Reinvestigation of the Upper Triassic (Norian-Rhaetian) Steinplatte “reef ? of the Northern Limestone Alps near Salzburg, Austria, revealed a clear facies zonation of this carbonate complex. The upper Kössen Beds are of a basinal facies and are composed of dark bedded limestones, poor in fossils, and partly intercalated with marls. The “Oberrhatkalk,” which is the time equivalent of the uppermost part of the Dachstein Limestone, can be subdivided into a fore-reef, reef, and back-reef facies. The fore-reef is characterized by crinoids and by reef derived particles, which were deposited on a slope with dips up to 35 degrees. The upper boundary of the fore?reef is marked by a coquina, built by shells of bivalves and brachiopods. The reef is represented by a narrow zone (less than 100 meters wide) which can be subdivided into: 1. reef-slope, with a diversified organism community of various corals, calcareous sponges, hydrozoans, tabulozoans/bryozoans, and microproblematica, embedded in a micritic matrix, and 2. reef-crest, represented as a belt of large phaceolid corals. Facies zonation consists of a back-reef facies, and a patch-reef facies which is characterized by two types of patch-reefs: one dominated by large dendroid corals, similar to the reef-crest, and the other with high organism diversity in a micritic matrix and similar to the reef-slope. Within this facies the most important organisms are megalodontid bivalves. Sediment composition changes rapidly, and the primary particles are oncoids, peloids, “lumps,” and various bioclasts. With increasing distance away from the reef, patch-reefs become scarcer and in the eastern lagoon various facies types occur, e.g., grape-stone, foraminiferal-algal, oncolithic, and oolitic. Near the eastern margin of the Steinplatte Platform intertidal algal stromatolites occur, indicating a transition into the Lofer facies of the bedded Dachstein Limestone.This facies interpretation differs widely from that postulated previously, in that a good reef zone is delineated, and the so called fore?reef breccias are now regarded as part of the back-reef sediments.
Investigations of adjacent carbonate platforms (Loferer, Leoganger Steinberge, and Steinernes Meer) demonstrate that these areas, which today are isolated by erosion and tectonics, represent the continuation of the shallow water lagoon of the eastern part of the Steinplatte Platform. The bedded Dachstein Limestone of all these platforms developed in Lofer facies with cycles of supra-, inter-, and subtidal members. The supratidal member is charac-terized by green or red marly limestones, the intertidal member by partly dolomitized Loferites (mainly algal stromatolites), and the subtidal member by megalodont limestones with various grain types. In the eastern part of Steinernes Meer the sediments change. Here, limestone was deposited in the subtidal zone and is composed of bioclastic arenites, which are mainly reef derived. The Hochkönig Massif is located eastward of Steinernes Meer and represents a large Dachstein Limestone reef, with a thickness of 700 meters. This reef fringes the carbonate platform on its southeastern edge, with well developed fore-reef breccias towards the basin. Therefore the Stein-platte Reef represents only a small part of an Upper Triassic shallow water carbonate platform, approximately 40 kilometers wide, which was fringed by reefs on the northwestern edge (Steinplatte), and on the southeastern edge (Hochkönig). Whereas the Hochkönig Reef existed throughout almost all of Norian-Rhaetian time, the Steinplatte Reef was initiated only in the uppermost Norian-Rhaetian. During early Norian-Rhaetian time the carbonate plat-form was connected to the land by widespread tidal-flats of the Hauptdolomite, and only when the Kössen Basin separated this land-connected platform did the Steinplatte Reef develop as a relatively small barrier on the margins of the Kössen Sea.
Figures & Tables
The voluminous amount of information presented in this Special Publication not only fills a gap in understanding the European approach to reef studies but also provides the necessary data base to allow us (in particular the North American geologist) to incorporate this information in our overall interpretive studies. These studies should serve as an impetus for new investigations and will broaden our understanding of the complex interrelationships that operate in the reef environment.