Landscape Development in the Western Transverse Ranges, California: Insights from Mapping, Geochronology, and Modeling

Abstract

Understanding how climate and tectonics have interacted to shape current landscape configuration requires application of the latest geomorphological techniques. This dissertation presents results from a combination of field mapping, geochronology, and numerical landscape development modeling. The papers contained here focus on studies from Cuyama Valley, California, at the junction of the Coast Ranges and Western Transverse Ranges in southern California.Combining field observation with three geochronological techniques has led to a detailed understanding of the late Quaternary alluvial history of Cuyama Valley. The alluvial history, in turn, allows for a better understanding of important events in the history of landscape development. In the western Cuyama Valley, the timing and morphology of alluvial fans record both climatic forcing in the form of variable sediment supply from drainage basins, and tectonic forcing from ongoing tectonically driven incision of the axial Cuyama River. Fan-terrace surfaces are subparallel (older surfaces are slightly steeper) and offset systematically in relation to their ages, suggesting response to ongoing base-level incision and fluctuation in sediment supply. The fans aggraded during relatively cool and wet climate of the last glacial period, which is out-of-phase with the regional model developed in nearby desert regions.In the upper Cuyama Valley, deposition in the Cuyama sedimentary basin ceased in the mid-Pleistocene, after which basin fill was uplifted, deformed, and beveled, forming a low-relief erosion surface on which the alluvium of San Emigdio Mesa was deposited. Subsequent fluvial drainage network development formed the Cuyama badlands by incising into the deformed Cuyama basin sediments.The history of the upper Cuyama Valley was used to calibrate a numerical landscape development model. Uplift rate U, bedrock erodibility K, and landslide threshold-slope Sc are related to steady-state relief, hypsometry, and drainage density for a wide range of synthetic topographies produced by a stream-power-based landscape development model. A combination of fluvial channels and threshold-slopes occurs for only a relatively narrow range of U/K between 10 and 5000 m·kyr/kyr. Using measured values for hypsometric integral, drainage density and relief, the U/K value can be further constrained, enabling K to be determined if U is known.