Bull Volcanol (1992) 54:253-266

Vol6i ology �9 Springer-Verlag 1992

Post-collapse volcanic history of calderas on a composite volcano: an example from Roccamonfina, southern Italy P D Cole 1 . , JE Guest 2, A M Duncan 1 , D K Chester 3, and R Bianchi 4

School of Geological & Environmental Science, Luton College of Higher Education, Luton, LU1 3JU, UK 2 University of London Observatory, University College London, Mill Hill Park, London, NW7 2QS, UK 3 Department of Geography, Liverpool University, Liverpool, L69 3BX, UK 4 Consiglio Nazionale delle Ricerche, IAS c/o Area Ricerca Frascati, Via G. Galilei CP 27, 1-00044 Frascati (Roma), Italy

Received March 18, 1991/Accepted November 15, 1991

Abstract. Roccamonfina, part of the Roman Potassic Volcanic Province, is an example of a composite vol- cano with a complex history of caldera development. The main caldera truncates a cone constructed predom- inantly of potassic lavas of the HK Series. The initial collapse of this caldera may have been associated with one of the ignimbritic eruptions of the Brown Leucitic Tuff (BLT) around 385000 yr BP. The Campagnola Tuff, the youngest ignimbrite of the BLT, however, drapes the caldera margin and must postdate at least the initial stages of collapse. During the subsequent his- tory of the caldera there were several major explosive eruptions. The largest of these was that of the Galluccio Tuff at about 300000 yr BP. It is likely that there was further collapse within the main caldera associated with these eruptions. It is of note that despite these sub- sequent major explosive eruptions later collapse oc- curred within the confines of the main caldera. Be- tween eruptions caldera lakes developed producing nu- merous lacustrine beds within the caldera fill Exten- sive phases of phreatomagmatic activity generated thick sequences of pyroclastic surge and fall deposits. Activ- ity within the main caldera ended with the growth of a large complex of basaltic trachyandesite lava domes around 150000 yr BP. Early in the history of Rocca- monfina sector collapse on the northern flank of the volcano formed the northern caldera. One of the youngest major events on Roccamonfina occurred at the head of this northern caldera with explosive activity producing the Conca Ignimbrite and associated cal- dera. There is no evidence that there was any linkage in the plumbing systems that fed eruptions in the main and northern calderas.

* Now at Dipartimento di Geofisica e Vulcanologia, University degli studi Napoli, Italy

Offprint requests to: AM Duncan

Introduction

The recent unrest exhibited by the Long Valley, Campi Flegrei and Rabaul calderas has promoted an increas- ing interest in the longterm post-collapse history of cal- deras of different types (Newhall and Dzurisin 1988). Knowledge of a caldera's activity following collapse is of profound importance not only to our understanding of sub-surface mechanisms and structure, but also in forecasting future activity especially for those calderas that contain or are surrounded by heavily populated ar- eas.

In this investigation techniques are employed which we have developed in earlier studies of caldera-like fea- tures on Mount Etna (Guest et al. 1984) and Monte Vulture (Guest et al. 1988). Analysis of relict slope sur- faces enables reconstruction of the palaeotopography of the volcano to be made at different stages in its his- tory. This, together with examination of the associated volcaniclastic deposits and their stratigraphy, provides an effective approach to understanding the volcano's history.

Roccamonf ina

Roccamonfina is a Quaternary volcano located approx- imately 60 km northwest of Vesuvius (Fig. 1). The vol- cano has been the focus of considerable geochemical interest (Appleton 1972), especially with reference to the petrogenesis of potassium-rich magmas. The pion- eering work of Bernardino Giannetti and his co-work- ers (Giannetti 1979a, 1979b; Giannetti and Luhr 1983; Luhr and Giannetti 1987) has established the main structural and stratigraphic elements of the volcano.

The base of Roccamonfina, which to the southwest rests on the coastal plain, has an average diameter of about 18 km. The lower slopes of the volcano are gener- ally less than 6 ~ up to an altitude of about 400 m asl where there is a gentle break in slope; the cone then rises at up to 20 ~ to the rim of the main caldera, which truncates the summit. The overall symmetry of the vol- cano is broken by an apparent sector collapse to the north, containing a small caldera at its head, and by

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deep dissection of the pyroclastic deposits of the north- ern and eastern flanks.

The history of the volcano, summarised in Table 1, may be divided into two stages (Giannetti 1979a). Dur- ing Stage I, a strato-volcano was constructed from leu- cite-bearing lavas and pyroclastics of the High-K Se- ries, together with reworked materials. Radiometric dating indicates that this first stage lasted from about 1.54my to 0.34my, during which time Giannetti (1979a) estimates that between 100 and 120 km 3 of ma- terial was produced. Near the end of Stage I, several distinct eruptions produced a number of ignimbrites, collectively labelled by Giannetti as the Brown Leucitic Tuff (BLT). Here the uppermost of these ignimbrites is named the Campagnola Tuff.

Products of Stage II differ from those of Stage I in that less potassic and leucite-free lavas and pyroclas- tics, typical of the K Series of the Roman Province, were erupted. Recent work (Radicati di Brozolo et al. 1988) has shown, however, that lavas of the High-K Se- ries continued to be erupted during Stage II, with the youngest dated lava of this series having an age of 53 000 BP. One of the first major events during this stage was the production of pyroclastic flows that em- placed several ignimbrite units referred to by Giannetti as the White Trachytic Tuff (WTT). Although the bulk of the WTT formed ignimbrites and was the product of one eruption, the presence of a soil within the upper part of the intracaldera sequence of the WTT, which is predominantly of lacustrine facies, (Fig. 6) indicates

255

Table 1. General stratigraphy of Roccamonfina Volcano

Age my Major explosive events Comments

0.035 Campanian lgnimbrite Thought to be erupted from the Campi Flegrei region. Fills radial (1) valleys and caldera on Roccamonflna. (Barberi et al. 1978)

0.3 (3) Conca lgnimbrite Domes of Mt Croce and Lattani possess unclear age relations, (formerly Yellow Trachytic Tuff) however probably represent final activity in main caldera

Conca lgnimbrite restricted to northern flank Upper caldera fill predominantly phreatomagmatic with abundant

Stage II pyroclastic surge deposits. Ephemeral caldera lakes Galluccio Tuff forms exposed base of caldera fill comprising sev- eral ignimbrites. Plinian airfall and extensive pyroclastic surges in extracaldera sequence only

Galluccio Tuff (part of White Trachytic Tuff)

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Campagnola Tuff (upper part of Brown Leucitic Tuff)

Several Brown Leucitophyric Ignimbrites separated by extensive soils

Initial caldera collapse? (Luhr and Giannetti 1987)

Early activity involving mainly effusive activity and minor pyro- clastics. Explosive activity predominated toward the end of Stage I, evidenced by the presence of several ignimbrites beneath the Campagnola Tuff

Source of dates: (1) radiocarbon, Rosi and Sbrana (1987), samples from Campi Flegrei; (2) K/At, Giannetti et al. (1979), lava from west flank; (3) K/Ar sanidine, Giannetti and Luhr (1983); (4) K/Ar biotite, Luhr and Giannetti (1987)

that the WTT, as defined by Giannetti and Luhr (1983), includes more than one eruption. On the basis of an intercalated palaeosol, Ballini et al. (1989a), also con- sider the WTT to be the product of more than one erup- tion. In this paper the ignimbrites of the White Tra- chytic Tuff which lie above two distinctive palaeosols (see Section R90/92, Fig. 7), the upper of which ap- pears to be the same as that of Ballini et al. are named the Galluccio Tuff. These we consider to be the product of one eruptive event. There then followed more local- ised activity, predominantly phreatomagmatic, in the main caldera and this ended with the construction of a large dome complex. At a late stage in the northern cal- dera small to intermediate volume pyroclastic flows were erupted to form the Conca Ignimbrite. This ignim- brite was called the Yellow Trachytic Tuff by Luhr and Giannetti (1987), but this is an inappropriate name as there are other trachytic ignimbrites of similar appear- ance on Roccamonfina.

Although not the focus of this work, a composite stratigraphic section of the Campagnola Tuff and younger major pyroclastic deposits on the flanks of the volcano is shown in Fig. 2. It is of interest to note that co-ignimbrite lithic breccias are present within both the Campagnola (upper part) and Galluccio Tuffs (Middle Unit). These are thicker and more extensive in the Campagnola Tuff and it is possible that these breccias are related to episodes of collapse of the main cal- dera.

In this paper we are concerned particularly with the styles of activity and morphological evolution of the volcano after the initial formation of the main caldera. Four phases in the volcano's history are considered: (1) immediately pre-main caldera; (2) immediately post in-

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itial caldera collapse and subsequent major pyroclastic eruptions; (3) the period of caldera filling by predomi- nantly phreatomagmatic activity and (4) late-stage ac- tivity with formation of the lava domes within the main caldera and explosive activity from the Conca Crater in the northern caldera.

1 Pre-main caldera (Fig. 3a)

The cone-building phase of Stage I involved mainly ef- fusive activity with the eruption of strongly undersatu- rated lavas of the High-K Series. A date of 1.54 + 0.06 my (Giannetti et al. 1979) for a lava on the western flank implies that this activity must have been initiated more than 1.5 my ago. The original slopes of this cone are well preserved on the western flank of the volcano. By interpolating contours from preserved relict surfaces of the cone it can be established that it had an elliptical shape with its longest axis orientated NW-SE (Fig. 3a). We suggest that the elliptical shape is due to there be- ing two overlapping cones as shown in the topographic reconstruction (Fig. 3a) or that there was a single main construct with a cluster of parasitic cones to the south- east.

The age and origin of the northern caldera is un- clear. Giannetti and Luhr (1983) referred to this feature as the 'gli Stagli' caldera but morphological examina- tion shows this to be a composite feature formed by at least two separate collapse events. Firstly, there is the large horseshoe-shaped amphitheatre open to the north and we consider that this formed by sector collapse. The presence of ignimbrite of the Brown Leucitic Tuff in this caldera supports the view that this is an old fea- ture. Subsequent collapse at the head of the northern caldera occurred at a later stage and is discussed be- low.

2 Immediately post main caldera and the major ignimbrite-forming eruptions (Fig. 3b)

The main caldera was elliptical in shape with the long- est axis, 6.5 km, aligned NW-SE and the shortest axis 5 km. Its rim is well preserved to the south and west and rises to a height of 286 m above the floor on the western edge. The north and eastern edge of the caldera is poorly preserved but its position can be established from detailed field observation and analysis of aerial photographs.

The timing of caldera formation is still unclear. To- wards the end of Stage I, however, a number of explo- sive eruptions generated ignimbrites of small to inter- mediate volume. Luhr and Giannetti (1987) collectively called these ignimbrites the Brown Leucitic Tuff (BLT)

Fig. 3. Palaeotopographic reconstruction of Roccamonfina before (a) and immediately after (b) the initial formation of the main caldera. Solid contours are derived from relict surfaces while the dashed ones are interpretations of pre-existing topography. The inset sketches provide an impression of the likely appearance of the volcano viewed from the east at each stage in its evolution

and they are typified by being brown-orange in colour and containing leucite-phyric pumice. It was suggested by Giannetti et al. (1979) that the ignimbrites of the BLT were related to the formation of the main caldera, but subsequently Luhr and Giannetti (1987) argued that the BLT postdated caldera formation. Palaeosols occur between a number of these ignimbrites demonstrating that they represent several distinct eruptions and it is the youngest of these ignimbrites that we have named the Campagnola Tuff. Near Mt Atano (Fig. 3), howev- er, the Campagnola Tuff can be seen to drape the east- ern rim of the main caldera and therefore this, at least, the youngest of the BLT ignimbrites, must have been emplaced after the initial caldera collapse. The pumice from the Campagnola Tuff is not strongly alkalic in terms of its major element chemistry (Table 2), typically giving a trachyandesitic composition. However, the pu- mice has been altered, with the leucite crystals replaced by analcite, and the composition of the pumice, there- fore, may not closely reflect that of the erupted mag- ma.

The Campagnola Tuff is a compound ignimbrite dis- playing typical features of pyroclastic flow deposits: coarse tail grading, associated ground surges and a val- ley-filling aspect. Near the rim of the caldera the Cam- pagnola Tuff shows numerous facies variations with abundant cross-bedded surge horizons interlayered with discontinuous lithic breccias and pumice concen- tration zones, which grade both vertically and horizon- tally into massive ignimbrite. Away from the rim, mov- ing eastwards onto the flank of the volcano, the Cam- pagnola Tuff may be traced laterally into homogeneous ignimbrite. It is considered that the heterogeneous fa- cies relationships on the caldera rim area were caused by the turbulent nature of the flow in the proximal re- gion, possibly accentuated by the effects of topography as the flow passed over the rim of the caldera (Cole 1990).

An exploratory geothermal borehole was recently drilled in the northern part of the main caldera and this has provided a core through the caldera fill (Watts 1987). Interpretation by Watts that the lower part of the fill is made up of 140 m of BLT is disputed by Ballini et al. (1989b) who consider this material to be a vent ag- glomerate. It is clear that caution needs to be exercised in the interpretation of borehole data on volcaniclastic lithologies which are likely to be heterogeneous on a large scale. Indeed, the so-called mud flow deposits identified near the base of the sequence may be mega- breccias related to caldera collapse of the type de- scribed by Lipman (1976).

Between the eruptions of the Campagnola Tuff and the Galluccio Tuff there was at least one major explo- sive event. On the lower flank of the volcano to the northeast, near Catailli, an ignimbrite rich in white pu- mice occurs stratigraphically below the Galluccio Tuff. This ignimbrite, named here the Saraceni Tuff (see Sec- tion R106, Fig. 7), has a trachytic composition similar to that of the later ignimbrites of the Galluccio Tuff (Table 2). Overlying this ignimbrite is a sequence of some 40m of debris flow and hyperconcentrated

258

Table 2. Whole rock geochemistry of pumice and scoria clasts

Sample 641 512 97c 533 63 69 92 435 418 268 246 252 51 108 709

SiO2 55.28 54 .64 55.99 58.42 60.68 63.82 61.22 63.21 58.71 58.71 56.1 61 60.42 51.5 57.57 TiO~ 0.59 0.64 0.52 0.4 0.3 0.25 0.29 0.22 0.34 0.44 0.56 0.37 0.45 0.84 0.41 A1203 24 19.32 19.12 20.31 17.82 1 7 . 4 18.22 17.43 20 19.9 18.08 18.09 17.75 18.01 17.63 Fe203t 3.84 4.83 4.06 2.59 2 1.66 2.19 2.04 2.40 3.23 5.38 3.69 3.95 8.3 3.5 MnO 0.14 0.17 0.12 0.14 0.17 0.16 0.19 0.21 0.1 0.09 0.12 0.14 0.14 0.15 0.12 MgO 0.4 1.19 0.82 0.15 0.18 0.04 0.14 0 0.19 0.49 2.3 0.79 1.02 3.99 1.51 CaO 2.31 3.84 3.84 2.04 1.91 1.24 1.78 1.14 2.1 3.06 5.34 3 3.52 8.73 4.15 Na20 1.42 1.66 2.02 3.69 4.41 5.37 4.48 4.97 4.24 3.37 3.14 3.72 4.25 2.79 0.79 K20 4.41 5.97 6.27 7.72 6.07 5.84 5.78 6.75 7.91 8.05 5.25 6.16 5.99 4.45 4.43 P205 0.12 0.14 0.13 0.11 0.05 0.01 0.04 0.09 0.13 0.16 0.26 0.2 0.19 0.45 0.13 LOI nd 7.16 5.88 4.47 4.54 2.8 4.77 3.54 3.45 2.11 3.07 3.09 2.24 0.77 9.31

Total 92 .51 99.57 98.77 100.0 98.13 98.56 99 .1 99 .59 99.58 99.62 99 .6 100.3 99 .93 99 .98 99.55

Ba 520 nd 900 nd 121 14 115 nd nd nd nd nd nd nd nd La 87 nd 88 nd 170 160 187 nd nd nd nd nd nd nd nd Rb 127 nd 289 547 438 413 484 538 436 373 234 372 428 227 411 Sr 767 nd 1230 264 171 49 157 55 386 882 766 556 512 1268 2173 Y 44 nd 33 41 54 48 57 40 35 30 35 44 44 30 36 Zr 439 nd 267 505 700 637 784 675 416 305 285 403 461 178 244

nd, not determined General location and grid reference for each sample in brackets Low total for Sample 641 due to LOI not being determined

641 Brown Leucitic Tuff (N caldera 134772) 512 Campagnola Tuff (E flank 187715) 97c Campagnola Tuff (By M Atano 175712)

533 Saraceni Tuff (NE flank 184794) 63 Lower Galluccio Tuff (M Caldera 162702) 69 Lower Galluccio Tuff (M Caldera 167707) 92 Lower Galluccio Tuff (M Caldera 153693)

709 Conca lgnimbrite (N Caldera 152775) 418 Middle Galluccio Tuff (SW flank 127668) 268 Upper Galluccio Tuff (SW flank 127675) 246 Upper Galluccio Tuff? (M Caldera 153693) 252 Garofali Formation (M Caldera 152699) 51 Garofali Formation (M Caldera 144696)

108 Fontanafredda Ignim. (M Caldera 130703) 435 Lower Galluccio Tuff (Galluccio 132777)

Samples 641, 97c, 63, 69, 92 analysed by XRF at Liverpool University using pressed pellets Samples 512, 533, 709, 435, 418, 268, 246, 252, 51, 108 analysed by XRF at Keele University using fused beads

stream deposits indicating considerable erosion in the northern caldera prior to the emplacement of the Gal- luccio Tuff.

The Galluccio Tuff is named after the quarry at Gal- luccio where these deposits are well exposed (Cole 1990). We have made this modification to the strati- graphic nomenclature of Giannetti (1979a) for two rea- sons. Firstly, the White Trachytic Tuff (as discussed earlier) has at least one intercalated soil horizon de- monstrat ing that it is the product of more than one eruption. Secondly, neither the colour nor the petrology are characteristics unique to this deposit on Roccamon- fina. The Galluccio Tuff forms the base of the exposed caldera fill and has been dated at 0.307 + 0.004 my com- pared with 0.385+0.023 my for the BLT (Dates f rom Giannetti and Luhr 1983 and Luhr and Giannett i 1987, see Table 1). Radicati di Brozolo et al. (1988) consider, on the basis of dating of an intra-caldera lava, that the main caldera is at least 400 000 years old. Therefore, it is likely that the Galluccio Tuff was erupted some 100000 years after the initial collapse of the main cal- dera. Further collapse of the caldera is likely to have occurred as a result of this eruption.

The Galluccio Tuff is made up of three eruptive units: the Lower, Middle and Upper Galluccio Tuff (LGT, M G T and UGT). The term 'eruptive unit ' is used here to mean a set of successive pyroclastic deposits

formed during a short time interval and which are com- positionally homogeneous (modified f rom Freundt and Schmincke 1985). The L G T contains frothy, crystal- poor ( ~ 5 wt%) pumice, the M G T frothy, well-vesicu- lated crystal-rich ( - 16 wt%) pumice and the U G T pre- dominant ly grey, relatively dense crystal-rich (12-19 wt%) pumice (Cole 1990). The ignimbrite is composi- t ionally zoned with the LGT being more evolved with higher abundances of SiO2 and Zr and lesser amounts of K20, MgO and CaO than the M G T and U G T (see Table 2). The account by Giannetti and Luhr (1983) on the petrogenesis of the White Trachytic Tuf f includes fuller geochemical data on the Galluccio Tuff. Only the L G T and a single thin flow unit which may be U G T have been recognised in the caldera-fill.

The intracaldera LGT occurs as massive unwelded ignimbrite with abundant well-vesiculated pumice. Sev- eral flow units have been documented and on the basis of the borehole evidence (Watts 1987), a possible 150 m of Galluccio Tuff is present beneath the present-day caldera floor. Surge deposits, interpreted as ground surges, showing sand-wave structures and bomb sags, are recognised between flow units. The total volume of magma (DRE) erupted during the Galluccio Tuff erup- t ion is estimated at 4.3 km 3 and this, with approxi- mately 1.7 km 3 of lithic material, forms a total volume of ~ 6 km 3.

259

3 The caldera-fill (Fig. 4a)

The main caldera-fill above the Galluccio Tuff crops out only in the southeastern part of the caldera (Fig. 5). Measured sections of the caldera-fill are shown in Fig. 6 and based on these data a composite stratigraphic succession of the main caldera-fill has been con- strueted. The caldera fill above the Galluccio Tuff con- sists in the main of lacustrine beds and phreatomag- matic deposits and we have called this sequence the Garofali Formation (it is best exposed around the vil- lage of Garofali). The presence of palaeosols overlying the Galluccio Tuff indicates a considerable period of quiescence before the onset of the phreatomagmatic ac- tivity. The products of the Garofali Formation are more basic than the preceding Galluccio Tuff, being mainly trachyandesitic in composition (Giannetti et al. 1979a).

Examination of the topography reveals that nested within the main caldera there are three small overlap- ping calderas, the northernmost of which cuts the outer rim of the main caldera to the northeast (Fig. 4a). The southernmost of these calderas is draped by the upper- most part of the Garofali Formation, whereas the west- ern sides of these calderas are obscured by the younger lava domes of Mt Croce and Mt Lattani. More than one episode of collapse is represented, but the lack of out- crop makes it impossible to relate these to known se- quences of volcanic deposits. It is likely, however, that these nested calderas relate to explosive deposits within the Garofali Formation.

Near Cese in the southern part of the caldera the Galluccio Tuff is overlain by up to 6 m of predomi- nantly surge deposits, here termed the Cese Member (see Section 1, Fig. 6). These surge deposits possess abundant sand-wave structures, of several types, as well as low-angle cross stratification. Accretionary lapilli, restricted to individual laminae, are present within these surge deposits. The orientation of bomb sags indi- cates a source to the north of Cese village.

In most places, however, the base of the Garofali Formation is marked by lacustrine beds up to 20 m thick indicating that a caldera lake was well developed at this stage. Lacustrine horizons occur within both the upper part of (Fig. 6) and beneath the Galluccio Tuff (Watts 1987), demonstrating that caldera lakes were de- veloped at several periods prior to the Garofali Forma- tion. The lacustrine facies of the Garofali Formation are composed typically of alternating fine-laminated ash beds and coarser lithic-rich material which are con- sidered to represent periods of eruptive quiescence and explosive activity respectively. The presence of both re- verse and growth faults within the lacustrine beds sug- gests that seismic activity occurred during the life of the caldera lake.

Prominent within the lacustrine sequence are a num- ber of coarse pumice horizons (pumice clasts up to 1 m) recognised by Giannetti and Luhr (1983). These pumice beds range from being clast- to matrix-supported, with the matrix made up of a diatomaceous clay/silt materi- al. The beds occur in individual horizons up to 10 m

thick. The presence of reverse grading within some of the units indicates that flotation of pumices probably occurred, where the smallest pumices became waterlog- ged and sank first followed by larger clasts. As the pu- mice blocks never exceed 1 m in diameter and occa- sional lithic fragments are included with the pumice, these beds are considered to be the proximal deposits of plinian-type eruptions which fell into the caldera lake rather than being the products of underwater dome eruptions as invoked for other coarse pumice beds (e.g. La Primavera, Mexico; Clough et al. 1981, and Lake Taupo, New Zealand; Wilson and Walker 1985). Pumice fragments of this size have been re- corded elsewhere in proximal regions of plinian depos- its, for example, the Askja eruption of 1875 (Sparks et al. 1981). Sparks et al. considered that pumice clasts greater than 50 cm in diameter were emplaced in a bal- listic fashion and that many of the larger clasts may have broken up on impact. At Roccamonfina preserva- tion of larger clasts will have been favoured by the cushioning effect of landing in the crater lake.

Localised lacustrine horizons at higher levels within the Garofali Formation indicate that, although a large sustained caldera lake was formed after the Galluccio Tuff eruption, as time progressed this lake was replaced by a number of localised ephemeral lakes. The abun- dance of fine-grained sub-cm material within the lacus- trine facies indicates that most of the contemporaneous volcanic activity was probably phreatomagmatic punc- tuated by occasional magmatic eruptions represented by the pumice horizons. A topographic reconstruction of the volcano at this time is shown in Fig. 4a.

The lacustrine beds are overlain by a sequence of al- ternating thin (< 5 cm) ashfall and rather massive beds (typically 10 cm) interdigitated with occasional coarser lithic and crystal-rich surge deposits. Collections of several massive beds sometimes form crude cross-stra- tification and sand-wave structures, suggestive of depo- sition by relatively dilute surges. Because of their fine grainsize and the abundance of accretionary lapilli in the fall beds, it is considered that these deposits are of phreatomagmatic origin, resulting in all probability from interaction between the magma and water of the caldera lake. This sequence of ashfall and surge is re- peated many times, forming a collective thickness of several metres and representing frequent repetitive ex- plosions of a similar style and magnitude. The lithic- and crystal-rich surge deposits, due to the absence of juvenile material, are interpreted to be the result of eruptions of a predominantly phreatic character.

The overlying Filorsi Member has a 5-m-thick lo- cally developed ignimbrite at the base overlain by up to 15 m of pyroclastic surge deposits which contain at least three types of sand-wave structure (Cole 1991). A decrease in accretionary lapilli and increase in bedform structures upwards suggest that with time the surges be- came drier and more dilute. The succeeding Torano Member (see Section 2, Fig. 6), a scoria bed of airfall origin up to 7 m thick, was probably generated by sus- tained strombolian activity and forms a distinctive hori- zon within the caldera fill

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Domes - Trachyandesitic laves

laeustdnal facies Formation

Galluccie Tuff

leucite free laves

Campagnola Tuff

~ , pest Collapse lava dome ' precaldera 4ayes and pyroclastics (leucite

§ Contours bearing) '

Dip of flow streams J -unit boundaries

1 Crater and caldera rims

(broken where inferred) Cesal Cone

�9 , ~

I ' . ' . ' . ' . ' . "

Fig. 5. Geological map of the southern half of the main caldera

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261

0 Scale f Km | . . . . J f

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The Upper Garofali Member is well exposed to the east of the Savone delle Ferriere gorge (Fig. 5 and Sec- tion 4, Fig. 6). The sequence is characterised by a lower coarse-grained surge deposit, which usually displays well-developed sand-wave structures, interlayered be- tween massive fine-grained units. This coarse-grained surge deposit is interpreted as a 'dry' surge and proba- bly the product of the passage of only one surge event. This is overlain by a sequence 5 m thick of fine-grained pyroclastic surge deposits which show several types of sand-wave structure and represent the passage of nu- merous surge clouds. Mantling ashfall beds are occa- sionally interbedded with the surge deposits. The abun- dance of accretionary lapilli and their fine grainsize supports a 'wet' phreatomagmatic origin for these surge deposits. Analysis of the orientation of bomb sags and sand-wave structures indicates that the Upper Garofali

Fig. 4. a Palaeotopographic reconstruction of Roccamonfina dur- ing the time of the Garofali Formation showing the presence of caldera lakes and coalescing nested calderas, b Palaeotopogra- phic reconstruction of Roccamonfina at a late stage after the for- mation of the domes in the main caldera and the collapse of the Conca Caldera to the north

Member was derived from a vent or vents somewhere in the region of the Mt Croce lava dome (Fig. 5). It may be that the Upper Garofali Member represents an early phreatomagmatic phase of dome building where upris- ing magma came into contact with water. With time, ac- cess of water to the vent ceased and dome formation continued. Just south of Roccamonfina village the rim of a partially buried cone can be just discerned. This is the Cesapiana cone (Fig. 5) which is composed of thin ashfall and pyroclastic surge deposits of a phreatomag- marie origin containing abundant bomb sags; it may form part of the the Upper Garofali Member.

In the northern caldera a distinctive set of alternat- ing pumice lapilli fall and phreatomagmatic ash beds occur overlying the Galluccio Tuff. These beds are par- ticularly well developed around the village of Cave and we have labelled this sequence the Cave Unit (see Sec- tion R620, Fig. 7). These beds, which have not been re- cognised previously, are not recorded in the main cal- dera fill. However, construction of relatively poorly constrained isopachs for the individual beds within the Cave Unit indicates a source vent within the main cal- dera. It is possible that the Cave Unit correlates with some of the coarse pumice horizons associated with the main caldera-fill lacustrine facies. The lowest horizon

262

S E C T I O N 1

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550.

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SECTION 3

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560

550

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massive surge

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[] [ ] [] [] [] ] PLINIAN AIRFALL

] REWORKED MATERIAL

] PALAEOSOIL

IGNIMBRITE

PYROCLASTIC SURGE

STROMBOLIAN AIRFALL

PHREATOMAOMATIC ASHFALL WITll ACCRETIONARY LAPILLI

LACUSTRINE MATERIAL (LAMINATED ASH & COARSE LAYERS) GIANT PU MICE LAYERS

giant pumice

slumped lacustr]nal clays COMPOSITE SECTION

Campanian Ignimbrite

Upper Garolali Member

Torano Member

Filorsi Member

1Live fall ~urge facies

;trine material

itive fall ~urge facies

mart atomagmatic fall

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white ignimbrites

repetitive fall and surge facies

giant pumices

lacustrine material

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lacustrine material

white ignimbrites

Fig. 6. Measured sections from the main caldera fill (for location of sections see Fig. 5). An interpretive composite section is shown

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264

of the Cave Unit is a thin gritty ash probably repre- senting a vent-opening phase. This was followed by finely comminuted khaki ash showing soft sediment de- formation and scattered accretionary lapilli. This bed was the product of phreatomagmatic activity which may have resulted from interaction between magma and water from the caldera lake. There then occurred an alternation between phreatomagmatic and magmatic explosive phases, generating beds of highly commin- uted ashfall interbedded with airfall lapilli beds of white pumice. With a thickness of over 6 m some 5 km from the source area, the Cave Unit, represents a major intracaldera eruptive event. One possibility is that the Cave Unit correlates with the intracaldera coarse pu- mice beds within the Garofali Formation. The upper surface of the Galluccio Tuff underlying the Cave Unit is heavily weathered, suggesting there was a significant time interval between these two events.

i!!iit!ii :i!t ti!

AIRFALL LAPILLI

PYROCLASTIC SURGE

AIRFALL LAPILLI

IGN1MBRITE (SCORIA FLOW)

ABUNDANT GAS PIPES

SURGE

AIRFALL LAPILLI

PYROCLASTIC SURGE

FLUVIALLY REWORKED

VOLCANICLASTICS

t I 0 1 2 3m,

Fig. 8. Measured section of the Fontanafredda Member (for loca- tion see Fig. 5)

4 Late-stage activity (Fig. 4b)

The dominant late-stage activity of the main caldera was the construction of the lava domes of Mt Croce and Mt Lattani. These basaltic trachyandesite lava domes rise to a height of some 400 m above the caldera floor and have an approximate total volume of 900 • 106 m 3. The Mt Croce lava dome has been dated at 0.155 my and represents the youngest dated activity within the caldera (Radicati di Brozolo et al. 1988).

A short but informative section through the upper part of the main caldera fill is exposed in the southwest part of the caldera just south of the village of Fontana- fredda. This sequence is termed here the Fontanafredda Member and is composed of fluvially reworked volcan- iclastics, pyroclastic surge deposits and a distinctive scoriaceous ignimbrite interbedded between airfall la- pilli beds (Fig. 8). The scoriaceous ignimbrite contains abundant normally graded scoria clasts of basaltic tra- chyandesite composition (Table 2) and numerous gas- escape pipes are developed within the upper part of the flow unit. Both these features are indicative that the py- roclastic flow was well fluidised. Lack of exposure frus- trates correlation of this sequence with the rest of the caldera fill, but it is likely, based on their similarity in composition and the fact that the sequence occurs at the top of the fill, that the Fontanafredda Member is related to an explosive phase in the growth of the lava domes.

The northern caldera (Fig. 9) has been the site of vig- orous volcanic activity in the latter part of the history of Roccamonfina. The floor of the northern caldera is dominated by the Friello scoria cone. Associated with the Friello Cone is a thick sequence of valley-fill vol- caniclastics which in large part are probably reworked from Friello airfall material. Overlying this sequence of valley-fill volcaniclastics are the volcanic products of the Conca Caldera.

The Conca Caldera lies at the head of the northern caldera and is flanked to the north by a subdued rim of ejecta. The first product of the Conca Caldera is a

coarse proximal airfall bed of mixed pumice and basal- tic scoria. This is overlain by a distinctive mildly indu- rated trachytic ignimbrite, the Conca Ignimbrite, the Yellow Trachytic Tuff of Luhr and Giannetti (1987). The pumice in this ignimbrite is much altered and the analysis given in Table 2 clearly represents a modifica- tion from the original composition. The Conca Ignim- brite is only found in the northern caldera and can be traced for a distance of about 5 km downslope from the source forming the fiat surface to the north of Friello. In the proximal region the Conca Ignimbrite is overlain by coarse, poorly sorted airfall and phreatomagmatic surge and airfall beds, the final products of the Conca Caldera (see Section R701, Fig. 7). It is of interest that at this late stage in the history of Roccamonfina, while activity in the main caldera was becoming more local- ised and more basic in the northern caldera, there was a phase of major explosive activity involving trachytic magma. Clearly the plumbing systems feeding the two calderas were not related at shallow depths.

The youngest volcanic product in the main caldera is a grey ignimbrite which fills recent valleys. This is con- sidered by Barberi et al. (1978) to be a distal part of the Campanian Ignimbrite which was erupted about 30000 BP from the Campi Flegrei region. The Campanian Ig- nimbrite fills radial valleys on the flanks of Roccamon- fina and occurs around the eastern and northern mar- gins.

Conclusions

Roccamonfina is an example of a composite volcano with a long and complex history of caldera formation. During the 100 000 years following the initial formation of the main caldera there were three episodes of major explosive activity involving the emplacement of small- to medium-volume ignimbrites. In the intervening, more quiescent, periods caldera lakes became estab- lished and the volcanic activity was predominantly

Conca caldera products

] Conca Ign imbdte -....-.-.2oo-.~..,.,

Fluvial valley fill Friello Cone complex

Garluccio Tuff j

~ Lahars J

i Saraceni Ignimbrite [

Contours (m)

Streams & Rivers

Crater and caldera rims (broken where inferred)

265

Saraceni

*•o•**o ~ r p';,"o* r .%: ,,, t * * * * * : ' Gallucoio Quan

+ + + + + + + + .t. "1" + t

+ + + -

Conca Caldera

Scale 0 1 km I I

%

Friello Cone Conca Caldera __

/.~ , ~ ' ~ , .o- ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -I- + 4- .,.t Cave Unit

Brown Leucitic Tuff i i

Note the Outcrop areas of the Cave Unit and the Brown Leucitic Tuff are too small for them to be represented on the geological map.

Not to scale

Fig. 9. Geology of the northern caldera and schematic cross section

266

phreatomagmatic . This phrea tomagmat ic activity led to the accumulat ion of pyroclastic surge deposits and as- sociated airfall material in the caldera intercalated with lacustrine beds. Plinian or sub-plinian magmatic explo- sive events punctuated the phrea tomagmat ic activity forming pumice lapilli beds both in the caldera and on the flanks of the volcano. The format ion of the small nested calderas within the main caldera testifies to the continued role of late-stage collapse. The final activity in the main caldera was the format ion of the lava domes. Though the formation of these domes involved predominant ly effusive activity there were more explo- sive events giving rise to scoriaceous pyroclastic flows. During the history of the caldera there was a general trend in the composi t ior ~f the erupted magmas f rom undersaturated leucite trachyte through trachyte to tra- chyandesite. However, the fact that there were at least four major trachytic eruptions over a period of about 100000 years suggests that the magma system with which the caldera was associated was being periodi- cally replenished. The northern caldera of Roccamon- fina appears to have had a separate and quite distinct history f rom the main caldera.

The idea that calderas may develop in an incremen- tal fashion was discussed by Walker (1984). In a study of Santorini, Druitt et al. (1989) describe several major explosive events associated with caldera collapse. More recently, Nappi et al. (1991) proposed that the Latera caldera of Vulsini Volcano, the northernmost volcano of the Roman Province, formed as a result of incremen- tal collapse involving numerous ignimbrite eruptions.

Is is clear that the current structure of the main cal- dera of Roccamonf ina is the result of incremental col- lapse involving several eruptive events which were well separated in time. During this period no major cone construction occurred and the activity continued to be predominant ly explosive, ranging f rom localised phrea- tomagmat ic activity through to plinian-type eruptions. The dome-building phase in the main caldera may rep- resent the final phase of activity as geothermal explora- tion (Watts 1987) indicates that the caldera is not un- derlain by a thermal anomaly.

This has implications for our understanding of the way in which relatively small calderas may develop. The format ion of the caldera cannot be modelled in terms of single eruptive and magma reservoir evacua- tion events.

Acknowledoements. We thank Donatella De Rita and Ruggero Casacchia for their help in the field and valuable discussions re- garding this work. We gratefully acknowledge financial support from a NATO Research Grant, CNR Research Grant (RB) and a Luton College Studentship (PC). We thank Sandra Mather for drafting the palaeographic reconstructions in Figs. 3 and 4. We acknowledge the constructive comments of two anonymous re- viewers which led to improvements in the paper.

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Editorial responsibility: H-U Schmincke