Fossilization processes of the microbial mat developing in clastic sediments of the Puck Bay (southern Baltic Sea, Poland)

Abstract

The modern microbial mat from clastic depostis of the Puck Bay coastal shallows and Holocene siliciclastic, laminated deposits were studied by means of sedimentological and geomicrobiological methods. It was discovered that the latter one originated as a result of burial of microbial mats. This is indicated by structural features of the deposits (i.e. relatively high organic matter content, lamination) and by species composition of microfossils of which cyanobacteria, green algae and diatoms are the most abundant. Amongst cyanobacteria there both coccoid and filamentous forms occurred. Filamentous forms were represented by genera Lyngbya, Oscillatoria, Phormidium and Spirulina. These are the main matforming microorganisms during the summer bloom of cyanobacteria (late August - early September). The predomination of Opephora olsenii amongst diatoms and relatively high content of filamentous cyanobacteria indicate that Holocene laminated deposits originated as a result of fossilization of microbial mats at their maximum development i.e. during the bloom of cyanobacteria. At that time the Puck Bay mat possesses the highest preservation potential. This is favored by high organic matter content, elimination of zoobenthos, low activity of hydrodynamic factors and rapid burial of the mat by the sediments. The origin of Holocene laminated deposits is connected with the Puck Bay water level rise during the last millenium.