Geoscience Reference
In-Depth Information
possible model variations. In spite of uncertainties of imprecise estimations of
erosion and accumulation rates in different areas, time-scale reconstructions provide
a good picture of the regional loading-unloading cycles.
Our modeling also assumes variability of erosion and accumulation rates in time
and space. For the Baltic area the largest short-term erosion rates are expected in the
case when sediments are incorporated into the ice or pushed in front of glacier on
initial advances in areas where intensive interglacial accumulation created uncon-
solidated extra-soft beds. Even on relatively hard argillaceous Late Vendian clays in
eastern Gulf of Finland, the zone of very intensive dislocation has a normal thick-
ness of 4-8 m with common thick slabs in overlaying tills. Increasing erosion rates
during rapid deglaciation are related to highly dynamic ice masses, fluvioglacial
processes, and outbursts from glacial lakes.
Modeling shows that the deepest sedimentary bedrock erosion is related to soft
formations in depressions, i.e., graben-like structures, proximal to ice-flow contact
zones between rocks of highly contrasting erodability. In such cases, hard abra-
sive material comes to the ice-bedrock contact zone, while the contact zone usually
forms a relatively steep slope, possibly providing rotational flow with a sufficient
supply of fresh firm abrasive. Major aquifers may serve as an additional factor in
bedrock removed by other mechanisms.
Knowledge of bedrock topography and measure of its overdeepening and lower-
ing from reconstructions of older geomorphic facets serve as important validation
steps in the determination of the erosion magnitude. However, it cannot be used
to judge erosion rates. In many cases, glacially shaped topography, with elon-
gated basins alternating with conformal ridges and riegels, produced multiple local
depocenters for interglacial (postglacial) sedimentation, partly being inherited. For
such basins, erosion and later sedimentation could be compared with a pendulum,
when the nature “masked its wounds.” Local zones of deep erosion appeared as
zones of thick sedimentation with maximum rates immediately after glacial retreat,
but roles reversed again on the next advance. The initial glaciation(s) affected the
bedrock, but later ones eroded glacial and interglacial deposits over wide areas
cesses could be productive, in spite of the multiple assumptions and imperfection of
our current simple tools.
The load redistribution caused by erosion and sedimentation is compensated iso-
statically. To assess this, sediment thicknesses must be converted to mass. Where
the conditions are submarine, the load is the equivalent buoyant load. Whether on
land or submerged, the porosity of the sediments must be taken into account. The
algorithms we have designed take these matters into account. Figure
3.6 (right)
shows the isostatic uplift and subsidence pattern that would be produced by the
sediment redistribution that we estimate occurred over the last glacial cycle. Full
isostatic equilibrium is assumed and the load is filtered by a lithosphere of flexu-
The modeling shows that the isostatic response to erosion and sediment loading
(Fig.
3.6 (right)
)
is significant compared to that caused by deglaciation and sea level
changes. The rise of sea level caused ca. 40 m of hydro isostatic subsidence under
