Burgess Shale-Type Deposits Worldwide Share a Common Paleoenvironmental Setting and Origin

Cambrian Burgess Shale-type (BST) deposits occur worldwide and capture the most important record of the initial Phanerozoic radiation of the Metazoa. Recently, it has been shown that BST preservation represents a single major taphonomic pathway that is shared among BST deposits worldwide, by which macrofossils were preserved as carbonaceous compressions. The specific conditions that led to organic preservation in marine sediments are not agreed upon. One hypothesis has suggested that primary conditions of the physical depositional and early diagenetic environment were responsible for exceptional fossilization, whereas others have invoked sediment mineralogy or ionic abundance as controls over fossilization. Here, I present petrographic, mineralogic, microanalytical and isotopic data from 9 BST deposits from North America and South China, which demonstrate that BST preservation occurs in a unique sedimentary and early diagenetic microfacies that is common to all deposits analyzed. These data suggest that primary controls of the physical and chemical depositional environment were responsible for the preservation of BST assemblages. The microfacies that contains BST assemblages accumulated below the influence of storm waves, and is characterized by: 1. ultra fine-grained clay sediments with all particles finer than 20 µm; 2. event-driven deposition of clay only without subsequent disturbance or re-working; 3. pervasive early carbonate cements that formed near the sediment-water interface. Event deposition of ultra fine-grained sediments resulted in entombment of organisms quickly in mm to cm-scale depositional beds that also provided minimum original porosity. Early calcite cements in pore spaces and at bed tops typically constitute a low weight percent of the sediments (>10%), but are interpreted to have formed an important permeability barrier that prevented diffusion of oxidants into the sediments and facilitated the preservation of BST assemblages as organic compressions within the ultra fine-grained muds.