Location and Physiography This report describesthe geology of a region containing the DesignatedArea of the Northwest , and is covered by parts of the Geological Survey 1:20 000 Map Sheets2 (), 5 (Tsing Shan) and 6 (). The area stretchesfrom the northern limits of Tuen Mun through Yuen Long and SanTin to . It hasan areaof some 6 000 ha and it is shown in Figure 1; it is referred to in this report as the district. The district occupiesa low-lying region much of which is only a few metresabove sea level. It is some 25 kIn in length and up to 6 kIn wide in the vicinity of Yuen Long town (plate 1). It is boundedto the southwestby the Tsing Shan(Castle Peak) range of hills (583 m) and to the eastby the foothills of Tai Mo Shan (957 m). The northernland boundary is formed by Deep Bay and the Sham Chun River.

Plate 1 -Yuen Long Looking Southfrom the Industrial Estate

Much of the area is agricultural, with fish and duck farming predominatingin the northeast(plate 2). Yuen Long, the principal town of the district, was designatedas a satellite new town some 15 yearsago and has a population approaching200 000. Another new town, for 135 000 people,is being developed at Tin Shui Wai, whilst Fairview Park, a low-densityresidential development,covers an area of nearly 200 ha. Many new roads are planned or under constructionin the area,and numerousold villages, such as Lau Fau Shanand Ying Lung Wai, are scheduledfor modemisation.

Previous Work The first geological investigations in Hong Kong were undertakenby Brock, Uglow, Schofield & Williams between 1923 and 1927 under an agreementbetween the Colonial Office and the University of British Columbia. A geologicalmap of Hong Kong was published at a scale of 1:84 480 in 1936, and several papersrelating to this work were published by Brock & Schofield (1926), Uglow (1926) Plate 2 -Fish and Duck PondsNortheast of Fairview Park

and Williams (1943; 1945). The first memoir, based largely on this work, was produced by Davis (1952), followed later by a detailed descriptionof the geologyof the Territory by Ruxton (1960). In 1971, Allen & Stephenspublished the first comprehensivegeological map at a 1:50 000 scale together with a descriptive report. This survey remained the definitive work on the geology of the Territory until 1982 when the Hong Kong Geological Survey, staffed initially by secondedofficers of the then Institute of Geological Sciences(now British Geological Survey), commencedthe 1:20 000 mapping programme. Bennett (1984a; 1984b; 1984c) reviewed the stratigraphyand tectonics of the Territory, but the first geological memoir to describethe was published by Langford et al (1989), with accompanying map Sheets 5 & 6 in 1988 and Sheet 2 in 1989. The terrain characteristics,superficial depositsand engineeringgeology aspectswere describedin the Geotechnical Areas StudiesProgramme Report No. IV, North West New Territories (GCO, 1988b).The first records of marble occurring in the district were from ground investigationboreholes put down at Fairview Park in 1977 (Langford et ai, 1989)and at Yuen Long in 1980 (Ha et ai, 1981). It is now known that marble occurs beneatha thick cover of superficial deposits,which masksthe complex solid geology of the area, but its wide extentwas not suspectedjudging by the accountsof the geologyof the Territory published prior to 1989. Ground investigationsconducted in 1987 for the new Light Rail Transport (LRT) depot in Yuen Long discoveredcavities within the marble some of which were greater than 20 m in vertical extent. Although the lateral extent of these cavities was unknown, there was concernthat their presencecould seriouslyaffect development.Further drilling on the LRT site proved more zonesof dissolvedand highly weatheredrock (plate 3).

The Designated Area Project In 1987 the GeotechnicalControl Office initiated a programmeof explorationand deepboring to helpdelineate the marble substrate,to assist prediction of the cavitous areas within it, and to study its hydrogeology. A new series of geological maps at a scale of 1:5 000 was proposed to record the fmdings of the survey. Some 40 boreholeswere drilled to various depths not exceeding 200 m while two extra boreholes were sunk to over 400 m in depth at the locations shown in Figure 3. Theseboreholes were continuously sampledby rotary coring in the superficial depositsand the solid rocks. Summary logs of all these boreholesare shown in Appendix 1. Additional information was obtained from the logs of many hundredsof public and private ground investigationboreholes. Where core was not available for inspection, interpretation of the lithology was sometimes possible from colourphotographs in the boreholelog reports.

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15 From a total of some 5 000 m of core drilled for the Project, some 100 specimenswere thin sectioned; 8 sampleswere sent to the Institute of Geological Sciencesof GuangdongProvince, People's Republic of China, and 2 samplesto the University of Toronto, Canada,for radiocarbondating. Eight samples were sent to the School of Physics,Newcastle University, for palaeomagneticassessment; 38 samples were sent to the Stratigraphyand TectonicsUnit of the British Geological Survey for miosporeanalysis, cathodeluminescence and heavy mineral separation(Appendix 2 & 3). Analysesof 27 marble samples to determine lead-oxygenisotope ratios were undertakenby the Isotope Geology Centre of the Natural EnvironmentResearch Council in London. A preliminary assessmentof the hydrogeology of the district was made by the Hydrogeological ResearchGroup (HRG) of the British Geological Survey. Some 35 water sampleswere analysedby HRG to assessgroundwater geochemistry,and a duplicate set of analyseswas made by the Public Works Central Laboratory in Hong Kong to facilitate comparisonand standardisationof the results.The Public Works Central Laboratory also carried out soil classificationtests, and the insoluble residuesof 12 marble sampleswere determined. As an aid to correlation of the strata,several boreholes were testedby a gamma-logprobe by Electronic & GeophysicalServices Ltd (EGS). EGS were also employed to conducta local gravity survey of the Yuen Long area to help elucidatethe structureand sedimentarysequence present. In 1988 a visit to the Karst Institute in Guilin, China, for an internationalconference enabled the author to make comparisonsbetween the limestone structuresin a type area of karst developmentand those buried beneaththe superficial depositsof Yuen Long. Exposuresof Carboniferouslimestone and marble were examinedat quarries in the ShenzhenSpecial Economic Zone which adjoins Hong Kong (plate4). The value of aerial photographsin a flat area dominatedby man's activities was limited, but some extrapolationof photogeologicallineamentswas possible from the surroundinguplands (Hansen, 1990). A Landsat image of the Pearl River Estuary, although at a small scale,gave an excellent overview of the district and supplied further evidenceof fault lines suspectedfrom boreholedata. The 1:5 000 geological maps which accompanythis report are based on the Hong Kong Geological Survey field surveys, now publishedas maps at a scale of 1:20 000. The presentproject not only added to their work but also produced a detailed interpretationof the sub-surfacegeology based largely on new information, mostly from boreholes, obtained since their survey. Each 1:5 000 sheet area is representedby two geologicalmaps, one showing the superficial geology(Series HGP5A) accompanied by contours on the base of the superficial depositsin meb"esrelative to principal datum, and another depicting the solid geology(Series HGP5B) with contourson the baseof completelyweathered rock. All the records from this project, including rock samples,thin sections,manuscript maps and analytical data. are held in the archives of the Hong Kong Geological Survey,Geotechnical Engineering Office. The cores from the two deepboreholes, and representativecores from the other BGS Seriesboreholes, are also retained in the Hong Kong Geological Surveyarchives. As a result of this project the DesignatedArea was redefined into ScheduledArea No.2 (Figure 2), which occupiesless than half of the territory previouslysuspected as underlainby soluble bedrock.

Plate 4 -Marble Quarry near ShenzhenShowing Palaeokarst Scarp and Dip Topography.A Small Doline Depressionon the ExtremeLeft is Filled with Supetficial Deposits

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