The newly defined Che Kwu Shan Formation (Campbell & Sewell, 1998) (Table 5.6) includes interbedded eutaxites and fine ash vitric tuffs formerly mapped as the Ap Lei Chau Formation or the Shing Mun Formation in eastern Kowloon and eastern Hong Kong Island (Figure 5.16). It also includes the eutaxitic Silverstrand Member (previously the Silverstrand Formation) as a distinct unit. The type area of the formation is at Razor Hill (Che Kwu Shan) to the east of Sai Kung (Figure 5.21) where the formation is approximately 1400 m thick.

Reclassification is based on general stratigraphic relationships, trace element geochemistry, petrographic interpretation, and high precision U–Pb zircon age-dating, which has returned an age of 142.5 ± 0.3 Ma for the formation (Davis et al., 1997). Although lithologically similar in many respects to the Ap Lei Chau Formation, the Che Kwu Shan Formation has a significantly lower SiO2 content than the Ap Lei Chau Formation.

In the Sai Kung area, fine ash tuffs and eutaxites typical of the Che Kwu Shan Formation interdigitate with coarse ash crystal tuffs characteristic of the Long Harbour and Mount Davis formations. Generally, however, the formation overlies the latter two formations. Locally, on the southern side of Hebe Haven, the formation includes block-bearing breccias. Both the blocks and matrix comprise coarse ash crystal tuff. These deposits are interpreted as having been deposited close to, or even infilling, their source vent.

On the south side of Silverstrand Bay, the Silverstrand Member comprises fine ash vitric tuff with a prominent and uniform eutaxitic fabric with abundant fiamme of flattened and stretched pumice fragments (Plate 5.28). Interbedded lenses of tuffaceous mudstone, up to 50 m thick, presumably developed at interflow boundaries, indicate that the member is a composite sequence of pyroclastic flow deposits. The member is estimated to be up to 500 m thick.

In the vicinity of Tseung Kwan O, the Silverstrand Member rests conformably on fine ash tuffs of the Che Kwu Shan Formation, but in the Sai Kung area it is unconformable on coarse ash tuffs of the Long Harbour Formation. The member resembles eutaxitic tuffs within the lower part of the Che Kwu Shan Formation, but is distinguished by its larger fiamme and flattened clasts, and greater and more uniform bed thickness. Typically, quartz and feldspar crystals are relatively common and fiamme are abundant within the fine ash vitric matrix.

The top of the member coincides with the top of the Che Kwu Shan Formation and is marked by an abrupt change into tuffaceous sedimentary rocks of the Mang Kung Uk Formation. The outcrop of the Silverstrand Member now includes an area previously mapped as Ap Lei Chau Formation to the north of the Ho Chung Valley.

Details

Siu Chai Wan to Big Wave Bay. New excavations at Siu Chai Wan (HK 578, 843610 813910 Krc-1) have revealed bluish grey fine to coarse ash welded tuff with abundant subhedral, often corroded, plagioclase crystals 1 to 2 mm in length, and more altered microperthite crystals, 2 to 3 mm across. On slopes forming the eastern side of Siu Chai Wan (844130 813850 Krc-2) scattered dark grey lava lapilli stand out on the white weathering tuff surface. Southwards to Cape Collinson Training Centre these tuffs are bluish black when fresh and are well exposed along the Cape Collinson Road. There (e.g. 844210 813010 Krc-3), a black tuff with a very fine matrix contains abundant angular to sub angular black aphanite lapilli up to 40 mm across (see also Plate 5.A22). In the road cutting 120 m north of the Training Centre gate (844200 812600 Krc-4) the lapilli become so abundant that the rock becomes a lapilli tuff. In thin section many of these tuffs show evidence of welding. In particular, HK 3311 (844220 812700 Krc-5) shows a streaky parataxitic fabric; in thin section the vitric matrix is very fine, but abundant 0.5 to 1 mm quartz crystals are present. Sample HK3314 (844020 813080 Krc-6) is a welded tuff that contains large angular lapilli of parataxitic banded tuff.

A strong feature forming welded tuff layer can be traced across Pottinger Peak, and dips steeply towards the south (843850 812790 Krc-7). At this location large boulders of welded tuff, many displaying a crude eutaxitic fabric, are strewn along the ridge. A similar, roughly horizontal layer of eutaxite forms a scarp along the catchwater east of the cemetery (843140 813670 Krc-8), but appears impersistent. Along the catchwater that skirts Mount Collinson a thick band of coarse ash tuff forms a strong feature striking NW-SE (842770 812570 Krc-9). 200 m farther southwest along this catchwater, greenish grey fine ash tuffs of the Che Kwu Shan Formation crop out.

Between Cape Collinson Training Centre (844160 812500 Krc-10) and Big Wave Bay (843620 811970 Krc-11) the volcanics are very variable, with layers of eutaxite interlayered with fine ash to coarse ash lapilli and block bearing crystal tuffs. Large sub-rounded blocks of aphanitic lava up to 3 m across are enclosed by variable tuff. Occasional very fine chert lenses, 3 to 4 m in length, are also present (843640 811940 Krc-12). Abundant lithic fragments are found in the fine ash to coarse ash tuffs at the southern end of Tso Tui Wan (843860 812120 Krc-13), and in places the rock becomes a polymictic tuff breccia.

In the vicinity of Hing Wah Estate, Chai Wan, there are exposures of fresh, dark grey to black, lapilli-bearing fine ash vitric tuffs in roadside cuttings (842080 813590 Krc-14). A thin section (HK 2926) shows a welded, faintly eutaxitic fabric present in the fine ash tuff. Scattered quartz, microperthite and plagioclase crystals are, also present, with some flow structure in the matrix visible around the larger crystals. Some 150 m farther west northwest, across a major fault line, the tuffs are highly altered close to the granite. In thin section (HK 2924, 841930 813610 Krc-15) the rock has a recrystallized matrix consisting of equigranular quartz and muscovite grains averaging 0.5 mm.

Tung Lung Island, Junk Island and Southern Clear Water Bay Peninsula. Che Kwu Shan Formation crops out over Tung Lung Island and the attitude of the strata seem to be fairly gently dipping or roughly horizontal in many places. The eutaxitic fabric is visible everywhere, particularly on weathered surfaces in the coastal exposures. Abundant fiamme are visible in coastal exposures, viz. the cliffs between Nam Tong and the northeast corner of the island at Fat Tong Mun. At Nam Tong (847900 813000 Krc-16) the fiamme are extremely abundant and make up over 30% of the rock. Small angular to sub-angular clasts of aphanitic pale grey rock possibly chert, are also present in the eutaxite, and these sho no signs of flattening. These angular clasts become more abundant towards the western side of the island and in places (847100 812600 Krc-17) form a tuff-breccia with an average clast size of 20 mm. Along the western coastline the eutaxitic fabric is still present, but in many places the rock is also packed with angular clasts of chert. Several quartzphyric rhyolite dykes are present, the largest striking at 45^o across the western side of the island. On the southeastern side of Tung Lung Island a rock of very similar appearance to the trachydacite lava of the Che Kwu Shan Formation underlies welded tuff containing a pebbly base.

Junk Island (Fat Tong Chau) and the smaller island (prior to reclamation) of Tit Cham Chau consist of very similar eutaxite to Tung Lung Island. Several quartzphyric rhyolite dykes are found on Tit Cham Chau (846600 813900 Krc-18) and a number of basalt dykes cut the Che Kwu Shan Formation on Junk Island where the northern part of the island is dominated by a fine-grained granite intrusion with a wide flow-banded quartzphyric rhyolite margin. On the western side of Junk Island, abundant angular and subangular aphanitic clasts are found together with the characteristic flattened fiamme. On the southeast shoreline (845500 815000 Krc-19), crude columnar jointing was noted perpendicular to the eutaxitic fabric.

The Che Kwu Shan Formation forms the high rugged slopes of Tin Ha Shan between Clear Water Bay and Joss House Bay (Tai Miu Wan). Here, the northerly contact with the lavas of the Pan Long Wan Formation is fault bounded. 300 m northwest of the Joss House Bay temple (847790 814980 Krc-20), the contact between the eutaxite and the Kau Sai Chau Volcanic Group (undifferentiated) is a marked unconformity. The Che Kwu Shan Formation in this area displays the typical eutaxitic fabric with stretched fiamme up to 50 mm in width and 400 mm in length. 300 m south of Tin Ha Wan (846800 815100 Krc-21) hillside boulders are rich in angular clasts, similar to those on the western side of Junk Island.

Silverstrand and Northern Clear Water Bay Peninsula. At Silverstrand, a continuous sequence of the Silverstrand Member through most of the formation can be seen along the coast to Pik Sha Wan (Bayside Beach). The overlying Mang Kung Uk Formation is found on the east side of Pik Sha Wan Beach. In the road cutting near the junction of Silverstrand Road and Clear Water Bay Road (845730 820580 Krc-22), feature-forming eutaxite overlies an homogeneous fine ash tuff with no visible welding fabric.

Abundant small fiamme are present in the eutaxite excavated from borrow areas for the Junk Bay Controlled Tip site (846100 818700 Krc-23), 1 km south of Hang Hau Town. Here the rock is hard and has a brittle fracture. When fresh the eutaxite is black in colour with a very fine glassy matrix; the characteristic fiamme are only faintly visible on fresh surfaces. To the north of Hang Hau, the Kohima Barracks site (now the Hong Kong University of Science and Technology), has numerous fresh rock excavations into the Che Kwu Shan Formation tuffs. Invariably black when fresh and with a brittle, almost conchoidal glassy fracture, the eutaxite here is typical of the fresh Che Kwu Shan Formation rocks. In places, for example 400 m north of the entrance to the film studios (845170 821750 Krc-24), the eutaxite is crystal rich with patchy concentrations of feldspar laths. Small, sub-angular aphanitic clasts were also noted in places.

At Mau Tin (845500 823100 Krc-25), a lense of tuffite and weathered fine ash tuff forms a negative topographic feature. The overlying eutaxite displays the characteristic fabric on weathered surfaces in the cliff sections (845650 823300 Krc-26) to the northeast of Camper's Beach where the large fiamme (up to 600 mm by 100 mm) closely resemble those found on the northern side of Tung Lung Island.

East Kowloon, Ho Chung and Junk Bay. A similar breccia crops out at Nam Wai (844500 823640 Krc-27), enclosed by eutaxite. Abundant pyrite is scattered throughout the eutaxite and breccia matrix.

The thickest eutaxite layer and strongest associated topographic feature in the area strikes north northwest along Razor Hill. Eutaxite blocks are present along the ridge crest but natural rock exposure is poor. The feature is broken and displaced by several faults (844400 821610 Krc-28 & 844480 821520 Krc-29) but can still be traced over a distance of at least 2.5 km.

White, fine grained sandstone is exposed in roadside cuttings on Hiram's Highway (844230 823250 Krc-30). This epiclastic layer is about 25 m thick, with several metres of pale cream siltstone present in the lower part.

Across Junk Bay (844940 820430 Krc-31) a similar rock passes laterally southeastwards into a thick pyroclastic breccia.

Hebe Haven and Inner Port Shelter. On the Ma Lam Wat peninsula, to the east of Hebe Haven, the Che Kwu Shan Formation overlies the coarse ash tuffs of the Long Harbour Formation, where a basal epiclastic breccia, 400 mm in thickness marks the base of the Che Kwu Shan Formation tuff (846130 825060 Krc-32). On the western side of the peninsula, at a similar basal horizon, blocks of streaky parataxitic fine ash tuff are strewn along the coastline (845800 824800 Krc-33). At Sha Tsui (845400 824340 Krc-34) a tuffaceous lens up to 50 m in thickness is found within the predominantly eutaxitic tuff. The tuffite have been excavated in the past for their clay content, with a processing and tile manufacturing works at the nearby Sha Tsui Pier (845400 824500 Krc-35). Similar tuffite outcrops along the coast 600 m northeast of Ma Lam Wat where multicoloured tuffaceous sandstone alternates with siltstone layers and thin breccia horizons. These banded rocks dip southeast between 20 and 35^o and are overlain by a thick eutaxite sequence (846500 824600 Krc-36) characterised by small fiamme with abundant sub angular aphanitic clasts in the vicinity of Pak Ma Tsui (846400 823500 Krc-37).

Tung Yeung Shan. The Che Kwu Shan Formation are exposed in the upper part of the sequences on Tung Yeung Shan ridge, immediately east of Tate's Pass. The rocks here are generally pale grey, lapilli bearing ash vitric tuffs. Lapilli are usually flattened pumice fragments (fiamme) which may be porphyritic. Crystals of quartz and feldspar 2 3 mm long are common. Vitric shards are commonly visible in hand specimen as grey brown, flattened vermiform shapes up to 2 mm long. The welding fabric represented by the flattened fiamme dips generally to the east at around 35^o (841110 824730 Krc-38).

Wong Ngau Shan, Shui Ngau Shan. The outcrops of these hills are structurally complex. Measurements of welding fabrics are steep and the outcrops are cut by northeast and northwest trending faults. Exposures of clearly recognisable welded fiamme and shard rich, ash tuffs occur (HK 1065, 842100 826240 Krc-39, Plate 5.A27) but stratigraphic relations with the similarly complex outcrops of the Mount Davis Formation, which form the lower slopes of the hills, are not clear. No contact with the overlying Long Harbour Formation has been observed.