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Quinqueloculina seminulum, which is also frequent at -20.8 mPD and common in the upper partof the formation. Although individual species were recorded rarely at -24.8 mPD, the genusElphidium is well represented and Ammonia and Rotalidium are also present. Ammonia beccariiis common between -22.8 and -20.8 mPD. Upward from these depths, diversity increases and anumber of miliolids and other foraminiferal species appear, including common Rotalidiumannectens, Elphidium advenum and frequent Hanzawala nipponica. In Borehole A5/1,foraminifera are extremely rare in the Sham Wat Formation. Where present they are generallyagglutinated forms (Wilkinson, 1991).Age and Sedimentary EnvironmentThe incised channels at the base of the formation are interpreted as marginal estuarine channelsystems associated with rapid downcutting. Their form may have been maintained by strong,semi-diurnal tidal streams and high runoff. Channel base levels are eroded down to at least -54mPD. The well-developed, sub-parallel draped reflectors suggest relatively rapid sedimentationfrom a highly sediment-charged water column, with little current disturbance duringsedimentation; a conclusion supported by the preservation of small and delicate foraminifera.High terrestrial runoff is likely to have continued during sedimentation. Foraminiferalassemblages in Borehole ESC 17 (Wilkinson, 1992a) suggest that deposition of the formationoccurred within an estuarine to marine environment, with rising sea levels and increasing salinitythrough time. Analyses of sediment at -24.8 mPD indicate that they were deposited in brackishwater in depths of less than 20 m (salinity c.30%o). Above -22.8 mPD, the sediments weredeposited in fully marine conditions and slightly deeper water. Thus sea levels attained elevationscomparable to the present day. The palynological evidence supports sedimentation in anestuarine environment.Three 14 C dates have been obtained from material within the formation. Two analyses fromBorehole ESC17 were of small, less than 6 mm, disarticulated bivalves using the accelerator(AMS) method. The highest sample analysed was at -17.8 mPD, about 2.5 m below the top ofthe formation, and gave a date of 41,700 ±1700 years BP, The other sample was 7.7 m above thebase of the formation, at a depth of-24.8 mPD, and yielded a date of greater than 43,000 yearsBP. Both dates are close to the limit of the method and the large error factor casts doubt on theresult. In addition, the possibility of contamination is high. The third date, in Borehole A5/1, wasfrom a wood fragment in a sandy clay horizon with peat fragments, located at the top of theformation at an elevation of -25,6 mPD. It gave an age of 19,580 1-320 years BP. AcceptedQuaternary chronologies place world sea levels at least 100 m below present levels about thistime, although Long and Huo (1990) present evidence from the Pearl River Delta for a sea-levelhigh from 28,000 to 21,000 years BP in the region. The age of the formation is equivocal andrequires further resolution.Channel and Transgressive DepositsThese deposits form a distinctive sequence of sediments within what has been interpreted as aproto-channel of the Pearl River system, and occur only in a limited area. Consequently, they areof indeterminate stratigraphical status. They arc composed predominantly of sand, although thereis variability both laterally and vertically throughout. Intcrbedded sequences are common, andinclude fine to coarse sand, some with gravel and cobbles, silty fine sand and sandy clayey silt,The deposits are characterized by distinctive high amplitude reflectors, They most commonlyoccur as sets of shallow-angled, dipping clinoforms and sub-parallel reflectors about 3 to 4 mthick, with continuous high amplitude planar reflectors at the bottom and top of these sets, Theclinoforms, particularly on the cross-channel sections, can dip in opposite directions and in someareas minor channelling and truncation is evident, Along the channel, the planar sets have a highdegree of continuity, with foresets appearing to have a dominant down-channel dip to the east,132