Abstract: The CUCC expedition area in Austria is situated in the Northern Calcareous Alps, the northern part of the Eastern Alps. The sedimentology and structural geology of the Northern Calcareous Alps is described.

Structure and Genesis of the Eastern Alps

The caving area is situated in the Eastern Alps. The northern part of this range is known as the Northern Calcareous Alps (Nordkalkalpen). This is a band of MEsozoic limestones about 50km wide which stretches from Voral in the east to Vienna in the west. To the south is the axial zone of the Eastern Alps, and to the north is the Tertiary Molasse (Fig. 1). The Eastern Alps are considered to be made up of piles of horizontal sheets which may extend laterally for hundreds of kilometres, but which may be only a few kilometres thick. Three, possibly four, structural levels can be recognised. The basement rocks (gneiss) are overlain by Palæozoic and then Mesozoic sediments. Repetition of the sequence of Mesozoic sediments is the result of overthrusting of the sheets during the Tertiary. The strike of the rocks tends to be east-west as they are arched about an axis in this direction. Generally, the dip direction in the Northern Calcareous Alps is northward, because they are to the north of the axis.

The thrusting and arching is related to a shortening of the crust in a north-south direction, which occurred during the Alpine mountain building period. The Alps were formed when the African tectonic plate ploughed into the Eurasian plate from the south. The Indian plate similarly ploughed into Tibet, creating the Himalayas. These processes of collision are still in progress; the Alps are still rising at about the same rate as they are being eroded. If uplift were to cease, and erosion continue at the present rate, the Alps would be denuded after just three million years. This is very little on the geological time scale.

Summary of the Geological History of the Calcareous Alps

Due to subsidence in the basement in the Permian, an east-west zone of sedimentation was established. In some areas, evaporitic rocks (gypsum and salt) were laid down. These are characteristic of shallow seas in a hot climate. The facies belts were established in the Triassic, these being controlled largely by how fast the basement was subsiding. Where the rate of subsidence was fast, it outpaced deposition and the sea bottom subsided below the photic zone, resulting in deep water sediments rather than reefs. It can be concluded that subsidence was more rapid in the south of the depositional area during the Triassic. In the Jurassic, the pattern of subsidence changed, affecting the facies belts. There may have been local areas of elevation. In some areas deposition continued into the Cretaceous. During Late Cretaceous time, parts of the Calcareous Alpine facies became detached and slid over others. The main northwards movement leaving the Calcareous Alps in their present position occurred during the Eocene.

The Northern Calcareous Alps: Sedimentary Rock Types

The oldest rocks seen in the area are of Triassic age, and were produced in a submarine reef environment. The existence of three distinct reef-related depositional environments (facies) can be inferred from the rocks (Fig. 2)

1. Haupt Dolomit facies. This rock type was laid down behind the main reef, in the quiet back reef lagoon. Cycles of deposition are apparent as are periods of aerial exposure. The facies is mostly characteristic of the northern part of the Kalkalpen. It interfingers to the south with the Dachstein Limestone (Dachsteinkalk) unit. The maximum thickness of the Haupt Dolomit is 2000m.
2. Dachstein Limestone facies (Dachsteinkalk). This is the cave-bearing formation and two lithologies are present. The first is a thinly bedded limestone of shallow water origin, again behind the main reef. The thinly bedded nature of the rock causes caverns to collapse before reaching any appreciable size. The second lithology is massively bedded limestone containing abundant fossils, and is extensively cavernous. It formed the main body of the reef. The maximum thickness, and hence cave depth potential, is 1800m and the unit occurs mostly in the middle region of the Kalkalpen.
3. Halstatt facies. This consists of red and black shales and pale deep water limestones, perhaps laid down further from the land than the other two types. It occurs to the south of, and interfingers with, the Dachstein Limestone facies.

The younger Jurassic rocks correspond only slightly to the underlying Triassic in terms of depositional environment. In the North there are marls and cherty limestones, whereas in the South, reef limestone are more important. Units of coarse clastics (sandstones and breccias) occur.

Structure of the Northern Calcareous Alps

Generally, the region is not intensely folded, being more characterised by broad open flexures with tight folding being confined to fault zones. The Triassic rocks have the form of almost horizontal thrust sheets. There is little disturbance along the thrust planes and these may be difficult to recognise. The sheets are broken into blocks by steeply dipping faults. Many of the 'hading rifts' in the caves have formed along such faults. Complex deviations from this structure occur where salt and anhydrite from the underlying Permian evaporite beds have penetrated the overlying Triassic strata in the form of Diapirs, as it has in the 'salt mountain' near Halstatt. It is thus generally agreed now that the rocks of the Northern Calcareous Alps are not in their original depositional location but have been brought in from the south by thrust faulting. This has been deduced from borehole information as well as from the outcrop pattern, though how far to the south the rocks were formed is not clear, nor is it agreed whether thrusting occurred mainly along one plane or equally along several.

Guide to the CUCC area: the Totengebirgs and the Loser Plateau

A map of the area is given (Fig. 3). The Halstatt facies crops out in a fault bounded basin. In places it is overlain by the Zlambach beds and Lias marls. To the north are the Totengebirgs and to the south the Dachstein, where the massive coralliferous Dachstein limestone crops out. The Loser Plateau is in the upper right of the map: the plateau is believed to be an Upper Miocene erosion surface. walking from the car park at the Loser mountain restaurant along the Stogerweg footpath to the east, a remarkable wall of rock is seen to stnad high above the plateau. This is composed of Jurassic clastic rocks, with some Zlambach beds (uppermost Triassic) at their base. They are more resistant to weathering than the limestone. The limestone in the area immediately around the Loser carpark is the thinly bedded type, which is poorly cavernous. To find any decent sized caves one has to walk some 3km to the east where the massively bedded type is found. Unfortunately these types are not differentiated on the map. The depth potential of the area is limited to slightly more than a kilometre by the water table rather than the base of the Dachstein limestone, this being at the level of the surface of the adjacent lake Altausseer See.

Acknowledgements: I am indebted to Hans Schönlaub of the Geologische Bundesanstalt, Wien for his help in sending literature, and to Professor Oxburgh of the Department of Earth Sciences, Cambridge, for allowing the reproduction of his diagrams.

Bibliography:

OXBURGH, E.R. (1968), The Geology of the Eastern Alps, The Geologists Association, London.

PLOCHINGER, B. (1984), Nordkalkalpen, Monograph of the Austrian Geological Survey, Wien.

TOLLMAN, A. (1985), Die Nordlichen Kalkalpen.