Geoscience Reference
In-Depth Information
varved clays cover the entire Baltic Sea floor. Local sediment transfer is common.
Numerous local overdeepenings of the late-glacial surface rapidly accumulated sed-
iments immediately after glacial retreat. As a result, a thick (tens of meters over wide
areas) veneer of sediments has been deposited. The impact on sediment loading is
less than might be expected, however, because these postglacial clays are relatively
uncompacted and have low density.
The central area of the Gulf of Bothnia is not a zone of low erosion as often
expected from its position in the central zone of maximum ice accumulation. Lower
erosion is expected in the northeastern part of the Bothnian Bay with adjacent
onshore areas. In the western offshore part of the Bothnian Sea area, along the
Swedish coast, erosion could be of the same order as in the Baltic Proper-White Sea
lowland.
rding to our time-slice computations, much of the erosion occurred
in the Early-Mid Weichselian, when ice marginal fluctuations occurred around the
modern northwestern coast of the Bothnian Sea. Erosion was also strong during the
piedmont phase and during glacial retreat.
In some ways the Swedish coast of the Bothnian Sea is comparable with the
northwestern rim of the Lake Ladoga basin and other areas where unmetamorphosed
These areas are zones of deep glacial erosion. A west-east seismic profile from
slope of the Northern Ladoga basin. Trends of bedrock topography are similar; even
comparable scarps are observed in the Bothnian Sea in connection with resistant
dolerite intrusions, but here they rise 25-30 m above the bedrock surface instead of
60-100 m in Lake Ladoga. The most intensive erosion resulted in the large negative
relief form that is today the Hörnösand Deep. The present day bottom depths here
range between 150 and 260 m, and the bedrock topography is slightly deeper. Such
values are similar to those in the deepest northern part of the Lake Ladoga basin
and to deeps in the steep coastal zone. Climate, the duration of ice activity, and ice
streams can account for lateral changes of bedrock overdeepening along the contact
zone between the crystalline rocks and the Riphean-Jotnian sediments.
Topographic similarities are connected with geological ones. In the authors'
opinion, the deeps have been formed by the selective erosion of Riphean-
Jotnian sandstones that fill tectonic basins. In the Hörnösand area erosion-resistant
Ordovician limestones, which armor the bedrock surface to the south, are absent,
thinning out at the southern slope. Here the erosion of the Cambrian-Ordovician
platform produced a composite 100-120 m scarp-like slope that faces to the north.
It is similar in form, magnitude, and lithology to the Cambrian-Ordovician slopes
and escarpments in the Baltic Proper. The axial part of the Hörnösand Deep has
sublongitudinal strike, joining to the south with a 100 m deep buried tunnel valley
expected southeast of the Hörnösand Deep and further toward the south, following
an elongated bedform with depths between 110 and 160 m below sea level.
Locally, especially around the northern slope, Quaternary deposits up to 100-
150 m thick occur in the Hörnösand Deep. A distinct acoustic appearance (Axberg
Asso
