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The Geology of Hong Kong (Interactive On-line)
Shing Mun Formation - Jts


The Shing Mun Formation (Table 5.4) crops out mainly in the vicinity of Tai Mo Shan and on the north side of Tolo Harbour (Figure 5.7). Smaller outcrops are also present on Tsing Yi, along the southern side of Lantau Island, and in the New Territories around Sha Tau Kok and Plover Cove. The formation is absent in the east and southeast of Hong Kong. Areas of outcrop formerly thought to be of the Shing Mun Formation to the south of the ‘Jordan Valley fault’ are now identified as the Mount Davis Formation (see below).


The Shing Mun Formation is the most complex and lithologically variable formation, both laterally and vertically, within the Tsuen Wan Volcanic Group. In its type area west of the Shing Mun Reservoir, the formation is approximately 400 m thick, but may reach up to 600 m thick in the western New Territories (Figure 5.10). It consists of units of block-bearing fine ash, coarse ash and lapilli–ash crystal tuffs, eutaxite, tuff breccia, pyroclastic breccia, tuffite and occasional conglomerate, breccia, sandstone, siltstone and mudstone. Crystals include quartz, alkali feldspar and plagioclase. Large areas of tuff and pyroclastic breccia, adjacent to areas of fine ash tuff on the northern side of Tolo Harbour, may represent the infills of substantial vents. Some of the crystal- and lapilli-bearing mudstones, tuffaceous sandstones and conglomerates within the sequence are likely to represent slump, debris flow and laharic deposits. Other sedimentary rocks are consistent with fluvial and lacustrine deposition, possibly in shallow intermontane basins. Occasional clasts and blocks of marble occur in the breccia deposits and these may represent limestones or calcareous siltstones that were metamorphosed adjacent to subvolcanic magma chambers or on vent walls. However, there is also the possibility that the metamorphism was caused by earlier intrusion into limestones that now form subcrops of marble belonging to the Yuen Long Formation in the northwestern New Territories. Three members have been recognized locally in the Shing Mun Formation.


The Shek Lung Kung Member (Langford et al., 1989) is a welded pyroclastic flow deposit, approximately 200 m thick, within the Shing Mun Formation. Its main outcrop is in the vicinity of the type locality at Shek Lung Kung, west of Tsuen Wan (Plate 5.9). Scattered outcrops are also present on northeast Lantau Island. The member comprises pyroclastic breccia, with blocks up to 6 m in diameter, and lapilli–ash crystal tuff with glassy and porphyritic lapilli. Strong elongation lineations within the deposit have been interpreted as being related to primary flow from northwest to southeast. The coarsest facies is very localized, suggesting its closeness to source, and over a distance of 100 m it grades laterally into tuff breccia.


The Ngau Liu Member (Langford et al., 1989), which crops out west of Tai Mo Shan, is a welded, structureless pyroclastic flow deposit (up to 250 m thick) near the top of the Shing Mun Formation. It comprises lapilli–ash to coarse ash crystal vitric tuff. The crystals, set in a microcrystalline matrix, are commonly quartz (variably resorbed) and plagioclase, with alkali feldspar, including characteristic microcline, biotite and hornblende.


The Cheung Shan Member (Langford et al., 1995), formerly part of the Lantau Formation (Langford et al., op.cit.), comprises eutaxite. In its type area on the northern flanks of Cheung Shan in western Lantau Island, the member is up to 500 m thick. The member has returned a U–Pb zircon age of 164.2 ± 0.3 Ma (GEO, unpublished data). This supports its correlation with the Tsuen Wan Volcanic Group. It is tentatively included within the Shing Mun Formation (Table 5.3) and is lithologically similar to the Ngau Liu Member in particular. The member directly overlies either the Shing Mun or Yim Tin Tsai formations on the southeast side of Lantau Island, between Cheung Sha and Tong Fuk. On the northwest side of Lantau Island, the member overlies possible Lower Jurassic sedimentary rocks of the Tai O Formation.


The lowermost 280 to 340 m of the Lai Chi Chong Formation, as originally described by Strange et al. (1990), was tentatively reassigned by Campbell and Sewell (1998) to the Shing Mun Formation (Table 5.3) on the basis of a U–Pb date of 164.7 ± 0.3 Ma (GEO, unpublished data). This sequence comprises coarse ash crystal tuff, tuffite and rhyolite, and unconfomably overlies sedimentary rocks including the Devonian Bluff Head Formation and the Early Jurassic Tolo Channel Formation.




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