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The Geology of Hong Kong (Interactive On-line)
Lok Ma Chau Formation - Csl

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.