Report No. : GEO Report No. 322

Report Title : Dating of Debris Fan Complexes from Lantau Island, Hong Kong (2016), 125 p.

Author : R.J. Sewell

Abstract

Five debris fan complexes bordering the coastal foothills of western Lantau Island, Hong Kong, have been the subject of a detailed stratigraphic and dating study to explore the potential relationship between past periods of climate change and landslide activity. The debris fan complexes generally comprise an upper sequence of dominantly loose, gravelly colluvium and alluvium overlying a lower sequence of dominantly cohesive, bouldery colluvium. Luminescence (OSL) ages from the debris fan complexes, supported by a few radiocarbon ages (14C) on duplicate samples, suggest six main periods of accumulation: 800 – 1,350 yr BP, 3,320 – 3,560 yr BP, 4,230 – 4,370 yr BP, 5,000 – 8,000 yr BP, 11,000 – 14,500 yr BP, and 20,000 – 28,000 yr BP. The younger periods (< 10,000 yr BP) appear to be dominated by relatively thin (0.5 – 1.5 m) colluvial units deposited by mainly 'watery' debris flow and debris flood events, whereas the older periods are dominated by thicker (1.0 – 3.5 m) colluvial units deposited by 'drier' debris flow events. We hypothesise that immediately prior to the Last Glacial Maximum (LGM, i.e. 21,000 yr BP), the climate was cool and dry. Upland areas were probably devoid of vegetation, providing favorable conditions for sediment production in source areas. These debris-laden source areas are likely to have been mobilized en masse during periodic rainstorm activity, rapidly channelised over relatively short drainage courses, and then deposited as thick, sediment-rich, debris flow deposits along the coastal foothills. During post-LGM, the climate was warmer and more humid. Pluvial conditions predominated; forests occupied the upland source areas, leading to possibly reduced sediment yields, more frequent flash floods and deposition dominated by 'watery' debris flows or debris floods. Compared with recent studies on the weathering and erosion history of the Pearl River Delta, our landslide age data suggest a potential link between increased landslide activity and intensification of the Asian Monsoon during the Early to Mid. Holocene.

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