Lofer cyclothema

from Wikipedia, the free encyclopedia

The Lofer cyclothema is a more or less rhythmically recurring sequence of layers in some Keuper formations in the Alps . It was first discovered in Dachstein limestone in the state of Salzburg , when a sequence of sedimentary rocks was repeated several times when they formed around 200 million years ago ( Upper Triassic ) .

The Lofer cyclotheme was scientifically described by the American geologist Alfred G. Fischer , who came to the Alps after the Second World War and saw and researched the layer sequences there in the light of the new cyclotheme theory.

It was named after the place Lofer in the Saalachtal or after the Loferer Steinberge . In this massif of the Northern Limestone Alps , the Lofer cyclothem is particularly clear in the facies of the banked Dachstein limestone . In a figurative sense, it also explains the banking (stratification) of other rocks of the Upper Triassic, as the change was probably caused by global eustatic sea ​​level fluctuations.

A typical cyclothema, for example, has a layer sequence of the form ACBACBACB. The rhythmically or irregularly changing quality of the rocks, in which a certain sequence of rock types is repeated in many cycles, is what gives rise to the striking layered structure of the mountains.

Occurrence in other mountains

The Lofer cyclothema of the Upper Triassic shows a. in the coarse banding of the following mountains of the Northern and Southern Limestone Alps :

Layer sequence of the typical Lofer cyclotheme in the Dachstein limestone

The limbs at the beginning of the cycle are only relatively thin. Often, but not always, a breccia appears in which material from the rock bank below has been worked up (e.g. by the impact of waves). This breccia is very often dolomitized . Whether present as a breccia or not, a dolomitic stratovolver typically occurs; overall, this dolomite is usually decimeter to about meter thick. Instead of it, a layer of marl appears, which can have any color: from black to pastel tones such as yellowish, greenish and reddish ( Kössen facies ), the clay content of the marl comes from the continent. Both dolomite and marl are brittle and fragile and form easily erupting layer joints in the landscape, which shape the rock mechanics of the entire mountains.

Mostly preserved as dolomite, sometimes as limestone, is the next layer of layers, which is very finely layered in parallel or laminated: millimeter-thin, flat, wavy layers of lighter and darker sediment alternate. They are interpreted as algae laminites (fossilized algae mats) that were created in the very shallow but calm water of a lagoon that is not disturbed by waves . This lagoon water heats up strongly in the sunshine and is not conducive to most marine life, so that only simple algae form a green carpet on the bottom. Petrified gas bubbles (“bird's eyes”) are embedded in the fine layers, which were subsequently filled with crystalline calcite from the underside . These laminites are, even if partly chalky, somewhat brittle due to their flat structure and form terraces in the rock walls. In other rocks of the same age, such as the main dolomite , they represent an essential layer element.

The lamination becomes lighter towards the hanging wall and finally disappears completely in a very massive lime that is at least one meter, on average three to five meters, and occasionally 20 meters thick. In this area the rock is chemically purest (CaCO 3 ), it forms steep rock walls and often bulbous overhangs over the broken- out parts of the layers below.
In these mighty banks of the Dachstein Limestone, fossils are found as gigantic shells, the megalodonts , as well as coral sticks and calcareous algae , which have often been preserved in a living position. This layer member must have been deposited on the shelf a little deeper and further away from the continent , so that the water for the coral growth was uniformly warm and unclouded and the waves in the lagoon and the tidal currents were high enough to keep the water fresh, but weak enough to not to stir up the muddy ground. On the other hand, the water depth cannot have been more than 50 meters, because a certain warming is necessary for the high level of lime production, and because organisms for which sunlight was important, namely calcium algae, still lived on the seabed itself. For megalodonts, too, it is assumed that, as with today's giant clams ( Tridacna ), a symbiosis with green algae is necessary in order to ensure the oxygen supply in warm water.

The uppermost parts of the strata banks may have been dolomitized afterwards, often they were apparently only eroded when the sea level fell. Elsewhere, the remains of this erosion are the breccias, even if they are not always clearly recognizable as they can be dolomitized afterwards.
With a bit of luck, cracks can be found in the upper parts of the massive rock banks, which reach to the surface and are filled with breccias, which are then not dolomitized but consist of white material, and their matrix is sometimes white, later limestone, typically but a clay-containing, brick-red or ocher-yellow mass, which makes these breccias nicely visible ("floating shards"). These breccias are likely to represent debris deposits that filled karst crevices and whose clasts were cemented with a loamy red earth. The red clay should have been transported by the wind and collected in the crevices of the cave. The crevices can rarely be seen right up to the top in the cut of the rock face, as they represent parts of small cave passages. Their formation through karstification requires an erosion phase in which the landscape came to lie above sea level or the sea level sank.

In less than a meter of rock sequence, the existing rock changes from a deposit area with 10 to 20 meters of water to an exposed karstified land area. The same strata of the shallower water are not present here backwards, because towards the end of the uplift the retreat phase was eroded or dolomitized. For this reason, the second shift member ("B member") is not retained in the withdrawal phase. The shift sequence is therefore “ABC ABC” and not “ABCBA”.

theory

According to today's interpretation, cyclothemes are caused by fluctuations in the sea ​​level and arise as follows: After each silting up, the sea level rises again, which leads to the silted up former seabed sinking back into the sea, initially completely flat. The sea gradually gets deeper and with it the quality of the sediment changes. Eventually the sea level will drop again and the sea floor will reappear. The uppermost part of the previously newly formed sediment can also be eroded again. Later the whole thing is gradually flooded again by the sea. In addition to the sea level fluctuations, a second condition for recording these events in the rock (“geolologic record”) must be met: more sediment must be deposited than is removed again in the subsequent erosion phases. This requires a continuous lowering of the subsoil ( subsidence ).

One might be tempted to explain the sedimentation fluctuations solely by the fact that the subsoil occasionally sinks jerkily due to earthquakes , but this explanation fails in the case of the Lofer cyclothema: Here, for example, one finds one meter above a layer that has been proven to have formed in more than 10 meters of water must be, already the breccia layer that was created by the erosion at sea level. Then, after each subsidence of the subsoil, a new uplift phase would have to take place before the subsoil subsides again. This possible explanation seems implausible.

One can also deduce sea level fluctuations from the facial peculiarities of other rocks in the world, for example in the Keuper of the Germanic Triassic (Upper Triassic Germany) one finds an alternation of sea and river sediments, also with delta formations and lateral erosion of older deltas, which indicates sea level fluctuations suggests.

In the main dolomite of the North Tyrolean facies there are considerably more cyclothems in the form of layers. Nevertheless, this is not a contradiction to the thesis of sea level fluctuations, since the main dolomite was deposited there in a higher environment and was therefore brought closer to the sea surface more often, even with smaller mirror fluctuations that left no trace in the Dachstein limestone.

The calcare cavernoso in Tuscany has an extremely holey structure. Here - with lower subsidence than in the Alps - (the entire Upper Triassic only covers a few hundred meters) the silting phases are particularly long. This is noticeable in the holey limestone facies, whereby marine life is found in the sediments of the limestone caves.

literature

  • Alfred G. Fischer: The Lofer Cyclothems of the Alpine Triassic . Kansas Geol. Surv. Bull. 1964

Web links