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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 56 (1972)

Issue: 3. (March)

First Page: 664

Last Page: 664

Title: Upper Triassic Carbonate Depositional Environments in Northern Limestone Alps: ABSTRACT

Author(s): H. Zankl

Article Type: Meeting abstract


The model of facies and facies transitions of the Late Triassic in the Northern Limestone Alps shows extended shallow-water platforms separated by small basins. The basin rocks consist of thin-bedded limestones and marls with a reduced thickness of 200 m compared with the more than 1,000-m thick platform carbonates. The most characteristic lithofacies consists of red nodular limestones with a fauna of pelagic origin. These rocks are widespread in all basin sequences of the Tethys.

Toward the platform, the basin sediments interfinger with the forereef breccias of the marginal reef complexes. Tension fractures at the forereef slope were filled by basin sediments forming neptunian dikes. The central reef area consists of small patch reefs (10%) surrounded by well-bedded reef debris (90%). Reef growth is controlled by: (1) growth of a rigid organic framework; (2) production of debris by predominantly biogenic or mechanical destruction; (3) syngenetic cementation and multiple reworking; (4) a balance between constructive and destructive factors and epeirogenic movements. The reef limestone is up to 1,200 m thick.

In the backreef is the Dachstein Limestone consisting of 1,200 m of washed coralgal sands near the reef and cyclic sequences of subtidal to supratidal carbonates far behind the reef. A cycle is 2-5 m thick and starts on top of a disconformity with an argillaceous residual sediment followed by an intratidal to supratidal member with laminated dolomites ("loferite"). The dolomite is overlain by a subtidal member consisting of calcilutites and calcarenites with many colonies of the pelecypod Megalodon. About 300 "lofer cycles" are present. The cycles originate from complex processes of subsidence, eustatic sea level fluctuations, and sediment accumulation and transportation.

Toward the center of the platform, completely dolomitized tidal flats with algal mats predominate. The most characteristic member is a laminated fine-grained dolomite with shrinkage and desiccation structures. Local intercalations of bituminous shales indicate small stagnant basins.

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