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| Titel |
Combined 40Ar/39Ar and OSL dating of Pleistocene pyroclastic and fluvial deposits of the Cagayan Valley Basin, Northern Luzon, Philippines |
| VerfasserIn |
Gitte M. Jensen, Michael Storey, Richard Roberts, Terry J. Lachlan, Kristina J. Thomsen, Andrew Murray |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034934
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| Zusammenfassung |
| The backarc Cagayan Valley Basin (250 x 80 km) of Northern Luzon, Philippines has a 10
km thick sedimentary infill deposited since the late Oligocene. The upper 900 m consists of
non-marine Plio-Pleistocene fluvial sediments with interbedded pyroclastic deposits. Fossil
vertebrates and stone artefacts occur within the upper part of this succession. The terrestrial
vertebrate fauna is thought to have migrated to Cagayan Valley Basin via one or more land
bridges during the Middle Pleistocene, however, the precise age of the fossil and artefact
sequences is poorly known. In this study, we present the first results of a dating program on
key sections of the Cagayan Pleistocene succession that is being carried out using two
complementary numerical-age dating methods - 40Ar/39Ar and optically stimulated
luminescence, OSL. In addition to providing numerical age information on the
Cagayan Valley Basin Pleistocene succession, our aim is to compare results from the
two different dating methods using data obtained on stratigraphically equivalent
samples.
We sampled below, within, and above the fossil and artefact section of the Pleistocene
succession. Samples for laser fusion, single crystal 40Ar/39Ar dating were taken from both
ignimbrites and airfall ash deposits. The analyses were carried out using a CO2 laser attached
to a low-volume (450 cc) cleanup system and a NU-Instruments Noblesse multi-collector
noble gas mass spectrometer.
In OSL dating the age is determined by dividing the total dose absorbed during burial by
the environmental dose rate. Samples for OSL dating were taken in sand sized fluvial
sediments. These samples contained very little quartz so OSL measurements were undertaken
using potassium-rich feldspar extracts. To determine the dose, conventional infrared (IR)
stimulation of feldspar is usually performed at or close to room temperature. However, this
signal suffers from a significant athermal loss with time (anomalous fading). It has
been shown previously that IR stimulation at elevated temperature following IR
stimulation at 50oC gives rise to signal that is much less subject to this undesirable
phenomenon. Here we use this post-IR IR signal to determine the burial dose in small
aliquots (each ~100 grains) and individual grains. By examining the resulting dose
distributions we are able to deduce the most probable burial dose. Environmental dose
rates were measured using gamma spectrometry both on site and in the laboratory. |
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