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The Geology of Hong Kong (Interactive On-line)
Hang Hau Formation - hh

The youngest sediments found offshore are those of the Hang Hau Formation, which is mainly Holocene in age, but may include latest Pleistocene southwest of Hong Kong. The formation has been subdivided into four members: the early channel-filling Tung Lung and Pok Liu members, the locally restricted Kwo Chau Member and the more widespread Tseung Kwan O Member (Table11.2) (Figure 11.3).

The formation as a whole is interpreted as having been laid down during the last post-glacial rise in global sea level, which commenced in the Late Weichselian at around 18 000 years BP. Fluvial entrenchment gave way to deposition of fluvial sediments, particularly in the east. This phase was succeeded by marine inundation of the area, firstly creating mudflats with intertidal channels, then a shallow shelf sea. The nature of this inundation varied in its timing and character across the area.

The beaches and intertidal, or estuarine, deposits are formed in the coastal and intertidal zones by the marine reworking of fluvially-derived sediments. These features were mapped on the Hong Kong Geological Survey 1:20 000-scale maps using a combination of aerial photograph interpretation and field identification. They were classified as lithological units and distinguished by their mode of formation. In this account, they are interpreted as two un-named members within the Hang Hau Formation.

The deposits of these two un-named members were classified by their genesis, and subdivided on the basis of variations in lithology and age. By virtue of their accessibility during field mapping, they were previously considered, and described in the Hong Kong Geological Survey memoirs, along with the onshore superficial deposits. In this account they are instead assigned in Fyfe et al., (2000) to the offshore Hang Hau Formation primarily because of the dominance of tidal reworking in their origin. Nevertheless, a brief description of the beaches and intertidal is included here owing to their close association, and continuity, with onshore superficial deposits.

Un-named beach members

Beaches are laterally extensive, usually narrow, deposits of loose sand and gravel that accumulate in embayments, extending landwards from the low water mark to the inland limit of wave influence. This limit may be a cliffline, rising hill slope or back beach deposit. The upper limit of wave action under prevailing conditions is usually up to about +3 mPD. Most beaches occur in relatively sheltered locations, in environments characterised by low energy, constructive wave activity. Sandy beaches have developed along most of the mainland coastline of Hong Kong, and around the majority of islands.

Beach deposits characteristically consist of clean, pale yellow sand, with occasional gravel and shell fragments. Seawards, these sands are usually laterally continuous with the littoral and sublittoral sands and muds of the Hang Hau Formation.

Beach deposits in the more exposed sections of coast, particularly southeasterly facing bays along southern coasts, may include pebbles and cobbles, and even boulders (Plate 11.15). Steep, cliffed shorelines are common along these high energy sections of the coastline. Cliffed coasts usually have only narrow boulder beaches fringing the bases of the cliffs. This material accumulates as aprons of coarse rockfall debris derived from the wave eroded cliffs, or from coastal exposures of colluvium. Strong wave action winnows out the fines leaving a lag of coarse debris.

In some areas, most commonly along exposed coasts with a southeasterly aspect, the upper limit of the beach may extend up to about +6 mPD. These deposits commonly comprise gravelly coarse sand with scattered pebbles, characteristics that suggest they formed in a high energy environment. Although several of these features have been mapped as raised beaches on 1:20 000-scale maps, they are probably storm beaches formed by strong wave surges associated with typhoon conditions. This interpretation is supported by the generally coarser grain size of the higher level beach deposits. Strictly, therefore, they should be regarded as storm beaches. True raised beaches, resulting from deposition during a period of higher sea level or from tectonic uplift, have not been unequivocally identified in Hong Kong.

Mostly inactive, high level storm beaches exist behind the active beaches. Diurnal tidal variations and seasonal wave activity rarely impinges upon them, except during typhoon storms. In some localities, such as at Pui O Wan (816058 811205 s-35) on Lantau Island, storm beach ridges form barriers across the mouths of rivers. Freshwater lagoons have developed behind the barriers and extensive alluviated tracts extend inland along the river courses.

Rock platforms, extending out from the bases of coastal cliffs or surrounding rocky islands, are a common feature of coastlines in the eastern part of Hong Kong. They usually have only a thin veneer of sediment and are characterised by scattered boulders, numerous rock pools, and occasionally small stacks or rock residuals. In these areas there is usually a distinctive boundary between the exposed rock platform and the offshore sand or mud deposits. The irregular rock surface, covered with a diversity of intertidal flora and fauna, usually appears as a darker and mottled phototone that ends abruptly at the boundary with the smoother, more homogeneous marine mud or sublittoral sand sheet at the low water mark.

The most extensive beaches occur at Tai Long Wan (856944 830566 s-36), Long Kei Wan (856671 826041 s-37) and Clear Water Bay (847696 816561 s-38) in the Sai Kung district, Deepwater Bay (837308 811857 s-39) and Big Wave Bay (843480 811999 s-40) on Hong Kong Island, Cheung Sha (813414 810607 s-41) , Pui O Wan (816105 810844 s-42) and Silver Mine Bay (817931 814346 s-43), on Lantau Island, and Butterfly Beach (815990 826683 s-44) and the Cafeteria beaches (816646 826133 s-45)in the western New Territories. Several of the largest beaches in Hong Kong have been gazetted for recreational uses and some, notably Repulse Bay, have been artificially replenished for this purpose, following recent erosion of the features.

Un-named intertidal members

Estuarine intertidal deposits are of mixed alluvial and marine origin. The deposits form a wide transitional zone in lowland areas where wide, shallow gradient river valleys meet the sea along sheltered sections of the coastline. Fine-grained fluvial sediments accumulate in deep coastal re-entrants or wide, sheltered embayments forming extensive mudflats. Intertidal flats extend between the high and low water marks.

Branching and anastomosing distributaries are commonly developed across these flats, with patterns of deeper base-flow master channels, and shallower high tide channels separated by wide, flat and very low interfluves. Current ripple patterns are usually developed on the muddy surface of the interfluves, created by the waters of the diurnal tidal inundations. The floors of the master channels are usually sandy. Sand splays and sand ribbons cross the flats, spreading out from the channels during times of increased fluvial discharge. Mangroves, and associated halophytes, are a common feature of the saline mudflats and channel margins.

Intertidal deposits usually consist of grey clayey silt over the mudflats. Grey silty sand characterises the channel floors, with the mud adjacent to the channels being sandy in places or containing sandy intercalations. Flaser bedding is a common sedimentary feature. Plant remains, dark grey, organic clayey silt lenses and shell fragments are common throughout the deposits.

Intertidal deposits are well developed around the coast in many parts of Hong Kong. The most extensive occurrences are around the shores of Deep Bay (816176 836369 s-46) and Starling Inlet (839961 843387 s-47) in the New Territories. Around the coast of Lantau Island, intertidal deposits occur at Tai O (803831 812475 s-48) and adjacent Yi O, at Sham Wat, and at Shui Hau Wan. Many small coastal streams in the eastern New Territories have muddy, mangrove-fringed, tidal lower courses.

Intertidal muds are difficult to identify unequivocally in borehole samples, appearing similar to sandy organic coastal muds. However, it is suspected that mangrove- clad intertidal mudflats were probably widely developed under the sites of many of the present New Town reclamations. Prior to development, they probably occurred at Tuen Mun, Tin Shui Wai, Tai Po, and fringing the Sha Tin Valley. Intertidal mudflats were also more extensive around the shores of what is now the Plover Cove Reservoir, before the freshwater level of the artificial lake was raised, and under the site of Tung Chung New Town on Lantau Island.