Forestepping-backstepping stacking pattern of volcaniclastic successions: Roccamonfina volcano, Italy

Journalof vokanology andgeothermalresearch

ELSEVIER Journal of Volcanology and Geothermal Research 80 (1998) 155-178

Erratum

Erratum to ‘ ‘Forestepping-backstepping stacking pattern of volcaniclastic successions: Roccamonfina volcano, Italy” [Journal of Volcanology and Geothermal Research 78 ! 1997) 267-2881 ’

D. De Rita a, *, G. Giordano a, S. Milli b

a Dipartimento di Scienze Geologiche, Terza Uniuersit& di Roma, V. Ostiense 169, 00154 Roma, Italy h Dipartimento di Scienze della Terra, Uniuersith degli Studi di Roma ‘La Sapienza’, P.le Aldo Moro 5, 00185 Roma, Italy

Due to a technical error that occurred during the printing of the issue, incorrect font symbols were used. The correct article is reprinted in full on the subsequent pages. The Publisher apoligizes for the inconvenience caused.

’ PII of original article: SO377-0273(97)00005-X.

PII SO377-0273(97)00069-3

ELSEVIER Journal of Volcanology and Geothermal Research 80 (1998) 155-178

Joumalof vokanology andgeothermalreseamh

Forestepping-backstepping stacking pattern of volcaniclastic successions: Roccamonfina volcano, Italy

D. De Rita a3 * , G. Giordano a, S. Milli b

a Dipartimento di Scienze Geologiche, Terza lJniuersit& di Roma, V. Ostiense 169, 00154 Roma, Italy b Dipartimento di Scienze della Terra, Uniuersitc3 degli Studi di Romn ‘La Sapienza’, P.le Aldo Moro 5, 00185 Roma, Italy

Received 1 July 1996; revised 3 December 1996; accepted 3 December 1996

Abstract

We use Unconformity Bounded Stratigraphic Units (UBSU) to reconstruct the chronostratigraphy of volcaniclastic deposits of Roccamonfina Volcano (southern Italy). Significant discontinuities of different hierarchical order in the stratigraphic succession of the volcano have allowed us to identify the UBSU supersynthems, synthems and subsynthems. The hierarchical order has been determined by the importance and duration of the discontinuities bounding such units and by their geographic distribution. Supersynthems are bounded by regional-scale unconformities that may correspond to important tectonic events, changes in the style of eruptive activity or volcano-tectonic collapses. Synthems are bounded by unconfotities marked by paleosols that are representative of considerable hiatus and extended all over the synthem. They are composed of complex sequences of depositional units. At Roccamonfina the synthems reveal cyclical eruptive activity. Unconformities bounding the subsynthems have been recognized through a detailed facies analysis. More than 100 stratigraphic sections have been analyzed and correlated, restoring them to single successive datum planes. It has been possible to reconstruct the original stratigraphic relationships, the geometry of the units, and the effects of syn-eruptive tectonic activity. This procedure revealed that the unconformity surfaces bounding depositional units show different physical expression in the proximal, medial and distal areas: (a) erosive surfaces may become amalgamation surfaces; (b) amalgamation of several depositional units can locally produce a single lithostratigraphic unit. In this case the unconformity surfaces bounding the units can be identified by the presence of discontinuous pumice and lithic concentration zones; (c) non-depositional surfaces in proximal areas (bypassing surfaces) or in interfluve areas may correspond to downslope deposits; (d) unconformity surfaces can bound different lithostratigraphic units that are the result of down-current facies variation.

The pyroclastic units at Roccamonfina are organized as forestepping-backstepping sequences. We interpret this organization as resulting from waxing-waning processes associated with eruption and emplacement. In fact, the internal geometry of each synthem indicates a forestepping phase until the maximum eruption rate is achieved; as it later declines deposition occurs upslope (backstepping). This mechanism is also reproduced at smaller scale within each subsynthem.

The forestepping-backstepping organization of the deposits provides an explanation for the variability of physical,

* Corresponding author. Fax: + 39-06-5737 2827; E-mail: derita@uniroma3it

0377-0273/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SO377-0273(97)00005-X

158 D. De Rita et al. /Journal oj Volcanology and Geothermal Research 80 (1998) 155-l 78

chemical and sedimentological parameters observed in the Roccamonfina volcaniclastic deposits. 0 1997 Elsevier Science

B.V.

Keywords: volcanology; stratigraphy; emplacement processes; Roccamonfina volcano; Italy

1. Introduction

Most pyroclastic-surge and -flow deposits show lateral and vertical facies variations that are related

to topographic control on the transport and deposi-

tion mechanisms (Sparks et al., 1978; Fisher and Heiken, 1982; Fisher, 1983; Fisher and Schmincke,

1984; Cas and Wright, 1987; Valentine, 1987; Bran-

ney and Kokelaar, 1992; Fisher, 1990, 1995). Pro- cesses such as gravitational settling of fragments, gas elutriation, and boundary-layer dynamics result in

flow evolution, density stratification, and decoupling of single gravity currents into separate flows with

different lithologies (Druitt, 1992; Fisher, 1995).

These processes suggest that the use of lithostratigraphic units to describe volcanic successions may be incorrect, because a single unit may include de-

posits related to subsequent eruptive episodes. Alter-

natively different lithostratigraphic units may correspond, in fact, to a single unit. This complicates

reconstruction of the volcanic history, solely from observation of the rock types. The relationships between eruptions and the preserved volcanic sequence

is further complicated by the fact that volcanic sequences are usually more complex than might ex-

pected from the eruptive phases that generated them [see, for instance, deposits from the 1980 Mount St.

Helens eruption (Rowley et al., 1981) and the 79 A.D. eruption of Vesuvio (Sigurdsson et al., 198S)l.

For these reasons, to reconstruct the stratigraphy of ancient volcanic deposits a more powerful tool is

needed that will allow the recognition of products from single volcanic events within the succession

(Fisher and Schmincke, 1984). Ideally, eruptions are the basic units for the study of volcaniclastic de-

posits. Sin&in et al. (198 1) define eruptions in term of preceding and following quiet periods, which are recorded in volcanic sequences by unconformities; that is erosion and/or non-deposition surfaces that represent significant hiatus in the rock sequence. Unconformities can be used to identify Unconform-

ity Bounded Stratigraphic Units (UBSU) (Salvador, 19871, which have been adopted by Servizio Geo-

logic0 Nazionale (1992). The UBSU have indirect

chronostratigraphic significance because all the rocks

below an unconformity are older than all of those above and time lines do not cross unconformity

surfaces. A hierarchy for the unconformities is con- strained by the relative temporal importance of the

depositional discontinuities and by their geographic

extends. This hierarchy allows subdivision of the

UBSU into supersyntherns, synthems and subsynthems.

In this paper we use UBSU to reconstruct the chronostratigraphy of volcaniclastic deposits of Roc-

camonfina Volcano. This is one of the first attempts at using the UBSU in volcanic stratigraphy, particu-

larly in the identification of the lowest hierarchical order units. Only by using the UBSU we were able

to subdivide eruptive sequences into depositional

units, each related to a discrete phase of eruption, and to reconstruct the 3-D geometry and the internal facies association within each depositional unit. Fi-

nally, we show that the stratigraphy has a forestepping and backstepping stacking pattern that we interpret as resulting from waxing and waning of the

volcanic activity.

2. Methodology

In the last years of field surveys of Roccamonfina

volcano (Giordano, 1995) we identified the major unconformities that separate the supersynthem and synthem rock units. The unconformities bounding

the supersynthems are at regional level and relate to important tectonic events that caused significant changes in the eruptive style of the volcano (Giordano, 199.5; De Rita and Giordano, 1996). The supersynthems are made up of many eruptive rock sequences and may correspond to the ‘eruptive epochs’ of Fisher and Schmincke (1984) because the time interval separating the units is comparable for both (Table 1).

D. De Rita et al./ Journal of Volcanology and Geothermal Research 80 (1998) 155-178 159

The discontinuities bounding the synthems are recognized over the whole volcanic area and are usually marked by thick paleosols and/or re-worked materials. They separate periods of cyclic activity of the volcano and may correspond to the ‘eruptions’ of Fisher and Schmincke (1984) (Table 1).

Unconformities bounding the subsynthems are not obvious; they usually correspond to hiatus of hours, days, or a few years and extend only locally. Gener- ally, they are identified by correlating many stratigraphic sections containing several unconformities of lowest order. The subsynthems may correspond to the ‘eruptive phases’ of Fisher and Schmincke (1984) (Table 1) and they may be represented by a single

bed (e.g., lava and pyroclastic-flow deposits) or a bedset (e.g., surge or fall deposits).

The correlation is done using standard stratigraphic methods, primarily constructing cross sections restored to a single datum plane (i.e., flattened isochronous surfaces or synchronous beds, see Krumbien and Sloss, 1963, pp. 439-443). In this method the beds that can be widely traced are used as the datum planes to correlate the stratigraphic sections. They are flattened to restore the original stratigraphic relationships among the units below and to reveal the synvolcanic tectonic deformations. The correlation is accepted when the units show a geometry and a facies association consistent with the pale-

Table 1

Comparison between volcanic activity units as proposed by Fisher and Schmincke (1984) and rock-stratigraphic units as proposed by

Salvador (1987) and adopted in this work

Volcanic activity Units

Eruptive pulse (seconds to minutes) Time interval Q between phase:

Eruptivz phase (minutes, hours, days)

) * . ERUPTION (davs, months. veals)

Time interval Q between eruptions of significant duration for soils or erosional breaks

ERUPTIVE EPOCH (tens, hundreds or thousands of years)

I Time interval Q sufficient for tectonic

events to occur

\ J

Eruptivx epoch ! *

. ERUPTIVE PERIOD

(thousands to millions of years)

Airfall Pyroclastic flows Lahars Lava flows

surfaces, ’ amalgamation surfaces, by-pass or non depositional surfaces

soils erosional surfaces

Association of products

base and top of a volcanic sequence soils, caldera collapse, sector collapse or surfaces that can be traced laterally into si

9 nificant

uncon ormittes of Volcanoes or pans of the surrounding a volcano sedimentary

environment

Volcanoes Volcanic fields and regions

Rock-stratigraphic Units

SUBSYNTHEMS

SYNTHEMS

SUPERSYNTHEMS

160 D. De Rita et ul./ Journal of Volcanology and Geothermal Research 80 (1998) 155-178

oto~graphy and with the emplacement mechanism. The same procedure is repeated sequentially from top to bottom of the sequences until all possible unconformities are identified.

The unconfo~ties funding the subsynthems need to be identified for their whole extent of their synthem. Unconformities that cannot be correlated within the whole extension of the synthem may, however, indicate internal d&continuities, such as

those formed by the supposition of lobes of a flow unit (Fisher, 19901, or by variations of the flow emplacement mechanism induced by paleotopogra- phy. In some cases, they may not be correlated only because of the limited number of outcrops.

The unconformities bounding subsynthems usually have local character directly related to the eruptive activity; for this reason their lateral extent is normally limited. In contrast, the unconformities

Fig. I. Simplified geology of R~camo~na volcano and dist~b~t~on of WTT deposits. Legend: 2 = su~rsyn~em of Vezzara products;

2 = supersyntbem of the Riardo Basin products, 2u = outcropping area of the four WTT syntbems; 3 = supersyntbem of Roccamonfina

products; 4 = smmit depression rim. Kilometric coordinates are from U.T.M. system, 33TVF zone.

D. De Rita et al. /Journal of Volcanology and Geothemal Research 80 (1998) 155-I 78 161

bounding major units (supersynthem and synthem), because they reflect regional events, coincide with unconformities in the coeval sedimentary successions and extend beyond the volcano area. For this reason, the UBSU are a more useful stratigraphic tool than the volcanic activity units of Fisher and Schmincke (19841, to study the history of a volcano in a regional context (see, e.g., De Rita et al., 1991, 1994, 1995).

3. Regional geology and volcanological history

Roccamonfina is a Pleistocene composite volcano, about 10 km in radius (Fig. l), belonging to the Roman Magmatic Province (Radicati Di Brozolo et al., 1988; Serri et al., 1991). From the rim of the summit depression, 933 m in elevation, the flanks slope 15” down to about 400 m in elevation, After a break in slope, they become on average less than 6” down to 100 m in elevation. The regional geology shows that Roccamonfrna lies at the intersection between a NE-elongated graben, the Garigliano graben (Ippolito et al., 19731, and the NW-trending regional faults. These faults determined the early structural and magmatic evolution of the volcano (De Rita and Giordano, 1996). More recently, the activity was controlled by a N-S-trending fault system.

The stratigraphy of Roccamonfina Volcano has been studied by Giannetti (19791, Giannetti and Luhr (19831, Ballini et al. (1989a,b) and Cole et al. (1992, 1993). These studies were all based on identification of lithostratigraphic units.

More recent study (De Rita and Giordano, 1996) has distinguished at Roccamonfina three main epochs of volcanic activity (Table 2; Fig. 1). They are characterized by distinct eruptive style, and coincide with climaxes in tectonic activity.

During the first epoch, from 630 to 385 ka (Su- persynthem of Roccamonfina), mainly high-K products erupted to form the central stratovolcano and several monogenic volcanic centers within the Garigliano graben. Northeast-oriented faulting of the stratovolcano by regional extension caused gravitational collapse of the volcano summit (De Rita and Giordano, 1996).

During the second epoch, from 385 to 230 ka, five intermediate-volume eruptions emplaced com- pound ignimbrites (Supersynthem of the Riardo Basin). The first one (the Brown Leucititic Tuff or BLT of Luhr and Giannetti, 1987) also belonged to the high-K series. The following ones (the White Trachytic Tuffs or WIT of Giordano, 1995) are silica-saturated (Giannetti and Luhr, 1983).

The third epoch developed from 230 to 53 ka and was predominantly characterized by phreatomagmatic activity. The distribution of these youngest units was strongly controlled by a N-trending regional fault system (Supersynthem of Vezzara).

4. Stratigraphic organization of the eruptive rock-sequences

In our stratigraphic reconstruction the three epochs of Roccamonfina coincide with supersynthems (Roc- camonfina, Riardo, and Vezzara supersynthems), whereas the eruptions represent the synthems (Tables 1 and 2).

Our work has been mainly focused on the stratigraphy of the Riardo supersynthem. This is subdi- vided into five synthems (BLT, WTT Cupa, WIT Aulpi, WIT S.Clemente, WTT Galluccio in Table 2) by the presence of well developed and extended paleosols. In turn, the second (WIT Cupa, identified by C symbol) and fifth synthems (WTT Galluccio, identified by G symbol) include distinctive subsynthems. These are bounded by minor (third-order) unconformities that have been defined on the basis of facies analyses and chronostratigraphic correlation: Ca, Cb, Cc, Cd and Ce for the Cupa synthem; and Ga, Gb, Gc, Gd, Ge, Gf, Gg, Gh and Gi for the Galluccio synthem.

We next present the analysis of four cross sections on the southwestern side of Roccamonfina volcano (Fig. 21, representing an area where stratigraphic relationships between synthems and subsynthems are more evident. Cross sections A and B (Figs. 3 and 4) are perpendicular to the southwest margin of the caldera rim. Cross sections C and D (Figs. 5 and 6) are parallel to the volcano slope. Our layers of correlation were: unconformity surfaces (Y and p (cross sections A), and fall layers S and y

162 D. De Rita et al. / Journal of‘ Volcanology and Geothermal Research 80 (1998) 155--l 78

Tabte 2 Stratigraphic organization of the Roccarnonfina volcanic deposits as presented in this work and comparison with previous stratigraphies.

First-order unconformities related to tectonic events separate supersynthems, whereas second-order unconformities bound synthems. They

are marked by the presence of paleosols

This work; Giordano, 1995; De Rita and Giordano, 1996

Age (ka) Su~~yn~em Synthem Comments

53 (1) Vezzara Yellow Trachytic Tuff (YTT) About 1 km3 of magma D.R.E. erupted.

(Epoch III) Monogenic activity along N-trending lineaments

Intracaldera lava domes

230 (2)

385 (3)

630 (4)

Riardo Basin WTT Galtuccio About 10 km3 of magma D.R.E. erupted.

(Epoch II) Highly explosive activity along NE-trending lineaments

WTT S. Clemente

WTT Aulpi

WTT Cupa

Brown Leucitic Tuff (BLT)

Roccamonfina Gravitational collapse of the volcano summit. About 100 km’ of mainly lavas erupted.

(Epoch I) Development of a NE-trending graben structure The stratovolcano develops into the NE-trending

along the eastern flank of the stratovolcano. Garigliano graben. SE of the Mt. Massico fault

Stratovolcano building. Strong parasitic activity a complex of small volcanic centers develops

Previous stratigraphies

Luhr and

Giannetti

(1987)

Cole et al. (1992) Ballini et al. (1989a)

Stage II YTT

Intracaldera

Conca Ignimbrite

Intracaldera lava domes

Stage I

lava domes

Intracaldera Intracaldera phreatomagmatic eruptions

phreato-

magmatic

eruptions

WIT Upper Galluccio Tuff WI-I sup.

Middle Galluccio Tuft’ WIT medio

Lower Galluccio Tuff WIT inf.

BLT Campagnola Tuff Lateral sector collapse?

Several leucitophyric ignimbrites BLT

Caldera collapse

related to

magma

withdrawal

Central Central stratocone building Central stratocone building

stratocone

building

The radiometric ages are from: (1) Radicati Di Brozolo et al. (1988); (2) Giannetti (1990); (3) Luhr and Giannetti (1987); (4) Balhni et al.

(1989a). Note that, in the stratigraphy of this work, the main caldera collapse event is not related to ignimbrite eruptions (see De Rita and

Giordano, 1996) whereas in Cole et al, (1992) the caldera collapse events are always connected to ignimbrite eruptions.

(cross sections B, C and D). The sedimentary characteristics of the identified subsynthe~ are synthe- sized in Table 3.

In cross section A (Fig. 3) the Ca and Cb subsyn-

thems mark the base of the WTT Cupa and were formed by pyroclastic surges that reached distal locations. The Cc subsynthem is massive and chaotic and shows lenticular geometry with maximum thickness

D. De Rita et al. /Journal of Volcanology and Geothermal Research 80 (1998) 155-I 78 163

4580

Roccamonfina I

b

Fig. 2. Distribution of the White Trachytic Tuff eruptive units and locations of the cross sections. n indicate locations of the measured

stratigraphic sections.

at the break in slope (section 359), where it consis- tently eroded Cc the underlying unit. Downslope (section 237) thins and is laterally equivalent to an ashy layer, 10 cm thick in section 346, which repre-

sents its more distal equivalent (possibly the co-

ignimbrite ash cloud). The Cd subsynthem shows lithological and geometrical characteristics very similar to Cc. The lateral equivalent upslope of the Cc and Cd subsynthems are bypassing surfaces (surfaces where there was no deposition or erosion at the time

of Cc and Cd emplacement). Neither the Cc no Cd subsynthems are present in the proximal areas, where

Ce lies directly up on Cb (section 249). It is worthy of note that our correlation allows identification of two subsequent depositional units (Cc and Cd sub-

synthems) even where (between sections 359 and 237) they appear as a single lithological unit because of amalgamation processes. Fig. 7 shows the unconformity between the Cc and Cd subsynthems. Fur- thermore, the presence of the Cc ash layer in the

Tab

le

3 L

ithol

ogic

al

and

sedi

men

tolo

gica

l ch

arac

teri

stic

s of

the

su

bsyn

them

s be

long

ing

to W

TT

C

upa

(C

sym

bol)

an

d W

TT

G

allu

ccio

(G

sy

mbo

l)

synt

hem

s,

resp

ectiv

ely

Synt

hem

Su

bsyn

them

L

ithol

ogy

Thi

ckne

ss

Sedi

men

tary

Fl

ow

Geo

met

ry

WT

r G

i pr

ey

pum

ices

(c

px +

snd

) an

d

Gal

lucc

io

Gh Gg

Gf

Ge

Gd Gc

Gb

Ga

lithi

cs

( >

50%

)-m

as

h m

atri

x;

brec

cia

laye

r at

the

top

grey

pu

mic

es

(cpx

+

snd)

an

d

lithi

cs

( >

50%

) in

ash

m

atri

x:

brec

cia

laye

r at

the

bas

e

whi

te

and

grey

pu

mic

es

and

lithi

cs

(40%

) in

ash

m

atri

x;

brec

cia

laye

r at

the

bas

e

whi

te

and

grey

pu

mic

es

and

lithi

cs

(40%

) in

ash

mat

rix

grey

as

h w

ith

abun

dant

accr

etio

nary

la

pilli

whi

te

and

wel

l ve

sicu

late

d

pum

ices

an

d lit

hics

(3

0%)

in a

sh

mat

rix

- __

whi

te

and

wel

l ve

sicu

late

d

(snd

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mic

es

and

ash

whi

te

and

wel

l ve

sicu

late

d

pum

ices

w

ith

ash

mat

rix

whi

te

ash

3-5

m

mas

scve

2-3

m

IlXlS

SiVe

3-l

m

mas

sive

3-4

m

clin

ostr

atif

ied

to m

assi

ve

0.3-

0.8

m

cros

s-st

ratif

icat

ion

O-3

m

m

assi

ve

O-3

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cr

oss-

stra

tific

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n

lm

coar

seni

ng

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ard

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sit

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ratif

ied

to t

he b

ase

and

mas

sive

to

the

top

0.4-

l .2

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an

gle

cros

s-st

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catio

n pl

ant

sags

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ar

lent

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ar

lent

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ar

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tabu

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with

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sens

itivi

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to t

he

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tic

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de

posi

t

pyro

clas

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flow

de

posi

t

pyro

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de

posi

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late

ral

and

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posi

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show

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and

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ical

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ansi

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once

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ted

base

su

rge

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grey

an

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l m

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ning

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t; le

ntic

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(c

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( >

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whi

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de

posi

t

(snd

+ c

px)

and

lithi

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(40%

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with

ab

unda

nt

ash

mat

rix

cc

Cb

Ca

whi

te

and

frot

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O

-4

m

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upw

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depo

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(3

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gene

rally

m

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with

with

ab

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es

of r

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;

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atiti

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tal

area

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pum

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d 0.

7-l

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to

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h an

gle

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cs

and

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rdin

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ion

whi

te

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an

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l

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tabu

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%

e Q

D. De Rita et al. / Journal of’ Volcanology and Geothermal Research 80 (1998) 155-l 78

CROSS-SECTION A

237 359 252

NE

52 249

d

Fig. 3. Cross section A. 2.7 km long, is roughly perpendicular to the southwestern caldera rim: the datum plane are the unconformity

surfaces LY and p. The backstepping organization of the Cc, Cd and Ce subsynthems of the W?T Cupa synthem is shown. See Appendix A

for the geographical coordinates of the sections in this and subsequent figures. Ca and Cb are surge deposits, whereas Cc. Cd and Ce are

ignimbrites.

most distal areas indicates that the Cc and Cd emplacement events were time-spaced. In cross section C the valley ponded geometry of subsynthem Cc is clearly visible. Subsynthem Cc has massive facies

and lithic enrichment within the valley where it had its maximum erosive ability; it is absent in interfluve

areas. In cross section B (Fig. 4a), the Ga, Gb, Gc and

Gd subsynthems closing as a wedge upslope and

downslope, have an offlap geometry. Stratigraphic relationships among Gf, Gg, Gh and Gi subsynthems (Fig. 4b) have been masked upslope by the presence of a syn-depositional fault (see Section 5 for discus-

sion). Comparing perpendicular and tangential cross sections (Figs. 3 and 4 vs. Figs. 5 and 61, it is possible to recognize that such deposits show, in

tangential cross section, an overall tabular geometry due to shifting of the subsequent units within valleys.

This kind of emplacement resembles that recognized

in turbidite systems where lobes overlap according to the principle of compensation (e.g., compensation cycles by Mutti and Sonnino, 198 1).

Our correlations stress that the third-order unconformities subdividing subsynthems show different physical characteristics in distal, medial, and proxi-

mal areas. In distal areas, where the record of the depositional units is more complete and best preserved, the unconformities are typically sharp with

low relief (Fig. 8). In medial areas, the unconformities are more irregular and show maximum erosional relief at breaks in slope (Fig. 9), where pyroclastic

flows exercized maximum dynamic pressure and strongest erosion of preexisting deposits (see, e.g., Clark, 1984; Levine and Kieffer, 1992; Giordano and Dobran, 1994). Where breaks in slope induce hydraulic jumps, the velocity suddenly decreases, caus- ing flow deposition immediately downslope and pos-

sible amalgamation of flow units. In proximal areas,

Fig. 4. Cross section B is about 3.5 km long and is also roughly perpendicular to the southwestern caldera rim. (a) The datum plane 6 is an

accretionary lapilli-rich fall layer (Gel. The Ga. Gb, Gc, and Gd subsynthems are mainly surges, show forestepping organization. (b)

Forestepping organization is also shown by the Gf, Gg and Gh subsynthems, whereas Gi represents the first backstepping subsynthem. The

downslope geometry (not in scale) of the Gf Gg and Gh subsynthems is hypothesized. The presence of the fault has been inferred by: (a)

the anomalous ~ckening of the units without evidence of erosion: (b) the impossibility to correlate Gi with Gj. This last subsyn~em, in

fact, shows the presence of large blocks of the Gc subsynthem in section 126, which suggests correspondence with an erosional bypassing surface upslope, whereas at the base of Gi there is no evidence of erosion.

D. De Rita et al. /Journal of Volcanology and Geothermal Research 80 (1998) 155-l 78 161

the lateral equivalents of the unconformity surfaces are bypassing surfaces. These may be erosional or simply non-depositional. Locally, on the unconform-

ity surface, a thin lag deposit is present. It is consti- tuted by the densest clasts of the flows and by reworked particles from the substrata (Fig. 10). This

CROSS-SECTION B (4

E

377

w

115 126 410

_______GB-________________ -_____-_____-_-_~..-__-- z”-_-____-_-_x;zr - -Gb - - - - - - - - - --..__ -____ --__

llm

(b) (- not in scale :I

Foresteptaing I

1’26 Backstepping I

_jlm 1OOfYl

168 D. De Rita et al. /Journal of Volcanology und Geothermal Research 80 f’19BI 155-178

N

404

CROSS-SECTIOIV C

387 126 3437 122

SE

359

llm

?%m

Fig. 5. Cross section C. 4.5 km long, is circumferential to the volcano. The datum plane y is a fail level at the base of the WTT S. Ciemente. A indicates the WTT Anipi synthem and C indicates the W’IT Cupa synthem. Ca. Cb and Cd as in Pig. 3. Note the valley-ponded Cc subsynthem. The fault adjacent to the 126 section has been inferred by the thicknesses of the units (see text for discussion).

N CROSS-SECTION D

390 377 329 306 346

[fi ---_-_

ef 3:: - --

__=?== -_ ,: _, = z 27 z = _z-;-_--_z_-- - Y - _----CL _; z =- z- r_-z-2

Gd ,_ ____T&_----;,- ------&-___=____ ---___

_=~~I~~___.-__~~~~____--____-_,~~~__..____-;___~, - --___ 1::: _,..._“_-_-

Fig. 6. Cross section D, is about 6 km long and lies 1.5 km downslope of cross section C. Datum plane is the same as in Fig. 5. At the surface a large topographic depression is aligned with the tectonic depression recognized in cross section B. Gc and Gd show an overall tab&r geometry according to the principle of com~nsation. Letters as in previous Figures.

Fig. 7. I Jncol nformity surface that separates subsynthems Cc and Cd (WIT Cupa), that became an amalgamation surface downr

left) (set :tion 359 of the cross sections A and C).

D. De Rita et al. / Journal of Volcanology and Geothermal Research 80 (1998) 155-178 169

Fig. 8 ;l’ ape (to the

Fig. 8. Shq unconformity with low relief between subsynthems Ca and Cb of the distal WlT Cupa. Section 237 of cross section A.

Fig. 9. Strongly erosive unconformity (see arrows), separating subsystems CD and Ce (WTT Cupa). Section 249 of cross section A at the

break in slope.

Fig. 10. Lag deposit at the base of subsynthem Cd (section 359, cross section A). The lag deposit, 2 cm thick, is mainly composed of lithics

and is mixed with rounded pumices belonging to the underlying unit (Cc subsynthem).

Fig.

subr

stral

Fig. 12. The unconformity surface separating subsynthems Cc and Cd. Cc onlaps (see arrows) the subsynthem Cb (section 359

sect ion A). Undulating (wavyjlines indicate the unconformity surfaces.

D. De Rita et al. /Journal of Volcanology and Geothermal Research 80 (1998) 155-I 78

Fig. 11

Fig. 12 11. Deposits with inclined stratification, showing progradation (from left to right) and forming a coarsening upward sequen

;ynthem Ce (WIT Cupa synthem). Section 237 of cross section A. Arrows indicate the unconformity bounding subsynthem Cc

ulating line marks the less evident unconformity separating subsynthem Ca from Cb; broken lines represent the surfaces of

Scation inside subsynthem Ce.

ce inside r; broken

inclined

of cross

172 D. De Rita et al. /Journal of Volcanology and Geothermal Research 80 (1998) 155-I 78

Ca,b CC Cd Ce Ga,b Gc Gd Ge Gf,g,h Gi t Ga,b,c,d Gf,g,h,i t

a b

Forestepping

Ca,b,c Cd,e Ga,b,c,d Gf,g,h,i t

C

40- . . . 4-

20- 2- . .

CaCbCcCdCe GaGbGcGdGeGf GgGhGi t I I I I I

58 60 62 64 66 SiOz (%)

Ic (%) K20 (%) Gf,g,h,i

80- 8_ ~c%J

60- . 6-

casl&

GX$X .

5 7 D (km)

d

e f

Fig. 13. Diagrams of sedimentological, physical and chemical parameters variation versus time. Time is expressed qualitatively in terms of

stratigraphic sequences for WIT Cupa (Ca, b, c, d, e) and WTT Calluccio (Ga, b, c, d, e, f, g, h, i) synthems. Dam from literature have been

revised to fit into the present stratigraphy by Giordano (1995). (a) Maximum distance bypassed (non-deposition) by flow units. (b) Pumice

vesicularity (Cole, 1990) cross indicates average value; vertical line indicates standard deviation. (c) Vitric loss (Cole, 1990). (d) Median

(0) diameter versus distance from the summit depression rim; values are indicated by dots (Ca, Cb, Cc), oblique crosses (Cd) and crosses

(Ce). (e) Average hthic content. (f) K,O/SiO, diagram from Ballini et al. (1989a).

D. De Rita et al./ Journal of Volcanology and Geothermal Research 80 (1998) 155-178

Fig. 14. Large blocks (shown by dashed lines) of subsynthem Gc inside Gf near the fault scarp (section 126 of cross sections B ar

indil cate :s the WlT Aulpi Synthem and S indicates the WlT S.Clemente Synthem. Geologist for scale.

td C). A

174 D. De Rita et al. /Journal of Volcanology and Geothermal Research 80 (1998) 155-l 78

lag deposit can be reworked by a subsequent flow losing its continuity. The lag, then, appears as a series of discontinuous pumice and lithic concentration zones. This suggests that the presence of lenses of pumices and lithics in homogeneous ignimbrite

deposits could indicate the amalgamation of more than one flow unit.

Within each synthem, the stratigraphic relation-

ships indicate a cyclical forestepping and backstepping organization, and the strong influence of pale-

otopography on the emplacement and transport processes. The forestepping organization is clearly evi- denced by the lateral and vertical accretion of sub-

synthems Ga, Gb, Gc and Gd (Fig. 4a) and Gf, Gg and Gh (Fig. 4b). We suppose that the same mechanism operated during the emplacement of subsynthems Ca and Cb (Fig. 9). Ca and Cb subsynthems show, in fact, similar facies association to Ga, Gb,

Gc and Gd subsynthems (mainly surge deposits, see Table 3) as well as a similar geometry.

Backstepping is clearly evident in the superposi-

tion of subsynthems Cc, Cd and Ce onlapping the underlying unconformity surface at the top of subsynthem Cb (cross section A, Figs. 3 and 11). The

same mechanism probably operated during emplacement of subsynthem Gi (Fig. 4b).

Tangential cross sections (Figs. 5 and 6) show that the forestepping subsynthems are less topo- graphically controlled than the backstepping subsynthems.

The forestepping-backstepping geometry is re- played at a smaller scale (progradation and retrogra-

dation) inside each subsynthem. In this case the progradation is evident in the presence of deposits with inclined stratification (Fig. 12) forming coarsening-upward sequences (CU). In some cases, such as

in subsynthem Cc, the inclined stratification takes the form of lenses of pumices and lithics more inclined than the subsynthem boundary. The ret- rogradation is generally shown by upward decreases in grain-size defining fining-upward (FIJI sequences at the top of each subsynthem (e.g., section 249 of Figs. 3 and 8). The FU sequences can locally be absent at the top of subsynthems where erosional processes were particularly active, such as at a major break in slope or in proximal areas. In such places only coarsening upward sequences (CU) are preserved (Fig. 3, section 359, subsynthem CC).

We suggest also that the FU sequences in the

backstepping subsynthems are better developed than in the forestepping subsynthems because of the general decrease of the transport energy.

This stratigraphical organization of synthems allows us to explain in terms of waxing-waning erup-

tive processes the variability of physical, chemical, and sedimentological parameters observed in the

Roccamonfina deposits (Fig. 13). The maximum pre-depositional (bypassing) distance shows an in-

creasing decreasing pattern which relates to the forestepping-backstepping stacking pattern (Fig. 13a) as did pumice vesicularity (Fig. 13b), vitric loss

(Fig. 13~) and median diameter (Fig. 13d), whereas the lithic content increased (Fig. 13f). Magma can become progressively more matic in composition (if the volume of magma involved implies the withdrawal of different levels of a compositionally strati-

tied magma chamber; Fig. 13f). Similar trends are

clearly evident for the 1980 eruption of the Mount St. Helens (Rowley et al., 1981) and for the A.D. 79 Vesuvio eruption (Sigurdsson et al., 1985).

5. Syn-eruptive faulting

Analysis of the cross sections shows that volcano tectonic faulting occurred throughout eruption of the

entire Roccamonfina volcanic succession. In cross section B (Fig. 4b) the correlation procedure revealed the presence of a fault active during the time

interval between datum S (Ge) and the emplacement of subsynthem Gi. None of the units Gf, Gg, or Gh

(sections 126, 115, 377) are cot-relatable with unit Gi (section 410). This is based on three lines of evidence: Gf, Gg and Gh show erosive bases, whereas the base of Gi is non-erosive. The former subsyn-

thems also incorporate large blocks eroded from the underlying units (Fig. 14), and these are not present within Gi; surge deposits on top of Gi are completely lacking in sections 126, 115 and 377; finally, Gi presents a lithic enrichment at the top, whereas Gg and Gh have lithic enrichment at the base.

We interpret this situation as due to the action of a fault that was active immediately after the emplacement of unit Ge (datum plane S) and lowered the downslope block. This created accommodation space filled by subsynthems Gf, Gg and Gh, which

D. De Rita et al. /Joumul of Volcanology and Geothermal Research 80 (1998) 155-178 17.5

a

by-passing surface

by-passing surface

\ d

erosional surface I amalgamation surface

176 D. De Rita et al. /Journal of Volcanology and Geothemml Research 80 (1998) 155-l 78

bypassed the sector west of the fault. The subsidence

caused by the faulting increased the gradient and here, each subsequent subsynthem (Gf, Gg, Gh) was able to erode at the newly formed break in slope until the topographic low was filled (sections 115

and 126). In the cross section C (Fig. 51, between the

sections 387 and 361, there is evidence for a depression that formed prior to the WTTC emplacement. This area is a preferential accumulation zone for all

the eruptive units. Here, the flow deposits are preserved to their maximum thickness (Giordano, 19951, indicating that little or no erosion occurred between

their emplacement. This tectonic depression was sub- jected to multiple collapses during the volcanic events. By the comparison between the sections and evaluating the general effect of erosive processes, we calculate that the area sank by about 15 m.

6. Discussions and conclusions

Use of UBSU to reconstruct the chronostratigraphy of complex volcaniclastic rock-sequences allowed identification of distinct depositional units. At

Roccamonfina the eruptive rock units are organized as overall forestepping-backstepping sequences. We interpret, this organization of the deposits as reflect- ing dynamic waxing-waning eruptive and emplace-

ment processes (Fig. IS>. The internal geometry of each synthem indicates a forestepping phase until the

eruption rate peaks; during the subsequent decline in

eruption energy, deposition occurs upslope (backstepping). We suggest that this forestepping-backstepping mechanism is reproduced at a smaller scale within each subsynthem.

Detailed stratigraphic correlations allow us to recognize that eruptive rock sequences are made up of several depositional units emplaced during discrete

events. Any emplacement model for pyroclastic currents should take into account the chronostrati-

graphic significance of the boundary surfaces, avoid- ing the risk of miscorrelating units that are not genetically related. One of the main implication of our study is in fact that graphical methods of correlation can also induce misinterpretations when attempt- ing lateral correlations. A striking example came

from cross section A (Fig. 31, that, if correlated by

flattening the base of the synthem (base of Ca) could show a ramp-flat stacking pattern (Fig. 15b). This kind of miscorrelation led, for example, Cole et al.

(1993) to interpret the same WTT Cupa sequence as emplaced from a continuously feeded pyroclastic

flow decelerating from the front to the rear, thus justifying the supposed ramp structures (cf. fig. 5 in Cole et al., 1993). As we have demonstrated by using more appropriate stratigraphic technique, the internal geometry of WTT Cupa can be restored by

flattening subsequent stratigraphic markers, and in- terpreted in terms of forestepping-backstepping

stacking of time-spaced pyroclastic flow deposits. This interpretation is also better in agreement with the presence in distal areas of co-ignimbritic ash

separating each subsynthem (Fig. 3) and occasionally of reworked materials (Giordano, 1995). This com- ment is not to contest the model of pyroclastic flow

emplacement proposed by Cole et al. (19931, that might well act for single subsynthems, but to stress the fundamental importance (up to date widely disre- garded) that methodologies of correlation, together with facies analysis, play in the understanding of

volcaniclastic successions.

Our study of the Roccamonfina sequences indicates that:

- an eruptive unit defined at the top and at the

base by paleosols is composed of a sequence of depositional units that are affected by the pre-exist- ing topography, eruption style, and the distance from the vent;

- the depositional units are bounded by chronostratigraphic surfaces that may have different lateral physical expressions: (a) an unconformity surface

Fig. 15. (a) Conceptual diagram showing the emplacement mechanisms of eruptive units in forestepping (a-c) backstepping (d-f)

sequences. The forestepping phase is related to high energy of the transport system, whereas the backstepping phase marks a decrease in the

eruptive and transport energy, with deposition occurring progressively upslope. (b) Example of how the graphic method of correlation can

affect stratigraphic interpretation. This is the same cross section shown in (f) correlated by flattening the base of the synthem. The internal

geometry of the subsynthems simulates a ramp-flat stmcture instead of the real offlap (forestepping), onlap (backstepping) geometry.

D. De Rita et al. / Journal of Volcanology and Geothermal Research 80 (1998) 155-l 78 117

can became an amalgamation surface; (b) a litho- logic unit could be the result of the amalgamation of several flow units. This process can be recognized by the presence of discontinuous pumice and lithic concentration zones; (c) a non-depositional surface in proximal areas or in interfluve areas may be related to a downslope deposit. Or the same surface can divide lithologically distinct units as the result of downslope facies variation;

- the preexisting topography strongly influences the transport and depositional system. It is necessary to reconstruct the preexisting topography for each unit, considering the relative thickness variations of each unit and accounting for the effects of possible successive faulting events.

- physical, chemical and sedimentological parameters are correlative with the forestepping-backstepping organization of the deposits. Similar stratigraphic organization has also been recognized in turbidite deposits where it is related to the frequency of gravity flows and to cyclical variations in their individual volumes (Mutti, 1992; Mutti et al., 1994).

Acknowledgements

A. Borgia provided constant support in writing this paper and together R. Cas and R.V. Fisher gave useful suggestions on the preliminary version of the manuscript and for improving the English. Special thanks to R. Cas for discussions in the field and to R.V. Fisher, and G. Valentine for their critical final reviews. The authors are particularly grateful to J. Luhr for his patient and careful review. This work has been performed with the financial support of the MURST funds and by the Dottorato di Ricerca in Scienze della Terra scholarship (GG).

Appendix A. Section locations by kilometric coordinates (U.T.M. system, 33TVF zone)

410 = 692 112 361= 678 098 237 = 655 123 126 = 685 091 122 = 669 104 346 = 648 155 115 = 687 081 359 = 663 124 306 = 651 091 377 = 683 075 252 = 667 125 329 = 663 087 387 = 699 084 S2 = 670 124 390 = 698 072 404 = 718 089 249 = 672 124

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Documents

Forestepping-backstepping stacking pattern of volcaniclastic successions: Roccamonfina volcano, Italy