| Chronology is the key issue Quraternary Geology and Geomorphology research. Reliable dating is a prerequisite to interpret tectonic movement, rate of Earth surface processes and climate change process. Among various dating targets in the geologic or geomorphologic settings, the Quaternary gravels and fluvial terrace deposit are notorioiusly difficult to date.Development of Accelerator mass spectrometry (AMS) technique in the past several decades permit the cosmogenic nuclides burial dating in many conditions from Pliocene to Pleistocene (0.3-5Ma). Although it has been applied to a variety of settings, especially successful in dating cave clastic sedimentation, it suffers from several shortcomings as it requires buried quickl and depths enough that limit its application to other conditions of geology and geomorphology. Recently, Granger developed a new isochron method for burial dating which sampled from a single stratigraphic horizon. All samples from the same burial depth will have an identical level of postburial production, but different samples may have different levels of the inherited component. A plot of26Al vs.10Be will yield a line whose slope is dependent on the burial age. This isochron method offers several advantages over previous burial dating techniques, it is able to remove any assumptions about burial depth and its changes over time, in addition, it allows one to test for internal consistency. So this method can be used in dating the late Cenozoic conglomerate and river terraces. However, this approach has not been verified with other absolute chronology methods.The famous two lava layers near Pulu, Xinjiang, with a gravel bed sandwiched in, provides an ideal place to carry out the cross checking. The upper and lower lava at Pulu has been dated by K-Ar and40Ar-39Ar method to be1.21±0.03Ma (1.20±0.05Ma) and1.43±0.03Ma (1.41±0.05Ma) respectivley (Liu et al.,1989; Li et al,2008). It provided a reliable age control for the gravel embedded in. We carried out both the simple and isochrom burial dating on the gravel bed between the lava layers. Purification of quartz were processed in Nanjing Normal University, Be and Al chemical extraction and subsequent AMS measurement were conducted at the PRIME Lab, Purdue University, USA.Three samples, PL-1, PL-2and PL-3demonstrated low current behavior in the Al target AMS measurement, the results of these three samples show large errors and were not discussed in the paper. PL-4, sampled at the bottom from the gravel bed sandwiched in the lava layers, is1.46(+0.46-0.40) Ma, largely agree with the K-Ar dating. The8individual gravels sampled from this gravel bed construct a reliable isochron regression, resulted the age of1.43±0.05Ma, fitting well with the K-Ar dating constrains. The intercept of the regression shows there are postburial nuclide concentration, equivalent to about2000years surface exposure at Pulu, with10Be production rate at22at/g/yr.This study provided a direct comparison of26Al/10Be burial dating with K-Ar dating. It shows under ideal burial conditions fulfilling the request of simple burial dating, simple burial dating can result reliable data. The isochron burial dating will provide broader use in the dating of late Cenozoic conglomerate and fluvial terrace, which generally suffer from complicated burial and exposure history, and not-enough burial. |
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