Introduction
This chapter describes the onshore Quaternary (Holocene and Pleistocene) superficial deposits and the historical development of land reclamation, which has modified the coast line of Hong Kong. The offshore Quaternary deposits are not dealt with in this interactive geological memoir.
The total area of Hong Kong is 2 755 km2. This comprises 1 104 km2of land area, which represents about 40% of the total area, while the remaining 1 651 km2, some 60% of the total area, is covered by sea. About 14% of the onshore area is covered by Quaternary deposits greater than 2 m thick (Figure 11.1). These deposits are up to 50 m thick, but generally they form thin veneers, and are referred to as 'superficial deposits' on the fifteen 1:20 000-scale geological maps and in the six memoirs produced by the Hong Kong Geological Survey (HKGS). The deposits are unlithified and were formed by a variety of processes in a wide range of environments. Almost 6% (67.4 km2) of the current land area is reclaimed land (Figure 11.2). The earliest reclamation dates back to the mid-1800s.
A lithostratigraphy has been proposed that encompasses all superficial deposits, regardless of whether they occur onshore or offshore. Wherever possible, the terminology used previously for offshore superficial deposits has been adapted to include onshore equivalents. Thus, the Pleistocene Chek Lap Kok Formation is extended onshore to embrace both alluvium and colluvium of Pleistocene age. The Fanling Formation includes all onshore Holocene alluvium and colluvium. Holocene beach, intertidal and estuarine deposits, however, are included within the offshore Hang Hau Formation.
Onshore Quaternary stratigraphy of Hong Kong
Several regional studies have produced maps that depict the broad distribution of Quaternary superficial deposits over the onshore areas of Hong Kong. Most notably, Allen and Stephens (1971) recognized, and mapped photogeologically, three classes of these deposits: alluvium, colluvium and, in littoral environments, marine sediments. Bennett (1984) carried out a review of published sources of information on the stratigraphy of the onshore superficial deposits. He concluded that they could be divided into two broad categories: fluvial deposits, which included alluvium and beach deposits, and mass wasting deposits (colluvium), which included debris flow and talus deposits. These two categories were adopted by the Hong Kong Geological Survey during the 1:20 000-scale geological mapping of the whole of Hong Kong.
Prior to the publication of The Quaternary Geology of Hong Kong (Fyfe et al., 2000), only a limited attempt had been made to erect a formal lithostratigraphy for the onshore Quaternary sequence in Hong Kong. The principal exception to this was the description of two Pleistocene alluvial formations, the Wong Kong Shan Formation and the Shan Ha Tsuen Formation, by Lai (1997, 1998) in the New Territories. With respect to chronostratigraphy, onshore Pleistocene deposits of various types have been differentiated from similar deposits of Holocene age on some, but not all, of the 1:20 000 geological maps of Hong Kong produced by the Hong Kong Geological Survey. Otherwise, the onshore Quaternary deposits have generally been described, and their distributions mapped, in terms of depositional processes and environments of deposition. This approach has been in contrast to that adopted for offshore Quaternary deposits, for which a robust lithostratigraphy has been developed, based on extensive seismic reflection and borehole data.
To redress the imbalance in treatment of the onshore and offshore Quaternary stratigraphy, Fyfe et al., (2000) presented a comprehensive lithostratigraphy that encompasses all Quaternary deposits. The proposed stratigraphy (Table 11.1 , Figure 11.3) is essentially an extension of the existing offshore stratigraphy with the addition of one new formation, the Holocene Fanling Formation. In addition, the two alluvial 'formations' previously described by Lai (1997, 1998) are incorporated in the lithostratigraphy, but as members within the Chek Lap Kok Formation: the Wong Kong Shan Member and the Shan Ha Tsuen Member.
Fill
The earliest reclamations were on tidal flats in the New Territories, in particular at Sha Tau Kok, Nam Chung, Luk Keng, Shuen Wan and Yuen Long (Guilford, 1997). These were carried out primarily to create new agricultural land. Following the land auctions of 1841, small urban reclamation began around Victoria Harbour and formal waterfront reclamation commenced in 1851. By 1886, an 8 km long strip had been formed between Kennedy Town and North Point and areas of the West Kowloon waterfront had been extended. Further phases of reclamation occurred in the 1890s, the early 1900s, the 1920s and the 1950s (Guilford, 1988). By 1967, a total area of 1050 hectares had been reclaimed (Figure 11.13).
Early reclamations were, in general, confined to relatively narrow coastal strips in shallow water. More extensive reclamation into deeper water began in the 1970s. By 1991 a further 3 050 hectares had been completed, most of which was in deeper water. Between 1991 and 1995, large reclamations were constructed in connection with the new airport and related infrastructure projects. Up to 1995 a total of 6 000 hectares of land had been reclaimed (Figure 11.13 and Figure 11.14). Up to 2005, a total of 6 739 hectares of land, or approximately 6 % of the onshore area of Hong Kong, had been reclaimed (Figure 11.2).
Initially the small, localized reclamations were completed using urban waste and spoil from nearby excavations. As urban development expanded on to the Kowloon peninsula during the 1900s, extensive hillside cutting was carried out to create building platforms. This produced a supply of fill material, mainly of residual weathered granitic rocks, that was used to reclaim onshore valleys and extend coastal sites. This material was also used for stabilisation work (Henry & Grace, 1948), construction of earth dams (Ruxton, 1957), and for mortar and aggregate (Redmond, 1936). Early reclamations were formed mainly by end-tipping directly onto the underlying marine deposits. In some instances, this practice led to displacements of the soft sediments and to the creation of mud-waves beneath the fill. Early reclamation also adopted balanced cutting and filling, creating additional development platforms on the borrow areas (Guilford, 1997). As Kowloon became more urbanized, fill had to be sought from farther afield, and land sources, usually in the New Territories, were identified.
Granular fill material from the seabed was first used to develop Kai Tak airfield (838903 820747 s-49). Kai Tak opened in 1924, but was extended in 1929 and 1931 partly using hydraulic fill from Victoria Harbour (Guilford, 1997). In the 1940s, the Causeway Bay Typhoon Shelter and Victoria Park reclamations (837572 816079 s-50) were both constructed using sand from Victoria Harbour off Hung Hom Bay (Guilford, 1988).
Offshore fill was used in increasing quantities during the 1950s (Table 11.3). Between 1956 and 1959, Kai Tak runway was realigned and extended seawards using 0.5 million cubic metres (Mm3) of granular seabed material, mostly obtained from Victoria Harbour (Henry et al., 1961). Subsequent major projects using marine sources of fill included the construction of the Plover Cove Reservoir main dam (842763 835942 s-51), and Tuen Mun New Town (814295 826300 s-52).
With the restricted availability of onshore fill sources, the expanding scale and capital cost of proposed reclamations, and the new generation of ocean-going trailing suction hopper dredgers (Plate 11.16), offshore sources of sand became increasingly attractive for both economic and environmental reasons. Two major reclamation projects, the construction of Container Terminal 6 and the Tin Shui Wai New Town, were turning points in the history of seabed sand use in Hong Kong. Container Terminal 6 at Kwai Chung was completed using about 8.6 Mm3 of seabed sand dredged from the area to the southwest of Tsing Yi (Wragge-Morley, 1988). Tin Shui Wai, a 400 hectare new town development site, was created using 24 Mm3 of reclamation fill obtained from off Black Point (Dutton, 1987). Since then, all major reclamations in Hong Kong have used suction-dredged offshore sand (Table 11.3). The largest reclamations are the Chek Lap Kok airport platform, north of Lantau Island, the West Kowloon Reclamation (834714 818874 s-53) (Plate 11.17), and the Penny's Bay in Lantau Island. Other important developments include Tung Chung New Town (811823 816755 s-55), Chek Lap Kok Airport and related infrastructure (809645 818288 s-56), Central and Wan Chai (835154 815636 s-57), Hung Hom Bay (836396 817936 s-58), and Tseung Kwan O (844768 819328 s-59).