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Pumice, glassCrystals,crystalfragmentsRockfragmentsFigure 7a - Classification ofPyroclastic Rocks Based on Composition (after Le Maitre, 1989)Blocks and bombsOver 60mmPYROCLASTIC/ BRECCIA, \/AGGLOMERATEX25,75TUFF-BRECCIA,TUFF-AGGLOMERATE75,25Lapilli2-60 mmLAPILLI \ ASH-LAPILLI TUFFTUFF25755050UPILLI-ASHTUFF7525ASH TUFFAshUnder 2 mm(coarst 0.06-2,fine under 0.06)Figure 7b-Classification of Pyroclastic Rocks Based on Grain Size (Adapted from Schmid 1981 andFischer & Schminke, 1984)38
are crudely stratified and welded in places, the latter fabrics being characterized by alignedcrystal pyroclasts and compressed lithic clasts.Crystals are commonly broken, locally flow-orientated, and mainly comprise plagioclase andmicrocline feldspars and quartz, with generally minor amounts of magmatic hornblende (variablypseudomorphed) and biotite. The matrix, which makes up nearly half of the rock, is a devitrifiedglass, and varies from microcrystalline quartz, feldspar, chlorite and sericite, to acryptocrystalline texture. On Ma Wan, thin tuffite bands also occur. The formation is, only seenin faulted contact with older rocks, but is gradationally overlain by the Shing Mun Formation.DetailsTsing Yi and Ma Wan. The Yim Tin Tsai Formation is the principal volcanic formation on Ma Wan and TsingYi. It is uniform throughout, and is well exposed along the shorelines and in cut-slopes. The rock compriseslapilli-coarse ash lithic crystal tuff, and varies from light to dark grey, depending on the mafic mineral content.Feldspars are the dominant crystal type, with common hornblende, and subordinate quartz and biotite. Lithic clastsof dark green siltstone are generally prevalent. Plagioclase and biotite are the most distinctive flow-orientedphenocrysts, and the tuffs commonly have a flattening-fabric, defined by compressed lithic clasts and alignedcrystal pyroclasts.Tuffs of the Yim Tin Tsai Formation crop out over much of the northwest, and western of central Tsing Yi. Theyare well exposed in the cut-slopes (2785 2270) around Sai Tso Wan, where they are transected by north-, andnortheast-trending faults, and intruded by numerous granitoid and basalt dykes. On the island's main ridge, thetuffs are flat-lying above granite, but locally, they dip steeply to the north. They are also intruded byeasterly-trending rhyolite dykes. The tuffs are well exposed in hillside tors (2750 2310) above Kam Chuk Kok, andare intruded by granodiorite, in cut-slopes to the west. On the headland (2650 2380) west of Shek Wan, the tuffsare partly recrystallised, but lithic clasts are still recognisable.The Yim Tin Tsai Formation is widespread on Ma Wan, forming extensive coastal outcrops in the north andsouthwest, and boulderfields inland. The lithology is identical to that on Tsing Yi, with a well-displayedflow-fabric in outcrops around Kau Po (2382 2388), Tung Wan (2438 2338) and Tung Wan Tsai (2476 2399).Commonly, the tuff has a well-developed welding fabric which is gently undulose. In the east, this fabric dips tothe northwest, at about 30°, while in the north it dips gently southwards. At Pak Wan (2437 2398), on the northcoast of the island, there is a thin, light greenish-grey, fine-grained, silty tuffite with pebbles of tuff up to 50 mm.The exposure is entirely surrounded by rhyolite dykes.Tsing Chau Tsai area. The crystal tuffs in this area contain a few large, diffiise-edged porphyritic lapilli, and havea generally coarse ash to lapilli grain size, glassy matrix and well-defined mafic crystals; all features which aretypical of the Yim Tin Tsai Formation. The tuffs are also characterized by aphanitic, pebble-like, lithic clasts, up to30 mm across, which, unlike typical clasts in the overlying Shing Mun Formation, do not have altered margins. Onthe coast (2265 2270) at Tsing Chau Tsai, a lapilli-bearing, coarse ash tuff is exposed. Locally to the west (22102232), the tuffs are crudely stratified, and contain quartz and feldspar crystals (pyroclasts) up to 5 mm across./Cheung Sha. Yim Tin Tsai Formation crystal tuffs, containing both large porphyritic lavl lapilli kid smaller, darkaphanitic lapilli, are exposed on the coast (140 105) near Cheung Sha Lower Village. Some of the dark lapilli havereaction (resorption or alteration) rims, although most have sharp contacts with the matrix. The porphyritic lapilligenerally have diffuse margins, The tuffs are cut by granodiorite and feldsparphyric rhyolite dykes. Bluish-grey,porphyritic, lapilli-bearing ash crystal tuff is exposed along the catchwater (133 111) above the South Lantau Road.It also contains small, dark grey, aphanitic lapilli.Exposures offshore (1196 0977) from Cheung Sha Beach include tuff and feldsparphyric rhyolite dykes. The tuffcontains many prominent lapilli and has a planar fabric as is typical of the Yim Tin Tsai Formation. A very smalloffshore exposure (1393 0944), about 1 km south of Cheung Sha Lower Village, comprises lithic lapilli-bearingtuff and probably belongs to the formation.Tong Fuk Grey lapilli-ash crystal tuffs are exposed east of Tong Fuk (1126 0973). A prominent planar fabric,dipping 80° to the south, is defined by elongate porphyritic lapilli and blocks up to 150 mm. Some lapilli havediffuse m&rgins, and appear to grade into the matrix. Smaller, dark aphanitic lapilli, up to 50 mm. across, haveclearly-defined margins arwi in some cases have rea6ti©n (resorption or alteration) rims. \Weathered surfaces expose quartz crystals (pyroclasts) from 1 to 5 mm across, and aligned mafic crystals between1 and 3 mm in length. Boulders of grey tuff (1036 0922), exposed offshore in the middle of Tong Fuk Mui Wan,39
Page 2 and 3: UNIVERSITY OF HONG KONGLIBRARIES
Page 4 and 5: © Government of Hong KongPublished
Page 7 and 8: CONTENTSTitle pagePage1ForewordCont
Page 9 and 10: Distribution and Lithology 73Detail
Page 11 and 12: Chapter 10 Economic Geology 149Intr
Page 13 and 14: (Plates)Plate 22 - Porphyritic Medi
Page 15: Table 1 — Grain Size Description
Page 18 and 19: 0 1 2 3 4 5 k mCONTOURS AT 100m INT
Page 20 and 21: properties of the marine deposits b
Page 22 and 23: Archival DataAll records from the s
Page 24 and 25: Chapter 2Outline of GeologyThe soli
Page 26 and 27: The Mesozoic volcanic rocks are div
Page 29 and 30: Chapter 3Palaeozoic Sedimentary Roc
Page 31 and 32: Carboniferous metasedimentaiy rocks
Page 33 and 34: Plate 1 - Fining-Upward Sequence of
Page 35 and 36: Plate 3 - Well-bedded Sandstones an
Page 37 and 38: Reef Island. The northern end of th
Page 39: Table 3. Evolution of Nomenclature
Page 43 and 44: Shing Mun FormationThe type localit
Page 45 and 46: Tai Che Tung. Crystal tuff, interpr
Page 47 and 48: Plate 7 • Lapilli-Ash Crystal Tuf
Page 49 and 50: Plate 11 - Pyroclastic Breccia (086
Page 51 and 52: appears fragmental, and contains mu
Page 53 and 54: exposures in the stream bed consist
Page 55 and 56: Lin Fa Shan. The steep eastern flan
Page 57 and 58: dipping northwest at 46°, is prese
Page 59 and 60: Sunset Peak MemberThe Sunset Peak M
Page 61 and 62: Plate 16- Block and Lapilli-bearing
Page 63 and 64: Chapter 5Major IntrusionsClassifica
Page 65 and 66: in grain size from 0.4 to 1.5 mm, a
Page 67 and 68: 20 - Megacrystic Medium-grained Gra
Page 69 and 70: Pui O Wan. Porphyritic medium-grain
Page 71 and 72: minimum age of c. 145 Ma (see earli
Page 73 and 74: The granite on Chek Lap Kok is most
Page 75 and 76: Southeast of Tin Sam, on the wester
Page 77 and 78: About 1 km east of Nam Shan, exposu
Page 79 and 80: Plate 25 - Thin Section of Porphyri
Page 81 and 82: Chapter 6Minor IntrusionsIntroducti
Page 83 and 84: The microgranite grades southwards
Page 85 and 86: the Lantau Formation, however, and
Page 87 and 88: Plate 29 - Flow-banded Quartzphyric
Page 89 and 90: also cut volcanic rocks of the Tsue
Page 91 and 92:
Silver Mine Bay. On the headland so
Page 93 and 94:
Plate 33 - Core stone Development i
Page 95:
Plate 37 - Thin Section ofLamprophy
Page 98 and 99:
Pre-Yensfaanlan StructureWithin the
Page 100 and 101:
separates areas of different lithol
Page 102 and 103:
NeotectonicsFault activity in the r
Page 105 and 106:
Chapter 8Metamorphism and Alteratio
Page 107 and 108:
the majority of rocks in the distri
Page 109:
Chapter 9Superficial GeologyIntrodu
Page 112 and 113:
Detailstal O. Alluvial deposits for
Page 114 and 115:
trains. The regolith on most steep
Page 116 and 117:
Plate 44 - Boulder Debris on the So
Page 118 and 119:
gazetted for recreational purposes.
Page 120 and 121:
DetailsTai O. Extensive intertidal
Page 122 and 123:
Plate 48 - Sandy Beach at Fan Lau S
Page 124 and 125:
Pre-Chek Lap Kok Formation Deposits
Page 126 and 127:
formation. The sequence is usually
Page 128 and 129:
BOREHOLE NO: ESC 17GRID REFERENCE:
Page 130 and 131:
BOREHOLE NO: A5/1GRID REFERENCE: 80
Page 132 and 133:
formation, thinly laminated clays o
Page 134 and 135:
Quinqueloculina seminulum, which is
Page 136 and 137:
Channel and Transgressive Deposits.
Page 138 and 139:
Figure 20 - Areas of Acoustic Turbi
Page 140 and 141:
West Lamma Channel These may be att
Page 142 and 143:
Figure 23 - Depth-Averaged, Maximum
Page 144 and 145:
The formation is Holocene in age. T
Page 146 and 147:
Boreholes (A5/1, A5/2, B2/1, ESC17
Page 148 and 149:
Plate 51 - Deep Weathering Profile
Page 151 and 152:
Chapter 10Economic GeologyIntroduct
Page 153 and 154:
Non-metalliferous MineralsFissure v
Page 155 and 156:
REFERENCESAddison, R. (1986). Geolo
Page 157 and 158:
James, J.W.C. (1993). The offshore
Page 159 and 160:
Sewell, R.J., Strange, PJ., Langfor
Page 161 and 162:
Appendix 1Microfossils identified f
Page 163 and 164:
Appendix 2 (continued)Sample HK9913
Page 165 and 166:
Appendix 2 (continued)Sample HK1054
Page 167 and 168:
Appendix 4aSelective normalized pal
Page 169 and 170:
,Appendix 5 -Radiocarbon and Other
Page 171 and 172:
construction materials 151contact m
Page 173 and 174:
molybdenite 149Mong Tung Hang 151Mo
Page 175 and 176:
talus 107,115Tarn Tsui Wan 71,87Tan
Page 177:
LB 555.125 G34 LGeology of Lantaii
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