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Author's personal copy Sedimentary Geology 220 (2009) 256–270
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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o
Intra- and extra-caldera volcaniclastic facies and geomorphic characteristics of a frequently active mafic island–arc volcano, Ambrym Island, Vanuatu Károly Németh a,⁎, Shane J. Cronin a, Robert B. Stewart a, Douglas Charley a,b a b
Institute of Natural Resources, Volcanic Risk Solutions, Massey University, Private Bag 11 222, Palmerston North, New Zealand Department of Geology, Mines and Water Resources, Port Vila, Republic of Vanuatu
a r t i c l e
i n f o
Article history: Received 9 November 2007 Accepted 23 March 2009 Keywords: Tephra Caldera Volcanic sedimentology Grain flow Hyperconcentrated flow Ambrym Vanuatu
a b s t r a c t Ambrym is one of the most voluminous active volcanoes in the Melanesian arc. It consists of a 35 by 50 km island elongated east–west, parallel with an active fissure zone. The central part of Ambrym, about 800 m above sea level, contains a 12 kilometre-wide caldera, with two active intra-caldera cone-complexes, Marum and Benbow. These frequently erupting complexes provide large volumes of tephra (lapilli and ash) to fill the surrounding caldera and create an exceptionally large devegetated plateau “ash plain”, as well as sediment-choked fluvial systems leading outward from the summit caldera. Deposits from fall, subordinate base surge and small-volume pyroclastic (scoria) flows dominate the volcaniclastic sequences in near vent regions. Frequent and high-intensity rainfall results in rapid erosion of freshly deposited tephra, forming small-scale debris flow- and modified grain flow-dominated deposits. Box-shaped channel systems are initially deep and narrow on the upper flanks of the composite cones and are filled bank-to-bank with lapilli-dominated debris flow deposits. These units spill out into larger channel systems forming debris aprons of thousands of overlapping and anastomosing long, narrow lobes of poorly sorted lapilli-dominated deposits. These deposits are typically remobilised by hyperconcentrated flows, debris-rich stream flows and rare debris flows that pass down increasingly shallower and broader box-shaped valleys. Lenses and lags of fines and primary fall deposits occur interbedded between the dominantly tabular hyperconcentrated flow deposits of these reaches. Aeolian sedimentation forms elongated sand dunes flanking the western rim of the ash-plain. Outside the caldera, initially steep-sided immature box-canyons are formed again, conveying dominantly hyperconcentrated flow deposits. These gradually pass into broad channels on lesser gradients in coastal areas and terminate at the coast in the form of prograding fans of ash-dominated deposits. The extra-caldera deposits are typically better sorted and contain other bedding features characteristic of more dilute fluvial flows and transitional hyperconcentrated flows. These outer flank volcaniclastics fill valleys to modify restricted portions of the dominantly constructional landscape (lava flows, and satellite cones) of Ambrym. Apparent maturity of the volcanic system has resulted in the subsidence of the present summit caldera at a similar rate to its infill by volcaniclastic deposits. © 2009 Elsevier B.V. All rights reserved.
1. Introduction A number of recent studies have focused on understanding volcaniclastic sedimentation around volcanoes (Cronin and Neall, 1997; Donoghue et al., 1997; Lecointre et al., 1998; Giordano et al., 2002). The majority of these studies concentrate on volcaniclastic sedimentation around large-volume intermediate or silicic volcanic centres (e.g. Manville et al., 2009-this volume). This type of volcanism generally produces large volumes of tephra that is often transported tens of kilometres away by pyroclastic falls, flows or surges (Fisher and Schmincke, 1984). The loose and commonly highly fragmented eruptive products are easily remobilised to form thick successions of reworked volcano-sedimentary units up to hundreds of kilometres
⁎ Corresponding author. E-mail address:
[email protected] (K. Németh). 0037-0738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2009.04.019
away from the source volcano (Vallance and Scott, 1997; Capra et al., 2004; Scott et al., 2005; Thouret, 2005;). Such remobilisation commonly alters the sedimentary budget of large areas around silicic centres, changing fluvial network patterns, forming lakes or draining large basins over brief periods (e.g. Manville et al., 2009-this volume). The scale of such volcaniclastic sedimentation triggered by explosive silicic eruptions is significantly larger than that typically associated with mafic explosive volcanism. Hence, examples and studied on large-scale volcaniclastic sedimentation in association with mafic volcanism are rare. On volcanic islands, volcaniclastic successions can also be traced for long distances in subaqueous settings (Ballance and Gregory, 1991; Karátson and Németh, 2001). The process of reworking primary tephra can be facilitated by external forces, such as heavy rainfall, wind or sediment transport through existing fluvial networks (Major et al., 2000; Manville et al., 2000; Torres et al., 2004). The net result of these processes is the buildup of vast volumes of volcaniclastic
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Fig. 1. A) Overview map of the Vanuatu (New Hebrides) archipelago. B) Overview map of Ambrym Island with a panoramic view from the south. Line marks the approximate position of the panoramic view.
sediments in a variety of textures and structures surrounding active volcanoes. These distal facies can sometimes be directly linked to primary explosive volcanic eruptions [such as block-and-ash fans that may contain both primary and secondary (lahar) units, Siebe et al., 1993)], while others are described more generally with terms such as volcaniclastic aprons or ring plains. In spite of the amount of current research focused on the interrelationship between primary pyroclastic and secondary volcaniclastic sedimentation around intermediate to silicic volcanoes,
relatively less attention has been paid to mafic volcano–sedimentary systems. The interrelationship of sedimentation and formation of mafic volcanic calderas is also not in all cases clearly understood, with caldera formation models including both explosive disruption triggered by violent phreatomagmatic and magmatic explosive eruptions (Braitseva et al., 1996), along with those of gradual subsidence due to lateral discharge of magma (Munro and Rowland, 1996). In addition, the role of intra-caldera active cone complexes in overall volcanic sediment production can also be significant for mafic calderas. These
Fig. 2. Oblique aerial view (looking east) of the western side of the caldera and ash plain of Ambrym Island. Note the de-vegetated “badlands” landscape in the foreground.
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Fig. 3. Small sub-Plinian eruption on 17 July 2005 from the Marum cone complex, within the summit caldera of Ambrym, feeding a sustained low ash plume and depositing fine ash on the ash plain.
“secondary” eruptive centres are comparable in size to large scoria cones in intra-continental settings. Eruptive products and redistributed sediments from these volcanoes are dominantly deposited and re-distributed within the closed caldera depression. Such mafic calderas are therefore commonly filled with thick piles of ash and lapilli entirely locally derived. The total volume of the accumulated tephra inside the mafic caldera depends on the eruption frequency and intensity and how efficiently it is removed from the caldera system. In this paper we describe the sedimentary and geomorphic characteristics of a large basaltic caldera system with an unusually high sediment input rate (through tephra deposition and redistribution). In this case the accumulation of intra-caldera volcaniclastic deposits appears to keep pace with gradual caldera subsidence, in part “drowning” the volcano–tectonic structure and leading to spilling-out of sediment and widespread extra-caldera sedimentation.
The 12 km-diameter caldera on Ambrym Island, Vanuatu, is an excellent example location where the transition from proximal primary pyroclastic deposits can be traced through several generations of reworking into volcaniclastic deposits of differing character both within and outside the caldera structure. In this example, frequent heavy rainfall is the main driving force behind redistribution of vast volumes of tephra toward the outer flanks of the volcanic island. 2. Geological setting Ambrym is one of the most voluminous active volcanic islands in the Vanuatu portion of the Melanesian arc (Fig. 1). Ambrym Island is 35 km wide, 50 km long and elongated in an east–west direction, with an active rift zone parallel to the elongation (Robin et al., 1993). Along this system are many scoria cones and fissure fed lava flows (McCall
Fig. 4. Interpreted volcaniclastic sedimentary environment on an active volcanic cone of Ambrym Island. Inset shows the SE flank of Benbow as a type location to demonstrate major sedimentary environment identified. 1) Crater rim crest with cracks and unstable collapse tephra beds, 2) box-shaped gully headwalls, 3) coarse-grained unsaturated grain-flow dominated mass-flow deposits in the upper gully floors, 4) granular debris flow deposits in the middle-level gully floors with well-developed levees, 5) overlapping distal saturated granular debris flow generated fans and lobes near the base of the cone, 6) major sediment artery between the active cones of Marum and Benbow dominated by shallow broad hyperconcentrated flow deposits, and 7) artery valley wall exposing older hyperconcentrated flow deposits cut by small stream valleys and topped by fresh airfall tephra. Sedimentation zone A represented by processes and features marked on 1 to 5. Sedimentation zone B is represented by 6 and sedimentation zone C represented by 7 on the figure.
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et al.,1970). The northern part of Ambrym is the oldest and it is inferred to be part of a series of coalescing strato-volcanoes and lava shields (McCall et al., 1970). The central part of Ambrym Island comprises a 12 kilometre-wide caldera structure that rises to between 600 and 800 m above present day sea level (Fig. 2). Recent and current activity is commonly but not exclusively concentrated at the two active vent complexes (Fig. 1) of Marum and Benbow (Purey-Cust, 1896; Frater, 1917; Gregory, 1917; Carniel et al., 2003). These systems regularly produce small-volume Strombolian style events, constant intense degassing, phreatomagmatic explosions and more rarely sub-Plinian and Vulcanian eruptions (Fig. 3). At the western and eastern extremities of Ambrym, many phreatomagmatic eruption centres have developed near sea level as a result of interaction of fissure-fed magma and near-surface water and/or water saturated sediments (Németh and Cronin, 2007, 2009a). The resulting tephra rings/cones typically have 1000 m diameter craters and low (b100 m) rims. The most recent shallow-submarine, phreatomagmatic and rift-edge fissure eruptions
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took place in 1894, 1913, and 1937 at the western edge of Ambrym (Purey-Cust, 1896; Frater, 1917; Gregory, 1917; Németh and Cronin, 2009b) and in 1954 in the eastern rift edge (McCall et al., 1970). These rift-edge phreatomagmatic volcanoes are similar in architecture, size and eruption mechanisms to those described in other volcanic islands in Vanuatu, such as Ambae (e.g. Németh et al. 2006). Ambrym eruptives are predominantly calc–alkaline basalts and include both high and low-K variants (Pickard et al., 1995). Few, if any, differentiated rocks occur. Analyses of more evolved rocks were reported by Pickard et al. (1995), but re-visiting of the sampling sites suggests that the geochemistry of these samples was modified by alteration (Cronin and Németh, 2005). The Ambrym caldera is here (and by McCall et al., 1970) interpreted to be a gradual (or episodic) subsidence feature associated with ongoing centrally focused basaltic volcanism and concomitant lateral fissure feeding, similar to mafic calderas described from Hawaii (Kilauea caldera) and the Galapagos Islands (Chadwick et al., 1991; Geist et al.,
Fig. 5. A) Overview of the SW flank of Benbow with dendritic valley network. B) Overview of the NW flank of Marum after a small sub-Plinian eruption in July 2005, which covered the dendritic valley and gully network with fresh ashfall.
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2002). The feature has apparently been in existence since c. 2200 years (McCall et al., 1970). It has also been alternatively proposed that a cataclysmic phreatomagmatic eruption led to the caldera formation and construction of an associated giant tuff cone (Robin et al., 1993). Recent field evidence does not support this interpretation; rather, it suggests that the island is a composite structure formed by many generations of coalescing monogenetic volcanic fields and broad lava shields topped by complex composite volcanic cones (Cronin and Németh, 2005). The type localities of the “giant tuff cone”, which were reported as dacitic pyroclastic flow deposits (Robin et al., 1993), have instead proven to be either (1) mafic, weathered and hydrothermally altered, phreatomagmatic fall and surge sequences from mafic magmatic and phreatomagmatic cone clusters, or (2) stacks of laharic and fluvial basaltic sediment derived from the caldera outflow and forming valley-fills (Cronin and Németh, 2005). However the role of phreatomagmatism in the central vent eruptions is evident similarly to other volcanic islands in Vanuatu such as Ambae (e.g. Németh et al 2006), but the scale, extent and style of such eruption mechanism still need more work in the near future in Ambrym. The early evolution of Ambrym is also purely understood and needs to apply comprehensive methods of geochronology (e.g. Balogh and Németh 2005) and geochemistry to distinguish certain evolutionary phases of the volcanic island growth. The island has a relatively low number of permanent streams. Many of these streams initiate outside and about 200–300 m below the margin of the flat summit plain. In coastal areas, permanent waterfalls lead water to the sea especially in the NW and E side of the island. Most of the major fluvial tributaries are active only during the rainy season and are located mostly in the southern part of the island.
4. Intra-caldera sedimentation at Ambrym Island The intra-caldera sedimentation at Ambrym Island can be separated into three major geographical facies associations related to the active and primary volcanic cones (Fig. 4); Zone A. Tephra cone and near-cone sequences (primary deposits, gullies, gully fills, deposit spills into arterial channels) — #1 to 5 in Fig. 4. Zone B. Arterial channels (main flow paths in flash floods) — #6 in Fig. 4. Zone C. Interfluve and marginal caldera (dunes, side valleys, etc.) — #7 in Fig. 4. These geographically distinct sedimentation zones include a broad range of volcaniclastic sediments produced from primary to secondary processes demonstrated on Fig. 4. Zone A represents the tephra cones formed by mafic explosive processes, where an edifice is constructed from proximal ballistic fall out and pyroclastic density current-deposited primary volcaniclastic successions. Secondary processes cut and erode the primary pyroclastic constructs (e.g. Benbow and Marum) and remove and accumulate pyroclasts in their vicinity (Fig. 4 — #1 to 5). Tephra cones are a pyroclastic volcanic construction that consist of stacked primary ash- and lapilli-dominated units. These volcanic edifices suffer flank rilling and erosion, leading to the development of complex gully network with associated gully fills all feeding spills leading to arterial
3. Climate and topography The island of Ambrym is situated in the centre part of the Vanuatu archipelago at latitude 16.25° south. The climate is oceanic tropical, moderated by southeast trade winds between May and October. Rainfalls are strongest during the wet season from November through April. The annual rainfall totals through Vanuatu vary from 1500 to 4200 mm from south to north. In Port Vila, Efate (120 km south of Ambrym), the average temperatures range between 26.7 °C (January) and 22.2 °C (July), and the average annual rainfall is 2103 mm. The climate varies from hot, very wet and humid with little seasonality in the north of Vanuatu. Much of the yearly rainfall is delivered by tropical fronts, depressions, heavy storms and cyclones. There is no permanent meteorological station in Ambrym, therefore no exact data can be provided for the areal distribution of rainfall over the island. However, dryer conditions prevail on the western flanks (rain shadow effect from the southeasterly trades) and wet conditions in the east and summit even during the “dry” season due to orographic effects in relation to the trade winds. Vanuatu lies in the cyclone belt, and cyclones may occur anywhere in the islands between November and April (Vanuatu Meteorological Service, 1994). Over the last 40 years, the islands have been struck by an average of 2.6 cyclones per year (Neil and Barrance, 1987; Longworth, 1991). Hurricane-force storms may occur, and in 1987, Vanuatu was ravaged by the worst storm in its modern history, which destroyed or damaged many houses (Vanuatu Meteorological Service, 1994). Severe tropical storms are accompanied by heavy rainfall and the low pressure may cause the sea to rise as much as 2 m (Longworth, 1991). Flooding, coastal inundation, land erosion, destruction of housing and gardens, loss of vegetation, pollution of water supplies and destruction of coral reefs and sea-grass beds are common impacts from tropical cyclones in Vanuatu. For example, Hurricane Betsy struck Ambrym in January 1992, with heavy rainfall occurring over 4 days, causing extensive flooding while wind speeds reached 46 m/s [90 knots] (Vanuatu Meteorological Service, 1994). Extreme rainfalls usually take place over a few days, and commonly trigger floods in low lying areas resulting in sudden erosion and sediment redistribution.
Fig. 6. A) Primary pyroclastic succession exposed in the upper flank of the Marum cone complex. B) Cauliflower-shaped bombs within fine-grained tephra in the flank of Marum indicate occasional phreatomagmatic explosive eruptions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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channels (Fig. 4 — #1 to 5). Arterial channels are defined as the main pathways of flash floods that carry sediment load away from the cones (Fig. 4 — #6). They evolve into wide and flat, poorly confined channel systems between interfluve areas in the marginal areas of the caldera. The Ambrym caldera appears to have existed as a depression for at least the last c. 2200 years, formed within a deeply eroded, older terrain. Scales and styles of volcanism on the island appear to show a steady state of activity, similar to detailed historic observations over the last c. 100 years, and occasional reports going back to 1774 when Captain Cook visited the region (Gregory, 1917; McCall et al., 1970). Historicallydescribed flank eruptions were always coincident with or followed initial summit eruptions, suggesting a strong coupling mechanism (Purey-Cust, 1896; Frater, 1917; Gregory, 1917; Németh and Cronin, 2007). Magma rise in the island's centre and the presence of a shallow magma storage region below the central edifice is indicated by the constant low-level lava lake activity in the intra-caldera vent systems as well as the huge rates of CO2, SO2 and halogen degassing from the centre (e.g. Bani et al., in review). During periodic larger eruptive episodes, magma ascent to near-surface levels is followed by intense summit degassing and intra-caldera explosive eruptions. Larger events lead to drainage of degassed magma along lateral dykes (east and westward along the rift system) to form fissures and effusion of lava on the lateral flanks (e.g. western arm — 1937; eastern arm — 1954; Wiart, 1995). These combined processes cause a gradual but cumulatively substantial volume loss from the stable central shallow magma system. Present-day intra-caldera activity is focused on the two large active pyroclastic cone complexes (Benbow and Marum; Fig. 5A,B), although at least one older dormant cone system within the caldera attests to alternative earlier focal points of intra-caldera volcanism. Near the two
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major active intra-caldera centres, deposits from fall and subordinate base surge and/or small volume scoria-and-ash flows dominate the volcaniclastic sequences (Fig. 6 A). The exposed interior walls of Benbow are mainly pyroclastic in nature, with minor interbedded lava flows, solidified lava-lake remnants, sills and dykes outcropping. In contrast, the outwardly similar Marum complex has a much larger proportion of shallow intrusive rocks, solidified lava-lake remnants, and occasional radiating lava flows (e.g. 1989 eruption; Wiart, 1995). Despite this variety of hard-rock lithologies, the bulk of the edifice comprises ubiquitous thick successions of phreatomagmatic and scoriaceous tephra (Németh and Cronin, 2008). Each vent/crater of Marum is large, up to 200 m deep and floored with either a lava lake or mud. The two intra-caldera eruptive centres produce frequent (weekly– monthly) small-volume eruptions ranging from Strombolian events through to phreatomagmatic explosions. More rarely (c. 30 year intervals) larger sub-Plinian and Vulcanian eruptions occur (Fig. 3). These centres hence supply huge volumes of tephra (lapilli and ash) to the surrounding summit area. Along with the intense acidic degassing from these vents (e.g., Bani et al., in review) vegetation growth is hindered, creating an exceptionally large badlands landscape or “ash plain” within the caldera (Fig. 2). 5. Tephra cones and near cone sequences — primary gullies, gully fills, deposit spills into arterial channels 5.1. Description This complex sedimentary environment is dominated by near-vent primary and proximal secondary volcaniclastic sediments forming zone
Fig. 7. A) Termination of dense shallow gully systems at the foot of the Marum cone complex. B) Gully headwall of a typical gully on the upper flank of Benbow cone. C) Disaggregating collapsed tephra wall in a gully in the upper flank of Benbow. D) Coarse-grained, granular debris flow deposits forming well-defined lobes in the middle section of one of the gullies on the Marum cone complex.
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A in Fig. 4. The primary pyroclastic beds surrounding the Benbow and Marum complexes are dominated by cm-to-dm thick layers of highly vesicular, basaltic, scoriaceous ash and lapilli with subordinate volcanic lithic fragments (Fig. 6A). The dominantly angular scoriaceous ash and lapilli have well-developed inter-connected vesicle networks with smooth intra-vesicle surfaces. Falls from minor “background” activity form intercalating sequences of beds dominated by Pele's tears and well developed Pele's hair up to 10 cm long. All juvenile ash and lapilli are crystal-poor, glassy, and black to dark red in colour. The scoria particles are typically blocky and equant, with very rare flattened particles. Free pyrogenetic crystals are rare and dominantly pyroxenes. In near-vent locations, occasional lobes of lava spatter/welded agglutinate also occur within the tephra-dominated sequences. Contrasting, pale-coloured, poorly sorted, fine-ash dominated surge-generated beds occur interspersed within the sequences. These matrix-supported tephra units contain more common accidental (weathered and hydrothermally altered) fragments, including lithic and cauliflower bombs up to 250 mm in diameter (Fig. 6B). Near the vents these beds commonly show low-angle cross-bedding, and plastering effects on the craterward side of large pre-existing blocks and bombs. The cone complexes are dissected by narrow, box-shaped gullies cut into their flanks (Fig. 4 — #1–5) that deepen and widen toward the lower slopes (Fig. 7A). The headward eroding gullies begin with a metre-scale abrupt step, around one-third of the way down the cones. The gullies are evenly distributed around the cone systems and are typically spaced c. 10–15 m apart (Fig. 4). Although there are many meanders, some systems meet and amalgamate. The outer slope sequences are exposed in the gully walls and dominated by ash-rich
fall beds, along with common intercalated scoria lapilli fall units at least 10 cm thick. Bedding surfaces in the successions are slopeparallel and reflect accumulation on slopes of between 10 and 25°. Less common matrix-supported surge beds are interspersed between the fall units. They contain ash aggregates and accretionary lapilli, and form megaripples, along with channel cut-and-fill structures as well as 10-centimetre-scale slumping structures. The floors of erosion gullies on the upper cones tend to contain little or no re-sedimented material until around two-thirds of the way down the cone, where the side walls collapse to form 45–50° wedges of talus, including intact blocks of semi-consolidated tephra. These collapsed tephra blocks are quickly disaggregated to create shallow fans of ash-lapilli dominated sediment (Fig. 7B and C), which overlap to eventually cover gully floors between rain-storm events. In the upper, and steep (25–30°) narrower part of the gullies the fill deposits form steep-margined 0.3 m thick lobate units, with convex upper surfaces that are traceable over tens of metres (Fig. 4 — #2–3). At the lower ends of the gully systems, toward the base of the cones, gullyfill deposits form a complex morphology of thousands of overlapping and anatomising long and narrow lobes (Fig. 4 — #5). Typically these units are on the scale of 1–5 m across and 50–100 m long with thicknesses up to 0.5 m. The lobes have well-defined lateral levee–channel surface forms at their tails, grading into flat-topped or gently humped frontal lobes, 0.2–0.3 m thick. They collectively stretch bank-to-bank in the c. 20–50 m wide channels (Fig. 7D). The levee structures typically comprise the coarsest-grained fragments and many individual units, including channel–levee structures cross-cut one another (Fig. 8A), or form mini-breakout lobes (Fig. 8B). Marginal levees are clearly clast-
Fig. 8. A) Well-developed levee in a small cross-cutting granular debris flow. The initiation point of this flow lobe is marked with a square. Hammer in circle is c. 30 cm long. B) Small side lobe initiated from the center lobe complex (dashed circle) of agranular debris flow deposit on the flank of the Marum cone complex. Hammer in circle is c. 30 cm long. C) Lower gully floor section on Benbow with wider debris flow deposits with well-developed lobe traceable over a few metres. Hammer in circle is c. 30 cm long. D) Lower gully floor overview on the flank of Benbow. Note the finer-grained channels associated with the meandering debris flow lobes, suggesting water saturation of the flow during transportation.
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supported, and apparently better sorted than central parts of the lobes. In the lower, flatter (10–15o) gully reaches the individual lobes are wider and flatter at only 0.1–0.2 m thick (Fig. 8C–D). At the base of the main cones these gullies terminate where they join into sub-perpendicular, broader secondary valleys that are up to 100–200 m wide. Grain-flow channel and levee structures protrude (between floods) into these second-generation channels, terminating in humpy fan structures. The remainder of these larger valleys are filled by broad flatter lobes (Fig. 9A) that are almost tabular. Tabularform deposits, 0.1–0.3 m thick, are exposed at channel margins and “island” banks (Fig. 9B). Some cross-bedded deposits occur, normally within well-sorted, clast-supported, matrix-poor ash and lapilli. Cross-sections exposed in side walls show mantle-bedded ash and lapilli fall units up to 15 cm thick that drape older side-gully morphologies (Fig. 9 C). In the immediate vicinity of Benbow and Marum, the gully networks form a dendritic pattern, feeding into progressively wider and shallower channels (Fig. 5A,B). Especially between Marum and Benbow, the base of the edifices is partially buried by ash and lapilli-dominated, flat-lying beds. The transition from the pyroclastic construct to their foothill area is represented by zone B, where volcaniclastics directly feed from the eroding pyroclastic construct (Fig. 4 — #5–6). These wide, sediment-choked shallow valleys formed in the foothills of the cones lead toward a flat “ash plain” region where they interconnect and form, shallow arterial channels N300 m wide (Fig. 4 — #6). 5.2. Synopsis — pyroclastic cones and near-source sediment redistribution structures The exposed gully walls on both cones demonstrate transportation and deposition dominated by tephra fall and subordinate base surge activity from mafic explosive eruptions. The style of fragmentation and transportation of pyroclasts and the resulting deposit morphology are very similar to those found around any mafic explosive volcanoes that develop complex scoria cones through intermittent phreatomagmatic activity (Houghton and Schmincke, 1989; Calvari and Pinkerton, 2004). Particle and deposit textures indicate Strombolian explosive activity (Mangan et al., 1993), Vulcanian eruptions (Self et al., 1979; Stix et al., 1997) and phreatomagmatic eruptions (Dzurisin et al., 1995). The limited distribution of phreatomagmatic base-surge beds indicates these were relatively low-energy events, with deposits confined to the cones, whereas many fall events were large enough to leave significant deposits well away from the cone structures. Given the high eruption rates of these two centres and the youthfulness of the cone-complex slopes, the well-developed dendritic erosion structures indicate rapid remobilisation of tephra during frequent and high-intensity rainfall events. The immediate erosion of freshly deposited tephra forms small granular avalanches or dense granular debris flows that are apparently self-confining and “freeze” into lobes. These features and scales of flow are similar to dense, granular debris flows generated in large-scale flume experiments (Major, 1997; Major and Iverson, 1999), albeit involving coarser and better sorted sediment at Ambrym. In flume experiments the resulting deposits exhibited lobate snouts, blunt marginal levees, arcuate surface ridges, clusters, and streaks of accumulated surface gravel, and preferentially aligned particles along surge perimeters (Major, 1997; Major, 1998; Major and Iverson, 1999). The distribution and freezing of these lobes appear to be volume-controlled, similar to larger-scale rock or scoria avalanche deposits (Lube et al., 2007). These units are notably fines-poor, and appear to be generated from rainfallinduced collapses of saturated lapilli and ash beds from the banks of box-gullies, and perhaps from steep talus toes of lapilli and ash sediment, similar to deposits reported from gullies in the upper section of Vulcano in Italy (Ferrucci et al., 2005). The structure of the cone flanks probably helps build excess hydrostatic pressures in coarse lapilli and ash beds that are sandwiched between poorly-sorted
Fig. 9. A) Typical debris flow lobes with well-developed levees traceable over tens of metres in the sites where cone-gullies spill into the main artery valley between Benbow and Marum. People in circle are for scale. B) Collapsed wall of the artery valley locally feeding debris into the main artery. The wall exposes hyperconcentrated flow deposits of earlier sedimentation around the active cones. Hammer in circle is c. 30 cm long. C) Older interfluve erosional surface covered by mantling tephra deposits (arrows).
and fine-grained surge deposits that act as aquitards. In addition, the pyroclastic sequences contain many “glazed” layers a few mm-thick, where fine-grained pyroclastic deposits are cemented and altered by precipitates from intense local degassing (Schiffman et al., 2006). This alternation of aquitard layers and loose lapilli and ash units leads to formation of the box-gully structures.
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The gully-filling granular lobes with minor matrix and moderate sorting suggest transportation and deposition by cohesionless debris flows (Pierson, 1980). The sedimentary textures (and size) of these lobes are strikingly similar to cohesionless granular debris flows produced in large-scale flume experiments (Major, 1997). The experimental large-scale debris flow deposits showed similar geometrical parameters in their width, length, levee dimensions, and ridge sizes to the upper gully lobes. In experimental debris flows textural differences have been identified between unsaturated and saturated granular debris flows (Major, 1997). Unsaturated flows produced relatively thick lobes with high aspect ratios and arcuate surface ridges, very similar by texture and dimension to those lobes described from the upper gully floors., The lobes in the upper gully floors at Ambrym tend to be pushed into each other and shoulder aside previously deposited sediments in a very similar fashion to those described in the experimental unsaturated debris flows. Small, rare holes occurring in the coarse-grained head and lower levee margins at Ambrym are interpreted to be “sieve holes” through which finer-grained sediment and water may have escaped from the otherwise coarse-grained and granular lobes (c.f., Krainer, 1988; Shakesby and Matthews, 2002). The Ambrym debris flow deposits are so coarse and well-sorted (due to a source from tephra fall deposits), as well as having a high clast vesicularity, that water and fines drainage is probably extreme without the need to generate many sieve holes (c.f., Shakesby and Matthews, 2002). In areas where the slope angle reduces to about 15°, complex lobes tend to override each other and/or younger lobes seemed to be deflected by earlier deposited lobes. These geometrical features have been identified predominantly in saturated debris flows in large-scale experiments (Major, 1997). In the lower wider gullies, lobes tend to develop broader coarse-grained snouts and lower levees, which seem to function as dam structures, holding finer-grained sediment behind. This textural characteristic has been reported from saturated debris flows generated by large-scale flume experiments and explained as water-rich fines-enriched sediment ponds behind the lobe heads (Major, 1997). This spatial distribution suggests that initial debris flows and grain flows were less saturated than those developed in the cone basal zones. This could be explained by the high porosity and coarseness of the individual grains (scoriaceous lapilli and ash) forming the steep upper flank, allowing rapid vertical drainage of pore water. In the lower zones, escaping shallow groundwater as well as surface run-off through the gully network are likely to have initiated saturated debris flows. In the lower gully network, primary finer tephra (ash grade) is more common as a local sediment source and therefore the resulting mass flows were more poorly sorted, similar to normal debris flows as described elsewhere (Major, 1997; Major and Iverson, 1999). The progression of overlapping lobes down the upper gully systems implies that generation of these flows is a sporadic but repeated process with many individual flow-triggering collapses occurring during any one rainstorm from multiple source areas along the sides of the box-cut valleys. These lobate deposits are eroded and incorporated into successive flows in places or otherwise just piled up. Larger flows gradually sweep the ash and lapilli material toward main arterial channels. At the base of the cones, the toes of these grain-flows spill into arterial channels, where greater amounts of water are focused. Here, during particularly intense or long-duration storms, debris flows and hyperconcentrated flows are formed, truncating tributary grain-flow lobes (that are presumably formed under a wider range of conditions). Initial debris flows rapidly transform into shallow hyperconcentrated flows as they flow down increasingly broad arterial channels and onto the ash plain. Lenses and lags of fines interbedded between the tabular hyperconcentrated flow deposits occur during periods of ponding, along with occasional fluvial reworking and sorting of deposits in localised areas.
6. Arterial channels (main flow paths in flash floods) 6.1. Description The volcaniclasts remobilized from the pyroclastic construct gradually reach the zone B sedimentation area at the relatively flat base of the cones (Fig. 4 — #6). The sedimentary system of the main ash plain shows significant differences from that in the gullies of the volcanic cones (Fig. 9A). The slope angle reduces to b10o, and channels either disappear or remain broad and shallow until reaching caldera exit points (sometimes having to skirt around lava-flow or other structural features near the ring-faulted margins of the caldera). The shallow valleys that remain on the ash plain form straight, box-formed and broad (100–300 m wide) features that are only a few metres deep. Remobilised tephra fills these valleys to form sheets, bars and lobes of mixed ash and lapilli deposits, slightly finer-grained than in the proximal valley systems. Gently lobate bars, 20–100 m long and a few metres wide, are formed of coarse ash and lapilli. Vertical or nearvertical channel margins expose coarse ash-dominated tabular scoriaceous fall deposits, interbedded with volumetrically dominant matrix-supported ash-rich beds containing lenses (several metres long and a few cm thick) of coarse lapilli (Fig. 9B). Periodic erosion in interfluve areas has also created an irregular surface, comprising a network of shallow (1–4 m deep) gullies. These systems appear to be episodically active, since mantle-bedded scoriaceous fall deposits overlie them in many instances (Fig. 9C). Toward the caldera margins channels widen and the fill deposits are of lower relief and more homogeneous, comprising broad and shallow tabular beds of fine ash and lapilli. 6.2. Interpretation The volcaniclastic facies identified in the main ash plain zone are interpreted as units transported and deposited by hyperconcentrated flows and debris flows in the main valley network running through the ash plain, similar to those described from other more desert-like environments (Smith and Lowe, 1991; Lirer et al., 2001; Giordano et al., 2002). These broad channels serve as the main arterial pathways for distributing cone-derived volcaniclastics to the outer caldera and beyond. They are also periodically inundated by pyroclastic fall. The wide valleys and their low slope angles mean that hyperconcentrated flows are shallow sheet-like flows. Visual observations in 1989 of such flows within the arterial channels indicated that they were no more than 30 cm in depth, appeared as sandy floods, were initiated rapidly during intense rainstorms and ceased within 30 min of the rain stopping. Both cut and fill processes are occurring within these arterial channels. However, despite channel fills of sandy hyperconcentrated flow and flood deposits, the presence of vertical channel walls and cross-sections of fall and flow deposits in the interfluves show that there is net downcutting and removal of sediment from these stretches. This downcutting is limited by the base level of the elevation of hard-rock rims in the main outlets from the caldera, especially on the western side. At least one consistent set of angular unconformities between flat-lying beds and mantling tephra fall units in the interfluve sedimentary sequences shows that periodic storm events involve severe erosion across broad areas of the SW caldera, presumably associated with deep erosion in the main arterial channels. These extraordinary events developed dense network gullies of a few metres depth in interfluve areas. The present day gully network cuts through these mantle-bedded sequences in places. Older paleosurfaces and sequences exposed in the present main valley walls suggest periodic shifts in locus of valleys and perhaps the sources and transport paths of sediment. An older extinct cone-complex to the north (Fig. 10) of Marum shows that intra-caldera sedimentary
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Fig. 10. A) Major tributary (arrows) carries dominantly scoriaceous sand-sized sediment, feeding volcaniclastic fans in coastal areas. B) Older, strongly eroded pyroclastic cone of Marumliglar with similar dense box-shaped gully network to those on the active cones of Benbow and Marum.
sources were not always from the currently active Marum and Benbow complexes. 7. Interfluve and marginal caldera — dunes and side valleys 7.1. Description Major artery valleys become wide and shallow over distances of 2–3 km from the base of the pyroclastic constructs (Fig. 4 — #7). In these areas, distal fallout tephra can uniformly blanket the major valleys and other slightly elevated parts of the ash plain. At the margins of the ash plain, especially in the SW quadrant, sediments are dominated by cross-bedded aeolian dune sands. Longitudinal ash dunes are a few metres high, and tens of metres long (Fig. 11A), forming an upward-migrating sequence toward the SW caldera rim. In some locations near the caldera margin a steep-sided and deep valley runs parallel to the outer wall, before passing into a hard-rockbased outflow channel. In other cases, sand dunes overtop the caldera
rim to accumulate on the uppermost outer flank, c. 100 m below the rim crest. Aside from the main arterial channels, small tributaries can be traced in the entire ash plain following the gently sloping local morphology toward the caldera margins. These side valleys are usually not more than a few metres deep and bounded by whaleback shaped hills covered by fresh fallout tephra. These side valleys commonly initiate with a few narrow (b0.5 m) erosion cuts of a metre or more in depth, that quickly develop to a few metre-wide, flat-bottom channels. Due to the low slope angles of these gully margins only occasional slide blocks of a few metre-scale coarse debris fans occur. The side valley floors are covered by fine-grained sediments and a few bars of coarser lapilli material. 7.2. Interpretation Due to strong winds at the high altitude of the ash plain, large cross-bedded dunes are formed on the western margin of the ash
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Fig. 11. A) Aeolian dune sands with fine volcaniclastic sand in the SW side of the caldera ash plain. Hammer is c. 30 cm long. B) Deep and narrow valley exits the caldera on its southern margin. C) Gently whaleback-shaped fill of scoriaceous sand in valley leading out of the western Ambrym caldera. Arrows point to small coarse-grained bar. D) Upper reaches of a typical volcaniclastic fan on the northern coastline of Ambrym Island.
plain (downwind of the dry-season easterly trade winds). These dunes are entirely composed of fine ash to fine lapilli derived from the freshly accumulated, and probably immediately remobilized, tephra cover that accumulates during eruptions. The dunes form climbing features especially in the SW part of the ash plain, where its flat plateau-like top gradually covers the caldera margin. Aeolian redistribution of tephra in the ash plain is probably an important sedimentary process due to constant winds at an elevation of 700– 1000 m. Side valleys in the interfluve areas appear to be significantly cut down only during the rarer largest rain storms. These potentially also supply significant volumes of fine-grained sediment to the main arterial channels. 8. Extra-caldera sequences The extra-caldera sedimentary system of Ambrym is similar to sedimentary systems described from other volcanic terrains with high annual rainfall in tropical climates, and can be separated into 3 major reaches; 1) narrow, steep and deep upper channels, 2) lava-based, broad and shallow lower channels, and 3) coastal prograding fans. The characteristic feature of all of these reaches is that surface water flow is ephemeral and occurs normally only during moderate-large rain storms. 8.1. Description The main channels leading out of the caldera are initially controlled by hard-rock (lava) rims at the caldera edge (Fig. 11B).
They tend to be narrow and deeply eroded into older pyroclastic deposits or strongly confined to smoothed lava channels. Typically ephemeral and mostly dry, any lower-gradient, near-caldera reaches of the channels are filled with volcanic sand-dominated sequences (Fig. 11C). Downslope, the channels pass into a steep-gradient reach where valleys alternate between stretches of 20–50 m deep, narrow (10–30 metre-wide) cuts into weathered older pyroclastic units, and cataracts and waterfalls at outcrops of hard rock (lava and cemented older pyroclastic rock sequences). Toward lower-gradient lower slopes the channels broaden again, forming broad, dominantly sandy expanses of low relief. These are cut into low banks exposing 1–3 m deep sequences of medium-to-coarse volcanic sands, horizontally bedded on a 10–30 centimetre-scale. Within 100 m of sea–level, the valleys become shallower and wider, discharging onto volcaniclastic fans along the coast line (Fig. 11D). These fans are dominated by well-sorted, black medium-to-coarse volcanic sands. High-energy beach environments cause strong rounding, abrasion and better sorting of the sands in comparison to deposits of the fluvial channels inland. In these environments, obviously scoriaceous particles are rare, free crystals (olivine and pyroxene) are common, and exotic lithologies of dense coherent lava clasts (from mid-slope channels) are over-represented. 8.2. Interpretation On the upper slopes of Ambrym Island, high slope angles result in active erosion or formation of stable channels in hard-rock, hence, typically little sedimentation takes place. These merely act as conduits of caldera-derived or occasional direct fall sediment to lowland areas.
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From c. 500 m below the caldera rim, the radiating valleys become more obvious, being wider and filled with volcanic sand-dominated deposits. In the lower valley reaches volcaniclastic sediments are typically better sorted and significantly finer-grained than intracaldera sequences. They contain other bedding features indicative of more dilute fluvial flows and possibly transitional hyperconcentrated flows (cf. Pierson and Scott, 1985; Lavigne and Suwa, 2004). In these normally dry channels flash floods were observed on several occasions to last for several hours to days and typically involve broad shallow (b1 m deep) flows with major bed-load motion and several metres of downcutting. 9. Discussion 9.1. Volcaniclastic sedimentary deposition systems at Ambrym Island The volcaniclastic sedimentary deposition systems identified on Ambrym Island can be summarised (Fig. 12) into four distinctive areas: (1) intra-caldera cone-complexes, where primary volcanic and locally reworked deposits overlap (A to E on Fig. 12); (2) intra-caldera arterial valleys, which are the main conduit for sediment transport from near-cone sequences to inner-caldera deposition sites and/or caldera outlets (F on Fig. 12); (3) intra-caldera storage areas where reworked sand-dominated deposits form stacked dune sequences in the SW caldera or sandy–gravel hyperconcentrated flow and fluvial units, intercalated with fall deposits, form a broad partly dissected “bad-lands” landscape (partially F,G and H on Fig. 12); and (4) near-
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coastal broad valley systems filled with sandy fluvial and hyperconcentrated flow deposits along with prograding fans of sediment, sorted and reworked on high-energy coastlines (I and J on Fig. 12). The western Ambrym caldera margin is obscured by thick fall deposits and the lower flanks of the Benbow edifice. In addition, parts of the SW margin are overtopped by dune sands, and in general the caldera is deeply filled by volcaniclastic sediments. The volcaniclastic sedimentary fill is estimated to be about 400 m on average, based on geophysical measurements (McCall et al., 1970). While supply of new material is presently concentrated at Marum and Benbow, previous point-source supply areas include Marumliglar (Fig. 10B) to the NE of Marum, as well as another cone-complex that appears to have been a proto-Benbow, indicated by pyroclastic sequences near the western caldera margin. The redistributed eruption products have gradually filled the central depression on Ambrym Island over the c. 2000 years since the inferred caldera inception (McCall et al., 1970). This intracaldera filling appears to have kept pace with subsidence in parts of the large caldera structure and contribute to its very distinctive form. 9.2. Triggering of debris and grain flows Ambrym volcano constantly produces large volumes of pyroclastic material that accumulates in the major cone complexes as well as to a lesser extent, throughout the entire caldera. This loose, scoriaceous (hence porous and low density) material is highly erodible through wind and rainfall, as well as the lack of vegetation which is exacerbated by ongoing gas and acid rain production. The tropical
Fig. 12. Diagrammatic representation of the volcaniclastic sedimentary system of Ambrym Island. A) Magmatic-fragmentation-driven tephra fall units in proximal areas of the active cones, B) fine-grained dune-bedded, pyroclastic density current deposits in proximal sections of the active cone complexes, C) upper granular, cohesionless, unsaturated debris-flow lobes initiated from the headwalls of box-shaped gullies of the active cone complexes, D) grain-flow dominated gravity mass flow deposited lobes in the headwall of gullies of the active cone complexes, E) complex saturated cohesionless debris-flow sediment lobes in the middle sections of the active cone complexes, F) artery valleys filled with tabular and low-wedge-form deposits from saturated debris flow and hyperconcentrated flows close to the exit points of the gullies into the main artery networks, G) aeolian dunes flank the inner caldera walls in downwind direction, H) caldera margin channels with whaleback formed volcaniclastic sand infills, and I) deep, and narrow channels lead out from the caldera with low sedimentation rates. The floor of these valleys is commonly coherent lava flows, J) coastal volcaniclastic fans and deltas.
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and humid climate on Ambrym volcano produces frequent storms of high-intensity and heavy rainfall events. These cause saturation and collapse of lapilli and ash beds (fall units) on the pyroclastic cones, particularly where aquitards of glazed surfaces or poorly-sorted, finegrained surge deposits generate overpressures of water in the intervening beds, and at times piping. Small-scale collapses and minor landslides eventually trigger debris flows and grain flows in the upper flanks of the bare volcanic cones. Despite the high average annual rainfall over Ambrym, no permanent watercourses occur in intra-caldera ash plain, resulting from the highly porous and permeable caldera-fill and cone complexes. Hence sediment transport is episodic and chaotic in relation to the occurrence of high-intensity continuous rainfalls that reactivate fluvial systems. Eyewitness accounts and historic records document that extremely heavy rainfall events occur both during cyclones and smaller tropical depressions. These produce flash floods and shallow, brief, sedimentgravity currents in the main tributaries of the ash plain. They are active for hours, followed by rapid water loss, presumably resulting from vertical drainage. These sediment movement events occur on a periodicity of reputedly 2–10 times a year. The infrequent large-scale sediment motion contributes to the choking of the tributaries near the active cones. The addition of tephra fall deposits, especially in the western areas, also contributes broad fans of finer ash-grade sediment that is periodically washed into intra- and extra-caldera tributaries during flash floods. The two active cone complexes with frequent activity and ongoing gas emission keep the central and upper western areas of Ambrym devegetated and therefore prone to erosion. The resulting thin soil cover and the coarse and porous nature of the primary pyroclastic deposits cause a sedimentary environment that is not unlike arid desert-like systems. This makes it similar to other non-volcanic (Church and Miles, 1987) or volcanic regions (Miyabuchi and Daimaru, 2004) where rainfall-generated debris flows dominate sediment transport. Debris flow triggering is generally expected to be a result of brief high-intensity rainfalls (Church and Miles, 1987). However, in addition, in maritime climates debris flows may also be initiated by persistent and long-term rainfall events, where saturation and pore pressures may build up gradually (Church and Miles, 1987). The summit area of Ambrym receives certainly N3000 mm/year of annual rainfall and in most times of the year, moist clouds cover the top of the island. Despite such conditions, constant high pore pressures in sediments on upper Ambrym do not exist. This is caused by (1) the total lack of vegetation and hence therefore little organic matter and soil cover, and (2) deep (up to 400 m) coarse, highly porous and highly permeable loose scoriaceous tephra. Many eyewitness accounts report, that despite low-intensity rain, no surface streams are active and sediment is immobile in all parts of the system. Triggering mechanisms for debris flows on steep slopes due to rainfall has been characterised by the intensity of the rainfall and the textural characteristics of the cover of the slope (Wieczorek, 1987). Many studies put a threshold of the rainfall intensity and duration to trigger landslides that may initiate debris flow however the numbers vary greatly (Caine, 1980; Wilson et al., 1992; Larsen and Simon, 1993). In a similar tropical climate to Ambrym in Puerto Rico, landslides and associated debris flows on steep slopes are reported to be initiated by intense and prolonged rainfalls (Larsen and Simon, 1993). In a 33-year period in Puerto Rico, 41 out of 256 intense rainfalls initiated landslides and debris flows (Larsen and Simon, 1993), resulting in a recurrence rate of 1.2 years. The brief high-intensity rainfalls generated only shallow soil slips and debris flow; longer period but lower-intensity rainfall led to deeper debris avalanches and slumps (Larsen and Simon, 1993). Reports from Puerto Rico also noted that rainfall duration up to 10 h is needed, with about three times the intensity of thresholds in temperate climates. Similarly high rainfall thresholds for widespread landsliding in other tropical countries has also been suggested (Carson and Kirkby, 1972), however the reason for
this is unclear. Climatic conditions from Puerto Rico are similar to those in Ambrym; however, the landslides reported occurred in areas with soil cover and associated vegetation. Therefore the reported data are unlikely to be directly relevant to characterise the threshold values for Ambrym due to its bare and highly permeable deposits. The implication of this for Ambrym is that cone-flank failure and therefore sediment movement are triggered only by prolonged, high intensity rainfalls, which is in good concert with eyewitness accounts of the low recurrence rates for flash flooding in the intra-caldera area. In non-volcanic terrains three basic types of slope failure have been recognised; 1) deep slumps, 2) shallow slumps and slides, and 3) very shallow slumps over bedrock (Wieczorek, 1987). However these recorded initial slumps were documented in areas with soil cover, and therefore cannot be directly compared with the bare loose pyroclast constructs of Ambrym. In non-volcanic examples, the depth and magnitude of landslides resulting from increases in pore-water pressure depend on the relative depth of the soil, position of hillside profile, hillside shape and the storm intensity and duration (Wieczorek, 1987). In theoretical models, relatively impermeable bedrock is assumed to exist beneath the soil cover in order to generate positive pore-pressures and initiate flank failure (Wieczorek, 1987). In the case of Ambrym, there is no soil cover. Continuous acidic and salt-rich volcanic degassing from the active vents however form a hard and impermeable “glazing” or coating that probably acts in similar way to a soil cover in terms of allowing pore pressures to build up. Its brittle and weak nature leads to sudden block-shaped failures over small scales as observed. 9.3. Mass flow evolution at Ambrym Identified grain- and debris-flow deposits in the gully network developed in the pyroclastic constructs of Ambrym are very similar in size, architecture and texture to those granular non-cohesive debris flows formed during large-scale flume experiments (Major, 1997). The significant difference is that the Ambrym deposits are better sorted. Many textural features of the Ambrym deposits indicate that they were initiated as unsaturated, non-cohesive debris flows in the upper reaches. The initial flows are replaced by more saturated equivalents in the lower reaches on the pyroclastic cones. The high vesicularity and hence low particle densities probably exacerbate sorting during movement and result in slightly graded deposits at flow fronts and levees. Higher vesicularity also facilitates rapid water infiltration into the substrate, causing flows to “freeze” once they slow. The lack of clear and abundant “sieve holes” can be explained by the lack of fines and high vesicularity and permeability of particles. In more distal areas, at low slope angles, and with local supplies of finer-grained sediment, the gravity currents are more similar to normal, saturated non-cohesive debris flows. In the broader arterial channels, both the deposits and rare observations indicate typical hyperconcentrated flows associated with normal high-concentration stream flows. In more distal areas, the sedimentary network is similar to small-scale braided fluvial systems produced by shallow, repeated hyperconcentrated flow deposits. The sedimentary system on the pyroclastic cone flanks of Ambrym shows similarities in textural characteristics to the gully fills and network of Vulcano in Italy (Ferrucci et al., 2005) despite the two settings being located in very different climates. The gully fills at Vulcano are dominated by debris flow deposits initiated from the upper third of the cone (Ferrucci et al., 2005), in a similar location to those described from Ambrym. The upper gully headwalls at Vulcano are neither as well-defined nor as box-shaped as at Ambrym. At Vulcano, collapses are clearly initiated from slips of coarse-grained tephra over mud-rich phreatomagmatic tephra substrates (Ferrucci et al., 2005), which is not the case at Ambrym. The debris flows in Vulcano hence show textural evidence of more cohesion and saturation. The top third of the Vulcano cone is inferred to be a
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water-supply zone collecting surface water during storm events, which flows out on the fine-grained phreatomagmatic rock units (Ferrucci et al., 2005). Shallow rills and gullies lead surface water to the lower part of the cone where debris flows are initiated. Ambrym does not have such an impermeable bedrock layer although boxshaped gullies are formed in a similar position, in the upper third of the cone. Between Ambrym and Vulcano, at least four variables can be identified that may cause differences in the initial cone-derived mass flows and their deposits: 1) existence of impermeable bedrock at Vulcano that can effectively drive surface water from the water supply zone of the cone; 2) poor sorting and higher fines content of deposits at Vulcano leading to more typical debris flows; 3) high vesicularity and permeability of clasts and substrate deposits at Ambrym leading to unsaturated flows and rapid deposit “freezing”; and 4) differences between the arid climate of Vulcano and tropical climate of Ambrym, in which intermittent flows may be more frequent at the latter. 10. Conclusion The volcaniclastic sedimentary facies of Ambrym Island demonstrate a step-wise and episodic volcaniclastic redistribution system developed in response to very high sediment production rates from the intra-caldera mafic cone-complexes (Fig. 12). Over at least the last 2000 years, steady high rates of tephra production from intra-caldera cones, lateral magma drainage into axial flank rift zones, and enormous degassing rates have produced a gradually subsiding broad summit caldera. Primary pyroclastic deposits from the intracaldera cone-complexes are redistributed to distal intra-caldera storage sites in a general order of (1) unsaturated debris flows, (2) saturated debris flows, (3) hyperconcentrated flows (during flashflood events in the stormy tropical climate), and to a lesser extent aeolian sand dunes. In addition to these storage areas, the bulk of volcaniclastic sands and scoriaceous gravels are moved out of the caldera into radial valley systems, accumulating in short coastal river flood plain and fan systems. The constant acid rain around active vents, along with rare production of fine-grained, poorly sorted pyroclastic surge deposits, forms very thin, hardened aquitard layers in the primary pyroclastic sequences of the cone. Saturation of the layers between these during periods of heavy rainfall causes flushing and en-masse collapse of blocks of tephra sequences from cone sides. These collapses feed small-volume debris flows. Stepwise remobilisation and collapse of these primary deposits and debris flow lobes lead to transport of volcaniclastic sediments into fans spilling into arterial valleys. Heavier or longer rainstorms generate sediment-rich floods and hyperconcentrated flows in all valley systems, to remobilise the stubby debris flow lobes and remove sediment to outer parts of the caldera or into channels leading out from the caldera. This extremely active redistribution system is exacerbated by the lack of vegetation on W–SW part of the caldera, which is caused by intense acid degassing from the active intra-caldera volcanic vents. The loose, highly vesicular, and relatively coarse (coarse ash to lapilli) nature of the bulk eruptive products limits flow and deposition processes in this sedimentary system to flash-flood events, where rainfalls of extremely high intensity are needed to overcome the rapid drainage of water through the loose volcaniclastic pile. Acknowledgements This work has been supported by the New Zealand FRST Postdoctoral research grant (KN) (MAUX0405) and FRST-PGST funding — MAUX0401 (SJC, RBS). We thank Esline Garae, the people of Lalinda, Ranon, Craig Cove and Endu villages and “kastom” landowners for assistance and permission of access to sites in carrying out the fieldwork. Helpful reviews by Prof Young Kwan Sohn and Prof. Hiromitsu Yamagishi are also thankfully acknowledged.
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