Quaternary stratigraphy of Hong Kong coastal sediments

Quaternary stratigraphy of Hong Kong coastal sediments

Journal of Asian Earth Sciences 17 (1999) 521±531 Quaternary stratigraphy of Hong Kong coastal sediments A.M. Davis* Department of Earth Sciences, Un...

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Journal of Asian Earth Sciences 17 (1999) 521±531

Quaternary stratigraphy of Hong Kong coastal sediments A.M. Davis* Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, SAR, China Accepted 3 February 1999

Abstract Geotechnical borehole data from around Hong Kong record a lithostratigraphic succession extending at least back to the early Pleistocene (Oxygen Isotope Stage 12). From these data, together with seismic data, four unconformity bounded units which accumulated between the mid Pleistocene and Holocene can be identi®ed. This permits the development of an integrated sequence stratigraphy which can be applied to regional investigations. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Hong Kong, Special Administration Region (SAR), is situated immediately south of the Tropic of Cancer on the South China coast, near the mouth of the Pearl River (Zhujiang) Delta (Fig. 1). Present-day climate is subtropical, characterized by hot, wet summers and cool, dry winters associated with the southwest and northeast monsoon, respectively. Waters around Hong Kong may be hydrologically divided into three main zones (Morton et al., 1996); a western zone which is predominantly estuarine due to the in¯uence of the Pearl River Delta, an eastern zone which is predominantly oceanic, and a central zone which is intermediate in nature. Onshore areas of Hong Kong are dominated by steep hills of Mesozoic granite and associated volcanic successions. The Pearl River is the major drainage feature in the area today and is an important factor in modern sedimentation patterns in the western territorial waters of Hong Kong. The Pearl River delta was formed during the Holocene through rapid progradation of several large tributaries of the river (Huang, 1984). At the beginning of the Holocene, the coastline lay to the north of the present-day Guangzhou City. The river presently discharges approximately 300  109 m3 of

* Fax: +852-25176912. E-mail address: [email protected] (A.M. Davis)

water and 90  106 tons of sediment annually into the estuary (Gu et al., 1990). The delta is estimated to be prograding an average 50 to 60 m/year with a maximum of 100 m/year (Huang and Song, 1981). The north±south orientation of the estuary and the Coriolis e€ect result in the highest sedimentation rates occurring in the west. Quaternary sediments are, for the most part, preserved in the o€shore waters, but they may be absent in areas where strong tidal scouring occurs such as in the Ma Wan Channel and Lei Yue Mun (Fig. 1). Development of Quaternary sediment distribution patterns in coastal waters around Hong Kong has been in¯uenced by climatic variations. Relic sediments have been identi®ed in further o€shore areas south of Hong Kong where areas of the sea ¯oor are covered by preHolocene sands and gravels (Niino and Emery, 1961). Large-scale coastal engineering projects resulting from an increasing demand for land in Hong Kong have led to an unprecedented amount of o€shore site investigation work. Two separate and distinct databases, one seismic, the other borehole log data, have been collected during geotechnical investigation associated with the extraordinary urban development of Hong Kong. These data present an opportunity for development of a high-resolution stratigraphic zonation for shallow marine sediments that can be applied elsewhere along the margin of South China. Major engineering projects to date include the construction of the new Hong Kong Airport at Chek Lap Kok, the

1367-9120/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 0 7 - 3

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Fig. 1. Map of the Hong Kong SAR area showing the localities and boreholes referred to in the text.

Mass Transit Railway, the Eastern and Western Harbour Crossing tunnels, coastal land reclamation including container terminals (both existing and planned) and coastal reservoirs at High Island and Plover Cove. Coupled with the exploration for and exploitation of o€shore sand and gravel deposits needed for reclamation ®ll, these projects provide exposures, dredged samples and boreholes which can be used to study the Quaternary sediments from this part of the inner shelf of the South China Sea. The earliest borehole descriptions in Hong Kong date back to at least the 1950 s when Kai Tak Airport was developed (Yim and Fyfe, 1992). An outline of the history of Quaternary stratigraphic investigation in Hong Kong was undertaken by Fyfe (1992) with supplementary information provided by Yim and Fyfe (1992). Initially, o€shore sediments were identi®ed as a basal alluvial deposit with an overlying marine deposit (Berry, 1959; Holt, 1962). Since the mid-eighties an intensive network of seismic surveys has been conducted (Fyfe et al., 1997) (Fig. 2). This pro®ling has provided further valuable information on the subsurface geology. Analysis of marine seismic data (Strange and Shaw, 1986; Evans, 1988; James, 1993), sedimentological, palaeontological and engineering properties (Yim and Li, 1983; Yim, 1984, 1992; Yim et al., 1988) indicate a complex situation. As a result of this com-

plexity, the scale at which sequences are studied, and di€erent approaches to the analysis of data, two stratigraphic schemes for the subdivision of Quaternary sequences are currently in use in Hong Kong. A seismic stratigraphy devised by the Hong Kong Geological Survey (Strange and Shaw, 1986) is based on the interpretation of seismic re¯ection pro®les (Figs. 3 and 4) has since been expanded to incorporate a sequence stratigraphy (Fyfe et al., 1997). A lithostratigraphic zonation (Fig. 5) was introduced by Yim (1984), and subsequently modi®ed (Yim et al., 1988; Yim, 1994), and is based on the identi®cation of discrete marine and terrestrial deposits which re¯ect eustatic sea-level changes linked to global glacial and interglacial cycles. An attempt is made to produce an integrated and more re®ned zonation. This approach will not only provide an improved stratigraphy, it should also lead to identi®cation of remaining problems and strategies to resolve them.

2. Seismic stratigraphy Approximately 5000 line kilometres marine seismic tracks have been obtained from the coastal waters of Hong Kong (Fig. 2). Surface-towed boomer systems, with a frequency range of 400±4000 Hz have been

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Fig. 2. Marine seismic tracks in the territorial waters of Hong Kong SAR (from Fyfe et al., 1997). Line A±A ' corresponds with that shown on Fig. 3.

used for much of the seismic pro®ling in Hong Kong. This con®guration is considered to o€er the best results in terms of resolution and penetration (Premchitt et al., 1992). For interpretation, a constant seismic velocity of about 1.6 km/s was chosen as this was considered to be the best approximation to the acoustic velocity of Quaternary sediments in Hong Kong (Evans et al., 1994). Seismic resolution may be hampered by acoustic turbidity. The presence and build-up of biogenic gas greatly attenuates seismic signals causing gas masking where large tracts of the sea ¯oor are rendered acoustically impenetrable or, at best, producing chaotic re¯ectors. However, recent work by Wong (1996) has shown that much more information can be obtained in areas of gas masking than previously thought. Deepseated gas release may be used to provide stratigraphic information. For example, low permeability Holocene sediments in some areas can be seen to act as an impermeable blanket for the gas released. Thus the build-up of gas beneath these sediments may be an additional useful criterion for recognizing the Holocene/ pre-Holocene boundary. Gas is also often associated

with development of channels in the Pok Liu member (Fyfe and Shaw, in press). From the seismic records four units have been de®ned by Fyfe et al. (1997) and are: Hang Hau, Waglan, Sham Wat, and Chek Lap Kok `formations' (Fig. 3). Distinct lithostratigraphic sections which correlate with the seismic units occur in borehole sections and the formations are de®ned from these. The Hang Hau Formation (Strange and Shaw, 1986) is widely distributed on the sea-bed of Hong Kong and the type section is the continuously sampled borehole JBS1/1A (Fig. 1) from Junk Bay. The formation was later subdivided into three members (Fyfe et al., 1997). The Tung Lung and Pok Liu members are the lowermost. They are restricted and are interpreted to represent channel ®ll deposits. The Tseung Kwan O member is more widespread and is characterized by extensive ¯at re¯ectors. It is comprised of soft to very soft greenish to bluish grey silty clay with shell debris. Locally the sediments may contain sand and gravel horizons (Strange and Shaw, 1986). The Hang Hau Formation is typically 10 to 20 m thick (Fyfe et al., 1997) and has an unconformable basal contact. The

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Fig. 3. Simpli®ed diagram illustrating the overall trend of seismic stratigraphy described by the Hong Kong Geological Survey (after Fyfe et al., 1997).

depositional environment has been interpreted as representing a transgressive estuarine to marine sequence with sediment derived from the Pearl River (James, 1993; Fyfe et al., 1997).

The Waglan Formation has recently been described (Fyfe and Shaw, in press) from southeastern waters. The type section is de®ned from borehole HKGS-A (Fig. 1) near Waglan Island. The formation consists of

Fig. 4. Log of borehole ESC17 showing corresponding seismic interpretation (after Fyfe and James 1995).

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Fig. 5. Lithostratigraphic interpretation of logged sections and boreholes from High Island, Sheung Wan, and WB7 (after Yim 1984; Yim et al., 1990, 1996).

two seismic units. The lower unit is comprised of chaotic re¯ectors and represented by 2±3 m of basal sand. The upper unit, characterized by sub-parallel low amplitude re¯ectors, comprises up to 10 m of soft grey clayey silt with mottled bands. The Sham Wat Formation (James, 1993; Fyfe and James, 1995) is named after the Sham Wat locality on the northwest coast of Lantau Island (Fig. 1). The type section is based on the borehole ESC17 located to the north of Sham Wat (Fig. 4). The formation is characterized by draped intraformational re¯ectors. It is not as extensively developed as the Hang Hau Formation (James, 1993) being con®ned to the western areas of Hong Kong. Sediments comprise ®rm grey silty clay with mottled horizons and some shell material. The upper boundary of the Sham Wat Formation is erosive and it is commonly overlain by the Hang Hau Formation (Fyfe and James, 1995). The lower boundary is distinctive and characterized by deep channels (James, 1993; Fyfe and James, 1995). The depositional environment is interpreted as a marginal estuarine channel system associated with rapid down-cutting during a fall in sea level (Fyfe and James, 1995). The exact relationship between the Waglan Formation and the Sham Wat Formation is uncertain. Fyfe et al. (1997) debate whether the

Waglan Formation is a later stage of the Sham Wat or if it was deposited during a last glacial interstadial. The Chek Lap Kok Formation type section is the continuously sampled borehole B13/13A drilled on the test embankment at Chek Lap Kok (Strange and Shaw, 1986) (Fig. 1). The formation is characterized by variable chaotic re¯ectors and is the thickest and most complex of the seismic formations. Sediments of the Chek Lap Kok Formation are widely distributed and comprise a mixture of silts, sands and gravels with plant debris and organics commonly found (Strange and Shaw, 1986). Massive sand horizons are also present. The formation is interpreted to have been deposited under submarine to subaerial conditions in estuarine, intertidal to ¯uvial environments (Strange and Shaw, 1986; James, 1993). Few age constraints are available for the seismically de®ned formations. The dates that are available are not always of high quality, thus there are few reliable age-based tie points to borehole stratigraphy. Shells from near the base of the Hang Hau Formation in borehole JBS1/1A (ÿ19.5 m PD) yielded a radiocarbon age of 8080 2 130 years BP (Strange and Shaw, 1986). Subsequent work has produced radiocarbon dates of 9500 to 9000 years BP for the overlying Pok Liu Member (ÿ34.5 m PD) and 7990 years BP for the

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Table 1 Stratigraphy of seismically de®ned units around Hong Kong SAR waters and chronostratigraphic constraints on their age of formation Lithological unit

Age (Ka)

Oxygen isotope stage

Control

Hang Hau Formation Waglan Formation Sham Wat Formation Chek Lap Kok Formation

< 8100 80,000 125,000 >135,000

1 5c 5e 6

14 C (Strange and Shaw, 1986) Inferred (Fyfe et al., 1997) Inferred (Fyfe et al., 1997) Fyfe et al. (1997)

Tseung Kwan O member (ÿ14.8 m PD) (Fyfe et al., 1997). Radiocarbon dates of 30,000 years and >40,000 years have been obtained from the Waglan Formation but are considered to be minimum dates (Fyfe et al., 1997) while radiocarbon dating of the Sham Wat Formation yielded dates of 40,000 (1,700 years BP and 43,000 years BP (James, 1993). In the absence of reliable dating of the Sham Wat Formation, Fyfe et al. (1997) have inferred a stage 5e age (Table 1) by comparison with the global sea-level curve of Chappell and Shackleton (1986) and Shackleton (1987). The Chek Lap Kok Formation is considered to be late-Pleistocene in age. The most recently published dates (Table 3) obtained for the Chek Lap Kok Formation are a thermoluminescence date of 78,000 2850 years BP and an optically stimulated luminescence date of 80,000 2 9,000 years BP (Owen et al., 1995) from borehole A5/2 in the West Lamma Channel (Fig. 1).

3. Sequence stratigraphy Three major unconformities have been identi®ed (Fyfe et al., 1997) and these coincide with formation boundaries. The oldest of these surfaces is correlated with the base of the Quaternary sequence onlapping bedrock. The two upper boundaries are related to ¯uvial/tidal entrenchment surfaces (Fyfe et al., 1997) during the Eemian between the Chek Lap Kok Formation and the Sham Wat Formation and the last marine transgression between the Waglan Formation and the Hang Hau Formation, respectively. Two ravinement surfaces are also recognized. The ®rst of these is correlated with the late-Weichselian between the Sham Wat Formation and the Waglan Formation and the latter with an intra-Holocene transgression that is considered to be diachronous across the region (Fyfe et al., 1997). This is a provisional sequence stratigraphy and has not been developed any further.

4. Lithological subdivision Yim (1984) and co-workers have established a lithostratigraphy based on the identi®cation of mul-

tiple marine and terrestrial horizons from boreholes and excavations. Borehole WB7 is located in the West Lamma Channel about 2 km east of Cheung Chau (Fig. 1) in water depths of 10.2 m below PD (principal datum for Hong Kong is about 1.23 m below mean sea-level). The borehole was drilled using a power swivel machine mounted on a barge. It was continuously sampled except for the upper 2 m using either undisturbed 1 m piston or 0.5 m U76 liner samples, while disturbed samples were collected at 0.5±1 m intervals. Borehole C/B151 was obtained by shell and rotary boring using a Longyear 34 rig on the seabed o€ the High Island, East Dam, site after the area between coffer dams was pumped dry. The borehole was at a depth of 5.92 m below PD and extended to 41.35 m below PD. The borehole was discontinuously sampled by mazier with a total recovery of 70%. Borehole JBS1/1A located in Junk Bay in the eastern New Territories was a continuously sampled borehole extending from 8.1 m below PD to 39.76 m below PD. A section at Sheung Wan (Fig. 1) exposed during land reclamation in Central District, Hong Kong Island, was continuously sampled from 0 m below PD to 30 m below PD. Yim (1994) proposed a model for the o€shore Quaternary geology of o€shore regions of Hong Kong based on a succession of alternating marine (M) and terrestrial (T) units. The most recent marine deposits are termed M1 while the youngest terrestrial deposits are termed T1 with subsequent underlying layers M2, T2, M3, etc. Not all horizons recognized by Yim (1994) are present at any one borehole locality. However, regional correlation of boreholes has allowed ®ve cycles to be identi®ed. Of the boreholes studied, WB7 (Figs. 5 and 6) provides the most continuous and longest record. The core is 60 m in length and was continuously sampled except for the top 2 metres. The stratigraphic units present include: Unit M1 comprises very soft to soft, grey green silty clay with abundant complete shells and shell fragments, many of which are well preserved. It has a total thickness of about 7 m. The upper boundary is the sea ¯oor and its contact with the underlying M2 unit is unconformable. Unit M2 is 6.6 m thick. It is characterized by green grey silty clay and silt that is usually ®rmer and paler

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Fig. 6. Borehole WB7 from the West Lamma Channel showing correlation between seismic interpretation and lithological subdivision. The only surface which clearly correlates is the entrenchment surface developed between the Hang Hau Formation and the Chek Lap Kok Formation which corresponds to the break between T2 and M3 of Yim et al. (1997). The ravinement surface between the Tseung Kwan O (TKO) member and the Pok Liu (PL) member of the Hang Hau Formation possibly correlates with the boundary between M2 and T2 of Yim et al. (1997).

in colour than M1 and has a noticeably lower calcareous shell content. Shells where present have a chalky appearance with little nacreous material preserved. Both upper and lower contacts are unconformable.

Unit T2 is 3.8 m thick and comprises yellowish brown and grey ®ne to coarse grained sand with coarse gravel common in places. The upper and lower contacts are unconformable.

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Table 2 Borehole based detailed subdivision of o€shore Hong Kong Quaternary lithostratigraphy. `M' and `T' units are inferred marine and terrestrial units respectively. Approximate chronological constraints on age of formation are given where available Lithological unit

Age (Ka)

Oxygen isotope stage

Control

M1 T1 M2 T2 M3 T3 M4 T4 M5 T5

< 8100

1

14

130,000±142,000

5 6 7 8 9 10 11 12

225,000±260,000

C (Strange and Shaw, 1986) Inferred Ur-series (Yim et al., 1990) Inferred Nanofossils (Yim et al., 1997) TL (Yim, written comm., 1996) Inferred Inferred Inferred Inferred

Table 3 Summary of available geochronological data for Quaternary coastal sediments o€shore Hong Kong Borehole no.

Location

A5/2 W. Lamma Ch. A5/2 W. Lamma Ch. A5/2 W. Lamma Ch. A5/2 W. Lamma Ch. B15 Chek Lap Kok ESC17 N. Lantau ESC17 N. Lantau ESC17 N. Lantau HKGS-A S.E. Hong Kong HKGS-A S.E. Hong Kong HKGS-A S.E. Hong Kong HKGS-A S.E. Hong Kong HKGS-A S.E. Hong Kong HKGS-B S.E. Lamma HKGS-B S.E. Lamma HKGS-B S.E. Lamma HKGS-B S.E Lamma HKGS-B S.E. Lamma JBS1/1A Junk Bay M28 Chek Lap Kok M28 Chek Lap Kok M35 Chek Lap Kok M67 Chek Lap Kok WBH1 Chek Lap Kok WBH1 Chek Lap Kok WBH1 Chek Lap Kok WBH1 Chek Lap Kok WBH1 Chek Lap Kok WBH1 Chek Lap Kok Grab sample East Harb Cross Grab sample East Harb Cross Grab sample East Harb Cross Grab sample East Harb Cross Grab sample East Harb Cross Grab sample East Harb Cross Grab sample East Harb Cross Section Sheung Wan Section Sheung Wan Section Sheung Wan 17.00 Section Sheung Wan Section Sheung Wan a

Depth (m) below PD 3.34 6.15 13.80 13.95 20.00

Sample material

Dating techniquea

Shell Shell Wood Sandy silt

14

C C 14 C TL 14 C 14 Foram C 14 15.10 Foram C 14 17.80 C 14 35.00 Shell C 14 C 36.50 Shell 14 40.00 Shell C 14 55.70 Shell C 14 52.40 Shell C 14 C 22.80 Shell 14 25.50 Shell C 14 27.00 Shell C 14 30.00 Shell C 14 C 34.00 Shell 14 18.00 Shell C 14 25.50 Wood C 14 24.20 Wood C 14 C 26.50 Wood 14 25.50 Wood C 12.28±12.90 Sandy silt TL 24.95±25.40 Sandy silt TL 26.85±27.40 Sandy silt TL 28.85±29.40 Sandy silt TL 32.45±32.90 Sandy silt TL 34.45±34.90 Sandy silt TL 14 20.80 Organic mud C 14 20.00 Organic mud C 14 20.00 Bivalve C 20.00 Crassostrea gigas 14C 14 20.00 Ostrea C 14 20.00 Andara sp. C 20.00 Andara sp. U-series 18.00 Crassostrea gigas 14C 14 15.50 Wood C 36,2302680 Crassostrea gigas 14C 14 17.00 Crassostrea C 20.00 Crassostrea gigas U-series

TL=Thermoluminescence, OSL= optically stimulated luminescence.

14

Age BP

Reference

39502 95 78402 120 35,0002 1250 78,0002 15% >41,000 21702 60 79602 85 41,7002 1700 20252 65 35752 60 48702 75 32,5002 1000 291002 1300 825265 39352 65 54252 85 89202 90 93102 80 80802 130 26,7702 840 37,5902 1590 33,4402 1740 276602 590 >53,000 247,0002 98 225,0002 50 260,0002 38 >48,000 >60,000 239802 2055 21,5802 1210 31,5002 2000 34,8802 1230 39,9102 24,60 39,4602 2320 142,0002 2000 30,5602 610 31,4502 610 Yim (1986) 45,7002 2000 130,5002 5300

Owen et al. (1995) Owen et al. (1995) Owen et al. (1995) Owen et al. (1995) James (1993) James (1993) James (1993) James (1993) Fyfe and Shaw (in press) Fyfe and Shaw (in press) Fyfe and Shaw (in press) Fyfe and Shaw (in press) Fyfe and Shaw (in press) Fyfe and Shaw (in press) Fyfe and Shaw (in press) Fyfe and Shaw (in press) Fyfe and Shaw (in press) Fyfe and Shaw (in press) Strange and Shaw (1986) R.M.P. Econ. (1982) R.M.P. Econ. (1982) R.M.P. Econ. (1982) R.M.P. Econ. (1982) Yim (written communication, Yim (written communication, Yim (written communication, Yim (written communication, Yim (written communication, Yim (written communication, Yim et al. (1990) Yim et al. (1990) Yim et al. (1996) Yim et al. (1996) Yim et al. (1996) Yim et al. (1996) Yim et al. (1996) Yim (1986) Yim (1986) Yim et al. (1990) Yim et al. (1990)

1996) 1996) 1996) 1996) 1996) 1996)

A.M. Davis / Journal of Asian Earth Sciences 17 (1999) 521±531

Unit M3 comprises 9.1 m of ®rm mottled greyish yellow and brown clayey silt. The mottled appearance persists for 4.5 m before changing to a dark grey clay silt. Gravel is present near the base of the unit. Upper and lower contacts are unconformable. Unit T3 is represented by a 1.25 m deposit of silty ®ne to coarse grey and white sand with sub-angular ®ne gravel towards the base. Unit M4 unit comprises 14.5 m of soft to ®rm mottled grey, yellow and brown clayey silt which changes to dark grey and grey clayey silt towards the base. The mottling indicates the presence of a desiccated crust which persists for 9 m. Sideritized plant rootlets are present at several levels. Unit M5 comprises 2 m of yellowish brown silty clay with grey silty clay towards the base of the unit. The upper and lower contacts are unconformable. The presence of mottled sediments in the upper 1.3 m is evidence of a desiccated crust. Unit T5 unit has a total thickness of 7 m, consisting of sti€ white pink-grey clay unconformable on top of completely decomposed granite. Finally a residual soil unit consisting of decomposed granite is developed on top of basement rocks. Age control is provided by uranium-series dates obtained by Yim et al. (1990). Specimens of Crassostrea gigas (H4894) and Anadara sp. (H5261) from the M2 unit at Sheung Wan have yielded dates of 130,500 2 5300 and 142,0002 20,000 years BP respectively (Tables 2 and 3). Uranium-series dates obtained from material other than unrecrystallized coral, e.g. shells, are open to question due to the diculty in identifying a closed system (Stearns, 1984). However, Yim et al. (1990) argued that the ®nite date obtained for the C. gigas specimen is within acceptable limits. The 230Th/232Th activity ratio is well in excess of 20 and no correction for detrital thorium was required. The shell material may have remained in a closed geochemical system because the initial 234 U/235U activity ratio is within two standard deviations of the 1.15 20.01 value for sea water recommended by Thurber et al. (1965). This age is corroborated by palynological results and the rare occurrence of the nannofossil Emiliana huxleyi in M2 (Yim et al., 1997). This calcareous nannofossil ®rst became abundant in deep sea deposits during oxygenisotope stage 4 (Thierstein et al., 1977). Three TL dates, 247,0002 98 (2s), 225,000 250 (2s) and 260,000 2 38 (2s) years BP, have recently been obtained from sandy silt horizons in borehole WBH1 from Chek Lap Kok (Yim, written communication, 1996). These ages constrain the fourth terrestrial unit to approximately oxygen isotope stage 8 (Table 2). The greatest diculty with a borehole-based approach to lithostratigraphy is the absence of a com-

529

plete sequence in any one borehole. This, combined with the lack of age control, means that even tentative correlation across the region is dicult. Absence of one or more marine or terrestrial units in any given borehole may cause discrepancies when a `counting backwards' approach is adopted (Table 2). For example the T1 unit between M1 and M2, while not represented in WB7, is present at the East Dam site of High Island and at the Sheung Wan section (Fig. 5), and is poorly developed elsewhere in the SAR. Similarly the T4 unit is absent in WB7 but is present in boreholes such as those from the Chek Lap Kok Airport site. The presence or absence of the `T' units may depend on whether or not alluvial deposition occurred during low sea-level stands of the glacial periods (Yim, 1994). Even if such deposition did occur, sediments may have been eroded during subsequent sea-level rise. Lateral continuity is a concern with regard to identi®cation and correlation of desiccation crusts. Desiccation crusts are not preserved uniformly at boundaries and are not seismically signi®cant. As most of the Quaternary sediments around Hong Kong developed in association with the growth of the Pearl River delta, caution is required when counting back through M and T layers and inferring correlation between boreholes. It is grossly simplistic to assume that all horizons should be laterally persistent and be present at all sites, especially as the locus of Pearl River deposition is likely to have shifted on numerous occasions. 5. Discussion Fyfe (1992) and Yim and Fyfe (1992) have discussed the diculties in reconciling the seismic and borehole data. The problem is largely related to the variability in physical properties of the sediments involved which make seismic interpretation, particularly of the Chek Lap Kok Formation, dicult. The discontinuous nature of desiccation crusts also presents problems for their correlation across the area. Such desiccation crusts form the basis of the stratigraphic correlation of Yim (1994), but are not seismically recognizable. Fig. 6 shows the seismic interpretation of the o€shore sequence from the West Lamma Channel. Also shown is the sedimentary log of borehole WB7 which intersects the seismic pro®le. The lithological subdivision shows the presence of two marine units M1 and M2 within the Hang Hau Formation and three marine units within the Chek Lap Kok Formation. The ravinement surface between the Tseung Kwan O and the Pok Liu members of the Hang Hau Formation (Fig. 6) is tentatively correlated with the boundary between the M1 and the T2 unit of the lithological subdivision.

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The entrenchment surface at the base of the Hang Hau Formation clearly correlates with the base of the T2 Unit. The boundary between the M1 and M2 units represents the Holocene/pre-Holocene boundary of Yim's stratigraphy but is not seismically signi®cant. This is one of the major discrepancies between the two stratigraphies. The implication here is that the T1 unit which represents last sea-level lowstand and is found in more nearshore localities (Fig. 5) is missing in the o€shore boreholes. Either it was never deposited in these localities or that the water depth in these areas was sucient during the lowstand to prevent exposure. Further di€erences in interpretation of the Holocene/ pre-Holocene boundary are suggested by Wong (1996). Evidence collected from areas prone to acoustic turbidity suggests that the M1 unit acts as an impermeable layer impeding the rise of gas from lower levels. It is suggested that variation in permeability between Holocene and pre-Holocene sediments is due to the di€erent diagenetic histories each unit has experienced. Desiccation crusts identi®ed from the borehole at the top of the M3, M4 and M5 units are not observed in the seismic re¯ection pro®le. Fyfe (written communication) suggests that there are re¯ectors within the Chep Lap Kok Formation but that they tend to be chaotic and impersistent. Whether or not these re¯ectors can be correlated with the presence of desiccation crusts will require further investigation. A better understanding of the succession relies on further integration of seismic sequences which provide the lateral correlation with borehole records which provide the vertical resolution. Only then can the sequence stratigraphy be further developed and used for regional comparison with stratigraphies from the Pearl River delta (Long and Huo, 1990) and the South China Sea (Feng et al., 1988).

Acknowledgements The author would like to thank Alan Fyfe and C. Evans for critical reviews of the manuscript and helpful comments. Thanks to Jonathan Aitchison and Peter Flood and Wyss Yim for comments on drafts of this manuscript. The author is grateful to governmental and non-governmental organizations in Hong Kong who have provided relevant information and borehole samples. This paper is a contribution to the International Geological Correlation Programme Project No. 396 `Continental Shelves in the Quaternary' and the Commission on Quaternary Shorelines, International Union for Quaternary Research. The research is supported by grants from the University of Hong Kong and the Hong Kong Research Grants Council.

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