The Lantau Volcanic Group (undifferentiated) (Table 5.5) is the name given for all rocks previously assigned to the Lantau Formation (Langford et al., 1995) on Lantau Island, with the exceptions of the Cheung Shan and Sunset Peak members.
On Lantau Island (Figure 5.13), the Lantau Volcanic Group (undifferentiated) comprises mainly porphyritic rhyolite lavas and welded and non-welded rhyolitic tuffs, with common tuffaceous sedimentary rocks. In the west, southwest and south of Lantau Island, the group rests unconformably, but without marked angular discordance, on the Yim Tin Tsai and Shing Mun formations. Elsewhere on Lantau Island, the contact of the group is faulted, occasionally against older volcanic formations, but mainly with Late Jurassic rhyolitic dykes and Middle Jurassic granitoids. At its type locality on Nei Lak Shan, the group is estimated to be more than 1700 m thick (Figure 5.14). High precision U–Pb zircon ages of 147.5 ± 0.2 Ma and 146.6 ± 0.2 Ma have been obtained from rhyolite on the Tung Chung Road (GEO, unpublished data) and near Ngong Ping (Davis et al., 1997). These ages are similar to U–Pb zircon ages (146.3 ± 0.3 Ma and 146.5 ± 0.2 Ma) obtained by Davis et al. (op. cit.) from the eastnortheast- to east-striking feldsparphyric and quartzphyric rhyolite dyke suite that extensively intrudes the granite terrain in northeast Lantau Island.
Sedimentary intercalations within the Lantau Volcanic Group (undifferentiated) vary in dip and dip direction. However, the sequence is generally inclined to the north or northwest at 20 to 30o.
Quartz monzonite has been intruded along part of the faulted northern contact west of Tung Chung, but the formation is thought to have been originally faulted against granite. East of Tung Chung, the eastnortheast-striking boundary of the Lantau Volcanic Group (undifferentiated) is interpreted as a younger, sinistral strike-slip fault. Farther east, the eastern contact of the formation with rhyolite dykes and granite may be a normal fault with a substantial downthrow of the volcanic sequence to the west against its subvolcanic and substantially contemporaneous equivalents.
The porphyritic lavas of the Lantau Volcanic Group (undifferentiated) contain phenocrysts of quartz and feldspar. Some of these crystals have been mechanically broken, either as a result of fragmentation during eruption or at their site of deposition (Plate 5.20). It is also possible that some of the lavas may have initially been erupted more explosively as pyroclastic flows but later became reconstituted as lava flows. Other volcanic deposits include banded rhyolite lava (Plate 5.21), rhyolitic tuff, and tuffaceous sedimentary rock, including fossiliferous mudstone. The well-banded lavas are characterised by large white euhedral feldspar crystals and do not contain lithic fragments, whereas the tuffs frequently contain lithic lapilli. Thin beds of tuffaceous mudstone and siltstone, some containing rare fossils, are commonly interbedded with the lava flows. These sedimentary interbeds are extensive in the west, but are restricted in the north.
The Pak Kok Member (Langford et al., 1995) crops out in the southwest of Lantau Island. At its type locality at Pak Kok, the member comprises bedded tuffaceous and sedimentary rocks and is at least 300 m thick. Poor sorting of the pyroclasts and lithic fragments may indicate local reworking of older volcaniclastic deposits. The limited lateral extent and thickness of the member indicates deposition in a restricted basin. This may have been close to an active eruptive centre, as indicated by local hydrothermal alteration of the sedimentary rocks. The Pak Kok Member passes westwards into siltstones of the Lantau Volcanic Group (undifferentiated), whereas towards the east it passes laterally into a sequence of tuffite, tuff, rhyolite and siltstone. These relationships imply an active volcanic source in the east, although the volcanism at this time was probably sporadic and on a comparatively small scale.
Within the Lantau Volcanic Group (undifferentiated) there appears to be a discrepancy between the absolute U–Pb zircon ages and the age based on palaeontological determination. The U–Pb ages vary between 147 and 146 Ma. The Jurassic–Cretaceous boundary has been constrained elsewhere in the world using U–Pb chronology to be approximately 144 Ma (Gradstein et al., 1995). Therefore, the ages obtained for the group so far indicate the Late Jurassic. This accords with some earlier interpretations of the age of the volcanic rocks on Lantau Island (in Langford et al., 1995) based on identifications of macroflora and miospores that indicated mainly Jurassic and possibly Late Jurassic ages. However, the most recent assessment of the age of these rocks based on miospores (Lee et al., 1998) has concluded that the sediments are Lower Cretaceous. This apparent inconsistency has yet to be resolved.
Details
Banded lava, or uniform grey porphyritic rhyolite lava, crops out over a wide area around Nei Lak Shan, and is occasionally interbedded with thin tuffite and siltstone layers. Typical lava occurs on the hills south of Ngong Ping, northwest and northeast of Nei Lak Shan. Overall, banding is subhorizontal, albeit with local variations. For example, on the summit of Nei Lak Shan (808880 813840 Jlu-1) and 500m southsoutheast, the lava displays roughly horizontal banding. Further south (809110 813070 Jlu-2), it may dip up to 30o to the northwest, and between 400 and 500 m southwest of the summit, it dips 18o to the southwest.
West of Nei Lak Shan, the succession is pervasively altered and metamorphosed, but primary banding in the lavas is often preserved. Contorted, or folded banding is well displayed near the crematorium (Plate 5.A14, 807810 813240 Jlu-3) east of Ngong Ping. There, across a distance of 50 m, the dip of the banding varies from 67o to the south, to 61o to the north. This folding is interpreted as primary, resulting from flow irregularities during emplacement of the lava, and not due to post-consolidation tectonic processes.
About 1 km north of Nei Lak Shan, there is a zone of autobrecciation in the rhyolite lava which lies stratigraphically below an impersistent pyroclastic breccia (Plate 5.A15). The autobrecciated lava (808980 814740 Jlu-4) has a pitted weathering surface on corestones, and occurs with evenly-banded lava and finely-banded lava. The banded lava (809250 814940 Jlu-5) crops out below the autobrecciated zone, and has feldspar crystals up to 8 mm in length, and bands less than 10 mm wide.
Tuffaceous sandstone and tuffite form an erosion-resistant feature on the northern slopes of Nei Lak Shan. The member also includes porphyritic rhyolite lava, with well-developed banding, and fine ash crystal tuff, and lies above a thin pyroclastic breccia. The tuffaceous member is less than 70 m thick, and appears to be laterally impersistent. The pyroclastic breccia may be as little as 10 m thick, but has not been traced laterally.
Sham Shek Tsuen. Exposures of banded lava occur in a stream (807180 815180 Jlu-6) about 80 m from the faulted contact with Jurassic Tai O Formation. The banding dips 50-60o to the southeast, close to the subvertical reverse fault. Adjacent to a narrow tuffaceous bed, there are exposures of a contorted and autobrecciated, finely-laminated lava, containing blocks up to 0.6 m across. Planar and folded flow-laminae in the rhyolite lava are less than 1 mm thick, and fissures perpendicular to this flow-banding pervade the rock. Overall, these features indicate a much more viscous lava than the coarsely porphyritic, planar-banded lavas at the type area on Nei Lak Shan.
Southwest Lantau Island. South of the summit (803350 809000 Jlu-7) of Tai Hom Shan, small exposures of finely banded, locally autobrecciated lava lie stratigraphically above a tuffite and tuffaceous siltstone member. Brecciated rhyolite lava, also appears overlie a fossiliferous mudstone bed on the ridge (802730 808160 Jlu-8) southwest of Sham Hang Lek, although the contact is not exposed. The breccia is interpreted as autobreccia formed at the base of a rhyolite lava flow, which is uniform and coarsely porphyritic elsewhere in the area. North of Sham Hang Lek (803130 808900 Jlu-9), fine banding in lava dips southwest at 18o.
Cheung Shan. At Cheung Shan (805300 813450 Jlu-10), fine banding, 10-50 mm wide, is well developed in lava. The banding undulates gently, but generally dips 20 to 50o to the south. About 300 m to the east, on the summit of Cheung Shan, relict banding dips steeply to the southeast, and on Fan Shui Au, banded rhyolite lava dips consistently to the south at 50o to 80o. Along the coast north of Pak Kok, banding in lavas is again well developed. It strikes roughly eastnortheast and dips variably 18 to 82o, but folds and angular unconformities also occur locally (805860 808860 Jlu-11).
Lantau Peak (Fung Wong Shan). Well-banded lavas dip 73 to 78o south on a knoll (809470 811500 Jlu-12) about 850 m south of Lantau Peak . They also crop out on the ridge (812540 811480 Jlu-13) extending to the west, with steep dips to the north and south. Corestones on a ridge north of Tong Fuk display well-developed planar banding in porphyritic rhyolite lava. The bands vary from 1 to 5 cm in thickness, and form thin, upstanding ribs on weathered surfaces. At this locality the banding dips 35o north, whereas to the north and south along the ridge-line, the dip can be as steep as 74o. Banded lavas are exposed in cut slopes along the road leading to Ma Po Ping Prison. At one locality (810460 810160 Jlu-14) the banding dips 43o southeast in highly weathered porphyritic lava.
Sunset Peak (Tai Tung Shan). Banding in corestones of lava on the lower southeastern flanks of Sunset Peak dips 57o to the southeast. The prominent eastnortheast-trending features traversing the hillside probably reflect bedding in the underlying sequence of banded lava, lava and sedimentary rocks. To the southeast of Lin Fa Shan (815800 813600 Jlu-15), steeply dipping banding in corestones and boulders strikes east. 2 km along the ridge (811900 814600 Jlu-16) which trends northwest from Sunset Peak, the banding dips steeply to the eastnortheast. On a small hill top (814870 815080 Jlu-17) about 900 m north of Lin Fa Shan, weathered surfaces of lava boulders display banding up to 0.1 m thick. Roadside exposures (811610 813860 Jlu-18) of weathered lava on the Tung Chung Road, near the Country Park Management Centre, display well-developed planar banding which dips 37o east. Banded lava, dipping 40 to 50o northeast, is also exposed between 100 and 400 m west of the Tung Chung Road at Pak Kung Au.
The outcrop consists of brownish-grey, fossiliferous siltstone interbedded with dark grey mudstone, and tuffaceous sandstone. The siltstone, exposed near the swimming pool (813940 813100 Jlu-19), contains abundant plant remains, most of which are reed-like stems. Dip variation of 38o north to 74o southeast is related to an eastnortheast-trending fault with a southerly downthrow, exposed at the west end of the pool.
A black to grey tuffaceous mudstone member is exposed on a footpath (813680 813550 Jlu-20) on the north side of Sunset Peak. The rocks are well laminated, with dark grey mudstone and lighter tuffitic laminae between 10 and 20 mm thick. Load structures in tuffite overlying the mudstone indicate that the strata are not inverted. Both the mudstone and tuffite contain subangular to subrounded, pebble-like clasts between 2 and 15 mm across. The member dips northwest at 65o, and is thin and impersistent.
Several thin siltstone and tuffite members, some of which persist laterally for more than 2 km, are exposed along the southern flanks of Sunset Peak. The best exposed occurs along and adjacent to the footpath that traverses the hillside at about 350 mPD. Lava is in contact with underlying black cherty mudstone at one locality (813820 812370 Jlu-21), and 500 m to the east (814340 812460 Jlu-22) a well-laminated, grey to dark grey mudstone lies close to light green tuffaceous sandstone. On a small-scale, emplacement of the tuffite disturbed the underlying mudstone. Hence, the strata, which typically dip 55o to the north, are the right way up.
East of Sunset Peak, there are exposures (814460 813890 Jlu-23) of lithic lapilli-bearing crystal tuff or tuff-breccia., similar to those on both Sunset Peak and Lin Fa Shan. The pock-marked surface of the light, brownish-grey boulders indicates the presence of abundant, rounded lithic clasts.
Lin Fa Shan. On the summit (815020 815140 Jlu-24), light grey, lapilli-ash crystal tuff with a fine ash matrix includes lithic lapilli which weather preferentially, leaving elongate hollows (Plate 5.A16) up to 50 mm long. Fabric in the corestones appears to dip to the east at 40o. Elsewhere on the summit, fabric is poorly developed, but the rock contains angular tuff blocks, up to 300 mm across, which weather preferentially. The slopes to the south are dominated by displaced boulders of elongate lapilli- and block-bearing tuff. The member is underlain by tuffaceous sandstone. Although the contact is not exposed, it may be locally unconformable. The succession is preserved in a synclinal core, whose northerly-trending axial plane passes through the summit.
Por Kai Shan. Banded lava and lava are exposed on the summit of Por Kai Shan, and on the ridges to the south and west. On the south side of the hill (813940 816260 Jlu-25), the light grey lava is autobrecciated and grades into a tuffaceous breccia. The relationship between the banded and brecciated lavas is uncertain because of the steep, but variable dip of the strata and the rarity of exposed contacts. However, the breccias are probably autobreccias, formed at the tops, bases and fronts of successive, banded lava flows. On Por Kai Shan (813980 816330 Jlu-26), the weathered porphyritic lava has pronounced, steeply inclined banding which dips at 60 to 90o, and generally strikes northwest. Banding, which dips approximately 50 to 60o to the north, is also well developed on a ridge (814700 816300 Jlu-27) 700 m east of Por Kai Shan. About 1 km south (814000 815400 Jlu-28) of Por Kai Shan, lava displays fine banding, about 5 mm thick, which dips about 60o south, and banding is also well developed at several localities between 400 and 800 m east of Pok To Yan, where it and varies from roughly horizontal, to a dip of 53o to the north.
The rocks forming Por Kai Shan are generally metamorphosed, with original textures masked or obscured. However, on the northwest flanks of the hill (813600 816500 Jlu-29), there are exposures of tuffaceous conglomerate, sandstone and mudstone, together with banded lava. This sequence is about 150 m thick, and dips 35o to the east and southeast. It forms part of a prominent spur as it is more erosion-resistant than the adjacent lavas. Uniform banded lavas directly underlie this tuffaceous unit, and it is overlain by what is probably the autobrecciated base of a lava.
Ngong Ping. The prominent ridges west of Ngong Ping are primarily composed of tuffite, but are locally interbedded with fossiliferous mudstone. Tuffite, tuffaceous siltstone, sandstone and rare mudstones were temporarily exposed along the Ngong Ping Road. Close to the Tai O Road (806980 811940 Jlu-30), two 0.1 to 0.5 m thick, subhorizontal ash bands can be seen in a cut slope, separated by 5 m of tuffite.
The ridge, about 400 m southwest of the statue of the Buddha, is composed of light grey siltstone and tuffaceous siltstone. Bedding is only locally displayed. The member dips southwest at 11 to 40o, and is probably less than 50 m thick. It thins rapidly to both north and southeast. At least 20 m of sandstone, siltstone and mudstone, in part tuffaceous, dipping east to southeast at 17 to 23o (808020 812540 Jlu-31), and containing some well-preserved plant fossils, have been recorded.
Sham Wat. Close to the weir (806550 813270 Jlu-32) at the start of the catchwater, a sequence, dipping 10o south, comprises grey, finely laminated, cherty tuffaceous siltstone and mudstone, and contains fining-upward, graded units, 1 to 4 mm thick. About 300 m to the northwest (806540 813490 Jlu-33), close to the Sham Wat Road, finely laminated mudstone comprising graded units, 0.5 to 3 mm thick (HK 9216), is overlain by a massive, light grey, cherty tuffaceous siltstone. Bedding dips 24o to the south, varying nearby to 28o to the north. Some of the rocks are brecciated (806290 813890 Jlu-34), but sedimentary laminae, dipping at up to 44o, can still be discerned.
On the hillside east of Sham Wat, a tuffite unit, less than 50 m thick, lies below autobrecciated rhyolite lava. It typically comprises grey, porcellanous, tuffaceous siltstone, but varies from tuffaceous sandstone to vitric tuff.
Man Cheung Po. Fossiliferous siltstone has been excavated in pits and trenches at the campsite (804280 810180 Jlu-35). The rock is typically weathered to a brownish-white colour, and contains small fern-like leaf impressions. Nearby exposures in the stream bed consist of tuffaceous siltstone and mudstone. Bedding is sometimes well-developed. Dips range from 8o northeast to 33o southeast.
Subhorizontal bedding of the sediments is well displayed about 1 km northwest of Man Cheung Po monastery. Laminated cherty siltstone, with laminae ranging from 2 to 20 mm in thickness, dips 16o to the east, and lies below adjacent exposures of lava (803900 810840 Jlu-36). A small outlier of lava (803900 810700 Jlu-37) crops out to the south of these siltstones, and the contact between the two forms a clear feature on the hills to the east, where finely laminated siltstones dip variously from 15o west to 12o east.
Sham Hang Lek. The sedimentary member north of Sham Hang Lek is interpreted as the southwesterly extension of outcrops seen around Man Cheung Po. However, the member appears to thin rapidly further southwest, and on the northern ridge of Sham Hang Lek (802500 808700 Jlu-38), it is probably only 20 m thick. Southwest of Sham Hang Lek, there are exposures of siltstone, mudstone and tuffaceous sandstone. In one locality (802730 808180 Jlu-39), there are the preserved remains of mostly reed-like plant stems, up to 100 mm long. The strata are well bedded and dip to the southeast. They overlie a porphyritic rhyolite lava. Bedding planes, dipping at about 35o, are exposed on the hillside to the south.
Towards Fan Lau Teng, the sedimentary member has an extensive outcrop, being up to 500 m wide and over 2 km long. The commonest rock type is a grey, finely laminated cherty siltstone, but there are also exposures of poorly bedded siltstone and tuffaceous sandstone. The strata dip generally southeast to eastsoutheast, roughly parallel to the hillside. Dips range from 10 to 60o, but average about 25o, giving a thickness for the member of about 100 m.
Pak Kok. On the east side of the headland (805600 808200 Jlu-40), finely bedded tuffite and tuffaceous siltstone are dominant. They weather to a light greenish-grey and can contain abundant quartz crystals up to 1 mm across. Bedding, which is defined by abrupt variations in the abundance of quartz grains, dips 57o to the southeast. Further south, the tuffite is stratigraphically overlain by quartz crystal, ash-bearing tuffs, and underlain to the north, by tuffites, sandstones and siltstones. On the coast west of Pak Kok, the member comprises grey to dark grey, laminated mudstones and siltstones. It is underlain by banded porphyritic lavas, but the overlying sequence has not been preserved.
Kau Ling Chung. Poorly bedded, pebbly tuffaceous sandstone, which is exposed along the footpath (803580 807500 Jlu-41) to Fan Lau, dips gently to the southeast. Overlying strata include lithic lapilli-bearing tuff and porphyritic lava, but the upper contact of the member can not be distinguished.
At Kau Ling Chung (804100 807100 Jlu-42), laminated siltstone and tuffaceous sandstone are overlain by rhyolite lava. The sequence is well bedded, dipping 21o to the southeast, and the common laminae are less than 1 mm thick. The siltstone displays small-scale sedimentary structures, including syn-depositional microfaults, flattened sand balls and graded beds. A narrow (<100 mm wide) cross-cutting, sandy tuffaceous body, is interpreted as a tuffisite dyke, intruded before compaction of the sediments. It probably fed a thin ash unit exposed at the base of conformably overlying lava.
The sequence is faulted along a northnorthwest-trending contact with lava to both southwest and northeast. A series of small faults, which downthrow to the southwest, suggest that these exposures may be a faulted continuation of both the sequences to the northeast, and hillside exposures to the northwest.