OSL Dating of Fluvioglacial and Post-Glacial Debris Flow Deposits in the Gredos Mountain Range, Central Spain

This report is concerned with optically stimulated luminescence (OSL) investigations of sediment collected from fluvio-glacial and post-glacial debris flows in the Gredos mountain range, Central Spain. The samples were collected with the aim of establishing a relative chronology of the fluvio-glacial and post-glacial events. Sampling was undertaken by E. Muñoz-Salinas. Analyses were performed at the Scottish Universities Environmental Research Centre, East Kilbride. In total four dating samples were submitted to the laboratory. All samples were subjected to laboratory preparation of sand-sized quartz. Dose rates for the bulk sediment were obtained by high resolution gamma spectrometry coupled with in-situ field gamma spectrometry and beta dose rate measurement using thick source beta counting. Equivalent doses were determined by OSL from 16 aliquots of quartz per sample using the quartz single-aliquot-regenerative (SAR) procedure. The material exhibited good OSL sensitivity and produced acceptable SAR internal quality control performance. Dose distributions from the aliquots were examined using radial plotting methods. Three of the four samples produced finite ages. Although each of these samples showed considerable scatter in the stored dose estimates from the 16 aliquots measured; this reflects poor bleaching at time of deposition and/or mixing of poorly bleached and unbleached material at deposition (some of which had large residual signals). The fourth sample showed natural luminescence signals in excess of the saturation signals for the regenerative dose curves. Age estimates of 15.7 ± 1.0 ka, 10.7 ± 1.6 ka and 8.1 ± 1.0 ka were obtained for the sampled fluvio-glacial and post-glacial debris flows in the Gredos and Las Pozas gorges. The Gredos mountain range was glaciated between ~ 21 and 15 ka. A relative chronology, based on field relationships, indicates that the sampled debris flow in the Gredos gorge is between 21 and 15 ka in age, and that the sampled debris flows in Las Pozas gorge are younger than 15 ka. The fourth sample did not yield a finite date. In an attempt to produce a finite age for this sample a thermally-transferred OSL (TT-OSL) protocol was run on a further set of 8 aliquots. The TT-OSL method is believed to extent the working dose range of quartz up to and above 1 kGy. TT-OSL signals in quartz extracted from the problematic sample were not adequately reset at time of deposition