The Mang Kung Uk Formation (Table 5.6) crops out mainly in the vicinity of the Clear Water Bay Peninsula (Figure 5.16) where it conformably overlies the Silverstrand Member of the Che Kwu Shan Formation. Further north, in the Sai Kung and Tai Long Wan areas, the formation has been mapped as conformably overlying the Long Harbour Formation.

The Mang Kung Uk Formation, which is up to 300 m thick, is dominated by alternations of well-bedded tuffite, epiclastic breccia, conglomerate, mudstone, siltstone and sandstone (Plate 5.29). Graded bedding is a feature of some sandstone units. Thick fine ash and coarse ash tuff units, with occasional intercalated rhyolite lavas, are also present (Figure 5.22). The type locality is at Mang Kung Uk Village, towards the northern end of the Clear Water Bay Peninsula.

A high precision U–Pb zircon age of 142.9 ± 0.2 Ma has recently been obtained from near the base of the formation at Bayside Beach (Pik Sha Wan) (GEO, unpublished data).

Details

Mang Kung Uk to Junk Bay. The lower part of the formation is well exposed along the coastline (prior to reclamation) in the vicinity of Tai Chik Sha Wan. On one location (846390 817150 Krm-1), interbedded sandstone, tuffaceous siltstone, volcanogenic conglomerate and bedded tuff dip east between 18 and 25^o. Bedding is well developed, with the sandstone also exhibiting cross bedding structures and distinctive graded bedding. Individual graded units vary in thickness from 300 mm to 1.0 m and can be traced laterally for between 20 and 40 m along the shoreline. The conglomerates contain clasts up to 200 mm, the majority being fine ash tuffs, but some siltstone, quartzite and chert fragments were noted. The well stratified units are overlain by a rhyolite lava (846500 817200 Krm-2) about 25 to 30 m in thickness. This lava is very fine-grained but contains prominent quartz and feldspar phenocrysts, and forms a feature which can be traced over a distance of 1.5 km. A heterolithic tuffite sequence, possible as much as 120 m thick, overlying the lava, has been proved in a number of boreholes at Tai Chik Sha (847000 817000 Krm-3). This pale grey to pale green rock contains abundant lithic fragments of mudstone, siltstone and welded tuff, averaging 10 mm in diameter. Occasional thin bands of accretionary lapilli have also been noted.

Numerous small exposures of tuffaceous siltstone, tuffaceous sandstone and crystal bearing fine ash tuff are found in the stream bed east of Siu Chik Sha (846700 817600 Krm-4). Northwards, similar material occupies the wide topographic hollow between Siu Chik Sha and Mang Kung Uk. Soft, grey, flaggy siltstone and sandstone, the basal beds of the formation, sit without apparent unconformity on eutaxite of the Che Kwu Shan Formation, 400 m south of Wo Tong Kong (846400 818980 Krm-5). A pale yellow sandstone band about 25 m in thickness forms a strong north south striking feature 250 m east of Wo Tong Kong (846500 819300 Krm-6).

Isolated exposures of the Mang Kung Uk Formation are seen along the coastline between Pik Sha Wan (846800 820100 Krm-7) and Little Palm Beach (847200 820300 Krm-8). The lower part of the formation contains breccia and a chaotic assemblage of clasts of all sizes in a tuffaceous siltstone matrix (846790 820100 Krm-9). This melange may possibly represent a lahar flow deposit. Bedded tuffite, exhibiting good graded bedding structures, outcrops further east (846940 820160 Krm-10) and contains thin impersistent epiclastic breccia horizons. Sandstone and mudstone bands with interlayered clast-bearing tuffite dominate the upper part of the formation (847070 820220 Krm-11) and are overlain by extremely contorted flow-banded lava of the Pan Long Wan Formation.

Isolated outcrops of Mang Kung Uk Formation occur on the western side of Sharp Island (Kiu Tsui Chau). On Kiu Tau (847500 824900 Krm-12) a faint eutaxitic fabric is seen, together with abundant small angular aphanitic clasts. A 30 m thick clast rich horizon has been mapped as tuff-breccia (847630 824870 Krm-13), with polymictic clasts including some sandstone and mudstone fragments. 500 m southeast of Kiu Tsui (848200 824700 Krm-14) clast-rich fine ash tuffs are overlain by alternating tuffaceous sandstone and siltstone of the Mang Kung Uk Formation dipping eastwards at about 45^o.

Sai Kung Hoi and Tai Mong Tsai. The Mang Kung Uk Formation outcrops on a number of small islands in Sai Kung Hoi (Inner Port Shelter). Alternating layers of siltstone and clast rich tuffite are exposed on the southwest side of Sharp Island (848200 824600 Krm-15). The strata dip eastwards at 45^o and are about 40 m thick between underlying fine ash welded tuff and overlying flow-banded lava. In the northern part of Sharp Island, thickly bedded pale greenish grey tuffaceous sandstone contains scattered green mudstone clasts (848320 826270 Krm-16). Thin epiclastic breccia horizons and dark grey mudstone bands are also present (848190 826160 Krm-17). The formation occupies the islands of Pak Sha Chau, Cham Tau Chau, and the smaller islands of Siu Tsan Chau and Tai Tsan Chau. On the latter (847870 826330 Krm-18), laminated siltstone is interbedded with mudstone and sandstone (Plate 5.A28), and dips 20^o towards the southeast. Cham Tau Chau (847800 826600 Krm-19) and Pak Sha Chau (847500 826200 Krm-20) are composed predominantly of thick coarse tuffite units with occasional epiclastic breccias containing subangular clasts of tuff and siltstone to 200 mm in diameter. Tuff-breccia, similar to the chaotic melange rocks north of Mang Kung Uk, is found on the east side of Pak Sha Chau (847550 826150 Krm-21), and on Cham Tau Chau (847690 826700 Krm-22), where large blocks of black mudstone and pale grey cherty siltstone are found.

Eastward dipping, well bedded pale grey tuffite is found along the shoreline south of the Outward Bound Training School at Tai Mong Tsai (848800 827200 Krm-23). Here, a series of minor intraformational unconformities between the tuffite units are exposed between high and low water marks. Black mudstone clasts are present in pale grey siltstone, which is finely laminated in part.

Nearby, penecontemporaneous slumping and small scale faulting is well seen in laminated beds (848950 827300 Krm-24). A black, fine ash welded tuff (HK6005, 849320 827510 Krm-25) overlies the tuffite sequence. This tuff is in turn overlain by autobrecciated rhyolitic lava, which is exposed on the shoreline 1.2 km southsoutheast of Tai Mong Tsai Village (849400 827200 Krm-26), where angular to subangular flow-banded lava blocks comprise the autobreccia. The feature-forming lava can be traced northwards as the surface is strewn with flow banded lava fragments and distinctive blocks of spherulitic rhyolite (849400 827600 Krm-27). Eastwards to the shores of Tsam Chuk Wan, tuff and tuffite predominate. The soft, white weathering tuffite is frequently crudely layered, and cherty lenses have been noted (849950 827480 Krm-28). Around Tsam Chuk Wan Village and on the small island of Muk Yue Tau (850100 827800 Krm-29), fine ash to coarse ash tuff occurs, with occasional small fiamme. The nearby islands of Ching Chau (850300 827800 Krm-30) and Wong Nai Chau Tsai (850400 827200 Krm-31) consist of well-bedded tuffaceous siltstone and finely laminated mudstone, which on the southwest of Wong Nai Chau Tsai (850420 827200 Krm-32) is seen overlying coarse ash tuff.

The Mang Kung Uk Formation extends northwards from Tai Mong Tsai, but thins rapidly north of Ping Tun, and has not been recognized in the vicinity of Kai Kung Shan (848200 830500 Krm-33), where both lava of the Clear Water Bay Formation and tuff of the Long Harbour Formation appear to overstep directly on to the coarse ash tuff of the Mang Kung Uk Formation.

At Ping Tun (848970 829820 Krm-34), bedded tuffaceous siltstone and sandstone lie unconformably on an irregular surface of the Mang Kung Uk Formation. Crystal-bearing fine ash to coarse ash tuff, at least 100 m in thickness, overlies the well-bedded tuffaceous sedimentary rocks, and is well seen in the stream course east of Tit Kam Hang Village (849300 829500 Krm-35). At She Tau (849250 828860 Krm-36), finely laminated, colour-banded siltstone and mudstone are exposed in the footpath leading to Tai Tun.

Finely laminated and banded, white to pale grey tuffaceous mudstone and siltstone rest unconformably on Long Harbour Formation coarse ash tuff to the west and south of Wong Mo Ying. Here, the Mang Kung Uk Formation is about 70 m thick. South of Wong Mo Ying (848300 828900 Krm-37) the formation is dominantly mudstone and siltstone, and is overlain by a distinctive pyroclastic flow deposit.

Chek Keng to Tai Long Wan. Small isolated outcrops of bedded tuffite are found underlying the Clear Water Bay lava in the Chek Keng area. These uppermost beds of the Mang Kung Uk Formation can be seen on the shoreline immediately northeast of Chek Keng Village (854200 831500 Krm-38). Eastwards to Nam She Au (856200 831900 Krm-39), a thick sequence of variable tuff, tuffite and occasional mudstone and siltstone layers underlie a hilltop capping of Clear Water Bay lava. The Mang Kung Uk Formation forms subdued topography on the east side of the Tai Long Valley (856700 831000 Krm-40). Pale greenish grey, fine ash to coarse ash tuff and tuffite is exposed in the footpath from Tai Wan to Tung Wan (857500 831100 Krm-41).