engleski

Microbial deposits in the Permian-Triassic boundary interval of the Slovenian Dinarides

Investigating the Dinaridic branch of the western Palaeotethys, microbial type limestone and dolostone have been found in the Permian– Triassic boundary interval at three localities in Slovenia. In the previously studied Lukač section, where the P/T boundary has been identified, no microbial facies were found, but rather shallow subtidal and evaporite types of sedimentation occurred. A new discovery of microbial type sediments in the Slovenian Dinarides represents a rare occurrence of this significant facies at the end-Permian extinction event. Microbial deposits can be described as grey, tiny laminated stromatolites with thickness ranging between 1.6 to 2.2 m. They represent ramp deposits, possibly its mid-part. Conodonts and foraminifers in the section are being currently studied to document the exact position of the P–T boundary. The grey microbialite comformably overlies black Bellerophon limestone that contains a rich biota of dasycladacean algae, foraminifers, echinoderms, sponges, bivalves, gastropods, ostracods, brachiopods, bryozoans and conodonts forming small skeletal buildups or reworked bioclastic limestone with bioclasts, intraclasts and coated bioclasts. The most characteristic textural feature in the laminated stromatolite bindstone is an alternation of light (microsparitic) and dark (microbial) lamina couplets. In the light laminae homogenous microspar represents enhanced calcite precipitates due to significant changes in the marine biochemistry during the P–T interval. Isolated tests of Hyperammina deformis found in these laminae suggest stressful depositional conditions which may have promoted calcite precipitation. Microbial lamina may contain a significant amount of pyrite cubes and framboids. Several distinctive microfabrics that suggest precipitation within the microbial mat are recognized within the dark organic rich laminae: i) microbial threads consist of tiny micritic threads that can be continuous, dissected or wrinkled, often lying above light gray microcrystalline calcitic laminae and sometimes encompassing large macrospar crystals ; ii) sphere clusters characterized by micrite-walled hollow spheres, sometimes scattered or forming continuous lamina that are considered to be coccoidal microbes preserved due to early mineralization ; iii) micrite clots and peloids that are elongate, completely micritic forms or have only a dark micrite rim with macrocrystalline spar interiors, and iv) sparitic microspheres that have fairly round or ovoidal shapes, sharp dark edges and a macrocrystalline interior and, due to their spheric shape and dark outer rim, resemble ooids. We are not sure about the origin of the microbial threads that envelope the macrospar crystals and the formation of peloids and finally the microspheres. Peloids with a micrite rim and macrocrystallyne interior may represent reworked irregular knots of micrite threads encompassing spar crystals, whereas microspheres may represent further reworking, microbial coating and the forming of fairly rounded detrital objects

microbial deposits; Permian-Triassic boundary; Dinarides; Slovenia

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