Steinplatte Ramp

The famous Steinplatte “reef”, whose crest is cropping out at the Sonnenwände hill north of Waidring and Lofer (Tyrolic Alps, SW Salzburg, Austria) was situated on the northern slope of the Upper Triassic Carbonate Platform (Fig. 18, 19).

Fig. 18. (A): Location of Steinplatte mountain. (B): Location of sampling sites at Steinplatte. (C): Schematic sketch of the outcrop "red wall" at Kammerköhr-Alpe (tourist trail along northern slope of Steinplatte/Plattenkogel, view to the south). Jurassic onlap sedimentation comprises red Adnet limestones with several slump folds, slided megabreccias (checkered) and debrites (dotted).

As facies analysis by Stanton and Flügel (1989; 1995) have shown, only a minor part of the Steinplatte buildup is formed by a real reef framework, thus the whole complex is rather described as an “accretionary distally steepened ramp” the top of which was partly overgrown by separate bushlike corals (capping facies) in a late Upper Raethian stage. At the end of Triassic, the coral growth stopped, whereas the paleo-relief of the carbonate platform still existed until Middle Liassic time. Just on top of the buildup the Triassic-Jurassic interval encompassing the decease of the coral fauna is concealed by a small sedimentary break. Only some traces of fresh-water diagenesis in the capping facies might indicate short-termed falls of the sea level at Steinplatte buildup (Stanton and Flügel 1989). In contrast to the elevated platform position, sedimentation continuously passed into grey cherty limestones in adjacent basins (Hettangian Kendlbach Formation and Hettangian-Sinemurian Scheibelberg Formation). Predominantly the latter are characterized by varying, often high amounts of siliceous sponges and/or siliceous bulbs (Mostler 1986; Krainer and Mostler 1997). They are frequently exposed at the toe of Steinplatte buildup slope at Kammerköhr Alpe and also at several outcrops in the Unken syncline (e.g. at Scheibelberg locality and at Karnergraben) that extends north of the Steinplatte buildup.

First Hettangian to Sinemurian sedimentation at the northern slope of Steinplatte “reef” shows an onlap of red condensed limestones indicating a drowning of the platform during Lower Liassic time. A wedge of these sediments is cut by a trail surrounding the Plattenkogel hill, north of Steinplatte summit (Fig. 18C, 19).

Fig. 19. Northern slope section of Steinplatte “Reef" (N of Waidring, Austria) and its paleogeographic position.

At the “red wall” of this site a big massflow of Adnet limestones crops out comprising slump folds and megabreccias that slided down onto grey limestones of the Scheibelberg Formation (Garrison and Fischer 1969; Wächter 1987). At the south-eastern margin of this outcrop the Adnet limestone pinches out along Triassic coral “reef” limestone. The onset of Liassic sedimentation is preserved in small crevices or interstices of the rough Triassic relief.

Samples of the spiculite facies were taken at the northeastern slope of the Plattenkogel hill (locality S1), where a spiculite covers a small clastic sequence that was also sampled to evaluate the emergence of the sponge fauna. The sequence most likely provides the oldest Jurassic limestones at Steinplatte buildup slope, thus giving a small insight into the sedimentary conditions closely after the Triassic-Jurassic boundary event. In the Alpine region, the T-J boundary interval is often characterized by sedimentary breaks and ferromanganese crusts. Since a small Fe/Mn crust was also found in association to the Liassic sequence at Steinplatte/Plattenkogel hill, it was of interest to compare its geochemistry with those of other Fe/Mn-crusts (see chapter 8.2.) from famous localities of the Northern Calcareous Alps (localities S3 a,b,c, see supplement 1.2).

Another site at Steinplatte, showing the contact between Triassic and Jurassic sedimentation is located at the Steinplatte summit, called “Fischer’s Coral Garden” (locality S2). It displays limestone of the capping facies in which gaps and interstices of the Triassic coral framework are partly filled with Liassic red sediments.

4.10.1. Plattenkogel hill (ST) Geological Setting:

§ The Plattenkogel hill (TK25 / Blatt 8341 Seegatterl, R⁴⁵4385, H⁵²7455) provides a small sequence of Lower Liassic sediments that is preserved in a shallow depression of the Upper Rhaetian coral limestone (locality S1, Fig. 18, 20, Pl. 17A). The section starts with a lumachelle layer and clastic sediments that fill up a small sink hole abutting a little fault. The lumachelle displays dissolved valves of transported, but often articulated Cardinia shells (3-4 cm in size, Pl. 17B). Sporadically manganese “Frutexites” structures grew into the mould cavities, before remaining space was closed by fine detrital sediments and blocky sparite (Pl. 17D, E).

Accessorial fauna comprises ostreoid, pterioid, and pectinoid bivalves and crinoidal fragments.

Intercalated and more matrix-dominated layers show fine-grained biopelsparites/micrites and intrasparites. The upper part of the sink hole is filled by a small succession of different clastic sediments, partly fractured, reworked or brecciated, and showing several discontinuities (Fig. 20).

Some of the layers display ferromangenese impregnations in small cavities of probably biogenous origin (Pl. 17C). Similar to the base of the clastic sequence, reworked and corroded clasts of a Pecten-lumachelle layer were found at the edge of the depression. The clasts are often covered by black to brown goethite crusts that consist of thin and curly laminae, alternating with 25-100 µm thick sedimentary layers. They grew in cauliflower-like to digitate structures of up to 5 mm thickness (Pl. 17F). From the crust surface black tree-like “Frutexites” structures protude non-geopetally into the surrounding sediment (Pl. 17G). Finally the whole clastic sequence in the sink hole as well as parts of the Triassic reef limestone are discordantly overlain by a conspicuous spiculite layer up to 80 cm in thickness pinching out laterally after a few meters. It is red to pink colored and pervaded by a network of “white” spar-filled stromatactis cavities. Due to the very similar facies that is known from the Adnet reef slope near Hallein, the sponge-stromatactis-biomicrites should also be attributed to the Schnöll-Formation (Böhm et al. 1999) instead to the Hierlatz Facies, as it has been assumed by Mazzullo et al. (1990) who focussed their studies on the cements of the cavities. Stable isotopes of this horizon were measured also by Turnšek et al. (1999). As a new road-cut in summer 2003 has shown, the succession above the

Fig. 20. Profiles of Triassic-Jurassic boundary sequences from Steinplatte localities: Plattenkogel hill (locality S1) and “Fischer’s Coral Garden” (locality S2). Numbers of layers/samples correspond with Figs. 21 + 43.

spiculite continues with some red crinoidal limestones, where a few isolated sponges appear but spicular mats are absent. They are followed by the marmorea crust (zone of Schlotheimia marmorea), an ammonite-rich and condensed marker horizon that is best known from several quarries in the Northern Calcareous Alps. The Sinemurian Adnet Formation above belongs to the Lienbacher Member (zone of Arnioceras semicostatum). The Liassic sequence ends in red nodular breccias.

§ A first profile from this site was published by Rakús and Lobitzer (1993).

Facies and Sponge Analyses:

The facies at Steinplatte/Plattenkogel exhibit a benthic sponge community with more or less autochthonous individuals of mostly non-rigid species. In contrast to the fauna at Adnet/Rot-Grau-Schnöll Quarry, nearly all specimens at Steinplatte are highly collapsed. Investigations were focused on the taphonomy of the sponges and the origin of related stromatactis cavities.

The results are given in chapter 5 and the following.

4.10.2. Fischer’s Coral Garden (CG)

§ Condensed red limestones that represent an advanced stage of Liassic onlap sedimentation were sampled from interstices of the Upper Rhaetian coral limestones (capping facies) at Steinplatte summit (TK25 / Blatt 8341 Seegatterl, R⁴⁵4375, H⁵²7415), called “Fischer’s Coral Garden” (locality S2, Fig. 20, 21). They are lacking sponge remains and presumably belong to the Adnet Limestone facies that is of Lower Sinemurian age.

Fig. 21. Drawing of a thin coral framework is closed by a succession of Jurassic sediments and radiaxial fibrous cements. Numbers refer to layers in Fig. 20 + 43.

4.11. Rettenbachalm/Jaglingbach (RJ)