CAMBRIAN FIRM MUDDY SUBSTRATES: SIGNIFICANCE FOR THE PRESERVATION OF TRACE FOSSILS

Although Skolithos, Diplocraterion, Arenicolites and other vertical trace fossils reflect the presence of deep tiers in sandy substrates, Early Cambrian fine-grained substrates typically had only a shallow-tiered infauna and thus a poorly developed mixed layer. A consequence of this is that these fine-grained sediments commonly developed into cohesive substrates close to the sediment-water interface which allowed for a style of trace fossil preservation that is not common in normal marine conditions later in the Phanerozoic.

The Early Cambrian Chapel Island Formation (Newfoundland), Wood Canyon Formation and Harkless Formation (western US), Mickwitzia sandstone (Sweden), Lükati Formation (Estonia), Cambro-Ordovician Bynguano Formation (New South Wales, Australia) all show direct evidence for cohesive substrates, which includes 1) the common preservation of Treptichnus pedum (and other treptichnids), which is interpreted to be a shallow tiered burrow; 2) exquisite detail of preservation of arthropod scratch marks and other trace fossil features and 3) the ubiquity of bed junction preservation (adhering burrows of Droser et al., in press).

With the development of a mixed layer in fine-grained sediments, this type of preservation presumably declined and it may, for example, have contributed to the temporal distribution of large well-preserved Rusophycus. Thus the Cambrian was not just a time of transition from matgrounds but also transition from cohesive fine-grained sediments to soft substrates characteristic of the rest of the Phanerozoic.