The recently defined Pan Long Wan Formation (Table 5.6), which is up to 420 m thick, crops out mainly to the south of Siu Chung Lam Wan on the Clear Water Bay Peninsula (Figure 5.16). It incorporates the Tai Miu Wan Member and the lower part of the Clear Water Bay Formation, as previously recognized by Strange et al., 1990. The formation probably extends for some distance beneath the seabed to the north of Siu Chung Lan Wan, but is not seen in exposure and probably diminishes rapidly in thickness in this direction. The formation comprises a basal trachydacite lava (Tai Miu Wan Member), and units of fine ash tuff, up to 140 m thick but generally <50 m thick, separating further trachydacite lavas (Figure 5.23).

The Tai Miu Wan Member (Strange et al., 1990), which is up to 150 m thick, is a distinctive bluish grey trachydacite lava with finely laminated flow banding and locally large hexagonal cooling columns. Characteristic feldspar phenocrysts, set in a very fine-grained groundmass, are common whereas quartz phenocrysts are rare. The member is thickest in the vicinity of the easterly-striking Tin Ha Shan Fault, towards the southern end of the Clear Water Bay Peninsula, and diminishes rapidly in thickness towards the north.

The fine ash tuff units of the Pan Long Wan Formation, which are weakly eutaxitic and contain small angular aphanitic clasts, decrease in thickness from south to north. Towards the north end of its outcrop, the formation mainly comprises dark bluish grey, very fine-grained lava with scattered small euhedral feldspar crystals. The lava is flow banded and locally flow folded in the north (Plate 5.30), whereas in the south it is mostly autobrecciated.

The formation has not yet been dated radiometrically, but is included as part of the Repulse Bay Volcanic Group on the basis of whole-rock geochemistry (Campbell & Sewell, 1998).

Details

Tai Miu Wan and Fat Tong Kok. Flow-banded trachydacite of the Tai Miu Wan Member of the formation forms vertical cliffs in the vicinity of Clear Water Bay Country Club and at Fat Tong Kok (848830 813600 Krp-1), where vertical fissure vents cut through the lavas. The trachydacite is overlain at Po Keng Teng (849180 814220 Krp-2) by fine ash vitric tuff. The topmost few metres of the trachydacite are purple-stained (848860 814600 Krp-3) and may represent exposure to subaerial weathering prior to deposition of the overlying tuffs. Within the lava flows, at Tai Miu Wan (847980 814500 Krp-4), a 50 m thick tuffite layer has been mapped. Flow-banding in the lavas is prominent, but is frequently highly contorted and is not a useful indicator of the attitude of the strata.

Clear Water Bay (Tsing Shui Wan) and Tai Au Mun. The Tai Miu Wan Member outcrops on the promontory immediately south of the main beach (847950 816280 Krp-5), where crude hexagonal columns 2 to 2.5 m in diameter indicate a gentle dip to the south. The trachydacite is greyish blue and is frequently iron-stained due to abundant pyrite crystals scattered throughout the rock, seen for example in the road cutting 250 m south of the bus terminus (847630 816190 Krp-6). On the hill summit, west of the main beach, the trachydacite outcrop is characterised by numerous rounded boulders on the surface, often pitted and iron-stained, and containing prominent feldspar laths up to 8 mm in length. The member produces a significant dip and scarp feature, resulting in crags along the slopes west of High Junk Peak (846950 817230 Krp-7).

Clear Water Bay Peninsula. Alternating layers of fine ash tuff and autobrecciated trachydacite to rhyolite lava form north striking dip and scarp features along the main High Junk Peak ridge. The lavas are more resistant to erosion and form isolated crags on the steep west facing slopes. A common feature of the lava is the highly contorted flow banding which is well seen on weathered surface blocks and particularly prominent in shoreline exposures as at Little Palm Beach (847440 820260 Krp-8) (Plate 5.A29). Autobrecciation becomes increasingly common southwards in the vicinity of Clear Water Bay Second Beach (847800 816700 Krp-9) and to the southwest of High Junk Peak (847300 817200 Krp-10). When fresh the rock is very fine grained and dark bluish grey, with faint flow banding and scattered small euhedral feldspar laths. The interlayered fine ash tuff contains small angular aphanitic clasts and in places displays a faint eutaxitic fabric. Within 50 m of the top of the formation, for example at Lung Ha Wan (848920 818970 Krp-11), the tuff and lava take on a mottled purple to pink coloration and are in places deeply weathered. It is possible this horizon represents the top of the Pan Long Wan Formation which was exposed to subaerial weathering prior to the deposition of the overlying Clear Water Bay Formation.

The summits of Miu Tsai Tun (847200 817800 Krp-12) and High Junk Peak (847500 817400 Krp-13) are capped by columnar jointed fine ash tuff which is very resistant to erosion. In this area the formation rests on soft, pink to purple fine ash tuff presumed to be the uppermost part of the Pan Long Wan Formation.

Southern Clear Water Bay Peninsula. Pale greenish grey tuff, tuffite and associated tuff-breccia is exposed on the coast around Shek Miu Wan (846900 816000 Krp-14). The formation is less than 100 m thick in this area. Similar, soft, weathered tuffite and tuff is found on the southern side of Clear Water Bay Second Beach (847700 816300 Krp-15) where the strata are intruded by quartzphyric rhyolite dykes and a small fine-grained granite body. The tuffite and tuff is overlain by a thick trachydacite lava flow.