Seismic stratigraphy of Quaternary deposits in Lake Peipsi, eastern Estonia

Shallow marine seismic reflection profiling in the western part of Lake Peipsi, eastern Estonia, has revealed new information about thickness, structure and distribution of the Quaternary deposits. Additional data from numerous drillings in the area around the lake have facilitated the interpretation of the seismic profiles. The thickness of the Quaternary deposits varies over a wide range, from a few metres in the northwestern part to over 50 m in the central part. Two seismic units have been subdivided in the Quaternary cover, interpreted as till followed by glaciolacustrine and postglacial lacustrine sediments. The till layer has in general a rather complicated structure. It reaches a thickness of about 33 m in the elongated bedrock depressions found in the western part of the lake, whereas within the flat bedrock areas, it has a more uniform structure and a thickness of 10-15 m. The till is only locally exposed in the lake bottom in the northwestern and northeastern areas. The glaciolacustrine and postglacial lacustrine deposits are deposited over the major part of the investigated area. The thickness of this upper seismic unit increases gradually towards the central part of the lake, where it locally reaches up to 38 m. In the deeper parts of the lake, at water depths below c. -9 m, it is usually possible to separate the postglacial lacustrine deposits from the glaciolacustrine deposits.