Glacial Erosion/Sedimentation of the Baltic Region and the Effect on the Postglacial Uplift - The Baltic Sea Basin - page 66

are remnants that survived in shadow position, as in the Lake Ladoga basin. If so,
this supports a scenario of excavating and shaping the major bedforms by the first
glaciations, with subsequent oscillation between sedimentation and “cleaning out”
of outlet zones.
In spite of the presence of pliable presumably Lower Cambrian-Upper Vendian
sedimentary formations, the erosion of the northern part of the Baltic, the Bothnian
Bay, is mild to moderate. This is supported by both bedrock topography and the pat-
tern of glacial accumulation. The bedrock surface is rarely deeper than 130-170 m
below sea level, and somewhat steeper along the Swedish coast. The southwest-
ern area seems to have eroded, especially to the southeast from Shellefteo, but the
erosion is mild compared to the Hörnösand Deep area. In the northeastern part
of the Bothnian Bay, the bedrock surface on the Riphean sediments is 50-120 m
lower than the surrounding crystalline rocks in the coastal area of Finland south-
east of Ouly, where the Riphean-Upper Vendian Muhos formation comprises the
half-graben appendage of the major Riphean-Jotnian basin. The bedrock is over-
lain by 50-80 m Quaternary sediments (Tynni and Uutela 1984 ) . Thus, the bedrock
surface is relatively deepened, as is noted everywhere where Riphean sediments are
surrounded by harder crystalline basement, but to a lesser degree. The Quaternary
sequence suggests moderate erosion prior to Weichselian. The total Quaternary sec-
tion attains great thickness, frequently 50-100 m, and pre-Weichselian till deposits
may be expected in the southwestern parts of the basin (Floden et al. 1979 ) . Survival
of the thick and complicated Quaternary succession in the subbottom area is in
agreement with onshore observations. In the continuation of the major lowland in
the Nordkalott area, north of the Bothnian Bay, the cover is comprised of two or
more till beds, Eemian sediments are common, and even Saalian and older deposits
occur (Hamborg et al. 1986 ) . The survival of these remnants is compatible with their
location in a complicated zone of ice divide, where the flow of ice was slow and its
direction complicated with a dominance to the southeast (Hirvas and Tynni 1976 ) .
The first glaciations significantly transformed the region, by strongly eroding pliable
terrigenous formations, which, together with the consequent isostatic adjustment,
separated central sedimentary outliers of the Bothnian Sea and Bothnian Bay from
each other and from the sedimentary domain of the Baltic Sea Proper.
3.4.1 Sediment accumulation and mass balance
Sediments accumulated around the areas glaciated as shown in Fig. 3.3 . This sedi-
ment mass must, of course, match the mass of material eroded, taking into account
the redistribution of material over a wider area. Our analysis assures that this is
the case, not only today but also for every increment of erosion that occurred over
the entire Quaternary (e.g., all the glacial cycles, including the last). There is great
uncertainty regarding how the erosion is distributed between the glacial cycles, but
we make an attempt to apportion it in a reasonable fashion.
The history of ice sheet development is relatively well known for the last 25,000
years, but uncertainties of earlier ice sheet oscillations are an important factor in