Microbially induced sedimentary structures (MISSs) from the Lower Triassic Kockatea Formation, northern Perth Basin, Western Australia: palaeoenvironmental implications

Like microbialites, microbially induced sedimentary structures (MISSs) are commonly observed in siliciclastic and mixed carbonate siliciclastic settings in the Lower Triassic, and are represented by several morphological types including wrinkle structures. However, their origin and their role in the Early Triassic ecosystem recovery is currently a hot topic of debate. Herein we report and describe abundant wrinkle structures from the Smithian Kockatea Shale Formation of Northampton area in the Perth Basin, Western Australia, on the basis of detailed field observations as well as petrographic and SEM examinations of rock thin sections. The surficial morphologies of wrinkles are characterized by closely spaced crests and ridges on the upper bedding surfaces, together with other forms such as pitted bulges and reticulated ridges. Detailed petrologic analysis revealed that the Smithian wrinkle structures from the northern Perth Basin possess a distinct vertical profile: (1) cross-stratified/lamianted sandstone, (2) alternating laminated layers and sandstone, (3) laminated layers, and (4) wrinkled layers, in an ascending order. The wrinkled layers are overlain by thin shale. Laminated layers beneath wrinkled surfaces comprise densely packed, pronounced, wavy laminae. Mica flakes in laminations show a preferred alignment parallel to bedding under SEM imaging, suggesting the binding and trapping of the former presence of a microbial matground. These evidences support that the Early Triassic wrinkle structures from the Smithian Kockatea Formation are biogenic in origin. Close association of both trace and body fossils with the wrinkle structure-bearing layers is interpreted to indicate the presence of a matground ecosystem in which some small gastropods and various trace-making animals lived, possibly taking the habitat as a refuge. The global temporal and spatial occurrences of Early Triassic MISS were tabulated and analyzed, revealing a wide range of palaeoenvironmental distributions and the seemingly selected association between metazoans and certain MISS. As to why only some MISSs have been found together with metazoans remains as an open question. It will require further investigation especially with regard to the specific type of microbes that had built the microbial matground ecosystems and their interplay with the hosted metazoans in the aftermath of the end-Permian mass extinction.