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The Geology of Hong Kong (Interactive On-line)
Tai Mo Shan Formation - Jtm

Previously, the Tai Mo Shan Formation (Table 5.4) was mapped over a wide area of Hong Kong, including much of the New Territories, and large outcrops in the Sai Kung and western districts. However, on the basis of detailed petrographic and geochemical study, supported by high precision isotopic dating, the outcrops in the Sai Kung and western districts have now been assigned to two younger formations: the Long Harbour Formation in the eastern New Territories, and the Mount Davis Formation on western Hong Kong Island. The bulk of the Tai Mo Shan Formation is now restricted to outcrops north and northwest of the Sha Tin Valley (Figure 5.7), including the high ground around Tai Mo Shan itself. The formation is thought to rest conformably on the Shing Mun Formation. Locally, on the western side of Tai Mo Shan, there is an impersistent eutaxite layer at this contact. At the type locality of Tai Mo Shan, the formation is at least 600 m thick (Figure 5.11), despite the absence of the top of the formation due to erosion. U–Pb dating of zircon crystals has yielded an age of <164.6 ± 0.7 Ma for the formation (Davis et al., 1997).

The formation is commonly a massive, largely featureless, pale grey to dark grey lapilli–ash or coarse ash crystal tuff (Plate 5.18). It contains mainly feldspar and quartz crystals, with some dark green biotite, and lithic lapilli of pale sandstone (up to 20 mm). Volcanic lithics are commonly flattened and sometimes aligned, resembling fiamme associated with a eutaxitic fabric. Welded lapilli–ash crystal tuffs are present near the summit of Tai Mo Shan along with rare bands of impersistent slightly tuffaceous sandstone, up to 30 m thick.

One possible source for the formation is the area around Tai Mo Shan. There may have been another source of eruption of the Tai Mo Shan Formation to the north of Tai Mo Shan. However, due to structural repetition associated with major thrusting in the northern New Territories, the sequence is not well established and interpretation is uncertain. Several sources of information suggest a possible volcanic source, or sources, north of Tai Mo Shan. These include: a concentration of granodiorite intrusions of similar composition around Shek Kong Camp; an outcrop of block-bearing tuff on Pak Tai To Yan; and evidence from gravity data of a localy thicker volcanic sequence. Given the apparently homogeneous nature of most of the tuffs, rapid accumulation is presumed from large pyroclastic flows.