Geological Structure of the Quaternary Sedimentary Sequence in the Klaipeda Strait, Southeastern Baltic - The Baltic Sea Basin - page 141

6.3.3 Other Investigations

In the Klaipeda Strait and surroundings, inter-till organogenic sediments were
found in 10 boreholes. Paleobotanical analysis was carried out in the Department
of Quaternary Researches of the Institute of Geology and Geography, Vilnius,
Lithuania. The pollen content of seven borehole sections was identified by
O. Kondratiene (including boreholes 36856, 36922 and 36888 shown in the geo-
logical cross-section, Fig. 6.2 ) . V. Šeiriene analysed the diatoms in three borehole
sections (not included in the presented geological cross-section).
In seven borehole sections, remnants of mollusc shells were found inside the
layers of organic sediments. Identification of mollusc species was carried out in the
Lithuanian Geological Survey by A. Damušyte.
The adaptation of other methods of absolute geochronology for determining
the absolute age of organogenic inter-till sediments was unsuccessful: the sedi-
ments were too old for the radiocarbon ( 14 C) method, whereas the uranium-thorium
(U-Th) method was unsuitable due to the very low content of organic matter in the
sediments.

6.4 Results

The results of IR-OSL analysis in the four dated borehole sections fall into a rela-
tively narrow time span: from 76.5

7.3 ka (Table 6.1 , Fig. 6.3 ) .
The average ages in the five inter-till layers are as follows: 95.6

± 4.9 to 114.1 ±

± 8.1 ka in borehole

35257, 82.7 ± 5.2 and 113.2 ± 7.3 ka in borehole 36856, 81.8 ± 5.2 ka in bore-

hole 36888 and 101.8 6.4 ka in borehole 36897. Each result presented here is an

average of three dating obtained on samples taken from three different sedimentary
levels. The single young data of 25.9

±

2.5 ka in borehole 35257 is probably an
anomaly due to mistakes in sampling or labelling or due to the influence of some
uncontrollable factor. Therefore, this date can be regarded as an outlier and omitted
from consideration.
The results of the investigations aimed at finding the features of glacigenic tills
which allow to temporally constrain Pleistocene tills in the Klaipeda Strait region
are discussed in our companion article (Molodkov et al. 2010 ) . The data of the
pollen analysis of organogenic inter-till sediments show that the sedimentation of
the examined deposits took place under interglacial conditions (Kondratiene and
Damušyte 2009 ) .
The results of diatom analysis of inter-till organogenic sediments indicate that
these sediments accumulated in a freshwater basin: for example, in borehole 36917
(depth 14.2-15.0 m) freshwater planktonic species like Aulacoseira granulata and
Aulacoseira ambigua prevail.
Molluscs were very poorly preserved. They were represented only by shell frag-
ments. As a result, in all investigated borehole sections it was possible to identify
only two species - Pisidium sp. and Bithynia tentaculata (Linnaeus 1758). Both
species are of freshwater origin.

±