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.

Details

Hong Lok Yuen to Fung Yuen Lo Tsuen. The lithic block-bearing, ash crystal tuff in this area surrounds a granodiorite intrusion and has been subjected to contact (thermal) metamorphism. At Hong Lok Yuen (834400 836000 Jts-1), the tuff is characterized by abundant lithic lapilli and blocks, whereas 600 m northwest of Fung Yuen Lo Tsuen (836000 836000 Jts-2), there are abundant sandstone and siltstone intercalations, 0.5 to 1.5 m thick.

Tai Po and Hong Lok Yuen. In borrow areas south and east of Tai Po and around Hong Lok Yuen (834600 835900 Jts-3), complexly faulted and intruded sequences are composed predominantly of block-bearing tuffites of presumed laharic origin. The sedimentary rocks in these borrow areas (836000 835500 Jts-4 and 837900 835900 Jts-5) are extremely complex and it is possible that they represent large dislocated or slumped blocks within a lahar deposit. Large (up to 3 m) rounded blocks of skarn, composed of chlorite, tremolite and grossular, with carbonate nodules, are another feature of these borrow areas.

Sheun Wan to Tai Mei Tuk. Between Shuen Wan (839200 836400 Jts-6) and Tai Mei Tuk (842100 836600 Jts-7), the coarse and fine ash crystal tuffs are characterized by the abundance and variety of lithic lapilli and blocks, and by intercalations, 0.5 to 3 m thick, of sandstone and siltstone. Marble clast-bearing fine ash tuff occurs 370 m west of Wai Ha (838570 836500 Jts-8) and 870 m west of Ha Tei Ha (838160 836110 Jts-9). The marble clasts are greyish green to dark grey and vary from cobble to gravel size. The fine ash crystal tuff is intruded by small bodies of granodiorite.

Tai To Yan. On the southern flanks of Tai To Yan the dynamic metamorphism of the tuffs and underlying granodiorite is often severe, resulting in a quartz sericite metatuff. Original texture is rarely seen, although it is possible to distinguish some sandstone and some lithic lapilli bearing ash crystal tuff. A possible tuffite can be seen on the path from Kadoorie Farm to Tai To Yan (829220 833210 Jts-10), containing scattered pyroclasts generally less than 1 mm. On the eastern flanks (828710 833390 Jts-11) is a pale brown weathered, medium grained sandstone which is probably conglomeratic in part; the conglomerate clasts have been dissolved away, leaving well rounded, slightly elongated holes. The clasts probably define bedding that dips roughly northwest at 56^o.

On the southern flanks of the hill (829800 833200 Jts-12) is an isolated exposure of markedly crystalline, grey lapilli-ash crystal tuff speckled with mafic minerals. The area is one of intense dynamic metamorphism, although original rock textures are preserved in isolated patches. The lithology is ascribed to the Ngau Liu Member, but cannot be distinguished on the map from the rest of the Shing Mun Formation.

The Tsuen Wan Volcanic Group outcrops of the Kadoorie Farm area are difficult to correlate lithologically since hand specimens and thin section (HK 341) invariably show some degree of thermal metamorphism or cataclastic deformation (830670 831830 Jts-13). The assemblage of rocks, however, includes tuffites, sandstones, siltstones and coarse ash tuffs; some tuffites are block-bearing (831650 833000 Jts-14) and correlate best with the Shing Mun Formation. This correlation is consistent with the interpretation of a syncline through Tai Mo Shan. The Shing Mun Formation is recognised on the southeastern flank of Tai To Yan, northwest of the Lam Tsuen Valley. The succession there is dominated by lapilli-ash crystal tuffs but includes block-bearing tuffites (HK 352, 830400 834600 Jts-15) and at least two horizons of siltstone and fine-grained sandstone (831750 835490 Jts-16). The tuffs and tuffites are rich in secondary calcite, which forms plates and veins. The boundary between the Shing Mun Formation and the overlying Tai Mo Shan Formation appears to be transitional.

Kap Lung. From Route Twisk northwest towards Kap Lung (828800 831100 Jts-17) the Shing Mun Formation outcrops below the Tai Mo Shan Formation. Dynamic metamorphism of the tuff, tuffite and sedimentary rocks resulted in recrystallization of the matrix to quartz and sericite, so differentiation of mappable rock units is not possible. The pyroclastic rocks are typically grey, lithic lapilli bearing ash crystal tuff containing quartz and feldspar crystals up to 3 mm across; small lithic lapilli with reaction haloes are common.

On the footpath west of the Shek Kong Lookout (829200 830450 Jts-18) is finely laminated tuffite dipping northnorthwest at 21^o. Along strike to the northeast (829700 830750 Jts-19) a pale greenish grey tuffite is exposed on Route Twisk, and this appears to dip southeast at 70^o. A dip of 69^o roughly southeast was recorded a short distance to the northeast (829780 830910 Jts-20) in a 1 m wide tuffaceous siltstone band within ash crystal tuffs.

Crystal tuff of the Ngau Liu Member outcrops about 1 km south of Kap Lung (828800 830700 Jts-21). Most of the outcrop is, however, dynamically metamorphosed, and the coarse ash crystal tuff is altered to a dark quartz relict bearing fine grained metatuff. This metamorphism obscures the relationship with the rest of the Shing Mun Formation below and with the granodiorite and fine grained granite surrounding the outcrop. The unmetamorphosed rock (828360 830890 Jts-22) has quartz and feldspar crystals up to 3 mm, abundant small mafic crystals and the characteristic crystalline appearance.

Ho Pui Reservoir. Lapilli bearing coarse ash crystal tuffs outcrop to the east of the reservoir, below crystal tuffs of the Ngau Liu Member. These are generally pale grey, with blocks up to 250 mm and dark lithic lapilli up to 20 mm; the lapilli may have reaction haloes. In parts of the outcrop (826340 830060 Jts-23) there are abundant large clasts and the rock is a tuff breccia. Contact metamorphism of these tuffs can be seen in the stream north of Ho Pui Reservoir (825550 830110 Jts-24).

North of Tin Fu Tsai, sandstone and tuff form an isolated outcrop bounded by Ngau Liu Member, fine grained granite and porphyritic fine grained granodiorite. There are exposures of deeply weathered reddish brown tuff, and the presence of epiclastic rocks is taken as evidence that this outcrop is stratigraphically part of the Shing Mun Formation. The sandstone is only seen as debris on the slopes west of the Fire Lookout (824700 828700 Jts-25).

North of the granite and granodiorite intrusions, towards the portal at Tsing Tam (827400 831300 Jts-26), lithic lapilli-bearing, lithic and crystal tuffs are interbedded with tuffaceous siltstone and sandstone. There were parts of the tunnel where sharp edged, pink porphyritic lapilli could be seen (HK 4671, 826620 829920 Jts-27), such as those typical of the Shek Lung Kung Member. However, for the most part the clasts in the tuffs are dark lithic lapilh up to 25 mm with reaction haloes or, more rarely, quartzite lapilli. The matrix contains quartz crystals up to 4 mm, and feldspar generally less than 3 mm.

Western Aqueduct. North of the portal at Yau Kom Tau (827800 825900 Jts-28) the succession passes upwards from Yim Tin Tsai Formation into undifferentiated Shing Mun Formation. The dips are very low, but tectonic disturbance at the granite contact produces a repetition of the Yim Tin Tsai Formation. However, from about 1.9 to 2.3 km from the portal the Shing Mun Formation occurs as dominantly fine ash tuff, with quartz up to 3 mm and feldspar up to 2 mm (827210 827730 Jts-29).

Crystal tuffs of this member have been exposed in the water tunnel in two places. About 1 km south of the portal at Ko Po Shan Tsuen is pale grey lapilli-ash crystal tuffs with a crystalline appearance. The tuff bears the characteristic small mafic crystals seen elsewhere in the member, and has quartz and feldspar crystal up to 4 mm across. In thin section they are known to have small microcline crystals, hornblende or biotite, and a finely crystaline matrix. About 250 m north of the portal at Fung Kat Heung were exposures of crystal metatuff finely speckled with mafic crystals.

Ngau Liu to Tin Fu Tsai. The type locality of the Nau Liu Member is east of the Forestry Post on Route Twisk, near Ngau Liu (828300 829500 Jts-30). A complex fault zone obscures stratigraphic relationships with the overlying Tai Mo Shan Formation. To the north and west of the outcrop are tuffs and sedimentary rocks of the lower part of the Shing Mun Formation, while granodiorite outcrops to the south. The rock is pale grey, almost white, and superficially resembles fine grained granite. It is a lapilli ash to coarse ash crystal tuff, dominated by crystals of quartz and white feldspar less than 4 mm across and mafic minerals less than 3 mm, set in a very fine grained matrix.

To the west of the type locality, around a Fire Lookout (827300 829300 Jts-31), the rock is pale crystal tuff with prominent mafics less than 1 mm, and it contains small, dark rounded lithic lapilli up to 50 mm across. In the hills about 1 km east of Tin Fu Tsai, at another Fire Lookout (825020 828640 Jts-32), is a pale grey lapilli bearing ash crystal tuff with small mafic crystals and pale, porphyritic lapilli less than 15 mm.

Tsing Tam Reservoir. Exposures of a pale grey granite-like tuff outcrop to the north of the typical undifferentiated Shing Mun Formation. The pale, lapilli-ash crystal tuff (826300 830790 Jts-33) has a scattering of mafics up to 2 mm, and quartz and feldspar crystals up to 5 mm; the matrix is very fine-grained.

Grassy Hill and Tai Po Kau. The assemblage of block-bearing lapilli-ash tuffs, tuffites and sedimentary rocks (Plates 5.A5 and 5.A6) is well exposed on the ridges between Grassy Hill and Needle Hill (835700 829350 Jts-34) and on the hill south of Tai Po Kau (837420 831960 Jts-35). In this area the base of the formation can be placed at 837000 829420 Jts-36, where mottled, grey-green and brown, lapilli-bearing ash tuff overlies a massive ash-lapilli crystal tuff of the Yim Tin Tsai Formation which is rich in euhedral biotite and hornblende. The overlying succession is dominated by block-bearing tuffs (835700 829350 Jts-34) with subordinate sandstone and siltstone. A number of contacts of block-bearing tuff resting on lapilli tuff or sandstone illustrate the complex, variable nature of the formation. A chemical analysis of tuff from the Shing Mun Formation near Tai Po Kau is given in Table 5.A1.

Tai Mo Shan to Tso Kung Tam. The succession cropping out on the southern flanks of Tai Mo Shan (831200 827340 Jts-37) is typical of the formation and is shown in Figure 5.A3. The base crops out on the ridge north of Fu Yung Shan (830200 827140 Jts-38), Kwai Chung, where pale grey, lapilli-ash crystal tuff, poor in mafic minerals, (HK 388) overlies an ash-lapilli tuff of the Yim Tin Tsai Formation rich in mafic minerals (biotite and hornblende) and conspicious feldspar (HK 389). Boulders from strata higher up the ridge line are of various lithologies, including interbedded tuffaceous sandstones and conglomerates in beds up to 3 m thick. Conglomerates are occasionally rich in limestone clasts up to 0.3 m set in a tuffaceous sandstone matrix (830250 827610 Jts-39). Elsewhere, rounded cobbles of quartzitic sandstone are more common (830420 828070 Jts-40) and the matrix is more clearly tuffaceous. A fossil from one of the limestone cobbles from the conglomerate has been identified as a brachiopod, probably a chonetacean of Carboniferous or Permian age (C.H.C. Brunton, written comm). The implication of this occurrence is that Carboniferous or Permian limestones underlie the volcanic source area. About 100 m thickness of conglomerates, siltstones, and lapilli- and block-bearing tuffs are estimated on the southern ridge of Tai Mo Shan, with coarse, block-bearing deposits occurring at two or possibly more levels (830220 827320 Jts-41 and 830560 828320 Jts-42).

On the southeastern ridge of Tai Mo Shan the lithotypes are more clearly defined. Block-bearing deposits with limestone clasts form crags above the catchwater (831440 827280 Jts-43) They are about 50 m thick and dip gently northwards. At their top they are bedded and are considered to have been alluvially re-worked; lower down they are composed of a chaotic assemblage of rounded and angular blocks of volcanic and sedimentary rock types, and are probably laharic in origin. The block-bearing deposits form a distinct shoulder on the ridge crest, above which siltstone and overlying quartzitic sandstone crop out (831540 827690 Jts-44). The siltstone may be continuous with the siltstone seen on the southern ridge of Tai Mo Shan (830200 827870 Jts-45) and in the valley of the Shing Mun River (833000 829000 Jts-46), where it can be seen to be interbedded with siliceous tuffites. The siltstone and quartzitic sandstone total some 40 m thickness and are overlain by pale grey, lapilli-ash and ash tuff which typifies the tuff deposits of the formation. In places, (e.g. 831600 828050 Jts-47), these tuffs include lenticular layers rich in angular blocks of tuff and rounded cobbles of quartzitic sandstone.

The uppermost beds of the formation are ash-lapilli tuffs with occasional small sandstone clasts, as seen for example at 831540 828180 Jts-48, and these are overlain at 831640 828420 Jts-49 by about 20 m of pale buff, ash crystal tuff which contains common biotite flakes and small fiamme in a faintly aligned, probably welded, fabric. This latter lithology crops out also on the southern ridge of Tai Mo Shan (830240 828700 Jts-50).

There are exposures of tuff, tuffite and sedimentary rocks on the southern and southwestern flanks of Tai Mo Shan. These lie above typical tuffs of the Yim Tin Tsai Formation. The dominant pyroclastic rocks in the area are block and lapilli bearing ash crystal tuffs, although there are also tuff breccias (Plate 5.A7). These lithologies are best seen on the catchwater north of Route Twisk (829000 827700 Jts-51), and were also seen in site formation works north of Tso Kung Tam (829000 827400 Jts-52). The rock is grey or, more usually, greenish grey, with lapilli and blocks up to 400 mm of crystal tuff, mudstone, limestone and sandstone. The matrix contains quartz crystals up to 4 mm, feldspar up to 2 mm and lithic clasts up to at least 5 mm.

Pale greenish grey, lithic lapilli bearing ash crystal tuffs are found around Chuen Lung (829000 828300 Jts-53) and east towards the Country Park Management Centre (829610 827970 Jts-54). The pyroclasts are dominantly quartz up to 3 mm, feldspar up to 2 mm and lithics ranging from 40 mm down to fine ash, but commonly around 5 mm. The rocks are slightly tuffaceous, and have the characteristic highly varied appearance of the Shing Mun Formation.

Epiclastic and tuffaceous horizons are common, such as in the area east of the Country Park Management Centre (829600 828000 Jts-55), where a tuffaceous siltstone up to 50 m thick outcrops. Along the catchwater above Route Twisk (829440 827380 Jts-56), finely laminated pale brown siltstone dips roughly westnorthwest at 15 to 25^o. On the catchwater west of Route Twisk (828820 827490 Jts-57), tuffaceous siltstone contains lithic lapilli up to 15 mm that sometimes display reaction haloes. North of Chuen Lung (828980 828780 Jts-58) is a grey medium grained sandstone with indistinct bedding dipping south at 55^o. A short way to the north of this exposure is tuffaceous siltstone (828950 828820 Jts-59) with fine irregular bedding dipping roughly northeast at 15^o.

Shek Lung Kung to Lin Fa Shan. The type area for the Shek Lung Kung Member is on Shek Lung Kung and on smaller hills to the west. The rock is dominantly either tuff breccia or lithic lapilli bearing lapilli ash crystal tuff. It is pale grey, and most of the lithic lapilli are of porphyritic rock with a pale pink glassy matrix. The glassy lapilli have sharp edges and are up to 70 mm long. They are elongated or flattened and define a linear or planar fabric. The pyroclasts are mostly coarse ash crystals of quartz up to 5 mm and feldspar up to 7 mm. There are also lithic lapilli and blocks of tuff or epiclastic material up to 150 mm. Westsouthwest of Shek Lung Kung (826770 826810 Jts-60) the welding fabric dips southsoutheast at 30^o, while nearby (826750 826640 Jts-61) it dips roughly northnorthwest at 18^o.

North of Shek Lung Kung (827080 827010 Jts-62) is an exposure of pyroclastic breccia with poor sub horizontal layering (Plate 5.A8). This rock contains blocks of crystal tuff up to 1.5 m across, although most clasts are 0.1 to 0.5 m. East of Ha Fa Shan (828200 826700 Jts-63) is a pale grey tuff breccia with some blocks up to 250 mm. There is a sub horizontal elongation in the blocks and lapilli. Some of the lapilli are glassy and porphyritic, as seen at the type area.

Eastsoutheast of Sheung Fa Shan (827900 827500 Jts-64) the member comprises pale grey lapilli bearing ash crystal tuffs characterized by the presence of pale porphyritic lapilli and blocks up to 100 mm. In the matrix are quartz crystals up to 4 mm and feldspar up to 3 mm.

On a summit north of the abandoned school (826720 827390 Jts-65) the clasts in the lithic lapilli and block bearing tuff display a rough layering which is either sub horizontal or dipping gently south (Plate 5.A9). The rock is grey, and contains quartz crystals up to 3 mm and dark lithic fragments around 6 mm across.

Tsing Yi to Yau Kom Tau. From the northern flanks of Tsing Yi to Western Aqueduct portal at Yau Kom Tau the outcrop of the Shing Mun Formation is dominated by sandstone. A related outcrop of sandstone lies northwest of Pun Shan Tsuen (828100 826300 Jts-66). This sandstone lies within the Shing Mun Formation, close to the underlying Yim Tin Tsai Formation and at the base of the Ngau Liu Member. Boreholes and large scale site formation have proved the sandstones and associated tuffites on north Tsing Yi at Fung Shue Wo (827400 824100 Jts-67). These dominantly epiclastic rocks form a near continuous band from Fung Shue Wo to the hill slopes at Yau Kom Tau. The rock is either grey or, more usually, it is epidotized and pale green or greenish white. At both Fung Shue Wo and Yau Kom Tau there are sandstones that are more tuffaceous in appearance.

Numerous boreholes south of Ngau Kok Wan (827000 824500 Jts-68) have proved a transition from the Yim Tin Tsai Formation into distinctively finer tuffaceous rocks of the Shing Mun Formation. However, no exposures were seen, although the contact between the formations, dipping at about 40^o to the north, forms a slight erosional feature.

South of Kam Tin. The outcrop is characterized by an abundance of epiclastic horizons relative to the main Shing Mun Formation outcrop to the east. Pale grey fine ash crystal tuff (823710 831310 Jts-69) contains quartz pyroclasts up to 3 mm and dark elongated lithics defining a sub horizontal fabric. Nearby, lithic lapilli bearing ash crystal tuff (823750 831250 Jts-70) contains scarce pebble like lapilli up to 15 mm across in a fine ash matrix that contains quartz crystals up to 2 mm.

In an eroded hollow (823630 830850 Jts-71), finely bedded siltstones can be seen overlying pinkish weathered tuffs dipping northnorthwest at 29^o. A thermally metamorphosed siltstone with sericitized chiastolite (823450 831020 Jts-72) is exposed to the northwest of this contact dipping eastnortheast at 12 to 22^o. These changes of dip result in a complex outcrop pattern, and are caused by the irregular intrusive surface of the nearby granite.

In the hills to the south of Kam Tin an outcrop of crystal tuff is overlain by lapilli ash crystal tuff of the Tai Mo Shan Formation. To the south are fine ash tuff, siltstone and sandstone of the underlying undifferentiated part of the Shing Mun Formation. Dynamic metamorphism has affected the tuffs of the Ngau Liu Member, but when unaltered the rock is pale grey with the characteristic granite like appearance.

Tai Lam. At one locality on the largest outlier (822910 829970 Jts-73) is grey coarse ash crystal tuff. To the north (822590 830800 Jts-74) is an outcrop containing siltstone, sandstone and crystal tuff. This assemblage is similar to that seen 1 km to the east, which is part of the Shing Mun Formation.

Ma On Shan. Northwest of the major northeast trending fault on Ma On Shan the outcrop of the volcanics is disturbed by subsidiary faults and folds. Lithologies are difficult to correlate but closest resemblances are with tuffs of the Shing Mun Formation. Fiamme rich welded tuff (HK 829) crops out on the ridge north of the summit of Ma On Shan (844050 830010 Jts-75). The base of the Tsuen Wan Volcanic Group on Ma On Shan can be seen on the ridge southwest of the summit (843850 829620 Jts-76), where thin tuffs, up to 2 m thick, are interbedded with dark grey argillaceous sandstones. However, lithologies are all somewhat altered and identification of rock types is problematical.

Sham Chung and Yung Shue O. 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 Middle Jurassic Tai O Formation and the Early Jurassic Tolo Channel Formation. The unit is a coarse ash tuff that outcrops along the coast of Tolo Channel northeast of Sham Chung Kok (846870 834150 Jts-77). The pale grey tuff contains clasts of dark grey mudstone, siltstone and sandstone, together with abundant quartz crystals.

These exposures commonly display narrow and sinuous dyke-like structures containing mudstone clasts which may have been formed by vaporization of water from beneath the tuff, resulting in upward streaming of gases which transported the clasts. In places the tuff contains numerous cobbles of quartzite and siltstone (847100 834540 Jts-78). The lowermost coarse ash tuff unit is also exposed south of Sham Chung Wan, where it is characterised by the conspicuous presence of dark grey mudstone clasts (847120 833050 Jts-79). Southwards, on Ngau Yue Tau (847480 832240 Jts-80), the coarse ash tuff contains two subdivisions, the lower one having been stained red and eroded prior to the deposition of the upper bomb- and block-bearing subdivision.

The coarse ash tuff is overlain by thinly bedded tuff and tuffite interbedded with siltstone and mudstone. These are well exposed on the foreshore north of Shek Ngau Tau (847350 834720 Jts-81), where beds of coarse ash tuff up to 20 m in thickness are intercalated with fine ash tuffite and siltstone (Plate 5.A11). The tuffite frequently displays penecontemporaneous deformation by faulting and slumping. These beds can be traced southwards to Sham Chung Wan (847060 833790 Jts-82), and on the coast south of Sham Chung Wan (847160 832940 Jts-83), where sandy tuffite with pumice fragments occupies a shallow channel structure cut into tuff containing accretionary lapilli. Further south, (847260 832860 Jts-84) the interbedded fine ash tuffite and mudstone have yielded plant remains. Near Yung Shue O, dark grey siliceous siltstone is indurated and in part contains organic structures which appear to be algal patches.

Between Sham Chung and Yung Shue O a thick sequence of coarse ash crystal tuff extends northwards to the coastline near Shek Ngau Tau (847600 834950 Jts-85). Here, the strata consist of wall-bedded coarse ash tuff with interbedded sandy tuffite that includes beds of accretionary lapilli which in places display cross-bedding. These beds dip east to southeast at about 35^o, but their relationship with the underlying rhyolite lava is not seen and the contact may be faulted out of the coastal succession.

Leng Pei Tsuen to Pak Kung Au. From Leng Pei Tsuen (836400 841700 Jts-86) to Pak Kung Au (839200 846200 Jts-87), the formation is grey to dark grey in fresh, unaltered outcrop, but pale grey to greenish grey where metamorphosed. The tuffs are characterized by the abundance and variety of lithic lapilli and large boulders, including sandstone, siltstone and mudstone fragments, typically 50 to 500 mm in size. The matrix is dominated by coarse to fine ash crystals. There are good exposures in a large cut slope at Au Ha (838000 841000 Jts-88), along a track between the Au Ha cut slope and Hung Fa Chai (838000 845000 Jts-89), and along the border road from San Kwai Tin (838000 846000 Jts-90) to Pak Kung Au and Sha Tau Kok. Sandstone and mudstone intercalations occur at Kung Chu Shan (836750 841870 Jts-91 and 837020 841620 Jts-92), 450 m west of Sheung Tam Shui Hang and 200 m northwest of Hung Fa Chai (837850 844950 Jts-93). Large blocks, up to 2 m across, are exposed 450 m southwest of Tong To (839400 844600 Jts-94). The faulted contact with the overlying Tai Mo Shan Formation is well exposed in a road cutting at Au Ha (838050 842950 Jts-95). The northeast-trending fault dips northwest at 40^o.

Tung Shan Ha to Hoi Pui Leng. Between Tung Shan Ha (836800 840600 Jts-96) and Hoi Pui Leng (839100 843100 Jts-97), the formation is bounded by northeast-trending faults, along which the tuff is mylonitized. Red beds of the Pat Sin Leng Formation overlie the tuffs. 500 m south of Sheung Wo Hang (838270 842150 Jts-98 and 838340 842000 Jts-99), the tuff contains lithic lapilli and blocks, and thinly bedded intercalations of light red sandstone and greyish white conglomerate.

Cheung Sha. The only unaltered exposures of the formation occur in a narrow outcrop in south Lantau Island which dominantly comprises eutaxite, with minor, coarse, ash-bearing vitric tuff. The eutaxitic fabric is well displayed in an old quarry (812140 810650 Jts-100) adjacent to the catchwater. White fiamme, which are coarsely porphyritic, and have sharp, ragged edges, dominate the grey tuff. They define a fabric which dips northwards at 70^o, and similarly steep, northerly dips also occur at several localities along the Tung Chung Road. Stratigraphically above the eutaxite, the tuffite and lava belong to the Lantau Volcanic Group (undifferentiated), whereas below, the tuffs are typical of the Yim Tin Tsai Formation, but contact relationships have not been seen. East of the Tung Chung Road, the member has not been seen, but to the west, boulders confirm its presence on the hillside above Tong Fuk.

Yi O. A sequence of lithic lapilli-bearing tuffs and tuffaceous sedimentary rocks is exposed on the flanks of Nga Ying Shan and Tai Hom Sham, east of Yi O. The tuffs contain relict pyroclasts, mostly of quartz, and up to 3 mm across. Some angular lithic lapilli, rarely up to 25 mm, also occur. The tuffs display a secondary foliation, and the matrix and feldspar pyroclasts have been altered. Hence, stratigraphic determination is difficult. However, the lithological diversity, which also includes tuffaceous sandstone, pebbly siltstone and sedimentary breccia, is characteristic of the Shing Mun Formation.

The sequence lies above strata typical of the Yim Tin Tsai Formation, and is in turn overlain by lavas and tuffs of the Lantau Volcanic Group (undifferentiated), including a local basal eutaxite. However, contacts are not exposed, and the rocks of both units are strongly altered and locally foliated.

On the coast (801500 809800 Jts-101) west of Yi O there are exposures of lithic lapilli-bearing ash crystal tuff, that contain dark grey, aphanitic lapilli up to 150 mm across, and small, pale, porphyritic lapilli similar to those found in the Shek Lung Kung Member. Elsewhere along the coast to the south, the tuff contains varying, often abundant amounts of dark lithic lapilli. In this area it has not proved possible to differentiate the Shek Lung Kung Member from the main body of the Shing Mun Formation. On Kai Kung Shan (801900 810200 Jts-102), tuffs of the formation are highly altered, veined and fractured. Relict quartz pyroclasts, up to 5 mm across, pseudomorphs after biotite and lithic lapilli are the only indicators of the original pyroclastic texture.

Cheung Shan. West of the summit of Cheung Shan, there are exposures of lithic lapilli-bearing tuff and eutaxite. The eutaxite is the main component of this pyroclastic Cheung Shan Member. The eutaxitic fabric is well preserved even though some exposures display a well-developed platy foliation in addition to alteration of feldspars and matrix. Exposures (805350 813900 Jts-103) on a knoll northwest of Cheung Shan consist of greyish-white eutaxite, weathering red and yellow, with abundant relict, wispy fiamme up to 10 mm wide and 40 mm long, and relict quartz crystals up to 2 mm across. Nearby exposures (805550 813700 Jts-104) of tuff contain small, pebble-like lithic lapilli set in a vitric matrix.

At Cheung Shan (805300 813450 Jts-105), fine banding, 10-50 mm wide, is well developed in lava. The banding undulates gently, but generally dips 20 to 50^o to the south. About 300 m to the east, on the summit of Cheung Shan, relict banding dips steeply to the southeast, and on Fan Shui Au, banded rhyolite lava dips consistently to the south at 50^o to 80^o. Along the coast north of Pak Kok, banding in lavas is again well developed. It strikes roughly eastnortheast and dips variably 18 to 82^o, but folds and angular unconformities also occur locally.

San Chau. On the ridge and headland west of San Chau, the structural association of the eutaxite and Jurassic sedimentary strata is complex. A knoll (804890 814170 Jts-106) south of the contact, consists of light grey, finely foliated eutaxite which weathers to red and yellow. The rock only occurs as boulders from which it is not possible to measure the dip of the primary pyroclastic fabric. To the north, there is an isolated outcrop (804800 814400 Jts-107) of eutaxite which is interpreted as being in faulted contact with Jurassic Tai O sedimentary strata on both sides. A streaky eutaxitic fabric, dipping northwest at 46^o, is present in a deeply weathered tuff in exposures (804800 813800 Jts-108) near the footpath to Tai O.

Tai O. Eutaxite boulders are common in debris deposits on the hills east of Tai O, together with less common boulders of vitric tuff and quartzitic sandstone. Exposures (804480 813400 Jts-109) of highly weathered vitric tuff can be seen above Tai O cemetery. The weathered rock is white and contains small, broken to euhedral quartz crystals. Nearby, there are boulders of eutaxite which are more typical of the member.

At San Tsuen, south of Tai O, a north-trending ridge is composed of quartz ash-bearing tuff with a fine fabric of relict fiamme. Near the Service Reservoir (804470 813960 Jts-110) the relict fiamme dip westsouthwest at 38^o. Further west, altered and mylonitized tuffs of the Shing Mun Formation are exposed across a northwest-trending fault.

Tsim Fung Shan - Tai Hom Shan. The Cheung Shan Member forms a narrow, laterally discontinuous outcrop which extends from Tsim Fung Shan (804100 811400 Jts-111), south through Nga Ying Sham, Shui Lo Cho and Man Cheung Po, to Tai Hom Shan (803300 809300 Jts-112). Most of the exposures on the northwest flanks of Tsim Fung Shan consist of crystal ash-bearing vitric tuff. At several localities, well-developed eutaxitic fiamme, 10 to 30 mm long, are preserved in the tuff. No clear base to the member has been defined, although tuff and tuffaceous sandstone to the west are probably part of the Shing Mun Formation. The member is overlain by lavas, tuffaceous sandstones and tuffites of the Lantau Volcanic Group (undifferentiated). West of Man Cheung Po, the member crops out below a fossiliferous siltstone sequence. The eutaxitic fabric is occasionally visible in the quartz and feldspar vitric tuffs. The pyroclasts are generally less than 2 mm across.

Southwest of Man Cheung Po, towards Tai Hom Shan, a faint streaky fabric is often visible, although its orientation is rarely clear. The relict fiamme are, atypically for the formation, generally less than 20 mm long, and the euhedral to subhedral quartz crystals rarely exceed 2 mm. The formation lies below siltstone of the Lantau Volcanic Group (undifferentiated). However, the contact is not exposed, and the stratigraphic relationships can only be inferred.