Peritidal depositional facies and carbon isotope variation across K/T boundary carbonates from NW Adriatic platform.

The K/T boundary in the Karst region (Italy and Slovenia) is well documented in correspondence to shallow water limestones. Its evidence is usually related to palaeontological data (disappearance of Cretaceous taxa and subsequent appearance of Tertiary ones), palaeomagnetism (Ch 29r) iridium anomaly and negative shift of δ13C. In Italy, the Padriciano section highlights the K/T boundary included in the basal breccia of a peritidal cycle. Recent investigations brought to light a short well exposed new stratigraphic sequence containing the K/T boundary, 100 m east of Padriciano, as demonstrated by lithological, palaeontological and new isotopic data. The recording of a new peritidal setting denotes stressed environmental conditions. The findings of the biota mostly correspond to the lagoonal limestones of the peritidal cycle. It consists of small opportunistic thin shelled foraminifers, ostracods and gastropods. At Padriciano, the environmental stability occurred after episodes of strong fresh water influence. The intraclasts, oncolites and stromatolites have been observed in the Padriciano section. The broken shell fragments, algae and foraminifera have been found associated with fenestral fabric. These features indicate deposition in a shallow marine (intertidal to supratidal) environment. Algal boundstones and microbial (stromatolitic) laminae, crenulated domal laminae and crenulated conical laminae have been studied in petrographic sections for microstructures. The K/T boundary section has been analysed for the carbon and oxygen isotopic ratios. The δ13C values range from −3.62 to −10.01‰ (V-PDB) and the δ18O values range from −3.85 to −5.47‰(V-PDB). This extreme depletion in the δ13C record has already been reported from other K/T boundary sections both in the Padriciano area (Italy) and in Slovenia (Dolenja Vas sections) and may be related to global climate change but also to local environmental changes occurring in the NWAdriatic