HOW DEEP WERE THE HALLSTATT BASINS OF THE NORTHERN CALCAREOUS ALPS?

The Northern Calcareous Alps (NCA) are the cradle of important stratigraphic and sedimentologic concepts but the intensive deformation of the allochthon frequently hampers reconstruction of depositional settings. The bathymetry of the Middle and Late Triassic Hallstatt Basins is a case in point. Since the 1960’s, two contrasting models have been applied. The deep-basin model assumes uniform subsidence between carbonate platforms and basins such that water depth is about equal to the difference in thickness between basins and platforms, i.e. 800 m or more. In the shallow-basin model, water depth is 200 m or less because the basin floor is assumed to have risen relative to the platforms, probably driven by salt-tectonics.

The shallow-basin model is supported by well-preserved transitions between outer platform and marginal basin sediments. Middle-Triassic examples draw additional support from the fact that the thickness variation of the overlying Raibl Formation (Carnian) that levels the platform-basin relief, is much less than predicted by the deep-basin model. Support for the deep-basin model is provided by the analogy with the well-preserved Middle-Triassic platforms of the Dolomites in the Southern Alps and the Permian Capitan platform in the USA, both exhibiting 800-meter high slopes and minimal differential movement between platform and basin. Recently, steep and over 600 m high clinoforms of Dachstein Limestone have been discovered at the Gosaukamm in the NCA. This shows that parts of the Hallstatt Basins were deep but does not exclude the possibility of a rise of the basin floor relative to the platforms. Slope bedding in the Dolomites and at Gosaukamm is mainly planar and close to the angle of repose of non-cohesive material. This geometry indicates that the platforms exported mainly sand and rubble with very little mud – in stark contrast to the mud-rich modern platforms of the Bahamas and the Caribbean. The steep slopes might serve as recorders of water stratification in the Hallstatt Basins using chemistry of marine carbonate cements and biota.