THE INFLUENCE OF BEDROCK WEATHERING ON THE RESPONSE OF DRAINAGE BASINS AND ASSOCIATED ALLUVIAL FANS TO HOLOCENE CLIMATES, SAN BERNARDINO MOUNTAINS CALIFORNIA

The magnitude of Holocene centennial and millennial climate fluctuations are relatively subtle and more frequent compared to those of glacial/interglacial transitions. Intrinsic factors such as rock type or basin size could thus moderate how Holocene climate and climate change influence alluvial fan processes. We examine variability in styles and rates of alluvial fan aggradation in six drainage basins of variable size and rock type (carbonates and granites) along a single mountain front in the San Bernardino Mountains of Southern California. The overall relationship between Holocene alluvial fan area and drainage basin area for the basins is relatively weak (R² = 0.50), suggesting variability in rock resistance between adjacent basins. In contrast, a relatively strong correlation (R² = 0.96) exists between Holocene fan area and the area underlain by granitic rock types. Holocene tectonic activity is minimal along the San Bernardino mountain front and is thus ruled out as a trigger for the two to three episodes of Holocene fan aggradation interpreted on the basis of stratigraphy and soils exposed in pits excavated on fan surfaces adjacent to the basins. We suggest that the particle size characteristics of grus produced by physical weathering of granitic rocks tends to foster sediment mobilization and extensive alluvial fan aggradation during previously documented episodes of increased precipitation in the Holocene. This sediment mobilization is also possibly enhanced by drought-related fires and vegetation loss which occurred during preceding dryer periods. In contrast to granite, carbonate outcrops mechanically weather to coarse sediment and/or marble grus depending on bedrock grain-size. Relatively rapid in-situ cementation of coarse talus precludes its transportation out onto the piedmont under almost all Holocene climate conditions. Marble grus behaves similarly to granitic grus, but rapid dissolution and minimum exposure of grus-producing marbles in the field area precludes significant Holocene fan aggradation. Before Holocene alluvial fan aggradation can be employed as an indicator of past climates or a predictor of future landscape response to global change, these mechanisms by which different rock types can exert dramatically different effects on how landscapes respond to climate and climate change must be documented.