Depositional environment and taphonomy of the 'strings of beads': Mesoproterozoic multicellular fossils in the Bangemall Supergroup, Western Australia

Enigmatic Mesoproterozoic bedding-plane structures, informally known as 'strings of beads', are common in the Bangemall Supergroup in Western Australia and the Belt Supergroup in North America, The Belt Supergroup structures are formally known as Horodyskia moniliformis, and age constraints show that they are more than 350 million years older than similar 1070 Ma structures in the Bangemall Supergroup. Strings of beads in the Bangemall Supergroup have previously been interpreted as fossils of multicellular organisms based on their morphology and on statistical comparisons with a range of organic and inorganic structures. However, details of their depositional environment and taphonomy, which can provide important insights into their origin, are poorly known. This study shows that strings of beads in the Backdoor Formation of the Bangemall Supergroup were deposited in a subtidal marine-shelf environment in association with microbial mats and enigmatic dimple marks. The structures are preserved at the base of fine sandstone beds within shoaling-upward shelfal mudstone and sandstone packages deposited below storm wave-base. Following deposition from suspension, the strings of beads were bound to the sea floor by microbial mats and their associated mucilage, preventing realignment by palaeocurrents. Relationships between the strings of beads and dimple marks in offshore sandstones suggest that the latter may represent holdfasts to which the strings were originally attached.