The influence of bedrock weathering on the response of drainage basins and associated alluvial fans to Holocene climates, San Bernardino Mountains, California, USA

Abstract

The primary factors that control alluvial fan evolution still remain in question particularly for the Holocene. Holocene centennial- and millennial-scale climate fluctuations are relatively subtle and more frequent than those of glacial/interglacial transitions, therefore intrinsic factors such as rock type or basin size are hypothesized to moderate significantly the influence of Holocene climate and climate change on alluvial fan processes. Here, we examine variability in styles and rates of alluvial fan aggradation along a single mountain front that is characterized by basins of varying size and rock type (carbonate versus granite). Basin rock type is more closely correlated to variability in the episodic nature and magnitude of alluvial fan aggradation than is basin area. Bedrock physical and chemical weathering properties control sediment delivery to the piedmont and thus influence alluvial fan aggradation. We suggest that the particle size of grus produced by weathering of granitic rocks fosters sediment mobilization and alluvial fan aggradation during episodes of increased precipitation in the Holocene. Sediment mobilization during wetter climates is also possibly enhanced by drought-related fires and vegetation loss that occurred during preceding drier periods. In contrast, carbonate outcrops weather to both dissolved materials and clastic sediment and relatively rapid cementation of talus precludes its transportation out onto the piedmont under almost all Holocene climatic conditions. If the scale of past Holocene climate change is the closest analogy to current global change, this study documents some mechanisms by which different rock types can exert dramatically different effects on landscape response to those changes.