Paolo Grandesso

Integrated biomagnetostratigraphy of the Alano section (NE Italy): A proposal for defining the middle-late Eocene boundary

The Alano section has been presented at the International Subcommission on Paleogene Stratigraphy (ISPS) as a potential candidate for defi ning the global boundary stratotype section and point (GSSP) of the late Eocene Priabonian Stage. The section is located in the Venetian Southern Alps of the Veneto region (NE Italy), which is the type area of the Priabonian, being exposed along the banks of the Calcino torrent, near the village of Alano di Piave. It consists of ~120–130 m of bathyal gray marls interrupted in the lower part by an 8-m-thick package of laminated dark to black marlstones. Intercalated in the section, there are eight prominent marker beds, six of which are crystal tuff layers, whereas the other two are bioclastic rudites. These distinctive layers are useful for regional correlation and for an easy recognition of the various intervals of the section. The section is easily accessible, crops out continuously, is unaffected by any structural deformation, is rich in calcareous plankton, and contains an expanded record of the critical interval for defi ning the GSSP of the Priabonian. In order to further check the stratigraphic completeness of the section and constrain in time the critical interval for defi ning the Priabonian Stage, we performed a high-resolution study of integrated calcareous plankton biostratigraphy and a detailed magnetostratigraphic analysis. Here, we present the results of these studies to open a discussion on the criteria for driving the “golden spike” that should define the middle Eocene–late Eocene boundary.

Use of trace fossils in delineating sequence stratigraphic surfaces (Tertiary Venetian Basin, northeastern Italy)

Abstract The lowermost third-order sequence of the Venetian Molasse Basin (Upper Chattian-Lower Aquitanian) in the Belluno syncline is composed of a transgressive systems tract, comprising a basal, condensed, glauconitic sand sheet deepening upwards into finer-grained offshore deposits, and a highstand systems tract mainly consisting of prograding, offshore mudstones capped by very fine sandstones representing offshore-transition deposits. An integrated sedimentological, ichnological and micropaleontological approach has allowed the recognition of the internal organization of the sequence, the subdivision into parasequences in absence of an obvious physical expression, and the characterization of significant discontinuity surfaces at both sequence and parasequence scales. The transgressive surface shows a complex geometry of bored and encrusted cavities produced by erosional undercutting and a system of neptunian dykes and sills, with hiatal shell concentrations as infill. Component parasequences in the texturally homogeneous transgressive sand sheet can be identified on the basis of the preservational state of trace fossil assemblages (softground versus firmground conditions) and subtle changes in grain-size and glauconite content. Omission surfaces bounding the parasequences are marked by increased glauconite content, and densely crowded, predominantly vertical or oblique, relatively large, very distinct, thin-walled or unwalled, and uncompacted burrows. The condensed section coincides with highest values in relative abundance of planktic foraminifers and in diversity of benthic foraminifers. Parasequence-bounding flooding surfaces in the muddy offshore portion of the highstand systems tract are recognized as firmground surfaces mantled by thin shell concentrations in a matrix of silty, highly burrowed, glauconitic sand.

Alternation of ecologic regimes in a deep-marine carbonate basin: calciturbidite trace fossils from the Cretaceous Scaglia Rossa, northeastern Italy

Abstract The mostly pelagic Scaglia Rossa limestone exposed in the Vich quarry near Belluno contains numerous light-colored turbidites, consisting of calcareous sandstone and mudstone derived from the adjacent Friuli Platform. The turbidites contain an ichnofauna that records short-lived departures from background conditions in this part of the Belluno Basin in the Late Cretaceous. At least seven distinctive ichnotaxa were recognized, some of which could be the parts or extensions of more complex burrow systems of deposit feeders that exploited the disturbances. Earliest exploitation of the individual turbidites is indicated by nondescript burrow mottling in the lowest sandstone divisions. This is followed by an ensemble of comparatively well preserved sand-filled and typically mud-lined/-mantled burrows produced as the mud fraction began to accumulate gradually from suspension. All of these structures are crosscut by a latest series of mud-filled tunnels. This is not exactly the pattern of siliciclastic turbidites of flysch successions, having post-event ichnoassociations consisting of traces that are more nearly contemporaneous and record either depth of penetration or post-depositional movement of the redox boundary. Because the mud component in the Vich turbidites may have required several months to settle out of suspension, the patterns of bioturbation were extended over months to years. But as in the case of siliciclastic turbidites, emplacement of the calciturbidites suppressed the background fauna and supported a post-event association of different endobenthic animals, by changing temporarily the physical properties of the substrate, providing a windfall of organic particles, and by ventilating the near-bottom water.

The Alano Section: The Candidate GSSP for the Priabonian Stage

The deep-water hemipelagic Alano section is located in the Southern Alps of northern–eastern Italy close to the village of Alano di Piave in the type area of the Priabonian. Since 2005, the section has been presented as the potential Global Boundary Stratotype Section and Point (GSSP) for the late Eocene Priabonian Stage at several meetings of the International Subcommission on Palaeogene Stratigraphy (ISPS). Because of the favourable qualities of the section, we have been asked to produce the data necessary for the formal definition of the GSSP. In 2011, Agnini et al. published most of the documentation needed (lithostratigraphy, calcareous plankton biostratigraphy, magnetostratigraphy, and chemostratigraphy, including oxygen and carbon isotopes and carbonate content). The Agnini et al. (2011) paper also contained a motivated proposal for the definition of the Priabonian at the base of a prominent crystal tuff layer, the Tiziano bed, located at the 63.57 m level in the Alano section. Following the publication of this proposal, a unanimous consensus on the Alano section as the GSSP of the Priabonian (i.e., Gradstein et al. 2012; Wade et al. 2012) has been reached during an ad hoc workshop held in Alano in June 2012. However, not unexpectedly, alternative criteria for the definition of the GSSP have been subsequently advanced (Gradstein et al. 2012; Wade et al. 2012; Less and Ozcan 2012). During 2012, we further refined the stratigraphy of the Alano section by obtaining cyclostratigraphic data. Moreover, U–Pb dating of zircons from seven levels has been undertaken. Here, we present the new cyclostratigraphic data and discuss various proposals for the definition of the Priabonian at Alano. We reiterate that our original proposal is the most balanced and in better keeping with Hedberg’s (1976) principles of chronostratigraphy.