Facje i geochemia dolnosarmackich raf z północnych obrzeży Paratetydy na Roztoczu (Polska) i Miodoborach (Ukraina): implikacje paleośrodowiskowe

Abstract

Facies and geochemistry of Lower Sarmatian reefs along the northern margins of the Paratethys in Roztocze (Poland) and Medobory (Ukraine) regions: paleoenvironmental implications.S u m m a r y. The Middle Miocene (Lower Sarmatian = Middle Serravallian) carbonate buildups called “serpulid-microbialite reefs” that occur in the Medobory and Roztocze regions (Paratethys Basin, western Ukraine and southeastern Poland) are composed mainly of calcite precipitates. Skeletal organisms represented by serpulid tubes and bryozoans (in places) are of minor importance and comprise merely a few percent of the rock volume. They are overgrown with micritic peloidal microbialites that are the major reef component. The microbialites and serpulids/bryozoans make together a porous reef framework that is usually filled up with abundant synsedimentary fibrous cements and micritic internal sediments. Other biota is taxonomically impoverished but often rich in individuals and comprises few species of bivalves, gastropods, benthic foraminifers. In places, the serpulid-microbialite limestone masses are incrusted with coralline algae, bryozoans and nubeculariid foraminifers. Bivalve coquinas, bioclastic limestones, breccias and conglomerates are associated with the reefs. The synsedimentary precipitates represented by microbialites and fibrous cements have quite unusual geochemical characteristics. They are composed of Mg-calcite with 5–6 mole % MgCO3 in average with a very high content of strontium (1000–1650 ppm Sr). The precipitates are enriched in heavy oxygen and carbon isotopes. Fibrous cements exhibit the highest values (δ18O ca. +1.5 [PDB] and δ13C 2.8 [PDB]. It is generally believed that in the Early Sarmatian the Paratethys was a brackish basin due to restricted connections to theWorld Ocean. This is indicated by taxonomically poor biotic assemblages that inhabited the basin. On the other hand, however, the predominance of calcitic precipitates (such as microbialites and synsedimentary cements) is indicative of water highly supersaturated in respect to calcite due to high carbonate alkalinity content. Oxygen isotopic composition of the precipitates indicates that the brackish water, in which the reefs originated, had to be enriched in the heavy oxygen isotope due to strong evaporation. The low Mg content in calcite (if compared to modern tropical settings) could be the result of a relatively low precipitation temperature coupled with lower Mg/Ca ratio in Miocene than today, and the elevated Sr content may be indicative of a high precipitation rate resulting from high supersaturation. It seems therefore that the origin of the serpulid-microbialite buildups is related to a peculiar interplay of regional and local controls in the Sarmatian Paratethys. First, the relative isolation of the entire Paratethys basin resulted in the brackish water environment that could be inhabited by taxonomically poor opportunistic biota only and simultaneously enabled blooming of microbial communities. Second, a strong evaporation of the brackish water with high alkalinity content caused a supersaturation in respect to calcium carbonate and consequently a widespread precipitation of calcite (including calcification of microbial mats).