LOFER CYCLOTHEMS IN THE DACHSTEIN LIMESTONE OF THE JULIAN ALPS (NE ITALY)

The Dachstein Limestone (Late Norian-Rhaetian) is exceptionally well exposed in the Julian Alps of northeastern Italy (Friuli region), and exhibits bedding similar to the classic Lofer cyclothem sequences of the Dachstein in the Northern Calcareous Alps. Here, the Dachstein is ca. 700 meters thick and lies conformably over the Dolomia Principale Fm (ca. 1000 m thick). At Monte Canin, 112 bedding cycles with a total thickness of 270 m were measured in the upper part of the formation. The basic meter-scale cycle is bound at the top and base by sharp subfacies boundaries; internally, gradational transitions between subfacies occur. Most of the subfacies that were originally described by Fischer in 1964 are present (A=paleosol, B=loferites, and C=megalodont limestone), who proposed that sea level oscillations were responsible for the ordering of subfacies into bedding cycles. The Monte Canin section, however, contains subfacies orderings that are indicative of ‘regression’ type cycles only, with cycle tops modified by paleokarst dissolution and infilling. No evidence was found for ‘transgression’ type cycles that have been described in the Northern Calcareous Alps and Transdanubian Central Range sections. Time series analysis was performed on the series of measured cycle thicknesses at Monte Canin, and on greyscale scans of field photographs of the section, and a stratigraphically equivalent section at nearby Picco Carnizza. The results reveal a pattern of stratigraphic bedding that suggests Milankovitch-forced sea levels were at play. Thickness bundling ratios (5:1 to 7:1) consistent with a precession index forcing signal are present over significant intervals of the section. In addition, a number of 4:1 and 3:1 thickness bundles occur especially toward the top of the section; these shorter bundles may be the result of “missed beats” that were caused by tectonic disturbances. These observations corroborate previous reports for non-random cyclic bedding in other Dachstein Limestone sequences, and do not support a stochastic origin for the Lofer cyclothems.