Trace Fossils of a Middle to Upper Ordovician Pelagic Deep-Ocean Bedded Chert in Southeastern Australia

The uppermost Middle to lowermost Upper Ordovician succession of interbedded radiolarian chert and shales exposed at Seal Creek in southeastern Australia contains a diverse, abundant, and well-preserved trace-fossil assemblage that inhabited pelagic, deep-oceanic sediments on an abandoned section of a submarine fan complex. Trace fossils occur throughout the measured section, but the types and abundance of the trace fossils vary from horizon to horizon. Planolites and Palaeophycus are common in all chert beds, while Zoophycos, Alcyonidiopsis, Compaginatichnus-like, and Teichichnus-like trace fossils are limited to the upper part of the succession. Shales interbedded with the cherts have a much less diverse and abundant ichnofauna than the siliceous layers. The appearance of Zoophycos on the deep-ocean floor in the earliest Late Ordovician may represent the first appearance of this ichnogenus in the pelagic realm, as benthic fauna migrated from shallow to deep water. The later emergence of Alcyonidiopsis at Seal Creek than in turbiditic mudstones elsewhere in the world perhaps reflects benthic migration from turbiditic to radiolarian chert facies. Similar chert facies in the Triassic-Jurassic succession of southwest Japan lack the common deep-sea ichnogenus Zoophycos but have peculiar zigzag-shaped burrows that are absent in the Seal Creek chert. This difference in ichnofauna may be an aftereffect of the mass extinction event at the Permian-Triassic boundary.