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

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 subject 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 oxygenation. 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 trilobites, 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 Burgess Shale-type preservation is strongly favoured by bottom water anoxia, but may not require it in all cases.