Facies associations of rain-generated versus crater lake-withdrawal lahar deposits from Quaternary volcanoes, central Italy

Journal of Volcanology and Geothermal Research 118 (2002) 145^159 www.elsevier.com/locate/jvolgeores

Facies associations of rain-generated versus crater lake-withdrawal lahar deposits from Quaternary volcanoes, central Italy G. Giordano  , D. De Rita, M. Fabbri, S. Rodani Dipartimento di Scienze Geologiche, Universita' degli Studi ‘Roma TRE’, L.go S. Leonardo Murialdo 1, 00146 Roma, Italy Received 13 December 2000; accepted 12 December 2001

Abstract Two syneruption lahar deposit successions from Quaternary Italian volcanoes are presented, displaying different facies associations interpreted to reflect different water sources. The lahar deposits associated with the White Trachytic Tuff Cupa (WTTC) ignimbrites from the Quaternary Roccamonfina volcano, located 150 km to the southeast of Rome, have been interpreted in terms of rain-generated lahars. The WTTC ignimbrites are made of more than 1 km3 of loose pumice and lava lithic debris emplaced along the hyperbolic slope of the volcano at ca. 300 ka during an interglacial period characterised by mild and wet climate. The lahar deposits are organised in a coarseningupward, aggradational, and back-stepping succession of medium- to thick-bedded, progressively juvenile-poorer, noncohesive debris flow to fluvial deposits. Box-shaped channels cut the WTTC ignimbrites along the steep upper slopes. Channels are filled with lava lithic-rich fluvial to hyperconcentrated-flow sand and conglomerate, which are interpreted as lag deposits related to processes of bulking due to the removal of light pumice and ash debris from the upper slope. Along the lower slopes of the volcano and in the surrounding ring plains where the average slope inclination decreases to few degrees, lahars emplaced an aggradational succession of bedded, ash-rich, hyperconcentrated-flow deposits entirely derived from WTTC components. The succession coarsens upward with increasing presence of lava-rich conglomerate lenses, fluvial in origin, interpreted to record the progressive restoration through time of the drainage network. The succession is cut by incised gullies filled with polygenetic fluvial deposits which indicate the restoration of intererruption condition. By contrast, the ca. 23-ka, small-volume, Peperino Albano phreatomagmatic eruption from Colli Albani volcano, located 30 km to the southeast of Rome, emplaced a valleyponded, block and ash ignimbrite, which, along the western slope of the volcano, grades laterally into a single, farreaching, thick lahar deposit. The lahar deposit coarsens upward from coarse-ash, hyperconcentrated-flow deposit into a lithic-block-rich, debris-flow deposit. This lahar deposit has been interpreted to be directly derived from a pyroclastic flow and particularly related to the entrance of the pyroclastic flow into a pre-existing maar crater lake along the pyroclastic-flow path. The basal sand-size, hyperconcentrated-flow deposit is interpreted to represent early deposition from the fast frontal flood wave, whereas the coarse lithic-rich debris-flow deposit at the top may represent the rear of the lahar. The separation of the two facies can be related to processes of ‘hydraulic sieving’ operated by the lake water, which couples with ash particles, leaving behind the coarser fraction. 5 2002 Elsevier Science B.V. All rights reserved.

* Corresponding author. Fax: +39-06-54888201.

E-mail address: [email protected] (G. Giordano).

0377-0273 / 02 / $ ^ see front matter 5 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 2 5 4 - 8

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Keywords: lahar; debris £ow; hyperconcentrated £ows; facies association; crater lake; hazard

1. Introduction Lahars are rapidly £owing, volcaniclastic sediment^water mixtures originating on or near volcanoes during or following eruptions (Smith and Fritz, 1989; Smith and Lowe, 1991). Lahar events associated with explosive eruptions are the most hazardous (e.g. Blong, 1984; Scott, 1989; Simkin and Siebert, 1994; Scott et al., 1995; Major et al., 1996; Rodolfo et al., 1996; Mastin and Witter, 2000). The rapid emplacement of loose pyroclastic debris on the landscape induces the abrupt transition from normal sedimentary condition (or intereruption condition) to quickly aggradational condition with removal of debris from upslope and its redistribution farther downslope, in the attempt to restore intereruption condition (Smith, 1991a,b). This perturbation of the normal sedimentary condition is called syneruption condition and includes not only the time interval of the eruption but also the relatively short period following the cessation of the activity (Smith, 1991a,b). ‘The transition from normal sedimentary condition and syneruption condition is very abrupt. The transition from syneruption to inter-eruption conditions proceeds through a series of adjustments in the geomorphic systems as sediment load and discharge variability diminishes’ (Smith, 1991a,b, p. 112). Direct observation and understanding of lahar events have increased since the 1980 eruption of Mt. St. Helens (Janda et al., 1981; Scott, 1988, 1989) and broadened during and after the 1991 eruption of Mt. Pinatubo (Major et al., 1996; Rodolfo et al., 1996). Lahars may occur during explosive eruptions when pyroclastic £ows or fall admix with water of crater lakes, snowmelt, and rivers. Lahars triggered by normal (or exceptional) rainfall may occur during the eruption and for several tens to hundreds of years after (Smith, 1991a,b). Facies associations of lahar deposits vary depending upon the type and size of the eruption, type of water involved, climate condition and topography. This paper focuses on the facies description of

two lahar deposit successions from Quaternary Italian volcanoes. The two lahar successions show di¡erent facies associations that have been used to evaluate the relative importance of climate and type of water involved, topography, volume and areal distribution of primary pyroclastic deposits as triggering factors. A facies model is proposed for lahar deposits related to the lateral transformation of pyroclastic £ows due to catastrophic mixing with crater-lake water and for lahars successions related to the post-depositional, climate-driven restoration of the £uvial network.

2. Terminology A lahar is an event; it can refer to one or more discrete processes such as debris £ows and hyperconcentrated £ows (Smith and Fritz, 1989). Hyperconcentrated £ows and debris £ows form a continuum of £ow processes (Smith and Lowe, 1991; Scott et al., 1995). During syneruption condition, clay-size particles available are mostly the ¢ne ash fraction of pyroclastic deposits, which, unlike clay minerals, have no adhesive properties. For this reason debris £ows generated during syneruption condition are generally non-cohesive (Scott et al., 1995). Non-cohesive debris £ows (less than 3^5% of clay by weight relative to the matrix fraction) are granular £ows that begin mainly as stream £ows then bulk by incorporating sediment to form ¢rst hyperconcentrated £ows and then a debris £ow; distally the sequence of £ow types reverses (Rodolfo and Arguden, 1991; Scott et al., 1995). Non-cohesive debris £ows leave deposits that are signi¢cantly thinner than the £ows themselves, and deposition does not occur en masse (Smith and Lowe, 1991). Volcanic, non-cohesive debris-£ow deposits share some facies characteristics with non-volcanic and cohesive debris-£ow deposits, that is a massive structure and a texture of commonly dispersed clasts, pebble-size or coarser, in a ¢ner-grained granular matrix (Smith and Lowe, 1991; Scott et al., 1995). Hyperconcentrated £ows are non-Newtonian

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£ows with little or no internal strength, turbulent enough to allow a degree of di¡erential settling of particles (Smith and Lowe, 1991). Hyperconcentrated-£ow deposits are generally sand-size to cobble-size, massive or crudely parallel strati¢ed, ungraded, normal or reverse graded, with reverse grading of light particles.

3. Lahar deposits related to the 300-ka White Trachytic Tu¡ Cupa (WTTC) eruption from Roccamon¢na volcano 3.1. Stratigraphy of the WTTC, topographic setting and climate The stratigraphy of the White Trachytic Tu¡ of Cupa is described in detail by Giordano (1998a,b) and De Rita et al. (1998). The WTTC was erupted from the Middle to Upper Pleistocene Roccamon¢na volcano at ca. 300 ka (Giannetti, 1990) during a stage of mostly explosive activity of the volcano which lasted from 385 to 230 ka (De Rita and Giordano, 1996). Roccamon¢na volcano

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is part of the back-arc-related, Quaternary to Recent, potassic volcanic belt that runs NW trending along the Tyrrhenian margin of the Italian peninsula and to which also the more famous Mt. Vesuvius belongs. The WTTC is composed of a succession of pumice-rich, unwelded ignimbrites overlying a widely dispersed base-surge deposit. The WTTC eruption emplaced more than 1 km3 of loose volcanic debris over an area of more than 200 km2 . The WTTC products are made of ash, porphyritic and well-vesiculated pumice, broken crystals of sanidine, clinopyroxene and biotite, and variable proportions of di¡erent lava xenoliths that range in composition from leucitite to trachyte. The WTTC onlaps the slopes of Roccamon¢na volcano (Fig. 1), which levels down from the 20‡ of inclination of the upper reaches (the highest point is 933 m a.s.l.) to the few degrees of inclination along the lower slopes and the surrounding ring plain at the sea level. The WTTC completely buried the middle and lower reaches of the volcano, destroying all vegetation to a distance of 10 km from vent, as indicated by the ubiquitous presence in the basal deposits of abun-

Fig. 1. Distribution map of the WTTCupa ignimbrite and related lahar deposits along the SW slope of Roccamon¢na volcano.

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dant stricken tree moulds, aligned with the £ow direction. Ignimbrites show lateral and vertical grading of components, and, in proximal areas, coarse facies are common and co-ignimbrite breccia is present, whereas distal outcrops are mostly made of ash. The breccia is made of decimetre- to metre-size, angular to rounded blocks of lava, leucitite to trachyte in composition. The WTTC was emplaced during a high stand of the sea level (stage 9 of the oxygen isotopic scale, Shackleton et al., 1990), suggesting a warm and humid climate, as is also suggested by the many tree moulds present at the base of the WTTC and the underlying thick and mature dark brown palaeosol (Giordano, 1998b). 3.2. Facies association of the lahar succession The lahar deposits associated with the WTTC are monogenic in that they derive entirely from the WTTC components (ash, pumice, crystals and lithics) and show signi¢cant lateral facies variation according to the palaeotopography and facies of the WTTC source (Fig. 2). 3.2.1. Proximal facies Along the steeply inclined upper slopes of the volcano, several cross-cutting, box-shaped channels cut the coarse and lithic-breccia-rich WTTC down to the underlying palaeosol (Facies B in Fig. 2A,B). Channels are ¢lled with sand- to cobble-size, lava-clast-rich, well-sorted deposits (Fig. 3A). The deposits are massive to gently crossstrati¢ed, and show normal grading of individual beds. Clasts range from angular to subrounded. Lava clasts compose more than 60% of the deposits (leucitite to trachyte in composition), the rest being crystals and subordinate pumice and ash. Downslope, sand-size, cross-bedded lithofacies are more common than conglomerate lithofacies (compare Fig. 2A,C). 3.2.2. Interpretation The deposits can be interpreted as £uvial and hyperconcentrated-£ow conglomerate and sand. The origin of such coarse and lithic-rich sediment at proximal locations may be related to processes of bulking of still dilute lahars. Bulking progres-

sively removed the light component of the proximal WTTC, i.e. mainly pumice and ash, but £ows were not able to carry most of the heavy, coarse and abundant lithic component, which therefore only travelled a short distance. This interpretation is supported by the similarity in lava clast, composition, size and rounding between the lahar deposits and the parent co-ignimbrite breccia of the WTTC. The box shape of the channels also suggests erosion by sediment-loaded, braided-stream £ows (Allen, 1982). 3.2.3. Medial facies Along the middle slopes, where the average inclination decreases to a few degrees, the WTTC is cut by a low-relief erosion surface, gently inclined, but more than the underlying palaeoslope, so that at a distance of 8^10 km from vent, WTTC is eroded away completely. The volcaniclastic deposits that onlap this erosion surface (Fig. 2D,E) form a coarsening-upward succession. The lower part (Facies A, 4^5 m thick in Fig. 2D,E) is made of tabular, purely aggradational, medium-bedded (10^30 cm in thickness), ash- and pumice-rich sand and silt deposits, with subordinate lithic clasts (Fig. 3B). The juvenile component makes up to 90^95% of the deposit, 80% of which is coarse and ¢ne ash, the rest being rounded pumice clasts. Individual beds are massive and generally coarse-tail graded, with pumice concentration zones at the top and lithic concentration zones toward the base, which is generally reverse graded (Fig. 3B). Crude lamination is occasionally present (top of Fig. 3C). No bedforms have been seen. Density and water-escape structures are common (Fig. 3C,D). Upward, the succession becomes progressively coarser, with increasing presence of well-sorted conglomerate lenses and sheets, inter-bedded with and cut within aggradational ash-rich deposits of Facies A (Fig. 3E). The conglomerate lenses are made of subangular to subrounded lava cobbles and pebbles, and in¢ll erosion channels (Facies B in Fig. 2C^E). The juvenile component decreases to less than 50% of the deposit, decreasing further upward (topmost metre of section E, Fig. 2), where cross-bedded, lava lithic-rich sand and conglomerate are found (Fig. 3F).

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Fig. 2. Stratigraphic logs measured through the WTTCupa-related lahar deposits along the slope of Roccamon¢na volcano. Numbers refer to locations as indicated in Fig. 1. See text for explanation.

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3.2.4. Interpretation The deposits in Facies A show individual beds, at maximum few decimetres in thickness, that can be followed laterally for hundreds of metres with little variations. This evidence, along with the mainly massive structure of the deposits, the aggradational geometry of the succession and the occurrence of water-escape structures, suggest the emplacement from Newtonian, non-cohesive, sediment-loaded £ows that quickly dumped the volcaniclastic debris eroded upslope. The deposits in Facies A can therefore be interpreted as emplaced from hyperconcentrated £ows to non-cohesive debris £ows (cf. Smith and Lowe, 1991; Scott et al., 1995). This occurs at medial locations where the decrease in slope inclination probably induced a signi¢cant velocity reduction of the lahars (cf. Pierson, 1985). Lahars’ high particle concentration, at least during the ¢nal depositional stage (cf. Postma, 1986), would have promoted both the common reverse grading observed at the base of many beds, as a result of dispersive pressure (e.g. Lowe, 1982), and the coarse-tail grading of lithics (normal grading) and pumice (reverse grading), as a result of hindered settling. Particle concentration induces buoyancy forces which result in strong hydraulic inequivalence between light and dense components (Druitt, 1995). The deposits in Facies B, which show alternated lithic-rich conglomerate lenses and ash- and pumice-rich sand and silt beds, suggest the occurrence through time of highly energetic £ood events. The overall reverse grading of the succession and the upward decrease of the juvenile component, along with the upward increase in £uvial deposits (lavaclast-rich, channelled conglomerate and crossbedded sand) may re£ect the progressive decrease in ash and pumice debris available upslope as a result of the progressive restoration of the drainage network, and consequently processes of dilution through time (cf. Smith, 1991a,b). In a section radial to the volcano slope, Facies A+Facies B onlap an erosion surface cut on the WTTC deposits, suggesting that deposition of lahar deposits at any site follows the removal of part of the ignimbrites and therefore proceeds in a backstepping succession of erosion^deposition events. Early deposited Facies A would form as a conse-

quence of the removal of the medial and distal facies of the WTTC, whereas Facies B would represent the remobilization of progressively more proximal (and therefore coarser) parts of the WTTC. In three dimensions the lahar succession forms a fan, laterally onlapping lobes of WTTC ignimbrites. 3.2.5. Distal facies In the open ring plains surrounding the volcano, lahar deposits reach farther than the WTTC and rest directly on a dark-brown palaeosol. The deposits are a succession of tabular, aggradational, roughly reverse-graded beds, up to 3 m thick. Individual beds are 1^20 cm thick, massive- to di¡usely to plane-parallel- and cross-laminated, ash- and pumice-rich sand and silt beds (Facies A, Fig. 2F, bottom) alternated with well-sorted, lithic-rich conglomerate lenses, the presence of which increases upwards (Facies B, Fig. 2F, top). 3.2.6. Interpretation The succession is interpreted as deposited by a sequence of hyperconcentrated £ow to £uvial events in a braided river £ood plain environment, which took the place of the previously non-depositional environment indicated by the underlying palaeosol. The absence of signi¢cant erosion surfaces, and the aggradational and tabular overall geometry of the succession suggest that several £ood events occurred closely spaced in time and that the amount of redistributed volcaniclastic sediment was high, although lahars were more diluted with respect to medial locations, as suggested by the reduced thickness of individual beds and the common planar and cross-lamination. 3.3. Discussion: a succession of rain-generated lahars The facies association shown by the WTTC-associated lahars is characterised by the following aspects: (a) components of the lahar deposits derive from the parent WTTC ignimbrites ; (b) the succession onlaps a gentle ‘listric’ erosion surface carved on top of the WTTC ignimbrites;

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4m

Pcz

WTTC Lcz A

C

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D B F

Channelled cross-stratified juvenile-poor conglomerate and sand

De-watering pipe Tabular aggradational juvenile-rich beds

E

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Fig. 3. Lithofacies of the WTTCupa-related lahar deposits; (A) box-shaped channels ¢lled with lava lithic-rich conglomerate and sand cutting the WTTCupa ignimbrites at proximal locations (Tuoro di Sessa, log A of Fig. 2); (B) tabular aggradational succession of medium-bedded, juvenile-rich deposits; individual beds are coarse-tail graded with pumice concentration zones (pcz) developed at the top and lithic concentration zones (lcz) developed toward the base, above few centimetres of reversely graded division. Facies A, Masseria Frazzano (log E of Fig. 2); (C) density structures developed between a lower ash-rich bed and the upper lithic-rich bed. Facies A, Masseria Frazzano (log E of Fig. 2); (D) pipe-like de-watering structure ¢lled with pumice lapilli; note also the crude strati¢cation in the deposit at the top part of the picture; Facies A, Masseria Frazzano (log E of Fig. 2); (E) lithic-rich and juvenile-poor, conglomerate lenses inter-bedded with deposits in Facies A; Facies B, Masseria Marotti (log D of Fig. 2); (F) channelled £uvial conglomerate at the top of the lahar succession; the topographic surface at this location is also the terraced palaeo-depositional surface; Facies B, Lauro locality.

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Conglomerate

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VOLGEO 2490 11-10-02 Fig. 4. Model for the generation and emplacement of rain-generated lahars at Roccamon¢na after the WTTCupa eruption. (a) early syneruption stage: erosion starts at the distal end of the WTTCupa ignimbrites producing ¢ne-grained and juvenile-rich, hyperconcentrated and debris £ows; excess of debris produces aggradational succession of deposits; (b) late syneruption stage: the drainage network is progressively restored upslope and lahar deposits are inter-bedded with £uvial conglomerates; (c) intereruption stage: the restoration of the drainage network and the removal and redistribution of most of the pyroclastic debris allows the £owage of clean streams that re-incide the lahar deposits.

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(c)lahar deposits form an aggradational succession of several tens tabular beds; (d) early deposited Facies A is made of deposits almost entirely composed of ash and pumice that are interpreted as non-cohesive debris-£ow and hyperconcentrated-£ow deposits; (e) later deposited Facies B is made of coarse lenses and sheets of £uvial conglomerate interbedded with deposits in Facies A, and onlaps the WTTC ignimbrites upslope with respect to Facies A. Fig. 4 illustrates our interpretation of the evolution through time of the drainage system after the 300-ka eruption of the WTTC at Roccamon¢na volcano. Climate during interglacial stage 9 (Shackleton et al., 1990) would be expected similar to the one experienced today in central Italy, i.e. temperate, with seasonal abundant rains (up to 2000 mm/yr and occasional peaks of up to 120 mm/day at 800 m a.s.l.; Min.LL.PP., 1918^1993). Facies A corresponds to the emplacement of lahars during an early syneruption stage (Fig. 4a). Erosion starts from the distal end of the WTTC and proceeds progressively more upslope. This erosion pattern is to be related to the geometry of the WTTC that for the most part was emplaced on the low-inclined ring plain. Early lahars would therefore be loaded of abundant ¢ne-grained juvenile debris (which makes up the distal facies of WTTC), explaining the tabular and aggradational geometry, ash-rich composition and the debris- to hyperconcentrated-£ow-inferred transport mechanisms of deposits in Facies A. Individual beds are interpreted to correspond to single lahar events and therefore suggest the occurrence of several tens of events, similar to observation of rain-generated lahars at Mt. Pinatubo, where, for example, 73 events were recorded during the ¢rst year after the eruption along the St. Tomas River (Rodolfo et al., 1996). Transition from Facies A to Facies B would record the progressive reintegration of the drainage system (late syneruption stage in Fig. 4b), although the amount of debris available was still large, explaining both the aggradational geometry and the occurrence of £uvial conglomerate and sand. A comparative idea of the time involved in the deposition of Facies A+Facies B can be given by lahar processes at Mt.

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Pinatubo. After the 1991 eruption, the drainage network was reintegrated in two months and most of the lahars were generated during the ¢rst three years after the eruption (Major et al., 1996; Rodolfo et al., 1996). The transition from syneruption to intereruption stage (Fig. 4c) is marked by the abandon of the depositional surface at the top of Facies B (Fig. 3F), which therefore became exposed to weathering to produce the palaeosol at the top of the succession visible today. The succession was then reincised leaving morphologic terraces (cf. Smith, 1991a,b).

4. Lahar deposits related to the ca. 29-ka Peperino Albano eruption from the composite Albano maar, Alban Hills volcano 4.1. Stratigraphy of the Peperino Albano, topographic setting and climate The Peperino Albano is a small-volume (ca. 0.1 km3 of products), phreatomagmatic ignimbrite, leucitic in composition, which is the product of the last eruption of the composite Albano maar located along the western £ank of the Colli Albani volcano (Fig. 5; De Rita et al., 1988b, 1995a). Like Roccamon¢na, also the Colli Albani volcano is part of the back-arc-related, Quaternary to Recent, potassic volcanic belt that runs NW trending along the Tyrrhenian margin of the Italian peninsula. The last epoch of activity of the Colli Albani volcano (ca. 100^20 ka, De Rita et al., 1995a) has been characterised by only phreatomagmatic activity from many di¡erent maars, scattered along the western slopes of the volcano. All these centres emplaced mainly very small-volume, and closely dispersed surge and fallout deposits. The Peperino Albano is the only ignimbrite erupted from these maars. It is a highaspect-ratio and valley-ponded ignimbrite that reached 7 km from vent toward NW and more than 3 km toward the south. Age determinations for the Peperino Albano span between 36 and 19 ka (Fornaseri and Cortesi, 1989; Mercier, 1993), i.e. during the Wu«rm glacial period (oxygen isotope stage 2; Shackleton et al., 1990). At the time of the eruption the £ank of the volcano was cut

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Fig. 5. Distribution map of the Peperino Albano ignimbrite and related lahar deposits along the SW slope of the Alban Hills volcano.

by several deep ravines along which the pyroclastic £ows were channelled radially outward from the maar centre to a maximum distance of 4 km from vent. The Peperino Albano is made of 80% ¢ne and coarse juvenile ash and crystals (leucite, biotite, clinopyroxene), the rest being blocks of lava, intrusive and sedimentary xenoliths. The ignimbrite is generally massive and zeolitised when con¢ned in the valleys, where it is also up to 30 m thick (De Rita et al., 1988a; Giordano et al., 2002). On the maar rim and along valley ridges, the Peperino Albano is instead strati¢ed (Giordano et al., 2002).

4.2. Facies association of the lahar succession Along Fosso di S. Procula, a SE-trending valley draining out from the Albano maar to the Tyrrhenian sea, the Peperino Albano ignimbrite laterally grades into a lahar deposit con¢ned to the valley, as much as 10 m thick, which reaches approximately 15 km from vent (Fig. 5). The deposit comprises a basal sand-size, ashand crystal-rich (leucite, clinopyroxene, biotite), massively to poorly strati¢ed division with dewatering dish structures (Fig. 6A) grading upward into a block and boulder (lava, tu¡, intrusive and

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ash and lapilli tu¡ clasts (Fig. 6B). This lithology, generally not found within the lithic population of the Peperino Albano ignimbrite, is comparable with either one of the early phreatomagmatic units from the Albano maar or with the phreatomagmatic deposits from the nearby Giuturna maar. The strati¢ed clasts are generally found at the gradational contact between lower and upper divisions and toward the base of the upper division. The contact between the lower and upper divisions is generally gradational and undulate, i.e. no erosion surface can be identi¢ed, and it develops across a thickness of about 1 m, where the deposit is di¡usely strati¢ed with upward increase in outsized clasts of strati¢ed lapilli tu¡s, up to 1 m in diameter. The vertical succession remains laterally constant even at distal locations, more than 15 km from the maar centre, on a gently inclined palaeoslope (1%). Thicknesses of the upper and lower divisions change laterally.

A

B Fig. 6. (A) Peperino Albano lahar, lower division: sand-size, ash-rich hyperconcentrated-£ow facies with de-watering structures; Pomezia locality. (B) Peperino Albano lahar, upper division: lava-lithic-rich conglomerate facies with a large clast of strati¢ed lapilli tu¡; Pomezia locality.

sedimentary lithics), generally matrix-poor, massive and chaotic division (Fig. 6B). The lower division is 90% juvenile component (coarse ash) and crystals, and 10% xenoliths of lava and limestone. Component proportions within the upper coarse division change drastically. The conglomerate varies from matrix supported to clast supported. The matrix, where present, is made of a small proportion of ash and crystals and up to 50% of millimetre- to centimetre-size lava and sedimentary lithic fragments. Large clasts ( s 64 mm) up to 1.3 m in size make up to 50% of the deposit, and at least 30% of this fraction is made of strati¢ed

4.2.1. Interpretation The succession was emplaced by a single lahar event, grading from hyperconcentrated £ow to debris £ow. This interpretation is based on the evidence that no clear stratigraphic surfaces can be identi¢ed within the lower and upper divisions. In addition, the sand constituting the lower division also constitutes part of the matrix of the coarse upper division, suggesting a drastic but gradual upward evolution from a lahar carrying almost only sand-size particles into a lahar carrying almost only cobble particles. Grain-size, structure and texture of the lower part are suggestive of deposition from a hyperconcentrated £ow (Smith and Lowe, 1991). The generally massive structure can be related both to a high rate of sedimentation (Druitt, 1995) and to large abundance of ash in the parent Peperino Albano ignimbrite (cf. Smith and Lowe, 1991), so that the lahar was in origin loaded with mostly siltand sand-size fraction made of ash and crystal fragments. The upper conglomerate is an intriguing deposit as it is generally massive and ungraded with buoyed large clasts, sharing the facies of debris-£ow deposits (e.g. Scott et al., 1995); however, it is also clast supported and was depos-

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ited as far as the lower unit, suggesting mechanisms of hyperconcentrated-£ow transport (Arguden and Rodolfo, 1986; Smith and Lowe, 1991) or at least the presence of large amount of water during the transport. 4.3. Discussion: a catastrophic lahar and implications for hazard assessment The facies association shown by the Peperino Albano-associated lahars is characterised by the following aspects : (a) components of the lahar deposits are derived from the parent Peperino Albano ignimbrites, with the noticeable exception of large clasts of strati¢ed ash and lapilli tu¡s within the coarse upper unit;

(b) the lahar is laterally contiguous to the Peperino Albano ignimbrite and never overlies it; (c) the lahar deposit is made up of two divisions that share the same components; the contact between the two is gradational ; (d) the lower division is almost entirely composed of ash and crystals and was deposited by a hyperconcentrated £ow; (e) the upper division is a coarse sheet of matrix- to clast-supported, lithic-rich conglomerate that can be interpreted to be emplaced by a hyperconcentrated to debris £ow. Fig. 7 illustrates our interpretation of the formation and emplacement of the Peperino Albanoassociated lahar. We believe that the two divisions are part of one single lahar event that rapidly segregated an ash-rich hyperconcentrated £ow

Fig. 7. Model for the generation and emplacement of the Peperino Albano lahar. The Peperino Albano pyroclastic debris mixes with large amount of water from either the Albano maar crater lake or the Giuturna maar crater lake. Water sieves the ash particle fraction and forms a frontal hyperconcentrated £ood £ow, then followed by a lithic-rich debris £ow which carries the coarse particle fraction of the Peperino Albano ignimbrite.

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carrying the ¢ne juvenile fraction of the Peperino Albano ignimbrite from a debris £ow carrying the coarse lithic fraction. To infer the origin of water it must be taken into account that the Peperino Albano ¢lls a valley just upslope the breached maar of Giuturna (Fig. 5 ; Servizio Geologico Nazionale, 1962). The occurrence of ‘exotic’ blocks of lapilli tu¡ within the lahar similar to those forming the Giuturna maar rim, led us to envisage the Giuturna crater lake breaching as the possible mechanism. The Peperino Albano would have ¢lled the Giuturna lake contributing to the slope failure of one £ank of the maar. The large amount of water available would have promoted a process of coupling between water and the ash fraction that we name ‘hydraulic sieving’, in accordance with the fact that particles of di¡erent sizes dispersed within a £uid have di¡erent hydraulic behaviour and are subject to di¡erent mechanisms of transport (Lowe, 1982). This process accounts for the formation of a fast frontal hyperconcentrated £ow that was channelled in the Santa Procula valley and deposited the lower division. The upper division would have been emplaced by the rear of the lahar impoverished in the light ash component and loaded with the lithic component of the Peperino Albano ignimbrite plus the large strati¢ed ash and lapilli tu¡ clasts deriving from the Giuturna maar rim. The distal end of the lahar deposit is not presently exposed (likely the lahar reached farther than the present coast line), so we cannot ascertain the occurrence of slope-failure-related blocks at the front of the deposit. However, lapilli tu¡ clasts are generally found at the gradational contact between lower and upper divisions. This position can be explained by processes of saltation and rolling of the large lapilli tu¡ clasts along a rheological depositional surface developed within the fast aggrading lower division (cf. Postma et al., 1988). According to this interpretation, the lapilli tu¡ clasts were incorporated at the front of the lahar and lifted to the present position during transport and sedimentation. An alternative source of water for this lahar is the crater lake water that ¢lls the Albano maar, i.e. the vent for the Peperino Albano ignimbrite. The Albano maar is composite and four strati¢ed

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ash and lapilli, phreatomagmatic units separated by palaeosols underlie the Peperino Albano. The eruption of the Peperino Albano was therefore likely to have occurred in a crater lake, as it is today. The lahar represented the mixture of the erupting tephra with crater lake water and its only south^westward distribution the result of a directional £ow. The large amount of water-prevented deposition along the upper slope of the Albano maar promoting turbulence within the lahar, and the lapilli tu¡ clasts were rip-up clasts of one of the underlying four phreatomagmatic units. The lahar was therefore the earliest unit erupted during the Peperino Albano eruption and would represent one of the few documented lahars directly extruded from vent (cf. Sheridan and Wohletz, 1983). A ¢nal important remark concerning hazard assessment is due. The Peperino Albano lahar deposit, whatever its interpretation, indicates that maar-related eruptions in the Roman area can trigger catastrophic and far-reaching lahar events. The time interval between today and the Peperino Albano eruption spans between 36 and 19 ka. This time interval is comparable with the quiescence periods shown by the chronostratigraphic record of the volcano, which therefore must be considered quiescent and not extinct (De Rita et al., 1995b). This notion, together with the persistent presence of hydrothermal and seismic activity in the area (cf. Giordano et al., 2000 and references therein), suggests that, however small, there is potential for future activity. The presence of many crater lakes in the area is therefore a potential hazard for the highly populated Roman hinterland and should be carefully considered for civil protection purposes.

5. Conclusions : catastrophic versus climate-driven lahar deposits In this paper we have presented two lahar deposit successions from Quaternary Italian volcanoes in di¡erent eruptive, climatic and topographic settings. The facies association of these lahar deposits show important di¡erences that can be referred to di¡erent triggers and can be summarised as follows.

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The WTTCupa lahar deposits are a coarseningupward and back-stepping succession of several low-aspect-ratio aggradational deposits onlapping an erosion surface cut on the WTTC, and forming aprons in the ring plain surrounding Roccamon¢na volcano. The lahar succession developed in a wet climate. The succession records post-eruption processes of reintegration of the ‘normal’ £uvial drainage condition in a humid environment; transport and deposition are controlled by retrogradational erosion processes and progressive ‘clearing’ of the £owing water. The excess of pyroclastic debris is redistributed in aggradational aprons wherein is possible to distinguish early syneruption deposits characterised mainly by hyperconcentrated £ood £ow deposits with s 90% of juvenile and late syneruption deposits characterised by upward increase of conglomerate lenses and £uvial deposits. Late syneruption deposits onlap the primary WTTC upslope respect to the early syneruption deposits. The Peperino Albano lahar deposit is a high-aspect-ratio, valley-ponded, coarsening-upward unit, which is transitional upslope to the Peperino Albano ignimbrite. The lahar developed on a relatively £at topography incised by gullies into which is con¢ned. The lahar deposit records the catastrophic mixture of pyroclastic debris with crater lake water during the eruption; transport and deposition are controlled by the e¡ect of ‘hydraulic sieving’ that allowed the formation of a fast frontal hyperconcentrated £ow with ash particles followed by a debris £ow with cobbles and blocks. The di¡erent facies associations described for the WTTCupa and the Peperino Albano lahar deposits suggest a set of key observations that can be used to infer the water source for ancient lahar successions. Rain-generated lahar deposits form aprons made of several individual low-aspect-ratio aggradational deposits onlapping erosion surfaces cut on the parent pyroclastic deposits and forming coarsening-upward and backstepping successions with upward decrease of juvenile material. The vertical and lateral grading of grain-size and components re£ect the evolution through time (in the order of years) of availability of volcaniclastic sediment versus rainwater. Crater lake-withdrawal lahar deposits form single, high-

aspect-ratio lobes, contiguous upslope to the parent pyroclastic deposits. The vertical and lateral grading of grain-size and components re£ect rapid processes (in the order of minutes/hours) of coupling/decoupling between water and particles according to their hydraulic behaviour and the amount of water available.

Acknowledgements This work has been funded partly by a C.N.R. grant for mapping of volcaniclastic deposits at Roccamon¢na volcano and partly by a National Geologic Survey grant for the new 1:50 000 scale map of Albano. We also thank N. Riggs and G. Alvaredo for useful reviews of this manuscript.

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