The Paleoredox Setting of Burgess Shale-type Deposits

Burgess Shale-type (BST) biotas occur globally in Lower and Middle Cambrian shales, and were preserved under specific and exceptional circumstances. Among the circumstances surrounding fossilization, benthic redox conditions have been the subject of particular disagreement. While most authors have assumed that the sites of preservation were anoxic and therefore inhospitable to metazoans, several have recently argued that BST assemblages were preserved in situ under habitable benthic conditions. Here, we use field and laboratory intensive, fine-scale methods to investigate the paleoredox settings of fossil assemblages in two Burgess Shale-type deposits, the Wheeler and Marjum Formations of western Utah (Cambrian Series 3), and review recently published data for other principal Burgess Shale-type deposits. In the Wheeler and Marjum Formations, fossil assemblages and ichnofabrics were evaluated at the millimeter scale. An ichnological model was then applied to reconstruct relative paleo-oxygen content of bottom waters at a bed-to-bed scale through multiple intervals sampled continuously. This approach permits fine-scale evaluation of the relationships of discrete fossil assemblages to paleoredox conditions. Over 7800 individual beds were logged in this study. These data demonstrate that the Wheeler and Marjum Formations were deposited under dynamic benthic redox conditions. In both deposits, beds representing deposition under dysoxic conditions may be interbedded at the millimeter–centimeter scale with those representing deposition under anoxic conditions. Although the beds deposited under both types of redox conditions may be richly fossiliferous, the distribution of fossil assemblages within the Wheeler and Marjum Formations was regulated by dissolved oxygen content of bottom waters as a first-order control. BST preservation occurs within a microfacies inferred to represent deposition under anoxic conditions, and contains transported fossils along a recognizable proximal–distal gradient. Under dysoxic benthic conditions, assemblages of skeletonized body fossils dominated by trilobites occur in both formations. In the Wheeler Formation, the junction of these two benthic redox environments is characterized by dense monospecific associations of the opportunistic trilobite Elrathia kingii, but a comparable exaerobic taxon does not appear to be present in the Marjum Formation. These results demonstrate the control of near-bottom anoxia over preservation of BST assemblages in both formations and are consistent with the findings from the Burgess Shale and the Chengjiang