Optically stimulated luminescence dating techniques and their application to dating the formation of late quaternary loess in southern North Island, New Zealand

Abstract

Three of the optically stimulated luminescence (OSL) dating techniques were used at the luminescence dating laboratory, Victoria University of Wellington. Each technique involved using 1.4 eV infrared light to stimulate K-feldspar in the fine grained (4-11 μm diameter) mineral fraction from samples of aeolian and fluvial silt. OSL ages were obtained from 11 samples using both the additive-dose with thermal transfer correction and regeneration methods and from 5 samples using both the additive-dose method and the single aliquot regeneration method. The study also showed that there was no anomalous fading in most of the samples and no significant radioactive disequilibrium in the dated samples. Loess samples were collected from the Rangitikei and Wairarapa valleys, southern North Island. In the Rangitikei area, loess was collected from previously mapped Porewa, Rata and Ohakea surface sections. Loess from a Porewa terrace yielded minimum age of 113.5±9.4 ka (using the regeneration method) which is older than the inferred maximum Porewa surface age of ~80 ka. Further dating of this or older loess sequences is required to confirm the age. Two samples of loess from a Rata surface from 0.7m below the base and 0.56m above the top of the Kawakawa tephra (calibrated 14C age of 24.5 ka) yielded ages of 21.9±1.2 ka and 16.3±1.4 ka, respectively. Comparison of these ages with OSL and TL ages in the published literature suggests that the ages may be too young by 5-6 ka, and this could be due to downward movement of U and Th by leaching over the last 26 ka or anomalous fading. An Ohakea terrace age of 10.4±0.6 ka is consistent with the age of the youngest Ohakea terrace (Oh1) subset in the Rangitikei area. In the central and southern Wairarapa Valley, seven OSL ages were derived from pockets of Ohakea loess that overlie the youngest (Waiohine) aggradation surface. The OSL ages range between 7.5±0.6 ka and 16.1±1.6 ka. with a mean age of 11.8 ka. OSL ages of 7.0±0.5 ka and 4.3±0.5 ka of loess derived from the Waiohine surface provide a minimum age for the Ohakea loess in the south Wairarapa Valley area. The youngest aggradation surface at Wharekauhau, Palliser Bay yielded an OSL age of 5.0±0.4 ka, and is thus considerably younger than the Waiohine surface further north. Loess from an inferred Porewa surface at Waingawa River yielded an age of 10.6±0.9 ka and this, together with the paucity of loess cover (absence of Rata loess and the Kawakawa tephra), indicates deflation during deposition of the Ohakea loess. An OSL age of 14.8±1.1 ka from a Rata terrace, also at Waingawa River, is consistent with the samples position about 0.8 metre above the Kawakawa tephra at this locality. The Wairarapa OSL ages were used to estimate slip rates on the Wairarapa Fault; a horizontal slip rate of 11.0±0.5 mm/yr at Waiohine River and variable vertical slip rates of 4 mm/yr in the southwest (Wharekauhau); 0.6 mm/yr (Pigeon Bush); 0.9 mm/yr (Tauwharenikau River); 1.7 mm/yr (Waiohine River). The variable vertical displacement, and therefore vertical slip rate of the Waiohine surface by the Wairarapa Fault, is thought to be controlled by the presence of growing bulges on the upthrown side of the fault caused by left-lateral sidesteps.