The Tolo Harbour Formation on Ma Shi Chau, the type area, has been divided into two lithofacies (Addison, 1986): a lower facies of calcareous siltstone, mudstone and sandstone and an upper facies of thickly- bedded siltstone, sandstone and conglomerate. However, this subdivision has not been formalized owing to difficulties in applying it to other Permian rock outcrops.

The lower facies on Ma Shi Chau comprises dark grey mudstone and pale grey to pink calcareous siltstone with lenses and interbeds of sandstone. The argillaceous rocks are generally well-laminated, with syn-sedimentary slump folding in places (Plate 3.8). The sandstone lenses exhibit low-angle cross bedding and are graded with occasional clasts along their bases (Jones, 1996). This unit has yielded a varied fossil assemblage, including a marine fauna and several plant fragments (Yim et al., 1981; Nau, 1980). Similar rocks are exposed on Centre Island where thin quartzitic sandstone is interbedded with dark argillaceous siltstone, which has yielded plant fossils (Nau, 1980).

The upper unit on Ma Shi Chau comprises a sequence of mudstone, siltstone, sandstone and conglomerate. The succession is distinctly different from the underlying unit and was previously assigned to the Bluff Head Formation by Allen and Stephens (1971). Nau (1981), however, reinterpreted the upper unit as Permian, and it is assigned to the Tolo Harbour Formation on Hong Kong Geological Survey maps. Thinly-bedded, unfossiliferous purplish grey siltstone interbedded with black shale and mudstone, dominates the lower part of the unit. In the middle and upper parts of the unit, locally cross-bedded, white to pale grey, micaceous quartz sandstone is intercalated with lenses of quartz - pebble conglomerate.

Metasedimentary rocks and their weathered products have recently been identified (Gillespie et al., 1998; Kirk, 2000; Fletcher et al., 2000) in boreholes in the vicinity of Tung Chung New Town. The rocks were provisionally named as the "Tung Chung Formation" (Lee et al., 1998), but have since been tentatively correlated with the Tolo Harbour Formation (Sewell et al., 2000). These rocks comprise metamudstone, metasandstone, marble, skarn, calcsilicate rock and quartzite and occur as large (up to 350 m across), isolated blocks within the medium-grained Lantau Granite (Middle Jurassic). In some of the largest metasedimentary blocks, fossiliferous limestone is preserved and has yielded an Early Permian fossil assemblage, including the fusilinid foraminifers Schwagerina callosa and Schwagerina spp, and the rugose coral Rugosachusenella hutiensis (Lee et al., 1998).

Details

Tung Chung. The metasedimentary rocks are well-bedded with lithologically distinct bands of skarn, calc-silicate, and rare magnetite-rich rock, usually no more than a few metres thick. Marble, up to 60 m thick, is known from one of the largest metasedimentary blocks (c. 812240 817160 Pt-1). The marble, together with some sandstone and siltstone, occurs as discrete 'blocks' within the granite, i.e. they are xenoliths (Figure 3.14) (Gillespie et al., 1998; Kirk, 2000). In the weathered state, the metasedimentary rocks are variably altered depending on the characteristics of the parent rock. Quartzite, metasandstone, calc-silicate and siliceous skarn are largely resistant to weathering whereas marble, impure marble and carbonate-bearing siliceous rocks are highly susceptible. Commonly, the carbonate-rich rocks have been dissolved by oxidizing waters to leave an insoluble residue of silt and clay minerals. Cavities are often formed and many of these have subsequently been infilled with superficial deposits. The occurrences of marble with cavities, in addition to the extremely deep weathering encountered in parts of the Tung Chung area (Kirk, 2000; Fyfe et al., 2000) have created some significant engineering problems in the development of the new town.

Ma Shi Chau (type area). The strata on Ma Shi Chau are structurally complex, and the interpretation of stratigraphic relations within the formation is extremely difficult. The strata are involved in syndepositional slump folds as well as a series of northerly plunging minor anticlines and synclines, the limbs of which are often sheared and overturned (Plate 3.A7).

The northeastern coast demonstrates the most complete sequence although the succession, which youngs to the northwest, is cut by faults at the eastern end of this section. A sequence of some 500 m thickness is estimated on Ma Shi Chau, although the tectonic structures make this figure somewhat speculative. The formation is uncomformably overlain by tuffs and sedimentary breccia which crop out along the western shoreline; the contact has not been observed.

The sequence is divided into two lithofacies following Nau (1980). However, the outcrop is considered to be structurally too complex to warrant formal sub-division. The lower facies consists of a sequence of pale grey, pink-weathered, slightly calcareous siltstones, dark grey mudstones, siltstones and sandstones (842150 835230 Pt-2). The sandstones form layers up to 0.5 m thick. They are moderately even-bedded and persistant along strike, except where faulted. The dark grey mudstones and siltstones are occasionally ferruginous and contain pyrite nodules and, according to Nau (1980), in places contain plant remains. The pink-weathered, pale grey calcareous siltstones contain assemblages of poorly preserved marine fossils including molluscs, corals, bryozoans, brachiopods and crinoids (Plate 3.A8). Interbedded strata have also yielded plant remains (Nau, 1980) (Plate 3.A8).

On the northern coast of Ma Shi Chau these beds pass upwards with gradual increase in sand content into an upper facies consisting of more thickly bedded siltstones and sandstones, with beds up to 1 m thick. Conglomerates occur within the sandstones near the top of the exposed succession. Boulders which occur on the northwestern shore of the island show signs of tectonic or in-situ brecciation. Where they crop out on the eastern coast of the island, Nau (1980) recorded plant fossils, crinoid remains and corals in places within these beds. The strata there are occasionally contorted in syndepositional slump folds. At one locality (841720 834640 Pt-3) slumped, folded siltstones are overlain by planar-bedded, limonitic fine-grained, sandstone.

Centre Island. Complexly faulted, contorted strata of similar lithologies to those of Ma Shi Chau occur across the entire island. Thin quartzitic sandstones are interbedded with dark siltstones at the southern tip of the island, while sandstones in beds up to 1.6 m occur in the north. About 150 m of strata are estimated to crop out on the island. Plant remains have been found in dark argillacous siltstones cropping out on the northeastern shore; no marine fossils have been found. Strata on the northwestern coast (840900 833420 Pt-4), which dip NE at 50^o, display a younging direction to the northeast. Farther south (840940 833240 Pt-5) strata dipping W at 58^o also young to the northeast. These attitudes are similar to those displayed on Ma Shi Chau.