The effect of paleotopography on lithic distribution and facies associations of small volume ignimbrites: the WTT Cupa (Roccamonfina volcano, Italy)

Ž .Journal of Volcanology and Geothermal Research 87 1998 255–273

The effect of paleotopography on lithic distribution and faciesassociations of small volume ignimbrites: the WTT Cupa

ž /Roccamonfina volcano, Italy

Guido Giordano )

Dipartimento di Scienze Geologiche, UniÕersita di Roma 3, Largo S. Leonardo Murialdo 1, 00154, Roma, Italy`

Received 20 May 1997; accepted 23 July 1998

Abstract

The distribution of lithic clasts within two trachytic, small volume, pumiceous ignimbrites are described from theQuaternary ‘White Trachytic Tuff Cupa’ formation of Roccamonfina volcano, Italy. The ignimbrites show a downslopegrading of lithics, with a maximum size where there is a major break in the volcano’s slope, rather than at proximallocations. This is also the location where ignimbrites are thickest and most massive. The break in slope is interpreted to havereduced flow capacity and velocity, increasing the sedimentation rate, so that massive ignimbrite formed by hindered settlingsedimentation.

Ignimbrite Cc, exhibits no vertical grading of lithics, though it does show downslope grading with maximum size at themajor break in slope and a rapid decrease further downslope. Ignimbrite Cc thins away from the break in slope, and showsan upward fining of the grain size within the topmost few decimeters of the unit. The ignimbrite is stratified proximally, andgrades to massive facies at the break in slope, and distally to stratified facies with numerous inverse-graded beds. Thesimplest mechanism accounting for these downslope variations is progressive aggradation from a quasi-steady, nonuniformpyroclastic density current. The changes in deposit thickness and facies are interpreted to record downcurrent changes insedimentation rate. The upward fining reflects waning flow. Inversely graded, bedded depositional facies in distal areas isinterpreted to reflect flow unsteadiness and a decrease in suspended sediment load.

Ignimbrite Cd shows vertical, as well as downslope grading of lithics. This characteristic, coupled with the widespreadmassive facies of the deposit and the tabular unit geometry are features that can be reconciled with both the debris

Ž . Žflowrplug analogy for pyroclastic flows Sparks, 1976 and the progressive aggradation model Branney and Kokelaar,.1992 . However, none of them appears to satisfy completely the field evidences, implying that when dealing with massive

ignimbrites, other evidence than lithic grading needs to be presented to better understand the related transport anddepositional processes. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: ignimbrite; lithic grading; facies association; break in slope; topography; progressive aggradation

) Fax: q39-06-54888201; E-mail: [email protected]

0377-0273r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0377-0273 98 00096-1

( )G. GiordanorJournal of Volcanology and Geothermal Research 87 1998 255–273256

1. Introduction

1.1. PreÕious studies

Pyroclastic flows are hot, high-concentration den-sity currents of pyroclasts dispersed in gas that aregenerated by explosive volcanic eruptions. During

Žthe last decade, early pyroclastic flow models e.g.,.Sparks, 1976 have been enriched and challenged by

Ž .the introduction of two key concepts: 1 distinctionbetween transport processes and depositional pro-

Žcesses in pyroclastic currents Valentine, 1987;. Ž .Fisher, 1990 ; and that 2 pyroclastic flow dynamics

lie in the continuum of pyroclastic density currents,where flow regime, density, and particle support

Ž .mechanisms can change vertically stratified , down-current, and through time depending on material

Žsupply and topography Valentine, 1987, 1988;.Branney and Kokelaar, 1992; Druitt, 1992 . At pre-

sent, a number of models for pyroclastic currentŽdynamics e.g., Fisher, 1966; Sparks, 1976; Sparks et

al., 1978; Wright and Walker, 1981; Wilson, 1985;Valentine, 1987; Fisher, 1990; Branney and Koke-laar, 1992; Druitt, 1992; Anilkumar et al., 1993;Battaglia, 1993; Palladino and Valentine, 1995;

.Walker et al., 1995; Dade and Huppert, 1996 ac-count for a variety of theoretical possibilities, butrequire better field and laboratory constraints. Thedistinction between deposit and the parent flow isimportant, because processes of deposition are dis-

Žtinct from those operating during transport Kokelaar.and Branney, 1996 , and deposition processes may

obliterate signatures of a particular transport mecha-nism. However, recognising transport mechanismsfrom deposits is an important aim in volcanic risk

Ž .evaluation studies e.g., Fisher, 1995 .Alternative models have been presented for the

formation of thick, massive, coarse tail graded ign-imbrites interpreted as emplaced ‘en masse’ by

Žhigh-concentration laminar flows e.g., Vulsinian ig-nimbrites; Sparks, 1976; Palladino and Valentine,

. Ž1995 , as progressively aggraded Branney and.Kokelaar, 1992; Druitt, 1992 , and as transported by

low-concentration stratified flows and then depositedŽ‘en masse’ e.g., Campanian Ignimbrite; Fisher et al.,

.1993 . Stratified pumiceous ignimbrites have beeninterpreted as a result of stepwise aggradation at the

Žbase e.g., Neapolitan Yellow Tuff; Cole and.Scarpati, 1993 , and as a result of syn- and post-em-

Žplacement shearing of the deposit e.g., AD 1912rhyolitic ignimbrite from Valley of Ten ThousandSmokes; Fierstein and Hildreth, 1992; McPhie et al.,

.1993 .ŽGiven that a deposit is not a flow Kokelaar and

.Branney, 1996 , can we tell something about trans-port from the deposit? What deposit features can beused to interpret the nature of transport, as well asprocesses of deposition?

This paper addresses the influence of break inslopes on pyroclastic flow dynamics and, in particu-lar, on pyroclastic flow capacity, inferred from de-posit characteristics. It focuses on the distributionand significance of lithic clasts within an ignimbrite,from which interpretations are made on transportprocesses during the emplacement of two small vol-ume ignimbrites belonging to the White Trachytic

Ž . Ž .Tuff WTT Cupa Giordano, 1998 at Roccamon-Ž .fina volcano, southern Italy Fig. 1 . The vertical and

downslope distribution of lithic clasts is comparedwithin and between the two ignimbrites and inter-preted in the light of unit geometry, facies analysis,volcano paleotopography and, in particular, breaks inslope.

2. WTT Cupa stratigraphy

ŽThe WTT Cupa eruption De Rita and Giordano,.1996 occurred at the Roccamonfina volcano in

Žsouthern Italy Roman Magmatic Province; Figs. 1. Ž .and 2 about 300 ka ago Giannetti and Luhr, 1983 ,

ejecting a total volume of ;0.75 km3 of trachyticŽ .magma dense rock equivalent; Giordano, 1998 .

The lowest units of the WTT Cupa stratigraphicŽsequence are two surge deposits Ca and Cb in Fig.

.1 . These are overlain by at least three ignimbritesŽ .Cc, Cd and Ce in Fig. 1 , that are arranged in a

Žforestepping–backstepping stacking pattern De Rita.et al., 1997 . This type of architecture implies that

Ž .earlier depositional units Fig. 1; from Ca to Cbprograde along the volcano slope, whereas later de-positional units onlap progressively closer to the ventŽ .Fig. 1; from Cc to Ce . The WTT Cupa crops out inproximal locations where it onlaps the volcano slope,

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Fig. 1. Location of Roccamonfina volcano and map of the WTT Cupa on the SW slopes, and locations of measured stratigraphic sections used to correlate the radial cross-sectionŽ .The cross-section is modified from fig. 8 in De Rita et al., 1997; sections 1 to 8 . Numbers annotated on the inner sides of map boxes, in this and in the next figures, arekilometric coordinates from U.T.M. system, 33TVF zone for the northern hemisphere.


Fig. 2. View of the SW slope of Roccamonfina volcano. Note the almost hyperbolic profile interrupted by the summit depression rim. TheŽ .upper slope is up to 158, whereas downslope of the main break in slope dashed line averages 58.

permitting a detailed study of downslope changes indeposit characteristics from proximal to distal locali-ties. The source of the lithics within the WTT Cupaignimbrites has been constrained to the vent area,which is located within the summit caldera of the

Ž .volcano Fig. 1 , based on the type of lithologiesrepresented compared with detailed mapping of the

Ž .Roccamonfina volcano Giordano, 1998 . The pre-sent paper deals with the lithic distribution withinIgnimbrite Cc and Ignimbrite Cd, the facies of whichare summarised as follows.

Ignimbrite Cc is made up of white, crystal-poor,well-rounded pumice lapilli and ash. Average lithic

Ž .content is ;10% by volume , but can be up to30%. Pumice clasts are generally smaller than lithicclasts. The ignimbrite is lenticular in longitudinal

Ž .section Fig. 1 . This geometry is assumed to be itsoriginal geometry, because Ignimbrite Cc is every-where covered by Ignimbrite Cd, with little sign oferosion between the two, excepting locally at thebreak in slope where Ignimbrite Cd lies on a low

Žrelief erosional base carved in the Ignimbrite Cc see.below . Ignimbrite Cc is exposed from ;3 km from

Žthe caldera rim ;5.5 km from the inferred vent.inside the summit depression; Fig. 1 , and pinches

out upslope. At its most proximal exposure, it isstratified and shows bedding planes onlapping onto

Ž .the underlying topography Fig. 3 . The bedding isdefined by alternations between lapilli- and coarseash-sized beds, either ungraded or symmetricallygraded. The ignimbrite attains its maximum thick-

Ž .ness 4 m 500 m further downslope, where it isŽ .massive Fig. 4 and rests on an erosional surface

Ž .Fig. 1 . Elongated and platy lithic and pumice clastslocally show poor imbrication. The topmost 15 cm ofthe ignimbrite grades upward into a finer facies.Further downslope, Ignimbrite Cc thins and showsmultiple inversely graded beds, the lowest two being0.8 m thick each, and the upper two 0.4 m thick eachŽ .Fig. 5 . At distal locations, the ignimbrite thinsfurther to 0.3 m and is capped by two ash layers afew centimetres thick. Pumice clasts exhibit reversedowncurrent coarse tail grading, being coarser atdistal locations than at proximal locations.

Ignimbrite Cd is massive and exposed ;1 kmŽ .further upslope than Ignimbrite Cc Fig. 1 , where it

Ž .pinches out upslope Fig. 6 . It contains similar, butŽ .less rounded pumice lapilli, and ;20% by volume

lithic lapilli, which locally are imbricated. Its thick-Ž .ness varies up to 9 m , but is generally 7–8 m at

Ž .medial and distal localities Fig. 1 . At medial loca-Ž .tions sections 4 and 6; Fig. 1 the base of the

ignimbrite rests on an erosional surface, that cuts10–40 cm in the underlying substrate.

3. Lithic clast distribution patterns and their rela-tionships with facies and paleotopography

The WTT Cupa allows comparison of the lithicdistributions and facies in successive, rapidly em-

( )G. GiordanorJournal of Volcanology and Geothermal Research 87 1998 255–273 259

Ž . Ž . Ž .Fig. 3. Ignimbrite Cc at section 5 deposit’s proximal edge . a The ignimbrite onlaps the pre-existing topography upslope, to the right. bŽ .Stratification is defined by alternate coarse and finer layers of very well-rounded pumice indicated by box . Flow direction is from right to

left.


Ž .Fig. 4. Ignimbrite Cc at section 6 about 500 m downslope from location shown in Fig. 3 is massive and fines upward within the topmostŽ . Ž .15 cm arrow , and then is capped by Ignimbrite Cd above dashed line .

placed, small volume ignimbrites: Ignimbrites Ccand Cd. The volume of each is in the order of 0.1km3; ignimbrite Cd is of somewhat larger volume

than Cc. Lithic distribution is shown graphically asthe vertical and downslope variations of the average

Ž .and maximum lithic clast size Figs. 7–11 . Average

Ž .Fig. 5. Ignimbrite Cc at section 7 distal location . The unit comprises an upward fining sequence of reversely graded beds, capped by theŽ .massive Ignimbrite Cd above dashed line . The bottom two beds are 0.8 m thick each, whereas the topmost two are about 0.4 m thick each.


Ž .Fig. 6. Ignimbrite Cd at section 2 proximal edge , where itŽ .pinches out upslope to the right , is massive and overlain by the

erosive, lithic-rich base of Ignimbrite Ce. The tree for scale isŽ .about 3 m. Unit Ca and Cb lie above the paleosol dashed line .

lithic size is taken as the mean of the largest sevenlithic clasts in 1 m2 measured at the base and the topof the ignimbrites. Maximum lithic clast size is themaximum seen at the outcrop, both at the base andtop of each ignimbrite.

3.1. Areal lithic distribution Õs. paleotopography

The paleotopography of the southeastern slope ofRoccamonfina has been reconstructed on the basis ofdetailed correlation of the WTT Cupa deposits, tak-ing also into account the occurrence of post-em-

Ž .placement faulting Fig. 7 . The paleotopographyshows a prominent break in slope at ;3 km from

Ž .the rim of the summit depression Figs. 7 and 8 .

This break in slope is generally normal to the in-ferred paths of pyroclastic flows. Second order to-pography comprises valleys parallel to flow direc-tions. The lithic distribution patterns of IgnimbritesCc and Cd are first compared with the paleotopogra-

Ž .phy Fig. 7 . Maximum lithic clast dimension gener-ally decreases with distance from source, but thelargest lithic clasts are not located at the most proxi-mal exposures; rather they are some hundreds ofmeters downslope of breaks in slope. The lithicdistribution pattern along the irregular topography ofthe southern slope of the volcano slope, with largeclasts present in paleovalley, indicating that rough-ness of the paleotopography plays an important role

Žon lithic segregation cf. Freundt and Schmincke,.1985; Robool et al., 1987 . Erosional bases occur

Žalmost at the same sites of maximum lithic size cf..Fig. 1 .

3.2. Lithic distribution with distance from Õent area

Downslope and vertical lithic distribution withinIgnimbrites Cc and Cd are shown by comparing theaverage and maximum values with distance from

Ž .vent area Figs. 9 and 10 . The average lithic size isŽalso compared with the highest which may or may

not include the maximum value recorded in each.outcrop and the lowest values of the seven largest

lithic clasts measured at each location to calculatethe average diameter.

Lithic clast dimensions both from top and base ofŽ .Ignimbrite Cc Fig. 9a,b increase from the most

Ž .proximal locality section 5; cf. Fig. 1 to a maxi-Ž .mum at the break in slope section 6; cf. Fig. 1 ,

where the deposit also shows its maximum thicknessand the underlying deposits are eroded. From sites atthe break in slope, lithic sizes decrease with distancefrom vent area. This pattern is also displayed by thehighest and lowest diameters of the seven measuredlithic clasts at each location. Lithic clast dimensions,both average and maximum, are almost identical atthe base and at the top of the ignimbrite, indicating

Ž .no vertical lithic grading Fig. 9c .Ignimbrite Cd shows a similar pattern, exceptŽ .that: 1 there is a normal vertical grading of lithicŽ . Ž .clasts Fig. 10c ; 2 localities where the largest

maximum and average values occur are displaced

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Fig. 7. Isopleths of maximum lithic clast for Ignimbrite Cc and Ignimbrite Cd compared with the reconstructed paleotopography of SW Roccamonfina volcano at the time of theWTT Cupa eruption. Note that maximum clasts do not occur at the proximal edge of the deposit, but occur just downslope of breaks in slope.


Fig. 8. Downslope topographic profiles of SW Roccamonfina. The approximate extension of the WTT Cupa deposits is also indicated and itschematically illustrates the relationship of the ignimbrites with the topography. Traces of the profiles are indicated in Fig. 1.

Ž .from the base Fig. 10b , to the top of the ignimbriteŽ . Ž .Fig. 10a ; 3 at break in slope, lithic clast sizedispersion is at its largest, recording a positive peakof the largest of the seven clasts measures at thatlocation, and a negative peak of the smallest onesŽ .Fig. 10a,b , which also are the largest and thesmallest clasts measured in the whole deposit; andŽ .4 at the downslope of the break in slope, lithicdimensions remain almost constant with distance.

From the lithic clast dimension vs. distance pro-files, there are three important features to point out:Ž .1 downslope lithic grading occurs in both ign-imbrite units, and in both cases, the lithic distributionpattern is characterised by a downslope

Ž .increasermaximumrdecrease I–M–D lithic varia-

Ž .tion; 2 vertical lithic grading occurs only in Cd;Ž .and 3 at the site of the break in slope, Ignimbrite

Cc is characterised by a general increase in lithicŽ .sizes the M region, Fig. 9a,b . By contrast, in

Ignimbrite Cd, the widest range of lithic clast sizeswas deposited, suggesting at this location that thewhole lithic load had been dumped out.

3.3. Maximum lithic size Õs. deposit thickness

Fig. 11a,b illustrate the relationship between max-imum lithic size and deposit thickness for IgnimbriteCc and Ignimbrite Cd, respectively. Ignimbrite Ccshows a positive relationship with increase both inmaximum lithic size and deposit thickness fromproximal exposures to the area of the break in slope


Ž . Ž . Ž . Ž .Fig. 9. Downslope lithic distribution from Cc top a and base b . Average, largest high and smallest low values of the seven largestlithic clasts measured in each stratigraphic section are shown in this figure and Figs. 10 and 11. The distribution pattern can be described as

Ž . Ž . Ž .a downcurrent increasermaximumrdecrease I–M–D see text . Comparison of average values from base and top of the unit c show theabsence of any vertical lithic grading. Numbers in this and next figures refer to stratigraphic sections as in Fig. 1.


Ž . Ž .Fig. 10. Downslope lithic distribution from Cd top a and base b . The distribution pattern can be described as a downcurrentŽ . Ž . Ž . Žincreasermaximumrdecrease I–M–D see text . However, within the maximum M region, lithic clast dispersion is at its largest see

. Ž .text . Comparison of average values from base and top of the unit c show the occurrence of vertical lithic grading, which appears todevelop within the M region.


Ž . Ž .Fig. 11. Maximum lithic clast dimension vs. deposit thickness for Cc a and Cd b . Cc shows an almost linear relationship, whereas CdŽ .from medial to distal locations sections 6 to 8 does not show a thickness reduction as lithic dimension decreases.

Ž .the I–M region , followed by a decrease of bothlithic clast size and deposit thickness with distance

Ž .from the vent area the M–D region . Ignimbrite Cdshows a similar increase in both maximum lithicclast size and deposit thickness from proximal expo-

Žsures to the break in slope I–M region; sections 2 to.6 , but, in contrast, decreasing lithic size does not

correspond to a decrease in deposit thickness furtherŽ .downslope sections 6 to 8 .

3.4. Standard deÕiation of the grain size Õs. distancefrom Õent area

Samples for grain size analysis have been takenfrom the very base and 1.5 m above the base for


Fig. 12. Standard deviation of matrix grain size vs. distance from the vent area. Samples have been taken from the very base of each unitand at about 1.5 m above the base. Note that lowest values occur between sections 2 and 6, which approximately correspond to the M region

Ž .for each of the analysed ignimbrites see text .

Ž .each ignimbrite cf. Fig. 1 . The standard deviationof the grain size fraction -1 mm is compared with

Ž .distance from the vent area Fig. 12 . Values equal toor lower than 2.6 occur only at the site of the breakin slope, between sections 3 and 6, indicating arelative better sorting of the matrix at those loca-tions.

4. Discussion

4.1. The effect of topographic irregularities ondownslope lithic distribution and deposit character-istics

Ž .On stratovolcanoes like Roccamonfina Fig. 2 ,the main break in slope of the edifice may exert a

Žfundamental influence on pyroclastic flows e.g.,Clark, 1984; Freundt and Schmincke, 1985; Robool

.et al., 1987; Giordano and Dobran, 1994 . WTTCupa ignimbrites are absent on the upper slopes of

Ž .the volcano Figs. 1, 7 and 8 , where the slopeaverages 158. It is unlikely that this absence is due topost-depositional erosion, as Ignimbrites Cc and Cdonlap the topography at their most proximal deposi-

Ž .tional site Figs. 1, 3 and 6 . The pyroclastic flowsalong the upper slopes were nondepositional andfully competent to transport the sediment load. Ign-imbrites are present downslope of the area of the

Ž .main break in the edifice slope Fig. 8 , where theyare thickest and contain the maximum lithic sizes.

The coupled occurrence of maximum lithic size andof maximum thickness suggests that pyroclastic flows

Žsuddenly decreased in capacity, as capacity i.e., theability of a flow to carry a certain amount of sedi-ment load irrespective of particle sizes; Hiscott,

.1994 , and not competence, is the parameter that cancontrol the simultaneous deposition of particles of

Ž .any size Hiscott, 1994 . A decrease in competenceof the flow would instead lead to deposition of a lagof lithics or a co-ignimbrite breccia at the break in

Žslope e.g., Freundt and Schmincke, 1985; Robool et.al., 1987 , but their occurrence bears no relationship

with the ignimbrite thickness. By contrast, Ign-imbrites Cc and Cd increase their thickness and theirmaximum lithic clast from their most proximal expo-

Ž .sure to the area of the break in slope Fig. 11 ,suggesting that maximum lithic size and thicknessare related to the same process of loss of capacity.Note that a simultaneous loss in competence is alsopossible.

The other important characteristic of the ign-imbrites at the break in slope area is that they aremassive. A possible explanation is that flows sud-denly decelerate at the break in slope inducing lossof capacity, in turn leading to increase the mixtureconcentration, and potentially to the development of

Ž .a hindered settling zone Druitt, 1995 . Sedimenta-Žtion rate may increase i.e., a rapid upward move of

.the depositional boundary layer, Fig. 13 , inhibitingŽdevelopment of layering cf. Kneller and Branney,

.1995 . However, experimental data on hindered set-


Fig. 13. Cartoon illustrating the possible effect of the break in slope on pyroclastic flows. Proximally, adjacent to the by-passing zone,tractional sedimentation causes layering, very well-rounded pumices and clasts imbrication. The gas-particles mixture is almost coupled andvelocity vectors are almost parallel. Breaks in slope induce loss of flow capacity and flow deceleration. As a result, particle sedimentation

Ž . Ž .rate increases i.e., particle velocity vectors are more inclined toward the topography , whereas gas and matrix is much less affected by theŽ . Ž .flow deceleration i.e., gas velocity vectors remain parallel to flow direction , resulting in a net upward gas and matrix flux, and then

Ž . Ž .self-fluidisation. The deposit surface dbl rises, more or less rapidly dbl2 and dbl1, respectively , and if proper mixture concentration isreached, hindered settling sedimentation takes place producing massive deposits, and possibly fines depletion. Further downslope, two

Ž .options have been reasoned. 1 As velocity and suspended sediment load progressively decreases, unsteadiness condition may arise andtractional sedimentation occurs. Deposit thickness thins with distance as well as lithic dimension. Time-lines within the deposit are almost

Ž . Ž .parallel to or more inclined than the topography T1–T4 . 2 If the hindered-settling zone affects most of the flow’s thickness, the flow maytravel independently as a very high concentration suspension under its own momentum. Increasing particle concentration promotes theeffective viscosity to increase, inhibiting further development of layering. Deposit thickness remains constant with distance, and norelationship between lithic clast dimension and thickness are expected. Time-lines within the deposit are less inclined than topography in

Ž . Ž .distal areas T1–T3 and almost parallel to the topography in proximal areas T3–T4 .

Ž .tling Druitt, 1995 concern vertically moving parti-cles and fluid, modeling stationary deposition ratherthan deposition during flow. Numerical simulations

Žof pyroclasts and gas dispersions Dobran et al.,.1993; Giordano and Dobran, 1994 have shown that

instantaneous velocity vectors for the two phases


diverge, the particles being more affected by gravityŽ .Fig. 13 . As a result, in real pyroclastic flows, a net

Ž .upward directed gas flux cf. Dobran et al., 1993Ž .occurs Fig. 13 , providing the pyroclastic flow with

an internal source of self-fluidisation. It is proposedhere that, as the break in slope induces flow deceler-ation and loss of flow capacity, the particle sedimen-

Žtation rate increases i.e., particle velocity vectors are.more inclined toward the base of the flow , whereas

Ž .gas and matrix is much less affected by flowŽdeceleration i.e., gas velocity vectors remain paral-

.lel to flow direction , resulting in a net upward gasŽ . Ž .and matrix flux Fig. 13 . Such a process canexplain the large clasts deposition, and its locationjust downslope of areas of main break in slope, themassive facies of the deposits, and possibly the

Ž .better sorting of the matrix Fig. 12 , due to finesŽdepletion cf. sedimentation-induced fluidisation of

Ž .Druitt 1995 . It is worth mentioning that the closeassociation of maximum lithic deposition, maximumdeposit thickness and fines depletionrgas segrega-

Žtion pipes characterises also other deposits e.g.,Tuscolano Artemisio’s second pyroclastic flow unit;

.De Rita et al., 1988; Giordano and Dobran, 1994and again in correspondence of break in slope androrpaleovalleys, i.e., where favorite conditions for hin-dered settling sedimentation may arise.

At the site of break in slope, ignimbrites locallyoverlie a scoured substrate. Pyroclastic flows mayerode at the break in slope as a result of the dynamicpressure exerted during the impact with the less

Žinclined topography see Giordano and Dobran,.1994 . The energy loss due to the erosion of the

substrate may contribute to the decrease in flowvelocity and hence capacity, immediately downslopeof the break in slope.

Lithic clast sizes decrease both in upslope anddownslope directions away from the break in slope.This downslope lithic distribution pattern has beencalled the downslope increasermaximumrdecreaseŽ .I–M–D pattern. This I–M–D pattern can be dis-cussed in terms of nonuniformity of pyroclastic cur-rents induced by the break in slope, which maypromote the flows to change from accumulative todepletive. ‘Accumulative’ and ‘depletive’ are terms

Ž .proposed by Kneller and Branney 1995 to describeflows characterised by a positive or a negative down-flow velocity gradient respectively. The rapid in-

crease in lithic clast dimension from the I to the MŽ .region Figs. 9 and 10 coupled with the absence of

the ignimbrites from the upper slopes of the volcanoedifice can be seen as a result of an accumulativecurrent which is accelerating along the steep upper

Ž .slope of the volcano Fig. 8 . The M region repre-sents a ‘singular point’ related to the break in slopeas discussed above. The M–D region appears toreflect, instead, the deposition from a depletive cur-rent, as the region of break in slope induces thepyroclastic flow to decelerate. Note that a densitycurrent can be depositional irrespectively of whether

Žit moves uniformly or not Kneller and Branney,.1995 .

( )4.2. Downslope but no or little Õertical lithic clastsgrading: progressiÕe aggradation of ignimbrite Cc

Ignimbrite Cc is characterised by downslope, butnot vertical lithic grading, that is, at each location,lithic clast dimension at the base and top of the

Ž .ignimbrite, are approximately equal Fig. 9c . Such alithic clast distribution can be interpreted either in

Žterms of a progressively aggraded ignimbrite Bran-.ney and Kokelaar, 1992 , where there is steady

supply of lithic material at the source, or in terms ofŽ .the pyroclastic flow model of Sparks 1976 , where

the internal yield strength is sufficient to hinder anygravitational settling of lithics. However, as shownbelow, the lithic distribution pattern, facies varia-tions, and paleotopography, show an interrelation-ship which places constraints on the likely emplace-ment mechanism.

The original geometry of Ignimbrite Cc is well-preserved, as it is everywhere capped by IgnimbriteCd. Ignimbrite Cc appears as a wedge. This geome-try is not consistent with a deposit related to a high

Ž .yield strength flow plug flow . In plug flows, inter-nal yield strength hinders the distal thinning of thedeposit, which is in contrast characterised by con-

Žstant thickness and a steep front e.g., Johnson,.1970 . The upslope and downslope wedging of the

deposit is, instead, in agreement with depositionfrom density currents which deposit progressively

Žthe suspended sediment load cf. Fisher, 1966; Bran-.ney and Kokelaar, 1992; Druitt, 1992 , as well as the

Žpositive correlation of thickness and maximum and. Ž .average lithic size Fig. 11a .


The ignimbrite is stratified where it is onlappingthe pre-existing topography in proximal locationsŽ .section 5; Fig. 3 . This records the aggradation ofthe deposit, probably under traction condition. Thedownflow transition from proximal stratified ign-

Žimbrite to massive at the break in slope section 6;.Fig. 4 is interpreted to record high sedimentation

rates from a steady or quasi-steady sustained pyro-clastic current. The lack of vertical lithic gradingsuggests constant, or almost constant, feeding condi-tions at the vent. The decrease in suspended sedi-ment load of the current in distal areas allows for

Ždevelopment of layering and bed-forms Druitt,.1992 . This can explain the formation of inversely

Ž .graded beds in distal areas Fig. 5 . Decreasingparticle concentration of the current also agrees with

Ž .the decrease in deposit thickness Fig. 13, case 1 .The topmost 15 cm of Ignimbrite Cc at medial

Ž .locations are finer in grain size section 6; Fig. 4 ,and at distal location, the deposit is a succession ofinversely graded layers with each layer which be-

Ž .come thinner upsection section 7; Fig. 5 . The fin-ing upward grainsize at the very top of the ign-imbrite is interpreted to represent the final stage ofdeposition, as expected for a progressively aggradeddeposit. A sustained depositional current, in fact,will of necessity eventually transform into waningstage flow characterised by decreasing velocity,

Žcompetence and sediment load cf. Kneller and Bran-.ney, 1995 .

4.3. Downslope and Õertical lithic grading: the prob-lem of ignimbrite Cd

Interpretation of the emplacement mechanism forIgnimbrite Cd remains somewhat equivocal. The ign-imbrite characteristics are similar to those ascribed to

Ž .the pyroclastic flow model of Sparks 1976 for theŽ .emplacement of ignimbrites, such as: 1 downslopeŽand vertical normal lithic grading Fig. 10c; cf.

. Ž .Palladino and Valentine, 1995 ; 2 a tabular depositŽ .geometry; 3 only a massive facies is observed; and

Ž .4 there is no relationship between deposit thicknessand maximum lithic size beyond the break in slopeŽ .Fig. 11b .

However, the formation of massive ignimbrite canalso be explained in terms of the progressive aggra-

Ž .dation model Branney and Kokelaar, 1992, 1997 ,

where the lithic clast distribution is interpreted toreflect time-related variation of lithic supply at thesource. Also erosion of the distal facies of the ign-imbrite appears likely, so that the downslope lithicclast grading and ignimbrite lateral facies and thick-ness variations cannot be fully appreciated.

The lithic clast distribution within Ignimbrite Cdsuggests that areas of break in slope controlled theflow capacity, as for Ignimbrite Cc. This is of coursenot theoretically in contrast with a conventionalmodel, but in such a case, the effects of a break inslope would be transferred further downslope, as thepyroclastic flow passes by. At the break in slopeIgnimbrite Cd, also shows the widest range of lithic

Ž .clast sizes Fig. 10a,b , which suggests the suddendeposition of the whole particle load, in agreementwith a loss in flow capacity. However, at the site ofthe break in slope, a vertical normal lithic grading

Ž .develops, which is ‘preserved’ downslope Fig. 10c .A possible explanation for these grading patternsmay be that the Ignimbrite Cd-related pyroclasticflow first impacted and eroded the pre-existing to-

Žpography at section 4 which is 0.5 km further.upslope than for Ignimbrite Cc , losing capacity and

then segregating lithics to the base, and dumping outŽalmost its entire sediment load i.e., the depositional

boundary layer rapidly moves upward; cf. dbl2 in.Fig. 13 . The sudden increase in concentration at the

base of the pyroclastic flow may produce normalŽ .lithic grading via hindered settling Druitt, 1995 . If

the hindered settling zone becomes thick enough toŽ .travel under its own momentum see Druitt, 1992 ,

the aggrading pyroclastic flow would have eventu-ally transformed into a ‘conventional’ pyroclasticflow further downslope, at proper particle concentra-tion, effective viscosity, and if enough gas is retained

Ž .within particles Fig. 13; case 2 . The normal lithicgrading developed within the break in slope areawould be preserved during laminar flow downslope,whereas the downslope lithic grading may be theresult of sinking through a laminar pyroclastic flowŽ .Palladino and Valentine, 1995 , andror of variationin the lithic supply, or of the initial downslope lithicdistribution inherited by the pyroclastic flow fromthe ballistic lithic distribution present in the collaps-

Žing eruptive column Fisher, 1979; Valentine and.Wohletz, 1989 . In such a case, the imaginary lines

joining particles through the ignimbrite deposited at


the same time would be less inclined than topogra-Žphy, downslope of section 6 Fig. 13; time-lines

.T1–T3 of case 2 . Upslope of section 4, the ign-imbrite would be, instead, still aggrading and thentime-lines should be almost parallel to the topogra-

Ž .phy Fig. 13; T4 of case 2 . The area of the break inslope comprised between sections 4 and 6 would be

Ž .a transitional region Fig. 13; T3 of case 2 . ThisŽinterpretation implies flow transformation cf. Fisher,

.1983 , and also presents spatial problems within thedeposit calling for other field evidence like ramping

Žor stretching structures cf. Cole et al., 1993; see alsoBranney and Kokelaar, 1992, for a full discussion of

.spatial problems .An alternative could be that Ignimbrite Cd was

aggraded along its entire extent where depositionwas dominated by hindered settling due to a particleconcentration at the base of the flow capable toincrease the effective fluid viscosity, and inhibitedthe development of bedding or tractional structuresŽ .Druitt, 1992; Kneller and Branney, 1995 . Thiscould better justify the downslope lithic I–M–Ddistribution pattern both at the base and top of theunit. However, the tabular geometry of the Ign-imbrite Cd and the absence even at distal locations

Žof any bedding cf. downcurrent decrease in sus-.pended sediment load, Druitt, 1992 , as well as the

absence of a fining upward of the average grain size,need to be explained as a result of post-depositionalerosion.

A corollary of this discussion is that unless we aresure that we are dealing with the entire originalextent of an ignimbrite, andror the field character-istics are unambiguous, it is speculative to infermodels for transport processes as the deposit charac-teristics of massive ignimbrites can be interpreted in

Ža number of ways e.g., Sparks, 1976; Wilson, 1985;Valentine, 1987; Branney and Kokelaar, 1992; Pal-

.ladino and Valentine, 1995 .

5. Conclusions and summary

Lithic clast distribution is a useful tool to interprettransport processes from deposit characteristics, inparticular, comparing the lithic distribution with the

Žfacies association which comprises the whole de-

.posit geometry and how these two characteristicsrelate to paleotopography.

A main conclusion is that the main break in slopeof a stratovolcano may exert a fundamental influenceon pyroclastic flow capacity. No ignimbrites have

Ž .been observed along upper and steep ;158 slopesof Roccamonfina volcano. The analysed ignimbrites

Žshow a downslope increasermaximumrdecrease I–.M–D pattern of the average lithic size values. This

is interpreted to reflect the combined effect of thedowncurrent variation of the flow capacity and inter-action with the pre-existing topography. In particu-lar, locations where ignimbrite are thickest and con-tain the maximum lithic sizes correspond with areasof breaks in slope. The downslope increase in lithicsizes from the most proximal ignimbrites’ exposuresdown to the area of the break in slope is interpretedto correspond to a positive velocity gradient, aspyroclastic flows accelerated along the steep upperslopes of Roccamonfina stratovolcano. The coupledmaximum lithic size and maximum thickness, occur-ring at the break in slope for all ignimbrites havebeen interpreted to reflect a decrease in capacity ofthe parent pyroclastic flows at such locations. Theignimbrites also are massive at the break in slope,and this is consistent with an increase in sedimenta-tion rate, and hindered settling conditions. The de-crease in lithic sizes further downslope of the area ofthe main break in slope has been interpreted toreflect flow deceleration and decreasing suspensionconcentration.

Ignimbrite Cc exhibits, besides downslope lithicgrading, no vertical lithic grading. This characteris-tic, coupled with the downslope thinning of theignimbrite, the downslope transitional facies changesfrom stratified in proximal areas, through massive in

Ž .medial locations that is at the break in slope , toinversely graded beds in distal areas, are interpretedas reflecting a progressive aggradation of the deposit.The fining upward of the grain size in Ignimbrite Ccis interpreted to record waning flow conditions.

Ignimbrite Cd, by contrast, shows vertical as wellas downslope lithic grading. This characteristic, thewidespread massive facies of the deposit, and thetabular unit geometry can be discussed in the light ofa conventional pyroclastic flow model or of a pro-gressive aggradation model. However, none of themappears to completely satisfy the field evidence.


Another main conclusion is that for massive ign-imbrites, the depositional characteristics remainequivocal and neither a progressive aggradationmodel nor a plug flow analogy can be confidentlyapplied to a deposit.

Acknowledgements

This research was supported by a MURST schol-arship and partly by a University of Pisa grant.Helpful reviews by M.J. Branney, S. Bryan, A.Freundt are gratefully acknowledged. I wish to thankthe Volcanology and Sedimentology Group of

ŽMonash University R. Cas, S. Bryan, S. Allen, L.. ŽMoore, C. Scutter , D. De Rita and S. Milli Univer-

.sity of Rome for discussion. Two Honour studentsof the University of Rome, Ms. I. Leschiutta and Mr.P. Miele, provided assistance in the field and inlaboratory.

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Documents

The effect of paleotopography on lithic distribution and facies associations of small volume ignimbrites: the WTT Cupa (Roccamonfina volcano, Italy)