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Book Chapter

Paleoecology and Facies of Upper Triassic Reefs in the Northern Calcareous Alps

By
Flügel Erik
Flügel Erik
Institute of Paleontology University Erlangen-NürnbergErlangen, West Germany
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Published:
January 01, 1981

ABSTRACT

In the Northern Calcareous Alps of Austria and Bavaria, Upper Triassic reefs are known from the Carnian (parts of the Wetterstein Reefs; Tisovec Limestones), and from the Norian and Rhaetian (Dachstein Reef Limestones; “upper Rhaetian” reef limestones; Kössen coral limestones). The Dachstein reefs developed predominantly on the southern exposed platform edges. The upper Rhaetian Reefs were formed upon shoals within the relatively shallow Kössen Basin (Rötelwand, Feichtenstein, Gruberalm), near the inner boundary of the Dachstein Platform (Stein-platte), or upon the Dachstein Platform (Adnet).

Dachstein Reefs and upper Rhaetian Reefs can be compared with respect to their generic and specific composition of the framebuilding biota, but striking differences are evident with regard to constructional types. Upper Rhaetian Reefs seem to have been developed with an initial mud-mound stage followed by a second stage in which an ecological reef formed in the turbulent zone. In the Dachstein Reefs only the second stage appears to be represented, and the distinction between a central reef area (upper reef-crest and reef-flat) with various framebuilding communities, a fore-reef slope with coarse reef breccias, and an extended back-reef area with open and restricted lagoons, is much more pronounced.

The organisms involved in the construction of the primary and secondary framework of Upper Triassic reefs consist of sessile foraminifers, segmented and non-segmented calcisponges, hydrozoans, corals, bryozoans, tab-ulozoans, calcareous algae, and many microproblematica. Reef dwelling organisms are vagile foraminifers, bra-chiopods, gastropods, lamellibranches, a few ammonites, serpulids, crustaceans, ostracodes, echinoderms, fishes, reptiles, and some algae. Our knowledge of the reef biota is strongly biased with respect to the framebuilding organisms, which have been studied in great detail during the last few years.

The most important framebuilders are corals and calcisponges, followed by hydrozoans and solenoporacean algae. Corals and calcisponges are generally restricted to different parts of the reefs, and in different zones of water energy. A critical review of the reef building organisms from the upper Rhaetian and Dachstein Reefs reveals great difficulties in their systematical treatment, especially corals, “hydrozoans,” bryozoans, and “tabulozoans,” but also clearly indicates the very strong facies control of the reef biota.

This facies control is expressed by the unique distributional patterns of the foraminifers, calcareous algae, and the microproblematica, by which different environments (and facies units) can be recognized. Another hint as to facies control is furnished by a rather regular distribution of the secondary framebuilders, and by the zonal distri-butional patterns of the various reef communities.

Using distributional patterns and microfacies types of the limestones, 12 “facies units” can be differentiated within the Upper Triassic reef and platform carbonates: these consist of restricted shelf-areas with tidal-flats, open-shelf environments, areas of winnowed edge sands, and reef complexes. The lateral arrangement of these facies units can change depending upon the paleogeographical setting in which the reefs were formed.

In spite of some success in understanding upper Rhaetian Reefs, some important questions still have to be resolved. These include the relationships between the Wetterstein Limestone reefs and the Dachstein Limestone reefs, constructional style of the larger Dachstein Reefs, and the evolution of the reef building communities through time. The latter is one of the topics which is presently being studied by the 4'Erlanger Reef Research Group,” comparing Upper Triassic reefs in the Alps with those in Sicily, Solvenia, and Greece.

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Contents

SEPM Special Publication

European Fossil Reef Models

Donald Francis Toomey
Donald Francis Toomey
Cities Service Company Midland Texas
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SEPM Society for Sedimentary Geology
Volume
30
ISBN electronic:
9781565761605
Publication date:
January 01, 1981

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