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THE UNIVERSITY OF HONG KONGLIBRARIESHong Kong Collectiongift fromHong Kong (China), Civil Engineering Dept


I and Northshore8 Lantau IslandS^1:5 000 Sheets 9-SE-A, 9-SE-BOffshore Part Sheets 9-NE-D, 10-NW-A, 10-NW-B, 10-NW-C &10-NW-D, & Study of Ground Conditions at lung Chung New TownRJ. Sewell & P.A. KirkinOOOo wOoooGeotechnical Engineering OfficeO Civil Engineering Departmentffi HONGKONG July 2002


,11 DEC'£• Government of the Hong Kong Special Administrative RegionFirst published July 2002ISSN 1022-6168Prepared by:Geotechnical Engineering OfficeCivil Engineering DepartmentCivil Engineering Building101 Princess Margaret RoadHomantin, KowloonHona KongCover: Oblique aerial view of Tung Chung New Town from above Chek Lap Kok taken in June 1998.This publication is available from:Chief Geotechnical Engineer/PlanningGeotechnical Engineering OfficeCivil Engineering DepartmentCivil Engineering Building101 Princess Margaret RoadHomantin, KowloonHong Kong


ForewordThis report and associated 1:5 000-scale maps specifically relate to development areas in the vicinity ofTung Chung New Town and parts of proposed development areas for Northshore Lantau Island.The report forms part of the published results of a programme of systematic geological mapping of HongKong that began in 1982. It complements geological information contained in the Hong Kong GeologicalSurvey Memoir No. 6 - Geology of Lantau District, and in two Hong Kong Geological Survey Sheet Reports(No. 2 - Geology of Chek Lap Kok, and No. 4 - Geology of North Lantau Island and Ma Wan). Thesereports have greatly enhanced our understanding of the stratigraphy, structure, and geological history ofHong Kong's rocks. At the same time, they have allowed a geological database, necessary for the continuingeconomic development of the Hong Kong Special Administrative Region, to be established and developed.The mapping programme was undertaken by the Hong Kong Geological Survey, which is a section of thePlanning Division of the Geotechnical Engineering Office, Civil Engineering Department. The section wasled by Dr C.J.N. Fletcher and the Division was under the direction of Dr R.P. Martin during the mappingproject reported here. The report was subsequently written and compiled by Dr R.J. Sewell and Mr P.A.Kirk, at which time the section was led by Dr S.D.G. Campbell and the Division was under the direction ofMr H.N. Wong.The 1:5 000-scale onshore geological survey of Tung Chung was conducted by Mr P.A. Kirk, who alsomanaged a consultancy, on behalf of the Territory Development Department, and undertaken by membersof the British Geological Survey, to study complex ground conditions at Tung Chung New Town. Some ofthe information from the consultant's study is incorporated in this report. Much helpful advice and reviewcomments were also given by GEO colleagues, including Mr K.W. Lai and Dr X.C. Li.The Survey benefitted from the co-operation of many organizations and individuals. In particular, the cooperationof Mott Macdonald (Hong Kong) Ltd, Scott Wilson (Hong Kong) Ltd, the Airport Authority,Electronic and Geophysical Services Ltd, and Cosine Ltd., is gratefully acknowledged.This report and the accompanying map sheets will be of interest and value to earth scientists, engineers,planners, developers and educators.R. K. S. ChanHead of the Geotechnical Engineering OfficeJuly 2002


CONTENTSPageTitle Page 1Foreword 3Contents 5Map and Report Series Notes 9Chapter 1 Introduction 11Location and Physiography 11Previous Work 12Complex Ground Conditions at Tung Chung 12The Northshore Lantau Development Feasibility Study 12Offshore Geophysical and Marine Ground Investigations 14Purpose and Scope of Study 14Data Sources 14Chapter 2 Outline of Geology 17Chapter 3 Palaeozoic Sedimentary Rocks 23San Tin Group (Carboniferous) 23Classification and Distribution 23Lok Ma Chau Formation 23Yuen Long Formation 23Tolo Harbour Formation (Permian) 24Chapter 4 Mesozoic Sedimentary and Volcanic Rocks 25Tai 0 Formation 25Tsuen Wan Volcanic Group 25Yim Tin Tsai Formation 25Lantau Volcanic Group 26Chapter 5 Intrusive Igneous Rocks 28Classification and Distribution 28Major Intrusions 28Granite 28Lantau Granite 29Chek Lap Kok Granite 29Quartz Monzonite 30Tong Fuk Quartz Monzonite 30Minor Intrusions 31Feldsparphyric Rhyolite • 31Porphyritic Microgranite 32Quartzphyric Rhyolite 32Aplite 32Quartz Veins 33Mafic Dykes 33


References 59Index62AppendixList of TablesLexicon of Terms Used for Drillcore Logging and Surface 64Mapping at Tung Chung, Lantau Island, Hong Kong by M.R.Gillespie, A.J. Humpage, and R.A. EllisonTable1 Grain-size Description and Classification of Rocks and Superficial 10Deposits in Hong Kong2 Summary of the Onshore and Offshore Stratigraphy of the District 183 Whole-rock Major- and Trace-element Geochemistry for Representative 20Rock Types in the Tung Chung District. Major Oxides in wt%, TraceElements in ppmList of FiguresFigure1 Location Map of Major Infrastructure Developments Related to Tung Chung 11New Town and Northshore Lantau Island2 Traverses Undertaken During 1968, 1988-89 and 1991-92 Field Surveys 133 Locations of Boreholes Drilled for Tung Chung New Town and Northshore 15Lantau Island Infrastructure Developments4 Locations of Seismic Lines around Northshore Lantau Island 165 Simplified Onshore Geology of the District 196 Simplified Seabed Sediment Map of the District 227 Generalized Classification and Nomenclature of Selected Major and Minor 28Intrusive Rocks (after Le Maitre, 1989)8 Principal Structural Features of Tung Chung and Northshore Lantau Island 359 Contoured Pole Plots of Joints Measured for All Rock Types on Tung Chung 37and Northshore Lantau Island (after Woods, 1993). a) Sha Lo Wan to HauHok Wan, b) Hau Hok Wan to Tin Sam, c) Tung Chung to Lau Fau Sha,d) Lau Fau Sha to Kei Tau Kok, and e) Kei Tau Kok to Pak Mong10 Schematic Section Showing the Relationship between Superficial Deposits 40and the Different Environments of Deposition


11 Location Map Showing the Distribution of the Tung Chung FormationDetermined from Interpretation of Gravity and Borehole Data (Modified afterKkk etal., 2000)4412 Distribution and Isopachs (in Metres) of the Hang Hau Formation 4613 Location Map of Part of the Tung Chung East Reclamation Potentially 49Underlain by Complex Geological Conditions14 Examples of Different Interpretations of Problematic Materials Encountered 51in Boreholes Drilled at various locations at Site 3 of the Tung Chung EastReclamation15 Schematic Representation of the Development of Karst Deposits Beneath 53Tower 5, Site 3, Tung Chung East Reclamation (after Figure 6 in Fletcher etah, 2000)16 Simplified Averaged Rockhead Models (Top of Grade III or Lower) for 54Offshore Parts of the District Based on 3-D Modelling of the TerrainCorrected Bouguer Anomaly (after EGS 1999a, 1999b)17 Volcanotectonic Map of Hong Kong (c. 146 Ma) Showing the Distribution of 56the Tung Chung and Ma On Shan Marble Subcrops and Major Faults thatWere Active During Emplacement of Rocks Belonging to the Lantau VolcanicGroup and Kwai Chung Suite (Modified after Kirk, 2000).List of PlatesPlate1 Steeply-dipping and Deformed, Fine-grained Metasandstone and 24Carbonaceous Metasiltstone of the Lok Ma Chau Formation Exposed on TszKan Chau (1588 2085)2 Altered, Silicified, Banded Coarse Ash Crystal Tuff Exposed on Pok To Yan 26(1350 1587)3 Fine- to Medium-grained Lantau Granite from a Borehole South of Ha Law 29Wan (1131 1716)4 Thin Section of Fine- to Medium-grained Lantau Granite from Pak Sha Tsui, 30Tung Chung (HK 9236, 1131 1698); XPL5 Fine-grained Chek Lap Kok Granite from Tin Sam (HK 9252, 0945 1709) 306 Fine-grained, Porphyritic Tong Fuk Quartz Monzonite from Sha Lo Wan (HK 3110067,0812 1633)7 Thin Section of a Mafic Dyke from Hau Hok Wan (HK 9237, 11561727) 338 Tung Chung Wan from the Northwest Showing Quaternary Superficial Deposits 43Composed of Intertidal Deposits in the Foreground, and Beach Deposits andAlluvial Deposits in the Background. (Photo taken on 5.12.1989)


to be associated with areas of new reclamation. In the past, some of the problems encountered in such majorinfrastructure developments in Hong Kong have been attributed to insufficient geotechnical and geologicalknowledge. The NLDFS included an initiative to examine all available site investigation data to confirmoffshore ground conditions. Five additional ground investigations, including boreholes and cone penetrationtests (CPTs) and two geophysical surveys, were earned out to study the geological conditions and sedimentquality in the area of proposed reclamation(s). An overview of the results of these investigations are reportedhere. They confirm the complexity of the geology in offshore areas along the North Lantau coast.Offshore Geophysical and Marine Ground InvestigationsAdditional offshore geophysical and marine ground investigations, undertaken for the NLDFS and completedin 1999, have proved depressions filled with siltstone or fine alluvial sediment to a depth of -162 mPDwithin 500 m of the North Lantau shoreline (Scott Wilson (HK) Ltd., 2001; Kirk et al., 2000) These dataindicate the presence of deep zones of weathering and marble with cavities beneath areas of proposedreclamation. The geological information to hand suggests that local areas of complex ground may beencountered beneath areas of proposed and planned infrastructure development along the North Lantaucoast.Purpose and Scope of StudyThe main purpose of the 1:5 000-scale mapping programme was to provide information on rock structure,texture and composition for use by engineers involved with infrastructure development. An additional aimwas to supplement existing published geological data (Langford et al, 1995; GEO, 1994, (Sheet 9)) withmore detailed information to help interpretation of broader aspects of the geology of Hong Kong.In addition to a description of the geology covered by 1:5 000-scale geological map sheets 9-SE-A and 9-SE-B, this report gives a detailed account of the complex ground conditions encountered at Tung ChungNew Town. It also covers the solid and superficial geology of the proposed infrastructure development areaat Siu Ho Wan and the offshore area immediately east of Chek Lap Kok.Data SourcesA large volume of borehole data for North Lantau and Tung Chung exists from previous site investigations,including those for the airport project (Figure 3). This large data set was compiled as part of a computeriseddatabase.Aerial photographs, particularly those taken in 1963, were invaluable in mapping of onshore superficialdeposits. A Landsat image of the Pearl River Estuary, although at a small scale, gave an excellent overviewof the district and supplied further evidence of structural trends determined from field mapping.From a total of 216 rock samples collected, some 171 specimens were thin sectioned and 15 samples weresent to the University of Nottingham for whole rock major and trace element geochemical analysis. Twosamples were collected from the district for Rb-Sr age determinations at the NERC Isotope GeosciencesLaboratory, Keyworth.The nature and distribution of offshore superficial sediments have been mapped using shallow seismicprofiles, borehole logs, and CPT traces from several surveys (Figure 4). These surveys were originallycarried out in preparation for the Port and Airport Development Strategy (PADS), offshore sand resourceexploration and site investigations for reclamations. Copies of seismic profiles used in this project are heldby the Hong Kong Geological Survey.The 1:5 000-scale geological maps which accompany this report are based on the Hong Kong GeologicalSurvey field surveys, now published as a map at a scale of 1:20 000. The present report has extended thiswork through a detailed interpretation of the sub-surface geology based mostly on recent borehole data andoffshore seismic traverses, obtained since the earlier survey. Each 1:5 000-scale sheet area is represented byone geological map showing both the solid and superficial geology.All the records from this project, including rock samples, thin sections, manuscript maps and analyticaldata, are held in the archives of the Hong Kong Geological Survey, Geotechnical Engineering Office, CivilEngineering Department. The powders used in geochemical analysis are also retained in the Hong KongGeological Survey archives, and a split is kept at the British Geological Survey, Keyworth.14


Seismic reflection line8Onshore areaaS3x m LULANTAU ISLAND


Chapter 2Outline of GeologyThe main part of the district lies across the northern faulted boundary of the Lantau Caldera (Langford etal,1995) which separates Mesozoic volcanic rocks in the south from Mesozoic sedimentary and intrusiverocks in the north. The volcanic rocks are composed mostly of rhyolite lava and tuff with minor intercalatedvolcaniclastic sandstone and siltstone, tuffite and tuff breccia. The intrusive rocks consist mostly of graniteand quartz monzonite plutons in the west and feldsparphyric and quartzphyric rhyolite dykes in the east(Table 2, Figure 5). Representative whole-rock geochemical analyses of the rock types are given in Table 3.Palaeozoic sedimentary rocks of the San Tin Group are the oldest rocks in the district. They are exposedonly on Tze Kan Chau (1585 2090) in the northeast, but are known to underlie parts of the offshore area tothe south and east of The Brothers islands. On Tze Kan Chau, the rocks comprise mainly quartz metasandstoneand graphitic metasiltstone of the Carboniferous Lok Ma Chau Formation, whereas offshore they consist ofmarble belonging to the Yuen Long Formation. Limestone of Permian age, together with sandstone andsiltstone, has been identified in boreholes at Tung Chung. These rocks have also been encountered in boreholesfarther north offshore and have been tentatively assigned to the Tolo Harbour Formation (Sewell et al.,2000).Metasiltstone and sandstone are exposed in the far west of the district against the fault marking the boundaryof the Lantau Caldera. Previously, these rocks were considered to be of Carboniferous age and assigned tothe Lok Ma Chau Formation (GEO, 1994; Langford et al, 1995). However, following the discovery ofJurassic fossils in these rocks near Tai O (Ng et al, 1997), they have been reassigned to the Lower JurassicTai O Formation (Sewell et al, 2000). The Tai O Formation forms a northeast-trending outcrop along thecoast of Lantau Island from Tai O to San Shek Wan.Lapilli-bearing coarse ash crystal tuff of the Yim Tin Tsai Formation (Tsuen Wan Volcanic Group) isstratigraphically the oldest Mesozoic volcanic rock in the district and is confined to offshore boreholesalong the North Lantau coast from Sham Shui Kok to Kwai Shek. These rocks have been dated elsewhere(Sewell et al, 2000) at 164.5 Ma and they conformably overlie lapilli-bearing coarse to fine ash crystal tuffof the Shing Mun Formation. They are intruded by porphyritic fine- to medium- and medium-grainedgranite belonging to the Tai Lam Granite and Lantau Granite plutons of the Lamma Suite (Sewell et al, op.cit). In the western part of the district, the fine-grained Chek Lap Kok Granite (Lamma Suite) intrudesporphyritic fine- to medium-grained Lantau Granite.Eastnortheast-trending feldsparphyric and quartzphyric rhyolite dykes of the Lantau Dyke Swarm intrudegranites of the Lamma Suite in the northeastern and northwestern parts of the district. The dykes have beendated at approximately 146 Ma and are truncated by the fault marking the boundary of the Lantau Caldera.The dykes were emplaced in at least two phases (Li et al, 2000) and may vary internally from feldsparphyricrhyolite to porphyritic microgranite. Quartzphyric rhyolite dykes are considered to have been emplacedmostly during the younger phase.Welded lapilli- to block-bearing crystal tuff, tuff breccia, tuffite and flow-banded rhyolite lava, withintercalated volcaniclastic sedimentary rocks infill the Lantau Caldera. Previously, these rocks were namedas the Lantau Formation (Langford et al, 1995), but they have now been reassigned to the Lantau VolcanicGroup (undifferentiated) (Sewell et al, 2000). These rocks have been dated at approximately 146 Ma andrepresent the products of a number of discrete eruptions.The volcanic and granitic rocks of the district are intruded along the caldera-bounding fault by quartzmonzonite belonging to the Lion Rock Suite. Minor basalt, microdiorite and aplite dykes occur sporadicallyin the northern part of the district intruding the granitic rocks and rhyolite dykes. These mafic and felsicdykes are considered to Cretaceous in age.The geological structure of the district is complex, owing to the close proximity of intersecting regionalfault trends and faults related to the caldera boundary. The dominant regional fault trend is to the northeast.17


Table 2 - Summary of the Onshore and Offshore Stratigraphy of the DistrictStratigraphic DivisionsLithostratigraphy andGenetic ClassificationPrincipalMaterialsMapSymSuperficial DepositsHoloceneFillAlluviumIntertidal depositsEstuarine depositsBeach depositsNatural earth and wasteSilt, sand and gravel with bouldersMud and sandMud and sand with peatSandQh F !Q a !Q 1 1Qh ie !Qh bS |QUATERNARYHoloceneandPleistocenePleistoceneHang HauFormationMarine mudMarine sandSlope debris andDebris flow deposits(Colluvium)Sham WatFormationChek LapKokFormationTung ChungFormationEstuarine tomarine depositsAlluvium,terraced alluvium,slope depositsKarst-relateddepositsVolcanic and Sedimentary RocksSoft to very soft mud; some sandSand, some gravel and mudSand, gravel, cobblesCobbles in fine matrixClay, silt and some sandClay, silt, sand, and gravelSand, gravel, cobblesSilt, sand and gravel with bouldersQhmMQh mSjjQ sD !QDCP j0p a !Qp alQP SDiiQp kD ;MESOZOICJurassicLantauVolcanicGroupTsuen WanVolcanicGroupUndifferentiatedYim Tin Tsai Fm.Tai 0 FormationTuff and lava, siltstone, sandstone,tuff and tufflte, crystal tuff, mudstoneCoarse ash crystal tuffMetasiltstone with metasandstoneJlu, si s, itt, t, mJty !Jo |PermianTolo Harbour Fm.MarblePt |CarboniferousSan TinGroupLok Ma Chau Fm.Yuen Long Fm.Metasiltstone with metasandstoneMarbleCsl !CsyMajor Intrusive RocksjMESOZOICJurassic-CretaceousFine-grained quartz monzonite,Fine-grained granite and fine- to medium-grained graniteMedium-grained granitemqfgf, gfmgm|Minor Intrusive RocksCRETACEOUSBasalt, lamprophyre, microdiorite and andesiteb, 1, JID, aMESOZOICJurassic-CretaceousFeldsparphyric and quartzphyric rhyolite,aplite, porphyritic micrograniterf, rq,ap,fig18


Mount Davis FormationLantau Volcanic GroupYim Tin Tsai FormationTai O FormationLok Ma Chau FormationFine-, Fine- to medium-, andMedium-grained GraniteFine-grained Quartz MonzoniteFeldsparphyric RhyolitePorphyritic MicrograniteQuartzphyric RhyoliteGeological BoundaryFaultMinor intrusiverocks (dykes)THEm 73BR0THERSmmmmmmHong KongInternational Airport


These faults offset the caldera boundary faults and an earlier set of north- to northnorthwest-trending faults.However, the northeast-trending faults have themselves been offset by reactivation of northnorthwest-trendingfaults. Northeast- to northwest-trending quartz veins are present in the northwestern part of the district,mainly close to granite intrusions. However, they are also found in close proximity to the caldera marginfault, intruding volcanic rocks, and commonly contain traces of mineralization including arsenopyrite,chalcopyrite, galena, magnetite, molybdenite, pyrite and wolframite.Superficial deposits within the district cover most of the low-lying ground and large tracts offshore (Figure6). The onshore deposits comprise dominantly debris flow deposits on the higher ground and alluvialsediments on the valley floors. In the larger valleys, these have been disturbed by cultivation. Close to thecoast, the superficial deposits comprise beach deposits of sand. Areas of deep weathering have been detectedin boreholes beneath the Tung Chung reclamation and in a northeast-trending zone subparallel to the NorthLantau coast extending from Tai Ho Wan to Ha Kok Tsui. These areas are associated with unusually thicksuperficial deposits (Tung Chung Formation) comprising an assortment of debris flow deposits, laminatedsediments, and block breccias. These deposits are thought to be associated with collapse of karst-relatedcavities (Kirk, 2000; Fletcher et al, 2000) formed from blocks of marble encased within granite and dykeintrusions.The offshore superficial deposits on the seabed in the district consist mostly of intertidal and estuarinedeposits of sand and mud, and marine mud and sand of the Hang Hau Formation. A small area of marinesand belonging to the Chek Lap Kok Formation has been identified in the extreme northeastern part of thedistrict north of Yam O Wan. Extensive subcrop of the Chek Lap Kok Fomiation, and of estuarine to marinedeposits of the Sham Wat Fomiation, have also been identified.21


Chapter 3Palaeozoic Sedimemtary RocksSan Tin Group (Carboniferous)Classification and DistributionMetasedimentary rocks are exposed in one small area of the district (Sheet 10-NW-C); Tsz Kan Chau (ReefIsland) to the north of Lantau Island. They form part of a steeply northeast-plunging syncline with westernand eastern limbs defined by The Brothers islands (Tai Mo To and Siu Mo To respectively) to the north ofthe Tsz Kan Chau.The metasedimentary rocks have lithological and palaeontological similarities with the main outcrop ofCarboniferous rocks in the northwest New Territories (Langford et al, 1989) and have been assigned to theSan Tin Group (Langford et al, 1995). The San Tin Group comprises two formations; the older Yuen LongFormation (Lee, 1985) and younger Lok Ma Chau Formation (Williams, 1943). Only the Lok Ma ChauFormation is exposed in the district although an extensive subcrop of marble, correlated with the Yuen LongFormation, has been proved by boreholes to the south and east of this island (NS1/14460, NS3/14460,T8/3/13951, M7/29998, Mll/29998, DH4/30023, DH6/30023).Lok Ma Chau FormationMai Po MemberThe Lok Ma Chau Formation comprises two members; the older Mai Po Member consisting of metamorphosedsiltstone, fine-grained sandstone and carbonaceous siltstone, and the younger Tai Shek Mo Member consistingof metamorphosed sandstone and conglomerate. The rocks exposed at Tsz Kan Chau have been assigned tothe Mai Po Member. Langford et al (1995) previously assigned metasedimentary rocks outcropping on thecoast between Tai 0 and San Shek Wan to the Mai Po Member of the Lok Ma Chau Formation. However,these rocks have since been reclassified as the Middle Jurassic Tai 0 Formation (Sewell et al, 2000) on thebasis of new fossil discoveries (Ng et al., 1997).At the southeastern end of Tze Kan Chau (1588, 2085), the sediments are composed of buff to brown, crossbedded(units 10s of mm), pebbly coarse sandstone and laminated fine quartz sandstone with sparseintercalated beds (0.5 - 1 m) of dark grey to black, massive to poorly laminated graphitic siltstone (Plate 1).Andalusite crystals are well-formed in the sandstone beds indicating that the succession has been contactmetamorphosed. To the northwest, the sediments grade upward into buff to reddish brown, massive finegrainedquartz sandstone which dips steeply (65 - 70°) to the northwest. The succession is considered tooverlie stratigraphically marble of the Yuen Long Formation found in offshore boreholes to the east, and tohave a minimum thickness of 300 m (Langford et al, 1995).Yuen Long FormationMa Tin MemberThe Yuen Long Formation consists of dark grey and white marble, and has been described principally fromthe Yuen Long area in the northwest New Territories. Frost (1992) divided the formation into two members:the lower Long Ping Member is a dark grey marble with complex internal structure, whereas the Ma TinMember is a massively-bedded white marble. The marble encountered in offshore boreholes is predominantlywhite and is assigned to the Ma Tin Member. Solution features have been encountered in three of theboreholes: NS1/14460 at -54mPD; DH4/30023 at -59mPD and -62mPD; and DH6/30023 at -65 mPD. Thesefeatures are karst-related cavities, up to 6 m across, and are commonly infilled with brown sandy silt.Sediments of the Lok Ma Chau Formation are considered (Lai et al, 1996) to have been deposited in a tidalswamp or fluvial-deltaic environment. The presence of small-scale cross-bedding in the fine-grainedsandstones suggests shallow water conditions whereas intercalated carbonaceous material may indicateneritic conditions. Marble of the Yuen Long Formation is considered (Frost, 1992; Sewell et al, 2000) tohave been originally a pure to slightly impure limestone.23


Chapter 4Mesozoic Sedimentary and Volcanic RocksTai O FormationMesozoic sedimentary rocks are only exposed in the extreme western corner of the district. They form theeastern limit of an outcrop of Mesozoic sedimentary rocks along the North Lantau coast extending from TaiO in the southwest to San Shck Wan in the northeast. These rocks were previously assigned to theCarboniferous on the basis of lithological similarity (Langford et ai, 1995), although the presence of arhyolitc lava flow intercalated with the sedimentary rocks at San Chau raised the possibility that the upperpart of the succession may be of Jurassic age. The recent discovery of macrofossils in the rocks near Tai 0have now confirmed a Middle Jurassic age.The Tai O Formation mostly comprises an interbedded succession of grey, fine-grained sandstone alternatingwith siltstone and sandy siltstone. Occasional beds of dark grey graphitic siltstone are also present. The TaiO Formation is in fault contact with volcanic rocks belonging to the Lantau Volcanic Group, and thesedimentary rocks are thermally metamorphosed within a 100 m zone near the contact. Little structural datais available from outcrops of Tai O Formation within the district. However, to the west at San Shek Wan, theformation is considered to dip moderately to the south. Farther west at Sham Wat Wan, the formation istightly folded into a series of anticlines and synclines. The Tai O Formation has an estimated thickness of c.400 m (Langford (Vc//., 1995).Jones (1996) considers that the Tai 0 Formation was deposited on an alluvial plain, crossed by small- tomoderately-sized rivers. Fossils recovered from the Tai 0 Formation at Tai 0 include several late Early toearly Middle Jurassic plants including Ptilophyllum contigitum, Otozamites hsiangchiensis, Tyrmia cf.nut harts L and Eretmophyllwn? sp.Tsuen Wan Volcanic GroupVolcanic rocks of the Tsuen Wan Volcanic Group are not exposed in the district but have been encounteredin marine boreholes along the North Lantau Coast from Sham Shui Kok to Kwai Shek. Although two volcanicformations were previously identified in onshore areas of North Lantau Island (Langford et al, 1995), thevolcanic rocks are now all considered to belong to one formation: the Yirn Tin Tsai Formation, comprisinglapilli-bearing coarse ash crystal tuff with minor intercalated fine ash crystal tuff. A sample of the fine ashcrystal tuff from Yam O Wan (HK11821) has yielded a U-Pb age of 164.5 ± 0.2 Ma {Davis et aU 1997).Yim Tin Tsai FormationThe dominant lithology is lapilli-bearing coarse ash crystal tuff. The tuff is generally rhyodacitic in compositionand is typically massive, although it may occasionally exhibit foliation. Minor intercalated fine ash tuffhorizons are present in isolated boreholes (DH10/30023, DH11/30023). The lapilli lithic volcaniclasts consistof dark aphanitic and strongly porphyritic lava. The clasts vary in size from less than 10 mm to 150 mm. Theaphanitic volcaniclasts commonly have sharp edges suggesting that they were fragmented during chilling,whereas the porphyritic volcaniclasts have diffuse edges suggesting that lava fragments' were still partlymolten at the time of deposition.Petrographically, the Yim Tin Tsai tuff has relatively equal proportions of quartz and alkali feldspar butplagioclase content may vary. The chief mafic minerals are biotite and amphibole, and accessory mineralscomprise zircon, apatite, monazite, and magnetite. In a typical thin section from north Lantau, the proportionsof these minerals are quartz 35%, alkali feldspar 25%, plagioclase 30%, biotite 7%, amphibole 3%, andtrace accessory minerals. Total crystal content (>0.06 mm) varies from 40 to 60%. The overall thickness ofthe Yim Tin Tsai Formation in the district is estimated to be 500m.The internally massive and structureless character of the Yim Tin Tsai Formation suggests that it representsthe product of large, relatively homogenous, ash flow eruption. Welded fabrics have not been observed,suggesting that the ash had cooled considerably by the. time the flow came to rest. The crystal-rich nature of25


the deposit indicates that cooling and crystallisation of the magma started well before eruption and this islikely to have been from a relatively shallow level magma chamber (e.g. Clemens & Wall, 1984).Lantau Volcanic GroupThe Lantau Volcanic Group largely occupies the Lantau Caldera (Langford et al, 1995) in central LantauIsland, although correlatives of the group (Lai Chi Chong Formation) are known from the eastern NewTerritories (Sewell et al, 2000). Formations have been not yet been formally identified on Lantau althoughone member, the Pak Kok Member, has been mapped (Langford et al, 1995). Within the district, the volcanicrocks remain largely undifferentiated except for locally prominent siltstone, sandstone, tuffite, crystal tuffand mudstone beds. Two U-Pb ages have been obtained from volcanic rocks on Lantau Island, but outsidethe district these have returned ages of 147.5 ± 0.2 Ma to 146.7 ± 0.2 Ma respectively (Davis et al, 1997;Sewell et al, 2000).Main LithologiesUndifferentiated. Most of the volcanic rocks mapped within the district remain undifferentiated. Includedwithin this grouping are large areas dominated by vitric tuff and porphyritic rhyolite lava. The porphyriticrhyolite lavas generally contain euhedral phenocrysts of quartz and feldspar, but some of these crystals havebeen mechanically broken as a result of fragmentation during eruption. Banded porphyritic rhyolite lava isexposed on the summit of Por Kai Shan (1398 1633) and on the south side of the hill (1394 1626), the lavagrades into rhyolite breccia. The volcanic lithologies in this part of the district, and to the northwest, areweakly metamorphosed. Metamorphosed tuff is commonly pale red to reddish white but may also vary tobrownish white. Metamorphism is considered to be dominated by hydrothermal alteration associated withintrusion of silicic magmas and channelling of silica-rich fluids along the northern boundary fault system ofthe Lantau Caldera (Langford et al, 1995). Minor fine ash crystal rhyolitic tuff and tuffite are intercalatedsporadically within the main succession, generally separating thick successions of rhyolite lava and bandedrhyolite lava.Crystal Tuff A feature-forming outcrop of crystal tuff has been mapped to the northwest of Por Kai Shan(1375 1650), where it conformably overlies a thick band of tuffite. The crystal tuff varies locally to crystalbearingvitric tuff and is weakly bedded. In places, a weak foliation has developed and the tuff also showsevidence of thermal metamorphism (Plate 2). The crystal tuff is estimated to be approximately 110 m thick.Plate 2 - Altered, Silicified, Banded Coarse Ash Crystal Tuff Exposed on Pok To Yan (1350 1587)26


Tuffite. A prominent outcrop of banded tuffite, comprising pale grey tuffaceous sandstone, dark greytuffaceous mudstone, and grey crystal-bearing vitric tuff has been mapped on the northwestern flanks of PorKai Shan (1360 1650). The tuffite dips at 35° to the east and is estimated to be approximately 140 m thick.Large corestones of vitric tuff occur within the tuffite, which has been weakly thermally metamorphosed. Asmall (0.002 knr) outcrop of tuffite has been mapped within the main Lantau volcanic succession in the farsoutheastern corner of the district. The tuffite consists mostly of cherty tuffaceous siltstone and sandstone.Siltstone. Grey, finely laminated tuffaceous siltstone is exposed to the east of Pok To Yan (1385 1570) inthe southeastern part of the district. The siltstone is weakly thermally metamorphosed and forms a prominentfeature-forming bed up to 60 m thick, dipping to the east at up to 36°. The siltstone bed is exposed forapproximately 750 m in a north-south orientation, and thins out rapidly to both north and south.Mudstone. Grey to light or reddish-grey, laminated mudstone with minor intercalated tuffaceous sandstoneis exposed as a feature-form ing unit 300 m to the southeast of Por Kai Shan (1420 1608) in the southeasternpart of the district. The mudstone unit can be traced southeastward for approximately 500 m and is possiblyup to 80 m thick. Load structures displayed by the mudstone unit indicate that it is the right way up and dipsapproximately 17° to the southeast. A second, less prominent, feature-forming mudstone unit is exposedapproximately 300 in upslope of the main mudstone outcrop. This upper mudstone unit is similar iithologicallyto the lower mudstone unit and displays a similar bedding attitude. The upper mudstone unit is exposed in aneast-west orientation and can be traced laterally for approximately 200 m. The unit is up to 50 m thick.Sandstone. A very minor outcrop of tuffaceous sandstone has been mapped between the upper and lowermudstone units southeast of Por Kai Shan (1455 1573). The unit is approximately 6 m thick, and can only betraced for approximately 25 m in an east west orientation. The tuffaceous sandstone is typically light-grey,and either massive or poorly bedded.27


Chapter 5Intrusive Igneous RocksClassification and DistributionThe intrusive igneous rocks of the district comprise major intrusions of granite and quartz monzonite, andminor intrusions of porphyritic microgranite, feldsparphyric rhyolite, quartzphyric rhyolite, aplite, quartzveins and mafic dykes. Compositional classification is based on Le Maitre (1989) (Figure 7). The majorintrusions are divided into medium-grained, fine- to medium-grained, and fine-grained lithologies. On thebasis of geochemical and mineralogical studies, the granites may be further divided into separate plutons orintrusive units. These plutons form the host rock to a swarm of rhyolite dykes (Lantau Dyke Swarm) thatdominates the geology in northeastern Lantau Island. Abundant textural variation is present within bothmajor and minor intrusions.Major IntrusionsGraniteTwo main varieties of granite are present in the district; megacrystic fine- to medium-grained and mediumgrainedgranite belonging to the Lantau Granite, and fine-grained granite belonging to the Chek Lap KokGranite (Sewell et al., 2000). The Lantau Granite crops out intermittently across the northern part of thedistrict between Pak Mong and Sha Lo Wan, whereas the Chek Lap Kok Granite crops out principally alongthe coast west of Tin Sam. U-Pb dating of zircon crystals has returned ages of 161.5 ± 0.2 Ma for the LantauGranite, and 160.4 ± 0.3 Ma for the Chek Lap Kok Granite (Davis et al, 1997). Both granites predateemplacement of the Lantau Dyke Swarm.Q = quartzA = alkali feldspar,including albite(AnO - An5)P = plagioclase feldspar,(An5-An100)QALKALI FELDSPARGRANITEalkali feldsparrhyolite(SYENO-GRANITE)QUARTZSYENITEquartz trachyte(MONZO-GRANITE)QUARTZMONZONITEquartz iatite2010/SYENITE trachyte jMQNZONITE Iatite35 65 90Figure 7 ~ Generalized Classification and Nomenclature of Selected Major and Minor Intrusive Rocks (afte{ JLe Maitre, 1989)28


Lantau GraniteThe Lantau Granite is typically megacrystic medium-grained with large megacrysts of pink alkali feldsparset in a granular matrix of quartz, plagioclase, alkali feldspar, amphibole and aggregates of biotite. Nonporphyriticfine- to medium-grained lithologies are also present (Plates 3 and 4). Accessory minerals includeconcentrically zoned allanite, apatite, titanite, zircon, rare fluorite, and Fe-Ti oxide.Euhedral to subhedral K-feldspar megacrysts (5-15 mm) are dominantly composed of perthitic orthoclasewith subordinate microcline. Two generations of plagioclase crystals (25 - 30%) are commonly present:strongly concentrically zoned species and relatively unzoned species. The strongly zoned plagioclase grainsare euhedral (2-5 mm) to subhedral with andesine-rich cores and oligoclase- to albite-rich rims. The coresof these crystals are commonly altered to sericite and sharp boundaries often exist between successivegrowth phases. In some rocks, residual cores of extensively altered anhedral alkali feldspar are mantled byconcentrically-zoned plagioclase. Unzoned to weakly zoned plagioclase is generally euhedral and composedmostly of oligoclase and albite. The cores are often weakly sericitized. Anhedral quartz (30 - 35%) showsundulose extinction showing that it is typically strained. Yellowish brown to green biotite usually comprisesbetween 2 - 10% of the mode and is sometimes accompanied by subordinate greenish brown amphibole(3-5%).Plate 3 - Fine- to Medium-grained Lantau Granite from a Borehole South of Ha Law Wan (1131 1716)V7Chek Lap Kok GraniteThe Chek Lap Kok Granite (Sewell et al, 2000) forms a subcircular pluton centred on the airport at ChekLap Kok. Prior to construction of the airport, the unit was exposed on the western half of the island Chek LapKok and on Lam Chau (Langford et al, 1995). Exposures are now confined to the area between Tin Sam andHau Hok Wan on the northern coast of Lantau Island. The type locality is designated as Hau Hok Wan.On the former island of Chek Lap Kok, the Chek Lap Kok Granite was seen (Langford et al, 1995) tointrude porphyritic fine- to medium-grained Lantau Granite. This relationship is still exposed on the northerncoast of Lantau Island at Hau Hok Wan indicating that the Chek Lap Kok Granite post-dates emplacementof the Lantau Granite. The U-Pb age-dating results are consistent with the observed field relationships.The Chek Lap Kok Granite consists of an equigranular, fine-grained, leucocratic monzogranite (Plate 5).Euhedral to anhedral alkali feldspar is composed principally of mesoperthite. Subordinate euhedral to subhedralalbite is unzoned and anhedral quartz is weakly strained. Interstitial late stage muscovite is commonly presentalong with trace amounts of euhedral fluorite. Biotite, zircon, and Fe-Ti oxide may be present in trace amounts.29


Plate 4 - Thin Section of Fine- to Medium-grained Lantau Granite from Pak Sha Tsui, Tung Chung (HK9236,1131 1698); XPLQuartz MonzoniteSmall intrusions of quartz monzonite crop out at the faulted contact between the Lantau Volcanic Group andother rocks south of Sha Lo Wan and Hau Hok Wan.Tong Fuk Quartz MonzoniteThe Tong Fuk Quartz Monzonite crops out in a series of discontinuous stocks along the faulted contactbetween the Lantau Volcanic Group and other rocks. Major outcrops occur at Sha Lo Wan on the westernside of Lantau Island and at Tong Fuk on the eastern side of Lantau Island (Plate 6). Smaller outcrops arefound at Fan Lau and Pui O.2i, i . t . i j l . i 1..A..1. i . j i3 l L 1,,,) .1. i .1 i44 l i ii . » i b lPlate 5 - Fine-grained Chek Lap Kok Granite from Tin Sam (HK9252, 0945 1709)30


" .-.": •' - • • *"» JI i I • i , I , I ,3| , I , I , J ,4| , I , I . ,.J._i?L6 - Fine-grained, Porphyritic Tong Fuk Quartz Monzonite from Sha Lo Wan (HK10067, 0812 1633)The Tong Fuk Quartz Monzonite is typically fine-grained and porphyritic, with roughly equal abundancesof alkali feldspar and plagioclase phenocrysts, although alkali feldspar usually predominates. Roundedmafic enclaves composed mostly of quartz microdiorite, are common. In the Sha Lo Wan area, the quartzmonzonite locally may vary to porphyritic fine-grained granite, but it is distinguished by the presence ofscattered alkali feldspar phenocrysts with prominent twinning. In places, the quartz monzonite has beenhighly weathered to a reddish brown soil.In thin section, euhedral to subhedral alkali feldspar phenocrysts (1-7 mm) display well-developed Carlsbadtwins and are weakly zoned. Plagioclase phenocrysts are strongly zoned with sericitized andesine cores andalbite rims. The matrix is generally granular and fine- to very fine-grained and comprises quartz and alkalifeldspar, with subordinate plagioclase. The mafic minerals consist of greenish brown biotite and rare brownamphibole. Biotite is commonly altered to chlorite. Accessory minerals include zircon, fluorite, titanite,apatite, allanite, epidote, and Fe-Ti oxide.A sample of quartz monzonite from Tong Fuk has yielded a U-Pb single zircon age of 140.4 ± 0.3 Ma.Minor IntrusionsWith the exception of mafic dykes, the minor intrusions of the district are divided on the basis of texture intofeldsparphyric rhyolite, porphyritic microgranite, and quartzphyric rhyolite. The felsic dykes are present asa dense swarm of strongly eastnortheast-westsouthwest-oriented multiple intrusions cutting granitic andvolcanic rocks. The dykes were intruded in at least three pulses and show evidence for compositional zoning(Li et ah, 2000). Composite intrusions, featuring a felsic phase injected into a more mafic one, are sometimesobserved. None of the dykes in the district have been dated. However, rhyolite dykes to the northeast of thedistrict have returned ages of 146 Ma which suggests that the age of emplacement is synchronous with thatof the Lantau Volcanic Group.Feldsparphyric RhyoliteFeldsparphyric rhyolite is the general term given to dykes ranging in composition from rhyodacite to highsilicarhyolite which contain abundant feldspar (±quartz) phenocrysts (


are usually composed of aphyric basaltic andesite to andesite. These compositions grade rapidly throughfeldsparphyric rhyolite to aphyric rhyolite in the cores of the dykes. Sometimes there is evidence for abruptchanges in composition which reflect separate pulses of magma intrusion within a single dyke (Li et al, op. cit).Fresh, feldsparphyricrhyolite has a distinctive dark grey appearance with white and grey phenocrysts. Thephenocrysts are made up of slightly pinkish grey euhedral alkali feldspar, with subordinate euhedral tosubhedral white plagioclase, dark grey bipyramidal quartz, aggregates of biotite, and rare crystals ofhornblende. With a slight degree of weathering, the groundmass turns to greyish green, or greyish purple,and ultimately to orangish brown in intensely weathered rocks.In thin section, the groundmass is dark grey, very fine-grained to cryptocrystalline and aphanitic, withoccasional granophyric intergrowths of quartz and feldspar, and biotite. Accessory minerals include zonedallanite, zircon and Fe-Ti oxide.Along with the granite, these blocks have been intruded by composite feldsparphyric rhyolite to porphyriticmicrogranite dykes of the Lantau Dyke Swarm.Porphyritic MicrogranitePorphyritic microgranite is the term given to a textural variant of feldsparphyric rhyolite in which individualcrystals in the groundmass are less than 2 mm diameter on average, but are still visible to the unaided eye.The rock is compositionally similar to feldsparphyric rhyolite and contains phenocrysts of feldspar (±quartz)up to about 25 mm in size. Porphyritic microgranite dykes are characterised by the presence of granophyrictexture in the groundmass. The cores of these dykes may be either finely feldsparphyric or aphyric andindistinguishable from normal fine-grained granite.Porphyritic microgranite dykes trend eastnortheast and have mainly been mapped to the east of Tung ChungWan between Tin Sam and Tung Hing. They vary from 2 to 30 m wide and intrude fine-grained granite(Chek Lap Kok Granite). The porphyritic microgranite dykes were probably intruded broadly at the sametime as the feldsparphyric rhyolite (c. 146 Ma) although slight differences in the age of emplacement aredisplayed by signs of chilling along some dyke to dyke contacts.Quartzphyric RhyoliteQuartzphyric rhyolite is the name given to rhyolite dykes in which large (up to 3 mm) bipyramidal quartzphenocrysts are more abundant than feldspar phenocrysts. Crystals in the groundmass are too fine-grainedto be distinguished by the unaided eye, and flow banding may be discernible. The cores of quartzphyricrhyolite dykes may be almost aphyric. Very fresh examples of quartzphyric rhyolite may be dark grey toblack, whereas with slight alteration, the colour is grey, purple or pink.Quartzphyric rhyolite dykes within the district trend northeast and eastnortheast and are confinedalmost entirely to the former island of Chek Lap Kok where they intrude granite. A few quartzphyricrhyolite dykes have been mapped along the coast northeast of Tung Chung. The dykes are typically 3to 5 m wide, but can be up to 30 m wide. The margins of the quartzphyric rhyolite dykes are commonlyflow banded.In thin section, the quartz phenocrysts are euhedral to subhedral with well-developed bipyramidal shapes,although occasionally they may be slightly embayed. Alkali feldspar is commonly microperthitic andplagioclase feldspar is mostly oligoclase. The fine- to very fine-grained groundmass contains accessoryminerals of muscovite, biotite and Fe-Ti oxide. Granophyric texture is sometimes present.ApliteSporadic northeast- and eastnortheast-trending dykes of aplite have been mapped along the coast betweenSha Lo Wan and Hau Hok Wan. These dykes vary from 25 to 50 mm wide and intrude fine- to mediumgrainedLantau Granite and Chek Lap Kok Granite. The aplite dykes are typically leucocratic and equigranular,and are thought to represent granitic fluid expelled along fractures at a late stage in the cooling history of thegranitic magma.32


Quartz VeinsQuartz veins are ubquitous in the northern part of the district where they are commonly seen intrudinggranitic rocks, rhyolite dykes, and volcanic rocks. The quartz veins vary from stringers a few millimetreswide to dykes up to 30 m wide. Ruxton (1958) divided the quartz veins into two types: 1) high temperature(pneumatolytic) type, typically of grey colour and medium-grained granular texture, found between graniteand country rock, and in its immediate vicinity, interlacing in all directions at the contact zone, and 2)moderate to low temperature (hydrothermal) type composed dominantly of glassy or milky quartz, withminor feldspar, and found in more regular linear zones which cross cut the type 1 veins. The pneumatolyticveins (type 1) are composed of quartz and bleached biotite with minor quantities of feldspar, topaz, fluorite,molybdenite, chalcopyrite, arsenopyrite, wolframite, pyrite, and beryl. The concentrations of the metallicores are sporadic. The hydrothermal veins have sharp boundaries with the country rock and appear to haveexploited structural planes of weakness, such as shearing, jointing and fracturing. These veins often carrypegmatitic patches composed of pink orthoclase. The type 2 quartz veins strike eastsoutheast-westnorthwestand dip about 60° to the southsouthwest. Chalcopyrite is the dominant ore mineral in these veins, with minoramounts of pyrite, arsenopyrite, galena, and wolframite. Veins of magnetite ore also occur on the coastbetween Sha Lo Wan and Hok Hau Wan.Numerous quartz veins have been mapped adjacent to the faulted contact between the Lantau VolcanicGroup and other rocks in the western part of the district. These quartz veins contain zones of tungstenmineralisation. The thickest quartz vein is exposed on the western side of Hau Hok Wan.Mafic DykesMafic dykes, ranging from 50 mm up to 7 m wide and with variable orientations, are present mainly in thenorthern part of the district. They intrude granitic rocks and rhyolite dykes. They are mostly of basalticandesite composition, but range from basalt to andesite. Several high-K varieties can be classified aslamprophyres. The mafic dykes are all strongly altered. Although none of these dykes has been radiometricallydated, they have similar composition to mafic dykes that occur in northeastern Lantau Island and whichhave returned Late Cretaceous ages (GEO, unpublished data).In thin section, the rocks are typically very fine-grained and microporphyritic. The basaltic andesite dykes havesericitized plagioclase and pyroxene phenocrysts, sometimes showing the development of sub-ophitic texture.The groundmass of the basaltic andesite dykes contains abundant hornblende, with plagioclase, epidote, and Fe-Ti oxide (Plate 7). Lamprophyric dykes generally have abundant augite and/or hornblende in the groundmass,and are distinguished from the basaltic andesite dykes by the absence of plagioclase phenocrysts.Plate 7 - Thin Section of a Mafic Dyke from Hau Hok Wan (HK9237, 1156 1727)33


Chapter 6StructureThe major structural features of the district include folds in Carboniferous strata surrounding The Brothersislands, regional scale Post-Yanshanian basement faults, Late Jurassic to Early Cretaceous faults related tovolcanic and plutonic activity, photolineaments and joints. The eastnortheast-striking Lantau Dyke Swarmalso exerts a strong geomorphological influence, especially in the eastern part of the district where therhyolite dykes are interspersed with less resistant granitic material. Many of the larger valleys within thedistrict are interpreted as being underlain by major faults. As found elsewhere in Hong Kong, these faultsare commonly associated with zones of weaker material which is more susceptible to weathering. However,some lineaments seen on aerial photographs may be joint related rather than fault related. Sometimes thereis no field evidence of fault or joint control of the photolineaments, and, in the case of volcanic and sedimentaryrocks, many of the lineaments are more closely related to the strike of bedding.FoldsThe metasedimentary rocks forming The Brothers islands are considered to belong to the southern continuationof an outcrop of Carboniferous strata extending from Man Kam To in the northern New Territories to TuenMun in the western New Territories. This zone has been intruded by the Middle Jurassic Tai Lam Granite.The variations in bedding attitude of the metasedimentary rocks on The Brothers islands, together with theiroutcrop pattern, indicate that the strata are tightly folded into an asymmetric plunging syncline with the foldaxial plane striking toward the northeast.Marble encountered in offshore boreholes to the south and east of East Brother island is con-elated with theYuen Long Formation in the western New Territories and stratigraphically underlies the graphitic siltstonesand sandstones of the Lok Ma Chau Formation. The tightness of the folding and proximity to the graniticintrusion suggests that the fold might be an annular structure, formed as a result of diapiric emplacement ofa granite pluton.FaultsThe major faults of the district can be broadly divided into those which are associated with Late Jurassic toEarly Cretaceous volcanism and those which are related to post-Yanshanian movement (Yanshanian Orogeny:190-80 Ma, Jahn et aL, 1976). Faults associated with the former generally reflect extension on easterly,and eastnortheast-trending structures, whereas those related to the latter mainly reflect transtension on majornortheast-, and northwest-trending structures (Campbell & Sewell, 1997).The most prominent volcanic-related structure of the district is a major fault marking the boundary of theLantau Caldera (Langford et aL, 1995; Campbell & Sewell, 1997; Sewell et aL, 2000) (Figure 8). This faultextends for approximately 5.7 km in a northwesterly direction from Tai Ho in the east to Sha Lo Wan in thewest, and is offset by both the Shek Pik Fault and the Por Kai Shan Fault. In the east, the northern boundaryof the caldera intersects the eastern boundary marked by a north-trending fault. In the west, the calderaboundary is partly intruded by the Tong Fuk Quartz Monzonite. Details of the caidera collapse are uncertain,but the collapse is thought to have occurred during the main episode of eruption of the Lantau VolcanicGroup (c. 146 Ma). In the east, the bounding fault is sometimes marked by the presence of a thick (up to 5m wide) quartz vein, and is easily distinguished in the field and on aerial photographs by a prominent breakin slope, the volcanic rocks being more resistant to erosion. Minor syndepositional faulting associated withsoft sediment deformation within the volcanic succession is marked by variably oriented faults on Por Kai Shan(1398 1633). Some of these have been inferred from rapid pinching out of intercalated sedimentary units.Post-Yanshanian faults dominate the structure of the district forming prominent northeast- and northnorthwesttrendingfeatures. The most pronounced feature is the northeast-trending Shek Pik Fault. The Shek PikFault, which is inferred to be a segment of the Sha Tau Kok Fault farther north, transects the district in asoutheastward direction from offshore northern Lantau Island to Shek Pik, passing onshore just to the southand east of Tung Chung. The Shek Pik Fault offsets the northern boundary of the Lantau Caldera by c.3.1 km in the vicinity of Tung Chung and provides an excellent example of sinistral displacement (Sewell et34


LEGENDOoI2PhotolineamentFaultConcealed or Inferred Fault_ — — Caldera Boundary• i • Volcanic Layering—>— Palaeodrainage Network— so — Rockhead Contours (mPD)IILANTAU /CALDERAI


aL 2000). Restoration of this sinistral displacement, along with a 250 m sinistral displacement on a subparallelnortheast-trending fault extending from Wong Lung Hang to Pak Mong (see Fletcher et aL, 2000), realignsthe northern caldera boundary, and also reveals a northward extension of the eastern caldera boundary faultat Kei Tau Kok, east of Tung Chung. Rockhead contours for the offshore area north and east of Tung Chunghave revealed a prominent ridge extending north from the small promontory at Kei Tau Kok. Gravity profilesfor the offshore area have also revealed a sharp east-west change in gradient across this ridge, separatingmetasedimentary rocks, including marble, in the Tung Chung area from granite farther east. Therefore, it islikely that the western margin of the ridge marks the offshore extension of the eastern caldera boundaryfault, which currently forms the axis of Tai Ho Wan valley.Restoration of the sinistral displacements also suggests the presence of some Post-Yanshanian faulting. Forexample, the northnorthwest to northwest-trending faults forming Tung Chung valley and Wong Lung Hangvalley have not been displaced by the Shek Pik Fault and, therefore, must represent a period of younger faultmovement.Rockhead contours, together with gravity (Kirk et aL, 2000) and boomer surveys, for the offshore area alongthe north Lantau coast have revealed areas of deep weathering which appear to coincide, in part, with theoffshore extension of the Shek Pik Fault. The rockhead contours and data on superficial deposits also suggesta series of westnorthwest- and northwest-trending buried drainage lines which have been captured by aburied northeast-trending valley. The northwest-trending drainage lines appear to be partly controlled bynorthwest-trending faults described above, whereas the westnorthwest-trending drainage lines are subparallelto the feldsparphyric rhyolite dyke orientations. The buried valley which marks the offshore extension of theShek Pik Fault along the north Lantau coast, diverges to the north adjacent to Ha Kok Tsui before intersectinga major buried drainage channel trending westward north of The Brothers islands. Gravity and boomersurveys, together with borehole data, suggest that to the north of Ha Kok Tsui, the buried valley follows theboundary between the Carboniferous metasedimentary rocks and Middle Jurassic to Early Cretaceous igneousrocks. The buried drainage channels are marked by deposits of marine sand which have been partly dredgedfor reclamation projects.PhotolineamentsLaterally continuous photolineaments within the district have mostly been mapped in the volcanic areaaround Pok To Yan (1360 1591). A northeast-trending photolineament in the southeastern corner of thedistrict follows a small valley, but there is no field evidence that this valley is fault controlled. Farther west,on the flanks of Pok To Yan, two relatively short (200 m) photolineaments have been mapped which coincidewith narrow (


No. of Joints measured=82No. of Joints measured=68No. of Joints measured=86No. of Joints measurecl=952%No. of Joints measured=87Figure 9 - Contoured Pole Plots of Joints Measured for All Rock Types on Tung Chung and Norths horeLantau Island (after Woods, 1993). a) Sha Lo Wan to Hau Hok Wan, b) Hau Hok Wan to Tin Sam,c) Tung Chung to Lau Fau Sha, d) Lau Fau Sha to Kei Tau Kok, and e) Kei Tau Kok to Pak Mong37


Rock Quality Designation (RQD) values for slightly weathered granite, feldsparphyric rhyolite and volcanicrock are 55%, 47% and 56% respectively, and these values generally reduce with increasing weatheringgrade. Average RQD values of 48%, 28% and 30% respectively are found in moderately weathered granite,feldsparphyric rhyolite and volcanic rock. Fracture index values range from 5 to 15 in all rocks, and there isno significant increase in. fracture index with increasing weathering grade, or measurable variation amongthe different rock types (Woods, 1993).Joint persistence in granitic rocks observed from coastal exposures indicates that the dominant joint sets(e.g. northeast- and eastnortheast-trending joints) may range up to to 30 m. Similar persistence has beenobserved in shallowly-dipping sheeting joints, whereas minor random joints have a persistence of only afew 10s of mm. Joint persistence in volcanic rocks observed in the water tunnel from Ngong Ping to ShekPik Au averages 10 m (Woods, 1993).Minor variations in joint regularity have been recorded among different rock types. For example, smoothand planar joints are generally a feature of volcanic rocks, whereas joints in the granites and feldsparphyricrhyolites are often irregular, and for gently dipping joints, often wavy.38


Chapter 7Metamorphism and AlterationMetamorphism is generally defined as a structural/textural modification and/or mineralogical modificationof a pre-existing rock involving crystallisation of new phases, recrystallisation of existing phases, and/orstrain.Thermal MetamorphismMetamorphic rocks in the district include thermally metamorphosed sedimentary and volcanic rocks closeto igneous contacts and incipiently metamorphosed rocks in zones of hydro themial alteration. Examples ofthemial metamorphism include: graphitic siltstones and metasandstones of the Lok Ma Chau Formation,marble of the Yuen Long Formation, and metamudstone, metasandstone, marble, skarn, calcsilicate rockand quartzite of the Tolo Harbour Formation. These metasedimentary rocks are mostly encountered inboreholes in the vicinity of The Brothers islands and at the Tung Chung reclamation. The rocks have beenmetamorphosed by the intrusion of granite.A narrow (100 m wide) zone of contact metamorphism within the Lantau Volcanic Group is present alongthe caldera boundary in the western part of the district between Tung Chung valley and Sha Lo Wan. In thisarea, the intrusion of quartz monzonite along the caldera boundary has not only metamorphosed the adjacentvolcanic rocks, but also has given rise to local pockets of mineralization.Hydrothermal MetamorphismHydrothermal metamorphism (or alteration), involving the addition of hot water and crystallisation of newminerals, has locally affected volcanic and granitic rocks close to fault zones. In granites, this process ischaracterised by chloritization of biotite and hornblende, and sericitization and clay mineral alteration offeldspars. A broad zone of hydrothermal alteration is present within the Lantau Volcanic Group in thenortheastern part of the district. The rocks are typically altered to a reddish grey colour, which weathers topinkish white.39


Chapter 8Superficial Deposits and WeatheringClassification and DistributionSuperficial deposits, comprising accumulations of sediment derived from the weathering and erosion ofolder rocks, are widespread in both onshore and offshore parts of the district. They mantle an often deeplyweathered rock surface and vary from discontinuous veneers of slope debris and alluvium in onshore areas,to laterally extensive mud and sand sheets up to several tens of metres thick in offshore areas. The environmentsof deposition are shown in Figure 10.In onshore areas, the superficial deposits on the higher ground consist mostly of colluvium (slopedebris, etc) much of which is derived from processes of mass movement. Downslope, the colluviumgrades into, and interdigitates with, alluvium which infills the major valleys. The boundary betweenthe two deposits is often difficult to define as many of the superficial deposits in the larger valleyshave been disturbed by cultivation.The offshore superficial deposits of the district comprise four formations; the Tung Chung Formation, theChek Lap Kok Formation, the Sham Wat Formation and the Hang Hau Formation. They have been identifiedon lithological, palaeontological and geotechnical evidence from borehole cores and on the basis of seismiccharacter as displayed on shallow seismic reflection records (Fyfe et al., 2000).Correlation between onshore and offshore superficial deposits is problematic. The onshore Pleistocene alluvialdeposits can generally be matched with Pleistocene alluvial deposits now offshore (Chek Lap Kok Formation).However, there is no such straight-forward correlation between onshore Pleistocene mass wasting deposits(slope debris, debris flow deposits, etc) and potential lateral equivalents now lying offshore. Fyfe et al.,(2000), have tentatively linked these onshore mass wasting deposits with occasional thin colluvial interbedsin the offshore Pleistocene alluvial deposits. Holocene colluvial deposits, described variously as debris flowdeposits, slope debris, slide deposits, and talus deposits on 1:20 000-scale geological maps, have recentlybeen assigned to the Fan Ling Formation (Fyfe et al., op. cit.) and are considered to be contemporaneouswith the Hang Hau Formation. However, on the 1:5 000-scale map series, the lithostratigraphic units are thesame as those in the 1:20 000-scale map series.fQl- lie QhbSCJfHWQhmSQ h mM———Qp a^W^^Rock andweathered rock- ^Figure 10 - Schematic Section Showing the Relationship between Superficial Deposits and the DifferentEnvironments of DepositionQp = Pleistocene; Qh = Holocene; Q = Quaternary (undivided);a = alluvial; b = beach; e = estuarine; i = intertidal; m = marine;D = debris; S = sand; M = mud; HW = high-water level40


Offshore superficial deposits within the district are dominated by mud and sand of the Hang Hay Formation.These marine deposits form a thick blanket overlying estuarine deposits of the Sham Wat Formation andalluvial sediments of the Chek Lap Kok Formation. The Chek Lap Kok Formation comprises sand banksand sand sheets to the north of Lantau Island. The sand banks have been extensively dredged and the sandused as fill material for reclamations. Beneath the Chek Lap Kok Formation, the deposits of the Tung ChungFormation infill local depressions in the weathered rock surface. The thickest accumulation of these coarseandfine-grained sediments lies in the vicinity of the type locality at the Tung Chung reclamation, andimmediately to the north of Lantau Island between Tung Chung and Yam O. At the type locality, the sedimentsinfill three separate sub-circular basins with diameters of c. 200 m, 300 m and 600 m, and reach up to96.5 m in thickness (Fyfe et ai, 2000). Subparallel with the coastline to the north of Lantau Island, thesediments locally infill a series of discontinuous deep basins which follow a northeast-trending fault-controlleddepression (Kirk et al, 2000). Contours on rockhead (Grade III or better) suggest that a palaeovalley onceformed the axis of the depression and this feature is thought to have captured water draining from northwesttrendingand west-trending valleys during a low sea level stand.Onshore Superficial DepositsSlope DebrisSlope debris consisting of locally derived material (debris flow deposits, talus deposits, etc,) is consideredto be the product of mass movement processes. This material, also referred to collectively as colluvium, iscommonly generated during periods of high rainfall. The mass movements, such as debris flows and debrisslides, may be initiated on slopes inclined at only a few degrees. Slope debris deposits commonly formaprons on the middle to lower slopes of high ground and may grade downslope into alluvium. They comprisea structureless mixture of silt, sand, gravel, cobbles and boulders, often embedded in a gravelly, sandy silt tosilty clay matrix. The colour of the matrix varies from yellowish brown, through brownish grey to pink;white kaolin streaks and spots are common. The rock fragments generally show a weathered rind consistentwith significant post-depositional weathering. The slope debris in the middle to lower slopes is generally upto 10 m thick, but a maximum thickness of 17.5 m has been recorded in a borehole (TRL103/3608, 12561662) near TaiPo.Distinction between Pleistocene (Chek Lap Kok Formation) and Holocene (Fan Ling Formation) slopedebris is based on the general criteria used by Lai & Taylor (1984) and Lai (1997, 1998) for the subdivisionof colluvium. These criteria include superposition, colour, and the degree of decomposition of the clasts. Ingeneral, Pleistocene slope debris are poorly sorted, with a reddish brown or dark yellowish brown to orangishred, slightly mottled slightly clayey sandy silty matrix containing subangular, slightly to moderatelydecomposed boulders and cobbles. In contrast, the Holocene colluvial deposits have a fresher appearance,without the distinctive mottling of the matrix. They are described as poorly sorted, with a light yellowishbrown, slightly clayey sandy silt to gravelly silty sand matrix containing subangular to angular slightlydecomposed boulders with weathering rinds only a few millimetres thick.The distribution of slope deposits within the district has been determined by surface morphology. Pleistoceneslope debris deposits have been mapped in the middle to lower reaches of the major valleys (e.g. Tin Sam,Tung Chung, and Wong Lung Hang), and as thick fan aprons on the northwest-facing slopes above Tai Po,east of Tung Chung. In the lower parts of the main valleys, the slope deposits are commonly gradationalinto, or overlain by, Pleistocene alluvium. In places (e.g. Tin Sam valley), the Pleistocene slope debrisdeposits extend almost to the coastline.Holocene slope debris within the district is mostly confined to the upland areas and heads of stream courses.In the upland areas, the Holocene debris forms discontinuous veneers on the weathered bedrock. In theheads of stream courses, the slope debris forms minor dendritic networks. Deposits of slope debris whichalmost reach sea level are thought to be the products of large debris flows.Alluvial SedimentsAlluvial deposits of the district are generally thicker and more widespread than the slope debris deposits.The alluvium infills the floors of many of the larger valleys and has, in many cases, been significantlymodified by cultivation.41


Both the Pleistocene and Holocene alluvial deposits have been mapped in the district mainly by aerialphotograph interpretation. In general the Pleistocene alluvial deposits form elevated terraces whichhave been incised by recent streams, whereas Holocene alluvial deposits are largely confined to thebeds of recent stream courses. The alluvial deposits consist mostly of well-sorted to semi-sorted clay,silt, gravel and sand.Pleistocene alluvial deposits have been mapped in several of the major valleys in the district, and are commonlyassociated with Pleistocene slope debris deposits. Extensive deposits of Pleistocene alluvium, now modifiedby cultivation, form the floors of valleys at Sha Lo Wan, Tin Sam, Tung Chung, Wong Lung Hang, and PakMong. In the Tung Chung area, the Pleistocene alluvium consists of cobbles and boulders surrounded by amottled red and yellow sandy silt with some lenses and layers of sand. Thicknesses of alluvium recorded inboreholes vary from 5 to 20 m. The Pleistocene alluvial deposits were laid down under various conditions ofdeposition, but mostly fomied as fans and deltas at the bases of extensive debris deposits. During periods ofactive erosion of nearby hills, coarse sandy material was deposited near to the source area. At other times, aquieter environment existed, allowing far-travelled silts and muds to be laid down.Holocene alluvial deposits within the district mainly occupy the beds of recent streams and form small fandeltas where these streams enter the sea. The largest deposits are found at Sha Lo Wan, Tung Chung, WongLung Hang and Pak Mong. The Holocene alluvium consists mostly of gravelly sand with subrounded cobblesand boulders in the stream courses, or yellowish-brown, well sorted clayey sand or silt in the deltas. In theTung Chung valley, Holocene alluvium forms a series of narrow strips along the present stream courses.These are incised into the Pleistocene alluvial terraces.Beach DepositsBeach deposits are exposed in many of the areas of remaining natural coastline, particularly the shelteredbays to the west of Tung Chung. These deposits consist of unconsolidated, mainly sand-sized material andoccupy the narrow strip of land extending from the low-water mark to the upper limit of wave action,usually delineated by either a cliff line, or storm beach. Prior to reclamation, beach deposits occurred alonga large portion of the natural coastline between Pak Mong and Lau Fau Sha.At Sha Lo Wan and Hau Hok Wan, broad (100 m wide) sand beaches have developed consisting of clean,yellowish brown, fine- to medium-grained sand. Small pockets of beach sand have also accumulated in gapsalong the rocky coastline between these two beaches. At Tin Sam, a narrow (25 m wide) sand beach hasdeveloped on the edge of the small delta.Intertidal DepositsIntertidal deposits of mixed alluvial and marine origin have been mapped between Tin Sam and Tung ChungWan (Plate 8). They consist of soft dark grey to yellowish brown clayey silty sands with plant remains, anddark grey, organic silt with shell fragments, and are generally confined between the low and high watermarks. At Tung Chung Wan, the intertidal deposits are characterised by mangrove-lined channels and sandysplays.Offshore Superficial DepositsTung Chung FormationThe Tung Chung Formation (Kirk, 2000; Fyfe et al, 2000) is the name given for coarse- and fine-grainedsediments which infill local depressions offshore in the weathered rock surface near Tung Chung and alongthe north coast of Lantau Island. These sediments were first identified on the basis of their seismic characterand were previously termed "Pre-Chek Lap Kok Formation Deposits" (Langford et al, 1995).The Tung Chung Formation has been formally defined from boreholes at the Tung Chung reclamation (Fyfeet ah, 2000), At the type borehole (CC10/WO2893, 1215 1704), the formation comprises boulders ofcompletely decomposed fine-grainedgranite and feldsparphyric rhyolite, intercalated with yellowish brownsilty sand, reddish brown sandy silt, matrix-supported gravel conglomerate, and brown, fme-grained micaceoussand with minor clay laminae. The formation unconformably overlies Mesozoic or older rocks, and isconformably overlain by the Chek Lap Kok Formation.42


Plate 8 - Tung Chung Wan from the Northwest Showing Quaternary Superficial Deposits Composed ofIntertidal Deposits in the Foreground, and Beach Deposits and Alluvial Deposits in theBackground. (Photo taken on 5.12.1989)Northwest of Yam O Wan, the sediments of the Tung Chung Formation partly infill a north-trending depressionor channel about 1100 m by 800 m in area (1954 2251). The deposits, which have been confirmed by boreholes,were originally identified on the basis of their seismic character (Sewell & James, 1995) and compriserelatively strong reflectors which tend to be parallel to the base of the unit, are synclinal in form in thedepression, and are more horizontal at the margins. Several marine boreholes, drilled as part of the Tai LamChung to Siu Ho Wan water mains project (GIU No. 16271), penetrated these sediments below the base ofthe Chek Lap Kok Formation alluvium (c. -38 mPD). They are interpreted here as fine alluvial sediment.The maximum thickness of the Tung Chung Formation in this area has been proved to be 50 m.Gravity and marine borehole data acquired during the Northshore Lantau Development Feasibility Study(Scott Wilson (HK) Ltd., 2001) have revealed that the Tung Chung Formation locally infills a fault-controlleddepression that trends northeastward from Tung Chung along the northern Lantau coast. The depressioncoincides with the boundary between Palaeozoic metamorphie rocks (marble of the Yuen Long Formation)and Mesozoic volcanic and granitic rocks. Between Tai Ho Wan and Ha Kok Tsui, the formation has so farbeen detected in three S-shaped basins (Figure 11). The largest of these lies in the north, and is in probablecontinuity with the north-trending depression described above. The thickness of the formation, proven byboreholes, varies from 40 m to over 110m. The Tung Chung Formation has been interpreted to be sedimentthat was deposited in a Late Tertiary to Middle Pleistocene karst environment (Kirk, 2000; Fyfe et al,2000). Solution of the marble/limestone (karst formation) is likely to have been facilitated by water flowingalong the faulted boundary between marble, and volcanic and granitic rocks.Chek Lap Kok FormationThe Chek Lap Kok Formation (Strange & Shaw, 1986) is widespread in the offshore parts of the district. Itcomprises a diverse assemblage of Pleistocene gravel, sand, silt and clay of dominantly alluvial origin, butalso includes intercalations of colluvium (e.g. debris flow deposits, etc.) that can be related to the onshorePleistocene succession. The formation mostly rests on bedrock, except for isolated areas at the Tung Chungreclamation and along the North Lantau coast, where it rests on the Tung Chung Formation.43


Siu Mo ToNorth ShoreStudy AreaTai Mo ToThe BrothersTsz Kan ChauCHEKLAPKOKHony KongAirportTung Chung -Tai HoReclamation/ \ Occurrence of Tung Chung Formationi *tir ) defined from closely-spaced seismic^--^ profilesTung Chung Reclamationo_l2 km. M5 Intersection of Tung Chung Formationin isolated borehole£P*Negative Gravity Anomaly (< -0.40 mGal)-~~ ~-~ Proposed Limited of ReclamationFigure 11 - Location Map Showing the Distribution of the Tung Chung Formation Determined fromInterpretation of Gravity and Borehole Data (Modified after Kirk et al, 2000)North of Lantau Island, the formation is generally between 15 and 20 m thick, but may vary from 7 m tomore than 40 m thick according to the form of the underlying rock surface. The formation is variable interms of its seismic character, which is often chaotic and complex, with indistinct channel forms, welldevelopederosional channels and strong parallel reflections. The sediments are generally light bluish greyto yellowish brown and comprise fine to medium gravel, coarse to fine quartz sand, sandy silt and silty clay.The type section of the formation has been designated as Borehole B13/B13A (Strange & Shaw, 1986, 18701029), drilled on the test embankment at Chek Lap Kok prior to the development of the airport. The lithologiesdescribed from this borehole show a wide range of grain sizes from gravel to clay. Unlike the overlyingHang Hau Formation (see below), the clay in the Chek Lap Kok Formation tends to be firmer withuncompressive strengths between 30 and 90 kPa. Overall, the succession fines upward from a basal sequenceof gravels, through silts, to clay-dominated layers with abundant organic material. However, there are alsointerbedded coarsening-upward layers, as well as sporadic massive, moderately sorted sands.Several large channels within the Chek Lap Kok Formation have been determined from seismic records andfrom boreholes. North of The Brothers islands and Yam O Wan, a large east-west trending channel abovethe formation has restricted its thickness from less than 5 m to 10 m. Around The Brothers islands itsthickness is governed by higher areas within the rock surface, as well as north-south trending channelsincised into its upper surface.Although the Chek Lap Kok Formation is considered to be dominantly fluvial, there is evidence of a marineinfluence suggesting that the sediments were deposited under variably estuarine, intertidal, and fluvialconditions. A floodplain environment is generally envisaged, with fluvial and alluvial deposits intercalatingwith colluvium on steeper slopes and the heads of valleys. Using both radiocarbon and luminescencetechniques, a number of absolute ages have been obtained for the formation from offshore boreholes. Theages range from 16 420 years BP at Chek Lap Kok to 80 000 ± 9000 years BP (Borehole A5/2) east of theSoko Islands (Fyfe et al., 2000).44


Sham Wat FormationThe formation, known only in subcrop, was first described in the district by James (1993) and Langford etal. (1995) during mapping of the area north of Lantau Island.The type section of the formation, designated as borehole ESC 17, lies two kilometres west of the district,where the sequence is 17 m thick. However, the formation thins rapidly towards the east, and occurs only inthe extreme western part of the district, where it is a few metres thick. The formation was proved in many ofthe boreholes in the western part of site now reclaimed for Chek Lap Kok airport and is interpreted tounderlie the area surrounding this part of the reclamation (Fyfe et al, 2000).The formation comprises soft to firm silty clay with some sand, and is medium grey in colour with paleyellowish grey oxidized patches, some laminations, nodules and sparse mottles. Shell fragments and sparseplant fragments are consistent with a marginal marine, probably estuarine environment varying to fullymarine conditions.The formation has a distinctive seismic character but intra-formational reflections may be confused withthose of the Pok Liu Member of the Hang Hau Formation, which also usually has an irregular channelledbase. Resolution of this problem depends on close examination of the stratigraphical relationships alongseismic reflection profiles.The base of the formation is represented by a strong reflection with a distinctive undulating form that in partdefines the seismic character. This basal surface is commonly deeply incised into the underlying Chek LapKok Formation as a series of channels. The interfluves between the channels exhibit a distinctly planatedcrest, representing the remnants of an older erosion surface. Intra-formational reflections are of low tomoderate amplitude and their geometry is controlled by the morphology of the channels. The reflections arecontinuous, sub-parallel to the planated crest of the interfluves and drape down into the deeper channels. Atthe top of the sequence, the internal reflections are truncated by the reflection marking the erosional base ofthe overlying Hang Hau Formation.The age of the formation has not been confidently established but available evidence is consistent with aLate Pleistocene age.Hang Hau FormationThe Hang Hau Formation (Strange & Shaw, 1986) forms a blanket of marine mud and sand over large partsof the offshore area (Figure 12). The formation is mostly Holocene in age and is the youngest offshoreQuaternary stratigraphic unit in the district. Fyfe et al (2000) described four member units within the HangHau Formation, although none of these members is distinguished in the district.The Hang Hau Formation comprises mostly very soft to soft, olive grey clayey silt and is relativelyhomogeneous throughout the district. Shear strengths in the undrained state vary from less than 3 to 20 kPa.Minor silt and sand lenses are locally present throughout the sequence. Disarticulated and articulated bivalvesare common, as is comminuted shell debris ranging in size from less than 0.1 to 20 mm.West of Chek Lap Kok, the formation is mostly between 5 and 10 m thick, but it reaches 15 m thick in thearea between Chek Lap Kok and The Brothers islands to the east. A zone of acoustic turbidity (or gasblanking) is present to the east of Chek Lap Kok and this has obscured the seismic reflectors. The gasblanking is thought to originate from the release into the sediment of biogenic gas produced by anaerobicbacteria feeding on decaying organic matter at depth.In Tung Chung Wan, the formation is generally 5 m thick and consists of very soft, dark grey clay and siltwith occasional shells. On the small delta at Tin Sam, a layer of firm, brown fine sandy clay interpreted as amarine deposit was found intercalated with alluvium in two boreholes. The marine deposit is approximately5 m thick and is encountered at a depth of c. -5.0 mPD.The seismic signature of the Hang Hau Formation is distinctive, with very extensive, laterally continuous,horizontal to subhorizontal, moderate to low amplitude reflectors. The base of the formation is marked by ahigh amplitude reflector. Long, low amplitude reflectors predominate at the margins of the main tidal currentchannels, and an extensive intra-formational reflector is present in many of the channels. The reflectors tend45


Figure 12 - Distribution andlsopachs (in Metres) of the Hang Hau Formation. Prior to Reclamation at the InternationalAirport, up to 10m of Hang Hau Formation was Dredged to form a New Seabed at -15mPD (Pinches et al, 2000)46


to infill depressions and channels with little or no indication of truncation and overlap.North of Lantau Island the thicker sequences of the formation infill a deep channel within Yam 0 Wan (20702130) to a maximum depth of over -22 mPD, and also infill a number of channels south of The Brothersislands to depths of-25 mPD. The formation also forms a number of bar-like features, over 20 m thick, onthe margins of the east-west trending tidal channel running from The Brothers islands to Kap Shui Mun.Sand banks, up to 15 m thick, formed of silty sand, occur sporadically within the formation. To the north ofYam O Wan, these have been dredged for reclamation works (Choot, 1988).Most of the sediment of the Hang Hau Formation is thought to have been derived from the Pearl River. Theformation is generally considered to be an estuarine to marine sequence with at least some sediment, at thebase of the formation, related to the marine transgression which occurred during the rise in sea level followingthe last glaciation. Most of the muddy sediments forming the bulk of the formation, appear, however, tohave been deposited in water depths similar to the present day.A radiocarbon age of 7960 ± 85 years BP was obtained for the base of the formation from a borehole nearChek Lap Kok (James, 1993). This is considered to be the maximum age of the formation in the district andis similar to that obtained from another borehole at Junk Bay (8080 ± 130 years BP, Strange & Shaw, 1986).WeatheringDifferential weathering among the different rock types exerts an important geomorphological control withinthe district. As described above, faults commonly host zones of weak rock which are more susceptible toweathering. Granite generally weathers more rapidly than volcanic rocks, which explains why the highestpeaks of the district are capped by volcanic rocks. Zones of hydrothermal alteration within the volcanicrocks are also more susceptible to weathering than unaltered zones. This might help to explain the largevalley system developed in the Wong Lung Hang area in the eastern part of the district, which hosts a broadzone of hydrothennally altered rock (see above). Localised deeep weathering in the vicinity of Tung Chungand along the north coast of Lantau Island is discussed in detail in chapter 9.


Chapter 9Complex Ground Beneath the TungChung ReclamationIntroductionThis chapter describes the geology associated with complex ground conditions at the Tung Chung reclamation.These complex ground conditions were first encountered during preliminary ground investigations for theTung Chung reclamation in 1991 and have had a significant impact there on the design and construction ofdeep foundations for infrastructure development.Much of the information presented here is taken from a report entitled "Geology of Tung Chung New Town"produced by the British Geological Survey (Gillespie et al, 1998) under a Consultancy Agreement managedby the Geotechnical Engineering Office for the Territory Development Department. This detailed study wasprompted by the need to establish a reliable geological model to assist further development in the area.The geological model developed for Tung Chung has emphasised how the combination of several distinctgeological processes, of widely differing age, has led to the development of the complex ground conditions.Useful related information is included in papers by Kirk (2000), Kirk et al (2000), and Fletcher et al. (2000).BackgroundPrior to its recent development, marble was unknown in the Tung Chung area. The nearest sedimentaryrocks had been mapped at The Brothers islands, and marble was proved in offshore boreholes nearby and atTai O. These marble occurrences had both been correlated with the marble-bearing Carboniferous sequencein the Yuen Long area. Skarn had also been reported from San Shek Wan and Sha Lo Wan (Langford et al.,1995). The solid geology in the vicinity of Tung Chung was known to be dominated by granitoid dykes(feldsparphyric rhyolite, quartzphyric rhyolite, and microgranite), with some small intervening slivers ofmedium-grained granite (GEO, 1994).According to GEO records, marble was first reported at Tung Chung reclamation in late 1992 during drillingat Housing Development Phase 1 ('Area 10'). Consequent geophysical surveys, undertaken by GEO during1993, suggested that sedimentary rocks occurred offshore to the north of Lantau Island. However, the resultsfor the existing reclaimed area remained inconclusive. In 1994, fragments of marble and small voids werenoted from several boreholes at the Pedestrian Bridge site in Tung Chung Phase 1. In 1996, drilling for TungChung Town Lot 3 ('Site 3', Figure 13) encountered a range of problematic geological conditions includingmarble with cavities, steep weathering profiles reaching to depths of more than 170 m, and unconsolidatedsediment to depths of at least 80 m. The complex geological conditions at Site 3, eventually led to theabandonment of a planned residential tower block in late 1996.The preliminary geological model suggested that the marble and associated metasedimentary rocks occurredas small slivers within a fault zone (GEO, unpublished correspondence) although the scale and extent of theproblematic ground conditions were unclear. Attempts to understand the geological controls on the distributionof the unusual materials were hindered at the time by inconsistent description and interpretation in boreholelogs undertaken by different contractors. This made correlation or comparison between adjacent sites verydifficult.Tung Chung StudyThe Geotechnical Engineering Office, on behalf of the Territory Development Department (TDD), andassisted by British Geological Survey consultants, carried out a geological study of the entire Tung ChungNew Town area between 1997 and 1999. The objectives of the study were to collate all available groundinvestigation (GI) data; develop a geological model for the occurrence of complex geological conditions;provide guidance on the most appropriate GI techniques to enable early recognition and effective delineationof affected areas; and to evaluate the possibility of other areas being similarly affected. Follow-up studies,48


ased on recommendations contained in the report (Gillespie et a!., 1998) included: further onshore andmarine ground investigation to test models and investigation techniques, onshore and marine geophysicalsurveys, and development of a database for all archival and new information.Data SourcesSome 2,500 boreholes have been drilled in the Tung Chung New Town area over the last 20 years. Most havebeen carried out within the last few years as the tempo of development has increased. Despite the volume ofinformation available, there were problems in developing a geological model for the site. Problems arosemainly from the inaccurate or inadequate descriptions of geological materials, and widely differinginterpretations of some of the less common lithologies. For example, the various logging geologists preparingthe numerous Gl reports from areas of the reclamation adopted a wide range of interpretations of the highlyvaried lithologies present. In some instances, several versions of logs existed for the same hole, probablyreflecting the difficulties that contractors and consultants were experiencing while attempting to interpret theunusual materials (e.g. Figure 14). Logging problems were further compounded by sampling strategies. Inmost cases the very thick soils at the site were only logged from Standard Penetration Test (SPT) and Maziercutting shoe samples. Numerous Mazier samples were reserved for future laboratory testing, but very fewwere used to help resolve the difficulties encountered in lithological description. Thus, it was not possible tobuild up a consistent geological model from the widely differing information, ranging across many sites.As part of the GEO study, logs (and particularly core photographs, where available) of about 2000 boreholeswere reviewed. Consistent terminology and classification were applied to a revised set of descriptions.About 400 boreholes were completely re-logged by re-examination of some or all of the samples from eachhole. However, it should be noted that not all the borehole logs have been reviewed by GEO.The re-logging undertaken in the GEO study is now stored digitally in Planning Division's GeologicalModelling System. The original paper records, most of which are stored in the CEL GIU, have not beenupdated. Future workers accessing "factual data" for the Tung Chung reclamation from the GIU shouldtherefore be aware of the difficulties of using the original paper records as outlined above.Developing the Geological ModelThe geological model developed from the reassessment of borehole logs during the Tung Chung study isdescribed below in terms of three stages:a) a description of the metasedimentary xenoliths, and their inclusion into the local granitic rocks;b) a description of the deep weathering of the granitic rocks in the vicinity of some of the xenoliths;andc) a description of the development of karstic features in and above those xenoliths that containcarbonate rocks.XenolithsThe area is underlain by granite, intruded by numerous granitoid dykes (mainly feldsparphyric rhyolite).The intrusive rocks enclose blocks (xenoliths) of metasedimentary rocks, predominantly of marble, sandstoneand siltstone. The blocks range in size from a metre across to more than 300 m across. It is inferred that theblocks were incorporated during (Jurassic) intrusion, as they commonly exhibit effects of contactmetamorphism. For example, skarn is well developed where carbonate rock (such as marble) has been incontact with the magma. The structural geology of the area is complex, and faulting further complicates theoriginal distribution of xenoliths.WeatheringExtreme depths of weathering are evident where the sedimentary rock xenoliths occur. In the Tung Chungarea, continuous Grade III material and better, typically occurs at depths of 40 to 50 m. However, in theimmediate vicinity of some xenoliths, many boreholes have been drilled to 150 m or more without reachingcontinuous Grade III or better material granite, and at least one borehole exceeds 200 m. These unusualweathering profiles are characterised by abrupt weathering fronts with transitions from CompletelyDecomposed Granite (Grade V) to Slightly or Moderately Decomposed Granite (grades II, and III) over only afew metres. The gradient on these transitions, also referred to as 'rockhead', is also locally very steep (up to 70°).50


most likely a collapse doline. These features exceed 100 m and 60 m in width respectively, and are up to130 m deep. Both are infilled with unconsolidated sediment, predominantly comprising boulders and cobblebearingsilt and sand. A new formation, the Tung Chung Formation, has been proposed (Kirk, 2000; Fyfe etal, 2000) for these thick, though laterally restricted, deposits. Pollen dates, from serial samples, have suggesteda mid-Pleistocene age (Fyfe et al., 2000). The Chek Lap Kok Formation unconformably overlies the TungChung Formation,Complex Ground Conditions at Site 3Gillespie et al., (1998), Kirk (2000), and Fletcher et al (2000) have recently described the complex groundconditions encountered at Site 3, Tung Chung reclamation, which led eventually to the abandonment ofProposed Tower 5. Continuous mazier samples through karst deposits below 100 m from ground surfacerevealed the presence of two sedimentary facies (Fletcher et al, 2000): a 'cavity-fill facies' and a 'collapsefades'. The 'cavity-fill' facies comprises laminated clay, silt and sand deposits, mixed debris-flow deposits,sedimentary breccias and colluvium (termed 'diamict'), and gravel. The 'collapse-facies' comprises bouldersof completely decomposed granite and rhyolite in a matrix of diamict (see below), clay, silt and sand.Laminated DepositsExceptionally well-preserved thin beds of clay, silt and fine sand, some of which have been highly contorted,are a feature of the laminated deposits in the cavity-fill facies. The laminations consist of orangish brownclay layers up to 20 mm thick, alternating with brown, silty clay layers (possibly organic-rich) and mediumto fine sand layers from a few millimetres to 30 mm thick. The sand layers are commonly normally graded,with coarser sand at the base and fine sand at the top. Normal faults and sand injection structures are alsopresent in the laminated deposits.DiamictDiamict deposits are present in both the cavity-fill and collapse facies, but are distinctive in terms of theirmatrix composition, grain size and the lithology of rock fragments. In all cases, the diamict deposits arechaotically arranged. In the cavity-fill facies, the diamict is characterised by completely decomposed granite,rhyolite and metasedimentary rock fragments, ranging from a few tens of millimetres to several metresacross, set in a matrix of buff clay to silty clay with small angular fragments. Diamict in the collapse-faciesconsists almost entirely of large blocks of completely decomposed granite and feldsparphyric rhyolite,(possibly up to 10m across), intercalated with fine-grained diamict, clay, silt and sand. A notable feature ofdiamict belonging to the collapse facies is that several of the decomposed granite boulders are cut by narrow,sub-vertical soil pipes, between 10 and 20 mm in diameter. A few small sub-rounded clasts of volcanic rockhave been recorded among the intercalated fine-grained diamict layers.Sand and GravelThin, discontinuous layers of sand are present throughout most of the cavity-fill and collapse facies (Fletcheret al, 2000). The sand is generally massive in appearance, and predominantly medium grained. Near thebase of the cavity-fill facies, the sand layers appear to be thicker (some greater than 10 m) and more laterallycontinuous. Fletcher et al (2000) have reported the presence of some deeply iron-stained gravels containingsub-angular rock fragments in a loose silty sand close to the bedrock surface.Model for Formation of Karst Deposits at Site 3Fletcher et al (2000) have proposed that the laminated clay, silt and sand, loose sand and gravel, anddiamict deposits accumulated in a cavity formed from dissolution of a marble block enclosed within thegranite. Sedimentary structures displayed by these cavity-fill deposits are interpreted as indicating depositionin an underground lake or water-filled cavern. The diamict deposits are thought to consist mostly of detritusderived from the roof and sides of the cavity. As the cavity was enlarged, the decomposed granite roofbecame unstable and eventually collapsed onto the cavity-fill deposits, forming a chaotic mixture of largegranite blocks and cavity-fill deposits. Voids between the large decomposed granite blocks were subsequentlyinfilled with fine sand, laminated clay and silt. A network of sub-vertical pipes formed as water was expelledunder pressure during compaction. These dewatering structures were later filledby fine sediment. A schematicrepresentation of the evolution of karst deposits beneath Tower 5 is shown in Figure 15.52


over a wider area. The survey results correlated well with seismic reflection profiles, and drilling confirmedinterpretations of both lithological variation and rockhead levels. Further details and background to theprocessing of the raw gravity data are given in Kirk et al. (2000).The anomaly maps and modelled depths (contour maps on rockhead) accurately identified the distribution ofdeep weathering, and correctly showed zones where the rockhead gradient was particularly steep (Figure 16).3D-models of the superficial strata were created locally and densities of each lithology were measured in thelaboratory. Fill, Hang Hau, Chek Lap Kok, and Tung Chung formations, together with the largest xenoliths,the shallower parts of the weathering profile and shallow man-made voids (MTR tubes) were all modelled.The component of gravity due to these materials was subtracted from the Terrain Corrected Bouguer Anomalyleaving a residual component due to deep weathering.The refined model of rockhead generated by further processing of the data described above has providedstructural information and details of variation in rockhead that would have been difficult to get by othermeans. In addition, detailed studies undertaken by Fletcher et al. (2000) have further refined the geologicalmodel for complex ground conditions beneath a single tower block on the Tung Chung reclamation. In thiscase, six new deep boreholes were drilled, and considerable care taken, to recover high quality samples.Kirk (2000) proposed a revised geological model for the geology beneath the Tung Chung reclamation ascomprising predominantly granite but with xenolithic blocks of meta-sedimentary rock, including marble.The igneous rocks adjacent to the xenoliths are unusually deeply weathered and characteristically haveabrupt weathering fronts and steep gradients on 'rockhead'. In places, carbonate dissolution has producedkarstic terrain (now buried) and cavities. Locally severe, dissolution has led to the development of sedimentfilledcollapse basins.Implications of the Revised Geological ModelThe revised geological model proposed for the Tung Chung reclamation implies local associations of materialsthat can pose geotechnical difficulties for some types of proposed development. A site investigation designthat places a heavy reliance on drilling may not encounter these materials at an early stage, and may onlyintersect them during dense drilling for final design of foundations. However, if any one of these materialsis intersected in any borehole, this should raise awareness as to the possibility of the occurrence of complexground conditions in the locale, and further investigation should be undertaken.Drilling and seismic reflection profiling have limitations as investigation methods in this geologicalenvironment but gravity surveying has proved effective in identifying the main occurrences of the lowdensity(e.g. marble) materials. The marble of Tung Chung is one of a series of local accumulations of metasedimentin the district along the northern coast of Lantau Island. Further accumulations are likely to occurelsewhere in this region, and this should be taken into account in preliminary site investigations for futureprojects.Complex Ground Conditions in Areas Adjacent to Tung ChungPreliminary investigations for the proposed Northshore Lantau reclamation (at the eastern end of the NorthLantau coast) have revealed the occurrence of deeply weathered structures associated with marble and akarstic palaeo environment similar to those found at Tung Chung (Kirk et al., 2000). The extent of themarble farther north is difficult to evaluate and must be viewed in the context of Hong Kong's regionalgeology. It is noted that the marble at Tung Chung is one of a series of small, localised occurrences of metasedimentaryrocks, including Ma On Shan, that lie along a northeast-trending structural zone extendingfrom North Lantau to Tolo Channel. The style of these occurrences of marble contrasts with the laterallyextensive Palaeozoic sequence of the Yuen Long area. The distribution of the marble at Tung Chung and MaOn Shan agrees well with the volcanotectonic reconstruction for the Late Jurassic of Campbell and Sewell(1997) (Figure 17, after Kirk, 2000). Campbell and Sewell (1997) have interpreted the volcanic and plutonicrocks of the c. 146 Ma volcanic event to represent magmatism during crustal extension or transtension.Incorporation of country rock into intrusive rocks (as at Tung Chung) and within controlling structures (asat Ma On Shan) also ties in well with this model Therefore, it can be inferred that marble may occur, eitherwithin fault structures or associated with magmatism, anywhere along the eastnortheast-trending zone shownin Figure 17.55


f820 EGuangdong Province860 EI1-8CHEK KENG FAULT / \ ^35OI.isII2 sifLEGEND^Marble/Skarn in North Lantau-Tolo ZoneCarboniferous metasedimentary rocksin Yuen Long Areanferred volcano-tectonic fault activeduring the 146 Ma. magmatic eventYounger faultLANTAU VOLCANIC GROUP| | OutcropKWAI CHUNG SUITEOutcrop[ | Inferred subcrop


Chapter 10Economic GeologyEconomic deposits of the district include non-metalliferous minerals and construction aggregates. Metalliferousmineral deposits are also known mainly from the Sha Lo Wan area and have been extensively prospected.However, none of these metalliferous mineral deposits is of economic or potentially economic proportion.Non-metalliferous MineralsFissure veins of quartz are common throughout the granitic rocks, particularly in those exposed betweenSha Lo Wan and Tung Chung. Many of these veins are weakly mineralized (see earlier), although Ruxton(1958) has also noted that some very wide quartz veins, often with open textures and cavities between thecrystals, are non-mineralized. The non-mineralized quartz veins are considered to belong to a separate,younger emplacement event.Quartz was mined from the Sha Lo Wan area between 6.8.55 and 6.3.59, and from 8.1.60 to 7.7.61. However,no records exist on production volumes. Similarly, mining licences for quartz were granted for the Hau HokWan area between 15.1.71 and 14.1.76 and again from 26.5.76 to 25.3.85, but there are no records onproduction volumes. The quartz at Hau Hok Wan was mined from a large (up to 30 m wide), non-mineralizedquartz vein on the western side of the bay (0857 1700). Prospecting licences for quartz were granted for theSha Lo Wan area between 17.10.67 to 16.4.68 and from 25.7.69 to 24.7.74, but no mining licences weresought.Feldspar was also prospected in the Sha Lo Wan area, mainly from sporadic pegmatites containing smallquantities of pink orthoclase. A prospecting licence for feldspar was granted, along with quartz, for the Sha LoWan area between 17.10.67 to 16.4.68, and from 25.7.69 to 24.7.74. A mining licence was granted to extractfeldspar from the Sha Lo Wan area between 8.1.60 and 7.7.61, but not records on production volumes exist.A prospecting licence granted for the Sha Lo Wan area between 25.7.69 and 24.7.74, included provision forkaolin prospecting. However, apart from minor deposits associated with weathered granitic rocks, there areno known major sources of kaolin within the district.Minor quantities of apatite, garnet, topaz, fluorite, diopside, muscovite, biotite and beryl have been reportedby Ruxton (1958) and Peng (1978) from veins adjacent to granitic intrusions in the Sha Lo Wan area. Theseoccur sporadically as rich, local concentrations of minerals, sometimes associated with skarn deposits. Ruxton(1958) has cautioned that such mineral occurrences should not be considered as representative of all quartzveins in the area.Metalliferous MineralsA variety of metalliferous minerals occurs within the district, mainly in the Sha Lo Wan area. The mineralizationoccurs as two main types: 1) High temperature (pneumatolityic) mineralization associated with interlacingquartz veins at the contacts between granite intrusions and host rocks, and 2) Moderate to low temperaturemineralization associated with eastsoutheast-westnorthwest trending quartz veins. The metalliferous mineralsinclude arsenopyrite, chalcopyrite, galena, magnetite, molybdenite, pyrite and wolframite. Minor quantitiesof pyrrhotite and scheelite have also been reported (Peng, 1978; On Yang, 1979). Ruxton (1958) describedsimilar minerals from both the high temperature and low temperature quartz veins, with the exception ofgalena which was only described from the low temperature veins. However, according to a stream sedimentgeochemical survey in the Sha Lo Wan area by Nau and Yim (1977), the sources for arsenic, tungsten, andcopper, were thought to be the contact between granite and country rocks, whereas that for iron, manganeseand zinc was considered to be farther to the southwest. The presence of a skarn deposit at San Shek Wan(Peng, 1978) immediately to the west of the district suggests a possible explanation for the source of thesemetals as well as for the variety of non-metalliferous minerals present. Sewell (1999) has reported similaranomalous concentrations of arsenic (3 - 35 ppm) and tungsten (9 - 80 ppm), as well as moderate to highlevels of antimony (3-4 ppm), bismuth (15-73 ppm), and tin (7 - 57 ppm). By contrast, concentrations ofiron (Fe 2 O 3


Prospecting licences were granted for tungsten and iron in the Sha Lo Wan area (0901 1584, 0896 1564,0885 1570) from 17.10.67 to 16.4.68 and from 25.7.69 to 24.7.74. Mining licences for the same area weregranted for wolframite and associated minerals from 8.11.54 to 7.5.58, and from 19.3.55 to 18.9.57, but norecords of production exist.Magnetite has been reported by Peng (1978) 1 km west of Tung Chung in a quartz vein in granite (09301710). A similar occurrence of magnetite has also been reported by Peng (1978) from Ngau Au (1010 1530)near Tung Chung.Construction MaterialsUrban development at Tung Chung, combined with the creation of the international airport and LantauExpressway, has involved large areas of new reclamation. Although the bulk of the material for reclamationhas come from outside the district, small borrow areas have been developed. The history of reclamation forthe international airport and the nature of the fill materials used, are described in detail in Pinches et al.(2000) and references therein.A regional seismic survey for offshore sources of sand for use in reclamation fill (Seamat Study, Cheung &Shaw, 1993) did not reveal any economic reserves of sand in the district.58


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Li, X.C., Sewell, RJ. & Fletcher, C.J.N. (2000). The Dykes of Northeast Lantern Island. Geological ReportNo. GR 6/2000, Geotechnical Engineering Office, 61 p.Nau, P.S. & Yim, W.S. (1978). Geochemical prospecting in the vicinity of Sha Lo Wan, Lantau Island.Annals of the Geographical Geological and Archaeological Society, University of Hong Kong,vol. 6, pp 26-38.Ng, S.H., Lee, CM., Lai, K.W., Ho, K.H. & Liu, C.T. (1997). Discovery of Early to Middle DevonianJurassic fossil plants at Tai O, Hong Kong. In: CM. Lee, K.W. Chan & K.H. Ho (editors)Palaeontology) and Stratigraphy of Hong Kong. Science Press, pp 163-174.On Yang, CM. (1979). Three different species of garnet in Sha Lo Wan, Lantau Island, Hong Kong. Annalsof the Geographical, Geological and Archaeological Society, University of Hong Kong, vol. 7,pp 42-^7.Peng, CJ. (1978). Hong Kong minerals. Hong Kong Government Printer, 100 p.Pinches, G., Tosen, R. & Thompson, J. (2000). The Contribution of Geology to the Engineering of HongKong International Airport. In: Page, A. & Reels, S J. (editors) The Urban Geology of HongKong. Geological Society of Hong Kong Bulletin No. 6, pp 21-42.Ruxton, B.P. (1958). The geology and ore minerals of the Sha Lo Wan area, Lan Tau Island, Hongkong. FarEastern Economic Review, vol. 25, pp 389-394.Ruxton, B.P. (1960). The Geology of Hong Kong. Quarterly Journal of the Geological Society of London,vol. 115, pp 233-260 (plus 2 plates & 1 map).Scott Wilson (H.K) Ltd. (2001). Agreement No. CE 60/96 Northshore Lantau Development - ExectiveSummary, March 2001, 16 p.Sewell, R.J. (1999). Geochemical atlas of Hong Kong. Geotechnical Engineering Office, Civil EngineeringDepartment, Hong Kong SAR Government, 110 p.Sewell, RJ. & James, J.W.C. (1995). Geology of North Lantau Island and Ma Wan. Hong Kong GeologicalSurvey Sheet Report No. 4, Geotechnical Engineering Office, Hong Kong Government, 46 p.Sewell, RJ., Campbell, S.D.G., Fletcher, C.J.N., Lai, K.W. 8i Kirk, PA. (2000). The Pre-Quaternary Geologyof Hong Kong. Geotechnical Engineering Office, Civil Engineering Department, Hong KongSAR Government, 181 p. plus 4 maps.Strange, PJ. & Shaw, R. (1986). Geology of Hong Kong Island and Kowloon. Hong Kong GeologicalSurvey Memoir No. 2. Geotechnical Control Office, Hong Kong, 134 p.Uglow, W.L. (1926). Geology? and Mineral Resources of the Colony of Hong Kong. Legislative Council ofHong Kong, Sessional Papers, No7/1926, pp 73-77.Vibro (HK) Ltd. (1996). Tung Chung Station Development. Site Investigation Works. Site III, Section II.Volume IB - Final Fieldwork Report.Williams, M.Y. (1943). The Stratigraphy and Palaeontology of Hong Kong and the New Territories.Transactions of the Royal Society of Canada, third series, vol. 37, sect. IV, pp 93-117.Williams, M.Y., Brock, R.W., Schofield, S.J. & Phemister, T.C. (1945). The Physiography and igneousgeology of Hong Kong and the New Territories. Transaction of the Royal Society of Canada,Vol. 39, pp 91-119.Woods, N.W. (1993). Engineering Geology Study of North Lantau, Tung Chung (Vols. I & 2). SpecialProjects Report No. SPR 1/93, Geotechnical Engineering Office, 95 p.61


foliation, 2:>, 26accessory minerals, 25, 32 geochemistry, 20acoustic turbidity, 45 granite, 17, 21, 28, 31, 32, 33, 34, 36, 38, 39,aerial photographs, 34 42, 48, 50, 51, 52, 53, 55, 57, 58albite 29, 31 granophyric texture, 32alkali feldspar, 25, 29, 31, 32 graphitic, 17, 23, 25, 34, 39allanite, 29, 31, 32 gravity survey, 53alluvium, 18, 40, 41, 42, 43, 45alteration, 26, 32, 36, 39, 47 Hang Hau Formation, 18, 21, 40, 41, 44, 45, 46,amphibole, 25, 29, 31 47andalusite, 23 Holocene, 9, 18, 40, 41, 42, 45apatite, 25, 29, 31, 57 hydrothermal alteration, 26, 36, 39, 47aplite, 17, 28, 32intertidal deposits, 18,42basaltic andesite, 32, 33 intrusive igneous rocks, 28beach sand, 42 isopachs, 46biotite, 25, 29, 31, 32, 33, 39, 57bipyramidal, 32 joints, 34, 36, 38boreholes, 14, 17, 21, 23, 24, 25, 34, 39, 42,43, 44, 45, 48, 50, 51, 55 kaolin, 41, 57British Geological Survey, 12, 14, 48, 60Lantau Dyke Swarm, 17, 28, 32, 34caldera, 17, 21, 34, 36, 39 Lantau Granite, 17, 24, 28, 29, 30, 32Carboniferous, 17, 18, 23, 25, 34, 36, 48 Lantau Island, 9, 11, 12, 15, 16, 17, 23, 25, 26,channel, 36, 43, 44, 47 28, 29, 30, 33, 34, 35, 37, 41, 42, 44,Chek Lap Kok Formation, 21, 40, 41, 42, 43, 44, 45, 47, 48, 55, 60, 6145 5 52 Lantau Volcanic Group, 17, 18, 25, 26, 30, 31,Chek Lap Kok Granite, 17, 18, 28, 29, 30, 32 33, 34, 39, 56chemical analyses, 17 lapilli, 17, 25chlorite, 31 lineaments, 34classification, 9, 28, 50 Lok Ma Chau Formation, 17, 18, 23, 24, 34, 39colluvium, 9, 18,40, 41, 43, 44, 52conglomerate, 23, 42 mafic dykes, 28, 31, 33corestones, 27 magma, 26, 32, 50crystallisation, 26, 39 magnetite, 21, 24, 25, 33, 57, 58Mai Po Member, 23debris flow, 21, 40, 41, 43 marble, 12, 14, 17, 18, 21, 23, 24, 36, 39, 43,dykes, 17, 28, 31, 32, 33, 34, 48, 50 48, 50, 51, 52, 53, 55marine deposits, 21, 41faults, 17, 21, 34, 36, 47, 52 marine mud, 18, 21, 45feldsparphyric rhyolite, 17, 28, 31, 32, 36, 38, marine sand, 18, 21, 3642, 48, 50, 52 mass wasting, 40fine- to medium-grained (granite), 17,28,29, 32,42 megacrystic, 28, 29fine-grained granite, 28, 31,32,42 Mesozoic, 9, 18, 17, 25, 42, 43, 59, 60flow banding, 32 metalliferous, 6, 5762


microgranite, 17, 28, 31, 32, 48 Tung Chung, 9, 11, 12, 14, 15, 17, 21, 24, 30,mineralisation, 33 32, 34, 35, 36, 37, 39, 40, 41, 42, 43,mining, 57 44, 45, 47, 48, 49, 50, 51, 52, 53, 55,mud, 9, 21, 40, 41, 45 56, 57, 58, 59, 60, 61muscovite, 29, 32, 57 Tung Chung Formation, 18, 40, 41, 42, 43, 44,52,55Northshore Lantau Development Feasibility Study, volcanic rocks, 17, 21, 25, 26, 31, 33, 34, 36,12, 43, 59 38, 39, 47weathering, 14, 21, 24, 32, 34, 36, 38, 40, 41,oligoclase, 29, 32 47, 48, 50, 51, 55photolineaments, 34, 36 YamO, 9, 12, 21, 25, 41, 43, 44, 47plagioclase, 25, 29, 31, 32, 33 Yim Tin Tsai Formation, 17, 18, 25Pleistocene, 9, 18, 40, 41, 42, 43, 45, 51, 52, 60 Yuen Long Formation, 17, 23, 34, 39, 43pluton, 29, 34zircon, 25, 28, 29, 31, 32quartz veins, 21, 28, 33, 57 zoned, 29, 31, 32quartzphyric rhyolite, 17, 28, 31, 32, 48Quaternary, 9, 18, 40, 43, 45, 59, 60, 61reclamation, 11, 12, 14, 21, 36, 39, 41, 42, 43,45, 47, 48, 50, 52, 55, 58reflector, 45rhyodacite, 31rhyolite, 17, 25, 26, 28, 31, 32, 33, 34, 36, 38,42, 48, 50, 52San Tin Group, 17, 18, 23sandstone, 17, 18,23, 25, 26, 27, 50sea level, 41, 47seismic, 14, 40, 42, 43, 44, 45, 55, 58Sham Wat Formation, 18,21, 40, 41, 45shell fragments, 42siltstone, 14, 17, 18,23, 25, 26, 27, 50slope deposits, 41Tai O Formation, 17, 18, 23, 25tectonic, 59, 60The Brothers Islands, 17, 23, 34, 36, 39, 44, 45,47, 48thin sections, 14Tolo Harbour Formation, 17, 18, 24, 39Tong Fuk Quartz Monzonite, 30, 31, 34Tsuen Wan Volcanic Group, 17, 18, 25TszKanChau, 23, 24tuff, 17, 18,25, 26, 27tuffite, 17, 18,26, 2763


AppendixLEXICON OF TERMS USED FOR DRILLCORE LOGGING AND SURFACEMAPPING AT TUNG CHUNG, LANTAU ISLAND, HONG KONG(Reproduced, with minor amendments, from a report prepared for theGEO, CED, by M. R. Gillespie, A. J. Humpage and R. A. Ellison, November 1998)IntroductionThis document outlines the geological terms recommended for use during drillcore logging and surfacemapping of rocks and superficial deposits at Tung Chung New Town, Lantau Island, Hong Kong. The termsand their definitions are based largely on those described in Hong Kong Geological Survey Memoir No. 6(Geology of Lantau District; Langford et al 1995) and in Geoguide 3, the 'Guide to Rock and SoilDescriptions' produced by the Geotechnical Engineering Office, Civil Engineering Department, Hong Kong.Descriptions of all the key lithologies and geological features encountered in the Tung Chung drillcores arepresented. These are based primarily on visual examination of drillcore in a wettened state, using a handlens where appropriate. Useful additional information is provided by the acid test (using 10% HC1 to detectcalcite), and a hardness (scratch) test using a steel knife.A list of key diagnostic features and, where appropriate, one or more photographs accompany the briefdescription provided for each recommended term. Significant differences in the character of lithologiesbetween the main sites (Sites 3,4 and 5) investigated within the Tung Chung New Town Area (Figure 1) arenoted. Grain size terms are defined in Table 5 of Geoguide 3.One or more reference samples of most of the Tung Chung lithologies has been collected and is stored by theHong Kong Geological Survey, Geotechnical Engineering Office, Civil Engineering Department, HongKong. These are mainly hand samples of drillcore, each of which has been assigned an identificationnumber (e.g. Reference sample no. 1). However, Mazier samples, whole drillcore boxes and several completedrillcores also form part of the reference sample collection. The reference sample numbers, or the boreholename and depth of the other types of reference samples, are referred to at appropriate points in the text.In photographs showing drillcore in boxes, or parts of boxes, the shallowest part of the core is at top left, thedeepest part at bottom right.LexiconFillDescriptionMan-made deposit composed mainly of sand, gravel and shell fragments, with occasional boulders, cobbles,wood and synthetic materials. Table 18 in Geoguide 3 summarises some of the features associated with fill.Key diagnostic features :(i) Mainly sand, gravel and shell fragments.Occasional boulders, cobbles, wood and synthetic materials.(ii) Unconsolidated.(iii) No structure.(iv) Usually light brown to grey.Reference sample no.: Fill is represented in all the whole reference drillcores.64


Marine sedimentDescriptionUnconsolidated mud, sand, silt, shell fragments. The marine sediments at Tung Chung belong to the HangHau Formation, details of which are summarised in Section A.6.4 and Plate 11C of Geoguide 3.Key diagnostic features :(i) Composed dominantly of mid- to dark grey, clayey/silty mud, often with shell fragments.(ii) The mud is soft and plastic, though it may dry out and become hard in storage.CommentsLight brown to grey sand may form part of the marine sediment locally and can be difficult to distinguishfrom fill, where the two are juxtaposed.Reference sample no.:Reference drillcore B-19, 20.5 - 27.7 mPhotograph:See Geoguide 3: Plates 9A & 11CAlluviumDescriptionUnconsolidated boulders, cobbles, gravel, sand and silt. Features characteristic of alluvium in Hong Kongare summarised in Section A.6.3 of Geoguide 3.Key diagnostic features :(i) Composed of unconsolidated boulders, cobbles, gravel, sand and silt.(ii) Clast surfaces are typically rounded.(iii) Clasts are generally not cemented.(iv) There is usually a variety of clast lithologies.CommentsAlluvium at Tung Chung belongs to the Chek Lap Kok (?and pre-Chek Lap Kok) Formation. Alluviumsequences in Tung Chung drillcores are typically dominated by boulders and cobbles, any finer matrixhaving been lost during drilling. Intervals up to several metres thick of dark grey to pale green mud and sandoccur locally in the alluvium of Site 3 and Site 4.Reference sample no.: Reference drillcores B-5 (21-27 m) and CB-46A (21-42 m)Plate A1 - Typical appearance of alluvium in Tung Chung drillcores. The five jar samples contain pale greenmud. Tung Chung, Site 3; Drillcore CB-49C; Box 1,0.0- 39.0 mSee also : Plate 265


Plate A2 - 30.30 to about 30.90 is typical alluvium: 30.90 to 33.50 is typical colluvium: the colluvium sitsdirectly on siltstone (see Section 2.6), which fills all the jar samples representing the cored interval 33.50to 66.0 m. Tung Chung, Site 4; Drillcore B-19; Box 2, 30.30 - 66.0 m. See also: Geoguide 3, Plate HEColluviumDescriptionA deposit of boulders, cobbles, gravel, sand and silt. Features characteristic of colluvium in Hong Kong aresummarised in Section A.6.2, Table 18 and Plate HE of Geoguide 3.Key diagnostic features :(i) Composed of boulders, cobbles, gravel, sand and silt.(ii) Clast surfaces are typically angular.(iii) Clasts are set in a silty sand matrix, at least part of which generally survives coring.(iv) A variety of clast lithologies usually distinguishes colluvium from a fault breccia.CommentsMost of the colluvium in Tung Chung drillcores belongs to the Chek Lap Kok (?and pre-Chek Lap Kok)Formation and occurs immediately beneath, or wholly within, alluvium. Colluvium often forms the basalpart of the superficial deposits. *Reference sample no.: Reference drillcores C-42 (36 - 55 m) and B-19 (31 - 33.5 m)Fine alluvial sedimentDescriptionTypically orangish to dark brown, unconsolidated, soft sediment of mainly sand-silt grade, with occasionalgravelly beds and little or no discernible structure.Key diagnostic features :(i) Orangish to dark brown colour.(ii) Generally fine grain size.(iii) Unconsolidated (though it may become hard and brittle after drying-out).(iv) Typically found immediately beneath alluvium/colluvium.(v) Contains very rare, small shell fragments.(vi) Sand to gravel sized fragments of a range of rock types typically occur in the fine alluvial sediment(see Plate 3), and serve to distinguish it from residual soil derived from igneous rock (see Section2.24).CommentsPrincipal occurrence of fine alluvial sediment at Tung Chung is in Site 3, where up to seventy metres hasbeen cored locally. Recovered material is almost entirely in jar and Mazier samples.Reference sample no.:Reference drillcores CC9 (40.5 - 72.5; 83 - 111 m) and CB46A (42 - 97 m);and all Mazier samples in these intervals.66


Plate Ala- Marble. The generally dark colour is typical of marble from this part of Site 4. See also Platelib. Tung Chung, Site 4; Drillcore C-42; Box 17, 145.9 mPlate A7b - Marble from Site 5, showing the typical pale colour and banded appearance. Tung Chung,Site 5; Drillcore E-74; Box 11, 61.65 mPlate A7c - Marble from Site 4. Note the textural heterogeneity and blue-grey to cream colour. Localiseddissolution along calcite veinlets has developed into a small cavity at about 52.6 m. Tung Chung, Site 4;Drillcore A-19A; Box 2, 53.08 mCalc-silicate rockDescriptionUsually pale-coloured, often texturally heterogeneous rocks characterised by the presence of calcium-silicateminerals. The most diagnostic and most readily identifiable calcium-silicate minerals in Tung Chung drillcoresare grossular (Ca-rich garnet, which typically forms round pink or purple crystals) and diopside (Ca-Mg-Fepyroxene, which typically forms shapeless masses of dark green crystals).Key diagnostic features :(i) Presence of calcium-silicate minerals, in particular grossular and/or diopside; theseminerals typically appear as irregular masses of pink or green crystals, respectively,(ii) Virtually always associated with other types of metasedimentary rocks.CommentsCalc-silicate rocks are metamorphosed shaly or quartz-bearing carbonate rocks or relatively pure carbonaterocks that have been metasomatized by siliceous solutions from contiguous granitic intrusions. They are acommon component in skarn sequences (see Section 2.14).Reference sample no.: Box 3 (117.5 - 121.5 m) from borehole BB-24; Reference drillcore C-6 (105 -110 m).71


MetamudstoneDescriptionMetamorphosed sedimentary rock composed dominantly of micas, though some quartz and/or feldspargrains may be present.Key diagnostic features :(i) Dark grey or dark green-grey, fine-grained rock.(ii) Often has discernible banding, although often poorly preserved.(iii) Often associated with other metasedimentary rocks.CommentsCare must be taken in distinguishing metamudstone from dark, fine-grained igneous rocks (see Sections 2.22 and 2.23). If the diagnostic features listed above are insufficient, the rock should be examined closely forphenocrysts (which may be present in the igneous rocks), and the hardness test should be applied(metamudstone will generally scratch more easily than igneous rock).Reference sample no.: No sample.Photograph:See Plate 9 and Geoguide 3, Plate Al [WJMagnetite-rich rockDescriptionRock rich in magnetite.Key diagnostic features :(i) Generally black and dense (heavy) relative to silicate and carbonate rocks.(ii) Usually occurs as thin bands in skarn sequences 9see Section 2.14).(iii) Likely to be magnetic.CommentsVery rare in Tung Chung drillcores; possibly confined to parts of Site 5.Reference sample no.: 10SkarnDescriptionThe term 'skarn' is generally used for rock composed of Ca-, Mg- and Fe-silicate minerals that has beenderived from nearly pure limestone or dolomite into which large amounts of Si, Al, Fe and Mg have beenintroduced by the activity of fluids because of proximity to an igneous intrusion. At Tung Chung the termis used in a broader context, to describe complex sequences of differing types of thermally metamorphosedrocks (calc-silicate, metamudstone etc.) The term 'skarn' can also be used where the nature of the originalmetasedimentary rock cannot be determined due, for example, to overprinting (see Plate 10a).Key diagnostic features :(i) Sequence of lithologically variable metasedimentary rocks with mineralogicaland/or textural features indicative of thermal metamorphism.(ii) Usually banded and/or texturally heterogeneous. Banding can be highly convoluted,(iii) Often associated with other metasedimentary rocks, such as quartzite and marble.CommentsIrregular masses and veins of purple fluorite are associated occasionally with skarn.Reference sample no.:10, 24, 33, 34; also Reference drillcores D-34 (118 - 120.5 m) and BB-24(117.5 - 121.5 m)73


Plate A9 - Skarn from Site 3 showing typical textural and mineralogical heterogeneity. Note the purplefluorite. The dark green rock at bottom left is Metamudstone with faint banding. Most of the remainingskarn assemblage is calc-silicate rock. Tung Chung, Site 3; Drillcore CC-20; Box 7, 168 mPlate A 10a - Skarn. A pale green mineral or mineral assemblage (probably mainly epidote) has overprinteddark green ?metamudstone during thermal metamorphism. The white mineral is quartz, filling a sub-verticalvein. Tung Chung, Site 3; Drillcore BB-24; Box 4, 122 m74


Plate A 10b - Typical skarnfrom Site 3. Note the textural heterogeneity and banding. Tung Chung, Site 3;Drillcore CC-15; Box 7, 138.20 - 143.96 mCavityDescriptionA void in the rock mass.Key diagnostic features :(i) In drillcores, cavities may be recognised by intervals of no recovery, usually in association withmetasedimentary rock.(ii) Cavities may be bounded by rock with a weathered or partly dissolved appearance,(iii) Drillers logs may record a drop of the coring tool when a cavity is encountered.Reference sample no. : No sampleCavity fill depositDescriptionA sediment of rock fragments, sand, silt, Fe, Mn and clay minerals which partly or wholly fills a cavity.Cavity-fill deposits range from unconsolidated to consolidated/lithified.Key diagnostic features :(i) A sediment of unsorted rock fragments, sand, silt, Fe and/or Mn minerals and clay.(ii) May be several tens of centimetres to several tens of metres thick.(iii) Bedding or lamination may be discernible.CommentsThe nature of cavity-fill deposits is highly variable. In Sites 3 and 5, cavity-fill deposits are up to 2-3 metresthick and consist of sand to silt grade, orange to dark brown, unconsolidated sediment, with or withoutpebbles. Cavity-fill deposits in Site 4 are commonly much thicker (up to about 40 metres) and consist ofsemi-lithified 'breccias' of coarse to fine, angular rock fragments set in a dark grey, silty matrix, with localreplacement and veining by gypsum.The considerable thickness of many cavity-fill deposits, the lack of polished surfaces or slickensides, andoccasional lamination in the matrix is generally sufficient to distinguish them from fault breccia.Reference sample no. :18; also in Reference drillcores D-28 (72 - 88 m), D-34 (105 -118 m),C-6 (discontinuously in the interval 95 - 160 m) and C-26(discontinuously in the interval 115 - 134 m)75


1 8Plate Alia - Cavity-fill deposit. This sample is typical of the thick intervals of consolidated cavity-filldeposit encountered in parts of Site 4. Angular fragments of various lithologies are set in a silty matrix.Following formation of the deposit, some of the calcareous rock fragments have partly dissolved, creatingpits and voids in the rock, and the deposit has been veined by white gypsum. Tung Chung, Site 4; DrillcoreC-13;Box6, 139.65 mPlate Allb - Cavity-fill deposit and marble. Dark blue-grey marble with white calcite veins separated byintervals of weakly consolidated cavity-fill deposit. In this case the cavity-fill deposit is dark grey andcontains assorted angular rock fragments set in a grey, silty matrix. The marble may be in situ, or it mayrepresent large detached blocks in the cavity-fill. Tung Chung, Site 4; Drillcore C-20; Box 5, 89.3 - 94.0 m76


GraniteDescriptionLight coloured, acid igneous rock composed principally of alkali-feldspar, quartz and biotite, with someplagioclase. Individual crystals in the groundmass are greater than 2 mm diameter on average. May containphenocrysts of feldspar (and quartz) in various proportions and up to about 25 mm diameter.Key diagnostic features :(i) Individual crystals are generally greater than 2 mm diameter.(ii) The mineral assemblage is dominated by quartz, alkali-feldspar and biotite, with some plagioclase.(iii) Generally massive (though locally sheared).CommentsThe granite is locally texturally heterogeneous, with abrupt variations in grain size and mineralogy. Veins ofvery coarse granite (pegmatite) and very fine granite (aplite) are observed occasionally. The freshest granitein Tung Chung drillcores has grey quartz, white feldspar and black biotite (Plate 12a). However, the biotitehas commonly altered to chlorite, giving the rock a greenish appearance. Pervasively weathered granite isstained brown (Plate 12b).Mineral and textural modifications of the granite are commonly observed in areas where it occurs close tometasedimentary rock. For example, in boreholes D-35, D-36 and D-37 in Site 5, the granite around thinintervals of metasedimentary rock has an intensely altered, 'bleached' appearance, contains small voidslined by euhedral crystals, and is mineralised by pyrite and galena. Much of the granite encountered in Site3 drillcores has a similarly 'modified' appearance, again generally in close association with metasedimentaryrock. Because of the close association with metasedimentary rocks, the modified granite has been termed'contact fades' granite (Plates 13a and 13b). It is generally pink, often contains small, primary cavities, andis rich in quartz, which occurs in the groundmass and also as discrete 'blobs' and veins.Reference sample no. : 5, 6, 25; also in Reference drillcore D-34 (120.5 - 139 m).1 8Plate A 12a - Typical unweathered granite from Site 5.Tung Chung, Site 5; Drillcore E-28; Box 3, 22.8 m11


Plate A12b - Pervasively weathered granite from Site 5. Tung Chung, Site 5; Drillcore E-28; Box 2, 17.3 mPlate A 13a - Typical contact fades granite from Site 3. Tung Chung, Site 3; Drillcore CC-14; Box 11, 137.81-141.63 m78


Plate A 13b - Contact fades granite and skarn. This is typical of granite-metasedimentary rock contacts inSite 3 drillcores : the granite, whose textural and mineralogical characteristics are clearly modified withrespect to 'normal' granite encountered in Tung Chung drillcores, has a sharp, unfaulted contact (at the leftside of the fourth drillcore stick in this view) with skarn. Tung Chung, Site 3; Drillcore CC-20; Box 7, 163.22-168.71mFeldsparphyric micrograniteDescriptionLight coloured, acid igneous rock which is compositionally identical to granite, but in which individualcrystals in the groundmass are less than 2 mm diameter on average, though still visible to the unaided eye;it is essentially a finer-grained version of granite. Contains phenocrysts of feldspar (±quartz) in variousproportions and up to about 25 mm diameter.Key diagnostic features :(i) Individual crystals are less than 2 mm diameter on average, but still visible to the unaided eye.(ii) The mineral assemblage is dominated by quartz, alkali-feldspar and biotite, with some plagioclase.CommentsIn some Tung Chung drillcores feldsparphyric microgranite grades into feldsparphyric rhyolite.Reference sample no. : 31 7 1 8 1 9Plate Al4 - Typical feldsparphyric m icrogran ite from Site 5.Tung Chung, Site 5; Drillcore G-12; Box 3, 35.0 m79


RhyoliteDescriptionAcid igneous rock which is compositionally identical to granite but in which individual crystals in thegroundmass are too fine grained to be distinguished by the unaided eye.Key diagnostic features :(i) Crystals in the groundmass are too fine grained to be distinguished by the unaided eye.(ii) No (or very few) phenocrysts.(iii) siliceous, does not scratch with a steel knife.(iv) Flow banding may be discernible.(v) Fresh rock is dark grey to black. Alteration changes the colour to grey, purple or pink.CommentsRhyolite containing no phenocrysts is relatively rare in the Tung Chung drillcores. It is observed mostcommonly along the 'chilled' margins of feldsparphyric rhyolite bodies.Reference sample no. : No sample.Coarsely feldsparphyric rhyoliteDescriptionRhyolite (see Section 2.19) with abundant phenocrysts of feldspar, mostly 5-25 mm.Key diagnostic features:(i) As rhyolite, but with abundant phenocrysts of feldspar (iquartz), mostly 5-25 mm.CommentsFresh rock has a distinctive appearance : white and grey phenocrysts in a dark grey groundmass (Plate 15a).With slight alteration, the phenocrysts become pinkish and the groundmass alters to greyish-green, or greyishpurple (Plates 15b and 15c). Pervasive weathering turns the groundmass to orange-brown (Plate 16a).Intensely altered coarsely feldsparphyric rhyolite containing pyrite and galena (Plate 16b) occurs in andnear to borehole A-18 in Site 4. A texturally modified variant of coarsely feldsparphyric rhyolite, in which'phenocrysts' of feldspar and quartz have irregular, curved outlines, occurs rarely in Tung Chung drillcores.Reference sample no. : 14, 7, 2, 13, 1980


Plate A 15a - Typical fresh coarsely feldsparphyric rhyolite. Tung Chung, Site 5; Drillcore D-23; Box 7,57.3 m% 3t a 19 2O i? 1P/ate A15b - Typical altered coarsely feldsparphyric rhyolite. Tung Chung, Site 5; Drillcore E-84; Box 3,48.05 mPtoe A15c - Typical altered coarsely feldsparphyric rhyolite. Tung Chung, Site 5; Drillcore E-44; Box 3,33.55 m81


Plate A 16a - Pervasively weathered coarsely feldsparphyric rhyolite. Tung Chung, Site 5; Drillcore D-31;Box 5, 66.5miXX 1 3 2 0 2 1 2 2Plate A 16b - Strongly altered and mineralised coarsely feldsparphyric rhyolite. Tung Chung, Site 4; DrillcoreA-18; Box 3, 39.93 m2 aPlate A 16c - Texturally modified coarsely feldsparphyric rhyolite. Tung Chung, Site 5; Drillcore G-23; Box3,58.7 m82


Finely feldsparphyric rhyoliteDescriptionRhyolite (see Section 2.19) with abundant small (mostly


1 f < 1 9 2 OPlate A 18b - A Itered finely feldsparphyric rhyolite. Tung Chung, Site 5; Drillcore D-23; Box 9, 63.3 mPlate A 19a - A Itered finely feldsparphyric rhyolite with epidotisation (green alteration) around fractures.Tung Chung, Site 3; Drillcore CB-42; Box 2, 60.0 m16 1 ? 18 19 2 O 2 1Plate A 19b - Pervasively weathered finely feldsparphyric rhyolite. Tung Chung, Site 5; Drillcore D-8; Box2,29.6 m84


Basaltic rockDescriptionDark grey, very fine grained, massive, intrusive igneous rock.Key diagnostic features :(i) Dark grey.(ii) Very fine grained (individual crystals cannot be discerned with the unaided eye).(iii) Massive (i.e. no banding).(iv) Sharp, cross-cutting (i.e. intrusive) contacts.(v) Typically fractures into many angular fragments.CommentsOften occurs at the boundary between two other rock units (e.g. coarsely feldsparphyric rhyolite andmetasedimentary rock). Also found occasionally as rounded 'blobs' up to about 50 centimetres diameter infeldsparphyric rhyolite.Reference sample no.: 371 4Plate A20 - Basaltic rock. Tung Chung, Site 5; Drillcore D-16; Box 6,29.6 m. See also: Geoguide 3, Plate A1[Q]Lamprophyric rockDescriptionGenerally dark, very fine grained igneous rock characterised by a high proportion of mafic minerals whichoften form phenocrysts and/or 'clots' of crystals.Key diagnostic features :(i) Generally dark grey.(ii) Fine to very fine grained.(iii) Massive (i.e. no banding).(iv) Intrusive contacts.(v) Small phenocrysts and/or 'clots' of mafic minerals are discernible occasionally.Reference sample no.: 2185


2 1 2 2Plate A21 - Lamprophyric rock. Tung Chung, Site 4; Drillcore A-44; Box 6, 90.88 m. See also : Geoguide 3,Plate AlfRJResidual soil derived from igneous rockDescriptionMaterial derived from an igneous rock (principally granite and feldsparphyric rhyolite) which has undergonechemical weathering to a point where the original rock texture cannot be discerned. Residual soil correspondsto rock weathering grade VI (see Table 4, Table 10 and Plate 3 in Geogudie 3).Key diagnostic features :(i) Original rock texture cannot be discerned.(ii) Consists of variable proportions of soft to firm clay and oxide minerals, usually with quartz crystals.(iii) Generally red, pink or orange, and often characterised by a range of coloured bands (Plate 23).(iv) Contains no 'exotic' clasts or fragments (which distinguishes it from fine alluvial sediment (seeSection 2.5).CommentsQuartz is generally the only primary mineral to survive intensive chemical weathering in igneous rocks. Thenature of the parent rock may be deduced by examining quartz crystals in the residual soil. In residual soilderived from granite, quartz crystals often have irregular form, whereas in residual soil derived from rhyolitethe quartz crystals are usually subhedral, typically with sub-spherical form. The size and abundance ofquartz crystals in residual soil may provide further clues as to the nature of the parent rock. For example, ifthe quartz crystals are relatively coarse and sub-spherical the parent rock was probably coarsely feldsparphyricrhyolite; if they are relatively fine and sub-spherical the parent rock was probably finely feldsparphyricrhyolite.Residual soil derived from igneous rock may contain discrete, boulder-like areas of relatively unweatheredrock, with weathered margins; these are 'corestones'. In drillcores, corestones appear as intervals of relativelyfresh rock of the host lithology above and below which is residual soil and/or pervasively weathered rock.Reference sample no.: Mazier samples: 64 - 65 m in borehole A-48; 76 -77 m in borehole A-8B; 41 -42 m in borehole CC9. Most of the Reference drillcores contain intervals ofresidual soil derived from igneous rock.86


Plate A22a - Residual soil derived from finely feldsparphyric rhyolite. Relict feldspar and fresh quartzphenocrysts can be discerned in the reddish clayey matric. The black and orange colouration is secondaryMn and Fe oxide minerals which developed originally in and around hairline fractures. Way-up is to theright. Tung Chung, Site 4; Drillcore A-48, Mazier sample, 64 - 65 mPlate A22b - Residual soil derived from (?) coarsely feldsparphyric rhyolite. Relict feldspar (white areas)and fresh quartz phenocrysts can be discerned in the orange-brown clayey matric. Way-up to the right. TungChung, Site 4; Drillcore A-8B, Mazier sample, 76 - 77 m87


Residual soil derived from metasedimentary rockDescriptionMaterial derived from metasedimentary rock which has undergone chemical weathering to a point wherethe original rock texture cannot be discerned. Residual soil corresponds to rock weathering grade VI (seeTable 4, Table 10 and Plate 3 in Geoguide 3).Key diagnostic features :(i) Usually dominated by dark brown, grey, purple or green clayey and oxide-rich material.(ii) The clayey material described in (i) often contains angular fragments of siliceous metasedimentaryrock, ranging from fine sand to cobble size. There are usually no discrete quartz crystals as inresidual soil derived from igneous rocks,(iii)The characteristics, in particular the colour, of residual soil derived from metasedimentary rocksusually vary considerably over short depth intervals, reflecting the variability in character of theparent rocks as well as secondary chemical processes.CommentsResidual soil derived from metasedimentary rock can have characteristics that are very similar to those ofunconsolidated cavity-fill deposits. Careful examination of Mazier samples may help to distinguish betweenthem.Reference sample no.:Reference drillcores C-42 (55 - 94.5 m) and CC-9 (118 - 130 m).FaultDescriptionA discontinuity in the rock containing fault breccia. Definition and description of faults and related featuresare given in Geoguide 3, Section A7.2.Key diagnostic features :(i) Presence of fault breccia.(ii) Fault breccias are unlikely to be wider than 1 metre, and are usually considerably less.CommentsHealed faults are those in which a (usually siliceous) cement has sealed the discontinuity and is contiguouswith the wallrock minerals. A healed fault is no longer a physical discontinuity, and therefore may not besignificantly weaker than the enclosing rock.Reference sample no.: Reference drillcore E-96 (80 - 92 m)Plate A23 - Healed fault and shearing (see Section 2.28) in contact fades granite. Tung Chung, Site 3;Drillcore CC-23; Box 4, 66 m


Vein brecciaDescriptionA brecciated rock in which the fragments form a 'jigsaw' texture which is cemented by carbonate or silicateminerals.Key diagnostic features :(i) Interval of rock with detached, angular wallrock fragments cemented by authigenic minerals.(ii) Usually less than 5 centimetres wide.(iii) The absence of fine or crushed wallrock fragments distinguishes vein breccia from fault breccia.CommentsSome vein breccias (and fault breccias) in Tung Chung drillcores are cemented by calcite. Where theseoccur below the weathered zone the breccia is intact. Where they occur above the base of the weatheredzone the calcite has dissolved partly or completely, leaving a 'collapsed' breccia characterised by an intervalof disaggregated drillcore. Such intervals may be indistinguishable from those created in highly fracturedand/or weakened rock by coring disturbance.Cf 13 142 2Plate A24 - Calcite-cemented vein-breccia in coarsely feldsparphyric rhyolite. Tung Chung, Site 5; DrillcoreE-72; Box 8, 123.3mChlorite seamsDescriptionGenerally thin (

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