Palaeoenvironmental changes preceding the Middle Miocene Badenian salinity crisis in the northern Polish Carpathian Foredeep Basin (Borkow quarry) inferred from foraminifers and dinoflagellate cysts

摘要

Study of foraminiferal and dinoflagellate cyst assemblages and palynofacies occurring in 2 m-thick marls cropping out beneath a giant gypsum intergrowth unit in the Borkow gypsum quarry in Southern Poland, one of the key Badenian evaporite sections in the Polish Carpathian Foredeep Basin, has shown the presence of 49 species of benthic foraminifers and 11 species of planktonic ones, and 51 dinoflagellate (including 8 redeposited ones). The composition of the foraminiferal fauna and its isotopic signal indicate temperaturestratified, nutrient-rich and thus less-oxygenated marine water. Changes in the relative abundance of epifaunal and infaunal species indicate a clear environmental change during the deposition of the marls. A middle to outer shelf marine, well-ventilated environment with moderate primary productivity existed during the deposition of the bottom part of the marls. Subsequently, infaunal bottom-dwellers became dominant due to a massive increase in food supply to the sea bottom and shallowing of the sea to inner – middle shelf depths, and than a decreasing trend of a relative abundance of the infaunal morphogroups is observed until the top of the marls that were deposited in an inner shelf environment with moderate primary productivity. The calculated palaeotemperatures for particular foraminifer taxa (Globigerina spp., Cibicidoides and Bulimina elongata) show a slight upsection decrease and a decrease in the temperature differences between the bottom and intermediate water beds. Palynofacies are composed of elements of mixed origin, including terrestrial, marine (mainly dinoflagellate cysts) and elements of uncertain derivation (structureless organic matter). The palynological content of most samples indicates their deposition in an open-marine marine environment, in the stable marine conditions of an open shelf basin with no salinity fluctuations. The sample just below the gypsum contains no dinoflagellate cysts, perhaps due to a drastic change in the photic zone leading to a complete collapse of the dinoflagellate flora. Very rare occurrence of planktonic foraminifers in that sample suggest the shallowing of the basin accompanied by a decrease in the temperature gradient between the upper (warmer) and lower (colder) water beds. A shallow, cold water marine environment is indicated for the topmost foraminiferal assemblage.