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formation, thinly laminated clays occur at a depth of-27.3 to -29.7 mPD. Plant fragments anddebris are rare within the main body of the sequence, but thin organic streaks are more prevalentbelow -29.7 mPD. The base of the formation lies at a reworked horizon around -32.5 mPD,where sand laminae and beds become dominant. Bulk XRD analyses of two samples fromBorehole ESC 17 showed major concentrations of quartz and kaolinite with minor mica and somesmectite; subordinate calcite, feldspar and pyrite were also noted. The fractions of the samplesless than 2|im are predominantly composed of kaolinite with minor smectite and mica.In Borehole A5/1 (Figures 13 and 15), east of the Soko Islands, the top of the formation lies at adepth of about -24.6 mPD. Some reworking of the top may have occurred during deposition ofthe overlying Hang Hau Formation muds. The sediments arc soft to firm, grey clayey silts.Lenses and mottled bands of pale yellow, oxidized clayey silt, which are up to 10 mm thick, arecommon below -25.7 mPD, as are occasional lenses of fine sand. There is abundant peaty debrisin a sandy matrix near the top of the formation. Sand and organic fragments also occur at itsbase in association with a 60 mm thick peat.It is clear from the lithologies described in these boreholes that the Sham Wat Formation has afew diagnostic features. The formation is firmer (40 - 60 kPa) and has a lower moisture content( occursbetween Cheung Chau and Shek Kum Chau. The third subcrop, south of Cheung Chau, is about 10 m thick and hasan area of 14.00 km 2 .West Lamma Channel. A continuous subcrop of unequivocal Sham Wat Formation sediments wedges out to thesoutheast of Cheung Chau, on the western edge of the West Lamma Channel. However, in the southern part of theWest Lamma Channel, channels in the surface of the Chek Lap Kok Formation, between 5 and 15 m deep, arefilled with laminated sandy silts and silty sands. In the larger channels, these have a wavy, sub-parallel drapedreflector style on seismic records, analogous to the Sham Wat Formation. The age and stratigraphical significanceof these extensive channel-fills in this part of the district have not been resolved,PalaeontologyDistinctive palynofloral associations occur in sediments from the type section in Borehole ESC 17(Appendix 4b) (Jolley, 1992b). Ptera-type spores are relatively abundant and dominate much ofthe sequence, from a depth of -30.8 to -15.7 mPD, as do other polypodiaceous fern spore-types130