The lowstand extensions of two fluvial systems from the Cascadia subduction margin are investigated using high-resolution seismic reflection data to determine the relative controls on paleodrainage morphology. We document distinct differences between the two systems, located ~40 km apart, attributed to underlying structure and lithology, shelf gradient, and drainage basin characteristics. A cross-shelf valley exists for only one of the drainage systems, the Umpqua River. The outer shelf part of the valley is a syncline, which formed a natural structural low for lowstand drainage, while the inner shelf part of the valley exhibits incision and is associated with a mid-shelf convex change is slope. The other drainage system, the Siuslaw River, did not develop an across-shelf valley and is only denoted in the seismic data by an erosional subaerial unconformity. Structural trends are perpendicular to cross-shelf drainage, supporting a more anastomosing stream morphology for the paleo-Siuslaw. The shelf edge for this system was not exposed for a long period of time during the last lowstand, preventing substantial outer shelf incision. Differences in fluvial elevation, discharge, and hinterland geology, also played a role in the cross-shelf morphologies of the paleo-Umpqua and paleo-Siuslaw. These results highlight how controls on lowstand paleodrainage morphology can change over short distances on an active margin.

利用高分辨率地震反射数据，研究了卡斯卡迪亚俯冲带边缘两个河流系统的低位扩展，以确定对古排水形态的相对控制。我们记录了相距约40 km的两个系统之间的明显差异，这归因于下伏的构造和岩性，层架坡度和流域盆地特征。只有一个排水系统（Umpqua河）存在跨架谷。山谷的外层部分为向斜，形成了低位排水的自然结构低点，而山谷的内层部分则显示出切口，并伴随着中层凸起的变化是坡度。另一个排水系统索斯劳河 没有形成跨架的谷地，仅在地震数据中以侵蚀性的地下不整合面表示。结构趋势垂直于跨架排水，支持了古苏斯拉夫河的形态更加吻合。在上次低潮期间，该系统的搁板边缘未长时间暴露，从而防止了大量的外部搁板切口。河流高程，流量和腹地的地质学差异在古翁布夸和古苏斯劳的跨架形态中也起作用。这些结果突显了对低位古土壤流变形态的控制如何在短距离内有效地改变。在上次低潮期间，该系统的搁板边缘未长时间暴露，从而防止了大量的外部搁板切口。河流高程，流量和腹地的地质学差异在古翁布夸和古苏斯劳的跨架形态中也起作用。这些结果突显了对低位古土壤流变形态的控制如何在短距离内有效地改变。在上次低潮期间，该系统的搁板边缘未长时间暴露，从而防止了大量的外部搁板切口。河流高程，流量和腹地的地质学差异在古翁布夸和古苏斯劳的跨架形态中也起作用。这些结果突显了对低位古土壤流变形态的控制如何在短距离内有效地改变。