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 (<1 Ma difference) the true time of deposition throughout the Sonsela Member. This model suggests a significant decrease in average sediment accumulation rate in the mid-Sonsela Member. Hence, the biotic turnover preserved in the mid-Sonsela Member at PFNP is also middle Norian in age, but may, at least partially, be an artifact of a condensed section. Model 2 following the magnetostratigraphic-based age model for the CPCP core 1A suggests instead that the ages from the lower and middle Sonsela Member are inherited populations of zircon crystals that are 1–3 Ma older than the true depositional age of the strata. This results in a model in which no sudden decrease in sediment accumulation rate is necessary and implies that the base of the Sonsela Member is no older than ca. 216 Ma. Independent of these alternatives, both age models agree that none of the preserved Chinle Formation in PFNP is Carnian (>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.

中文翻译：

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三叠纪上层三叠纪Chinle组的U-Pb锆石年代学和沉积年龄模型（美国亚利桑那州石化森林国家公园）：对三叠纪晚期古生态和古环境变化的影响

上三叠世Chinle组是北美西南部低古生物和环境变化的重要非海洋档案。在亚利桑那州的石化森林国家公园（PFNP）中，经过充分研究和高度化石化的Chinle地层，保存了脊椎动物和palynomorph化石记录的生物更新事件，或者将其假设为与构造驱动的气候变化或Manicouagan撞击事件相吻合。在约。215.5马。以前基于露头的年代学年龄限制很难放入准确的地层学框架中，因为侧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）的主要目标是以明确的叠加方式检索连续记录，以解决这种情况。我们对CPCP的520米长岩心1A进行了采样，通过结合U-Pb锆石年龄和地磁地层学，毫无疑问地叠加了准确的年龄模型。从13个火山碎屑富集的粉砂岩和砂岩中，我们使用激光烧蚀-电感耦合等离子体-质谱法筛选出每个样品中约300颗锆石晶体，随后使用化学磨蚀-同位素分析了19个年龄最小的晶体。稀释-热电离质谱（CA-ID-TIMS）光谱法。这些数据为Moenkopi组顶部（约241 Ma），下部Blue Mesa成员（约222 Ma），下部（约218至217 Ma）和上部（约213.5）提供了新的最大沉积年龄。 Ma）Sonsela成员。上层Chinle组获得的最大沉积年龄恰好在先前提出的年龄限制内，而下层Chinle组的最大沉积年龄则比先前提出的露头年龄要年轻。但是，露头与地层的相关性仍不确定。通过将我们的新年龄与磁心地层学联系起来，可以提出两个可行的年龄模型解决方案。模型1假设每个样品的最年轻，连贯的U-Pb年龄簇代表最大沉积年龄，并且接近整个Sonsela成员的真实沉积时间（相差小于1 Ma）。该模型表明，Sonsela中段的平均沉积物沉积速率显着下降。因此，PFNP的Sonsela中部成员所保存的生物更新年龄也处于诺里安时代中期，但可能至少部分是凝结部分的人工产物。遵循CPCP岩心1A的基于磁地层年龄模型的模型2反而表明，来自Sonsela下部和中部的年龄是锆石晶体的遗传种群，其年龄比地层的真实沉积年龄大了1-3 Ma。这样就形成了一个模型，在该模型中，不需要突然降低沉积物积聚速率，这意味着Sonsela成员的基数不超过ca。216毫安。不管这些选择如何，这两个年龄模型都认为PFNP中保留的Chinle形成年龄都不是Carnian（> 227 Ma），因此，生物更新事件不能与Carnian-Norian边界相关，而只是Norian中部事件。我们的年龄模型证明了将碎屑CA-ID-TIMS年龄与地磁地层数据相集成以正确解释复杂的沉积序列的能力，同时也面临着挑战。