Glacial Erosion/Sedimentation of the Baltic Region and the Effect on the Postglacial Uplift - The Baltic Sea Basin - page 64

The sub-Late Vendian or sub-Cambrian (basement) peneplain is an important ref-
erence for erosion up to where it is covered by sediments that are not penetrated by
glacial erosion (Amantov 1992 ) . It can be reconstructed on the adjacent shield area
by interpolating preserved fragments of exposed peneplain under sedimentary cover
and connecting summit highs of the Archean-Early Proterozoic crystalline bedrock.
The slope of the stripped and slightly dissected peneplain presents one flange of the
lowland onshore slope of parts of Sweden and Finland (Lidmar-Bergström 1992 ) .
The exhumed surface usually has shallow dip relative to its cover and is not signifi-
cantly affected by faults. This contrasts strongly with the rugged (30-80 m) bedrock
topography on crystalline rocks on the periphery of glaciated areas. There the topog-
raphy is controlled by crystalline rock properties and structural peculiarities, like
faults and fracture zones.
Bedrock depressions are often localized in the more erodable formations. They
are separated by minor asymmetric basement highs whose steeper side faces the
shield. The shallowest and narrowest lowland of this kind occurs in the Lake Onega-
Vetryany Poyas region and on the Karelic peninsula, where elevation of the bedrock
roof is 20-40 mbsl (meters below sea level), and locally overdeepened troughs with
erosible lithology or structures can extend to 300-400 mbsl. On average the base-
ment lies 50-200 mbsl. The depth to basement gradually increases from 55 mbsl in
the eastern part of the Gulf of Finland to more than 200 mbsl in the Central Baltic
Proper, where a paleo ice stream could have been located in the Gotland Deep.
Smoothed onshore scarps and slopes often bound the lowland. They are consid-
ered to be products of selective glacial denudation. Scarps and slopes usually face
the shield, and their outline roughly corresponds to the outline of the ice at a partic-
ular glacial stage. The bounding is not distinct in areas where bedrock seems to be
worn down and smoothed by ice streams. The more resistant strata control the plains
between scarps and slopes. Evidence of their origin is provided by escarpments that
can be traced in overdeepened locations like the >100 m scarps in the zone of max-
imal erosion in northern Lake Ladoga. These scarps can be seen to be localized by
Riphean gabbroic sills that penetrated the sedimentary sequence (Amantov 1992 ,
Amantov et al. 1995 ) .
A significant percentage of the glacial erosion occurs in negative structures filled
by more erodable, usually Riphean-Jotnian sequences. Examples are the Landsort,
Åland, and Lake Ladoga deeps where the bedrock surface has been overdeep-
ened by hundreds of meters (Amantov 1995 ) . Rare thick Quaternary remnants
in protected positions indicate the decisive role of first glaciations in excavating
the troughs (Amantov 1992 ) and suggest that the subsequent glaciations mainly
removed Quaternary sediments left by their predecessors and affected the bedrock
surface in only a minor fashion. As a result, in zones of deepest erosion the tills now
present belong mainly to the last glaciation and are overlain by the late-postglacial
mantle. The last glacier could have removed 20-50 m of rocks of different density
over wide zones of maximum erosion. Locally, in narrow overdeepenings, hollows,
and glacial valleys, this figure increases to 70-90 m.
Tills, fluvioglacial, and other relevant sediments cover the peripheral accu-
mulation belt. Late Pleistocene-Holocene uncompacted sediments starting with