The Long Harbour Formation (Table 5.6) crops out in two areas of the eastern New Territories: Long Harbour and Sai Kung (Figure 5.16). The formation is dominantly rhyodacite to rhyolite in composition and comprises mainly coarse ash crystal tuff with distinctive pink alkali feldspar crystals (Plate 5.24).

In the Long Harbour area, the rocks are either massive or crudely layered with some hexagonal cooling columns. The locally eutaxitic, coarse ash crystal tuff is commonly lapilli lithic-bearing (Figure 5.17). The lithics include angular sharp-edged black aphanitic fragments and pink crystalline lithics with diffuse edges in roughly equal proportions. Fine- and medium-grained granite lithics are also present. The diffuse-edged lithics are similar to those seen in the Yim Tin Tsai Formation. These suggest that there was disequilibrium, probably while the deposit was still hot (i.e at the time of their emplacement), between the lithics and their matrix. West of Shek Uk Shan, the formation rests unconformably on a distinctive rhyolite lava and an associated impersistent eutaxitic fine ash tuff at the top of the Lai Chi Chong Formation. The northern boundary of the formation is marked by the northeast-trending Tolo Channel Fault.

In the Sai Kung area, an eastnortheast-trending ellipsoidal outcrop of coarse ash crystal tuff, formerly mapped at 1:20 000 scale as part of the Tai Mo Shan Formation, has now been assigned to the Long Harbour Formation. This has been done on the basis of its whole-rock geochemistry, petrography and absolute age (Campbell & Sewell, 1998). In this area, the formation comprises a uniform, greenish grey, coarse ash crystal tuff, and is at least 600 m thick. Scattered black aphanitic angular clasts, with an average size of 20 mm, and black biotite crystals (some as small aggregates) are present throughout. Welding fabrics occur, especially towards the western end of the outcrop. Quartz crystals are abundant and are usually brown. The matrix is often characterised by a very finely spaced welding fabric. The base of the formation in the Sai Kung area appears to be faulted. The formation here is overlain with apparent unconformity, by the Silverstrand Member of the Che Kwu Shan Formation, and with markedly angular unconformity by the Mang Kung Uk Formation.

The formation has yielded U–Pb zircon ages of 142.8 ± 0.2 Ma and 142.7 ± 0.2 Ma from the Long Harbour and Sai Kung areas respectively (Davis et al., 1997).

Details

Ngau Kwo Lo and Tai Leng Tun. Mainly lithic-bearing lapilli-coarse ash crystal tuff with lithic tuff and lithic vitric tuff are exposed between Ngau Kwo Lo (851100 836200 Krl-1) and Tai Leng Tun (Goliath Hill) (853400 836200 Krl-2). Sample HK8401 (850190 836970 Krl-3), from Ngau Kwo Lo, contains mainly crystals (52 %), including plagioclase, alkali feldspar, quartz and biotite, and also rhyolite (3 %) and vitric (15 %) clasts. The cement (30 %) is volcanic dust, recrystallized to a microcrystalline aggregate of feldspar and quartz. Fine ash vitric tuff at Chuen Lo Kok Tsui, represented by sample HK8407 (852110 836830 Krl-4), contains 64.5 % vitric pyroclasts, 10.5 % crystal tuff and 25 % fine ash matrix. A quartzphyric rhyolite dyke intrudes the formation at Chuen Lo Kok Tsui.

Tap Mun Chau (Grass Island), Chek Chau (Port Island) and Kung Chau. The rocks mainly comprise lapilli-bearing coarse ash lithic crystal tuff with crystal lithic tuff and crystal vitric tuff. Sample HK7996 (854660 840030 Krl-5) from 100 m south of Tai Wan, is a vitric crystal tuff, containing 52 % crystal pyroclasts, including plagioclase, alkali feldspar, quartz and biotite, vitric clasts (24 %), rock fragments (4 %), and cement (20 %). Rock sample HK8000 (854870 839690 Krl-6), from 80 m north of Hung Ying Tsui, is a grey to pale grey, lapilli-bearing vitric lithic tuff containing almost equal proportions of rhyolite and tuff clasts, vitric clasts, crystals, and cement. Columnar joints are always evident. A reddish brown palaeo-weathering crust, up to 12 m thick, is developed in the tuffs at the top of the formation, which is overlain by red beds of the Upper Cretaceous Port Island Formation.

Lai Chi Chong to Cheung Sheung and Ko Tong. To the south of Lai Chi Chong (849450 834220 Krl-7), massive coarse ash tuff rests on weathered purple lava. This contact can be traced southwards to the western flanks of Shek Uk Shan where eutaxitic fine ash tuff lies between the lava and the coarse ash tuff. At Ngo Keng Tsui (849190 835920 Krl-8), at the extreme northern margin of the district, coarse ash tuff rests on easterly dipping tuffite and fine ash welded tuff. The coarse ash tuff extends eastwards to the shores of Long Harbour, and is fairly uniform with occasional angular clasts to 200 mm in diameter. These are polymictic, with dark grey or black aphanitic fragments being the most common. The matrix contains angular bipyramidal quartz grains up to 5 mm across, and abundant pink feldspar crystals, and is speckled throughout with small biotite flakes. A crude bedding and columnar jointing was observed on the tors of Mount Hallowes (850670 835910 Krl-9). Occasional diffuse-edged granitic clasts were noted at Nam Shan Tung (850090 834650 Krl-10). Examination of the High Island Water Scheme tunnels from Nam Shan Tung southwards to Ko Tong (851300 832240 Krl-11) revealed a remarkably uniform body of coarse ash tuff over the entire tunnel length. At Ko Tong the Long Harbour Formation ends abruptly against Clear Water Bay lavas, along the line of the Cheung Sheung - Chek Keng Fault.

Wong Shek Pier and Long Harbour. Excellent exposures of the Long Harbour Formation are seen around the coastline of Long Harbour. A clast-rich zone, in places almost a tuff-breccia, extends from Tai Tan (853100 833600 Krl-12) southwards to Wong Shek Pier (852700 832900 Krl-13) and eastwards to include the Tung Sam Kei Peninsula. Within this zone, 600 m north of along Shek Pier (852860 833420 Krl-14), 100 to 300 mm diameter clasts of equigranular medium-grained granite were noted, and nearby, fine-grained granite clasts are also present. Siltstone, sandstone, feldsparphyric rhyolite and welded tuff clasts were noted at Tung Sam Kei (853700 833300 Krl-15). 400 m northeast of Sz Tei (853200 834180 Krl-16), hexagonal columns, 1 to 2 m in diameter in clast rich coarse ash tuff, suggest the strata here dip eastwards at about 30^o. Good exposures of pinkish grey coarse ash tuff, with only occasional clasts, were seen along the coastline from Wong Mau Kok (854400 834400 Krl-17) to Kau Lo Wan (853300 835600 Krl-18). Quartzphyric rhyolite dykes cut the tuff, but are usually impersistent. Fiamme, averaging 20 mm in width and 250 mm in length, were seen on weathered rock surfaces 2 km south of Kau Lo Wan (855410 833810 Krl-19).

Sharp Peak. The Long Harbour Formation rests with angular unconformity on Mang Kung Uk Formation tuffite and Clear Water Bay Formation lava along the northern flanks of Sharp Peak. Massive, pinkish grey coarse ash tuff, with only rare clasts, outcrops along the coast from Nam She Wan eastwards to Mun Wan (858700 832900 Krl-20). The tuff appears to be banked up against the lava, and crude bedding suggests northerly dips of up to 40^o in the Long Harbour Formation. Similar crudely bedded tuff was seen on Wong Mau Chau (858650 834320 Krl-21).

Pak Kong and Ma On Shan Tsuen. As on Tai Mo Shan, the outcrop of the Pak Kong area is of grey or dark grey, lapilli ash crystal tuff rich in quartz, feldspar, biotite and flattened lithic fragments of volcanic glass. Horizons of dark grey tuff conspicuously rich in mafic minerals (biotite and hornblende) are moderately common in the lower part of the succession exposed on Fu Yung Pit hill (842600 825890 Krl-22) and on the ridge extending northwards from there to Ngong Ping (844000 827850 Krl-23). To the southeast the tuffs are paler grey and less rich in mafic minerals, although still containing conspicuous crystals of quartz and feldspar. Little variation was noted across this area eitherin field characters or thin section.

Pak Sha Wan, Sai Kung and Wong Chuk Yeung. To the east of Pak Sha Wan, coarse ash crystal tuff contains small angular aphanitic clasts of vitric tuff and pale grey chert fragments between 10 and 30 mm across. A sedimentary breccia, 400 mm thick, marks the base of the overlying Che Kwu Shan Formation on the east side of the peninsula (846150 825040 Krl-24). Exposures are poor away from coastal sections and inland. For example at Tsui Hang, 1 km south of Sai Kung town (845910 825970 Krl-25), where the coarse ash tuff is deeply weathered with rounded corestones protruding from the reddish brown saprolite. In a former borrow area at this locality the weathered material has been excavated to a depth of 10 m. Similar deep weathering with associated reddish brown soil and corestones of coarse ash tuff is found to the southeast of Wong Chuk Yeung (845950 828960 Krl-26). Large blocks of the tuff form a debris veneer to the hill slopes at Shui Long Wo, particularly well seen at the Star Lookout near the Country Park Management Centre (846500 829500 Krl-27).

Coarse ash crystal tuff is exposed on the shoreline between Sai Kung and Tso Wo Hang (847800 828000 Krl-28) and on numerous small islands within Sai Kung Hoi. On Yeung Chau (846890 826790 Krl-29), a north-south striking quartzphyric rhyolite dyke intrudes the tuff. Near Tai Wan Village (846970 828160 Krl-30), abundant lapilli of angular aphanite are set in a crudely banded coarse ash tuff, with faint fiamme. Inland, at Lung Mei (845690 828480 Krl-31), the tuff is finer grained but still possesses the same characteristics of the typical coarse ash tuff. Small felty biotite patches were noted and occasional pink feldspar crystals were scattered through the rock.

Tai Mong Tsai to Kai Kung Shan. Between Tso Wo Hang and Tai Mong Tsai, road cuttings have exposed fresh tuff. 200 m southeast of the Marine Police Base (847950 827990 Krl-32), a thick layer of fine to coarse ash crystal tuff, with a purplish grey colour, is exposed. Angular quartz crystals up to 3 mm across are set in a fine mafic rich matrix. Further southeast, large fiamme are visible on the weathered rock surface and indicate an easterly dip of 20^o (848100 827800 Krl-33). Fiamme are also visible in coastal outcrops near the Outward Bound School (848700 827670 Krl-34), where the coarse ash tuff is overlain by the Mang Kung Uk Formation. The boundary of the two formations can be traced northwards along the eastern side of the Tai Mong Tsai Valley. Fine ash to coarse ash crystal tuff is seen in stream exposures at Shek Hang (848510 829740 Krl-35), where crude columnar jointing was noted. Uniform coarse ash crystal tuff was seen throughout the Water Supplies tunnel extending from Sheet 7 eastwards to Ping Tun (849050 829770 Krl-36).