Geological Structure of the Quaternary Sedimentary Sequence in the Klaipeda Strait, Southeastern Baltic - The Baltic Sea Basin - page 137

the Klaipeda Strait were generally attributed to the middle Pleistocene Warthanian
(Medininkai, MIS 6) glaciation, and in some cases they were attributed to the
first glacial advance of late Weichselian (late Nemunas, MIS 2) glaciation. The
uppermost part of glacigenic sediments along the Baltic Sea coast (including the
vicinities of the Klaipeda Strait as well) is covered by sediments of the Baltic Ice
Lake, Ancylus Lake, Littorina and Post-Littorina Seas and also by recent aeolian
sediments (Gelumbauskaite and Šeckus 2005 , Kabailien˙eetal. 2009 ) .
The boreholes in the Klaipeda Strait and surroundings drilled during the
engineering geological mapping generally uncovered only the upper part of the
Quaternary sequence to an altitude of 30, in some cases 50 m below sea level
(Fig. 6.3 ) . Alternating till and inter-till sediments were established in this part of
the Quaternary. According to visual description of borehole cores, two types of
till layers were distinguished in the geological sections: brown-grey or grey-brown
till and dark grey till (at intervals with a greenish tinge). The inter-till sediments
are represented by laminated silt, sandy or clayey silt and fine-grained sand with
inter-layers of organogenic sediments - dark grey or black gyttja and dark brown
peat. Traces of glaciotectonic disturbances (micro-folds, thrust faults, micro-rafts,
i.e. glaciodislocations) were observed in the cores of inter-till sediments (Figs. 6.4
and 6.5 ) . The structure of sediments was possible to establish only in the compact
laminated (sand, silt, clay) sediments, because the cores of incompact powdery-like
sandy sediments were withdrawn disordered due to drilling technology. This tech-
nology does not enable to collect the samples in plastic tubes, so the textures of
the sand samples removed from the core barrel are destroyed. The upper part of the
borehole sections (to the depth of 2-8 m below sea level) is composed of sandy sed-
iments of the Baltic Ice Lake, organogenic sediments (like gyttja, clayey gyttja, etc.)
formed in the lagoons of the Ancylus Lake and the Littorina Sea, as well as layers of
marine sand with molluscs formed in the Littorina and Post-Littorina Seas. Recent
aeolian sediments are widely prevalent on the western coast of the Klaipeda Strait -
the Curonian Spit. In some places, 2-3 m thick layers of anthropogenic sediments
occur in the uppermost parts of the borehole sections (Fig. 6.3 ) .
6.3 Methods
6.3.1 Sampling
Fine-grained inter-till sand, in some intervals with minor inclusions of tiny parti-
cles of organic matter (limnic sediments), was sampled for IR-OSL analysis in four
borehole sections (Fig. 6.3 ) . It was very important to establish the absolute age of
organic (gyttja and peat) sediments. Therefore, three samples beneath and three sam-
ples above them were taken for IR-OSL dating in borehole 36856. Sampling of sand
layer only above the organic sediments was available in borehole 36888. In the other
two boreholes (35257 and 36897) where the samples were taken from sandy sedi-
ments, the borehole sections did not contain inter-layers of organic sediments. All