Volcaniclastic aggradation in a semiarid environment, northwestern Vulcano Island, Italy

Geological Society of America Bulletin

doi: 10.1130/0016-7606(1998)110<0630:VAIASE>2.3.CO;2 1998;110, no. 5;630-643Geological Society of America Bulletin

Greg A. Valentine, Danilo M. Palladino, Emanuela Agosta, Jacopo Taddeucci and Raffaello Trigila Island, ItalyVolcaniclastic aggradation in a semiarid environment, northwestern Vulcano

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ABSTRACT

Most studies of volcaniclastic facies andaggradation cycles have been conducted in re-gions with humid climates, and on regionalscales. Here we document a volcaniclastic suc-cession that formed in a semiarid climate char-acterized by rare, heavy rains onto a relativelybarren volcanic landscape on Vulcano Island,Italy. The deposits, which we informally callthe Cuesta succession, occur in a small valleybetween two rhyolite domes, and consist of asequence of pyroclastic surge and fallout de-posits interbedded with their reworked equiv-alents. A phase of eruptive activity character-ized by sporadic hydrovolcanic explosionssupplied ash to the valley and its flanks. Runoffevents during the eruptive phase continuallywashed ash down to the valley floor in the formof hyperconcentrated flows. The inferredtransport mechanism and depositional faciesof these beds reflect the control of primary vol-canic fragmentation processes on sedimenta-tion; the abundant, medium-to-coarse ash sup-plied by eruption was incorporated sufficientlyeasily into the runoff to hyperconcentrate theflows, but not sufficiently fine grained to makethe flows cohesive. These flows moved out ontothe floor, merged, and transported ash downthe valley axis, depositing the ash as a monoto-nous succession of massive to faintly laminatedbeds. The combination of primary depositsand the deposits washed off the valley flanks

led to aggradation of the valley floor. As erup-tions waned, ash was eroded off the flanksfaster than it was replenished, leading to astratigraphic upward increase in lithic clastsderived from the underlying lavas and a widerrange of sedimentary facies as the ash load be-came more variable. After eruptions ceasedand all remaining ash was removed from theflanks, aggradation gave way to degradation toform small canyons that expose the succession.

INTRODUCTION

A focus of geologic research in the pastdecade has been the relationships between vol-canism and sedimentation (e.g., Fisher andSmith, 1991). These relationships are importantto understand for a variety of reasons. A signif-icant aspect of volcanic hazards, often the mostsignificant aspect, is catastrophic sedimentaryprocesses accompanying and succeeding erup-tions. Such processes include debris flows, hy-perconcentrated flows, aggradation of drainagebasins, and general disruption of hydrologicsystems in the region surrounding the activevolcano. These hazards may continue for manyyears to decades after a large explosive erup-tion. A second reason for studying sedimenta-tion in volcanic terrains is that volcaniclasticrocks are often important components of sedi-mentary basins and can provide useful informa-tion on basin evolution (Fisher, 1984). Finally,most subaerial volcanoes form topographichighs that are subsequently eroded away anddeposited by sedimentary processes. Thus

much of the record of volcanism on Earth isrecorded in sedimentary successions.

In this paper we describe a succession of vol-caniclastic deposits on Vulcano Island, Italy, thatrecords deposition of ash from fallout and pyro-clastic surges associated with explosive eruptionsand contemporaneous reworking of that ash byrunoff to cause aggradation within a small valley.We informally refer to these deposits as theCuesta succession. Unlike most previous studiesof volcaniclastic aggradation, which have fo-cused on regional processes in relatively wet cli-mates (e.g., maritime; G. A. Smith, 1991), ourwork centers on local sedimentary processes in asemiarid climate and a barren landscape thatlikely had no permanent streams. We show thatthe style of sedimentation was strongly influ-enced by the primary grain size, which was de-termined by eruptive processes, and by the natureof precipitation on the island (rare, heavy rainsand large runoff). After discussing terminologyand the geologic setting on Vulcano Island, wediscuss the likely age and composition of theCuesta succession. This is necessary because thesuccession has not been previously defined in theliterature. We then present descriptions of the in-ternal stratigraphy of the Cuesta succession, fol-lowed by interpretation and discussion.

TERMINOLOGY

For clarity, we first define some terms that areused in this paper. We use the word primary to re-fer to deposits emplaced directly by a volcanicprocess. These processes include fallout, pyro-

630

Volcaniclastic aggradation in a semiarid environment, northwestern Vulcano Island, Italy

Greg A. Valentine* Geoanalysis Group, M.S. F665, Los Alamos National Laboratory,Los Alamos, New Mexico 87545

Danilo M. Palladino Dipartimento di Scienze della Terra, Università degli Studi di Roma “La Sapienza,”Piazzale Aldo Moro, 5, 00185 Roma, Italy

Emanuela Agosta Highline College, 2400 South 240th Street, P.O. Box 98000, Des Moines, Washington 98198

Jacopo Taddeucci Dottorato di Ricerca in Scienze della Terra, Università degli Studi di Roma “La Sapienza,”Piazzale Aldo Moro, 5, 00185 Roma, Italy

Raffaello Trigila Dipartimento di Scienze della Terra, Università degli Studi di Roma “La Sapienza,”Piazzale Aldo Moro, 5, 00185 Roma, Italy

GSA Bulletin; May 1998; v. 110; no. 5; p. 630–643; 9 figures; 5 tables.

*E-mail: [email protected]



clastic surge (relatively dilute pyroclastic densitycurrent), and pyroclastic flow (relatively highparticle concentration, or sustained, pyroclasticdensity current); the reader is referred to Francis(1993) for further definitions of these processes.Reworked deposits have been mobilized and re-deposited by the action of wind and/or water onthe primary deposits; we avoid the term epiclas-tic because it is normally associated with materialderived from weathering of rocks rather thanfragmental material. For consistency, we use thevolcanological terminology for clast sizes in bothprimary and reworked deposits, instead of mix-ing volcanological and sedimentological terms,following the size definitions of Fisher andSchmincke (1984). Ash refers to pyroclasticgrains as much as 2 mm in diameter. Fine ashcorresponds to clay and silt sizes in the sedimen-tology literature (to 1/16 mm), and coarse ashcorresponds to sand sizes (1/16–2 mm). Lapilli(singular lapillus) refers to clasts in the gravel topebble range (2–64 mm). Block refers to cobbleand coarser sizes (>64 mm). We follow the usageof Smith (1986) and Smith and Lowe (1991) forflow processes and facies of reworked deposits.These include debris flows (highly concentratedclast dispersions where transport is dominated bymatrix strength, buoyancy, and dispersive pres-sure), normal stream flows (traction and turbulentsuspension transport), and hyperconcentratedflows (intermediate between the former two,transport is a combination of dampened turbu-lence, buoyancy, and grain interaction). Charac-teristics of facies pertinent to our study are listedin Table 1, along with interpretations in terms ofthe primary and reworked transport mechanisms.

GEOLOGIC AND CLIMATIC SETTING

Vulcano is the southernmost island of the Ae-olian volcanic arc (Fig. 1), which includes twoactive volcanoes, Stromboli and La Fossa di Vul-cano. Volcanic activity on Vulcano began in latePleistocene time (the age of the oldest exposedproducts is about 113 ka, according to Frazzettaet al., 1985; Keller, 1980) with the building ofthe southern part of the island (Fig. 1), which isa stratovolcano composed mainly of trachy-basaltic and trachyandesitic lava flows interlay-ered with scoria horizons, locally intruded bydikes (Keller, 1980). Subsidence of the upperpart of the stratovolcano produced the Calderadel Piano (Fig. 1); this subsidence occurred overan extended period of time and was caused pri-marily by tectonic activity rather than a largeeruption (Ventura, 1994). Between 99 and 8 kathis depression was filled by the products of ef-fusive and explosive activity (De Astis et al.,1989) ranging from trachybasaltic to trachyan-desitic in composition, interrupted by many per-iods of rest recorded by sharp erosional uncon-formities. The late Caldera del Piano–fillingactivity was almost contemporaneous with for-mation of the Lentia Complex in the northwest-ern part of the island (Fig. 1). The Lentia Com-plex is a complicated succession of rocks thatcan be divided into two main parts (De Astis etal., 1989). The first part includes early palagoni-tized Strombolian deposits (scoria, bombs, andlava clasts) that erupted along the shoreline,lavas, wet and dry pyroclastic surge deposits,and trachytic and rhyolitic lavas. The second partof the complex was built by rhyolitic effusive ac-

tivity along north-south fissures, that formedlava flows and domes composed of thick centralmassive porphyritic zones overlain by vitro-phyric massive to flow-layered to folded zones,and underlain by massive vitrophyric and/or au-tobrecciated zones generally covered by debrisfans. The latest Lentia Complex activity is char-acterized by trachytic scoria and lava flows (15.5ka, Frazzetta et al., 1985).

The Lentia Complex and the Caldera del Pi-ano–filling sequences were dissected by the sub-sidence of the Caldera della Fossa. According toinferences from deep drilling (Gioncada andSbrana, 1991), the subsidence occurred in severalphases. The earliest was contemporaneous withtrachybasaltic lava-flow activity from the north-ern part of the south stratovolcano (42–13 ka).The main phase of the subsidence occurred be-tween 15.5 and 7.3 ka, after the emplacement ofthe products of the Lentia Complex. Subsidenceoccurred in the southeastern part, and succes-sively migrated toward north-northeast. As withCaldera del Piano, subsidence of Caldera dellaFossa was more a result of tectonism than rapiddrainage of a magma reservoir (Ventura, 1994).Currently, the main feature of the Caldera dellaFossa area is the active La Fossa di Vulcano, asteep 391-m-high cone, rising from the calderafloor which is only a few meters above sea level.La Fossa di Vulcano, which began growing about6 ka, has been described by many workers (e.g.,Mercalli et al., 1891; Frazzetta et al., 1977, 1982,1983; Keller, 1980; Sheridan et al., 1987; Dellinoet al., 1990; Frazzetta and La Volpe, 1991) and isnot discussed further here. A tephritic lava-floweruption began the construction of the island of

VULCANO ISLAND, ITALY

Geological Society of America Bulletin, May 1998 631

TABLE 1. CHARACTERISTICS AND INTERPRETATION OF PRIMARY AND REWORKED DEPOSITS IN CUESTA SUCCESSION

Bedding features Internal structures Grain size Interpretation

Very thinly to thinly bedded, parallel to Massive, commonly vesicular. Fine to medium ash. Fallout or/and pyroclastic surgedepositional surface, laterally continuous beds.where not disrupted by overlying layers. Primary

Thinly to medium bedded, plane-parallel to Laminated to cross laminated. Fine to coarse ash. Pyroclastic surge deposits.wavy and lenticular. Dune, antidune, and Primarychute-and-pool bedforms common.

Thick to very thick bed. Massive, possible coarse-tail reverse Ash matrix with dispersed black pumice Pyroclastic flow deposit.grading of black pumice clasts. lapilli and blocks. PrimaryDeformed and imbricated pumice clasts.

Thinly to thickly bedded, laterally continuous Massive to faintly laminated and Medium to coarse ash. Isolated Hyperconcentrated flow deposit.to lenticular over several meters. Mainly cross-laminated. Some beds outsized black pumice blocks Reworkedplane-parallel top and bottom contacts, contain imbricated lithic lapilli, and lithic lapilli.locally bottom contacts form U-shaped in others lapilli have no preferredchannels into underlying beds. orientation.

Medium to very thickly bedded, erosional to Massive matrix supported lithic Ash matrix with angular lithic blocks Debris flow.planar basal contact, irregular top breccia. and lapilli. Reworked(eroded).

Very thinly to medium bedded, lenticular. Cross-laminated, lithic lapilli Mainly coarse ash, sparse small lithic Normal (dilute) stream-flowOccurs as lenses on planar depositional commonly imbricated. lapilli. deposits.surfaces, in the lee of large lithic clasts or Reworkedbed undulations, and fills V-shaped channels.



632 Geological Society of America Bulletin, May 1998

Figure 1. Map showing the general geology of Vulcano Island. Outlined study area is shown in detail in Figure 2.



Vulcanello in 183 B.C. It was joined to VulcanoIsland during the sixteenth century A.D. by accu-mulation of volcaniclastic material (Fig. 1).

Paleoclimate of the Vulcano region has beeninferred from pollen and oxygen isotope records(Rossignol-Strick and Planchais, 1989; Follieriet al., 1990), which consistently indicate thatduring the early period of deglaciation (startingat 15 ka) the climate was drier even than the pre-sent semiarid conditions. Prior to this, whileglaciers were common in northern Italy, thesouthern part of the country was arid. Duringthese times it is likely that precipitation fell asrare, but heavy, rains.

CUESTA SUCCESSION

In the following sections we describe theCuesta succession in detail. The first two sectionsdefine the succession in terms of its stratigraphicrelationship to older and younger deposits, and itsprobable age and composition. This is necessarybecause the succession has not, to our knowl-edge, been described in the literature. We thendescribe the internal features of the succession.

Stratigraphic and Geomorphic Setting

The Cuesta succession is in a small valleybounded on the north and south by two rhyolitedomes of the Lentia Complex (Figs. 2 and 3).The valley is the low area between the two domesand is not primarily an erosional feature. Thenorthern dome is Mount Lentia, and the southerndome is unnamed. Prior to deposition of theCuesta succession the valley was floored by rhy-olite lava, and locally by debris flow deposits,sloping westward to the Tyrrhenian Sea. In theupper reaches of the valley, where the Cuestasuccession is present, the slope of the valley flooris about 12°W (assuming that there has been norotation, we infer that the paleoslope at the timeof Cuesta deposition was the same); in the lowerreaches the floor drops rapidly to sea level in a se-ries of two cliffs (Fig. 3). The valley-boundingdomes have been dissected by subsidence alongthe north-south–trending scarp of Caldera dellaFossa (Figs. 2, 3, and 4; Ventura, 1994). Simi-larly, the valley has been disconnected from itshead by the subsidence and is in a sense a“stranded” valley; the valley head now is buried

beneath younger pyroclastic deposits of La Fossadi Vulcano. The valley walls and floor had rough,irregular surfaces locally covered by the looserubble typical of rhyolite lavas; this can be seenwhere erosion has exposed the basal contact ofthe Cuesta succession. The northern valley wallwas (and is) particularly steep, and has local ver-tical cliffs; the wall rises 60 m from the valleyfloor over a lateral distance of only 100 m. Thesouthern valley wall has similar relief in the east-ern part, but becomes less steep toward the west.A debris-flow–lahar deposit that predates theCuesta succession is exposed at station 1. Thisdeposit is rich in angular rhyolite blocks andsparse black pumice blocks and lapilli. It accu-mulated locally in low spots in the valley (Fig. 3),and has an immature paleosol at its top.

The Cuesta succession is composed of mod-erately indurated volcaniclastic deposits, andhas an originally smooth, west-sloping uppersurface. The surface is now incised by smallcanyons, so that the sequence forms two slop-ing cuestas (Fig. 4). The canyon walls presentexcellent exposures of the Cuesta succession,but a (nearly) continuous vertical exposure



Figure 2. Geologic map of studyarea, showing stations and locationof cross sections A–A′ and B–B′(see Fig. 3).



from the base to the top of the succession ispresent at one place (station 1, Figs. 5 and 6).Stratigraphy of the sequence is discussed in de-tail in the following. We did not find paleosolswithin the Cuesta succession, and other thansmall local channels there are no unconformi-ties within the succession, suggesting that dep-osition was relatively continuous.

Post-Cuesta deposits rest in angular unconfor-mity (locally in disconformity) on the walls of thecanyons that incise the Cuesta succession at sta-tions 5 and 6, and along the southern valley wall(vicinity of stations 7, 8, and 9; Fig. 2). Thesepale-colored ash deposits are plastered againststeep surfaces of the unconformity and appear tobe the result of wet pyroclastic surges. Similardeposits are found at station 15, where they are

plastered against a steep cliff wall and around alava pinnacle, and we infer that they correlatewith the deposits that rest directly on the Cuestasuccession upslope. The most recent deposits inthe study area (colluvium in Figs. 2 and 3) are acombination of talus at the foot of the steep val-ley walls and ash from La Fossa di Vulcano thathas been reworked during runoff events. Thesedeposits are in the bottoms of the small canyonsincised in the Cuesta succession and on the wallof Caldera della Fossa (Fig. 2).

Age, Composition, and Source Vent

Geologic maps of the island (e.g., Keller,1980; De Astis et al., 1989; Ventura, 1994)commonly have included the Cuesta succes-

sion as products of La Fossa di Vulcano, themajor active cone that has formed since 6 ka,but we have found no descriptions of the se-quence in the literature. We suggest insteadthat the Cuesta succession predates subsidenceof Caldera della Fossa, which began about 15ka at this location (Ventura, 1994). This con-clusion is based on the following lines of evi-dence. (1) The geometry of the succession sug-gests that the caldera rim did not yet exist as atopographic high at the time of Cuesta deposi-tion. If this had been the case the sequencewould pinch out at the rim, which is not sup-ported by projecting the surface of the Cuestadeposits eastward over the rim (Fig. 3, crosssection B–B′). (2) Reworked ash deposits,most of which probably result from a form of

VALENTINE ET AL.


Figure 3. (Top): Cross section A–A′, taken across the valley, showing major units of the Cuesta succession and reconstructed cross section at theend of aggradation of the Cuesta succession. (Bottom): Cross section B–B′, taken along the valley axis, schematically showing interfingering be-tween primary pyroclastic surge and fallout deposits. Most proximal (to volcanic source) deposits are to the right on the cross section. Dashed lineextending from east end of Cuesta top is projected top of sequence before subsidence of Caldera della Fossa.



hyperconcentrated flood flow (see following)are interbedded with primary pyroclastic-surgeand fallout deposits clear to the bottom of thesequence where it is exposed on the moderncaldera rim. This indicates that the location ofthe modern rim was a depositional site at thetime of the Cuesta succession, which would beunusual if it were a topographic high as it is today. (There is also evidence for intrasucces-sion erosion at the site, but the net effect was depositional.) (3) Finally, the small, coarse-pumice–rich deposit within the Cuesta succes-sion (black pumice flow, see following) proba-bly originated as a relatively low energypyroclastic flow confined to the valley (strati-graphically equivalent deposits have not beenfound elsewhere on the island). If the Cuestasuccession postdated caldera subsidence, thispyroclastic flow would have had to climb thecaldera wall (>100 m high), which seems un-likely. Rather, we think that it flowed down acontinuous valley from its source vent; the val-ley and the Cuesta succession were truncatedby caldera subsidence. There is no evidencefor significant postsubsidence morphologicalretreat of the caldera rim; the caldera scarp isgenerally linear at this location, whether it cutsLentia domes or the paleovalley between them.

Major element and selected trace element

compositions of juvenile lapilli in the pyro-clastic flow deposit are provided in Table 2.The pumice lapilli are latites on a total alkali-silica diagram. We compared these data to a re-cent survey of the geochemistry of Vulcano Is-land (De Astis, 1995) for possible correlationwith other eruptive products. Some of the ma-jor and trace element abundances (e.g., K2O,Rb, Th, Ba, Sr, and Hf) are similar to trachyan-desites from several eruptive cycles of LaFossa di Vulcano. Other elements such as MgOand Zr, and some element ratios like Zr/Nb andRb/Sr, however, indicate a strong associationwith latites and trachytes of the earliest ex-posed units of the Lentia complex (exposedalong the coast 1 km north of the Cuesta loca-tion, at Cala di Mastro Minico). We think thatthe Cuesta magma was related to that of theLentia complex, predating subsidence ofCaldera della Fossa, and consistent with ourstratigraphic arguments.

We infer that during Cuesta deposition thevalley that contains the sequence continued torise to the east beyond the modern caldera rim,probably between the prefaulted extensions ofMount Lentia and the unnamed dome. Thesource of the Cuesta pyroclastic material was avent at or near the head of this valley (probablyno farther than about 1 km east of B′, Fig. 3), but

all of these features subsequently subsidedwithin Caldera della Fossa and were buried byproducts of La Fossa di Vulcano.

Main Features of Primary and Reworked Deposits

A problem that is nearly always present in vol-canic terrains is distinguishing between mecha-nisms of primary deposition (pyroclastic flow,surge, and fallout; Valentine and Giannetti, 1995)and between primary and reworked volcaniclas-tic deposits (Smith and Katzman, 1991). Pyro-clastic-surge deposits share many characteristicswith fluvial or eolian deposits, such as cross-bed-ding and the range of planar, dune, ripple, and an-tidune bedforms. Most pyroclastic-flow depositsshare characteristics with debris-flow deposits,such as massive beds, poor sorting, and coarsetail grading. Recent books discuss various as-pects of these problems (Fisher and Schmincke,1984; Cas and Wright, 1987; Carey, 1991; Fran-cis, 1993; Chester, 1993). Table 1 describes char-acteristics of primary and reworked deposits inthe Cuesta succession. The interpretive criteriaare not always clear in the field, and as in all fa-cies analyses, it is the association between differ-ent deposits that establishes the depositionalmechanism for some types. For example, takenby themselves pyroclastic surge and stream-flowdeposits share many characteristics, such ascross-lamination and lenticular bedding. How-ever, in the Cuesta succession the stream flow de-posits are always associated with debris flow andhyperconcentrated flood-flow deposits (Piersonand Scott, 1985; Smith, 1986), which are rela-tively unambiguously interpreted in this area, andthe pyroclastic surge deposits are associated withother primary deposits such as vesicular ash beds(Rosi, 1992; Capaccioni and Coniglio, 1995). Inthe Cuesta succession we are aided by the factthat all lithic clasts are derived from the valleywalls (Lentia rhyolite). This enables us to distin-guish debris flows derived from the valley wallsfrom the pyroclastic flow deposit that is rich injuvenile, plastically deformed, black pumice.

In order to reduce repetition in the descriptionsbelow, we discuss the stratigraphy in terms of theinferred emplacement mechanisms. Except forspecific features pointed out in the descriptions,deposits of a given inferred emplacement mecha-nism are as described in Table 1.

Stratigraphy of the Cuesta Succession

For discussion purposes we divide theCuesta succession into lower, middle, and up-per units. The units are defined by two mainvariables: (1) the relative importance of pri-mary vs. reworked deposits; and (2) lithic con-



Figure 4. Photograph taken from Mount Lentia looking south to unnamed dome (round hillin middle ground). Right (west)-sloping mesas, or cuestas, between Mount Lentia and unnameddome are the Cuesta succession. The surface of the cuestas is the original surface of the sequenceat the end of aggradation of volcaniclastic debris in the valley. The length of the main cuesta inthe middle ground (left to right) is ~300 m.




Figure 5. Stratigraphic column for station 1. See Figure 2 for station location.

Station 1 stratigraphic column

CARTography by A. Kron 2/20/96



tent. Each unit grades into the overlying unit sothat there are no clear boundaries betweenthem in the field. At station 1 the boundariescoincide with breaks in slope (Fig. 5). Figure 6shows correlations within the succession be-tween different stations.

Lower Unit. The lower unit is best exposed atstation 1, a distal location. Exposure of the lowerunit is moderate to poor in proximal areas(around stations 2 and 4), where it occurs aroundthe bases of two small rhyolite lava ridges (Figs.2 and 3). In both proximal and distal locations thelower unit is dominated by ash beds that we in-terpret as having been deposited by hypercon-centrated flows (Table 1). These beds are decime-ter thick, greenish-brown, massive to faintlylaminated, contain isolated outsized lithic and

pumice clasts, and are commonly capped with avery thin (<0.5 cm) layer of fine ash (Fig. 7A)representing deposition from the waning floodwaters. Many of the beds form shallow U-shapedchannels that cut into underlying beds; thesechannels are typically on the order of a fewdecimeters to a meter wide, and 5–30 cm deep. Inproximal locations, where bedding planes are ex-posed, raindrop impressions are common on bedtops. The unit is 4–6 m thick in proximal areas,and 7–8 m thick in distal areas.

Everywhere in the lower unit, primary vol-canic beds are subordinate to reworked deposits.Some stratified and low-angle, cross-stratifiedash beds that may record pyroclastic surges oc-cur at the very base of the sequence near station1. Other possible primary beds in the lower unit

are thin beds of fine ash, in some cases vesicular,and of varying shades of brown, interpreted asfallout deposits.

Middle Unit. The middle unit is similar to thelower unit in the distal parts of the mesas, where itis as thick as 6 m (e.g., station 1), but in proximalareas as much as 50% of its thickness is primarypyroclastic surge and fallout deposits (Fig. 3,cross section B–B′; characteristics are describedin Table 1). At stations 2–4 these primary depositsare very well exposed in trail cuts; they are char-acterized by laminated and cross-laminated fineto coarse ash and reach a total thickness of about3 m (Fig. 7B). Color variations from bed to bed,ranging from tan to gray and slightly purple, givethe deposits a similar appearance to “varicolored”tuffs that have been described elsewhere on the is-



Figure 6. Correlation of inter-nal stratigraphy in the Cuestasuccession between stations. Thelower correlation line (betweencolumns) is the base of pyroclas-tic surge deposits in the middleunit, and the upper line corre-lates the base of the pyroclasticflow unit. See Figure 2 for stationlocations.



land (De Astis et al., 1989; Capaccioni andConiglio, 1995). Within the upper part of thishorizon local erosional channels are cut into theprimary deposits. These are exposed both on bed-ding planes and in cross section. Channels filledwith massive tuff, probably representing debris-flow material (or possibly surge deposits; Fisher,1977), have shallow U-shaped cross sections;other channels approach a V-shaped cross section,as much as 2 m wide and ~0.5 m deep. The flanksof the latter contain smaller (~10 cm wide) chan-nels that feed into the bottom of the main channelin the form of tiny rivulets. The V-shaped chan-nels probably represent runoff erosion from rain-fall directly on the pyroclastic deposits, and werefilled in by subsequent surge and fallout ash.

At stations 6 and 9 the pyroclastic surge andfallout horizon ranges from 50 to 65 cm and 75 to110 cm thick, respectively. At station 6 exposure ofthe horizon is poor but it appears to be mainly pla-nar, thinly bedded ash. At station 9 a variety of pla-nar and cross-stratified bed forms are present, in-cluding a chute-and-pool structure. Thin, vesicularash beds are present in the horizon. At these andother equally distal locations, the middle unit isdominated (>75% of total thickness) by reworkeddeposits, mainly from hyperconcentrated flows.

The middle unit is capped by a massive de-posit, as thick as 120 cm, that has a tan to yel-lowish, indurated ash matrix and is rich inhighly vesiculated, black pumice lapilli andblocks (up to 40 cm diameter in proximal areas,

e.g., station 5). Many of the pumice lapilli areplastically deformed and imbricated, indicatinghot emplacement. We interpret this deposit to bea small pyroclastic flow (referred to as the blackpumice flow in the figures; chemical analysesare in Table 2).

Upper Unit. The upper unit is similar to thelower unit in that it is everywhere dominated bydeposits of reworked material, but it is muchricher in rhyolite lithic clasts and contains a widervariety of sedimentary structures close to the footof Mount Lentia, becoming more like the lowerunit toward the south. The upper unit forms thecap rock of the cuestas; it is well exposed alongmost of their lengths, and is between 3 and 4 mthick. Along the traverse from stations 12 to 13and 14, the unit displays a variety of sedimentaryfacies that record a full range of transport and dep-osition mechanisms, from debris flow to hyper-concentrated flow to dilute, “normal” stream flow,as described in Table 1. A debris-flow deposit ex-posed along the traverse is rich in large (up to 60cm long) rhyolite blocks, and ranges to ~80 cmthick; it fills steep sided channels cut into under-lying deposits. The debris-flow deposit is in turnoverlain by a discontinuous, strongly laminatedand cross-laminated deposit that fills V-shapedchannels cut into the debris-flow deposit (bottomof Fig. 7C). We interpret the debris flow to havebeen derived from a steep slope of lava rubble onthe valley wall. This rubble was mixed with asmall amount of primary Cuesta ash during arunoff event to form a coarse debris flow. Contin-ued runoff (possibly during a single rain storm)partly reworked the debris flow and deposited astream flow bed. Elongate lithic lapilli and blocksare imbricated in many hyperconcentrated flowbeds. Thin (no more than several centimeters) lay-ers of vesicular ash, representing possible pyro-clastic surge and fallout deposits, suggest that sporadic eruptive activity continued during theformation of the upper unit. There are raindropimpressions on the tops of some beds, but thesesurfaces are rarely exposed in the upper unit.

The top surface of the upper unit is the originaltop of the Cuesta succession. This surface is lit-tered with rhyolite blocks, especially toward thenorth; these blocks rolled off the steep slopes ofMount Lentia. Many of the blocks are partlyimbedded in the underlying deposits, suggestingthat the deposits were not indurated when theblocks first rolled onto the valley bottom.

Paleocurrent Data

Paleocurrent directions in primary pyroclasticsurge deposits can be determined from duneforms and sole marks on bedding planes. In thestudy area sole marks include small flutes andlinear scour marks, which allow accurate paleo-

flow determination (bedforms indicate flow di-rections that are consistent with these, but are notas tightly constrained because of exposure limita-tions). In reworked deposits the main paleocur-rent indicators were channel orientations.

Paleocurrent directions (Fig. 8) show that flowwas dominantly westward toward the sea. At sta-tion 1 all but one of the paleocurrent measure-ments are channels in reworked deposits. Most ofthese show that flow was toward the northwest,consistent with the fact that a lava ridge partlyblocked the valley 30 m downstream; the (mainlywestward) flow was deflected northward througha notch between the paleoridge and the foot ofMount Lentia. Two of the channels at station 1 in-dicate flow to the southwest. These were proba-bly drainage channels coming off the slopes ofMount Lentia and merging with the dominantwestward flow in the valley bottom. Near station2, west-northwest trends for channels in re-worked deposits are consistent with the paleoslope there. Paleocurrent directions for the pyro-clastic surges are west-southwest, slightly trans-verse to the paleoslope, and probably indicate theinfluence of their source-vent location. Station 6is located in the paleovalley center, away fromsteep valley walls. Paleocurrent directions in thereworked deposits there are consistently west-northwest. Pyroclastic surges flowed in approxi-mately the same direction, but show less scatterin paleocurrent directions.

Scanning Electron Microscope Data

We performed scanning electron microscope(SEM) analyses on 11 samples from the Cuestasuccession, and one from recently reworked de-posits of La Fossa di Vulcano (collected in mod-ern channel at station 1), to compare particlesfrom different deposit types and locations (Fig. 9and Table 3). For each sample 50 vitric particlesin the 125–250 µm size fraction were character-ized in terms of five morphological parameters(shape, contour, structures, vesicle size, and vesi-cle shape), each expressed numerically in termsof five classes (Table 4). These data produced a50 × 5 matrix for each sample. The arithmeticmean for each parameter was computed for eachsample based on the 50 grains. We then per-formed cluster analysis on these data using theapproach in Davis (1986). In this approach thecorrelation or “distance” between two samples iscalculated. A small “distance” indicates that thereis a close match of all five parameters betweenthe two samples, and that they are morphologi-cally similar. Larger distances indicate that sam-ples are different. The analyses were performedwithout the operator knowing sample identity inorder to eliminate any prejudice.

Table 5 reports the degree of sample correla-

VALENTINE ET AL.


TABLE 2. MAJOR-ELEMENT AND SELECTED TRACE-ELEMENT COMPOSITIONS OF JUVENILECLASTS FROM BLACK PUMICE

FLOW DEPOSIT

Station 16 Station 4Major elements(wt%)SiO2 58.96 58.13Al2O3 14.00 14.54Fe2O3 6.18 7.09MnO 0.12 0.14MgO 3.45 4.06CaO 5.82 6.80Na2O 3.29 3.29K2O 4.72 4.74TiO2 0.43 0.51P2O5 0.22 0.30LOI* 1.53 0.81Total 98.72 100.41Trace elements(ppm)Rb 184 176Th 1 1Ba 647 689Sr 701 863Zr 153 143Nb 23 21Hf 5 4

Note: Major elements determined by X-ray fluorescence, trace elements by aninductively coupled plasma (ICP) massspectrometry.

*LOI—loss on ignition.



tion obtained by means of cluster analysis. Twomajor groups of samples can be recognized, rep-resented by samples V4-V5-V8-V9-V10 and V3-V6-V7-V11. The first group includes samplesfrom pyroclastic surge horizons of stations 2 (V9,V10), 16 (V8), 1 (V5), and 6 (V4). The secondgroup includes beds interpreted as derived fromhyperconcentrated flow at stations 2 (V7), 17(V11), and 1 (V3-V6). Samples V1 (massive de-posit from station 1) and V2 (debris-flow matrixfrom stop 12) show closer affinity to the lattergroup. Note that the degree of correlation in thepyroclastic surge group decays with increasingdistance between stations, possibly indicatingsome effect of syneruptive reworking processesdown the valley (e.g.,V5 and V4 show some cor-relation to the reworked material group). SampleV12, which represents present-day reworked ma-terial from the A.D. 1888–1890 eruption at LaFossa di Vulcano, broadly correlates to the re-worked Cuesta deposits rather than to primarypyroclastic surges.

Overall, particles from both surge layers and

reworked beds have similar general morpholog-ical features: they are dominantly dense, poorlyvesicular, equant to somewhat platy, often dis-play adhering particles, and tend to havecracked hydration skins. All this indicates thatthe eruptions that supplied the ash were drivenmainly by explosive magma-water interaction(Heiken and Wohletz, 1985). There are someslight (but significant) differences: (1) in somecases primary grain features (i.e., degree ofvesiculation, vesicle size pattern, and vesicleshape) suggest that primary source materialsupplying reworked beds was slightly differentfrom the primary surge deposits, due to preerup-tive conditions (vesiculation and/or fragmenta-tion processes); (2) reworked grains show moreirregular three-dimensional shapes and sharperedges than surge grains, apparently in contrastwith a surplus of transport in the reworking sys-tem. This likely indicates that such differencesreflect different primary transport systems thatthe superposition of a short-distance reworkingprocess was unable to change significantly. In

these cases, a larger component of deposition byfallout, rather than by surge, of material feedingreworked beds would be consistent with thesefeatures and minor abundance of adhering par-ticles. Primary grain features of hyperconcen-trated flow deposit from the 1888–1890 erup-tion material are different from both surges andreworked beds in the Cuesta succession, clearlyindicating its magmatic origin.

INTERPRETATION

We infer that the Cuesta succession was de-posited by sporadic hydrovolcanic explosionsand contemporaneous reworking processes. Ashwas deposited in the valley and on its flanks frompyroclastic surges and fallout. Pyroclastic surgesare density currents and would have left most oftheir deposits on the valley floor, while the flankswould have received a larger proportion of fall-out. During explosive eruptions or soon there-after, rain storms remobilized this unconsolidatedmaterial in ash-charged runoff that flowed down



A

C

B

Figure 7. (A) Beds in the lower unit at station 1, showing typical mas-sive to faintly laminated structure. Each bed is separated from theoverlying bed by a thin layer of fine ash. Trowel (~20 cm) for scale. (B)Primary pyroclastic deposits in middle unit, proximal part of studyarea, showing laminated and cross-laminated gray ash deposits frompyroclastic surge and fallout at station 2. Just to the right of the note-book is a rhyolite block that rolled from the valley wall during em-placement of the primary horizon. It was subsequently mantled by fall-out ash layers, and cross-bedding formed around it during passage ofpyroclastic surges. Hammer is 30 cm long. (C) The upper unit at sta-tion 12. Thin debris-flow deposit rich in angular rhyolite blocks (nearbottom of photograph) is eroded on top, locally forming channels thatare filled with cross-bedded and laminated ash that record dilutestream flow. These are overlain by more massive to faintly laminatedbeds that record hyperconcentrated flood flow, more typical of theCuesta succession. Width of photograph is ~2 m.



the valley walls, and small alluvial fans probablyformed at the foot of Mount Lentia and the un-named dome to the south. By analogy withSegerstrom’s (1950) descriptions of ash rework-ing near Parícutin volcano, these alluvial fansmay have been 5–20 m long. Paleocurrent direc-tions in reworked beds at station 1, which is about20 m from the foot of Mount Lentia, suggest thatthe station may coincide with the location of thefoot of such an alluvial fan, where runoff mergedwith floodwaters running down the valley axis.The abundance of unconsolidated, relatively uni-formly sized ash consistently led to hypercon-centrated flow during these runoff events, pro-ducing a monotonous sequence of massive tofaintly laminated and cross-laminated beds. Lo-cally derived lithic clasts (from underlying rhyo-lite lavas) are sparse in the early deposits (lowerand middle units) because the lava surface wasburied by ash.

Sporadic eruptive activity, alternating with pe-riods of reworking, continued throughout most ofthe deposition of the lower and middle units. Theash supply from eruptions kept pace with erosionduring runoff events, so that little of the underly-ing Lentia lava was exposed and the reworkeddeposits remained lithic poor. During formationof the middle unit, eruptions increased in inten-sity. Surges flowed down the upper reaches of thevalley, depositing ash at a rate that exceeded re-working and leaving as much as 3 m of continu-ous primary deposits. Toward the end of this timea relatively large eruption deposited the blackpumice flow.

After deposition of the black pumice flow, erup-tion frequency decreased and erosion of ash fromthe valley walls began to exceed the eruptive sup-ply of ash. The upper unit records these changes intwo main ways: (1) many beds in the upper unithave much higher lithic contents than underlyingunits, suggesting that more of the valley-boundingLentia lavas were exposed, and (2) a wider varietyof sedimentary facies suggests that runoff eventswere no longer consistently hyperconcentratedwith ash. The lithic-rich debris-flow deposit de-scribed here also indicates the presence of surfaceexposures of Lentia lava breccias.

During the entire eruptive phase and for ashort time thereafter, the ash supplied to the val-ley by eruptions was remobilized by runoff, car-rying the ash from the valley walls to the floor ata rate that exceeded sediment transport along thevalley axis to the sea. This resulted in aggrada-tion of sediments (with a component of primarydeposits) in the valley floor. When the eruptionsended, ash continued to wash onto the valleyfloor until it was all removed. After this, largeblocks of Lentia lavas rolled onto the floor of thevalley, littering the top of the Cuesta succession.

At this time incision into the Cuesta succes-

sion began, ultimately forming the small canyonsthat we see today. The location of these canyonswas largely controlled by erosionally resistantlava highs at what is now the eastern end of thevalley (Fig. 2).

DISCUSSION

The Cuesta succession is an excellent, small-scale example of volcaniclastic aggradation con-trolled entirely by influx of ash from nearby ex-

plosive eruptions (e.g., G. A. Smith, 1991). Thissuccession is on a small, semiarid island wherethere are no permanent streams; it is unlikely thata permanent stream occupied the paleovalley atCuesta time due to the dry climate then and thesmall catchment area that would have been avail-able. Reworking was therefore controlled byrunoff events associated with individual rainstorms, rather than by a permanent stream orlarge-scale alluvial system, which has been thecase with most previous studies of volcaniclastic

VALENTINE ET AL.


Figure 8. Orientation of channel structures and flute casts on bedding planes at stations 1, 2,and 6. Arrow heads point downslope.



Geological S

ociety of Am

erica Bulletin,M

ay 1998641

Figure 9. Scanning electron microphotographs of representative samples from pri-mary and reworked deposits in the Cuesta succession. (a) Several clasts from pyro-clastic surge deposits at Station 2. (b) Close-up of pyroclastic surge clast from station1. Note well-developed hydration cracks and skin. (c) Several clasts from hypercon-

centrated flood-flow deposits at station 9. Elongate clast with fluidal texture (leftcenter) is quite rare in these deposits. (d) Closeup of clast from same deposit as c.Note poorly developed hydration cracks.

on August 2, 2012

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aggradation-degradation cycles (e.g., Vessel andDavies, 1981; G. A. Smith, 1987a, 1987b, 1991;R. C. M. Smith, 1991a, 1991b; Turbeville, 1991).

As discussed by Smith and Lotosky (1995),volcanic fragmentation plays a fundamental role,alongside weathering and sedimentary transportprocesses, in determining the grain size and com-position of reworked volcaniclastic deposits. Inthe Cuesta succession the volcanic fragmentationprocess controlled both grain size and sediment-transport mechanisms. The primary pyroclasticdeposits were composed of fairly well-sorted ashfrom explosive hydrovolcanic eruptions and fromfallout and surge emplacement. These depositswere fine grained enough to be easily remobi-lized by rainfall and to hyperconcentrate therunoff, but coarse enough to be noncohesive.Runoff transported the material from the valleyflanks to its floor, where much of the sedimentload was deposited (probably a large fraction wascarried down the valley to the sea as well). Be-cause of the short transport distance (< 200 m),overall grain size was not significantly modifiedby abrasion and sorting (which was also ham-pered by the high sediment load of the hypercon-centrated flows), so that the reworked deposits inthe Cuesta succession essentially have the samegrain size as the interbedded, primary pyroclasticmaterials. Lithic clasts in the succession are de-rived from foliated lava flows and flow brecciasthat were exposed on the valley flanks, especiallytoward the end of the eruptive phase. These clastsalso underwent little modification in size over theshort transport distance, and therefore their sizesreflect autoclastic processes in their parent lavaflows and/or erosional processes on the domes.

The most common deposits in the Cuesta suc-cession are similar to sand-sized, hyperconcen-trated flow deposits that have been describedelsewhere, being mainly massive to faintly lami-nated or low-angle cross-laminated (Pierson andScott, 1985; G. A. Smith, 1991). We observe arange of grading styles of lithic lapilli in thesebeds, including ungraded, normally graded, re-verse to normally graded, and reversely graded.Such grading patterns are difficult to assess interms of emplacement mechanisms because it isnot known whether clast supply to a given bedvaried over time. For example, reverse grading oflithic clasts could have been the result of an in-crease in the lithic content of runoff waters at agiven place during an event, rather than processeswithin the sediment flow.

Because of the simplicity in source materials,size, and sedimentary processes, the Cuesta suc-cession can be described in terms of three or-thogonal axes. An east to west axis defines faciesvariations from proximal to distal relative to thevolcanic source of material, such that primary py-roclastic facies (mainly surge deposits) are rela-

tively more abundant toward the east end of theaxis. A north-south axis crosses the valley; faciesvariations along this axis reflect the proximity tosources of reworked material (sediment sources),namely the valley walls. This is especially appar-ent as one moves south from the foot of MountLentia. The lithic content of the upper unit de-creases away from the relatively steep MountLentia. In addition, the abundance of blocks lit-tering the top of the sequence decreases awayfrom Mount Lentia. The vertical axis representstime and records the gradual waning of ash sup-ply and hence eruptive activity.

We speculate now on the time scale for deposi-tion of the Cuesta succession. The runoff eventsthat caused reworking may have been fromweather-related, heavy rains. If this is the case, thelow frequency of such events (of the order of oneevent per year) on the semiarid island would sug-gest that the eruptive phase had a duration of sev-eral years to decades. Rains could have accompa-nied the eruptions. SEM and facies data indicatethat the eruptions were driven mainly by magma-water interaction, and therefore the eruption

columns may have been wet. Also, the columnsmay have entrained moist air from the marine sur-face layer, and raised it to a level where condensa-tion and subsequent precipitation occurred (e.g.,Woods, 1993). However, the predominance of re-worked deposits compared to primary pyroclasticdeposits suggests that this was a minor process.

Therefore, from this study some implica-tions arise for volcanic hazard assessment inthe area of La Fossa di Vulcano. The time scaleof intra-eruptive periods is a crucial point forthe evaluation of eruption frequency, intensity,and duration. The Cuesta succession, due to itspeculiar geomorphic and climatic setting,records a phase of eruptive activity that proba-bly lasted several years or decades. During thistime sporadic eruptive episodes alternated withrepose periods on the order of years, character-ized by sedimentation from weather-relatedhyperconcentrated flow events. Evidence ofsuch year-scale repose periods might be miss-ing in the eruptive record in areas dominatedby apparently continuous primary pyroclasticsuccessions, especially in semiarid or arid

VALENTINE ET AL.


TABLE 4. FIVE DESCRIPTIVE FEATURES, EACH WITH VALUES RANGING FROM 1–5, USED IN CLUSTER ANALYSIS OF ASH USING SCANNING ELECTRON MICROSCOPE OBSERVATIONS

Value Shape Contour Structures Vesicle size Vesicle shape1 Equant/ Smooth None None None

blocky2 Planar Sharp Adhering particles Few small (<0.32 mm) Spherical3 Elongated Concave Fractures Many small Elliptic4 Acicular Concave/ Impact marks Only large (>0.32 mm) Elongated

convex5 Irregular Irregular Hollows Large and small Irregular

TABLE 3. SCANNING ELECTRON MICROSCOPE SAMPLE NUMBERS, STATION NUMBERS, AND FACIES DESCRIPTIONS

Sample Station DescriptionV1 1 1.1 m thick massive ash bed with sparse lithic lapilli near base,

near base of lower unit (hyperconcetrated flow).V2 12 10–25-cm-thick, matrix supported, massive bed rich in angular

rhyolite blocks, near base of upper unit (debris flow).V3 1 Massive to faintly laminated ash bed, ~10-cm-thick, near base of

middle unit (hyperconcentrated flow).V4 6 Thin planar bed, middle of the middle unit (pyroclastic surge).V5 1 7–10-cm-thick, cross-laminated ash bed, near top of middle unit

(pyroclastic surge).V6 1 ~20-cm-thick, massive to faintly laminated ash bed, near middle of

lower unit (hyperconcentrated flow).V7 2 10–50-cm-thick massive ash bed with isolated lithic and black-

pumice lapilli, upper part of middle unit (hyperconcentrated flow or possibly debris flow).

V8 16 8–10-cm-thick bed of laminated and cross-laminated, fine to medium ash, upper part of lower unit (pyroclastic surge).

V9 2 Cross-laminated gray to tan, fine to medium ash, near top of surge horizon in middle unit (pyroclastic surge).

V10 2 Cross- and planar-laminated, gray, fine to medium ash, near base of surge horizon in middle unit (pyroclastic surge).

V11 17 10–20-cm-thick, massive to faintly laminated ash bed, middle of lower unit (hyperconcentrated flow).

V12 1 10–20-cm-thick, massive to faintly laminated ash bed in modern stream channel, reworked La Fossa di Vulcano ash, probably 1888 eruption (hyperconcentrated flow).

Note: Inferred deposition mechanisms are in parentheses.



lands, where soil formation between pyroclas-tic units is slow or absent.

ACKNOWLEDGMENTS

Valentine’s work was funded by the Univer-sità degli Studi di Roma “La Sapienza,” as a vis-iting scientist, and partly by the U.S. Depart-ment of Energy. Palladino’s and Trigila’s workwas funded by C.N.R. (National ResearchCouncil of Italy), Gruppo Nazionale per la Vol-canologia. We thank Grant Heiken, NancyRiggs, Ray Cas, Kevin Scott, and Guido Gior-dano for very helpful reviews of the manuscript.

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MANUSCRIPTRECEIVED BY THESOCIETYAPRIL 28, 1997REVISEDMANUSCRIPTRECEIVEDSEPTEMBER22, 1997MANUSCRIPTACCEPTEDSEPTEMBER24, 1997



TABLE 5. DEGREE OF CORRELATION OF SAMPLES FROM THE CUESTA SUCCESSION, OBTAINED BY MEANS OF CLUSTER ANALYSIS

———— V1———— 0.2919 V2

———— 0.2009 0.2289 V3———— 0.1905 0.2143 0.2637 V4

———— 0.1505 0.2710 0.2307 0.3025 V5———— 0.2180 0.1691 0.0804 0.1469 0.2211 V6

———— 0.1339 0.2514 0.2281 0.1390 0.2614 0.1663 V7————- 0.2597 0.2603 0.1542 0.1891 0.3103 0.3526 0.3153 V8

———— 0.0941 0.3164 0.3386 0.2050 0.2383 0.3566 0.4102 0.3879 V9———— 0.1333 0.0744 0.3188 0.3025 0.1496 0.2050 0.3387 0.3516 0.3081 V10

———— 0.3032 0.3285 0.2701 0.1225 0.1096 0.2360 0.2334 0.1559 0.2256 0.2562 V11———— 0.2455 0.4294 0.3989 0.3856 0.2261 0.3057 0.4085 0.4006 0.2826 0.4328 0.3762 V12

V12 V11 V10 V9 V8 V7 V6 V5 V4 V3 V2 V1

Note: Morphological data from scanning electron microscope analyses are expressed as a “distance”: the lower is the “distance” the higher is the degree of correlation between any of two samples (sample identification on bottom row and right column of table). These data have been used to define thegroups of samples discussed in the text.

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Volcaniclastic aggradation in a semiarid environment, northwestern Vulcano Island, Italy