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Chapter 4Mesozoic Volcanic and SedimentaryRocksClassification and DistributionAddison (1986) and Strange & Shaw (1986) provided comprehensive summaries of earlyresearch (Uglow, 1926; Brock & Schofield, 1926; Williams et al, 1945; Ruxton, 1960; Allen &Stephens, 1971; Table 3) on the stratigraphy of the Mesozoic volcanic and sedimentary rocks ofHong Kong. Allen & Stephens (1971) described these rocks, occurring across the Territory, asthe Upper Jurassic Repulse Bay Formation. Addison (1986) later amended their lithostratigraphyin the Sha Tin district and proposed the Repulse Bay Volcanic Group. However, the volcanicrocks of Hong Kong were subsequently divided into two groups, the Tsucn Wan and RepulseBay volcanic groups, on the basis of their petrography, geochemistry, and chronology. The olderTsuen Wan Volcanic Group (Geotechnical Engineering Office, 1992; Sewell et al, 1993), whichis doniinantly composed of rhyodacitic tuffs, includes three formations that were formerlyassigned to the Repulse Bay Volcanic Group by Addison (1986). It crops out mostly in the northof the Territory. Only two of these three formations, the Yim Tin Tsai and Shing Mun, occur inthe Lantau district. The younger Repulse Bay Volcanic Group largely comprises rhyolitic tuffand lava, and is found mainly in the east and south of the Territory. It is represented in thedistrict by the Lantau and Mount Davis formations, both defined here for the first time. Theformer crops out over large areas of the district, whereas the Mount Davis Formation, which maybe the younger of the two formations, occurs only in the extreme cast, where the LantauFormation is unrecognised. As well as the formations themselves, the present survey hasidentified various named members and other lithological units, including cpiclastic rocks(conglomerate, sandstone, siltstone), tuffaceous sedimentary rocks, eutaxite, tuff-breccia andcrystal tuffs. The general volcanic sequence for the district is presented in Table 3. Thenomenclature and classification of the pyroclastic rocks used below, and summarized in Figures7a and b, are based on International Union of Geological Sciences recommendations (Schmid,1981; Le Maitre, 1989), and the work of Fisher & Schmincke (1984). Representative whole-rockgeochemical analyses of the major volcanic rock types are presented in Appendix 2.Tsuen Wan Volcanic GroupYim Tin Tsai FormationThe Yim Tin Tsai Formation is the oldest formation in the Tsuen Wan Volcanic Group (Sewellet a/., 1993). Its type locality is northeast of the district on Yim Tin Tsai, in Tolo Harbour, whereapproximately 200 m of strata overlie a basal sedimentary breccia (Addison, 1986), In thedistrict, the formation crops out in several areas: in the east on Tsing Yi and Ma Wan; innorthern Lantau Island, near Tsing Chau Tsai; in south Lantau Island between Pui 0 and TongFuk; and at Yi 0 in west Lantau Island.The lithology, which weathers to a light brown, is a uniform grey to dark grey, lapilli-ash crystal,or vitric crystal tuff, characterized by elongate, or sub-rounded lapilli of coarsely porphyriticlava, typically with diffuse margins. Aphanitic lapilli, some with diffuse margins up to 30 mmthick, and dark green siltstone clasts also occur. Lithic blocks may be as large 150 mm. The tuffs36