Peter A. Allison, Carlton E. Brett; In situ benthos and paleo-oxygenation in the Middle Cambrian Burgess Shale, British Columbia, Canada. Geology ; 23 (12): 1079–1082. doi: https://doi.org/10.1130/0091-7613(1995)023<1079:ISBAPO>2.3.CO;2
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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.