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

Abstract

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 colluvial loess deposits underlying and upslope of individual boulders. The quartz OSL and K-feldspar pIRIR 290 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 and oldest loess colluvium unit must have been emplaced < 13 ka ago. A comparison of luminescence ages with cosmogenic 3 He surface-exposure ages from the surfaces of each boulder shows that two out of four boulders contain pre-deposition 3 He inheritance.Overall, the optical ages are consistent with both a prehistoric rockfall event at ~8-6 ka and a possible preceding event at ~14-13 ka, although the temporal resolution of the time of emplacement of individual boulders is ca. 3-5 ka. This resolution is not limited by age uncertainties but rather by the stratigraphy. This study is the first to show a successful application of luminescence dating to New

Chronology and processes of late Quaternary hillslope sedimentation in the eastern South Island, New Zealand

“…Although the consistency between quartz and feldspar suggests (Table , Fig. and Sohbati et al ., ) these may reflect in situ accumulation ages, we cannot dismiss the possibility that they result from resetting of the luminescence signal during near surface mixing. Further investigations involving single grain luminescence methods would be required to resolve whether these reflect deposition ages or not.…”

“…Luminescence sample preparation and analytical details are provided in Sohbati et al . (). Optical ages are labeled on corresponding trench logs (Figs C, 6C, 7C, 9C and 10B) and presented in Table .…”

“…Thirteen samples were collected for luminescence dating in (i) loess deposits underlying the prehistoric boulders and (ii) loess-colluvium accumulated upslope (i.e. colluvial wedge) of the boulders after emplacement on the hillside (Sohbati et al, 2016). Sampling involved pushing 5-cm-diameter stainless-steel tubes (15 cm in length) into cleaned sections of the trench walls.…”

“…In combination with Sohbati et al (2016) we present the first successful (i.e. reliable luminescence ages) optical dating of coarse-grained (i.e.…”

Hebei loess section in the Anyemaqen Mountains, northeast Tibetan Plateau: a high‐resolution luminescence chronology

“…6A). Either all samples have been incompletely bleached to the same degree or, more likely, all samples have been completely bleached (see also Sohbati et al 2016). 6A).…”