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of uncertain stratigraphical significance have been recognised, associated principally withlarge-scale erosional and depositional events during the last sea-level lowstand and thesucceeding transgression. However, the basal part of this unit may include sediments thatpre-date these events,The major formation- or sequence-bounding reflectors have been correlated with borehole, conepenetration test, and vane test data. The reflectors generally correspond to significant changes ina number of parameters, including lithology, geotechnical properties, palaeontology andgeochemistry. However, difficulties of interpretation and correlation arise where minor variationsin facies and lithology occur laterally across, and vertically through a deposit, but these changesare not clearly displayed on the seismic records. These subtle changes within and acrossformation and sequence boundaries have led to the inclusion of two facies within the offshoredeposits that are based on their genetic character; marine sand sheets and marine sand banks.Within the Pearl River (Zhujiang) Estuary, a twofold stratigraphical division has been describedby Long and Huo (1990), comprising a lower Zhongshan Formation (40 000 to 12 000 BP) andan upper Shunde Formation (12 000 BP to Present). Both formations have been subdivided intothree units. From the limited published descriptions, the lower unit of the Zhongshan Formationappears to be the equivalent of the Chek Lap Kok Formation, the middle unit of the ZhongshanFormation matches the features of the Sham Wat Formation, and the middle and upper units ofthe Shunde Formation are similar to those of the Hang Hau Formation (Shaw & Fyfe, 1992).Over most of the offshore area of the district there is a thick mantle of superficial deposits, withrock exposures at the sea bed confined mainly to shoals such as Pun San Shek (2510 2030), tonearshore exposures, and to areas swept by currents with relatively high velocities, such as thenarrow channels of Kap Shui Mun (2400 2230) and around Tsing Yi (2630 2300). Thesuperficial deposits generally lie on a rock surface that is steeply inclined near the coast,continuing the generally steep coastal slopes. The slopes tend to flatten out at c. -20 to -30 mPD,although in some places close to the coastline they descend to lower than -50 mPD. Away fromthe coast, the superficial deposits commonly infill broad, relatively smooth depressions, althoughthere are areas, such as around Sha Chau and The Brothers islands, where the superficialdeposits onlap and thin over local rock highs.A number of factors control the thickness of the superficial sediments. These include the formand elevation of the rock surface, tidal currents, the sediment sources and their depositionalenvironment. These are in turn influenced by the proximity of the coastline. For example, in arelatively narrow tidal channel north of Lantau Island, between Ma Wan and Sha Chau,superficial sediment thickness varies from zero around Ma Wan to about 50 m at the Territory'sboundary west of Chek Lap Kok, Within the central part of the tidal channel, superficialsediments can be relatively thin (