Superficial Geology

Methods of Survey
The offshore deposits have been investigated by reference to data from boreholes, shallow seismic
profiles and sea-bed sampling programmes. Information from some 1 600 boreholes has been
collected from site investigation reports, while new stratigraphical detail has been gathered from
the logging and analysis of a few continuously sampled boreholes commissioned by the
Geotechnical Control Office (GCO) for this survey (Appendix). The sites of these continuously
sampled boreholes and the general distribution of site investigation boreholes are shown in
Figure 24.
Seismic data, obtained using Boomer equipment, combined with site investigation borehole
information, formed the basis of a general appraisal of the offshore geology of this district
commissioned by the GCO (EGS, 1985). In this survey the original seismic traces have been
re-appraised to elucidate stratigraphical detail. In addition, the survey has benefitted from data
collected by geophysical investigations commissioned by the GCO for this Geological Survey, and
carried out by Port Works Division, Civil Engineering Services Department using Finger and
side-scan sonar equipment. A track plot showing the distribution of the geophysical data is given
in Figure 25.
The Boomer seismic records have generally allowed unambiguous recognition of the base of the
Hang Hau Formation except in parts of the central and eastern Harbour. Also, they give a clear
indication of the stratification within that formation. By contrast, the boundary between the Chek
Lap Kok Formation and rock or weathered rock is commonly poorly defined. The presence of
biogenic gas in places within the Hang Hau Formation has a masking effect on the seismic traces,
preventing resolution of all underlying reflectors. Disturbance of the sea-bed by dredging or other
engineering work may also produce a masking effect.
Chek Lap Kok Formation
Distribution and Physical Characteristics
The Chek Lap Kok Formation has a widespread distribution but is masked by the Hang Hau
Formation in all but a few localities. Its thickness is generally in the range 10 to 30 m. It is absent
around parts of the coastal fringe, in a contemporary deep water channel at Lei Yue Mun, and
over several rock knolls in the Harbour area and East Lamma Channel. The type section of the
formation is the continuously sampled borehole B13/13A at Chek Lap Kok, an island off
northern Lantau (Sheet 9) (Shaw, 1985; Shaw et al, 1986).
The formation is lithologically diverse, boreholes proving sequences of mainly firm to stiff clay as
well as silt and dense sand and gravel and, in a few cases, boulders. Some of the clays and silts are
laminated and some include grey or dark grey organic layers with wood fragments (Plate 44). The
more typical colours, however, are bluish grey to light bluish grey, with sporadic yellow, brownochre
and red mottling (Plate 44). Plots of particle size distribution analyses for samples from the
formation are given in Figure 27.
Unlike the continuous sub-parallel reflectors that characterise the seismic signature of the marine
muds of the Hang Hau Formation, the reflectors in the Chek Lap Kok Formation are generally
impersistent to chaotic, though rather stronger than those in the marine muds (Plate 51).
Marine fossils have been recorded from the formation within the district by Yim (1984) and
Howat (1985). The top of the formation commonly carries desiccation cracks filled with muddy
sand of the Hang Hau Formation.
The Chek Lap Kok Formation rests unconformably on rock in various states of weathering. The
surface of unconformity is generally irregular (Plate 51), and the formation may thin out against,
or be confined to, channels or gullies in rock or weathered rock. Its base is probably markedly
diachronous, but, in the absence of continuous seismic reflectors and of documented,
stratigraphically marked horizons, such diachronism remains unproved. The formation is
estimated to extend to a depth of at least -60 mPD in the East Lamma Channel. Seismic records
in the central Harbour area indicate that weathered rock there lies at a maximum of about
-60 mPD.
The top of the formation is an erosion surface over wide areas, particularly in the East Lamma
Channel, in the central and eastern Harbour areas, and at Lei Yue Mun in the deeper parts of the
contemporary channel, where the formation has been completely eroded. In the western Harbour
area the top forms a gently undulating, gullied or channelled surface that falls gently
southwestwards. The upper surface of the formation forms closed basins under the Hang Hau
Formation off Tsim Sha Tsui and at Lei Yue Mun; these hollows are considered to be of marine
origin, scoured by localised, high velocity tidal streams.
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