Tuff Member Research Articles

Identification of the source caldera for the Middle Miocene ash‐flow tuffs in the Kii Peninsula based on apatite trace‐element composition

AbstractA large caldera cluster consisting of at least four calderas (Omine, Odai, Kumano‐North and Kumano calderas) existed in the central–southern part of the Kii Peninsula approximately 14–15 Ma. On the other hand, thick Middle Miocene ash‐flow tuffs, referred to as the Muro Ash‐flow Tuff and the Sekibutsu Tuff Member, are distributed in the northern part of the Kii Peninsula. Although these tuffs are considered to have erupted from the caldera cluster in the central‐southern Kii Peninsula, identifying the source caldera in the cluster has been controversial because of similarities in the petrological characteristics and identical radiometric ages of the volcaniclastic rocks of these calderas. We successfully discriminated the characteristics of the eruptive products of each caldera in the caldera cluster based on the apatite trace‐element compositions of the pyroclastic dikes and ash‐flow tuffs of the calderas. We also demonstrated that the source caldera of at least the lower main part of the Muro Ash‐flow Tuff and the Sekibutsu Tuff Member was the Odai Caldera, which is located in the central Kii Peninsula. Our findings show possible correlations among the pyroclastic conduits and ash‐flow tuffs of the caldera‐fill and/or outflow deposits, even in cases where they have been densely welded and diagenetically altered. This method is useful for the study of deeply eroded ancient calderas.

Timing of deformation along the Iron Springs thrust, southern Sevier fold-and-thrust belt, Utah: Evidence for an extensive thrusting event in the mid-Cretaceous

ABSTRACT The temporal and spatial distribution of strain associated with the Sevier orogeny in western North America is significantly different in the southern end of the belt, at the latitude of Las Vegas, Nevada, than farther to the north at the latitude of Salt Lake City, Utah. Reasons for these differences have been speculative as a lack of temporal constraints on thrusting in the intervening region hindered along-strike correlation across the belt. We determined a crystallization age of 100.18 ± 0.04 Ma for zircons extracted from a recently recognized dacite lapilli ash-fall tuff near the base of the synorogenic Iron Springs Formation. We propose the name “Three Peaks Tuff Member” for this unit, and identify a type stratigraphic section on the western flank of the “Three Peaks,” a topographic landmark in Iron County, Utah. Field relationships and this age constrain movement on the Iron Springs thrust and the end of the sub-Cretaceous unconformity in the critical intervening area to latest Albian/earliest Cenomanian. Movement on the Iron Springs thrust was synchronous with movement on multiple Sevier thrusts at ~100 Ma, indicating that the mid-Cretaceous was a period of extensive thrust-fault movement. This mid-Cretaceous thrusting event coincided with a period of global plate reorganization and increased convergence, and hence an increased subduction rate for the Farallon Plate beneath North America. The accelerated subduction contributed to a Cordilleran arc flare-up event and steepening of the orogenic wedge, which triggered widespread thrusting across the retroarc Sevier deformation belts. Additionally, based on temporal constraints and the strong spatial connection of mid-Cretaceous thrusts to lineaments interpreted as pre-orogenic transform faults, we suggest that temporal and spatial variations along the strike of the orogenic belt reflect tectonic inheritance of basement structures associated with the edge of the rifted Precambrian craton.

Split-grain 40Ar/39Ar dating: Integrating temporal and geochemical data from crystal cargoes

Large sanidine crystals from the Mesa Falls Tuff (MFT), Yellowstone volcanic field, have been split and individually dated via high-precision 40Ar/39Ar geochronology with the undated portions further analysed for major elements, trace elements, Pb and Sr isotopes in the sanidine and trace elements in the melt inclusions. This allows the geochemical and geochronological identity of an individual sanidine to be combined. Our MFT sanidines return a preferred eruption age of 1.3011±0.0015/0.0016 Ma (2-sigma, n=56, MSWD 0.8, analytical/full external) with a significant component of subtly older (up to ~2Ma) crystals. Combined with recent results (Rivera et al., 2016, Journal of Petrology 57, 9, 1677–1704) our data define a global mean sanidine 40Ar/39Ar age for the MFT of 1.3022±0.0006/0.0008 Ma (2 sigma, analytical/full external) relative to Alder Creek sanidine at 1.1891 Ma and total λ40Ar 5.5305e-10, which gives RACsMFT: 1.09542±0.00050. The ability to couple geochemistry and geochronology from a single grain allows us, for the first time, to evaluate the origin of the subtly older sanidines present in the same pumices as juvenile sanidines. Melt inclusions from all Mesa Falls sanidines represent extremely fractionated melts with low Sr contents (max. 12ppm, n=39), and rare earth element patterns which require that they be formed from an A-type magma rather than the preceding subduction-related Eocene volcanism as previously suggested. 87Sr/86Sr from juvenile and subtly older sanidines shows the same range of 0.7073 to 0.7096, illustrating the susceptibility of such low-Sr melts to slight degrees of assimilation. Pb isotopic compositions are more restricted and identical between the juvenile and subtly older sanidines in the Mesa Falls Tuff (207Pb/206Pb 0.900–0.903, 208Pb/206Pb 2.217–2.226, n=83) and these compositions rule out the underlying Huckleberry Ridge Tuff member B as a potential source for the subtly older sanidine. LA-ICPMS 206Pb/238U dating of Mesa Falls zircons supports no role for the Huckleberry Ridge Tuff. Rather, these subtly older sanidines are interpreted as containing excess mantle-derived Ar. The ability to couple the geochemical and geochronological records within individual sanidine crystals that we demonstrate here has potential to provide new insights for a variety of petrological studies such as diffusional modelling.

Age of the Yarrabee and accessory tuffs: implications for the upper Permian sediment-accumulation rates across the Bowen Basin

ABSTRACTThe Yarrabee Tuff is a stratigraphically significant marker across the Bowen Basin separating the Fort Cooper–Burngrove–Fair Hill formations from the overlying Rangal and equivalent coal measures. At least three to four persistent tuffs (referred here as accessary tuffs) beneath the Yarrabee Tuff were recognised in the Fort Cooper Coal Measures as suitable for regional stratigraphic correlations. In this study, we determined the ages of the Yarrabee and accessary tuffs across different morphotectonic zones of the basin through high-precision U–Pb dating of zircon with the CA-IDTIMS technique. The age of the Yarrabee Tuff is found to be 252.69 ± 0.16 Ma in the Duckworth 11 well, 253.07 ± 0.22 Ma in the Crocker Gully 2 well and <252.58 ± 0.23 Ma in the Peat 1 well. The age range of the Yarrabee Tuff coincides with the previously published date of the Kaloola Tuff Member in Meeleebee 5 suggesting that the tuffs are stratigraphically equivalent. The age range for the accessory tuff 1 is 253.12 ± 0.12 Ma to 252.85 ± 0.16 Ma, 253.45 ± 0.08 Ma for accessory tuff 2 and 253.77 ± 0.17 Ma to 253.57± 0.18 Ma for accessory tuff 3, placing them in the upper Changhsingian Stage. The age of the accessory tuff 6 (less laterally consistent in the basin) from the Fair Hill Formation is 254.03 ± 0.03 Ma, placing it in the lower Changhsingian Stage. The age-constrained intervals allow the estimation of sedimentation rates using decompacted coal and clastic sediment thickness. In the Taroom Trough, the temporal variation in sedimentation rates is found to be 902 m/Ma in the Fair Hill Formation decreasing to 234.5 m/Ma in the overlying Burngrove Formation, reflecting a decrease in accommodation or sediment supply upwards in the sequence. Across the basin, the sedimentation rates for the Burngrove Formation are consistently higher in the Taroom Trough ranging between 234.5 and 224.5 m/Ma and lower rates of 112 m/Ma in the Roma Shelf. This regional variation reflects areas of high sedimentation rates that are high accommodation sites recognised by split coal seams and increased interburden. Conversely, low sedimentation rates reflect low accommodation sites, such as the Roma Shelf and the Burunga Anticline that are characterised by coalesced coal seams. The results help to understand stratal relationships across variable accommodation sites, basin-fill history of the basin including extent of sediment supply and paleotopographic controls during the evolution of the Bowen Basin. We also discuss criteria for interpreting the results of CA-IDTIMS U–Pb dating and consider the possible geological uncertainties related to either the primary magmatic processes or secondary reworking of tuffs at the site of deposition.

Volcaniclastic rocks of the Miocene Sekibutsu Tuff Member in the northern Kii Peninsula, central Japan

Volcaniclastic rocks of the Miocene Sekibutsu Tuff Member in the northern Kii Peninsula, central Japan

New isotopic and geochemical data from the Palaeoproterozoic Pechenga Greenstone Belt, NW Russia: Implication for basin development and duration of the volcanism

The North Pechenga Group of the Pechenga Greenstone Belt comprises a thick supracrustal succession developed within a time interval of ca. 500Ma in the early part of the Palaeoproterozoic Era. It records several important global events leading to the onset of modern-style Earth system, but the temporal evolution of the belt is still insufficiently established. New Nd isotope data clearly indicate an abrupt change from a strong crustal signature in the lower two volcanic units to a depleted-mantle like signature in the upper two volcanic units, which conforms with the coeval shift of the volcanism from subaerial to submarine, indicating a significant thinning of the lithosphere due to continental rifting. The timing of the change in the geotectonic regime can be bracketed between 2058±6Ma (U-Pb data) and 2018±54Ma (Sm-Nd data).In situ ion microprobe analyses of zircon grains from a thin felsic tuff member in the upper part of the succession (Pilgujärvi Volcanic Formation) produced an age of 1988±3Ma, which is within error the same as has been reported previously for the ferropicritic magmatism that produced the Pechenga Ni-Cu ores. It is concluded that the upper 3-km-thick volcanic pile dominated by tholeiitic basalts and minor ferropicrites was generated within a short time span unresolvable with current geochronological tools. Geochemical and isotopic data suggest that the felsic tuffs were generated from a ferropicritic parental magma via extensive fractional crystallisation, and the presence of gneiss xenoliths in the tuffs reveals that this happened in a magma chamber located in the underlying Archaean basement. Thus, although the mafic subaqueous volcanism of the upper part of the belt does not bear Nd isotopic or trace element evidence for contamination with ancient crustal material, the rifting of the basement did not yet proceed to a total break-up of the Archaean craton.

Spatial distribution of damage around faults in the Joe Lott Tuff Member of the Mount Belknap Volcanics, Utah: A mechanical analog for faulting in pyroclastic deposits on Mars

Volcanic ash is thought to comprise a large fraction of the Martian equatorial layered deposits and much new insight into the process of faulting and related fluid flow in these deposits can be gained through the study of analogous terrestrial tuffs. This study identifies a set of fault‐related processes that are pertinent to understanding the evolution of fault systems in fine‐grained, poorly indurated volcanic ash by investigating exposures of faults in the Miocene‐aged Joe Lott Tuff Member of the Mount Belknap Volcanics, Utah. The porosity and granularity of the host rock are found to control the style of localized strain that occurs prior to and contemporaneous with faulting. Deformation bands occur in tuff that was porous and granular at the time of deformation, while fractures formed where the tuff lost its porous and granular nature due to silicic alteration. Non‐localized deformation of the host rock is also prominent and occurs through compaction of void space, including crushing of pumice clasts. Significant off‐fault damage of the host rock, resembling fault pulverization, is recognized adjacent to one analog fault and may reflect the strain rate dependence of the resulting fault zone architecture. These findings provide important new guidelines for future structural analyses and numerical modeling of faulting and subsurface fluid flow through volcanic ash deposits on Mars.

Stratigraphy and correlation of the Sekibutsu Tuff Member, Nara, Japan.

Stratigraphy and correlation of the Sekibutsu Tuff Member, Nara, Japan.

Tephrostratigraphy of the Bedded Tuff Member (Kapthurin Formation, Kenya) and the nature of archaeological change in the later middle Pleistocene

Correlation of volcaniclastic deposits of the Bedded Tuff Member (K4) of the Kapthurin Formation (Kenya) provides the means to assess the nature of archaeological change during the later middle Pleistocene, a time period critical to human evolution but poorly represented at other African localities. Field stratigraphic evidence, and petrographic and electron microprobe geochemical analyses of volcanic glass and phenocrysts, define eight subdivisions of K4 tephra. These include a succession of deposits from a local volcanic source that erupted intermittently, as well as other tuffs likely from different sources outside the Baringo basin. Upper portions of the Bedded Tuff Member date to ∼235,000 yr. The Bedded Tuff Member is underlain by sediments that include the Grey Tuff, dated to 509,000 ± 9000 yr. The tephrostratigraphic framework defined here is used to place Acheulian and Middle Stone Age (MSA) archaeological sites in chronological order. Results show the persistence of Acheulian large cutting tool manufacture after the advent of points, considered an MSA artifact type. Two assemblages from the site of Koimilot record the appearance at ∼200,000–250,000 yr of a variety of Levallois flake production methods, an integral if incompletely understood feature of the MSA, here likely derived from local technological antecedents. Combined evidence from the tools and flake production methods suggest an incremental and mosaic pattern of change in hominin adaptive strategies during the Acheulian–MSA transition.

Middle Archean volcano-hydrothermal sequence: Bacterial microfossil-bearing 3.2 Ga Dixon Island Formation, coastal Pilbara terrane, Australia

The 3.2 Ga Dixon Island Formation in the Cleaverville Group of the coastal Pilbara terrane, Australia, is one of the most complete and best-preserved examples of middle Archean oceanic stratigraphy and contains possible microbial material. Field observations and geochemical evidence suggest that this formation contains a low-temperature hydrothermal vent system with a biogenic microbial colony from the Archean ocean. The Dixon Island Formation is ∼350 m thick and consists of the Rhyolite Tuff, Black Chert, and Varicolored Chert Members, in ascending order. The Rhyolite Tuff Member contains many vein swarms, such as quartz and black chert veins, and highly altered rhyolite tuff layers, which are identified as an underground bypass zone for circulating hydrothermal fluid. Many black chert vein swarms in the Rhyolite Tuff Member imply intensive low-temperature hydrothermal activity during deposition of the Black Chert Member, which is 10–15 m thick. The Black Chert Member is composed of massive black chert, laminated black chert, dark-greenish siliceous shale and tuffaceous laminated chert, which are mainly composed of very fine quartz. Abundant pseudomorphs of silica after aragonite, barite, and gypsum, and a distinctly continuous, stromatolite-like biomat layer (10–20 cm thick), are preserved within the laminated black chert bed. The stromatolite-like biomat bed is formed of fine iron or iron-coated quartz pisolite within fine-grained silica. The absence of detrital sediment of continental origin and the many vein injections imply that this sedimentary facies represents a pelagic hydrothermal environment at ∼500–2000 m paleodepth, and may have been on the slope of an immature island arc. Microbial material has been preserved well in the lower part of Black Chert Member. The massive black chert has carbonaceous peloids (0.3–2.0 mm in diameter), which are similar to those in the black chert veins. The massive black chert contains spiral-, rod, and dendrite-shaped bacterial material. The total organic carbon (TOC) value of massive black chert in the lower part of the Black Chert Member is higher (TOC = 0.15–0.45%) than that of the overlying laminated chert section (TOC = 0.02–0.15%) and the black chert vein (TOC = 0.1–0.13%), and the carbon isotope (δ 13 C) values of this lithology (−33‰ to ∼−27‰) are also lighter than for the black chert veins (−29‰ to ∼−26‰) and the laminated black chert in the upper part of the Black Chert Member and the Varicolored Chert Member (−27‰ to ∼−13‰). This evidence suggests that the carbonaceous grains and bacteria-shaped material in the lower part of the Black Chert Member are of bio-genic origin and were formed close to a low-temperature hydrothermal vent system. The microbial colony may have been rapidly fossilized by silicification related to hydrothermal activity. Laminated black chert in the upper part of the Black Chert and the Varicolored Chert Members may have formed by cyanobacterial sedimentation from the ocean surface.

Geochemistry and origin of the Şaphane alunite deposit, Western Anatolia, Turkey

A replacement type alunite deposit occurs in the Şaphane area, western Anatolia. The orebody within the rhyolitic–rhyodacitic Civanadag Tuff Member is mainly composed of alunite, with quartz, kaolinite and small amounts of pyrite. The deposit is laterally zoned from inner advanced argillic alteration to an outer propylitic–argillic zone. Vertical zoning is expressed in a transition from a silicified zone downwards through silica–alunite (alunite–kaolinite–quartz–opal-CT), limonite and propylitic–argillic zones (K-feldspar–quartz–smectite–illite). A barren silicified zone at the north of Main pit forms a silica cap over the alunitization. Due to strong acid sulfate alteration, K-feldspar and volcanic glass in the host rock were altered and released K and Al, facilitating the formation of alunite. Silica was transported upwards and deposited at the surface as quartz and/or opal-CT. Similar K–Ar ages (12 to 13 Ma) obtained for both the deposit and volcanic host rocks indicate that volcanism and alunite mineralization are cogenetic. Isotope analyses reveal that sulfur in the deposits has a magmatic origin. Acid sulfate alteration resulted in strong leaching of major elements, but most of the trace elements were generally retained in the original rock.

440 Ma igneous activity in the Meguma Terrane, Nova Scotia, Canada; part of the Appalachian overstep sequence?

Abraded zircons from the basal rhyolitic tuff member of the White Rock Formation, which disconformably overlies the Cambrian-Early Ordovician Meguma Group, have yielded a nearly concordant U-Pb age of 442+ or -4 Ma interpreted as the age of extrusion. This age straddles the approximately 443 Ma Ordovician-Silurian boundary. Abraded zircons from the Brenton granite lie on a chord with an upper intercept age of 439+4/-3 Ma, which is interpreted to be the age of intrusion, thus supporting its inferred subvolcanic nature. On the other hand, monazite from the Brenton pluton yielded a nearly concordant analysis with a 207 Pb/ 206 Pb age of 380+ or -3 Ma, which is interpreted to be the time of the low pressure, high temperature metamorphism. Using these new data, the following observations suggest that the Meguma and Avalon terranes were neighbors during the Silurian-Early Devonian: (1) both have latest Ordovician-earliest Silurian, bimodal, subaerial, alkalic-tholeiitic, rift-related, volcanic rocks with Nd signatures indicating a similar continental basement source; (2) both show a similar progression of depositional environments: subaerial in the earliest Silurian, progressively deeper-water marine strata in the Llandovery and Wenlock, switching to gradually shallowing marine environments in the Ludlow, and reverting to subaerial in the Pragian; and (3) both contain Rhenish-Bohemian Early Devonian fauna. Furthermore, the southeast to northwest transition from an offshore sandbar to a beach sand in the Silurian White Rock Formation suggests the presence of land to the north, now recognized as Avalonia. These conclusions support published suggestions that the Siluro-Devonian successions in the Meguma and Avalon terranes form part of the overstep sequence that extends across most of the northern Appalachians. Published data indicate that Avalonia was adjacent to Gondwana in the Neoproterozoic, that it separated from Gondwana in the Early Ordovician, and was accreted to eastern Laurentia in the Late Ordovician-Early Silurian. On the basis of the data and correlations presented here, we suggest that the Meguma Terrane travelled with Avalonia. This is consistent with the absence of a phase of deformation between the Cambro-Ordovician Meguma Group and the Silurian White Rock Formation.

Miocene stratigraphy and tectonic process of the Echigo-Yuzawa Basin, Northern Fossa Magna region, central Japan.

This study clarified the stratigraphy of the Miocene series that occur in contact with pre-Neogene basement rocks in the Echigo-Yuzawa region. Based on new lithologic assemblages, the Miocene stratigraphy and the tectonic process of the Echigo-Yuzawa Basin were reconstructed. The Miocene formations are distributed in the Shiozawa and Tsuchitaru areas in the Echigo-Yuzawa region. The Shiozawa area comprises the newly proposed Shiozawa Formation (Maiko Breccia Member, Yoshiyama Shale Member and Komatsuzawa Basalt Member) and the Tsuchitaru area comprises the redefined Matsukawa Formation (newly proposed Tsuchitaru Sandstone and Basalt Member, Nakazato Tuff Member and Kandatsu Shale Member). A new record of planktonic foraminifer indicative of Blow's N.9 (Blow, 1969) was discovered from the Kandatsu Shale Member. The tectonic development of the Miocene Echigo-Yuzawa Basin is as follows : [Stage I] Basin generation due to collapse, forming a lacustrine lake (18-17 Ma).[Stage II] Subsidence accompanied with marine transgression and bimodal volcanism (17-15Ma).[Stage III] Continuous subsidence depositing mud (15 Ma).

熊本県人吉盆地に分布する人吉層中の凝灰岩のＫ‐Ａｒ年代

The well documented Hitoyoshi Formation, consisting mainly of lacustrine clastic sediments distributed in the Hitoyoshi Basin in southern Kyushu, has well preserved tuff beds. The Funato Tuff Member intercalated in the basal part of the formation, and the Yamada-1 and -2 Tuffs in the upper part were examined to estimate timings of the sedimentation. The former consists of the lower pumice layer and the upper pyroclastic flow deposit, and plagioclase separated from the middle part of the pyroclastic flow deposit gives 2.58±0.08 Ma in K-Ar age. The later consists of chiefly volcanic ash and pumice, and hornblende separated from the lower part of the Yamada-2 Tuff is dated at 2.72±0.25 Ma in K-Ar age. These results clearly indicate that the Hitoyoshi Formation is Late Pliocene in age.

The Dixon Island Formation of the Western Pilbara Coastal Greenstone Terrane

Hydrothermal activity is considered to be responsible for ubiquitous silicification of Archean supracrustal rocks. Here, we show well-preserved evidence of hydrothermal silicification with in the Dixon Island Formation (3.2Ga) of Pilbara Craton, Australia. The formation is a part of immature oceanic island arc origin Cleaverville Group consists of three cycles of volcano?sedimentary sequence (Kiyokawa and Taira, in press). Each sequence consists of thick volcanics (basalt, rhyolite), black shale, ferruginous chert and BIF to the top. Especially, the Dixon Island Formation, which is the best exposed sequence, is composed of the Rhyolite Tuff, Black Chert and Varicolored Chert Members. The Rhyolite Tuff Member contains the multiple black chert veins which is feeders of the hydrothermal vent. These veins probably produced the chemogenic sediments of the upper members.

Evolution of the early devonian bindook volcanic complex, wollondilly basin, eastern lachlan fold belt

The Early Devonian Bindook Volcanic Complex consists of a thick silicic volcanic and associated sedimentary succession filling the extensional Wollondilly Basin in the northeastern Lachlan Fold Belt. The basal part of the succession (Tangerang Formation) is exposed in the central and southeastern Wollondilly Basin where it unconformably overlies Ordovician rocks or conformably overlies the Late Silurian to Early Devonian Bungonia Limestone. Six volcanic members, including three new members, are now recognised in the Tangerang Formation and three major facies have been delineated in the associated sedimentary sequence. The oldest part of the sequence near Windellama consists of a quartz turbidite facies deposited at moderate water depths together with the shallow‐marine shelf Windellama Limestone and Brooklyn Conglomerate Members deposited close to the eastern margin of the basin. Farther north the shelf facies consists of marine shale and sandstone which become progressively more tuffaceous northwards towards Marulan. The Devils Pulpit Member (new unit) is a shallow‐marine volcaniclastic unit marking the first major volcanic eruptions in the region. The overlying shallow‐marine sedimentary facies is tuffaceous in the north, contains a central Ordovician‐derived quartzose (?deltaic) facies and a predominantly mixed facies farther south. The initial volcanism occurred in an undefined area north of Marulan. A period of non‐marine exposure, erosion and later deposition of quartzose rocks marked a considerable break in volcanic activity. Volcanism recommenced with the widespread emplacement of the Kerillon Tuff Member (new unit), a thick, non‐welded rhyolitic ignimbrite followed by dacitic welded ignimbrite and air‐fall tuff produced by a large magnitude eruption leading to caldera collapse in the central part of the Bindook Volcanic Complex, together with an additional small eruptive centre near Lumley Park. The overlying Kerrawarra Dacite Member (new unit) is lava‐like in character but it also has the dimensions of an ignimbrite and covers a large part of the central Bindook Volcanic Complex. The Carne Dacite Member is interpreted as a series of subvolcanic intrusions including laccoliths, cryptodomes and sills. The Tangerang Formation is overlain by the extensive crystal‐rich Joaramin Ignimbrite (new unit) that was erupted from an undefined centre in the central or northern Bindook Volcanic Complex. The volcanic units at Wombeyan and the Kowmung Volcaniclastics in the northwestern part of the complex are probably lateral time‐equivalents of the Tangerang Formation and Joaramin Ignimbrite. All three successions pre‐date the major subaerial volcanic plateau‐forming eruptions represented by the Barrallier Ignimbrite (new unit). The latter post‐dated folding and an extensive erosional phase, and unconformably overlies many of the older units in the Bindook Volcanic Complex. This ignimbrite was probably erupted from a large caldera in the northern part of the complex and probably represents surface expressions of part of the intruding Marulan Batholith. The final volcanic episode is represented by the volcanic units at Yerranderie which formed around a crater at the northern end of the exposed Bindook Volcanic Complex.

ＦＴ法の最近の成果と石油探鉱へのジルコンの応用

Zircon Fission-Track Analysis (ZFTA) as a geological clock is based on zeta age calibration and track length measurements. Fission-track dating samples are classified into four types according to combination of two factors of 1) homogeneity in fission track age and 2) heating effect to fission track age. The four types of samples are (a) homogeneous and unheated, (b) heterogeneous and unheated, (c) homogeneous and heated and (d) heterogeneous and heated. As an example of ZFTA applied to the four-type samples, the author reviewed recent fission track dating studies in Japan as follows: (a) volcanic ash layers of the Kazusa Group in the Boso Peninsula, (b) the Pliocene Hirosegawa Tuff Member of the Sendai Group in Northeast Honshu, (c) the MITI “Honjo-oki” well and (d) the MITI “Mishima” well, respectively. Studies on the two exploratory test wells show ZFTA is a useful tool for petroleum exploration in Japan.

K-Ar age and zircon crystal morphology of the Junitoge Tuff Member in the Yuzawa area, Niigata Prefecture, central Japan

K-Ar age and zircon crystal morphology of the Junitoge Tuff Member in the Yuzawa area, Niigata Prefecture, central Japan

Correlational study of the Pliocene Hirosegawa Tuff Member of the Sendai Group in Northeast Honshu; heavy mineral composition and refractive index of volcanic glass.

Correlational study of the Pliocene Hirosegawa Tuff Member of the Sendai Group in Northeast Honshu; heavy mineral composition and refractive index of volcanic glass.

High precision radiometric ages from the northern Sydney Basin and their implication for the Permian time interval and sedimentation rates

Three pyroclastic samples that bracket the coal‐bearing Permian System of the northern Sydney Basin have been dated using the zircon U/Pb and hornblende K/Ar methods. The Matthews Gap Dacitic Tuff Member, situated 170 m below the base of the Permian System, gives a best estimate of 309 ± 3 Ma. Its age correlates well with the Paterson Volcanics which suggests that the immediately overlying clastic sediments are equivalent to the Seaham Formation. The Awaba Tuff, which at the sample locality is located 50 m below the top of the Permian System, gives a best estimate of 256 ± 4 Ma. An intervening horizon, the Thornton Claystone of the Tomago Coal Measures, gives a best estimate of 266 ± 0.4 Ma. The ages indicate an earlier beginning (∼ 299 Ma BP), an earlier termination (∼ 255 Ma BP) and slightly longer duration (44 ± 13 Ma) of the Permian System in the Hunter Valley than previously suggested. Sedimentation rates of ∼ 65 m/Ma, calculated from proximal sequence thicknesses, are only half the rate calculated f...