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The district is situatedin a complexzone of intrusionson the boundarybetweentwo contrastingstructural domains. The rocks are composeddominantlyof Mesozoicgranitesand volcanics(Table 2; Figure 5) which are intrudedby a stronglyENE-WSWorientedporphyritic rhyolite dyke swarm. Palaeozoicsedimentaryrocks of the San Tin Groupare the oldestrocks in the district and are only exposed on Tsz Kan Chau in the westernextremity of the district. Theserocks are thermally metamorphosedand belong to the Lok Ma ChauFormationof Carboniferousage. They are thoughtto occupya northeasterly-curvingbut irregular belt up to 4 km wide extendingfrom ChekLap Kok in the southwestto EastBrother in the northeast. Coarse-ashcrystaltuffs of the Vim Tin Tsai Formation(TsuenWan Volcanic Group)are stratigraphically the oldestvolcanic rocks in the district althoughtheyhave not beendated. Theserocks are unconformably overlain by lapilli-bearing coarse-to fine-ashcrystaltuff of the Shing Mun Formation. The internally massiveand structurelesscharacterof the volcanic depositssuggestthat they are the product of large, relatively homogenous,ash-floweruptions. Weldedlapilli- to block-bearingcrystaltuff, tuff brecciaandtuffite of the LantauFonnation(RepulseBay Volcanic Group) is exposedin the extremesouthwestern part of the district. Theserocks are thoughtto occupy an ancientcaldera fonning the centralpart of LantauIsland. They are assignedto the Repulse Bay Volcanic Groupbasedon whole rock geochemistryand mineralogicalcharacteristics. The volcanic rocks of the district are intruded by granite, granodiorite,porphyritic rhyolite and mafic dykes. The granodioriteis the oldestof the majorintrusionsand subcropsin boreholesin the northeastern part ofMa Wan. Graniteforms two majorplutonic bodiesin the southernand easternparts of north Lantau Island. It is mostly medium-grainedwith a pronouncedporphyritic texture apparentin granite outcrops in the south. Eastnortheast-trending porphyritic micrograniteand feldsparphyricrhyolite dykesare the dominantminor intrusiverocks in the district. They areintruded by quartzphyricrhyolite and mafic dykes in the central and northernparts of the district. Mafic dykes up to 5 m wide are generallythe youngestintrusiverocks intruding all lithologies exceptfor somequartzphyricrhyolite dykes. The geologicalstructureof the district is complexowing to the close proximity of intersectingregional fault trends. The dominant fault trend is to the northwestalthoughthese faults have been offset by a younger set of north-trendingfaults in the centralregion. Quartz veins are commonlyassociatedwith thesefaults and maybe up to 5 m thick. Thereis no evidencefor recentmovementalong any of the faults within the district. Superficialdepositswithin the district covermost of the low-lying groundand large tractsoffshore (Figure 6). The onshoredepositscomprisedominantlydebris flow depositson the highergroundand alluvial sedimentson the valley floors. In the largervalleys,thesehavebeendisturbedby cultivation. The offshore superficialdepositsconsistmostly of marinesandof the Chek Lap Kok Formationand marine mud of the Hang Hau Formation. A smallarea of Pre-ChekLap Kok Formationdepositshas been identified in the district. 16 Table 2 -Summary of the Onshore and Offshore Stratigraphy of the District Stratigraphic Divisions Principal Materials Map Sym Natural earthandwaste QhF Mud andsand Sand;somegravelandmud Qhie Qhbs Silt, sand,gravel Sand,gravel,cobbles QhmM QhmS Lithostratigraphy and Genetic Classification Superficial Deposits Fill Estuarinedeposits Beachsand Holocene QUATERNARY Holocene Hang Hau and Formation Marinemud Marine sand Pleistocene silt, sandand gravel .Clay, Debris AlluvIum flow deposits Sand Pleistocene ChekLap Alluvium, debris flow Kok deposits,estuarineand Formation marinesediments " gravel cobbles Clay, silt, sand,and gravel Sand,gravel,cobbles Q' QDf Qp' QpDf Qpie Qpms Volcanic and SedimentaryRocks Repulse Bay Volcanic Group MESOZOIC LantauFormation Rhyolite lava andtuff JLT Jurassic- Cretaceous Tsuen ShingMun Formation Lapilli coarse-ash crystaltuff JSM Wan Volcanic Group Vim Tin Tsai Formation PALAEOZOIC Carboniferous Coarse-ash crystaltuff JYT SanTin Lok Ma ChauFormation Metasiltstone,metasandstone; Group Mai Po Member graphite-bearing Cmp Major Intrusive Rocks MESOZOIC JurassicCretaceous Granite Granodiorite gf,gm gdf Minor Intrusive Rocks TERTIARY MESOZOIC JurassicCretaceous Basaltandlamprophyre b Feldsparphyricandquartzphyricrhyolite Porphyriticmicrograniteandfme-grainedgranite rf,rq ~g 17
