Aggradation and fluvial incision controlled by downstream base-level changes at timescales of 10 to 500 kyr is incorporated in classic sequence stratigraphic models. However, upstream climate control on sediment supply and discharge variability causes fluvial incision and aggradation as well. Orbital forcing often regulates climate change at 10 to 500 kyr timescales while tectonic processes such as flexural (un)loading exert a dominant control at timescales longer than 500 kyr. It remains challenging to attribute fluvial incision and aggradation to upstream or downstream processes or disentangle allogenic from autogenic forcing, because time control is mostly limited in fluvial successions. The Palaeocene outcrops of the fluvial Lebo Shale Member in north-eastern Montana (Williston Basin, USA) constitute an exception. This study uses a distinctive tephra layer and two geomagnetic polarity reversals to create a 15 km long chronostratigraphic framework based on the correlation of twelve sections. Three aggradation–incision sequences are identified with durations of approximately 400 kyr, suggesting a relation with long-eccentricity. This age control further reveals that incision occurred during the approach of – or during – a 405 kyr long-eccentricity minimum. A long-term relaxation of the hydrological cycle related to such an orbital phasing potentially exerts an upstream climate control on river incision. Upstream, an expanding vegetation cover is expected because of an increasingly constant moisture supply to source areas. Entrapping by vegetation led to a significantly reduced sediment supply relative to discharge, especially at times of low evapotranspiration. Hence, high discharges resulted in incision. This study assesses the long-eccentricity regulated climate control on fluvial aggradation and incision in a new aggradation–incision sequence model.

中文翻译：

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美国蒙大拿州东北部古新世河流切割和加成的长偏心率调控气候控制

在 10 到 500 kyr 的时间尺度上，由下游基线变化控制的堆积和河流切口被纳入经典的层序地层模型。然而，上游气候对沉积物供应和排放变异的控制也会导致河流的切割和积聚。轨道强迫通常在 10 到 500 kyr 时间尺度上调节气候变化，而弯曲（卸载）载荷等构造过程在超过 500 kyr 的时间尺度上发挥主导作用。将河流切口和加积归因于上游或下游过程或从自生强迫中解开异体仍然具有挑战性，因为时间控制在河流演替中大多受到限制。蒙大拿州东北部（美国威利斯顿盆地）河流 Lebo 页岩段的古新世露头是一个例外。本研究使用独特的火山灰层和两次地磁极性反转，基于 12 个剖面的相关性创建了一个 15 公里长的年代地层框架。确定了三个持续时间约为 400 kyr 的加深 - 切口序列，表明与长离心率有关。这种年龄控制进一步表明，切口发生在接近 405 kyr 长偏心最小值期间或期间。与这种轨道相位相关的水文循环的长期松弛可能会对河流切口施加上游气候控制。在上游，由于源区的水分供应越来越稳定，预计植被覆盖范围会扩大。植被截留导致沉积物供应相对于排放显着减少，尤其是在低蒸腾量的时候。因此，高放电导致切口。本研究在新的堆积-切口序列模型中评估了对河流堆积和切口的长偏心调节气候控制。