Pumice Fragments Research Articles

New evidence of Holocene pyroclastic density currents at Galeras volcano, Colombia

The Galeras Volcanic Complex (GVC) is a composite volcano located between the Central and Western cordilleras in southern Colombia. It is considered one of the most active volcanoes in Colombia, with typically Vulcanian eruptions. The complex has been divided into different stages based on stratigraphic and geochronological differences. The most recent one is called the Galeras stage and is considered to have started ca. 4500 years. This study presents the results of new stratigraphic and geochronological analysis of the deposits that outcrop along the El Barranco river valley, NW of the GVC, which allowed us to identify newer and older deposits within the most recent stage (i.e., the Galeras stage). The deposits were analyzed via distribution, stratigraphy, geochronology, componentry and granulometry, as well as vesicularity and microtexture of the pumice fragments. We identified three previously unreported deposits, which results in a total of eight pyroclastic density current (PDC) deposits in the valley, which discordantly overlie andesitic lava flows of the previous stage (i.e., Genoy stage; 150–40 ka) of the GVC. They were named from Unit U1 to Unit U8, with units U1, U2 and U8 representing events not previously recorded in the eruptive history of Galeras volcano. Units U1 (8303 ± 97 and 8284.5 ± 90.5 cal BP), U2 (7667 ± 78 cal BP), U3 (5082 ± 198, 5096.5 ± 226.5, 5801.5 ± 481.5 and 5966 ± 235 cal BP) and U6 (2215.5 ± 101.5 and 2033 ± 91 cal BP) are interpreted as formed by PDCs generated by pumice-rich flows linked to the collapse of Subplinian eruptive columns, while units U5 (3201.5 ± 129 cal BP), U7 (1066.5 ± 109.5 and 944.5 ± 115.4 cal BP) and U8 (390.5 ± 80.5 cal BP) are interpreted as formed by PDCs generated by block and ash flows linked to the destruction of intracrater domes and the collapse of Vulcanian eruptive columns. U4 (4625 ± 181 cal BP) is associated with a “blast” type event, caused by the overpressure exerted by the gases of a viscous magma. The above mentioned record allows us to propose that the eruptive history of Galeras volcano began at the beginning of the Holocene (∼8300 years BP), and not 4500 BP, as previously established, which has hazard implications. Unit U8 in the town of La Florida as well as the nearby recorded deposits, indicates a probable PDC hazard for this population.

New evidence of syn-eruptive magma-carbonate interaction: the case study of the Pomici di Avellino eruption at Somma-Vesuvius (Italy)

Calcareous lithics are commonly found within the products of some explosive eruptions of Somma-Vesuvius. The pumice fragments from the final phase of the Plinian fallout event of the Pomici di Avellino eruption contain abundant calcareous xenoliths. Previous work on that eruption, including numerical simulations, suggested that the release of CO2 from the entrapment of carbonates may have prolonged the magmatic phase of the eruption by maintaining sufficient driving pressure in the feeding dike. The texture and thermo-metamorphic reactions of carbonate xenolith-bearing pumice fragments of the Pomici di Avellino eruption are analyzed through petrography, scanning electron microscope images, energy dispersive spectrometer analyses, and micro-computed X-ray tomography to deduce the behavior of short-term carbonate-magma interaction and its contribution to the eruption dynamics. Results show that calcareous xenoliths experienced short-term magma-carbonate interaction, which took place in three steps: (i) entrainment, i.e., the mechanical process of carbonate xenoliths entrapment into a magma; (ii) decarbonation, related to high-temperature decomposition reaction of the xenoliths; and (iii) digestion or dissolution of the incorporated calcareous xenoliths into the melt with diffusion of Ca and Mg. The CO2 released during the syn-eruptive decarbonation process thus provided extra volatiles to the rising magma, which may have maintained magma buoyancy longer than expected if only magmatic volatiles were involved in the eruption.

Aquifer Mapping Using Geo-Electrical Resistivity Survey for Seepage Mitigation in Kuwil Kawangkoan Dam, North Minahasa, North Sulawesi, Indonesia

Abstract The dam built in an area with a productive aquifer system has potential seepage, influenced by groundwater presence. Seepage can cause instability in the main dams or slopes around the dam. An initial assessment of subsurface geological conditions needs to be done to identify groundwater potential as information in mitigating seepage potential in the dam. Kuwil Kawangkoan Dam is located in the Manado groundwater basin with a high groundwater potential. The 2D electrical resistivity approach with the Wenner configuration was applied in this study. Geoelectric data was acquired on 12 lines spread across the dam area. The measurement results show a resistivity value of less than 300 Ωm, indicating the layer is composed of unconsolidated pyroclastic sediments in the form of sand–gravel. This layer has a depth of around 35 m from the surface. The lithology found is dominated by tuff lapilli with pumice fragments that have the potential as an aquifer.

Compositional zoning of the Otowi Member of the Bandelier Tuff, Valles caldera, New Mexico, USA

Abstract The Otowi Member of the Bandelier Tuff erupted at ca. 1.60 Ma from the Valles caldera (New Mexico, USA). It consists of as much as 400 km3 (dense rock equivalent) of strongly differentiated high-silica rhyolite and shows systematic upward variations in crystallinity, mineral chemistry, and trace element concentrations through its thickness, but the major element composition is almost constant and is near the low-pressure granite minimum. Incompatible trace elements in whole pumice fragments and glasses show well-correlated linear covariations. Upward zoning to lower abundances of incompatible trace elements is accompanied by development of overgrowths on quartz and alkali feldspar, although earlier-formed interiors of quartz and feldspar have near-constant compositions throughout the tuff, modified by cation diffusion in the case of feldspar. Melt inclusions in remnant quartz cores show diverse Pb isotope ratios, pointing to a wide range of distinct protoliths that contributed rhyolitic melt to the Otowi magma. Mineral thermometers suggest a modest temperature gradient through the melt body, perhaps of 40 °C, at the time of eruption. Chemical, textural, and mineralogical variations and volume-composition relations through the tuff are consistent with an origin for zoning by melting of a high-crystallinity cumulate layer beneath cognate supernatant liquid to produce denser, remobilized liquid of accumulative composition (i.e., the “modified mush model”). Melting may have occurred in several episodes. The latest of these episodes, probably thousands of years prior to eruption, introduced new rhyolitic liquid into the system and was associated with a thermal excursion, recorded in core compositions of pyroxene, during which much of the earlier crystal mass was dissolved. This left inherited cores and interiors of accumulated quartz and feldspar mantled with new growth having less-evolved compositions (higher Ti, Sr, and Ba). Changing solubility of zircon during cumulate melting produced a reversal of Zr concentrations. There is no clear petrologic evidence of a recharge eruption trigger; nonetheless, compositional zoning resulted mainly from repeated recharge-induced remobilization of quartz-feldspar cumulate. The Otowi system was built, evolved, and modified by several events over the course of a few hundred thousand years.

New evidence of the Green Tuff deposits and post-caldera, recent explosive volcanic activity at Pantelleria volcano (Sicily Channel, Italy) recorded in near-vent marine areas

The record of the explosive activity of Pantelleria volcano is well documented by several distal tephra collected in various areas in and around the Mediterranean, while few tephrochronological studies exist on proximal marine areas. In this paper, we investigate three selected coring sites from the northern shoulder of the Pantelleria graben, about 15–30 km from the island, in near-vent position with respect to that volcanic source. Our multiproxy analyses revealed nine tephra layers, totally composed of juvenile materials, mostly ash and pumice fragments, as well as a coarse-grained tephra deposit at the bottom of one of the cores. The major element composition of glass shards indicates a very homogeneous geochemical composition for the tephra layers, suggesting their primary origin. Using litho-stratigraphic and paleoenvironmental proxies, we associated the tephra layers to the latest (<∼13.7 ka) explosive activity of Pantelleria. The coarse-grained deposit, instead, due to its distinctive composition is considered to represent the near-vent, marine equivalent of the distal Y-6 tephra layer, related to the ∼45 ka old co-ignimbrite fallout deposit of the Green Tuff event. The geochemical characterization of such deposit, mostly rhyolitic and partly trachytic, appears to enlarge the bimodal composition recorded in other distal tephra (Y-6) reported in the literature, highlighting the potential of near-vent record in providing additional information on the eruptive history.

Ultra-crystalline pyroclastic deposits and rhyolitic lavas controlled by crystal mushes: insights from the Acoculco Caldera Complex, México

AbstractThe Acoculco Caldera Complex (ACC), located in eastern Mexico, began its activity during the Pleistocene ~ 2.7 Ma. One of the most relevant and largest rhyolitic eruption in the complex, the Piedras Encimadas Ignimbrite (PEI), occurred during the late post-caldera phase at ~ 1.2 Ma. This ignimbrite is unique with respect to the other caldera products and other contemporaneous ignimbrites in the Trans-Mexican Volcanic Belt (TMVB) because of its ultra-high crystallinity and the absence of pumice fragments. The PEI is made almost entirely of crystals where the main constituents are k-feldspars and silica polymorphs that range from ≤ 5 µm to tens of centimeters in size. XRD on bulk rock, geochemical modeling, FTIR, Raman, and EPMA analyses were carried out in all mineral phases to assess the origin and the causes of high crystallinity within the PEI. We interpret the high crystallinity on the basis of magmatic crystallization of a magma body that was remobilized and altered by post-depositional hydrothermal alteration processes. We suggest that ACC rhyolites are geochemically influenced by at least one crystal mush established during the Pleistocene. We suggest that the PEI could be the result of an erupted crystal mush (melt + crystals), or a cumulate, or an ancient and crystallized reservoir generated after the first ACC collapse due to intrusion or underplating of mafic hot magmas. Extensional episodes within the ACC facilitated the ascent of mafic magmas. This interaction increased the liquid fraction of the mush through partial melting/crystal dissolution, generating a drop in density and viscosity in the mush, thus triggering eruption. The PEI provides evidences for an association between the geochemically-diverse ACC rhyolites with the complex interaction between mafic transitional alkaline magmas and a crustal mush system, promoted by continuous changes in the stress field during the Pleistocene.

Transportation of sediments by floating processes from southern Europe to the coastal regions of eastern Libya: Pumice rock fragments from at Tariyah and Qasr Libya coasts1

Abstract&#x0D; Fragments of pumice of various sizes and roundness were found mixed with recent coastal sediments of calcareous sand, calcareous granules, and shells in At Tariyah coast southwest of Benghazi, and in the coastal area of Qasr-Libya, Al Jabal Al Akhdar, northeastern Libya. The source area for the pumice fragments appears to be the active volcanic regions of eastern and southern Sicily; Etna volcano and submarine volcano known as the CampiFlegrei del Mar di Sicilia respectively. &#x0D; The pumice must have traveled a distance of at least 1000km across the Mediterranean Sea as a floating load. &#x0D; The recent coastal sediments of the Al Jabal Al Akhdar area are derived mainly from two different sources; a southern proximal source consisting of a well-exposed sequence of Tertiary carbonate rocks that produced well-sorted sand-size fragments and a northern distal source that produced cobble-size fragments of pumice. &#x0D; The mechanism of transporting sediments by floating as a floating load, from one sedimentary basin to another is completely different from the other well-known and documented methods of bedload and suspended load. Floating fragments can be transported for long distances and finally deposited without appreciable changes in size, roundness, shape, or sorting. Pumice deposited on the eastern coast of Libya indicates that long-distance transport produces minimal or negligible changes in the textural parameters of floating load components.

Characteristics of Mengger Formation’s volcaniclastic in Cisaat River, Brebes, Central Java, Indonesia

The Cisaat River in Brebes Regency, Central Java, is one of the sites on Java Island where vertebrate fossils were found. Mengger Formation, composed of volcaniclastics material such as tuff and pumice fragments, contains these fossils. The presence of vertebrate fossils along with volcanic deposits has remained unclear of its deposition history. This research characterizes the volcaniclastic rocks in Mengger Formation based on geological field observation and petrography analysis. The study aims to identify and distinguish its primary and secondary volcaniclastic depositional processes. The measured stratigraphic sections reveal that the formation has 7 lithofacies from older to younger, namely grain-supported conglomerate, sandstone, siltstone, sandy mudstone, matrix-supported conglomerate, tuffaceous sandstone, and tuffaceous conglomerate. The primary volcaniclastic materials comprise pumice clast, plagioclase, pyroxene, hornblende, quartz, biotite, lithic fragments, and volcanic glass. In contrast, secondary volcaniclastic or epiclastic-sedimentary materials contain lithic fragments, quartz, plagioclase, pumice clast, and clay matrix. The origin of the resedimented volcaniclastic material was probably derived from a large volume of ignimbrite near the volcano. From the deposit and measurement section, it can be concluded that the type of the Mengger volcaniclastic.

Mineralogická charakteristika sekundárních změn miocenních vulkanoklastických hornin z jižní a střední Moravy (Česká republika)

Bentonite is a significant industrial raw material which deposits deserve protection. The initiative for elaborating this study was the decision from November 2019 of the current owner (the town of Ivančice) to remediate the entire area of the deposit Ivančice-Réna. Lower Miocene volcaniclastic rocks from the Ivančice-Réna (Ivančice) and Middle Miocene sediments in surrounding area of Lipník nad Bečvou are relatively rich in clay minerals (smectites). The mineralogical characteristics of the volcaniclastic rocks were examined using several methods: polarized-light microscopy, X-ray powder diffractometry (XRD), electron microprobe (SEM–WDX), and whole-rocks chemical analysis (ICP-MS). Acid ash-fall tuff and/or very fine-grained tuffite form layers with variable thickness (several cm to m). Grains larger than clay fraction are formed by K-feldspars, plagioclase, biotite, muscovite and quartz crystals accompanied by fragments of pumice and glass shards. The whole-rocks chemical composition of the investigated volcaniclastic rocks indicates similar magmatic sources for both examined samples (Y = 28 and 29 ppm, Yb = 2.8 and 3.2 ppm, Nb = 12 and 14 ppm, Ta = 1.1 and 1.2 ppm, Zr/Ti = 0.16 and 0.17 and Nb/Y = 0.4 and 0.5). The formation of smectite from alteration of rhyolite volcanic glass is accompanied by an increased loss on ignition (LOI: 9.4 and 20.2 wt. %) together with a decrease in K2O (0.7 and 2.1 wt. %) and Na2O (0.6 and 2.1 wt. %). The absence of a negative Ce anomaly suggests that secondary alterations took place under suboxic or anoxic conditions. Smectite and minor opal partially replace volcanic glass and pumice shards. Plagioclase (Ab59–60) and K-feldspar (Or62–78 Ab21–33 An0–1) crystals are relatively fresh (weak sericitic alteration). Biotite is partly to wholly oxidized and chloritized (4.9–6.2 wt. % K2O). Iron oxyhydroxides are present only as minor components. Variation of intensity of secondary alteration (bentonitization) within the studied volcanoclastic layer from the Ivančice-Réna suggests sedimentation in lacustrine environment. In contrast to the case of the sample from Lipník nad Bečvou, the smectite was formed authigenetically from volcanic glass in interaction with seawater or during an early diagenetic process.

Chrono-stratigraphy of the youngest (last 1500 years) rhyolitic eruptions of Lipari (Aeolian Islands, Southern Italy) and implications for distal tephra correlations

The youngest (last 1500 years) volcanic eruptions of Lipari, within the Aeolian Archipelago, produced the prominent pumice cone of Monte Pilato and the obsidian lava flows of Rocche Rosse and Forgia Vecchia, concentrated in the north-eastern sector of the island as well as highly dispersed white-coloured, fine-grained tephra layers of rhyolitic composition in terrestrial and marine settings on the regional scale. Here we describe in detail the stratigraphy of pyroclastic successions and lava flows erupted by different vents - Monte Pilato, Forgia Vecchia, Lami, and Rocche Rosse - combining field observations, sedimentological characteristics of the tephra deposits, and major and trace element compositions of the volcanic glass. All the pyroclastic materials consist of aphyric pumice lapilli and ash with a largely homogeneous rhyolitic composition. The Monte Pilato and Forgia Vecchia deposits primarily consist of highly vesicular pumice fragments and subordinate obsidian clasts, whilst Rocche Rosse and Lami are characterized by moderately vesicular juvenile fragments with a more significant fraction of obsidian. The Lami tephra also contains peculiar pumice clasts with a fibrous texture and breadcrust bombs.Stratigraphic relationships, and paleomagnetic and 14C ages of the lava and pyroclastic deposits are combined with the archaeological information and historical reports, enabling us to provide an accurate chrono-stratigraphic framework for the youngest eruptions of Lipari. Following the 8th century CE eruption of Monte Pilato, which produced a pumice cone and a obsidian lava flow, activity resumed in the second half of 13th century CE with the explosive eruption of Forgia Vecchia that culminated in the emission of a bilobate obsidian lava flow. This eruption was shortly followed by the explosive eruptions of Lami and Rocche Rosse, the latter concluded with the emission of the widely renowned obsidian lava flow.By integrating stratigraphy and geochemistry of tephra deposits with a new chronological scheme, our work facilitates the refinement of proximal-to-distal correlation of Lipari's rhyolitic tephra in continental marine environments of the central Mediterranean area in the last 1500 years. A fine-grained, rhyolitic ash found on Stromboli (~40 km NE from Lipari) has an origin from the Monte Pilato and thus, constrains tephra dispersion towards the NE. Very similar ash beds dispersed southwards and interlayered within the near-source deposits of La Fossa, Vulcano island (~10 km from Lipari) exhibit features that are consistent with the younger activities of the Rocche Rosse eruption. A possible link between previously identified rhyolitic ash layers identified in marine cores of the Ionian Sea and the Forgia Vecchia eruption are postulated, although the age and textural characteristics of these distal tephra are not univocal in indicating a correlation to either Monte Pilato or Forgia Vecchia.

Extremely high-grade, lava-like rhyolitic ignimbrites at Osham Hill, Saurashtra, northwestern Deccan Traps: Stratigraphy, structures, textures, and physical volcanology

In many continental flood basalt (CFB) provinces of the world, widespread silicic lavas and ignimbrites overlie the mafic lavas. Our knowledge of this post-flood basalt silicic volcanism in the Deccan Traps CFB province of India remains limited. Here we describe the stratigraphy, structures, and textures of the rhyolites of Osham Hill in Saurashtra, in the northwestern Deccan Traps. Basaltic lavas forming the lower parts of the hill are overlain by the “Lower Rhyolite” (LR), which is tens of meters thick and shows intense rheomorphic deformation (flow banding and folding) at microscopic, outcrop, and tens of meters scales. The LR is overlain by a “Green Tuff and Pitchstone” (GTP) band 20–40 m thick, containing massive green tuff to strongly rheomorphic, commonly spherulitic pitchstone, both types showing sharp but irregular contacts. The GTP is overlain by the “Upper Rhyolite” (UR), with an ~100 m preserved thickness. This resembles the LR in many structural features, and also contains a thin (~1 m), laterally discontinuous basal layer of unbedded, nonwelded lapilli tuff. The LR and UR locally contain basal autobreccias which are unlike crumble breccias of rhyolitic lava flows. In thin section the LR and UR are microcrystalline and lava-like, though they contain many relict vitroclasts (glass shards and pumice fragments). The LR also shows rare millimeter-size basaltic lithic clasts. The GTP contains abundant vitroclasts. We interpret the LR and UR as extremely high-grade, intensely welded, lava-like ignimbrites, and the GTP as a low-grade ignimbrite with varying degrees of welding and rheomorphic deformation. The absence of sedimentary interbeds and soil horizons suggests that the whole Osham ignimbrite sequence was deposited subaerially and rapidly. The eruptive vent locations are unknown. However, a rhyolite dyke intruding basalts at the southeastern end of Osham Hill is pyroclastic, showing a eutaxitic to microcrystalline interior and margins of welded and rheomorphic tuff-pitchstone. Basaltic lithic fragments found in the dyke margins suggest that it may be the feeder of the LR. The LR and UR show many close similarities to “Snake River-type” ignimbrites (large-volume, metaluminous, high-temperature, lithic- and crystal-poor, intensely welded, rheomorphic and lava-like ignimbrites). These render Osham Hill a newly recognised and important locality of such ignimbrites.

Diagenesis and alteration of subsurface volcanic ash beds of the Vaca Muerta Formation, Argentina

The Vaca Muerta is a marine source rock with excellent petroleum potential containing a large range of heterogeneities such as natural fractures filled with fibrous calcite (“beef”), concretions and ash beds. An exhaustive diagenetic study of ash beds samples was carried out, based on petrographic, mineralogical and geochemical investigations on five wells from the Neuquén Basin with different burial (1.1–1.6% VRo Eq). Ash beds usually display two facies, the clay-rich and the calcite-cemented one, both with complex alterations. The clay-rich ash beds are characterized by predominance of illite/smectite assemblage, added mainly to albite, pyrite and Fe–Ti oxides. They are interpreted to correspond to the first, denser and less porous ash emitted by the volcanoes, with a high amount of micro-crystals transported over relatively short distances. The calcite-cemented ash beds consist mostly of argillitized glass shards and pumice fragments cemented by calcite. They may represent late eruptive stage, wind-blown tephra deposited at greater distance from the volcanic source, with higher initial porosity and lower crystal content favoring early calcite cementation. Diagenetic alteration has completely modified the initial minerals of both facies, which makes it difficult to establish the precursor mineral composition. However, geochemical analyses demonstrate that both facies display well marked calc-alkaline magmatic signature (andesite, dacite/rhyodacite), and similar conditions of alteration during burial.

Lithological, physical and chemical attributes of primary volcanic ash of YTT, Purna alluvial basin, Central India

Youngest Toba Tuff (YTT) ash preserved at two different localities in the Quaternary sediments of Purna alluvial basin have been categorised as of the primary nature on the basis of for their lithological, physical and geochemical characteristics. These ash are light grey, massive to faintly laminated and occur as discontinuous beds of 15–20 cm thickness, extending laterally for 70–100 m that are hosted in yellowish‐brown, laminated silty‐clay lithounits. It consists of high‐quantity vitric grains represented by bubble wall, blocky and pumice shards in decreasing order of percentage, light and heavy minerals and fine‐grained unidentifiable admixture. Major oxides data through EPMA of glass shards show rhyolitic composition being rich in SiO2, K2O and Na2O percentage, whereas ICP‐MS data exhibit enriched LREE, depleted HREE and negative Eu anomaly. Pre‐tephra successions at both the localities show preservations of various biogenic and non‐biogenic structures having control of specific environmental conditions on their formation, that is, pedogenic calcretes, rhizolith balls and insect agriculture activities in the form of calcretised bodies.

Davut – Tazekent Civarındaki Kayaçların Stratigrafik ve Petrografik Özellikleri, Diyadin-Ağrı-Türkiye

The study is located in vicinity of Taşbasamak, Tazekent, Davut, Boyalan, Kuşburnu and Ulukent villages in the south of Diyadin district of Ağrı city. In the study area, Paleozoic aged Batıbeyli metamorphites which consist of crystallized limestones which contain calc-schist interlevels are observed at the bottom. The Upper Miocene aged Alibonca formation composed of conglomerate, trachyandesitic tuff-agglomerate, sandy limestone, marl, dolomite-dolomitic limestone and lacustrine limestone are located unconformably on these metamorphites. Upper Miocene aged Sekirdağ volcanites consisting of dacitic - rhyodacitic tuffs, agglomerates and lavas are observed by lateral - vertical transition over the Alibonca formation. Higher up, Lower Pliocene aged Solhan volcanites are composed of alkaline basaltic lavas are observed. The Solhan volcanites are covered by Upper Pliocene aged Hamur volcanites consisting of trachyandesitic lava and ignimbrite. Higher up, Upper Pliocene-Pleistocene aged Kale pyroclastics consisting of lapilli tuff, ash, volcanic bomb, slag and agglomerate with alkaline basalt composition are observed. These pyrolastics are covered by Upper Pliocene-Pleistocene aged Tutak volcanites which are composed of alkaline basalts. At the top, travertine and alluvium are found. Calcite + dolomite ± quartz ± muscovite mineral paragenesis is observed in metacarbonates belonging to Batıbeyli metamorphites. The tuffs belonging to the Kuşburnu pyroclastic member include quartz, plagioclase, sanidine, clinopyroxene (diopside/augite), biotite, amphibole (hornblende), sphene, opaque mineral, volcanic glass and litic fragment. These tuffs were named as lapillistone and ash tuff “according to grain size”, trachyandesite tuff “according to composition and grain size”, lapillistone, ash tuff and lapilli tuff “accoding to percentage distribution of grains” and vitric tuff “according to glass-crystals-lithic fragments composition”. The tuffs of Sekirdağ volcanites contain quartz, plagioclase, sanidine, biotite, amphibole (hornblende), opaque mineral, volcanic glass and litic fragment. The dacitic and rhyodacitic lavas belonging to these volcantecs include quartz, plagioclase, sanidine, biotite, amphibole (hornblende), opaque mineral, plagioclase microlites and volcanic glass. Tuffs belonging to Sekirdağ volcanites were named as lapillistone and ash tuff “according to grain size”, rhyodacitic tuff and dacitic tuff “according to composition and grain size”, lapillistone and ash tuff “according to percentage distribution of grains” and vitric tuff “according to glass-crystals-lithic fragments”. Alkaline basalts of Solhan volcanites include quartz, olivine, clinopyroxene (diopside/augite), apatite, opaque mineral and plagioclase microlites. Plagioclase, sanidine, amphibole (hornblende), clinopyroxene (diopside/augite), apatite, opaque mineral, plagioclase and sanidine microlites are observed in trachyandesites belonging to Tazekent trachyandesite member. Plagioclase, hornblende, clinopyroxene, sanidine, pumice fragments, litic fragments and volcanic glass are located in ignimbirites belonging to Dalören ignimbirite member. The tuffs of Kale pyroclastics include plagioclase, clinopyroxene (diopside/augite), olivine, opaque mineral, volcanic glass and litic fragment. Agglomerates of Kale pyroclastics include plagioclase, olivine, clinopyroxen (diopside/augite), orthopyroxen (enstatite), apatite, opaque mineral, volcanic glass and litic fragment. These tuffs were named as lapillistone “according to grain size”, basaltic tuff “according to composition and grain size”, lapilli tuff “according to percentage distribution of grains” and vitric tuff “according to glass-crystals-lithic fragments composition”. The agglomerate is in basaltic agglomerate composition “according to composition and grain size”. Alkali basalts belonging to Tutak volcanites include plagioclase, olivine, clinopyroxen (diopside/augite), orthoproxen (enstatite), apatite, opaque mineral and plagioclase microlites.

Sandstone petrography and provenance of the Chubut Group (Cretaceous) in the Cañadón Matasiete (Golfo San Jorge Basin, central Patagonia): Implications for basin evolution and alluvial organization

Sandstone petrographic data have been used to investigate the provenance of the Matasiete, Pozo D-129, and Castillo formations (Chubut Group) in the Cañadón Matasiete, San Bernardo Fold Belt at Golfo San Jorge Basin, Argentina. Available petrographical data provides evidence of several distinctive volcanic source areas, with minor contributions of metamorphic, plutonic, or sedimentary components. Paleo-volcanic contributions from basic-intermediate (lathwork and microlithic textures) and acidic compositions (felsitic, seriate, granular, and vitric textures) were mainly derived from the Middle-Upper Jurassic rocks of the Lonco Trapial Formation. Pyroclastic components (glass shards, pumice fragments, and eutaxitic textures) derived from coeval volcanic activity in the Andes Cordillera, generated either gradual or abrupt changes in the source of the lithic fragments along the study succession. The observed trends in detrital lithic provenance can be integrated with previous sedimentological and architectural studies of the Matasiete and Castillo formations, revealing that narrow-sheet geometries of the channel fill characterize stratigraphic intervals dominated by pyroclastic components, whereas stratigraphic intervals that contain a low content of pyroclastic materials in the channel fill show ribbon-shaped geometries. Sandstone petrography in volcaniclastic basins can be used to complement other indicators of the evolution of the fluvial systems, providing reliable information about the nature of the changes in the sediment supply within the drainage catchment.

El Escondido tuff cone (38 ka): a hidden history of monogenetic eruptions in the northernmost volcanic chain in the Colombian Andes

El Escondido is a dacitic monogenetic volcano situated in the Samana monogenetic volcanic field, within the Central Cordillera of Colombia. The tuff cone was emplaced in a deeply incised and rainy mountainous zone, ca. 38 ky ago by an explosive eruption that affected not only the metamorphic and igneous basement but also the remnants of the ~ 154 ka Pela Huevos volcano. The El Escondido volcaniclastic deposits are composed of juvenile pumice and lithic fragments including dense volcanic rocks from the Pela Huevos volcano, as well as metamorphic and igneous rocks from the basement. The pumice shows tubes and spongy textures. The volcanic lithics are dominantly angular and fresh, and exhibit different mineralogy and whole-rock geochemistry in comparison to the pumice. Plagioclase and amphibole are ubiquitous; however, biotite and quartz crystals occur only in the pumice fragments (~ 70 wt% SiO2 volatile-free), whereas olivine and pyroxene crystals are only found in the volcanic lithics (~ 65 wt% SiO2 volatile-free). The El Escondido tuff cone is strongly eroded and Pela Huevos is a dome-like remnant in the SE sector. Because of this, along with the highly vegetated tropical zone where the volcanoes are emplaced as well as difficult political issues in the region, the edifices were not recognized until recently; this is why the younger cone was named “El Escondido” (which means “The Hidden”). These eruptions evidence that recent volcanism has occurred in a zone of the Central Cordillera that has been considered as non-volcanogenic in recent studies.

Nature, Occurrence, and Lithological Setup of Youngest Toba Tuff Volcanic Ash, Purna Alluvial Basin, Central India

Youngest Toba Tuff ashes in the Purna alluvial basin of central India have been encountered at various places and are represented by light-gray, yellowish-brown, grayish-orange-pink, loosely packed, fine-grained material. These are preserved as pockets, lenticles, and discontinuous beds of 0.5–3.0-m thickness in the arenoargillaceous sediments of fluvial environments. Based on color, thickness, and sedimentological features, this distal material is divisible into primary ash and reworked ash. Microscopically, the ashes consist predominantly of glass shards with bubble-wall morphologies, minor minerals, unidentifiable fine-grain admixture, and, rarely, channel sediments. The glass shards are colorless, transparent, angular, unfractured, and unaltered in nature. A total of six morphotypes have been identified: (1) uniradiate, or two-bubble-wall, shards, (2) Y-shaped, or triradiate, shards, (3) platy shards, (4) blocky shards, (5) shards with elongated vesicles, and (6) pumice shards. Facies architecture of various successions shows broadly low-energy fluviolacustrine depositional environments. Major-element analysis of the glass shards shows a high percentage of silica, followed by alumina, potassium, and sodium. In general, high percentages of major ions in the glass shards reflect rhyolitic magma composition. The minerals in the ash, though minor in quantity, are represented by quartz, feldspar, biotite, amphibole, pyroxene, olivine, allanites, and Fe-Ti oxides. Preservations of various paleoclimatic proxies are also recorded from pretephra successions and are represented by rhizoliths, rhizolith balls, rhizosphere, ant traces, and vertebrate remains.

Generation of block-and-ash flows at the onset of silicic volcanism in the Paraná Magmatic Province (Brazil): evidence from photoanalysis of Caxias do Sul breccias

In this work, we perform textural analysis on volcanic breccias related to the silicic magmatism in the region of Sao Marcos (southern Brazil), Parana Magmatic Province. The breccias are matrix-supported, coarse-grained, poorly sorted with angular to rounded fragments in a fine-grained matrix. Juvenile fragments are dense to pumiceous, massive to banded dacites; lithic fragments were not recognised. The analyses were performed on field photographs. Grain size distribution (GSD) analysis showed maximum fragment sizes between − 8 and − 9.5 ϕ and smallest recognised sizes between − 3 and 1.5 ϕ. Due to the spatial resolution limit of the photographs and the weathered condition of the matrix, the fine fragments from the matrix were not analysed. The GSD fractality could be quantified by a single power law, with low/moderate D values of 1.9–2.3. Vesicle size distribution (VSD) and shape parameters of both fragments and vesicles were also measured. The fragments are typically low vesicularity, with median vesicle area fractions around 5% for all samples, but pumiceous fragments with up to 43% area are found. VSDs exhibit single exponential, concave upwards curves, with vesicles becoming more deformed/complex with increasing size, suggesting a single continuous process of nucleation and growth under low degrees of shear. Regarding the fragments’ shape parameters, all samples have high values of solidity which, together with the low-vesicularity of the fragments, suggest brittle fragmentation of poorly vesicular material and the operation of secondary fragmentation processes during transport that round and smooth the clasts. However, field evidence for local conduit systems together with the GSD fractality suggest that these may be proximal deposits, without substantial transport history. Based on a comparison of the deposits’ characteristics with several types of volcanic breccias, we propose a block and ash flow source for these breccias that indicate the presence of nearby lava domes, providing evidence for high-viscosity low-flux dacitic volcanism in the Parana Magmatic Province.

Architecture of intereruptive and syneruptive facies in an Andean Quaternary palaeovalley: the Huarenchenque Formation, western Argentina

The Huarenchenque Formation is a volcano sedimentary unit deposited to the east of the Plio-Quaternary Andean Magmatic Arc. In order to define depositional settings, two lithofacies associations (fluvial and pyroclastic) were defined. The fluvial facies association is composed of polymictic conglomerates with the predominance of basalt- dominated clasts, coarse- medium-grained conglomeratic sandstones and medium- to coarse-grained sandstones. These deposits occur as stacked or single bodies, display both sheet and channelized geometries, and contain a range of internal sedimentary structures, such as planar, low angle stratification and cross-bedding. This facies association is interpreted as the deposit of a multichannel fluvial system characterized by high bed load, steep gradient and non-cohesive bank materials. Facies and architecture of the fluvial deposits are the result of high bank full discharge related to rapid deglaciation of the Andean Last Glacial Maximum. The pyroclastic facies association is characterized by lapilli and ash tuffs deposited from air fall, pyroclastic density current, and density stratified surge mechanisms. In the Huarenchenque Formation the fluvial and the pyroclastic facies associations show a clear physical separation, suggesting that sedimentation occurred in two distinct (intereruptive and syneruptive) phases. During the long-lived intereruptive phases the sedimentary record corresponds mainly to the deposits of the gravelly braided fluvial system, whereas during syneruptive phases the fluvial valley was almost entirely occupied by primary pyroclastic deposits related to high-explosive episodes of the neighbor Andean strato-volcanoes.&#x0D; Although most of the cross-bedded sandstones and conglomerate sandstones are rich in basaltic fragments, some strata are composed almost entirely of pumiceous fragments, while in others there is a marked alternation between “basalt” and “pumiceous” foresets. These attributes reflect the preservation of intrabasinal pyroclastic fragments and allow suggest that: i. explosive volcanic events could be more frequent than reflected by the pyroclastic deposits themselves; ii. syneruptive pyroclastic materials could be eroded (even eliminated) by the fluvial system; iii. contributions of primary pyroclastic material persisted during intereruptive (fluvial-dominated) phases.

The ancient pozzolanic mortars of the Thermal complex of Baia (Campi Flegrei, Italy)

Abstract Ancient pozzolanic mortars show the high technological quality achieved by Roman construction workers, due to their ‘excellent state’ of preservation in every environment. These workers well knew that thanks to the combination of lime with specific volcanic products (pozzolana), mortar and concrete become hydraulic, allowing underwater hardening and increasing mechanical strength. The use of pozzolana in a mortar provides the underwater curing (hydraulic limes) of whatever construction with higher speed compared to carbonation processes of slaked lime. Whenever pozzolana is not available, it is substituted by ceramic fragments, which possess similar hydraulic properties. This research focuses, for the first time, on the detailed characterization of mortars coming from the Thermal Complex of Baia, which represents one of the most important archaeological sites in the Campania region. Thanks to several thermal springs, the ancient city of Baiae (Campi Flegrei) was the holiday resort of the Roman aristocracy. The former Soprintendenza Archeologia della Campania, allowed us to perform non-invasive, but representative, sampling of mortars that were characterised by multianalytical methodologies (POM, XRPD, SEM-EDS, TGA, and MIP) providing useful information on possible future activities of restoration. Results confirmed the expertise of Roman workers, who skilfully combined volcanic tuff aggregate, hydrated lime, and ceramic fragments. In particular, the typical zeolitic mineral association of phillipsite > chabazite > analcime found in the tuff aggregate pointed out their provenance from the Neapolitan Yellow Tuff related to the volcanic activity of Campi Flegrei of ca. 15 ka BP. The most relevant characteristic detected in all studied samples is the mortar hydraulicity testified by evidences such as reaction rims between pozzolana and binder, Hydraulicity Index (HI), and thermal analyses investigation. Also, composition of secondary mineralogical phases in the cementiceous matrix is particularly relevant. Distinctive is the contemporary presence of C-A-S-H gel, calcite and gypsum. C-A-S-H gel is derived from lime/ceramic fragments reaction; calcite is likely related to the partial reaction of underburned lime; and gypsum could be ascribable to the sulphation process of calcite. These secondary minerogenetic products fill pore space and enhance bonding in pumice fragments, thus contributing to long-term durability of mortars.