Abstract
Dr Howat has raised two issues relating to the offshore sediments and proposed reclamation work in the vicinity of North Lantau. The first relates to the stratigraphy of the offshore sediments, the second to their engineering behaviour. The terminology used by Dr Howat to describe the offshore sediments is the simplified stratigraphical model current during the early 1980s.
Over the past ten years considerable effort has been directed by the Hong Kong Geological Survey (HKGS) of this department into refining this early model. The stratigraphy has been defined from boreholes and mapped from a detailed study of high resolution seismic profiles. In general the offshore sequence of sediments over which reclamation is planned within this region comprises very soft to soft marine clays and clayey silts (Hang Hau Formation) unconformably overlying intercalated sands, gravels and firm to stiff silty clays (Chek Lap Kok Formation). Recent study of offshore boreholes and seismic surveys to the south-west of Chek Lap Kok has revealed another formation (Sham Wat Formation), albeit limited in extent within the area to be reclaimed, at the base of the Hang Hau Formation comprising a soft to firm clayey silt.
Available evidence suggests the Chek Lap Kok Formation underlying the marine Hang Hau Formation, is predominantly alluvial in origin as shown by detailed analysis of continuously sampled boreholes (Shaw, Zhou, Gervais & Allen 1986). Recent detailed micropalaeontological analysis of sediments from off-shore boreholes has not revealed any definitive evidence of marine conditions within the Chek Lap Kok Formation. However,