Permian rock garden of Hong Kong － Ma Shi Chau Special Area
Background Information
The 61-hectare Ma Shi Chau Special Area is made up of Ma Shi Chau, Yeung Chau, Centre Island and an unnamed islet to the northeast of Sam Mun Tsai New Village. Ma Shi Chau is the key site of Hong Kong's Permian Tolo Harbour Formation. Home to well-outcropped rocks, the island is an ideal location for studying strata, rock properties, sedimentation features and different geological compositions because here you can find faults and folds as well as deformed and displaced rock mass. The unnamed islet near Yim Tin Tsai, vegetated with dense woodlands, is a popular roosting ground for egrets and herons. Given such outstanding conservation value, the Ma Shi Chau region was declared a protected Special Area in 1999.
The Ma Shi Chau Special Area nature trail runs along the southeastern shore of the island. 1.5 km in length, this route presents 16 attractions which highlight local geological features. By observing the rocks on site and reading trailside interpretation plates, visitors can understand how tombolos and various rocks are formed, as well as natural phenomena like wave erosion, weathering and folding .
Geological Information
Outcropped strata dating back to the Permian (290,000,000 to 250,000,000 years ago) are extremely rare and isolated in Hong Kong. Tolo Harbour Formation is a particularly prominent example. This formation is mainly distributed across Tolo Harbour and its environs, such as Ma Shi Chau, Centre Island, the Chinese University of Hong Kong and the western shore of Three Fathoms Cove. Ma Shi Chau is a representative showcase of this formation. The well-outcropped rocks on the island are composed of mudstone, siltstone and sandstone deformed by folding . Centre Island to its south is made up of the same rocks. The sandstone there exhibit ripple marks corrugated and flaser-bedding, while the siltstone often manifests bioturbation.
Extensive Permian sedimentary rock layers are not the only geological attraction of Ma Shi Chau. Here on the island, you can also find faults and folds, deformations and displacements. On the northwestern edge, some outcrops are part of the Early Jurassic Tolo Channel Formation. These include dark siltstone, mudstone and Late Jurassic sedimentary breccia, tuff and tuffite.
The Permian rock at Ma Shi Chau is estimated to be approximately 500 metres thick. It has two lithofacies. The lower facies in the southeastern part of the island is an assembly of powdery grey to pale red calcareous siltstone and charcoal-tone alternation beds of mudstone, siltstone and sandstone. The sand content increases upwards progressively and transits to the approximately 0.5m thick sandstone beds. The upper facies in the northern part of the island is mainly thick layers of siltstone, sandstone and conglomerate. Depositional environment is stable and each layer is as thick as 1 metre. Fossils like mollusks, corals, bryozoans, brachiopodas and crinoids have been discovered both in the upper and lower facies.
Judging from the rock facies and their features, as well as the presence of both marine fauna fossils and terrestrial flora fossils, we can make a conjecture that the Permian rocks in Tolo Harbour and nearby regions were formed in a shallow estuary. The well developed stratification, alternate sandstone and siltstone beddings, as well as cross bedding or flaser-bedding, are evidences of frequent changes in sediment sources and hydrodynamic conditions. All these are characteristics of a shallow estuary environment.
Landform Video
How to get there
Take bus No.74K or minibus 20K at the Tai Po MTR Station for Sam Mun Tsai. After getting off, walk across the hillock in Yim Tin Tsai and reach Ma Shi Chau via the tombolo. The hike is about 2 km in distance.
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