Pace of Landscape Change and Pediment Development in the Northeastern Sonoran Desert, United States

Pediments of the Sonoran Desert in the United States have intrigued physical geographers and geomorphologists for nearly a century. These gently sloping bedrock landforms are a staple of the desert landscape that millions visit each year. Despite the long-lived scientific curiosity, an understanding of the processes operating on the pediment has remained elusive. In this study we revisit the extensive history of pediment research. We then apply geospatial, field, and laboratory cosmogenic ¹⁰Be nuclide dating and back-scattered electron microscopy methods to assess the pace and processes of landscape change on pediment systems abutting the Salt River in Arizona. Our study focuses on the Usery pediments linked to base-level fluctuations (river terraces) of the Salt River. Relict pediment surfaces were reconstructed with dGPS data and kriging methodologies utilized in ArcGIS—based on preserved evidence of ancient pediment surfaces. ¹⁰Be ages of Salt River terraces established a chronology of incision events, where calculating the volume between the reconstructed relict pediment and modern surface topography established minimum erosion rates (∼41 mm/ka to ∼415 mm/ka). Pediment area and length appear to have a positive correlation to erosion rate and development of planar pediment surfaces. Field and laboratory observations reveal that pediment systems adjust and stabilize at each Salt River terrace. Relief reduction across the pediment begins with pediment channel incision via headward erosion. Next, tributary drainage capture begins and collapses interfluves. Lateral stream erosion promotes planation where the porosity of decayed granite along channel banks exceeds the bedrock underneath ephemeral channels.

近一个世纪以来，美国索诺兰沙漠（Sonoran Desert）的山麓夷平面（pediment）始终是自然地理学家与地貌学家关注的研究对象。这类缓倾斜基岩地貌是沙漠景观的典型组成部分，每年吸引数百万游客造访。尽管学界对其抱有持久的科学兴趣，但始终未能清晰阐明驱动山麓夷平面演化的具体过程。本研究首先系统梳理了山麓夷平面研究的漫长发展历程，随后结合地理空间分析技术、野外考察与实验室宇宙成因¹⁰Be核素测年、背散射电子显微镜（back-scattered electron microscopy）方法，对毗邻亚利桑那州盐河（Salt River）的山麓夷平面系统的景观变化速率与过程进行评估。本研究聚焦于与盐河基准面波动（河流阶地）相关的尤塞里山麓夷平面。研究团队依托残存的古山麓夷平面遗迹，通过差分GPS（dGPS）数据与ArcGIS平台的克里金插值法，重建了古夷平面的表面形态。基于盐河阶地的¹⁰Be测年结果，研究建立了盐河下切事件的年代序列；通过计算重建的残存夷平面与现代地表地形之间的体积差，得出了研究区的最小侵蚀速率范围为约41 mm/ka至约415 mm/ka。分析表明，山麓夷平面的面积与长度似乎与侵蚀速率及平面状夷平面的发育程度呈正相关。野外与实验室观测结果显示，山麓夷平面系统会随盐河各阶地的形成发生调整并趋于稳定。山麓夷平面的地形起伏消减过程首先始于溯源侵蚀引发的夷平面河道下切；随后支流袭夺作用开始发生，河间地块逐渐被侵蚀坍塌；侧向河流侵蚀推动了夷平作用的进行，此时季节性河道岸带的风化花岗岩孔隙度高于河道下方的基岩。