Biostratigraphy and conditions of formation of Albian and Cenomanian deposits on the south-western slope of the Ukrainian Shield (Middle Dnister region)

Abstract

Data from a comprehensive study into the lithological composition and lateral distribution of Albian and Cenomanian deposits in the Middle Dnister region (Ukraine), in the watersheds of the Smotrych–Tarnava–Studenytsia–Ushytsia–Kalyus–Zhvan–Lyadova–Nemiya–Murafa–Rusava–Markovka rivers, are presented in the paper. Cretaceous deposits of Volyn-Podillia are part of the terrigenous-carbonate subformation of the Jurassic-Cretaceous carbonate formation (J3–K1) and the glauconite Cretaceous formation (K1al3–K2), which includes gaizeous (K1al3–K2s1), siliceous-chalk (K2s2–K2k) and marl-gaizeous (K2sn–K2m) subformations. Cretaceous deposits of the Middle Dnister region are represented by the lower (Albian stage) and upper parts (Cenomanian stage).The study is focused on the distribution of micro- and macrofauna (e.g. molluscs, calcareous nannoplankton) in the Albian and Cenomanian deposits and their facies affiliation, as well as on the quantitative analysis of organic remains and palaeoecological conditions. Differences in homogeneous conditions of sedimentation within the epicontinental sea basin of the modern Middle Dnister region at the Early-Late Cretaceous boundary were insignificant. Due to a study of Cretaceous macro- and microfauna, its facies distribution, quantitative analysis, and palaeoecological conditions, it is possible to trace slight differences between close homogeneous conditions of sedimentation within the relatively homogeneous sea basin of the Middle Dnister region on the verge of the Early and Late Cretaceous. This shallow epicontinental sea was characterized by normal salinity and well-aerated warm waters with temporary strong bottom currents within shallow water and with maximum depths up to 150–200 m in some areas and soft muddy bottom of deep-water areas. Phosphatisation of the early-middle Cenomanian fauna is evidence for the important role of the Carpathian upwelling at that time. Geochemically active phosphorus is concentrated on shelf due to changes in salinity, temperature, pH, CO2 content and other parameters of the aquatic environment, which were characteristic for this region of Volyn-Podillia.

Keywords

• stratigraphy
• biochronology
• palaeoecology
• sea
• cretaceous
• ukraine