The northern edge of the Alps, which consists mainly of non-metamorphic sedimentary rocks, is called the "Northern Limestone Alps". These rock sequences form all kinds of landscapes, from high mountains to forested hills, are very often rich in fossils.

In our area south of Munich (between Mittenwald in the west and Kufstein in the east and between Bad Tölz in the north and Innsbruck in the south), the mountains can be divided quite well. During the folding of the Alps, the rocks were sheared from the bedrock, transported northwards and pushed over several times and are now present in larger tectonic units. This means that identical strata can be found partly on top of each other or rocks of the same age from completely different depositional areas can be found directly next to each other. Almost anything is possible here.

The age of the rocks ranges from the oldest Triassic to the Miocene or the present.

Tectonic units

In dieser Zone, die den weitaus größten Bereich der Nördlichen Kalkalpen bildet, liegen die Gesteine in bis zu vier übereinandergestapelten Abfolgen (=“Decken“) vor. Diese Gesteinsabfolgen sind nicht immer vollständig und die obersten natürlich bereits stark abgetragen. Man unterscheidet die Allgäudecke (unten), darauf die Lechtaldecke und zuoberst die Inntaldecke. An der Nordstirn dieser Decken sind noch Gesteine angepresst, die als „Randschuppe“ bezeichnet werden. Die tektonischen Abläufe und die Alpenentstehung allgemein ist sehr spannend, soll aber hier nicht vertieft werden. Es geht hier mehr um die paläontologischen Aspekte: Gesteine und deren Fossilien. Die Gesteine wurden überwiegend in einem nicht zu tiefen Randmeer auf dem südlichen Kontinent („alpiner Schelf“) abgelagert.

Age: Beginning of the Triassic until the Paleocene

  • Hazel Mountains (Late Permian - Scythian): Salt rocks, mostly tectonically squeezed out.
  • Reichenhaller Schichten (Scyth - Anis): predominantly dark grey limestones and marls. Shelf sea sediment, basin area.
  • Wetterstein limestone (Ladin): light grey, brown and very pure limestone. The shallow water equivalent ("reef sediment") to the Reichenhaller layers.
  • Raibler layers (Ladin): predominantly clastic sediments - sandstones, marls, but also evaporation series with calcium sulphate (gypsum), dolomite and limestones. There was a crisis in the earth's history so that the limestone deposit temporarily collapsed.
  • Main dolomite (Nor): brittle rock that breaks into angular pieces when weathered. Formed in a very shallow sea (tidal flat area). As a special facies, Seefelder Schichten - bitumen-rich rocks formed in anaerobic, enclosed areas.
  • Plattenkalk (Nor): grey and brown limestones from a subsiding shelf area without intermediate marl layers.
  • Kössen strata (Rhät): dark grey limestone-clay alternation sequences, deposited in basin areas. There are also corresponding massive reef sediments ("Kössener Kalke").

In the Jurassic, the alpine shelf was very strongly structured and several different depositional areas can be distinguished. Depending on the depth or other conditions, the most diverse rocks are present.

  • Allgäu strata (Hettangian - Aalenian): grey spotted limestones and marls, often in alternating sequences. Formed in a basin area.
  • Kendlbach strata (Rhät - Hettangium): grey, brown, partly sandy clays and marls, calcareous in the upper part. Formed at the transition from the basin to the sill.
  • Schnöllkalk (lower - middle Hettangian): characteristic, colourful, partly iron-rich limestone.
  • Adnet strata (upper Hettangian - Aalenian): reddish brown, mostly nodular limestones with marl interbeds. Formed as deep swell sediment.
  • Hierlatzkalk (predominantly Sinemurium): echinoderm debris lime, sill.
  • Scheibelberg limestones (upper Hettangian - Sinemurian): grey, brown, red, siliceous limestones, formed at the transition from sill to basin.
  • Klauskalk & Kieselkalk (Middle Jurassic): reddish brown siliceous rocks, deposited at great depths.
  • Ruhpolding radiolarite (upper Jurassic): red, purple and green deep-sea sediment, mainly consisting of radiolarians. Breaks into small pieces.
  • Ammergau strata (Upper Jurassic - Lower Cretaceous): brown limestones with siliceous concretions in some areas. Also called "aptych layers". Predominantly deposited at a depth where ammonite shells dissolve but the calcitic aptychs do not (CCD = calcite compensation depth).

  • Schrambach strata: dark grey marls. With these rocks, the stratification sequence in the Karwendel and pre-Karwendel mountains often ends.
  • Other chalk sediments: in the Cenomanian, another marine advance took place on the alpine blankets. Conglomerates are occasionally found. The same applies to the Tannheimer and Losensteiner strata, which are only found with a very low thickness.
  • Gosau: in various areas of the Northern Limestone Alps sediments of the Upper Cretaceous can be found, which have been deposited in small basins and are still preserved. These so-called Gosau deposits are shallow water deposits - marl predominates. The nearest deposit is in Brandenberg am Rofan.
  • Tratenbach layers: (see Tertiary)
  • Tratenbach strata: The youngest rocks preserved on the northern edge of the rim scarp - sandy limestones and sandstones that can be placed from the Upper Cretaceous to the Tertiary (Palaeocene) on the basis of microfossils.

The rocks of flysch are deposits from a former deep-sea trench that formed in the Cretaceous on the northern front of the approaching alpine blankets. A recent example would be the deep-sea trench off the coastal Cordillera in South America. A small part was included in the mountain building and is present today at the surface north of the calcareous alpine zone.

Age: Chalk

  • Tristel Formation: Sedimentary cycles of limestone-marlstone and mudstone predominate. The thick beds may also contain calcareous rock fragments and coarser biogenic debris. Quartz-bearing sandstones occur only subordinately. The rocks of the Tristel Formation are very resistant to weathering. Age classification: Lower Cretaceous.
  • Rehbreingraben Formation: Formerly called "Flysch-Gault", the series consists of dark grey and greenish grey pebbles whose limestone content increases steadily from the older to the younger sequences. The benches are not infrequently up to 3m thick! Characteristic are the greasy-oily shiny grey-black sandstones occurring in the lower part of a cycle, which are also called "oil quartzites". Age classification: Upper Lower Cretaceous.
  • Lahnegraben Formation: Characteristic are red, green, grey and black clays and clay marls. Sandstones occur again in a subordinate position. Age classification: Upper Lower Cretaceous - Lower Upper Cretaceous.
  • Ofterschwang Formation: This is dominated by grey limestone marl up to several metres thick, which is conspicuously light grey and silky-glossy and breaks in a slate-like to lath-like manner. Sandstones and mudstones occur subordinately. Age classification: uppermost Lower Cretaceous - lowermost Upper Cretaceous.
  • Reiselsberg Formation: Characterised by sandstones of varying grain size that weather rust-coloured. They are rich in quartz, muscovite and rock fragments. Banks several decimetres thick are typical, but more than 10m are possible! Calcareous and clayey deposits from turbidity currents also occur subordinately between the sandstone layers. Age classification: lowest Upper Cretaceous.
  • Piesenkopf Formation: The oldest rocks of the former "cement marl series" are platy, thin-banked (20cm at most) sandstones, marls and grey to variegated mudstones (red, green). They are also deposited from turbidity currents, with the typical greenish deep-sea clay after each cycle. Age classification: Upper Cretaceous.
  • Limestone Graben Formation: Compared to the overlying Hällritz Formation, the sedimentation cycles are often incomplete - before one submarine avalanche could be completely deposited, the next one already came. The hard beds are thinner, finer-grained, the calcareous marls are often thicker than 50 cm and weather conspicuously ochre. Glauconite is only present to a lesser extent. Age classification: Upper Cretaceous.
  • Hällritz Formation: The uppermost unit of the former "cement marl series" consists of sandstones, which sometimes also contain cherts, as well as marls and mudstones. The rocks are often up to 3m thick recurrent sequences (so-called bouma cycles), which are not infrequently complete. The deposits originate almost exclusively from submarine avalanches: loose sediment from the shelf, probably triggered by earthquakes, slid off and into the deep-sea trench. In the process, more and more material was swept away. This avalanche was then deposited in the deep sea, sorted by gravity. In the lowest, thick banks of the cycles, one often recognises a graded stratification, i.e.: coarse material at the bottom, fine at the top, with all transitions. This is followed by finer-grained rocks such as limestones and calcareous marls. The top rock of such a cycle is always a grey-green deep-sea clay, the actual sediment in such a deep depositional space. In the Hällritz Formation, the hard beds are particularly thick and the components within them particularly large. Typical is a clear glauconite lead in the sandstones. The limestone marls are never thicker than 50cm. Age classification: Upper Cretaceous.
  • Altlengbach Formation: Characteristic is the common occurrence of calcareous sandstones and muscovite-bearing "Mürbsandsteins". This youngest unit of the local flysch zone is only rarely preserved and the upper part has been cut by erosion. Age classification: Upper Cretaceous to Old Tertiary.

The few surviving Helvetic rocks were deposited on the northern continent, onto which the calcareous-alpine sequences of the southern were eventually pushed. In the process, some successions were passed down. They were welded to the northern edge of the flysch zone.

Age: Upper Cretaceous and Old Tertiary (until Eocene)

  • Seewen Formation (Seewerkalk, Cenoman-Turon): grey limestone, very rarely exposed (e.g. at Tegernsee).
  • Amden Formation (Coniac-unt. Santon): grey, partly sandy marls.
    • Stallauer Grünsandstein-Formation (oberes Santon bis unteres Campan): grüner feinkörniger Sandstein, der zu 50% aus Glaukonit besteht.
    • Pattenau Formation (upper Campan): grey marls.
    • Gerhardsreit Formation strata (Maastrichtian): dark grey, sandy, micaceous marls.
    • Hachau Formation (Maastrichtian): Sandstones and sandy marls.
    • Glauconitic sandstone (Palaeocene): glauconite-rich quartz sandstone.
    • Alveoline layers: (lower Eocene): grey-brown sandstone.
    • Assiline sandstone (Eocene): brownish micaceous sandstone with assilines.
    • Enzenau marble (Eocene): reddish brown quartz-rich limestone with rock-forming nummulites.
    • Stockletten (Eocene - Oligocene): grey marls.

    When the Alpine blanket stack arrived at the northern continent, the deep-sea trench (flysch zone) was closed. However, due to the uplift, a shallow marginal sea formed in the north instead, into which the erosion debris of the early Alps was poured. This so-called Molasse Sea existed for many millions of years. Due to constant subsidence, there are several thousand metres of sediment in this basin today. The southern edge of the Molasse Basin was also affected by mountain building and is folded.

    Age: Tertiary (Oligocene - Miocene)

    • Clay marl layers (Rupel): marine mudstones and marls.
    • Building stone layers (older chatt): predominantly marine sandstones.
    • Cyrenenschichten (Chatt): brackische Mergelkalke und Mergel, wechsellagernd mit Kohleflözen.
    • Lower Coloured Molasse (Chatt): marine sandstones and marls.
    • Promberger Schichten (younger Chatt): marine sandy mudstones and marls.
    • Upper Variegated Molasse (Aquitan): marine to brackish sandstones and marls.

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      hettangian ammonites

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