A strategy for cross-calibrating U-Pb chronology and astrochronology of sedimentary sequences: An example from the Green River Formation, Wyoming, USA
A strategy for cross-calibrating U-Pb chronology and astrochronology of sedimentary sequences: An example from the Green River Formation, Wyoming, USA
复制标题
DOI：
10.1016/j.epsl.2014.12.009
复制
发表时间：
2015-03
影响因子：
5.3
通讯作者：
M. Machlus;J. Ramezani;S. Bowring;S. Hemming;K. Tsukui;W. Clyde
中科院分区：
文献类型：
--
作者：
M. Machlus;J. Ramezani;S. Bowring;S. Hemming;K. Tsukui;W. Clyde
直到最近，对地质时间刻度的天文校准一直受到放射性同位素日期的精确度和准确度的限制，特别是新近纪以前的记录。较老地层的放射性同位素测年的不确定性通常比单一进动周期大得多，而且日期往往很稀少，因此需要对旋回地层进行轨道调谐，以便天文校准所需的间隔。理想情况下，为了用地质年代学检验天文定标的假设，放射性同位素日期的精确度必须与被测试的周期相当。新的U-PbCA-TIMS(化学磨损-热电离质谱)锆石年龄符合这一精度要求，来自绿河组威尔金斯峰段的7个火山灰的σ分析不确定度为±11000~±52000年。锆石年龄分布简单，离群值很少，可以准确估计喷发年龄，潜在误差小于进动周期。始新世绿河组(美国怀俄明州)长期以来一直被认为是旋回沉积的湖相沉积的记录，丰富的夹层火山灰使其成为测试天文标定旋回地层的新方法的合适场所。不同类型标志层的丰度，包括与沉积旋回夹层的凝灰岩，保证了盆地边缘和旋回性最发达的盆地中心的凝灰岩采样位置之间的明确相关性，从而将任何地层不确定性减少到(假设的)进动旋回的一小部分。被以前的地质年代学接受的基于调谐的轨道年龄模型与新的地质年代学显著背离，而以前被拒绝的假设短偏心周期为125Ky的模型现在被允许。为了检验对表观125Ky周期的可能解释，例如轨道周期的变化或沉积记录中的空白，我们提出了一种迭代策略来选择未来的火山灰进行测年，以便优化天文校准/测试。我们迭代地对比了两个特别的年龄模型，它们包含了日期骨灰之间的线性内插。进一步测年的最佳时间间隔位于模型之间的偏差超过我们报告的不确定性的地方。我们建议，在存在足够的火山灰的情况下，这里描述的迭代方法应该成为建立严格的地层记录轨道校准的标准。
Astronomical calibration of the geological timescale has been limited until recently by the precision and accuracy of radioisotopic dates, especially for pre-Neogene records. Uncertainties for radioisotopic dates of older strata were typically much larger than a single precessional cycle, and dates were often sparse, leading to the practice of orbital tuning of cyclic strata in order to astronomically calibrate the desired interval. Ideally, in order to test the assumptions of astronomical calibration with geochronology, it is necessary that the precision of radioisotopic dates be comparable to the period of the cycle being tested. The new U–Pb CA-TIMS (chemical abrasion–thermal ionization mass spectrometry) zircon dates reported here conform to this precision requirement, with 2σ analytical uncertainties from±11 000 to±52 000 years for seven volcanic ashes from the Wilkins Peak Member of the Green River Formation. The zircon dates have simple distributions with few outliers and allow accurate estimations of the eruption ages with potential inaccuracies of less than precessional cycle. The Eocene Green River Formation (Wyoming, USA) has long been recognized as a record of cyclicly-deposited lacustrine sediments, and the abundant intercalated volcanic ashes make it a suitable place to test new approaches to astronomical calibration of cyclic strata. The abundance of different types of marker beds, including tuffs that are intercalated with the sedimentary cycles, guarantee an unambiguous correlation between sampling locations of dated tuffs on the margins of the basin and the basin center where the cyclicity is best developed, thus reducing any stratigraphic uncertainties to a fraction of (hypothesized) precession cycle. Tuning-based orbital age models, accepted by the previous geochronology, significantly deviate from the new geochronology, whereas a previously rejected model that assumes a short eccentricity period of 125 ky is now allowed. In order to test possible explanations for the apparent 125 ky period, such as changes in orbital periods, or gaps in the sedimentary record, we present an iterative strategy to select future ashes for dating such that the astronomical calibration/testing is optimized. We iteratively contrast two ad-hoc age models that bracket the linear interpolation between the dated ashes. The optimal intervals for further dating are located where the deviations between the models exceed our reported uncertainties. We propose that the iterative approach described here should become the standard for establishing a rigorous orbital calibration of the stratigraphic record where sufficient ashes exist.