{'fraction_non_alphanumeric': 0.02397105382180009, 'fraction_numerical': 0.005427408412483039, 'mean_word_length': 5.4678362573099415, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Eocene animal trace fossils in 1.7-billion-year-old metaquartzites

Significance Strained 1.7-billion-year-old metasedimentary rocks in southwestern Australia contain traces of burrowing animals, structures only known from the last half billion years of Earth history. As metamorphic events had made the sediment too hard for burrowing by ∼1.2 billion years ago, it has been suggested that the burrows were made during the Paleoproterozoic by early animals with no other fossil record. We found, however, that the quartzite had been deeply weathered by about 50 Mya, allowing burrowing during an Eocene flooding. Subsequent rehardening of the sediment by silica precipitation restored the impenetrable quartzite with its metamorphic fabric. However, the burrows lack that fabric and contain detrital grains that are much younger than the matrix in which they occur. The Paleoproterozoic (1.7 Ga [billion years ago]) metasedimentary rocks of the Mount Barren Group in southwestern Australia contain burrows indistinguishable from ichnogenera Thalassinoides, Ophiomorpha, Teichichnus, and Taenidium, known from firmgrounds and softgrounds. The metamorphic fabric in the host rock is largely retained, and because the most resilient rocks in the sequence, the metaquartzites, are too hard for animal burrowing, the trace fossils have been interpreted as predating the last metamorphic event in the region. Since this event is dated at 1.2 Ga, this would bestow advanced animals an anomalously early age. We have studied the field relationships, petrographic fabric, and geochronology of the rocks and demonstrate that the burrowing took place during an Eocene transgression over a weathered regolith. At this time, the metaquartzites of the inundated surface had been weathered to friable sandstones or loose sands (arenized), allowing for animal burrowing. Subsequent to this event, there was a resilicification of the quartzites, filling the pore space with syntaxial quartz cement forming silcretes. Where the sand grains had not been dislocated during weathering, the metamorphic fabric was seemingly restored, and the rocks again assumed the appearance of hard metaquartzites impenetrable to animal burrowing.