Paleobotanical Estimates of the Eocene Elevation of the Sierra Nevada

现在北美科迪勒拉山脉的大部分地区在白垩纪早期接近海平面。知道何时隆起到现在的高度，对于理解导致其形成的大规模构造过程至关重要。对于加利福尼亚的内华达山脉，自19世纪90年代以来，人们一直认为地貌学和其他证据要求中新世中期和后来的山脉隆起，尽管晚白垩纪停止了主要的岩浆活动。新的地球物理、地球化学和古高程数据对这种晚隆升的概念提出了质疑。我们提出了新的古植物学研究来确定内华达山脉的始新世海拔，这将使我们能够在早期和晚期隆起模式之间做出决定。对现代树叶组合的物理形态的分析能够对当今的气候和海拔高度（750米）作出相当准确的估计。这种关系在北美尤其精准。叶片形态学研究表明，内华达西部在16 Ma（中新世中期）比现在高，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16ma之前的海拔历史是非常不确定的。我们拟收集和分析该地区出现的大量始新世叶片组合，年龄在45 - 50 Ma之间。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集中在被称为白垩崖植物群的化石叶子上，这些叶子在含金砾石的下部被发现。这些砾石可能是始新世早期和（或）中期的砾石，由祖先的玉巴河沉积而成。推断出的古地温差异可能是由全球气候变化引起的，也可能是由海拔变化引起的局部气候变化引起的。西部有相同年龄的低海拔叶片组合，可作为控制区域气候变化和估计内陆组合古海拔的标准。并将其与太平洋沿岸20纬度范围内其他早始新世至中始新世低空叶片组合进行比较。我们希望考察的一些地区正受到发展的威胁；在限制内华达山脉北部和中部早第三纪海拔的机会消失之前，进行这些收集和分析是非常紧迫的。

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.