ELSEVIER Sedimentary Geology 129 (1999) 127–141

Facies analysis and depositional mechanisms of hydroclastite breccias,Acicastello, eastern Sicily

Lawrence H. Tanner a,Ł, Sonia Calvari b

a Department of Geography and Earth Science, Bloomsburg University, Bloomsburg, PA 17815, USAb Istituto Internazionale di Vulcanologia, Consiglio Nazionale delle Ricerche, Piazza Roma 2, 95123 Catania, Italy

Received 16 October 1998; accepted 2 August 1999

Abstract

Hydroclastite beds are interbedded with pillow lavas in an overturned section exposed near Acicastello, on the Ioniancoast of Sicily. Two distinct hydroclastite facies are recognized in a continuous 15-m section in a cliff face overlying pillowlavas. The first, an hyaloclastite facies, comprises black hyaloclasts and pillow fragments, many of which retain a partialpillow shape and glassy rim, in a matrix of smaller, partially devitrified and palagonitized hyaloclasts. Beds of this faciesare predominantly inversely graded, exhibit both matrix and clast support, and commonly contain outsize clasts at or nearthe bed top. Beds of the hyaloclastite facies beds are interpreted as the deposits of noncohesive submarine debris flowstransporting fragments formed primarily by cooling-contraction granulation. The second, a pillow-fragment breccia facies,forms wedge-shaped beds that are interbedded with the hyaloclastite facies. Pillow-fragment breccias comprise non-gradedbreccias of basalt blocks that formed by spalling of joint blocks from pillow lavas. Beds of this facies are clast-supportedand contain a matrix of marly sediment. This facies formed by accumulation of pillow fragments along the sides ofpillow-lava ridges, creating wedges of talus which interfinger with the debris-flow deposits of the hyaloclastite facies. 1999 Elsevier Science B.V. All rights reserved.

Keywords: hyaloclastite; hydroclastite; debris flow; volcaniclastic; pillow lava

1. Introduction

This paper describes hydroclastite breccia bedsthat are interbedded with pillow lavas at Acicastello,on the eastern coast of Sicily. This outcrop has longbeen known for the splendid exposure of pillow lavasand associated hydroclastite breccias (Chester et al.,1985; Lajoie, 1986). However, little attention hasbeen paid to the sedimentology of the breccias whichappear to have resulted from two separate mass-flow

Ł Corresponding author. Fax: C1-570-389-3028; E-mail:lhtann@planetx.bloomu.edu

depositional processes: cohesionless debris flow, orgrain flow; and talus accumulation.

The term hydroclast refers to all debris pro-duced by magma–water interactions, both explosiveand nonexplosive (Fisher and Schmincke, 1994).Hyaloclastites are volcaniclastic deposits compris-ing glassy or formerly glassy volcanic fragmentsformed by the cooling-contraction granulation of theglassy outer coatings of subaqueously erupted lavas(Staudigel and Schmincke, 1984; Kokelaar, 1986).These deposits are generally associated with effu-sive eruptions of mafic lavas in marine settings, andare particularly well known from seamounts (Lons-

0037-0738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 9 7 - 4

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dale and Batiza, 1980; Batiza et al., 1984; Staudi-gel and Schmincke, 1984). Felsic hyaloclastites havealso been described (Pichler, 1965), particularly fromsubglacial environments (Furnes et al., 1980). Hon-norez (1963) described three genetic types of hyalo-clastite deposits: (1) those formed by the crumblingof the glassy crusts and associated with pillow lavas;(2) deposits not associated with pillow lavas, butformed by the direct granulation of lava in contactwith water; (3) and stratified deposits formed by re-working of clasts by submarine currents or overlyinglava flows. Batiza et al. (1984) defined four mech-anisms for the formation of the glassy fragments:(1) spalling of the glassy rinds from the pillows; (2)granulation of the lava by thermal shock; (3) vesic-ulation from gas expansion; (4) and fragmentationby steam explosion. Cooling-contraction granulationis now considered the primary mechanism for theformation of hyaloclastite at any depth (Kokelaar,1986).

Many hyaloclastite deposits are non-bedded(Carlisle, 1963; MacDonald, 1972; Higgins andSoper, 1981; Aalto, 1986; McPhie, 1995) and, whereassociated with pillows, can be regarded as in situaccumulations of the clasts. Normal grading is alsowidely reported (Tasse et al., 1978; Floyd, 1980;Smith and Batiza, 1989; Bergh and Sigvaldason,1991; Garcia, 1996) and is generally attributed toredeposition by turbidity currents at greater depths.Less commonly reported inversely graded hyalo-clastites are interpreted as the result of depositionby debris flows (Batiza et al., 1984; Bergh andSigvaldason, 1991; Yamagishi, 1991). Hyaloclastitedebris, formed by cooling-contraction granulationat the summit of seamounts, apparently is trans-ported by debris flows down the flanks (Lonsdaleand Batiza, 1980; Cas and Wright, 1988). Beddedhyaloclastites deposited in a shallow-marine settinghave been interpreted by Schmincke et al. (1997) asforeset beds at the toe of a lava delta.

Pillow-fragment breccias comprise accumulationsof lapilli- to block-sized clasts (Staudigel andSchmincke, 1984) with shapes varying from bul-bous or spherical, to blocky or pyramidal (Kokelaar,1986). This debris may be formed by explosive frag-mentation of lavas at depths less than the volatilefragmentation depth (VFD) (Kokelaar, 1986). Alter-natively, the formation of pillow-fragment breccia by

nonexplosive spalling of pillow-lava blocks has beenobserved along modern seamounts at water depthsexceeding 1800 m (Lonsdale and Batiza, 1980). Anupward grading sequence from pillows to pillowbreccia to hyaloclastite often occurs as pillows be-come buried by the debris of their fragmentation(Carlisle, 1963; Tasse et al., 1978).

2. Geologic setting

The Pleistocene pillow lavas cropping out alongthe Ionian coast at Acicastello, nearby Acitrezza,and inland at Ficarazzi (Fig. 1), and the associatedvolcaniclastic breccias and intrusions at Acitrezzaand the Cyclopean Islands, are widely regarded asthe initial manifestations of the igneous activity thatcreated the Etna volcanic edifice (Rittmann, 1973;Tanguy, 1978; Romano, 1982; Gillot et al., 1994).The initial submarine lavas and shallow intrusionswere subalkaline tholeiites (Cristofolini, 1972, 1974;Corsaro and Cristofolini, 1997a,b), gradually evolv-ing toward the modern, mildly alkaline-sodic associ-ation as eruptive activity was changing from fissureeruptions to central-vent eruptions (Rittmann, 1973;Tanguy, 1978; Romano, 1982; Armienti et al., 1994).Dating of the clay-rich marine sediments intimatelyassociated with the early igneous products, on whichthe Etna volcanic pile rests, as well as K–Ar datingof the volcanics, indicates that submarine volcanicactivity occurred in this region between 500 and600 ka BP (Sicilian age) (Wezel, 1967; Gillot etal., 1994). This eruptive activity took place on theseafloor in a region uplifted by nappe-thrusting of theApennine–Maghrebian chain (McKenzie, 1970; Bar-beri et al., 1973, 1974; Rust and Neri, 1996). Thisuplift has elevated the pillow lavas at Ficarazzi to100 m a.s.l. and the Pleistocene marine sediments toas much as 700 m a.s.l. on the northwest side of Etna(Ogniben, 1966). Stewart et al. (1993) have demon-strated coastal uplift of the coastal region north ofCatania of 6 to 11 m during the last 7700 years.

The submarine volcanics, volcaniclastics, andshallow intrusions in this region are grouped intofour related units by Sturiale (1968). (1) The LacheaFormation comprises columnar-jointed basalts rep-resenting shallow intrusions into the Sicilian-ageArgille marnose azzure marine sediments. These are

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Fig. 1. Location of study area on the eastern coast of Sicily. The stratigraphic section was measured on Castle Rock at Acicastello.Topographic contours are given in meters.

exposed in the Cyclopean Islands, offshore from thevillage of Acitrezza. The intrusive nature is demon-strated by the contact metamorphism of the intrudedmarine sediments. (2) The Acicastello Formationcomprises pillow lavas and interbedded breccias,exposed in the cliff (Castle Rock) and wave-cutplatform at Acicastello (Fig. 2). (3) The AcitrezzaFormation constitutes a breccia of pillow fragmentsin a matrix of hyaloclasts exposed inland near the vil-lage of Acitrezza. (4) The Ficarazzi Formation com-prises a breccia of oxidized lava blocks and pillowfragments exposed near Ficarazzi. The Acicastello–Acitrezza–Ficarazzi sequence is interpreted by Sturi-ale (1968) and Chester et al. (1985) as representinga deep- to shallow-water sequence. The association

of intact pillows and hyaloclastite at Acicastello in-dicates greater water depth, whereas the oxidizedand vesicular lavas at Ficarazzi indicate very shallowwater depth with subaqueous to subaerial eruptions.

Bedded hydroclastite breccia is found only at Cas-tle Rock, a sea cliff on the coast at Acicastello. Thestructural aspects of this exposure have generatedsome debate. Di Re (1963) recognized that the shapeof the pillow lavas indicates that the entire section issteeply dipping and overturned. This structural atti-tude contrasts with the nearby outcrops of intrusionsof the Lachea Formation in the Cyclopean Islands,where the sills and laccoliths are right-side-up andhave dips of up to 15º. Di Re (1963) proposed amodel involving the foundering of an isolated fault-

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Fig. 2. Map of the outcrop exposure at Castle Rock. The outlineof the cliff is indicated by the dashed line. Adapted from Corsaroand Cristofolini (1997b).

block in an underlying magma mass to explain thisunique attitude. Romano (1982) suggested, instead,that the force of intruding magma during continuedpillow formation caused local tilting of an isolatedblock. Chester et al. (1985) interpret the Acicastelloblock as an isolated slump block transported by asubmarine landslide. Borgia et al. (1992) also recog-nized the overturned nature of the section and invokegravitational spreading caused by loading of the Et-nean volcanic edifice as the cause of tilting. Most re-cently, Corsaro and Cristofolini (1997b) interpretedthe section as gently dipping and interpreted thenear-vertical hydroclastite beds as vertical fissureswithin the volcanic pile that became infilled by theproducts of subaqueous explosive activity. However,this interpretation ignores the bedding clearly visi-ble in the hydroclastite facies. This paper describestwo facies of hydroclastites interbedded with pillowlavas at Castle Rock. These facies display beddingand sedimentary fabrics that indicate deposition bymass-flow processes, forming a stratigraphic sectionthat has been since overturned.

3. Methods

The sedimentology of the outcrop section wasstudied to determine the origin of the hydroclastitebeds. Only the stratigraphic section exposed on the

stack is sufficiently exposed for detailed measure-ment. This section was measured perpendicular toformation dip. The fabric of the beds was exam-ined by measuring relative grain size and distributionwithin beds. The proportions of clasts and matrixin the breccia beds were determined by counting100 points on the outcrop surface at 10 cm intervalson straight-line transects within beds. For the pur-poses of this study, clasts are defined as fragmentsgreater than 1 cm long (long axis), and matrix asall material 1 cm or smaller. The rationale for thisoperational definition is based on the composition ofthe breccias. Almost all pillow fragments and blackhyaloclasts are >1 cm long, while brown, palagoni-tized hyaloclast fragments form a grain size contin-uum from 1 cm to submillimeter-size. Petrographicanalysis was performed on standard 30-µm thin sec-tions. The composition of the matrix was examinedby analysis of bulk powder samples and <2-µmclay separates by X-ray diffraction (XRD) utilizingCu-Kα radiation. Clay separates were analyzed bothair-dried and solvated with ethylene glycol.

4. Description of the Castle Rock section

The Castle Rock outcrop lies along the shore ofAcicastello, a village 5 km north of the city of Cata-nia on the Ionian coast of Sicily. The outcrop consistsof a near-vertical stack protruding from the shore andcomprises pillows and bedded hydroclastics (Figs. 2and 3). The stack is about 15 m high and roughly 50m across. A wave-cut platform surrounds the stack,separating it from the sea by 30 to 50 m on all butthe northwest side which is attached to the mainland.A lava flow, possibly emplaced in 1169 (Sturiale,1968), covers the entire section on the mainland inAcicastello. The west side of the outcrop consistsentirely of pillow lavas, typically 1 to 2 m long, upto a maximum of about 10 m. Glassy margins arepreserved on many pillow lobes and radial jointingis well displayed in broken pillows. Pipe vesicles inthe pillows are mainly concentrated in the outer shelland are filled by zeolites, chiefly phillipsite. Signifi-cantly, pillow lobes on the northeast and east sides ofthe outcrop are oriented with a concave surface to thenorthwest. The concavity is the basal surface of thepillow lava, formed by draping of plastic lava over

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Fig. 3. Castle Rock at Acicastello viewed from the southwest. Pillow lavas comprise the left side of the stack, overlain to the right bybedded hydroclastite breccias. The breccia sequence exposed on the cliff is overlain by a second pillow-lava sequence, exposed on thewave-cut platform. An additional breccia sequence overlies this pillow sequence, seen at the far right on the wave-cut platform.

a previously solidified pillow (MacDonald, 1972).This orientation indicates that stratigraphic top in thesection is toward the southeast.

The southeast side of the stack exposes a 15-msection of breccia beds striking roughly N65ºE withan apparent dip of about 68º to the northwest (Figs. 2and 4). The orientation of the pillow lobes describedabove, however, demonstrates that the section is over-turned, with dips of approximately 112º to the south-east. The breccia section exposed in the cliff face restson the irregular surface of the pillow lavas, filling ir-regular depressions on the surface. The stratigraphicsection cropping out in the cliff face comprises 25breccia beds, easily defined by abrupt change in clastsize or fabric, overlying the irregular pillow-lava sur-face (Figs. 4 and 5). Relief on this surface is up to6 m. Breccia beds overlying the pillow-lava surfacetotal 15 m in thickness and comprise two distinct fa-cies. The wave-cut platform exposes an additional 30m stratigraphic thickness of interlayered pillow lavasand breccia (Figs. 2 and 3), but the exposure is notclear enough to permit the measurement of the thick-ness and characteristics of individual beds. The se-

quence can be traced offshore to a water depth of atleast 15 m, where modern sediments bury the section(Corsaro and Cristofolini, 1997b).

4.1. Hyaloclastite facies

The predominant facies in the section, making up19 of the 25 beds, comprises a poorly sorted brec-cia of grayish-weathering pillow fragments and rare,complete miniature pillows, up to 90 cm in diame-ter, and black, glassy hyaloclasts up to 5 cm long(Fig. 6). The pillow fragments, which outnumberthe hyaloclasts in most beds, are extremely vari-able in size, ranging up to 60 cm (long axis), butwith a maximum size of 30 cm in most beds. Thepillow fragments are mainly angular to subangularand equidimensional in shape and massive to finelyvesicular with zeolite-filled vesicles. Larger pillowfragments typically retain a partial curved shapeand black glassy coating, locally altered to goldenbrown. The hyaloclasts are black shiny clasts, 1 to 5cm long, with a subangular to angular and elongateto subequidimensional shape. Elongate hyaloclasts

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Fig. 4. (a, b) The breccia sequence on the northeast side of Castle Rock measured and described in the text; (b) is annotated to indicatesome bed contacts. The top of a bench in the foreground is indicated by the solid line. Numbers of the beds correspond to the numbersin Fig. 5 and in the text. Bed 1, for example, fills an irregular 6-m depression on the pillow-lava surface.

commonly have a curved shape. Pillow fragmentsand hyaloclasts together comprise 27% to 78% ofthe volume of individual beds.

The black hyaloclasts and pillow fragments areset in a ‘matrix’ of brown, non-vesicular fragments

from roughly 1 cm (long axis) grading continuouslydownward to submillimeter size, with a yellow-brown groundmass. Within this matrix, the brownfragments and groundmass are typically subequalcomponents. In thin section, the brown fragments

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Fig. 5. The measured section for the breccia sequence exposed in the stack at Castle Rock. Inverse and normal grading and the relativemean grain size for individual beds are indicated by the horizontal scale.

are vitreous yellow clasts, isotropic under crossed-polarizers, containing common subhedral to euhedralolivine and augite phenocrysts up to 0.5 mm long and

plagioclase laths up to 2 mm long. The surroundinggroundmass is orange- to reddish-brown in plain-polarized light, dark brown and weakly birefringe-

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Fig. 6. Typical exposure of hyaloclastite facies on cliff face. h D dark hyaloclasts; pf D gray, basalt pillow fragments. Pillow fragmentbelow the lens cap (5 cm) is curved and has a partial glassy rim. Matrix between these clasts consists of small (�1 cm) brownpalagonitized clasts and groundmass.

ment under crossed-polarizers. The groundmass con-tains many smaller, angular vitreous fragments upto 2 mm long, vugs up to 2 mm long, and iso-lated crystals of olivine, augite, and plagioclase. Thevugs are lined with thin rims of brightly birefringentclay, identified by XRD as smectite (nontronite), andare filled by zeolites, identified by XRD as mainlyphillipsite, with minor herschellite.

The matrix of the hyaloclastite facies was formedby the initial stages of alteration, or palagonitiza-tion, of the smaller fragments from the sideromelane(basaltic glass) rims of the pillows, primarily throughalteration of olivine phenocrysts, and release and ox-idation of iron. The groundmass consists of the min-eraloid palagonite and a mixture of clay, Fe-oxides,and zeolite. An upward increase within the matrix ofmany beds of the size and number of zeolite-filledvugs is apparent on the outcrop face.

Beds of the hyaloclastite facies range from 0.2to 1.95 m in thickness, with a mean of 0.6 m. Thethickness of the beds does not appear to vary sys-tematically in the vertical sequence. The fabric in thehyaloclastite facies is primarily matrix-supported,

using the parameters of matrix defined in this paper,although the smaller palagonitized fragments thatconstitute much of the matrix are actually the sup-porting agents of the fabric in most beds. Of the 19beds of this facies, 17 are inversely graded (Figs. 5and 7), one is normally graded (at 4.8 m), and theremaining bed is non-graded (at 0 m). The inversegrading is exhibited primarily by un upward increasein size and concentration of pillow fragments andhyaloclasts. Size of the palagonitized fragments alsofrequently increases upwards within the beds. Thefabric grades upward from matrix supported to clastsupported in four of these beds. Outsize clasts, whichare significantly larger than the mean clast size in thebed, occur at or near the tops in seven of the beds,and project above the top of the bed in five of thebeds (Fig. 8). There appears to be a weak correlationbetween the maximum particle size (MPS), taken asthe mean of the ten largest clasts visible in the bed,and the bed thickness (Bth) (Fig. 9). The slope of theregression line is 0.164 (b), but the correlation index(Pierson) of 0.375 (R) indicates that the correlationis not statistically valid.

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Fig. 7. Portion of sequence exposed on cliff face. The field book rests on the contact between pillow-fragment breccia bed (right) andoverlying hyaloclastite bed (arrow). Basal portion of the bed displays inverse grading. Stratigraphic top is to the left.

Fig. 8. Portion of sequence exposed on the southwest side of the cliff. Stratigraphic height increases to the right in this view. A pillowjust above the hammer projects downward (to the left) from the pillow-fragment breccia bed in which it occurs to the underlyinghyaloclastite bed. Higher on the cliff in the same bed, several large clasts and small pillows project above the bed top.

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Fig. 9. Plot of maximum particle size (MPS) against bed thickness (BTh) for hyaloclastite beds. b D slope of regression line, R D Piersoncorrelation coefficient.

4.2. Pillow-fragment breccia facies

This facies, accounting for 6 of the 25 beds (Fig. 5),comprises boulder breccias that are moderately- towell-sorted accumulations of cm- to tens of cm-sizepillow fragments and whole pillows up to 90 cm indiameter (Figs. 8 and 10). Pillow-fragment brecciabeds range in thickness from 60 cm to 2 m, averaging1 m, but display abrupt lateral thickness variations andpinch-outs. The fabric of these beds is clast-supportedand ungraded. The matrix, comprising 12% to 22%of the beds, consists of marly claystone that is zeo-lite-cemented, and is mostly free of the palagonitizedfragments that typify the matrix of the hyaloclastitefacies. The bases and tops are mostly irregular and, inplaces, the largest boulders penetrate downward intothe underlying bed (Fig. 5 between 7 and 8 m).

4.3. Vertical sequence

The section was measured on the northeast faceof the cliff. Bed 1 fills a 6-m depression on thepillow-lava surface. This bed is atypical, consistingof non-graded pillow-fragment breccia, but with a

matrix containing palagonitized hyaloclasts that isabsent in other beds of pillow-fragment breccia. Thevertical sequence continues with the first laterallycontinuous bed overlying the pillow-lavas surface.Locally, bed 2 (hyaloclastite facies) is draped overthe irregular pillow-lava surface and overlies the 6-mfilled depression. This bed is overlain by a wedgeof pillow-fragment breccia that contains pillows upto 90 cm in diameter and thins toward the platform.Bed 3 is overlain by another pillow-fragment brec-cia bed (bed 4). Bed 6 is a pillow-fragment brecciabed containing whole pillows up to 1 m in diame-ter. The remainder of the vertical sequence consistsprimarily of beds of the hyaloclastite facies withtwo interbeds of pillow-fragment breccia (beds 13and 15) in conformable (non-erosional) succession.The southwest face of the stack exposes the same se-quence of facies, although the thickness of individualbeds differs, particularly those of pillow-fragmentbreccia. Thin (<0.5 m) irregularly shaped basalticdykes intrude and cross-cut the breccia section onthe southwestern face of the stack.

The measured section comprises a single breccialayer sequence exposed at Castle Rock. This se-

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Fig. 10. Pillow-fragment breccia bed consisting of roughly equidimensional, subangular basalt blocks with marly matrix exposed on cliffface. Lens cap D 55 mm.

quence is overlain by pillow lavas up to 15 m thickcontaining laterally interfingering m-scale brecciabeds. Another breccia sequence, with an irregularbase, overlies the pillow lavas and has a thicknessof at least 15 m. The exposure is truncated by thesouthern edge of the wave-cut platform. The brecciabeds occupy swales on the platform that are filledby water or salt, obscuring the sedimentary features,while the pillow lavas form the intervening ridge.

5. Interpretation

The hyaloclastite facies consists of pillow frag-ments and hyaloclasts in a matrix of smaller hyalo-clast debris that has been slightly to greatly palag-onitized. The degree of alteration appears a func-tion of surface area, causing more extensive alter-ation of the smaller (<1 cm) fragments, the mostpronounced effect being the characteristic Fe-oxidestaining, whereas the larger hyaloclasts appear onlyslightly altered. The greater zeolite cementation atthe tops of upward-coarsening beds probably re-sults from greater porosity and permeability in the

coarser, upper portion of individual beds. The hyalo-clastite facies association of pillow fragments, manyof which retain a partial pillow shape and siderome-lane rim, with hyaloclasts of varying sizes suggestsformation of the debris by a combination of explo-sion fragmentation and contraction-cooling granula-tion.

The depositional processes responsible for theoutcrop sequence clearly operated in a submarine en-vironment, based on the association of pillow lavasand marine sediments. The vesicularity of the pillowlavas and the presence of pillow fragments, whichform from explosive fragmentation, preclude a waterdepth greater than the VFD, which may be as shal-low as 100 to 200 m for tholeiitic lavas (Kokelaar,1986). Lonsdale and Batiza (1980) have previouslyinvoked mass flow to explain hyaloclastite ‘stonestreams’ on the flanks of seamounts. In the marineenvironment, mass flow is often dominated by tur-bidites, but the non-erosive nature of the contactsbetween the breccia beds suggests deposition by anon-turbulent flow mechanism. More conclusive isthe predominance of inverse grading (17 of the 19hyaloclastite facies beds) over normal grading, which

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typifies turbidites. Debris flows, in contrast, com-monly produce deposits with basal inverse gradingresulting from grain interactions and dispersive pres-sure in the sheared basal layer of the flow (Larsenand Steele, 1978; Nemec and Steele, 1984; Hubertand Filipov, 1989). An ungraded flow body and out-size clasts projecting from the flow top result fromthe support strength provided by a cohesive matrix(Nemec and Steele, 1984; Hubert and Filipov, 1989).Dilution of the upper surface of subaqueous flows,however, causes a loss in competence and may resultin a normally graded flow top (Gloppen and Steele,1980; Nemec and Steele, 1984). A positive corre-lation between bed thickness and maximum clastsize is considered a characteristic of both subaerialand subaqueous debris flows (Gloppen and Steele,1980; Nemec and Steele, 1984). Subaqueous flows,however, generally exhibit lower support strengthand, consequently, display a lower slope of correla-tion (Gloppen and Steele, 1980; Nemec and Steele,1984).

Inverse grading is also typical of the class of sed-iment-gravity flows lacking cohesive matrix termedgrain flow by Lowe (1976, 1982) and cohesionlessdebris flow by Nemec and Steele (1984). Both termsrefer to a transport process in which the dispersivepressure from grain collision keeps larger grains insuspension, producing beds that may be substantiallyor entirely inversely graded. Yet another process thatproduces deposits that are partially to completelyinversely graded is the traction carpet. This is a high-concentration sediment-gravity flow layer driven byoverlying turbulent flow that may also be dominatedby collisional forces (Sohn, 1997).

The beds of the hyaloclastite facies display manyof the features of debris-flow deposits: inverse grad-ing (Fig. 7), a tentative correlation between bedthickness and maximum clast size (Fig. 9), and out-size clasts near the bed top (Fig. 8). However, thesebeds lack the cohesive matrix that provides yieldstrength in cohesive debris flows. Additionally, theinverse grading associated with cohesive debris flowsis typically limited to the base of the flow, whereasthe main body of the flow consists of the ungraded,unsheared plug. Beds of the hyaloclastite facies,however, often display inverse grading through muchof the bed, exhibited by a continuous upward in-crease in the size of and=or abundance of clasts

(particles > 1 cm), relative to matrix (particles �1 cm). This fabric suggests that dispersive pressure,caused by grain-to-grain collision during transport,was active throughout most of the flow. Also, giventhe absence of evidence of an overriding turbulentlayer, we rule out transport by traction carpet. Webelieve that masses of mixed pillow fragments andhyaloclasts accumulated on the steep slope of agrowing volcanic pile until instability, resulting fromoversteepening or seismic shock, caused transport ascohesionless debris flows, or grain flows.

The pillow-fragment breccia facies comprisesnon-graded, clast-supported beds of basaltic pil-low fragments and whole pillows in a matrix con-sisting mainly of pelagic sediment, and generallylacks hyaloclasts or smaller hyaloclast matrix debris(Fig. 10). Clast support, sedimentary matrix, and alack of grading have been recognized previously ascriteria for differentiation of talus and debris-flowbeds (Tanner and Hubert, 1991). Talus deposits formwedges onlapping the source slope, thinning in thedirection of transport. Talus deposits may be subjectto ‘gravity sorting,’ caused by the similar travel dis-tances of clasts with similar potential energy (Tannerand Hubert, 1991). The wedge-shaped nature andsorting of the pillow-fragment breccia beds are con-sistent with a model of talus accumulation against asteep slope formed by the buildup of pillow lavas.Lonsdale and Batiza (1980) observed talus rampsof pillow joint blocks at the feet of long, narrowpillow-lava walls. We interpret beds of the pillow-fragment breccia facies as formed by the gradualaccumulation of talus made up of spalled-off jointblocks from pillow-lava walls, similar to those de-scribed by Lonsdale and Batiza (1980). The pillowfragments fell freely downslope for short distances,and pelagic marine sediments filled the intersticesbetween clasts. Larger fragments and whole pillowstravelled farther downslope, but smaller fragmentswere left behind, resulting in gravity sorting.

6. Depositional model

The sequence of hyaloclastite and pillow-brecciabeds was formed by two different depositional pro-cesses operating contemporaneously on the south-east-facing slope of an initially submerged volcanic

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Fig. 11. Interpreted model for deposition of hyaloclastite and pillow-fragment breccia facies. The hyaloclastite facies was deposited bycohesionless debris flows following troughs or channels between pillow-lava buildups. Interfingering wedges of pillow-fragment brecciawere deposited by accumulation of falling blocks of broken pillows derived from laterally adjacent pillow ridges.

pile. Beds of the hyaloclastite facies were formed bydebris-flow transport of pillow fragments and hyalo-clasts from the shallower flanks of the growing vol-canic pile. These flows initially followed topographiclows, or natural channels created by adjacent ridgesof pillow lavas, gradually burying the topography ofthe pillow lavas (Fig. 11). Although no paleocurrentfeatures are preserved in these beds, the outcropsof what appear to be in situ shallower-water faciesof pillow breccias (sensu Staudigel and Schmincke,1984) at Ficarazzi and Acitrezza indicate that waterdepth shallowed to the west and northeast where ex-plosive processes dominated (Sturiale, 1968; Chesteret al., 1985). Pillow-fragment breccia was derivedlocally from the joint-fragmentation of pillows onsteep ridges adjacent to the channel. Gradual accu-mulation of talus during hiatuses in deposition of thehyaloclastite facies formed discontinuous wedges ofnon-graded blocks and boulders that interfinger lat-erally with the more continuous hyaloclastite facies.The lack of pillow-fragment breccia in the upper-most part of the measured section may result fromthe burial of adjacent pillow-lava ridges by flows ofthe hyaloclastite breccia, eliminating topographicalrelief. Periodically, the entire sequence was covered

by the eruption of fresh pillow lavas, as the outcropon the wave-cut platform exhibits.

Acknowledgements

Field work by Tanner was funded by a FacultyProfessional Development Grant from the Pennsyl-vania State System of Higher Education. This paperbenefitted substantially from thoughtful reviews byW.J. Fritz and H. Yamagishi.

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