Ch. Spötl
Sedimentologisch-fazielle Analyse tektonisierter Evaporitserien – eine Fallstudie des Alpinen Haselgebirges (Permoskyth, Nördliche Kalkalpen)

GPM 15, p. 59-69

Zusammenfassung:
Aus dem permoskythischen Salinar der Nördlichen Kalkalpen Österreichs (Alpine Haselgebirge Fm., Reichenhall Fm. partim) werden primär-sedimentäre Faziesentwicklungen beschrieben und schwefelisotopengeochemisch datiert. Der volumsmäßig überwiegende Oberperm-Anteil (Alpine Haselgebirge Fm, gekennzeichnet durch δ34S-Werte von +11.6‰ CDT (Mittel aus 179 Proben), wurde in einem E-W-streichenden Tethys-Grabenbruch unter größtenteils
extrem seichten bis supratidalen Bedingungen sedimentiert. Der oberskythisch bis tiefanisische Anteil (Reichenhall Fm.) entstammt einem karbonatisch-evaporitischen Flachstwasserbereich, dessen marine Sulfate die weltweit hohen δ34S-Signale von + 25.3‰ CDT besitzen (Mittel aus 44 Proben).
Es wird die generelle Problematik eines derart tektonisierten Evaporitkomplexes aufgezeigt und offene Fragestellungen diskutiert.

Abstract:
The Alpine Haselgebirge Fm. constitutes a wide-spread evaporite horizon at the base of the Northern Calcareous Alps (Austria). S-isotope and palynological data point to an Upper Permian and locally Upper Scythian age. According to illite-crystallinity measurements and metamorphic index minerals (in accompanying volcanics) the whole rock suite (composed of shales, silt/sandstones, carbonates, anhydrites/polyhalites, halite, scarce potash salts and basic volcanics) endured very low grade metamorphic overprint (reaching low grade metamorphic rank in the southernmost parts) during Cretaceous time.
Due to its high percentage of incompetent rocks the Alpine Haselgebirge Fm. acted as one of the main detachment horizons in the course of the Alpidic orogeny (commencing with large-scale Upper Jurassic gravitational sliding processes). The present status of these sediments is characterized by a highly tectonized appearance: Brecciated siliciclastic, evaporitic and volcanic components float in an argillaceous halite-bearing matrix. A sedimentary origin of the Haselgebirge-texture and structure can be ruled out because transitional stages between almost undisturbed sediments and highly deformed
and tectonized areas could be observed.
Investigations in the salt mines of Hall (abandoned 1967), Hallein and Hallstatt (both under production since the Iron Age) revealed undisturbed m-sized sections inside the tectonite complex, bordered by faults which have been studied in detail. S-isotope measurements of intercalated sulfates (anhydrite, gypsum, polyhalite) permit a stratigraphie differentiation of an Upper Permian and Upper Scythian evaporitic phase.
On the basis of a few dozens of sections a first evaluation of the depositional environment of the Alpine Haselgebirge Fm.
can be given.
Upper Permian siliciclastics and evaporites:
During Upper Permian time an approximately E-W-trending rift-system was invaded by the Tethys ocean from SE. Subtidal up to supratidal marine conditions prevailed during deposition of siliciclastic sediments grading up into anhydrites and halite beds (Alpine Haselgebirge Fm.). Alluvial fans, flood plains and playas (Präbichl Fm., Mitterbérg Fm., "Alpine Verrucano Fm.") engulf ed the graben to the north, west and south. Sulfate and chloride precipitation was repeatedly interrupted by mud and sand flat sediments during stages of high continental meteoric water charge.
Upper Scythian carbonates and evaporites:
During Lower and Middle Scythian time shallow marine subtidal and intertidal siliciclastic sedimentation of the Werfen Fm. prevailed in most parts of the Northern Calcareous Alps (except in the westernmost part, where the fluvial Buntsandsteinregime dominated). Due to a Tethys-wide regressional event in the Upper Scythian hypersaline conditions developed which led to gypsum and halite sedimentation in sabkha- and salina-like settings (Alpine Haselgebirge Fm. partim, Reichenhall Fm.). Rauhwackes, collapse-breccias and vuggy dolomites (containing a low-diversity fauna) in evaporite-free areas can be correlated throughout all Eastalpine tectonic units.


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