SEISMIC STRATIGRAPHY OF QUATERNARY DEPOSITS IN LAKE-PEIPSI, EASTERN ESTONIA

Shallow marine seismic reflection profiling in the western part of Lak e Peipsi, eastern Estonia, has revealed new information about thicknes s, structure and distribution of the Quaternary deposits. Additional d ata from numerous drillings in the area around the lake have facilitat ed the interpretation of the seismic profiles. The thickness of the Qu aternary deposits varies over a wide range, from a few metres in the n orthwestern part to over 50 m in the central part. Two seismic units h ave been subdivided in the Quaternary cover, interpreted as till follo wed by glaciolacustrine and postglacial lacustrine sediments. The till layer has in general a rather complicated structure. It reaches a thi ckness of about 33 m in the elongated bedrock depressions found in the western part of the lake, whereas within the flat bedrock areas, it h as a more uniform structure and a thickness of 10-15 m. The till is on ly locally exposed in the lake bottom in the northwestern and northeas tern areas. The glaciolacustrine and postglacial lacustrine deposits a re deposited over the major part of the investigated area. The thickne ss 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 p arts of the lake, at water depths below c. -9 m, it is usually possibl e to separate the postglacial lacustrine deposits from the glaciolacus trine deposits.