MICRO AND ULTRAFACIES OF A MICROBIAL REEF FROM THE MULDE EVENT INTERVAL (HOMERIAN, MIDDLE SILURIAN) FROM PODOLIA, WESTERN UKRAINE

The Mulde event (lower Homerian, Wenlock) of the Silurian is characterized by a positive δ13C isotope excursion, a stepwise extinction of the hemipelagic fauna (such as graptolites and conodonts) and an increase in microbial deposits. There is little known whether the proliferation of microbialites is due to reduced grazing, increased seawater saturation state or an increase in the nutrient supply. We have studied a Mulde Event-associated reef from the Muksha Formation in Bagovytsya, Podolia, Western Ukraine. The goal of this study is to identify the main components and structure of this reef and to further investigate the microproblematica present. The samples collected are being processed through the analyses of thin sections and with SEM. Preliminary results show some unusual characteristics for a Silurian reef, such as low abundance of rugose and tabulate corals. The framework of this reef is dominated by different constructors, such as stromatoporoids and heliolitid corals and the binding by calcimicrobes, which are found encrusting the stromatoporoids and shells and also forming oncoids. Several microproblematica are present, such as Girvanella, Rothpletzella and Hedstroemia, along with porostromate problematica. This study will attempt to further identify Hedstroemia and the porostromate problematica through SEM analyses which have revealed a recrystallized structure with possible microdolomite, indicating an originally high-Mg calcite composition. Dwellers include ostracods, trilobites, crinoids, brachiopods, bryozoans and rostroconchs. The reef is characterized by elevated δ¹³C values, suggesting that the atypical composition resulted from altered seawater chemistry. Our observations may help to constrain the environmental controls on the development of microbial- and microproblematica-dominated deposits during the Mulde Event. This is a contribution to the International Geoscience Programme (IGCP) Project 591 – The Early to Middle Paleozoic Revolution.