Pumice Layer Research Articles

Rainfall infiltration process on steep slopes mantled with a pumice layer

Rainfall infiltration process on steep slopes mantled with a pumice layer

The age and stratigraphic significance of sea-rafted Loisels Pumice in northern New Zealand

Abstract Abstract Loisels Pumice is a sea-rafted pumice of uncertain origin, found on the east coast of the North Island of New Zealand. Since a radiocarbon age range was compiled from different and indirectly dated pumice layers by McFadgen (1982), Loisels Pumice has been used as a stratigraphic marker layer of known age in late Holocene coastal deposits. We contend that the date of primary deposition has been incorrectly determined and applied. We examined field sections from the far north of the North Island to identify and date the primary deposition of Loisels Pumice and to describe the physical characteristics and value of such a deposit. We also critically examined sites identified in the literature. Only one deposit was potentially primary, and the age of pumice deposition could be closely bracketed. From this section, we propose an age range of 915–1030 cal AD for the first stranding of Loisels Pumice in northern New Zealand. Analysis of sections shows that stratigraphic and geomorphic interpretation may be used to identify sites with the potential to be primary, but primary and secondary deposits cannot be identified by descriptive statistics. We conclude that the age of first stranding of Loisels Pumice cannot be reliably derived by compiling an age range bracket using maximum and minimum dates from different sections. Loisels Pumice should not be used as a coastal marker layer to establish synchrony between sites, and recent interpretations based on this method should be re-appraised. Keywords:: coastal landformspumice layersmarker layersradiocarbon datinggeomorphologyarchaeologyHolocenedeposition

中国・四国地方の第四紀テフラ研究 広域テフラを中心として

A tephrochronological study has been carried out in the Chugoku and Shikoku Districts, paying special attentions to the widespread marker tephras from Mts. Sanbe and Daisen, as well as tephras from volcanoes in Kyushu. Chemical analysis of the major element composition of glass shards and/or magnetite grains collected from each tephra layer, using the electron microprobe technique, has been quite useful in identifying the individual tephra. The stratigraphy, chronology, and distribution of tephras, and their implications for Late Quaternary events are summarized as follows: (1) Aira Tn (AT; erupted about 25, 000BP), and Kikai Akahoya (K-Ah; erupted about 6, 300BP) ashes, of Kyushu origin, occur as the most distinctive markers. Both are easily identified by the major element composition of their glass shards. The Aso-4 ash is another maker tephra of Kyushu origin, distinguishable by the major element composition of its magnetite. (2) Of the many tephras erupted from Sanbe volcano, the Sanbe Kisuki (SKP), Unnan (SUn), and Ikeda (SIP) pumice layers are the most extensive and representative. The SUn and SIP pumice layers are identified in a marine core taken in the Japan Sea to the north of Tottori, 200km away from Mt. Sanbe. (3) Of the Daisen tephras, the Matsue (DMP), Kurayoshi (DKP) and Kusadanihara (KsP) pumice layers occur most extensively. KsP, of ca 18ka, was recognized in a marine core from the Japan Sea to the west of Sakata City, 600km away from Mt. Daisen. This correlates with the San'in 2 ash. (4) The followings are evident from the relationship of tephrochronology and geomorphic development in the Chugoku and Shikoku Districts: maximum sea level regression in the Late Pleistocene had occurred before the deposition of AT ash in the Izumo Plain; there were three stages of dune sand formation in the San'in District: the first was prior to the deposition of DMP, the second was between the eruptions of DMP and Aso-4, and the third was after the deposition of K-Ah; (5) Some of the unidentified tephras were examined and showed that an ash layer sampled from Pleistocene gravels near Ozu City, Ehime Pref., and one from the lowermost part of the Oetakayama pyroclastic formation in the Tsunozu Group are similar to the Guminoki ash and Yellow III of the Osaka Group respectively, from the viewpoint of the major element composition of glass shards or glassy matrix.

Failure Mechanism of Landslide with “squeezing-out” deformation

The Nebukawa-station Landslide was triggered by the Kanto earthquake in 1923. Both geological and geotechnical studies point out that the landslide failed rapidly by shearing (compression failure) in the weak pumice layer which was accompanied with the unique deformation named “squeezing-out” that is, the upper layer, mainly massive lava flow, subsided and squeezed out the underlying pumice layer to the front of slide, and the slope collapsed successively.The elastic-plastic FEM analyses both on the landslide and the simplified three-layers model indicate that the “squeezing-out” is one of the common deformation type on slope which consists of thin weak intertrappean layer (e. g. pumice) and gentle dipping (less than 10 degree) thick rigid layers (e. g. lava).Several geologic evidences support the idea that the landslide with the “squeezing-out” deformation occurs in dissected slope of stratovolcano, and sometimes leads to the run-off of debris, high speed and long distance; these will then provide basic information useful for disaster prevention planning.

夕張山地南部・日高山脈西部における最終氷期から完新世初頭にかけての斜面安定性の垂直的変遷

This study reconstructed the vertical change of slope stability in the southern part of the Yubari Mountains and the western part of the Hidaka Range, Hokkaido, Japan, by using three eolian pumice layers deposited during the Last Glacial and early Holocene; Spfal, 33, 000-32, 000y.B.P.; En-a, 19, 000-16, 000y.B.P.; Ta-d, about 8, 000y.B.P. A pumice layer overlying a mountain slope indicates that the slope has been stabilized since the pumice fell. In this view, the examination of each pumice layer in about 330 outcrops on the mountain slope suggests a belt of slope stability in the study area, as follows:Spfal; crest slope lower than about 250m high.En-a; crest slope lower than about 500m and side slope lower than about 300m high.Ta-d; crest-side slope lower than about 1, 250m high.On the other hand, the lack of a pumice layer and/or the absence of a pumice layer which is usually incorporated in the colluvium suggest slope instability after the fall of the pumice. Several outcrops on the mountain slope suggest that the removal of the pumice layer was mainly caused by periglacial slope processes, associated with the frost shattering of the underlying bedrock. The belt higher than the belt of slope stability, in which each pumice layer was removed, therefore, corresponds to that of the periglacial belt in the period after the fall of each pumice layer. According to this view, the lower limit of the periglacial belt rose from about 250m during the 33, 000-18, 000y.B.P., to about 500m during 18, 000-8, 000y.B.P., to about 1, 250m after 8, 000y.B.P. This vertical change in slope stability is parallel to that of the timber-line and snowline of the study area which has been indicated by other data.

宮城県中部および北部に分布する後期更新世広域テフラとその層位

Widespread Tephra is a valuable time marker for tephrochronology and archaeology. Several fine ash fall deposit are distributed in central and northern part of Miyagi Pref. The authors have correlated them to widespread tephras by means of following methods. They are lithological description of tephras, measurements of refractive indices of glass shards and heavy minerals, and analyses of major elements chemical composition of glass shards using a microprobe analyzer. As a results, four late Pleistocene widespr ead tephras are discovered in this study area. They are AT, Aso-4, On-PmI and Toya. The authors described the stratigraphic positions of those widespread tephras in detail. And furthermore, they mentioned the significance that four late Pleistocene widespread tephras were discovered in this study area. The results are summarized as follows.1) In central part of Miyagi Pref., the stratigrafic sequence of AT ash, Kawasaki scoria layer, Aso-4 ash and Medeshima pumice layer occur in ascending order is confirmed. Kawasaki scoria and Medeshima Pumice are valuable marker tephras in that region.In northern part of Miyagi Pref., 10 tephras or tephra formations and their stratigraphic positions are recognized. They are, in ascending order, Hijiori pumice layer, Narugo·Katanuma-Uehara tephra, AT ash, Narugo-Yanagisawa tephra layer, Aso-4, Narugo-Nisaka tephra layer, Kitahara ash layer, On-PmI, Toya ash and Ichihasama pumice layer. Consequently, the late Pleistocene tephra stratigraphy in Miyagi Prefecture is linked with those in central and southwestern part of Japan.2) The stratigraphic relation between On-PmI and Toya ash is revealed for the first time to implicate the occurrence of marine terrace developed in ca 100 ka in a tectonically active region.

６世紀における榛名火山の２回の噴火とその災害

Haruna volcano, situated in the central part of Japan, erupted twice in the 6th century, from Futatsu-dake crater. The first eruption, which occurred in the early part of the 6th century, was set set off by a low-temperature phreatomagmatic eruption. The initially ejected very fine ash accumulated as accretionary lapilli and muddy rainfalls. Later, the eruption changed to hot pyroclastic flow effusions, which contained many essential lithics. These pyroclastic flow effusions included small-scale phreatic eruptions. The ash had formed ash clouds that then accumulated on each pyroclastic flow deposit. This tephra sequence was named the Haruna-Shibukawa tephra formation(Hr-S).These pyroclastic flow encroached on an older village, Nakasuji, situated on the eastern flank of Haruna volcano. The pyroclastic flow (S-5) burned and destroyed many houses. Because its deposit was very thinly laminated, it took the form of a hot pyroclastic surge, which spread over the eastern side of Haruna volcano, causing widespread damage there before changing to mud flows and floods and damaging rice fields in the area.The second eruption, which occurred in the middle or later part of the 6th century, is characterized by plinian eruptions and pyroclastic flow effusions. This tephra sequence was named the Haruna-lkaho tephra formation (Hr-I). The pumice ejected in the plinian eruptions was deposited, in a layer about 3cm thick, on Soma city, 200km from the vent.An older village, Kuroimine, situated about 10km from the vent, was buried by a layer of pumice about 200cm thick. Because pumice oxidized by the flames of burning houses is observed from the bottom to near the top of the pumice fall deposit, we can confirm that the greater part of the pumice accumulated during a period of hours. A house was crushed by the coarser part of the pumice fall deposit (1-6). The pyroclastic flows, which caused columns to collapse, moved and accumulated along the valleys before changing into mud flows and floods. They also caused heavy damage to rice fields and farms.In Gunma Prefecture, it may well be that villages, rice fields, and farms damaged by volcanic eruptions in the same way as Nakasuji village and Kuroimine village were damaged will be discovered. The data in relation to past volcanic hazards, obtained by joint research between archaeology and volcanology, will contribute to predicting volcanic disasters.

完新世における日高山脈北部の周氷河性斜面堆積物の移動期

Observation of a series of more than 50 exposures of the gentle and smooth mountain slope in the Northern Hidaka Mountains, central Hokkaido, revealed that the slope is composed of ill-sorted angular debris which is covered and/or intercalated with Holocene pumice layers. Fabric analysis of the slope deposits, including measurement of vector mean and vector magnitude of the a-axis, which is nearly parallel to the azimuth of the slope, strongly suggests that the deposit is of periglacial origin, probably transported by solifluction. The vector magnitude of the slope deposits in the study area generally is more than 65%, while that of the fluvial, debris-flow, and other non-periglacial slope deposits is below 65%.Dating the pumice layers and Holocene andosol horizon allows us to determine the period of solifluction on the slope as follows:Stage 1: before the fall of Ta-d pumice (9, 000y.B.P.)Stage 2: the fall of Ta-d-short periods (not confirmed)Stage 3: before the fall of To-c2 pumice (3, 000y.B.P.) or Ta-c1 pumice (2, 000y.B.P.)Stage 4: after the fall of To-c2 or Ta-c1Stage 5: after the andosol horizon (7, 000-3, 000y.B.P.)The periods of solifluction differ with rock types and slope altitude. Granite area: most solifluction ceased in stage 1 or 2 in the area lower than about 1, 000m a.s.l., while the slope was unstable until stage 3 in the area above about 1, 000m a.s.l. Hornfels and shale (Hidaka Group) area: many localities show continuation of the solifluction until stages 3, 4, and 5, while the movement ceased in stage 1 or 2 in some localities.In the granite area, an altitude of about 1, 000m a.s.l. seems to correspond to the lowest horizon where the solifluction occurred in stage 3.The fact that the solifluction continued for a longer time in the hornfels and shale area than in the granite one may explain by the finer debris production in the former.

A study on element-stratigraphy of Okinawa Trough

Regular distribution patterns of elements Ca, Sr, K, Na, Mg, Fe, Mn, Ti, Cu, Co, Ni, Zn, Pb, Cr, Rb, Ba, Cd, and U in Late Quaternary strata have been studied in detail. The regional variation patterns of elements in these strata are similar to those in recent surface sediments, showing that the sedimentary environment has been relatively stable since Late Quaternary. Element distribution changes with the variations of lithological characters. Contents of Ca and Sr increase in coarse sediments, while those of other elements decrease. Na content increases in the pumice layer. Mn, content tends to increase sharply towards the surface layer. Ca content decreases sharply in strata where water depth is more than 2,000 m. The strata can be divided based on the distribution curves of element assemblages. The variations in contents of most elements in Holocene Series are smaller than those in Late Pleistocene Series, which might be related to the factors of glacial and interglacial changes, sea level fluctuations and material source differences.

Geology of the San Cristobal volcanic complex, Nicaragua

The San Cristobal volcanic Complex is located in northwestern Nicaragua, near the city of Chinandego. Five volcanic centers occur within this group. The youngest and tallest volcano is San Cristobal, which erupted repeatedly throughout the 16th and 17th centuries. Renewed low-level volcanic activity, beginning in 1971, continues to the present. Examination of prehistoric deposits indicates that strombolian to moderately strong vulcanian eruptions are typical of San Cristobal, with effusion of lava possible from both the summit and lower flanks of the cone. Of potential volcanic hazards associated with this mountain, the most serious to neighboring urban areas is heavy tephra fall. The four other volcanoes of the San Cristobal complex are in various stages of development and erosion. Only La Casita shows evidence of possible future eruptive activity. Extrusive rocks of the volcanic complex are dominantly tholeiitic to calc-alkaline basalts and andesites showing chemical affinity with island-arc lavas. Dacitic domes and pumice layers are associated with at least two other volcanoes in the complex.

High velocity debris avalanche at Lastarria volcano in the north Chilean Andes

A pre-historic collapse of the southeast flank of Lastarria volcano (∼ 5700 m) in the north Chilean Andes (25° 10′ S), produced a fluidized volcanic debris avalanche whose morphology and surface structures are exceptionally well preserved. The avalanche travelled to the east-south-east, covering an area of 9.3 km2, and came to rest after climbing and over-riding a 125 m high older scoria cone. The 0.091 km3 avalanche has an apparent coefficient of friction (H/L) of 0.15 and an excessive travel distance index (Le) of 5.1 km, indicating high emplacement velocity, perhaps of the order of 80 m s−1. An important cause of the high mobility may have been the predominance of low-density, poorly cohesive scoriaceous and pumiceous layers in the source region. The flow may have had properties similar to those of a small ignimbrite.

The Mid-Pleistocene Marker-tephras in the Southern Central Parts of Miyagl Prefecture, Northeastern Japan

This paper describes the characteristics of the marker-tephras in the tephra sequence overlain by the Medeshima Pumice layer and the relationships between the markers and the geomorphic surfaces in the area to the south of Sendai.Five marker-tephras are found in the tephra sequence overlain by the Medeshima Pumice. They are named Tsubonuma-1 Pumice layer (TbP 1), Tsubonuma Lithicfragments layer (Tblf), Tsubonuma-2 Pumice layer (TbP 2), Tsubonuma-3 Pumice layer (TbP 3), and Tsubonuma-4 Pumice layer (TbP 4), in ascending order. The marker-tephras are identified by both chemical analysis of ferromagnetic minerals and X-ray diffraction data. In chemical composition of ferromagnetic minerals, TbP 1 and Tblf are of andesitic to dacitic, and TbP 2 is of rhyolitic, while TbP 3 and TbP 4 are of dacitic.Both the isopack maps and petrographic characteristics indicate that source volcanoes of TbP 1 and Tblf are inferred to be around the Aoso and the South Zao volcanoes, while those of TbP 2 and TbP4 to be in the areas extending from the north Zao to the Funagata Ranges. Moreover the relationships between the marker-tephras and geomorphic surfaces make clear that the Aobayama Surfaces are divided at least into the following two groups of different ages: the Aobayama I and II surfaces and the Aobayama III and IV surfaces, and that the hill-top surfaces of the Motoisago Hills and the Medeshima Hills are older than the Aobayama Surfaces.The five marker-tephras overlying the surfaces as above are considered to be of mid-Pleistocene age.

新期御岳テフラ層の層序と時代 中部日本における編年上の意義

Ontake Volcano was one of the most active volcanoes in Japan, producing many extensive sheets of tephras during the late Pleistocene. These sheets are very useful for the cor- relation and chronology of the late Pleistocene deposits distributed in central Japan.The authors established the stratigraphy of the Younger Ontake tephra group by careful investigation along the Kiso valley and on the eastern foot of Ontake volcano. The Younger Ontake tephra group is classified into 17 formations by buried soils and intervening unconformities. In addition, the tephra ages are estimated using the radiometric ages of related sediments, the stratigraphic positions of widespread marker tephras, and the geologic histories of well-investigated areas in central Japan.The following are the new results obtained from this investigation.1) Among the many marker tephras originating from the Ontake volcano, Katamachi pumice layer (On-KtP) extends to the Takada coastal plain on the Sea of Japan, Ina pumice layer (On-Inp) to southern Kanto, Nagawa pumice layer (On-NgP) to the northern part of Gunma Prefecture and Tatsuno pumice layer (On-Ttp) to the Nobi plain. Therefore, these extensive pumice layers make it possible to correlate the geomorphic surfaces and the late Pleistocene deposits which are distributed in areas separated by great distances.2) The upper part of the Atsuta formation in the Nobi plain is correlated to the upper part of the Katamachi sand bed in the Takada plain. These two beds were deposited during the marine regression stage from 90, 000 to 60, 000 years ago, after the culmination of the last interglacial transgression.3) Kisogawa mudflow deposits from the Ontake and Komaki gravels in the Nobi plain are considered to have been accumulated about 45, 000 years, which makes them much older than the previously estimated age of 27, 000 years.4) The maximum phase of the Kumazawa glacial stadial in the Central Japan Alps is estimated to be older than 57, 000 years.

Chemical differences between the island and submarine pumice layers of Thera

Investigations on tephra layers of deep-sea cores from near the island of Santorini showed that only the pumice clasts from the west of the island can be attributed to the Minoan eruption. The tephra layers within the caldera are of uncertain origin.

Fission-track ages of late Pleistocene tephra on the Bay of Plenty coast, North Island, New Zealand

Abstract In the area surrounding Opotiki on the Bay of Plenty coast, tephra deposits are classified into 3 units: post-Hamilton tephras, Hamilton Ash, and Tablelands Tephra Formation. Each is separated by well-developed paleosols or unconformities. The Rotoehu Ash and the Ohinewai Tephra are located at the bases of the post-Hamilton tephras and the Hamilton Ash, respectively. Tablelands Tephra Formation (new) is defined and tentatively subdivided into upper and lower members by paleosols or unconformities that commonly occur at different exposures. This formation contains several characteristic pumice layers which are very useful for tephrochronological correlation. Obsidian fragments from 3 of the pumice layers in the lower member were collected for fission-track dating, which yielded ages between 0.21 and 0.18 m.y. The fission-track ages of these tephras are consistent with their stratigraphy. Among the marine terraces along the Bay of Plenty coast, the Otamaroa and Matakaoa Terraces are inferred to have been formed in the last and the penultimate interglacial stages, respectively, judging from their geomorphological positions, tephro-chronological horizons, and fission-track ages of tephras overlying the terrace deposits. The late Quaternary uplift rate in the eastern Bay of Plenty, North Island, New Zealand, is much higher than that estimated by Chappell in 1975. The lower member of the Tablelands Tephra Formation resulted from violent volcanic activity which distributed a large amount of rhyolitic tephra over an extensive area in a short time. The pumice layer Ta-3 of the lower member is probably correlated to the Mount Curl Tephra and the Ohariu tephra, based on their similar ages and mineral compositions.

The Eruption of Vesuvius in A.D. 79: Reconstruction from Historical and Volcanological Evidence

Reinterpretation of the volcanological and historical evidence shows that the eruption of Vesuvius in A.D. 79 consisted of two main phases. initial 18 to 20hour Plinian phase caused extensive pumice-fall south of the volcano, resulting in the slow accumulation of a pumice layer up to 2.8 m. thick over Pompeii and other regions to the south. Much of the population fled the area during this non-lethal phase. On the second day of activity the Pelkan phase occurred, when a series of nudes ardentes or hot ash-avalanches swept down the south and west flanks of the volcano, affecting the region as far as Misenum, 30 km. to the west. first of two nudes ardentes which inundated Pompeii overwhelmed and asphyxiated those who remained above ground in the city and their bodies were immediately interred in the fine-grained deposit. effects of the Pelkan activity were even more severe west and northwest of Pompeii, resulting in burial of the cities of Oplontis and Herculaneum by a series of nue A. Maiuri, Herculaneum (Rome 1977) 13. 2 S.R.J. Sparks and G.P.L. Walker, The Ground Surge Deposit: a Third Type of Pyroclastic Rock, Nature 241 (1973) 62-64; S.R.J. Sparks, S. Self and G.P.L. Walker, Geology 1 (1973) 115-18; S.R.J. Sparks, Dusts of Destruction, New Scientist (1973) 134-36. 3A. Rittmann, Vulkane und ihre Tiitigkeit (Stuttgart 1960) 48. 4 Maiuri 1958 (supra n. 1) 69. 5G.A. Macdonald, Volcanoes (Englewood Cliffs 1972) 234; F.M. Bullard, Volcanoes of the Earth (Austin 1976) 200; P. Francis, Volcanoes (New York 1976) 63. 6 A. Helprin, Mont Pelke and the Tragedy of Martinique (New York 1903) 120; E.T. Merrill, Notes on the Eruption of Vesuvius in 79 A.D., AJA 22 (1918) 304-309; E.T. Merrill, Further Note on the Eruption of Vesuvius in 79 A.D., AJA 24 (1920) 262-68. This content downloaded from 157.55.39.35 on Mon, 29 Aug 2016 05:31:16 UTC All use subject to http://about.jstor.org/terms 40 H. SIGURDSSON, S. CASHDOLLAR AND S.R.J. SPARKS [AJA 86 after the disastrous nu&e ardente eruption of Mt. Pel&e in 1902, and later revived by Sparks and Walker,7 and Lirer et al.8 VOLCANOLOGICAL EVIDENCE Products of the eruption of Vesuvius in A.D. 79 form distinct deposits on the southwestern and southern slopes of the volcano and on the Sarno plain as far south as Castellammare (ill. 1). We r. Air-fall deposits accumulate gradually during the explosive phase of a volcanic eruption and characteristically mantle the topography with a wellbedded, uniform pumice layer which shows an exponential decrease in thickness with increasing distance from the volcano. Their distribution is entirely controlled by the effect of the prevailing wind direction on the vertical eruption column, which may reach a height of tens of kilometers above the vol-

FORMATIVE AGE AND RATE OF DEFORMATION OF FLUVIAL TERRACES ALONG THE LOWER STREAM OF THE SHINANO RIVER, NORTHEAST JAPAN

Fluvial terraces are widely distributed along the lower stream of the Shinano River from Ojiya city to Nagaoka city, Niigata prefecture, northeast Japan. This area has experienced active crustal movement since the Tertiary period, and the deformations of terrace plains by active folds and faults are distinctly recognized. Therefore, this area is well known as one of the best fields in Japan for studies on active crustal movement, especially on active folds. Many works have been published about fluvial terraces and active crustal movement in this area since pioneer works by Ikebe (1942) and Otsuka (1942), but few detailed studies about formative age of fluvial terraces have been published except Ota (1969). Tephrochronological approach is usually effective for regional geologic chronology. The purposes of this paper are to determine the formative age of fluvial terraces by means of tephrochronological approach, and to calculate the deformation rate of terrace plains. Main results are summarized as follows. 1. Three dated marker tephra layers were found out in this area. They are Aira-Tn ash (AT, ca. 21, 000 22_??_000 y. B. P.), Daisen Kurayoshi pumice (DKP, ca. 45, 000_??_47, 000 y. B. P.), and Nakago pumice layer (NG, ca. 130, 000_??_150, 000 y. B. P.). 2. Based on the stratigraphic relationships between terraces and these marker tephra layers, formative ages of fluvial terraces were determined as follows; A Terrace (Koshijippara terrace): ca. 130, 000_??_150, 000 y. B. P., B Terrace (Kowadappara Terrace): ca. 50, 000_??_60, 000 y. B. P., C Terrace (Funaokayama Terrace): ca. 21, 000_??_25, 000 y. B. P., D_??_F Terraces: younger than 20, 000 y. B. P., perhaps Holocene. 3. The average rate of the northward tilting of the fluvial terraces in the region is recalculated as about 0.15_??_0.17mm/km•year during the past 130, 000_??_150, 000 years, and about 0.13_??_0.16mm/km•year during the past 50, 000_??_60, 000 years. Both values correspond well with the rate of the tilting deduced from the precise relevelings (0.19 or 0.15mm/km•year, Mizoue et at., 1980). 4. The average rates of the tilting at the west wing of the Ojiya syncline (Ikebe, 1940) during the past 130, 000_??_150, 000 years and 50, 000_??_60, 000 years are 0.4‰•1, 000 years and 0.4_??_0.5‰•1, 000 years, respectively. 5. The above-mentioned facts in 3 and 4 imply that the active folding in this area has proceeded at a nearly constant rate during at least the past 150, 000 years. 6. The Katakai faults (Ota, 1969) cutting A Terrace have displaced at the order of 0.01_??_0.1m/1, 000 years.

ACTIVE FAULTS IN WESTERN AND SOUTHERN MARGINS OF THE KRFU BASIN, CENTRAL JAPAN

The author investigated topography and geology in western and southern margins of the Kofu Basin, in order to consider a characteristic of active faults through late Quaternary in these areas, on the basis of deformed landforms. The Kofu Basin, about 20km accross, is one of the tectonic basins in Central Japan, and consists of a flight of dissected fans in the area less than 500 m high (Fig. 1). In the western (Ichinose Upland) and the southern (Bone Hill) margins of the basin, dissected fans are well developed and classified into several steps. As a time marker tephra for the classification of terraces, Pm-I, one of widely distributed pumice layer (ca. 80, 000 y. B. P., Machida and Suzuki, 1971) which was erupted from Ontake Volcano, and Nirasaki mud flow deposits (ca. 300, 000y. B. P.) which were erupted from Yatsugatake Volcano, were used (Table 1). In the Ichinose Upland, geomorphic surfaces are classified into five levels, Higher terraces, Ia, Ib, II and III surfaces in descending order (Fig. 3). In the Sone Hill, they are classified into six levels (Ia, Ib, II, IIIa, IIIb and IV surfaces in descending order) as shown in Fig. 5. Table 1 shows correlation of terraces in both areas. Many active faults which displace these geomorphic surfaces are observed in the Kofu Basin, especially in the both areas of the Ichinose Upland and Sone Hill (Figs. 1, 3 and 5). Active faults of the Kofu Basin are dip slip faults without strike slip component. Most of them are distributed in margins of the Upland and the Hill. Detailed features of deformed surfaces and terrace deposits are illustrated in the maps and cross sections (Figs. 6-13). All the active faults are expressed as fault scarps, scarplets and flexure scarps, and most of them are estimated to be reverse faults or thrust faults. Warped terrace surfaces and reverse tilting towards mountain area are the typical types of deformed landforms in these areas. Rate of faulting of active faults in these areas are listed in Table 2. Since older terraces are usually much more deformed than the younger ones, progressive faulting in the late Quaternary is indicated. Average rate of vertical displacement is calculated to be 0.3_??_1.4m/1, 000 yrs. for the Ichinose Upland and less than 0.6m/1, 000 yrs. for the Sone Hill. It is interesting to note that major active faults are located at the outer margins of terraces far away from the piedmont lines themselves, both in the Ichinose Upland and Sone Hill, although some fault scarplets can be observed along the piedmont in the Ichinose Upland. Due to such arrangement of the active faults, the oldest terraces with reverse tilting are located immediately adjacent to the modern alluvial fans, and their apex area are often covered with the younger terraces. Above-mentioned characteristics of fault topographies may be explained by “a bending model of fault plane” (Ikeda and Yonekura, 1979) (Fig. 14). In the Sone Hill, this model can be applied (Fig. 15-C). But, in the Ichinose Upland, another models are devised because active faults in piedmont area were not inactive (Figs. 15-A, B).

Metamorphic carbonate ejecta from vesuvius plinian eruptions: Evidence of the occurrence of shallow magma chambers

Petrological, mineralogical and chemical data of 46 ejecta deriving from the sedimentary basement beneath Somma-Vesuvius volcano are reported. The ejecta samples were collected in pumice deposits formed during two major Plinian eruptions. One of these pumice deposits was formed during the well-known 79 A.D. eruption, and the other one — the so-called Avellino pumice — during an eruption occurred about 3,500 years B.P. Most of the ejecta from both the layers are fragments of contact-metamorphosed carbonate rocks. For the ejecta of the 79 A.D. Plinian eruption, the mineralogical parageneses of the metamorphosed carbonate rocks (dolomite-Mg calcite-periclase, and dolomite-Mg calcite-brucite) allow the evaluation of the conditions under which contact metamorphism developed. Temperatures, estimated by the Mg content in the calcite coexisting with dolomite, ranged from 360° to 790°C, whereas total fluid pressure would not have exceeded 1,500–2,000 bars with a maximum depth of metamorphism (and hence of the magma chambers) of 5,000–6,000 m. The ejecta from the so-called Avellino pumice layer (characzerized prevalently by a dolomite-Mg calcite assemblage) show that contact metamorphism occurred under the same temperature range as that of the 79 A.D. ejecta, but at an higherP CO2 partial pressure and probably at an higher total fluid pressure. These differences in physico-chemical conditions of metamorphism seem to indicate that the two Plinian eruptions were fed probably by two different magma chambers. Comparison between chemical composition of the carbonate ejecta and carbonate samples of the Mesozoic sedimentary series outcropping near the volcano indicates that fragmentation of almost all the sequence were brought to the surface by the explosive Plinian eruptions. Although the data at our disposal do not provide any information on the size of the 79 A.D. eruption magma chamber, this probably had an important vertical length component of at least 2,000 m

Evidence of the Obanazawa Ash Layer in the Chronological Study of the Late Pleistocene along the Eastern Fringe of the Yamagata Basin

Tephra is often effective for chronological studies as time-marker beds as long as identification of the tephra is completed. This paper describes the age of the Mukaibara river terrace which is along the Midare River. The age is based on evidence of the Obanazawa ash layer.The Obanazawa ash layer is composed of yellow-brown ash and underlies the Obanazawa pumice layer. The pumice layer erupted from the Hijiori caldera depression which was followed by the Hijiori pyroclastic flow about 10, 000yBP (14C age).The heavy mineral composition of the Obanazawa ash layer and the Obanazawa pumice layer are characterized by a high content of hornblend. There is no tephra layer containing high content of hornblende without being associated with the Obanazawa ash layer and the Obanazawa pumice layer in the Midare River valley along the eastern fringe of the Yamagata basin. Accordingly, identification of the Obanazawa ash layer was determined by a high content of hornblende in the heavy mineral composition.Archeological data at the Kanohara D site near Mt. Yakurai leads to an estimation of the age of the Obanazawa ash layer. At the Kanohara D site, evidence of tanged points accompanied by linear relief potteries between the Obanazawa pumice layer and the Obanazawa ash layer and a micro blade within the Obanazawa ash layer shows that the deposition of the Obanazawa ash layer was completed between 14, 000yBP and 12, 000yBP. 14C age of the layer containing the tanged point accompanied by linear relief potteries and the micro blade in Japan is 12, 200-12, 700yBP and 13, 200-14, 300yBP, respectively.In the Midare River basin, the fact that the Obanazawa ash layer is deposited on the Mukaibara terrace leads to the conclusion that the Mukaibara terrace, which has been considered to have been completed between 10, 000yBP and the present, was completed at least by 12, 000yBP.