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The Geology of Hong Kong (Interactive On-line)
Tolo Harbour Formation - Pt

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).