Geology in the mining areas of Imst-Tarrenz-Nassereith
The mining areas of Imst, Tarrenz and Nassereith are in the rocks of the Northern Limestone Alps, the uppermost “storey” of the East Alp rocks, in the upper East Alps.
They were deposited (sedimented) on the Northern edge of the African plate and shunted onto the European plate – in the course of the alpine orogeny – after a far push to the North – put simply. The Inn valley floor, which was shunted onto the Lechtal valley floor (now below the Inn Valley floor but according to the stone, the same age) is a several thousand metre deep rock package. These mountain forming movements were most active in the upper cretaceous and early tertiary periods (100 to 30 million years) and led to small and large folding, as well as corresponding cutting of the rocks. Almost all ore deposits and repositories can be found in the rocks of the Inn valley floor.
The rock strata (not uncovered here) start with the rocks of the youngest permian over 250 million years ago and reach into the upper cretaceous period in a time around 70 million years ago. Schematic layer profile: over a mountain forming related sedimentation pause at the time of the carbon/permian period alpine red sand stone with its base sediments and Reichenhall layers formed the deepest part of the stack of sediment – transition of river delta sediments into a very flat coastal area.
Above is the stratum of alpine shell limestone – this is the limestone that is formed increasingly further away from the coast and rare copper ore. Above that comes the 2000 metre thick weather stone as the main rock formation – it is the main carrier of the lead–zinc–molybdenum ore. In our region for example in the area of Heiterwand, Wannig or Tschirgant. The sediments first emerged in a reef area and above follows the transition into the accompanying lagoon. At the chronological level of the older weather stone limestone, shale clay and limestone can also appear, which are described as part afterlayers and were formed in not so shallow sea areas.
The next youngest rock package is the stratum of the Raibler layers, a mostly colourful alternating sequence of limestone, dolomite stone, cell limestone, shale clay, marl and typical sandstones. The occurrence of layers of gypsum is a typical feature and locally thin beds of lignite can be intercalated. Additionally flat water deposits can occur, lead and zinc ore are very rare.
The main dolomite is even younger, which equally can be up to 2000 metres thick and on the whole forms the less sharp and rather lower summits – for example Simmering or Rauchberg. This is called the lagunar milieu. Especially in the Muttekopf area stronger bituminous deposits appear, which were probably not used for mining purposes here. In the highest part a transition into the platy limestone occurs.
The highest parts of the Triassic rock strata are the Kössen layers, an alternating sequence of limestone and shale clays (only uncovered in the Lechtal valley floor) and/or the reef limestone. The rocks in the next highest system, the Jurassic are also only found in the Lechtal valley floor. At the time of the Gosau, after a time of erosion, the Gosau layers were deposited in the Muttekopf area: sandstones, marl, breccia, conglomerate. The unconsolidated sediment deposited on the bed rock emerged during the ice ages and later: moraines, breccia, conglomerate, boulders, sands, clays etc. The landslide on Tschirgant and on the Fernpass significantly altered the landscape. Even in the most recent times the looks of our alps is changing, as their formation is not yet complete. In any case people are also involved in this process now. From the 15th to 18th century it was the miners and now increasingly the stone quarry and boulders pit operators as well as the street and piste builders amongst others.
Geologist Dr. Peter Gstrein
