The Lok Ma Chau Formation was first defined by Williams (1943) and was formerly considered to be Permian (Heim, 1929). The formation was later grouped with Early Jurassic strata by Allen and Stephens (1971). Lai (1977), however, interpreted it as Carboniferous on the basis of correlation with sequences in Shenzhen. This age has been substantiated and further refined to Early Carboniferous by Ha et al., (1981), Lai and Mui (1984), Lee (1985) and Frost (1992).
The contact with the underlying Yuen Long Formation is variable. In places, the boundary is gradational with beds of marble and intercalated siltstones of the Lok Ma Chau Formation, while elsewhere it is sharp (Frost, 1992).
The Lok Ma Chau Formation is generally thicker than the Yuen Long Formation, reaching 700 m in the western New Territories. On West Brother island, a graphite mine extended to -90 mPD (Woods & Langford, 1991), yet did not reach the bottom of the succession, which is estimated to be at least 300 m thick.
The Lok Ma Chau Formation has been divided into the lower Mai Po Member, comprising dominantly metamorphosed siltstone with fine grained sandstone and carbonaceous siltstone and the upper Tai Shek Mo Member, which is a coarser grained facies of metamorphosed sandstone with conglomerate (Langford et al., 1989). The type section of the Lok Ma Chau Formation is designated here as Borehole BGS27 from the Yuen Long area (Figure 3.11).
The rocks of the Lok Ma Chau Formation are all metamorphosed. In particular, the conversion of carbonaceous material to graphite and the schistose foliation are obvious metamorphic features. Thermal and dynamic metamorphism combined to generate generally low-grade greenschist facies conditions. Evidence of both the thermal and dynamic metamorphism can be seen in a carbonaceous sandstone exposed on Reef Island adjacent to The Brothers islands (Langford et al., 1995). The rocks contain well-developed andalusite crystals, with chiastolite crosses and sericitized margins. In addition, the quartz grains are recrystallized and display undulose extinction.
The Mai Po Member comprises metamorphosed siltstone, phyllite with graphitic schist, and sandstone. Near the base of the succession, the argillaceous rocks are carbonaceous, often having been later metamorphosed to graphitic siltstone or schist (Plate 3.7). This lithology has a clastic component mainly of silt-grade quartz, with a few heavy minerals such as tourmaline and magnetite. The quartz grains are often strained and flattened, and show undulose extinction. The cement comprises recrystallized sericite with dispersed microgranular haematite and pyrite. The graphitic schist occurs as thin layers or lenses that are dark grey to black when fresh, with the well-developed schistocity being defined by preferred orientation of sericite.
The sandstones are generally silver - grey to yellowish grey in colour, though those associated with the carbonaceous mudstones are often stained red and yellow, reflecting the presence of haematite and its weathered product limonite. They are mostly fine- to medium-grained and comprise dominantly quartz with minor haematite, feldspar and heavy minerals. In thin section, fine- and coarse-sized quartz grains may be interlaminated, tend to be subangular and are often stretched into lenses or ribbons with undulose extinction. The cement is dominantly clay, recrystallized as sericite and displaying a preferred orientation. The complex structure and metamorphism have largely masked sedimentary structures within the rocks, though washouts and channel fills have been recorded (Frost, 1992).
The dominant lithology of the younger Tai Shek Mo Member is greyish white to yellowish white, medium to fine grained sandstone, with subordinate conglomeratic sandstone and phyllite. The conglomeratic sandstone comprises pebbles, mainly of subrounded quartzite and vein quartz, varying from 5 to 30 mm in size, set in a fine- to coarse-grained sandy matrix. The clasts, which make up 40 to 60% of the rock, are flattened and elongated with a preferred orientation. The matrix comprises quartz, feldspar, muscovite, magnetite and haematite with a microgranular quartz and sericitic cement. The sandstone comprises mainly silt to medium-grained quartz, together with haematite and minor feldspar and pyrite. In both the sandstone and conglomeratic sandstone, the cement is recrystallized to sericite and also displays a preferred orientation.
Details
The member is exposed northwest of Ho Sheung Heung (829100 841600 Csl-1), where the succession can be divided into three parts. The lower part consists mainly of grey to dark grey phyllite and sericite schist, with two layers 1 to 2 m thick, of fine grained metasandstone, and four thin layers of graphite schist. In the middle part of the succession there are phyllite and metasiltstone, with a 15 m thick greyish white, fine grained quartzitic metasandstone, while in the upper part the main rock type is metasiltstone, with some sericite schist and several layers of thick bedded fine grained metasandstone.
Two to four thin layers or lenses of graphite schist occur intermittently southwest of Lo Wu Camp (829300 841800 Csl-2) towards Fung Kong Shan (828400 841700 Csl-3), each varying in thickness from 0.1 to 0.5 m. North of Fung Kong (828700 841500 Csl-4) there are two layers of graphite schist, 1.2 and 1.5 m thick.
The unit attains a total thickness of 300 m to the southeast of Tit Hang. The lower part of the member here is predominantly phyllite, with five layers of metasandstone 1 to 12 m thick; these rocks form a northeast-trending low hill (827600 841200 Csl-5). The middle part of the succession comprises phyllite with three layers of graphite schist which are 2 m, 5 m and 8 m thick respectively. The thickest graphite schist outcrops southeast of Tit Hang (827600 841400 Csl-6). To the south a 1.8 m thick graphite schist forms a minor syncline south of Pak Shek Au (827500 841000 Csl-7) and extends east of Chau Tau (827100 840800 Csl-8), where the upper part of the member is phyllite, with three layers of fine grained metasandstone.
The Mai Po Member is exposed on isolated low hills stretching from Sheung Ma Lei Yue near Lo Wu, west of Liu Pok (828400 843100 Csl-9), and from northeast of Ping Hang (827300 842500 Csl-10) to Ha Wan Tsuen (825800 841400 Csl-11). The outcrops are repeated by a northeast trending reverse fault.
At Sheung Ma Lai Yue and west of Liu Pok the dominant rocks are greenish grey phyllite and metasiltstone with metasandstone. Northeast of Ping Hang the phyllite is the only rock type present. At Ha Wan Tsuen a greenish grey phyllite with good schistosity is exposed on the south side of the Shenzhen River near Lok Ma Chau Road. In a borehole D246/03335 (825910 841690 Csl-12) the metasiltstone of the Mai Po Member was found overlying the marble of Yuen Long Formation at 21.69 mPD.
Mai Po to Fairview Park. The Mai Po Member exposed on Mai Po Hill comprises silver grey to greenish grey phyllite with metasandstone and four layers of graphite schist, all with an excellent schistosity. The member overlies schistose metatuff of the Tai Mo Shan Formation, with a reverse fault at the contact. A stratigraphic section was examined east of Mai Po Lo Wai (824400 839300 Csl-13) towards Mai Po San Tsuen (824700 839200 Csl-14), the details of which are shown in (Figure 3.A1).
The lithology of the member changes towards the northwest, with arenaceous rocks becoming dominant. Graphite schist forms in four layers extending east of Mai Po San Tsuen (824400 839200 Csl-15), and can be seen in a new road cutting on the southeast side of Mai Po Hill. However, the geological structure is complicated and the strata may be repeated by thrusting.
At Fairview Park the member was encountered in the boreholes in the northwestern part of the estate (820400 838000 Csl-16). The main rock type is dark grey phyllite, with some fine grained metasandstone. Two layers of graphite schist 0.5 m and 7 m thick were found in a borehole (822460 837530 Csl-17), respectively at 19 and 42 m beneath the surface.
An isolated exposure of the Mai Po Member, surrounded by superficial deposits, can be seen at Tam Kon Chau (823300 839400 Csl-18). The strata comprise greenish grey phyllite with greyish white metasandstone, white thick-bedded quartzite and graphite schist. Between 1966 and 1974, six boreholes and two shafts were sunk to investigate the graphite at Tam Kon Chau. At least four layers of graphite schist were found, varying in thickness from 1 to 12 m. The deepest borehole (220 m) failed to penetrate the base of the Mai Po Member.
Yuen Long. Exposures of the Mai Po Member occur from Ng Uk Tsuen to Ping Shan, north and west of Yuen Long, and at Tung Tau Tsuen, northeast of Yuen Long.
At Chu Wong Ling the strata are greyish white, thickly bedded fine grained metasandstone, with metasiltstone and thin layers of graphite schist; the graphite schist is exposed at the top of Chu Wong Ling (820800 835300 Csl-19). At Tung Tau Tsuen, fine grained metasandstone with metasiltstone is displaced by a northwest-trending fault (821800 834500 Csl-20).
There are no exposures in Yuen Long, but ground investigation reports indicate that the Mai Po Member outcrops beneath the alluvium in the town area. In the eastern part of the town the strata consist of metasiltstone with some metasandstone and graphite schist, and are characterized by being rich in carbonaceous material; up to four layers of graphite schist, varying in thickness from 1 to 22 m, have been found in boreholes. In the western part of the town the strata are less carbonaceous and more arenaceous, with one or two layers of metasandstone or quartzite which vary in thickness from 10 to 40 m. There are several layers of calcareous metasiltstone varying from from 1 to 2 m in thickness near the base of the member.
At Long Ping Estate (820600 834400 Csl-21), including Kit Yeung Tsuen, the strata are similar to those in the western part of Yuen Long, in this case with two to five layers of metasandstone or quartzite. The thickness of each sandstone layer ranges from 4 to 22 m.
To the southwest, at Ng Uk Tsuen (820300 836300 Csl-22), the rock is mainly yellowish brown, thick bedded fine grained metasandstone with phyllite. The member is intruded by granitic rocks to the southwest of Ng Uk Tsuen (820200 836200 Csl-23). Fine grained metasandstone forms the top of low hills northeast of Fung Ka Wai (819700 835500 Csl-24). Dark grey to greenish grey metasiltstone and phyllite underly Ping Shan (819200 834000 Csl-25), Wang Chau (819700 835400 Csl-26), and Shui Ngau Leng (820000 833500 Csl-27).
Tin Shui Wai to Tuen Mun. The Mai Po Member is covered by between 6 and 20 m of superficial deposits. The main rock type is metasiltstone or phyllite, with subordinate metasandstone and quartzite. There are four to six intercalations of sandstone with thicknesses varying from 1 to 10 m. Between Tin Shui Wai and Ha Tsuen San Wai, boreholes indicate that the metasiltstone of the Mai Po Member is conformable with the underlying Yuen Long Formation, and an anticlinal structure can be defined. West of Ha Tsuen San Wai, thin lenses of calcareous siltstone were found in the lower part of the Mai Po Member in seven boreholes (817800 834100 Csl-28). Rhyolite dykes intrude the member, but are only found in boreholes (817800 834100 Csl-28).
The member outcrops in the Tuen Mun valley from Ha Tsuen (817300 834200 Csl-29) to Tseng Tau Sheung Tsuen (815800 829400 Csl-30), and again at Mouse Island, south of Tuen Mun (prior to reclamation) (815500 827100 Csl-31). For the most part it is concealed beneath alluvium, with exposures only along the northeast trending faulted margin of the Tai Lam Granite. The strata comprise greenish grey phyllite and dark grey metasandstone with a strong metamorphic fabric, well seen at Lo Fu Han (817000 830500 Csl-32) and Tuen Mun Treatment Works (816300 829700 Csl-33, Plate 3.A5). The southernmost outcrop, at Mouse Island, is isolated from the main outcrop, and is intruded by the adjacent granite. The metasandstone here forms a low hill, with phyllite and graphite schist exposed at the northeast end of the island. Colour banded siltstone has been found in boreholes south of the island 1918D/07183 (prior to reclamation)(815460 827170 Csl-34).
Sheung Shui Water Treatment Works to Cheung Po Tau. The formation is best exposed on the small hill (830620 842240 Csl-35) northwest of the Water Treatment Works and at Cheung Po Tau (831260 842430 Csl-36), where it comprises phyllite and metasiltstone with metasandstone, calcareous metasiltsone and graphitic schist. The latter HK1151 (830430 842310 Csl-37); HK6984 (830610 842210 Csl-38) occurs in five discrete beds, varying from 0.1 to 2 m in thickness and dipping at 30-40o to the northwest. The calcareous metasiltstone (HK6981) is present in Borehole BH7/5922 (830810 842770 Csl-39). The contact between the formation and the tuff of the Tai Mo Shan Formation is faulted and can be seen at Fu Tei Au Road (830520 841860 Csl-40) and below the service reservoir (831460 842500 Csl-41) at Cheung Po Tau. The metasedimentary rocks are thrust over the tuff, the thrust dipping northwest at 28-40o. Rocks on both sides of the fault are intensely mylonitised.
Kong Nga Po to Lo Shue Ling. All the rocks in this area are metamorphosed, and quartzphyric rhyolite dykes trending 020o that intrude the formation are also foliated. A thin bed of graphitic schist occurs at Kong Nga Po HK6935 (831860 842880 Csl-42), but the main lithology is phyllite. The outcrop of the formation continues northeast across the Sham Chun River into Shenzhen. At Kong Nga Po, Borehole SL3/6 GIU report no. 6681 (831920 842910 Csl-43) penetrates metasiltstone (HK3417) overlying mylonitised tuff (HK3418) of the Tai Mo Shan Formation. The contact at 13 m is interpreted as a thrust, with the overlying rocks thrust from northwest to southeast.
Cheung Shek Tsui, Kat O Chau (Crooked Island). Two outcrops occur on Kat O Chau, one north of Tung O Wan (848150 845950 Csl-44) and one south of Cheung Shek Tsui (848300 846030 Csl-45). Both are bounded by faults and comprise dark grey siltstone with thinly-bedded limestones. The latter occur as eight beds between 50 and 200 mm thick, are grey to dark grey with a micritic texture, and have yielded the foramaniniferan Archaesphaera magna Suleymanov (identified by Nanjing Institute of Geology and Palaeontology, Academia Sinica). The rocks in this outcrop are believed to belong to the lowermost part of the Mai Po Member.
West Brother (prior to development). The island is dominantly composed of silty sandstone, but also contains prominent beds of graphitic siltstone. Graphite-rich horizons are also present, and were mined in the 1950s and 60s. These horizons are highly eroded and are not well exposed on the shoreline, but have been mapped underground (Ruxton, 1957; Woods & Langford, 1991). Neither the base nor top of the succession were seen, but the siltstone and sandstone sequence is at least 200 m thick.
The west coast of the island is dominated by graphitic siltstone and graphite-rich horizons. Near the abandoned mines and associated buildings, the beach is dominantly composed of pebbles and cobbles of graphitic siltstone. Near the northern tip of the island, graphitic siltstone, is well bedded, dips northeast at 52o , and underlies brown-weathering, silty sandstone. The east coast is dominated by this massive, silty sandstone, in which bedding is absent, or poorly defined. The finely interlaminated graphitic siltstone and siltstone include load structures which confirm that the sequence is the right way-up. Cross-lamination also occurs, but was not observed in situ. Pure graphite was only seen on spoil heaps beside the old mine workings (814560 821170 Csl-46). The graphite probably came from the axial zone of the northeast-plunging syncline which forms the island. The rock has lustrous foliation surfaces, and has probably been thermally metamorphosed.
The northern and eastern parts of the island are dominated by banded and massive, silty sandstone, with locally extensive mineral boxworks. Bedding is obscure in the east, but nearly vertical erosional hollows, striking eastnortheast, probably lie along softer strata.
East Brother (prior to development). The island is dominated by red-weathering, finely laminated siltstone and sandstone (Plate 3.A6, massive siltstone and fine-grained sandstone. Thin quartzitic sandstone beds at the southern end of the island are less than 2 m thick. Quartzitic sandstone also occurs at the northern end, and may be partly responsible for the hog-back appearance of the island. Well-defined bedding dips steeply on the west coast of the island, and can also be seen occasionally on the east coast in the more massive siltstone and silty sandstone. The succession is at least 100 m thick, but no top or base has been observed.
Reef Island. The northern end of the island is mainly composed of quartzite, which forms the island's highest point. The strata are massive, or poorly bedded, but a steeply inclined, northerly-dipping, graphitic parting occurs. The silty sandstone to the south contains nodules, possibly iron-cemented, and pebbly patches. Bedding, which dips 71o northnorthwest, is well defined in these red-weathering sandstones. The centre of the island is also comprised of red-weathering silty sandstone, whereas the southern end is composed of finely bedded, or massive quartzitic sandstone and graphitic siltstone. The graphitic rock is locally spotted, and may be hornfelsed. Generally, the strata dip very shallowly, varying locally up to 29o, towards the northwest or northnorthwest. There are two distinct graphite siltstone beds, each several metres thick.
Tai Shek Mo to Lok Ma Chau. The Tai Shek Mo Member type section occurs in a section northeast of Ho Sheung Heung and west of Fuk Tak Kung (829500 843000 Csl-47); the succession is summarized in Figure 3.A1. Metaconglomerate and conglomeratic metasandstone are well exposed on the ridges around Tai Shek Mo (829100 842400 Csl-48, 829200 842700 Csl-49, 828700 842600 Csl-50), at Ma Tso Lung Police Station (828200 842800 Csl-51). The metasandstone with conglomeratic sandstone also occurs on hills southeast of Lo Wu (830000 843100 Csl-52).
The coarser grained clastic rocks are well developed between Tit Hang (827500 841600 Csl-53) and Ping Hang (827100 842200 Csl-54). Generally there are one to two beds of metaconglomerate intercalated in the metasandstone but in some areas five beds of metaconglomerate occur, for example from the northeast of Pak Shek Au (827600 841000 Csl-55) to the east of Lok Ma Chau (826600 841600 Csl-56). A metaconglomerate bed approximately 2 to 3 m thick extends for 1 km from southeast of Tai Law Hau (826900 841700 Csl-57) to the southwest of Ma Tso Lung (827600 842300 Csl-58); metaconglomerate is also exposed northeast of Pun Uk Tsuen (826600 841200 Csl-59).
San Tin. The Tai Shek Mo Member is covered by superficial deposits in this area and only isolated outcrops occur, for example at Shek Shan (822300 840000 Csl-60) where white, quartzitic sandstone forms a small rocky hill at the river mouth of Shenzhen River.
Sha Leng (Sandy Ridge). The dominant lithologies in the area are metasandstone and conglomeratic metasandstone with phyllite and metaconglomerate. The sandstone is mainly fine to medium-grained, whereas the sand fraction of the conglomeratic metasandstone is normally coarse. A bed of metaconglomerate, cropping out on a hill 500 m east of Lo Wu (830110 843330 Csl-61) dips at 27o to the northeast.
Muk Wu. The main lithologies in this area are thickly bedded yellowish brown metasandstone and greyish white conglomeratic metasandstone with thinly-bedded metasiltstone, phyllite and sericitic schist. Most of the rocks have been foliated with the structural fabric dipping at between 23o and 40o to the northwest. The rocks of the formation are highly to completely weathered and are widely covered by superficial sediments.