Cristina Stefani

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.

Detrital Fission-Track Analysis and Sedimentary Petrofacies as Keys of Alpine Exhumation: The Example of the Venetian Foreland (European Southern Alps, Italy)

Sedimentary petrography and detrital apatite fission-track analysis of sediments from the Oligo-Miocene Venetian foreland succession (NE Italy) reveal a multi-stage evolution of the Alpine source areas. During the first stage of sedimentation (Chattian to Langhian), the foreland basin was filled by sediments derived from erosion of mainly crystalline Austroalpine units exhumed north of the Periadriatic Lineament. These sediments are characterized by litharenites with mean quartz content of about 61% and by a mean detrital apatite fission-track peak of about 30 Ma. The Permian-Paleogene sedimentary cover of the Southalpine crystalline basement (Dolomite region) was bypassed by the sediments and made only a minor input. From the Serravallian onwards, the foreland basin was incorporated into the Southalpine thrust belt. The compressional events of the Valsugana phase resulted in thrusting and uplift of the Southalpine domain, as documented by Tortonian fission-track ages in the hanging wall of the major regional thrust (Valsugana thrust fault). The Southalpine crystalline basement, unaffected by Alpine metamorphism, and its sedimentary cover became the main source of sediments, as revealed by the increase in extrabasinal carbonate grains and detrital apatite grains with Late Triassic-Jurassic fission-track ages. During this final stage of basin evolution, about 3000 m of sediments were deposited. These strata subsequently have been uplifted and eroded since the Pliocene, indicating a southward migration of the Alpine deformation front.

Late Cretaceous Carbonate Turbidites of the Northern Apennines: Shaking Adria at the Onset of Alpine Collision

Thick and regionally widespread deepwater carbonate turbidite successions, the Upper Cretaceous Helminthoid Flysch, were deposited during the initial stages of Alpine collision. Despite the dominant intrabasinal origin, bulk composition and heavy minerals of minor terrigenous mode indicate provenance from different crustal levels of the European and Adria plates. These data allow us to place the Helminthoid Flysch units within a Late Cretaceous paleogeographic reconstruction of the Northern Apennine Tethys. The depositional characters of these turbidites indicate the coexistence of an active tectonic setting, a paleogeographic location prone to carbonate ooze production and storage, and limited terrigenous sources outside the basin.

Variation in terrigenous supplies in the Upper Pliocene to Recent deposits of the Venice area

Abstract The subject of this work is a gross composition and rock fragment analysis of sands drilled by the Venice 1 and 1 bis wells from the depth of 947.70 to 19.33 m. The obtained data allow the delineation of Upper Pliocene to Pleistocene terrigenous supplies in an area of great environmental interest. From the bottom until about 513 m in depth, the sands are litharenitic, characterised by a high amount of carbonate rock fragments and are considered indicative of an eastern South Alpine provenance. From 512.59 to 438.43 m, the sand composition is quartzolithic with large amounts of quartz and metamorphic rock fragments, indicating a provenance from the western Alps/northern Apennines. This quite abrupt change in composition is thought to reflect either the progressive infill of the Apennine foredeep and the consequent northward migration of the Po fluvial system, or tectonic movements affecting the Apennine chain and/or the eastern Southern Alps during the Middle Pleistocene. In the shallowest interval, 438.43 to 19.33 m below the surface, the sand composition results from the mixing of these two kinds of sediments. This reflects both a persistence and simultaneous activity of the sources, and reworking by marine littoral processes related to repeated sea-level oscillations.

Palaeogeography of the Upper Cretaceous-Eocene carbonate turbidites of the Northern Apennines from provenance studies

Abstract The Upper Cretaceous Helminthoid Flysch (HF) of the Northern Apennines consists of thick and regionally widespread deep-water carbonate turbidite successions, deposited during the initial stages of Alpine collision. The HF spans the time from Turonian to Early Eocene and is mainly composed of intrabasinal carbonate ooze mixed with clay; siliciclastic terrigenous beds are also present, but they are a volumetrically minor component of the successions. Petrographic and sedimentological signatures indicate that the HF was deposited in distinct basins located below the carbonate compensation depth. Bulk composition and heavy minerals of terrigenous beds indicate provenance from different crustal levels of the European and Adria plates. The petrographic and palaeobathymetric characteristics of these turbidites indicate the coexistence of an active-margin tectonic setting, a palaeogeographical position suitable for carbonate ooze production and storage, and limited supply of terrigenous detritus into the basin. Palaeotectonic reconstructions and stratigraphic data suggest that Adria represented a vast repository of penecon-temporaneous carbonate mud; the presumably intense seismic activity related to the pre-collisional Alpine orogeny promoted large-scale failures of shelf and/or slope biogenic muddy sediments, resulting in the deposition of a large volume of carbonate turbidites. Only occasionally, turbidity currents probably linked to exceptional fluvial floods generated pure terrigenous beds with different petrographic signatures for each HF succession.

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.

Correlation between shallow benthic and calcareous plankton zones at the Bartonian-Priabonian transition (Varignano section, northern Italy)

Abstract from 88th Congress of the Italian Geological Society, 2016-09-07 - 2016-09-09, NaplesAbstract from 88th Congress of the Italian Geological Society, 2016-09-07, 2016-09-09, Naplesbook Edited by D. Calcaterra, S. Mazzoli, F.M. Petti, B. Carmina & A. Zuccari doi: 10.3301/ROL.2016.79

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.