Optical dating of loessic hillslope sediments constrains timing of prehistoric rockfalls, Christchurch, New Zealand

Developing a robust chronology for mass-movement events is of crucial importance to understanding triggering mechanisms and assessing hazards. We constrain the emplacement time of four palaeorockfall boulders near Christchurch, New Zealand, using optically stimulated luminescence (OSL) of quartz and infrared stimulated luminescence dating (IRSL) of K-feldspar from loessic hillslope deposits underlying and upslope of individual boulders. The quartz OSL and K-feldspar pIRIR290 ages are all consistent with the stratigraphy and in excellent agreement with each other, indicating that all the boulders that overlie the in situ loess must have been emplaced <13 ka ago. A comparison of luminescence ages with cosmogenic 3He surface-exposure ages from the surfaces of each boulder shows that two out of four boulders contain pre-deposition 3He inheritance. Overall, the optical ages are consistent with both a prehistoric rockfall event at 6–8 ka and a possible preceding event at 12–14 ka. This study is the first to show a successful application of luminescence dating to New Zealand colluvium loess and demonstrates the great advantage of a multi-technique approach in mass-movement dating.