Alluvial fans are a significant part of the sediment routing system, forming distinctive steep, fan-shaped deposits of coarse-grained detritus where rivers lose flow velocity after exiting confined mountain drainages. Processes on the fan are influenced by both internal (autogenic) feedback cycles like channel avulsion and by external (allogenic) conditions such as climate and tectonics. These conditions in turn influence the stratigraphic architecture (i.e., the pattern of channel stacking and sizes) within the fan. Studying stratigraphic architecture of alluvial fans can, therefore, provide insight into controls on fan deposition. We employ UAV-based photogrammetric models to analyze the stratigraphic architecture of two well-exposed ancient alluvial fans in the western US – the Eocene Richards Mountain Conglomerate and the Cretaceous Echo Canyon Conglomerate. Both fans were deposited under relatively warm, wet climates and compressional tectonic regimes. We use a seven-fold hierarchy of bounding surfaces and associated lithosomes to describe alluvial fan architecture. First- through fourth-order surfaces and lithosomes represent bedform to channel-scale features influenced primarily by autogenic processes on the fan. Controls on fifth-order surfaces/lithosomes have historically been poorly understood, but probably represent fanhead trench migration and lobe construction. Sixth-order surfaces bound individual alluvial fans and seventh-order surfaces correspond to formation boundaries. These are controlled primarily by tectonics. The fifth-order architectural style of the deposits in our two study areas is significantly different and we use this difference to try to isolate a primary control on fifth-order alluvial architecture. Average width:height ratios of fifth-order lithosomes are nearly twice as high for Echo Canyon (112:1) than for Richards Mountain (64:1). This indicates that active channels on the Echo Canyon fan were more mobile than those on the Richards Mountain fan. We attribute this to a more seasonal climate and less vegetation during the deposition of the Echo Canyon Conglomerate (relative to Richards Mountain). This would have increased lateral migration by destabilizing channels through increased sediment flux and flood events. Our results imply that fifth-order stratigraphic architecture of ancient alluvial fans may provide insight into allogenic processes related to paleoclimate. They also indicate risk of increased geologic hazards on alluvial fans where anthropogenic climate change increases future climate variability.

冲积扇是沉积物输送系统的重要组成部分，形成了独特的陡峭，扇形的粗粒碎屑沉积物，河流在离开受限的山间排水系统后失去了流速。风扇上的过程受内部（自成因）反馈周期（如通道撕裂）和外部（同源）条件（如气候和构造）的影响。这些条件继而影响风扇内的地层结构（即，通道堆积的样式和尺寸）。因此，研究冲积扇的地层结构可以深入了解扇沉积的控制方法。我们使用基于无人机的摄影测量模型来分析美国西部两个暴晒的古代冲积扇-始新世的理查兹山砾岩和白垩纪的回声峡谷砾岩的地层结构。两个扇形体都沉积在相对温暖，潮湿的气候和压缩构造条件下。我们使用边界表面和相关的脂质体的七层结构来描述冲积扇构造。一阶到四阶表面和脂质体代表床形到通道尺度的特征，这些特征主要受风扇上的自生过程影响。历史上对五阶表面/脂质体的控制知之甚少，但可能代表了扇形沟槽的迁移和叶的构造。约束单个冲积扇的六阶表面和七阶表面对应于地层边界。这些主要由构造控制。在我们两个研究区中，沉积物的五阶建筑风格存在显着差异，我们利用这种差异来尝试隔离对五阶冲积建筑的主要控制。Echo Canyon（112：1）的五阶岩体的平均宽高比几乎是Richards Mountain（64：1）的两倍。这表明Echo Canyon风扇上的活动频道比Richards Mountain风扇上的活动频道更具移动性。我们将其归因于回声峡谷砾岩（相对于理查兹山）的沉积过程中气候更为季节性，植被较少。通过增加泥沙通量和洪水事件来破坏河道，将会增加横向迁移。我们的结果表明，古代冲积扇的五阶地层构造可能提供与古气候有关的异源过程的见识。它们还表明人为气候变化会增加未来气候变异性的冲积扇具有增加地质灾害的风险。