Quantifying the Temporal Evolution of Eocene Lake Gosiute

摘要与其他陆地环境相比，湖泊沉积物提供了许多关键地表过程的最佳可用档案，因为湖泊中的沉积相对连续。除了保存各种古气候代用物外，旋回沉积相序列还可以提供周期性气候强迫的记录。湖相地层也为研究气候、景观剥蚀和上升造山带地形之间复杂的相互作用提供了一个独特的、未被充分利用的窗口。在大型湖泊中可能发生的快速生物物种形成可能使它们成为重要的进化孵化器。然而，湖泊沉积物在测量所有这些过程的变化率方面的显著效用受到普遍较差的年代地层控制的阻碍。始新世绿河组（GRF）可能是世界上研究最多的湖相沉积体系。始新世的气候记录特别令人感兴趣，因为始新世早期代表了新生代最温暖的时期，因此可能阐明现代全球变暖的潜在长期影响。最近的研究表明，GRF的沉积也可能与相邻的Laramide山脉意外的快速隆起同时发生。长期以来，湖相中互层的大量泥层一直有助于将这些经典沉积物进行对比，但直到最近，最好的年龄分辨率是100万年或更久。我们最初由美国国家科学基金会资助的项目通过确定7块凝灰岩中的硅和/或黑云母的激光聚变40Ar/ 39Ar年龄，极大地改善了怀俄明州GRF的地质年代学框架。其中6个凝灰岩的年龄在50.700.14 ~ 48.940.12 Ma （2 sigma）之间，因此我们可以在大部分2.3 myr的年代跨度中解决100年的时间差异。为了准确地将这些凝灰岩置于更广泛的沉积学和地层背景中，我们还构建了威尔金斯峰和兰尼段的盆地尺度截面。我们的研究结果已经得出了几个重要的新结论：1) GRF的沉积时间比以往的K-Ar年龄早3-4 myr；2)布里杰世—瓦萨奇世动物群转变的年龄现在可以限制在50.55 - 0.43 Ma。3)威尔金斯峰段蒸发相的积累速度比蒂普顿段和兰尼段的深湖相快3倍。4)蒂普顿段和兰尼段湖泊旋回的平均持续时间与旋进一致。但以前在威尔金斯峰段解释的“岁差”周期实际上持续时间要短得多（~10 kyr）。5)虽然我们最初的研究集中在气候问题上，但我们偶然发现了一个干燥事件，似乎记录了心脏山支队的灾难性安置。我们已经向顶级期刊提交了三篇论文来记录我们之前的研究结果。为了充分利用这些成果，我们提出了以下新目标：_通过U-Pb和40Ar/ 39Ar测年提高精度和时间分辨率。通过对火山锆石颗粒进行U-Pb分析，我们希望将我们已经很小的相对定年误差减少一半，并将完全传播的误差减少一个数量级。_测试统计模型描绘湖泊旋回的能力。放射性同位素测量的累积速率将用于将床层厚度光谱按时间进行标度，并确定不同GRF成员的允许循环周期。此外，我们将评估不同地层“完整性”对这种光谱的影响。_探索盆地边缘冲积扇的时间记录。与GRF相交的粗粒冲积地层提供了独特的盆地边界隆起时间记录。我们将把这些沉积物与年代久远的凝灰岩联系起来，以提供拉腊米断层运动的高分辨率记录。_将时间相关性扩展到相邻盆地。通过对来自于unta、Piceance Creek、Fossil和Bighorn盆地的精选tephras定年，我们将建立一个区域一致的年代地层框架，这将有助于重要的构造、气候和生物事件的相关性。该项目将利用沉积学、地层学和地质年代学领域的跨学科专门知识来解决有关地球表面过程速率的基本问题。这种对世界上最著名的湖泊沉积物之一的综合方法将为阅读保存在大型湖泊沉积物中的古气候陆地记录提供新的基准。

ABSTRACTLacustrine deposits provide some of the best available archives of many critical Earth surface processes, due to the relatively continuous sedimentation in lakes compared to other terrestrial environments. In addition to preserving various paleoclimatic proxies, cyclic sedimentary facies successions may provide records of periodic forcing of climate. Lacustrine strata also offer a unique and underutilized window on the complex interactions between climate, landscape denudation, and rising orogenic topography. The rapid biological speciation that can occur in large lakes may make them important evolutionary incubators. However, the remarkable utility of lake deposits to measure rates of change in all of these processes has been hampered by generally poor chronostratigraphic control. The Eocene Green River Formation (GRF) is perhaps the most studied system of lacustrine deposits in the world. Eocene climate records are of particular interest because the Early Eocene represents the warmest interval of the Cenozoic, and as such may shed light on the potential long-term effects of modern global warming. Recent studies suggest that deposition of the GRF may have also coincided with unexpectedly rapid uplift of adjacent Laramide mountain ranges. Numerous tephras interbedded in lacustrine facies have long aided in correlating these classic deposits, but until recently the best age resolution was one million years or more. Our intitial NSF-funded project has vastly improved the geochronologic framework for the GRF in Wyoming by determining laser fusion 40Ar/ 39Ar ages of sanidine and/or biotite from seven tuffs. Ages for six of the tuffs are between 50.700.14 and 48.940.12 Ma (2 sigma), thus we can resolve temporal differences on the order of 100 kyrs over most of the 2.3 myr span of the dated deposits. To accurately place these tuffs within their broader sedimentologic and stratigraphic context, we have also constructed basin-scale cross sections of the Wilkins Peak and Laney Members. Our results have already permitted several significant new conclusions: 1) We have found that the GRF was deposited 3-4 myr earlier than suggested by previous K-Ar ages, and therefore coincides with the the latter half of the globally warmest period in the Cenozoic. 2) The age of the Bridgerian-Wasatchian faunal transition can now be constrained to 50.55 0.43 Ma. 3) The evaporative facies of the Wilkins Peak Member accumulated three times more quickly than the deeper lacustrine facies of the Tipton and Laney Members. 4) The average duration of lake cycles in the Tipton and Laney Members is consistent with precession, but previously interpreted "precession" cycles in the Wilkins Peak Member are actually of much shorter duration (~10 kyr). 5) Although our initial study focused on climatic issues, we fortuitously discovered a desiccation event that appears to record catastrophic emplacement of the Heart Mountain Detachment. We have submitted three papers to top journals to document our prior results. To capitalize and build upon these achievements, we propose the following new objectives:_ Increase accuracy and temporal resolution via U-Pb and 40Ar/ 39Ar dating. By conducting U-Pb analysis of volcanic zircon grains we hope to cut our already small relative dating errors in half, and to reduce fully propagated errors by an order of magnitude._ Test the ability of statistical models to delineate lacustrine cycles. Radioisotopically-measured accumulation rates will be used to scale bed thickness spectra into time, and to determine permissible cycle periods for different GRF Members. Also, we will evaluate the effect of varying stratigraphic "completeness" on such spectra._ Explore the temporal record of basin-margin alluvial fans. Coarse-grained alluvial strata that interfinger with the GRF offer a unique record of the timing of uplift of basin-bounding ranges. We will correlate these deposits with dated tuffs to provide a high-resolution record of Laramide fault movements._ Extend temporal correlations into adjacent basins. By dating selected tephras from the Uinta, Piceance Creek, Fossil, and Bighorn basins we will establish a regionally consistent chronostratigraphic framework that will facilitate the correlation of important tectonic, climatic, and biologic events.This project will draw upon cross-disciplinary expertise from the fields of sedimentology, stratigraphy, and geochronology to address fundamental questions concerning rates of Earth surface processes. This integrated approach to one of the worl's best known lacustrine deposits will provide a new benchmark for reading the terrestrial record of paleoclimate preserved in the deposits of large lakes.