Report No. : GEO Report No. 170

Report Title : Report on the Dating of Natural Terrain Landslides in Hong Kong (2005), 151 p.

Author : R.J. Sewell & S.D.G. Campbell

Abstract

This report describes the results of a detailed dating study of natural terrain landslides in Hong Kong undertaken by the Planning Division as part of an overall objective to advance the understanding of the causes and mechanisms of natural terrain landslides. Specifically, the aims of the study were to accumulate sufficient knowledge on the precision and accuracy of various dating methods to be able to produce guidelines on dating of natural terrain landslides, and to obtain sufficient numerical and relative age data in order to categorise relict natural terrain landslides in broad ages.

Knowledge of numerical (absolute) and relative ages of relict natural terrain landslides in the Hong Kong Special Administrative Region (HKSAR) is important in assessing natural terrain landslide hazards and the design of mitigation measures. Relative ages of relict landslide clusters can be suggested by Aerial Photograph Interpretation (API). Previous determinations of numerical ages of relict landslides have mostly relied on radiocarbon dating of large, readily visible, but rare, organic fragments/debris, and some optically stimulated luminescence (OSL) dating.

In November 2001, the Geotechnical Engineering Office commenced an integrated pilot study of relative and direct dating techniques to assess the accuracy and reliability of previously used techniques and the applicability of recently developed techniques. Recent developments include 1) Accelerated Mass Spectrometry (AMS), enabling radiocarbon dating of extremely small fragments of organic debris (5 mg), and extending radiocarbon dating back to c. 50,000 yrs BP, and 2) in-situ produced cosmogenic isotopes, apparently not previously used in the HKSAR, enabling the time rocks have been exposed to cosmic radiation, i.e. within c.3 m of the surface, to be established. Results from these new techniques can be compared with numerical results from OSL, and relative results from API.

Dating was undertaken at 19 sites around the HKSAR following detailed geomorphological observations based on API. Datable features included in situ materials from landslide sources, landslide debris, and fluvial/alluvial material associated with, or influenced by, landslides. The results have revealed that direct dating of carefully selected, relict natural terrain landslide scars in Hong Kong is viable. The new techniques provide several examples of confirmation of relative ages determined from API.

With respect to radiocarbon dating, it is possible to date natural terrain landslides using fragments of disseminated charcoal down to a minimum weight of 5 mg identified and separated with the aid of a binocular microscope.

With respect to in-situ cosmogenic nuclide analysis using ¹⁰Be and ²⁶Al, the sensitivity of the AMS technique is sufficiently high to measure exposure ages of surfaces associated with landslides of a few thousand years, despite the unfavourable circumstances of low latitude and low altitude.

With respect to luminescence dating, it is possible to successfully date large natural terrain landslide deposits up to 17 m thick using both the finegrain and Single Aliquot Regenerative (SAR) techniques despite potentially unfavourable circumstances affecting bleaching of sediment grains.

The numerical ages indicate that relict natural terrain landslides classified as large in the NTLI and LLS appear to be generally thousands and tens of thousands, rather than hundreds of years old. Smaller relict landslide features (e.g. relict open hillslope features) appear to be generally hundreds rather than thousands of years old. However, there are insufficient chronometric data on relict natural terrain landslides with which to determine landslide frequency magnitudes on a territory-wide basis.

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