Superficial Geology

Plate 51 - Seismic Trace of Superficial Deposits and Rock in Western Victoria Harbour showing
Muds of Hang Hau Formation Filling and Overlapping Gullies in Chek Lap Kok
Formation, Trace by EGS Ltd for Harbour Reclamations and Urban Growth Study
(Figures Indicate Approximate Depths in Metres below Principal Datum)
The overlying East Lamina Channel Member is seismically similar, but characterised by lower amplitude
internal reflectors. Its main feature is its position as an inset sequence that lies on a locally steep surface
truncating the underlying Telegraph Bay Member. At its base it has a seisrmcally distinct deposit considered
to be a lag deposit of sand or gravel. A seismically similar layer, 1 to 3 m thick, occupies the floor of the
present channel at the top of the East Lamma Channel Member. To the southwest, reflectors within this
member downlap northeastwards into the centre of the channel, while to the northeast, reflectors lap onto
the steep northeast erosion surface. A rounded, levee-like feature, 100 m wide, flanks the southern margin of
the present channel, rising a few metres above the adjacent channel margin and displaying strongly
downlapping internal reflectors.
The seismic characteristics of the Hang Hau Formation in the East Lamma Channel indicate that the muds
of the Telegraph Bay Member accumulated by downlapping out from the coast of Hong Kong Island, and
that they were subsequently removed in much of the central part of the Channel by a major erosional event.
Subsequently, mud accumulation resumed, depositing the overlying East Lamma Channel Member.
Western and Central Harbour. The Hang Hau Formation forms a uniform mud plain under about 10 m of
water. The maximum thickness is about 18 m (EGS, 1985) but the general thickness is of the order of 10 m.
The base of the formation is characterised by a layer of sand or muddy sand, and the formation is
represented by littoral sand around the pre-reclamation coasts. Thickness variations are mostly attributable
to variations in the underlying surface of the Chek Lap Kok Formation, which is extensively channelled or
gullied across this area (Plate 51). The formation thins and becomes slightly sandier eastwards towards Tsim
Sha Tsui, as it laps onto a topographic rise formed by the Chek Lap Kok Formation. Off Tsim Sha Tsui the
formation is incised by a contemporary channel. On the northern flank of this channel the formation is
represented by a wedge-shaped deposit of sand that is seismically transparent, exhibiting prograding
reflectors that dip towards the channel The sand attains a maximum thickness of 8.5 m. Sand also occurs on
the floor of the present channel. To the east of Tsim Sha Tsui, in a narrow tract south of Hung Horn and
Kowloon Bay, the Hang Hau Formation is absent or represented only by a thin veneer of mobile sand that
directly overlies the Chek Lap Kok Formation.
A comparison between hydrographic surveys carried out in 1978, 1982 and 1985, (I. J. Ayson, written
communication, 1986) has indicated that the level of the sea-bed has been lowered by scouring by about 0.5 m
over the area of Kellett Bank to the south of Stonecutters Island. In a small area in the North Green Fairway, to
the northeast of Little Green Island, the sea-bed has been lowered by up to 1 m during this interval. This
represents erosion of about 2.3 million cubic metres of Hang Hau Formation muds over the period.
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