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Dynamic Palaeoredox and Exceptional Preservation in the Cambrian Spence Shale of Utah

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Dynamic Palaeoredox and Exceptional Preservation in the Cambrian Spence Shale of Utah Dynamic palaeoredox and exceptional preservation in the Cambrian 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: Dynamic palaeoredox 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 metazoan life during the Cambrian. Although anoxia has long been thought to be a pre-requisite for this particular type of soft-bodied preservation, the palaeoenvironmental conditions that 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, ichnological approach to determine bottom water oxygen conditions under, which Burgess Shale-type biotas were preserved in the Middle Cambrian Spence Shale of Utah. Mudstones of the Spence Shale are characterized by fine scale (mm-cm) alternation between laminated and bioturbated intervals, suggesting high-frequency fluctuations in bottom water oxygena- tion. Whilst background oxygen levels were not high enough to support continuous infaunal activity, brief intervals of improved bottom water oxygen conditions punctuate the succession. A diverse skeletonized benthic fauna, including various polymerid trilo- bites, hyolithids, brachiopods and ctenocystoids suggests that complex dysoxic benthic community was established during times when bottom water oxygen conditions were permissive. Burgess Shale-type preservation within the Spence Shale is largely confined to non-bioturbated horizons, suggesting that benthic anoxia prevailed in intervals, where these fossils were preserved. However, some soft-bodied fossils are found within weakly to 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 not require it in all cases. h Anoxia, Burgess Shale, Burgess Shale type-preservation, Langston Formation, Spence Shale Member, Utah. Daniel E. Garson [[email protected]], Department of Earth Sciences, University of California, Riverside, CA 92521, USA; Robert R. Gaines [[email protected]], Geology Department, Pomona College, 185 E. Sixth St., Claremont, CA 91711, USA; Mary L. Droser [[email protected]], Department of Earth Sciences, University of California, Riverside, CA 92521, USA; W. David Liddel [[email protected]], Department of Geology, Utah State University, Logan, UT 84322-4505, USA; Aaron Sappenfield [aaron.sappenfi[email protected]], Department of Earth Sciences, University of California, Riverside, CA 92521, USA; manuscript received on 14 September 2010; manuscript accepted on 04 February 2011. Burgess Shale-type (BST) biotas are named after the preservation of BST assemblages may be just as world famous locality from which they were first importantastheexceptionalfossilsthemselvesasit described, but represent a global, if rare, phenomenon. speaks directly to the unique environmental condi- Well-described BST localities are known from North tions that were widespread in the marine realm at the America, Europe, Siberia, Asia and Australia and are time. largely confined to Series 2 and Series 3 of the Cam- Anoxia, at least within the sediments, has been con- brian (Conway Morris 1989a; Butterfield 1995). These sidered a necessary pre-requisite for BST preservation deposits provide a unique window on the early diver- (Allison & Brett 1995; Butterfield 1995; Gaines et al. sification of the Metazoa (Conway Morris1989a, 2005). Anoxia may increase the preservation potential 1992). BST assemblages are characterized by a com- of soft tissues in two ways: by preventing the direct mon preservational style, in which soft tissues of scavenging of tissues by animals, and by potentially organisms were conserved as carbonaceous compres- helping to slow the normal processes of microbial sions in fine-grained marine sediments (Butterfield decay (Allison & Briggs 1991). Oxygen is the most 1995; Gaines et al. 2008). However, the mechanisms energetically favourable oxidant for degradation of controlling this taphonomic pathway, referred to as organic matter (Berner 1981) and soft tissues are BST preservation, and this pathway’s restriction in degraded quickly under oxic conditions. However, time are not fully understood. Insight into the anoxia alone is not sufficient to explain the DOI 10.1111/j.1502-3931.2011.00266.x Ó 2011 The Authors, Lethaia Ó 2011 The Lethaia Foundation 2 Garson et al. LETHAIA 10.1111/j.1502-3931.2011.00266.x preservation of soft tissues as oxidation of organic Hou 2007), which again would have required preser- matter by sulphate-reducing bacteria can occur at vation in close temporal and spatial association with comparable rates to oxic degradation (Foree & at least marginally oxygenated bottom waters. McCarty 1970; Henrichs & Reeburgh 1987; Allison Gaines & Droser (2005) described three distinct 1988; Lee 1992). Actualistic experiments using shrimp oxygen related microfacies within superficially monot- and other non-mineralizing taxa have also shown that onous claystones of the Middle Cambrian Wheeler degradation of soft tissue occurs rapidly under anoxic Formation, a BST deposit, which occurs in the House conditions (Allison 1988; Briggs & Kear 1994). Range and Drum Mountains of Utah, based on ich- Although anoxia is generally considered as a necessary nofabric index (i.i.), a semi-quantitative method to pre-requisite, anoxia alone is not sufficient to account distinguish relative amounts of bioturbation (Droser for BST preservation. Some additional mechanism is & Bottjer 1986) and diagnostic body fossil assem- required for the early diagenetic stabilization of labile blages. This model was subsequently applied to the tissues. This preservational mechanism remains con- Marjum Formation, which also occurs in the House troversial (Butterfield 1995; Petrovich 2001; Gaines Range (Gaines & Droser 2010). These studies demon- et al. 2005). strated that nearly all occurrences of BST preservation The necessity of anoxic conditions as a pre-requisite in both formations occur within laminated intervals, for BST preservation is not universally accepted. On entirely lacking bioturbation (i.i.1), consistent with the basis of trace metal ratios interpreted as palaeore- benthic anoxia. In addition, this anaerobic microfacies dox indicators, Powell et al. (2003) argued for mini- includes some skeletal body fossils, dominantly agnos- mally oxic conditions during deposition of much of tid trilobites (Gaines & Droser 2005). A dysoxic the Burgess Shale, including intervals containing BST microfacies was defined by the presence of weakly preservation. In addition, Caron & Jackson (2006, developed, shallow ichnofabrics (i.i.2 and i.i.3) and it 2008) have argued for in situ preservation of Burgess includes a low-diversity shelly benthic fauna. At the Shale assemblages; obviously, consistently oxic or dys- margin of the dysoxic environment, the trilobite Elra- oxic bottom water conditions would be required to thia kingii occurs in dense (up to 500 individuals ⁄m2) sustain a benthic fauna. This interpretation was based monospecific assemblages that represent an exaerobic on taphonomic evidence suggesting ‘minimal’ trans- microfacies (Savrda & Bottjer 1987) at the transition port of the Burgess Shale biota (Caron & Jackson between laminated (i.i.1) and weakly bioturbated 2006). (i.i.2) sediments (Gaines & Droser 2003). These three Allison & Brett (1995) reported that bioturbation microfacies, recognized at the millimetre scale, may be and BST preservation occur in mutually exclusive closely interbedded (mm-cm scale), and represent horizons in the Burgess Shale and argued for oscilla- oxygen levels that alternated between anoxic environ- tions between anoxic and dysoxic bottom waters dur- ments conducive to the preservation and low-oxygen ing its deposition. Anoxic benthic conditions were environments conducive to the establishment of ben- interpreted to have prevailed with periodic and strati- thic metazoan communities. Applied more broadly, graphically brief (cm) episodes of bottom water oxy- this model suggests that BST preservation occurred genation that allowed colonization by an in situ primarily through transport from the living environ- benthic fauna. ment to the anoxic preservational trap. Gaines & Dro- Evidence from the Lower Cambrian Chengjiang ser (2005, 2010) recognized a proximal–distal gradient BST deposit of South China suggests a complex palae- in composition and articulation of BST assemblages, oredox history. Dornbos et al. (2005) found that sedi- andalsodocumentedexamplesofin situ preservation, ments of the Maotianshan shale in the Yu’anshan although such occurrences are most rare. Formation, which contain BST preservation, are Here, we apply a micro-stratigraphical approach to almost entirely unbioturbated whilst underlying sedi- the Spence Shale Member of the Langston Formation, ments of the Shiyantou Formation, which is not a a BST deposit of early ‘Middle’ Cambrian age, occur- BST deposit, are consistently moderately bioturbated. ring in the lower part of the yet unnamed Cambrian However, Zhang et al. (2007) found examples of trace Series 3 in the Wellsville Mountains of Utah (Fig. 1), fossils in direct contact with soft-bodied
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