Volcanic rocks thought to be pre-Middle Jurassic, are restricted to the northwest of the New Territories where they outcrop to the west and north of Tuen Mun (Figure 5.7). Previously, on 1:20 000-scale geological map Sheets 5 and 6 and in Memoir 3, two formations were recognized: the Tsing Shan Formation in the west and the Tuen Mun Formation in the east. However, on the basis of extensive new borehole data from poorly exposed parts of the Tuen Mun area, it is proposed here to rationalize the lithostratigraphy into one formation only, the Tuen Mun Formation.

The Tuen Mun Formation is fault bounded and commonly strongly foliated suggesting that it may have been tectonically emplaced as a single block. Ar–Ar dating of a small rock fragment and a single amphibole crystal from the Tuen Mun Formation have indicated ages of 181 ± 3 Ma and 182 ± 35 Ma respectively. However, this interpretation is complicated by a strong thermal overprint (Evensen & York, 1998) so that significant uncertainty remains over the age of formation.

The lower part of the Tuen Mun Formation comprises a largely epiclastic and volcaniclastic sequence (Figure 5.8). In the west, fine-grained, cross-bedded, well-graded quartzitic sandstone, metasiltstone and phyllite predominate, with subordinate tuff, tuffite and conglomerate. In the east, tuff and tuffite are more pronounced and the sandstones and siltstones include a minor volcaniclastic component. A silica-rich detrital source is therefore inferred for the lower part of the formation, with fluvial deposition localized in narrow basins or channels. Some volcaniclastic material was derived from distant or small volume andesitic stratovolcanoes.

The upper part of the formation (>2000 m thick) is mainly composed of a sequence of andesitic lavas (Plate 5.17) with some interbedded lapilli-bearing ash crystal tuffs (Figure 5.8). These indicate that, compared with the lower part of the formation, the volcanic sources were either nearer, or that their activity had increased. There may be an unconformity at the base of this upper part of the formation. This is marked locally by conglomerates and breccias, but the contact is also faulted in places. A source vent has been inferred for some of the lavas. This lies to the north of Tuen Mun, where marble and other Palaeozoic clasts also occur in the lavas.

Details

Tsing Shan (Castle Peak). Quartzitic sandstone, siltstone and phyllite, mostly steeply inclined, are well exposed in the steep stream courses draining the eastern flanks of Tsing Shan. A section in the southern part of the outcrop, near San Shek Wan Tsuen, is representative (813530 826700 Ju-1). Massive to thick bedded fine-grained quartzitic sandstone showing traces of sedimentary lamination and cross lamination alternates with beds and partings of grey or pale grey phyllitic siltstone. A more northerly section (813840 827600 Ju-2) includes coarse- to fine-grained sandstone and foliated sandy siltstone, with fining upward sedimentary units that indicate that the strata are overturned. A bed of light greyish green, slightly foliated tuffaceous siltstone is exposed upstream (813760 827610 Ju-3).

Sandstone and siltstone associated with tuffite and conglomerate were proved under mass wasting deposits in boreholes near San Shek Wan San Tsuen. Borehole 1960D (813750 827260 Ju-4) passed from conglomerate into grey tuff, tuffaceous fine-grained sandstone and siltstone, with layers of coarse-grained quartz sandstone and tuffite as well as scattered coarse sand grains; cross bedding was noted. Another hole (1962D, 813790 827500 Ju-5) encountered greenish grey metasiltstone, with lithic clasts in some layers.

The conglomerate proved in these boreholes was mainly massive but included some beds of sandstone and tuffite. The lithological features recorded in boreholes 1965D (813830 827240 Ju-6) and 1971D (813850 827300 Ju-7) are representative. Clasts up to about 50 mm across were noted; these were sub angular to sub rounded and comprised sandstone, vein quartz and lava. The rock is greenish grey with irregular light brown and white tones in the matrix. Where weathered, the rock was generally ochre brown, though some clasts were very light grey, white or yellow. Joint surfaces and cavities within the weathered rock fabric formed by the selective corrosion of clasts commonly carried a dark brown or black coating.

The carious weathering of the conglomerate is conspicuous, for example, on the hillside west of the Shan King Estate (813690 828860 Ju-8). In a section of almost vertical strata, conglomerate forms beds up to 2 m thick. The clasts range up to 0.3 m across and comprise coarse to fine grained sandstone, vein quartz and dark greenish grey porphyritic lava; the matrix is limonitic and veined with quartz. A fining upward sequence from conglomerate through pebbly coarse grained sandstone to fine grained sandstone was noted, indicating that the strata there are younger towards the east.

Grey to greenish grey, lithic bearing quartz rich ash and lapilli tuff, and tuffite are associated with conglomerate along the northeastern margin of the outcrop. A hillside exposure near Tsing Shan Monastery (813920 828020 Ju-9) shows a layer of tuff including rounded boulders of sandstone and abundant dark grey clasts of lava. Similar rocks are well exposed in a stream section southwest of Leung Tin Estate (813630 829200 Ju-10). Tuffites recorded in the boreholes near San Shek Wan San Tsuen (813700 827300 Ju-11) were light greenish grey and included lapilli and coarse ash debris, mainly of quartz, with angular to sub rounded lithic clasts up to 40 mm across (1962D, 813790 827250 Ju-12; 1969D, 813830 827240 Ju-6). These tuffites are interbedded variously with sandstone, siltstone and conglomerate.

Northwestern Tuen Mun. The site formation for the Leung Tin Estate involved the excavation of mainly weathered andesites that were well exposed in temporary trenches at the time of survey. Zones of foliation were noted, as were thin veins of quartz and greisen. Sub angular to rounded clasts of coarser textured andesite, with feldspar and mafic phenocrysts, were noted, these lithic clasts being up to 70 mm across.

Andesite is well exposed in a stream to the southwest of this site (813800 829440 Ju-13, 813490 829260 Ju-14), and includes clasts of a similar rock type up to 0.25 m. There are also clasts up to 80 mm that appear to consist of epidotized quartzitic sandstone. Foliation was noted in parts of this section, as were veins of epidote and quartz.

Further east, towards Tai Hing Estate, metamorphosed andesites form an irregular ridge feature, largely free from superficial deposits. The alteration is extreme on the hillside beside the Shan King Estate (814470 828890 Ju-15) The rock there is schistose and shows tectonic discontinuities with associated contortion, crenulation and quartz veins. In roadside rock cuts at Shek Pai Tau the formation comprises andesites altered to various degrees; the rock is dark greyish green, slightly foliated and aphanitic (814930 828820 Ju-16). These dark volcanic rocks are cut at intervals by steeply inclined shear zones trending eastnortheast. The rock in the shear zones is sericite schist with a conspicuous light greyish green colour.

An outcrop of tuffs within the formation has been identified some 300 m west of Tai Hing Estate. Like the surrounding andesites, the tuffs are metamorphosed; the rock consists of quartz lapilli up to 8 mm across in a fine grained sericitized matrix (814640 829180 Ju-17).

Western Tuen Mun. Andesites were recorded in several temporary sections, notably in excavations for a factory building (814780 828280 Ju-18), and in a site formation (814220 827920 Ju-19) and a roadside trench (814530 827950 Ju-20) near Tsing Shan Tsuen. In excavations for road construction (814500 828230 Ju-21) near Yeung Siu Hang the andesites are foliated.

Meta andesites are exposed in former sea cliffs and rock cuts at Tuen Mun Kau Hui. They are dark grey or, where epidotized, green. Temporary sections recorded during road construction nearby showed outcrops of very light greyish green to purplish grey sericite schist (HK 0989, 814730 828040 Ju-22) as well as light grey lithic lapilli bearing metatuff; the metatuff (814710 828050 Ju-23) is foliated, with lithic clasts streaked out in the plane of foliation.

Southwestern Tuen Mun. Andesite is widespread between the foothills of Tsing Shan and Lung Mun Road, although it is generally deeply weathered and largely concealed by superficial deposits. A major borrow area northeast of San Shek Wan Tsuen (New Town Development Area 19) has been excavated into these weathered andesites, and temporary sections examined during this survey showed khaki silt with white flecks after feldspar phenocrysts and, in places, dark brown blotches after mafic phenocrysts. The fresh rock is exposed in a few places as a result of excavation (e.g. 814020 826850 Ju-24), and consists of dark grey or greyish green andesite containing sporadic clasts of andesite and, rarely, quartzitic sandstone (813800 826850 Ju-25). Epidote staining is common, as is the presence of secondary pyrite. Calcite has been observed as joint coatings. Foliation was noted in some boreholes, notably in those near the contact with the Tsing Shan Formation.

An outcrop of massive, lapilli bearing ash crystal tuff within the formation was exposed near the southern limit of the Area 19 borrow area (813830 826930 Ju-26) The tuff is light grey and includes lithic and quartz lapilli up to 20 mm across, the lithics comprising siltstone and quartzitic sandstone.

Metamorphosed volcanic rocks included in the Tuen Mun Formation have a wide outcrop both east of Lung Mun Road and to the west of this road in the Hung Lau Pak Kok area. They are poorly exposed in the low hills to the north of Hung Lau. The rock (813990 826620 Ju-27) is light brown with a crude foliation. Further southwest, at Hung Lau, there are hillside exposures of strongly foliated, very light brown rock.

Strongly foliated, metamorphosed volcanic rocks resembling siltstones are exposed in a stream bed near Pak Kok (813910 826300 Ju-28). Similar rocks, in contact with granite, can be seen at low tide on the shore at the western end of Wu Tip Wan (Butterfly Beach) (813100 825500 Ju-29), and consist of contorted, highly foliated, partly phyllitic sericite rock, with secondary quartz and iron ore.

Metamorphosed volcanic rocks assigned to the Tuen Mun Formation have been encountered in many boreholes at Pak Kok and in the reclamation area to the east. At the Pak Kok Sewage Pumping Station (813850 826130 Ju-30) is a greenish grey, highly foliated, sericitized rock with pods of andesite aligned in the plane of foliation.

A steeply inclined sheet of volcanic rocks up to 25 m wide outcrops within the Tsing Shan Granite and is well exposed in a valley (810960 826640 Ju-31, 810950 826190 Ju-32) between Tap Shek Kok and Mong Hau Shek (Pillar Point). The rocks, classified arbitrarily as Tuen Mun Formation, are grey and greenish grey, with pink patches in places. They are partly silicified and foliated in the upstream part of the section, and locally strongly foliated and contorted. Further downstream (810930 826240 Ju-33) there are conspicuous lithic clasts of pink and light green altered andesite and dark grey, fine grained sandstone; clasts up to 150 mm across were noted.

Northern Tuen Mun. Around and to the north of Wong Fung Lek (815800 829500 Ju-34) are exposures of foliated andesitic volcanic rocks. The foliated rock is exposed on the southern end of the hill (815930 829520 Ju-35) dipping southeast at 63^o. The rock is grey, with the foliation well displayed in secondary biotite. To the northwest, numerous ground investigation boreholes have found a marble clast bearing andesite. This rock is typically foliated, and the marble clasts are streaked out in the plane of the schistosity. In other boreholes the marble forms angular clasts up to 0.1 m across set in a fine grained matrix. This block bearing tuff is probably a vent, as its extent is limited to this small area of northern Tuen Mun.

San Tin. Fine-grained metasandstone with phyllite is the main rock type at Mong Tseng Wai (818600 837700 Ju-36), while conglomeratic metasandstone occurs to the northeast of Mong Tseng Wai (818600 837900 Ju-37).

Eastern Tuen Mun. Volcanic rocks are nowhere exposed in the area south of the Pumping Station at Wong Fung Lek (815800 829500 Ju-34). However, numerous ground investigation boreholes have penetrated andesitic volcanic and volcaniclastic rocks.