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Figure 23 - Depth-Averaged, Maximum Current Velocities in ms' 1 for Dry Season Spring Tide Conditions. Data from HongKong Government WAHMO Model. Analyses by Hydraulics & Water Research (Asia) Ltd.
West Lamma Channel The Hang Hau Formation forms a thick blanket over the entire West Lamma Channelarea. It ranges in thickness from 6 to 20 m, being thickest in the north and thinning southwards towards thesouthern limits of Territorial Waters. The base of the formation, as seen on the seismic records, is fairly uniform, orgently undulating, It forms an almost horizontal plain, falling gently from about -25 mPD in the north to about -30mPD in the south. A thick accumulation of Hang Hau Formation muds occurs southeast of Cheung Chau ? filling achannel which runs northwest to southeast between Hei Ling Chau and Cheung Chau. Near the southwestern end, athicker channel-fill tongue of Hang Hau Formation muds is situated between Hei Ling Chau and tfte northern endof Lamma Island. The thinnest area of Hang Hau Formation muds, about 7 m, occurs at the southern limit ofTerritorial Waters. Acoustic turbidity, or occluded gas bubbles, affect almost 50 km 2 of the muds in the WestLamma Channel.The surface of the formation declines gently southwards, and the sea bed forms a wide, shallow channel Waterdepths shelve to 5 m deep within 0.5 km of the coastlines. To the east of Hei Ling Chau, the floor of the WestLamma Channel is less than 10 m deep, reaching 10 m between Cheung Chau and Lamma Island, 15 m south ofLamma Island and 20 m near the limits of Territorial Waters. The smooth-surfaced, gently southward-shelving seabed indicates the lack of strong currents or erosive tidal streams in the West Lamma Channel. This is in contrast tothe rock-floored Kap Shui Mun Channel to the north and the East Lamma Channel, which is 30 m deep andscoured to 40 m in places. This quiescent picture is also emphasised by the thick, uniform mud deposits that floorthe channel, which slowly settled out from suspension in the water column.Age and Sedimentary EnvironmentThe formation contains abundant macro- and microfauna. Calcareous disarticulated andarticulated bivalves are common, with, some gastropods, scaphopods and echinoids. There is arich diatom flora (Owen et al, 1995) and a varied foraminiferal fauna, although diversity ismoderate, A bimodal distribution of Elphidium hispidulum is evident (Wilkinson, 1990, 1992aand 1992b) and the population is also dominated by Rotalidium annectens, Quinqueloculinaseminulum, Ammonia beccarii, E. advenum and Crib onion tikutoensis (Figure 17). A similarrange of calcareous microfauna was noted by Shaw et al (1986) and also by Yim & Yu (1991).Wilkinson (1990) found the ostracod fauna to be generally of a lower diversity compared withthe foraminifera, with very little variation through the sequence analysed in Borehole B15. Thespecies were euryhaline. Sinocytherida impressa, Keijella hodgii and Stigmatocythere spinosadominate the assemblage. In Borehole ESC 17, diatoms were rare to moderately common(R.B.Owen, written communication, 1992) with a restricted fauna dominated by Cydotellastriata. In Borehole A5/2, Owen et al, (1995) distinguished five diatom zones in the Holocenesequence, and related them to sea level variations, and the fluctuating influence of fresh andbrackish water at the site.Shaw et al (1986) and Yim et al. (1987) recorded palynofloradominated by Castanopsis and Quercus, although Yim et al, (1987) emphasised theRhizophoracean component in the formation. Jolley (1992a and b) found a comparable record inboreholes ESC 17 and B15 (Appendices 4a and 4b) with Castanopsis/Lithocarpus and Quercusoccurring, commonly accompanied by pollen of the Rhizophoraceae. Cibotium barometz-typsswere also regularly recorded.The seismic, lithological and palaeontological evidence suggests a regressive estuarine to marinesequence, with most of the sediment probably derived from the Pearl River. There are likely to bethin sequences at the base of the formation associated with the Holocene marine transgression,but the bulk of sedimentation occurred in water depths at, or rising towards, those of the presentday. The relatively homogenous nature of the sediments down to the base of the formation andthick foreset developments in some areas, suggests rapid initiation of sedimentation with tidalcurrent streams similar to the present regime prevailing throughout the sedimentation cycle.Palaeontological evidence indicates fluctuations in marine influence during sedimentation withvariations in salinity due to increased flows of fresh water in the system. Whether these indicateannual monsoonal influx of fresh water or short term climatic oscillations has not been resolved.141
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UNIVERSITY OF HONG KONGLIBRARIES
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© Government of Hong KongPublished
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CONTENTSTitle pagePage1ForewordCont
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Distribution and Lithology 73Detail
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Chapter 10 Economic Geology 149Intr
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(Plates)Plate 22 - Porphyritic Medi
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Table 1 — Grain Size Description
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0 1 2 3 4 5 k mCONTOURS AT 100m INT
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properties of the marine deposits b
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Archival DataAll records from the s
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Chapter 2Outline of GeologyThe soli
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The Mesozoic volcanic rocks are div
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Chapter 3Palaeozoic Sedimentary Roc
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Carboniferous metasedimentaiy rocks
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Plate 1 - Fining-Upward Sequence of
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Plate 3 - Well-bedded Sandstones an
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Reef Island. The northern end of th
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Table 3. Evolution of Nomenclature
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are crudely stratified and welded i
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Shing Mun FormationThe type localit
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Tai Che Tung. Crystal tuff, interpr
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Plate 7 • Lapilli-Ash Crystal Tuf
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Plate 11 - Pyroclastic Breccia (086
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appears fragmental, and contains mu
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exposures in the stream bed consist
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Lin Fa Shan. The steep eastern flan
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dipping northwest at 46°, is prese
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Sunset Peak MemberThe Sunset Peak M
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Plate 16- Block and Lapilli-bearing
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Chapter 5Major IntrusionsClassifica
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in grain size from 0.4 to 1.5 mm, a
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20 - Megacrystic Medium-grained Gra
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Pui O Wan. Porphyritic medium-grain
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minimum age of c. 145 Ma (see earli
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The granite on Chek Lap Kok is most
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Southeast of Tin Sam, on the wester
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About 1 km east of Nam Shan, exposu
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Plate 25 - Thin Section of Porphyri
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Chapter 6Minor IntrusionsIntroducti
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The microgranite grades southwards
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the Lantau Formation, however, and
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Plate 29 - Flow-banded Quartzphyric
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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 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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