Since the publication of 40Ar/39Ar dates from Cretaceous bentonites in the Western Interior Basin by J.D. Obradovich in 1993 and in Japan by J.D. Obradovich and colleagues in 2002, improvements in the 40Ar/39Ar method have included a shift to astronomically calibrated ages for standard minerals and development of a new generation of multi-collector mass spectrometers. Thus, the 40Ar/39Ar chronometer can yield results that are synchronous with U-Pb zircon dates and astrochronologic age models for Cretaceous strata. Ages determined by Obradovich have ± 2σ analytical uncertainties of ± 400 ka (excluding J value or systematic contributions) that have been used to discriminate stratigraphic events at ca. 1 Ma resolution. From among several dozen sanidine samples, 32 of which were dated by Obradovich in 1993, we present new multi-collector 40Ar/39Ar ages that reduce the average analytical uncertainties by nearly an order of magnitude. These new ages (where the uncertainties also include the contribution of the neutron fluence J value) include:Topmost Bentonite, Mowry Shale, Kaycee, Wyoming, USA, 97.52 ± 0.09 MaClay Spur Bentonite, Mowry Shale, Casper, Wyoming, 98.17 ± 0.11 MaArrow Creek Bentonite, Colorado Shale, Montana, USA, 99.12 ± 0.14 MaUpper Newcastle Sandstone, Black Hills, Wyoming, 99.49 ± 0.07 MaMiddle Newcastle Sandstone, Black Hills, Wyoming, 99.58 ± 0.12 MaShell Creek Shale, Bighorn Basin, Crow Reservation, Wyoming, 99.62 ± 0.07 MaShell Creek Shale, Bighorn Basin, Greybull, Wyoming, 99.67 ± 0.13 MaShell Creek Shale, Bighorn Basin, Lander, Montana, 100.07 ± 0.07 MaMuddy Sandstone, Wind River Basin, Wyoming, 101.23 ± 0.09 MaThermopolis Shale, Bighorn Basin, Wyoming, 101.36 ± 0.11 MaVaughn Member, Blackleaf Formation, Sweetgrass Arch, Montana, 102.68 ± 0.07 MaTaft Hill Member, Blackleaf Formation, Sweetgrass Arch, Montana, 103.08 ± 0.11 MaBase of the Skull Creek Shale, Black Hills, Wyoming, 104.87 ± 0.10 MaThermopolis Shale, Bighorn Basin, Wyoming, 106.37 ± 0.11 MaTopmost Bentonite, Mowry Shale, Kaycee, Wyoming, USA, 97.52 ± 0.09 MaClay Spur Bentonite, Mowry Shale, Casper, Wyoming, 98.17 ± 0.11 MaArrow Creek Bentonite, Colorado Shale, Montana, USA, 99.12 ± 0.14 MaUpper Newcastle Sandstone, Black Hills, Wyoming, 99.49 ± 0.07 MaMiddle Newcastle Sandstone, Black Hills, Wyoming, 99.58 ± 0.12 MaShell Creek Shale, Bighorn Basin, Crow Reservation, Wyoming, 99.62 ± 0.07 MaShell Creek Shale, Bighorn Basin, Greybull, Wyoming, 99.67 ± 0.13 MaShell Creek Shale, Bighorn Basin, Lander, Montana, 100.07 ± 0.07 MaMuddy Sandstone, Wind River Basin, Wyoming, 101.23 ± 0.09 MaThermopolis Shale, Bighorn Basin, Wyoming, 101.36 ± 0.11 MaVaughn Member, Blackleaf Formation, Sweetgrass Arch, Montana, 102.68 ± 0.07 MaTaft Hill Member, Blackleaf Formation, Sweetgrass Arch, Montana, 103.08 ± 0.11 MaBase of the Skull Creek Shale, Black Hills, Wyoming, 104.87 ± 0.10 MaThermopolis Shale, Bighorn Basin, Wyoming, 106.37 ± 0.11 MaA new U-Pb zircon age of 104.69 ± 0.07 Ma from the Skull Creek Shale at Dinosaur Ridge, Colorado, USA, is close to the new 40Ar/39Ar age of the Skull Creek Shale in the Black Hills, Wyoming, but 5 m.y. is missing in the unconformity between the Skull Creek Shale of the Black Hills and the overlying Newcastle Sandstone. Considering the average total uncertainties that include decay constant and standard age or tracer composition for the 40Ar/39Ar (± 0.19 Ma) and the U-Pb (± 0.13 Ma) ages does not alter this finding. Moreover, the lower Thermopolis Shale in the Bighorn Basin is 1.5 Ma older than the Skull Creek Shale in the Black Hills. The 100.07 ± 0.07 Ma Shell Creek Bentonite in Montana is close to the Albian–Cenomanian boundary age of 100.2 ± 0.2 Ma of Obradovich and colleagues from Hokkaido, Japan, and 100.5 ± 0.5 Ma adopted in the 2012 geological time scale of J.G. Ogg and L.A. Hinnov. Our findings indicate that correlations based on similarity of lithology, without independent radioisotopic ages or detailed biostratigraphic constraints, can be problematic or invalid. There is much more time missing in unconformities than has been previously recognized in these important, petroleum-bearing reservoir strata.

中文翻译：

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美国西部内陆晚阿尔比-塞诺曼阶地层年代学

自从 1993 年 JD Obradovich 和日本 JD Obradovich 及其同事于 2002 年分别从西部内陆盆地的白垩纪膨润土发表了 40Ar/39Ar 日期以来，40Ar/39Ar 方法的改进包括向标准矿物的天文校准年龄的转变新一代多收集器质谱仪的研制。因此，40Ar/39Ar 天文钟可以产生与 U-Pb 锆石日期和白垩纪地层天体年代学年龄模型同步的结果。奥布拉多维奇确定的年龄具有 ± 2σ 的 ± 400 ka 分析不确定性（不包括 J 值或系统贡献），已被用于区分约 1 毫安分辨率。在几十个桑尼丁样品中，其中 32 个由奥布拉多维奇于 1993 年定年，我们展示了新的多收集器 40Ar/39Ar 年龄，可将平均分析不确定性降低近一个数量级。这些新时代（其中的不确定性还包括中子注量 J 值的贡献）包括：Topmost Bentonite, Mowry Shale, Kaycee, Wyoming, USA, 97.52 ± 0.09 MaClay Spur Bentonite, Mowry Shale, Casper, Wyoming, 98.17 ± 0.11 MaArrow Creek Bentonite, Colorado Shale, Montana, USA, 99.12 ± 0.14 MaUpper Newcastle Sandstone, Black Hills, Wyoming, 99.49 ± 0.07 MaMiddle Newcastle Sandstone, Black Hills, Wyoming, 99.58 ± 0.12 MaShell Creek Shale, Wyoming29.58 ± 0.12 MaShell Creek Shale, Wyoming29 ± 0.07 MaShell Creek 页岩，Bighorn 盆地，怀俄明州 Greybull，99.67 ± 0.13 MaShell Creek 页岩，Bighorn 盆地，蒙大拿州兰德，100.07 ± 0.07 MaMuddy Sandstone，Wind River 盆地，怀俄明州，101.293 Ma 考虑到包括衰减常数和标准年龄或 40Ar/39Ar (± 0.19 Ma) 和 U-Pb (± 0.13 Ma) 年龄的示踪剂成分在内的平均总不确定性，不会改变这一发现。此外，比格霍恩盆地下部的 Thermopolis 页岩比 Black Hills 的 Skull Creek 页岩年龄 1.5 Ma。蒙大拿州100.07±0.07 Ma Shell Creek膨润土接近日本北海道Obradovich及其同事100.2±0.2 Ma的Albian-Cenomanian边界年龄，以及JG Ogg和LA 2012年地质时间尺度采用的100.5±0.5 Ma欣诺夫。我们的研究结果表明，基于岩性相似性的相关性，没有独立的放射性同位素年龄或详细的生物地层限制，可能有问题或无效。