Occurrence of inter-eruption debris flow and hyperconcentrated flood-flow deposits on Vesuvio volcano, Italy

Occurrence of inter-eruption debris ¯ow and hyperconcentrated¯ood-¯ow deposits on Vesuvio volcano, Italy

L. Lirera,*, A. Vincib, I. Albericoc, T. Gifunia, F. Belluccia, P. Petrosinoa, R. Tinterrib

aDipartimento di Geo®sica e Vulcanologia, Via Mezzocannone 8, UniversitaÁ di Napoli Federico II, 80134 Napoli, ItalybDipartimento di Scienze della Terra, Viale delle Scienze 157/a, UniversitaÁ di Parma, 43100 Parma, Italy

cCentro Interdipartimentale di Ricerca ªAmbienteº Via Mezzocannone 16, UniversitaÁ di Napoli Federico II, 80134 Napoli, Italy

Received 1 December 1999; accepted 14 September 2000

Abstract

In the period between AD 79 and AD 472 eruptions, inter-eruption debris ¯ow and hyperconcentrated-¯ood-¯ow deposits

were deposited in the Somma-Vesuvio areas. These deposits, forming cliffs at the Torre Bassano and Torre Annunziata, were

generated by highly erosive ¯oods, whose erosive capacity was enhanced by acceleration due to the steepness of the volcano

slopes. In this type of deposits were distinguished ®ve depositional facies (from A to E) outcropping well at Torre Bassano

where they are stacked in three ®ning-upward (FU) sequences, probably representing three forestepping Ð backstepping

episodes in the emplacement area of gravity ¯ows. These ®ve facies from coarse to ®ne are interpreted to represent the

downcurrent evolution of particular composite sediment gravity ¯ows characterized by horizontal segregation of the main

grain-size population. The blocking of these highly concentrated composite parent ¯ows would ®rst produce the deposition of

the coarse front part to form facies A and then the overriding of this deposit by the bipartite ¯ow, which constitutes the body of

the ¯ow. This ¯ow is composed of a highly concentrated basal inertia carpet responsible for the deposition of facies B, C and D

and an upper hyperconcentrated ¯ood ¯ow that forms facies E, through traction plus fallout processes, respectively. Finally, the

occurrence of ªlaharº type events at Somma-Vesuvio region even at present times is discussed. q 2001 Elsevier Science B.V.

All rights reserved.

Keywords: Vesuvio; Laharic deposits; Facies analysis; Debris ¯ow; Hyperconcentrated ¯ow; Composite sediment gravity ¯ow

1. Introduction

Volcanism-induced sedimentation has been recog-

nized in ancient volcaniclastic sequences as well as in

numerous modern active volcanoes (e.g. Mount St.

Helens-1980, Nevado del Ruiz-1985, Mayon-1984,

Pinatubo-1991). When pyroclastic deposits are

mainly made up of pyroclastic debris emplaced from

strato-volcanoes whose steep slopes rapidly degrade

towards plain surfaces or coast-lines, they are

expected to make a large contribution to sedimenta-

tion of epiclastic deposits, as a consequence of the

rapidity of erosion and transport of loose particles

by heavy rains of short duration. These climatic

phenomena are quite typical of temperate climates

in Mediterranean areas, where rains are concentrated

in some parts of the year (mostly autumn and spring)

and can reach high values in few hours (De Vita and

Vallario, 1996).

In the Somma-Vesuvio region, and mostly in the

Torre del Greco area (Fig. 1), volcaniclastic deposits

Sedimentary Geology 139 (2001) 151±167

0037-0738/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.

PII: S0037-0738(00)00162-7

www.elsevier.nl/locate/sedgeo

* Corresponding author. Fax: 139-81-5525739.

E-mail address: [email protected] (L. Lirer).

crop out, which Di Girolamo (1970) and Rolandi and

Russo (1989) interpreted as reworked deposits related

to the heavy rain fall that occurred after the AD 79

eruption or the 472 eruption, respectively. These

deposits consist of silt-size to Rudite-size layers,

massive to strati®ed, including blocks and boulders,

mostly made up of lithic fragments.

Volcaniclastic deposits in subaerial settings are

commonly linked to sediment-laden ¯ood and

debris-¯ow processes. The term ªlaharº, considered

neither as a depositional mechanism nor as a deposit,

but simply as an event ªcan refer to one or more

discrete processesº (Smith and Fritz, Penrose Confer-

ence Ð GSA, 1989); this term encompasses a variety

of rheological behavior patterns.

This ªlaharº event involves several processes caus-

ing the emplacement of various deposits whose

features vary with the sediment/water ratio. The mini-

mum value of the sediment/water ratio refers to dilute

stream ¯ow, where the ¯ow is fully turbulent. The

maximum ratio coincides with debris ¯ow in which

cohesive matrix strength dominates and is de®ned as

highly concentrated ¯owing mixtures of sediment and

water that commonly consist of a broad distribution of

grain sizes and exhibit resistance to shear.

Hyperconcentrated ¯ood ¯ow (HFF) represents an

intermediate state between these extremes (Smith and

Lowe, 1991) and is a high-discharge ¯ow in which

turbulence is not the only sediment-support mechan-

ism and in which deposition does not occur en masse

(Smith, 1986). HFF cover a wide range of transport

processes and are characterized by a turbulent transi-

tional-to-laminar support mechanism (Costa, 1988).

The deposits range from pebble to sand-grade, exhi-

biting generally massive or crude horizontal strati®ca-

tion; outsize cobbles and boulders, with no scour and

®ll structure, are common. Cross-bedding or ripple

lamination, typical features of more dilute, tractional

stream ¯ow, are absent.

HFFs have three different origins: (1) as the direct

result of a ¯ood event (Costa, 1988; Smith and Lowe,

1991); (2) from the dilution of a debris ¯ow (Smith

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167152

National roadsN. 268, N. 18

Administrative municipalboundaryHighwayA3

•

S.Anastasia

Ottaviano

Napoli

Portici

Ercolano

Torre delGreco

TorreAnnunziata

2

Vesuvio

1

3

SommaVesuviana

TerzignoN.268

N. 18

A3

4•

Pozzelle

•65

Oplonti

Pompei

TorreBassano

CapoOncino

VillaRegina

Tyrrheniansea

Location sites1: Ranieri quarry2: Torre Bassano3: Torre Annunziata - Capo Oncino4: Torre Annunziata - National roadN. 185: Oplonti6: VillaRegina

0 2 4 Km

••

•

•

Fig. 1. Location map of the investigated sites.

and Lowe, 1991); and (3) as a part of sediment-laden

composite ¯ows characterized by a longitudinal

segregation of grain-sizes where a coarser front part,

formed by highly concentrated debris ¯ow, and ®ner

body and tail, formed by an increasingly diluted over-

riding ¯ow, can be recognized (Sohn et al., 1999). In

the ®rst case, while the water delivery and the relative

transport capacity increase, the ¯ood can entrain

enough sediment to assume the characteristics of an

hyperconcentrated ¯ow; in the second case hypercon-

centrated ¯ows can form when a debris ¯ow front runs

into the perennial waters of a river and dilutes (Smith

and Lowe, 1991; Sohn et al., 1999). Such a process

was well documented during a 1982 eruption-related

lahar event at Mount St. Helens (Collins and Dunne,

1986; Pierson and Scott, 1985; Scott, 1988), as well as

in Mayon Volcano, Philippines (Rodolfo and

Arguden, 1991) and at Mount Pinatubo (Major et

al., 1996; Rodolfo et al., 1996). Finally, in the third

case, hyperconcentrated ¯ows can constitute the body

or tail of composite sediment-laden ¯ow (Sohn et al.,

1999).

Volcaniclastic deposits can be emplaced syn-

eruption, or during inter-eruption times. The ®rst is

the interval during an eruption and immediately

following cessation of activity, in which sediment

delivery is abundant. The second is the interval

when the geomorphic system has returned to a normal

state.

The syn-eruption deposits are essentially instanta-

neously emplaced sediments produced at short-term

rates. They generally show little lithologic diversity

and tend to be rich in sand- and ash-size pyroclasts.

Inter-eruption volcaniclastic sediments, on the

contrary, generally show a considerable lithological

diversity and gravel-bedload facies are very abundant.

The former are laterally extensive sheets, whereas the

latter are typically much thinner and are con®ned to

valley systems cut into more ancient eruptive products

(Smith, 1991).

In this paper the sedimentary characteristics of the

¯ood-deposits from Vesuvio volcano Ð Italy, that

were emplaced in the period between the AD 79 and

AD 472 eruptions (Figs. 1 and 2), are described and

genetic features are pointed out. Different facies were

distinguished on the basis of texture and sedimentary

structures and explained in terms of transformations

which occurred in the ¯ows.

2. An outline of the AD 79 and AD 472 eruptions

The AD 79 deposits of Vesuvio were generated by

the type example of a Plinian eruption. In proximal

areas these deposits range between 3 and 30 m in

thickness. Stratigraphic correlations indicate that in

perivolcanic areas the lowest unit is a thin ash layer

which dispersed eastward; grain-size distribution of

this layer displays two modes (11 phi and 16 phi)

and high sorting values (Lirer et al., 1993); this unit is

overlain by a white pumice fall deposit which shows

an upward increase in the mean diameter. Mean value

sorting is about 1.0 phi (Lirer et al., 1993). The next

overlying unit is a grey pumice fall which contains

strati®ed surge deposits in all perivolcanic sections

(the term perivolcanic indicates the geographical

zone located on the slopes or in the plain just around

the volcano) except at Pompeii (Lirer et al., 1993). At

the passage between the white and grey fall deposits

lithic content is about 30%. The grain-size character-

istics of the lower part of the grey pumice unit are

similar to those of the white pumice unit. In contrast,

the upper part of the grey pumice fall displays a

marked decrease in mean size and sorting. The

grain-size distributions of dry surge deposits inter-

bedded within the grey pumice fall are polymodal

and have sorting values ranging between 2 and 3.5.

Two distinct chemical compositions discriminate the

white pumice (phonolite) from the grey one (tephritic

phonolite) (Lirer et al., 1993).

Stratigraphic correlations also demonstrate that the

lower and middle parts of pyroclastic ¯ow deposits at

Ercolano were contemporaneous with the alternating

fall and surge horizons in the grey deposits at Oplonti

and Villa Regina (Lirer et al., 1993). The dry surge

deposits interbedded within grey fall units have a

larger lithic content than their associated fall deposits.

These thin basal layers are ®ne-grained and display a

polymodal distribution with a negative skewness

(Lirer et al., 1993). The structural features of pyro-

clastic ¯ow deposits at the Ercolano excavations

range from massive to gently cross-bedded. The

upper parts of stratigraphic outcrops in the perivolca-

nic area show textural characteristics of debris ¯ow,

pyroclastic ¯ow, and surge deposits with associated

accretionary lapilli and are characterized by low

sorting values. A striking characteristic of these

upper deposits is an abrupt decrease in the juvenile

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167 153

fraction (to less than 10%, see sample PZ36, VR77,

PE92, OP62, ER25 in Lirer et al., 1993, 1997) and a

corresponding increase (to 70±80%) in the lithic (lava

and limestone) fraction. This is consistent with a tran-

sition from dominantly dry explosive to dominantly

hydromagmatic activity (Lirer et al., 1993).

In AD 472 another Somma-Vesuvio energetic

historic eruption occurred. This event generated wide-

spread tephra fallout and associate devastating pyro-

clastic ¯ows, debris ¯ows, and pyroclastic surges. The

tephra fall layers exhibit different patterns of

characters in the different sectors of the volcano,

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167154

Fig. 2. Stratigraphic correlations among investigated sequences. The lahar deposits overlie the AD 79 ones and underlie the AD 472 ash layer or

medieval fall deposits or lava ¯ow. Circled numbers above columns relate to site locations in Fig. 1.

both in the proximal and distal outcrops. On the

whole, four main lithic-enriched dark grey pumice

fall layers can be distinguished: the ®rst, best repre-

sented in north eastern area (Somma Vesuviana) and

disappearing toward east; the others, best represented

in eastern sector (Ottaviano), and made up of many

sublayers varying in sorting values �1:1 , s , 3:3�(Postiglione, 1998).

The fall unit is overlain by pyroclastic ¯ow and

associated dry surge layers, covering an area of

60 km2. Major deposition occurred in the Pollena

Valley, north-west of Somma, where the deposit

consists of three pyroclastic ¯ow units (lower, inter-

mediate, upper; 2:7 , s , 3:7� which overlie the

thick pyroclastic ¯ow deposit of AD 79 eruption.

The intermediate unit represents a nuee ardent

deposit, mainly constituted of juvenile bread crust,

low vesiculated, porphyritic blocks. Wet surge deposit

�1:5 , s , 2:5�; mainly dispersed toward E and SE,

close the sequence, except in the northern area where

a lahar unit can be found topmost (Postiglione, 1998).

On the whole, the products of the eruption are

mainly made up of juvenile fragments, lava lithic

fragments (often exceeding 70% weight), scarce lime-

stone and marble fragments.

The emplacement of the AD 79 and AD 472

products covered perivolcanic areas and surrounding

plains with a noticeable thickness of deposits of pyro-

clastic debris that affected the morphological aspect of

the Somma volcano, emplacing great volumes of

coarse-silt to coarse-gravel sized fragments, which

are distributed as hillslope-mantling or valley-®lling

deposits, that changed the preexisting topography.

Fig. 3 represents a digital-terrain model of the volcano

reconstructed to just after the AD 79 eruption. This

reconstruction, obtained using the topographic heights

of the top of the AD 79 products in outcrops and drill-

holes, shows that, apart from the presence of the Vesu-

vio cone within the Somma caldera, the height of the

Somma volcano and the slope values are similar to the

present values.

How rapidly erosion and remobilization at present

affects loose pyroclastic debris emplaced on the

volcano slopes has been studied in detail by De Vita

and Vallario (1996) who infer the different solid loads

with varying of ¯ood intensity. Taking into account

present rain distributions in the Vesuvio area (De Vita

and Vallario, 1996) and supposing that no signi®cant

change of climate happened during the last 1500

years, we can hypothesize that in the time-span

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167 155

Fig. 3. Post-eruption AD 79 digital terrain model of the Vesuvio.

between the AD 79 and AD 472 explosive eruptions

the volcano underwent a morphological evolution

during which a new rill network was developed,

representing a phase of erosion, transport and deposi-

tion of volcaniclastic material. The ®rst products

involved in such a morpho-dynamic evolution

would have been the ®nal hydromagmatic deposits

of the AD 79 eruption that were widely widespread

over perivolcanic areas (Lirer et al., 1997), composi-

tionally displaying a very low juvenile fraction

percent (about 10%, Lirer et al., 1993).

3. Types and distributions of post-AD 79volcaniclastic deposits

The investigated volcaniclastic deposits are

exposed in the perivolcanic area in a discontinuous

manner due to the intense urbanization and the recent

building of walls along the Napoli-Salerno railway.

These make it impossible to survey the lateral strati-

graphic variation of the volcaniclastic deposits. This

limitation, however, did not prevent both stratigraphic

characterization and recognition of volcaniclastic

facies.

Field survey indicated two main depositional areas,

Terzigno, in the SE sector, and Torre del Greco and

Torre Annunziata, in the S sector of the volcano (Fig.

1). In these areas deposits showing a generally

con®ned distribution, partly due to the presence of

preexisting topographic depressions, overlie the AD

79 pyroclastic deposits. Everywhere thick paleosol

and/or erosional surfaces separate these volcaniclastic

deposits from the underlying AD 79 pyroclastics

(Fig. 4). Minor outcrops of the same deposits, in

erosional contact with the AD 79 surge deposits, are

present in the Ercolano archeological excavations and

below the railway on the western side of the

Granatello harbour (Portici). Field investigation

points out that these lahar events are almost comple-

tely con®ned to the eastern and southern sectors of the

volcano, where the AD 79 products were mainly

dispersed.

Three stratigraphic sequences (TER� Terzigno,

TB� Torre Bassano, TA� Torre Annunziata;

Fig. 5), were studied and sampled to de®ne the grain

size and lithologic features of the deposits.

Microprobe analyses were performed on the minor

juvenile fraction of the volcaniclastic deposits (white-

light gray pumice and black scoria fragments) in order

to determine, from their chemical composition, the

Vesuvio explosive event to which juvenile fraction

found in volcaniclastic deposits corresponds. Fig. 6

(TAS classi®cative diagram, Le Bas et al., 1986)

shows the chemical composition of these products

and compares them with those of the protohistoric

eruptions, occurred between the 3800 BP Avellino

and AD 79 eruptions (Rolandi et al., 1998), the AD

79 eruption (Lirer et al., 1993) and AD 472 eruption

deposits (Postiglione, 1998). These data show that the

main part of glassy juvenile fraction of the investi-

gated samples resembles chemical composition of

white and gray pumice fragments of AD 79 eruption.

Only one sample, TA10, displays a glass composition

corresponding to juvenile fragments of AD 472 erup-

tion. Besides the strong similarity between the chemi-

cal composition of the juvenile fraction and that of the

AD 79 white and gray pumice, an even more striking

connection emerges from the coinciding lithological

component distribution for HFF and the ®nal

hydromagmatic deposits of the AD 79 eruption (see

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167156

Fig. 4. Erosional contact between the hydromagmatic deposits of

the AD 79 eruption (beneath the dashed line) and debris and hyper-

concentrated ¯ood ¯ows deposits.

Fig. 5. Stratigraphic successions, lithological components and grain-size distribution at the three investigated sites. Numbers besides strati-

graphic sections represent sampled layers. Circled numbers above stratigraphic columns indicate sites locations as in Fig. 1.

L.

Lirer

eta

l./

Sed

imen

tary

Geo

logy

139

(2001)

151

±167

157

samples PZ36, VR77, PE92, OP62, ER25 in Lirer et

al., 1993, 1997).

3.1. Facies description

Among the investigated sites, the most complete

facies sequence of volcaniclastic deposits can be

observed at Torre Bassano, along the shoreline of

Torre del Greco, and this section has here been chosen

as a type section to represent and describe the whole

set of outcropping facies. The ®ve principal facies, as

indicated in the Figs. 7 and 8, from the coarsest to

®nest, are:

(1) Facies A Ð Clast-supported conglomerate

composed of rounded or subangular boulders, cobbles

and coarse pebbles. The matrix is composed of small

pebbles and coarse sand. Clasts range in size from a

few centimeters to one meter in diameter. Boulders

and cobbles are mostly leucititic, whereas metamor-

phosed limestone lithics comprise 5±10% clasts and

have smaller sizes. Most beds are laterally persistent

for tens of meters; internal strati®cation is absent; they

are generally poorly sorted and show both normal and

inverse grading. Top and bottom contacts are sharp.

Sometimes the bottom contact can be highly erosive,

being characterized by deep scours (Fig. 9).

(2) Facies B Ð Clast-supported conglomerate

composed of subangular coarse pebbles and small-

pebbles. This facies, which can be massive or can

show a well-developed inverse grading, is character-

ized by medium to thick beds with a well-de®ned

lateral continuity. Facies B, being ®ner than A, repre-

sents a more evolved stage of the ¯ow. It has a sharp

bottom with common scours (Fig. 10).

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167158

6

8

10

12

14

16

18

48 49 50 51 52 53 54 55 56 57 58

TA

TER

TB

TA10

SiO2

Na2

O+K

2O

Fig. 6. Chemical composition ranges of protohistorical (dashed

lines), AD 79 (full lines), AD 472 (dashed and dotted lines) pumice

fragments. Full symbols represent the pumice fragments of hyper-

concentrated ¯ood ¯ow (HFF). The TA10 ash sample falls in the

AD 472 ®eld. (TA� Torre Annunziata, TER� Terzigno, TB�Torre Bassano).

Fig. 7. Spectacular outcrop showing the different facies (A±E)

deposited by debris ¯ows and hyperconcentrated ¯ood ¯ows

(Torre Bassano, site 2). The three lines (FU) show the three

®ning-up cycles (see text for explanation) whereas the long line

shows the trace of detailed stratigraphic section of Fig. 8.

Fig. 8. Detailed stratigraphic section (Torre Bassano outcrop) showing the principal facies and interpretation of relevant sedimentary processes

related to debris ¯ows and hyperconcentrated ¯ood ¯ows. The trace of the section is shown in Fig. 7.

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167 159

Types of faciesand

sedimentary structures

Facies B:Conglomerate clast-supported with

inverse grading

Depositional processes

Facies C:Pebbly sandstone with

normal grading

Facies C:Pebbly sandstone with

normal grading

Facies D :Massive pebbly conglomerate

Frictional freezing frombasal inertia carpet of the bipartite flow

that bypass the facies A deposits

Facies E: Low angle laminae

Facies E: Low angle laminae

Traction plus fallout

Traction plus fallout

Density interface

Density interface

Density interface

Bypass surface

Bypass surface

Bypass surface

Facies D:Massive pebbly conglomerate

Facies D: Massive pebbly conglomerate

Facies AMassive clast-supported

conglomerate

Frictional freezing frompreceding debris flow of the

composite sediment flow

Facies BMassive clast-supported

conglomerate

Density interface Bypass surface

Facies D :Massive pebbly conglomerate

Facies E: Low angle laminae Traction plus fallout

Facies D :Massive pebbly conglomerate

Frictional freezing withtraction

Frictional freezing with traction

Frictional freezing with traction

En-masse deposition frombasal part of overtaking

hyperconcentrated flood flow

Frictional freezing frombasal inertia carpet of the bipartite flow

that bypass the facies A deposits

Frictional freezing frombasal inertia carpet of the bipartite flow

that bypass the facies A deposits

Frictional freezing frombasal inertia carpet of the bipartite flow

that bypass the facies A deposits

En-masse deposition frombasal part of overtaking

hyperconcentrated flood flow

60cm

Pe fS mS cS vcS Gr sP cP Co Bo

Stratigraphic section

(3) Facies C Ð Medium to thick graded beds char-

acterized by coarse pebbles to coarse sand, the

presence of angular mudstone and pisolites eroded

from the underlying substratum (hydromagmatic

deposits from AD 79 eruption). Erosional features,

such as scours (Fig. 11), can be associated with

more evolved ¯ows (facies C), which also develop

turbulence at various intensities.

(4) Facies D Ð Thin to medium massive beds

composed of granules and small pebbles. This facies

is characterized by lenticular to low angle sigmoidal

units in which it is possible to ®nd outsized clasts

dragged from coarser deposits (facies A). The bottom

of these units can be erosive, whereas the top some-

times can show undulated pro®les (Fig. 10).

(5) Facies E Ð Thin to medium laminated beds

composed of medium to coarse sand. This facies is

characterized essentially by planar and undulating to

low angle laminae (Fig. 10).

3.2. Single sites analysis

3.2.1. Terzigno site

Volcaniclastic deposits are well exposed in several

quarry-cuts in the Caprai al Mauro area; the surveyed

area is about 1 km2 (Fig. 12). The deposits overlie,

with erosional surfaces, accretionary lapilli rich

surge deposits of the AD 79 eruption and are overlain

by the fall products of the third mediaeval eruption

(named Formazione di Terzigno by Rolandi et al.,

1998, and by the same authors 14C dated 1140 ^ 60

years BP). The 1834 lava ¯ows (Santacroce, 1987)

cover the whole quarry area.

In the SW sector the deposits show their maximum

thickness (about 20 m); they are sand-sized, generally

massive and in places contain lenses of angular,

coarse, lava fragments. Elsewhere, rare lenses

containing ¯oating pumice fragments occur; thickness

of individual beds varies between few centimeters up

to 50 cm. In the NW sector the thickness of the whole

deposit decreases (5 m) and the silt-sized deposits are

massive and sometimes contain Roman brick frag-

ments.

In the SE sector (Roman Villa dei Doli ruin) the

thickness of the deposit reaches about 8 m. Further-

more, these deposits show scour and ®ll structures in

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167160

Fig. 9. Highly erosive clast-supported conglomeratic deposits essen-

tially made up of cobbles and boulders (Facies A, Torre del Greco,

site 2). The dashed lines indicate the erosive surfaces of different

events.

Fig. 10. Detail of Fig. 7. Facies B±E are present and two density

interfaces, characterized by clear steps of grain size, are shown

(Torre del Greco, site 2). Ruler is 60 cm long.

Fig. 11. Example of cut and ®ll structure (arrow) associated with the

facies C, (Terzigno, site 1).

the underlying AD 79 products (Fig. 11). Commonly,

pinching-out of pumice-rich lenses occurs at the top of

the deposits. In the N quarry sector the deposits

abruptly disappear and the AD 79 surge deposits are

directly overlain by fall products of medieval Vesuvio

eruptions and lava.

The outcrop geometry and the variable thickness of

the volcaniclastic successions, as deduced by the ®eld

survey (Fig. 12) indicate that the emplacement of

these products occurred on the slopes of the volcano

in a pre-existing NW±SE directed paleovalley. In this

sector facies C and D are the best represented.

Component analysis, reported in Fig. 5, indicates a

juvenile fraction of about 2%, a lava lithic fraction

ranging between 80 and 85%, a limestone lithic frac-

tion between 7 and 10% and crystal fraction between

5 and 15%. Grain-size data show a sorting value

around 2, de®ning these deposits as poorly sorted.

Finally, chemical analysis of juvenile fragments indi-

cates a composition resembling that of white pumice

fragments from AD 79 products. (Fig. 6).

3.2.2. Torre del Greco site

The deposits, extending about 300 m along the

shoreline of Torre del Greco, make up the Torre

Bassano cliff. The contact between the sequence of

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167 161

•

•

•

•

•

•

••

•

••

•

•

•

•

•

•

•

•

•

•

•

Reworked pyroclasticmaterial1834and 1701 (or1817)lava flows

Lahar deposits

A. D. 79 deposits

64 Elevation points

Archeological sites

*

*

*

72.1

72.2

70.177.7

90.8

64

6399.1

92

71.5

70.3

81.9 83.8

77.6

83

62.5

56.9

7563

64.8

75.3

Quarryfloor

Quarryfloor

National

road

N.268

Contour lines(equidistance 10m)

N

0 50 100mt

Fig. 12. Map showing distribution of eruptive products in the Ranieri quarry, Terzigno (site 1).

volcaniclastic products and the underlying AD 79

accretionary lapilli-rich surge deposits is well de®ned

on the beach under the ruin of a 16th century tower.

The medieval lava ¯ows (Rolandi and Russo, 1989)

overlie the volcaniclastic sequence.

The maximum thickness of the volcaniclastic

products in the Torre del Greco shoreline sector is

12 m at Torre Bassano and decreases progressively

to 5 m in a NW direction. They are mainly sand-

and gravel-sized massive lithic layers whose thickness

varies between 0.5 and 1.5 m, in which matrix-

supported blocks and boulders are scattered. In

some basal beds of the sequence, a crude lamination

and lenses of pebbles and cobbles are present. In

this sector all the investigated facies are well

represented.

In Fig. 5 component analysis reveals lithological

component distributions similar to the distributions

reported for Terzigno stratigraphic section, whereas

the sorting values, ranging between 2 and 3, indicate

a very poorly sorted deposit. Chemical composition of

juvenile fragments strongly resembles the AD 79

white and gray pumice compositions (Fig. 6).

3.2.3. Torre Annunziata site

The volcaniclastic deposits crop out along the Lido

Azzurro Ð Villa Filangieri cliff. The erosional

contact with the underlying AD 79 accretionary

lapilli-rich surge deposits is well exposed along the

coast road. Stratigraphic correlations (Rolandi and

Russo, 1989) identify the 13th±14th century Capo

Oncino lava above the volcaniclastic deposits,

although direct contact is not visible on the cliffs.

Drill-hole stratigraphic sequences in the Torre Annun-

ziata area (Fig. 13) show that the volcaniclastic depos-

its thicken in a N±S directed paleo-valley.

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167162

Fig. 13. Reconstruction of stratigraphic relationships from drill-holes in the Torre Annunziata area.

The thickness of the volcaniclastic sequence ranges

from about 10 m at the Villa Filangieri site to 3 m

along the coast road. These deposits are massive to

strati®ed, normal to inversely graded, silts to Rudites,

commonly including coarse block lenses. At the

Lido Azzurro site the basal beds contain pumice

lenses. In this sector all the investigated facies are

well represented.

In Fig. 5 the results of component analyses in

some samples representing directly graded layers

are reported: from base to top, the increase of

juvenile light and ¯oating fraction can be noticed,

showing the opposite behavior to lithic fragments

which tend to be concentrated at the bottom. Chemi-

cal analysis indicates that the juvenile fraction has

the same composition as the AD 79 white fall pumice

(Fig. 6).

In the upper part of Torre Annunziata sequence a

20-cm thick, continuous primary ash-fall deposit is

present (Fig. 14). This sample (TA10) shows 32.1%

juvenile fragments, 22.8% crystals and 45.1% lithic

fragments (Fig. 5). Its lateral continuous distribution,

lithologic composition and ®eld features are evidence

of the primary nature of this pyroclastic layer. Chemi-

cal data on the glass fraction suggest its correlation

with the AD 472 eruption fall deposits (Fig. 6). The

presence of this layer interbedded in the volcaniclastic

deposits demonstrates both that most of their

emplacement in the Torre Annunziata sector occurred

in the AD 79±AD 472 time-span, and that it contin-

ued, though to a lesser extent, also after the AD 472

eruption.

3.3. Facies interpretation

During its down-slope motion a gravity ¯ow

progressively deposits different facies determined by

different stages of development. Indeed, the ¯ow,

undergoing transformations (Fisher, 1983), segregates

progressively different grain-size assemblages.

Consequently lateral associations of these distinct

facies represent the record of progressive transforma-

tions of the ¯ows in a down-current direction (Lowe,

1982; Mutti, 1992).

Smith (1986) outlined the characteristics of the

deposits and proposed a facies nomenclature to

describe coarse-grained volcaniclastic-sediments.

More recent studies carried out by Smith (1988);

Sohn and Chough (1990, 1992) on the facies analysis

of pyroclastic deposits emphasize the importance of

¯ow transformations occurring in the downslope

motion of gravity ¯ows as a cause of facies changes.

The remobilization of this loose material by ¯uvial

processes occurs in short periods of time and through

successive events. Observations in Guatemala

(Volcan Fuego) indicate that eruptions producing

sediment load in excess of geomorphic thresholds

permitting aggradation, may control sedimentation

for several decades following an eruption (Kuenzi et

al., 1979; Vessel and Davies, 1981). On the contrary,

Smith (1987) believes that depositional episodes may

have had longer duration for the Deschutes Basin in

Oregon, where climate conditions are different.

A recent study of the Sakurajima volcano (Ministry

of Construction, 1988), carried out between 1961 and

1981, revealed the magnitude of the landscape

response to events such as heavy rainfalls of short

duration which completely changed the drainage

area of the southern volcano slope. Similar events

could have happened on the Vesuvio volcano land-

scape after AD 79, where the initial environment

created by that eruption was modi®ed by post-erup-

tion processes.

The facies described in the previous section are

often stacked in ®ning-upward (FU) sequences as

shown in the Torre Bassano outcrop (Figs. 7 and 8).

We can observe three FU sequences probably repre-

senting three forestepping±backstepping episodes of

the depositional zone of gravity ¯ow due to high

erosive ¯oods. These trends are related to the

frequency of gravity ¯ows and to cyclic variations

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167 163

Fig. 14. Continuous primary ash-fall pyroclastic deposit (arrow) of

the AD 472 eruption interbedded to the volcaniclastic deposits at

Lido Azzurro cliff (Torre Annunziata, site 3).

in their capacity or ef®ciency of transport (Mutti et al.,

1994, 1996). In this way, because lateral variations are

not visible along continuous exposures, it is consid-

ered that at each location the vertical variations may

express the kind of lateral facies variation that would

have developed within a single ¯ow. Therefore, the

coarser and ®ner deposits of the studied sequences

(facies A±E) can be considered as deposited by the

same kind of gravity ¯ow, with high and low transport

capacities, respectively. The absence in the outcrops

of ®ner grained facies representing the deposit of more

evolved ¯ows can be noticed and we stress they could

be emplaced in more distal depositional zones, prob-

ably in the sea (Fig. 1).

The facies presented here are interpreted as the

result of the down-current evolution of a composite

sediment-laden ¯ow (Fig. 15) due to a highly erosive

¯ash ¯ood.

Floods, in fact, accelerating down the volcano

slopes, progressively gain velocity and erosive

power, to the extent that they form highly concen-

trated sediment gravity ¯ows that may be character-

ized by both vertical and longitudinal grain-size

segregation. More in detail, these ¯ows can be consid-

ered as a type of composite sediment gravity ¯ow

(Mutti et al., 1999, 2000; Sohn et al., 1999) formed

by a frontal debris-¯ow where the coarsest grain-sizes

are concentrated and by a body and a tail that, becom-

ing more and more diluted down current, can be made

up of hyperconcentrated ¯ows and stream ¯ows,

respectively (Fig. 15). The body, displaying features

between the frontal debris ¯ow and the diluted tail,

can be vertically bipartite with a basal coarse inertial

carpet joining the head down-current and an upper

turbulent hyperconcentrated ¯ow (Fig. 15). When

the front of such a ¯ow slows down or stops, the

turbulent ¯ow and the basal inertial carpet can over-

ride the front deposit giving rise to a ¯ood wave or to

increasingly ®ner ¯ood waves propagating down-

current. So the ¯ood ¯ow bypassing the coarse front

deposit can be a sharply bipartite strati®ed ¯ow

(Todd, 1989; Postma et al., 1988; Sohn et al., 1999).

Every horizontal ¯ood wave overrides the previous

along a density interface that represents a bypass

surface. These surfaces and so these overriding events

are recorded by sharp grain-size changes in the depos-

its (Mutti et al., 1999, 2000). A sedimentation model

of this kind was also successfully tested in the analysis

of ¯uvio-deltaic and turbiditic sediment facies in

tectonically active basins (Mutti et al., 1996, 1999,

2000). Furthermore geomorphologic studies in allu-

vial fan and volcanic environments prove that in this

type of environments high density sediment gravity

¯ows with such features are very common (Sharp and

Nobles, 1953; Pierson, 1995; Sohn et al., 1999).

In this regard, the facies A (Figs. 7 and 8), that is the

coarsest facies, can be interpreted as the deposit of the

coarse frontal part of a composite sediment ¯ow

(Fig. 15). In particular, taking into account the deposit

features, we can stress that the front part of this

deposit represents an intermediate stage between a

cohesive debris-¯ow and an inertial grain ¯ow (Pier-

son and Costa, 1987) where dispersive pressure and

frictional grain interactions tend to predominate

during motion and depositional phase, respectively.

Moreover, the strong erosion phenomena associated

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167164

GRAIN SIZE OF FACIES AGRAIN SIZE OF FACIES B

GRAIN SIZE OF FACIES D

GRAIN SIZE OF FACIES C

GRAIN SIZE OF FACIES E

not to scale

Stream flow Hyperconcentrated flow non cohesive debris flow or granular flow

HEADBODYTAIL

Fig. 15. Example of composite sediment gravity ¯ow in the sense of Sohn et al., 1999 (see also Mutti et al., 1999, 2000). In the ®gure is also

indicated the distribution of the main grain-size populations concerning the facies described in Fig. 8.

with these facies are interpreted as due to the blocking

of the high momentum head of the composite ¯ow

front (Fig. 9).

Facies B±E (Fig. 8) which, on the contrary, are

®ner facies, represent the deposit of the more diluted

bipartite ¯ow, characterizing the body of the compo-

site ¯ow that bypasses the coarse front deposits once

they stop. In particular, B, C and D facies are the basal

inertial carpet deposits, whereas E facies is the deposit

of upper turbulent HFF (Fig. 15). Furthermore, facies

B and D were interpreted as a granular ¯ow in which

during motion a dispersive pressure dominates, as the

reverse grading, well developed mainly in facies B,

points out (Fig. 8). Facies C, which is a normal graded

pebbly sandstone, can be variously interpreted: it

may be the ®ner portion of the composite sediment

¯ow basal inertia carpet where overpressure or

lique®ed condition (Mutti et al., 1999, 2000) predo-

minate or the deposits of the highly concentrated basal

part of a hyperconcentrated ¯ood ¯ow in which

en-masse sedimentation and/or traction processes

predominate (Postma et al., 1988). The last facies E,

characterized by a traction carpet and low angle lami-

nae, represents the deposit of the low concentration

upper part of the overriding hyperconcentrated ¯ood

¯ow where traction plus fallout processes predomi-

nate. In some case, facies D contains outsized clasts

transported by traction caused by the overlying

turbulent sediment-laden gravity ¯ow along the

rheological interface that develops within the gravity

¯ow (Figs. 7 and 8).

4. Discussions and conclusions

This study suggests that, during the AD 79±AD 472

period, morphologic and lithological were ideal

conditions for lahar activity in the Somma-Vesuvio

area, since:

² The volcano was covered by large amounts of

highly erodible pyroclastic material deposited

during the AD 79 eruption, as well as during the

previous protohistoric (3200 years BP) and Avel-

lino (3500 years BP) eruptions.

² At Torre Annunziata the occurrence of the AD 472

ash-fall layer interbedded in volcaniclastic depos-

its, indicates that these inter-eruption sediments

were mobilized episodically and, at the most,

over 400 years (Fig. 2).

² The position of the volcano, located in a plain near

the seaside, and the rapid aggradation of pyroclasts,

like the destruction of the vegetation, could prob-

ably have favored degradational processes and

contributed to quick morphologic modi®cations

of the volcano.

² Along the volcano ¯anks textural characteristics of

the AD 79 ®nal hydromagmatic deposits probably

allowed the growth of a network of more or less

deep rills and gullies that reached the sea. Their

presence favored the piling-up of debris-¯ow

deposits and hyperconcentrated ¯ood ¯ow deposits

in narrow areas, as a consequence of rainfall events

of strong intensity and short duration. These rain-

fall events triggered sediment-laden stream ¯ows

from zones with a high elevation drainage basin

and a short distance between drainage and sedi-

mentation areas, as in the Somma-Vesuvio region.

The sedimentological analyses of the stratigraphic

sequences at Torre Bassano and Terzigno make it

possible to characterize a facies tract that can repre-

sent deposits of debris ¯ow and hyperconcentrated

¯ood ¯ows (Smith and Lowe, 1991). These facies

have been generated by high energy ¯ash ¯oods

that, bulking volcaniclastic sediments deposited in

former times, increased their concentration until

they transformed into highly concentrated composite

sediment gravity ¯ows. They were characterized by a

longitudinal segregation of grain-size classes where a

coarse front part, formed by non-cohesive debris ¯ow-

type ¯ows, and a body and a tail, formed by ¯ows

which become increasingly ®ner and diluted up-

current, can be recognized (Fig. 15; Mutti et al.,

1999, 2000; Sohn et al., 1999). The blocking of

these highly concentrated composite parent ¯ows

would ®rst produce the deposition of the coarse

front part to form facies A and then the overriding

of this deposit by the bipartite ¯ow, which constitutes

the body of the ¯ow. This ¯ow is composed of a

highly concentrated basal inertia carpet responsible

for the deposition of facies B±D and an upper hyper-

concentrated ¯ood ¯ow that forms facies E through

traction plus fallout processes, respectively (Fig. 15).

The occurrence of these inter-eruption deposits in

the Vesuvio area suggests that lahar events may be

L. Lirer et al. / Sedimentary Geology 139 (2001) 151±167 165

more frequent than previously recognized. Conse-

quently, the type of hazard linked to these gravity

processes, related to high erosive ¯ood events, is

generally underestimated. The effect of the erosive

process, connected with heavy rains of short duration,

is strongly dependent on the surface lithology of the

volcano. At the present time in the perivolcanic area

the strong hazard of gravity processes is con®ned

mainly to the northern slope of the volcanic complex,

where a thick coarse-grained pyroclastic deposit

covers the oldest Somma lava ¯ows. These outcrop

only at the base of deep gullies related to an extensive

rill network, whereas on the southern side lava ¯ows

mainly outcrop at a depth of few meters.

In the inter-eruptive phase, similar events can

verify following rains of strong intensity and short

duration that can trigger landslides because of the

motion of old pyroclastic deposits outcropping

above the limestone layers on the slopes of the Apen-

nine chain surrounding the plains nearby Somma-

Vesuvio. During the Vesuvio present quiescence

phase, in fact, the ¯ood landslides happened in May

1998 in the Sarno town area, where at least 150 people

died, reported to this phenomenon. Consequently, also

the plains nearby the Vesuvio are still presently

subjected to an inter-eruptive ªlaharº hazard that is

not negligible.

Acknowledgements

The authors wish to thank Prof. Mutti for the help-

ful contribution to data interpretation and the

reviewers Cas, Smith and Rowland whose suggestions

greatly improved the manuscript.

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