The Burgess Shale of the Canadian Rocky Mts is one of the most well-known occurrences of Cambrian strata exhibiting exceptional preservation of a soft-bodied fauna. While Lagerstätten of the Burgess Shale-type fauna have been recognized in almost all parts of the world and in all Cambrian series, the circumstances and mechanisms allowing for such a unique mode of preservation have remained enigmatic. Of particular debate has been the reconstruction of prevailing redox conditions. This brief contribution overviews the fauna and recent advances towards understanding the sedimentary and taphonomic conditions prevailing during deposition.
Ongoing magnetostratigraphic and geochemical studies (including major and trace element geochemistry and stable isotopes of C, N, O) in the Canadian Rocky Mountains are providing insight into the paleoenvironmental changes of the mid-Frasnian punctata Event, a prominent marine geochemical perturbation. The data is evaluated with 1) a regional sequence stratigraphic perspective and 2) within the context of a rapidly changing Late Devonian world, characterized by numerous sedimentological and faunal perturbations leading up to the eventual Frasnian-Fammenian (F/F) mass extinction. Proxies for bottom water paleoredox conditions (Mo, V, U), oceanic primary productivity (δ13Corg, δ15Nbulku, Cu, Ni, Ba), changes in detrital input (Si, Al, K, Ti, Zr), and magnetic susceptibility display similar trends, indicating that these proxies and MS variations are inherently linked. The observed excursions suggest that changes in detrital input were the main driver of a bioproductivity increase. Elevated organic matter export from the photic zone likely led to the deposition and later preservation of organic-carbon rich facies under facilitated conditions of bottom water suboxia-anoxia. These geochemical trends were likely influenced by eustatic sea level change, but may have been enhanced by pulses of coincident orogenic activity and pulses of terrestrial afforestation. The rise and expansion of the first true forests is thought to have drastically altered nutrient fluxes to the oceans via increases in pedogenesis and the expansion of a mature soil profile. Our work is intended to complement the growing body of research aimed at elucidating the causes and understanding the effects of terrestrial and marine events of the P. punctata biozone and, more broadly, at understanding the Earth-system changes of the Late Devonian leading up to the F/F boundary.
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