Jurassic Austroalpine (Page 2).doc

On top of the Rhaetian shallow-water carbonates red condensed limestones of the Adnet Group (Hettangian to Toarcian; Böhm, 1992, 2003) were deposited, partly with a gap. On top of the Rhaetian Kössen Formation cherty and marly bedded limestones (Scheibelberg Formation - Hettangian to Toarcian, Kendlbach Formation - Hettangian; Böhm, 1992, 2003; Krainer & Mostler, 1997; Ebli, 1997) were deposited, in transitional areas to the Rhaetian Kössen Basin the crinoidal or sponge-spicule rich limestones of the Enzesfeld Formation (Hettangian to Sinemurian; Böhm, 1992).

In Late Pliensbachian and Early Toarcian a horst and graben morphology developed (Bernoulli & Jenkyns, 1974; Krainer et al., 1994) and triggered breccia formation along submarine slopes and escarpments (Böhm et al., 1995).

The Toarcian and most of the Middle Jurassic is characterised by starving sedimentation on the horsts, and ferro-manganese crusts or by a hiatus, whereas the grabens were filled with deep-water carbonates and breccias, which were formed at near fault scarps. Neptunian dykes were formed in various places.

In these newly formed basinal areas grey bedded limestones of the Allgäu Formation were deposited, whereas on top of the topographic highs condensed red limestones of the Klaus Formation were deposited (e.g., Krystyn, 1972) (Fig. 1).

In the Bajocian the sedimentary evolution in the southern part of the Tirolic unit differed from that in the northern part. Deep-water trenches were formed in front of advancing nappes. The southern parts of the Northern Calcareous Alps received mass-flow deposits and large slides derived from the Hallstatt zone (Gawlick & Frisch, 2003).

The thickness of the basin fills may reach 2000 m (Gawlick, 1996; Gawlick et al., 2007a). The Tauglboden trench in the north was subjected to high subsidence and sedimentation rates in the Oxfordian (Schlager & Schlager, 1973; Gawlick & Frisch, 2003).

A rise was eroded and supplied the Tauglboden trench to its north with mass-flow deposits and slides. These two basins groups differ, since the Hallstatt Mélange trenches formed earlier and exhibited a different composition of its huge mass-flows. However, both basins formed syntectonically suggesting a substantial relief between the basin axis and the source area.

A third type of radiolarite basin, the Sillenkopf Basin (Missoni et al., 2001a), remained as starved basin in the Kimmeridgian in the southern Northern Calcareous Alps (Fig. 1). This basin contains the earliest ophiolitic detritus from the accreted or obducted(?) Neotethys Ocean floor (Missoni, 2003).

This orogenic event (e.g., Lein, 1985, 1987a) was related to the closure of parts of the Neotethys Ocean. According to other authors (e.g., Wächter, 1987; Channell et al., 1992; Frank & Schlager, 2006) these Late Jurassic coarse clastic sediments should be related to strike-slip faulting.

The Hallstatt Mélange as erosional product of the today eroded Juvavic nappes was formed in the late Early to early Late Jurassic interval as a result of a successive shortening of the Triassic to early Jurassic distal shelf area (Hallstatt Zone).

In front of advancing and rising nappes a lot of different trenches were formed and filled up by various deposits. These trenches were overthrusted and incorporated into the accretionary prism subsequently.

In the Tirolic units of the Northern Calcareous Alps establishment of the shallow-water Plassen Carbonate Platform started on the frontal parts of the rising and advancing nappes (Gawlick et al., 1999a, 2005).

From there, these platforms prograded towards the adjacent radiolarite basins (Gawlick & Frisch, 2003; Gawlick et al., 2005). This resulted in a complex basin and rise topography with shallow-water and deep-water areas with different types of sediments (Gawlick & Schlagintweit, 2006).