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

ELSEVIER

Sedimentary Geology 129 (1999) 143–163

Distinguishing primary and resedimented vitric volcaniclastic layers in the Burdigalian carbonate shelf deposits in Monferrato (NW Italy) A. d’Atri a,Ł , F. Dela Pierre a,1 , R. Lanza b , R. Ruffini a a

CNR – Centro di Studi sulla Geodinamica delle Catene Collisionali, V. Accademia delle Scienze 5, 10123 Torino, Italy b Dipartimento di Scienze della Terra, V. Valperga Caluso 35, 10125 Torino, Italy Received 5 October 1998; accepted 16 July 1999

Abstract A multidisciplinary study, including stratigraphic, sedimentological, biostratigraphic, petrographic, magnetic fabric and SEM analyses, has been performed on six volcaniclastic layers (VLs) interbedded in the Burdigalian shelf succession of the Monferrato (NW Italy). The aim was to distinguish between the volcanic and sedimentary processes that produced these deposits, to suggest a depositional model for volcaniclastic sedimentation in a shelf environment 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 sorted vitric siltites, mainly consisting of volcanic components and minor terrigenous and intrabasinal grains; the vitric fraction mainly consists of blocky fragments. They show a very low magnetic anisotropy degree and a prominent magnetic lineation. These VLs are interbedded in outer shelf marls and are interpreted as primary pyroclastic fall deposits. MVLs, which can be up to 10 m thick, show limited lateral continuity and are made up of several-decimetre-thick graded beds, separated by erosional surfaces and consisting of vitric arenites and siltites with about 15% non-volcanic components. Two kinds of MVLs have been distinguished: (1) Type 1 MVLs, interbedded in storm-dominated glaucony-rich calcarenites and showing rough, low-angle cross-stratification (hummocky cross stratification), water escape and load structures. These deposits are characterized by a slightly foliated magnetic fabric and are interpreted as storm layers, deposited between fairweather and storm wave base. (2) Type 2 MVLs are interbedded in outer shelf marls, and 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 resulting deposits are type 1 MVLs. Below storm wave base, primary pyroclastic fall deposits were preserved, corresponding to SVLs. Storm-induced turbidity currents 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.  1999 Elsevier Science B.V. All rights reserved. Keywords: volcaniclastic layers; Burdigalian; shelf sedimentation; Monferrato

Ł Corresponding 1

author. Tel.: C39 011 5621179; Fax: C39 011 541755; E-mail: [email protected] Present address: Dipartimento di Scienze della Terra, V. Valperga Caluso 35, 10125, Torino, Italy.

0037-0738/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 9 8 - 6

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1. Introduction Volcaniclastic layers (VLs) have been extensively investigated in recent years (Fisher and Schmincke, 1984; Cas and Wright, 1987; Izett et al., 1988; Bitschene and Schmincke, 1991; Cas and Busby Spera, 1991), since they are the best means available to stratigraphers for long-range physical and geochronological correlations (Sarna-Wojcicki et al., 1984; Schmincke and van den Bogaard, 1991; Calanchi et al., 1998). Moreover, they provide information about past volcanic activity. Most of the preserved ancient VLs, however, were deposited in a marine environment, where the interaction between volcanic and sedimentary processes may produce a strong convergence of lithofacies among volcanic products of distinct origin, thus hindering their correct interpretation. For example, primary pyroclastic products as ash falls, pyroclastic flow and surge deposits may macroscopically resemble pyroclast-rich detrital sediments that result from epiclastic processes involving freshly erupted pyroclasts (Wright and Mutti, 1981; Cas and Wright, 1987; Cole and Stanley, 1994). Many studies have focused on VLs interbedded in deep water successions (Sparks and Walker, 1977; Carey and Sigurdsson, 1978; Sparks and Huang, 1980; Cas and Wright, 1987), whereas few examples of VLs deposited in shelf environments are described in the literature (Di Marco and Lowe, 1989; Kano, 1991; Fritz and Howells, 1991; Heikoop et al., 1997; Stow et al., 1998). In this case, the correct interpretation of the depositional mechanism is even more crucial, because primary pyroclastic deposits can be easily reworked due to the high-energy conditions of the shelf. The Burdigalian carbonate shelf succession of Monferrato (northwestern Italy) offers good examples of different types of rhyodacitic to rhyolitic VLs with unusual thickness and a variety of sedimentological and petrographic features. This paper reports on a multidisciplinary investigation of these VLs, including stratigraphic, sedimentological, petrographic, magnetic fabric and SEM analyses. The main purposes of the investigation were: (a) to document their composition, distribution and sedimentology; (b) to distinguish between the volcanic and sedimentary processes that originated these deposits;

(c) to suggest a depositional model for volcaniclastic deposits in a shallow water environment; (d) to discuss the use of VLs in stratigraphic correlations.

2. Geological setting During Oligocene–Miocene times, a post-collisional magmatic cycle developed throughout the Alps and Apennines systems. Remnants of this magmatic activity, whose volcanic centres are unknown, include: (a) andesitic clasts in the external flyschs of the Alps (Taveyanne and Champsaur Sandstones, Martini, 1968; Ruffini et al., 1995b); (b) calc-alkaline plutons and dykes intruded along or close to the faults of the periadriatic system (Dal Piaz and Venturelli, 1983); (c) very rare lava and pyroclastic flows in the internal part of the Alpine belt (Zingg et al., 1976); (d) volcanic andesitic complex (Mortara) buried under the Po Plain sediments (Cassano et al., 1986); and (e) VLs interbedded in the sedimentary successions of the Northern and Central Apennines (Guerrera and Veneri, 1989; Tateo, 1992), the Tertiary Piemonte Basin s.str. (Galbiati, 1976; d’Atri and Tateo, 1994) and the Monferrato (Clari et al., 1988). The Monferrato (Fig. 1) corresponds to the northwest termination of the Apennines thrust belt and consists of an Eocene–Miocene mainly terrigenous succession resting unconformably on Upper Cretaceous and Eocene Ligurian flyschs (Clari et al., 1995). It is separated from the Torino Hill, which consists of a coeval terrigenous succession resting on buried Alpine metamorphic units, by a km-wide transpressional deformation zone (Rio Freddo deformation zone, Piana and Polino, 1995). Recent stratigraphic and structural works on Monferrato (Clari et al., 1995; Piana and Polino, 1995) resulted in the recognition of two main tectonostratigraphic units (Fig. 1), separated by a NNE– SSW-trending fault zone and characterized by distinct pre-Langhian stratigraphic successions: (a) The Western Monferrato, characterized by Oligocene to upper Burdigalian coarse terrigenous sediments deposited both in strongly subsiding basins and structural highs. (b) The Eastern Monferrato, characterized by Oligocene to Aquitanian deep water terrigenous sed-

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Fig. 1. Structural sketch map of northwestern Italy. IL D Insubric line; SVZ D Sestri–Voltaggio Zone; VVL D Villalvernia–Varzi line; RFDZ D Rio Freddo Deformation Zone; WM D Western Monferrato; EM D Eastern Monferrato; TPB s.s. D Tertiary Piemonte Basin s.str. Plio–Quaternary deposits are indicated in white. Structural Model of Italy (1990), modified.

iments followed unconformably by Burdigalian carbonate shelf sediments, known as the Pietra da Cantoni (PDC) Group. In the PDC Group, three facies have been recognized (Clari et al., 1995): (1) massive biocalcarenites and biocalcirudites (maximum thickness 60 m), containing shallow water biota — these sediments are referable to a ‘foramol’ platform depositional environment (sensu Carannante et al., 1988); (2) highly burrowed planktonic foraminif-

era and glaucony-rich coarse calcarenites, with rare intercalations of calcareous marls (maximum thickness 30 m) — these sediments are interpreted as deposited in a storm dominated shelf; (3) calcareous marls, with interbedded siliceous beds (maximum thickness 200 m), that are indicative of an outer shelf depositional environment. Lateral facies distribution patterns suggest a progressive westward deepening of the depositional en-

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Fig. 2. Schematic cross-section of the Miocene succession of the Eastern Monferrato, showing the position of the volcaniclastic layers studied. The trace of the cross-section is shown in the inset map, together with the location of the study volcaniclastic layers: 1 D Cornale; 2 D Camino; 3 D Castel S. Pietro; 4 D Varengo; 5 D Villadeati; 6 D Godio.

vironment, whilst the vertical facies evolution shows a transgressive trend culminating in the drowning of the ‘foramol’ carbonate platform (Fig. 2). In both the Eastern and Western Monferrato, Burdigalian deposits are followed unconformably by Langhian–Serravallian shelf calcarenites (Tonengo Calcarenites), suggesting that the different tectonostratigraphic units composing the Monferrato domain were tectonically amalgamated by Langhian time (Clari et al., 1995).

3. The Burdigalian volcaniclastic layers of the Monferrato Six VLs have been recognized in the Eastern Monferrato, where they are interbedded in the more distal facies of the Pietra da Cantoni Group (facies 2 and 3, Fig. 2). On the contrary, no VLs are present in the coeval facies 1 of the PDC Group and in the terrigenous succession of Western Monferrato. These VLs are the only indirect evidence of a Burdigalian volcanic activity. Some of these layers (Camino,

Cornale and Castel S. Pietro) are new discoveries, the others (Villadeati, Godio, Varengo) were previously studied from petrographic, geochemical and biostratigraphic points of view (Ruffini et al., 1995a; Ruffini, 1995). The mineral chemistry and glass composition of the VLs suggest that they are highly differentiated products from a high-K calc-alkaline original magma (Ruffini et al., 1995a). On the basis of mineralogy and microprobe analyses of glass shards, the VLs can be subdivided as follows: Camino and Cornale VLs have a rhyodacitic composition, while Castel S. Pietro, Villadeati, Godio and Varengo show a rhyolitic composition (Ruffini, 1995; Ruffini, unpublished data). On the basis of quantitative studies performed on calcareous nannofossil associations (Ruffini et al., 1995a; Polino et al., 1995; A. d’Atri, unpublished data) the analyzed VLs have been subdivided in three groups referable to three chronostratigraphic intervals (Fig. 3): (1) Camino and Cornale: Early Burdigalian (Zone MNN2b of Fornaciari and Rio, 1996).

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Fig. 3. Bio-chronostratigraphic scheme of the studied VLs based upon unpublished data (d’Atri) (Cornale, Camino, Castel S. Pietro) and after Ruffini et al. (1995a) (Villadeati, Godio, Varengo). Biochronology after Young et al. (1994) and Fornaciari and Rio (1996). LO D Last Occurrence; FO D First Occurrence; LCO D Last Common and Continuous Occurrence; FCO D First Common and Continuous Occurrence; AE D Acme end; PB D Paracme beginning.

(2) Villadeati, Godio and Castel S. Pietro: Late Burdigalian (Zone MNN3a of Fornaciari and Rio, 1996). (3) Varengo: referable to the Burdigalian–Langhian boundary (upper part of Zone MNN4a of Fornaciari et al., 1996). In addition, 40 Ar=39 Ar analyses performed with the step heating method on amphibole and biotite from Villadeati and Varengo VLs (Ruffini, 1995; d’Atri et al., 1997) yielded plateau ages of 18:7 š 0:1 Ma and 16:4 š 0:2 Ma, respectively. The result of these geochronological analyses are in good agreement with the biostratigraphic data. A palaeomagnetic study was also done on the Camino, Villadeati and Varengo VLs (Bresso, 1997). The measurements of isothermal remanent magnetization and its thermal demagnetization according to Lowrie’s method (Lowrie, 1990) done in this study,

showed that the main ferromagnetic mineral in the Monferrato VLs is a titanomagnetite.

4. Methodology Facies analyses have been carried out on all the VLs studied. Samples were collected in the VLs every 50–60 cm and were studied in thin section. Modal petrographic analyses have been performed using a transmitted light microscope equipped with a point counter. 400 points for each section were counted according to the criteria proposed by Zuffa (1985). SEM observations of glass shards were performed in order to relate their morphology and size to the mechanism of fragmentation that produced the ashes. In fact, recent studies (e.g. Heiken and Wohletz, 1985) demonstrated that the grain shapes, the sizes

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and vesicle characteristics of the pumice and volcanic ash provide information about the eruption process. For this reason, glass and pumiceous shards of VLs were mounted on stubs, coated with gold and observed using a Cambridge Instrument S360 scanning electron microscope (SEM) operating at an accelerating voltage of 20–25 kV, a current of 200 pA, and a working distance of 5 to 12 mm. Chemical analyses on the main volcanic phases and the glass shards were carried out with an EDS Spectrometer QX 2000 of Link Analytical. The magnetic fabric was investigated measuring the anisotropy of magnetic susceptibility (AMS). The fabric of a detrital rock mirrors the arrangement of its grains, which in turn traces the forces working in the depositional environment. Gravitational settling causes grains to lie with their largest sizes in the bedding plane and the longest axes either randomly distributed within the plane or aligned by hydrodynamic forces. Preferred orientation results in anisotropy of many physical properties; in particular, the AMS of a magnetite-bearing sediment mainly depends on the preferred orientation of its magnetite grains. Magnetite exhibits shape-anisotropy, i.e. susceptibility is highest when measured parallel to the longest axis of a grain, smallest when measured parallel to the shortest. The magnetic fabric of a rock may be described by the direction and relative strength of the three principal susceptibility axes (maximum k1 > intermediate k2 > minimum k3 ). The direction of k1 is called magnetic lineation and the plane defined by k1 and k2 magnetic foliation. The fabric is further characterized by various parameters, such as (Tarling and Hrouda, 1993): degree of anisotropy, P D k1 =k3 ; magnetic lineation, L D k1 =k2 ;

magnetic foliation, F D k2 =k3Ł; shape parameter, ð q D .k1 k2 /= .k1 C k2 /=2 k3 . The magnetic fabric of a fine-grained, magnetite-bearing sediment is typically oblate, with k1 ' k2 > k3 . Foliation, therefore, prevails over lineation, F > L, and the q value is less than 0.7. The simplest model presumes the foliation plane to be slightly inclined with respect to the bedding and the lineation either parallel or orthogonal to the flow direction, if there is any. Samples for magnetic fabric were cored from the Camino, Castel S. Pietro and Varengo VLs using a battery-powered drill, and cut to standard-size cylindrical specimens in the laboratory. The Cornale VL could not be sampled since it was covered by a landslide in the period intervened between its discovery and the rock-magnetism sampling. Villadeati and Godio were excluded because of their strong tectonization which could have overprinted the primary sedimentary fabric. AMS was measured using a KLY-2 bridge and the mean site values of directional and anisotropy parameters were calculated using the tensorial method of Jelinek (1978). The site mean values of the most representative AMS parameters are reported in Table 1 and discussed in the next sections.

5. Results Sedimentological features, petrographic characteristics, SEM observations of glass shards and magnetic fabric signature allowed two classes of deposits to be distinguished: single volcaniclastic layers (SVLs) and multiple volcaniclastic layers (MVLs). To describe these layers terminology proposed by Cas (1991) is adopted. The main characteristics of VLs are summarized in Table 2.

Table 1 Mean site values of AMS parameters Site Camino Castel S. Pietro Varengo 1 Varengo 2 Varengo 3

n 12 17 8 10 17

k SI ð 10 331 71 82 74 77

P

q

Lineation D, I

Foliation pole D, I

1.004 1.011 1.011 1.012 1.016

0.88 0.59 0.44 0.58 0.34

231, 40 == 321, 10 329, 6 110, 9

== == 295, 21 325, 57 303, 36

6

Symbols: n D number of specimens; k D bulk susceptibility; P D k1 =k3 , degree of anisotropy (Nagata, 1961); q D .k1 k2 /= [0:5.k1 C k2 / k3 ], shape parameter (Granar, 1958); D, I D declination, inclination.

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Table 2 Main characteristics of the three types of Monferrato volcaniclastic layers Volcaniclastic facies

Single volcaniclastic layers (SVL)

Type I multiple volcaniclastic layers (type 1 MVL)

Type 2 multiple volcaniclastic layers (type 2 MVL)

Encasing carbonate deposits

Outer shelf calcareous and siliceous marls

Storm-dominated glaucony-rich calcarenites

Outer shelf calcareous and siliceous marls

Bed thickness Single bed Number of beds Total thickness

180 cm 1 1.80 m

35–120 cm 7 3–6 m

25–280 cm 7–10 6–10 m

Bed contact

Sharp, non erosive with underlying deposits. Gradual or diffuse with overlying non volcanic deposits (bioturbation)

Sharp, erosive with underlying deposits; sharp and erosive between successive beds. Gradual with overlying non volcanic sediments (bioturbation)

Sharp, erosive with underlying deposits. Sharp and erosive between successive beds. Gradual with overlying non-volcanic deposits

Internal structures

Normal grading

Normal grading within single beds. Hummocky cross-stratification; pervasive bioturbation; water escape and load structures

Normal grading and parallel lamination within single beds. Density grading within individual beds

Sorting

Very good

(Poor to) moderate

(Poor to) moderate

Mean grain size (mm)

0.2–0.02

0.6–0.05

0.6–0.05

Major constituents

Glass shards

Glass shards, pumice, crystals, intrabasinal grains (bioclasts, glaucony, phosphates)

Glass shards, pumice, crystals, intrabasinal grains (bioclasts, glaucony, phosphates)

Percentage of major constituents

Volcanic grains (glass and crystals): 94%; terrigenous grains: 2%; intrabasinal grains: 4%

Volcanic grains (glass and crystals): 87%; terrigenous grains: 4%; intrabasinal grains: 9%

Volcanic grains (glass and crystals): 88%; terrigenous grains: 4%; intrabasinal grains: 8%

Magnetic fabric

Very low degree of anisotropy. Lineation well developed and close to remanent magnetization

Principal susceptibility directions dispersed

Foliation well developed and inclined with respect to the bedding. Lineation at right angle to the foliation’s plunge

5.1. Single volcaniclastic layers (SVLs) 5.1.1. Sedimentological features In Monferrato, this type of deposit has been found only in one locality (Camino), where it is interbedded in outer shelf calcareous and siliceous marls (facies 3 of the PDC Group). Due to poor outcrop conditions, this VL can be traced laterally only for about ten metres. The Camino volcaniclastic layer (Fig. 4a) consists of well sorted vitric siltites, in a single bed about 180 cm thick with a sharp, non-erosional basal contact. The upper boundary is more diffuse because of bioturbation. The Camino VL is characterized by

the lack of current-induced sedimentary structures. The bed is normally graded and the basal 70 cm are massive and slightly bioturbated. No evidence of welding has been recognized. Moreover, centimetre-wide firm ground burrows filled by the overlying carbonate deposits have been observed in the upper part of the bed. 5.1.2. Petrography The Camino VL corresponds to a fine to very fine vitric siltite (Fig. 5a). It consists of (Fig. 6) volcanic components (94%), intrabasinal (4%) and minor terrigenous (2%) grains. The volcanic minerals are represented by common plagioclase, minor

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Fig. 4. (a) Vitric siltite. Camino SVL. (b) Alternating glaucony-rich (darker colour) and impure vitric (lighter colour) arenites, followed by a slumped interval containing deformed clasts of both the sediments. Castel S. Pietro type 1 MVL. (c) Erosional surface separating two beds composing Villadeati type 2 MVL. (d) Elongated decimetric fragment of unconsolidated vitric sediment (‘tuff chips’, a) in the middle part of a bed composing Villadeati type 2 MVL. (e) Varengo type 2 MVL. Four distinct single beds are clearly recognizable. (f) Detail of Varengo type 2 MVL showing two superimposed graded beds, separated by a sharp, erosional surface. Rough parallel laminae are recognizable in the upper part of the second bed.

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Fig. 5. Microphotographs of the main petrographic textures of the volcaniclastic layers. (a) Vitroclastic structure from Camino SVL. Note the great abundance of fine to very fine glass shards and a crystal of plagioclase (arrow). The bar corresponds to 1 mm. (b) Medium to fine vitroclastic structure consisting of glass shards, minor magmatic crystals (B) and small volcanic lithics (L). Note the absence of fine volcanic matrix and the presence of calcite (CC) filling the pores and the greater holes. Cornale Type 1 MVL. The bar corresponds to 1 mm. (c) Well preserved vitroclastic texture with abundant Y-shaped glass shards, magmatic crystals as amphibole (A), biotite (B), plagioclase (P) and planktonic foraminifers often filled by glaucony. Varengo Type 2 MVL. The bar corresponds to 1 mm. (d) Well preserved vitroclastic texture with pumiceous glass shards and abundant bioclasts. Villadeati Type 2 MVL. The bar corresponds to 0.5 mm.

clinopyroxene, quartz and Ti-magnetite. Plagioclase appears both as quenched microlites within the vitric groundmass of the shards and isolated grains. It is slightly zoned from labradorite at the core to andesine at the rim. Pyroxene occurs in fragments and is augitic in composition. The vitric fraction is up to 90% of the volcanic components and is made of fresh, colourless, undeformed fine glass shards. 5.1.3. Glass morphology The Camino VL is dominated by fine ash particles (<50 µm). The prevailing morphology of the glass shards consists of angular blocky types with a low

degree of vesicularity (Fig. 7a). Some ashes are elongated suggesting a foliation formed during injection prior to fragmentation (Wohletz, 1983). The surface of the finest ashes is slightly hydrated and shows some alteration effects such as small cracks and tiny pits, most of them filled by secondary small crystals (Fig. 7d). 5.1.4. Magnetic fabric The magnetic fabric of the Camino VL is characterized by a very low anisotropy degree paired with a prominent lineation (Table 1). The maximum axes are well grouped and close to the bedding, whereas

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Fig. 6. Pie diagrams showing the percentages of different grains for the three types of volcaniclastic layers. SVL D single volcaniclastic layer; MVLs D multiple volcaniclastic layers. Legend: white D volcanic component; dots D terrigenous component; vertical lines D intrabasinal component. The table shows the recalculated modal point-count data. Number 1 includes the grain categories classified as volcanic component, number 2 D terrigenous component, number 3 D intrabasinal component.

the intermediate and minimum axes are dispersed along a girdle (Fig. 8A). These unusual features suggest that the fabric was acquired under the influence of a weak aligning force acting in a quiet depositional environment. A tentative explanation may be advanced taking into account the closeness between

the directions of the magnetic lineation (D D 231º, I D 40º) and the characteristic remanent magnetization (D D 249º, I D 63º) (Bresso, 1997). This means that remanence of each grain is close to its longest axis and both were oriented by the same process, whichever it was.

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The emplacement model we propose is that cold, already magnetized volcanic material was deposited grain by grain in still water and the torque exerted by the Earth’s field aligned the remanence vectors and hence the longest axes. The grains’ remanent magnetization was acquired before deposition, while they cooled as free particles suspended in the atmosphere. It was parallel to the longest axis because this corresponds to the minimum magnetostatic energy, and hence to the more favourable configuration in the absence of other influences. 5.1.5. Interpretation The absence of welding or viscous deformation of vitric fragments points to an emplacement at low temperature on the sea bottom for the Camino SVL pyroclastic fraction (Cas and Wright, 1987). Furthermore, the absence of zeolites in the deposits suggests an emplacement at temperatures lower than 200ºC (De Gennaro and Colella, 1991). The good sorting of the sediment, the lack of current-induced sedimentary structures and the magnetic fabric suggest deposition by gravity settling in still water (Fisher and Schmincke, 1984); after deposition, the sediment was slightly bioturbated. The petrographic composition, characterized by the predominance of glass shards, the scarcity of non-volcanic (terrigenous and intrabasinal) components (<6%) and the normal grading, allow us to interpret this VL as a primary pyroclastic fall deposit (Sigurdsson et al., 1980; Cas and Wright, 1987; Schmincke and van den Bogaard, 1991; Cousineau, 1994; Perkins et al., 1995). Moreover, angular and blocky equant types of glass shards, together with fine grain size, suggest that the pyroclastic debris was produced by explosive hydromagmatic fragmentation processes (Heiken and Wohletz, 1985). However, the collected data are not sufficient to assess if the volcanic ash resulted from eruption columns accompanying explosive eruptions or from ash cloud associated to pyroclastic flows (Cas and Wright, 1987). 5.2. Multiple volcaniclastic layers (MVLs) 5.2.1. Sedimentological features The most remarkable feature of these layers is their unusual thickness (up to 10 m). Each layer is made up of a succession of several decimetre-thick

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distinct beds characterized by erosional basal contacts and normal grading, and consisting of coarse to fine vitric arenites and siltites. On the basis of their internal structures and the characteristics of the encasing rocks two types of MVLs can be recognized. 5.2.1.1 Type 1 MVLs. These have been found at Cornale and Castel S. Pietro, interbedded in storm-dominated glaucony-rich shelf calcarenites referable to the facies 2 of the PDC Group (Fig. 2). Their lateral continuity is limited (up to 10 m). The total thickness of the Cornale VL is 6 m. It consists of seven superimposed beds whose thicknesses range from 35 to 120 cm. Single beds show a sharp and erosional basal contact and are normally graded; the base is an impure vitric arenite grading upward to a fine arenite or siltite. Sedimentary structures are almost completely obliterated by bioturbation. Rough, low-angle cross-lamination (hummocky cross-stratification?), water escape and load structures have been locally observed. Cm-thick hybrid glauconarenites are interbedded with the volcaniclastic beds. At the top of the MVLs firm ground burrows occur. The Castel S. Pietro MVL (Fig. 4b) is about 3 m thick. The basal 50 cm are made up of a succession of cm-thick, normally graded beds consisting almost exclusively of very fine vitric arenites. They are followed by alternating impure graded vitric arenites (about 40 cm thick) and glaucony-rich arenites (10– 15 cm thick). At the base of the latter beds load structures and large burrows are present. At the top of the layer, a slump deposit about 1 m thick occurs; it contains deformed clasts of both impure vitric and glaucony-rich arenites. This chaotic level indicates that submarine slides involving fragments of semi-consolidated volcaniclastic sediments occurred in unstable sectors of the basin. 5.2.1.2 Type 2 MVLs. These deposits crop out at Villadeati, Godio and Varengo. At Villadeati the layer is interbedded in outer shelf marls (facies 3 of the PDC group) and can be traced laterally for hundreds of metres. It is up to 10 m thick and consists of seven superimposed and amalgamated beds, whose thicknesses range from 25 to 280 cm. Each bed consists of medium to fine

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Fig. 8. Equal-area projection of the principal susceptibility axes. Symbols: squares D k1 ; triangles D k2 ; dots D k3 ; star D magnetic lineation; light circle D bedding; heavy circle D magnetic foliation.

Fig. 7. Scanning electron micrographs showing different types of glass shards recognized in the Monferrato volcaniclastic layers. (a) Equant blocky glass fragment with sharp edges. This type is characterized by low vesicularity. Camino SVL. The bar corresponds to 50 µm. (b) Bubble-wall shard which has almost intact spherical vesicle in its centre. Cornale Type 1 MVL. The bar corresponds to 50 µm. (c) Pumice fragment with large vesicles. Fine dust adheres on its surface. Villadeati Type 2 MVL. The bar corresponds to 100 µm. (d) Blocky vesicle-poor shard with abundant adhered dust and slight surface alteration. Camino SVL. The bar corresponds to 20 µm, the zoom is 6ð. (e) Alteration effects such as tiny pits due to hydration on an elongated pumice glass. Villadeati Type 2 MVL. The bar corresponds to 100 µm, the zoom is 8ð. (f) Curve-shaped fragment of broken bubble walls. In thin section they are commonly Y-shaped representing remnants of wall between three adjoining bubbles. Villadeati Type 2 MVL. The bar corresponds to 200 µm. (g) Elongated highly pumiceous glass type. The overall grain shape is determined by vesicle shape which in this case is fibrous. Varengo Type 2 MVL. The bar corresponds to 200 µm. (h) Angular blocky type with low vesicularity formed by interaction between magma and water. Cornale Type 1 MVL. The bar corresponds to 100 µm. (i) Flat and cuspate bubble wall with medium vesicularity. Varengo Type 2 MVL. The bar corresponds to 50 µm.

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impure vitric-crystal arenites, grading upward to impure vitric siltites. The base is sharp and erosional (Fig. 4c), and the lowermost centimetres of the bed are enriched in biotite flakes. Parallel laminae and elongated dm-sized fragments of unconsolidated vitric sediments (‘tuff chips’, Fig. 4d) have been locally observed in the middle part of the beds, together with cm-sized firm ground burrows at the top. The Godio VL shows the same characteristics as that of Villadeati. The Varengo MVL (Fig. 4e) is located at the boundary between facies 3 of the PDC Group and the overlying Langhian deposits. Its lateral continuity is about 10 m and its total thickness is estimated to be 6 m. It consists of six superimposed and amalgamated beds, 60–120-cm thick (Fig. 4f); they show a massive coarse-grained basal portion (20–30-cm thick) with cm-sized flattened ‘tuff chips’, a middle portion characterized by cm thick parallel laminae and a fine-grained homogeneous upper part where centimetre-sized firm and soft ground burrows occur. 5.2.2. Petrography Both MVL types are coarse to medium impure vitric arenites, grading upward to fine arenites and siltites. They are composed of a prevalent volcaniclastic material, together with intrabasinal and extrabasinal grains (Fig. 6). The volcanic fraction consists of glass shards and minor volcanogenic minerals and represents up to 88% of the components. Glass shards are colourless and have predominantly a Y shape; pumices are common (Fig. 5d). The volcanic minerals are plagioclase, biotite, quartz, sanidine, amphibole and magnetite. Plagioclase mainly occurs as isolated slightly zoned crystals, whose composition ranges from labradorite to andesine. Sanidine is scarce. Biotite occurs as brown flakes, sometimes kinked and bent. Quartz is present in anhedral crystals and only rarely shows embayed edges. Amphibole and Ti-rich magnetite are common accessory phases. In some VLs (Varengo) amphibole is the dominant mafic phase and, when analysed at EDS, shows an edenitic composition (sensu Leake, 1997). Intrabasinal grains consist of bioclasts (benthonic and planktonic foraminifera, echinoid spines and bryozoan fragments), glaucony, phosphates and pyrite and represent up to 9% of the volume

(Fig. 5c,d). The foraminifera are often filled by glaucony, phosphates or amorphous silica. The green grains show different stages of maturity and are concentrated in the lower part of the beds. Extrabasinal grains are mainly represented by metamorphic minerals (blue amphibole and epidote) and their content can reach 4% of the volume. There are remarkable differences in composition between the base and the top of each bed, evidencing a density grading. The base is characterized by the presence of heavier grains, such as coarse-sandsized grains of magmatic biotite, feldspar and glass shards and heavy metamorphic minerals. Intrabasinal grains are normally scarce; when present in high percentage (Varengo), they consist of non-carbonate intrabasinal grains (glaucony, phosphates, pyrite) and glauconitized, phosphatized or silicified foraminifera. On the contrary, the tops of the single layers are dominated by the lighter fraction: fine- to very fine-sand-sized glass shards, pumices and carbonate intrabasinal grains such as foraminifera filled by fine sediment or secondary calcite. There are slight differences in the petrographic composition between the two types of MVLs. Generally, the layers classified as type 1 MVLs are devoid of fine to very fine glass particles that constitute the matrix (Fig. 5b); intergranular pores are sometimes completely filled by secondary calcite (Cornale). Type 2 MVLs show a major amount of non-volcanic extrabasinal and intrabasinal grains compared to type 1, and have a considerable amount of matrix consisting of fine-grained glass shards (Fig. 5c). 5.2.3. Glass morphology SEM observations revealed that type 2 MVLs are constituted by two subpopulations of glass shards showing different grain sizes. The predominant subpopulation (600–200 µm), present also in type 1, is represented by a bubble wall type form (Fig. 7b), with also a discrete amount of highly vesiculated and coarse-grained pumice fragments with tubular or ovoidal vesicles (Fig. 7c,e,g). Different types of bubble wall glass can be distinguished: flat and cuspate (Fig. 7i), curved and junction shards (Fig. 7f). Generally the pumices are coarser than bubble wall glass shards. The finer subpopulation (200–20 µm), absent in type 1 MVLs, represents the matrix of type 2 MVLs

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and consists of blocky angular types, with low vesicularity (Fig. 7h). Tiny pits due to a slight alteration are recognizable on all the glass shards. All transitions from highly vesicular through slightly vesicular with scallopped edges to blocky types can be observed in the glass shards. 5.2.4. Magnetic fabric The specimens from the Castel S. Pietro MVL are characterized by a slightly foliated fabric (Table 2). At the site level, however, the principal susceptibility directions are rather dispersed (Fig. 8B) and only roughly consistent with the bedding. This dispersion points to an extensive reworking subsequent to deposition. The good outcrop conditions of the Varengo MVL along the face of an abandoned quarry allowed a detailed sampling: 35 specimens were cored at a 5-cm interval from three different beds. The fabric characteristics are similar both within and between the beds and witness a similar depositional environment. Magnetic foliation is well developed, the minimum axes are always well grouped and the maximum and intermediate axes either grouped (Fig. 8C) or more or less dispersed within the foliation plane (Fig. 8D). The attitudes of the foliation and bedding planes are similar. The directions are close and foliation is inclined by 3º–4º to 30º–35º with respect to the bedding (Fig. 8C,D). The magnetic lineation is orthogonal to the foliation’s plunge. According to theoretical models (Tarling and Hrouda, 1993) and observations on deep-sea turbidites (Ellwood and Ledbetter, 1977; Ledbetter and Ellwood, 1980), this fabric may be interpreted as caused by deposition from high-velocity currents. The magnetic lineation develops at a right angle to the flow direction and grain imbrication results in foliation inclined with respect to the bedding. The different inclinations in the three beds might correspond to different strengths of the palaeocurrents. 5.2.5. Interpretation As in the Camino SVL, in MVLs there is also no evidence of emplacement at high temperatures. The volcanic component is dominated by two subpopulations of glass shards with different morphoscopic features: (a) bubble-wall and vesiculated shards and (b) blocky equant fragments. This indicates that

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these grains are apparently formed by a combination of vesiculating magma and quenching by water. Deposits made up of such shards are characteristics of shallow water eruption (Fisher and Schmincke, 1984). The relatively high proportion of intrabasinal grains (8–9%) suggests that both type 1 and type 2 MVLs are not primary pyroclastic deposits but resulted from redistribution and redeposition in distinct sectors of the carbonate shelf of loose pyroclastic detritus mixed with intrabasinal grains (e.g. Stix, 1991). The reworking mechanisms of these primary volcaniclastic deposits are different in type 1 and type 2 MVLs. Type 1 MVLs show the following characteristics: (1) the occurrence of a poorly defined hummocky cross-stratification; (2) the absence of features characteristic of wave activity; (3) the normal grading in the single, dm-thick beds, their erosional basal contact and the mixed composition of the sediments; (4) the dispersion of the principal susceptibility directions; and (5) the encasing carbonate sediments, consisting of glaucony-rich calcarenites deposited in a storm-dominated shelf (Clari et al., 1995). These features allow type 1 MVLs to be interpreted as storm layers, deposited between fairweather and storm wave base. Moreover, the lack of fine-grained matrix indicates that these deposits have been intensively winnowed by submarine currents; this is consistent with a high-energy depositional environment, such as a storm-dominated shelf. Finally, the occurrence of water escape structures observed both at Castel S. Pietro and Cornale, indicates a liquefaction of the sediments possibly induced by subsequent major storms or rapid deposition (e.g. Molina et al., 1998). Facies characteristics of the individual beds composing type 2 MVLs are summarized as follows: (1) normal grading and erosional basal contacts; (2) tripartition with a lower massive portion, a middle part showing parallel laminae and an upper homogeneous portion; (3) occurrence of flattened decimetre ‘tuff chips’ in the lower or middle part of the beds; (4) density grading and mixed composition of the components; and (5) magnetic lineation at a right angle to the foliation’s plunge. These features are consistent with deposition by

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turbidity currents involving both pyroclastic detritus and intrabasinal grains. Amalgamation of the beds composing the MVLs indicates repeated and closely spaced downslope flow of single turbidity currents. Type 2 MVLs represent storm-induced, distal shelf turbidites, triggered by storm activity acting in the more internal part of the shelf (e.g. Benton and Gray, 1981; Walker, 1984; Myrow and Southard, 1996). During major storms, cyclic wave loading of unconsolidated sediments may cause the liquefaction of the substrate; as these liquified sediments flow downslope, the combination of flow acceleration and expulsion of the pore fluid may be sufficient to generate turbidity currents, involving both intrabasinal and volcaniclastic sediments (Walker, 1984). This interpretation is supported by the occurrence of liquefaction structures in the more proximal sediments and in type 1 MVLs. The resulting turbidite deposits can be preserved only below storm wave base, where they are not reworked by storms. The occurrence of firm ground burrows at the top of single turbidite beds suggests that each resedimentation event was separated by short periods of hemipelagic sedimentation and bioturbation of the sea bottom.

be originally deposited by fall-out mechanisms (Cas and Wright, 1987). Furthermore, the abundance of unaltered shards and pumice fragments without abrasion features suggests a direct emplacement and=or a rapid resedimentation of the volcanic detritus after the eruption without the possibility of its strong alteration (e.g. Niem, 1977; Bull and Cas, 1991; Cousineau, 1994). The volcanic edifices responsible for the fallouts were presumably located outside of the basin, since no direct evidence of volcanic activity has been recorded in Monferrato. In absence of primary records of volcanic centres, thickness and mean grain size of pyroclastic fall deposits are the first parameters to be taken into account to constrain the distance of the source area, as they exponentially decrease from the vent (Fisher, 1965; Fisher and Schmincke, 1984; Cas and Wright, 1987; Pyle, 1989; Bitschene and Schmincke, 1991; Schmincke and van den Bogaard, 1991). In the case of Monferrato, the thickness and the grain size of the vitric fraction of the only primary pyroclastic deposit so far discovered (Camino SVL) are consistent with an origin from nearby (10 to 30 km) eruptions (Fisher and Schmincke, 1984).

6. Discussion

6.2. Depositional mechanisms

The analyses of the Monferrato VLs resulted in the discrimination between primary volcanic and marine sedimentary processes. The main steps of the VLs formation can be summarized as follows.

The explosive volcanic activity produced an extensive tuff blanket that was uniformly deposited on a carbonate-dominated shelf (Fig. 9a). The volcaniclastic sediments were immediately reworked and redistributed by the sedimentary processes acting in the basin (Fig. 9b). They can presently be detected as discrete layers only in the sectors where the energy conditions on the shelf were low. Above storm wave base, storm activity mixed the volcaniclastic fraction with a minor intrabasinal and extrabasinal component, accumulating them in morphological depressions of the shelf, where the sediments were more protected from storm currents; the resulting deposits are represented by type 1 MVLs. Below storm wave base, low-energy conditions allowed the preservation of the primary pyroclastic fall deposits that were only slightly bioturbated. These deposits correspond to SVLs. During large and infrequent storms, liquefaction of sediments in more internal portions of the shelf triggered turbidity currents transporting down-

6.1. Volcanic fragmentation processes Glass morphologies of the Monferrato VLs suggest that the juvenile fraction has been produced by explosive eruptions. Hydromagmatic and mixed hydromagmatic=magmatic fragmentation processes may be inferred respectively for the volcanic component of the SVLs and the MVLs. The absence of charcoal remains, welding, plastic deformation, columnar jointing, indicative of emplacement at high temperature or heat retention (Ross and Smith, 1961; Cas, 1979; Cas and Wright, 1987), suggests that the pyroclastic detritus was cold when it reached the sea bottom. This excludes an emplacement by pyroclastic flows and indicates that the volcanic grains had to

A. d’Atri et al. / Sedimentary Geology 129 (1999) 143–163 Fig. 9. Sketches showing the hypothetical depositional mechanisms of the Monferrato VLs. (a) Deposition of a pyroclastic fall. (b) Redistribution of the primary pyroclastic deposits by the sedimentary processes acting in the platform. For more details, see the text. SVL D single volcaniclastic layer; MVL D multiple volcaniclastic layer; 1 D undifferentiated Oligo–Miocene sediments; 2 D biocalcarenites and biocalcirudites; 3 D planktonic foraminifera and glaucony-rich calcarenites; 4 D calcareous and siliceous marls.

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slope volcaniclastic and intrabasinal grains. These sediments were deposited below storm wave base. Repeated resedimentation events, separated by periods of scarce hemipelagic sedimentation, originated type 2 MVLs. The proposed model indicates that the lateral extension of the VLs on a shelf depends on their preservation potential, that in turn is strongly controlled by the following factors: (a) Energy conditions of the depositional setting. High energy conditions do not permit the preservation of VLs. In the sectors where bottom and storm-induced currents intensively swept the shelf, the loose pyroclastic detritus was prone to be resedimented and the only primary pyroclastic products that could be eventually preserved were massive pyroclastic flow deposits that hardly could be completely reworked. On the contrary, laterally extensive primary fall deposits are likely to be found only below storm wave base, where the low-energy conditions allowed their preservation. No volcaniclastic sediments (either primary or resedimented) are to be expected in the high-energy inner shelf sediments. For this reason, the ‘foramol’ deposits of the Eastern Monferrato (facies 1 of the PDC Group) are lacking VLs. (b) Amount of sediment supply. The common finding of VLs in sediments characterized by low sedimentation rate suggests that they can be more easily preserved where the sediment supply is low enough to hamper the dilution of the volcanic fraction with the non-volcanic sediments. According to this hypothesis, the lack of VLs (both SVLs and MVLs) in the Western Monferrato succession, which is close and coeval to the Eastern Monferrato one, may be explained by the high terrigenous supply that prevented the preservation of high concentrations of the volcanic fraction in discrete volcaniclastic beds. The foregoing statements lead to important consequences: (1) The use of VLs as physical correlation tools for shelf successions may be misleading. In the Monferrato, the VLs represent only some limited remnants of volcaniclastic deposits of different ages. Therefore, they cannot be used for large-scale stratigraphic correlations. (2) The propensity to resedimentation of pyroclastic deposits in a shelf environment must be taken into consideration when these sediments are used

for biochronological studies: the radiometric age obtained from the resedimented volcanic grains may be older than the biostratigraphic age of the encasing sediments. However, the occurrence of unaltered glass shards indicates a narrow time span between eruption and final deposition by resedimentation. This has been checked for two Burdigalian VLs of Monferrato (Villadeati and Varengo), which show a good agreement between radiometric and biostratigraphic ages (d’Atri et al., 1997).

7. Conclusions The integration of stratigraphic, sedimentological, petrographic, and magnetic fabric studies and SEM analyses on six VLs interbedded in the Burdigalian shelf succession of Monferrato resulted in the discrimination between primary volcanic and marine sedimentary processes acting on the formation of these deposits. The characteristics of the volcanic fraction suggest that the pyroclastic detritus was produced by explosive volcanic activity and reached the basin by fall-out. The sedimentological and magnetic fabric data allowed to distinguish three types of VLs: (a) SVLs, corresponding to primary pyroclastic fall-out deposits. (b) MVLs originated by reworking of loose pyroclastic detritus. They can be divided into two types: Type 1 can be interpreted as storm layers, whereas Type 2 represent storm-induced, distal shelf turbidites, triggered by storm activity acting in the more internal part of the shelf. The proposed depositional model requires a strict interaction of volcanic and sedimentary processes that must be separately considered when VLs are studied. In a shelf environment the preservation of VLs is controlled by energy conditions and sedimentation rates. Therefore, the shelf VLs are typically sedimentary bodies with very scarce lateral continuity. Consequently, their use as marker beds in physical correlations may be misleading and must be checked by detailed sedimentological and biostratigraphic investigations.

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Acknowledgements This work was supported by ‘CNR–C.S. geodinamica delle catene collisionali (Torino)’ and was divided up as follows: lithostratigraphic framework F. Dela Pierre; biostratigraphy A. d’Atri; sedimentology A. d’Atri and F. Dela Pierre; petrography and morphological SEM analyses R. Ruffini; magnetic fabric study R. Lanza. The authors are very grateful to P. Clari, L. Martire and F. Piana for their constructive reviews of the manuscript. Many thanks to R. Cas and two anonymous referees whose critical reviews greatly improved the quality of this paper. J. Griffin revised the English text.

References Benton, M.J., Gray, D.I., 1981. Lower Silurian distal shelf storm-induced turbidites in the Welsh Borders: sediments, tool marks and trace fossils. J. Geol. Soc. London 138, 675–694. Bitschene, P.R., Schmincke, H.U., 1991. Fallout tephra layers: composition and significance. In: Heling, D., Rothe, P., Forstner, U., Stoffers P. (Eds.), Sediments and Environmental Geochemistry. Springer, Berlin, pp. 48–82. Bresso, C., 1997. Paleomagnetismo di rocce oligoceniche nell’Italia nord-occidentale. Unpublished Thesis, University of Torino, 199 pp. Bull, S.W., Cas, R.A.F., 1991. Depositional controls and characteristics of subaqueous bedded volcaniclastics of the Lower Devonian Snowy River Volcanics. Sediment. Geol. 74, 189– 215. Calanchi, N., Cattaneo, A., Dinelli, E., Gasparotto, G., Lucchini, F., 1998. Tephra layers in Late Quaternary sediments of the central Adriatic Sea. Mar. Geol. 149, 191–209. Carannante, G., Esteban, M., Milliman, J.D., Simone, L., 1988. Carbonate lithofacies as paleolatitude indicators: problems and limitations. Sediment. Geol. 60, 333–346. Carey, S.N., Sigurdsson, H., 1978. Deep-sea evidence for distribution of tephra from the mixed magma eruption of the Soufrie`re on St. Vincent, 1902: ash turbidites and air-fall. Geology 6, 271–274. Cas, R.A.F., 1979. Mass-flow arenites from a Paleozoic interarc basin, New South Wales, Australia: mode and environment of emplacement. J. Sediment. Petrol. 49, 29–44. Cas, R.A.F., 1991. Nomenclature of volcaniclastics. IAVCEI Newsl., pp. 3–6. Cas, R.A.F., Busby Spera, C. (Eds.), 1991. Volcaniclastic Sedimentation. Sediment. Geol. 74, 362 pp. Cas, R.A.F., Wright, J.V., 1987. Volcanic Successions: Ancient and Modern. Allen and Unwin, London, 528 pp. Cassano, E., Anelli, R., Fichera, R., Cappelli, V., 1986. Pianura Padana: interpretazione integrata di dati geofisici e geologici. 73o Congr. Soc. Geol. Ital., Roma, AGIP (ed).

161

Clari, P., Pirini, C., Ricci, B., Ruffini, R., Valleri, G., 1988. Livelli cineritici miocenici nel Monferrato. Rend. Soc. Geol. Ital. 11, 293–296. Clari, P., Dela Pierre, F., Novaretti, A., Timpanelli, M., 1995. Late Oligocene–Miocene sedimentary evolution at the Alps=Apennines junction: the Monferrato area, North Western Italy. Terra Nova 7, 144–152. Cole, R.B., Stanley, R.G., 1994. Sedimentology of subaqueous sediment gravity flows in the Neogene Santa Maria basin, California. Sedimentology 41, 37–54. Cousineau, P.A., 1994. Subaqueous pyroclastic deposits in an Ordovician fore-arc basin: an example from the Saint-Victor Formation, Quebec Appalachians, Canada. J. Sediment. Res. A64, 867–880. Dal Piaz, G.V., Venturelli, G., 1983. Brevi riflessioni sul magmatismo post-ofiolitico nel quadro dell’evoluzione spazio– temporale delle Alpi. Mem. Soc. Geol. Ital. 26, 5–19. d’Atri, A., Tateo, F., 1994. Volcano-sedimentary beds of Oligocene age from Tertiary Piedmont Basin (NW Italy): biostratigraphy and mineralogy. G. Geol. 56, 79–95. d’Atri, A., Dela Pierre, F., Novaretti, A., Ruffini, R., Cosca, M.A., Hunziker, J.C., 1997. Calcareous plankton biostratigraphy and 40 Ar=39 Ar dating of Miocene volcanic ash layers from Monferrato (NW Italy). Interim Colloquium R.C.M.N.S., Catania, 4–9 November 1997, Abstracts, p. 47. De Gennaro, M., Colella, C., 1991. The critical role of temperature in the natural zeolitization of volcanic glass. Neues Jahrb. Mineral. Monatsh. 8, 335–362. Di Marco, M.J., Lowe, D.R., 1989. Shallow water volcaniclastic deposition in the Early Archean Panorama Formation, Warrawoona Group, eastern Pilbara block, Western Australia. Sediment. Geol. 64, 43–63. Ellwood, B.B., Ledbetter, M.T., 1977. Antartic bottom water fluctuations in the Vema Channel: effects of velocity changes on particle alignment and size. Earth Planet. Sci. Lett. 35, 189–198. Fisher, R.V., 1965. Settling velocity of glass shards. Deep Sea Res. 12, 345–353. Fisher, R.V., Schmincke, H.U., 1984. Pyroclastic Rocks. Springer, Berlin, 472 pp. Fornaciari, E., Rio, D., 1996. Latest Oligocene to early middle Miocene quantitative calcareous nannofossil biostratigraphy in the Mediterranean region. Micropaleontology 42, 1–36. Fornaciari, E., Di Stefano, A., Rio, D., Negri, A., 1996. Middle Miocene quantitative calcareous nannofossil biostratigraphy in the Mediterranean region. Micropaleontology 42, 37–63. Fritz, W.J., Howells, M.F., 1991. A shallow marine volcaniclastic facies model: an example from sedimentary rocks bounding the subaqueously welded Ordovician Garth Tuff, North Wales, U.K. Sediment. Geol. 74, 217–240. Galbiati, B., 1976. La successione oligo–miocenica tra Rigoroso e Carrosio (Bacino ligure–piemontese). Atti Ist. Geol. Univ. Pavia 26, 30–48. Granar, L., 1958. Magnetic measurements on Swedish varved sediments. Arkiv. Geofys. 3, 1–40. Guerrera, F., Veneri, F., 1989. Evidenze di attivita` vulcanica nei

162

A. d’Atri et al. / Sedimentary Geology 129 (1999) 143–163

sedimenti neogenici e pleistocenici dell’Appennino: stato delle conoscenze. Boll. Soc. Geol. Ital. 108, 121–160. Heiken, G.H., Wohletz, K., 1985. Volcanic Ash. Univ. California Press, Berkeley, CA, 246 pp. Heikoop, J.M., Tsujita, C.J., Heikoop, C.E., Risk, M.J., Dickin, A.P., 1997. Effects of volcanic ashfall recorded in ancient marine benthic communities: comparison of a nearshore and an offshore environment. Lethaia 29, 125–139. Izett, G.A., Obradovich, J.D., Menhert, H.H., 1988. The Bishop Ash Bed (Middle Pleistocene) and some older (Pliocene and Pleistocene) chemically and mineralogically similar ash beds in California, Nevada and Utah. U.S. Geol. Surv. Bull. 1675, 37 pp. Jelinek, V., 1978. Statistical processing of magnetic susceptibility measured in group of specimens. Stud. Geophys. Geod. 22, 50–62. Kano, K., 1991. Volcaniclastic sedimentation in a shallow water marginal basin: the Early Miocene Koura Formation, SW Japan. Sediment. Geol. 74, 309–322. Leake, B.E., 1997. Nomenclature of amphiboles: report of the subcommittee on amphiboles of the international mineralogical association, commission on new minerals and mineral names. Canad. Mineral. 35, 219–246. Ledbetter, M.T., Ellwood, B.B., 1980. Spatial and temporal changes in bottom water velocity from the analysis of particle size and alignment in deep-sea sediments. Mar. Geol. 38, 245–261. Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties. Geophys. Res. Lett. 17, 159–162. Martini, J., 1968. Etude pe´trographique des Gre`s de Taveyanne entre Arve et Giffre (Haute Savoie, France). Schweiz. Mineral. Petrogr. Mitt. 48, 539–654. Molina, J.M., Alfaro, P., Moretti, M., Soria, J.M., 1998. Soft-sediment deformation structures induced by cyclic stress of storm waves in tempestites (Miocene, Guadalquivir Basin, Spain). Terra Nova 10, 145–150. Myrow, P.M., Southard, J.B., 1996. Tempestite deposition. J. Sediment. Res. 66, 875–887. Nagata, T., 1961. Rock Magnetism. Maruzen, Tokyo, 350 pp. Niem, A.R., 1977. Missisipian pyroclastic flow and ash-fall deposits in the deep marine Ouachita flysch basin, Oklahoma and Arkansas. Geol. Soc. Am. Bull. 88, 49–61. Perkins, M.E., Nash, W.P., Brown, F.H., Fleck, R.J., 1995. Fallout tuffs of Trapper Creek, Idaho — a record of Miocene explosive volcanism in the Snake River Plain volcanic province. Geol. Soc. Am. Bull. 107, 1484–1506. Piana, F., Polino, R., 1995. Tertiary structural relationships between Alps and Apennines: the critical Torino Hill and Monferrato area, Northwestern Italy. Terra Nova 7, 138–143. Polino, R., Clari, P., Crispini, L., d’Atri, A., Dela Pierre, F., Novaretti, A., Piana, F., Ruffini, R., Timpanelli, M., 1995. Relazioni tra zone di taglio crostali e bacini sedimentari: l’esempio della giunzione alpino–appenninica durante il terziario. Guida all’escursione in Monferrato e nella Zona Sestri–Voltaggio. Acc. Naz. Sci., Scritti Doc. 14, 531–593.

Pyle, D.M., 1989. The thickness, volume and grainsize of tephra fall deposits. Bull. Volcanol. 51, 1–15. Ross, C.S., Smith, R.L., 1961. Ash-flow tuffs: their origin, geologic relations and identification. U.S. Geol. Surv. Prof. Pap. 366, 81 pp. Ruffini, R., 1995. Evidenze di attivita` vulcanica terziaria nelle Alpi occidentali: problemi ed ipotesi. Ph.D. Thesis, University of Turin, 160 pp. Ruffini, R., Cadoppi, P., d’Atri, A., Novaretti, A., 1995a. Ash layers in the Monferrato (NW Italy): records of two types of magmatic source in Oligocene–Miocene time. Eclogae Geol. Helv. 88, 347–363. Ruffini, R., Cosca, M.A., d’Atri, A., Hunziker, J.C., Polino, R., 1995b. The volcanic supply of the Taveyanne turbidites (Savoie, France): a riddle for Tertiary Alpine volcanism. Acc. Naz. Sci., Scritti Doc. 14, 359–376. Sarna-Wojcicki, A.M., Bowman, H.R., Meyer, C.E., Russell, P.C., Woodward, M.J., McCoy, G., Rowe, J.J., Baedecker, P.A., Asaro, F., Michael, H., 1984. Chemical analyses, correlations, and ages of Upper Pliocene and Pleistocene ash layers of East-Central and Southern California. U.S. Geol. Prof. Pap. 1293, 40 pp. Schmincke, H.U., van den Bogaard, P., 1991. Tephra layers and tephra events, In: Einsele, G., Ricken, W., Seilacher, A. (Eds.), Cycles and Events in Stratigraphy. Springer, Berlin, pp. 392– 429. Sigurdsson, H., Sparks, R.S.J., Carey, S.N., Huang, T.C., 1980. Volcanogenic sedimentation in the Lesser Antilles arc. J. Geol. 88, 523–540. Sparks, R.S.J., Huang, T.C., 1980. The volcanological significance of deep-sea ash layers associated with ignimbrites. Geol. Mag. 117, 425–436. Sparks, R.S.J., Walker, G.P.L., 1977. The significance of vitric-enriched air-fall ashes associated with crystal-enriched ignimbrites. J. Volcanol. Geotherm. Res. 2, 329–341. Stix, J., 1991. Subaqueous, intermediate to silicic-composition explosive volcanism: a review. Earth Sci. Rev. 31, 21–53. Stow, D.A.V., Taira, A., Ogawa, Y., Soh, W., Taniguchi, H., Pickering, K.T., 1998. Volcaniclastic sediments, process interaction and depositional setting of the Mio–Pliocene Miura Group, SE Japan. Sediment. Geol. 115, 351–381. Structural Model of Italy, sheet 1, 1990. Geodynamic Project, C.N.R., SELCA, Firenze. Tarling, D.H., Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. Chapman and Hall, London, 217 pp. Tateo, F., 1992. Studio mineralogico e geochimico di sedimenti vulcanoderivati (Oligocene–Miocene inferiore) dell’Appennino settentrionale. Ph.D. Thesis, Bologna University, Barghegiani Ed., 216 pp. Walker, R.G., 1984. Shelf and shallow marine sands. In: Walker, R.G. (Ed.), Facies Models, 2nd ed. Geosci. Can., Reprint Ser. 1, 141–170. Wohletz, K.H., 1983. Mechanisms of hydrovolcanic pyroclast formation: grain-size, scanning electron microscopy, and experimental studies. J. Volcanol. Geotherm. Res. 17, 31–63. Wright, J.V., Mutti, E., 1981. The Dali ash, Island of Rhodes,

A. d’Atri et al. / Sedimentary Geology 129 (1999) 143–163 Greece: a problem in interpreting submarine volcanigenic sediments. Bull. Volcanol. 44, 153–167. Young, J.R., Flores, J., Wei, W., 1994. A summary chart of Neogene nannofossil magnetobiostratigraphy. J. Nannoplankton Res. 16, 21–27. Zingg, A., Hunziker, J.C., Frey, M., Ahrendt, H., 1976. Age and

163

degree of metamorphism of the Canavese Zone and of the sedimentary cover of Sesia Zone. Schweiz. Mineral. Petrogr. Mitt. 56, 361–375. Zuffa, G.G., 1985. Optical analyses of arenites: influence of methodology on compositional results. In: Zuffa, G.G. (ed.) Provenance of Arenites. NATO-ASI Ser., pp. 165–189.