Factors controlling sequence development on Quaternary fluvial fans, San Joaquin Basin, California, USA

Abstract

AbstractVariable geometry and distribution of stratigraphic sequences of fluvial fans in the eastern San Joaquin Basin, California, were controlled by tectonics, through basin subsidence and basin width, and response to Quaternary climate change, related to the degree of change in sediment supply to stream discharge ratios and local base-level elevation changes. Three fluvial fans — the Kings River, Tuolumne River and Chowchilla River fans — illustrate the influence of these factors on ultimate sequence geometry. In areas with high subsidence rates (e.g. the Kings River fluvial fan) sequences are relatively thick and apices of subsequent sequences are vertically stacked. Areas with relatively low subsidence rates (e.g. the Tuolumne River fan) produced laterally stacked sequences. Rivers that experienced a significant increase in sediment supply to stream discharge ratios due to direct connection to outwash from glaciated portions of the Sierra Nevada developed high accommodation space and relatively thick sequences with deep incised valleys. Conversely, rivers that were not connected to glaciated regions (e.g. the Chowchilla River fan) and, thus, experienced a relatively minor change in sediment supply to discharge ratios during climate change events, produced thinner sequences that lack deep incised valleys. Local base-level connection to sea level, via the axial San Joaquin River, produced deeper incised valleys than those of internally drained rivers. Finally, narrow basin width allowed glacially connected fans to completely fill available accommodation space, thus producing smaller fans that lack preservation of distal, interglacial deposits. Evaluation of these controls allows prediction of sequence geometries and facies distributions for other San Joaquin Basin fans for input into future hydrogeological models.