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Pre-Chek Lap Kok Formation DepositsThese deposits are of limited areal extent, and are defined solely on their seismic character. Theyhave not been assigned formation or member status The lithology of the deposits is not kno\vnbut their seismic character suggests that they are likely to be sand and gravel rather than finergrained sediments. The sequence displays well-developed, relatively strong reflectors parallel tothe base of the unit. Within the main Brothers - Ma Wan Channel they have a trough-like form,but become horizontal over the platform, or terrace, at its margins. The top of the sequence isdefined by a planar, horizontal reflector marking the base of the overlying Chek Lap KokFormation.DetailsThe Brothers islands - Ma Wan. These sediments have a very limited areal extent in the centre of the Brothers -Ma Wan Channel northwest of Yam O Wan (Figure 13). They cover an area of approximately 300 m by 500 m andfill a small channel, or basin, aligned parallel to the modern channel. The deposit has a maximum infill of about 15m beneath the overlying Chek Lap Kok Formation, The deepest part of the deposit has a symmetrical, V-shapedcross-section about 180 m wide, and its base is below -50 mPD. The southern margin is characterized by a 5m-thick platform, or terrace, with a horizontal base.Age and Sedimentary EnvironmentAn over-deepened fluvial or estuarine channel is the most probable environment of deposition.The exact age of the deposit is uncertain.Chek Lap Kok FormationThe type section of the formation is the continuously sampled borehole, B13/B13A (10292.818701.5), drilled on the test embankment at Chek Lap Kok (Strange & Shaw, 1986). Five otherboreholes, B15, ESC17, A5/1, A5/2 and B2/1, all penetrating sediments assigned to the ChekLap Kok Formation, have also been logged and analysed in detail (Figure 13). Of the offshoreQuaternary formations, it is the most complex and heterogeneous physically, lithologically andseismically. Sequences that are internally complex on seismic records and in boreholes aregenerally classed as Chek Lap Kok Formation. However, there arc several diagnosticcharacteristics which help to identify it, such as complex, small-scale channelling on seismicrecords, intense oxidation, and the presence of organic and plant debris in borehole samples.The Chek Lap Kok Formation is widely distributed throughout the area. Lithologically, itcomprises a wide range of grain sizes, from gravel to clay, and sediment type varies extensivelyboth laterally and vertically throughout. Clays tend to be firmer than in overlying formations,with unconfmed compressive strengths ranging from 30 to 90 kPa. Deposits have a variety offorms, including upward-fining sequences from basal gravels through to silts and clays withabundant organic debris. The organic material, which includes comminuted plant debris as wellas wood fragments, can occur either within the silt or clay matrix, or as thin beds within silt, clayand sand. Upward-coarsening sequences are also a feature, as are massive, moderately sortedsands^Interbedded with the sandy deposits, firm silts are common, some of which arc massive,though many contain thin flaser laminae and lenses of sand and silty sand,Oxidized horizons are characteristic of the formation, and oxidized palacosols commonly occurat the top of the formation. Around Chek Lap Kok, the top of the formation is a deeply oxidizedpaiaeosol extending to 1.0 m below its upper surface in Borehole B13/B13A (Shaw