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The Geology of Hong Kong (Interactive On-line)
Ping Chau Formation - Ep


The Ping Chau Formation comprises a gently (10–20º) northeastward-dipping succession of thinly-bedded dolomitic and calcareous siltstones with rare chert interbeds (Plate 8.6). The upper and lower contacts are not exposed and the formation has a minimum thickness of 450 m. Lai et al. (1996) have divided the Ping Chau Formation into three members. The lower and upper members are composed predominantly of thinly-bedded siltstone and dolomitic siltstone and mudstone. The middle member consists chiefly of dark grey, thinly- bedded aegirine- and zeolite-bearing siltstone with interbedded dolomitic siltstone and mudstone. The middle member has been further subdivided into three units, the lower unit comprising thinly-bedded aegirine-bearing siltstone with dolomitic siltstone and a layer of chert, the middle unit consisting of thinly-bedded siltstone with mudstone, and the upper unit consisting of thinly-bedded zeolite-bearing siltstone with aegirine-bearing siltstone. The designated type section is between Lung Lun Tsui and Wong Ye Kok (Figure 8.5).


The Ping Chau Formation is typically characterised by thinly-bedded strata. Individual beds are only 20 to 60 mm thick and can be traced laterally for up to 100 m. The beds are mostly composed of thin, alternating planar laminae formed from detrital or chemical sedimentary rocks. Aegirine, calcite and zeolite crystals occur locally on some horizons and are possible pseudomorphs of gypsum. Carbonaceous fragments, which are abundant on many bedding planes, represent bitumunized plant fragments. Sedimentary structures include low-angle cross lamination, ripple marks, raindrop impressions, and load structures. Multiple thin, recumbent, slump folds and pyrite and limonite concretions are also common throughout the succession. Zeolite, in the form of natrolite and analcime, is found abundantly as rosette-shaped crystal aggregates on bedding planes. Rare bands of chert (up to 1.2 m thick) are present in the succession, the most prominent of which occurs at Lung Lun Tsui (Plate 8.7) on the western coast of Ping Chau.


Individual facies have not yet been recognized within the Ping Chau Formation.




Details

Ping Chau. The strata on Ping Chau belong to the middle stratigraphic unit of the formation. The formation is characterized by thinly bedded strata. In general, each of the beds is only 2 to 6 cm wide but can be traced laterally for a distance of several hundred metres. Most beds are composed of very thin, alternating planar laminae, each less than 1 mm thick, of detrital and chemical sedimentary rocks (Plate 8.A5). The lowest unit of the member consists of dark grey, thinly bedded aegirine-bearing siltstone with clayey dolomitic siltstone and a layer of chert and exceed 60 m in thickness. Intercalated, pale yellowish brown, dolomitic siltstone is well exposed at Cham Keng Chau (861700 845400 Ep-1) and Chau Bei (862500 844400 Ep-2). There are also several thin layers containing coarse calcite aggregates that crop out at Hok Ngam Teng beach (863200 844300 Ep-3). Calcite veinlets, 0.5 to 1 cm thick, fill minor faults at Cham Keng Chau (861800 845400 Ep-4). Aegirine, calcite and pyrite crystals, some possibly pseudomorphic after gypsum, occur locally (861850 845700 Ep-5, Plate 8.A6). Bitumenized plant remains are abundant on bedding planes or scattered within beds as carbonaceous fragments. A large fossil tree branch, some 60 cm in length, was found at the beach near Tsau Uk (861900 845100 Ep-6). Sedimentary structures, including sub-aqueous and sub-aerial shrinkage cracks, ripple marks and the development of pyrite nodules are common. Cross-bedding, load structures and other forms of soft-sediment deformation (Plate 8.A7) also occur. A greyish white, massive chert forms a well-defined marker bed along the west coast (Plate 8.A8) and creates a hard and narrow ridge about 1 m thick (Plate 8.A9). Northwards, this horizon thins from 1.2 to 0.6 m.


The middle part of the member is 46 m thick, and comprises dark grey, thinly bedded siltstone with intercalated dolomitic siltstone and mudstone. Laminae within each of the siltstone beds are also very thin but contain more detrital minerals. Pyrite is abundant, ranging in size from 0.1 mm cubic crystals to 120 mm nodules. Shrinkage cracks, cross-bedding, rain pitting prints and trace fossils are present in the rocks, and fossil plants are also abundant. A fossil palm leaf, 60 cm long and 50 cm wide, occurs at Nan Kwo Shui (863380 844450 Ep-7)


The upper part of the member is 92 m thick and consists of dark grey, thinly bedded, zeolite- and aegirine-bearing siltstone (Plate 8.A10). The zeolite-bearing siltstone is well developed along the east coast between Wong Ye Kok (862700 844900 Ep-8) and Lam Uk (862700 844600 Ep-9). The zeolite includes natrolite and analcime, and occurs as abundant, rosette-shaped crystal aggregates lying on bedding planes, or forms prismatic and granular crystal aggregates that fill cracks and "birds eye" structures. Pyrite is present as subspherical nodules and laminae. Cross-bedding is common at Wong Ye Kok.


At Chau Mei Kok (862300 846000 Ep-10), the rocks display a lithofacies change to dark grey, thinly bedded calcareous and dolomitic siltstone, showing a conspicuous decrease of sodium-rich minerals (aegirine and zeolite).


Seabed West of Ping Chau. Seismic interpretation indicates that rock is exposed on the seabed within 200 m of the island. Further west, the Ping Chau Formation is overlain by Quaternary sediments. The seismic profiles suggest that the formation offshore comprises mainly thinly bedded, gently dipping strata that resemble the siltstones exposed on Ping Chau. The strata offshore, estimated as being more than 150 m thick, are assigned to the lowermost member of the Ping Chau Formation. They overlie the Port Island Formation, though the nature of the contact between the two formations is unknown.


Seabed East of Ping Chau. The upper member of the Ping Chau Formation is overlain by Quaternary sediments except near the coast and is presumed to be dominated by siltstone. The thickness of the sequence is more than 100 m in this area.