Paleobotanical Estimates of the Eocene Elevation of the Sierra Nevada

现在的北美科迪勒拉的大部分地区在白垩纪早期接近海平面。知道何时隆升到现在的高度，对于了解导致其形成的大规模构造过程至关重要。对于加利福尼亚州的内华达山脉，自19世纪90年代以来，人们一直认为，地貌和其他证据需要中新世中期和后来的山脉抬升，尽管主要的岩浆活动在晚白垩世停止。新的地球物理、地球化学和古高程测量数据对这一晚期隆起的概念提出了质疑。我们建议进行新的古植物学研究，以确定内华达山脉始新世的海拔，这将使我们能够在早期和晚期的隆升模式之间做出决定。对现代叶片组合的物理形态的分析能够相当准确地估计当今的气候和海拔(750米)。这种关系在北美尤其得到了很好的调整。叶形态研究表明，内华达西部在16 Ma(中中新世)比现在更高，这一结论得到了其他证据的支持。然而，内华达山脉-内华达西部地区在16 Ma之前的抬升历史是非常不确定的。我们建议收集和分析该地区大量始新世叶片组合，年龄为45-50 Ma。这可能是内华达山脉“含金砾石”下部的年龄，这里是加州淘金热的焦点。古植物学工作将首先关注在含金砾石下部发现的被称为白垩崖植物群的化石叶子。这些砾石可能是始新世早期和(或)中期的，是由原始的浴坝河沉积的。推断的古温度差异可能是全球气候变化的结果，也可能是由于海拔变化引起的局部气候变化的结果。西面有相同年龄的低海拔叶组合，我们将以此为标准来控制区域气候变化，并估计内陆组合的古海拔。低海拔组合还将与太平洋沿岸20纬度范围内早至中始新世的其他低海拔叶组合进行比较。我们希望抽样的一些地区受到发展的威胁；在限制内华达山脉北部和中部第三纪早期高程的机会溜走之前，进行这些收集和分析具有一定的紧迫性。

Much of what is now the North American Cordillera was near sea level in the Early Cretaceous. Knowing when uplift to its present elevation occurred is essential to understanding the large-scale tectonic processes that led to its formation. For the Sierra Nevada of California, it has been thought since the 1890s that geomorphic and other evidence required middle Miocene and later uplift of the range, despite cessation of major magmatic activity in the Late Cretaceous. New geophysical, geochemical, and paleoaltimetric data have cast doubt on this concept of late uplift. We propose new paleobotanical studies to determine the Eocene elevation of the Sierra Nevada, which will allow us to decide between early and late uplift models. Analyses of the physical morphology of modern leaf assemblages are able to produce reasonably accurate estimates of present-day climate and altitude (750 m). The relationship is especially well calibrated in North America. Leaf morphology studies have indicated that western Nevada was higher at 16 Ma (middle Miocene) than at the present, and this conclusion is supported by other lines of evidence. The history of elevation of the Sierra Nevada - western Nevada region prior to 16 Ma, however, is very uncertain. We propose to collect and analyze numerous Eocene leaf assemblages that occur in this region with ages from 45 - 50 Ma. This is the probable age of the lower part of the "auriferous gravels" of the Sierra Nevada, the focus of so much of the California gold rush. Paleobotanical work will focus first on fossil leaves known as the Chalk Bluffs flora found in the lower part of the auriferous gravels. These gravels are probably of early and/or middle Eocene age and were deposited by the ancestral Yuba River. Differences in inferred paleotemperature could result from global climatic change or from local climatic change due to altitudinal change. There are low-altitude leaf assemblages of the same age to the west that we will use as standards to control for regional climate change and estimate paleoaltitudes for the inland assemblages. The low altitude assemblages will also be compared to other low-altitude early to middle Eocene leaf assemblages in a range of 20 latitude along the Pacific Coast. Some of the localities we wish to sample are threatened by development; there is some urgency in making these collections and analyses before the opportunity to constrain the early Tertiary elevation of the north and central Sierra Nevada slips away.