Volcaniclastic Fan Research Articles

Origin and crustal contamination of magmas of the Chiapanecan Volcanic Arc in Southern Mexico: Constraints from new geochemical, isotopic, and geochronologic data

The processes related to the heterogeneous magma compositions of the Chiapanecan Volcanic Arc (CVA) are still poorly understood. Here we present new field descriptions combined with UPb zircon geochronology and whole-rock major, trace-element, and isotopic (Sr, Nd, Pb) data on poorly studied volcanic structures that surround the active El Chichón volcano in the northwestern part of this arc. These consist of the Santa Fe and La Mina intrusive bodies, volcaniclastic fans (La Catedral, and newly discovered Juárez and El Azufre) as well as Miocene volcanics. The mineralogy of the studied rocks consists of plagioclase + orthopyroxene + clinopyroxene + amphibole + biotite, ± FeTi oxides, ± olivine, ± quartz (this latter one only in the Santa Fe intrusive), and zircon as accessory phase. Young populations of zircons from El Azufre and Juárez lahar deposits were dated by UPb at ~2.1 Ma, being among the oldest yet-found magmatic products of the CVA. Strikingly, the Miocene volcanics have similar chemical characteristics as products of the modern arc. Based on geochemical data, we deduce that magma genesis in the CVA takes place in the mantle wedge, where it is constantly modified by aqueous fluids from the slab, causing relatively high Ba/La and Th/Yb ratios. The 87Sr/86Sr and 143Nd/144Nd ratios, zircon geochronology, and trace elements strongly suggest that crustal contamination was the dominant mechanism of differentiation of the magmas, supported also by Ce/Pb and Nb/U ratios. Furthermore, inherited zircons suggest the presence of Grenvillian and Permian (Chiapas Massif) rocks underneath the CVA. The contamination of the magmas by these basement rocks may be responsible for the K2O-enrichment in all CVA rocks.

Chapter 3.1a Antarctic Peninsula and South Shetland Islands: volcanology

Abstract The voluminous continental margin volcanic arc of the Antarctic Peninsula is one of the major tectonic features of West Antarctica. It extends from the Trinity Peninsula and the South Shetland Islands in the north to Alexander Island and Palmer Land in the south, a distance of c. 1300 km, and was related to east-directed subduction beneath the continental margin. Thicknesses of exposed volcanic rocks are up to c. 1.5 km, and the terrain is highly dissected by erosion and heavily glacierized. The arc was active from Late Jurassic or Early Cretaceous times until the Early Miocene, a period of climate cooling from subtropical to glacial. The migration of the volcanic axis was towards the trench over time along most of the length of the arc. Early volcanism was commonly submarine but most of the volcanism was subaerial. Basaltic–andesitic stratocones and large silicic composite volcanoes with calderas can be identified. Other rock associations include volcaniclastic fans, distal tuff accumulations, coastal wetlands and glacio-marine eruptions. Other groups of volcanic rocks of Jurassic age in Alexander Island comprise accreted oceanic basalts within an accretionary complex and volcanic rocks erupted within a rift basin along the continental margin that apparently predate subduction.

Volcaniclastic fan facies and reservoir characteristics: a case study of Guantao Formation in the No. 1 and No. 2 structures in the Nanpu Sag, Bohai Bay Basin, East China

This paper investigates volcaniclastic fan facies and reservoir characteristics of Guantao Formation in the No. 1 and No. 2 structures in the Nanpu Sag, Bohai Bay basin, Eastern China. Core and log data from the Guantao 3 and 4 Formations were analyzed in order to characterize volcaniclastic fan facies. Comprehensive analyses show that volcaniclastic fan sediments have been characterized as rapid proximal deposition of well-rounded and poorly sorted large-grain sediments. During the developmental processes of the volcanic fans, shallow to moderately deep lacustrine mudstones are deposited on the volcanoes. They vertically form superimposed sedimentary bodies that were characterized by laterally alternating fingers of volcaniclastic fan sediments and interchannel mudstone. Differences in pyroclastic supply and the volcanic terrain during the deposition account for the variations in volcanic rocks distribution. Distribution of volcaniclastic fans also exhibited characteristic features. Volcaniclastic fans around the volcanic highlands show variable distribution of volcanic rocks. The deep valley sizes of the debris flows and streams indicate variable sediment supply. The extent and thickness of the lahar deposits from the volcaniclastic debris flows indicate variable size of sedimentation processes. Most volcaniclastic fans are composed of debris-transported deposits having small clasts. The bed thickness increases with increasing distance from the volcanic source. The volcaniclastic rocks show high content of intermediate–basic volcanic rocks debris. The original and dissolved intergranular and dissolved intragranular porosities in the volcaniclastic rocks provide the best reservoir properties. Identification of lithofacies and their assemblages, and volcaniclastic sedimentary facies buried within volcanos and adjacent regions provide a better understanding that can be applied to similar rift basins. A method to identify depositional geometry and accumulation patterns of volcaniclastic fans was employed. This method assisted in constructing depositional models, reconstructing the depositional history of volcaniclastic fans and the related history of volcanic activity based on geological and geophysical data within volcanic areas in rift basins.

In situ ∼2.0 Ma trees discovered as fossil rooted stumps, lowermost Bed I, Olduvai Gorge, Tanzania

The discovery of fossil rooted tree stumps in lowermost Lower Bed I from the western Olduvai Basin, Tanzania, age-bracketed by the Naabi Ignimbrite (2.038 ± 0.005 Ma) and Tuff IA (1.88 ± 0.05 Ma), provides the first direct, in situ, and to date oldest evidence of living trees at Olduvai Gorge. The tree relicts occur in an interval dominated by low-viscosity mass flow and braided fluvial sediments, deposited at the toe of a largely Ngorongoro Volcano-sourced volcaniclastic fan apron that comprised a widely spaced network of ephemeral braided streams draining northward into the Olduvai Basin. Preservation of the trees occurred through their engulfment by mass flows, post-mortem mold formation resulting from differential decay of woody tissues, and subsequent fluvially-related sediment infill, calcite precipitation, and cast formation. Rhizolith preservation was triggered by the interaction of root-induced organic and inorganic processes to form rhizocretionary calcareous root casts. Phytolith analyses were carried out to complete the paleoenvironmental reconstruction. They imply a pronounced seasonality and indicate a wooded landscape with grasses, shrubs, and sedges growing nearby, comparable to the low, open riverine woodland (unit 4c) along the Garusi River and tributaries in the Laetoli area. Among the tree stump cluster were found outsized lithic clasts and those consisting of quartzite were identified as Oldowan stone tool artifacts. In the context of hominin activity, the identification of wooded grassland in association with nearby freshwater drainages and Oldowan artifacts significantly extends our paleoenvironmental purview on the basal parts of Lower Bed I, and highlights the hitherto underrated role of the yet poorly explored western Olduvai Gorge area as a potential ecologically attractive setting and habitat for early hominins.

GPR-derived architecture of a lahar-generated fan at Cotopaxi volcano, Ecuador

The internal geometry of volcaniclastic fans produced by aggradation during lahar events is difficult to examine in modern settings because of the frequent lack of three-dimensional exposures. This makes it challenging to (i) reconstruct the spatial and temporal evolution of such fans; and (ii) interpret observed facies stratigraphy in the context of lahar flow dynamics from proximal to distal fan reaches. This research therefore presents the results of a ground penetrating radar (GPR) survey of the Rumipamba fan at the mouth of the Burrohuaycu quebrada on the southwestern flank of Cotopaxi volcano. A survey grid consisting of 50 individual GPR profiles representing a total length of 19.4km was constructed covering most of the 4-km2 large fan surface. All GPR profiles were collected using a PulseEKKO 100 with a 400V transmitter. Fan sediments consist of sandy and gravelly lahar deposits, alternating with volcanic fallout including ash and pumice lapilli, at times reworked by fluvial processes. Deposits could be ground-truthed to a depth of ~3m, whereas GPR penetration depth reaches 15m. Data interpretation was based on classification into 15 distinct radar facies characterized by the nature of their bounding surfaces and/or internal features, cross-referenced where possible with shallow exposures. Three main facies were identified: parallel, irregular, and clinoform. Erosional contacts were distinguished from aggradational ones (vertical, channel fill, and lateral accretion). Flow parallel versus flow transverse and proximal–distal variations in deposit architecture were featured. The results of this study confirm the existence of two major channel systems in the northern and southern extremities of the fan and the more recent formation of a smaller central fan channel system. Deposit architecture is complex and facies chronologies illustrate that lahars have affected the entire survey area.

Syneruptive and intereruptive lithofacies in lacustrine environments: The Cretaceous Beolkeum Member, Wido Island, Korea

The Cretaceous Beolkeum Member was deposited in a lacustrine environment affected by explosive volcanism, and a number of syneruptive lithofacies are intercalated with lacustrine mudstones. The syneruptive lithofacies were formed by primary volcanic processes and resedimented depositional processes during and after eruptions and exhibit unique depositional features, which can be grouped into four syneruptive lithofacies assemblages (SLA-1–to SLA-4, from bottom to top).Primary syneruptive lithofacies are represented by welded massive lapilli tuff in SLA-1 and a normally graded lapilli tuff in SLA-2. In SLA-1, sustained pyroclastic density currents were able to displace lake water and rarely interacted with lake water, resulting in the formation of a welded texture and columnar joints. In SLA-2, unsteady pyroclastic density currents rapidly disintegrated, mixed with ambient water, and transformed into high-density turbidity currents, depositing thick normally graded lapilli tuff.Resedimented syneruptive lithofacies were primarily controlled by volcaniclastic sediment supply after eruptions. When volcaniclastic sediments were continuously supplied into the lake by debris flows, a volcaniclastic fan can be formed (SLA-1), resulting in a coarsening-upward trend and progradational geometry. In case of relatively small amounts of volcaniclastic sediment supply, turbidity currents would be a main depositional process, depositing a series of normally graded tuff on the primary syneruptive lithofacies (SLA-2), showing a fining-upward trend. In SLA-3 and SLA-4, there are only a few cm thick, normally graded tuff, reflecting minor volcaniclastic sediment supply. However, overlying normally graded sandstones in SLA-3, showing a coarsening-upward trend, and a thick normally graded sandstone in SLA-4 suggest favorable conditions for the generation of the sediment gravity flows, probably due to an increase in volcaniclastic sediment supply after eruptions.

Submarine structure of Vulcano volcano (Aeolian Islands) revealed by high-resolution bathymetry and seismo-acoustic data

A new definition of the morpho-structural submarine setting of the Vulcano volcano is presented based on the integration of multibeam swath bathymetry, high-resolution seismic profiles and sea-floor sampling. A number of unknown volcanic features have been mapped on the submarine flanks of Vulcano Island as volcanic outcrops and isolated cones. For most of them a relatively old age is suggested (especially to the west of the island), while evidence of relatively recent volcanic activity characterizes the north-eastern submarine sector, offshore Vulcanello. The morphology and distribution of these features, either aligned along NNW–SSE trends or radially elongated, suggest the prevalence of regional structural control or local stresses related to the volcano structure, respectively, in the different sectors of the volcanic edifice. Shallow-water (< 120 mbsl) insular shelves, related to wave action during Late Quaternary sea-level fluctuations, were recognized in correspondence of the oldest sectors of the edifice, while they are lacking in the younger sectors. Beyond the shelf edge, a strong increase of slope gradients characterizes the submarine flanks, promoting the formation of erosive-depositional and mass-wasting features at different scale, such as landslide scars, gullies, headless channelized features, crescent-shaped bedforms and volcaniclastic fans. In particular, two large volcaniclastic fans were identified on the north-eastern and south-western flanks, where marine retrogressive processes are very active and sediment is funnelled from the coasts to the foot of the volcano. These fans are created by the gradual stacking of gravity flows fed by repeated small- and medium-scale mass-wasting events, instead of being related to large-scale flank instability events. The lack of large-scale lateral collapse on the submarine flanks of Vulcano volcano has been related to the morpho-structural and volcano-tectonic setting of the volcanic edifice, characterized by the occurrence of summit caldera collapses limiting its total height and by the absence of well-defined rift zones. ► The submarine portions of Vulcano account for ≈ 85% of its areal extent. ► Multibeam and seismic data show new details of the submarine flanks. ► Interacting constructional and erosive-depositional processes are documented. ► Insular shelf development gives constraints on the evolution of the edifice. ► No large-scale lateral collapse deposits have been recognized on submarine flanks.

Tectonostratigraphy of the Cenozoic Tumaco forearc basin (Colombian Pacific) and its relationship with the northern Andes orogenic build up

The new tectono-stratigraphic setting of the Tumaco forearc basin based on outcrop logging, cutting description from deep oil wells, new biostratigraphy on calcareous nanofossils and sandstone petrography allows a margin scale comparison of the basin response to the Caribbean and Farallón/Nazca subduction under the South American margin. The results are compared to the laterally continuous Ecuadorian Borbón forearc basin and other southern Colombian basins: Patía sub-basin, Upper Magdalena Valley and southern Putumayo-Caguán basins. The proposed basement of the Tumaco basin is a Colombian Caribbean Oceanic Plateau (CCOP) sliver docked with Santonian-Campanian island arcs that was incorporated into the Colombian Pacific forearc during the Paleocene to Eocene. The filling of the Tumaco basin started with the Oligocene Unidad 1 Sur and the Early-Middle Miocene Cayapas/Viche/Angostura/Formations in a bathyal depositional setting. At Late Miocene to Holocene, a succession of volcaniclastic units was deposited in shallower environments: the Chagüí, San Agustín and Cascajal formations, and the recent volcaniclastic fans. The Late Cretaceous evolution of Northern Andes in Colombia was influenced by the collision and fragmentation of the Colombian Caribbean Oceanic Plateau, producing in the west the Tumaco block basement and an oceanic remnant basin in Patía Valley. The convergence between the Farallón/Nazca and South American plates since Paleocene allowed the development of the Pacific forearc as well as shortening leading to the uplift of the Central Cordillera and formation of the foreland basin system, which later was divided into the Upper Magdalena Valley broken foreland basin and the southern part of the Putumayo-Caguán foreland basin. Since Miocene, events in addition to plate convergence as the collision of the Baudó-Panamá Arc and the subduction of Carnegie Ridge perturbed the subduction zone in southern Colombia. The integration of all of these tectonic events offers a new improved dynamic framework for the evolution of this region. Un Nuevo marco tectono-estratigráfico para la cuenca de antearco de Tumaco basado en la descripción de afloramientos y de ripios de pozos profundos, nueva bioestratigrafía de nanofósiles calcáreos y petrografía de areniscas permite inferir cómo es la respuesta de la cuenca a la subducción de las placas Caribe y Farallón/Nazca bajo el margen de Suramérica. Los resultados son comparados con la colindante cuenca de Borbón en Ecuador y otras cuencas del sur de Colombia: sub-cuenca de Patía y las cuencas del Valle Superior del Magdalena y del Putumayo-Caguán en su parte sur. El basamento propuesto para la Cuenca de Tumaco es una astilla del Plateau Oceánico Colombo-Caribeño (CCOP) acoplada con arcos de isla de edad Santoniano-Campaniano que fue incorporada dentro del antearco Pacífico Colombiano entre el Paleoceno y el Eoceno. El relleno de la cuenca de Tumaco comenzó con la Formación Unidad 1 Sur en el Oligoceno y las Formaciones Cayapas/Viche/Angostura en el Mioceno medio-tardío en un ambiente de depósito batial. Entre el Mioceno tardío y el Holoceno, una secuencia de unidades volcaniclásticas definidas por las Formaciones Chagüí, San Agustín y Cascajal y los abanicos volcaniclásticos recientes fue depositada en ambientes más someros. La evolución de los Andes Norandinos en Colombia durante el Cretácico tardío estuvo influenciada por la colisión y fragmentación del Plateau Oceánico Colombo-Caribeño que produjo en el oeste el bloque de basamento de la cuenca de Tumaco y una cuenca oceánica remanente en el valle del Patía. La convergencia de las placas Farallón/Nazca permitieron el desarrollo del antearco Pacífico, así como también del acortamiento cortical que permitió el levantamiento de la Cordillera Central y el desarrollo del sistema de cuencas antepaís, el cual fue dividido en la cuenca rota de antepaís del valle superior del Magdalena y la parte sur de la cuenca de antepaís del Putumayo-Caguán. Desde el Mioceno, eventos adicionales a la convergencia de placas, tales como la colisión del arco de Baudó-Panamá y la subducción del ridge de Carnegie modificaron la zona de subducción en el sur de Colombia. La integración de todos estos eventos tectónicos permite establecer un marco dinámico mejorado para entender la evolución en esta región.

La Fossa Caldera breaching and submarine erosion (Vulcano island, Italy)

La Fossa Caldera (LFC) on Vulcano island, in the Aeolian archipelago (Italy), is a multi-collapse caldera whose north-eastern part lies underwater. Through the integration of marine geophysical surveys and sampling carried out in the last decade, its submarine extension and offshore areas have been reconstructed in unprecedented detail. Strong erosive processes breached the submarine portion of LFC and gradually dismantled the caldera rims and infill, leading to the emplacement of a wide volcaniclastic fan from the base of LFC down to over 1000m. The bathy-morphologic setting, seismoacoustic facies and analyses of sediment samples from the fan, together with volumetric considerations, suggest that this feature is the pathway/deposit of submarine unconfined gravity flows and turbidity currents deriving from the gradual dismantling of LFC. Submarine erosive processes are still active and the progressive erosion of the northern caldera sector threatens the stability of the active La Fossa Cone and the nearby coastal settlements.

Structural imprints at the front of the Chocó-Panamá indenter: Field data from the North Cauca Valley Basin, Central Colombia

The northern Andes are a complex area where tectonics is dominated by the interaction between three major plates and accessory blocks, in particular, the Chocó-Panamá and Northern Andes Blocks. The studied Cauca Valley Basin is located at the front of the Chocó-Panamá Indenter, where the major Romeral Fault System, active since the Cretaceous, changes its kinematics from right-lateral in the south to left-lateral in the north. Structural studies were performed at various scales: DEM observations in the Central Cordillera between 4 and 5.7°N, aerial photograph analyses, and field work in the folded Oligo-Miocene rocks of the Serranía de Santa Barbara and in the flat-lying, Pleistocene Quindío-Risaralda volcaniclastic sediments interfingering with the lacustrine to fluviatile sediments of the Zarzal Formation. The data acquired allowed the detection of structures with a similar orientation at every scale and in all lithologies. These families of structures are arranged similarly to Riedel shears in a right-lateral shear zone and are superimposed on the Cretaceous Romeral suture. They appear in the Central Cordillera north of 4.5°N, and define a broad zone where 060-oriented right-lateral distributed shear strain affects the continental crust. The Romeral Fault System stays active and strain partitioning occurs among both systems. The southern limit of the distributed shear strain affecting the Central Cordillera corresponds to the E–W trending Garrapatas–Ibagué shear zone, constituted by several right-stepping, en-échelon, right-lateral, active faults and some lineaments. North of this shear zone, the Romeral Fault System strike changes from NNE to N. Paleostress calculations gave a WNW–ESE trending, maximum horizontal stress, and 69% of compressive tensors. The orientation of σ1 is consistent with the orientation of the right-lateral distributed shear strain and the compressive state characterizing the Romeral Fault System in the area: it bisects the synthetic and antithetic Riedels and is (sub)-perpendicular to the active Romeral Fault System. It is proposed that the continued movement of the Chocó–Panamá Indenter may be responsible for the 060-oriented right-lateral distributed shear strain, and may have closed the northern part of the Cauca Valley, thereby forming the Cauca Valley Basin. Conjugate extensional faults observed at surface in the flat-lying sediments of the Zarzal Formation and Quindío-Risaralda volcaniclastic Fan are associatedwith soft-sediment deformations. These faults are attributed to lateral spreading of the superficial layers during earthquakes and testify to the continuous tectonic activity from Pleistocene to Present. Finally, results presented here bring newinformation about the understanding of the seismic hazard in this area: whereas the Romeral Fault Systemwas so far thought to be themost likely source of earthquakes, themore recent cross-cutting fault systems described herein are another potential hazard to be considered.

(Plio-)Pleistocene alluvial-lacustrine basin infill evolution in a strike-slip active zone (Northern Andes, Western-Central Cordilleras, Colombia)

The (Plio)-Pleistocene Zarzal Formation was deposited in the Cauca Depression and Quindio-Risaralda Basin between the Western and Central Cordilleras (Northern Andes). This area is structurally located on the transcurrent Romeral Fault System (RFS). Because of the interaction between the Nazca plate and the Choco-Panama block (an active indenter), the RFS strike-slip component changes direction around the study zone (dextral in the south, senestral in the north). Zarzal sediments are the oldest ones not to have been affected by the Pliocene Andean tectonic phase. Their study aims at better understanding the subsidence of these two interandean sedimentary basins within a regional compressional regime. The two basins are separated by the the Serrania de Santa Barbara (SSB), made of Tertiary sediments thrusted during the Pliocene tectonic phase. Zarzal sediments are fluvio-lacustrine: diatomites are encountered on both sides of the SSB and are alternating with braided stream or alluvial deposits. Sedimentation was strongly influenced by volcanic processes which led to the deposition of the Quindio-Risaralda and Cartago volcaniclastic fans sourced from the Central Cordillera. These volcaniclastic mass flows mixed with the Zarzal sediments, and even dammed and infilled a lake in the Quindio-Risaralda Basin (east of the SSB). Numerous extensional features affect Zarzal sediments with a mean extensional trend subparallel to the SSW-NNE trending cordilleras. The syndepositional tectonic activity is demonstrated by numerous seismites. In the Cauca Depression, sediments are infilling the basin more rapidly than it subsides or than the relief lifts up, thereby drowning the topography. This might be related to the tectonically-induced, downstream damming of the Cauca River valley to the north by the Choco-Panama block.

Fingerprinting facies of the Tuff IF marker, with implications for early hominin palaeoecology, Olduvai Gorge, Tanzania

The top of Tuff IF marks the upper boundary of the Plio-/Pleistocene Bed I succession exposed in Olduvai Gorge, NE Tanzania, a tephrostratigraphic interval that is well known for remarkable Oldowan archaeological and vertebrate fossil assemblages, including early hominins. Geochemically, Tuff IF is characterized by a relatively consistent silica-undersaturated trachytic to phonolitic composition that relates both in terms of numerical ages (1.79 Ma) and compositional constraints to Olmoti volcano, located 22–38 km E of Olduvai Gorge. Measured Tuff IF sections are characterized by a thick unwelded pyroclastic flow in proximal settings and a succession of surges and ashfalls that interfingers with fluvio-lacustrine deposits of the Olduvai Basin in the medial to distal settings. Only in those areas with a relatively low topographic gradient, in distal reaches and beyond the toe of an Olmoti-sourced volcaniclastic fan, did the Tuff IF marker develop a typical threefold subdivision comprising: (1) primary surges and minor fallout, (2) reworked pumice units as lateral correlatives of proximally emplaced pyroclastic flows and (3) a succession of mass flows in conjunction with aeolian and fluvially reworked units. The pyroclastic marker unit is thus highly heterogeneous with regard to its vertical and lateral facies architecture and this has immediate effects on its preservation potential and in situ burial of fossils and stone artifacts. Even though the volcanically-related environmental perturbations were probably more severe at the eastern lake margin, the evidence for at least temporary freshwater sources and trees suggests environments conducive to hominin activities, but not during emplacement of Tuff IF pyroclastic flows and surges. This inference is supported by the presence of rich Oldowan stone artifact assemblages immediately preceding and following the deposition of Tuff IF, but only extremely sparse archaeological traces from one site in Tuff IF, restricted to the upper, fluvially reworked portion of Tuff IF. Tuff IF facies record the maximum of a regressive (drying) cycle that correlates with regional marine arid indicators. An ecological crisis covering ∼ 2000–3500 a of time during Tuff IF deposition resulted not only from the lethal effects of explosive volcanism but also from its coincidence with a pronounced period of climate induced drought. These combined effects appear to have made at least the eastern basin uninhabitable by hominins and other vertebrates for most of the time during which Tuff IF accumulated.

A facies model for a submarine volcaniclastic apron: the Miocene Manukau volcanic complex, Northland, New Zealand

The Manukau Subgroup, New Zealand comprises a Miocene submarine volcaniclastic apron consisting of the products eroded and erupted from a large offshore partially emergent basaltic to andesitic volcanic complex. The submarine volcaniclastic apron is well exposed for over 30 km along the coast. The apron records the depositional history of a marine basin at bathyal water depths and the onset of fan progradation due to volcanism. The exceptional exposure, variation in fan architecture (distal, lobe, upper slope and channel), facies (e.g. sandstone, pumice breccias, conglomerates and pillow lavas) and origin (primary, resedimented or epiclastic), allows analysis of a facies model for submarine volcaniclastic fans. The facies analysis shows that submarine volcaniclastic fans differ from other clastic sedimentary fan models in the origin, supply and types of clasts.

Nature and origin of toreva remnants and volcaniclastics from La Palma, Canary Islands

New data pertaining to the origin of volcaniclastic fan delta deposits (the El Time group) and previously unreported toreva remnants (intact landslide blocks) are presented in terms of their relationships with the Quaternary flank destruction of Bejenado volcano on La Palma, Canary Islands. Bejenado volcano is the heavily dismantled relic of a stratocone which developed at the confluence of the Cumbre Nueva embayment and the Caldera de Taburiente collapse amphitheatre. Both of these giant landslide structures formed at around 0.55 Ma after ∼0.2 Ma of gravitational spreading on the western and eastern flanks of the Taburiente–Cumbre Nueva shield volcanic complex. Evidence is presented on the basis of stratigraphic, photogeologic and Geographic Information System studies that both the toreva remnants and the El Time group can be related to the sequential collapses of the north flank and summit region of Bejenado inside the Caldera de Taburiente and the northernmost part of the adjoining Cumbre Nueva embayment. The collapses probably occurred between 0.44 and 0.4 Ma and they involved a volume of around 5 km 3. The toreva remnants display a deformed volcanic architecture, traversed by discontinuities which developed during impact and post-collapse dislocation during slip. They are composed of layer-differentiated scoria with ancillary ankaramite and basanite–tephrite lavas and occasional dikes. They rest upon a thin veneer of sheared volcaniclastic deposits in relative proximity to the disfigured north flank of Bejenado. The collapse event(s) is distinguished by its extremely low mobility due to the deceleration of the debris avalanche mass within the confines of the Caldera de Taburiente. The high friction coefficient ( H/ L=0.22) reflects this topographic factor. Reworking of the avalanche matrix gave rise to the accumulation and offshore progradation of the fan delta deposits at the SW base of Bejenado. Contemporaneous fluvial sediments, sourced by the retrogressive erosion of the pre-Bejenado collapse scarps, were fed into the subaerial confines of the Cumbre Nueva embayment. Piecemeal scarp failure and the extrusion of substratum from beneath the scarps also contributed to the sediment budget. The fluvial sedimentation rates subsequently increased with the onset of Cumbre Vieja volcanism prior to 0.125 Ma. These processes have contributed to the development of the flat-floored submarine channels which extend onto the volcaniclastic apron west of La Palma. These new data give a fresh insight into the role of epiclastic processes, landsliding and volcanism in the morphology and sediment mass balance of an oceanic island volcano system.

The eruptive history and volcanic hazards of Savo, Solomon Islands

Savo Island is the 6-km-diameter emergent summit of an andesitic-dacitic stratovolcano, rising from the Iron Bottom Sound, 35 km NW of Honiara, Solomon Islands. Savo has erupted at least three times within recorded history and the 3,000 inhabitants maintain extensive oral traditions of past events. Through description and interpretation of the volcaniclastic sequences on the island, in conjunction with historical accounts and oral traditions, we reconstruct the eruptive processes on Savo. Block-and-ash flow (BAF) deposits are volumetrically dominant on the island within three main depositional environments: near-vent sequences, thick medial channel sequences and distal fan sequences. The deposits comprise universally non-vesicular and highly porphyritic (40–70% phenocrysts), high-silica andesite and dacite clasts. These appear to have been derived from collapsing lava domes during an 1560–1570 A.D. eruption. However, eyewitness descriptions and crater morphology suggest that similar deposits formed from dome explosions or collapses of eruption columns during later eruptions (1830–1840 A.D.). The high-sodium magmas (ca. 5–7 wt% Na2O) apparently crystallised and strongly degassed prior to eruption. Shallow explosions were possibly caused by entrapment of magmatic gases beneath a dome or conduit plug of highly crystalline, near solid magma. Repeated sealing of the vent may have been due to inward collapse of the highly altered rocks of the surrounding hydrothermal system; these rocks probably were saturated due to contemporaneous high intensity rainfall events. BAFs were hot enough to char vegetation and attain aligned clast TRM (thermal remnant magnetism) up to 3 km from the vent, many being accompanied by ash-cloud surges. Changes with distance in the BAF deposits appear mostly dependent on flow confinement and are limited to an overall decrease in thickness and maximum clast size, and an increased definition of weak planar fabrics. In distal fan sequences, there is strong evidence for syn- and post-eruptive redeposition of primary deposits. Since the Savo population is concentrated on coastal volcaniclastic fans, we consider the greatest volcanic risk to life is from BAFs, associated ash-cloud surges and lahars. Hence, the main channels and fans are designated as the highest of three relative hazard zones on a simple map prepared to aid local education and planning initiatives on Savo.

Les lahars du strato-volcan du Cantal (Massif central, France); stratigraphie, modes de mise en place et implications paleo-geomorphologiques

Les lahars du strato-volcan du Cantal (Massif central, France); stratigraphie, modes de mise en place et implications paleo-geomorphologiques

Volcanic facies associations in a modern volcaniclastic apron (Lipari and Vulcano offshore, Aeolian Island Arc)

Multibeam bathymetric and reflectivity data, high-resolution sidescan sonar mapping and ROV images have allowed reconstruction of the present-day facies distribution within the volcaniclastic apron offshore from Lipari and Vulcano. Primary volcanic, resedimented volcaniclastic and volcanogenic sedimentary facies have been distinguished. Primary volcanic facies are volumetrically subordinate but play a major role in controlling the distribution of the other facies and the development of the main submarine canyons, and as the source of volcanic detritus in resedimented volcaniclastic facies. Erosion and transport prevail in the canyons. Conversely, deposition of volcanogenic sediments, in response to flow spreading and gradient reduction, occurs at the canyon mouths. The influence of the adjacent islands on the offshore processes is particularly evident in the area facing the active sector of Vulcano Island. Here, large volumes of volcanogenic sediment from the emergent volcanic edifice are transferred downslope by subaqueous gravity flows allowing the construction of a volcaniclastic fan.

A detailed tephrostratigraphic framework at Merapi Volcano, Central Java, Indonesia: implications for eruption predictions and hazard assessment

The upper part of Merapi Volcano is characterised by a summit crater breached to the southwest and occupied by an unstable lava dome. Episodes of dome growth have usually resulted in partial dome collapse events that have generated pyroclastic flows. In recent years, these have inundated the southwestern lower flanks, and in the past, other sectors of the volcano have been affected. This recent pattern of volcanic activity since the 1880s is well known from direct observations. In this study the eruptive record extending back to mid-Holocene times is deduced from the stratigraphy and chronology of volcaniclastic deposits. This record forms an important basis from which the average periodicity of eruptions at Merapi, and the severity and the extent of future eruptions can be assessed. The most complete record of eruptive activity from Merapi Volcano is found in the deeply dissected volcaniclastic fans of the lower flanks. Here, successions of numerous coarse ash and lapilli beds are interbedded with andic soil material, thin pyroclastic flow and surge deposits, and a hitherto unrecognised inter-regional tephra marker bed. Many of these tephra beds can be reliably correlated around the flanks of Merapi Volcano and can be distinguished on the basis of physical appearance and stratigraphic association. These formations can be reliably used to correlate units around the flanks of the volcano. In the deeper parts of this stratigraphic succession, pyroclastic products originating from the adjacent Merbabu Volcano are also recognised and provide evidence of concurrent activity at both volcanoes. It is clear from the stratigraphic record that relatively large magnitude (eruptive volume to 3.7×10 8 m 3) eruptions have been generated at Merapi Volcano. These eruptive deposits are typically produced from pyroclastic flow and/or pyroclastic fall events. Most of the fall beds are characterised by single to multiple, upward-fining, pumiceous or scoriaeous units, sometimes overlain by ash-cloud surge beds. Isopach maps have been produced for several of these tephra beds and most of them show either west–southwest, south and southeast dispersion, except for the Ngrangkah tephra which shows no systematic direction or distribution. Sub-plinian or plinian to vulcanian open-vent styles of activity appear to have been common at Merapi Volcano. Such eruptive episodes are in contrast with the style of activity that had been occurring at Merapi Volcano in the last two centuries. Clearly, the range of eruptive events likely to be experienced at Merapi Volcano needs to be more critically considered in any existing or future hazard assessment and contingency planning programme.

Deep‐sea volcaniclastic sedimentary systems: an example from La Fournaise volcano, Réunion Island, Indian Ocean

A volcaniclastic sedimentary fan extending to water depths of 4000 m is characterized using gravity cores, camera surveys, high‐resolution sonar images, seismic records and bathymetry from the submarine portion of La Fournaise volcano, Réunion Island, a basaltic shield volcano in the SW Indian Ocean.Three main areas are identified from the study: (1) the proximal fan extending from 500 m water depth down to 2000 m water depth; (2) the outer fan extending from 2000 m water depth down to 3600 m water depth; (3) the basin extending beyond 3600 m water depth. Within these three main areas, seven distinct submarine environments are defined: the proximal fan is characterized by volcanic basement outcrops, sedimentary slides, deep‐water deltas, debris‐avalanche deposits, and eroded floor in the valley outlets; the outer fan is characterized by a discontinuous fine‐grained sedimentary cover overlying coarse‐grained turbidites or undifferentiated volcanic basement; the basin is characterized by hemipelagic muds and fine‐grained turbidites interbedded with sandy and gravelly turbidite lobes.The evolution of the deep‐sea volcaniclastic fan is strongly influenced by sector collapses, such as the one which occurred 0·0042 Ma ago. This collapse produced a minimum of 6 km3 of debris‐avalanche deposit in the proximal area. The feeding regime of the deep‐sea fan is ‘alluvial dominated’ before the occurrence of any sector collapse and ‘lava‐dominated’ after the occurrence of a sector collapse.The main deep‐water lava‐fed delta is prograding among the blocks of the debris‐avalanche deposits as a result of turbidity flows occurring on the delta slope. These turbidity flows are triggered routinely by wave‐action, earthquakes and accumulation of new volcanic debris on top of the deltas.Both turbidity currents triggered on the deep‐water delta slope, and those triggered by debris avalanche reworked volcaniclastic material as far as 100 km from the shore line.

Nearshore, temperate, carbonate depositional systems (lower Tortonian, Agua Amarga Basin, southern Spain): implications for carbonate sequence stratigraphy

The bryozoan-rich lower Tortonian carbonates of the Agua Amarga Basin in southern Spain (Province of Almeria) provide an example of sediments formed in a nearshore, non-tropical depositional setting. Based on data derived from logging of sections and from field mapping, these lower Tortonian carbonates form a depositional sequence, which is subdivided into several depositional systems. A lowstand systems tract, which consists of volcaniclastic fan deltas and washover deposits, formed on the leeward side of a basement shoal which delimited the basin towards the south. A transgressive systems tract, which is characterised by a landward encroachment of deposits, is represented by submarine bars and dune deposits. A highstand systems tract consists of progradational beach deposits. Owing to strong reworking and re-distribution of the particles by currents and waves, microfacies differentiation of the deposits in poor. These Tortonian non-tropical carbonates display a style of deposition similar to siliciclastics suggesting that sequence stratigraphic concepts derived from tropical carbonates do not apply to non-tropical carbonates.