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).
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).
