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West Lamma Channel These may be attributed to increased pore (gas) pressure in theunderlying sediments, or, more likely, to sea bed dumping. A study to assess the effect of shallowgas on the engineering properties of sediments (Premchitt et al. 9 1990) indicated that someoccluded gas bubbles exist in the areas of acoustic turbidity. The free gas mainly consisted ofnitrogen and methane, while carbon dioxide was the dominant dissolved gas. Free gas was notdetected where acoustic turbidity was not seen on seismic records. It was concluded that the gasdid not adversely effect the geotechnical properties of the sediments.Seismically, the formation consists of very extensive, laterally continuous, horizontal tosub-horizontal, moderate to low amplitude reflectors. The base of the formation is generallyplaced at a well-developed high amplitude reflector. The horizontal to sub-horizontal reflectorstend to infill any small hollows and channels, with little or no evidence of truncation and overlap.In some areas, such as south of Lantau Island, the amplitude of reflectors increases within thebasal section of the formation with a more transparent unit above. An intra-formational reflectorhas been identified in many of the channels. This high amplitude reflector runs across the tops ofchannels, joining each channel crest (Figure 22). It marks the change in current activity as thechannels filled up with sediment and the sea bed became relatively planar. The reflector couldmark a hiatus in sedimentation due to erosion or non-deposition, or may form part of aravinement surface truncating the surrounding formations.The banks at the margins of the main tidal current channels display a characteristic internalreflector style. Long, low-angled reflectors predominate, with low-angled truncations formed byreactivation. An extensive, prominent reflector with a very long, sinuous wavelength has beenidentified south of the Urmston Road - Brothers proto-channel. This reflector may be the resultof a major hiatus or a change in current activity. South of Sha Chau, the formation exhibitsstrong clinoform reflectors dipping to the south. These are inclined at a high angle directly abovethe basal reflector of the formation. The whole formation in this area is virtually one foreset andtopset unit, with no bottomset development, indicating very rapid sedimentation.DetailsNorth and east of Lantau Island. Over most of the area, the sea bed comprises soil, clayey silts of the formation.Coarser sandy sediment is associated with the tidal current pathway that runs along the Urmston Road - BrothersChannel through Kap Shui Mun (240 225) and southwards past Green Island (290 160). Tidal current velocitiesexceed 1 m s" 1 in Kap Shui Mun where the sea bed is floored by rock. As the current velocity gradient decreases tothe west and south of Kap Shui Mun (Figure 23), so sedimentation has occurred, firstly as sand banks, and withcontinuing loss of velocity the sand has given way to the ubiquitous mud blanket. A sand bank occurs off thenortheast coast of Lantau, to the north of which the sea bed is rock floored, This sand bank widens and thinswestwards to be concealed by a thickening cover of marine mud. A second sand bank occurs immediately south ofKap Shui Mun, that is subsumed southwards by a thickening mud cover. A third sand bank occurred southwest ofTsing Yi, extending under a mud cover to Green Island, but was removed for reclamation fill Considerablemodification of the sea bed has taken place in this area. Much of the sand from the Kap Shui Mun bank has beendredged for reclamation fill, the Urmston Road Channel is regularly dredged for navigation, and the site of the newChek Lap Kok airport has had most of the marine mud removed, and replaced by reclamation fill. A number oflarge seabed pits have been dredged east of Sha Chau as a repository for dredged, contaminated marine mud andmost of the northeast Lantau coast and littoral zone has been extensively modified by reclamation works for theNorth Lantau Expressway.South Lantau Island. To the south of Lantau Island, the sea bed is a layer of marine mud that is generally up toc.l 5 m thick. A sand bank, which has now been dredged, occurred west of the Soko Islands, and there is some sandat the seabed immediately to the east of the Soko Islands. Both of these occurrences are related to accelerating tidalcurrents passing the islands, keeping finer material in suspension while sandier material is deposited on the leeside. A large gazetted dumping ground to the south of Cheung Chau (Sheet 1A) has received a variety of dumpedwaste, and most recently the dredged mud from the Chek Lap Kok airport and other reclamations throughout theTerritory. A former sea bed dumping ground (240 170) is located north of Siu Kan Yi Chau,138
10-Sea bedHang Hau Formation20-OLCD00IQ.30-Chek Lap Kok FormationT12.5msec40-Figure 22 - Boomer Seismic Profile along Line NL3 Showing Major, Interpreted Seismic Reflectors inthe Area West of The Brothers Islands, and Highlighting a High Amplitude Reflector,S5, within the Hang Hau Formation, which Runs across the Channel Tops, and Joins theChannel Crests,139
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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
Page 11 and 12:
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
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 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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