Fossil traces of Eocene animals in 1.7 billion-year-old meta-martzites
1.7 billion-year-old strained metasedimentary rocks in southwest Australia contain traces of burrowing animals, structures known only from the last half a billion years of Earth’s history . As metamorphic events made the sediments too difficult to dig about 1.2 billion years ago, it has been suggested that burrows were dug in the Paleoproterozoic by early animals without further fossil records. We found, however, that the quartzite had been deeply altered by about 50 Mya, allowing burial during an Eocene flood. Subsequent hardening of the sediment by precipitation of silica restored the impenetrable quartzite with its metamorphic tissue. However, burrows lack this tissue and contain detrital grains that are much younger than the matrix they are in.
The Paleoproterozoic (1.7 Ga [billion years ago]) metasedimentary rocks of the Mount Barren group in southwest Australia contain burrows indistinguishable from ichnogenera Thalassinoids, Ophiomorphic, Teichichnus, and Taenidium, known to firmgrounds and softgrounds. The metamorphic tissue of the host rock is largely preserved, and because the strongest rocks in the sequence, the meta-martzites, are too hard for animal burial, the fossil traces have been interpreted to predate the last metamorphic event. of the region. Since this event is dated 1.2 Ga, this would give advanced animals an unusually early age. We have studied the relationships on the ground, the petrographic tissue and the geochronology of the rocks and demonstrate that the burial took place during an Eocene transgression on an altered regolith. At that time, the metaquartzites of the flooded surface had been altered into friable sandstone or loose sands (arenized), allowing animals to be buried. As a result of this event, there was a resilicification of the quartzites, filling the interstitial space with syntactic quartz cement forming silcretes. Where the sand grains had not been dislocated during weathering, the metamorphic tissue was apparently restored, and the rocks again took on the appearance of hard meta-martzites impenetrable to burrowing animals.
Author contributions: research designed by SB and B. Rasmussen; SB and B. Rasmussen carried out research; SB, B. Rasmussen, J.-WZ, IRF, JGG and B. Runegar analyzed the data; J.-WZ brought new reagents / analysis tools; and SB and B. Rasmussen wrote the article.
The authors declare no competing interests.
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