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首页> 外文期刊>Quaternary geochronology >Luminescence dating without sand lenses: An application of OSL to coarse-grained alluvial fan deposits of the Lost River Range, Idaho, USA
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Luminescence dating without sand lenses: An application of OSL to coarse-grained alluvial fan deposits of the Lost River Range, Idaho, USA

机译：不带沙透镜的发光测年：OSL在美国爱达荷州失落河山脉的粗粒冲积扇沉积物中的应用

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Optically stimulated luminescence (OSL) dating is increasingly used to estimate the age of fluvial deposits. A significant limitation, however, has been that conventional techniques of sampling and dose rate estimation are suitable only for thick (>60 cm) layers consisting of sand size or finer grains. Application of OSL dating to deposits lacking such layers remains a significant challenge. Alluvial fans along the western front of the Lost River Range in east-central Idaho, USA are one example. Deposits are typically pebble to cobble sheetflood gravels with a sandy matrix but thin to absent sand lenses. As a result, the majority of samples for this project were collected by excavating matrix material from gravelly deposits under light-safe tarps or at night. To examine the contributions of different grain-size fractions to calculated dose-rates, multiple grain-size fractions were analyzed using ICPeMS, high resolution gamma spectrometry and XRF. Dose rates from bulk sediment samples were 0.4-40% (mean of 18%) lower than dose-rate estimates from the sand-size fractions alone, illustrating the importance of representative sampling for dose rate determination. We attribute the difference to the low dose-rate contribution from radio-nuclide poor carbonate pebbles and cobbles that occur disproportionately in clast sizes larger than sand. Where possible, dose rates were based on bulk sediment samples since they integrate the dose-rate contribution from all grain sizes. Equivalent dose distributions showed little evidence for partial bleaching. However, many samples had significant kurtosis and/or overdispersion, possibly due to grain-size related microdosimetry effects, accumulation of pedogenic carbonate or postdepositional sediment mixing. Our OSL age estimates range from 4 to 120 ka, preserve stratigraphic and geomorphic order, and show good agreement with independent ages from tephra correlation and Useries dating of pedogenic carbonate. Furthermore, multiple samples from the same deposit produced ages in good agreement. This study demonstrates that with modified sampling methods and careful consideration of the dose rate, OSL dating can be successfully applied to coarse-grained deposits of climatic and tectonic significance that may be difficult to date by other methods.
机译：光激发发光（OSL）测年法越来越多地用于估算河流沉积物的年龄。但是，一个明显的限制是，常规的采样和剂量率估算技术仅适用于由沙粒或更细的颗粒组成的厚（> 60 cm）层。将OSL测年法应用于缺乏这种层的沉积物仍然是一个重大挑战。一个例子就是美国爱达荷州中东部失落河山脉西侧的冲积扇。沉积物通常是卵石，卵石砾石铺平砂砾状基质，但砂砾较薄至无。结果，该项目的大多数样本是通过在光安全篷布下或晚上从砾石沉积物中挖掘基质材料而收集的。为了检查不同粒度分数对计算的剂量率的贡献，使用ICPeMS，高分辨率伽马能谱仪和XRF分析了多个粒度分数。散装沉积物样品的剂量率比仅砂粒级分的剂量率估计值低0.4-40％（平均值为18％），这说明了代表性采样对于确定剂量率的重要性。我们将差异归因于放射性核素差的碳酸盐小卵石和卵石的剂量率贡献低，这些卵石不成比例地发生在比沙子大的碎屑中。在可能的情况下，剂量率基于散装沉积物样品，因为它们综合了所有晶粒尺寸的剂量率贡献。等效剂量分布几乎没有显示出部分漂白的证据。但是，许多样品具有明显的峰度和/或过度分散，这可能是由于与粒度相关的微剂量学效应，成岩碳酸盐的堆积或沉积后沉积物的混合。我们的OSL年龄估计范围为4至120 ka，保留了地层和地貌顺序，并且显示了与独立的年龄相关联，这些年龄来自特非拉相关性和成岩碳酸盐的Useries日期。此外，来自同一矿床的多个样品产生的年龄一致。这项研究表明，使用改进的采样方法并仔细考虑剂量率，OSL测年可以成功地应用于具有气候和构造意义的粗粒矿床，而其他方法可能很难确定该年龄。

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