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filling channels and transporting sandy sediments. These sediments in Borehole A5/2 are veryorganic,with cosmopolitan diatoms and sponge spicules present in low numbers, and were deposited(Owen et al, 1995) in a brackish, coastal marsh setting succeeded by shallow mud flats, orpossibly estuarine conditions.Within the Chek Lap Kok Formation diatoms, foraminifera, ostracods and sponge spicules areabsent or sparse and, where present, are of very low diversity (Shaw et a!., 1986; Owen et al,1995). The foraminifera are generally planktonic forms (Wilkinson, 1990). Palynologically, theformation in Borehole B15 contains a large preponderance of terrestrially-derived organicmaterial and dinoflagellate cysts are scarce (Harland, 1990). The sediments are characterized bycysts of Protoperidinium with Spiniferites spp. and Polykrikos. Terrestrial pollen in BoreholeA5/1 (Appendix 4a), which lies just to the east of the Soko Islands, comprises a number ofpalynofloral associations (Jolley, 1992a). These include mangrove pollen, such as Sonneratiacaseolaris, as well as the marine microplankton Spiniferites ramosus and Pterosperma spp.Castanopsis/Lithocarpus and Quercus pollen, derived from a mixed mesophytic forestcommunity (Wang, 1961), are the background pollen in some of these associations that aredominated by Alsophyla spp, graminids and Pteris-typQ spores. There is also an associationcharacterized by common specimens of Liquidamber formosana type and frequent specimens ofRhizophora type.The lack of calcareous microfauna in sediments analysed from the Chek Lap Kok Formationdoes not entirely preclude a marine influence during some period of deposition; decalcificationcould account for the lack of fauna (Yim, 1984, 1994). Mangrove and other pollen associations,at depth in boreholes A5/1 and B13/13A (Shaw et al, 1986), suggest a littoral or sublittoralenvironment in a restricted estuary with a significant marine influence. The sediments withinBorehole A5/1 may record a cycle of rising and falling sea level, with intertidal deposits at thebase of the formation passing up into massive estuarine sands that were subsequently oxidized bypercolating groundwater during subaerial exposure. From Borehole WB7 (23999.4 08000.2) inthe West Lamma Channel, two lower marine units, separated by terrestrial deposits, have beendescribed (Yim & Yu, 1993). Fluvial and suprahttoral environments are also indicated by local,well-channelled architecture, upward-fining sedimentary sequences in boreholes B15 and A5/2,and extensive interbedded plant and organic horizons, which probably formed as overbankdeposits in a fluvial system. The formation was deposited in an environment that varied in timeand place from submarine to subaerial in an estuarine, intertidal to fluvial setting. Changes inlocal base level were likely, leading to changes in the location of sediment sources and sinks anda diversity of sediment lithology and architecture.There are various problems in determining the age of the formation. They stem largely fromtrying to reconcile the seismic evidence for two regional base level changes and sedimentaccretion cycles, which deposited the overlying Sham Wat and Hang Hau formations, withradiometric dates obtained from material within the Chek Lap Kok Formation in the district.Dates range from 16,420 years BP to more than 41,000 years BP (around the maximum age ofresolution of the 14 C method), Three dates were obtained near the top of the formation inBorehole A5/2 (Owen et a/., 1995), in the seismic unit characterised by poorly developed,laterally persistent reflectors. A radiocarbon date from a wood fragment, 6.6 m below the top ofthe formation, gave an age of 35,000 ± 1250 years BP. An Optically Stimulated Luminescence(OSL) date of 80,000 ± 9000 years BP and a Thermoluminescence (TL) date of 78,000 ± 850years BP were obtained 5 cm and 10 cm respectively below this level Further dating ofsediments within a well established seismic stratigraphy is needed to resolve the absolutechronology.125