The K/T boundary record in the Southern Alps: a multidisciplinary approach

An integrated paleontologic, mineralogical and stable isotope investigation was conducted on a previously undescribed central western Tethys pelagic section (Forada section) across the Cretaceous/Tertiary (K-T) boundary outcropping in the Piave River Valley (Southern Alps, Belluno Province, NE Italy). It is generally held that the K-T boundary is poorly preserved in the area where hardgrounds are widespread in the late Cretaceous to early Paleocene. The sediments of the investigated section were deposited at bathyal depth in the a basinal setting (Belluno basin). The quantitative study of the calcareous plankton suggests an apparently complete record of the K-T boundary and an expanded basalmost Danian. The K-T boundary is marked by a 1-2 cm-thick clay interval intercalated in the pelagic carbonates of the "Scaglia Rossa" formation. The "boundary clay" is constituted by a basal 1-2 mm-thick green lamina followed by a red interval where a weak iridium anomaly is present. The green lamina contains well preserved calcareous nannofossils and agglutinated benthic foraminifera. In the red interval an assemblage of the foraminiferal P0 Zone occurs together with scarce Thoracosphaera spp. (a disaster species) and common to abundant Cretaceous nannofossils. Below the "boundary clay", the late Maastrichtian calcareous nannofossil Micula prinsii Zone and the planktonic foramnifera Plummerita hankteninoides Zone have been identified. Some 13-14 calcareous plankton biohorizon have been identified in the ca. 4 meters thick Danian interval examined. Their ranking is the same observed in other complete mid-latitude Danian sections. The first appearance of Cruciplacolithus primus and the LO of Parvularugoglobigerina eugubina occur at ca. 120 cm and ca. 165 cm above the "boundary clay" respectively. These results point to a remarkable expansion of the basalmost Danian in comparison with other well-known complete K-T boundary sections in pelagic setting. We have examined the recovery pattern of calcareous nannofossil assemblage after the K-T event finding a good correspondence with previous similar studies. Our results suggest that the pelagic ecosystem remained unstable for a significant interval of time in basal Danian.