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The Geology of Hong Kong (Interactive On-line)
Tai O Formation - Jo

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 (40–60o) 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.