Abstract
Abstract. Wave-transported boulders represent important records of storm and tsunami impact over geological timescales. Their use for hazard assessment requires chronological information on their displacement that in many cases cannot be achieved by established dating approaches. To fill this gap, this study investigated, for the first time, the potential of optically stimulated luminescence rock surface exposure dating (OSL-RSED) for estimating cliff-detachment ages of wave-transported coastal boulders. The approach was tested on calcarenite clasts at the Rabat coast, Morocco. Calibration of the OSL-RSED model was based on samples with rock surfaces exposed to sunlight for ∼ 2 years, and OSL exposure ages were evaluated against age control deduced from satellite images. Our results show that the dating precision is limited for all targeted boulders due to the local source rock lithology which has low amounts of quartz and feldspar. The dating accuracy may be affected by erosion rates on boulder surfaces of 0.02–0.18 mm yr−1. Nevertheless, we propose a robust relative chronology for boulders that are not affected by significant post-depositional erosion and that share surface angles of inclination with the calibration samples. The relative chronology indicates that (i) most boulders were detached from the cliff by storm waves; (ii) these storms lifted boulders with masses of up to ∼ 24 t; and (iii) the role of storms in the formation of boulder deposits along the Rabat coast is more significant than previously assumed. Although OSL-RSED cannot provide reliable absolute exposure ages for the coastal boulders in this study, the approach has large potential for boulder deposits composed of rocks with larger amounts of quartz or feldspar and less susceptibility to erosion.
Highlights
Coastal boulders with masses of up to tens or hundreds of tons, located well above high tide level or far inland from the shoreline, are impressive evidence for the occurrence and impact of tsunamis and extreme storms (e.g. Engel and May, 2012; May et al, 2015; Cox et al, 2019)
The optically stimulated luminescence (OSL) surface exposure ages derived for boulders and niches from the Rabat coast show two striking characteristics: (1) all exposure ages are associated with relatively large dating uncertainties compared to previous applications of OSL-RSED (e.g. Sohbati et al, 2012; Lehmann et al, 2018); and (2) 5 of the 13 dated boulder samples yield OSL exposure ages that underestimate minimum ages deduced from satellite imagery and rock-pool depth, even when their uncertainties are considered (Fig. 6c)
OSL rock surface exposure dating was for the first time tested on coastal boulders overturned during wave transport to evaluate its reliability as a dating approach in this setting
Summary
Coastal boulders with masses of up to tens or hundreds of tons, located well above high tide level or far inland from the shoreline, are impressive evidence for the occurrence and impact of tsunamis and extreme storms (e.g. Engel and May, 2012; May et al, 2015; Cox et al, 2019). Engel and May, 2012; May et al, 2015; Cox et al, 2019) Such geological imprints may be preserved over periods that significantly exceed instrumental and historical records (Yu et al, 2009; Ramalho et al, 2015), making them valuable records for longterm hazard assessment. Compared to sandy tsunami and storm deposits, which are used more commonly for this purpose, wave-transported boulders are abundant along rocky coastlines and can be preserved over geological timescales even in settings dominated by erosion (Paris et al, 2011). Palaeomagnetic dating still suffers from a number of intrinsic methodological limitations, and cosmogenic nuclide dating cannot provide sufficient resolution on Late Holocene timescales and is of limited benefit for the vast majority of coastal boulders
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