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 The GEO (unpublished data) has obtained a high precision U–Pb single crystal zircon age from a pyroclastic unit near the top of the formation of 146.2 ± 0.3 Ma.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.

Details

Lai Chi Chong. Coarse ash tuff forms the headland at Pak Kok Chai (848230 835390 Jll-1). The tuff is similar in general appearance to that exposed around the flanks of Shek Ngau Tau (800 m to the southwest), but occasionally possesses a faint eutaxitic fabric dipping east at about 25^o. On the headland at Pak Kok Chai the tuff is vertically banded, and this is interpreted as rheomorphic flow-banding. Overlying the coarse ash tuff is a sequence consisting of mudstone, silty mudstone and interbedded tuffite which is in turn overlain by pale grey coarse ash tuff. Within the mudstone, thin fine-grained tuffite layers show classical wet-sediment deformation, usually resulting from slumping or seismic disturbance of unconsolidated strata. One 7 m thick, coarse-grained tuffite bed occurs in this sequence (848650 835190 Jll-2) and contains blocks up to 400 mm across near the base, but is fine-grained and cross-laminated near the top. Plant remains, including large fragments of silicified wood, have been found near Lai Chi Chong Ferry Pier (848800 8352200 Jll-3) within these mudstones, siltstones and tuffites (Dale & Nash, 1984). In places the strata are highly contorted (Plate 5.A17), but the overall dip is approximately 30^o towards the southeast.

Exposures of nodular and structureless chert occur at the river mouth south of Pak Kok Chai (848440 835160 Jll-4). These are identical to exposures on Knob Reef (848000 836000 Jll-5) which are associated with bedded sandy tuffite.

Cheung Sheung. A thick sequence of coarse ash tuff is overlain to the west of Cheung Sheung by dark grey mudstone containing two significant feature-forming conglomerate layers (Plate 5.A18). The mudstone has yielded abundant plant remains and several fossilized wood logs (849340 832440 Jll-6) (Wai, 1986). Rhyolite lava overlies the mudstone (849320 832520 Jll-7) and is about 35 m in thickness. A fairly thin impersistent fine ash welded tuff with a prominent eutaxitic fabric separates the lava from the overlying coarse ash tuff of the Long Harbour Formation (849400 832600 Jll-8).