Dynamic palaeoredox and exceptional preservation in the Cambrian Spence Shale of Utah

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.xBurgess 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, ichnologicalapproachtodeterminebottom wateroxygen conditionsunder,which BurgessShale-typebiotas were preserved in the Middle Cambrian Spence Shale of Utah. Mudstones of theSpence Shale are characterizedby 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 [daniel.garson@gmail.com], Department of Earth Sciences, University ofCalifornia, Riverside, CA 92521, USA; Robert R. Gaines [Robert.Gaines@pomona.edu],Geology Department, Pomona College, 185 E. Sixth St., Claremont, CA 91711, USA; MaryL. Droser [mary.droser@ucr.edu], Department of Earth Sciences, University of California,Riverside, CA 92521, USA; W. David Liddel [dave.liddell@usu.edu], Department ofGeology, Utah State University, Logan, UT 84322-4505, USA; Aaron Sappenfield[aaron.sappenfield@email.ucr.edu], Department of Earth Sciences, University of California,Riverside, CA 92521, USA; manuscript received on 14 September 2010; manuscript acceptedon 04 February 2011.