Wind-induced mat deformation structures in recent tidal flats and sabkhas of SE-Tunisia and their significance for environmental interpretation of fossil structures

Abstract

Physical processes acting on leathery and cohesive microbial mats that grow in tidal flats produce a large variety of mat deformation structures (MDS). Among these processes are strong winds which sweep episodically or continuously wide and protected areas of intertidal–supratidal zones covered with microbial mats. Wind-induced MDS occur when a mat layer covering the intertidal zone is floating or loosely attached to the underlying sedimentary layers. Observed MDS triggered by wind shear in recent intertidal to supratidal flats include: i) tearing and breaking up of mats into fragments and pieces of distinct size and shape, ii) network of folds and crumpled structures related to warping and creeping of soft mats, iii) flipped-over edges along shrinkage cracks and tears, iv) rolled-up mat edges and v) wind-blown mat fragments, scattered over the supratidal zone. The observed structures association forms a succession starting from simple tearing and breaking of a mat by wind forces and subsequent crumpling and folding. With continuous strong wind shear acting upon mat surfaces, most of the flipped-over edges are oriented in the direction of wind and form along tears and crack margins; they may evolve into rolled-up edges forming thick cigar-like bodies including both mat and thin sediment layers (‘jelly roll’). Dried and non-biostabilised mat fragments are ripped off, transported landward and scattered over upper supratidal and sabkha zones. Within and intertidal-supratidal profile, the structures display a zonality which is controlled by the cohesive behaviour of mats and water-saturation of both mats and underlying sediment substrate. In the absence of recorded physical sedimentary features within the peritidal deposits, recognition and preservation of similar wind-induced mat deformation structures appear critical for environmental interpretation and indicate aeolian processes and an intertidal to supratidal flat setting, flooded intermittently during spring tide or storm events and periodically during high tide.