Interpreting the Paleoenvironmental Context of Marine Shales Deposited During the Cambrian Radiation: Global Insights from Sedimentology, Paleoecology, and Geochemistry

Abstract

Many Cambrian marine shales are traditionally thought to have been deposited under dysoxic or anoxic conditions based upon interpretations of sedimentological and ichnological fabrics. Recently a number of geochemical studies have suggested the opposite, that some Cambrian marine shales were deposited under slightly oxic to well-oxygenated conditions. Despite the significant implications redox conditions have for the paleoenvironment, paleoecology, and taphonomy of Cambrian shales, rarely are these disparate sedimentological and geochemical approaches combined for direct comparisons in a singular study of Cambrian shales. A multidisciplinary approach is used here, combining sedimentological, paleoecological, and geochemical approaches, for re-interpreting the paleoenvironmental conditions of numerous Cambrian marine shales associated with exceptional fossil preservation. Sedimentological analyses indicate that most shales generally lack bioturbation (average ichnofabric index ~1), with some evidence for the presence of microbial mats, suggesting that depositional environments were dominated by Proterozoic-style substrates. An analysis of the morphological adaptations of marine benthic communities preserved in Cambrian shales demonstrates that most faunas exhibit a mixture of Proterozoic- and Phanerozoic-style adaptive strategies, which is consistent with expectations from the Cambrian substrate revolution. Geochemical analysis indicates that Cambrian shales were deposited under generally oxic conditions and not under anoxic conditions as traditionally thought. This result supports the sedimentological and paleoecological interpretations for the presence of Proterozoic-style substrates in these paleoenvironments. Oxic depositional conditions pose difficulties for taphonomic models where anoxia is a prerequisite for BST preservation. As these geochemical results are consistent with similar geochemical studies of the Burgess, Kinzers, and Emu Bay Shales, serious consideration must be made as to the validity of anoxia-based taphonomic models and oxic-based taphonomic models should be considered. The unique geochemical conditions found within microbially mediated substrates may offer a viable pathway for BST preservation under oxic conditions, as microbial mats were likely associated with Proterozoic-style substrates in Cambrian shales.