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formation. The sequence is usually about 5 m thick, but may reach 10 m locally. It can also betruncated by more than one generation of channelling, and be laterally impersistent, or passlaterally into the ambient chaotic reflector style. The sequence appears to be predominantly mud,and is characterized by the relatively ordered appearance of its reflectors compared to the morechaotic sequences beneath. Much of the sequence is commonly well oxidized, as in boreholesB15 and A5/2. There is no evidence of a change in seismic character due to changes in chemistryand mineralogy associated with oxidation.DetailsNorth of Lantau Island. Sediments of the Chek Lap Kok Formation are particularly well developed off the northLantau coast, where sequences over 40 m thick occur, lying unconformably on bedrock. Their onshore equivalentsare locally well exposed behind the foreshore east of Tai O, where they unconformably overlie Carboniferoussedimentary rocks (Plate 42). Several large, intraformational channels have been identified on seismic records andfrom boreholes. West of Chek Lap Kok, a large basal channel evident on seismic records, has been confirmed bythe well-developed, fining-upward sequence recorded at the base of Borehole IB 15. Another large channel, withsub-horizontal reflectors, runs north from Tung Chung Wan (105 170) and merges into a more complex channelsystem with overlapping bars north of Lam Chau (090 190). Two stages of extensive erosion have incised channelsdeep into the upper surface of the formation. They are associated with the low base-level erosional events thatpre-date deposition of the Sham Wat and Hang Hau formations. These channels are common off the North LantauIsland coast and around Chek Lap Kok, and some feed into the main Urmston Road - Brothers proto-channel. Themargins of the proto-channel form distinct breaks of slope at the limit of the Chek Lap Kok Formation, which sitsas a high sub-horizontal terrace on its southern side. Within the proto-channel, the formation either appears as athin basal deposit, or is absent. North of the proto-channel, the formation is not as well developed and is generallyless than 15 m thick, mainly because the rock surface is at a higher elevation. West of Chek Lap Kok, an intensivechannel complex, draining westwards into the Lantau Channel, has deeply eroded the formation.South of Lantau Island. The Chek Lap Kok Formation is over 40 m thick in this area, and lies unconformably onweathered bedrock, which occurs at about -80 rnPD. On seismic records, large-scale channelling of the uppersurface of the formation is clear, and around the Soko Islands, these channels are filled with sediments of the ShamWat Formation.West Lamma Channel. The formation ranges in thickness from a few metres, near the coasts and bedrock highs,to over 80 m thick near the southern part of the West Lamma Channel. On seismic records, the formation has acharacteristically confused and chaotic pattern of high amplitude, generally discontinuous reflectors that containseveral, distinctly cross-bedded, or channel-form units, confirming the fluvial origin of the sediments. Theformation lies with major unconformity on a weathered rock mantle that is commonly 10 m, or more, thick. Thebedrock surface is a gently undulating, or almost planar, surface, that suggests significant erosional planation priorto deposition of the Chek Lap Kok Formation.Characteristically, bedrock appears on the seismic records near the coasts or islands, and commonly crops out onthe seabed. Away from the coasts, the bedrock surface slopes gently. Sediments of the formation fill what appearsto be a deep and broad channel into the underlying bedrock. Over most of the southern part of the West LaminaChannel, bedrock occurs at more than 75 m to 85 m below seabed, or greater than -95 mPD.Further north, bedrock appears at about 65 to 70 m below seabed, or at about -75 to -80 mPI). This indicates a lowgradient valley with a southwards fall of about 20 m in 7 km. The surface of the formation falls gently from northto south. It is intensely channelled, indicating extensive erosion. The channels are up to 350 m wide and 15mdeep, and form a dendritic pattern that converges southwards. Near the top of the formation, the seismic recordsare characterized by laterally continuous, sub-parallel, high amplitude reflectors in a sequence up to 10 m thick.The sediments in Borehole A5/2 generally comprise a muddy sequence with sand laminae and bands, and organicfragments, and indicate deposition in an estuarine environment,Age and Sedimentary EnvironmentSedimentary features recognised from seismic profiles include channel-fill, exhibiting truncationand overlapping during vertical accretion, aaid lateral accretion foresets associated withmigrating channels. Sediment onlap across the underlying rock surface is apparent in a numberof areas, and indicates vertical accretion. The poorly developed, laterally persistent reflectorsnoted in some areas in the upper part of the formation may indicate sedimentation in relativelyextensive and undisturbed water, although subject to currents that were capable of cutting and124