USING INFRARED STIMULATED LUMINESCENCE TO BETTER CONSTRAIN TERRACE AGES ALONG THE CALIFORNIA COAST

Recent luminescence dates from several marine terraces in southern California (e.g., Kelty and Onderdonk, 2022, Morel et al., 2021) indicate younger terrace ages than previous estimates that were based on mollusk faunal assemblages, amino-acid racemization, and inferred regional correlations with terraces dated with Uranium series methods on shells and corals. To evaluate this difference in geochronological results, we used pIR-IRSL luminescence dating at two locations along the California coast that have well-established ages based on Uranium series methods on solitary corals. We dated three samples from paleo-beach deposits of the lowest marine terrace along the Central Coast at Cayucos where previous U-series dates on solitary corals indicated an age of 120 ka (Muhs et al., 2002). Two additional samples of overlying alluvium deposits were sampled to evaluate the influence of soil development on the ages and the timing of alluvial fan progradation across the terrace. The ages of these five samples were determined using the pIR-IRSL (50, 230°C) protocol. The paleo-beach deposits yielded ages of 42.3 ± 4.1 ka, 42.8 ± 4.4 ka, and 46.4 ± 5.6 ka (using a Central Age Model). A fading test was conducted on one sample, which showed no signs of fading. The overlying terrace alluvium ages ranged from 34.8 ± 6.2 to 42.1 ± 6.1 ka (using a Minimum Age Model). Our dates agree with previous OSL dates of 45 ± 3 ka from Stokes and Garcia (2009), who collected samples from alluvium overlying paleo-beach deposits on the same terrace 10 km north of the Cayucos site. Our pIR-IRSL dates are less than half the age of previous U-series dating methods of the Cayucos terrace. If the pIR-IRSL dates are correct this would imply that tectonic uplift rates in the area are significantly faster than previously interpreted.