The type locality of the Mount Davis Formation (Langford et al., 1995) is on Mount Davis, in the northwest of Hong Kong Island (Figure 5.16). The Mount Davis Formation (Table 5.6) includes rocks that were previously mapped as the Tai Mo Shan and Shing Mun formations on Hong Kong Island, and as the Ap Lei Chau and Shing Mun formations in eastern Kowloon (GCO, 1986b). The revised stratigraphy has been based on general rock relationships, detailed whole-rock geochemistry and age-dating.
At the type locality, the formation is at least 500 m thick and comprises variably lapilli-bearing coarse ash crystal tuff (Plate 5.25), with some eutaxite and sandstone beds (Figure 5.18). The base of the formation is marked by the first appearance of a massive coarse ash crystal tuff layer above the mainly fine ash vitric tuff of the Ap Lei Chau Formation. However, the Mount Davis Formation also interdigitates with eutaxitic vitric tuffs of the Ap Lei Chau Formation.
In eastern Kowloon, the formation is approximately 950 m thick and includes several thick eutaxite units, which are more typical of the Ap Lei Chau Formation. Together with the evidence from western Hong Kong Island, there is a strong possibility that the source of the Ap Lei Chau Formation continued to erupt periodically during the accumulation of the Mount Davis Formation.
Recently, Campbell and Sewell (1998) have suggested that the outcrop of the Mount Davis Formation extends farther west to include similar rocks on Lantau Island. This correlation is supported by a U–Pb zircon age of 142.8 ± 0.2 Ma obtained from a coarse ash crystal tuff unit on Lin Fa Shan. On the basis also of geochemical comparison and lithological similarity, outcrops previously mapped as the Sunset Peak Member by Langford et al. (1995) capping the peaks of Lin Fa Shan and Sunset Peak in the centre of Lantau Island, and as Lantau Formation on Lantau Peak, are all best correlated with the Mount Davis Formation. Here, the Mount Davis Formation is between 50 and 100 m thick and includes lapilli lithic-bearing crystal tuffs, tuff breccias and eutaxitic tuff.
Details
Near one cottage (814320 813350 Krd-4) east of the summit, tuffaceous, laminated siltstone dips southwards at 20o. A small reverse fault has thrust the adjacent lavas to the east over the siltstone. In tuffs (814320 813320 Krd-5) to the west, rounded lapilli of vitric tuff, or lava are up to 30 mm across. The matrix is comprised mainly of coarse ash crystals, and there is an indeterminate layering defined by clast orientation.
Lin Fa Shan. The steep eastern flanks of Lin Fa Shan are composed of alternating rhyolite lava flows and beds of tuffaceous sandstone, or siltstone. The sedimentary rocks form cliffs, most of which are inaccessible. However, on the west side of the hill, extensive exposures of the highest beds include massive, or poorly bedded, tuffaceous sandstone and scarce, finely laminated mudstone. Sandstone and siltstone beds, 200 m west of the summit (814820 814200 Krd-6), dip to the north at 34o, and graded units indicate they are the right way up. West of the summit, the beds generally dip steeply to the east or northeast, and may form part of a syncline whose axial plane trace trends northwards along the ridge. The ridge is capped in places by lapilli-ash tuff of the Mount Davis Formation, but their stratigraphic relationship to underlying strata is uncertain.
Kau Yi Chau. Lapilli-ash crystal tuff, with elongate, dark, lithic lapilli, is exposed along the east coast of the island. The lapilli and mafic crystals define a prominent lineation. There may also be small, sharp-edged porphyritic lapilli. Blocks of eutaxite on the beach are probably derived from a higher, but unexposed stratigraphic level within the formation. The eutaxite is black, with abundant crystals of quartz and feldspar up to 5 mm. The fiamme are up to 300 mm by 50 mm, and the rock resembles eutaxite partings in the Mount Davis Formation. The tuffs are cut by quartzphyric and feldsparphyric rhyolite dykes. At the northern end of the beach (826200 816240 Krd-7), there is an easterly-trending contact between lapilli-ash crystal tuff to the south, and a chilled, largely aphanitic dyke to the north.
Western Hong Kong Island. The Mount Davis Formation occurs as an isolated faulted block of coarse ash crystal tuff immediately to the east of the Wong Chuk Hang Housing Estate (835500 812050 Krd-8). At the western end of the Wong Chuk Hang Industrial Estate (834540 812220 Krd-9) the formation is exposed in roadside cuttings and can be traced northwestwards through the Shek Pai Wan Estate, occupying the core of the north west striking syncline. Here (HK 2472, 834350 812500 Krd-10, Plate 5.A20), the rock is dark grey, massive coarse ash tuff, containing occasional dark streaks of flattened pumice, and scattered black, subangular, aphanitic lapilli. Nearby (834270 812150 Krd-11), the coarse ash tuff passes abruptly downwards into eutaxitic structured rocks of the Ap Lei Chau Formation.
The syncline can be traced northwestwards through the Aberdeen Country Park, and coarse ash tuff again crops out between Mount Kellett and Peak Road (833600 814000 Krd-12), with very fresh uniform, dark bluish grey coarse ash tuff exposed in small cuttings along Mount Kellett Road (833630 814040 Krd-13). In the headwaters of the Pok Fu Lam Reservoir valley, isolated exposures of coarse ash tuff are highly weathered. In the col between High West and Victoria Peak (832370 814930 Krd-14) similar uniform coarse ash tuff prevails. Coarse ash tuff is exposed along the coastline between Telegraph Bay and Sandy Bay, dipping towards the west (831000 813950 Krd-15). In this area an arbitrary boundary with the Ap Lei Chau Formation has been drawn due to lack of exposure.
An extensive outcrop of Mount Davis Formation occurs to the north of a major fault that extends east-northeastwards from Sandy Bay to the Mid levels. The formation also occurs on nearby Green Island (Figure 5.A5). The rocks are predominantly coarse ash crystal tuff, as exposed in the extensive cut slopes behind the Mount Davis High Level Service Reservoir (830760 815060 Krd-16). They are dark grey when fresh, with biotite flakes visible in the matrix. Quartz and feldspar crystals up to 3 mm are common, but average around 1 to 1.5 mm. Lithic lapilli fragments and small blocks are occasionally present. In thin section (HK 2856, 830980 815020 Krd-17) the crystals are subangular to sub rounded, and set in a very flne but subordinate matrix. Nearby, exposures in Mount Davis Road (830810 814900 Krd-18) and Victoria Road (830870 814850 Krd-19) have revealed an almost vertical, white quartzitic sandstone layer which, in thin section (HK 2227, 830880 814860 Krd-20), is seen to be completely recrystallised. Near the summit of Mount Davis, four impersistent light yellowish brown micaceous sandstone bands have been mapped. Similar sandstone, thermally metamorphosed near the granite contact, is well exposed on the northern side of Green Island (829630 816360 Krd-21). Here the rock shows distinctive banding produced by preferential quartz recrystallisation along certain sedimentary layers.
A thick eutaxite layer, dipping westwards at 40 to 65o, appears to overlie the coarse ash tuff on the western flanks of Mount Davis. It is exposed in roadside cuttings at the junction of Mount Davis Road and Victoria Road (830170 815140 Krd-22), where a prominent parataxitic fabric is developed. The eutaxite is seen along the coast north of Sandy Bay, where it is thermally metamorphosed in the proximity of the granite. The small irregular granite intrusions exposed at Sandy Bay (830320 814810 Krd-23) may represent the roof of a much larger granite pluton.
Uniform, massive coarse ash tuff predominates in the Kennedy Town area, but, in Belcher's Street (831670 816270 Krd-24), sandstone is developed as a thick lens, up to 50 m in thickness and extending southwards over a distance of 400 m. Sandstone, dipping eastwards at 75o and part of the same horizon, is exposed in roadside cuttings along Pok Fu Lam Road (831740 816070 Krd-25). These light yellowish brown sandstone lenses have been thermally metamorphosed by the Kowloon Hong Kong granite pluton, and contain abundant muscovite mica.
Between Pok Fu Lam Road (831550 815580 Krd-26) and the Hill Above Belcher's (831950 815650 Krd-27), weathered fine ash tuff alternates with coarse ash tuff, and on the summit (831960 815650 Krd-28) the tuff has been intruded by a 30 to 50 m wide feldsparphyric rhyolite dyke.
Ma On Shan Tsuen. Northwest of Ngong Ping, east and southeast of Ma On Shan Tsuen (843750 829120 Krd-29 and 843200 828550 Krd-30), are two hillsides which differ from surrounding terrain in that they are capped by tors and boulder fields. The lithology of the rocks in both cases is lapilli ash crystal tuff, rich in conspicuous plagioclase and in places containing lithic clasts of pale grey sandstone and dark grey mudstone. Occasional lithic clasts of porphyritic volcanic rock up to 150 mm occur and some of these are welded. The top of these lapilli ash tuffs does not crop out in these areas, and the basal contact of the main outcrop is faulted along most of its length.
The western contact of the tuffs cropping out east of Ma On Shan Mine (843630 829180 Krd-31) is taken to be faulted, since thin sections of the tuffs there show no sign of any biotite regrowth, normally the first indication of metamorphism of such tuffs. The sedimentary rocks in the mine immediately west of the fault, on the other hand, are quite strongly baked by the granite. The contact at the southern end of this crop is intruded by the porphyritic rhyolite (842850 828160 Krd-32) or faulted against pale grey, welded fine ash tuffs of the Clear Water Bay Formation (843350 828070 Krd-33).
The Mount Davis Formation also crops out on dissected ridges above Mau Ping Lo Uk (843200 827860 Krd-34). where it is intruded by aphanitic rhyolites, and to the south, where it is faulted against tuffs of the Clear Water Bay Formation lying to the northwest.
Near Mau Ping Lo Uk (843000 827500 Krd-35) flow banded rhyolite intrudes tuffs of the Mount Davis Formation, and, as on Shek Nga Shan, quartz monzonites are associated with the contact zones. Northeast of Mau Ping Lo Uk and on Ngong Ping Shan the rhyolite intrudes the Mount Davis Formation. The southeastern flanks of the flow banded rhyolite outcrops are defined by the Ngong Ping Shan Fault, which also forms the locus for intrusion of quartz monzonite.
Southeast of the Sha Tin Valley. The Mount Davis Formation crops out on the ridge of Tate's Cairn (840200 824080 Krd-36) and on the ridges north of Wong Ngau Shan and Shui Ngau Shan where outcrops are complexly faulted and intruded. The lithologies represented are generally dark, bluish grey, ash crystal tuffs with common thin (up to c.5 m) units of laminated, dark grey mudstones and crystal-bearing mudstones. These units of sedimentary rock are discontinuous and disturbed, probably syndepositionally, as can be seen at Tate's Pass (840950 824000 Krd-37).
East Kowloon, Ho Chung and Junk Bay. To the east of the Pak Kung Au Ho Chung valley the strata dip fairly consistently to the east, and impersistent, feature forming eutaxite layers produce north south striking ridges, well seen on Tsim Fung Shan. There (842680 822390 Krd-38) the eutaxite is a dark greyish blue fine ash vitric tuff containing abundant flattened pumice lapilli, and fiamme are commonly visible in hand specimen. The thin section (HK 1215, Plate 5.A21) shows the fiamme clearly and also the characteristic, slender, wispy glass shards. The matrix is extremely fine and is most probably devitrified glass debris. Angular to sub rounded quartz crystals, 0.5 to 1 mm across, are common, and there are occasional altered microperthite and plagioclase crystals. Wavy flow patterns in the matrix around the larger crystals show stronger compaction near the crystal edges. HK 1216 (842870 822860 Krd-39) in thin section shows less prominent flow structures but a welding compaction fabric is faintly visible. More abundant crystals of quartz and microperthite, averaging 0.4 to 0.6 mm, are present in this sample. Nearby (842970 822920 Krd-40), a 15 m thick monomictic pyroclastic breccia crops out and forms a discontinuous feature extending for about 200 m diagonally across the ridge. The breccia consists of angular of sub rounded lava blocks 0.2 to 1.5 m across.
In the cuttings along Clear Water Bay Road, 400 m northeast of Pik Uk Prison (843780 822660 Krd-41), the rock is very hard, brittle and black in colour. No eutaxitic fabric was discernible in fresh outcrop but the thin section (HK 1199, 843780 822660 Krd-41) clearly possesses a welded fabric in the very fine vitric groundmass. Larger quartz crystals, 0.5 to 1 mm across, are angular to sub rounded and unaltered.
The tuffs are well exposed in the upper levels of the Anderson Road Quarries near Tai Sheung Tok (842730 820360 Krd-42), where they have been intensely baked close to the irregular granite contact. Small scattered lapilli are visible but the matrix has been completely recrystallized within 30 m of the contact. A prominent eutaxite band crosses Tai Sheung Tok (842510 820690 Krd-43) and, in thin section (HK 1211, 842630 820560 Krd-44), its eutaxitic fabric is very clear. Quartz and alkali feldspar are present as larger crystals, 0.4 to 0.7 mm across, and smaller plagioclase grains are scattered throughout. The collapse of porous pumice has formed dense, glassy fiamme. Minute quartz veinlets cut the rock and probably originate from the nearby granite, but no thermal metamorphism is apparent.
East of Anderson Road and Ma Yau Tong (843220 820170 Krd-45), fine ash vitric tuff with scattered crystals and lapilli predominates, with discontinuous north south striking eutaxite bands forming characteristic ridge features. At Ma Wu Tsai the eutaxite has a prominent flow fabric dipping variably to the east. In thin section (HK 1473, 844000 819510 Krd-46), black glassy fiamme are present and roughly parallel to a faint flow fabric that waves around the randomly scattered, 0.75 to 1.25 mm quartz crystals. 500 m to the east (844470 819280 Krd-47) a roughly horizontal polymictic, matrix supported breccia is exposed. Nearby (844540 819420 Krd-48), chert lenses several metres in length occur within crystal bearing fine ash tuff, which contains scattered rounded bombs with tails up to 0.45 m in length. Near the foundry (844450 819910 Krd-49) an exposure of breccia with clasts 20 mm to 0.2 m across is interlayered with fine ash tuff.
Tate's Cairn to Fei Ngo Shan. Lapilli bearing fine ash lithic and crystal tuffs are found to the east of Grasscutter's Pass (840400 824000 Krd-50), but these have been altered in the vicinity of the granite intrusions. Eastwards to Tate's Cairn the tuffs are variable from fine ash to coarse ash tuffs, and contain scattered lithic blocks. Near Tate's Pass (840660 823830 Krd-51), mudstone bands have been mapped. The black mudstones are finely laminated and have been thermally metamorphosed; they are impersistent and dip steeply towards the southeast. From Middle Hill (841000 823170 Krd-52) to Fei Ngo Shan (841020 822200 Krd-53) the strata strike approximately north south, and fine ash to coarse ash tuffs at Middle Hill display a crude bedding dipping towards the east at about 30o. In thin section (HK572, 840930 823290 Krd-54) the rock has a very fine vitric matrix enclosing large broken quartz crystals up to 2.5 mm. There are a few wispy shards and concentrations of small equigranular quartz grains along preferred planes indicating that the tuff is welded.
On the southeastern slopes of Fei Ngo Shan a tuff breccia and pyroclastic breccia layer has been exposed in excavations for a new service reservoir (841530 822220 Krd-55). This polymictic rock contains angular to sub rounded lithic blocks, mainly dark greenish grey aphanitic lava, averaging 0.35 m but with some up to 3 m across. Some of the smaller blocks are almond shaped with a tail, and resemble volcanic bombs. The matrix is a coarse ash crystal tuff. Overlying the tuff breccia is a thick lapilli bearing fine ash tuff layer. At Fei Ngo Shan summit (841100 822300 Krd-56) a prominent, vertical, N-S striking rhyolite is present. This rock displays distinctive highly contorted flow banding in exposures 50 m east of the transmitting tower (841100 822410 Krd-57). The rhyolite closely resembles the quartzphyric dyke varieties, which are intrusive. Although this rock unit parallels the strike of the volcanic strata, it is assumed that it is a dyke rather than an extrusive body. Feldsparphyric rhyolite dykes, striking NW-SE, intrude the tuffs to the northwest of Fei Ngo Shan (840540 823340 Krd-58 and 840550 822860 Krd-59). Above Jat's Incline (840340 822970 Krd-60) a light pink grey, fine grained, banded sandstone, approximately 15 m in thickness, has been thermally metamorphosed by the nearby granite.
Southeastern Kowloon - Devil's Peak. Two isolated outcrops of Mount Davis Formation, surrounded by granite, have been mapped between Yau Tong and Junk Bay. Much of the rock has been thermally metamorphosed by the granite. There is also hydrothermal alteration (greisenisation) of the granite that, to a lesser extent, has affected the volcanics lying within 20 m (thickness) of the easterly dipping contact.
In thin section (HK677, 843110 817130 Krd-61) the matrix of equigranular quartz grains is entirely recrystallised and encloses corroded quartz and microperthite crystals, which average 1 mm. Some muscovite is present and small secondary biotite crystals are scattered throughout the matrix. Along the coastline immediately northeast of Lei Yue Mun Point (843140 816360 Krd-62), subangular lithic blocks up 0.6 m across are present in the fine to coarse ash tuffs.
Hong Kong Island - Shau Kei Wan to Chai Wan. There are excellent exposures of the Mount Davis Formation along the western side of Lei Yue Mun Bay (now reclaimed) and southwards to Chai Wan. Near Pak Sha Wan at the railway tunnel portals (842590 815580 Krd-63), an irregular contact with the granite is well exposed, with little visible alteration of the tuffs. The tuffs are fairly uniform fine ash to coarse ash lapilli and block-bearing crystal tuffs with no sign of stratification. Nearby, the tuffs have been intruded by quartz syenite and in turn by the main granite pluton.
On the hill summit at Lei Yue Mun Fort (842250 814880 Krd-64), pyroclastic breccia is present, interlayered with fine to coarse ash tuff bands apparently dipping eastward at about 10o. Angular to sub rounded lithic blocks up to 1.5 m across are common and lie on an irregular load-casted tuff surface. A similar breccia crops out on the hill top 400 m to the west (841800 814880 Krd-65), above Aldrich Village. Here, the breccia layer is roughly horizontal and forms a flat capping to the hill. Block bearing tuff is present in cuttings (842520 815280 Krd-66) east of the old fort, and this may be equivalent to the breccia on the hill summit. Southwards, along this cut slope, the tuff becomes coarser until a considerable thickness of coarse ash tuff is exposed (842670 814910 Krd-67). In thin section (HK 2949) the crystal matrix is fine but abundant plagioclase and quartz crystals averaging 1 mm are present, with some of the plagioclase showing distinct zoning. Chlorite and iron oxide patches are common in the matrix. Similar dark purplish grey fine ash to coarse ash tuff has been exposed in excavations for the new Eastern Hospital (e.g. 842400 814530 Krd-68), where scattered black aphanitic lapilli are also present (Plate 5.A22). On this site (842460 814620 Krd-69) volcaniclastic rocks, consisting of laminated, brown and grey, mudstone and tuffite have been noted in temporary exposures (Plate 5.A23). These display graded bedding and appear to dip northeast at 35o. Some 50 m to the north, the volcanics have been cut by a 50 to 100 m wide quartz monzonite dyke, and within 3 m of the contact the tuffs have been bleached light grey and are closely jointed. On the northern slopes of Mount Parker (840860 814510 Krd-70) a layer of light brown, tuffaceous sandstone and siltstone crops out but is laterally impersistent. The surrounding rocks are fine ash tuffs weathered to light grey. Dark grey coarse ash tuffs form the hill summit at the western radar station (840850 814320 Krd-71).
