Amanjit S. Sandhu

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

Abstract 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.

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

Abstract 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

Optimizing the isothermal plateau fission-track dating method for volcanic glass shards

Abstract The dating of volcanic hydrated glass shards presents considerable difficulties. However, within recent years the fission-track method based on the isothermal plateau technique has been successfully applied to such glasses. Age estimates obtained by this means compare very well with those based on co-existing mineral phases using the K-Ar, 40 Ar/ 39 Ar and thermoluminescence techniques. Annealing studies of nuclear tracks in the temperature range of 100–300°C have been made for various volcanic glass samples. The activation energy, E a , for track annealing is determined in the frame of a model involving a unique value of E a for a given glass sample. These annealing studies suggest that the thermal stability of nuclear tracks in volcanic glasses is mainly controlled by the SiO 2 /Al 2 O 3 ratio.

Recurring middle Pleistocene outburst floods in east-central Alaska

Abstract Recurring glacial outburst floods from the Yukon-Tanana Upland are inferred from sediments exposed along the Yukon River near the mouth of Charley River in east-central Alaska. Deposits range from imbricate gravel and granules indicating flow locally extending up the Yukon valley, to more distal sediments consisting of at least 10 couplets of planar sands, granules, and climbing ripples with up-valley paleocurrent indicators overlain by massive silt. An interglacial organic silt, occurring within the sequence, indicates at least two flood events are associated with an earlier glaciation, and at least three flood events are associated with a later glaciation which postdates the organic silt. A minimum age for the floods is provided by a glass fission track age of 560,000 ± 80,000 yr on the GI tephra, which occurs 8 m above the flood beds. A maximum age of 780,000 yr for the floods is based on normal magnetic polarity of the sediments. These age constraints allow us to correlate the flood events to the early-middle Pleistocene. And further, the outburst floods indicate extensive glaciation of the Yukon-Tanana Upland during the early-middle Pleistocene, likely representing the most extensive Pleistocene glaciation of the area.

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

Abstract 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.