BURGESS SHALE-TYPE PRESERVATION AND DETRITAL CLAY MINERALOGY: A TEST OF THE “REACTIVE-CLAY” HYPOTHESIS

Burgess Shale-type biotas occur globally in the early and middle Cambrian and offer our best window on the ‘Cambrian explosion’. The circumstances surrounding this pathway of exceptional preservation, in which whole soft-bodied assemblages were preserved as carbonaceous compressions, have been the subject of much debate. One hypothesis, advanced by Butterfield (1995), proposed that Burgess Shale-type (BST) preservation resulted from a preponderance of highly-reactive clay mineral species in Cambrian sedimentary environments. Unusually reactive clay minerals are suggested to have absorbed bacterial and autolytic enzymes and thus prevented organic decay. Although this hypothesis was refuted for the Burgess Shale based on whole rock geochemical data from a small sample set (n=4; Powell, 2003), this hypothesis has not been systematically tested in Burgess Shale type deposits globally. We analyzed a new data set, consisting of 108 samples from the Burgess Shale (n=10), Chengjiang (n=43), and eight other Burgess Shale-type deposits from Laurentia and South China (n=55) for whole rock geochemistry by XRF. Following the approach of Powell, we tested the possibility that highly-reactive clay minerals (Fe- or Na- smectites) might have been present in the original detrital clay mineral assemblage, prior to burial transformation of mineralogy. We also determined the present clay mineral assemblage of each of these deposits using XRD analysis of clay separates. Neither mineralogical nor geochemical data support the possibility that unusually-reactive clay minerals might have been present in the sediments. XRF data indicates that BST mudstones are depleted in Fe, Si, and Na relative to the PAAS average shale standard and are enriched in Ca. Whole rock geochemical data confirm previous observations that BST mudstones are ultra fine claystones with a significant authigenic carbonate component, and exclude a role for clay mineral composition of the sediments in BST preservation.