First U-Pb zircon ages for late Miocene Ashfall Konservat-Lagerstätte and Grove Lake ashes from eastern Great Plains, USA

This paper documents the first U-Pb zircon ages for Ashfall Fossil Beds (Nebraska, USA), a terrestrial Konservat-Lagerstätte mass-death assemblage that is arguably the most diverse of its type and age. The Ashfall tephra was correlated with ignimbrites from the Bruneau-Jarbidge volcanic field (12.7–10.5 Ma) in southwest Idaho based on geochemical analysis. The methods and geochemical data supporting the original age assessment of the ash bed, however, were never published, and there has been a persistent misconception that dateable heavy minerals (e.g., zircon) are absent. Notwithstanding, we recovered abundant zircons from Ashfall Fossil Beds, and from an ash bed ~6 km to the southeast at Grove Lake, Nebraska, and analyzed them through LA-ICP-MS. Our new zircon U-Pb age of 11.86 ± 0.13 Ma substantiates correlation of the Ashfall Fossil Beds deposit to tuffs originating from the Bruneau-Jarbidge caldera (~12.7–10.5 Ma). Our U-Pb zircon age of 6.42 ± 0.06 Ma for the Grove Lake ash bed coincides with supervolcanic activity in the Heise volcanic field (6.6–4.3 Ma) in eastern Idaho. These new dates improve age constraints of strata comprising the Ogallala Group and the important paleontological site. Moreover, we find that detrital and airfall zircons are unevenly distributed in the stratified ash beds we describe herein and presumably in similar deposits worldwide. Therefore, a higher-resolution sampling scheme is necessary in such cases.本论文首次报道了美国内布拉斯加州阿什福斯化石床的铀-铅（U-Pb）锆石年龄数据。该遗址是一处陆生特异埋藏化石库（Konservat-Lagerstätte）集群死亡组合，被认为是同类和同年代标本中多样性最高的一处。研究人员曾通过地球化学分析，将阿什福斯火山灰与爱达荷州西南部布鲁诺-贾比奇火山岩区（12.7~10.5 Ma）的熔结凝灰岩进行了对比。然而，支撑该火山灰层原始年龄评估的方法学与地球化学数据从未公开发表，且长期存在一种错误认知，认为该遗址中不存在可用于定年的重矿物（如锆石）。尽管如此，我们仍从阿什福斯化石床，以及内布拉斯加州格罗夫湖附近东南约6公里处的一处火山灰层中分离得到了大量锆石，并通过激光剥蚀电感耦合等离子体质谱法（LA-ICP-MS）对其开展了分析。我们测得的新锆石U-Pb年龄为11.86±0.13 Ma，证实了阿什福斯化石床沉积物可与布鲁诺-贾比奇破火山口（~12.7~10.5 Ma）形成的凝灰岩相对比。我们对格罗夫湖火山灰层测得的U-Pb锆石年龄为6.42±0.06 Ma，与爱达荷州东部Heise火山岩区（6.6~4.3 Ma）的超级火山活动时间相吻合。这些新的定年结果优化了奥加拉拉群（Ogallala Group）地层以及这一重要古生物遗址的年代约束。此外，我们发现在所描述的层理火山灰层中，碎屑锆石与大气沉降锆石的分布并不均匀，推测全球范围内类似沉积地层中也存在这一现象。因此，在这类研究中采用更高分辨率的采样方案是十分必要的。