Quaternary stratigraphy of Hong Kong coastal sediments

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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 identified. This permits the development of an integrated sequence stratigraphy which can be applied to regional investigations.

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 influence 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 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 effect result in the highest sedimentation rates occurring in the west.

Quaternary sediments are, for the most part, preserved in the offshore 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 influenced by climatic variations. Relic sediments have been identified in further offshore areas south of Hong Kong where areas of the sea floor are covered by pre-Holocene 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 offshore 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 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 offshore sand and gravel deposits needed for reclamation fill, 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, offshore sediments were identified 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 profiling 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, Yim, 1992, Yim et al., 1988) indicate a complex situation. As a result of this complexity, the scale at which sequences are studied, and different 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 reflection profiles (Fig. 3, Fig. 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 modified (Yim et al., 1988, Yim, 1994), and is based on the identification of discrete marine and terrestrial deposits which reflect eustatic sea-level changes linked to global glacial and interglacial cycles. An attempt is made to produce an integrated and more refined zonation. This approach will not only provide an improved stratigraphy, it should also lead to identification of remaining problems and strategies to resolve them.

Section snippets

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 used for much of the seismic profiling in Hong Kong. This configuration is considered to offer 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

Sequence stratigraphy

Three major unconformities have been identified (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 fluvial/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.

Lithological subdivision

Yim (1984) and co-workers have established a lithostratigraphy based on the identification of multiple 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

Discussion

Fyfe, 1992, Yim and Fyfe, 1992 have discussed the difficulties 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, difficult. 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

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

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