The Sai Lau Kong Formation (Table 5.4) is restricted to a northwest-trending outcrop in the northeastern New Territories where it conformably overlies the Tai Mo Shan Formation (Figure 5.7). At the type locality of Sai Lau Kong, the formation comprises dominantly dacite lava (Plate 5.19) with intercalated tuff-breccia, tuffaceous siltstone and sandstone (Figure 5.12). The Sai Lau Kong Formation is thought to be the final expression of Middle Jurassic volcanism belonging to the Tsuen Wan Volcanic Group. Hence, the U–Pb date of 164.1 ± 0.2 Ma obtained from the formation (Sewell et al., 1998) constrains the youngest age of the Tsuen Wan Volcanic Group. However, there is evidence of later intrusive activity related to this magmatism (see below).

Details

Sai Lau Kong to Lai Chi Wo. In the southern area of its outcrop, the formation comprises dacite lavas with rhyolite, tuff, and tuffaceous sedimentary rocks. The feldspar-bearing blocky lava, interbedded with reddish brown to dark grey siltstone and mudstone, occurs at the base of the formation south of Sai Lau Kong (847440 842800 Jtl-1). Dacite lava of this formation overlying lithic crystal tuff of the Tai Mo Shan Formation is well exposed 300 m southeast of Chung Wan Tsui (HK9350, 846460 843200 Jtl-2). Tuff breccia, 100 m northeast of Ngau Shi Wu, is interpreted as infilling a volcanic vent. A tuff breccia dyke, 6 to 20 m wide, is present on both sides of a bay at Lai Chi Wo (845630 843060 Jtl-3 to 845080 843170 Jtl-4). It may represent the infill of a fissure vent. Some small volcanic fissures infilled by veins of lapilli tuff also occur 300 m east of Lai Chi Wo (HK7577, 845250 842910 Jtl-5), and at Lo Chi Pai (846100 843700 Jtl-6), Fung Chau (846400 843760 Jtl-7) and Tsing Chau (846900 843700 Jtl-8). Fine ash tuff, sandstone and siltstone of the formation overlie coarse ash tuff of Tai Mo Shan Formation east of Lai Chi Wo and form the core of a plunging anticline at Chung Wan (845950 842860 Jtl-9).

Tai Nim Chau to Sam Kok Tsui. The northern area of outcrop of the formation includes dacite and rhyolite lavas with crystal tuff and siltstone intercalations. Lava occurs between Cheung Shek Tsui (844980 845160 Jtl-10) and Sam Kok Tsui (843960 845310 Jtl-11) while tuff is exposed on Tai Nim Chau (845200 844700 Jtl-12), Siu Nim Chau (845600 844300 Jtl-13) and Ap Tau Pai (846080 844020 Jtl-14). Siltstone occurs west and north of Kau Ma Shek (845560 843740 Jtl-15).