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~u D 0 ~ 0 ~ D D Z ~0 ~ z Geotechnical Engineering Office 0 :I: Ci viI Enginee ring D epartmen t August 1992 @ Government of Hong KongPublished August 1992

Preparedby: GeotechnicalEngineering Office 11fF Civil EngineeringBuilding 101 PrincessMargaret Road Homantin, Hong Kong

Cover: Oblique aerial view of and in 1987

This publicationis availablefrom: Hong Kong GeologicalSurvey GeotechnicalEngineering Office 11fFCivil EngineeringBuilding 101 PrincessMargaret Road Homantin Kowloon Hong Kong This report specifically relates to the Designated Area, which is the part of the northwest defined by the Hong Kong Geological Survey in 1987as possibly underlainby marble. The geology of the area is depicted on twenty sheetsof the 1:5 000 Geological Map Series HGP5A and HGP5B. The 1:20 000 Geological Map Sheets2 (), 5 (Tsing Shan) and 6 (Yuen Long), and Memoir No.3, Geologyof the WesternNew Territories,also include information aboutthe geology of the area coveredby the report. This report forms part of the published resultsof a programmeof systematicgeological mapping that began in 1982. This work has greatly enhancedour understandingof Hong Kong's stratigraphyand structure, and is establishing the geological database necessary for the continuing economic development of the Territory. The mapping programme is being undertakenby the Hong Kong Geological Survey, which is a section of the Planning Division of the GCO. The section was led by Mr P. J. Strange and the Division was under the direction of Dr A. D. Burnett during the mapping project reportedhere. The 1:5 000 scale geological survey of the DesignatedArea was conductedby Dr D. V. Frost of the British Geological Survey under a consultancyagreement between Hong Kong Governmentand the Natural Environment ResearchCouncil, United Kingdom. Dr Frost was assistedby Ms N. J. Darigo, Mr D.K.M. Yuen and Mr A.H.P. Chau of Dames & Moore (Hong Kong), and the project was undertakenbetween May 1988 and April 1990. Prior to this, preliminary work in the DesignatedArea had beencarried out by Mr K. W. Lai and Dr R. L. Langford of the Hong Kong GeologicalSurvey. The Survey benefitted from the co-operationof many organizationsand individuals. The co-operation of Dr L. J. Endicott and staff of Dames & Moore (Hong Kong), Acer FreemanFox Ltd, Binnie & Partners,Fugro (Hong Kong) Ltd, Maunsell GeotechnicalServices Ltd, Ove Amp & Partners(Hong Kong) Ltd, and Electronic and GeophysicalServices Ltd is gratefully acknowledged. This report and the accompanyingmap sheetswill be of interestand value to earth scientists,engineers, planners,developers and educationalists.

A. W. Malone Principal GovernmentGeotechnical Engineer April 1992

3 Title Page 1 Foreword 3 Contents 5 Map and Report SeriesNotes 9 Chapter1 Introduction 13 Location and Physiography 13 Previous Work 13 The DesignatedArea Project 14 Chapter2 Outline of Geology 18 Chapter3 Sedimentary and Volcanic Rocks 23 PalaeozoicRocks of the SanTin Group 23 Classificationand Distribution 23 Y uenLong Formation 23 Long Ping Member 25 Ma Tin Member 25 Formation 26 Mai Po Member 26 Tai ShekMo Member 27 Mesozoic Sedimentaryand Volcanic Rocks 27 Chapter4 Metamorphic and Igneous Rocks 29 MetamorphicRocks 29 Regional Metamorphism 30 Thermal Metamorphism 30 Metasomatism 30 Intrusive IgneousRocks 30 Granitoid Rocks and Heat Generation 31 Chapter5 Structure 32 GranitePlutons 32 Volcanic Rocks 32 Folds 37 Faults and Joints 37 Chapter 6 Superficial Deposits 41 Classificationand Distribution 41 Clay Residues 41 Debris Flow Deposits 41 Alluvium 41 Marine Deposits 44 EstuarineDeposits 47 Chapter7 Karst 48 Reactivationof Karst Features 54

5Page Page Chapter8 Hydrogeology 56 Inuoduction 56 GroundwaterCharacteristics 56 Water Quality 56

References 59

62

Index 67

Appendix 1 AbbreviatedBorehole Logs 62 2 Classificationof Zircons by Colour and Morphology 64 3 Heavy Minerals in Cavity Fill and AssociatedSediments 64 in the Marble of Yuen Long

4 Microfossil Determinationsof Samplesfrom the CarboniferousSan Tin Group 65 5 Minerals from Insoluble Residuesfrom Yuen Long Marbles 65 6 Heavy Minerals from Sandstonesin the Yuen Long Area 66 7 Carbon 14 Dating of QuaternaryDeposits in the District 66 List of Tables Table Grain Size Descriptionand Classificationof Rocks and Superficial Deposits 10 in Hong Kong 2 Solid Rocks and Superficial Depositsof the District 19

Figure 1 Location Map of the Original DesignatedArea, as at 30 October 1987 11 2 The RedefinedDesignated Area in the NorthwestNew Territories 12 (ScheduledArea No.2) 3 Location of Boreholes(BGS) Drilled for the Marble Project 4 GeneralisedSolid Geology (a) Yuen Long 20 (b) Lok Ma Chau 21 5 GeneralisedSection of the SanTin Group 24 6 Ribbon SectionShowing the StructuralFramework of the DesignatedArea 33 7 Main StructuralFeatures of the District (a) Yuen Long 34 (b) Lok Ma Chau 35 8 Marble/GraniteAssociation in a Site Near Tin Shui Wai 36 9 Structureof the Long Ping Estate,Yuen Long 38

6

15 1011 Rose Diagram of Faults of the Yuen Long District 39 Graph Showing Frequencyand Inclination of Joints in the 40 Ma Tin Road Borehole Contours on the Base of the Superficial Deposits 42 Ribbon SectionShowing the Superficial DepositsTypical of the District 43 14 Contour Map Showing the Baseof the Alluvial DepositsSuggesting a 44 Palaeogeographyof MeanderingChannels Contoursand Isopachytesof Superficial DepositsOverlying Marble of 45 Site YLTL 313 on the Yuen Long Industrial Estate

GeneralisedThickness of Marine Depositsin the District 46 17 GeneralisedRockhead Contours of the CarboniferousStrata around Yuen Long 47 BetweenMarble and GranodioriteLocated at Site YLTL313, Yuen Long Industrial Estate Karst Featureson Part of Site YLTL 313, Wang Lok Street, Yuen Long Industrial Estate 19 RockheadContours and SectionShowing a Doline Developedalong the Faulted Boundary 20 Contour Map of the Top Karstic Surfaceof the Marble at Site YLTL 442 52 adjoining Hop Hoi Street,Yuen Long Cavity Infill Depositsfrom a Mazier Samplein BGS 2Large 53 Area of Soft Silt Formed at the Thrust Boundary Betweenthe Mai Po and 55 Ma Tin Members of the SanTin Group. Site YLTL 446, Yuen Long The Shan Poi River Catchment,Yuen Long 57 24 SchematicModel of GroundwaterFlow BeneathYuen Long 58 List of Plates

1 Yuen Long Looking South from the Industrial Estate 13 Fish and Duck PondsNortheast of Fairview Park 14 Large Block of Karst Marble (invertedMa Tin Member)Excavated from the 16 Rockheadat the LRT Site, Yuen Long

4 Marble Quarry near ShenzhenShowing PalaeokarstScarp and Dip Topography. 17 A Small Doline Depressionon the ExtremeLeft is Filled with Superficial Deposits 5 Highly to CompletelyWeathered Metasiltstone of the Mai Po MemberShowing LiesgangRings in FerruginousConcretions; BGS 17,52.90-53.90 m 6 Highly to CompletelyWeathered Metasiltstone of the Mai Po MemberShowing 22 SecondaryGrowth of ManganeseDioxide (Pyrolusite); BGS17,42.90-43.90 m 7 Marble From the Long Ping Member, YuenLong Fonnation; BGS2 8 Marble From the Ma Tin Member, Yuen Long Fonnation; BGS3 9 Metasiltstoneand Metasandstoneof the Mai Po Member,Lok Ma Chau Fonnation; 27 BGSll,112.55-117.50 m 10 Graphite Schist of the Mai Po Member,Lok Ma ChauFonnation, with Prominent 27 White Quartz Veins; BGS27,51.10-54.75 m Highly to CompletelyWeatherd Tuffaceous Siltstone with Clastsof Marble, Tin Shui Wai Member, Tuen Mun Formation;BGS8, 91.80-97.40m

InterbeddedTuffite and Marble-bearingBreccia, Tin Shui Wai Member, 28 Tuen Mun Fonnation; BGS18,40.62-44.24 m

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1121222228Page1213151618505123Plate2526123 Page 13 Veined and FracturedMarble CloselyOverlying Granodiorite;BGS9. 45.25-49.46m 29 14 GranodioriteShowing StainedJoints; BGS6. 22.66-26.17m 29 15 Karst Featuresin BoreholeCores: SolutionFeature Showing Feffilginous Staining 48 16 Karst Featuresin BoreholeCores: SolutionWidened Joint ShowingFretted Surface 48

8 .This report describes the geology of part of the western New Territories and should be read in conjunction with the 1:5 000 Geological Maps 2-NE-D, 2-SW-C, 2-SW-D, 2-SE-A, 2-SE-B, 3-NW-C, 6-NW-A, 6-NW-B, 6-NW-C and 6-NW-D. The 1:20000 Geological Map Sheets2 (San Tin), 5 (Tsing Shan) and 6 (Yuen Long), and Memoir No.3, Geology of the Western New Territories, also include relevant information on the geologyof the YuenLong area. .The report forms one of a series that recordsthe findings of the Hong Kong Geological Survey. An index of the 1:5 000 Geological Maps to which this report relates is shown below. This report specifically relates to the DesignatedArea (Figure 1), which is an area defined by the Hong Kong Geological Survey as an envelope within which the presenceof a marble subcropcontaining cavities was possible. The boundaries of the DesignatedArea were based upon the geological information available on 30 October 1987, comprising both borehole data from ground investigationsas well as surfaceexposures. On completionof this presentsurvey the envelopeof marble subcropwas redefined, and following amendmentof the Buildings Ordinanceand Regulationsis known as ScheduledArea No.2 (Figure 2).

'2-NE-D~.+. .' 3-NW-C I

2-SE-B

6-NW-A 6-NW-B I

6-NW-C 6-NW-D I

.Onshore superficial depositsare not generallyconsidered mappable if less than 2 m thick. .Grid referencesare based on the Hong Kong 1980 Metric Grid as shown on the 1:5 000 Geological Maps. Eight-figure references indicate positions to the nearest 10 m, with Eastings followed by Northings, eg, 26722739. Six-figure referencesindicate positions to the nearest100 m. .Hong Kong Principal Datum (PD) is 1.2 m below Mean Sea Level and 0.15 m above Admiralty Chart Datum. .Samples in the Territory-wide rock collection archived by the Hong Kong Geological Survey are prefixed HK followed by the serial number,eg, HK 2263. .Boreholes are generallyreferred to by the drilling contractor's numberfollowed by the Geotechnical Information Unit (GIU) accessionnumber for the relevantground investigationreport, eg 1201D/03412. Boreholes with the prefix BGS were specifically drilled for the Hong Kong Geological Survey. The GIU is located in the Civil EngineeringLibrary of the Civil Engineering Departmentand is maintained by the GeotechnicalEngineering Office. .The systemused in this report for grain size descriptionand classificationis summarizedin Table 1

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