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The Geology of Hong Kong (Interactive On-line)
Lai Chi Chong Formation - Jll

The Lai Chi Chong Formation (Table 5.5) crops out north of the Chek Keng Fault in the northeastern New Territories (Figure 5.1). It was first described by Strange et al. (1990) as a single unit cropping out between Three Fathoms Cove in the south and Lai Chi Chong in the north. However, new U–Pb age-dating (Campbell & Sewell, 1998), suggests that the formation is better restricted to the uppermost 130 to 180 m of the succession.

The revised Lai Chi Chong Formation comprises a well-bedded succession of pale grey cherty tuffite, coarse ash crystal tuff, thin eutaxitic fine ash tuff, flow-banded porphyritic rhyolite, conglomerate, tuffaceous sandstone, and dark grey laminated silty mudstone (Figure 5.15). Beds vary from 0.1 m to 6 m thick. Normal grading is common. The rhyolites, originally interpreted as lavas (Strange et al., 1990), appear locally to cut across beds and may instead be sills intruded at shallow levels. However, there are also some flow-banded lava pebbles within the conglomerates. Silty mudstone intraclasts are common within some of the thicker tuffaceous sandstones and coarse ash tuffs. These, together with common flame structures, load casts, and other features typical of soft sediment deformation within the underlying beds (Plate 5.22), are consistent with rapid emplacement from mass flow (low and high concentration turbidites and debris flows) onto unconsolidated substrate. The presence of common, and sometimes large, intra-formational slump folds (Plate 5.23) and related syn-sedimentary faults and dewatering structures, further confirms the unconsolidated nature of the substrate during deposition.

Fossil plant fragments (including conifers and cycadophyles) and tree-trunk remains have been reported from several locations in the formation, including the type locality at Lai Chi Chong (Dale & Nash, 1984; Wai, 1986; Atherton, 1989; Lee et al., 1997). These fossil assemblages have indicated an Early Cretaceous age. The fossil assemblage is also very similar to that from the Lantau Volcanic Group (undifferentiated). Indurated dark grey siltstone with organic structures of possible algal origin have also been observed (Strange et al. 1990) in the Lai Chi Chong Formation, but no marine fossils have been identified. The GEO (unpublished data) has obtained a high precision U–Pb single crystal zircon age from a pyroclastic unit near the top of the formation of 146.2 ± 0.3 Ma. This correlates closely with ages obtained from the Lantau Volcanic Group on Lantau Island. A similar U–Pb zircon age of 146.6 ± 0.2 Ma (GEO, unpublished data) was obtained from a rhyolite, interpreted as a possible sill, within the underlying Shing Mun Formation at Sham Chung west of Lai Chi Chong.

As was the case with the Lantau Volcanic Group (undifferentiated) there is an apparent discrepancy, yet to be resolved, between the absolute U–Pb age, which suggests that the age is Late Jurassic, and the Early Cretaceous age based on palaeontological determination.