PERITIDAL M-SCALE CYCLES OF THE DACHSTEIN LIMESTONE IN THE JULIAN ALPS (NE ITALY): NEW INSIGHTS ON THE LOFER CYCLOTHEM CONTROVERSY

The classic Lofer cyclothem proposed by Fischer (1964) to interpret the m-scale cyclicity of the shallow water Dachstein Limestone of the Northern Calcareous Alps (Late Norian-Rhaetian) is a deepening-upward cycle. After more than 50 years since its first formulation, no consensus has yet been established on the true polarity of the Lofer cyclothem, with current views varying from shallowing upward, to symmetric or to any combination of the above. The Dachstein Limestone Fm (Late Norian-Rhaetian) is exceptionally well exposed in the Julian Alps of northeastern Italy (Friuli region) thanks to recent deglaciation, and exhibits bedding similar to the classic Lofer cyclothem sequences of the Dachstein in the Northern Calcareous Alps. At Monte Canin meter-scale bedding cycles for a total thickness of 270 m were measured in the upper part of the formation and similar field studies were conducted in the classic Fischer’s localities of the Northern Calcareous Alps (Austria). Most of the sedimentary subfacies originally described by Fischer (1964) are present (A=paleosol, B=loferites, C=megalodont limestone), with the exception of well-developed mud-cracks, prism-cracks and neptunian dikes. The basic m-scale cycle is bound at the top and at the base by sharp subfacies boundaries, while internally mostly gradational contacts between subfacies exist. Subfacies C overlies with a sharp erosional contact either laminated and homogeneous loferites (B), or soil breccia (A) or more rarely the megalodont limestone (B). Subfacies B then passes gradually upward into first homogeneous and then laminated loferites (B). These can be overlain in sharp contact by either subfacies A or B. Subfacies A is interpreted as a soil breccia made of a green argillaceous mudstone matrix containing angular clasts of the underlying loferites. The green breccia fills in irregular dissolution cavities that can extend downwards into each cycle by a few meters. Whenever soil breccia was found underneath subfacies B, lateral tracing of this key surface showed that the green breccia occurred also above the loferites but due to cracks and partial dissolution had infiltrated downward. Therefore, the m-scale Dachstein cycles are interpreted as shallowing upward cycles with their tops modified by paleokarst dissolution and infilling.