The Ngo Mei Chau Formation (Table 5.6) occurs only in the northeast New Territories (Figure 5.16), and is exposed chiefly on Kat O Chau and at the type locality of Ngo Mei Chau in the northwest of Mirs Bay. It is the only representative of the Repulse Bay Volcanic Group exposed north of Tolo Channel. The formation, which is up to 450 m thick, comprises dominantly welded fine ash vitric tuff (Plate 5.26), with subordinate lapilli–ash tuff, coarse ash crystal tuff and rhyolite lava (Figure 5.19). Intercalated sedimentary rocks, including black siltstones and thin limestone beds are present on the east coast of Kat O Chau. A northwest-trending fault separates the main outcrop from the Tai Mo Shan Formation to the west and it is likely that this fault was active during deposition of the sequence. Although stratigraphic relationships with other formations are not observed, the formation has yielded an imprecise U–Pb zircon age of <142.7 ± 0.1 Ma (Sewell et al., 1998) suggesting that it post-dates the Tai Mo Shan and Sai Lau Kong formations, and is probably coeval with the Ap Lei Chau Formation.

Details

Ngo Mei Chau (Crescent Island) and Wong Wan Chau (Double Island). The rocks on Ngo Mei Chau are mainly fine ash vitric welded (eutaxitic) rhyolitic tuff with rhyolite lava, lapilli tuff, lithic crystal tuff and tuffaceous siltstone and sandstone. At Chik Mun Tau (850150 843100 Krn-1), sandstone at the base of the formation is interbedded with lapilli-bearing ash lithic crystal tuff. Sedimentary rocks at Lo Kei Wan and Ngau Mei Wan, dip northeast at 30-40^o and include minor plunging folds. Fossil wood is present in the dark grey carbonaceous rocks west of Lo Kei Wan (850660 842850 Krn-2). Rhyolite only occurs (850800 843400 Krn-3) 100 m southwest of Wan Tsai .

The formation also occurs on the northeast coast of Wong Wan Chau (850330 842740 Krn-4, 850640 842300 Krn-5. The base of the formation includes fossil wood-bearing dark grey carbonaceous sandstone and siltstone and overlies coarse ash tuff of the Tai Mo Shan Formation.

Kat O Chau and Tui Min Chau (Yeung Chau). In the north and east of Kat O Chau, and on Tui Min Chau (849700 844500 Krn-6) and Wong Nai Chau (850100 844700 Krn-7), the rocks are mainly eutaxitic lapilli-bearing lithic crystal welded tuffs with lapilli-bearing lithic tuffs and a few sedimentary rock intercalations. Fiamme within the eutaxitic bands are 10 to 200 mm long. Lapilli-bearing crystal lithic tuffs at Mun Tsai Wan (849100 844000 Krn-8), west (848400 844700 Krn-9) of O Pui Wan, Kai Kung Leng (849000 845500 Krn-10) and Yung Kok (849500 845700 Krn-11) mainly comprise pyroclasts (30-60%), including lithic fragments (Plate 5.A24), 10-100 mm across and occasionally up to 800 mm, of rhyolitic lava, crystal tuff, siltstone and sandstone. Sedimentary rock intercalations, including sandstone, siltstone, carbonaceous siltstone and chert, are exposed at Tau Tun (849140 845620 Krn-12), 300 m east (850440 845520 Krn-13) of Chung Wan Teng , and west (848170 845850 Krn-14) of Tung O Wan. The contact between the Ngo Mei Chau Formation and the Tai Mo Shan Formation is a northwest-trending fault in this area.