Geological Structure of the Quaternary Sedimentary Sequence in the Klaipeda Strait, Southeastern Baltic - The Baltic Sea Basin - page 136

issue has been investigated only superficially and is still waiting for solution. A
reliable geochronology of Pleistocene deposits and their stratigraphic correlation is
another so far unsolved problem: a precise number of glacial advances and their
stratigraphic rank have been the objects of intensive scientific discussions until
the present time. The petrographic composition of the gravel part of glacigenic
sediments (tills) which has traditionally been applied for stratigraphic subdivision
and correlation of Pleistocene tills in Lithuania has proved to be poorly effec-
tive (Gaigalas et al. 1987 , 1997 ) . Other lithostratigraphic methods and criterions,
such as geochemical composition of fine-grained part of tills (less than 1 mm) or
variation of well-rounded hornblende grains in tills (fraction Ø 0.25-0.1 mm), are
more effective, but, unfortunately, also do not give a clear-cut answer (Majore et al.
1997 , Bitinas et al. 1999 ) ; the fabric measurements of tills are available only in a
few cliff sections on the Baltic Sea coast (Bitinas 1997 ) . The only most positive
results were obtained by using thermoluminescence (TL) dating of inter-till sed-
iments: the Pamarys Interstadial sediments which were formed at the end of the
Medininkai (Varthanian, MIS 6) glaciation (i.e. around 140-160 kyears BP) were
identified in the big part of Lithuanian Coastal Area (Satkunas and Bitinas 2002 ) .
The Pamarys stratigraphic unit mentioned separates sediments of the middle and
upper Pleistocene, but these sediments have not been used for solution of strati-
graphic problems because they are not prevalent in the area of the Klaipeda Strait.
Notwithstanding this factor, the methods of absolute geochronology were used in
determining the stratigraphy and geological structure of Pleistocene sediments in
the Klaipeda Strait area. This chapter presents the results of IR-OSL dating of the
lacustrine inter-till sandy sediments. Besides, investigations aimed at finding out the
possibilities of till age estimation by the IR-OSL method were also carried out. For
this purpose, two till layers from boreholes dislocated along the Klaipeda Strait and
three till layers from the Olando Kepure outcrop (the Baltic Sea cliff), dislocated a
few kilometres to the north from the strait, were examined (Molodkov et al. 2010 ) .
Additionally, a number of Pleistocene inter-till sections containing organic sedi-
ments were examined paleobotanically by pollen and diatom analysis. Identification
of mollusc species was carried out.
6.2 Geological Setting
The thickness of the Quaternary cover in the Klaipeda Strait and surroundings
varies from 60 to 90 m. The upper Jurassic and lower Cretaceous sediments are
outcropping beneath the Quaternary sedimentary sequence. The Pleistocene sedi-
mentary sequence is generally composed of alternating till and inter-till sediments.
According to the results of previous geological investigations - state geological
mapping at a scale of 1:50,000 - the sediments of four different glaciations have
been detected in the sequence of Quaternary sediments (unpublished data, report
in the archive of Lithuanian Geological Survey). Till layers beneath the bottom of