Thin Volcanic Ash Research Articles

Temporal variations in the composition of Cretaceous Cordilleran arc volcanism through a high-frequency record of bentonites

Continental arc volcanism generates a wide diversity of magma compositions, but the tempos of compositional variation are unclear. Here, we investigate a 7-million year record of volcanic ash layers in the Cretaceous Eagle Ford Group, which was deposited in the Western Interior Seaway in western North America. Low oxygen conditions prevented significant bioturbation, allowing for the preservation of hundreds of thin volcanic ash layers in the form of bentonites. A drill core with an established high-resolution age model provides an opportunity to investigate temporal changes in ash composition on <100 kyr timescales. Because of intense alteration during diagenesis, ash protolith compositions were reconstructed from Ti/Zr. We first show that Ti and Zr had limited mobility during diagenesis. We then show, based on a compilation of data from unaltered volcanic rocks from six continental arcs, that the relationship between Ti/Zr and SiO2 is similar across continental arcs from different times and geographic locations, confirming that Ti/Zr is a robust differentiation index and indirect measure of SiO2. Applying an empirical Ti/Zr-SiO2 relationship to Ti/Zr measurements of 52 Eagle Ford bentonites allowed for reconstruction of ash protolith SiO2. Ash compositions vary from basalt/basaltic andesite to dacite/rhyolite, but the variations are not random. Ash compositions fluctuate between periods of high silica (>60 wt. % SiO2) and low silica (<60 wt. %) volcanism over ∼100 kyr timescales. If the temporal variability of these ashes represents broad snapshots of the Cordilleran continental arc, these results suggest that continental arc systems may undergo episodic changes in the extent of magmatic differentiation or the nature of eruption on rapid (<100 kyr) timescales.

Age and Paleoenvironmental Significance of the Frazer Beach Member—A New Lithostratigraphic Unit Overlying the End-Permian Extinction Horizon in the Sydney Basin, Australia

The newly defined Frazer Beach Member of the Moon Island Beach Formation is identified widely across the Sydney Basin in both outcrop and exploration wells. This thin unit was deposited immediately after extinction of the Glossopteris flora (defining the terrestrial end-Permian extinction event). The unit rests conformably on the uppermost Permian coal seam in most places. A distinctive granule-microbreccia bed is locally represented at the base of the member. The unit otherwise consists of dark gray to black siltstone, shale, mudstone and, locally, thin lenses of fine-grained sandstone and tuff. The member represents the topmost unit of the Newcastle Coal Measures and is overlain gradationally by the Dooralong Shale or with a scoured (disconformable) contact by coarse-grained sandstones to conglomerates of the Coal Cliff Sandstone, Munmorah Conglomerate and laterally equivalent units. The member is characterized by a palynological “dead zone” represented by a high proportion of degraded wood fragments, charcoal, amorphous organic matter and fungal spores. Abundant freshwater algal remains and the initial stages of a terrestrial vascular plant recovery flora are represented by low-diversity spore-pollen suites in the upper part of the unit in some areas. These assemblages are referable to the Playfordiaspora crenulata Palynozone interpreted as latest Permian in age on the basis of high precision Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry (CA-IDTIMS) dating of thin volcanic ash beds within and stratigraphically bracketing the unit. Plant macrofossils recovered from the upper Frazer Beach Member and immediately succeeding strata are dominated by Lepidopteris (Peltaspermaceae) and Voltziopsis (Voltziales) with subsidiary pleuromeian lycopsids, sphenophytes, and ferns. Sparse vertebrate and invertebrate ichnofossils are also represented in the Frazer Beach Member or in beds immediately overlying this unit. The Frazer Beach Member is correlative, in part, with a thin interval of organic-rich mudrocks, commonly known as the “marker mudstone” capping the Permian succession further to the north in the Bowen, Galilee and Cooper basins. The broad geographic distribution of this generally &amp;lt;5-m-thick mudrock unit highlights the development in eastern Gondwana of extensive, short-lived, shallow lacustrine systems with impoverished biotas in alluvial plain settings in the immediate aftermath of the end-Permian biotic crisis.

Analysis of properties of the 19 February 2018 volcanic eruption of Mount Sinabung in S5P/TROPOMI and Himawari-8 satellite data

Abstract. This study presents an analysis of TROPOMI cloud heights as a proxy for volcanic plume heights in the presence of absorbing aerosols and sulfur dioxide for the 19 February 2018 eruption plume of the Sinabung volcano on Sumatra, Indonesia. Comparison with CALIPSO satellite data shows that all three TROPOMI cloud height data products based on oxygen absorption which are considered here (FRESCO, ROCINN, O22CLD) provide volcanic ash cloud heights comparable to heights measured by CALIPSO for optically thick volcanic ash clouds. FRESCO and ROCINN heights are very similar, with the only differences for FRESCO cloud top heights above 14 km altitude. O22CLD cloud top heights unsurprisingly fall below those of FRESCO and ROCINN, as the O22CLD retrieval is less sensitive to cloud top heights above 10 km altitude. For optically thin volcanic ash clouds, i.e., when Earth's surface or clouds at lower altitudes shine through the volcanic ash cloud, retrieved heights fall below the volcanic ash cloud heights derived from CALIPSO data. Evaluation of corresponding Himawari-8 geostationary infrared (IR) brightness temperature differences (ΔBTs) – a signature for detection of volcanic ash clouds in geostationary satellite data and widely used as input for quantitative volcanic ash cloud retrievals – reveals that for this particular eruption the ΔBT volcanic ash signature changes to a ΔBT ice crystal signature for the part of the ash plume reaching the upper troposphere beyond 10 km altitude several hours after the start of the eruption and which TROPOMI clearly characterizes as volcanic (SO2 &gt; 1 DU – Dobson units – and AAI &gt; 4 – absorbing aerosol index – or, more conservatively, SO2 &gt; 10). The presence of ice in volcanic ash clouds is known to prevent the detection of volcanic ash clouds based on broadband geostationary satellite data. TROPOMI does not suffer from this effect and can provide valuable and accurate information about volcanic ash clouds and ash top heights in cases where commonly used geostationary IR measurements of volcanic ash clouds fail.

Retrieval of the Eyjafjallajökull volcanic aerosol optical and microphysical properties from POLDER/PARASOL measurements

Abstract. Total and polarized radiances provided by the Polarization and Directionality of Earth Reflectances (POLDER) satellite sensor are used to retrieve the microphysical and optical properties of the volcanic plume observed during the Eyjafjallajökull volcano eruption in 2010, over cloud-free and cloudy ocean scenes. We selected two plume conditions, fresh aerosols near the sources (three cases) and a downwind volcanic plume observed over the North Sea 30 h after its injection into the atmosphere (aged aerosols). In the near-source conditions, the aerosol properties depend on the distance to the plume. Within the near-source plume, aerosols are mainly non-spherical and in the coarse mode with an effective radius equal to 1.75 (±0.25) μm and an Ångström exponent (AE) close to 0.0. Far from the plume, in addition to the coarse mode, there are particles retrieved in the accumulation mode, suggesting a mixture of sulfate aerosols and volcanic dust, resulting in an AE around 0.8. The properties of the aerosols also depend on whether the plume is fresh or aged. For the downwind (aged) plume, if non-spherical coarse particles as well as some fine mode particles are retrieved, the AE is higher, around ~0.4. In addition, rather low values for the real part of the refractive index (RR) were retrieved for the fresh plume (1.38 &lt; RR &lt; 1.48). Single scattering albedo (SSA) values ranging between 0.92 and 0.98 were retrieved over some parts of the near-source plume; despite the low accuracy of our retrievals, the derived SSA values suggest that the ash particles are rather absorbing. To consider the particle shape, a combination of spheroid models was used. Although the employed model enabled accurate modeling of the POLDER signal in the case of non-spherical ash, our approach failed to model the signal over the optically thickest parts of the near-source plume. The most probable reason for this is the presence of ice crystals within the plume. For the aerosol above clouds (AAC) scenes, polarized measurements allowed the retrieval of the optical thickness (OT) and the AE of optically thin volcanic ash. We found that all the cloud parameters retrieved by passive sensors were biased due to the presence of the elevated volcanic plumes. Finally, thermal infrared measurements were used to identify the type of multilayer scene (cirrus clouds or volcanic dust above liquid clouds) and the retrieval method also provided the OT of thin cirrus layers above the clouds near Iceland.

U-Pb (SHRIMP II) age of zircons from ash beds of the chernokamen formation, Vendian Sylvitsa group (Central Urals)

The Serebryanka and Sylvitsa groups on the western slope of the Urals characterize the Vendian section of the eastern folded framework of the East European Platform [1]. They are conspicuous against the background of the coeval platformal sequences by their relatively complete sedimentary record, sufficiently good outcrops, and opportunities for detailed sedimentological study both along and across the regional paleoslope. The tillite units serve as key elements of the stratigraphic framework of the Serebryanka Group. However, their number and continuity along the strike remain a matter of debate [1, 2]. The Sylvitsa Group comprises the following formations recognized on a regional scale (from bottom to top): Staropechny Formation (with a thin tillite unit at the base), Perevalok, Chernokamen, and Ust’-Sylvitsa formations. Their specific structural features serve as criteria for largescale correlation with Upper Vendian platformal sequences [1‐3]. The Ediacaran-type soft-bodied fossil biota is abundant in the Chernokamen Formation. The great density of fossil populations, the high taxonomic diversity of this biota, and the presence of endemic (for the Southeast White Sea region) taxons give grounds to suggest biogeographic links between the Central Ural and White Sea segments of the Late Vendian paleobasin [4]. The fossil biota from sedimentary rocks of the Chernokamen Formation comprises Cyclomedusa davidi Sprigg, Dickinsonia sp., Dickinsonia tenuis Glaessner et Wade, Ediacaria flindersi Sprigg, Eoporpita medusa Wade, Inaria khatyspytia (Vodanjuk), Irridinitus multiradiatus Fedonkin, Medusinites mawsoni (Sprigg), Nemiana simplex Palij, Paliella patelliformis Fedonkin, Palaeopascichnus delicatus Palij, Protodipleurosoma wardi Sprigg, Vaizitsinia sophia Sokolov et Fedonkin, and Yorgia sp. These data make it possible to correlate the Chernokamen Formation of the Central Urals with the reference section of the southeastern White Sea region. The Upper Vendian of the White Sea region includes a very characteristic taxon Yorgia , which appears for the first time in the Zimnegorsk Formation. However, insufficiently reliable stratigraphic and facies affiliations of the Sylvitsa Group and the absence of isotopic datings remained serious obstacles for such correlation until recently. In 2002, we found thin volcanic ash beds in the upper part of the Staropechny Formation. These beds are associated with the chocolate brown thin-bedded mudstones that crop out on the left bank of the Sylvitsa River downstream of the Kernos Creek. In 2003, volcanic ash beds, which are also closely related to the chocolate brown mudstones, were revealed in the Staropechny and Perevalok formations and in the lower subformation of the Chernokamen Formation in the Vilukha ravine on the left and right banks of the Us’va River. In 2004, ash beds were traced for 4.5 km in the lower subformation of the Chernokamen Formation along the Mezhevaya Utka River. In all the above cases, ash beds were readily discernible as a poorly cemented light gray (with pinkish, cream, and greenish hues) rock among the host chocolate brown mudstones. Members of chocolate brown mudstones with volcanic ash beds are important for Upper Vendian stratigraphy of the East European Platform. The tuffaceous rocks serve as key units for subdivision and correlation of outcrops and borehole sections [2, 3, 5, 6]. The most complete Upper Vendian platformal sections of the Mezen syneclise and the southeastern White Sea region

Lithology, Biostratigraphy, and Magneto stratigraphy of Gas Hydrate‐Bearing Sediments in the Eastern Nankai Trough

Abstract: Stratigraphic controls on the formation and distribution of gas hydrates were examined for sediments from a BH‐1 well drilled in the landward slope of the Nankai Trough, approximately 60 km off Omaezaki, Japan. Three lithologic units were recognized in the 250 m‐thick sequence of sediments: Unit 1 (0–70 mbsf) consists of calcareous silt and clay with thin volcanic ash layers, Unit 2 (70–150 mbsf) consists of calcareous silt and clay with volcanic ash and thin sand layers, and Unit 3 (150–250 mbsf) consists of weakly consolidated calcareous silt and clay with thick and frequent sand layers. Soupy structures and gas bubbles in the sediments indicate the presence of two hydrate zones between 40 and 130 mbsf and below 195 mbsf. Nannofossil biostratigraphy and magnetostratigraphy indicate that the sequence recovered at the BH‐1 well is mostly continuous and represents sediments deposited from 0 to 1.5 Ma. Calculation of the sedimentation rate reveals a condensed section between 65 and 90 mbsf. The inferred distribution of gas hydrates in the BH‐1 well appears to be strongly controlled by the stratigraphy and lithology of the sediments. Thick, gently inclined sand layers in Unit 3 provide a conduit for the migration of gases from deeper regions, and are considered responsible for the formation of the hydrate zone below 195 mbsf. At shallower levels, thin, gently inclined sand layers are also considered to allow for the migration of gases, leading to the formation of the upper hydrate zone between 40 and 130 mbsf. The overlying sub‐horizontal silt and clay of the condensed section, truncating the underlying gently inclined sand and silt/clay layers, may provide an effective trap for gases supplied through the sand layers, further contributing to hydrate formation in the upper hydrate zone.

Tuffaceous deposition in the Newcastle Coal Measures: challenging existing concepts of peat formation in the Sydney Basin, New South Wales, Australia

The Late Permian Newcastle Coal Measures of the northern Sydney Basin, Australia, contain numerous volcanic ash deposits occurring as both interseam tuffs and intraseam tonsteins. These tuffaceous units have been used to correlate seams of the Newcastle Coal Measures with the stratigraphically equivalent Wollombi Coal Measures in the adjacent Hunter Coalfield. Such correlations across the width of the northern Sydney Basin are based not on only tuffaceous markers, but also on chemical and maceral profiles within individual coal plies and carbonaceous shales. These correlations imply a lateral continuity of the peat-forming environment previously unrecognised in the Sydney Basin, and have led directly to the discovery of a new open-cut coal resource in the Hunter Coalfield. The uniform distribution of tonsteins and coal plies also challenges many existing concepts of peat formation. It is proposed that the peat surface was predominantly below the water table, and the term “lowered mire” is proposed to describe this environment. A subaqueous peat surface would protect thin volcanic ash deposits from subsequent redistribution by rainfall and surface runoff, and is consistent with a lack of tree preservation within the intraseam tonsteins. It is envisaged that trees were generally restricted to the peat margins, and to specific horizons where the peat surface was exposed by a fall in the water table. Long-distance correlations of tonsteins also contradict models of coal formation incorporating lateral peat migration, where time constant horizons such as tonsteins should transgress the coal seam profile over large distances.

Late‐ to post‐orogenic basins of the Pan‐African – Brasiliano collision orogen in southern Africa and southern Brazil

Late‐ to post‐orogenic basins formed on both sides of the Pan‐African – Brasiliano orogen when the Congo and Kalahari Cratons collided with the Rio de la Plata Craton during the formation of western Gondwana. Trace fossil evidence and radiometric age dating indicate that deposits on both sides are coeval and span the Cambrian–Precambrian boundary.A peripheral foreland basin, the Nama Basin, developed on the subducting southern African plate. Lower, craton‐derived fluviomarine clastics are overlain by marine platform carbonates and deltaic flysch derived in part from the rising subduction complex along the northern (Damara Belt) and western (Gariep Belt) orogenic margins. Rare, thin volcanic ash layers (tuffs and cherts) are present. Upper sediments consist of unconformable red molasse related to collisional orogenesis. Orogenic loading from the north and west led to crustal flexure and the formation of a remnant ocean that drained to the south and closed progressively from north to south. During final collision SE‐, E‐ and NE‐verging nappes overrode the active basin margins. Although younger than most of the post‐orogenic magmatism, its setting on the cratonic edge of the subducting plate precluded marked volcanism or granitic intrusion, the only exception being the youngest intrusions of the Kuboos‐Bremen Suite dated at 521±6 Ma to 491±8 Ma.Two foreland‐type basins, perhaps faulted remnants of a much larger NE–SW elongated retroarc foreland basin, are found west of the Dom Feliciano Belt on the edge of the Rio de la Plata Craton in southern Brazil. In the southern Camaqua Basin, basal fluvial deposits are followed by cyclical marine and coarsening‐up deltaic deposits with a notable volcanic and volcaniclastic component. This lower deformed succession, comprising mainly red beds, contain stratabound Cu and Pb–Zn deposits and is overlain unconformably by a fluviodeltaic to aeolian succession of sandstones and conglomerates (with minor andesitic volcanics), derived primarily from an eastern orogenic source and showing southerly longitudinal transport. In the northern Itajaí Basin, sediments range from basal fluvial and platform sediments to fining‐up submarine fan and turbidite deposits with intercalated acid tuffs. The Brazilian basins had faulted margins off which alluvial fans were shed. They also overlie parts of the Ribeira Belt. Thrust deformation along the orogenic margin bordering the Dom Feliciano Belt was directed westward in the Camaqua and Itajaí basins, but reactivated strike‐slip and normal faults are also present. Late‐ to post‐orogenic granitoids and volcanics of the Dom Feliciano Belt, ranging in age from 568±6 Ma to 529±4 Ma, occur in the foreland basins and are geochemically related to some of the synsedimentary volcanics.

Progressive glaciotectonic deformation in Weichselian and Palaeogene deposits at Feggeklit, northern Denmark

Structural analysis of the glaciotectonic deformations at Feggeklit, Mors, Denmark, provide a unique record of succesive deformation phases in a progressive glaciotectonic deformation. The Feggeklit profile displays glaciotectonically folded, thrust-faulted and sheared Palaeogene diatomite with thin volcanic ash layers, the Fur Formation, overlain by a glacigene succession. The combination of stratigraphical and structural analysis shows that the Feggeklit was affected by three glaciodynamic events. The first event is of Saalian age and is represented by the deposition of a till and the formation of a para-authochthonous glacitectonite in the top of the Fur Formation deposits. The second event is only represented by the deposition of a till, probably of Saalian age. The third event is of Late Weichselian age. It includes: 1) deposition of proglacial glaciolacustrine and -fluvial sediments, 2) the formation of a glaciotectonic unit (the Feggeklit deformation complex) and 3) deposition of a till resting on a tectonic uncon-formity formed subglacially. A detailed structural analysis of the glaciotectonic unit provides a subdivision into five succesive deformation phases. The first four phases are related to the proglacial deformation and comprise 1) anastamosing jointing, 2) conjugate faulting, 3) buckle folding and listric thrust faulting, and 4) large scale ramp thrusting. The final phase (5) is related to subglacial shear deformation and loading which produced an allochthonous diatomiteglacitectonite at the sole of the overlying lodgement till. The formation of the structural complex at Feggeklit was caused by two glaciotectonic mechanisms: 1) a proglacial gravity spreading deformation, and 2) a subglacial cataclastic shearing. The balanced cross-section of the fold structures related to the first deformation mechanism indicates that the detachment of the dislocation is situated below the base of the diatomite formation in the plastic clay at a depth of 80-100 m below the surface. Based on the glaciodynamic analysis and considerations on the dating of regional glacigenic setting the velocity of the advancing ice is estimated at 10 m per year. This advance created the gravity spreading deformation reflected in the glaciotectonic structures preserved in the Feggeklit.

Origin and palaeogeographical distribution of the Viruan iron and phosphate ooids in Estonia: evidence from mineralogical and chemical compositions

The occurrence of iron ooid beds in the Ordovician sequence of Baltoscandia has been known for a long time. In Estonia, the best developed ooid beds are known to occur at two levels in the Lower (Oelandian) to Middle Ordovician (Viruan) boundary interval, being restricted to the Kunda and Aseri-Lasnamägi Stages, respectively. According to outcrop and subsurface data, these ooid beds can be traced from Hiiumaa island, Estonia to the St. Petersburg area, Russia, in the east. The geographic and stratigraphic distribution of the Viruan oolite body indicates that the source material was derived from a land area located northwest to north-northwest of Tallinn. The presence of well developed hardgrounds at the base of Viruan oolite suggests that ooid formation took place during a deepening event. Two mineralogically different ooid groups have been identified in this study—brown ooids, composed of iron oxy-hydroxides and apatite, and light-grey ooids, composed primarily of carbonate-fluorapatite. The latter probably formed through early diagenetic alteration of the brown ooids. Brown ooids contain over 60% of iron oxy-hydroxide and remnants of an unidentified silicate mineral. The concentric lamination is composed of alternating laminae of phosphate (francolite) and iron oxy-hydroxide. The trace element abundances in the ooids are strongly influenced by the phosphate content, as is the observed REE distribution pattern. The chondrite-normalized REE patterns are characterized by enrichment of the lighter REE, and have a typical negative Eu-anomaly of about 0.7. Shale-normalized REE patterns have a “hat shape” similar to REE patterns found in conodonts and other biogenic apatite. Thin volcanic ash beds have been recorded at this level in Sweden. In Estonia, however, there are no visible indications of volcanic material.

愛知県町田・松河戸遺跡から発見された縄文時代後・晩期の境界付近に位置する火山灰層について

In 1988, a thin volcanic ash layer was found in the peat layer from nine survey areas of the Choda and Matsukawado archaeological sites, Kasugai City, Aichi prefecture.This layer, about 4mm thick, is composed of light reddish brown to pinkish white and fine to medium-grained ash. It consists chiefly of glass (mainly C-type shards with refractive index of 1.499-1.504; mode 1.501-1.502), which small amounts of plagioclase, orthopyroxene (γ=1.703-1.705), amphibole, biotite and clinopyroxene.The 14C age of the peat layer from the horizon immediately beneath the ash layer is 3, 120±120y.B.P.; this ash layer was found above a bed containing late Jomon potsherds and below a bed containing final Jomon potsherds. So it can be said that this volcanic ash fell around the fine of the boundary between the late and final Jomon Period.This is probably the first time in the lowland Kinki and Chubu district that the ash layer of this period has been recognized as a visible stratum.The authors named the ash the “Matsukawado Volcanic Ash Layer” (MT) after the layer was discovered.

秋田県男鹿半島で発見された姶良Ｔｎ火山灰

A thin volcanic ash intercalation was found in a 0.2 to 1.4m thick peat layer in the basal part of the sedimentary layer, provisionally named the Hakoi Formation, unconformably overlying the Katanishi Formation; this formation yielded warm, near-shore marine and brackish water fauna and flora and has been correlated with the Last Interglaciation in the glaciated regions. The refractive index of glass shards of the ash (1.499-1.501) coincides with the Aira-Tn ash (AT), and the glass has almost the same major element composition as the AT detected in Dekizima, western Aomori Prefecture. The carbon-14 age of fossil wood from a horizon slightly above the ash is 15, 470±620y.B.P. (I-14, 646). These characteristics indicate that the ash is Aira-Tn ash, ejected from the Aira Caldera, southern Kyushu, the most prominent marker tephra of the Late Pleistocene in Japan. The pollen assemblage of the peat indicates a cold climate during the time of deposition of the ash.

北海道中央高地，大雪山における高地湿原の起源とその植生変遷 ＩＩ

Two different types of upland bogs in the Daisetsu Mountains, central Hokkaido, were studied in order to examine their origin and vegetational succession.The Numano-hara bog (1, 424m, a.s.l.) which is located on a lava plateau in the southeastern part of the Mountains, formed around 4, 000y.B.P. in a cool and moist climate. The result of pollen analysis suggests the following 3 pollen assemblage zones. During the period of Zone Nu-III (ca. 4, 000-2, 000y.B.P.), Betula ermanii was most dominant. Zone Nu-II (ca. 2, 000-400y.B.P.) is characterized by the decrease of Betula ermanii and the dominance of Picea glehnii as a rerult of the expansion of the bog in a cooler and moister climate. Also, a needle-leaved forest composed of Picea glehnii and/or Picea jezoensis and Abies sacharinensis spread to the lower areas. Finally, the bog decreased in size under a slightly drier condition, and Betula ermanii invaded again at the period of Zone Nu-I (ca. 400y.B.P.-present).The bogs around the Midorino-numa and the Basho-numa (1, 365m, a.s.l.) located near the eastern margin of Takanega-hara in the central part of the Mountains were observed in some places around the depressions formed by landslide. According to tephrochronology of 5 thin volcanic ash layers, the beginning of the Midorino-numa bog is a little prior to 1, 300y.B.P., whereas the Basho-numa bog is considered to have originated between 1, 300 and 5, 000y.B.P. Although Betula ermanii invaded into the landslide area in an early stage, Picea glehnii increased after the bog formed (Zone Mi-II). At about 1, 300y.B.P. the forest around the bog reached the climax Picea glehnii forest just like the present one (Zone Mi-I).Four other upland bogs developed in Hokkaido are classified into 2 types in the same way as the bogs in the Daisetsu Mountains. The Pankenai bog in the Teshio Mountains and the Ukishima bog in the Takinoue Plateau on flat plains like Numanohara formed in a cool and moist climate (Zone Nu-II). On the other hand, the bogs developed around the depression by landslide such as the Midorino-numa and the Basho-numa belong to different ages. The age is Orochiga-hara bog on Mt. Muine is estimated to be approximately 6, 000y.B.P. and the age of a small bog located on the southeastern slope of Mt. Onnebetsu-dake is approximately 300y.B.P.. Therefore, their difference in age probably depends on the formation of the depressions.

北海道中央高地，大雪山における高地湿原の起源とその植生変遷 Ｉ

Two different types of upland bogs in the Daisetsu Mountains, central Hokkaido, were studied in order to examine their origin and vegetational succession.The Numano-hara bog (1, 424m, a.s.l.) which is located on a lava plateau in the southeastern part of the Mountains, formed around 4, 000y.B.P. in a cool and moist climate. The result of pollen analysis suggests the following 3 pollen assemblage zones. During the period of Zone Nu-III (ca. 4, 000-2, 000y.B.P.), Betula ermanii was most dominant. Zone Nu-II (ca. 2, 000-400y.B.P.) is characterized by the decrease of Betula ermanii and the dominance of Picea glehnii as a rerult of the expansion of the bog in a cooler and moister climate. Also, a needle-leaved forest composed of Picea glehnii and/or Picea jezoensis and Abies sacharinensis spread to the lower areas. Finally, the bog decreased in size under a slightly drier condition, and Betula ermanii invaded again at the period of Zone Nu-I (ca. 400y.B.P.-present).The bogs around the Midorino-numa and the Basho-numa (1, 365m, a.s.l.) located near the eastern margin of Takanega-hara in the central part of the Mountains were observed in some places around the depressions formed by landslide. According to tephrochronology of 5 thin volcanic ash layers, the beginning of the Midorino-numa bog is a little prior to 1, 300y.B.P., whereas the Basho-numa bog is considered to have originated between 1, 300 and 5, 000y.B.P. Although Betula ermanii invaded into the landslide area in an early stage, Picea glehnii increased after the bog formed (Zone Mi-II). At about 1, 300y.B.P. the forest around the bog reached the climax Picea glehnii forest just like the present one (Zone Mi-I).Four other upland bogs developed in Hokkaido are classified into 2 types in the same way as the bogs in the Daisetsu Mountains. The Pankenai bog in the Teshio Mountains and the Ukishima bog in the Takinoue Plateau on flat plains like Numanohara formed in a cool and moist climate (Zone Nu-II). On the other hand, the bogs developed around the depression by landslide such as the Midorino-numa and the Basho-numa belong to different ages. The age is Orochiga-hara bog on Mt. Muine is estimated to be approximately 6, 000y.B.P. and the age of a small bog located on the southeastern slope of Mt. Onnebetsu-dake is approximately 300y.B.P.. Therefore, their difference in age probably depends on the formation of the depressions.

A summary of Deep Sea Drilling Project Leg 67 shipboard results from the Mid-America Trench transect off Guatemala

Summary The Middle America Trench off Guatemala was transected by 24-channel seismic reflection surveys, seismic-refraction surveys, and drilling with the Glomar Challenger . The drilling was done at three sites on the oceanic Cocos plate and four sites on the Caribbean plate. These plates converge at about 10 cm yr −1 . At all drill sites sediment of upper Miocene to Quaternary age is almost entirely hemipelagic mud with interbedded thin volcanic ash, except in the trench where mud and fine sand turbidites less than 400 000 yr old are ponded. However, the underlying rocks are very different. On the oceanic Cocos plate a basal chalk sequence of lower and middle Miocene age is overlain by a thin section of abyssal clay. At a site only 3 km landward of the trench axis where drilling penetrated the slope deposits we recovered a Cretaceous to lower Miocene claystone sequence resting on a section containing igneous rock of continental affinity. A large net subduction of sediment along with ocean crust has occurred during the present (Miocene-Quaternary) episode of subduction and perhaps parts of the continental framework have been subducted as well. However, no current model satisfactorily explains the surprising occurrence of Cretaceous-Miocene claystone at the foot of the trench slope.

Discrimination of the Volcanic Ash Layers by Means of the Features of Volcanic Glass, with Special Reference to the Difference of the Titanium Contents in the Volcanic Glass of the Plio-Pleistocene Osaka Group, Japan

The correctly identified pyroclastic layers are the best keys for the geologic chronology. The Plio-Pleistocene Osaka and Kobiwako groups are good sediments for the tephrochronology because of the high frequency of intercalations with many thin volcanic ash layers.The foundation of the tephrochronology is correct identification of tephra. Especially, the features of the volcanic glass are very important for identification, since main volcanic ash of the Osaka and Kobiwako groups consists chiefly of volcanic glass. The writer tried to determine the titanium content of volcanic glass by polarogram, in order to identify some tephra.In this paper, the method of polarographic determination of titanium used in cast iron with good accuracy, reported by Fukui & Arita (1962), was applied to the volcanic glass in a little modification. The sample was dissolved by heating in sulfuric acid-fluoric acid mixture. The fluoric acid was driven off by heating. Then, Fe3+ was reduced to Fe2+ by addition of metallic aluminum. The solution was treated with oxalic acid and the determination of titanium was carried out by recording a polarogram obtained in the range of 0--0.8V.The results are shown in Table 4. The individual volcanic ash layer has one's own titanium content. It may be sure that the determination of various metalic elements in the volcanic glass is a good method for identification of tephra.