362
C. Hoselmann . H. Streif
The study is a contribution to a Priority Program of the Deutsche Forschungsgemeinschaft with the title Changes of the Geo-Biosphere in the Course of the
last 15000 Years. Under this aspect, the mass-balance study can also be regarded
as a contribution to the Core Project Land Ocean Interaction in the Coastal Zone
of the International Geosphere Biosphere Program.
2
Evolution of the Coastal Holocene
A relative sea-level rise of 100 to 130 m affected the southern part of the North
Sea basin and adjacent lowland areas in the course of the last 18000 years of the
Weichselian late glacial and Holocene periods. Radiocarbon-dated transgression levels, however, exist only for the past 8600 years and permit a detailed reconstruction of the sea-level rise from about -45 m to its present-day elevation
(Behre et al. 1984; Streif 1990; Menke 1996). The evolution of the coastal landscape took place in the final stage of the Holocene transgression under the influence of a sea-level rise of about 25 m during the last 7500 years.
Prior to 8000 B.P., a hilly Pleistocene landscape covered with forest and local
peat bogs existed in the present day coastal lowland area. At about 7500 B.P., the
first incursions of brackish water reached the area now occupied by the East Frisian Islands and were followed by the inundation of the entire coastal region. In
the course of this event, which took only 5500 years, tidal erosion affected mainly the seaward part of the inundated area, locally forming deeply incised gully
systems. In extensive areas, however, the original relief of the pre-existing landscape is preserved underneath a wedge-shaped body of Holocene coastal deposits. The transgression and the accompanying sediment accumulation were almost completed around 2000 B.P. Since then, repeated storm surges affected various locations along the coast and formed tidal bays which expanded far into the
coastal marshes. Most of these bays silted up under natural conditions or were
systematically dyked during medieval and modern times.
The general geological features of the wedge-shaped body of coastal
Holocene deposits are depicted in a schematic cross section from the North
Sea to the Pleistocene hinterland (Fig. 2). At its seaward margin, in the region
of the East Frisian barrier islands, the Holocene is made up of 25 to 35 m thick
marine and beach sands. They are overlain by locally up to 25-m-thick dune
sands derived from adjacent beach areas. Thinner Holocene sequences consisting of sandy, silty, and clayey tidal-flat to brackish deposits occur beneath
the present-day tidal-flat area and coastal marshland. The fine-grained allochthonous clastic material stems from a marine or fluvial source and accumulated in the coastal zone in the course of the Holocene transgression. Additionally, peat layers, which were formed in coastal fens, occur at the base of the sequence of tidal clastics or are intercalated in it. These autochthonous organic
deposits become thicker towards the landward margin of the coastal lowland
area, and in many places continuous peat sequences abut onto the Pleistocene
hinterland.
C. Hoselmann . H. Streif
The study is a contribution to a Priority Program of the Deutsche Forschungsgemeinschaft with the title Changes of the Geo-Biosphere in the Course of the
last 15000 Years. Under this aspect, the mass-balance study can also be regarded
as a contribution to the Core Project Land Ocean Interaction in the Coastal Zone
of the International Geosphere Biosphere Program.
2
Evolution of the Coastal Holocene
A relative sea-level rise of 100 to 130 m affected the southern part of the North
Sea basin and adjacent lowland areas in the course of the last 18000 years of the
Weichselian late glacial and Holocene periods. Radiocarbon-dated transgression levels, however, exist only for the past 8600 years and permit a detailed reconstruction of the sea-level rise from about -45 m to its present-day elevation
(Behre et al. 1984; Streif 1990; Menke 1996). The evolution of the coastal landscape took place in the final stage of the Holocene transgression under the influence of a sea-level rise of about 25 m during the last 7500 years.
Prior to 8000 B.P., a hilly Pleistocene landscape covered with forest and local
peat bogs existed in the present day coastal lowland area. At about 7500 B.P., the
first incursions of brackish water reached the area now occupied by the East Frisian Islands and were followed by the inundation of the entire coastal region. In
the course of this event, which took only 5500 years, tidal erosion affected mainly the seaward part of the inundated area, locally forming deeply incised gully
systems. In extensive areas, however, the original relief of the pre-existing landscape is preserved underneath a wedge-shaped body of Holocene coastal deposits. The transgression and the accompanying sediment accumulation were almost completed around 2000 B.P. Since then, repeated storm surges affected various locations along the coast and formed tidal bays which expanded far into the
coastal marshes. Most of these bays silted up under natural conditions or were
systematically dyked during medieval and modern times.
The general geological features of the wedge-shaped body of coastal
Holocene deposits are depicted in a schematic cross section from the North
Sea to the Pleistocene hinterland (Fig. 2). At its seaward margin, in the region
of the East Frisian barrier islands, the Holocene is made up of 25 to 35 m thick
marine and beach sands. They are overlain by locally up to 25-m-thick dune
sands derived from adjacent beach areas. Thinner Holocene sequences consisting of sandy, silty, and clayey tidal-flat to brackish deposits occur beneath
the present-day tidal-flat area and coastal marshland. The fine-grained allochthonous clastic material stems from a marine or fluvial source and accumulated in the coastal zone in the course of the Holocene transgression. Additionally, peat layers, which were formed in coastal fens, occur at the base of the sequence of tidal clastics or are intercalated in it. These autochthonous organic
deposits become thicker towards the landward margin of the coastal lowland
area, and in many places continuous peat sequences abut onto the Pleistocene
hinterland.