The Bluff Head Formation consists of pale grey, fine- to coarse-grained quartzitic sandstone, with subordinate reddish brown and purple sericitic siltstone and greyish white, quartz - pebble conglomerate (Plate 3.1). The formation has a minimum thickness of 800 m (Lai et al., 1996) and its type section is at Bluff Head at the mouth of Tolo Channel (Figure 3.3).

North of Tolo Channel, the formation occurs as a fault-bounded block between strata of the Early Jurassic Tolo Channel Formation to the south and Mesozoic tuffs and sedimentary rocks to the north. Although the sequence dips steeply to the northwest, fossil evidence indicates that the beds nearest Tolo Channel are dominantly Middle Devonian in age, whereas those adjacent to Plover Cove are of Early Devonian age. It is probable that a fault separates the Early Devonian succession from the Middle Devonian succession. The southern faulted contact adjacent to Tolo Channel appears to be a high-angle reverse fault. The Tolo Channel Formation is present only as fault wedges between the Devonian and the main offshore expression of the Tolo Channel Fault. To the north, the Early Devonian succession is faulted against the Jurassic Tai Mo Shan Formation, but in the northeast, the Middle Devonian sedimentary rocks are in faulted contact with the Cretaceous Pat Sin Leng Formation.

At Bluff Head, the cliffs provide the most complete sequence through the formation. The strata are structurally complex, with the bedding oversteepened and locally overturned adjacent to the southern faulted margin (Plate 3.2). To the north, the strata are tightly folded in a series of northnortheast-plunging anticlines and synclines (Plate 3.3). In places the dip decreases to as little as 30^o, though a more normal angle is around 45^o to the northwest. This is the only comprehensive section through the sequence and it is possible that the rocks are folded in a similar fashion along the entire peninsula.

In the vicinity of Ma On Shan, Late Devonian sedimentary rocks dip 30^o to the southeast and form a succession at least 100 m thick. They were best exposed along a temporary road section excavated during site formation works for the Kwun Hang Water Treatment Works. Minor exposures of the Bluff Head Formation have also been reported from Wu Kwai Sha Tsui (Sewell, 1996). These sedimentary rocks have been correlated with the sequence at Harbour Island and are presumed to be Middle Devonian in age.

Details

Cheung Pai Tun. The formation here is best exposed on the northern and southern shores (845800 837250 Db-1, 845460 836010 Db-2) of Cheung Pai Tun. The stratigraphically lower strata are dominantly greyish white, thickly bedded, fine- to medium-grained sandstone with two beds of pebbly conglomerate. The conglomerates, which vary from 8 to 13 m thick, have erosive bases and well-developed trough and planar cross-bedding, and represent channel features. The conglomerate exposed on the southern sea shore is overturned, dipping at 60^o to the southeast. Higher in the succession, medium- to fine-grained sandstones are intercalated with siltstone and mudstone. The siltstone varies from reddish brown to reddish purple, greyish white and greyish green. The yellowish brown mudstone and fine-grained sandstone exposed along the northern shore contain bivalve, brachiopod and fish fragments and plant stems. The sedimentary rocks represent a deltaic facies association (Jones, 1995). The formation in this section is 690 m thick, but neither top nor base of the formation are seen. At (845150 835900 Db-3) the sandstone strata dip steeply, varying from fine- to coarse-grained, and show cross-stratification.

Wong Wan Tsai to Ngor Kai Teng. The formation is up to 900 m thick in this area. Stratigraphically lower strata are mainly greyish white, quartzitic sandstone with three beds of conglomerate: the lowermost bed, exposed near the base of the formation on the shore of Chek Mun Hoi Hap (Tolo Channel), is a massive, thickly bedded, quartz-pebble conglomerate, 12 m thick; the middle bed is also quartz pebble-bearing and is 3 m thick; and the uppermost bed, 0.5 m thick, is a pale reddish brown, fine-grained, polygenetic conglomerate with subrounded to subangular clasts including quartzite, metasiltstone, and less abundant vein quartz. The upper part of the sequence comprises dominantly medium- to fine-grained, quartzitic and clayey sandstone with intercalated sericitic siltstone, mudstone and two lensoid beds of conglomerate. There are around 40 individual siltstone beds, each being 0.5 to 10 m thick, and the proportion of siltstone in the sequence is nearly 10%. A thick pebbly conglomerate (848380 838650 Db-4) occurs 100 m north of Ngor Kai Teng. The pebbles are mainly of quartz, varying from 2 to 150 mm in diameter. A further 1 m thick, fine conglomerate occurs 5 m above this bed.

South of Pak Kok Shan. Between the seashore, 550 m south of Pak Kok Shan (850940 839450 Db-5) and 400 m northeast of Fung Wong Wat (850780 839200 Db-6), the formation is 670 m thick. The lower strata are overturned and comprise greyish white sandstone with a thin breccia and a thick conglomerate. These are succeeded by fine-grained quartzitic and clayey sandstones with subordinate sericitic siltstone and a lensoid conglomerate. These sedimentary rocks are considered by Jones (1995) to be dominantly sheetflood deposits. At Fung Wong Wat (849880 838400 Db-7), the lower part of the succession includes a 20 m thick unit of well-bedded white sandstone, interpreted by Jones (1995) as including aeolian facies.

Wong Chuk Kok Tsui (Bluff Head). The formation is well exposed (Plate 3.A1) along the shore of Wong Chuk Kok Tsui (852700 841100 Db-8), where it is 260 m thick. The lower strata comprise greyish white, quartzitic sandstones with two units of thickly-bedded conglomerate, up to 20 m thick. A quartzphyric rhyolite dyke is intruded into the sandstone. The central part of the formation comprises sandstone with siltstone and a lens of conglomerate, while a series of minor folds is developed in sandstone with siltstone in the upper part. The top of the formation is faulted against tuffs of the Upper Jurassic Tai Mo Shan Formation. These sedimentary rocks are dominantly a channel facies association (Jones, 1995), with several major channels and a number of single and multi-storey minor channels. They are interbedded with palaeosols, which are more common in this locality than elsewhere in the district.

Harbour Island and Ma Shi Chau. The formation consists of interbedded pale grey, medium - and coarse-grained, quartzitic sandstones (orthoquartzites) and slightly darker, fine-grained sandstones and siltstones (protoquartzites) with occasional beds and lenses of white, quartz-pebble conglomerate. About 800 m of strata are exposed on Harbour Island but account has not been taken of the possibility of these strata being repeated by strike-slip faulting or isoclinal folding. The base of this formation is not seen within the district.

The formation is best exposed on the southern shore of Harbour Island between (843480 834480 Db-9) and (844680 835120 Db-10). In this section the strata are dominantly fine- and coarse-grained quartz-rich sandstones. At (843480 834480 Db-9) strata show cross-stratification and this feature, along with graded bedding at (844400 834920 Db-11), indicate a northwestward younging direction. Farther north, at (844540 834960 Db-12), vertically disposed lenses of quartz-pebble conglomerate display further younging evidence (Plate 3.A2). At the southern tip of Harbour Island (843500 834520 Db-13) siltstones interbedded with quartzitic sandstones have yielded fossils identified as placoderm fish remains of probable late Middle Devonian age (Lee, 1982) (Plate 3.A3).

Sai O and Ma On Shan. A sequence of sedimentary rocks at least 300 m thick underlies the volcanic rocks which form the summits of Ma On Shan and The Hunch Backs. Plant fossils diagnostic of the Late Devonian have been recovered from this succession (Hu et al., 1997) and grouping of this succession with the Bluff Head Formation is favoured on palaeontological, lithological and structural grounds.

The rocks are not well exposed except at the following localities: on the western ridge of The Hunch Backs (842800 830400 Db-14); in a stream southwest of Sai O (843730 831500 Db-15); and in Ma On Shan Mine (843100 829380 Db-16), where they are much altered by thermal metamorphism and mineralisation. However, a water tunnel that was being driven southwards from a portal near Sai O (844100 832050 Db-17) in 1985 has provided excellent sections of fresh rock. The rocks of this outcrop are rather poorly bedded and comprise white quartzite, purple siliceous siltstone and quartz-pebble conglomerate (843900 831650 Db-18). Records from Ma On Shan Mine refer to limestones inferred by Davis (1964) to be of Permo-Carboniferous age. C. M. Lee (oral communication) considers that there are magnetite-bearing, metamorphosed sedimentary rocks in the mine that are Devonian in age.

On the western flanks of Ngau Ngak Shan (842950 830450 Db-19; Tiu Shau Ngau), the Bluff Head Formation is thermally metamorphosed and mineralized close to the contact with fine-grained granite. The intensity of metamorphism and mineralisation appears to increase abruptly on the southwestern side of a prominent NW-trending cross-fault (843280 830500 Db-20). The sandstone is typically hornfelsic and cut by numerous aplite dykes and quartz veins hosting iron mineralisation. In road cuttings at Sai Keng (845440 831390 Db-21), pale grey to white cross-bedded sandstones are seen.

At Wu Kwai Sha Tsui (843560 832840 Db-22) the Bluff Head Formation consists of white to cream, centimeter-bedded, medium- to coarse-grained quartz sandstone and pebble conglomerate (Plate 3.A4) dipping 26^o to the northeast. The sequence is estimated as 10 metres thick and the lithology is very similar to exposures of Bluff Head Formation on nearby Harbour Island (Addison, 1986).