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Urmston Road - Brothers proto - channelFigure 13 - Sub crops ofPre-Chek Lap Kok Formation Deposits and Sham Wat Formation (Stippled),and Locations of Boreholes A5/1 A5/2, B2/1, B13, B13A, B15 and ESC 17, Containing Typeand Other Sections Through Offshore Superficial Deposit Formations.palaeosols, but can also be seen at depth in massive sands and flaser sand lenses. The oxidationin these sandy sequences probably results from percolating iron-rich groundwater rather thandirect subaerial exposure.Bulk X-ray diffraction analyses on a number of samples in Borehole ESC 17 indicate that theChek Lap Kok Formation sediments are composed of quartz, kaolinite and illite, with subordinatecalcite, siderite and feldspar. In samples from the Chek Lap Kok airport site, Tovey (1991) foundthat illite is the most common clay mineral, comprising 44 to 50% of the clays. Kaolinite is alsopresent at a concentration of 29 to 36%, and smectite comprises only about 5%. Quartz is themain component of the >2jim fraction.The seismic character of the formation confirms the complexity of the lithologies and faciesassociations. Generally, there is low acoustic impedance contrast and poor to non-existentreflector development in much of the underlying rock, so that the base is commonly difficult todelineate accurately. The formation appears to have no single, diagnostic seismic character.Relatively short, laterally impersistent, truncated bounding reflectors and channel forms arecommon, with large variations in background seismic tones and internal reflector characteristics.However, on a local scale,, correlation can be made between the seismic reflectors and lithologiesin the Chek Lap Kok Formation, For example, the units of clayey silt and sand and gravel seen inboreholes B15, A5/1 and A5/2 can be matched crudely to changes in seismic character andlaterally persistent reflectors. Seismic features such as lateral accretion foreset reflectors, can becorrelated with sand sequences in boreholes B15 and A5/2. On some seismic records, a sequenceof sub-horizontal, relatively persistent, high amplitude reflectors occurs near the top of the123