Stratigraphic signature of the Vesuvius 79 AD event off the Sarno prodelta system, Naples Bay

Marine Geology 222–223 (2005) 443 – 469 www.elsevier.com/locate/margeo

Stratigraphic signature of the Vesuvius 79 AD event off the Sarno prodelta system, Naples Bay M. Sacchi a,*, D. Insinga a, A. Milia a, F. Molisso a, A. Raspini a, M.M. Torrente b, A. Conforti a a

Istituto per l’Ambiente Marino Costiero (IAMC) CNR, Napoli, Calata P.ta di Massa, Porto di Napoli, 80133, Napoli, Italy Dipartimento di Studi Geologici ed Ambientali (DSGA), Universita` del Sannio, Via Portarsa, 11- 82100, Benevento, Italy

b

Accepted 15 June 2005

Abstract Sedimentological and chemical analysis of gravity core samples, along with the interpretation of very high resolution, single channel seismic reflection profiles acquired off the Sarno prodelta system (southeastern Naples Bay), document the sedimentary facies and seismic stratigraphic signature of the tephra deposit erupted by Vesuvius during the plinian eruption of 79 AD. The 79 AD pyroclastic deposits sampled off the Sarno prodelta system revealed significantly different characters with respect to both the subaerial succession documented from the type sections of Pompeii, Herculaneum and Oplontis and the more distal pyroclastic fallout deposits that were recovered from the outer continental shelf off the northern Salerno Bay. This was likely to be expected as, in proximal subaqueous settings, both primary deposition and reworking of tephra are known to be influenced by the hydrodynamics of the water itself (induced by the pyroclastic currents entering the sea) as well as by the early instability of pyroclastic bedforms due to the exceptionally high sediment yield during volcanic eruption. According to the data illustrated and discussed in this study, we suggest that the 79 AD tephra deposit sampled off the Sarno prodelta is likely to be the result of a number of syn-eruptive genetically-related depositional and/or erosional mechanisms associated with (1) anomalous waves (tsunami) and currents generated by the entrance of pyroclastic flows and surges into the sea; (2) subaqueous density flows evolved from the impact of hot pyroclastic currents into the seawater; (3) failure of water-logged, cohesionless pumice deposits, due to instability of pyroclastic bedforms rapidly accumulating on the seafloor. Seismic interpretation revealed that the Upper Holocene sequence off the Sarno prodelta system is affected by extensive creep involving the whole post-79 AD succession. Deformation due to creep is documented by slumping of semiconsolidated strata over a basal surface represented by the lithologic discontinuity between the base of the 79 AD tephra deposits and the underlying hemipelagite. Seismic data also suggest that gravitational instability of this area has been induced, or enhanced, by significant volcanotectonic deformation and local uplift of the seafloor that ostensibly predated the eruptive event.

* Corresponding author. E-mail address: [email protected] (M. Sacchi). 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.06.014

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The results of this research may be relevant to studies of other eruption events in close proximity to coastlines in terms of wave- and/or current-generated sedimentary features as a possible explanation of unusual subaqueous facies architecture. D 2005 Elsevier B.V. All rights reserved. Keywords: Vesuvius; plinian eruption; Naples Bay; tephra deposits

1. Introduction It is generally accepted that prodelta sediments are of particular interest in the study of both modern and ancient marine deltaic environments because of their relatively high preservation potential associated with good vertical and lateral continuity of stratigraphic successions deposited below the fair-weather wave base (Miall, 1984; Einsele, 2000). Prodelta sediments are represented typically by fine-grained deposits delivered by the river, reflecting episodes of minor and major river supply. The morphology and sedimentary facies of marine deltas are controlled to a large degree by the sediment input and the hydrodynamic regime of the receiving marine basin (Prior and Coleman, 1978; Coleman, 1981; Swift et al., 1991). The mineralogy of siliciclastic deposits, as well as sedimentation rates off the river mouth, may vary significantly according to the geology and size of the drainage river basin inland (Reading, 1996). The prodelta is also disturbed commonly by softsediment failure and, in some deltas, prodelta sediments are dominated by products of mass movements deriving from the delta front (Coleman and Prior, 1982; Linsay et al., 1984; Coleman, 1988). The principal reasons for sediment-induced deformation include (a) the relatively high sedimentation rate on the delta front which causes undercompaction and high pore fluid pressures, leading in turn to loss of shear strength within the deposits; (b) biodegradation of organic debris and associated free methane gas which weakens the sediment stability; (c) shocking of accumulated sediment by storm wave action; (d) sediment instability induced by earthquakes. Among the factors that may have significant impact on delta construction is the frequency of recurrence of exceptional river floods, mudflows and explosive eruptions (pyroclastic falls, surges and flows) from coastal volcanoes. These can all induce the supply of large volumes of loose sediment into the

delta system and over vast areas of the continental shelf. In this paper, we describe the offshore facies of the volcaniclastic deposits associated with the sudden input of large volumes of pyroclastic material deposited offshore the Sarno prodelta during the 79 AD plinian eruption of Vesuvius. The research work was based on an integrated stratigraphic approach, including sequence stratigraphic interpretation of very high resolution single channel seismic reflection profiles (sub-bottom Chirp), sedimentological analysis of gravity cores, and chemical analysis of major and trace elements of the 79 AD tephra. The scope of the study is the understanding of the syn-eruptive depositional mechanism of pyroclastic deposits off the Naples Bay continental shelf and the impact of the input of large volumes of pyroclastic sediments in the Late Holocene evolution of the Sarno river prodelta system.

2. Geological outline The Campania plain and the Naples Bay are an integral part of a large Quaternary extensional basin located between the western flank of southern Italy and the eastern Tyrrhenian margin (Fig. 1). This area is the result of large-scale Late Neogene to Pleistocene lithospheric extension that accompanied the eastward accretion of the Apenninic fold and thrust belt during the roll-back of the subducting Adria foreland plate. Extensional processes within the Apennine–Tyrrhenian system basin began in the late Miocene and migrated progressively in time from NW to SE (Malinverno and Ryan, 1986; Kastens et al., 1988; Doglioni, 1995; Ferranti et al., 1996). The Quaternary evolution of the eastern Tyrrhenian margin is characterized by the development of halfgraben basins (e.g., Volturno basin, Naples Bay–Campania plain, Salerno Bay–Sele basin) and intervening

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Fig. 1. Physiography of the northern Campania margin showing the location of volcanic districts (Campi Flegrei, Vesuvius), the Sarno river basin and the data set. Seafloor bathymetry and onland topography are after D’Argenio et al. (in press); shelf break in the Naples Bay is from Milia (1999a,b); isopachs of the 79 AD pumice fall deposits and pyroclastic flow deposits are reported (and slightly modified) after Sigurdsson et al. (1985) and Lirer et al. (1993), respectively. Insert (a) displays the full grid of high-resolution (subbottom Chirp) profiles used in this study.

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structural highs (e.g., Sorrento Peninsula, M. Massico) that trend perpendicular to the main axis of the Apenninic belt (Mariani and Prato, 1988; Romano et al., 1994; Sacchi et al., 1994; Milia and Torrente, 1997, 1999; Milia et al., 2003a). The mountains that bound the Campania coastal region may reach an elevation of 1500 m and are composed mainly of carbonate rocks (Middle Triassic to Late Cretaceous) and subordinately siliciclastic units (Upper Neogene) that formed thrust sheets and imbricates during the late Miocene–early Pliocene. Multichannel reflection seismic profiles and deep borehole data (Ippolito et al., 1973; Finetti and Morelli, 1974; Bartole et al., 1984; Mariani and Prato, 1988; Sacchi et al., 1994) have documented that the Campania continental margin is characterized by a Quaternary basin fill that may exceed 3000 m. The sedimentary succession is represented by siliciclastic deposits interbedded with lavas and volcaniclastic rocks erupted by a number of Quaternary volcanic districts (Roccamonfina, Campi Flegrei–Procida, Ischia, and Vesuvius). 2.1. Late Quaternary evolution of the Campania Plain–Naples Bay basin The Campania Plain has been characterized by pronounced subsidence throughout the Late Quaternary (Brancaccio et al., 1991). The lowering of the sea level that accompanied the onset of the last glaciation between 125 ka and 18 ka resulted in a seaward shift of the shoreline, parallel with the forced regression of paralic-shallow marine depositional systems (Milia, 1996, 1999a,b, 2000; Cinque, 1999; Conforti, 2003). During this time of low base level, the whole continental margin of Campania was exposed and the modern submarine canyon system of the Naples Bay (Magnaghi and Dohrn canyons) evolved rapidly across the ancient continental slope. The onset of widespread volcanic activity and the consequent formation of a number of volcanic centres in the coastal zone of Campania (Campi Flegrei, Vesuvius) played a major role in shaping the morphology of the coastal landscape and continental shelf. The major eruption of the Campania Ignimbrite at 39 ka (De Vivo et al., 2001) floored the whole coastal plain and the continental shelf of the Naples Bay (Fusi et al., 1991; Milia et al., 1998; Milia,

1996, 2000). With the onset of the Somma–Vesuvius activity, since at least 25 ka (Santacroce, 1987; Santacroce and Sbrana, 2003), a paleo-Sarno alluvial plain began to form south of the volcanic edifice (Cinque, 1999). Seismic stratigraphic evidence off the modern Sarno river delta suggests that around the last glacial lowstand (ca. 25 ka–13 ka), the continental drainage system incised alluvial valleys and channels across the present-day continental shelf (Milia, 1996; Conforti, 2003). Later volcanic activity of the Campi Flegrei was accompanied by the eruption of the Neapolitan Yellow Tuff (ca. 15 ka) (Insinga, 2003; Deino et al., 2004) and the formation of a number of subaerial and submarine vents (tuff cones and tuff rings) across the Campi Flegrei coastal plain and the Pozzuoli Bay continental shelf between 10 ka and 1538 AD. The integration of geomorphologic evidence on land, subsurface data, and radiocarbon dating permitted reconstruction of the extent of the Holocene (Flandrian) transgression on the Sarno river plain (Cinque, 1999). During the Latest Pleistocene–Early Holocene (ca. 15–6 ka), the rising sea level gradually flooded the lower part of the Plain and caused a progressive broadening of the inner shelf by cutting into Upper Pleistocene alluvial fans. Erosional terraces of late Early Holocene age are found at 20 to 13 m water depth. About 6 ka ago, the termination of the transgressive phase marked a turnaround point within the coastline migration trend and the onset of the Late Holocene progradation (Barra et al., 1989; Conforti, 2003). In the last 5 ka, both the lower delta plain and the delta system have shifted sites of deposition accompanying an overall seaward migration of the shoreline in the order of 3–6 km (Cinque et al., 1997), notwithstanding the fact that the area was still subsiding with rates of approximately 2 mm/yr. Beach deposits of 5.6 ka to 4.5 ka BP occur at 3.5 to 10 m below the present-day sea level, whereas the beach deposits of Roman Age are found at 4 m below the present sea level. 2.2. Physiography of the Naples Bay The southeastern Naples Bay is characterized by a 10–15 km wide continental shelf, cut in its distal part by two branches of the Dohrn canyon. The general physiography of the Bay is influenced deeply by the

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interplay of volcanism and sedimentary processes during the Late Quaternary (Milia, 1999a,b; Milia and Torrente, 2000, 2003; Aiello et al., 2001) (Fig. 1). In particular, the shelf area is characterized by an irregular morphology due to the occurrence of numerous submarine volcanoes, with a shelf break lying at water depths between 70 and 150 m. Recent studies have shown that most of submarine and partly submerged volcanoes of the Naples Bay (e.g., Gaia Bank (GB), Pia Bank (PB), Miseno Bank (MB), Cape Miseno tuff cone (MC), Porto Miseno tuff ring (MR), Penta Palummo Bank (PP), Nisida Bank (NB), Nisida tuff cone (NC)), formed between 150 ka and 5 ka (Milia, 1996, 1999; Milia and Torrente, 2000, 2003; Insinga, 2003) (Fig. 1). The inner shelf depositional system of the eastern Naples Bay is controlled by the occurrence of two small coalescent deltas, namely formed by the Sebeto and Sarno rivers that are partly separated by an irregular relief south of Vesuvius, down to a depth of 100 m, associated with a 18-ka old volcanic debris avalanche (Milia et al., 2003b) (Da1 in Fig. 1). The southeastern sector of the Naples Bay continental shelf dips from the Sorrento Peninsula toward the northwest. The morphology of this area is the result of the erosional and depositional features formed by sea-level fluctuations over the last 39 ka (Milia, 1996, 1999b). Offshore the Sarno river mouth, the occurrence of extensive seafloor instability of Upper Holocene deposits has also been reported (Aiello et al., 2001; Conforti, 2003).

3. The 79 AD eruption of Vesuvius and its deposits The plinian eruption of Vesuvius on 24th and 25th August 79 AD is one of the best documented examples of a volcanic event in the literature. (Lirer et al., 1973, 1993; Sigurdsson et al., 1985; Carey and Sigurdsson, 1987; Cioni et al., 1990, 1992, 1995, 2000; Yokoyama and Marturano, 1997; Luongo et al., 2003a,b). It has been calculated that the eruptive event produced about 4 km3 of pyroclastic material (Sigurdsson et al., 1985; Cioni et al., 1999). The eruption started with a short and relatively weak phreatomagmatic explosion that covered the flanks of the volcano with ash deposits. This phase was followed by a second, very intense explosive

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(plinian) phase, accompanied by the formation of a volcanic plume that reached about 32 km altitude and was blown towards the southeast by stratospheric winds (Carey and Sigurdsson, 1987). As a result, within about 7 h, pyroclastic fall deposits, formed by a first layer of white pumice followed by grey pumice, covered vast areas south of the volcano, reaching an average thickness of 3 m in Pompeii. The colour transition between the lower and the upper part of the pyroclastic fall deposits (white to grey) corresponds to a variation in chemical composition of the magma (phonolitic to tephri-phonolitic, respectively) and also reflects a significant change in the grape density of the tephra deposit, namely 0.5– 0.6 g/cm3 on average for the white pumice deposits to 0.9 g/cm3 in the middle part and 1.10 g/cm3 at the base of the grey pumice layer (Lirer et al., 1973; Carey and Sigurdsson, 1987). Towards the end of the first day of the eruption, due to the interaction of the ascending magma with the phreatic zone, the volcanic activity entered a third phase characterized by the emission of pyroclastic flows and surges that repeatedly spread down the slopes of Vesuvius at high velocity (100 km/h) with temperatures in the order of 500 8C at Herculaneum (Kent et al., 1981; Mastrolorenzo et al., 2001; Gurioli et al., 2002) and reached the Sarno river Plain. These highly destructive pyroclastic currents deposited a suite of thin layers made up of poorly sorted ash with small and large-scale cross-bedding, alternated with thick, coarse-grained and massive pyroclastic beds (Sigurdsson et al., 1985; Cioni et al., 1992; Luongo et al., 2003a). The thickness of the pyroclastic fall deposits range from a maximum of 3–5 m on the southeastern flank of the Vesuvius to 1.0–0.5 m in the Sarno Plain and the northern coastal zone of the Salerno Bay, and to a few centimetres in the southern Campania region (Sigurdsson et al., 1985). Pyroclastic currents display a main flow direction towards the southeast, but were deposited over a much more restricted area compared to the fall deposits. Pyroclastic flow deposits reach a maximum thickness of 23 m in the vicinity of Herculaneum, ca. 4 m at Oplontis, 4–2 m in Pompeii, and 1– 2 m all over the Sarno river alluvial plain. The isopleth maps for both pyroclastic fall (white and grey pumice) and current deposits (flows and surges) reported in Fig. 1 have been constructed from onshore

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data obtained in the literature (Sigurdsson et al., 1985; Lirer et al., 1997). In the marine setting, the occurrence of the Vesuvius 79 AD fall products has been reported from the study of gravity core samples both in proximal and distal areas (Carbone et al., 1984; Buccheri et al., 2002). Recent detailed studies conducted off the Campania margin, based on a tephrostratigraphic approach (Insinga, 2003) and sequence stratigraphy (Conforti, 2003), have documented the occurrence of the Vesuvius 79 AD deposits in the southeastern sector of the Naples Bay and in the Salerno Bay. Evidence of the ash fallout that followed the 79 AD eruption of Vesuvius, after several days to a few weeks of residence in the atmosphere, has been reported as far as the central Mediterranean (Paterne et al., 1986, 1988).

Recently, flow-derived deposits have also been recognised in the Naples Bay off the archaeological site of Herculaneum and interpreted as the seaward modification of a series of pyroclastic flows that entered the seawater during the eruption of Vesuvius of 79 AD (Milia, personal communication).

4. Materials and methods The data set used for this study includes a grid of 400 km very high resolution single channel seismic reflection profiles (sub-bottom Chirp) and seven gravity cores acquired in the southern Naples Bay, along with a gravity core sampled in the northern Salerno Bay (Fig. 1). Ship position during geophysical acqui-

Fig. 2. Sub-bottom Chirp profile CsC-03 (a) and relative interpretation. The prominent reflector(s) within the HST deposits correlates with the Vesuvius 79 AD tephra.

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sition and coring was determined routinely using a differential GPS positioning system with accuracy in the order of 1 m. The interpreted seismic grid consisted of three sets of parallel profiles with different orientation and average distance between tracks of 500 m. The sub-bottom profiler operated in a wide frequency band (3–7 kHz), with a long pulse (20–30 ms). Maximum penetration of the seismic signal beneath the seafloor was about 25 ms in the middle–outer continental shelf. Conversion of two-way travel time to depth was obtained by assuming an average seismic wave velocity of 1600 ms 1 within the first 100 ms of seismic record beneath the seafloor, as a result

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of test-calibration (e.g., Carlson et al., 1986). Maximum vertical resolution was in the order of 30–40 cm. The interpretation of seismic profiles provided the general stratigraphic architecture of the shelf system, also supporting the construction of a chronostratigraphic framework and facies analysis of core samples. Gravity cores were described at centimetre-scale using a 10 lens together with microscope observation of sieved wet sediment (63-Am and 30-Am sieves) from selected samples. Sedimentological analysis included the recognition of major lithologic intervals, sedimentary textures and structures, chemical determination of major and trace elements of volcaniclastic

Fig. 3. Sub-bottom Chirp profile CsC-03 (b) across the outer continental shelf of southeastern Naples Bay and relative interpretation. Note the erosional character of surface (e) down to 90 m water depth and the remarkable thickening of the Vesuvius 79 AD deposits associated with wavy to chaotic reflections at a water depth of 110–120 m. See text for further explanation.

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within 3% for major elements and for Rb, Sr, Y, Zr, Nb, Zn, V, and 5–10% for the remaining trace elements excluding Sc, for which precision is closer to the XRF detection limits.

layers interbedded within the sampled succession, and grain size analysis of selected stratigraphic intervals by laser diffractometry. Four fresh pumice samples, representative of the Naples Bay tephra associated with the 79 AD event, were carefully handpicked in cores C82 (sample C82/ D/79), C4 (sample C4/B/12, C4/B/33) and C70 (sample C70/B/44A) and processed for chemical composition. Major and trace elements content was obtained with wavelength-dispersive X-ray fluorescence (XRF) analysis utilizing a Philips PW1400 spectrometer, according to methods described in detail elsewhere (e.g., Melluso et al., 1995). Typical uncertainties are

5. Data analysis 5.1. Seismic interpretation Interpretation of the selected grid of sub-bottom Chirp profiles allowed reconstruction of relationship between depositional elements of the Upper Holocene

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Fig. 4. Sub-bottom Chirp profile CsC-03 (detail) across the continental shelf edge of the southeastern Naples Bay and relative interpretation. Note the wavy to chaotic seismic facies and associated thickening of the Vesuvius 79 AD tephra in the central part of the profile. See also Fig. 3 for location of profile.

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sedimentary succession off the southern Naples Bay, and the stratal architecture of the Sarno prodelta system (Figs. 2–11). The seismic signal imaged the seafloor subsurface to a maximum depth of 30–35 m down to the acoustic basement, which is often represented in this area by a well-developed reflector (r) corresponding to a major erosional surface (Figs. 2 Figs. 3, 8 and 11). Reflector (r) can be followed readily all across the study area and has been interpreted as the Holocene ravinement surface (Conforti, 2003). This erosional surface is likely to mark a significant stratigraphic gap (or incomplete record)

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over the shelf, associated with the time-transgressive landward shift of the fair-weather wave-base during the rapid sea-level rise that accompanied the Holocene deglaciation. Seismic evidence from adjacent sectors of the midshelf north of the Sorrento Peninsula documents the occurrence, beneath the ravinement surface, of forestepping strata coupled with a relict alluvial system that probably represented the coastal drainage system on the exposed continental shelf during the Late Quaternary lowering of base level (Conforti, 2003) (Figs. 3 and 11).

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Fig. 5. Sub-bottom Chirp profile CsC-18 off the Sarno river mouth and relative interpretation. Note the remarkable erosive character of the base of the 79 AD tephra layer (e) down to 50–60 m water depth and the wavy to chaotic reflections within the post-79 AD deposits between 100 and 120 m water depth.

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Profile CsC-18 (detail-a)

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Fig. 6. Sub-bottom Chirp profile CsC-18 (detail-a) off the Sarno River mouth and relative interpretation. Note the erosional scours at the base of Vesuvius 79 AD tephra (e) between 40 and 60 m water depth. See also Fig. 5 for location of profile.

Above the ravinement surface, seismic profiles show relatively continuous, parallel trending reflectors corresponding to upper Transgressive Systems Tract (TST) and Highstand Systems Tract (HST) deposits. The maximum flooding surface (msf) has been detected on seismic profiles by the recognition of downlap stratal terminations at the base of the HST deposits (Figs. 2 and 3), and regarded as a virtually isochronous surface in the construction of the time-stratigraphic framework. Correlation of the mfs across the study area has been calibrated by sedimentological analysis of gravity core samples that display typically a significant decrease in the average grain size as well as high bioturbation at this stratigraphic level.

The thickness of the Holocene shelf wedge varies in this area from 25 m, at depth of ca. 20 m, about 2 km off the Sarno river delta front foresets, to about 1 m in the distal prodelta (detected from core C69) at ca. 140 m water depth (Figs. 2–8). It should be noted that the thickness of the Holocene wedge does not decrease regularly with depth everywhere across the shelf. This is due either to the uneven morphology of the ravinement surface which created conditions of variable accommodation space (and thickness) for the TST deposits, or to actual changes in thickness of the HST deposits only (Figs. 2–11). The seismic signature of HST deposits off the Sarno prodelta, as well as off the northern Salerno Bay, is characterized by a thick, prominent reflector

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Profile CsC-18 (detail-b)

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Fig. 7. Sub-bottom Chirp profile CsC-18 (detail-b) and relative interpretation, showing wavy to chaotic reflections within the post-79 AD succession between 100 and 120 m water depth. See also Fig. 5 for location of profile.

that consistently develops in the mid-upper part of this stratigraphic unit and can be correlated with the volcaniclastic layer deposited offshore during the 79 AD plinian eruption of the Vesuvius (Conforti, 2003; Insinga, 2003) (Figs. 2–11). The thickness of the 79 AD pyroclastic deposits ranges from a maximum of about 200 cm in the shoreface–foreshore area off the Sarno river mouth, to some 10 cm at the shelf edge. The significant decrease in the thickness of the tephra layer attained below 70–80 m water depth mirrors the large-scale architecture of the Sarno prodelta slope which displays most of its basal downlap stratal terminations within the same bathymetric range (Figs. 2 and 3). The post-79 AD succession is characterized off the Sarno river mouth by a low-angle sigmoidal pattern of reflectors and displays a thickness between 7 m at the shelf edge and 1 m towards the bottomset. The base of the 79 AD Vesuvius deposits shows an almost ubiquitous erosive character towards the inner shelf of the Naples Bay that is particularly evident off the Sarno river delta front and slope

and is often associated with erosional truncation of the underlying strata (Figs. 2, 6, 9 and 11). The erosional surface is well developed down to a water depth of 140 m and carries evidence of scours that are tens of metres wide and up to 2.5 m deep until a depth of 65 m (Fig. 6). Seismic profiles display a prominent convex upward flexure (morphologic high) in the mid-outer shelf (between 50 m and 125 m) that is associated with a significant decrease in thickness of both TST and HST deposits (down to less than 0.5 m) and onlap stratal terminations on the flanks of the morphologic high (Figs. 3 and 11). On the other hand, a significant thickening (up to about 2 m), often associated with wavy to chaotic reflections of the post-79 AD deposits, is observed towards the outer shelf at the base of the morphologic high (Figs. 3–5, 7, 8 and 10). The post-79 AD sequence is characterized by wavy bedforms, associated with internal wave-like to chaotic structure that have been previously interpreted as sediment failure due to creep (Aiello et al., 2001).

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Profile CsC-20

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125

Fig. 8. Sub-bottom Chirp profile CsC-20 and relative interpretation. Note the wavy to chaotic seismic reflectors within the post-79 AD succession at 110–120 m water depth, possibly due to gravity mass instability (creep) of the post-79 AD deposits.

Indeed gravity mass instability resulting from constant strain and creeping, or sliding and slumping is frequently observed on continental slopes or at the edge of the continental shelf (e.g., Prior and Coleman, 1978; Lee et al., 1992, 2002; Hampton et al., 1996; Correggiari et al., 2001). In fact, seismic interpretation suggests that downslope sediment instability may be postulated for a wide area of the eastern Naples Bay. Diagnostic indicators of gravity mass transport are (1) an upslope zone of evacuation, mostly represented by a significant thinning of the post-79 AD succession and (2) a basal weak layer represented by the sharp lithologic discontinuity at the base of the 79 AD pyroclastic deposits (Figs. 3–5, 7, 8 and 10).

5.2. Stratigraphy of gravity cores Gravity cores C81, C82, C4, C70, C73, C71 and C69, sampled in the Naples Bay at water depths of between 63 m and 141 m, provide a transect across the mid and outer shelf off the Sarno river mouth. Core C90 was collected at 103 m water depth, in the proximity of the shelf break in northern Salerno Bay (Figs. 1 and 12). The length of recovered core sections ranges from 584 cm in core C82 to 176 cm in core C71 (Table 1). Hence, depending on the variable thickness of the Holocene wedge, the core samples represent the stratigraphic successions of the last few thousand years towards the mid-inner

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455

Fig. 9. Sub-bottom Chirp profile CsC-20 (detail) and relative interpretation. Note the erosional truncation of reflectors at the base of Vesuvius 79 AD tephra (e). See also Fig. 8 for location of profile.

shelf, but may outreach the base of the Holocene off the shelf break. The HST (Upper Holocene) succession recovered off the Sarno river mouth consists mostly of bioturbated prodelta mud (sandy silt) punctuated by tephra deposits at various stratigraphic levels. Different tephra may vary significantly in terms of thickness and average grain size. The thinnest ash layers often become obliterated by bioturbation, making correla-

tion from site to site difficult. As recently documented by Conforti (2003) and Insinga (2003), the most prominent volcaniclastic horizon and seismic reflector within the HST shelf sediments of the southern Naples Bay and the northern Salerno Bay is represented by the tephra deposited by Vesuvius during the plinian eruption of 79 AD (tephra N4 and S1, Insinga, 2003). The tephra layer displays measured thickness ranging from 40–90 cm towards the Sarno river delta front

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Fig. 10. Detail of sub-bottom Chirp profile So-10 and relative interpretation suggesting gravity mass instability (creep) of post-79 AD deposits over a weak surface represented by the lithologic discontinuity at the base of the Vesuvius 79 AD tephra (e) (modified after Aiello et al., 2001).

(C81, C82 and C4) and on the shelf margin of the northern Salerno Bay (C90) to about 10 cm close to the continental shelf break in the southern Naples Bay (C69) (Figs. 1 and 12). A maximum estimated thickness of 200 cm is attained off the Sarno river mouth, as illustrated by seismic data. All the successions recovered from Naples Bay show that the 79 AD tephra overlies an erosional surface of older strata consisting of bioturbated sandy silt (Figs. 12 and 13). Locally, reworked soft pebbles of silty sediments, deriving from the erosion of the underlying deposits, have been found. The analysis of gravity cores showed that the 79 D tephra sampled offshore displays the same basic constituents reported from the type sections documented on land. Particularly, it consists of several distinct layers often represented by (1) normally graded coarse to medium sand and/or gravel in proximal-median areas; and (2) sandy silt with fine-grained lithics (volcanic and carbonate xenoliths) and bioclasts in more distal areas. The gravely layers commonly form the lower part of the 79 AD volcaniclastic deposit. They are made up of heterometric, mostly homogeneous or faintly graded,

greenish-grey pumice and lithics set in a coarse–medium dark grey sandy matrix, represented by pumice, scoriae, minerals, and rare bioclasts (benthonic foraminifera and small mollusc fragments). Leucite phenocrysts also occur on juvenile elements. Pumice clasts are angular to rounded. Carbonate xenoliths frequently occur and may reach 3 cm in size. Coarse to medium sand layers also consist of a suite of volcaniclasts, lithoclasts and subordinate bioclasts, and frequently display normal grading. At C70 and C82 core sites, the middle–upper part of sandy layers exhibits parallel lamination (Figs. 12 and 13). In core C81, the 79 AD deposit consist of 5to 15-cm-thick beds showing a sharp erosional base and normal grading from gravel to silty sand or sandy silt. The gravel consists mostly of rounded, locally fragmented, pumice in a matrix of poorly sorted medium–coarse silty sand with volcaniclasts, carbonate xenoliths, and small mollusc fragments. In addition to pumice clasts, the gravely fraction also includes large (up to 5 cm) flattened, poorly rounded pebbles consisting of fine-grained sandstone and fragmented lithoclasts, likely reworked from the underlying sub-

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457

Fig. 11. Detail of sub-bottom Chirp profile CsC-03 and relative interpretation. The convex upward morphology of the seafloor at water depth of 70–100 m may be interpreted as a consequence of pre-79 AD volcanotectonic deformation and seafloor uplift. See also Fig. 3 for location of profile.

stratum. The lithoclast fragments are made up of millimetre-thick grey, subrounded pumice and lithics, in a matrix of pumice lapilli, including dark volcanic minerals, and subrounded lithics often showing a reddish alteration patina. The middle part of the beds is characterized by a sandy fraction consisting of light and dark pumice, scoriae, minerals, carbonate xenoliths and rare bioclasts (small mollusc fragments; Posidonia oceanica remains; and benthic foramini-

fera). The uppermost part of these graded beds is represented by a pumiceous sandy silt that is barren of fossils and includes locally medium to medium– fine sand lenses with bioclasts (small mollusc fragments) and volcaniclasts (Figs 12 and 13). The 79 AD pyroclastic deposit sampled off the northern Salerno Bay at the C90 core site display remarkably different features from those recognised in the Naples Bay. It consists of three major strati-

458

Legend

C69

Gravity core

Oxidate pumices

(141 m)

water depth of coring station (m)

Bioturbation (2D, 3/4D, 5D)

Depth to top of 79 AD tephra below the sea floor (cm)

30

C4 Benthonic

Silty sand, sandy silt

Foraminifera

Mollusc fragments

C90

C82

C70

C90

C73

217

C81

82

(116 m)

Lithoclasts

(103 m)

196

(129 m)

Calcareous lithoclasts 40

Salerno Bay

(110 m)

Erosional surface

30

C69

79

Parallel lamination Normal gradation Lens

20

C81 C4

C73

N

Silt; sandy, clayey

10

C71

(114 m)

Planktonic

Gravel; sandy, silty, clayey 0 cm

C70

c

(63 m) 155

50

78

b

60

C71 70 80

(128 m)

C69 34

(141 m)

90 30

a

M

Cl Si

Sand

Gv

? 100 110

m

Cl Si Sand

m

m

Gv

Cl Si Sand

Gv

Cl Si Sand

Gv

m

Cl Si

Sand

Gv

Naples Bay

M

M

Cl Si

Sand

Gv

Cl Si

Sand

Gv

M

Cl Si

Sand

Gv

Salerno Bay

Fig. 12. The Vesuvius 79 AD tephra in the studied cores. Note the change in thickness of the deposit and the striking difference of facies in the Naples and Salerno Bays. See text for further explanation (Bioturbation Data from Droser and Bottjer, 1991).

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Thickness (cm)

C82

Naples Bay

(sensu Droser and Bottjer, 1991)

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459

Table 1 Summary of gravity core data Core (n8)

C C C C C C C C

81 82 4 70 73 71 69 90

Latitude N deg (8)

Longitude E deg (8)

40843,26V 40843,36V 40841,10V 40841,10V 40839,60V 40838,30V 40836,38V 40835,46V

Depth b.s.l. (m)

14825,11V 14822,41V 14821,78V 14819,91V 14820,90V 14816,86V 14813,46V 14842,38V

63 110 114 129 116 128 141 103

Core length (cm)

196 584 313 325 293 176 181 485

Depth below sea floor (cm) Bottom

Top

– 270 169 148 125 55 39 290

155 196 79 82 78 34 30 217

Thickness

40 74 90 66 47 21 9 73

12). The lithologic suite of horizon (c), with the exception of the bioclastic component, is represented by the same material forming the matrix of horizon (b) and, subordinately, horizon (a). The base of the pyroclastic succession of the northern Salerno Bay (a) is characterized by a sharp non-erosive contact above pre-79 AD hemipelagic deposits. The top of the unit is characterized by the gradual transition of parallel-

graphic horizons represented from bottom to top by (a) 25-cm-thick, reversely graded, subrounded, white pumice and lapilli; (b) 45-cm-thick, normally graded, sub-angular, coarse grey pumice lapilli and lithics; and (c) 10-cm-thick, parallel-laminated coarse pumiceous and scoriaceous lapilli, including loose crystals (biotite and pyroxenes), abundant small mollusc fragments, carbonate lithoclasts, and other lithics (Fig.

(cm)

Vesuvius 79 AD tephra layer

C 70

C 69

C 81

0

5

10

15

20

25

Naples C70 Bay

C81

30 C69

35

Salerno Bay

Fig. 13. Features of the lower part of the Vesuvius 79 AD tephra deposit from the distal to the proximal sector of the Naples Bay. Note the occurrence of large pebbles (C81) and smaller lithoclasts (C69) reworked from the underlying sustratum. See text for further explanation.

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laminated lapilli and ash of horizon (c) to bioturbated sandy silt deposits representing the onset of normal depositional conditions after the termination of the eruptive event.

11.25% in sample C70, to 18.71% and 13.41% in sample C4 (Table 2). TiO2 values are roughly constant in all samples, ranging from 0.50% (C4 core site) to 0.53% (C82 core site). Fe2O3 content varies from 4.95% in sample C4 to 5.31% in sample C82. Trace elements also have a limited range: immobile ones such as Zr and Nb, range from 234 ppm and 41 ppm in sample C70 to 284 ppm and 52 ppm in sample C4, respectively. Y and V display concentrations from 14 ppm and 87 ppm in sample C70 to 26 ppm and 105 ppm in sample C8, respectively. These petrochemical features are typical of highly silica undersaturated series and evidence a clear vesuvian origin (Fig. 14). The marine samples analysed in this study display substantially the same chemical composition reported for the 79 AD grey pumice on land (Table 2 and Fig. 14), although higher values of Na2O and lower values of K2O characterize the

5.3. Vesuvius 79 AD marine tephra chemical composition The 79 AD marine tephra of the Naples Bay (N4, in Insinga, 2003) is characterized by a tephri-phonolitic composition according to the chemical classification of De La Roche et al. (1980) (Fig. 14 and Table 2). Sample C70/B/44A has slightly higher MgO and is classified as phonolitic tephrite. A mild oxide chemical variation can be observed among the samples: SiO2 content ranges from 54.96% in sample C82 to 55.75% in sample C4. Al2O3 and total alkali range from 17.44% and

Vesuvius 79 AD marine tephra (this study)

R2 = 1000(6Ca+2Mg+Al)

3000

C4/B/12

C70/B/44A

C4/B/33

C82/D/79

Post-1631 activity

1631 event Pollena (472 AD)

2000

basanites basalts

phonolitic tephrites

tephritic phonolites

tephrites shoshonites latites

1000

Interplinian activity (3.4 ka-79 AD)

trachyphonolites

phonolites

trachytes

Pompei,79 AD

-1000

0

1000

2000

3000

R1 = 1000[4Si-11(Na+K)-2(Fe+Ti)] Fig. 14. Classification of the Vesuvius 79 AD marine tephra in the R1–R2 diagram (De La Roche et al., 1980). The composition of subaerial Vesuvius products from major eruptions of the last millennia is also shown (reference data after Santacroce, 1987; Civetta and Santacroce, 1992; Cioni et al., 1995; De Vivo et al., 2003 and references therein).

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461

Table 2 Major (wt.%) and trace (ppm) element composition of pumice samples from the Vesuvius 79 AD marine tephra (N4, in Insinga, 2003) by XRF Vesuvius 79 AD tephra Marine core samples (this study)

Outcrop data (from literature)

Sample

C4/B/33

C70/B/44A

C82/D/7

C4/B/12

C2g

M

Glg

Lg

B2g

Grey 5

Grey 6

Classification

Te-P

Te-P

Te-P

Te-P

Te-P

Te-P

Te-P

Te-P

Te-P

Te-P

Te-P

55.03 0.47 18.90 4.59 0.15 2.15 4.87 4.38 9.19 0.27 100.00 n.d. 6 87

54.73 0.45 19.21 4.37 0.14 2.03 4.48 5.10 9.24 0.24 100.00 n.d. 7 84

54.74 0.45 18.92 4.37 0.14 2.15 5.13 4.71 9.14 0.26 100.00 n.d. 8 87

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Sum LOI Sc V Cr Cu Zn Rb Sr Y Zr Nb Ba La Ce R1 R2

55.75 0.50 18.71 4.95 0.15 1.82 4.54 6.97 6.44 0.16 100.00 4.00 4 96 19

54.96 0.52 17.44 5.07 0.16 3.76 6.65 5.83 5.42 0.18 100.00 8.70 8 87 27

87 233 827 24 284 52 996

77 182 753 14 234 41 938

404 943

184 1240

55.50 0.53 18.06 5.31 0.16 2.77 5.12 6.87 5.49 0.20 100.00 5.20 7 105 33 23 82 218 860 26 268 47 1135

56.39 0.45 19.09 4.38 0.15 1.56 4.09 7.11 6.62 0.15 100.00 4.00 9 78 19

54.39 0.49 18.61 4.71 0.14 2.55 5.36 4.43 8.98 0.33 100.00 n.d. 4 94

54.58 0.45 19.08 4.44 0.15 2.04 5.22 6.03 7.51 0.50 100.00 n.d.

89 248 692 16 304 59 773

75 330 837 28 250 50 1078

73 379 763 25 260 52

77 334 805 26 258 51 1008

76 343 795 27 255 51 974

72 335 777 26 258 51 910

172 1039

437 890

181 1065

384 1034

166 998

447 957

282 1026

54.75 0.55 19.03 4.74 0.14 2.04 5.61 4.39 8.57 0.20 100.02 2.73

54.88 0.56 18.57 4.81 0.13 2.31 5.87 4.26 8.37 0.23 100.00

102 45

109 48

354 835 27 257 53 1046 86 144 48 1075

349 855 26 244 50 1084 82 142 52 1107

All analyses recalculated water-free to 100. Chemical composition of 79 AD grey pumice onland has been also reported for comparison (XRF data of samples C2g, M, G1g, Lg and B2g are from Morra and Melluso, personal communication; XFR data of samples Grey 5 and Grey 6 from Cioni et al., 1995). Loss on ignition (LOI) obtained with gravimetric method; R1, R2 classification parameters are calculated according to De La Roche et al. (1980); Te-P—Tephri-phonolite; n.d.—not detected.

signature of the marine tephra with respect to its subaerial counterpart. This difference in alkali content is observed in all the vesuvian products recovered from the Naples and Salerno Bays (Insinga, 2003) and is possibly related to post-depositional alteration processes (e.g., analcimization).

6. Discussion The facies associations, sedimentary structures, and stratal patterns documented in this study off the eastern Naples Bay indicate that the subaqueous tephra deposit associated with the 79 AD Vesuvius

event displays significantly different characteristics with respect to the subaerial succession documented on land, as well as to the more distal pyroclastic fallout deposits that we have recovered from the outer continental shelf off the northern Salerno Bay. This was expected as, in proximal subaqueous settings, both primary deposition and reworking of tephra are influenced by hydrodynamics as an active factor within the sedimentary process. Moreover, a succession representing subaqueous proximal pumice fall may differ significantly from its subaerial counterparts due to hydrodynamic behaviour of the particles themselves which may cause a portion, such as highly vesicular pumice, to float on the water surface and

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remain in suspension for a considerable time (Whitham and Sparks, 1986; Manville et al., 1998). As it readily appears from the sedimentological analysis, the volcaniclastic succession sampled in the northern Salerno Bay is very similar to the subaerial pyroclastic fall documented in the type sections of Pompeii and Oplontis. It consists of three layers represented from the bottom to the top by (a) white pumice lapilli; (b) grey coarse pumice lapilli and lithics; and (c) parallel-laminated lapilli, mollusc fragments, and lithics. The lithofacies pattern and sedimentary textures suggest a straightforward correlation of layers (a) and (b) of the Salerno Bay with the white pumice and grey pumice subaerial fallout deposits described in the type section of the 79 AD Vesuvius tephra on land (Sigurdsson et al., 1985; Cioni et al., 1992). The thin, parallel-laminated tephra at the top of the coarse-grained pumice deposits (layer c) may be interpreted as the result of the late-stage fallout of the fine-grained tail of the convective (plinian) plume and eventually minor ash clouds associated with pyroclastic currents (Fig. 12). Thickness of the AD Vesuvius tephra in Salerno Bay measured on seismic profiles, and calibrated by core samples, yields values that are in good agreement with the isopach map extrapolated by Sigurdsson et al. (1985) for the white/grey pumice layer couplet, on the basis of stratigraphic data on land (Fig. 1). This reinforces the suggestion that fallout from the volcanic plume was likely the dominant depositional mechanism for the 79 AD tephra in distal offshore areas. The 79 AD Vesuvius tephra deposit sampled in Naples Bay, instead, displays a remarkably different facies and depositional setting. It is mostly represented by greyish, faintly graded, sub-angular, coarse pumice lapilli as well as matrix-rich, normally graded coarse-grained beds, medium-grained beds with parallel laminations, and faint parallel laminations within fine-lapilli and ash layers (Figs. 12 and 13). Moreover, a number of distinctive features can be detected, including (1) the lack of the white pumice layer in the deposit; (2) a prominent erosive base; (3) a basal layer with rip-up clasts eroded from the substrate, and (4) several normally graded layers with basal erosional surface. Seismic profiles have shown that the basal erosive surface is well developed down to a water depth 140 m and displays scours that are tens of metres wide and up to 2.5 m deep, until a depth of

65 m (Fig. 6). Measured thickness of the whole pyroclastic layer varies from 200 cm at the Sarno river mouth, to 40–90 cm towards the Sarno delta front (C81, C82 and C4), and about 10 cm close to the continental shelf break in the southern Naples Bay (C69) (Figs. 1 and 12, Table 1). The above features clearly do not fit the depositional setting and thickness extrapolated by Sigurdsson et al. (1985) for the fallout deposits offshore. The absence of the white pumice unit off the Naples Bay was ostensibly not the result of a lack of primary deposition, as the pyroclastic fallout associated with the early phase of the 79 AD eruption (white and grey pumice layers) was homogeneously deposited on land both in proximal and distal areas by the volcanic plume (Sigurdsson et al., 1985; Lirer et al., 1997). At the same time, the Sorrento Peninsula mountain range likely acted as a morphologic barrier with respect to pyroclastic currents and confined the propagation of pyroclastic flows and surges within the Sarno Plain towards the coastline. In fact, the city of Herculaneum and the Sarno river mouth were the most important sites for the entrance of pyroclastic currents into the seawater (Fig. 1). One possible explanation for the lack of the Vesuvius 79 AD white pumice layer off the Naples Bay relies on the low average density of white pumice clasts (ca. 0.6 g/cm3) compared to the grey pumice (ca. 1.1 g/cm3) (Carey and Sigurdsson, 1987). The case history of Oplontis section, where archaeological excavations have revealed that the deposits of the 79 AD eruption covered a Roman villa, along with an adjacent large swimming pool (17 m wide and about 50 m long, with a depth of between 1.35 and 1.42 m), provides useful insights in understanding the interaction between pyroclastic fall and surge deposits with a water body (Sigurdsson et al., 1985). In their interpretation, the authors had concluded that most of the low-density white pumice that entered the swimming pool formed a floating bpumice raftQ that was partly dragged down by the denser grey pumice and lithic fragments during deposition of subsequent tephra beds (derived from both pyroclastic fall and currents) to form mixed layers in the section. Provided the significant density contrast between with pumice and grey pumice clasts, it may be proposed, accordingly, that the white pumice deposited by pyroclastic fallout off the Naples Bay mostly

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floated (and probably travelled) for some time over the sea surface, thus hampering preservation of this layer in proximal subaqueous settings. On the contrary, the recovery of this unit in the distal sector of Salerno Bay (Buccheri et al., 2002; Insinga, 2003) can be explained by the average smaller grain size of white pumice clasts, corresponding, in turn, to higher density values which may have caused early sinking and deposition of the pyroclastic bed. The stratigraphic signature of the Vesuvius 79 AD event off the Naples Bay indicates that this tephra deposit is unlikely to be the product of an undisturbed fallout but rather represents the result of a complex amalgamation of different types of pyroclastic deposits, and suggests that some additional sedimentary process, acting only in that Bay is required. According to the data illustrated in this study, the 79 AD tephra of the Naples Bay may be interpreted as the result of substantial reworking of primary pyroclastic deposits induced by three basic syn-eruptive, geneticallyrelated depositional and/or erosional mechanisms: (1) resuspension and transport by anomalous waves (tsunami) or currents generated by pyroclastic flows entering the sea; (2) marine gravity flows generated by, or evolved from, the impact and/or propagation of pyroclastic currents within the seawater; and (3) the failure of water-logged, cohesionless pumice deposits, due to instability of pyroclastic bedforms rapidly accumulating on the seafloor. The occurrence of a sharp erosional base and ripup intraclasts, clusters of graded beds showing planar-to-cross-lamination and an overall fining upward trend of the deposit are commonly regarded as useful criteria for the identification of shallow water tsunami deposits (e.g., Bondevik et al., 1997; Fujiwara et al., 2000; Massari and D’Alessandro, 2000; van den Bergh et al., 2003). Owing to the peculiar hydraulic behaviour of unconsolidated pumice deposits, and their mobility under much lower energy conditions than would be required for average-density siliciclastic material, it may be speculated that impact waves, or tsunami generated by the entrance of pyroclastic currents into the seawater, may have induced erosional and/or depositional features on the sea floor as a correlated but independent mechanism (Cas and Wright, 1991; Legros and Druitt, 2000; De Lange et al., 2001; Freundt, 2003; Tinti et al., 2003). Indeed, the occurrence of an

463

anomalous wave in the Pozzuoli Bay during the AD 79 eruption was reported by Pliny the Younger in his letters to Tacitus and a tsunami model for the Naples Bay, during the 79 AD event has been recently proposed by Tinti et al. (2003). A complex succession of events is likely to occur when land-generated pyroclastic currents enter the seawater, partly dependent upon the flow rheology (Sparks et al., 1980; Mandeville et al., 1994, 1996; Carey et al. 1996, 2000; Legros and Druitt, 2000; Freundt, 2003). Many pyroclastic flows separate into a denser portion, which may mix with water, and a more diluted part which can keep travelling on the water surface for several kilometres. Moreover, the entrance of the hot pyroclastic current into the sea may induce near shore steam explosions, thus generating additional mass flow deposits and produce impact waves and tsunami travelling rapidly away from shore (e.g., Cas and Wright, 1991; Legros and Druitt, 2000; De Lange et al., 2001; Freundt, 2003; Tinti et al., 2003). Another interesting implication, in the hypothesis of a significant tsunami-induced lowering of the base level, may be the downslope currents eventually associated with the subsiding water mass and their potential in terms of erosion and/or deposition. According to this scenario, the scoured valleys that characterize the base of the 79 AD layer in the inner mid-shelf (until 65 m water depth) may reflect a tsunami-induced lowering of the wave base level, while the deeper part of the erosional surface (down to 140 m) may be due to bottom currents triggered by the tsunami-induced lowering of the water mass. Owing to the multitude of possible nearshore events, the subaqueous modification of pyroclastic currents often produces complex facies relations consisting of a succession of gravity flow units which rarely correlate from section to section (e.g., Whitham, 1989). In particular, the 5- to 15-cm-thick normally graded deposits of core C81 may be interpreted as the products of transport by currents with excess suspended sediment load (Valentine, 1987; Druitt 1992). In contrast, the parallel-laminated fine-grained facies of the Vesuvius 79 AD tephra sampled off the outer shelf of the southern Naples Bay (C69, C71) may be regarded as the result of deposition by distal gravity currents, evolved from

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relatively low density flows travelling on the water surface after the entrance of pyroclastic currents into the sea (Carey et al., 2000; Legros and Druitt, 2000; Freundt, 2003). The matrix-rich coarse-grained pumice found in the upper part of the pyroclastic succession of core C4 is interpreted as a matrixsupported debris flow, evolving either from a subaqueous suspension of dense pyroclastic flows or

induced by gravity currents due to sediment instability over the shelf slope (Figs. 12 and 13). Gravity core analysis has shown the base of the 79 AD tephra corresponds to an ubiquitous erosive surface that often includes significant mixing of pumice deposits with the underlying hemipelagite and fragments (rip-up clasts) deriving from the eroded substrate (Figs. 12 and 13). The erosional character of the

Fig. 15. Paleobathymetry and paleocoastline of the southeastern Naples Bay before the Vesuvius 79 AD eruption, showing areas involved by volcanotectonic uplift of the seafloor (Castellammare anticline) and creep of the Upper Holocene sequence over a basal surface represented by the lithologic discontinuity at the base of the 79 AD tephra. The 79 AD paleo-isobaths are derived by seismic data discussed in this study. The 79 AD coastline of the Sarno river plain is after Pescatore et al. (2001).

M. Sacchi et al. / Marine Geology 222–223 (2005) 443–469

base of the Vesuvius 79 AD tephra appears even more dramatic on the basis of seismic evidence which documents erosional truncation of the underlying strata towards the inner shelf, off the Sarno delta front and proximal prodelta (Figs. 3, 5, 6, 9, 11). At the same time, off the Sarno river prodelta system, seismic profiles suggest the occurrence of pronounced seafloor instability by creep of the whole post-79 AD succession over a basal surface that coincides with the lithologic discontinuity between the base of the tephra deposits and the underlying hemipelagite (Fig. 10). As the gravitational instability involves the whole succession above the 79 AD tephra deposits, it can be concluded that the creep deformation largely postdates the eruptive event. Although the erosive character and other sedimentary structures and textures associated with this stratigraphic level can probably be best explained as a result of erosion due to tractive currents during the syn-eruptive phase, it cannot be excluded that the erosional surface at the base of 79 AD tephra may have been overprinted locally by shear deformation due to the creep of semi-consolidated sediments along the lithologic discontinuity at the base of the pyroclastic layer itself. The stratal architecture and acoustic facies imaged by seismic profiles off the Sarno prodelta system have shown the occurrence of significant thinning of the 79 AD tephra, over a convex upward flexure of the bathymetric profile of the mid-outer shelf (between 50 m and 125 ms) as well as thickening of the pyroclastic succession in more distal areas (sometimes also associated with chaotic reflections and seafloor deformation). In the hypothesis of an advanced stage of creep deformation of the seafloor evolving towards slumping, these features might be seen, respectively, as areas of denudation and accumulation of semiconsolidated strata by gravity slide. Seismic evidence also suggest that the morphologic high, associated with convex upward flexure of the bathymetric profile of the mid-outer shelf (between 50 m and 125 ms), may be interpreted as due to syn-eruptive uplift of seafloor (Fig. 15). The occurrence of such a volcanotectonic deformation and uplift of the seafloor off Vesuvius is in agreement with the generalised ground deformation and the abrupt seaward shift of the shoreline reported by historical documents and discussed in the literature (Pescatore et al., 2001). Moreover, it

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could better explain the pronounced gravitational instability by creep of the whole Upper Holocene succession over a significant area of the continental shelf off the Sarno river mouth.

7. Conclusions According to the data illustrated and discussed in this study, a number of conclusions can be drawn as follows: 1) Sedimentary structures and textures suggest a straightforward correlation of the 79 AD tephra deposit sampled off the northern Salerno Bay with the white pumice and grey pumice subaerial fallout deposits described in type section of Pompeii, Herculaneum and Oplontis. 2) The 79 AD tephra layer sampled off the southeastern Naples Bay is interpreted as the result of a number of syn-eruptive genetically-related mechanisms including (a) anomalous waves (tsunami) and currents generated by the entrance of pyroclastic flows and surges into the sea; (b) subaqueous density flows evolved from the impact of hot pyroclastic currents into the seawater; and (c) the failure of water-logged, cohesionless pumice deposits, due to instability of pyroclastic bedforms rapidly accumulating on the seafloor. 3) Thickness and areal distribution of the Vesuvius 79 AD pyroclastic deposits off the northern Salerno Bay show a close fit with isopleths maps reported in the literature (Sigurdsson et al., 1985), while substantial mismatch characterizes the distribution of the 79 AD deposits off the eastern Naples Bay when compared to the extrapolated thickness of fallout deposits only. 4) Seismic interpretation showed that the Upper Holocene sequence off the Sarno prodelta system is affected by extensive creep involving the whole post-79 AD succession. Deformation due to creep is documented by slumping of semi-consolidated strata over a basal surface represented by the lithologic discontinuity between the base of the 79 AD tephra deposits and the underlying hemipelagite. Seismic data also suggest that gravitational instability of this area has been induced, or enhanced, by significant volcanotectonic deforma-

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tion and local uplift of the seafloor that ostensibly predated the eruptive event.

Acknowledgements Geophysical data and gravity cores used in this study were acquired by the Istituto per l’Ambiente Marino Costiero (IAMC) of the National Research Council of Italy (CNR) of Naples during Oceanographic cruises GMS97_01, GMS98_01, and GMS00_05 on board of the R/V Urania, within the frame of a national project aimed to the geologic mapping of the coastal zone of Campania at 1:50,000 scale (CARG project) coordinated by Bruno D’Argenio. XRF analyses on tephra deposits were conducted at CISAG, University of Naples, Federico II. The authors gratefully acknowledge Simon Blockley, Patrick Friend, and Fabio Trincardi for their critical reviews which greatly improved the manuscript. Sincere thanks are also due to Vincenzo Morra and Leone Melluso for helpful discussion we had with them at various stages of the research work, to Monica Capodanno for her precious support to the sedimentological laboratory work at IAMC-CNR, and to Patricia Sclafani for the proofreading of the English text.

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