Luminescence Dating of Samples from Raised Beaches in Tanzania

Kinnaird, T., Cresswell, A. , Bishop, P. and Sanderson, D. (2010) Luminescence Dating of Samples from Raised Beaches in Tanzania. Technical Report. SUERC, Edinburgh, UK.

Abstract

This report presents results of optically stimulated luminescence (OSL) dating of eight sediment samples collected from raised beaches in the Pwani district of the southern Tanzanian coast. Six samples were collected in a vertically stratified sequence from Tundwisogani Quarry, approximately 10km inland from the present coast line. The remaining two samples were collected a further 8-9 km inland at Maragoro-Mkuranga and Koga-Mimba. The purpose of the study was to appraise the suitability of the material for OSL dating, with a view to constraining the rates and periods of recent uplift in this area due to tectonic activity. The sampling was conducted on behalf of Heritage Oil, and the analyses performed at the Scottish Universities Environmental Research Centre in East Kilbride, Scotland. All samples were subjected to laboratory preparation of sand-sized quartz, and purity checked using scanning electron microscopy. Dose rates for the bulk sediment were evaluated using analyses of the uranium, thorium and potassium concentrations obtained by high resolution gamma spectrometry coupled with 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, which revealed some heterogeneity. The uppermost sample from Tundwisogani Quarry, approximately 10 km from the present coastline, produced an age estimate of 14 ± 2 ka. Samples from Maragoro-Mkuranga and Koga-Mimba, approximately 17 km and 20 km from the present coastline, produced age estimates of 13 ± 2 and 20 ± 2 ka respectively. If these dates are taken as indicative of the depositional age the corresponding uplift rates would range from 5.3 to 11.5 mm per year, which seem remarkably high. However, the spread in equivalent doses raises the possibility that these upper layers may have been reworked since deposition, in which case the SAR ages will under-estimate the original depositional age and hence over-estimate uplift rates. The remaining five samples, from lower strata at Tundwisogani Quarry, showed natural quartz OSL signals in excess of the saturation values from the regenerative dose curves, resulting in minimum age estimates in excess of 45 ka. Additional work to explore the feasibility of applying other luminescence approaches to determining finite ages was also undertaken for two of these deeper samples. This further work confirmed that post-OSL thermally stimulated signals have the potential for extending the quartz dose response to several hundred Gy doses, as do elevated temperature infra-red stimulated luminescence signals recorded from K feldspars after removal of conventional IR-stimulated luminescence. The sediments from Tundwisogani quarry were shown to contain K feldspars. Exploratory studies confirm that both the high temperature quartz approaches and the post-IR IRSL feldspar signals are capable of extending the luminescence dating age range to several hundred ka. The age range for the lower strata at Tundwisogani obtained in the exploratory study is between 90 and 60 ka. The uplift rates implied for the lower layers at Tundwisogani based on these exploratory analyses would be 0.53 – 1.3 mm per year. Having established the suitability of both the quartz OSL method for samples younger than 100-150 ka, and the potential of the high temperature methods for longer timescales in the region it would now be possible to undertake a further programme of work to examine the geographical distribution of depositional ages and post-depositional alterations to further constrain the uplift rates of the region, and to provide an observational framework for further understanding the episodic nature of the regional movements that have formed the marine terraces in the area.

• Item Type: Research Reports or Papers (Technical Report)
• Status: Published
• Glasgow Author(s) Enlighten ID: Kinnaird, Dr Timothy and Sanderson, Professor David and Cresswell, Dr Alan and Bishop, Professor Paul
• Authors: Kinnaird, T., Cresswell, A., Bishop, P., and Sanderson, D.
• Subjects: G Geography. Anthropology. Recreation > GB Physical geography
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre; College of Science and Engineering > School of Geographical and Earth Sciences
• Publisher: SUERC