007:Diminutive trace fossils in the Chengjiang Lagerstatte. TerraNova 19, 407–412.


Documents

Dynamic palaeoredox and exceptional preservation in the ... et al 2011.pdfthe succession. A diverse skeletonized benthic fauna, including various polymerid trilo-bites, hyolithids,