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The Geology of Hong Kong (Interactive On-line)
Yuen Long Formation (Ma On Shan Formation) - Csy

The Yuen Long Formation was first encountered in boreholes and caissons during development of the new town of Yuen Long, in the northwest New Territories (Langford et al., 1989; Frost, 1992). It has not been possible to date the marble sequence palaeontologically in Hong Kong. However, similar carbonate sequences are found in southern China (Huang, 1978; Bennett, 1984) and these are interpreted as Early Carboniferous in age (Lai & Mui, 1984). In the Yuen Long area, Frost (1992) divided the formation into two members: the lower Long Ping Member is a dark grey marble, commonly with complex internal structure, while the upper Ma Tin Member is a massively bedded, white marble. The thickness of the Yuen Long Formation in the type area is around 600 m (Frost, 1992) although elsewhere in Hong Kong, thicknesses are less well constrained (Busby & Langford, 1995). On the basis of steep dips in the overlying, tightly folded Mai Po Member, the marble probably reaches a similar thickness on The Brothers islands (Langford et al., 1995). Frost (1992) has designated the type sections of the Long Ping and Ma Tin members respectively as boreholes BGS2 and BGS3 (Figures 3.6 & 3.7). In some areas, and especially on the north coast of Lantau Island, the marble exhibits skarn mineralization with diopside and garnet in a matrix of interstitial granular calcite. The presence of skarn mineralization suggests that igneous rocks lie nearby at depth.

The Long Ping Member dominantly comprises grey to dark grey fine- to medium-grained crystalline marble with minor chert (Plate 3.5). Stylolitic structures indicate dissolution at grain boundaries (Yuen, 1990). Metamorphism has been both thermal and regional. The original rocks are considered to have been an interbedded sequence of thin limestones, and calcareous mudstones and siltstones (Frost, 1992). The dark colour is caused by the content of mainly fine silt clasts, which typically make up 8% of the rock. This clastic component varied with the supply of terrigenous sediment into the original sedimentary basin in which the calcareous strata were deposited. Other lithologies, including sandstone (now chert), graphitic siltstone, and dolomite, occur within the Long Ping Member. The dolomite is thought to represent periods of partial shallowing of the basin.

In contrast to the dark clastic-rich carbonates of the Long Ping Member, the Ma Tin Member comprises massively bedded, pure, white to greyish white, medium- to coarse-grained crystalline marble, comprising over 99% calcium carbonate (Plate 3.6). Sporadic solution features (see below) and boxwork structures are common (Yuen, 1990). Accessory minerals include dolomite, tremolite, epidote and wollastonite. The metamorphic grade is therefore considered to have had both a thermal and regional component. The original limestone has been entirely recrystallized and any traces of sedimentary structures have been completely destroyed (Frost, 1992). In the same way, there are no indications of the preservation of fossils and microfossils.

The top of the marble comprises a zone of karstic development with an uneven and pinnacled surface (Figure 3.8), as well as cavities within the rock mass. Acidic rainwater has produced a 30 m-thick zone of epikarst, a very irregular karstic development comprising uneven solution features. Cavities also occur beneath this zone. The presence of palaeokarst beneath Yuen Long has caused significant engineering problems in the development of the new town (Holmes & Keung, 1990), especially in relation to foundation design and construction. Similar complex ground conditions have been encountered in developments to the north and south of Yuen Long at Pak Hok Chau and Tai Tong respectively, and more locally to the west of Yuen Long at Tin Shui Wai.

Marble encountered in boreholes on the reclamation at Ma On Shan (Figure 3.9), and in the Ma On Shan mine, has been assigned to the Ma On Shan Formation (Frost, 1991; Sewell, 1996). Detailed petrographic study of the Yuen Long and Ma On Shan formations concluded that there are no discernible differences between the two marbles.