Oral Presentation

The luminescence signals in quartz grains from tectonically active regions, such as Southern California, are usually dim. This precludes us from dating single-grain quartz luminescence from these regions. The K-feldspar samples are usually brighter when stimulated, responsive, and often do not show significant fading at higher temperatures (e.g., 225–230° C). The single-grain K-feldspar infrared stimulated luminescence (IRSL) has shown great promise in recent years, making it a valuable tool for burial dating for various sedimentary deposits collected from pits, trenches, exposures, and sediment cores from southern California. However, depending on the transport conditions or bleachability of the targeted luminescence signal, complete signal resetting of all feldspar grains prior to burial is not guaranteed in fluvial settings. Therefore, the K-feldspar grain population in concentrated flows often produces multiple single-grain subpopulations at a higher temperature (i.e., signals from more stable traps). Our recent findings from the upper and lower Mission Creek catchment in Southern California show that depending on the depositional settings (e.g., flood plain vs. alluvial fans), either the youngest or more dominant single-grain subpopulation produces the age of the deposit comparable to the other geochronometers. For example, in the lower flood plain deposits at the Mission Creek, the minimum age model (i.e., youngest subpopulation) offers exceptionally consistent age constraints with the stratigraphy and corresponds well with the youngest detrital C-14 ages at ±1σ. In contrast, the relatively old, upstream nested alluvial fan deposits show that the luminescence subpopulations with the most grains or the second-most grains, and not the youngest subpopulation, produce the ages consistent (±1σ) with the recalculated ages derived from previously published Be-10 depth profile and boulder ages of Owen et al. (2014). These case studies suggest that depending on the depositional settings, such as alluvial fans, finding the true burial age requires more rigorous sampling procedures and multiple dating methods. This is particularly critical since these fan surfaces are regularly targeted to constrain past earthquake events and estimate geologic slip rates. Additionally, the increased resolution offered by the single-grain method can also be used to extract additional information about the transport and depositional history of each grain within the catchment.