Microbial Mats in a Lower Triassic Siliciclastic Playa Environment (Middle Buntsandstein, North Sea)

The Lower Triassic siliciclastic sequence of the Germanic Basin is predominantly composed of terrestrial sediments deposited under semiarid to arid climatic conditions. In the Lower Buntsandstein unit, (non-?) marine stromatolites and oolites occur in the central part of the basin. Here we first describe the formation of microbial mats in the Middle Buntsandstein unit (Helgoland Island, southern North Sea) in a playa paleoenvironment. Microbial mats are intercalated in a depositional sequence of reddish silt- to fine-grained sandstones. Internal sedimentary fabric is dominated by even laminites, desiccation cracks, dewatering structures, internal brecciation, mm size graded layers, and small-scale oscillation ripples. Additionally, some distinct sandstone beds point towards eolian sedimentation. Units of variable thickness (mm to several cm) contain signatures of microbial mats, e.g., internal fine lamination. Several laminae show a wavy– crinkly character well known of modern biolaminites. Various laminae indicate internal spongy fenestrate fabrics comparable to fabrics produced by modern mats dominated by photoautotroph microbes. Elongated bulges related to polygons, and domical build-ups are microbial mat structures typical for sabkha or playa settings. In addition, heterogeneously shaped fragments showing fine laminations can be interpreted as chips released from the mats. In distinct layers, also ooids and other coated grains occur. Prolific microbial mats are known from a variety of ancient and modern sabkha and playa environments. In such settings, a wealth of sedimentary structures develops from growth response of mat-forming microbes to environmental disturbances.