Depositional evidence of water-level changes of the Baltic Ice Lake in southern Finland during the Younger Dryas/Holocene transition

Abstract

Two natural clay sections were described from southern Finland. The sites, Jokela and Koria, are located 5-10 km south of the 1st Salpausselkä end moraine. Six sedimentary units were identified and interpreted: (1) Baltic Ice Lake (BIL) varved sediments – interbedded, laminated clay and silt with load structures and sand layers in places; (2) homogenous, partly deformed clay; (3) BIL varved sediments, deposited in slightly shallower water – interbedded, laminated clay and silt; (4) BIL drainage from Billingen and the 25-m base-level fall 11 600-11 700 BP – homogenous clay with sand pods and discontinuous sand layers; (5) freshwater varves – clay and silt couplets with lenticular and flaser bedding and load structures and (6) brackish water varves – thick clay and thin silt couplets. These units can be associated with previously described units of the Finnish clay-varve chronology. The results confirm the importance of the drainage unit as a basin-wide key horizon and illustrate the differences in sedimentation in different parts of the Baltic Sea basin. This is evidenced by the diachronous nature of the incursion of brackish water and associated facies change in sediments.