A note of caution on the use of boulders for exposure dating of depositional surfaces
A note of caution on the use of boulders for exposure dating of depositional surfaces
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DOI：
10.1016/j.epsl.2010.11.039
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发表时间：
2011-02
影响因子：
5.3
通讯作者：
Silke R. Schmidt;R. Hetzel;J. Kuhlmann;F. Mingorance;V. Ramos
中科院分区：
文献类型：
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作者：
Silke R. Schmidt;R. Hetzel;J. Kuhlmann;F. Mingorance;V. Ramos
在大多数漂砾中沉积前的核素成分可以忽略不计的假设下，漂砾的暴露测年法已被广泛应用于确定沉积表面的年龄。在这里，我们提出了一个案例研究的河流阶地在活跃的安第斯山脉东部，在这种假设显然是无效的，因为砂岩巨石（n=13）从梯田在两个网站包含一个高度可变的inherited 10 Be组件和有明显的10 Be年龄超过各自的表面的年龄高达~ 90万年。同样，来自活跃河道的漂砾（n=5）含有大量的继承10 Be成分，相当于5- 48 ka的暴露量。河流阶地的年龄可以通过两种方法来确定，这两种方法可以纠正沉积前的核素成分：10 Be测年的混合卵石和10 Be的深度剖面的砂样。在1号站，三个阶地的~（10）Be年龄分别为3- 5 ka（T_2）、11- 13 ka（T_3）和16- 20 ka（T_4），与阶地地层学一致。T_3阶地的年龄由木样~（14）C年龄（12.61± 0.20kaBP）确定。T3阶地的~（10）Be年龄为13- 16 ka。从深度剖面和河流沉积物中确定的沙粒的平均继承10 Be浓度很小，相当于1- 3 ka的暴露量。相比之下，卵石和巨砾的平均遗传更高，分别相当于约10 ka和约30 ka的暴露时间。沉积前核素组分的这些差异与砂、卵石和巨砾的不同物源和搬运历史有关。砂来自于暴露在山前附近的中新世沉积物的快速侵蚀，而卵石和巨砾则来自于褶皱冲断带内部的三叠纪砂岩。在前往山前的途中，巨石和卵石被暂时储存在冲积扇中，并在目前正在重新加工的冲积扇中进行辐照。由于在安第斯山脉和其他褶皱冲断带，在山内盆地沉积沉积物并随后进行挖掘是很常见的，因此，在对活动的山前地带进行暴露测年时，应评估沉积前核素成分的存在。
Exposure dating of boulders has been widely applied to determine the age of depositional surfaces under the assumption that the pre-depositional nuclide component in most boulders is negligible. Here we present a case study on fluvial terraces at the active mountain front of the eastern Andes, where this assumption is clearly invalid, because sandstone boulders (n=13) from terraces at two sites contain a highly variable inherited10Be component and have apparent10Be ages that exceed the age of the respective surface by up to ~90ka. Likewise, boulders from active stream channels (n=5) contain a substantial inherited10Be component, equivalent to 5–48ka of exposure. The age of the fluvial terraces is well determined by two approaches that allow to correct for the pre-depositional nuclide component:10Be dating of amalgamated pebbles and10Be depth profiles on sand samples. At site 1, three terraces have10Be ages of 3–5ka (T2), 11–13ka (T3), and 16–20ka (T4), which are consistent with the terrace stratigraphy. The age of terrace T3is confirmed by a calibrated14C age of 12.61±0.20ka BP obtained from a wood sample. At site 2, terrace T3has a10Be age of 13–16ka. The average inherited10Be concentration of sand grains – determined from depth profiles and stream sediments – is small and equivalent to 1–3ka of exposure. In contrast, the mean inheritance of pebbles and boulders is higher and equivalent to exposure times of ~10ka and ~30ka, respectively. These differences in the pre-depositional nuclide component are related to the different provenance and transport history of sand, pebbles, and boulders. The sand is derived from rapidly eroding Miocene sediments exposed near the mountain front, whereas the pebbles and boulders originate from Triassic sandstones in the internal part of the fold-and-thrust belt. On their way to the mountain front, boulders and pebbles were temporarily stored and irradiated in alluvial fans that are currently reworked. As sediment deposition in intramontane basins and their subsequent excavation is common in the Andes and other fold-and-thrust belts, the presence of pre-depositional nuclide components should be evaluated when applying exposure dating at active mountain fronts.