Explanatory notes to the digital geological map of the Rax ... - KATER

KATER II Geology of the Rax-Schneeberg-region
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Abstract
Within the karst water system the geological setting is responsible for the collection, filtering,
storage and distribution of karst water on its way down from the surface through permeable
rocks to the spring. The geological setting provides boundary conditions to apply general
models on specific aquifers as well as to interpret measurements of water parameters (e.g.
discharge curves, water chemistry, isotopes and others).
Due to the complex geological genesis of the project area the geological mapping had to be
accompanied by additional investigations in lithostratigraphy, biostratigraphy, micropalaentology,
carbonate facies (lateral transitions) and tectonics.
Mesozoic carbonates are the dominant rock type within the Northern Calcareous Alps,
whereas siliciclastic sediments are restricted to a few stratigraphic levels and act as layers of
reduced permeability. The following sedimentary succession forms the Schneeberg/Rax area
(the mentioned thicknesses may vary considerable due to tectonic cutting):
The sedimentary succession starts with Permian continental red beds (Prebichl-Fm.; 0-150
meters), transgressing discordant over Early Paleozoic rocks of the Greywacke Zone. A
marine clay-evaporite association is locally developed and may affect the water quality with
sulfates. The Early Triassic is characterized by deposition of some 100 meters of marine
shallow shelf siliciclastics (Werfen-Fm.). They represent the main layer of low permeability.
In Middle Triassic times carbonate sedimentation became prevailing with well bedded
(Gutenstein-Formation; 150 m) and massive limestones and dolomites (Steinalm-Formation;
80-100 m). During the Middle Anisian a rapid deepening of the marin environment was
associated with blockfaulting. This has generated a pronounced relief of the sea-floor, which
is the reason for the subsequent development of different sedimentary facies: shallow-water
carbonate platforms (Wetterstein-Formation and lateral slope sediments) coexist with basins
of different shape, water depth and circulation. Four types of basinal sediments occur in the
investigated area: black, thin bedded allodapic limestones (Grafensteig-Fm., 300 m), nodular
limestones with chert (Reifling-Fm.; 20-30 m), light grey to reddish, nodular, bedded or
massive limestones (Hallstatt Limestones; 40 m) and bedded to massive multicolored
allodapic limestones (up to 250 m).
In general, the Wetterstein platforms show a rapid progradation of reef and reef breccias
over the adjacent basinal sediments. The thick platform carbonates (up to 1000 m) build the
main aquifer in the Hochschwab and Schneeberg/Rax region. The primary porosity of these
bioclastic sediments has been lost due to early diagenetic cementation. During diagenesis
also parts of the Wetterstein limestone as well as parts of the adjacent basinal limestones
became transformed into dolomite. Extended bodies of dolomitic rocks within the aquifer are
responsible for lower percolation rates and a more regular discharge of some springs.
The prevailing carbonate producing organisms within the platform lagoon were calcareous
green algae (dasycladaceans). The knowledge about their distinct evolution during Triassic
Times has been proved recently by correlation with conodont zonation and can be used now
as a powerfull tool for biostratigraphic dating. Tectonic offset along fault zones within
monotonous platform carbonates could be recognized and sometimes quantified in this way.
In the early Upper Triassic (Carnian) the platforms emerged and the basins received
sandstones and shales (10-30 m). In the Late Carnian a sea-level rise has initiated the
development of the Norian carbonate platform (up to 1000m) characterized by Dachstein
limestone (reef and lagoonal facies) and Hauptdolomite (intertidal facies).
Most of the following younger rocks are of minor significance for the lithological composition
of the karst aquifer but relevant for revealing the structural history.
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