Exceptionally preserved microbially induced sedimentary structures from the Ediacaran post-glacial successions in the Kimberley region, northwestern Australia

Abstract

A set of exceptionally preserved microbially induced sedimentary structures (MISS) is described from the post-glacial Yurabi Formation of late Ediacaran age in the Kimberley region, northwestern Australia. These MISS comprise oriented grains, bamboo leaf-like cracks, polygonal cracks, worm-like cracks, sinuous cracks, gas domes, pyrite concentrated laminae, and micrite laminae. Most of the Yurabi MISS are morphologically comparable with their ancient and modern counterparts. Micrometre or nannometre-scale bacteria- or biofilm-like inorganic objects are detected from most MISS, and are interpreted as bacterial bodies or remains that have been replaced with inorganic minerals upon fossilization. Oriented grains indirectly support microbial control on their accumulation. Both pyrite conformable laminae and micrite laminae suggest degradation and decay of original microbial mats upon diagenesis. Sinuous cracks were likely generated in a soft sediment deformation process, whereas other sand cracks (bamboo leaf-like cracks, worm-like cracks, polygonal cracks) could have formed from sediment surface layer microbial organics shrinkage upon desiccation. Gas domes may have formed as the result of anoxic redox reactions and being filled with gases and sediments transported from below. Two proxies bedding plane coverage percentage and accumulative thickness of single MISS-bearing beds are developed to semi-quantitatively evaluate MISS abundance on bedding plane and vertical outcrop, respectively. The Yurabi MISS have substratum preference to fine sandstone facies and intertidal settings. The Yurabi Formation yields the most abundant and diverse post-glacial MISS assemblages during the Late Ediacaran worldwide. Both Kimberley and global fossil records indicate that MISS may have proliferated in shallow marine ecosystems soon after the retreat of Ediacaran/Gaskiers glaciation during late Neoproterozoic.