Volcaniclastic Beds Research Articles

Geochemistry, Sr-Nd isotopes and zircon U-Pb dating of magmatic rocks from the Talesh range, western Alborz: New insights into Late Cretaceous evolution of the southern Eurasian margin

The eastern flank of the Talesh range of western Alborz, northwestern Iran, exposes volumetrically significant Late Cretaceous volcanic and volcaniclastic associations as well as cross-cutting dykes and minor subvolcanic equivalents. The volcanic units appear as massive to pillowed flows interlayered with volcaniclastic beds and subordinate limestones. We present new field data, zircon UPb ages, bulk-rock major and trace element geochemistry, and SrNd isotopes on the igneous rock suite. Geochemical data show that the lava units and dykes are mainly basaltic in composition with minor basaltic andesites and trachytes. They form two contrasting series of arc-related calc-alkaline (Group 1) and OIB-like (Group 2) rocks. The zircon UPb age of Group 1 rocks is 95.6 ± 1.8 Ma (2σ), whereas for Group 2 rocks, cross-cutting relationships and stratigraphic constraints indicate a somewhat younger age. Group 1 rocks have 87Sr/86Sr(i) varying from 0.7038 to 0.7070 and εNd(t) values ranging from +0.5 to +5.0. Group 2 rocks have 87Sr/86Sr(i) = 0.7040–0.7065 and εNd(t) from −3.0 to +3.7. Trace element and isotopic modelling shows that the primitive Group 1 magmas were derived from a metasomatized mantle source enriched by the addition of ∼6–10 % sediment melt component, while Group 2 rocks are consistent with melts of an asthenospheric mantle source enriched by ∼1–5 % EMII component. Trace element modelling indicates that Group 1 rocks formed from ∼6–13 % partial melts of a spinel-garnet lherzolite with garnet:spinel ratios of 45:55 to 10:90, whereas primitive Group 2 melts were generated through ∼1–3 % partial melting of a garnet-spinel-lherzolite with garnet:spinel ratios of 90:10 to 80:20. We propose a geodynamic model in which a north-dipping Neotethyan slab beneath the western Alborz (Talesh area) continental margin produced arc assemblages of Group 1 rocks and subsequent slab rollback led to the upwelling of an asthenospheric mantle to generate OIB-like magmatism of Group 2 rocks in an extensional intra-arc to back-arc setting.

Katian volcanism in Central Armorican Domain (France) and Central Iberian Zone (Portugal): a single monogenetic basaltic field?

The Crozon peninsula and Buçaco syncline belong respectively to the Central Armorican Domain and the Central Iberian Zone, located to either side of the Variscan ‘Ibero-Armorican Arc’ structure at present. The Ordovician sedimentary successions at both sites are known to display identical lithofacies. The paired Upper Katian formations also exhibit comparable volcanic successions of anorogenic affinity, including volcaniclastic beds and pillow lavas. The tephra deposits are inferred to have resulted from the activity of subaqueous monogenetic basaltic tuff cones above a <100 m deep calcareous soft substrate, in the context of a passive margin. The Surtseyan-style hydrovolcanic eruptions were followed by effusion of pillow lavas. Both sites also display a large range of similar peperite facies, sometimes related to magmatic segregation structures likely to derive from vapour differentiation. The deeper doleritic dykes and sills, comparable in terms of mineralogy and geochemistry, contain identical metasedimentary xenoliths, whose high-temperature phyllosilicates display a very restricted range of compositions. In the Late Ordovician, Crozon and Buçaco were probably nearby volcanic edifices belonging to a single monogenetic field inside the Armorica microplate. The sudden appearance of high-latitude Katian carbonates, associated with volcanism only at these two peri-Gondwanan sites, just preceded the Gondwanan Hirnantian glaciation. Supplementary material: Analytical data for minerals and whole-rock samples and an M + –4Si–R 2+ diagram are available at https://doi.org/10.6084/m9.figshare.c.6096737

Age of magmatism and alteration of basaltic rocks cored at the base of IODP Site U1513, Naturaliste Plateau, southwestern Australia

An 82.2 m-thick sequence of basaltic rocks was recovered from a deep-sea core on the eastern flank of the Naturaliste Plateau, offshore southwestern Australia during International Ocean Discovery Program (IODP) Expedition 369. The basaltic rocks were cored at the base of IODP Site U1513 and represent the acoustic basement of the Mentelle Basin. The recovered materials consist of subaerial to shallow-water basalt flows interbedded with volcaniclastic beds and cross-cutting dolerite dykes, all of which have been altered to some degree. Existing paleomagnetic data obtained from the overlying sedimentary sequences indicate that the basaltic sequence cored at Site U1513 was deposited before chron M10Nn.1n (which could be as young as 130.9 Ma or as old as 134.2 Ma, depending on the time scale used). Here, we present the results of 40Ar/39Ar step-heating experiments conducted on whole-rock samples of two basalt flows and six dolerite dykes to provide greater certainty of the timing of magmatism and subsequent alteration. Each sample was also characterised using optical and scanning electron microscopy together with X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses. The dolerite dykes are the only rocks that were sampled that record a remnant estimate of the primary cooling age of the magmatic event at ca 134 Ma. This magmatism was contemporaneous with rifting that preceded the break-up of Greater India from Australia/Antarctica and is potentially associated with the Greater Kerguelen large igneous province. The basalt flows record younger apparent ages (between 118 and 110 Ma) that cannot represent extrusive ages given their stratigraphic context, as well as older ages obtained from cross-cutting dykes. We interpret the younger apparent ages to reflect a phase of hydrothermal alteration that reset the argon isotopic system. A tectonic reconstruction, which shows the distribution of magmatism associated with the Greater Kerguelen large igneous province, indicates that the earliest phases of magmatism occurred within an ellipse with a NW–SE-oriented long axis. The reconstruction also shows that the Naturaliste Plateau was approximately 500 km from a known source of magmatism at ca 115 Ma. This distance is within the radius of a mantle plume head and indicates that hydrothermal and/or magmatic activity responsible for resetting the argon isotopic system of the dated basalt flows may have been associated with a mantle plume or was otherwise driven by a local source of magmatism that remained active during the Aptian (126.3–113.0 Ma). KEY POINTS 40Ar/39Ar dating indicates Naturaliste Plateau magmatism occurred between 141.3 and 130.9 Ma. The basaltic rocks potentially record a phase of hydrothermal alteration at ca 118–110 Ma. Greater Kerguelen large igneous province magmatism occurred within an ellipse with a NW–SE oriented long axis.

The Ypresian to Lutetian marine record in NW Turkey: a revised biostratigraphy and chronostratigraphy and implications for Eocene paleogeography

The Eocene sedimentary units in the Kocaeli and Armutlu peninsulas and İznik-Geyve region (northwest Turkey) consist of marine siliciclastic and carbonate deposits, locally intercalated with volcaniclastic rocks and tuffs (Middle Eocene Magmatic Rocks-MEMR). The record of larger benthic fauna (LBF) and pelagic microfossils, such as planktonic foraminifera and calcareous nannoplankton, combined with lithological description, indicate a wide spectrum of depositional environments ranging from shallowto deep-marine. These deposits contain in situ and/or allochthonous assemblages of orthophragminids, nummulitids, subordinate rotaliids, and sparse alveolinids. The in situ assemblages (mainly orthophragminids) point to several episodes of shallow marine depositional conditions during a) early Ypresian (SBZ 5-7 and 7-9), b) late Ypresian (SBZ 10-11), c) early Lutetian (SBZ 13), and d) middle Lutetian (SBZ 14-15). Upper Ypresian (Cuisian) deposits with rich LBF accumulations are the most ubiquitous, recognized across a wide region from the Thrace Basin to Central Anatolia. Lower Lutetian deposits, on the other hand, occur in a limited area only near the town of Şile, on the coast of the Black Sea. Orthophragminids immediately below the first volcaniclastic beds of the MEMR constrain the initiation of the volcanism to the latest Ypresian or around the Ypresian/Lutetian boundary. The LBF assemblages from the clastic intercalations of the MEMR are referred to SBZ 14-15 and indicate that the marine realm in the Armutlu and Kocaeli peninsulas persisted until middle Lutetian. Since then, continental conditions have prevailed in the region, while a marine realm has developed in the Thrace Basin during the late Middle to Late Eocene and Oligocene. The deep-marine Eocene deposits with pelagic microfossils and allochthonous LBF are mostly confined to the Ypresian. The allochthonous orthophragminids in these deep-marine deposits are also presented here to give a broader picture of their occurrence in NW Turkey.

Diatreme-forming volcanism in a deep-water faulted basin margin: Lower Cretaceous outcrops from the Basque-Cantabrian Basin, western Pyrenees

Deep-water diatremes and related eruption products are rare and they have been mainly interpreted from seismic-based data. We present lithofacies and geochemistry analysis of two Lower Cretaceous (Albian) deep-water diatremes and associated extra-diatreme volcaniclastic deposits at a well-exposed outcrop of the northern margin of the Basque-Cantabrian Basin (north Iberia). The studied diatremes are located along a N-S trending Albian fault and present sub-circular to elongate sections, inward-dipping steep walls and smooth to very irregular contacts with the host rocks. They are filled by un-bedded mixed breccias constituted by juvenile and lithic (sedimentary, igneous and metamorphic) clasts. Their textural and structural characteristics indicate that they represent lower diatreme and root zones of the volcanic system. Mapping, geochemical and petrologic data from diatreme-fills support their genetic relationship with the extra-diatreme volcaniclastic beds, which would be generated by the eruption of an incipiently vesicular trachytic magma. Studied diatremes result from multiple explosions that lasted over an estimated period of 65k.y. during the Late Albian (H. varicosum ammonite Zone, pro parte), and reached up to a maximum subsurface depth of ca. 370m, whereas extra-diatreme volcaniclastic beds were formed by eruption-fed gravity-driven flows on the deep-water (200–500m) paleoseabed. Petrological features suggest that these diatremes and related extra-diatreme deposits resulted mainly from phreatomagmatic explosions. In addition, organic geochemistry data indicate that the thermal effect of the trachytic melts on the sedimentary host caused the conversion of the abundant organic matter to methane and CO2 gases, which could also contribute significantly to the overpressure necessary for the explosive fragmentation of the magma and the host rocks. Considering the inferred confining pressures (ca. 8–11MPa) and the possible participation of unvesiculated (or degassed) melts, our results stress the importance of active hydraulic structures (e.g. faults) and the supplementary driving forces (e.g. thermogenic gases) in the formation of deep-water marine diatremes.

Geochemistry of Woranso–Mille Pliocene basalts from west-central Afar, Ethiopia: Implications for mantle source characteristics and rift evolution

The Woranso–Mille (WORMIL) area in the west-central Afar, Ethiopia, contains several Pliocene basalt flows, tuffs, and fossiliferous volcaniclastic beds. We present whole-rock major- and trace-element data including REE, and Sr–Nd–Pb isotope ratios from these basalts to characterize the geochemistry, constrain petrogenetic processes, and infer mantle sources. Six basalt groups are distinguished stratigraphically and geochemically within the interval from ~3.8 to ~3Ma. The elemental and isotopic data show intra- and inter-group variations derived primarily from source heterogeneity and polybaric crystallization±crustal inputs. The combined Sr–Nd–Pb isotope data indicate the involvement of three main reservoirs: the Afar plume, depleted mantle, and enriched continental lithosphere (mantle±crust). Trace element patterns and ratios further indicate the basalts were generated from spinel-dominated shallow melting, consistent with significantly thinned Pliocene lithosphere in western Afar. The on-land continuation of the Aden rift into western Afar during the Pliocene is reexamined in the context of the new geochemistry and age constraints of the WORMIL basalts. The new data reinforce previous interpretations that progressive rifting and transformation of the continental lithosphere to oceanic lithosphere allows for increasing asthenospheric inputs through time as the continental lithosphere is thinned.

Hydrogeology of Wadi Qudaid Area, Northeast Jeddah, West Central Arabian Shield, Saudi Arabia

The study area is a part of the Arabian Shield rocks of west central part of Saudi Arabia (150 km to the northeast of Jeddah). Geologically, the study area comprises five main geologic units i.e. 1) Layered basic volcanics and related volcaniclastics which are composed mainly from intercalated basalts and andesites and the related volcaniclastic derivatives, 2) Acidic volcanics and related volcaniclastics which are composed from layered and laminated dark and light acidic to intermediate igneous rocks, quartz and chert and marbles, 3) The Tertiary sedimentary succession which of volcaniclastic red beds and the intercalated clays; 5) Tertiary volcanics of Harrart, and 4) The Quaternary wadi fill deposits which are composed from friable pebble supported conglomerates, sandstones and clays. Hydrogeologically, the groundwater aquifer of Wadi Qudaid is present mainly in two main horizons i.e. i) unconfined shallow aquifer (13 - 37 m) within the well porous and permeable conglomerates of the Quaternary Wadi deposits, ii) The deep confined aquifer of the bedded tuffaceous sandstones and mudstone of the Tertiary sedimentary succession of Ash Shumaysi Formation. The water samples are analyzed for major elements i.e. Ca, Mg, Na, Cl, SO4, HCO3 and the results show the normal content of these elements. The study related the addition and depletion of many elements during the running trip of the groundwater from the northeast (recharge area) to the southwest (downstream) area.

Petrographic Characterization of the Different Types of Basalts of Harrat Al Fatih, Ablah Area, West Central Arabian Shield, Saudi Arabia

Harrat Al Fatih is located southwest of Ablah area, Assir terrain, southwestern Arabian Shield. It is present as black laterally extended horizontal basaltic sheet just overlying folded strata of the volcano-sedimentary succession of Ablah Formation (green mudstones, dolostones and green and red volcaniclastic red beds). These basalts are rift-related and represent part of the Oligo-Miocene basic volcanics of the western part of Saudi Arabia. The detailed field and microscopic description of these basalts enable the author to classify it into two main types: 1) Grey tuffaceous glassy basalts that present overlying the Ablah Formation. This basalt type is generally soft, bedded and intercalated with grey, green and red tuffaceous mudstones. Microscopically it composed of minute lath-like plagioclase and pyroxene crystals embedded in glassy groundmass. 2) Black olivine-rich basalts which are present in the topmost part of harrat Al Fatih are generally black, hard and contain remarkable reddish brown oxidized olivine spots and some calcite and amorphous quartz domains. Under the microscope, this basalt type is microcrystalline and composed mainly of lamellar twined Ca-plagioclase and colored olivines and pyroxenes. The olivines show different stages of oxidation and formation of amorphous blood red iron-oxyhydroxides and black hematite. The present study revealed the formation of the grey basalts at the initial stages of the volcanic eruption in ephemeral lakes and the second type of basalt was during the consolidation of proper basic magmas at the final stages of the volcanic eruption.

Reservoir architectures of interlava systems: a 3D photogrammetrical study of Eocene cliff sections, Faroe Islands

Abstract Following the intra-volcanic Rosebank discovery in the Faroe–Shetland Basin, NE Atlantic, there has been a need to find suitable analogues to characterize reservoir architectures, connectivities and compartmentalization of interlava clastic beds. The Faroe Islands, situated c. 160–190 km to the NW of this discovery, are an exposed remnant of the Palaeogene lava field host and the near-vertical cliff sections afford the opportunity to map lateral variations over many kilometres. This was achieved through 3D photogrammetry based on high-resolution digital photographs taken from a helicopter. The study focused on the Eocene Enni Formation, which is dominated by a mixture of simple and compound lava flows commonly separated by minor volcaniclastic beds, including the widespread Argir Beds. In general, the interlava beds are tabular shaped and <1 to c. 6 m thick. Locally they thicken in depressions formed by the wedging out or differential erosion of underlying lava flow lobes. Connectivity may be caused by the wedging out of successive lava flows leading to the lateral merging of interlava beds or more rarely be hydraulically connected via conglomeratic fills of major channels. Lateral compartmentalization may be caused by the intersection of interlava beds by dykes, lava tubes, lava-filled channels or reverse faults.

Volcano-sedimentary characteristics in the Abu Treifiya Basin, Cairo–Suez District, Egypt: Example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn-volcanic basaltic rocks

This paper describes the Neogene lava–sediment mingling from the Abu Treifiya Basin, Cairo–Suez district, Egypt. The lava–sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators. The identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the Abu Treifiya Basin. Characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops. The peperites and sedimentary rocks are up to 350m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area.Lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive/intrusive basaltic rocks forming peperitic breccias to lava–sediment contacts at a large to small scales, respectively. Peperites encompass five main facies types ascribed to: (i) carbonate sediments-hosted fluidal and blocky peperites, (ii) lava flow-hosted blocky peperites, (iii) volcaniclastics-hosted fluidal and blocky peperites, (iv) sandstone/siltstone rocks-hosted blocky peperites, and (iv) debris-flows-hosted blocky peperites. Soft sediment deformation structures, vesiculated sediments, sediments filled-vesicles, and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments. All the peperites in this study could be described as blocky or fluidal, but mixtures of different clast shapes occur regardless of the host sediment. The presence of fluidal and blocky juvenile clasts elucidates different eruptive styles, reflecting a ductile and brittle fragmentation. The gradual variation from fluidal to blocky peperite texture, producing the vertical grading is affected by influencing factors, e.g., the viscosity, magma temperature, confining pressure, sediment fluidization, and vapor film at the magma–sediment interactions.Peperites in the study area record deposition within a shallow marine and fluvio-lacustrine environment accumulated in a rift-related basin developed during pre- to syn-rift phase, respectively. The facies transitions (peperites) in this area resulted from the explosive and sediment depositional processes, which were mingled separately by volcanism under contrast geological conditions. The development of such contrast in the depositional sequences reflects variation in the accommodation to sediment supply in the same accumulation space inside the depocenters during the rifting of the Abu Treifiya Basin. Hydrothermal mineralizations comprising quartz and carbonate are restricted to peperites and lava flows.

WITHDRAWN: Volcano-sedimentary characteristics in the Abu Treifiya Basin, Cairo–Suez District, Egypt: Example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn-volcanic basaltic rocks

Abstract This paper describes the Neogene lava–sediment mingling from the Abu Treifiya Basin, Cairo–Suez district, Egypt. The lava–sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators. The identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the Abu Treifiya Basin. Characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops. The peperites and sedimentary rocks are up to 350 m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area. Lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive/intrusive basaltic rocks forming peperitic breccias to lava–sediment contacts at a large to small scales, respectively. Peperites encompass five main facies types ascribed to: (i) carbonate sediments-hosted fluidal and blocky peperites, (ii) lava flow-hosted blocky peperites, (iii) volcaniclastics-hosted fluidal and blocky peperites, (iv) sandstone/siltstone rocks-hosted blocky peperites, and (iv) debris-flows-hosted blocky peperites. Soft sediment deformation structures, vesiculated sediments, sediments filled-vesicles, and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments. All the peperites in this study could be described as blocky or fluidal, but mixtures of different clast shapes occur regardless of the host sediment. The presence of fluidal and blocky juvenile clasts elucidates different eruptive styles, reflecting a ductile and brittle fragmentation. The gradual variation from fluidal to blocky peperite texture, producing the vertical grading is affected by influencing factors, e.g., the viscosity, magma temperature, confining pressure, sediment fluidization, and vapor film at the magma–sediment interactions. Peperites in the study area record deposition within a shallow marine and fluvio-lacustrine environment accumulated in a rift-related basin developed during pre- to syn-rift phase, respectively. The facies transitions (peperites) in this area resulted from the explosive and sediment depositional processes, which were mingled separately by volcanism under contrast geological conditions. The development of such contrast in the depositional sequences reflects variation in the accommodation to sediment supply in the same accumulation space inside the depocenters during the rifting of the Abu Treifiya Basin. Hydrothermal mineralizations comprising quartz and carbonate are restricted to peperites and lava flows.

Origin and geochemistry of the Late Proterozoic intra-arc rift-related volcaniclastic red and green beds of Tayibit El Esm area, Ablah district, south central Arabian shield, Saudi Arabia

The volcaniclastic red beds of the study area are present overlying the rhyolite-dolostone succession of wadi Girshah area, Ablah district. It is composed of seven shallowing-upward cycles. The lower part of this sequence is dominated by gray tuffaceous mudstone which grades upward into mudstone/siltstone. The middle part is dominated by red hematitic volcaniclastic siltstone-sandstone while the upper part is dominated by thinly to thickly bedded silicified stromatolitic dolostone. This vertical distribution reflects the gradation from deeper conditions of high volcaniclastic input into shallower conditions of very low volcaniclastic input into more shallower and highly silicified restricted depositional environments dominated during the deposition of the uppermost silicicfied dolostones of the upper parts of the succession. The field, mega, microscopic, and geochemical results conclude the formation of the present volcaniclastic red beds during the following stages, these are: (1) deposition of the basic and intermediate volcanic ashes in slightly deeper back-arc setting, (2) the degradation of the deposited volcanic ash and the diagenetic authigenesis of green celadonitic clays either along the sediment/water interface or beneath the sea floor by the interaction between Fe2+, Mg2+, Si4+, and Al3+, (3) the diagenetic hematitization of the formed green celadonitic clays of the cycles of the middle and upper parts of the succession and the formation of the iron-oxyhydroxide mineral, i.e., goethite and hematite as a result of the change in the pore water sediments from reducing to oxidizing conditions, and (4) finally, the direct hematitization of the original tuffaceous materials and formation of iron minerals especially in the upper parts of the volcaniclastic red bed succession.

Long‐term (17 Ma) turbidite record of the timing and frequency of large flank collapses of the Canary Islands

AbstractVolcaniclastic turbidites on the Madeira Abyssal Plain provide a record of large‐volume volcanic island flank collapses from the Canary Islands. This long‐term record spans 17 Ma, and comprises 125 volcaniclastic beds. Determining the timing, provenance and volumes of these turbidites provides key information about the occurrence of mass wasting from the Canary Islands, especially the western islands of Tenerife, La Palma and El Hierro. These turbidite records demonstrate that landslides often coincide with protracted periods of volcanic edifice growth, suggesting that loading of the volcanic edifices may be a key preconditioning factor for landslide triggers. Furthermore, the last large‐volume failures from Tenerife coincide with explosive volcanism at the end of eruptive cycles. Many large‐volume Canary Island landslides also occurred during periods of warmer and wetter climates associated with sea‐level rise and subsequent highstand. However, these turbidites are not serially dependent and any association with climate or sea level change is not statistically significant.

Facies architecture of a continental, below-wave-base volcaniclastic basin: The Ohanapecosh Formation, Ancestral Cascades arc (Washington, USA)

The >800-m-thick, Oligocene Ohanapecosh Formation records voluminous sedimentation of volcanic clasts in the Ancestral Cascades arc (Washington State, USA). Most volcaniclastic beds are dominated by angular pumice clasts and fiamme of andesitic composition, now entirely devitrified and altered. All beds are laterally continuous and have uniform thickness; fine sandstone and mudstone beds have features typical of deposits from low-density turbidity currents and suspension settling. Erosion surfaces, cross-beds, and evidence of bi-directional oscillatory currents (i.e., wave ripples and swaley and hummocky cross-stratification) are almost entirely absent. We infer that the setting was subaqueous and below wave base. The abundance of angular pumice clasts, crystals and dense volcanic clasts, and the extreme thickness of several facies, suggest they were derived from magmatic volatile-driven explosive eruptions. The extremely thick beds are ungraded or weakly graded, and lack evidence of hot emplacement, suggesting deposition from subaqueous, water-supported, high-concentration volcaniclastic density currents. Some of the thickest beds contain coarse, rounded, dense clasts at their base and are interbedded with accretionary lapilli–bearing mudstone; these beds are interpreted to be deposits from subaqueous density currents fed by subaerial pyroclastic flows that crossed the shoreline. Shallow basaltic intrusions and mafic volcanic breccia composed of scoria lapilli indicate the presence of intra-basinal scoria cones that may have been partly subaerial. The range in facies in the Ohanapecosh Formation is typical of below-wave-base, continental (lacustrine) basins that form in proximity to active volcanic arcs, and includes eruption-fed and resedimented facies. Extreme instantaneous aggradation rates are related directly to explosive eruptions, and sediment pathways reflect the locations of active volcanoes, in contrast to conventional sedimentation processes acting in non-volcanic environments.

Morphology of the Faial Island shelf (Azores): The interplay between volcanic, erosional, depositional, tectonic and mass‐wasting processes

The extents of volcanic island shelves result from surf erosion, which enlarges them, and volcanic progradation, which reduces them. However, mass‐wasting, tectonics and sediment deposition also contribute to their morphology. In order to assess the relative significance of these various processes, we have mapped in detail Faial Island's shelf in the Azores archipelago based on interpretation of geophysical and geological data. The nearshore substrates of the island, down to 30–50 m depth, are rocky and covered by volcaniclastic boulder deposits formed by surf action on now‐submerged lava flows. Below those depths, sandy and gravel volcaniclastic beds dominate, building clinoforms up to the shelf edge. In some sectors of the coast, prograding lava has narrowed the shelf, but, in contrast to nearby Pico Island, we find fewer submarine‐emplaced lavas on the shelf. In this island, we interpret the distance between the coastline and the shelf edge as almost entirely a result of a straightforward competition between surf erosion and lava progradation, in which erosion dominates. Therefore shelf width can be used as a proxy for coastline age as well as for wave energy exposure. The stratigraphy of shelf deposits in boomer seismic data is examined in detail to assess the roles of different sediment sources, accommodation space and wave exposure in creating these deposits. We also show evidence of mass‐wasting at the shelf edge and discuss the possible origins of slope instability. Finally, we discuss the contributing role of tectonics for the development of the shelf.

Volcaniclastic gravity flow deposits in the Dezadeash Formation (Jura-Cretaceous), Yukon, Canada: Implications regarding the tectonomagmatic evolution of the Chitina arc in the northern Cordillera of North America

The Chitina arc in the northern Cordillera of North America evolved during the accretion of the Wrangellia composite terrane to the western margin of North America in the Jurassic and Cretaceous. The Dezadeash Formation, a 3000m thick sequence of deepwater turbidites in southwestern Yukon, was deposited as a submarine fan in a backarc basin to the Chitina arc. Only limited geochemistry of altered volcanic rocks associated with the arc are available from southern Alaska. However, three thick volcaniclastic beds occur in the Dezadeash Formation. These consist of fine- to medium-grained vitric to crystal tuffs that are interpreted as resedimented syn-eruptive volcaniclastics. A U–Pb zircon age (149.4±0.3Ma) indicates they are contemporaneous with the Chitina arc. Petrographic examination shows the volcaniclastic rocks are altered mainly by albite and locally calcite, with a chemical index of alteration suggesting the rocks are relatively unweathered, or have undergone only incipient weathering. Harker diagrams indicate that CaO, K2O and Rb were mobile. Based on the Th vs. Co compositional diagram, the volcaniclastic rocks are classified as dacites, and according to various trace element plots they are calk-alkaline and adakite-like in composition. The volcaniclastic rocks plot in the active continental margin field on a variety of tectonic discriminant diagrams, and chondrite-normalized multi-element plots display parallel, listric-shaped profiles with significant light rare-earth element enrichment and minor heavy rare-earth element enrichment. Sm–Nd systematics indicate the volcaniclastic rocks represent mixing of a depleted mantle source and an older crustal source. These data suggest mainly slab dehydration with low pressure fractionation controlled by amphibole, with possibly minor input by slab melting. The Chitina arc has previously been interpreted as an oceanic island arc, and the continental arc signature of volcaniclastic rocks in the Dezadeash Formation, together with a continental arc signature of altered volcanic rocks in southern Alaska, is attributed to arc magmas erupting through Paleozoic volcanic, sedimentary and plutonic rocks of the Wrangellia composite terrane proxying for continental crust.

Quaternary transpression and lacustrine sedimentation in the San Lorenzo area (Sant'Arcangelo Basin, Italy)

Quaternary lacustrine sedimentation of the San Lorenzo Succession represents one of the more recent steps in the evolution of the northern part of the Sant'Arcangelo Basin. The latter is a large Pliocene to Pleistocene wedge-top depozone of the southern Apennines foreland-basin system that developed close to the front of the thrust belt in southern Italy. The lower to middle Pleistocene lacustrine deposits are composed of siltstone and claystone interbedded with sandstone, carbonate and volcaniclastic beds, arranged in fining-upward sequences. The overall stratigraphical, sedimentological, and structural data collected demonstrate that the San Lorenzo lacustrine deposits formed when the northern sector of the Sant'Arcangelo Basin was undergoing tectonic deformation. In particular, new data reveal the presence of strike-slip syndepositional faults. These faults belong to positive flower-type structures, which developed in a left-lateral transpressive tectonic regime characterized by a E–W horizontal component of shear stress. The lacustrine deposits recorded the activity of these faults showing growth structures represented mainly by gentle synclines, as well as other minor folds. The tectonic activity of the main faults formed an endorheic area with at least two different depocentres accumulating lacustrine successions.

The Fukuyama volcanic rocks: Submarine composite volcano in the Late Miocene to Early Pliocene Akita–Yamagata back-arc basin, northeast Honshu, Japan

The Fukuyama Volcanic Rocks are composed of pyroxene andesite (FKV-1), hornblende-pyroxene andesite (FKV-2), biotite-hornblende dacite (FKV-3) and volcaniclastic debris-flow deposits and/or turbidites. FKV-1, FKV-2 and FKV-3 are medium-K calc-alkaline rocks depleted in Nd, similar to other back-arc volcanic rocks of the northeast Japan arc and constitute a dome cluster at Fukuyama. Volcaniclastic beds surround the dome cluster and thin and fine upwards. The predominant clast type in the volcaniclastic beds changes upwards from pyroxene andesite, through hornblende–pyroxene andesite, to biotite–hornblende dacite, consistent with the stratigraphic relationships of FKV-1, FKV-2 and FKV-3 lavas. All the siltstones inter-bedded with the volcaniclastic beds and overlying the whole succession contain diatom fossils indicative of the lower part of the Thalassionema schraderi zone (7.8 Ma to 8.5 Ma), compatible with the isotopic ages of FKV-1, FKV-2 and FKV-3. The Fukuyama volcano has a total eruption volume of 60–100 km 3, with a lifetime of the order of 10 5 years, as typically observed for volcanoes in the present back-arc region of northeast Honshu. FKV-1 erupted in deep water and partly disintegrated into hyaloclastite breccias due to direct contact with water. FKV-2 lava repeatedly effused over the FKV-1 lava and produced a volcanic apron of breccias that eventually grew above wave base and was eroded by wave action. The magma of FKV-3 was probably hydrous as it contains biotite and hornblende. The FKV-3 magma could have explosively erupted from a shallow-water dome or vent emergent above the wave base, followed by growth of a degassing lava dome. Repose between eruptions allowed accumulation of silt, and after the Fukuyama eruptions ceased silt entirely mantled the volcano. A small magma supply rate perhaps allowed a relatively long period of quiescence between eruptions of FKV-1, FKV-2 and FKV-3 magmas, resulting in abrasion and reworking of volcanic fragments and accumulation of non-volcanic sediments that constitute part of the volcaniclastic apron. Oil generated in the contemporaneous or underlying sediments, migrated to the upper levels of the volcaniclastic apron and accumulated there under the thick cover of fine-grained sediments that mantle the volcano.

Impacts of explosive volcanism on distal alluvial sedimentation: Examples from the Pliocene–Holocene volcaniclastic successions of Japan

Volcanic activities can create cataclysmic hazards to surrounding environments and human life not only during the eruption but also by hydrologic remobilisation (lahar) processes after the cessation of eruptive activity. Although there are many studies dealing with the assessment and mitigation of volcanic hazards, these are mostly concentrated on primary eruptive processes in areas proximal to active volcanoes. However, the influence of volcaniclastic resedimentation may surpass the impacts of primary eruptive activity in terms of both extent and persistence, and can ultimately result in severe hazards in downstream areas. Examination of the volcaniclastic successions of non-marine Pliocene–Holocene sedimentary basins in Japan has revealed hydrological volcaniclastic sedimentation in fluvial and lacustrine environments hundreds of kilometres from the inferred source volcano. Impacts on these distal and often spatially separated basins included drastic changes in depositional systems caused by sudden massive influxes of remobilised pyroclastic material. Typical volcaniclastic beds comprise centimetre- to decimetre-thick primary pyroclastic fall deposits overlain by metre- to 10s of metres-thick resedimented volcaniclastic deposits, intercalated in sedimentary successions of non-volcanic provenance. The relatively low component of primary pyroclastic fall deposits in the volcaniclastic beds suggests that: 1) potential volcanic hazards would be underestimated on the basis of primary pyroclastic fall events alone; and 2) the majority of resedimented material was likely derived from erosion of non-welded pyroclastic flow deposits in catchment areas rather than remobilisation of local fallout deposits from surrounding hillslopes. The nature, distribution and sequence of facies developed by distal volcaniclastic sediments reflect the influence of: 1) proximity to ignimbrite, but not directly with the distance to the eruptive centre; 2) ignimbrite nature (non-welded or welded) and volume; 3) temporal changes in sediment flux from the source area; 4) the physiography and drainage patterns of the source area and the receiving basin, and any intervening areas; and 5) the formation of ephemeral dam-lakes and intra-caldera lakes whose potential catastrophic failure can impact distal areas. Models of the styles and timing of distal volcaniclastic resedimentation are thus more complicated than those developed for proximal settings of stratovolcanoes and their volcaniclastic aprons and hence present different challenges for hazard assessment and mitigation.

The Neoproterozoic Mwashya–Kansuki sedimentary rock succession in the central African Copperbelt, its Cu–Co mineralisation, and regional correlations

Rocks of the Neoproterozoic Mwashya Subgroup (former Upper Mwashya) form the uppermost sedimentary unit of the Roan Group. Based on new field and drill hole observations, the Mwashya is subdivided into three formations: (1) Kamoya, characterized by dolomitic silty shales/siltstones/sandstones and containing a regional marker (the “Conglomerate de Mwashya” bed or complex); (2) Kafubu, formed by finely bedded black carbonaceous shales; and (3) Kanzadi, marked by feldspathic sandstones. Rocks of the Mwashya Subgroup are overlain by the Sturtian age Grand Conglomérat diamictite (equivalent to the Varianto/Brazil and Chuos/Namibia diamictites), and conformably overlie rocks of the Kansuki Formation (former Lower Mwashya), a carbonate unit containing volcaniclastic beds. New geochemical data confirm the continental rift context of this magmatism, which is contemporaneous with rift-related volcanism of the Askevold Formation (Nosib Group, Namibia). A gradational lithological transition between rocks of the Kansuki and the underlying Kanwangungu Formations, and similar petrological composition of these two formations, support the hypothesis that the Kansuki is the uppermost unit of the carbonate-dominated Dipeta/Kanwangungu sequence, and does not form part of the Mwashya Subgroup. Base metal deposits, mostly hosted in rocks of the Kansuki Formation, include weakly disseminated early-stage low-grade Cu–Co mineralisation, which was reworked and enriched, or initially deposited, by metamorphic fluids associated with the Lufilian orogenic event.