Palaeokarst of Yuen Long, north west New Territories, Hong Kong

A project in the north west New Territories of Hong Kong, undertaken by the Geological Survey of the Hong Kong Government, has provided geological maps at a scale of 1:5 000 of a region known as the Designated Area which is largely earmarked for new-town “high-rise” development. It has proved a complexly faulted area of Carboniferous metasediments associated with Jurassic and Cretaceous granites and volcanic rocks. The rocks have been faulted and metamorphosed and are now buried beneath a superficial cover of late Tertiary and Quaternary superficial deposits to depths of 8 0 m. Carboniferous limestone has been metamorphosed into marble and at some stage during the Tertiary was exposed to sub-aerial denudation resulting in extensive karstification with an epikarst zone some 10 to 30 in thick. This zone contains all sizes of solution features from widened joints to caverns 20 m high. The cavities contain clays, silts and sands, the dating of which is in progress. The epikarst surface is highly irregular with a relief of some 60 m showing hollows which are interpreted as dolines. The maximum frequency of dolines coincides with the underlying Ma Tin Member of the Yuen Long Formation - a unit of particularly pure, white massive marble. The largest cavities also occur in this Member in close proximity to major structural lines of weakness often associated with intruded igneous rocks. The project has provided an accurate basis for answering geotechnical enquiries in the Designated Area. The area originally thought to be underlain by cavitous marble now has been reduced by a half. Further research into the hydrogeology of the area and geochemisty and isotope geology of the rocks is planned.