EP029-0008
Examining single-grain luminescence dating uncertainties between upstream fans and downstream sediment deposits in the seismically active southern California
Thursday, 10 December 2020
Poster
Sourav Saha, University of California Los Angeles, Los Angeles, CA, United States, Seulgi Moon, University of California Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States, Nathan Brown, University of California Berkeley, Department of Earth and Planetary Science, Berkeley, CA, United States and Edward Rhodes, University of Sheffield, Department of Geography, Sheffield, S10, United Kingdom
Abstract:
The fidelity of paleoseismic records relies heavily on stratigraphic completeness and robust chronological controls. Stratigraphies with limited organic matter and reworked detrital material that are poorly suited for radiocarbon dating can often be reliably dated with luminescence techniques. While luminescence dating has proven to be a useful tool in dating sedimentary deposits in tectonically active southern California, issues like variable athermal fading of feldspars grains, grain-size dependent dose rates heterogeneity in fanglomerates, and the presence of unusually young grains in older deposits often make it challenging to date depositional sequences with accuracy. For example, researchers who used post-Infrared Infrared Stimulated Luminescence (p-IR IRSL) measurements to date the older upstream alluvial fans at the Mission Creek catchment cannot reconcile dose rate estimates measured using different techniques. Previous work has also shown p-IR IRSL signal fading at 225°C. However, feldspar grains recently dated using p-IR IRSL
at 225°C using a similar procedure from a downstream trench (~10 km) on the Banning strand in the catchment, show no fading. While these discrepancies may be caused by variable source area lithology, a detailed comparison of the single-grain luminescence ages or characteristics with the published ages and with the downstream luminesce signals may provide insights into the dose rate, signal fading, and the choice of appropriate age models.
In this study, we collected 16 luminescence samples from the same four sets of alluvial fans at the Mission Creek that have already been independently dated using 10Be (boulders/depth profiles), optically stimulated luminescence, and p-IR IRSL. These samples are mostly collected from ~1 m-deep pits close to the location of previous samples. These samples are currently being measured using the single grain K-feldspar p-IR IRSL method. A detailed dose rate measurement is also undertaken. Any deviation from the expected ages will be examined in detail to understand the source of uncertainties better. After determining ages accurately, we will use the single-grain distribution of the upstream fans and the downstream trench deposits to infer the periods of sediment aggradation, fan formation, and its relation to tectonics and climate.
