Products of micritization: evidences of microbial activity at and below the seafloor of the Upper Moscovian epicontinental basin of central European Russia

摘要

Thin sections of the Upper Moscovian shallow-marine limestones of central European Russia exhibit various products of synsedimentary and earliest diagenetic degradation of carbonate allochems. Micritized grains and related features (microcrystalline overgrowths, oncoids, and endolithic borings) are very similar to modern micritized grains forming on the shallow protected seafloors of warm-water carbonate basins. Surface-sediment micritization in the late Moscovian epicontinental basin of central European Russia is expressed in microendolithic destruction, micrite-minimicrite precipitation in intraskeletal voids, and non-selective replacements of the original skeletal structures. Most conspicuous products of surface-sediment micritization are peloidized large foraminifers (fusulinoids and palaeotextulariids), micritic-minimicritic envelopes on brachiopod and echinoderm bioclasts, and internal micritization in echinoderm stereomal pores. The last feature is structurally controlled and where not related to other surface-sediment alterations, may be attributed to intrasedimentary degradation. Surface-sediment micritization was produced by a syntrophic microbial community that must have included phototrophic cyanobacteria and/or chlorophytes and heterotrophic bacteria and fungae responsible for the intragranular microcrystalline recrystallization and internal cementation. Microborings are diverse, include microbial and metazoan varieties. Microendolithic destruction and subsequent microcrystalline occlusion of borings was locally important in micritizing processes, although cinder-like peripheral replacements in massive bioclasts and persistence of test walls in deeply peloidized foraminifers suggest that recrystallization and internal cementation was equally or more important. Many skeletal packstones, wackestones, mudstones, and tempestites irrespectible of their inferred paleobathymetric position exhibit the signs of selective intrasedimentary degradation: peripheral mergence of massive bioclasts to micritic matrix, degraded paleotextulariid tests, and recrystallized matrix. Dense micritic-minimicritic allochems were most resistant to the degradation. These alterations are recognized as intrasedimentary micritization, the process also known in modern carbonate sediments. Possible relation to, or contemporaneity with, early intrasedimentary hardening, as well as scattered mud intraclasts with signs of grain degradation in storm basal lags, suggest that intrasedimentary micritization started very early within the muddy sediment. Environments of these alterations must have been dark, stagnant, enriched in sapropelic organic matter and decomposing it bacteria, oxygen-deficient or anoxic, reducing, with elevated pH. Intrasedimentary micritization is supposed to be largely bacterial, with possible role of heterotrophic microendoliths (primarily fungae). Early generations of dolomite formed deeper in the sediment than intrasedimentary degradation but before the sediment compaction. They tend to replace marly subtidal lithofacies. Presumably, the early dolomitization is explained by the bacterial dolomitization model.