Publi.complète - Musée national d'histoire naturelle

R. Mikuláš Features of sandstone palaeorelief preserved in the Osek area, Czech Republic
Fig. 3: Uppermost block of the whole sandstone area at
Osek (called Letohrádek) showing features of the Pleistocene
eolian erosion ("areoxysts").
preserved near these surfaces must be very old,
very probably of late Neogene age (Fig. 2).
Uppermost parts of the present outcrops (the socalled
Letohrádek Rock) show surfaces strongly
modelled by eolian erosion. Such a process is of
marginal importance on recent sandstone surfaces
of European temperate zones (cf. Mikuláš 2001),
even in the case of poorly lithified sandstones. In
addition, the eolian erosion, augmented by shallow
pits elongated in roughly north-south direction, is
limited to a small part of the present rock surface.
Therefore, it must represent a yet another palaeorelief
feature. The eolian erosion is expected to be
strong during the Pleistocene glacial periods; this
possibility fits well with the geological position of
the Salesius Hill area, as regards possible changes
of the mesorelief as well as the degree of preservation
of the "aeroxysts" (Fig. 3).
Ferrantia • 44 / 2005
Discussion
The unique paleontological content of the Salesius
sandstones, i.e. the root traces, shows that, during
a certain part of the Miocene period, (1) the present
rocks formed surfaces of sandstone mesorelief, and
(2) the degree of their lithification was probably
comparable to the present "soft sandstones" of the
Bohemian Cretaceous Basin (e.g., Mikuláš 2001).
Such a situation could have been transferred into
the fossil record by means of rapid silicification
of porous substrates (i.e., sand fissures and the
surrounding sandstone body). Therefore, in the
Osek area, the silicification must have taken place
on the Earth surface (because of the presence of
roots). Coupled with the fact that the whole thick
sandstone body is cemented more-or-less equally,
we can conclude that the siliceous cement probably
originated through tropical exogenous processes
(silcretization; cf. Summerfield 1983) rather than
by the effect of hydrothermal processes.
Not only karst features can be fossilized: also
features falling within the pseudokarst or
sandstone phenomenon have a certain fossilization
potential. Impregnation of porous rocks
by SiO solutions can be the process augmenting
2
the fossilization potential of an ancient sandstone
landscape. Root traces preserved in a "silicified
sandstone landscape" can be considered a reliable
indicator of a former substrate hardness (i.e. degree
of lithification) and a proximity to the surface.
Acknowledgement. The study is financially
supported by the Grant Agency of the Academy
of Sciences of the Czech Republic, Project No
A3013302 "Tectonic and volcanic controls on
hydrothermal silicification in marginal zones of
the Ohře Rift".
References
Mikuláš R. 2001. - Gravity and orientated pressure
as factors controlling «honeycomb weathering»
of the Cretaceous castellated sandstones
(northern Bohemia, Czech Republic). Bulletin
of the Czech Geological Survey 76(4): 217-226.
Summerfield M. A. 1983. - Silcrete, in A.S. Goudie
and K. Pye (ed.), Chemical Sediments and
Geomorphology: Precipitates and Residua in
the Near Surface Environment: 59-91, Academic
Press, London.
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