The Fanling Formation is a mixed alluvial and colluvial formation of Holocene age (Fyfe et al., 2000). The formation occurs almost exclusively in onshore areas of Hong Kong where its main areas of development are in the northern New Territories on the Fanling and Yuen Long plains. The formation is also present in many other areas of Hong Kong. Alluvial components of the formation mainly occur in low-lying areas whereas colluvium is largely distributed on more steeply sloping ground. However, the alluvium and colluvium merge laterally and interdigitate, especially at the bases of slopes adjacent to floodplains.

Sediments of the formation are terrestrial in origin, and were deposited following the Holocene marine transgression. They are contemporaneous with the marine facies of the Hang Hau Formation defined offshore (Figure 11.3), and locally, especially in inshore areas, they may underlie, and interdigitate with, Hang Hau Formation sediments.

The formation has been mapped in detail on most, but not all, of the 1:20 000-scale geological maps produced by the Hong Kong Geological Survey and is described in accompanying memoirs. Depending on their origin, these onshore deposits have been variously mapped and described as Holocene alluvium, debris flow deposits, talus (rockfall) deposits, mixed debris flow and talus deposits, slide deposits, and slope debris. Deposits of the Fanling Formation often overlie sediments of the Chek Lap Kok Formation, although in some areas they may directly overlie bedrock in various states of decomposition. However, on some of the 1:20 000-scale geological maps, alluvial deposits of Holocene age, i.e. the Fanling Formation have not been differentiated from their Pleistocene equivalents, now included within the Chek Lap Kok Formation.

Alluvial deposits of the formation typically consist of well-sorted to semi-sorted clay, silt, sand and gravel. Consistency is generally soft to firm, with many coarser deposits being loose and unstable in vertical sections. The colour usually ranges from light grey in areas of higher water table, indicating mild reducing conditions, to yellowish brown, indicating oxidising conditions above the permanent water table. Mottled sediments are rarely found, this being a more diagnostic characteristic of the older, more indurated sediments of the Pleistocene Chek Lap Kok Formation.

The Fanling Formation is absent in many onshore areas such as on the steepest slopes and hill summits. Where present onshore, the alluvial deposits of the formation may be up to 10 m thick in the lower courses of the rivers, but they are generally less than 5 m thick.

Near the bases of slopes, colluvial deposits interdigitate with, and to varying extents grade laterally and vertically into, alluvial deposits of the Fanling Formation. Consequently, they are an integral part of the formation, with the status of un-named members. Colluvium characteristically occurs as sheets on open hillsides or infills drainage lines, such as those concentrated in steep tributary valleys in the present topography.

The colluvial deposits of the Fanling Formation have not been systematically mapped. Only where they are more than 2 m thick, have the deposits been depicted on the 1:20 000-scale geological maps. They are shown on the maps and described in the accompanying memoirs variously as debris flow deposits, slope debris and slide deposits. In some areas they have been differentiated from similar deposits of Pleistocene age that form part of the Chek Lap Kok Formation. On some maps in the 1:20 000-scale series, the Holocene colluvial deposits include talus deposits, but elsewhere, talus is mapped separately. Some of the colluvial deposits overlie colluvial and, or alluvial deposits, of the Pleistocene Chek Lap Kok Formation. Locally, they overlie, or interdigitate with, Holocene marine deposits of the Hang Hau Formation.

The Holocene colluvial deposits are generally poorly sorted and commonly comprise a light yellowish brown, slightly clayey sandy silt to gravelly silty sand matrix containing subangular to angular slightly decomposed boulders with thin weathering rinds a few millimetres thick. The grain size typically varies with the topography, parent rock type and distance transported from source. Strange and Shaw (1986) described colluvial deposits containing boulders averaging 0.3 m to 0.8 m in diameter, with individual boulders of up to 10 m diameter.

Few figures are available for the maximum thickness of Holocene colluvial deposits that have accumulated over the undeveloped upland areas of Hong Kong. However, colluvial deposits that are 5 to 10 m thick are frequently encountered in boreholes. More substantial deposits, ranging up to 15 m in thickness, occur, for example on the slopes at Castle Peak, Tsz Wan Shan and Ap Lei Chau.

Type section

The Fanling Formation is formally defined with the type section occurring on the low ground below Wong Kong Shan (833093 839647 s-24), at the site of the Fanling Centre development in Fanling in the northern New Territories. The formation here consists of 0.8 m of fill overlying a 1.55 m sequence of fine-grained, organic alluvial deposits, which in turn overlie an older mottled reddish brown clayey silt alluvium of the Chek Lap Kok Formation. In adjacent sections, the formation is up to 3 m thick.

The formation has an erosive base, which is immediately overlain by unmottled sandy, clayey silt sediments of the lower part of the formation. Sediments are generally grey in colour, with white sand and dark organic lenses and inclusions.

Radiocarbon dating of a sample of dark grey organic mud at 1.9 m below ground surface (i.e. 1.1 m below the top of the formation) has yielded a date of 2 100 ± 90 years BP (Guangdong Institute of Geological Science, Sample No. 934).

Depositional environment

The Fanling Formation is an actively accumulating formation. Older depositional environments associated with the formation are thought to have been broadly similar to those at present. However, changes in the fluvial discharge during the Late Pleistocene climatic amelioration and the Holocene climatic optimum at c.5 500 years BP would have probably caused variations in sediment load volumes and grain sizes.

The colluvium comprises accumulations of material transported by a broad spectrum of gravitational processes, many of which are water mobilized. They therefore include primary landslide deposits, accumulations related to the secondary mobilization of landslide deposits, and other mass movement processes. Most of the colluvium probably originated from sources on slopes with gradients of between 30 and 40 degrees, similar to recent natural terrain landslides in Hong Kong (Evans & King, 1998). Some colluvial debris has probably been derived directly from the failure of in situ rock (of weathering grade III or better), generating rock fall, rock slide or rock flow deposits. However, most colluvium is probably derived from the failure of slopes comprising mainly soil and, or, colluvium. Analogy with recent natural terrain landslides in Hong Kong (Evans & King, 1998) suggests that the dominant mechanisms of failure and transport are likely to have been debris slides, debris avalanches and debris flows, with some compound and other more complex mechanisms. Certain rock types appear more susceptible to slope failure than others. In a comprehensive review of the distribution of recent and relict landslides observed on high level aerial photography and incorporated within the Natural Terrain Landslide Inventory, Evans and King (1998) concluded that certain lithologies, such as those of the Lai Chi Chong Formation, have higher susceptibility to landslides than others. Consequently, it can be inferred that these same lithologies have also been more susceptible to generating colluvium during the evolution of the Fanling Formation.

Some colluvium is concentrated within pre-existing stream channels, where there was an increased likelihood of the debris mixing with water, enhancing its mobility and so generating debris avalanches or debris flows, further expanding the distribution of the material.

Downslope, the colluvium generally grades downslope into, and interdigitates with, fluvial and alluvial deposits. In the headwaters of the streams, these are typically high energy deposits laid down within the narrow channels of ephemeral streams during flash floods. Further down stream, as the gradient decreases, the channels broaden and begin to meander. Seasonal fluctuations in discharge, coupled with the more extreme short-term variations during typhoons and tropical depressions, have resulted in extreme variations in depositional environment and hence in the grain size and bedforms of the deposits. The importance of alluvial floodplain deposition, compared with fluvial deposition within the main distributary channels, increases downstream. Floodplain deposits reach their maximum development in the northwestern New Territories. Temporary seasonal lakes, some developed as a result of the abandonment of channels while others have developed on the floodplains, have resulted in the accumulation of organic-rich clays (Plate 11.9). Similarly, lacustrine deposits have also accumulated in lagoons developed near the coastline inland from sand bars and spits.

Age

Radiocarbon ages have been reported from the formation at several localities, mainly in the New Territories. The ages have been obtained from a variety of materials including rootlets, wood fragments, organic mud and a shell fragment. Ages range between 520 ± 112 years BP (South China Sea Institute of Oceanology) from the mud flat at San Tin to 6 760 ± 130 years BP (Guangdong Institution of Geological Sciences) from the mud flat at Yuen Long.

At Sheung Wan, Howat (1985a) identified marine mud of the Hang Hau Formation, underlying a possible colluvial lobe of the Fanling Formation The marine mud contained a fragment of wood that yielded a radiocarbon age of 8 600 ± 270 years BP.

Recently, King (1998) has reported two luminescence dates for material obtained from within a very large colluvial lobe at Sham Wat in northwest Lantau Island. The dates, which were quoted as 'best estimates', are 1 400 ± 200 years BP and 2 200 ± 300 years BP (Nordic Laboratory, Denmark).

Stratigraphical variation

No members are formally described within the formation, although there are many discrete colluvial deposits that have the status of un-named members. However, they have not yet been systematically mapped in all parts of Hong Kong enable them to be distinguished from colluvial deposits of Pleistocene age.

Regional lithological variation

- New Territories

In the New Territories, alluvium of the Fanling Formation occurs mainly as narrow deposits adjacent to the recent stream courses. These channels are incised some 1 to 7 m into the broad Pleistocene alluvial terraces on the Yuen Long, Kam Tin and San Tin plains (Figure 11.12) along the Tuen Mun and Ngau Tam Mei valleys in the northwest New Territories. They also occur on the Sheung Shui and Fanling (Plate 11.10) plains, and along the Sham Chun and Ng Tung Ho valleys (830224 841336 s-25) in the northern New Territories. Alluvium up to 10 m thick occurs in the Ho Chung Valley (843632 823821 s-26) in the eastern New Territories.

Deposits comprise semi-sorted subrounded boulders and gravelly sand in upper stream courses, and yellowish brown, well-sorted clayey sand or silt in lower courses.

Most areas of alluvium of the Fanling Formation are low-lying, the surfaces being typically at less than +10 mPD. However, some small patches of hill alluvium are present, for example on areas of granitic outcrop, between Tsing Shan and Lin Fa Shan in the northwestern New Territories, and in several parts of the eastern New Territories. This alluvium occurs in narrow, confined areas near the headwaters, especially where valley constrictions caused by geological factors have acted as barriers across the stream courses. These alluvial deposits are generally only 50 to 100 m wide and usually extend for no more than a few hundred metres.

In places along the coast, streams are dammed by sand bars or spits, behind which freshwater or brackish lagoons have developed and lagoonal or back shore deposits have accumulated (Plate 11.11).

The colluvial deposits are light yellowish brown, slightly clayey sandy silt to gravelly silty sand. The clasts are slightly to moderately decomposed, subangular to angular boulders and cobbles (Plate 11.12) with weathered rinds on the larger boulders never more than a few millimetres thick. Deposits commonly vary from 1 to 3 m thick, but are up to 15 m thick, for example on the slopes of Castle Peak, and typically overlie Late Pleistocene alluvial deposits. Offshore, they may be intercalated with, or overlie, Holocene marine deposits of the Hang Hau Formation.

- Kowloon, Hong Kong Island and neighbouring islands

Widespread deposits of Fanling Formation alluvium, up to 10 m thick, border Kowloon Bay (839282 821760 s-27) and occupy river valleys at Shek Kip Mei (834336 821980 s-28) and Kowloon Tong (836287 821171 s-29) in Kowloon, and at Happy Valley (836657 814870 s-30) and Wong Chuk Hang (835910 812125 s-31) on Hong Kong Island. These flat or gently sloping areas have mostly been obscured by buildings and other developments. Therefore, details about the distribution of alluvium have been mostly derived from aerial photograph interpretation and borehole records. Where undeveloped, the alluvial plain is either a major recreational asset, as at Happy Valley on Hong Kong Island, or is used for intensive agriculture, as in the Ho Chung Valley. The present alluvial surface generally lies 3 to 7 m below that of the Pleistocene alluvial terraces.

The alluvium typically comprises well sorted to semi-sorted grey to yellowish brown clay, silt, sand and gravel. Sand-size quartz grains are abundant where mainly derived from a granitic source area, as in Kowloon.

Small patches of alluvium are common in upland valleys. Some valleys along the coast are dammed either by sand bars, as at Shek O on Hong Kong Island, or by beach boulder barriers, as on the eastern side of Lamma Island. As a result, lagoons have formed within which organic, silty lacustrine sediments have accumulated.

The colluvium is widespread in the area but has only been differentiated locally from Pleistocene colluvium. Most of the colluvium has been interpreted as debris flow deposits (Strange & Shaw, 1986). The deposits are described as poorly sorted and comprise boulders and cobbles in a gravelly clay matrix (Plate 11.13). The boulders are up to 10 m in diameter but generally less than 1 m in size. Granitic boulders, which are corestones exhumed from deep weathering profiles, are commonly subrounded whereas volcanic boulders and cobbles are typically more angular.

The colluvium usually infills valleys in the present landscape, in contrast to the Pleistocene colluvium that commonly occurs in spurs. On the northern slopes of Victoria Peak (832687 816014 s-32), the colluvium comprises soft to firm, uniform, pale brown clayey sandy silt with angular to subrounded, slightly to moderately decomposed cobbles and boulders. The deposits extend to the former coastline and possibly beyond where occasional boreholes indicate that the colluvium interdigitates with marine deposits of the Hang Hau Formation. For example, at Sheung Wan a tunnelling excavation revealed a colluvial lobe within the marine mud (Howat, 1985b).

- Lantau Island and neighbouring islands

Because of the generally steep and rugged topography on Lantau Island, alluvium of the Fanling Formation is only developed locally. It occurs in recent stream courses incised into Pleistocene fluvial terraces (Plate 11.14). The valleys are short, steep sided and mostly choked with bouldery colluvium. The alluvium comprises gravelly sand with some subrounded boulders in upper stream courses and yellowish-brown well sorted clayey sand or silt in lower stream courses.

With the exception of the Tung Chung Valley (812642 814939 s-33) and the Shek Pik Valley (807633 810675 s-34), alluvium is largely confined to the low-lying seaward ends of valleys. The surfaces of the deposits rarely exceed +10 mPD. The deposits therefore occur either as short linear valley fills, or as wide fans along the coastal zone. At several localities along the coast, such as at Pui O, sand spits have dammed the streams and lagoonal deposits have accumulated behind the barriers.

In the Tung Chung Valley, alluvium forms a series of narrow strips, less than 100 m wide, adjacent to the present stream channels incised into the older terrace by up to 3 m. At Shek Pik an extensive alluvial deposit fills the valley now occupied by the Shek Pik Reservoir. A series of coalescing lacustrine fans mantle its eastern slopes.