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Valley pond and ignimbrite veneer deposits in the small-volume phreatomagmatic ‘Peperino Albano’ basic ignimbrite, Lago Albano maar, Colli Albani volcano, Italy: influence of topography
Journal of Volcanology and Geothermal Research 118 (2002) 131^144 www.elsevier.com/locate/jvolgeores
Valley pond and ignimbrite veneer deposits in the small-volume phreatomagmatic ‘Peperino Albano’ basic ignimbrite, Lago Albano maar, Colli Albani volcano, Italy: in£uence of topography G. Giordano a; , D. De Rita a , R. Cas b , S. Rodani a a
Dipartimento di Scienze Geologiche, Universita' di Roma TRE, L.go S. Leonardo Murialdo 1, 00146 Roma, Italy b Department of Earth Sciences, Monash University, Clayton, Vic. 3168, Australia Received 21 May 2001; accepted 2 November 2001
Abstract The ca. 23-ka, small-volume, basic phreatomagmatic Peperino Albano ignimbrite, from the polygenetic Albano maar (Colli Albani volcano, central Italy) shows valley pond facies as well as veneer deposits along the maar rim and along topographic ridges. Valley pond facies is characterised mainly by massive structure and chaotic texture and can be up to 30 m thick. Veneer deposit facies is characterised by parallel to low-angle cross-stratified bedforms alternating fines-depleted lapilli-sized layers, and massive, matrix-supported beds. Occurrence of uncharred wood remains and accretionary lapilli suggests temperature of emplacement comprised between 246‡ and 100‡C. We have interpreted the lateral facies variations in terms of temporal and spatial variations of the sediment supply from the transport system to the depositional system of the pyroclastic flow. Ignimbrite veneer facies at the maar rim may reflect pulsatory eruption dynamics, whereas valley pond facies may reflect the bulking of the pyroclastic flow inside the valleys and consequent high sedimentation rates. Ignimbrite veneer facies at topographic ridges has been interpreted to reflect detachment processes of more concentrated undercurrents draining within valleys from the more dilute upper part of the pyroclastic flow that climbs the ridges. The present interpretation suggests that processes of pyroclastic flow transformation downcurrent and induced by topography are not necessarily peculiar of hot, highmobility pyroclastic density currents. The more likely source of water interacting with magma is interpreted to be groundwater contained within the karstic aquifer located at approximately 1 km below the ground level. This is inferred by both the large amount of limestone xenoliths present in the Peperino Albano and the absence of vesicular juvenile clasts, the latter implying that magma^water interaction occurred before gas exolution processes were significant. < 2002 Elsevier Science B.V. All rights reserved. Keywords: Colli Albani; ignimbrite; phreatomagmatism; valley pond; veneer deposit
1. Introduction * Corresponding author. Tel.: +39-06-54888061; Fax: +39-06-54888201. E-mail address: [email protected] (G. Giordano).
Most documented pyroclastic £ows and their deposits are felsic in composition, and originate
0377-0273 / 02 / $ ^ see front matter < 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 2 5 3 - 6
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from central vent systems of large caldera volcanoes and stratovolcanoes. Two end-member models for relatively large-volume felsic pyroclastic £ow deposits have emerged. Lower-mobility, high-aspect-ratio ignimbrites tend to be massive with only minor strati¢cation developed (e.g. Miyabuchi, 1999). By contrast, higher-mobility, low-aspect-ratio ignimbrites tend to develop thick, massive valley pond ignimbrite facies, and thinner, frequently strati¢ed ignimbrite veneer deposits. Ignimbrite veneer deposit is developed over ridges, and is considered to be the result of the interaction between the upper parts of high-velocity pyroclastic £ows and topography (e.g. Wilson and Walker, 1985). By contrast, relatively little is known about the characteristics of basic pyroclastic £ow deposits, nor of the behaviour of phreatomagmatic pyroclastic £ows (e.g., Giordano and Dobran, 1994; De Rita et al., 1995). The Colli Albani volcano, located 25 km to the east^southeast of Rome, Italy, is a large, lava and pyroclastic caldera complex. It has been active episodically for the last 600 000 years, and emplaced both large-volume and small-volume basic ignimbrites (De Rita et al., 1995). The large-volume £ows were erupted from a summit collapse caldera, and are the subject of many studies (De Rita et al., 1995 and references therein ; Watkins et al., 2000). By contrast, small-volume ignimbrites have been erupted by parasitic maars on the slope of the volcano edi¢ce (De Rita et al., 1988b) and have received little attention to date. In this study we document the characteristics of the small-volume phreatomagmatic basic ignimbrite erupted from the polygenetic Lago di Albano maar (Fornaseri et al., 1963; De Rita et al., 1988a), during its last eruptive cycle which occurred approximately 29 000 years ago (Fornaseri and Cortesi, 1989). We document valley pond and ignimbrite veneer facies, as well as other facies, and consider the eruption and transport processes and their in£uences on the ignimbrite facies. 1.1. Geological setting Along the Tyrrhenian coast of central Italy, a NW-trending volcanic belt has been active since
Pliocene time, related mainly to back arc extensional processes (Ferrari and Manetti, 1993 and references therein). During the last 600 000 years several stratovolcanoes and caldera complexes have been active, characterised by the eruption of ma¢c magmas belonging to both the K serie and high-K serie that constitute the Roman Magmatic Province (e.g. Serri, 1990 and references therein). The Colli Albani volcano is located about 25 km southeast of the city centre of Rome. It has been active between 600 000 years ago and the Holocene (Funiciello et al., 2002). The style of eruptive activity of the Colli Albani volcano changed through time and the stratigraphy has been subdivided into three successions corresponding to as many cycles of activity (De Rita et al., 1995). The Tuscolano^Artemisio succession was emplaced between 600 000 and 330 000 years ago, and is made up mostly by six large-volume, low-aspect-ratio, caldera-forming ignimbrites interstrati¢ed with extensive leucitic lavas. The last ignimbrite forming eruption caused the ¢nal collapse of the central 10 kmU10 km wide caldera at 350 ka (Fig. 1). After the collapse, the small ‘Le Faete’ stratovolcano formed at the centre of the caldera, mostly made up of lavas and strombolian deposits. The last period of activity at Colli Albani was characterised by di¡use phreatomagmatic activity from several eccentric maars. Phreatomagmatism resulted from the interaction between the ascending magma and groundwater largely that is present in the highly permeable Mesozoic^Cenozoic karstic aquifers that underlie the volcano (Funiciello and Parotto, 1978; De Rita et al., 1988a). One of the most recent products from Colli Albani is the Peperino Albano phreatomagmatic ignimbrite, which is the subject of the present paper. Age determinations for the Peperino Albano range from 36 000 to 19 000 years ago (Fornaseri and Cortesi, 1989; Mercier, 1993). 14 C age determinations on branches of Ulmus and Quercus ilex cluster around 29^30 ka (Fornaseri and Cortesi, 1989). More recent U/Th dating of a carbonatic layer that underlies the Peperino Albano suggests an age younger than 23 O 6.7 ka, which is the most reliable determination (Soligo et al., in press). This range is in agreement with
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Fig. 1. Schematic distribution map of the Peperino Albano around the Albano polygenetic maar.
Ar/Ar age determination from sediment-cores drilled at the centre of the lake (Chondrigianni et al., 1996). The Lago Albano maar is located along the western slope of the Colli Albani edi¢ce (Fig. 1). It is elongated toward the NW, and has a maximum diameter of 2.5 km. It is interpreted as a polygenetic maar that consists of ¢ve nested craters, which formed as many phreatomagmatic stratigraphic units that are separated by palaeosols (De Rita et al., 1988a). The ¢rst four units
are mostly made of parallel to cross-bedded, ashto lapilli-sized surge and fallout deposits with abundant accretionary lapilli and bomb sags. Abundant limestone xenoliths are present and allowed to suggest that the groundwater interacting with the rising magma was stored within the Mesozoic^Cenozoic carbonate succession that underlies the volcanic succession at depths varying between 1.5 km and 3^4 km (Funiciello and Parotto, 1978). The products are strongly zeolitised in proximal areas (within 500^1000 m from
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Fig. 2. Photomicrographs of the Peperino Albano in thin section. (A^C) The juvenile is highly fragmented and zeolitised (15U). Note the occurrence of accretionary lapilli (A, white dashed line) and ash lumps (B, white dashed line); (D) No vesicular pyroclasts can be identi¢ed at high migni¢cation because of pervasive zeolitisazion (75U). (E,F) Scanning electron microprobe photomicrographs of individual shards, characterised by blocky shape and very low vesicularity.
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the maar rim), but rarely in medial and distal areas (maximum 3.5 km from vent). The Peperino Albano ignimbrite is the ¢fth unit and is one of the youngest products erupted from the Lago Albano maar. In contrast with the underlying units, it is an ignimbrite, and it mostly shows valley-ponded deposits. It is zeolitised throughout and has been quarried since the Roman Age as building and decorative stone.
2. The Peperino Albano ignimbrite 2.1. An overview The Peperino Albano ignimbrite is radially distributed around the Lago Albano maar within 2 km from the edge of the crater rim. Farther than that, the ignimbrite is only present as a lobate valley pond unit along the Petrare valley to the NW of the crater, as far as 7 km (Fig. 1). The Peperino Albano covers an area of approximately 40 km2 and its thickness varies from a few metres to up to 30 m within palaeovalleys. A total minimum volume of the deposit is approximately 0.2 km3 . The intense zeolitisation of the deposit makes it di⁄cult to extract good samples for accurate grain size, component and chemical data. Field analysis and point counting on thin sections show that the juvenile component of the deposit, which makes up the 90%+ of the unit, is consistently ¢ne-grained, with a maximum size of coarse ash (Fig. 2). Juvenile ash is typically grey in colour. The ¢ne ash matrix is pervasively zeolitised, giving the unit the lithi¢ed mechanical characteristics for which it has been long used as a construction stone. Scattered in the ash matrix there are up to 10% of leucite, clinopyroxene and biotite crystals and up to 10% by volume of lava, sedimentary (limestone) and intrusive lapilli- to bomb-sized xenoliths. Existing chemical analyses of the Peperino Albano ignimbrite plot in the leucitite ¢eld in the TAS diagram (Trigila et al., 1995 and references therein). However, chemical analyses refer to the bulk rock composition, because of the absence of fresh juvenile large clasts of scoria
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or pumice and the pervasive zeolitisation. X-Ray di¡ractometry indicates the presence of abundant mica, clinopyroxene, leucite, plagioclase and chabasite (Fig. 3). The ignimbrite shows unburnt clusters of £attened in situ steppe grass that grew in the underlying palaeosol. Clusters of arboreal vegetation showing minimal conversion to charcoal suggest that the ignimbrite had a minimal thermal e¡ect on the vegetation. The presence of unburnt plants indicate that the temperature of the pyroclastic £ow must have been lower than the temperature of ignition of wood, i.e. lower than or around 246‡C (Macdonald, 1972). A perfect mould of a vulture has also been reported complete with its plumage (Fornaseri et al., 1963). On the other hand, the subordinate presence of accretionary lapilli or of other common occurrence of agglutinated ash-particles (although locally present) suggest that temperature was higher or around 100‡C. This range of temperature is also well in agreement with temperature required for the pervasive zeolitisation of glass shown by the ignimbrite (cf. de’ Gennaro et al., 1999). The most common zeolites throughout the ignimbrite are chabasite and herschelite (Fig. 3). However, the lowest 2 m of the deposit show a progressive increase in chabasite. This can be related to the temperature pro¢le developed within the ignimbrite after the emplacement, characterised by lower temperature at the base that favoured the formation of clay minerals, and progressively higher temperature upward, which would have favoured the formation of chabasite and herschelite, which has a higher temperature of formation (C. Giampaolo, Universita' Roma TRE, personal communication). Assessing the vesicularity of pyroclasts of the Peperino Albano ignimbrite is a very di⁄cult task because of the pervasive transformation of glass particles into zeolite minerals (Fig. 2D). Thin sections do not show well-preserved shards helpful in de¢ning the vesicularity. However, it is clear that the juvenile component is entirely ¢ne ash (Fig. 2) throughout the deposit, which implies a very e⁄cient mechanism of fragmentation. SEM analysis allows to recognise some non-altered shard morphologies that show blocky shape and
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Fig. 3. X-Ray Di¡ractometry analysis of the Peperino Albano (courtesy of Ciro Giampaolo and Sergio Lo Mastro, Univ. Roma TRE). lc = leucite; cpx = clinopyroxene; bt = biotite and phlogopite; ch = chabasite and herscelite; cc = calcite; pl = plagioclase.
hydrofractures cross-cutting gas bubbles, typical of magma^water interaction (Fig. 2E,F).
3. Facies characteristics of the Peperino Albano ignimbrite We subdivide the Peperino Albano ignimbrite into three facies that di¡er in the structural and textural features of deposit, related to di¡erent palaeotopographic locations. (a) Valley pond facies (VPI). At the onset of the Peperino Albano eruption, the outer slopes of the Lago Albano maar were incised by deep valleys that channelled the Peperino Albano pyroclastic £ows. The most common facies shown by the Peperino Albano ignimbrite within the palaeovalleys is the VPI, which can be up to 30 m thick. It is characterised mainly by a massive, matrix-sup-
ported structure. Deposits are poorly sorted, with bombs and blocks scattered in the ash matrix (Fig. 4A). The ignimbrite generally does not show evident grading of coarse clasts. However, the base of the deposit may show reverse grading of lapilli-sized xenoliths and at some location planeparallel bedding is also present (Fig. 4B; Petrara Valley, km 2.5 km from the vent). The ignimbrite is generally homogeneous throughout its thickness. However, at some locations it is possible to distinguish up to ¢ve depositional units separated by several-centimetre-thick ¢ne-gained layers (Petrara Valley, 2 km from the vent). At the Marino Town train station (1 km from the vent, along the Petrara Valley), the ignimbrite is 30 m thick and is massive throughout its thickness without evidence of multiple depositional units. However, 25 m above the base of the unit, a matrix-supported lithic concentration zone occurs,
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Fig. 4. (A) Valley pond massive facies of the Peperino Albano. Note the occurrence of abundant, white xenoclasts of limestones. (B) Strati¢ed base to the Peperino Albano at 2.5 km from vent. (C) Lithic concentration zone occurring 25 m above the base of Peperino Albano in VPI at 1 km from vent, Petrara Valley. (D) Di¡usely strati¢ed facies of the Peperino Albano along the £ank of the Petrara Valley.
with clasts up to 1.3 m in diameter (Fig. 4C). The typical massive and chaotic VPI grades laterally into a di¡usely strati¢ed facies where the ignimbrite laterally onlaps the steep sides of the palaeovalleys (Marino Town). Strati¢cation is de¢ned by several ¢nes-depleted, elongated lenses of milli-
metre- to centimetre-sized xenoliths and juvenile clasts generally occurring up to a distance of 1^5 m away from the palaeovalley side-slope (Fig. 4D). Fines-depleted lenses are fan shaped in cross section, being almost parallel to the ground surface at the base and progressively
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Fig. 5. (A) Strati¢ed IVD of the Peperino Albano along the Marino ridge. (B) Agglutinated ash lumps making up one bed of the IVD. (C) Dunes within the IVD along the Marino ridge. Note that dunes are stationary (i.e. crests grew vertically) and are shaped on the morphology of the palaeoslope. (D) Scoria layer at the base of the Peperino Albano along the western rim of the Albano maar. The scoria layer either represents an early fallout phase or belongs to a di¡erent volcanic centre. (E) Massive intracrater facies of the Peperino Albano, which drapes the inner wall of the Albano maar. Xenoliths are larger than the average dimension found in the out£ow deposits and concentrated toward the base.
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more horizontal upward. The most distal outcrop of the Peperino Albano ignimbrite is at 7 km from vent, where the VPI is 3 m thick and shows normal grading of coarse clasts that concentrate toward the base. Coarse clasts are rounded and mostly made of leucitite lava up to 1 m in diameter derived from the nearby Capo di Bove lava £ow (Fig. 1). Clasts were therefore ripped-up in the Peperino Albano from the bottom of the talweg. (b) Ignimbrite veneer facies (IVD). The IVD is commonly found around the crater rim and along valley ridges. The IVD is parallel to low-angle cross-strati¢ed (Fig. 5A). Bedforms are generally aggradational and mirror the underlying topography. Bedding and strati¢cation are de¢ned by the alternation of ¢nes-depleted, non-zeolitised to poorly-zeolitised, lapilli-sized layers, rich in xenoliths, crystals and agglutinated ash droplets (Fig. 5B) and massive, matrix-supported, zeolitised beds up to 1 m in thickness that display a facies similar to the VPI. Elongated clasts may be oriented along the £ow direction. Average xenolith content generally exceeds 10% by volume although maximum diameters are always smaller than those measured in the VPI at similar distances from the vent. The thickness of this facies varies according to palaeotopography, but generally is less than 10 m thick. IVD developed at the crater rim is characterised by thicker beds that are lenticular in shape. Fines-depleted, xenolith-rich beds can be up to 80 cm thick and generally display a crude, low-angle, cross-bedded strati¢cation. Massive, matrix-rich beds can be up to 1 m thick (Fig. 5A). Ballistic bombs and related impact sags are usually present at the base of the unit very close to vent and are found along the crater rim. The Marino Town section (1 km from vent, Fig. 1) is an excellent example of IVD developed on a ridge transversal to £ow direction. The Valle Petrara is almost perpendicular to £ow direction in proximal location. Dunes are developed along the stoss side of the topographic ridge, i.e. where pyroclastic £ows climbed the topography. Dunes are aggradational and have wavelength exceeding 10 m and height of 2^3 m. Massive beds thicken on the stoss sides of the dunes (Fig. 5C). Ash-pellet-rich layers are common at
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this site. Unburnt blades of grass bent in the £ow direction away from the vent are present at the very base of the unit. Maximum lithic size is in the order of few centimetres. At Castelgandolfo location, the crater rim IVD is underlain by a several-centimetre-thick brown scoria layer, which rests on the palaeosoil (Fig. 5D). Scoria clasts are highy vesicular and well sorted with an average diameter of 1 cm. The deposit appears to be a strombolian fallout deposit. This is the only instance where a vesicular and non-zeolitised pyroclastic deposit is found at the base of the Peperino Albano succession. It is unclear whether this deposit belongs to the Peperino Albano succession or, more likely, was erupted earlier from one of the scoria cones surrounding the Albano maar. However, these scoria clasts give a reasonable idea of the style of magma fragmentation in absence of or with little water involved in the eruptive mechanism. (c) Intracrater facies. Several tongues of Peperino Albano ignimbrite drape the internal walls of the Lago Albano maar from the crater rim down to the lake level, showing a quaquaversae dipping. The intracrater facies of the Peperino Albano ignimbrite is characterised by a massive structure and chaotic texture (Fig. 5E) and is very similar to the VPI. The ignimbrite is generally less than 15 m thick. The body of the ignimbrite is generally homogeneous throughout its thickness. The ignimbrite is made up of up to 85% by volume of juvenile ash-sized component, with approximately 5% of leucite, clinopyroxene and biotite crystals and up to 15% of xenoliths. Compared to the VPI, xenoliths are more abundant, larger in size (up to 2 m in diameter) and intrusive types are more abundant. At one location (Fonte S. Pietro), xenolith bombs are normal graded and concentrated at the base of the unit as a lag deposit or a co-ignimbrite breccia. At some locations it is possible to recognise some internal layering parallel to the slope. The intracrater facies is laterally continuous with the IVD at the crater rim. 3.1. Discussion The Peperino Albano ignimbrite is a typical
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Fig. 6. Schematic cross section through the di¡erent facies of the Peperino Albano ignimbrite (the trace A^B is annotated in Fig. 1) and model of emplacement. Valleys act as traps for the denser parts of pyroclastic £ows which become more concentrated and gravity controlled, so that £ow direction might vary through a vertical section of the same £ow. Flow’s concentration and uniformity control the rate of deposition and development of massive versus strati¢ed facies.
high-aspect-ratio, valley-ponded, small-volume ignimbrite. All these characteristics are generally suggestive of low-mobility, high-concentration pyroclastic £ows perhaps close to the plug-£ow analogue end member for pyroclastic density currents (e.g. Sparks, 1976; Battaglia, 1993). However, the Peperino Albano also shows an IVD, indicating the ability to climb topographic highs. Bedforms and structures of the IVD suggest a more diluted transport system closer to the surge end member for pyroclastic density currents (e.g. Valentine, 1987; Fisher, 1990). The occurrence of the IVD points out that processes of pyroclastic £ow transformation laterally and related to topography are not necessarily peculiar to high-energy, low-as-
pect-ratio acidic ignimbrites as generally assumed, but also to low-temperature phreatomagmatic ignimbrites of basic composition. A key point for the understanding of the transport and depositional mechanisms associated with the Peperino Albano emplacement is to analyse a detailed cross section across the crater rim^Petrara Valley^Marino ridge (Fig. 6). Along this cross section, thanks to continuous exposure, it is possible to observe in the ¢eld the lateral transition from the IVD along the crater rim to the VPI in the Petrara valley, again to the IVD on the Marino ridge. The fewmetre-thick IVD is characterised by well-developed bedforms, which are made up of massive and poorly sorted beds alternated with coarse-
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and ¢ne-grained well-sorted beds (Figs. 6A and 5A). It laterally passes to a crudely strati¢ed deposit where the ignimbrite thickens along the valley margins (Figs. 6B and 4D). The transition to the VPI is marked by lateral thickening and amalgamation of the several massive and unsorted beds and the contemporaneous thinning and progressive disappearance of the well-sorted layers, which laterally become thin lenses and further within the valley disappear completely (Figs. 6C and 4C). The only evidence of layering within the VPI is marked by block and lapilli lithic concentration zones that occur at di¡erent heights through the deposit (Fig. 4C). We interpret the origin of this facies transition in terms of temporal and spatial variations of the sediment supply from the transport system to the depositional system of the Peperino Albano pyroclastic density current (cf. Valentine, 1987; Fisher, 1990; Branney and Kokelaar, 1992; Druitt, 1992). IVD facies suggests the vertical aggradation of several beds. The alternation of massive and unsorted beds with cross-bedded and sorted layers can be interpreted to either re£ect variations in sediment supply to the depositional system fed by the density current or the occurrence of several independent depositional events. The two cases are in fact not so di¡erent. The deposition of a massive and unsorted bed can be interpreted as re£ecting a process of high sedimentation rate, precluding the formation of bed structures (Lowe, 1988; Kneller and Branney, 1995). Particle sedimentation is driven by processes such as hindered settling (e.g. Druitt, 1992). By contrast, the deposition of a sorted and strati¢ed bed can be interpreted as re£ecting a lower sediment supply to the depositional system wherein bed structures can be developed (Allen, 1982; Cas and Wright, 1987). Particle sedimentation is driven mainly by grain to grain deposition (Allen, 1982). The alternation of such beds at the maar rim can be reasonably related to variations in the eruption rate which in turn can a¡ect the particle concentration and the transport energy of the pyroclastic density current (or currents) through time. Eyewitness observations indicate that phreatomagmatic eruptions are commonly characterised by high instability of the eruption rate and the occurrence of
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several hundred to thousands of high-frequency explosions during a single eruptive phase (e.g. Moore et al., 1966 ; Waters and Fisher, 1971). This information is important in that it may relate to the deposition of the strati¢ed IVD at the maar rim and points out that the distinction between several rapid depositional events and one depositional event characterised by high instability is a semantic one. The lateral transition to the VPI may indicate the lateral and topography-induced increase in particle concentration within the pyroclastic £ow. The increase in thickness of the massive and unsorted beds is suggestive of an increased tendency to deposit at higher sedimentation rates (Kneller and Branney, 1995; White and Schmincke, 1999). This also indicates that part of the erupted particles by-passed the crater rim and fell within the depositional system where the pyroclastic £ow was forced within the valley because correlatable packages have di¡erent thickness, i.e. at any time, the volume of debris deposited within the valley was greater than that deposited on the crater rim (Fig. 6). Lithic concentration zones should therefore correlate with variations through time of the eruption dynamics, such as conduit wall collapse, or vent widening etc. Furthermore, the disappearance of the layering and of sorted lenses within the massive VPI facies suggests that the pyroclastic density current was sustained through time and sediment supply high enough to hinder the formation of bedforms (Lowe, 1988; Kneller and Branney, 1995). It is therefore more likely that the IVD bedding and layering at the maar rim is to be related to the £ow dynamics rather than the occurrence of several discrete depositional events. This interpretation leads to the consequence that a pyroclastic £ow can start as a highly pulsatory £ow and can laterally transform into a uniform £ow if topographic conditions, such as the channellisation inside a deep ravine induce a rapid increase in £ow concentration and in its tendency to deposit (cf. Kneller and Branney, 1995; Giordano, 1998b). This story reverses where the pyroclastic £ow was forced to climb a ridge like at the Marino ridge, where the VPI laterally grades into the IVD facies. In this case the layering and the development of alternate massive and unsorted beds with sorted
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lapilli beds should relate to variations in sediment supply to the depositional system induced by the rapid rise of the underlying topography which probably forced most of the debris along the valley, detaching the upper and more diluted part of the £ow in order to climb the ridge (Fisher, 1990, 1995; Giordano and Dobran, 1994; Woods et al., 1998; Legros and Kelfoun, 2000). It is important to underline that maximum lithic size is larger within the VPI compared with maximum lithic size in the IVD. It is also important to point out that bedforms are found in phase with the underlying topography (Fig. 5C), suggesting that variations in £ow dynamics are intimately related with topographic e¡ects. The intracrater facies is somewhat intriguing as it is massive and poorly sorted, forms tongues that drape the inner wall of the Albano maar, it is lithic-rich and displays a basal breccia. The formation of the VPI-like intracrater facies can be related to processes of £ow bulking similar to that described for the VPI. However, it remains unclear whether this facies represents the back£ow of a pyroclastic density current that was not able to climb the rim, or the deposit of a pyroclastic density current that was able to climb the rim, or a portion of a collapsing column that collapsed inside the crater (cf. Valentine et al., 1992). In any of the above cases the intracrater facies shows the largest percentage of lithic clasts, as well as the largest lithic dimensions and this is in accordance with the proximality of this facies. Another issue for the Peperino Albano eruption is the source of water involved in the phreatomagmatic eruption. The abundance of limestone xenoliths suggest that a possible source of water is groundwater contained within the Mesozoic^Cenozoic carbonatic karstic aquifer underlying at approximately 1 km below the surface of the Albano maar (Funiciello and Parotto, 1978). We cannot exclude, however, a contribution from crater lake water at least during the earlier phases of the eruption, considering that the Albano maar was already in existence at the time of the Peperino Albano eruption and therefore likely to host a lake. Unfortunately we cannot ascertain whether the scoria layer found at Castelgandolfo locality underlying the Peperino Albano is to be related to
the Peperino Albano eruption or not. However, the Peperino Albano certainly does not show vesicular clasts throughout, and this is suggestive of magma^water interaction at a depth greater than the magma vesiculation depth, so that phreatomagmatic fragmentation occurred when exolution processes were not signi¢cant.
4. Conclusions We presented the facies association of the ca. 29-ka, small-volume, basic phreatomagmatic Peperino Albano ignimbrite, from the polygenetic Albano maar (Colli Albani volcano, central Italy). The ignimbrite shows VPI as well as veneer deposits along the maar rim and along topographic ridges. We have interpreted the lateral facies variations in terms of temporal and spatial variations of the sediment supply from the transport system to the depositional system of the Peperino Albano pyroclastic £ow. IVD at the maar rim may re£ect variations in the eruption dynamics, whereas VPI may re£ect the bulking of the pyroclastic £ow inside the valleys and consequent high sedimentation rates. IVD at topographic ridges has been interpreted to re£ect variations in sediment supply to the depositional system of the £ow related to detachment of more concentrated undercurrents draining within valleys from the more dilute upper part of the pyroclastic £ow that climbs the ridges and interacts with them, resulting in increased turbulence. The present interpretation for the facies association shown by the Peperino Albano ignimbrite suggests that processes of pyroclastic £ow transformation laterally and induced by topography are not necessarily peculiar of high-energy, low-aspect-ratio acidic ignimbrites as generally assumed. The more likely source of water interacting with magma is interpreted to be groundwater contained within the karstic aquifer located at approximately 1 km below the ground level, as is suggested by the large amount of limestone xenoliths present in the Peperino Albano. The absence of vesicular juvenile clasts suggests that magma^ water interaction occurred when gas exolution processes were not signi¢cant.
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Acknowledgements This work has been partly funded by a bilateral cultural agreement between Universita' Roma TRE (Italy) and Monash University (Australia). We gratefully thank Ciriaco Giampaolo and Sergio Lo Mastro (Universita’ Roma TRE) for XRD analyses. We also thank Piero Dellino for the careful review of the paper. References Allen, J.R.L., 1982. Developments in sedimentology, vol. 30 A^B, Sedimentary structures. Their Character and Physical Basis. Elsevier, Amsterdam. Battaglia, M., 1993. On pyroclastic £ow emplacement. J. Geophys. Res. 98, 22269^22272. Branney, M.J., Kokelaar, P., 1992. A reappraisal of ignimbrite emplacement: Progressive aggradation and particulate to non-particulate £ow transition during emplacement of high-grade ignimbrite. Bull. Volcanol. 54, 504^520. Cas, R.A.F., Wright, J.V., 1987. Volcanic Successions: Modern and Ancient. Unwin and Hyman Ltd., London, pp. 1^ 525. Chondrigianni, C., Aritzegui, D., Niessen, F., Ohlendorf, C., Lister, G.S., 1996. Late Pleistocene and Holocene sedimentation in Lake Albano and Lake Nemi (central Italy). In: Guilizzoni, P., Oldi¢eld, F. (Eds.), Paleoenvironmental analysis of Italian crater lake and Adriatic sediments. Mem. Ist. Ital. Idrobiol. 55, 23^38. de’ Gennaro, M., Incoronato, A., Mastrolorenzo, G., Adabbo, M., Spina, G., 1999. Depositional mechanisms and alteration processes in di¡erent types of pyroclastic deposits from Campi Flegrei volcanic ¢eld (Southern Italy). J. Volcanol. Geotherm. Res. 91, 303^320. De Rita, D., Funiciello, R., Pantosti, D., 1988a. Dynamics and evolution of the Albano crater, south of Roma. Proc. of the Kagoshima Int. Conference on Volcanoes, pp. 502^505. De Rita, D., Funiciello, R., Parotto, M., 1988b. Geological Map of the Colli Albani Volcanic Complex. CNR, Rome. De Rita, D., Faccenna, C., Funiciello, R., Rosa, C., 1995. Stratigraphy. In: Trigila, R. (Ed.), The Volcano of the Alban Hills. Tipogra¢a SGS, Rome, pp. 33^73. Druitt, T.H., 1992. Emplacement of the 18 May 1980 lateral blast deposit ENE of Mount St. Helens, Washington. Bull. Volcanol. 54, 554^572. Ferrari, L., Manetti, P., 1993. Geodynamic framework of the thyrrenian volcanism: a review. Acta Vulcanol. 3, 1^9. Fisher, R.V., 1990. Transport and deposition of a pyroclastic surge across an area of high relief: the 18 May 1980 eruption of Mount St. Helens, Washington. Geol. Soc. Am. Bull. 102, 1038^1054. Fisher, R.V., 1995. Decoupling of pyroclastic currents: hazard assessment. J. Volcanol. Geotherm. Res. 66, 257^263.
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Fornaseri, M., Ventriglia, U., Scherillo, A., 1963. La Regione Vulcanica dei Colli Albani. Eredi dott. Bardi editore, 549 pp. Fornaseri, M., Cortesi, C., 1989. Recenti acquisizioni sull’eta del ‘Peperino’ di Albano. Doc. Albana, II Ser. 11, 7^10. Funiciello, R., Parotto, M., 1978. Il substrato sedimentario dell’area dei Colli Albani (vulcano laziale). Geol. Rom. 17, 831^849. Funiciello, R., Giordano, G., De Rita, D., 2002. The Albano maar lake (Colli Albani volcano, Italy): Recent volcanic activity and evidences of pre-Roman age catastrophic lahar events. J. Volcanol. Geotherm. Res. 118, in press. Giordano, G., Dobran, F., 1994. Computer simulations of the Tuscolano Artemisio’s second pyroclastic £ow unit (Alban Hills, Latium Italy). J. Volcanol. Geotherm. Res. 61, 69^94. Giordano, G., 1998b. The e¡ect of paleotopography on lithic distribution and facies associations, of small volume ignimbrites: the WTT Cupa (Roccamon¢na volcano Italy). J. Volcanol. Geotherm. Res. 87, 255^273. Kneller, B.C., Branney, M.J., 1995. Sustained high-density turbidity currents and the deposition of thick massive sands. Sedimentology 42, 607^616. Legros, F., Kelfoun, K., 2000. On the ability of pyroclastic £ows to scale topographic obstacles. J. Volcanol. Geotherm. Res. 98, 235^241. Lowe, D.R., 1988. Suspended-load fallout rate as an independent variable in the analysis of current structures. Sedimentology 35, 765^776. Macdonald, G.A., 1972. Volcanoes. Prentice Hall, Englewood Cli¡s, NJ, 510 pp. Mercier, N., 1993. The thermo-luminescence dating technique: applications and possibilities. Symposium on Quaternary Stratigraphy in Volcanic Area, Rome, September 20^22, Abstract. Miyabuchi, Y., 1999. Deposits associated with the 1990^1995 eruption of Unzen volcano, Japan. J. Volcanol. Geotherm. Res. 89, 139^158. Moore, J.G., Nakamura, K., Alcaraz, A., 1966. The 1965 eruption of Taal volcano. Science 151, 955^960. Serri, G., 1990. Neogene-Quaternary magmatism of the Tyrrhenian Region: characterization of magma sources and geodynamic implications. Mem. Soc. Geol. It. 41, 219^242. Soligo, M., Tuccimei, P., Giordano, G., Funiciello, R., De Rita, D., in press. New U-series dating of a carbonate level underlying the Peperino Albano phreatomagmatic ignimbrite (Colli Albani, Italy), I1 Quaternario (It. J. Quat. Sci.). Sparks, R.S.J., 1976. Grain size variation in ignimbrites and implications for the transport of pyroclastic £ows. Sedimentology 23, 147^188. Trigila, R., Agosta, E., Currado, C., De Benedetti, A.A., Freda, C., Gaeta, M., Palladino, D.M., Rosa, C., 1995. Petrology. In: Trigila, R. (Ed.), The Volcano of the Alban Hills. Tipogra¢a SGS, pp. 267^283. Valentine, G.A., 1987. Strati¢ed £ow in pyroclastic surges. Bull. Volcanol. 49, 616^630. Valentine, G.A., Wohletz, K., Kie¡er, S.W., 1992. E¡ects of topography on facies and compositional variation in caldera related ignimbrite. Geol. Soc. Am. Bull. 104, 154^165.
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Waters, A.C., Fisher, R.V., 1971. Base surges and their deposits: Capelinhos and Taal volcanoes. J. Geophys. Res. 76, 5596^5614. Watkins, S.D., Giordano, G., Cas, R.A.F., De Rita, D., 2000. Internal Facies Changes In Ma¢c Pyroclastic Density Current Deposits: A Record Of Temporal Changes In The Eruption Style Of The Villa Senni Eruption Unit, Alban Hills Volcano, Rome, Italy. IAVCEI General Assembly 2000, Abstract volume.
White, J.D.L., Schmincke, H.-U., 1999. Phreatomagmatic eruptive and depositional processes during the 1949 eruption on La Palma (Canary Islands). J. Volcanol. Geotherm. Res. 94, 283^304. Wilson, C.J.N., Walker, G.P.L., 1985. The Taupo eruption. New Zealand: I. General aspects. Philos. Trans. R. Soc. Lond. A314, 199^228. Woods, A.W., Bursik, M.I., Kurbatov, A.V., 1998. The interaction of ash £ows with ridges. Bull. Volcanol. 60, 38^51.
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Valley pond and ignimbrite veneer deposits in the small-volume phreatomagmatic ‘Peperino Albano’ basic ignimbrite, Lago Albano maar, Colli Albani volcano, Italy: in£uence of topography G. Giordano a; , D. De Rita a , R. Cas b , S. Rodani a a
Dipartimento di Scienze Geologiche, Universita' di Roma TRE, L.go S. Leonardo Murialdo 1, 00146 Roma, Italy b Department of Earth Sciences, Monash University, Clayton, Vic. 3168, Australia Received 21 May 2001; accepted 2 November 2001
Abstract The ca. 23-ka, small-volume, basic phreatomagmatic Peperino Albano ignimbrite, from the polygenetic Albano maar (Colli Albani volcano, central Italy) shows valley pond facies as well as veneer deposits along the maar rim and along topographic ridges. Valley pond facies is characterised mainly by massive structure and chaotic texture and can be up to 30 m thick. Veneer deposit facies is characterised by parallel to low-angle cross-stratified bedforms alternating fines-depleted lapilli-sized layers, and massive, matrix-supported beds. Occurrence of uncharred wood remains and accretionary lapilli suggests temperature of emplacement comprised between 246‡ and 100‡C. We have interpreted the lateral facies variations in terms of temporal and spatial variations of the sediment supply from the transport system to the depositional system of the pyroclastic flow. Ignimbrite veneer facies at the maar rim may reflect pulsatory eruption dynamics, whereas valley pond facies may reflect the bulking of the pyroclastic flow inside the valleys and consequent high sedimentation rates. Ignimbrite veneer facies at topographic ridges has been interpreted to reflect detachment processes of more concentrated undercurrents draining within valleys from the more dilute upper part of the pyroclastic flow that climbs the ridges. The present interpretation suggests that processes of pyroclastic flow transformation downcurrent and induced by topography are not necessarily peculiar of hot, highmobility pyroclastic density currents. The more likely source of water interacting with magma is interpreted to be groundwater contained within the karstic aquifer located at approximately 1 km below the ground level. This is inferred by both the large amount of limestone xenoliths present in the Peperino Albano and the absence of vesicular juvenile clasts, the latter implying that magma^water interaction occurred before gas exolution processes were significant. < 2002 Elsevier Science B.V. All rights reserved. Keywords: Colli Albani; ignimbrite; phreatomagmatism; valley pond; veneer deposit
1. Introduction * Corresponding author. Tel.: +39-06-54888061; Fax: +39-06-54888201. E-mail address: [email protected] (G. Giordano).
Most documented pyroclastic £ows and their deposits are felsic in composition, and originate
0377-0273 / 02 / $ ^ see front matter < 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 2 5 3 - 6
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from central vent systems of large caldera volcanoes and stratovolcanoes. Two end-member models for relatively large-volume felsic pyroclastic £ow deposits have emerged. Lower-mobility, high-aspect-ratio ignimbrites tend to be massive with only minor strati¢cation developed (e.g. Miyabuchi, 1999). By contrast, higher-mobility, low-aspect-ratio ignimbrites tend to develop thick, massive valley pond ignimbrite facies, and thinner, frequently strati¢ed ignimbrite veneer deposits. Ignimbrite veneer deposit is developed over ridges, and is considered to be the result of the interaction between the upper parts of high-velocity pyroclastic £ows and topography (e.g. Wilson and Walker, 1985). By contrast, relatively little is known about the characteristics of basic pyroclastic £ow deposits, nor of the behaviour of phreatomagmatic pyroclastic £ows (e.g., Giordano and Dobran, 1994; De Rita et al., 1995). The Colli Albani volcano, located 25 km to the east^southeast of Rome, Italy, is a large, lava and pyroclastic caldera complex. It has been active episodically for the last 600 000 years, and emplaced both large-volume and small-volume basic ignimbrites (De Rita et al., 1995). The large-volume £ows were erupted from a summit collapse caldera, and are the subject of many studies (De Rita et al., 1995 and references therein ; Watkins et al., 2000). By contrast, small-volume ignimbrites have been erupted by parasitic maars on the slope of the volcano edi¢ce (De Rita et al., 1988b) and have received little attention to date. In this study we document the characteristics of the small-volume phreatomagmatic basic ignimbrite erupted from the polygenetic Lago di Albano maar (Fornaseri et al., 1963; De Rita et al., 1988a), during its last eruptive cycle which occurred approximately 29 000 years ago (Fornaseri and Cortesi, 1989). We document valley pond and ignimbrite veneer facies, as well as other facies, and consider the eruption and transport processes and their in£uences on the ignimbrite facies. 1.1. Geological setting Along the Tyrrhenian coast of central Italy, a NW-trending volcanic belt has been active since
Pliocene time, related mainly to back arc extensional processes (Ferrari and Manetti, 1993 and references therein). During the last 600 000 years several stratovolcanoes and caldera complexes have been active, characterised by the eruption of ma¢c magmas belonging to both the K serie and high-K serie that constitute the Roman Magmatic Province (e.g. Serri, 1990 and references therein). The Colli Albani volcano is located about 25 km southeast of the city centre of Rome. It has been active between 600 000 years ago and the Holocene (Funiciello et al., 2002). The style of eruptive activity of the Colli Albani volcano changed through time and the stratigraphy has been subdivided into three successions corresponding to as many cycles of activity (De Rita et al., 1995). The Tuscolano^Artemisio succession was emplaced between 600 000 and 330 000 years ago, and is made up mostly by six large-volume, low-aspect-ratio, caldera-forming ignimbrites interstrati¢ed with extensive leucitic lavas. The last ignimbrite forming eruption caused the ¢nal collapse of the central 10 kmU10 km wide caldera at 350 ka (Fig. 1). After the collapse, the small ‘Le Faete’ stratovolcano formed at the centre of the caldera, mostly made up of lavas and strombolian deposits. The last period of activity at Colli Albani was characterised by di¡use phreatomagmatic activity from several eccentric maars. Phreatomagmatism resulted from the interaction between the ascending magma and groundwater largely that is present in the highly permeable Mesozoic^Cenozoic karstic aquifers that underlie the volcano (Funiciello and Parotto, 1978; De Rita et al., 1988a). One of the most recent products from Colli Albani is the Peperino Albano phreatomagmatic ignimbrite, which is the subject of the present paper. Age determinations for the Peperino Albano range from 36 000 to 19 000 years ago (Fornaseri and Cortesi, 1989; Mercier, 1993). 14 C age determinations on branches of Ulmus and Quercus ilex cluster around 29^30 ka (Fornaseri and Cortesi, 1989). More recent U/Th dating of a carbonatic layer that underlies the Peperino Albano suggests an age younger than 23 O 6.7 ka, which is the most reliable determination (Soligo et al., in press). This range is in agreement with
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Fig. 1. Schematic distribution map of the Peperino Albano around the Albano polygenetic maar.
Ar/Ar age determination from sediment-cores drilled at the centre of the lake (Chondrigianni et al., 1996). The Lago Albano maar is located along the western slope of the Colli Albani edi¢ce (Fig. 1). It is elongated toward the NW, and has a maximum diameter of 2.5 km. It is interpreted as a polygenetic maar that consists of ¢ve nested craters, which formed as many phreatomagmatic stratigraphic units that are separated by palaeosols (De Rita et al., 1988a). The ¢rst four units
are mostly made of parallel to cross-bedded, ashto lapilli-sized surge and fallout deposits with abundant accretionary lapilli and bomb sags. Abundant limestone xenoliths are present and allowed to suggest that the groundwater interacting with the rising magma was stored within the Mesozoic^Cenozoic carbonate succession that underlies the volcanic succession at depths varying between 1.5 km and 3^4 km (Funiciello and Parotto, 1978). The products are strongly zeolitised in proximal areas (within 500^1000 m from
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Fig. 2. Photomicrographs of the Peperino Albano in thin section. (A^C) The juvenile is highly fragmented and zeolitised (15U). Note the occurrence of accretionary lapilli (A, white dashed line) and ash lumps (B, white dashed line); (D) No vesicular pyroclasts can be identi¢ed at high migni¢cation because of pervasive zeolitisazion (75U). (E,F) Scanning electron microprobe photomicrographs of individual shards, characterised by blocky shape and very low vesicularity.
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the maar rim), but rarely in medial and distal areas (maximum 3.5 km from vent). The Peperino Albano ignimbrite is the ¢fth unit and is one of the youngest products erupted from the Lago Albano maar. In contrast with the underlying units, it is an ignimbrite, and it mostly shows valley-ponded deposits. It is zeolitised throughout and has been quarried since the Roman Age as building and decorative stone.
2. The Peperino Albano ignimbrite 2.1. An overview The Peperino Albano ignimbrite is radially distributed around the Lago Albano maar within 2 km from the edge of the crater rim. Farther than that, the ignimbrite is only present as a lobate valley pond unit along the Petrare valley to the NW of the crater, as far as 7 km (Fig. 1). The Peperino Albano covers an area of approximately 40 km2 and its thickness varies from a few metres to up to 30 m within palaeovalleys. A total minimum volume of the deposit is approximately 0.2 km3 . The intense zeolitisation of the deposit makes it di⁄cult to extract good samples for accurate grain size, component and chemical data. Field analysis and point counting on thin sections show that the juvenile component of the deposit, which makes up the 90%+ of the unit, is consistently ¢ne-grained, with a maximum size of coarse ash (Fig. 2). Juvenile ash is typically grey in colour. The ¢ne ash matrix is pervasively zeolitised, giving the unit the lithi¢ed mechanical characteristics for which it has been long used as a construction stone. Scattered in the ash matrix there are up to 10% of leucite, clinopyroxene and biotite crystals and up to 10% by volume of lava, sedimentary (limestone) and intrusive lapilli- to bomb-sized xenoliths. Existing chemical analyses of the Peperino Albano ignimbrite plot in the leucitite ¢eld in the TAS diagram (Trigila et al., 1995 and references therein). However, chemical analyses refer to the bulk rock composition, because of the absence of fresh juvenile large clasts of scoria
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or pumice and the pervasive zeolitisation. X-Ray di¡ractometry indicates the presence of abundant mica, clinopyroxene, leucite, plagioclase and chabasite (Fig. 3). The ignimbrite shows unburnt clusters of £attened in situ steppe grass that grew in the underlying palaeosol. Clusters of arboreal vegetation showing minimal conversion to charcoal suggest that the ignimbrite had a minimal thermal e¡ect on the vegetation. The presence of unburnt plants indicate that the temperature of the pyroclastic £ow must have been lower than the temperature of ignition of wood, i.e. lower than or around 246‡C (Macdonald, 1972). A perfect mould of a vulture has also been reported complete with its plumage (Fornaseri et al., 1963). On the other hand, the subordinate presence of accretionary lapilli or of other common occurrence of agglutinated ash-particles (although locally present) suggest that temperature was higher or around 100‡C. This range of temperature is also well in agreement with temperature required for the pervasive zeolitisation of glass shown by the ignimbrite (cf. de’ Gennaro et al., 1999). The most common zeolites throughout the ignimbrite are chabasite and herschelite (Fig. 3). However, the lowest 2 m of the deposit show a progressive increase in chabasite. This can be related to the temperature pro¢le developed within the ignimbrite after the emplacement, characterised by lower temperature at the base that favoured the formation of clay minerals, and progressively higher temperature upward, which would have favoured the formation of chabasite and herschelite, which has a higher temperature of formation (C. Giampaolo, Universita' Roma TRE, personal communication). Assessing the vesicularity of pyroclasts of the Peperino Albano ignimbrite is a very di⁄cult task because of the pervasive transformation of glass particles into zeolite minerals (Fig. 2D). Thin sections do not show well-preserved shards helpful in de¢ning the vesicularity. However, it is clear that the juvenile component is entirely ¢ne ash (Fig. 2) throughout the deposit, which implies a very e⁄cient mechanism of fragmentation. SEM analysis allows to recognise some non-altered shard morphologies that show blocky shape and
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Fig. 3. X-Ray Di¡ractometry analysis of the Peperino Albano (courtesy of Ciro Giampaolo and Sergio Lo Mastro, Univ. Roma TRE). lc = leucite; cpx = clinopyroxene; bt = biotite and phlogopite; ch = chabasite and herscelite; cc = calcite; pl = plagioclase.
hydrofractures cross-cutting gas bubbles, typical of magma^water interaction (Fig. 2E,F).
3. Facies characteristics of the Peperino Albano ignimbrite We subdivide the Peperino Albano ignimbrite into three facies that di¡er in the structural and textural features of deposit, related to di¡erent palaeotopographic locations. (a) Valley pond facies (VPI). At the onset of the Peperino Albano eruption, the outer slopes of the Lago Albano maar were incised by deep valleys that channelled the Peperino Albano pyroclastic £ows. The most common facies shown by the Peperino Albano ignimbrite within the palaeovalleys is the VPI, which can be up to 30 m thick. It is characterised mainly by a massive, matrix-sup-
ported structure. Deposits are poorly sorted, with bombs and blocks scattered in the ash matrix (Fig. 4A). The ignimbrite generally does not show evident grading of coarse clasts. However, the base of the deposit may show reverse grading of lapilli-sized xenoliths and at some location planeparallel bedding is also present (Fig. 4B; Petrara Valley, km 2.5 km from the vent). The ignimbrite is generally homogeneous throughout its thickness. However, at some locations it is possible to distinguish up to ¢ve depositional units separated by several-centimetre-thick ¢ne-gained layers (Petrara Valley, 2 km from the vent). At the Marino Town train station (1 km from the vent, along the Petrara Valley), the ignimbrite is 30 m thick and is massive throughout its thickness without evidence of multiple depositional units. However, 25 m above the base of the unit, a matrix-supported lithic concentration zone occurs,
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Fig. 4. (A) Valley pond massive facies of the Peperino Albano. Note the occurrence of abundant, white xenoclasts of limestones. (B) Strati¢ed base to the Peperino Albano at 2.5 km from vent. (C) Lithic concentration zone occurring 25 m above the base of Peperino Albano in VPI at 1 km from vent, Petrara Valley. (D) Di¡usely strati¢ed facies of the Peperino Albano along the £ank of the Petrara Valley.
with clasts up to 1.3 m in diameter (Fig. 4C). The typical massive and chaotic VPI grades laterally into a di¡usely strati¢ed facies where the ignimbrite laterally onlaps the steep sides of the palaeovalleys (Marino Town). Strati¢cation is de¢ned by several ¢nes-depleted, elongated lenses of milli-
metre- to centimetre-sized xenoliths and juvenile clasts generally occurring up to a distance of 1^5 m away from the palaeovalley side-slope (Fig. 4D). Fines-depleted lenses are fan shaped in cross section, being almost parallel to the ground surface at the base and progressively
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Fig. 5. (A) Strati¢ed IVD of the Peperino Albano along the Marino ridge. (B) Agglutinated ash lumps making up one bed of the IVD. (C) Dunes within the IVD along the Marino ridge. Note that dunes are stationary (i.e. crests grew vertically) and are shaped on the morphology of the palaeoslope. (D) Scoria layer at the base of the Peperino Albano along the western rim of the Albano maar. The scoria layer either represents an early fallout phase or belongs to a di¡erent volcanic centre. (E) Massive intracrater facies of the Peperino Albano, which drapes the inner wall of the Albano maar. Xenoliths are larger than the average dimension found in the out£ow deposits and concentrated toward the base.
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more horizontal upward. The most distal outcrop of the Peperino Albano ignimbrite is at 7 km from vent, where the VPI is 3 m thick and shows normal grading of coarse clasts that concentrate toward the base. Coarse clasts are rounded and mostly made of leucitite lava up to 1 m in diameter derived from the nearby Capo di Bove lava £ow (Fig. 1). Clasts were therefore ripped-up in the Peperino Albano from the bottom of the talweg. (b) Ignimbrite veneer facies (IVD). The IVD is commonly found around the crater rim and along valley ridges. The IVD is parallel to low-angle cross-strati¢ed (Fig. 5A). Bedforms are generally aggradational and mirror the underlying topography. Bedding and strati¢cation are de¢ned by the alternation of ¢nes-depleted, non-zeolitised to poorly-zeolitised, lapilli-sized layers, rich in xenoliths, crystals and agglutinated ash droplets (Fig. 5B) and massive, matrix-supported, zeolitised beds up to 1 m in thickness that display a facies similar to the VPI. Elongated clasts may be oriented along the £ow direction. Average xenolith content generally exceeds 10% by volume although maximum diameters are always smaller than those measured in the VPI at similar distances from the vent. The thickness of this facies varies according to palaeotopography, but generally is less than 10 m thick. IVD developed at the crater rim is characterised by thicker beds that are lenticular in shape. Fines-depleted, xenolith-rich beds can be up to 80 cm thick and generally display a crude, low-angle, cross-bedded strati¢cation. Massive, matrix-rich beds can be up to 1 m thick (Fig. 5A). Ballistic bombs and related impact sags are usually present at the base of the unit very close to vent and are found along the crater rim. The Marino Town section (1 km from vent, Fig. 1) is an excellent example of IVD developed on a ridge transversal to £ow direction. The Valle Petrara is almost perpendicular to £ow direction in proximal location. Dunes are developed along the stoss side of the topographic ridge, i.e. where pyroclastic £ows climbed the topography. Dunes are aggradational and have wavelength exceeding 10 m and height of 2^3 m. Massive beds thicken on the stoss sides of the dunes (Fig. 5C). Ash-pellet-rich layers are common at
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this site. Unburnt blades of grass bent in the £ow direction away from the vent are present at the very base of the unit. Maximum lithic size is in the order of few centimetres. At Castelgandolfo location, the crater rim IVD is underlain by a several-centimetre-thick brown scoria layer, which rests on the palaeosoil (Fig. 5D). Scoria clasts are highy vesicular and well sorted with an average diameter of 1 cm. The deposit appears to be a strombolian fallout deposit. This is the only instance where a vesicular and non-zeolitised pyroclastic deposit is found at the base of the Peperino Albano succession. It is unclear whether this deposit belongs to the Peperino Albano succession or, more likely, was erupted earlier from one of the scoria cones surrounding the Albano maar. However, these scoria clasts give a reasonable idea of the style of magma fragmentation in absence of or with little water involved in the eruptive mechanism. (c) Intracrater facies. Several tongues of Peperino Albano ignimbrite drape the internal walls of the Lago Albano maar from the crater rim down to the lake level, showing a quaquaversae dipping. The intracrater facies of the Peperino Albano ignimbrite is characterised by a massive structure and chaotic texture (Fig. 5E) and is very similar to the VPI. The ignimbrite is generally less than 15 m thick. The body of the ignimbrite is generally homogeneous throughout its thickness. The ignimbrite is made up of up to 85% by volume of juvenile ash-sized component, with approximately 5% of leucite, clinopyroxene and biotite crystals and up to 15% of xenoliths. Compared to the VPI, xenoliths are more abundant, larger in size (up to 2 m in diameter) and intrusive types are more abundant. At one location (Fonte S. Pietro), xenolith bombs are normal graded and concentrated at the base of the unit as a lag deposit or a co-ignimbrite breccia. At some locations it is possible to recognise some internal layering parallel to the slope. The intracrater facies is laterally continuous with the IVD at the crater rim. 3.1. Discussion The Peperino Albano ignimbrite is a typical
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Fig. 6. Schematic cross section through the di¡erent facies of the Peperino Albano ignimbrite (the trace A^B is annotated in Fig. 1) and model of emplacement. Valleys act as traps for the denser parts of pyroclastic £ows which become more concentrated and gravity controlled, so that £ow direction might vary through a vertical section of the same £ow. Flow’s concentration and uniformity control the rate of deposition and development of massive versus strati¢ed facies.
high-aspect-ratio, valley-ponded, small-volume ignimbrite. All these characteristics are generally suggestive of low-mobility, high-concentration pyroclastic £ows perhaps close to the plug-£ow analogue end member for pyroclastic density currents (e.g. Sparks, 1976; Battaglia, 1993). However, the Peperino Albano also shows an IVD, indicating the ability to climb topographic highs. Bedforms and structures of the IVD suggest a more diluted transport system closer to the surge end member for pyroclastic density currents (e.g. Valentine, 1987; Fisher, 1990). The occurrence of the IVD points out that processes of pyroclastic £ow transformation laterally and related to topography are not necessarily peculiar to high-energy, low-as-
pect-ratio acidic ignimbrites as generally assumed, but also to low-temperature phreatomagmatic ignimbrites of basic composition. A key point for the understanding of the transport and depositional mechanisms associated with the Peperino Albano emplacement is to analyse a detailed cross section across the crater rim^Petrara Valley^Marino ridge (Fig. 6). Along this cross section, thanks to continuous exposure, it is possible to observe in the ¢eld the lateral transition from the IVD along the crater rim to the VPI in the Petrara valley, again to the IVD on the Marino ridge. The fewmetre-thick IVD is characterised by well-developed bedforms, which are made up of massive and poorly sorted beds alternated with coarse-
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and ¢ne-grained well-sorted beds (Figs. 6A and 5A). It laterally passes to a crudely strati¢ed deposit where the ignimbrite thickens along the valley margins (Figs. 6B and 4D). The transition to the VPI is marked by lateral thickening and amalgamation of the several massive and unsorted beds and the contemporaneous thinning and progressive disappearance of the well-sorted layers, which laterally become thin lenses and further within the valley disappear completely (Figs. 6C and 4C). The only evidence of layering within the VPI is marked by block and lapilli lithic concentration zones that occur at di¡erent heights through the deposit (Fig. 4C). We interpret the origin of this facies transition in terms of temporal and spatial variations of the sediment supply from the transport system to the depositional system of the Peperino Albano pyroclastic density current (cf. Valentine, 1987; Fisher, 1990; Branney and Kokelaar, 1992; Druitt, 1992). IVD facies suggests the vertical aggradation of several beds. The alternation of massive and unsorted beds with cross-bedded and sorted layers can be interpreted to either re£ect variations in sediment supply to the depositional system fed by the density current or the occurrence of several independent depositional events. The two cases are in fact not so di¡erent. The deposition of a massive and unsorted bed can be interpreted as re£ecting a process of high sedimentation rate, precluding the formation of bed structures (Lowe, 1988; Kneller and Branney, 1995). Particle sedimentation is driven by processes such as hindered settling (e.g. Druitt, 1992). By contrast, the deposition of a sorted and strati¢ed bed can be interpreted as re£ecting a lower sediment supply to the depositional system wherein bed structures can be developed (Allen, 1982; Cas and Wright, 1987). Particle sedimentation is driven mainly by grain to grain deposition (Allen, 1982). The alternation of such beds at the maar rim can be reasonably related to variations in the eruption rate which in turn can a¡ect the particle concentration and the transport energy of the pyroclastic density current (or currents) through time. Eyewitness observations indicate that phreatomagmatic eruptions are commonly characterised by high instability of the eruption rate and the occurrence of
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several hundred to thousands of high-frequency explosions during a single eruptive phase (e.g. Moore et al., 1966 ; Waters and Fisher, 1971). This information is important in that it may relate to the deposition of the strati¢ed IVD at the maar rim and points out that the distinction between several rapid depositional events and one depositional event characterised by high instability is a semantic one. The lateral transition to the VPI may indicate the lateral and topography-induced increase in particle concentration within the pyroclastic £ow. The increase in thickness of the massive and unsorted beds is suggestive of an increased tendency to deposit at higher sedimentation rates (Kneller and Branney, 1995; White and Schmincke, 1999). This also indicates that part of the erupted particles by-passed the crater rim and fell within the depositional system where the pyroclastic £ow was forced within the valley because correlatable packages have di¡erent thickness, i.e. at any time, the volume of debris deposited within the valley was greater than that deposited on the crater rim (Fig. 6). Lithic concentration zones should therefore correlate with variations through time of the eruption dynamics, such as conduit wall collapse, or vent widening etc. Furthermore, the disappearance of the layering and of sorted lenses within the massive VPI facies suggests that the pyroclastic density current was sustained through time and sediment supply high enough to hinder the formation of bedforms (Lowe, 1988; Kneller and Branney, 1995). It is therefore more likely that the IVD bedding and layering at the maar rim is to be related to the £ow dynamics rather than the occurrence of several discrete depositional events. This interpretation leads to the consequence that a pyroclastic £ow can start as a highly pulsatory £ow and can laterally transform into a uniform £ow if topographic conditions, such as the channellisation inside a deep ravine induce a rapid increase in £ow concentration and in its tendency to deposit (cf. Kneller and Branney, 1995; Giordano, 1998b). This story reverses where the pyroclastic £ow was forced to climb a ridge like at the Marino ridge, where the VPI laterally grades into the IVD facies. In this case the layering and the development of alternate massive and unsorted beds with sorted
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lapilli beds should relate to variations in sediment supply to the depositional system induced by the rapid rise of the underlying topography which probably forced most of the debris along the valley, detaching the upper and more diluted part of the £ow in order to climb the ridge (Fisher, 1990, 1995; Giordano and Dobran, 1994; Woods et al., 1998; Legros and Kelfoun, 2000). It is important to underline that maximum lithic size is larger within the VPI compared with maximum lithic size in the IVD. It is also important to point out that bedforms are found in phase with the underlying topography (Fig. 5C), suggesting that variations in £ow dynamics are intimately related with topographic e¡ects. The intracrater facies is somewhat intriguing as it is massive and poorly sorted, forms tongues that drape the inner wall of the Albano maar, it is lithic-rich and displays a basal breccia. The formation of the VPI-like intracrater facies can be related to processes of £ow bulking similar to that described for the VPI. However, it remains unclear whether this facies represents the back£ow of a pyroclastic density current that was not able to climb the rim, or the deposit of a pyroclastic density current that was able to climb the rim, or a portion of a collapsing column that collapsed inside the crater (cf. Valentine et al., 1992). In any of the above cases the intracrater facies shows the largest percentage of lithic clasts, as well as the largest lithic dimensions and this is in accordance with the proximality of this facies. Another issue for the Peperino Albano eruption is the source of water involved in the phreatomagmatic eruption. The abundance of limestone xenoliths suggest that a possible source of water is groundwater contained within the Mesozoic^Cenozoic carbonatic karstic aquifer underlying at approximately 1 km below the surface of the Albano maar (Funiciello and Parotto, 1978). We cannot exclude, however, a contribution from crater lake water at least during the earlier phases of the eruption, considering that the Albano maar was already in existence at the time of the Peperino Albano eruption and therefore likely to host a lake. Unfortunately we cannot ascertain whether the scoria layer found at Castelgandolfo locality underlying the Peperino Albano is to be related to
the Peperino Albano eruption or not. However, the Peperino Albano certainly does not show vesicular clasts throughout, and this is suggestive of magma^water interaction at a depth greater than the magma vesiculation depth, so that phreatomagmatic fragmentation occurred when exolution processes were not signi¢cant.
4. Conclusions We presented the facies association of the ca. 29-ka, small-volume, basic phreatomagmatic Peperino Albano ignimbrite, from the polygenetic Albano maar (Colli Albani volcano, central Italy). The ignimbrite shows VPI as well as veneer deposits along the maar rim and along topographic ridges. We have interpreted the lateral facies variations in terms of temporal and spatial variations of the sediment supply from the transport system to the depositional system of the Peperino Albano pyroclastic £ow. IVD at the maar rim may re£ect variations in the eruption dynamics, whereas VPI may re£ect the bulking of the pyroclastic £ow inside the valleys and consequent high sedimentation rates. IVD at topographic ridges has been interpreted to re£ect variations in sediment supply to the depositional system of the £ow related to detachment of more concentrated undercurrents draining within valleys from the more dilute upper part of the pyroclastic £ow that climbs the ridges and interacts with them, resulting in increased turbulence. The present interpretation for the facies association shown by the Peperino Albano ignimbrite suggests that processes of pyroclastic £ow transformation laterally and induced by topography are not necessarily peculiar of high-energy, low-aspect-ratio acidic ignimbrites as generally assumed. The more likely source of water interacting with magma is interpreted to be groundwater contained within the karstic aquifer located at approximately 1 km below the ground level, as is suggested by the large amount of limestone xenoliths present in the Peperino Albano. The absence of vesicular juvenile clasts suggests that magma^ water interaction occurred when gas exolution processes were not signi¢cant.
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