Organic shales in some Polish Eemian deposits: the case study of the Srebrna palaeolake

Abstract

Lacustrine Eemian Interglacial deposits are widely preserved in Poland, mainly beyond the maximum extent of the Vistula Glaciation. In some profiles, specific deposits are present, characterized by a black colour, a very high amount of organic matter and very hard shale-like texture. We describe these organic shales from a newly located occurrence, the Srebrna site (N Mazovia). The presence of fossil aquatic plants including algae suggests that these shales formed in shallow, eutrophic lakes that were gradually overgrown and/or transforming into peat bogs. The organic-rich sediments were subjected to repeated drying and compaction, leading to the formation of these hard, shale-like layers. No clear correlation was found between the occurrence of these deposits and the lithology of the overburden and underlying strata. The formation of these shales is also not dependent on the thickness of, and hence compactional pressure exerted by, the deposits overlying them. This particular lithology is not restricted to one time interval, but global factors occurring in the E5 zone of the Eemian Interglacial may have contributed to water level fluctuations at many locations. This suggests that factors such as short-term drainage and flooding cycles may have influenced the formation of these organic shales.