Abstract

The Late Triassic/Liassic boundary in the Ligurian Alps (Prepiemontese domain, Arnasco-Castelbianco unit) is marked by a sharp transition between carbonate tidal deposit and pelagites. The interbedded layers are characterized by omissions, condensed ammonite-rich horizons (johnstoni and rotiforme Biozones, i.e. Lower Hettangian and Early Sinemurian), Fe-encrusted surfaces with borings. Besides, two further hardgrounds have been identified; they lie immediately under the pelagic spongolites. Similar depositional setting are peculiar also of coeval successions in the Sestri-Voltaggio zone (M. Gazzo-Isoverde unit). Evidences or synsedimentary extensional tectonics, fauna condensation and early lithification beds led to topographic high and offshore shallow shelves already isolated in late Hettangian age.

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