The Clear Water Bay Formation (Table 5.7) crops out in eastern Kowloon and the eastern New Territories south of the Chek Keng Fault (Figure 5.24). The formation is at least 400 m thick and contains several trachydacite to high silica rhyolite lava flows (Plate 5.31) interbedded with fine ash tuffs, tuffaceous mudstones, and tuffites (Figure 5.25). Two members are recognized: the Tai Tun Member and the Lan Nai Wan Member. High precision U–Pb zircon age-dating has returned an age of 140.7 ± 0.2 Ma for the formation (Davis et al., 1997).

On Clear Water Bay Peninsula, the formation unconformably overlies the Pan Long Wan Formation, but farther north on Kiu Tsui Chau and in the Sai Kung Country Park, it overlies the Mang Kung Uk Formation. Adjacent to the Chek Keng Fault, the formation rests with angular unconformity on older stratigraphic units. Here, the easterly-dipping, moderately inclined flow banded rhyolites can be mapped in continuity with narrow outcrops of intrusive rhyolite with subvertical flow banding (Plate 5.32), which bound the fault to the north around Shek Nga Shan. High precision U–Pb zircon age-dating of the intrusive rhyolite has yielded a date of 140.9 ± 0.2 Ma (GEO, unpublished data). This is within error of the age obtained for the Clear Water Bay Formation and supports correlation of the intrusive and extrusive rhyolites.

Further west, narrow rhyolite dykes lie along a continuation of the same trend as the fault-bounded zone. These relationships suggest that the fault zone was a major conduit for the eruption of the rhyolitic magma that formed the formation (Figure 5.28). This is also consistent with the thickest development of the formation being northwest of High Island Reservoir. An inferred westerly-trending structure, passing through the area of the High Island Reservoir and Sai Kung, may have limited the southerly spread of the rhyolite lavas (Figure 5.28).

The Tai Tun Member (Strange et al., 1990), which crops out east of Sai Kung, is a highly distinctive fine ash vitric tuff packed with fiamme and a few angular clasts (Plate 5.33). An extreme parataxitic fabric is locally developed and the member has been interpreted (Strange et al., 1990) as a pyroclastic flow deposit. At the type locality at Tai Tun, the unit is about 120 m thick.

The Lan Nai Wan Member (Strange et al., 1990) is a distinctive tuffaceous unit comprising tuff, tuff-breccia, tuffite and epiclastic breccia at the top of the Clear Water Bay Formation. It conformably overlies red to purple autobrecciated lava and laps onto the Long Harbour Formation. At its type locality on High Island (West Col Dam), the member is 37 m thick. A small outcrop of the member is also exposed on South Ninepin Island at the contact with the overlying High Island Formation. Flow-banded rhyolite predominates in the Sai Kung Country Park area and comprises a very fine-grained microcrystalline groundmass, variably spherulitic and recrystallized, with flow-aligned euhedral feldspar crystals, some in clusters.

Details

Clear Water Bay Peninsula and Ninepins Group. In the Tai Au Mun area a significant fault can be traced parallel to the coastline, downthrowing the Clear Water Bay Formation against trachydacite lava. Columnar jointing is well developed, and close to the fault (848770 817050 Kkw-1) the columns have been bent through 120^o. Between Tai Au Mun (848300 817300 Kkw-2) and Lung Ha Wan (849300 818900 Kkw-3) exposures are few, except along the coast, where columnar jointing is commonly seen in the steep cliffs.

Ngau Mei Hoi (Port Shelter) and Sai Kung Hoi (Inner Port Shelter). Intensely contorted flow-banding is present in purplish grey lava occupying the northern part of Shelter Island. Similar rocks have been collected underwater at Tai Pai (848430 822590 Kkw-4) and are present striking northwards across Sharp Island. On Yim Tin Tsai (849020 826310 Kkw-5), purple lava, often autobrecciated, is overlain by High Island Formation tuff.

Sai Kung East Country Park, including Sharp Peak. The lava exceeds 400 m in thickness to the east of Pak Tam Chung, where a chemical analysis (HK6001, Table 5.A2) indicates it has a rhyolitic composition with some alkali enrichment. The dark grey to black, very fine grained rhyolite has fine flow-banding with sparse euhedral feldspar phenocrysts. The rock gradually changes in colour to reddish purple as the contact with the overlying Lan Nai Wan Member is approached. Near the High Island west col dam (852610 826910 Kkw-6) the lava is intensely autobrecciated with fragments of flow-banded rhyolite set in a highly silicified red groundmass. This autobreccia is present in the basal part of borehole DH8 (Figure 5.A6). A continuous exposure of black rhyolite is seen along the length of the water tunnel between Pak Tam Au (852370 830220 Kkw-7) and the High Island Reservoir (852640 828430 Kkw-8); the rock is flow banded in places but no autobrecciated layers were observed. Close to the Sai Wan road, 2 km east of Pak Tam Chung (853600 828400 Kkw-9), reddish grey flow banded lava is exposed in the stream bed (Plate 5.A30). This is overlain by autobreccia consisting of a chaotic assemblage of flow-banded rhyolite blocks of all sizes up to 1.2 m. Flow-banding is also seen in the groundmass separating the large blocks. This rock forms a large tor beside the road (853840 828010 Kkw-10) and is overlain by the High Island Formation 100 m to the east.

A faulted block of the formation is found on the south side of Tai Long Wan, where tuff, tuff-breccia and eutaxite layers have been noted within the lava sequence. On the nearby island Tsim Chau (857830 829120 Kkw-11) numerous small vertical silicified veins, apparently injected upwards into the tuff from the flow-banded lava, are presence. These may possibly represent late-stage emanations from the cooling lava body which had already been overlain by thick pyroclastic flow material. Autobrecciated reddish brown lava outcrops at Mong Yue Kok (857000 830100 Kkw-12). Altered and silicified lava and tuff are found on Sharp Peak (856800 832400 Kkw-13), where a steeply dipping eutaxite horizon has been delineated. Eastwards, at Bate Head (859400 833000 Kkw-14), autobreccia predominates (Plate 5.A31), with common large blocks to 1.5 m in diameter. At Tung Wan (857850 831380 Kkw-15) the lava is markedly spherulitic although traces of the flow-banding are still visible. On the Tai Long Tsui peninsula the lava is overlain by tuff and tuffite of the Lan Nai Wan Member.

Tai Long Tsui Peninsula. The basal beds of the Lan Nai Wan Member consist of tuffite and a feature-forming tuff breccia (859280 830480 Kkw-16) about 40 m in thickness. Large rounded clasts of tuff up to 400 mm in diameter are set in a fine ash tuff matrix. This is overlain by a thin fine ash vitric tuff containing small scale fiamme (859340 830490 Kkw-17) and a thick pale green tuffite (859500 830420 Kkw-18) at least 120 m thick. The tuffite shows signs of hydrothermal alteration with abundant quartz veining and iron-staining. On the eastern side of the peninsula, polymictic tuff-breccia and clast bearing tuffite contains angular fragments of green mudstone, banded rhyolite and fine ash tuff (859650 830840 Kkw-19).

Tai Tun and Pak Tam Au. The type locality of the Tai Tun Member is at Tai Tun (850100 828900 Kkw-20), where the member forms a hard feature around the summit. Abundant fiamme are present throughout and these indicate a roughly horizontal nature to the strata. The eutaxite rests on soft tuffite at the eastern side of Tai Tun. It is estimated that the member reaches 120 m in this area. To the north of Tai Tun, it reaches a thickness of almost 200 m as a feature-forming, roughly horizontal layer skirting the southern flanks of Ngau Yee Shek Shan. Typical exposures of the member reveal the highly distinctive eutaxitic nature. 800 m north of Pak Tam (850540 831000 Kkw-21), the eutaxite forms crags which also display an autobrecciated appearance. Along the Pak Tam Road (852170 831910 Kkw-22) the horizontal eutaxite is seen lying on purple rhyolite lava. On Ngau Yee Shek Shan (851360 830870 Kkw-23) the eutaxite is overlain by a thick autobrecciated lava.

Sharp Island and Wong Yi Chau. The eutaxitic character is well seen in the coastal exposures around Wong Yi Chau (850800 828090 Kkw-24), where steep easterly dips are seen in the strata. Occasional aphanitic clasts up to 20 mm across stand out, with the fiamme forming a flow fabric around these clasts (Plate 5.A32). In places the fiamme are stretched out to give a streaky parataxitic fabric. On Sharp Island the Tai Tun Member is steeply dipping and layered between massive flow-banded lavas. The eutaxite is well exposed along the shoreline, where it rests on a brecciated lava surface (848600 824400 Kkw-25).

Shek Nga Shan - Ngong Ping Shan. The rhyolites of Shek Nga Shan (842200 827000 Kkw-26) and Ngong Ping Shan (844400 829200 Kkw-27) differ from the more usual varieties of feldsparphyric rhyolite in being generally finer grained, darker coloured, sparsely porphyritic and flow banded (Plate 5.A33). In thin section the rocks have a finely recrystallised felsic matrix which in places develops perlitic cracking and spherulitic growths, and encloses resorbed quartz crystals and slender alkali feldspar crystals (sanidine). In hand specimen feldspar crystals show a blue schiller effect, a feature which has also been noted in welded tuffs of the Clear Water Bay Formation on Ma On Shan. The lithological indentification of the rocks as rhyolites is clear; whether they are extrusive or intrusive in origin is less obvious. The fine grain size and flow banding are perhaps more consistent with an extrusive origin. Chemical analysis, and particularly trace element analysis, of this rhyolite from an exposure on the Sai Kung Nai Chung road (adjacent to this district at 846050 830460 Kkw-28) shows similarities with volcanic rocks (Table 5.A3). However, other factors mitigate against an extrusive interpretation; interpreted contacts are mostly steep; the geological settings of the different outcrops are complex and varied; flow banding is inconsistent; and blocky or autobrecciated textures, such as would be developed in extrusive flows, are absent.

On Shek Nga Shan (842300 827000 Kkw-29) flow banded rhyolite intrudes tuffs of the Clear Water Bay Formation, but at Ngau Au (841700 827600 Kkw-30) and on the ridges northwest and southeast of Shek Nga Shan a quartz monzonite sheet intrudes along the contact. At Shek Nga Shan and at Shek Lung Tsai (842600 828300 Kkw-31) the Ma On Shan Fault forms the northwestern margin of the rhyolite intrusion.

Three Fathoms Cove to Kai Kung Shan. The Clear Water Bay Formation, consisting of the typical fiamme bearing fine ash vitric tuff, appears to occupy a roughly east-west striking trough which may represent infilling of a pre-depositional topographic hollow.

In the vicinity of Kai Kung Shan, the formation rests unconformably on coarse ash tuff of the Long Harbour Formation. South of Kai Kung Shan (847970 830570 Kkw-32), crystal-bearing fine ash vitric tuff with occasional fiamme lies on the coarse ash tuff. Immediately above, a tuff-breccia horizon, several metres thick, contains angular lithic fragments, mainly of fine ash and coarse ash tuff.

Crystal-bearing fine ash vitric tuff is intruded by quartz monzonite along the southern shores of Three Fathoms Cove, and this appears to mark the northern boundary of the Clear Water Bay Formation in this area. To the southwest, road cuttings in the vicinity of Kei Ling Ha Lo Wai (846100 830300 Kkw-33) show aphanitic crystal bearing welded tuffs, often with segregations of abundant crystals (HK6867, 846000 830200 Kkw-34), and bounded to the north and the south by flow banded rhyolite. At the northern contact the rhyolite is reddened as if weathered prior to deposition of the Clear Water Bay Formation tuffs. Paradoxically, however, the eutaxitic fabric in the tuffs is overturned and dips at 40^o towards the contact.

The outcrops on Ma On Shan are divided by a major northeast trending fault which downthrows stata to the southeast. Exposures of tuffs on this downthrow side are pale grey, coarse ash bearing fine ash tuffs which contain common vitric shards, sometimes visible in hand specimen. Crystals are mainly of feldspar, some of which show a blue iridescent schiller effect. Welding is well developed and in places (844000 829500 Kkw-35) columnar jointing can be seen. Welding fabric develops into a strong parataxitic fabric which resembles flow folding (844180 829400 Kkw-36) but thin sections can be used to distinguish these tuffs (HK 822, Plate 5.A10) from adjacent flow banded rhyolite lava (HK 823, 844510 829150 Kkw-37).