The suitability of a low temperature post-IR IRSL signal for dating alluvial and colluvial “cut and fill” sequences in the Great Karoo, South Africa

Abstract

Abstract Alluvial and colluvial “cut and fill” deposits preserved in valleys of the Karoo, South Africa, reflect basin-scale adjustments in fluvial process-regime. Such deposits in the Wilgerbosch catchment have previously proven difficult to date using radiocarbon (14C) and optically stimulated luminescence (OSL) methods. In this paper, we test the suitability of K-feldspar post-IR infrared (pIRIR) methods on 19 samples from Africanders Kloof, a low-order tributary of the Sundays River. Using three carefully screened quartz OSL ages, radiocarbon dating and site stratigraphic considerations we argue that the pIRIR170 protocol can be used to produce reliable age estimates. Fading rates for the pIRIR170 signal are consistently low (average g2days: 0.81 ± 0.58). The pIRIR170 residuals are dose dependent (r2 = 0.58); but are consistently low as a proportion (e.g. 1–4%) of sample equivalent dose (De). Despite the water-lain depositional context, single aliquot De distributions tend toward normality (for 11/19 samples) irrespective of aliquot size (2 mm or micro-aliquots containing 2–30 grains) with only a few statistical outliers per sample (max. n = 3) and overdispersion (OD) ranging from 1.6 to 30% excluding the two youngest (late Holocene) samples (OD: 37–87%). The resulting pIRIR170 ages are in the correct stratigraphic order and illustrate the potential of pIRIR170 luminescence dating to investigate the timing, processes and drivers of fluvial system adjustments across the Karoo.