Luminescence dating of sediments from central Atacama Desert, northern Chile

Abstract

Abstract This study examines the feasibility of applying luminescence dating methods to quartz and potassium feldspar (K-feldspar) grains from Quaternary continental deposits of the Mejillones Peninsula and Coastal Cordillera in Central Atacama, northern Chile. Sediment burial ages were calculated using single-aliquot regenerative-dose (SAR) protocols applied to the optically stimulated luminescence (OSL) signal of quartz as well as infrared stimulated luminescence (IRSL) and post-infrared infrared stimulated luminescence (post-IR IRSL) signals of K-feldspar. Considering that the target sedimentary deposits comprise the Late Pleistocene age range, K-feldspar grains were dated using fading-corrected IRSL signals measured at 50 °C (IR50) and post-IR IRSL measured at 225 °C (pIRIR225) to minimize the potential effect of residual doses on calculated ages. The results of the analytical procedures indicate that quartz grains extracted from the studied sediments present a very weak or even no fast OSL component. The combination of a low OSL sensitivity, signal instability and equivalent dose distributions with high overdispersion (>40% for most samples) hinder reliable age estimation using quartz aliquots. Bleaching test results show that the IR50 signal from K-feldspar aliquots is well reset after 20–24 h of light exposure, while the pIRIR225 signal may present residual doses corresponding to between 10 and 15% of the natural signal. IR50 fading rates are ∼6–7%/decade for most studied samples, with exception of one sample which yielded a g-value of 18.77 ± 2.06%/decade. Fading rates for the pIRIR225 signal yielded variable results among sampling sites, with g-values ranging between 0.70 ± 0.24 and 6.77 ± 1.05%/decade. The dating results point out that quartz OSL ages are largely underestimated in relation to K-feldspar fading-corrected IR50 and pIRIR225 ages. Fading-corrected pIRIR225 K-feldspar ages indicate that the alluvial sedimentation in the downthrown block of an active normal fault in Mejillones Peninsula occurred between 163.4 ± 18.4 ka and 87.4 ± 6.6 ka. K-feldspar ages also allow to constrain the age of fault scarp degradation and fault reactivation in two main branches of the Atacama Fault System. In the Naguayan Fault, pIRIR225 ages indicate that faulting occurred after 21.4 ± 3.2 ka while in the Salar del Carmen Fault, faulting is recorded after 14.7 ± 1.0 ka.