Tephra Beds Research Articles

斑晶鉱物の屈折率に基づく八ケ岳新期テフラ群川上テフラの給源火口推定

This study attempts to estimate the source craters of the Yatsugatake-Kawakami Tephra Bed (Yt-Kw), one of the Yatsugatake Younger Tephra Beds (Yt-Kw, Yt-Pm1, Yt-Pm2, Yt-Pm3, and Yt-Pm4, in ascending order), using the refractive indices of orthopyroxene and plagioclase phenocrysts, for the purpose of constructing an eruptive history of the Yatsugatake Volcanic Chain in central Japan. Previous studies have already clarified that the source of the youngest Yt-Pm4 bed is located around the summit of Yokodake, north of the Yatsugatake Volcanic Chain, whereas the craters of the four other tephra beds have not been identified. Therefore, this study tries to find a correlation between distal tephras and proximal deposits, such as welded tuffs, to estimate the sources of the Yatsugatake Younger Tephra Beds. In addition, the lavas distributed in the proximal area are analyzed to examine whether they have any common refractive indices according to the area. Based on the results, the proximal deposits are classified into four groups: those from around Ioudake (group (1)), from around Tengudake (group (2)), from craters between Mugikusa Pass and Nakayama Pass (group (3)), and others. These results suggest that the Yt-Kw bed erupted from around Ioudake, and the Yt-Pm1, Yt-Pm2, and Yt-Pm3 beds from around Tengudake. Furthermore, the Yt-Kw bed is estimated to correlate with the Ioudake welded tuff because both pyroclastic products contain olivine phenocryst, and the refractive indices of the orthopyroxene and plagioclase phenocrysts are similar. The distribution pattern of the Yt-Kw bed also supports this correlation. Based on the similarity of refractive indices of phenocrysts, Mikaburiyama welded tuff may correlate with one of Yt-Pm1-3. These tephras may erupt in the near ages of Tengudake lower lava, Tengudake middle lava, and Tengudake upper lava.

Tephra beds from the Older Ontake Volcano found from the Middle Pleistocene, Tsumagoi Group on the southeastern foot of the Azumaya Volcano, Central Japan

Tephra beds from the Older Ontake Volcano found from the Middle Pleistocene, Tsumagoi Group on the southeastern foot of the Azumaya Volcano, Central Japan

Gold Run tephra: a Middle Pleistocene stratigraphic and paleoenvironmental marker across west-central Yukon Territory, Canada

Gold Run tephra has been found at Thistle Creek, Sixtymile River, and the Klondike goldfields of west-central Yukon, Canada. It is a hornblende-bearing rhyolitic tephra with thicknesses of up to 10 cm at each site, suggesting a widespread distribution across interior Alaska and the Yukon Territory, given the long distance to the nearest volcanic centre. Old Crow, Flat Creek, and TA tephra beds are stratigraphically associated with Gold Run tephra at our study sites and have distinctive compositions. Gold Run tephra is not accommodated by the current classification scheme for late Cenozoic distal tephra beds in Alaska and the Yukon Territory — a scheme based on the physical and chemical attributes — so that its provenance is unknown. An early-Middle Pleistocene age is supported by a glass fission-track age of 0.74 ± 0.06 Ma, a normal remanent magnetic polarity for the enclosing loess, and the presence of a late Irvingtonian faunal assemblage in the associated organic-rich silts at Thistle Creek. Hence, Gold Run tephra was deposited during the very early part of the Brunhes Chron, at which time a shrub tundra environment prevailed across west-central Yukon.

Sedimentary signatures of cyclic growth and destruction of stratovolcanoes: A case study from Mt. Taranaki, New Zealand

The long-term behaviour of andesite stratovolcanoes is characterised by a repetition of edifice growth phases followed by collapse. This cyclic pattern represents a natural frequency at varying timescales in the growth dynamics of stratovolcanoes worldwide. Around the > 130 ka Mt. Taranaki (Egmont volcano), New Zealand, coastal–cliff successions at 20–40 km distance comprise repeating packages of lithologically and sedimentologically distinctive mass-flow deposits. Varying depositional mechanisms and source properties of these units record growth and collapse cycles of the central edifice. These are used to construct a model for cyclic volcaniclastic sedimentation in the surrounds of stratovolcanoes. During edifice-construction phases, thick packages of tabular, predominantly monolithologic, hyperconcentrated-flow and debris-flow deposits accumulate with intercalated tephra beds. The mass-flow units commonly contain large proportions of fresh pumice or juvenile-lithic andesite. Intervals of quiescence separating eruptive periods are characterised by landscape re-adjustment, accompanied by deposition of fluvial and aeolian sediments, along with steady accretion of medial ash. In contrast, brief episodes of destruction are marked by wide-spread, distinctively clay-rich, polylithologic debris-avalanche deposits and related marginal debris flow units. The growth stages can be terminated by an eruption-triggered sector collapse, or by external forces once the edifice exceeds a critical stable height or profile (dependent on eruptive style and local geo-tectonic conditions). Once the edifice becomes metastable, regional tectonic earthquakes or shallow-level intrusion events are likely triggers for collapse. Although the resulting debris avalanches represent the greatest individual hazard from such andesite stratovolcanoes, their frequency is relatively low compared with other types of mass-flows generated during edifice-growth phases. Accurate forecasts of future hazard from mass-flow events are therefore dependent on recognition of both the frequency of a stratovolcano's growth cycle and its current position in that cycle.

A 90,000-year tephrostratigraphic framework of Aso Volcano, Japan

A detailed 90,000-year tephrostratigraphic framework of Aso Volcano, southwestern Japan, has been constructed to understand the post-caldera eruptive history of the volcano. Post-caldera central cones were initiated soon after the last caldera-forming pyroclastic-flow eruption (90 ka), and have produced voluminous tephra and lava flows. The tephrostratigraphic sequence preserved above the caldera-forming stage deposits reaches a total thickness of 100 m near the eastern caldera rim. The sequence is composed mainly of mafic scoria-fall and ash-fall deposits but 36 silicic pumice-fall deposits are very useful key beds for correlation of the stratigraphic sequence. Explosive, silicic pumice-fall deposits that fell far beyond the caldera have occurred at intervals of about 2500 years in the post-caldera activity. Three pumice-fall deposits could be correlated with lava flows or an edifice in the western part of the central cones, although the other silicic tephra beds were erupted at unknown vents, which are probably buried by the younger products from the present central cones. Most of silicic eruptions produced deposits smaller than 0.1 km 3, but bulk volumes of two silicic eruptions producing the Nojiri pumice (84 ka) and Kusasenrigahama pumice (Kpfa; 30 ka) were on the order of 1 km 3 (VEI 5). The largest pyroclastic eruption occurred at the Kusasenrigahama crater about 30 ka. This catastrophic eruption began with a dacitic lava flow and thereafter produced Kpfa (2.2 km 3). Total tephra volume in the past 90,000 years is estimated at about 18.1 km 3 (dense rock equivalent: DRE), whereas total volume for edifices of the post-caldera central cones is calculated at about 112 km 3, which is six times greater than the former. Therefore, the average magma discharge rate during the post-caldera stage of Aso Volcano is estimated at about 1.5 km 3/ky, which is similar to the rates of other Quaternary volcanoes in Japan.

四阿火山を起源とする噴出物の岩石記載的特徴とテフラ分布

Azumaya Volcano is a stratovolcano located slightly at the back arc side of the volcanic front in central Japan. Previous studies led to the publication of a geologic map and the K-Ar age of some lavas, but the stratigraphic relationships of the volcanic products and their eruptive history are still unclear. Some tephras that possibly erupted from Azumaya Volcano have been found in the North Kanto region. But, there are relatively few descriptions of pyroclastic products. This study presents the lithological and petrographical characteristics of products from Azumaya Volcano, especially the refractive indices of phenocrysts, because these data are very important for identifying products and reconstructing eruptive history. This study examined the adequacy of the identification of tephras of the previous studies and whether the source of these tephras is Azumaya Volcano from the distribution of tephra and comparing mineral composition and refractive indices of orthopyroxene, plagioclase, hornblende, and cummingtonite phenocrysts in proximal products and distal tephras. Because the refractive indices of the phenocrysts in the products distributed in Azumaya Volcano have unique characteristics, refractive indices are useful for identifying and correlating products. Distal tephras correlated in previous studies have similar characteristics, increasing the probability that the source of the SgP.2 tephra bed is the Azumaya Volcano. In addition, the volume of the SgP.2 tephra bed was calculated to be 0.85 km3 dense-rock equivalents (DRE) using its distribution.

愛媛県松山平野における更新統および完新統の層序と火山灰の対比

愛媛県松山平野における更新統および完新統の層序と火山灰の対比

Major- and trace-element characterization, expanded distribution, and a new chronology for the latest Pleistocene Glacier Peak tephras in western North America

The Glacier Peak tephra beds are among the most widespread and arguably some of the most important late Pleistocene chronostratigraphic markers in western North America. These beds represent a series of closely-spaced Plinian and sub-Plinian eruptions from Glacier Peak, Washington. The two most widespread beds, Glacier Peak ‘G’ and ‘B’, are reliably distinguished by their glass major and trace element abundances. These beds are also more broadly distributed than previously considered, covering at least 550,000 and 260,000 km 2, respectively. A third bed, the Irvine bed, known only from southern Alberta, is similar in its major-element composition to the Glacier Peak G bed, but it shows considerable differences in trace element concentrations. The Irvine bed is likely considerably older than the G and B tephras and probably records an additional Plinian eruption, perhaps also from Glacier Peak but from a different magma than G through B. A review of the published radiocarbon ages, new ages in this study, and consideration in a Bayesian framework suggest that the widespread G and B beds are several hundred years older than widely assumed. Our revised age is about 11,600 14C yr BP or a calibrated age (at 2 sigma) of 13,710–13,410 cal yr BP.

Volcanic Tephra Bed Formation and Condensation Processes: A Review and Examination from Devonian Stratigraphic Sequences

The formation and preservation of airfall volcanogenic tephras is the result of various interacting factors. Explanations of the depositional history of individual tephra beds lie along a spectrum between two end-member models of a primary single event and a complex depositional history. Nonrecognition of the history of a tephra bed may lead to errors in geochronology, correlation, and interpretation of the record of explosive volcanism in sedimentary successions. Increased understanding of sequence stratigraphic principles and condensation processes favors the model of tephras with a complex depositional history. This is supported by studies that indicate many tephras in the rock record result from amalgamation and/or reworking in the depositional environment. A prediction of increased tephra bed formation/preservation within transgressive systems tracts due to condensation processes is tested through 11 Lochkovian to Eifelian (Lower to lower Middle Devonian) depositional sequences. The character and distribution of over 80 Devonian K-bentonites and tuffs in the Appalachian Basin (eastern United States) indicates a complex history of tephra deposition and preservation. Of 11 sequences, four have numerous tephra beds, one sequence yields one tephra bed, and six sequences yield no tephra beds. In total, 38 individual beds cumulatively occur within transgressive systems tracts, 39 in early highstand systems tracts, one in late highstand systems tracts, and one in lowstand systems tracts, noting that lowstands are poorly preserved or not recognized along the outcrop belt. This indicates that airfall tephras are concentrated in the transgressive and early highstand systems tracts, not just in condensed intervals of transgressive systems tracts; they do not consistently occur through all sequences. A simple sequence stratigraphic condensation model for the deposition and preservation of volcanic tephras is not applicable.

Fingerprinting ash deposits of small scale eruptions by their physical and textural features

Correlation of distal ash deposits with their proximal counterparts mainly relies on chemical and mineralogical characterization of bulk rock and matrix glasses. However, the study of juvenile fragments often reveals the heterogeneity in terms of clast shape, external surface, groundmass texture and composition. This is particularly evident in small scale eruptions, characterized by a strong variability in texture and relative abundance of juvenile fragments. This heterogeneity introduces an inherent uncertainty, that makes the compositional data alone inadequate to unequivocally characterize the tephra bed. Pyroclast characteristics, if described and quantified, can represent an additional clue for the correct identification of the tephra. The paper presents morphological, textural and compositional data on the products of an ash eruption from Middle Age activity of Vesuvius, to demonstrate the information that can be extracted from the proposed type of analysis. Juvenile fragments from five ash layers throughout the studied products were randomly hand-picked and fully characterized in terms of external morphology, particle outline parameterization, groundmass texture and glass composition. Statistical analysis of shape parameters characterized groups of fragments that can be compared with the other textural and physical parameters. The main result is that the data do not show important cross-correlation so suggesting that all of these parameters, together with accurate field data are needed for the complete fingerprinting of a tephra bed. We suggest that this approach is especially important for characterizing the products of small scale, compositionally undistinguishable, eruptions and represents the necessary step to deal with before going into more detailed compositional analyses.

The volcanic environment for 40,000 years of human occupation on the Willaumez Isthmus, West New Britain, Papua New Guinea

The stratigraphic sequence of 22 tephra beds and their associated paleosols at two sites selected from our studies in West New Britain, Papua New Guinea provide the essential environmental background for understanding human responses to c. 40 ka years of volcanic activity. Lithology, grain size, and mineralogy of the tephras, interpreted for the first time, elucidate the varying volcanic provenance, especially of the Pleistocene beds, and the environmental conditions which former inhabitants experienced. This sequence provides an important long-term record of how humans coped with volcanic adversity in Papua New Guinea. The stratigraphic record contains abundant evidence for human settlement during the intervals between volcanic eruptions. Following plinian and subplinian eruptions, people abandoned the region for significant periods of time. In some cases, these cataclysmic events caused major cultural disasters characterised by local population extinction and loss of some significant types of cultural behaviour. Despite the punctuated record of settlement and changes in material culture, over the long-term, human populations found effective ways to maintain themselves within an active volcanic environment. Key responses included persistence of mobile and flexible settlement patterns, planned and staged uses of raw materials, social exchange, and increasing levels of landscape management.

Occurrence, stratigraphy and geochemistry of Late Quaternary tephra layers in the Aegean Sea and the Marmara Sea

Several discrete tephra layers occur in 2.5–10.3 m-long cores from the Aegean Sea, whereas a single tephra is identified in 3.5–9.7 m-long cores from the Marmara Sea. Major and trace-element data on these tephras delineate eight distinct groups. The geochemical data and multivariate statistics allow the correlation of the tephra layers with six previously described marine tephras from the eastern Mediterranean Sea; potential terrestrial sources are suggested for the others. The chronostratigraphy of the tephra layers was established using several radiocarbon dates and the oxygen isotope stratigraphy from two long piston cores. Four widespread tephra layers occur within the upper Quaternary sediments of the Aegean Sea. The two youngest widespread tephra layers were derived from the Santorini volcano of south-central Aegean Sea: Z2 (the Minoan eruption, ca. 3.6 ka) and Y2 (the Cape Riva eruption, ca. 21.7 ka). The Y2 tephra is also identified in three piston cores from the Marmara Sea. The two older widespread tephra layers originated from the eruptions which emplaced the Campanian Ignimbrite of the Italian Volcanic Provinces (i.e., the Y5 tephra, ca. 39.3 ka) and from the Nisyros volcano of the southeastern Aegean Sea. The consistent occurrence of the Nisyros tephra below the Y5 tephra confirms similar findings in a lake core on the Island of Lesbos. Linear extrapolations using the oxygen isotope stratigraphy suggest an age of 42–44 ka for the Nisyros eruption. Three additional tephra layers with a limited regional distribution are identified in the Aegean Sea cores. Two of these tephras originated from the Yali volcano of the southeastern Aegean Sea and from the Aeolian Islands (i.e., X1 tephra). A series of six mugearite tephra beds in the South Skiros Basin is interpreted as a set of redeposited turbidites.

Deep water gravity core from the Marsili Basin (Tyrrhenian Sea) records Pleistocenic–Holocenic explosive events and instability of the Aeolian Archipelago, (Italy)

A 4.8 m long gravity core was recovered on a relative topographic high in the northern part of the Marsili Basin (southern Tyrrhenian Sea) at a water depth of 3200 m. The core was taken in order to decipher the sedimentary record of the past volcanic events of the nearby Aeolian arc. A succession of thin (2 cm to 5 cm thick) fine-grained turbidites, mainly of volcaniclastic origin, topped by hemipelagic mud layers and a number of primary tephra layers were recovered by the core. The most prominent turbidite occurs in the lower part of the core at 385 cm. It consists of a 20 cm-thick, thinning-upward, pebble to sand-sized bed. Grain-size analysis and component compositions in the 0.063–0.250 mm size fractions were determined on thirty samples taken from primary tephra beds and the silty–sandy basal part of the volcaniclastic turbidite units. SEM scans and glass fraction chemical analyses were successively carried out on a selection of 17 samples. To aid source correlation and comparison, sub-aerial tephras of the Lower Pollara (Salina, 24 ± 3.6 ka), Gabellotto-Fiumebianco (Lipari, 8.5 or 11.5 ka), Monte Pilato (Lipari, 749 or 580 AD) and Secche di Lazzaro (Stromboli, ~ 5 ka) eruptions were also analyzed with the same procedure. Primary tephra respectively belonging to the eruptions of Lower Pollara, Gabellotto-Fiumebianco and Vesuvius (AP eruptions 3.5 ka–79 AD) were identified in the core at the expected relative stratigraphic depths. Two turbidite beds composed of monogenic glass shards were also identified and interpreted as the remobilisation of primary tephras of Secche di Lazzaro (Stromboli, 5 ka) and Pilato (Lipari, 580 or 749 AD). Tephrochronology results indicate that the cored sequence formed in the last 30 ka suggesting an average sedimentation rate of 0.15–0.17 mm/y. The thick pebbly sandy turbidite unit in the lower part of the core has component and glass composition compatible with the Lower Pollara volcanic sequence of Salina Island. In view of the grain-size and thickness of the turbidite unit, we suggest that it represents the deposit of a large failure event. The tephra corresponding to the Lower Pollara event lies below the turbidite unit, separated by 16 cm of hemipelagic mud, indicating that the collapse took place sometime after the eruption.

Two Middle Holocene Marker Beds in Vertically Accreted Floodplain Deposits, Lower Fraser River, British Columbia

Des carottes recueillies dans la partie est du delta du Fraser et de la plaine d'inondation adjacente montrent deux lits repères datant du milieu de l'Holocène, un lit de tourbe datant d'à peu près 6000 BP et un lit de tephra, correspondant à l'éruption du mont Mazama (6800 BP). Ces lits repères, ainsi que les datations au radiocarbone obtenues dans les sédiments de crue, montrent que la plaine d'inondation du Fraser a progressé en réponse à une hausse du niveau marin survenue entre environ 8000 et 2250 BP et que l'aggradation suivait la hausse du niveau marin. L'accumulation de sédiments, de forme triangulaire, est composée principalement de limons riches en matière organique et de sables fins d'origine alluviale. Le triangle s'étend jusqu'à au moins 20 km dans la vallée inférieure du Fraser. La conservation des lits repères démontre la grande stabilité des biefs du Fraser inférieur depuis environ 7000 ans.

An extensive middle to late Pleistocene tephrochronologic record from east-central Alaska

Chester Bluff (CB), in Yukon Charley Rivers National Preserve (YCNP), Alaska, contains an extensive middle to late Pleistocene tephra record within thick loess deposited on a terrace. The regionally prominent Old Crow (OC) tephra (140±10 ka), GI tephra (560±80 ka) of Fairbanks, the Preido Hill (PrH) tephra of the Klondike, and VT tephra (77.8±4.1 ka) of both the Klondike and Fairbanks were found at CB. The presence of these tephra beds will aid in correlating disparate paleoenvironmental records across eastern Beringia. There are 15 new tephra beds; 11 from the Wrangell volcanic field (WVF) and/or Hayes volcano, one from the Aleutian-Arc Alaska Peninsula (AAAP) and three unclassified beds. CB is normally magnetized, indicating the entire sequence is of Bruhnes age (<780 ka). Also present are at least two interglacials, one late–Middle Pleistocene, the other early–Middle Pleistocene. At least five more organic horizons representing interstadials or interglacials are present. Collectively, sediments at CB span most of the middle to late Pleistocene (i.e. ∼780 to <77.8±4.1 ka). The bluffs represent the most extensive middle-to-late Pleistocene sedimentary record yet established for Yukon or Alaska.

A widespread ca. 1.1 Ma TVZ silicic tephra preserved near Wellington, New Zealand: Implications for regional reconstruction of mid-Pleistocene vegetation

A sequence of terrestrial pollen-bearing sediments exposed in Transmission Gully (TG), near Wellington, New Zealand contains a prominent ca. 20-cm-thick silicic tephra bed dated at 1.09±0.12 Ma based on ITPFT dating of glass shards. Such occurrences are rare in the Wellington region. The associated palynology, overlying TG tephra, represents vegetation totally different from any modern assemblage with both warm climate taxa, e.g. Lygodium articulatum, Toronia toru, and cool climate taxa, e.g. Acaena, Apiaceae, Epilobium, Gentiana, Lycopodium australianum, Taraxacum, and Wahlenbergia, occurring together. A pollen assemblage 0.4 m above the base of TG tephra comes from a southern beech forest existing under warm temperate conditions, with both Fuscospora and Nothofagus menziesii beeches, and an abundance of ferns and lycopods represented. Beeches rapidly disappeared from around the site of deposition until at 0.6 m above the base of TG tephra beech pollen were rare and the pollen assemblages were dominated by grass. Ferns and lycopods were still well represented but the dramatic increase in grass pollen suggests that the environment outside the immediate depositional site had become much drier. A ca. 0.64-m-thick gravel layer was then deposited which impeded drainage. Associated pollen assemblages were then dominated by sedges and grasses in approximately equal percentages, although conditions may have still been relatively dry. Overall, the pollen assemblages are indicative of a sedge swamp surrounded by grassland with distant beech forest. Since the glass ITPFT-age range (±1 SD) for TG tephra extends over Oxygen Isotope Stages (OIS) 37 to 26, the fossiliferous sequence overlying the tephra should be any of the five periods of relative warmth, in spite of the lack of tree pollen and dominance in the profile of herbaceous taxa. However, geochemical comparison of TG tephra with those preserved in ODP site 1124, and two equivalent-age sequences in Auckland region (Patiki Core Site, Okariha) suggest a correlation with OIS 35. On this basis, the presence of Beaupreaidites diversiformis 0.4 m above the base of TG tephra provides a date for the final extinction of the modern New Caledonian genus Beauprea in New Zealand. Pollen from Toronia toru occurs throughout the sequence, far further south than in any other Pleistocene sequence so far analysed.

Changing ideas on the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America

Sheep Creek tephra (SCt) consists of inflated, rhyolitic pumice with abundant plagioclase and hornblende, lesser amounts of ferrian ilmenite and magnetite, and trace quantities of basaltic hornblende, augite, quartz, apatite, and zircon. It has an adakitic composition; the glass has low Y, Yb, and high La/Yb and Sr/Y values. Previously, compositional differences between samples were known to exist and thought to be due to derivation from a compositionally zoned magma involving several closely spaced eruptions. SCt was recognized as a useful stratigraphic marker across central Alaska and western Yukon with an age of ∼190 ka. New OSL dates on a SCt occurrence in western Yukon are ∼80 ka and force a reassessment of the earlier work. SCt is now known to be made up of several stratigraphic units: SCt-F (Fairbanks, Alaska; ∼200 ka), SCt-CC (Canyon Creek, Alaska), and in the western Yukon, SCt-C, SCt-K (∼80 ka), and SCt-A. Beds C, K, and A must be similar in age because of their close stratigraphic association at Ash Bend in Yukon. Compositional and temporal controls, including Sr isotopes, suggest Mount Drum in the Wrangell volcanic field as the source of the SCt beds. Revision of the age of the SCt beds in western Yukon means that the Reid Glaciation of Yukon is likely of MIS 6 age and probably correlates with the Delta Glaciation of Alaska. Further, the buried forest bed just below SCt-K in the Klondike goldfields must be of MIS 5 age, not MIS 7, as previously thought.

Mid-Pleistocene silicic tephra beds in the Auckland region, New Zealand: Their correlation and origins based on the trace element analyses of single glass shards

The trace element composition of individual glass shards from twelve distinct mid-Pleistocene silicic tephra beds from the Auckland region, New Zealand, has been determined by laser ablation (LA) ICP-MS analysis. This is the first application of such data to tephra correlation studies in New Zealand. These analyses support earlier major-element based correlations between three surface sections (Oruarangi and Okariha at Port Waikato, and St Kentigern's at east Auckland) and a long core (Patiki Road, west Auckland). The correlations provide a higher resolution record of Taupo Volcanic Zone (TVZ) activity than is preserved in more proximal deposits, where burial and erosion obscure much of the volcanic record. A combination of major and trace element analysis enables the correlation of three units exposed in the Auckland region with their proximal equivalents (ignimbrites) from the TVZ, namely deposits associated with (1) the Ongatiti ignimbrite (1.21±0.04 Ma), (2) an un-named, post-Ahuroa/pre-Unit E ignimbrite (sample UT1274) from the Rocky Hill section, King Country and (3) the Unit E ignimbrite (∼1 Ma). Combined major and trace element data show a marked evolution in magma chemistry, with the large Ongatiti and Unit E ignimbrite eruptions occurring after a change in composition caused by replenishment/reactivation of the magma chamber. Changes in magma chemistry between these deposits resulted from a combination of assimilation of metasedimentary country rocks and periods of fractional crystallisation of a plagioclase-dominated assemblage. Trace element data and the wider dispersal of the silicic tephras in the Auckland region suggests that they are all sourced from the Mangakino Volcanic Centre (MVC) in the TVZ. This study illustrates for the first time in a New Zealand context the potential for correlating tephra deposits using single glass shard trace element chemistry.

New glass fission-track ages of Middle Pleistocene tephras on Yakushima Island, southern Japan

The ages of two hornblende-bearing Middle Pleistocene rhyolitic tephra beds on Yakushima Island, southern Japan, have been determined by the glass-ITPFT method. Koseda pyroclastic flow deposit is 0.58±0.08 Ma and Anbo tephra (An) is 0.78±0.09 Ma. The normal geomagnetic polarity of An demonstrates a stratigraphic position just above the Bruhnes/Matuyama Boundary. Both volcanic units are the result of large-magnitude eruptions from a source that is most probably located at one of the calderas south of the central part of the Kagoshima graben, southern Kyushu, given the low total alkali content of their glass shards. They must have been widely distributed across Kyushu and central Japan, although no other occurrences are presently known. New information on their mineralogy, glass composition, and age will greatly facilitate recognition of their distal correlatives. Although not explored in this study, these two tephra beds will undoubtedly play a useful role in future studies aimed at elucidating the Quaternary paleoenvironmental history of Yakushima Island.

Stratigraphy of the Pliocene to Lower Pleistocene Marine Formations in Japan on the Basis of Tephra Beds Correlation

A number of studies on lithostratigraphy and marine microfossil biostratigraphy has been conducted on the representative Pliocene to Lower Pleistocene formations in Japan. In recent years, new widespread tephra beds connecting the Pliocene to Lower Pleistocene formations in various areas have been discovered. The present work introduces the latest stratigraphic studies on the Pliocene to Lower Pleistocene formations in various regions. Further, it examines the relationship between extensive tephra beds as contemporaneous marker horizons and biostratigraphy of calcareous nannofossils and diatom fossils.