The Tai O Formation consists chiefly of an interbedded succession of grey fine-grained sandstone alternating with siltstone and sandy siltstone (Plate 4.3). The type section of the formation is along the coast northeast of Tai O (Figure 4.3) where the dip is moderate (40–60^o) to the south. The strata are well bedded and generally display a variety of sedimentary structures. Sandstone is the dominant lithology. Units vary from 4 to 12 m thick and are frequently cross bedded. The finer lithologies are thinner and occur in successions up to 3 m thick. They have been extensively modified due to bioturbation and shrinkage cracks, to the extent that most lamination formed during deposition would probably have been destroyed.

Jones (1996) identified five depositional facies in the sequence along the coast northeast of Tai O. These are channel sandstone, floodplain sandy siltstone, sheetflood sandstone, minor delta sandstone, and polygenetic palaeosol.

Channel sandstones form about 50% of the succession, have sharp or erosive bases, and are 4 to 7 m thick, but are rarely up to 12 m. These grey to red sandbodies, which are yellowish brown when weathered, are of consistent thickness across exposures. Individual sandstones are mostly medium- to coarse-grained with the top few centimetres fining upward to siltstone. The basal few centimetres are also slightly coarser grained and may contain mudstone pebbles. Trough cross bedding is common and shows consistent palaeocurrent directions towards the south-southeast. The upper part of each sandbody is thinner bedded and contains smaller sets of cross bedding. Bioturbation is also common. Several sandbodies contain pyrite crystals and these are mainly concentrated at the base and tops of certain sandbodies.

The floodplain sandy siltstone facies comprises approximately 45% of the exposed rock succession and consists of grey to red siltstone and sandy siltstone in beds up to 3 m thick. There are rare cross-laminated lenses and streaks of sandstone but fine bedding structure is generally absent. Bioturbation is extensive and burrows are simple vertical or subhorizontal tubes. Polygonal shrinkage cracks are pervasive, are either sandstone- or mudstone-filled and always occur in a coarse-grained siltstone or fine-grained sandstone. Plant debris is present but uncommon. Locally, the floodplain facies passes up into a 3 m thick minor delta facies, with an upward-coarsening silty sandstone and medium-grained sandstone succession, with current ripple cross lamination (flow directions to the southwest) and common bioturbation.

The sheetflood sandstone facies forms a comparatively minor part of the succession, interbedded with the floodplain facies. It comprises fine- to medium-grained sandstone, typically in sheet-like beds from 0.1 to 0.4 m thick. The beds have sharp bases, commonly fine upwards and are cross laminated.

The polygenetic palaeosol comprises pinkish red coarse-grained siltstone that is massive (or destratified), apart form occasional thin, impersistent lamination.

The heavy mineral assemblages of sedimentary rocks exposed at the Chinese University, Three Fathoms Cove and Tai O are characterized by a high proportion of rounded and subhedral purple zircon, low tourmaline, and low rutile contents. The similarity in heavy mineral contents from these locations is compelling evidence of relationship.

Details

Tai O. The Tai O Formation on the western end of Tai O Island strikes mainly eastnortheast. Steep dips to both north and south indicate that tight, to isoclinal folding may exist (e.g. at the northeastern tip of Tai O, near the sewage treatment works). Younging directions, determined from upwardly-fining units, consistently indicate that the strata are not inverted. Along the northern coast of Tai O the formation comprises finely laminated siltstones, and fine-grained sandstones with abundant sedimentary structures. Lenticular bedding occurs in interbedded red to grey siltstone and fine sandstone near the western tip of the island. Adjacent exposures show finely bedded sandstone and siltstone in upward-fining, graded units. These units dip and young to the southeast, whereas to the west, similar units of light grey to light red, fine-grained sandstone and siltstone, about 20 mm thick, young and dip to the northwest.

The eastnortheast-trending ridge through the summit at Fu Shan (803180 813310 Jo-1) is dominantly composed of massive, fine-grained quartzitic sandstone and siltstone. Similar rocks, together with finely laminated sandstone, are found to the east, on the island of Po Chue Tam (803900 813400 Jo-2).

Near the Sewage Treatment Works (803480 813610 Jo-3), siltstone and fine-grained sandstone dominate the sequence, and include some pebbly horizons. Rarely, the pebbles are up to 50 mm across, but more typically they range from 1 to 4 mm, and are angular to sub-rounded.

Tai O Cemetery. On the hillside above the cemetery, quartzitic sandstone and siltstone crop out close to eutaxite and tuff of the Cheung Shan Member of the Shing Mun Formation. A stream (804400 813650 Jo-4) to the north exposes reddish-white, fine-grained quartzitic sandstone, reddish siltstone and mudstone. However, the hillside is dominated by slope debris comprising eutaxite and tuff boulders from the hillside to the east.

San Chau. The footpath running east-west along the hillside at San Chau is underlain by reddish-white, fine-grained sandstone and siltstone, some of which has wavy, or lenticular bedding. Near the faulted contact between the Carboniferous and Jurassic, there are exposures of finely foliated, cherty, pale grey sandstone and siltstone (805030 814340 Jo-5). The foliation is parallel to the fault and dips northnorthwest at 60^o. Dark grey graphitic siltstone and grey, fine-grained sandstone and siltstone are exposed on the headland west of San Chau. The graphitic siltstone is up to 2 m thick, and probably occurs as several beds in the succession, rather than as a single bed repeated by tight folding. On the west side of the headland (804590 814200 Jo-6), porphyritic rhyolite lava directly overlies sandstone (Plate 4.A3), which in turn overlies a sequence including well-bedded, variably massive to well-laminated sandstones and grey to dark grey and carbonaceous siltstones (Plate 4.A4) (804750 814440 Jo-7). The lava appears to be intercalated with the underlying sandstone, and suggests that the sandstone is Jurassic in age.

Sham Wat Wan. There are two main groups of rocks in the Sham Wat area. The older, forming the low ridge of hills along the coast, comprises sedimentary rocks of the Tai O Formation. High hills to the south are composed of largely metamorphosed (altered) Jurassic volcanic rocks. The contact, exposed on the west side of the bay (Plate 4.A5) (806260 814540 Jo-8) between easterly-dipping siltstones of the Tai O Formation and Jurassic eutaxite, is conformable.

The headland at Hung Fan Shek is composed of fine-grained sandstone and silty sandstone, with occasional beds of dark grey graphitic siltstone. One of these beds (805910 814850 Jo-9) is tightly folded into a northerly-plunging syncline. A tight fold in a 3 m-thick graphitic bed also occurs north of Sham Wat Wan (806260 815260 Jo-10). The outcrop is dominated by grey siltstone and sandy siltstone, and structural features are only evident where scarce beds of dark grey graphitic siltstone are present.

San Shek Wan. Coastal exposures (Plate 4.A6) (806780 815700 Jo-11) west of San Shek Wan comprise upward-fining, cross-bedded, fine-grained, quartzitic sandstone. The sandstone youngs and dips to the east, although elsewhere in the area both the younging- and dip-directions vary markedly. The sequence is dominantly composed of interbedded sandstone and siltstone, with some coarser gritty beds.

The contact between foliated sandstone and megacrystic, fine-grained granite is exposed at the northeastern end of the bay (807140 816120 Jo-12). The sandstone is also cut by a thin, eastnortheast-trending quartz monzonite dyke. A skarn deposit has been recorded at this locality. It comprises a zone, about 5 m wide, which contains garnet, vesuvianite, diopside and epidote, with scattered magnetite.

Chinese University. The lithologies are dominantly grey to dark grey, fine-to medium-grained sandstones and laminated sandy siltstones. No fossils have been found but the laminated siltstones commonly show signs of intense bioturbation (838760 831580 Jo-13). Results of examinations for pollen and spores were negative.

In the stream section above Tai Po Mei (838140 831360 Jo-14) easterly dipping, faintly laminated sandstones up to 2 m thick occur, while upstream, close to the intrusions which separate the sedimentary and the volcanic rocks, westward dipping, well-laminated sandstones show ripple marks and flaser-bedding. These features indicate a younging direction to the west. Elsewhere in the area, dips are generally around 70-80^o to the E or S, and sedimentary features (worm burrows and flaser-bedding) indicate a younging in the direction of the dip, that is, away from the contact with the gently NW dipping Jurassic volcanic rocks lying to the west. From the general dip and the width of the outcrop, a thickness of up to 1 000 m of strata might be inferred, but the possibility of structural repetition of the sequence should be taken into account when estimating the overall thickness.

Three Fathoms Cove. Near the pier on the shore opposite Sam Pui Chau (845830 832430 Jo-15), and on the headland northeast of Che Ha (845650 832780 Jo-16), the strata are cut by faults that exhibit marked shear/breccia zones, some 700 mm in width. On Sam Pui Chau, the strata are tightly folded and exhibit various sedimentary features such as graded sandstone beds with cross-laminated silty tops that imply a westward younging direction on an eastern limb of a syncline (846130 832630 Jo-17).

Sham Chung Kok. At Sham Chung Kok (846600 833800 Jo-18), pink weathered phyllitic siltstones and sandstones are tightly folded. A thin section of sandstone (HK5769) (846660 833760 Jo-19) possessed a marked metamorphic fabric of sericitic micas. A sample of sandstone collected nearby (HK3681) (846600 833850 Jo-20), analysed for heavy minerals, contained assemblages of zircons similar to those identified from the Tai O Formation in Tai O, the Chinese University and Three Fathoms Cove (GEO Report GR 3/2005).