DEPOSITIONAL STYLES OF ALLUVIAL FANS SOURCED FROM PRIMARILY BASALTIC CATCHMENTS

Alluvial fans are primarily formed by two different sediment-transport processes: debris flows and sheet floods. Previous work has shown that catchment lithology plays a role in determining whether an alluvial fan is primarily driven by debris-flow or sheet-flood processes, or a mix of both. Debate exists about how much of a controlling influence catchment bedrock has on depositional styles relative to other influencing factors such as catchment shape. Catchment bedrock that produces clay minerals through chemical weathering or recycling of preexisting mudstones is more likely to form debris-flow dominated alluvial fans. Alluvial fans examined in previous studies also typically have mixed-lithology catchments, which makes it difficult to determine the influence of a specific lithology on depositional style. In this study we controlled for catchment lithology by selecting only alluvial fans sourced from primarily basaltic bedrock. The studied fans have a range of catchment relief and area to test whether a homogenous catchment lithology will create alluvial fans with the same depositional style or if other factors have enough influence to produce differing depositional styles. We investigated the primary depositional process of five basalt-sourced alluvial fans in southern Nevada and southeast California. At multiple locations across the each of the five alluvial fans, vertical sections in channel cuts show primarily poorly-sorted, disorganized, muddy, framework- and matrix-supported deposits consistent with deposition by debris flows. Where depositional contacts can be identified, individual debris-flow deposits range from 30 cm to 1.5 m thick and locally show inverse grading. A few sections contain uncommon bedded sandstone that comprise up to 20% of the total section thickness. Sandstones interpreted as fluvial deposits are consistent with expected fan surface reworking between debris-flow events. Our results show that despite varying catchment sizes and relief, all of the basaltic-sourced alluvial fans are dominated by debris-flow deposits, which indicates that catchment bedrock is the primary control on depositional style for these alluvial fans, and perhaps in general.