Isothermal Plateau Fission-track Research Articles

Onshore–offshore correlation of Pleistocene rhyolitic eruptions from New Zealand: implications for TVZ eruptive history and paleoenvironmental construction

Taupo Volcanic Zone (TVZ), in the North Island, New Zealand, is arguably the most active Quaternary rhyolitic system in the world. Numerous and widespread rhyolitic tephra layers, sourced from the TVZ, form valuable chronostratigraphic markers in onshore and offshore sedimentary sequences. In deep-sea cores from Ocean Drilling Program (ODP) Leg 181 Sites 1125, 1124, 1123 and 1122, located east of New Zealand, ca 100 tephra beds are recognised post-dating the Plio-Pleistocene boundary at 1.81 Ma. These tephras have been dated by a combination of magnetostratigraphy, orbitally tuned stable-isotope data and isothermal plateau fission track ages. The widespread occurrence of ash offshore to the east of New Zealand is favoured by the small size of New Zealand, the explosivity of the mainly plinian and ignimbritic eruptions and the prevailing westerly wind field. Although some tephras can be directly attributed to known TVZ eruptions, there are many more tephras represented within ODP-cores that have yet to be recognised in near-source on-land sequences. This is due to proximal source area erosion and/or deep burial as well as the adverse effect of vapour phase alteration and devitrification within near-source welded ignimbrites. Despite these difficulties, a number of key deep-sea tephras can be reliably correlated to equivalent-aged tephra exposed in uplifted marine back-arc successions of Wanganui Basin where an excellent chronology has been developed based on magnetostratigraphy, orbitally calibrated sedimentary cycles and isothermal plateau fission track ages on tephra. Significant Pleistocene tephra markers include: the Kawakawa, Omataroa, Rangitawa/Onepuhi, Kaukatea, Kidnappers-B, Potaka, Unit D/Ahuroa, Ongatiti, Rewa, Sub-Rewa, Pakihikura, Ototoka and Table Flat Tephras. Six other tephra layers are correlated between ODP-core sites but have yet to be recognised within onshore records. The identification of Pleistocene TVZ-sourced tephras within the ODP record, and their correlation to Wanganui Basin and other onshore sites is a significant advance as it provides: (1) an even more detailed history of the TVZ than can be currently achieved from the near-source record, (2) a high-resolution tephrochronologic framework for future onshore-offshore paleoenvironmental reconstructions, and (3) well-dated tephra beds correlated from the offshore ODP sites with astronomically tuned timescales provide an opportunity to critically evaluate the chronostratigraphic framework for onshore Plio-Pleistocene sedimentary sequences (e.g. Wanganui Basin, cf. Naish et al. [1998. Quaternary Science Reviews 17 695–710].

Silicic tephras in Pleistocene shallow‐marine sediments of Wanganui Basin, New Zealand

Vitric‐rich volcaniclastic horizons are important for correlation of glacio‐eustatic sedimentary cycles, both within the well known shallow‐marine record of Wanganui Basin, and other New Zealand terrestrial and deep marine records. They also record distal major rhyolitic eruptions from the Taupo (TVZ) and Coromandel (CVZ) Volcanic Zones that are lacking in proximal source areas. Twenty‐eight volcaniclastic horizons are recognised in the Castlecliffian and late Nukumaruan strata of Wanganui Basin from glass shard major element geochemistry and stratigraphic position, and are dated using magnetostratigraphy, orbitally tuned cyclostratigraphy and isothermal plateau fission track (ITPFT) ages. The major named volcaniclastic horizons (with ITPFT and/or astronomical ages, respectively) are: Onepuhi (0.57 Ma), Kupe (0.63 ± 0.08 Ma; 0.65 Ma), Kaukatea (0.86 ± 0.08 Ma; 0.90 Ma), Potaka (1.00 ± 0.03 Ma; 0.99 Ma), Rewa (1.20 ± 0.14 Ma; 1.19 Ma), Mangapipi (1.51 ± 0.16 Ma, 1.54 Ma), Ridge (1.56 Ma), Pakihikura (1.58 ± 0.08 Ma; 1.58 Ma), Birdgrove (1.60 Ma), Mangahou (1.63 Ma), Maranoa (1.63 Ma), Ototoka (1.72 ± 0.32 Ma; 1.64 Ma), Table Flat (1.71 ± 0.12 Ma; 1.65 Ma), Vinegar Hill (1.75 ± 0.20 Ma; 1.75 Ma), and Waipuru (1.79 ± 0.15 Ma; 1.83 Ma). The ITPFT ages are consistent with the astronomically tuned Geomagnetic Polarity Timescale. Volcaniclastic horizons in Wanganui Basin have been emplaced through a variety of primary and secondary processes, including direct tephra‐fall as well as transitional water supported mass flow through to hyperconcentrated flow. No gas supported flow deposits have yet been recognised. Only some horizons from Wanganui Basin can be chemically and chronologically linked to known TVZ eruptions, while others remain uncorrelated owing to proximal source area erosion and/or burial as well as vapour phase alteration and devitrification within near‐source welded ignimbrites. Nevertheless, many volcaniclastic deposits in Wanganui Basin can be reliably correlated to distal sedimentary successions in Auckland Region, Hawke's Bay and in Ocean Drilling Program (ODP) cores 1123 and 1124, to the east of New Zealand. The orbitally tuned chronology for ODP cores, which is calibrated by numeric ages on tephras and magnetostratigraphy, enhances inter‐regional correlation, providing an important framework for future pal‐aeoenvironmental reconstructions.

A late Early Pleistocene tephrochronological and pollen record from Auckland, New Zealand

Abstract A late Early Pleistocene pollen record was obtained from a coastal site in Auckland, New Zealand. A combination of isothermal plateau fission track ages on interbedded tephras, palaeomagnetism, palynostratigraphy and orbital tuning to the marine oxygen isotope record of Ocean Drilling Program Site 677 constrained the age of the topmost 28 m of sediments to c. 1.4–1.0 Ma (Marine Isotope Stages (MIS) 45–28). For this interval a diverse pollen record consisting of mostly extant pollen types shows multiple compositional shifts from a Nothofagus -dominated to conifer-dominated regional vegetation. These shifts are broadly correlated to changes in the marine oxygen isotope record. The inferred climate was moist, temperate, stable, and cooler than at present, but never as cool as the last glacial maximum. A permanent increase in Nothofagus forest in the region after MIS 35 seems to be related to a long-term palaeoclimatic shift that probably included greater temperature extremes between warm and cool stages and decreases in humidity and increased seasonality during cool stages. Although the Patiki pollen record predates the mid-Pleistocene revolution by c. 100 ka, the nature of climate change itself was already in transition, and becoming more similar to the climate regime experienced in northern New Zealand in the Late Pleistocene.

Onshore-Offshore Correlation of Pleistocene Rhyolitic Eruptions from New Zealand: Implications for TVZ Eruptive History and Paleoenvironmental Construction

Abstract Taupo Volcanic Zone (TVZ), in the North Island, New Zealand, is arguably the most active Quaternary rhyolitic system in the world. Numerous and widespread rhyolitic tephra layers, sourced from the TVZ, form valuable chronostratigraphic markers in onshore and offshore sedimentary sequences. In deep-sea cores from Ocean Drilling Program (ODP) Leg 181 Sites 1125, 1124, 1123 and 1122, located east of New Zealand, ca 100 tephra beds are recognised post-dating the Plio-Pleistocene boundary at 1.81 Ma. These tephras have been dated by a combination of magnetostratigraphy, orbitally tuned stable-isotope data and isothermal plateau fission track ages. The widespread occurrence of ash offshore to the east of New Zealand is favoured by the small size of New Zealand, the explosivity of the mainly plinian and ignimbritic eruptions and the prevailing westerly wind field. Although some tephras can be directly attributed to known TVZ eruptions, there are many more tephras represented within ODP-cores that have yet to be recognised in near-source on-land sequences. This is due to proximal source area erosion and/or deep burial as well as the adverse effect of vapour phase alteration and devitrification within near-source welded ignimbrites. Despite these difficulties, a number of key deep-sea tephras can be reliably correlated to equivalent-aged tephra exposed in uplifted marine back-arc successions of Wanganui Basin where an excellent chronology has been developed based on magnetostratigraphy, orbitally calibrated sedimentary cycles and isothermal plateau fission track ages on tephra. Significant Pleistocene tephra markers include: the Kawakawa, Omataroa, Rangitawa/Onepuhi, Kaukatea, Kidnappers-B, Potaka, Unit D/Ahuroa, Ongatiti, Rewa, Sub-Rewa, Pakihikura, Ototoka and Table Flat Tephras. Six other tephra layers are correlated between ODP-core sites but have yet to be recognised within onshore records. The identification of Pleistocene TVZ-sourced tephras within the ODP record, and their correlation to Wanganui Basin and other onshore sites is a significant advance as it provides: (1) an even more detailed history of the TVZ than can be currently achieved from the near-source record, (2) a high-resolution tephrochronologic framework for future onshore-offshore paleoenvironmental reconstructions, and (3) well-dated tephra beds correlated from the offshore ODP sites with astronomically tuned timescales provide an opportunity to critically evaluate the chronostratigraphic framework for onshore Plio-Pleistocene sedimentary sequences (e.g. Wanganui Basin, cf. Naish et al. [1998. Quaternary Science Reviews 17 695–710].

Deep‐ocean record of major late Cenozoic rhyolitic eruptions from New Zealand

A 12 m.y. record of large rhyolitic eruptions from the Coromandel (CVZ) and Taupo (TVZ) Volcanic Zones of New Zealand is contained in cores retrieved by Leg 181 of the Ocean Drilling Program. Site 1124, located 670 km from the TVZ, has a maximum of 134 macroscopic tephra layers with a total thickness of 13.18 m. These units, along with between 7 and 63 tephras from 3 other sites, were dated by a combination of magnetostratigraphy, biostratigraphy, isothermal plateau fission track determinations, and geochemical correlation with onshore tephra deposits. Additional time control for the last 3 m.y. came from an orbitally tuned, benthic, oxygen isotope profile for Site 1123. Results extend the incomplete terrestrial record of volcanism by placing the first major rhyolitic eruption in the CVZ at c. 12 Ma, c. 1.6–1 m.y. earlier than previously known. Tephras became thicker and more frequent from the late Miocene into the Quaternary—a trend that probably reflected (1) more frequent and intense volcanism and (2) reduced distances between sources and depositional sites on the evolving Australian/Pacific plate system. The passage from CVZ to Quaternary TVZ occurred without a major hiatus in activity, suggesting the transition was gradational. The ensuing TVZ volcanism was more continuous than known previously from the onshore geology. Ash dispersal was primarily eastward, highlighting the dominance of westerly winds since the middle Miocene. Nevertheless, variations in dispersal patterns suggest periodic changes in wind direction/speed and/or ejection of ash beyond the Roaring Forties.

Tephrochronology: a New Zealand case study

Tephrochronology is the study of volcanic ash (tephra) beds for the purpose of correlating and dating volcanic and other geologic events. Large magnitude silicic eruptions can disperse tephra up to thousands of kilometres from the vent, producing a near instantaneous marker horizon. In addition to their geochronological value, tephra beds are a major source of data on the eruption frequency and geochemistry of large rhyolitic volcanoes. The Quaternary Taupo volcanic zone (TVZ) in New Zealand is one of the most frequently active rhyolitic centres on Earth, and for much of the 20th century its tephra beds have been the focus of study. As a result, fingerprinting and dating techniques have been fine-tuned, and tephra beds now under-pin the late Cenozoic chronology for a wide variety of disciplines from volcanology to sequence stratigraphy and archaeology. The key to tephrochronology is the identification and correlation of tephra horizons. In the proximal setting (<50 km from vent), tephra beds can often be identified by their lithology, stratigraphic position and ferromagnesian mineralogy. Farther from source these features became less diagnostic as units become thinner and mineral depleted, and geochemical fingerprinting must then be employed. Grain-specific techniques are commonly required to assess the compositional homogeneity of the tephra, and to avoid xenolithic and detrital contaminants. Criteria that are valuable for identifying both source volcano and individual eruptive events from grain-specific electron microprobe techniques include: glass and Fe–Ti oxide chemistry, and eruption temperature and oxygen fugacity (estimated from oxide equilibrium pairs). Deposits of the contemporaneously active Taupo, Okataina, Maroa and Mayor Island caldera centres in the TVZ can be distinguished on the basis of such criteria. If a tephra lacks contaminants, trace- and rare earth element (REE) compositions of purified glass separates provide additional criteria. Similar approaches can be used on samples of the chilled, nonwelded bases of ignimbrites, while welded zones often display a characteristic thermal remnant magnetism (TRM) direction that can assist in correlation. Late Miocene and Pliocene rhyolitic tephra from the Coromandel region in New Zealand are compositionally indistinguishable from Quaternary TVZ deposits, but deposits from other SW Pacific provinces such as Antarctica and the Tonga–Kermadec arc are compositionally distinct. In the TVZ, rhyolitic tephra erupted during intervals of 10–20 ka from the same volcano are compositionally similar, while over longer periods or following large-volume eruptions the tephra are compositionally and mineralogically distinct. Most TVZ fall deposits are compositionally homogeneous, and there are no temporal compositional trends for individual volcanoes. Tephra beds are also valuable because they can be dated by a variety of methods, and therefore can provide direct age control for sequences that are not normally amenable to radiometric dating. For late Cenozoic tephra beds that are beyond the range of 14 C (ca. 40 ka), and are fine-grained and/or lack K-rich phenocrysts, the isothermal plateau fission-track (ITPFT) method using glass can provide an accurate and precise age. 40 Ar – 39 Ar can be employed for high-resolution dating, however this may require single crystal laser fusion to detect relict and partially degassed crystals that are common in pyroclastic deposits, even within single pumice blocks. The New Zealand tephrostratigraphic record is not complete in space or time. Only the post-64 ka record is well established. However, there are chronologically well-constrained tephra beds dating back to 10 Ma. Some tephra beds cover most of the North Island of New Zealand, are found in the South Island, and extend as much as 1400 km from vent into the Pacific Ocean. Widespread and/or stratigraphically important units include: Kaharoa (665 years BP); Kawakawa (22 ka); Rangitawa (0.33 Ma); Potaka (1 Ma) and Pakihikura (1.6 Ma). Ages on these and other tephra beds provide a framework for global climatic and sea-level change recorded in New Zealand, and allow direct correlation between the marine and terrestrial realms.

New zircon fission-track ages from New Zealand Quaternary tephra: an interlaboratory experiment and recommendations for the determination of young ages

Fission-track dating, of zircons from five Pleistocene tephra in the Wanganui Basin, North Island, New Zealand, has been carried out independently in two different laboratories. The results from each are statistically identical even though there were some differences in laboratory techniques. The resulting ages confirm recent isothermal plateau fission-track (ITPFT) ages on two of these tephra, but with increased precision. The weighted mean ages (± 1σ) are: Potaka Tephra, 0.97 ± 0.04 Ma (4 determinations); Rewa Pumice, 1.00 ± 0.06 Ma (3 determinations); Mangapipi Ash, 1.26 ± 0.08 Ma (2 determinations); Pakihikura Pumice, 1.66 ± 0.07 Ma (4 determinations); and the Vinegar Hill Tephra, 1.73 ± 0.08 Ma (3 determinations). These new results further constrain the time control in this important Pleistocene section which contains many cyclothems reflecting climatic and tectonic influences. This study illustrates well the pitfalls that can occur when dating such young material using single-crystal techniques. The difficulties of determining when etching is optimal, the presence of detrital grains and their subsequent removal from the data set are described, as well as a recommendation that a minimum of 15 crystals, but optimally at least 20, be counted before calculating the final age. It may also be necessary to plot running means of the single-crystal ages to determine when the final age has been approached.

Chronological and palaeomagnetic constraints on widespread welded ignimbrites of the Taupo volcanic zone, New Zealand

Large volume (100–1000 km3), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the non-welded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25±0.12 Ma), Whakamaru group (0.34±0.03 Ma), Matahina (0.34±0.02 Ma), Chimp (0.33±0.02 Ma), Kaingaroa (0.31±0.01 Ma) and Mamaku (0.23±0.01 Ma) ignimbrites. These glasses show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation. The glass ages we have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases. The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of 40Ar/39Ar. Errors as limited as 10–30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of caldera formation at 0.34–0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed by the formation of the Okataina and Reporoa calderas in rapid succession.

Isothermal plateau fission-track ages of tephra beds in an Early-Middle Pleistocene marine and terrestrial sequence, Cape Kidnappers, New Zealand

Abstract At least nine rhyolitic eruptive events or sequences are interbedded with alternating marine and terrestrial sediments of Early-Middle Pleistocene age at Cape Kidnappers, New Zealand. The sediments represent one of the few high resolution glacio-eustatic sea level records in New Zealand. We have obtained ages of 0.99±0.09, 1.00±0.10, 0.86±0.05, and 0.79±0.06 Ma (in ascending stratigraphic order) for four prominent tephra beds, from isothermal plateau fission track dating of their glass shards. These ages are older than previous fission track determinations, but are in agreement with tephra correlations to other sections, Ar/Ar data, and magnetostratigraphy. However, the upper part of the sequence, previously interpreted as the Brunhes Chron, may have been affected by normal overprinting and is instead older. The tephra record suggests a higher frequency of eruptions than is indicated by the proximal ignimbrite record in the Taupo Volcanic Zone. The new tephra ages indicate that the section was deposited in the interval 1.0-0.65 Ma at a rapid average sedimentation rate of ca. 1 m/ka.

Multi-method dating comparison for mid-pleistocene Rangitawa Tephra, New Zealand

The Rangitawa Tephra, a widespread mid-Pleistocene marker in the New Zealand region, is dated by fission-track, thermoluminescence (TL), infrared stimulated luminescence (IRSL), K Ar and single crystal laser fusion (SCLF) 40 Ar 39 Ar methods, as well as by matching to the astronomical timescale via oxygen isotope stratigraphy in deep sea cores. Fifty one independent numerical age estimates for Rangitawa Tephra range from 173 to 456 ka. However, only 15 of these results are considered to be reliable: five zircon fission-track results, four isothermal plateau fission-track (ITPFT) age estimates on glass shards, four SCLF 40 Ar 39 Ar results, and two astronomically calibrated age estimates from deep sea cores. The weighted mean of the nine reliable fission-track age determinations is 345 ± 12 ka, in excellent agreement with the weighted mean of 340 ± 7 ka for the two reliable astronomically calibrated age estimates. The weighted mean of the four reliable 40 Ar 39 Ar results (302 ± 8 ka) is significantly younger, and further Ar Ar dating is recommended to investigate this difference. Since the TL and IRSL dating methods are experimental for sediments beyond 200 ka, we have excluded these from the group of reliable age results. TL and IRSL results on dunesand appear to be systematically younger than TL results on loess, and loess results depend upon analytical procedures.

Integrated tephrochronology and magnetostratigraphy for cyclothemic marine strata, Wanganui Basin: Implications for the Pliocene-Pleistocene boundary in New Zealand

Abstract The middle Pliocene to earliest Pleistocene part of the Rangitikei River section in Wanganui Basin contains at least six distal tephras—five are rhyolitic (Kowhai, Eagle Hill, Ohingaiti, Waipura and Vinegar Hill tephras) and one is andesitic (Mangamako Tephra). All tephras occur as macroscopic layers and are enclosed within cyclothemic marine sediments that accumulated in shoreface to outer shelf water depths under conditions of oscillating sea level driven by glacio-eustacy. The glass and ferromagnesian compositions of the silicic tephras imply derivation from calc-alkaline rhyolitic sources, either or both of Taupo Volcanic Zone and southern Coromandel Volcanic Zone. The Waipuru and Vinegar Hill tephras have isothermal plateau fission-track (ITPFT) ages of 1.87 ± 0.15 Ma and 1.75 ± 0.13 Ma, respectively. Paleomagnetic results allow identification of (i) the Gauss/Matuyama boundary located ca. 70 m below the top of the Mangaweka Mudstone; and (ii) the top of the Olduvai Subchron, placed at the base of the Waipuru Shellbed. The ITPFT ages of the tephras are consistent with the interpreted magnetostratigraphy and also with astronomically tuned timescales. Correlation of the Rangitikei sequences to the ODP site 846 δ18O record indicates that each sequence corresponds to a glacial/interglacial isotope stage couplet, and allows ages to be interpolated for tephras not dated directly. Integration of the new ages for the Waipuru and Vinegar Hill tephras, the magnetostratigraphy, and the sequence correlations to the astronomically tuned oxygen isotope timescale, lead us to place the Pliocene-Pleistocene boundary, as defined at the Vrica stratotype, at the base of the highstand systems tract siltstone of Sequence 17 in the Rangitikei section, 60 m below the Vinegar Hill Tephra.

The correlation between reduction in fission-track diameter and areal track density in volcanic glass shards and its application in dating tephra beds

The relationship between the reduction in fission-track diameter and the reduction in areal track density in hydrated glass shards has been investigated under varying annealing and etching conditions. A near 1:1 relation is observed for adequately etched tracks. Following laboratory heating of induced tracks, the partially reduced areal track density was renormalized (with error < 10%) using the simple 1:1 relation between reduced diameter and reduced areal density. A similar approach was employed to correct for natural fading in spontaneous tracks. The apparent ages of 11 tephra beds, determined by applying the population-subtraction fission-track method to their hydrated glass shards, were corrected for partially lowered spontaneous track density in this way. These corrected ages are in good agreement with those obtained by the isothermal plateau fission track (ITPFT) method and other independent dating techniques.

Chronology of Pliocene and Quaternary bioevents and climatic events from fission-track ages on tephra beds, Wairarapa, New Zealand

High-resolution Pliocene and Pleistocene sequences exposed on land in New Zealand are some of the few detailed records of widepread marine bioevents and paleoclimatic changes in the Southern Hemisphere. Marine biostratigraphy calibrated in deep-sea cores by paleomagnetic reversals has been the primary basis for the chronology of these sequences. We have determined ages for several tephra beds which now provide an independent numerical age calibration for a well-studied marine and terrestrial section in Wairarapa. By using the isothermal plateau fission track (ITPFT) method on volcanic glass we have overcome the problems of partial track fading and detrital mineral contamination, which hindered earlier studies, to reveal a new chronology extending back to nearly 5 Ma. Our ages for the Hikawera Tuff (4.91 ± 0.25 Ma) and Spooner Tuff (3.44 ± 0.13 Ma) are consistent with the appearance and disappearance of many early Pliocene foraminiferial species, validating their age calibration in New Zealand. However, some fossil occurrences, including coccoliths, differ temporally by as much as 0.55 Ma, perhaps due to local tectonic-induced recycling. Four Pleistocene tephra beds (Potaka tephra (1.00 ± 0.03 Ma), Kaukatea tephra (0.87 ± 0.05 Ma), Rangitawa tephra (ca. 0.35 Ma) and Kawakawa tephra (ca. 0.22 Ma)) are now recognised in the Wairarapa sequence via stratigraphic and new geochemical and age data. These beds allow direct correlation to other marine and terrestrial basins, as well as volcanic regions in New Zealand, and will ultimately aid in a regional paleoenvironmental reconstruction where bioevents are absent. The tephra ages indicate that the marine sediment accumulation rates varied from 90 to 250 m/Ma between different sections of the Pliocene and reached ca. 350 m/Ma in the last 2.4 Ma, when the sequence displays pronounced glacioeustatic cyclic deposition. In the terrestrial realm, the oldest loess in New Zealand is now constrained to between 1.00 and 0.87 Ma.

Isothermal plateau fission‐Track age for a paleomagnetic excursion in the Mamaku Ignimbrite, New Zealand, and implications for Late Quaternary stratigraphy

Mamaku Ignimbrite, the youngest of the large, welded ignimbrite sheet‐forming eruptions from the Taupo Volcanic Zone in New Zealand, is dated at 230±12 ka by the isothermal plateau fission‐track (ITPFT) method on glass. This age is older and more precise than that indicated by previous studies, requiring the revision of loess/paleosol coverbed chronologies. The eruption occurred during a paleomagnetic excursion allowing the ignimbrite to be easily distinguished from other eruptive events by paleomagnetic methods. This excursion is one of the few such events recorded in igneous rocks in the Southern Hemisphere, may be widely recorded in New Zealand, and is now temporally well constrained.

Magnetostratigraphic, lithostratigraphic and tephrostratigraphic constraints on Lower and Middle Pleistocene sea-level changes, Wanganui Basin, New Zealand

Wanganui Basin, North Island, New Zealand, contains a complex sedimentary record of Lower and Middle Pleistocene sea-level changes. Palaeomagnetic results allow identification of the Matuyama/Brunhes transition, the Jaramillo Subchron and the Cobb Mountain Subchron. Correlations of rhyolitic tuff horizons across the basin are consistent with magnetostratigraphic correlations between sections. Isothermal plateau fission-track (ITPFT) ages of 1.05 ± 0.05 Ma and 1.63 ± 0.15 Ma on two tuffs (Potaka Pumice and Pakihikura Pumice, respectively) are consistent with the interpreted magnetostratigraphy and also with the astronomically tuned timescale of ODP Site 677. Magnetostratigraphy and ITPFT ages allow correlation of sedimentary cycles at Wanganui with odd-numbered oxygen isotope stages 17–31 in deep-sea cores. The stratotype section for the New Zealand Castlecliffian Stage is shown to be incomplete relative to other studied sections in the basin. Below stage 31, the character of the cyclothems changes at Wanganui, from marine dominated, to a greater representation of non-marine and estuarine strata. This change may be partly related to a change in amplitude and frequency of climatic cycles identified in oxygen isotope records, but the primary cause is likely to be increased basinal uplift.

Isothermal plateau fission track age determinations on volcanic glass from pleistocene tephra in New Zealand : Alloway, B.V., Sandhu, A.S., Westgate, J.A., Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 3B1; Walter, R.C. Geochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley, CA 94709

Isothermal plateau fission track age determinations on volcanic glass from pleistocene tephra in New Zealand : Alloway, B.V., Sandhu, A.S., Westgate, J.A., Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 3B1; Walter, R.C. Geochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley, CA 94709

Revision of the marine chronology in the Wanganui Basin, New Zealand, based on the isothermal plateau fission-track dating of tephra horizons

The occurrence of tephra horizons in basins adjacent to volcanic arcs provide an excellent opportunity for establishing a reliable chronostratigraphic framework for detailed sedimentological studies. In this study, three widespread and stratigraphically important rhyolitic tephra horizons interbedded in Plio/Pleistocene strata of the Wanganui Basin, New Zealand, are dated by application of the isothermal plateau fission-track (ITPFT) technique to hydrated glass shards. All glass samples were corrected for annealing and consequently yield reliable ages. Rangitawa Tephra yielded statistically indistinguishable ages from three localities that are in excellent agreement with recently determined zircon fission-track age estimates of ca. 0.35 Ma. ITPFT ages of 1.05 ± 0.05 and 1.63 ± 0.15 Ma for Potaka Pumice and Pakihikura Pumice, respectively, are considerably older than previous FT estimates but consistent with new magnetostratigraphic data that places the Potaka within the Jaramillo Subchron, and Pakihikura within the Matuyama Chron between the Cobb Mountain and Olduvai Subchrons. Combining our fission-track ages with the magnetostratigraphy, the true age of sediments within the Wanganui Basin is found to be significantly underestimated. Sedimentation rates of between ca. 680-630 m/Ma from 1.63 Ma to 0.35 Ma are calculated in the eastern part of the basin and are much lower than those calculated using the previous FT chronology. This new ITPFT-age data demonstrates that the existing Plio/Pleistocene marine chronology in New Zealand will require age revision and has important implications when considering the evolution of several other sedimentary basins in southern North Island that contain the same ITPFT-dated tephra horizons.

Isothermal plateau fission‐track age and revised distribution of the widespread mid‐Pleistocene rockland tephra in west‐central United States

A tephra bed in southeastern Idaho is correlated to Rockland tephra, a widespread silicic air‐fall unit previously identified in northern California and western Nevada. Correlation of proximal Rockland tephra and its distal correlative is based on glass chemistry determined by electron microprobe and neutron activation analysis. An age estimate of proximal Rockland tephra is precisely determined by application of the isothermal plateau fission‐track (ITPFT) technique to its hydrated glass shards. A weighted mean age of 0.47 ± 0.04 Ma compares well with age estimates based on co‐existing mineral phases using the external detector fission‐track and additive‐dose thermoluminescence techniques. The presence of Rockland tephra 800 km east of its presumed eruptive source will improve the development of detailed chronologies and correlation of Pleistocene sedimentary sequences located in coalescent intermontane basins of west‐central United States.

Isothermal plateau correction for partial fading of fission tracks in hydrated glass shards

The age of hydrated glass shards from the Pleistocene Ester Ash Bed at Fairbanks, Alaska, is determined using the population-subtraction fission-track method. The age so determined is then corrected for partial annealing by use of the isothermal plateau fission-track (ITP-FT) technique. In this paper, we describe the mathermatical basis of a procedure for obtaining the correct fission-track age using the ITP-FT technique.

Miocene tephra beds in the Cypress Hills of Saskatchewan, Canada

Six rhyolitic tephra layers from ancient loess and related detritus in the Cypress Hills, Saskatchewan, represent separate volcanic eruptions from the Snake River Plain. Idaho, U.S.A. The weighted mean age and uncertainty of the youngest tephra bed is 8.3 ± 0.2 Ma, using the isothermal plateau fissiontrack technique on its hydrated glass shards. The loess that hosts five of these tephra beds extends across the Cypress Plain, which is the oldest (Middle Miocene) and highest depositional surface in the Interior Plains, and also occurs on four juxtaposed erosion surfaces. It appears that the first and maybe the second erosion surface began forming before 10 Ma, and that formation of the second, third, and fourth erosion surfaces was completed between 10 and 8.3 Ma.