Geochemical and ecological aspects of lower Frasnian pyrite−ammonoid level at Kostomłoty (Holy Cross Mountains, Poland). Geological Quarterly 48

The lower Frasnian (transitans Zone with Ancyrodella priamosica = MN 4 Zone) rhythmic basin succession of marly limestones and shales (upper Szydówek Beds) at Kostomoty, western Holy Cross Mts., Central Poland, contains a record of the transgressive-hypoxic Timan Event in this drowned part of southern Laurussian shelf. The unique facies consists of organic-rich marly shales and a distinctive pyritic, goniatite level, 1.6 m thick. The faunal assemblage is dominated by pyritized shells of diminutive mollusks with cephalopods (in-cluding goniatites Epitornoceras and Acanthoclymenia), buchioline bivalves (Glyptohallicardia) and styliolinids. This interval is marked by moderately low Th/U ratios and pyrite framboid size distributions suggestive of dysoxic rather than permanent euxinic condi-tions. The scarcity of infauna and bioturbation resulted in finely laminated sedimentary fabrics, as well as the low diversity of the pre-sumed pioneer benthos (mostly brachiopods). In the topmost part of the Szydówek Beds, distinguished by the Styliolina coquina interbedded between limestone-biodetrital layers, the above geochemical proxies and C-isotope positive shift indicate a tendency to somewhat increased bottom oxygen deficiency and higher carbon burial rate linked with a bloom of pelagic biota during high-productiv-ity pulse. The geochemical and community changes are a complex regional record of the initial phase of a major perturbation in the earth-ocean system during a phase of intermittently rising sea level in the early to middle Frasnian, and associated with the highest posi