Benthic microbial mats from deep-marine flysch deposits (Oligocene Menilite Formation from S Poland): Palaeoenvironmental controls on the MISS types

Sedimentary structures produced by benthic microbial mats have been reported from various ancient shallow-marine deposits, but examples from deep-marine settings are rare. This study focusses on the microbially induced sedimentary structures (MISS) from fine-grained clastic deposits and carbonates of the Oligocene Menilite Formation that were deposited in a deep-marine environment periodically affected by distal turbidity currents. At the study location, the MISS-bearing deposits are represented by grey to black shales with intercalations of fine-grained sandstones, siltstones and limestones. The MISS were documented in thin sections and using scanning electron microscopy with elemental mapping. Four of the recognised MISS record periods of in-situ formation, which are attributed to a significant sedimentation slowdown, ranging approximately from a few weeks to more than a year. These four MISS include laterally continuous irregularly laminated layers, carbonaceous laminae regularly alternating with calcareous laminae, networks of thin carbonaceous laminae anastomosing between peloids, and irregularly laminated lenses occurring within siltstone-sandstone layers. The other MISS reveal signatures of erosion and redeposition by turbidity currents. They include small lenses representing mat chips and irregularly laminated lenses interfingering with the surrounding mudstone, which represent torn and re-established fragments of mats. The exceptional preservation of articulated fish skeletons that form Konservat-Lagerstätten within the studied succession is explained by the presence of epibenthic mats, which covered the fossil remnants. Microbial overgrowths protected fish carcasses against decay and floating, and facilitated precipitation of early diagenetic carbonates, which enabled fast fossilisation of the remnants. This study indicates that the interpretation of depositional conditions prevailing in a deep-marine setting may benefit considerably from the analysis of MISS. It also shows that the sedimentation rate of flysch deposits may be more variable than commonly assumed, as hiatus intervals are hidden within both hemipelagic deposits and amalgamated turbidite beds. The identification of various MISS types may be significant in distinguishing turbidites from hemipelagic deposits within homogenous muddy successions.