The Osli Formation – a Holocene lithostratigraphic unit in the Danube/Kisalföld Basin, eastern Austria and northwestern Hungary

In the course of field investigations and formalisation of Quaternary deposits in the Lake Neusiedl/Seewinkel and Hanság area the Osli Formation is designated as new formal lithostratigraphic unit. It covers an area of ~200 square kilometres and, in historic times, wetlands such as swamps and peat bogs extended from Waasen in the south-eastern Seewinkel Plain to the Hanság (Kisalföld). Due to missing stratigraphic data this formation cannot be formally divided into two members but into a lower and upper section instead. The lower section of the Osli Formation was cored in the Seewinkel Plain and consists of lacustrine deposits of up to 10 metres in thickness that were presumably deposited during Preboreal. Despite the draining of the Hanság over centuries and decades of peat mining, the upper section of the Osli Formation nowadays still consists of an at least one-meter-thick succession of peat intercalated with fluvio-lacustrine deposits. The investigated peat layers at Tétényi-Hany (~5 km north of Osli) were ¹⁴C-dated, ranging in age from ~2,400 BC to 1,500 AD. ¹⁴C ages of peat profiles at Osli-Tőzegbánya (Fövenyes-tó), located ~2,5 km northeast of Osli, even date back to ~4,000 BC. Hence the 10 to 12 m thick Osli Formation can be dated as Holocene. It is underlain by Quaternary deposits of the Illmitz Formation.