­­­­MICROBIAL SHAPING OF WRINKLE STRUCTURES IN SILICICLASTIC DEPOSITS

Wrinkle structures are millimeter- to centimeter-scale elongated or reticulate sedimentary structures that resemble symmetric ripples. Sharp-crested and flat-topped wrinkle structures up to 1 cm wide occur on numerous bedding planes in the Neoproterozoic and Cambrian, as well as in some Archean and Phanerozoic siliciclastic deposits. Because similar, but unlithified structures occur in some modern, microbially-colonized sands, wrinkle structures are typically interpreted as microbially induced sedimentary structures. However, it is unclear if physical processes, such as the motion of suspended sand grains, can produce similar features in sand even before microbial colonization. We introduced mat fragments to the surface of silica sand in wave tanks and generated sharp-crested, flat-topped and pitted wrinkle structures. The abrasion of the sandy surface by rolling, low density, millimeter-size fragments of microbial mats produces wrinkle structures at extremely weak orbital velocities that cannot move sand grains in the absence of light particles. Wrinkle structures form in a few hours and can become colonized by microbial mats within weeks. Thus, wrinkle structures are patterns formed by microbially mediated sand motion at low orbital velocities in the absence of bioturbation. Once formed, wrinkle structures can be colonized and stabilized by microbial mats, but the shape of these mats does not dictate the shape of wrinkle structures. These experiments bolster the interpretation of wrinkle structures as morphological signatures of organic particles and early life in Archean and Proterozoic siliciclastic deposits.