Revised age and regional correlations of Cenozoic strata on Bat Mountain, Death Valley region, California, USA, from zircon U-Pb geochronology of sandstones and ash-fall tuffs
Revised age and regional correlations of Cenozoic strata on Bat Mountain, Death Valley region, California, USA, from zircon U-Pb geochronology of sandstones and ash-fall tuffs
复制标题
根据砂岩和落灰凝灰岩的锆石 U-Pb 地质年代学修正美国加利福尼亚州死亡谷地区蝙蝠山新生代地层的年龄和区域相关性
作者：
T. Schwartz;A. K. Souders;Jens;A. Gilmer;Ren A. Thompson
美国加州的葡萄藤和Funeral山暴露的许多地层序列中缺乏火山（凝灰岩）年龄控制，阻碍了加州死亡谷地区新生代历史的盆地分析和构造重建。虽然从碎屑锆石U-Pb数据解释的最大沉积年龄（MDA）可能是火山年龄的一个有前途的替代方法，但关于MDA的计算仍然存在争议，包括但不限于要考虑的“年轻”颗粒的数量（即，用于计算MDA的日期谱）;哪些颗粒（如果有的话）应该被忽略;哪些方法产生的结果在统计上是严格的;以及最终，哪些方法产生的年龄在地质上是合理的。我们比较了加利福尼亚州南部葬礼山蝙蝠山新生代地层中暴露的五个砂岩样品的碎屑锆石MDA的常用指标，即，最年轻的单个晶粒（YSG）、在1σ不确定度下的两个或更多个晶粒的最年轻晶粒簇（YC 1 σ（2+））和在2σ不确定度下的三个或更多个晶粒的最年轻晶粒簇（YC 2 σ（3+））的加权平均值、最年轻的图形峰值（YPP）和最大似然年龄（MLA）。每一个砂岩样品产生了丰富的新生代锆石U-Pb年龄，形成单峰，近正常的年龄分布，清楚地区别于下一个最古老的颗粒在每个样品中，并表现出明显的上升部分的峰值年龄下降。以已发表的落灰凝灰岩的K/Ar和40 Ar/39 Ar年龄以及5个新的锆石U-Pb年龄为基准，我们的分析与前人的研究相平行，并表明许多MDA指标YSG、YC 1 σ（2+）、YC 2 σ（3+）和YPP-向不合理的年轻或古老值漂移。与此相反，最大似然估计方法和由此产生的MLA度量一致地产生地质适当的估计MDA没有任意遗漏任何年轻（或老）的锆石日期。利用砂岩的MLA和层间灰凝灰岩的锆石U-Pb年龄，建立了渐新世-中新世阿马戈萨谷组（沉积于约1000年）的新的年龄模式。28.5-18.5 Ma）和中新世蝙蝠山组（沉积于约28.5-18.5 Ma）。15.5-13.5 Ma），并修订了与整个东部死亡谷地区新生代地层的相关性。
Basin analysis and tectonic reconstructions of the Cenozoic history of the Death Valley region, California, USA, are hindered by a lack of volcanic (tuff) age control in many stratigraphic successions exposed in the Grapevine and Funeral Mountains of California, USA. Although maximum depositional ages (MDAs) interpreted from detrital zircon U-Pb data may be a promising alternative to volcanic ages, arguments remain regarding the calculation of MDAs including, but not limited to, the number of “young” grains to consider (i.e., the spectrum of dates used to calculate the MDA); which grains, if any, should be ignored; which approaches yield results that are statistically rigorous; and ultimately, which approaches result in ages that are geologically reasonable. We compare commonly used metrics of detrital zircon MDA for five sandstone samples from the Cenozoic strata exposed on Bat Mountain in the southern Funeral Mountains of California—i.e., the youngest single grain (YSG), the weighted mean of the youngest grain cluster of two or more grains at 1σ uncertainty (YC1σ(2+)) and of three or more grains at 2σ uncertainty (YC2σ(3+)), the youngest graphical peak (YPP), and the maximum likelihood age (MLA). Every sandstone sample yielded abundant Cenozoic zircon U-Pb dates that formed unimodal, near-normal age distributions that were clearly distinguishable from the next-oldest grains in each sample and showed an apparent up-section decrease in peak age. Benchmarked against published K/Ar and 40Ar/39Ar ages and five new zircon U-Pb ages of ash-fall tuffs, our analysis parallels prior studies and demonstrates that many MDA metrics—YSG, YC1σ(2+), YC2σ(3+), and YPP—drift toward unreasonably young or old values. In contrast, the maximum likelihood estimation approach and the resulting MLA metric consistently produce geologically appropriate estimates of MDA without arbitrary omission of any young (or old) zircon dates. Using the MLAs of sandstones and zircon U-Pb ages of interbedded ash-fall tuffs, we develop a new age model for the Oligocene–Miocene Amargosa Valley Formation (deposited ca. 28.5–18.5 Ma) and the Miocene Bat Mountain Formation (deposited ca. 15.5–13.5 Ma) and revise correlations to Cenozoic strata across the eastern Death Valley region.