Superficial Geology

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Chapter 2 Outline of Geology The solid geology of the district is dominated by Mesozoic volcanic and intrusive igneous rocks (Table 2, Figure 3). The volcanic rocks are those of the Upper Jurassic Repulse Bay Volcanic Group. The oldest rocks of this group exposed in the district are tuffs of the Yim Tin Tsai Formation. These are lithologically uniform, consisting of massive coarse ash tuff. The overlying Shing Mun Formation is variable, containing interlayered tuff-breccia, tuffite and tuff, and minor epiclastic bands. The dominant rock type is poorly sorted, lapilli-bearing, coarse ash crystal tuff. The Shing Mun Formation is overlain by up to 1 800 m of Ap Lei Chau Formation, composed of welded fine ash vitric tuff with well-defined eutaxite layers that form positive topographic features. The uppermost unit of the volcanic rocks within the district is the Tai Mo Shan Formation, formed of coarse ash to lapilli crystal tuff and impersistent layers of tuffaceous sandstone and siltstone. The Repulse Bay Volcanic Group is intruded by granitoid rocks of Upper Jurassic to Lower Cretaceous age. Granodiorite and the highly modified coarse-grained granites forming the plutons of northern Lamma Island and Lai Chi Kok - Sha Tin are the oldest intrusions. The younger plutons forming southern Lamma Island and Kowloon - northern Hong Kong island include fine-, fine- to medium-, and medium-grained granite varieties. In northern and eastern Lamma Island, and in the King's Park area of Kowloon, the granites are markedly megacrystic (Strange, 1985). Quartz syenite forms a small plutonic body on the D'Aguilar Peninsula of Hong Kong Island, as well as impersistent dykes extending across the island from Aberdeen to Chai Wan. The granites are cut by minor acidic intrusions, mostly dykes of feldsparphyric rhyolite and quartzphyric rhyolite. Several feldsparphyric rhyodacite dykes are present on southern Lamma Island. Basaltic dykes of probable Tertiary age occur throughout the district, cutting volcanic rocks and granite plutons alike. Thermal metamorphism around the granitic intrusions has altered tuffs and epiclastic rocks to hornfels. Late stage emanations of hydrothermal fluids from the granite plutons have resulted in greisenisation of the granite and the metasomatism of adjoining volcanic rocks in southeastern Kowloon and in northern Lamma Island. A regolith, or mantle of weathered rock, forms much of the outcrop, effectively masking the fresh rock solid geology. The effect of weathering upon the different rock types is broadly reflected in the topographic relief. Volcanic rocks, especially the finer grained tuffs, tend to form higher and sharper peaks with few surface boulders, whereas granites form lower and more rounded hills with abundant surface boulders (exhumed corestones). In general, the fine-grained granites are more resistant to weathering than the coarser grained varieties. Superficial deposits of Quaternary age conceal solid rock, in various states of weathering, over much of the district, particularly in low-lying areas and offshore (Table 2). In the hilly areas, for example on the slopes of Fei Ngo Shan, debris flow deposits (colluvium) are common, older deposits mantling interfluves and younger ones lining present-day valleys. Alluvium is widespread on the low ground of Kowloon and the Ho Chung Valley. Beach deposits fringe the natural coastline. The offshore Quaternary deposits are laterally extensive and generally comprise Holocene marine mud (Hang Hau Formation) overlying a complex stratified alluvial sequence (Chek Lap Kok Formation) of presumed Late Pleistocene age. These older alluvial deposits comprise clay and silt as well as sand and gravel; some of them are mottled yellow and red, while others are dark grey and organic. Around most of the Kowloon peninsula, in Junk Bay and along the northern shores of Hong Kong Island, the Quaternary deposits have been disturbed and concealed by reclamation. 17
Table 2 - Solid Rocks and Superficial Deposits of the District Superficial Deposits (Onshore) Age Genetic Classification Quaternary Holocene Holocene and Pleistocene Fill (made ground) Beach deposits Alluvium Terrace deposits Debris flow deposits | Talus (rockfall) deposits > colluvium Mixed debris flow and talus deposits J Superficial Deposits (Offshore) £ « ! § o> Age Solid Rocks Holocene Pleistocene Age Named Divisions Hang Hau Formation including East Lamma Channel Member and Telegraph Bay Member Chek Lap Kok Formation Named Volcanic Divisions Principal Materials Mainly mud Clay, silt, sand and gravel Principal Rock Types .- i Upper Jurassic Repulse Bay Volcanic Group Tai Mo Shan Formation Ap Lei Chau Formation Shing Mun Formation Yim Tin Tsai Formation Coarse ash crystal tuff Fine ash vitric tuff with eutaxite Lithic and crystal tuffs, tuffbreccia and tuffite Coarse ash crystal tuff Major Intrusive Igneous Rocks .« 1 Upper Jurassic — Lower Cretaceous Fine-grained granite Fine- to medium-grained granite Medium-grained granite Coarse-grained granite Quartz syenite and quartz monzonite Granodiorite Minor Intrusive Igneous Rocks ^IM 1 Palaeocene Basalt Lamprophyre 1 Upper Jurassic — Lower Cretaceous Quartzphyric rhyolite Feldsparphyric rhyolite Feldsparphyric rhyodacite Quartz trachyte Aplite Pegmatite 18
Page 2 and 3: 5 andKowloon Geoloovof 1:20 000 She
Page 4: Foreword This Memoir describes the
Page 7 and 8: Page Chapter 6 Structure 75 Folds 7
Page 9 and 10: Page List of Plates Plate 1 - Coars
Page 11 and 12: Plate 48 Plate 49 Plate 50 Plate 51
Page 13 and 14: Table 1 - Grain Size Description an
Page 15 and 16: TATE'S CAIRN • HO CHUNC 0 1 2 3 4
Page 17: The Present Survey Geological field
Page 21 and 22: Pumice, glass- Crystals, crystal fr
Page 23 and 24: (2) Western Hong Kong Island (4) Fe
Page 25 and 26: T-HT+T + T+T+T VT+-TV*V' YUNG SHUE
Page 27 and 28: Plate 3 - Thin Section of Lapilli-b
Page 29 and 30: Plate 7 - Coarse Ash Tuff in Tai Mo
Page 31 and 32: Details Tate's Cairn to Fei Ngo Sha
Page 33 and 34: Siu Chai Wan to Big Wave Bay. New e
Page 35 and 36: South of Rennie's Mill (4410 1760),
Page 37 and 38: Victoria Peak to Aberdeen, Mount Ca
Page 39 and 40: An extensive outcrop of Tai Mo Shan
Page 41 and 42: The field criteria used by Allen &
Page 43 and 44: dark biotite-rich groundmass. Bioti
Page 45 and 46: On Stonecutters Island the rock is
Page 47 and 48: Plate 17 - Coarse-grained Granite (
Page 49 and 50: Plate 21 - Fine- to Medium-grained
Page 51 and 52: «, , , y 25 - Megacrystic Medium-g
Page 53 and 54: with the older coarse-grained grani
Page 55 and 56: semi-liquid state. Near the contact
Page 57 and 58: along Tai Hang Road near its juncti
Page 59 and 60: 1 HK4040, 2. HK4221, 3. HK4222, 4.
Page 61 and 62: follows the observed emplacement se
Page 63 and 64: The geochronology of the granites h
Page 65 and 66: Abundant Feldspar / Sparsely Figure
Page 67 and 68: Plate 30 - Quartz Syenite (HK 4019)
Page 69 and 70:
Plate 34 - Thin Section of Quartzph
Page 71 and 72:
in diameter. Biotite is present as
Page 73 and 74:
Basalt dykes have also been noted i
Page 75 and 76:
m J^TV ; Plate 39 - Pegmatite Nest
Page 77 and 78:
Anticline axis Synclme axis Fault,
Page 79 and 80:
Figure 17 - Circular Histograms of
Page 81 and 82:
11 NE n = 109 11 SW 11 SE n = 165 1
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East Hong Kong Island (USE). A prom
Page 85 and 86:
quartz crystals in a preferred orie
Page 87 and 88:
Plate 42 - Thin Section of Medium G
Page 89 and 90:
gm JTM JAC VI VI ; ; II n Plate 45
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Chapter 8 Superficial Geology The s
Page 93 and 94:
Clear Water Bay Road and Ho Chung.
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of Lai (1982) the Mid-levels colluv
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containing subrounded cobbles and b
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Fill The coastline and topography o
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Methods of Survey The offshore depo
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SAND/GRAVEL + Junk Bay JBS1/1A n Kw
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CHEUNG SHA WAN Figure 28 - Generali
Page 107 and 108:
Particle size distribution analyses
Page 109 and 110:
Plate 51 - Seismic Trace of Superfi
Page 111 and 112:
Plate 53 - Seismic Trace of Superfi
Page 113 and 114:
fault zone SHEET ( weatherin § JOI
Page 115 and 116:
Plate 57 - Cut Slope in Medium-grai
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Plate 60 - Cut Slope in Completely
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MINERAL PROSPECTS AND MINOR WORKING
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altered. There is little quartz pre
Page 123 and 124:
Cobbing, E. J., Mallick, D. I. J.,
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Watt, J. C. Y. (1970). A Han Tomb a
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Borehole Grid Ref. Surface level Su
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Borehole Grid Ref. Surface level Su
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Cho Yiu Chuen 41 Cho Yiu Estate 54
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Mount Butler Drive 55 Mount Butler
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Tate'sPass'30 Tau Chau 34 Telegraph
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LB 555.125 S89 Strange, P. J. Geolo
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Superficial Geology

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