Evidence of epibenthic microbial mats in Early Jurassic (Sinemurian) tidal deposits, Kulla Gunnarstorp, southern Sweden

Abstract: Wrinkle structures, a type of microbially induced sedimentary structures (MISS) were found in Jurassic tidally deposited heterolites of Kulla Gunnarstorp, southern Sweden. Wrinkle structures are characterizeded by minute, commonly elongate crinkles on the upper bedding planes of fine grained arenites. These structures, as well as MISS in general, form in siliciclastic sediments due to bacterial secretion of extracellular polymeric substance (EPS), which is strongly cohesive. The wrinkle structures of the present study originated as thick epibenthic microbial mats, and are of the non-transparent type, meaning that underlying bedforms are completely concealed by the bacterial remains. Where the mat grew, EPS hindered erosion of the substrate, thus changing the premises of the physical distribution of sediments. The wrinkle structures were exclusively found on the upper bedding planes of wavy bedded micaceous arenites whose bioturbation indices range from 2–4. Microbial mat formation and preservation was facilitated since bioturbation and grazing was restricted by the fluctuating conditions (for instance regarding salinity) in the tidal depositional environment.