Distinguishing primary and resedimented vitric volcaniclastic layers in the Burdigalian carbonate shelf deposits in Monferrato (NW Italy)

A multidisciplinary study, including stratigraphic, sedimentological, biost ratigraphic, petrographic, magnetic fabric and SEM analyses, has been perfo rmed on six volcaniclastic layers (VLs) interbedded in the Burdigalian shel f succession of the Monferrato (NW Italy). The aim was to distinguish betwe en the volcanic and sedimentary processes that produced these deposits, to suggest a depositional model for volcaniclastic sedimentation in a shelf en vironment and to discuss the use of VLs for stratigraphic correlations. Two kinds of VLs have been distinguished: single volcaniclastic layers (SVLs) and multiple volcaniclastic layers (MVLs), SVLs are single beds of well sor ted vitric siltites, mainly consisting of volcanic components and minor ter rigenous and intrabasinal grains; the vitric fraction mainly consists of bl ocky fragments. They show a very low magnetic anisotropy degree and a promi nent magnetic lineation. These VLs are interbedded in outer shelf marls and are interpreted as primary pyroclastic fall deposits. MVLs, which can be u p to 10 m thick, show limited lateral continuity and are made up of several -decimetre-thick graded beds, separated by erosional surfaces and consistin g of vitric arenites and siltites with about 15% non-volcanic components. T wo kinds of MVLs have been distinguished: (1) Type 1 MVLs, interbedded in s torm-dominated glaucony-rich calcarenites and showing rough, low-angle cros s-stratification (hummocky cross stratification), water escape and load str uctures. These deposits are characterized by a slightly foliated magnetic f abric and are interpreted as storm layers, deposited between fairweather an d storm wave base. (2) Type 2 MVLs are interbedded in outer shelf marls, an d are characterized by parallel lamination and by a well developed magnetic foliation. They are interpreted as storm-induced, distal shelf turbidites, triggered by storm activity acting in the more internal part of the shelf. The Monferrato VLs resulted from explosive eruptions of volcanic edifices, located outside of the basin, that produced an extensive tuff blanket that was uniformly distributed on a carbonate-dominated shelf. Above storm wave base the VLs were immediately reworked by storm activity, and the resultin g deposits are type 1 MVLs. Below storm wave base, primary pyroclastic fall deposits were preserved, corresponding res SVLs. Storm-induced turbidity c urrents gave rise to type 2 MVLs, that were deposited below storm wave base . Preservation of VLs in a shelf environment is hampered by the high-energy conditions of the shelf. Consequently, these deposits are characterized by a restricted lateral continuity and their use as correlation tools may be misleading. (C) 1999 Elsevier Science B,V, All rights reserved.