Faunal and facies changes at the Early-Middle Frasnian boundary in the north-western East European Platform

摘要: Multidisciplinary study of the Early–Middle Frasnian boundary in north−western East European Platform (Main De− vonian Field) allows evaluation changes facies, brachiopod, ostracod, and conodont associations, as well carbon isotope composition. Brachiopod ostracod faunas, characterized by predominance rhynchonellids, spiriferids, podocopids, platycopids Early Frasnian, palaeocopids, kloedenello− copids Middle Frasinan, demonstrate significant dominance diversity likely caused regression− transgression couplets extremely shallow−water environment. Changes represented mainly polygnathids spathognathids, are controlled sea−level fluctuations well; however evolutionary turnover is absent. Deepening pulses lead to an increasing both ben− thic nectic groups, whilst regressions cause fall due progressive habitat reduction. Most prominent de− creasing fauna observed late (Dubnik time) coinciding with regressive phase Late Givetian eustatic cycle. Positive−negative 13C excursion, detected brachiopod calcite from early interval can be correlated global isotopic perturbations near substage boundary: positive 2.6‰ excursion probably linked enhanced primary production high−nutrient re− gimes epeiric sea.