The Middle Cambrian Burgess Shale was deposited under variable levels of oxygenation. During periods of oxygenation, low-diversity shell beds were formed and the muds were colonized by infauna. Under these circumstances pyrite was restricted to anoxic microenvironments and formed small (1 mm) discrete aggregated masses. Soft-bodied organisms were preserved under low-oxygen conditions typified by evenly dispersed pyrite, and an absence of trace fossils and shell beds.

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