Oldest rock in Hong Kong － Wong Chuk Kok Tsui

Oldest rock in Hong Kong － Wong Chuk Kok Tsui

Background Information

The oldest stratum in Hong Kong, as proven by existing fossil evidence, is a bed of Devonian sedimentary rock. It is mainly outcropped on both sides of Tolo Channel. Sedimentary pyroclastic rocks found around Pak Sha Tau and Wong Chuk Kok Tsui on the northern shore are called Wong Chuk Kok Tsui Formation. They are primarily sandstone, pebbly sandstone and conglomerate. The best outcropped areas of this formation can be found around Wong Chuk Kok Tsui at the northern tip of the channel. Influenced by the folding and staggering action of geological movements, the beddings are almost vertical. It is a breathtaking sight. The rich Devonian fossils found in Tolo Channel provide a strong basis for determining geological age and sequence stratigraphy. They also help us understand how Hong Kong's natural environment evolved during that time.

Geological Information

The oldest rocks of Hong Kong (Devonian) are distributed on both sides of Tolo Channel, i.e. Ma On Shan on the southern shore and the region south of Plover Cove Reservoir on the northern shore. The latter are known as Wong Chuk Kok Tsui Formation. The faults of this formation, some 800 metres thick, have contacts with other strata both in the north and south. The entire Wong Chuk Kok Tsui Formation is made up of clastic sedimentary rocks like sandstone, siltstone and conglomerate. While rock property variations are insignificant from bottom to surface, ages of fossils within cover the entire Devonian sedimentary sequence, from its early stage to the late stage. By analysing the sedimentation and living organisms of the Devonian in depth, we can see that the region around Plover Cove Reservoir on the northern shore of Tolo Channel today was primarily foreshore, river delta or inshore sedimentary shallow sea during the Early and Middle Devonian. Bivalvia, archaeostraca and conchostracan fossils have thus been discovered in this area. Fossils from the Late Devonian are mainly terrestrial primitive ferns. This indicates the sedimentary environment changed at this stage. The sea receded and land was exposed.