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Sham Wat FormationThe Sham Wat Formation is a newly-defined formation (James, 1993; Fyfe & James, 1995),named after the locality of Sham Wat (0620 1500) on the northwest coast of Lantau Island. Thetype section of the formation is the continuously sampled Borehole ESC 17 (04798 18402)(Figure 14) located north of Sham Wat. The borehole sampled thick deposits of the Sham WatFormation which are overlain by Hang Hau Formation sediments and underlain by thick ChekLap Kok Formation sediments. At this site, the formation is bounded by reflectors interpreted aserosional unconformities. The borehole was sited on the intersection of two seismic lines, B35and B39, allowing the formations and their bounding unconformities to be traced north-south andeast-west. Borehole B2/1, to the north, and Borehole A5/1 (Figure 15), which lies to the east ofthe Soko Islands, also intersected these formations.The Sham Wat Formation is very restricted in its occurrence, with a total subcrop area of 96 km 2in the district. The most extensive deposits are confined to the west of the area, both northand south of Lantau Island (Figure 13). The formation is not exposed at sea bed, as it is alwaysoverlain by Hang Hau Formation sediments.The diagnostic morphology of the formation includes the form of its lower bounding surface. Inthis district, this distinctive lower surface has a high-density, deeply-channelled form (Figure 16).Channel separation varies from 35 to 150 m., with depths of less than 5 m to greater than 20 m.The floors of the channels may reach a level of-54 mPD. On seismic records, the channels aregenerally symmetrical in cross-section, commonly with deep, V-shaped forms and convexmargins. Because of the high channel density many of the channel interfluves are no more thanrounded humps, but in some areas they may be planar, with the line of an older erosion surfacediscernible across the interfluves. Conversely, the upper bounding surface is a planarsub-horizontal erosive contact, although some channelling at the base of the Hang Hau Formationhas occurred locally and this has incised the upper surface of the Sham Wat Formation.The channelled morphology of much of the Sham Wat Formation has controlled the distinctiveseismic character of the formation in this district. The primary reflector style is of continuous,sub-parallel units that drape over channel interfluves and down into channel deeps. In the deeperchannels the reflectors are parallel, U-shaped infills that onlap the channel margins and across theinterfluves. In some channels, especially the widest and deepest, there is evidence of reflectoroverlap and truncation due to erosion and current activity. A transparent, monotonous seismicresponse characterises the basal infill of some channels, but these are covered by thick sequencesof well-developed, parallel-draped reflectors. The draped nature of the sequence suggests thatconsolidation of the sediments occurred since initial deposition. Although some consolidation islikely, the absence of significant thinning between the principal reflectors over most channelmargins, suggests it is not the principal cause (James, 1993). The drapes are probably primaryphenomena resulting from siltation in shallow waters. The sub-parallel draping is generallycontinuous to the top of the formation, although there is some decrease in the amplitude of thedrapes.Few boreholes penetrate the thick sequences of the Sham Wat Formation around the Soko Islandsand north of Sham Wat, The most complete sequence proved is in the type borehole, ESC 17(Figures 13, 14 and 16), The formation comprises generally firm (undrained shear strength 40 -60 kPa), grey clayey silt. Lenses and mottled bands of brittle and denser, pale olive, oxidized,clayey silt up to 8 mm thick, arc common. Leached, soft and corroded white shell fragments,generally less than 2 mm, but occasionally up to 6 mm, occur and very small disarticulatedbivalves, up to 6 mm across, are also present. Black spots and lenses occur randomly, as doinfrequent, small, organic fragments. Within the upper part of the channel, at the base of the127