U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change
U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change
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DOI：
10.1130/b35485.1
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发表时间：
2020-07
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上三叠统秦岭组是北美西南部低古纬度生物和环境变化的重要非海相档案。亚利桑那州石化森林国家公园（PFNP）的Chinle地层研究充分，化石丰富，保存了脊椎动物和苔藓化石记录的生物更替事件，该事件被假设与构造驱动的气候变化或约215.5 Ma的Manicouagan撞击事件相吻合。以往以露头为基础的地质年代学约束很难建立准确的地层格架，因为侧向相变化和不连续的露头允许多种解释。科罗拉多高原取心项目（CPCP）的一个主要目标是在明确的叠加中检索连续记录，以纠正这种情况。我们对CPCP的520 m长的1A岩心进行了采样，结合U-Pb锆石年龄和磁地层学，在无可置疑的叠加中建立了精确的年龄模型。从13个富含火山碎屑的粉砂岩和砂岩层位中，我们使用激光烧蚀-电感耦合等离子体质谱法筛选了每个样品多达300个锆石晶体，随后使用化学磨损-同位素稀释-热电离质量（CA-ID-TIMS）谱法分析了最年轻年龄人群的多达19个晶体。这些资料为Moenkopi组顶部（约241 Ma）、Blue Mesa组下部（约222 Ma）和Sonsela组下部（约218 ~ 217 Ma）和上部（约213.5 Ma）提供了新的最大沉积年龄。上秦岭组的最大沉积年龄完全符合先前提出的年龄限制，而下秦岭组的最大沉积年龄相对于先前提出的露头年龄要小；然而，岩心与露头地层对比仍然不确定。通过将我们的新时代与岩心的磁地层学联系起来，可以提出两种可行的时代模型解。模型1假设每个样品的最年轻、连贯的U-Pb年龄群代表了最大沉积年龄，年龄接近（227 Ma），因此生物转换事件不能与卡尼亚-诺里期边界相关，而是一个中诺里期事件。我们的年龄模型展示了将碎屑CA-ID-TIMS年龄与磁地层数据相结合以正确解释复杂沉积序列的能力，但也面临挑战。
The Upper Triassic Chinle Formation is a critical non-marine archive of low-paleolatitude biotic and environmental change in southwestern North America. The well-studied and highly fossiliferous Chinle strata at Petrified Forest National Park (PFNP), Arizona, preserve a biotic turnover event recorded by vertebrate and palynomorph fossils, which has been alternatively hypothesized to coincide with tectonically driven climate change or with the Manicouagan impact event at ca. 215.5 Ma. Previous outcrop-based geochronologic age constraints are difficult to put in an accurate stratigraphic framework because lateral facies changes and discontinuous outcrops allow for multiple interpretations. A major goal of the Colorado Plateau Coring Project (CPCP) was to retrieve a continuous record in unambiguous superposition designed to remedy this situation. We sampled the 520-m-long core 1A of the CPCP to develop an accurate age model in unquestionable superposition by combining U-Pb zircon ages and magnetostratigraphy. From 13 horizons of volcanic detritus-rich siltstone and sandstone, we screened up to ∼300 zircon crystals per sample using laser ablation–inductively coupled plasma–mass spectrometry and subsequently analyzed up to 19 crystals of the youngest age population using the chemical abrasion–isotope dilution–thermal ionization mass (CA-ID-TIMS) spectrometry method. These data provide new maximum depositional ages for the top of the Moenkopi Formation (ca. 241 Ma), the lower Blue Mesa Member (ca. 222 Ma), and the lower (ca. 218 to 217 Ma) and upper (ca. 213.5 Ma) Sonsela Member. The maximum depositional ages obtained for the upper Chinle Formation fall well within previously proposed age constraints, whereas the maximum depositional ages for the lower Chinle Formation are relatively younger than previously proposed ages from outcrop; however, core to outcrop stratigraphic correlations remain uncertain. By correlating our new ages with the magnetostratigraphy of the core, two feasible age model solutions can be proposed. Model 1 assumes that the youngest, coherent U-Pb age clusters of each sample are representative of the maximum depositional ages and are close to (227 Ma) in age, and hence the biotic turnover event cannot be correlated to the Carnian–Norian boundary but is rather a mid-Norian event. Our age models demonstrate the powers, but also the challenges, of integrating detrital CA-ID-TIMS ages with magnetostratigraphic data to properly interpret complex sedimentary sequences.