Poznań Clays versus Poznań Formation in the upper Neogene of the Polish Lowlands: controversy about meaning, origin and age

Abstract

The Poznań Clays, deposited during the late Neogene, form one of the most famous lithological units in Poland, and the Poznań Formation is a lithostratigraphic unit which includes these fine-grained deposits. This formation is divided into the lower Grey Clay Member (MPLS-1 – the first Mid-Polish lignite seam and grey clays) and the upper Wielkopolska Member (green and flame-coloured clays). Nevertheless, the Poznań Clays comprise the uppermost parts of the lower Grey Clay Member and the entire upper Wielkopolska Member. This often causes confusion among researchers who incorrectly assign the position of the Poznań Clays in the lithostratigraphic scheme of the Neogene of the Polish Lowlands. This also results in incorrect determination of the age of the Poznań Clays, which in reality span the time interval between the late Middle Miocene and the earliest Early Pliocene. Such a range of meaning, covering two lithostratigraphic members, impacts the interpretation of the origin of the Poznań Clays. An additional factor complicating the understanding of the problem is the connection between the colour of the Poznań Clays and their stratigraphic position. The Poznań Clays and the Poznań Formation are often ambiguously understood, and these issues require clarification and discussion, as provided in this study.