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The Geology of Hong Kong (Interactive On-line)
Yim Tin Tsai Formation - Jty

The Yim Tin Tsai Formation (Table 5.4) is exposed mainly in the vicinity of Tolo Harbour, but isolated outcrops occur in the western New Territories, and on Lantau Island, Lamma Island and Hong Kong Island (Figure 5.7). With the exception of tuffite developed at the base of the succession, the Yim Tin Tsai Formation is composed predominantly of lapilli–ash and lapilli-bearing coarse ash crystal tuff. Occasional, thin (<1 m), interbedded tuffite and volcanic breccia layers are the only indications of significant stratification. Towards the top of the formation, the appearance of interbedded epiclastic units marks a transition into the overlying Shing Mun Formation. At its type locality on Yim Tin Tsai, the formation is 200 m thick and unconformably overlies the Tolo Channel Formation (Early Jurassic) (Figure 5.9). The formation includes occasional blocks, up to 0.2 m, of crystal-rich, aphanitic volcanic rock. The weak alignment of crystals, together with occasional fiamme, indicate welding throughout the formation. In the western New Territories, the formation is slightly thicker (up to 300 m thick) and comprises a uniform grey to black, lapilli–ash crystal tuff characterised by elongate or subround lapilli of coarsely porphyritic lava (Plate 5.8). The lapilli typically have diffuse margins and are surrounded by white reaction rims, probably formed when the deposit was still hot. The crystals are mainly of feldspar with subordinate quartz, hornblende and biotite. U–Pb dating of zircon crystals has yielded an age of 164.5 ± 0.2 Ma for the formation (Davis et al., 1997).