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J. Jung Sandstone-Saprolite and its relation to geomorphological processes in Spessart, Germany
Sandstone-Saprolite and its relation to geomorphological
processes - examples from Spessart/
Germany as a sandstone-dominated highland-region
Ferrantia • 44 / 2005
Jürgen JUNG
Research Station for Highlands, Research Institute Senckenberg
Lochmühle 2, D-63599 Biebergemünd/Bieber
juergen.jung@senckenberg.de
Keywords: Spessart; Germany; Triassic; Sandstone; Sandstone-Saprolite; Geomorphology; Tertiary
The recent period of a temperate climate is characterized
by more or less morphological stability.
Changes in the landscapes are mainly connected
with the influence of human activities, e.g. farming
(agriculture). Actually weathering processes cover
a thin layer of the geological underground, influenced
by wetness, temperature, geomorphology,
geology, human activities and time. The results
are different soil-types, which emanate from
physical and chemical weathering in consideration
of the named factors. Deeply grounded
weathering, which affects the rocks 10 meters
and more, represents an geochemical hangover
from further geological periods. This weathering
processes depend on an palaeoenvironment,
which is characterized by an para-tropic climate.
These conditions predominated in Central Europe
nearly in the whole Mesozoic period. The red
beds of the lower Triassic unit, which are spread
over the investigation area Spessart, stand for
that palaeoenvironment (Schwarzbach 1993).
The period of sedimentation, which starts in the
upper Permian and has been continued with the
mentioned red beds, proceeded up to Jurassic,
mainly with marine layers. At the end of the
Jurassic period the marine environment changed
to continental conditions because of tectonic
activities, which led to an regional uplifting. From
the time of this geological event, morphological
activities started in the investigation area. The hothumid-climate,
differentiated in climatic optima,
held in the Cretaceous and lasted up to the end of
Tertiary. The surface was to be exposed to weathering
processes and erosion activities. The history
of the natural landscape "Spessart" started.
The current highland region Spessart is set within a
natural frame, outlined by the rivers Main, Kinzig
and Sinn. In total the Spessart spans an area of
2,260 km², mainly covered by forests. This makes
the Spessart the largest continuous forest region in
the central German highlands. With the view on
the cultural landscape the Archaeological Spessart
Project (ASP, www.spessartprojekt.de, www.pcleu.de)
acts in the Spessart. The Research Institute
Senckenberg (www.senckenberg.de) operates
with a numerous regional scientific studies. The
regional scientific interests are well founded,
because the Spessart mountains can be representative
for a big part of the German highlands.
Comparable with other regions, e.g. Odenwald,
Rhön, Schwarzwald, Solling, a.s.o., the Spessart is
dominated by sandstones as fluvial deposits from
the lower Triassic period (Buntsandstein; Fig. 1).
The Sandstones are characterized by a reddish
colour, which results from a patina of iron-oxids.
The Tertiary weathering effected an intensive and
deep-grounded decolourisation. Clay minerals,
especially caolinites were synthesised from disassociated
weathering products. Structures of
sediment-rocks, e.g. bedding and stratification
are maintained (Fig. 2), but the physical hardness
is significantly reduced. Sandstones with these
characters are defined according to Felix-
Henningsen (1990) as "Sandstone-Saprolite". They
are assigned to the pallid zone (mC(j)ew) of the
deeply grounded weathering-mantle as relicts of
the Tertiary landscape. Sandstone-Saprolites do not
exist area-wide. Some are concentrated on special
tectonic structures, which protect them against
erosion processes (Jung 1996). Furthermore they
have been found in very exposed positions. They
exist e.g. along the escarpment and the preliminary
hills. This fact is not easy to explain from a
morphological point of view. The escarpment is
adapted to the layers of the lower triassic units,
which are named as "Heigenbrücker Sandstein"
and "Miltenberger Sandstein". Palaeozoic basement
rocks, which are spread over the western part of
the Spessart, are overlaid by this sandstone-layers.
The sandstone-dominated area, which is about 70
% of the hole Spessart, is labelled by the sequence
from old units in the west and younger units in
the eastern part. This constellation communicates
with the general subsidence of Mesozoic layers,
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