The type locality of the Ap Lei Chau Formation (Table 5.6) lies along the southwest foreshore of Ap Lei Chau (Figure 5.16). The formation comprises mainly welded fine ash vitric tuffs with interlayered eutaxitic bands, typically defined by chloritized fiamme (Plate 5.27). It also includes tuff breccias, pyroclastic breccias, tuffites and coarse ash tuffs, thin lavas, and impersistent epiclastic layers. On the western side of Hong Kong Island, the formation is variably folded and is at least 1250 m thick, although the base has not been observed (Figure 5.20). A U–Pb zircon age of 142.7 ± 0.2 Ma indicates an Early Cretaceous age for the formation (Davis et al., 1997).
Previously, the Ap Lei Chau Formation was considered to outcrop over much of eastern Kowloon (GCO, 1986b). However, the rocks in this area have now been mainly reassigned to either the Mount Davis Formation or the new Che Kwu Shan Formation on the basis of petrographic interpretation and whole-rock geochemistry (Campbell & Sewell, 1998). Some of the outcrop in eastern Hong Kong Island has also been reassigned to the Mount Davis Formation. A pale buff vitric tuff with small fiamme on the southern flanks of Tai Mo Shan, formerly considered to be the thin northern equivalent of the Ap Lei Chau Formation (Addison, 1986), is now included within the substantially older Shing Mun Formation.
On the coast west of Yuk Kwai Shan (834000 810750 Kra-1) the eutaxite is well exposed in the cliffs, with the weathered surface of the rock showing the strong colour contrast between the fiamme and flattened discontinuous blocks and the vitric tuff matrix (Plate 5.A25). The pumice fragments have been flattened, stretched out and often contorted by the pyroclastic flow process. The darker areas are completely collapsed pumice fragments, but occasional unflattened non pumice lithic lapilli are present. Nearby (833930 810840 Kra-2), tuff breccia 50 to 70 m thick occupies the core of a syncline. All the blocks are of sub angular pumice averaging 0.5 m across, but with occasional blocks up to 2 m across set haphazardly in a coarse ash matrix.
Around much of southern Ap Lei Chau, Ap Lei Pai and the small islands of Fo Yeuk Chau (Magazine Island) (832540 811650 Kra-3) and Lung Shan Pai (832830 811340 Kra-4), eutaxite predominates. Along the eastern shoreline of Ap Lei Chau a fine grained, pink to brown sandstone, approximately 25 m in thickness underlies a thick eutaxite development (834600 811080 Kra-5), and is in turn underlain by about 10 m of uniform coarse ash tuff containing scattered dark aphanitic lithic lapilli (834620 811080 Kra-6). Discontinuous sandstone and mudstone layers have been exposed during the excavations for the new Housing Authority estate (834050 811380 Kra-7). In Ap Lei Chau Bridge Road (833960 811540 Kra-8) a fine grained sandstone grading upwards into mudstone forms a 5 m thick unit within fine ash tuff. In the fresh rock cuttings at the new housing estate (833890 811320 Kra-9), and in the road cuttings near the oil depot (833350 811390 Kra-10), the eutaxite is fresh and dark bluish black with little or no sign of the eutaxitic fabric seen so clearly on weathered surfaces.
Victoria Peak to Aberdeen, Mount Cameron and Mount Nicholson. The Ap Lei Chau Formation crops out over a wide area of western Hong Kong Island. The strata consists predominantly of crystal bearing fine ash vitric tuff with interlayered eutaxite bands, folded into a series of northwest trending anticlines and synclines. The eutaxite forms strong positive topographic features, for example High West and Mount Kellett.
In the vicinity of Victoria Peak the tuffs and minor sediments have been thermally metamorphosed by the Kowloon-Hong Kong granite pluton. Eutaxite is present as an impersistent layer on the summit of the Peak at the radio station (832990 815220 Kra-11). Nearby, fine grained sandstone forms a strong ridge feature striking NW-SE (833120 815120 Kra-12). Between the Peak and Pok Fu Lam the alternating eutaxite and fine ash tuff sequence is repeated at outcrop by the folding. Mount Davies Formation coarse ash tuffs occupy the synclinal core extending from the col between High West and Victoria Peak (832520 815000 Kra-13), southeastwards to Aberdeen (835450 812220 Kra-14). The transition to the dominantly coarse ash tuff of the Mount Davis Formation is obscured by superficial deposits in the valleys to the northeast of Pok Fu Lam Reservoir. The thickest eutaxite development extends from High West to Aberdeen, and crops out on both limbs of the anticline. On the eastern limb it is well exposed in the Mount Kellett area. Here the eutaxitic structure is displayed in the road cuttings near Matilda Hospital (833400 813450 Kra-15), where some fiamme are so stretched out that the texture is parataxitic. The rock is extremely hard and brittle when fresh. On the western limb of the anticline on the southern side of Pok Fu Lam Reservoir the eutaxite outcrop was mapped by Allen & Stephens (1971) as a rhyolite lava. Although spherulitic in part (HK 2435, 832350 814060 Kra-16), the eutaxitic fabric and lapilli fragments present suggest it is a pyroclastic flow deposit. No lava has been recognised in the present survey of the Ap Lei Chau Formation on Hong Kong Island. Coarse ash tuffs and thin lenticular mudstone layers (832600 812460 Kra-17, 832770 812280 Kra-18) were seen in cuttings along Shek Pai Wan Road, and a finely laminated volcaniclastic mudstone was noted along the catchwater north of Tin Wan Estate (833390 812740 Kra-19), where it appears to occupy a channel a few metres wide cut into fine ash tuff.
In the vicinity of Mount Gough, eutaxite layers become laterally impersistent, and homogeneous fine ash vitric tuff containing scattered dark aphanitic lapilli is dominant. Within the Aberdeen Country Park, to the south of Magazine Gap, a number of eutaxite bands form strong features, displaced by faulting. Such eutaxites are seen on Mount Cameron (836100 813710 Kra-20), with a prominent eutaxitic fabric dipping steeply to the west. A similar eutaxite band forms the sharp ridge feature striking southeastwards from near Wan Chai Gap (836000 814200 Kra-21) to Middle Gap. (836250 813760 Kra-22). This layer can be traced to the east of Middle Gap, striking eastwards across the summit of Mount Nicholson (837200 813400 Kra-23).
Cut slopes to the north of the Wong Chuk Hang factory area, around Tong Bin Lane (835340 812360 Kra-24), have exposed fresh, homogeneous fine ash vitric tuff which, in thin section (HK 2479, 835340 812360 Kra-24), is seen to have a welded fabric. The matrix is devitrified glass, and contains scattered angular to sub angular quartz crystals averaging 1 mm.
Deep Water Bay, Middle Island and Ocean Park. On Middle Island and the neighbouring coastline at the eastern end of Deep water Bay, excellent exposures of eutaxite are found in the cliffs. On the southern coastline of Middle Island (837140 810410 Kra-25), the eutaxite contains large, partially flattened vesicular pumiceous blocks that are less compressed than is usual in these deposits (Plate 5.A26). Fine ash vitric tuffs are interlayered with the eutaxite to the west of Deep Water Bay beach, and these are intruded by a small body of quartz monzonite near the Victoria Recreational Club (837170 811860 Kra-26).
Along the coast immediately to the southeast of the main Ocean Park development (836430 811670 Kra-27), fine ash tuff contains coarse ash lenses 0.2 to 0.3 m thick and 0.5 to 1.5 m long. The tuff is locally brecciated. Mudstone crops out nearby (836450 811680 Kra-28), and extends inland where old clay workings in this layer (836260 811840 Kra-29) are now part of the ornamental ponds of Ocean Park. This was the site of the Green Island Brick and Tile Works described by Davis (1952), who considered the clay deposit to be part of the Tolo Formation occurring as a large inclusion in the volcanics. It seems certain, however, that the mudstone is simply an epiclastic lens within the tuffs.
On Brick Hill and southwards to the peninsula at Sham Shui Kok (835800 810200 Kra-30), eutaxite predominates. Numerous exposures along Nam Long Shan Road and at the peninsula end of Ocean Park display excellent eutaxitic structures, and a series of roughly east west striking anticlines and synclines has been recognised. The road skirting the coast from Po Chung Wan to Tai Shue Wan provides good sections through the folded, predominantly eutaxite sequence. Intercalations of uniform, lapilli bearing coarse ash tuff (835050 811080 Kra-31) and a tuff breccia (835070 810910 Kra-32) are associated with a welded fine ash vitric tuff that lacks a eutaxitic fabric.
Tai Tam, Repulse Bay and Stanley. Apart from a capping of eutaxite on the summit of Violet Hill (838460 812280 Kra-33), much of the area of volcanic rocks cropping out in the Tai Tam Country Park consists of homogeneous fine ash tuff displaying little variation between exposures. A roughly horizontal fine grained sandstone band about 15 m thick skirts the hill 500 m southeast of Wong Nai Chung Gap (838330 812720 Kra-34). The thin section of this sandstone (HK 3531) shows that considerable thermal metamorphism has taken place, with abundant muscovite and recrystallised quartz present.
Between Repulse Bay and Tai Tam Tuk Reservoir the Ap Lei Chau Formation displays a monotonous lithology of crystal bearing fine ash vitric tuff with varying amounts of lithic lapilli scattered through the rock. In thin section most samples show a very fine vitric matrix with a faint welding fabric and containing a scatter of 0.5 to 1.5 mm crystals. These are usually subangular quartz grains, but smaller plagioclase and iron oxide grains may be present.
On Red Hill (841500 810750 Kra-35) the tuffs are welded, but lack the large scale eutaxitic fabric. Sample HK 3643 (841290 810900 Kra-36) is packed with welded glass shards and a good compaction fabric is seen under plain light. In polarised light the shardic fabric is less clear, but much of the glassy material is seen to be devitrified. Other nearby samples, for example HK 3644 (841160 810580 Kra-37), show a similar lithology and fabric. In hand specimen and outcrop, however, these rocks appear as fairly uniform, crystal bearing fine ash tuff without any visible evidence of welding.
A thick development of eutaxite crops out around Ma Hang Shan (840000 810000 Kra-38), dipping gently to the northeast. Lower down the hill, towards Stanley, another eutaxite layer some 40 m thick roughly follows the Stanley Gap Road (839220 809570 Kra-39), where there are good exposures in roadside cuttings. A gentle anticlinal fold crosses the Chung Hom Kok Service Reservoir hill, and the feature forming eutaxite is exposed in the nearby road cutting (839030 809500 Kra-40). This can be traced northwestwards to the coast between South Bay and Middle Bay (838380 810070 Kra-41). On Tau Chau, fine ash tuffs are interlayered with fine grained sandstone (837750 809540 Kra-42) and a breccia containing abundant chert fragments in a glassy matrix (837900 809530 Kra-43). From the tip of the peninsula opposite Tau Chau, as far east as South Bay beach, quartzphyric rhyolite has been noted. Near the point (838030 809500 Kra-44) flow banding is visible, but it has not been possible to determine whether the rhyolite is extrusive or intrusive. Allen & Stephens (1971) referred to this area as rhyolite lava in the text but it is not delineated as such on their map. This is the southernmost part of their Repulse Bay Formation type section, which extends northwards along the coast to Repulse Bay beach (838500 810620 Kra-45). North of South Bay beach (838310 809500 Kra-46), fine ash tuffs predominate, but several prominent eutaxite bands are present around Middle Bay. Although there are grain size variations in the tuffs at several localities, the entire section falls clearly into the Ap Lei Chau Formation.
Mount Parker to Shek O. Crystal bearing fine ash vitric tuff crops out on the hill 300 m southwest of the Mount Parker western radar station (840650 814000 Kra-47). The strata are fairly flat lying in this area. A series of eutaxite bands form strong features around the southerly part of the hill, with distinctive crags of eutaxite at 840820 813560 Kra-48. Several eutaxite bands contain pyroclastic breccia and tuff breccia horizons, similar to the pyroclastic breccias enclosed by eutaxite in the northeast of the district at Ho Chung. It is possible these blocks have been transported by the pyroclastic flow; Fisher & Schmincke (1984) cited several examples of this phenomenon. Between the strong eutaxite features, fine ash vitric tuff predominates, often with abundant aphanitic lithic lapilli. These lapilli are locally concentrated, and the rock appears almost a breccia, as for example along the catchwater, 550 m northwest of Boa Vista (840670 813180 Kra-49). Southwards to Mount Collinson, crystal bearing fine ash tuff is present over a wide area. A discontinuous eutaxite layer crops out at Mount Collinson (842290 812470 Kra-50). Road cuttings along the Tai Tam and Shek O roads contain excellent exposures of lapilli bearing fine ash vitric tuff. These are black when fresh but have a characteristic greenish grey colour where slightly weathered, and white where moderately to highly weathered. The lapilli content appears to increase towards the D'Aguilar Peninsula.
Along the coast south of Big Wave Bay, variably dipping eutaxite is interlayered with homogeneous fine ash tuff. Inland it has not been possible to trace the eutaxite bands beyond a north south fault (843390 811630 Kra-51). The contact with the granite is exposed in the road cutting east of Ngan Hang Village (842720 810120 Kra-52), where it dips 50o to the north.