Holocene Evolution of the Southern Baltic Sea Coast and Interplay of Sea-Level Variation, Isostasy, Accommodation and Sediment Supply - The Baltic Sea Basin - page 242

of the sandy deposits undulates only slightly and points to a connected Late Glacial
fluvio-lacustrine system in the Pomeranian Bight, the Rügen lagoons and the Darss-
Zingst area (Lampe 2005 ) . Shallow water-filled depressions, which remained after
the final drainage of this system, accumulated freshwater mud or lake marl during
the Early Holocene and mostly silted up in the mid-Holocene.
By contrast, in the area west of the Fischland, the coastal relief is widely char-
acterized by higher ground with long till cliff sections. The abrasion platforms in
front of the cliffs are covered mostly by lag sediments and show a steeper inclina-
tion towards the Baltic basin. Sandy sediments are rare. Only in the Wismar Bay
a more complex palaeo-valley system determines the relief, incised in the loamy
ground moraine (Harff et al. 2007 ) . The main differences between the coastal sec-
tions located east and west from the Fischland, therefore, are (i) the existence of
depressions reaching far below the recent sea level, (ii) the availability of sandy
material from both offshore and onshore sources and (iii) the inclination of the
palaeo-relief towards the proper Baltic basins.
12.4.3 Structure and Volume of Coastal Barriers
The rising Baltic Sea caused groundwater rise in the adjacent coastal mainland
and thus favoured peat accumulation upon the land surface. This 'basal peat',
mostly some centimetres to two decimetres thick, was later inundated due to the
landward-migrating shoreline. The transgression contact is often marked by a hia-
tus of several hundred years due to the erosion of the peat top layer. Usually,
a clear nearshore/beach facies cannot be observed or consists only of 1-cm-
thick sandy layer. These deposits are overlain by muddy sediments consisting of
silt with different admixtures of fine sand and up to 25% organic matter. The
lowermost section is strikingly enriched with shells of Hydrobia sp., Cerastoderma
sp. and Scrobicularia sp., indicating an evolutionary stage where the barriers not
yet existed and the present lagoon areas were still bays of the Baltic. The mud grad-
ually changes into sand which built the main base of the barrier. At the seaside
of the barriers the grain size is coarser, sometimes gravelly, while at the lagoon
side, mud-sand interlayerings are observed, which change upwards into fine sand
(Fig. 12.6 ) .
Peat is never intercalated in the siliciclastic sequence; only allochthonous flo-
ral detritus layers occur. This is an important difference from the coastal sediment
sequence described for the southern North Sea (Behre 2003 , Streif 2004 ) and
underlines the statement that no significant sea-level fluctuation occurred during
its accumulation. At the sea coast the surface of the marine sand package is covered
by progradational beach ridges and dunes, and on the lagoon side the barrier surface
is flat and covered with fenland peat ('cover peat'). Radiocarbon datings show that
the cover peat accretion began at about 800-1,200 year cal BP (Jeschke and Lange
1992 , Lampe and Janke 2004 ) .
The dense net of boreholes in combination with about 80-km GPR tracks, which
facilitate interpolation between the drilling profiles, allows for modelling of the base