The Reifling event in the Northern Calcareous Alps and in the Aggtelek Mountains (Middle Triassic)
Abstract
Based on selected sections of the Northern Calcareous Alps (NCA) and Aggtelek Mountains, this study compares the time and the reason for the drowning of the Steinalm ramp. The investigated sections were chosen from the NCA (Upper Austoalpine Nappes),
1) Juvavikum: Schreiergraben, Schreierkogel/Schreyeralm
The Steinalm Limestone Formation is light grey, bedded and in dasycladaleans (Physoporella pauciforata, Poncetella hexaster, Oligoporella pilosa, Teutloporella peniculiformis) rich grainstone, Meandrospira dinarica is also present. This formation is Pelsonian in age and signs of terrestrial influence (meteoric diagenesis, carstification) were not detected.
In the Schreiergraben outcrop the lowermost layers of the Schreyeralm Limestone Formation consist of breccia. The lithoclasts originate from the Steinalm Limestone Fm. Above the breccia the limestone, bedded, homogenous ”filament” wackestone can be found. This might represent a sediment of the lower slope or toe-of-slope. Conodonts from the matrix (Gondolella bulgarica, Nicoraella germanica) indicate a Pelsonian age. 20 m above the base Gondolella cornuta, G. liebermanni and Gladigondolella tethydis suggest a Lower–Middle Illyrian age (age interval between Trinodosus–Liepoldti Subzones)
Schreierkogel: the lower part of the Schreyeralm Limestone Fm was preserved only in red neptunian dykes of the Steinalm Limestone. The microfacies is “filament” wackestone. The ages are Late-Pelsonian (Gondolella bulgarica, G. bifurcata) and Early Illyrian (Gondolella cornuta, G. praeszaboi, G. excelsa, Gladigondolella budurovi).
2) Bajuvaricum:
Bajuvaricum: Nixhöhle/Frankenfels. The Annaberg Limestone is thick-bedded, light grey, slightly dolomitized limestone with pelsparitic microfacies. Upsection it is followed by 10 m- thick, thick-bedded Reifling Limestone. The microfacies is ”filament” wackestone. The Gondolella bulgarica at its base is indicative of a Pelsonian age. Hocheck/Annaberg. The grey, thick bedded Annaberg Limestone consists of an alternation of mudstones and packstones containing Teutloporella peniculiformis. Meandrospira dinarica indicates a Pelsonian age. At the base of the
overlaying Reifling Limestone Gondolella bulgarica also suggests a Pelsonian age.
Palfau. The uppermost part of the Steinalm Limestone consists of light grey, oncoidal beds. Above it can be found thin-bedded cherty limestone (Reifling Limestone); this is very rich in conodonts although only one species (Gondolella bifurcata) is present. The age is Pelsonian.
In the Aggtelek Mountains (Silica Nappe, Alcapa Megaunit), the drowning of the Steinalm platform was studied in two sections 1) Baradla Cave, 2) Nagy-Jenei Hill. The thickness of the Steinalm Limestone and the succession of the microfacies is identical across the whole area. The microfacies succession can be followed along a strike. The lower part is represented by cyclic peritidal sediments; in the upper part there are calcarenites which are rich in dasycladaleans (Physoporella pauciforata, Teutloporella penicu liformis, Poncetella hexaster, Anisoporella anisica) and foraminifera (Meandrospira dinarica, Glomospirella semiplana) alternate with oncoidal layers. The fossils indicate a Pelsonian age.
The Steinalm Limestone is dissected by numerous neptunian dykes. According to conodont findings three age intervals can be determined: 1) Binodosus Subzone (late Pelsonian). In the insoluble residue idiomoph orthopyroxene, magnetite, ilmenite and limonite were found these indicate coeval volcanic activity, 2) conodonts of upper Pelsonian and early–middle Illyrian ages occur together. 3) Trinodosus – and Reitzi Zones.
In the Baradla Cave above the Steinalm Limestone the Schreyeralm Limestone starts with a 25 cm-thick ”filament” mudstone, followed by crinoidal packstone and an ammonoid layer. The formation contains two conodont associations: 1) in the lowermost part there are species of the Gondolella bulgarica group (G. bulgarica, G. hanbulogi, G. bifurcata),Neospathodus kockeli, G. preszaboi bystrickyi, G. presz. preszaboi occur, indicating the Binodosus Subzone. The age is also confirmed by globose Ptychitid ammonites. In the insoluble residue of the lowermost part of the Schreyeralm Limestone idiomoph orthopyroxene, magnetite, ilmenite and limonite were found. In the upper part of the ammonoid layer, there are conodonts (Gondolella liebermani, G. constricta cornuta, G. szaboi, G. excelsa) which indicate an early–middle Illyrian age (Trinodosus Zone — most part of Reitzi Zone). The gladigondolelloids in the samples are younger than the Trinodosus Subzone and suggest a connection with the open sea. On the NE slope of the Nagy Jenei Hill, above the uneven surface of the Steinalm Limestone, a red, micritic, “filament”-rich Schreyeralm Limestone was deposited and this indicates condensated sedimentation.
In both the investigated areas -- which today lie several km apart from each other -- the Steinalm ramp was drowned coevaly in the Late Pelsonian. Due to the rifting of the Neotethys Ocean the crust was thinned and the basement subsided. This caused the drowning of the Steinalm ramp in the Nothern Calcareous Alps and in the Aggtelek Mountains. Coevally with the subsidence, the upper part of the crust was dissected and the blocks were rotated, and half grabens were also formed. The Schreyeralm and Reifling Limestones were deposited on a morphologically differentiated basement.
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