Much progress has been made in recent years towards a set of recognition criteria for river discharge variability in river channel deposits, and thus sedimentary proxies for precipitation variability. Despite this progress, there is currently no consensus on how different styles of discharge variability are reflected in river sedimentary records, and whether variable-discharge river records from different climate types can be distinguished. Herein, river discharge and precipitation variability in the Paleogene is investigated using associations between river channel and floodplain deposits across the Paleocene–Eocene boundary from the Paleocene upper Nacimiento Formation and the early Eocene San Jose Formation in the San Juan Basin, New Mexico, USA.The succession is identified as deposits of variable-discharge river systems based on shared channel-deposit characteristics with modern and ancient variable-discharge river systems and the proposed facies models, in addition to alternations of poorly drained and well-drained floodplain deposits and/or slickensides indicating alternating wet–dry cycles. A long-term stratigraphic trend toward increasingly well-drained floodplain deposits is also observed and hypothesized to indicate successively more arid conditions from the Paleocene into the early Eocene. Comparisons with modern rivers from various climate zones suggest a long-term shift from a monsoonal climate in the Paleocene, to a fluctuating subhumid climate, ultimately leading to semiarid to arid conditions in the early Eocene. These observations suggest that floodplain deposits may be a better indicator of ambient climate, whereas channel deposits are records for frequency and magnitude of high-intensity precipitation events. Therefore, the existing facies models for variable-discharge rivers that consider only channel facies may not capture critical information needed to make accurate interpretations of paleoclimatic conditions. This study also adds to a growing body of evidence from geologic records of mid-latitude Paleogene river systems suggesting increases in the magnitude or variability of river discharge coinciding with established climate perturbations.

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关于美国新墨西哥州圣胡安盆地古近纪河流和洪泛区沉积物中降水变化特征的检验假设

近年来，在为河道沉积物中的河水流量变化确定一套识别标准方面已经取得了很大的进展，因此在确定降水量变化的沉积代理方面也取得了很多进展。尽管取得了这一进展，但对于如何在河流沉积记录中反映出不同的流量变化形式，以及是否可以区分不同气候类型的流量排放记录，目前尚无共识。本文中，利用美国新墨西哥州圣胡安盆地的古新世上层纳西门托组和始新世早期的圣何塞组的古新世-始新世边界的河道与洪泛区沉积物之间的联系，研究了古近纪的河流流量和降水变化。根据排水通道的共享特征，结合现代和古代的可变流量河流系统以及拟议的相模型，确定排泄量为可变流量河流系统的沉积物，此外还包括排水不良和排水良好的洪泛区沉积物和/或光滑表示干湿交替循环。长期的地层趋势表明，洪泛区沉积物的排水量越来越高，并被认为可以指示从古新世到始新世的连续干旱条件。与来自不同气候区的现代河流的比较表明，从古新世的季风气候到不稳定的半湿润气候的长期转变，最终导致始新世半干旱至干旱。这些观察结果表明，洪泛区沉积物可能是环境气候的更好指标，而河道沉积物是高强度降水事件发生频率和强度的记录。因此，仅考虑河道相的可变流量河流的现有相模型可能无法捕获对古气候条件进行准确解释所需的关键信息。这项研究还从中纬度古近纪河流系统的地质记录中增加了越来越多的证据，表明随着既定的气候扰动，河流流量的大小或变异性增加。现有的仅考虑河道相的可变流量河流相模型可能无法捕获对古气候条件进行准确解释所需的关键信息。这项研究还从中纬度古近纪河流系统的地质记录中增加了越来越多的证据，表明随着既定的气候扰动，河流流量的大小或变异性增加。现有的仅考虑河道相的可变流量河流相模型可能无法捕获对古气候条件进行准确解释所需的关键信息。这项研究还从中纬度古近纪河流系统的地质记录中增加了越来越多的证据，表明随着既定的气候扰动，河流流量的大小或变异性增加。