Tephrochronology in the La Sal Mountains, Utah, USA

在美国La Sal山脉的一条公路上发现了一层tephra层，这是美国新墨西哥州Jemez山脉约1.65 Ma前爆发的Guaje tephra的一个管道。先前的工作已经提供了证据，表明这个温层可能在第二个位置，即它可能在第一次沉积后很久才被重新加工。这些成分——包括重矿物、火山玻璃和颗粒大小——很可能来自热毯，它们在剖面上的位置比热毯本身要深得多。尽管有这种假定的改造，但该层并没有受到周围材料的显著污染。这种纯净的热液通常被解释为首次沉积。如果这一层实际上已经被重新加工过，它就不适合作为一个色年代学工具。本项目拟通过电子探针、EDX和锆石测年等手段，明确该火山层是在喷发和首次堆积后很久才被改造和沉积，还是只是不同火山喷出物的剪切巧合。如果有证据支持这一假说，那么我们必须比迄今为止更加谨慎地对待这一研究结果。

A tephra layer in a roadcut in the La Sal Mountains, USA, was identified as an educt of the Guaje Tephra, which erupted some 1.65 Ma ago in the Jemez Mountains, New Mexico, USA. Previous work has provided evidence that this tephra layer might be in a second position, i.e. it might have been reworked long after its first deposition. Components - involving heavy minerals, volcanic glasses, and grain sizes - that likely have been derived from the tephra were identified much deeper in the profile than the distinct tephra layer itself. Despite of this presumed reworking, the layer has not been remarkably contaminated by surrounding materials. Such pure tephra is usually interpreted to be in first deposition. If this layer actually has been reworked, it is not suitable as a tephrochronological tool. The project intends to clearify whether this tephra layer has been reworked and deposited long after its eruption and first accumulation or whether there is just shear coincidence of different volcanic ejecta using electron microprobe, EDX, and zircon dating. Should there be support for the reworking hypothesis, tephrochronological findings must be treaded much more warily than to date.