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EASC1XXX Introduction to Hong Kong Geology Self-guided Field Trip to Ping Chau, Mirs Bay Department of Earth Sciences, The University of Hong Kong Objectives This fieldtrip will focus on the following topics. (1) History of High Island Reservoir and geographical factors considered during the construction (2) Physiography and Geology of Sai Kung (3) Coastal landforms General Physiography and Geology of Sai Kung Sai Kung area is a hilly terrain deprived of lowlands (Fig. 1), indicative of a submerged landscape with dismembered rivers and drowned valleys. The original topography of the area prior to the construction of the reservoir (Fig. 2) shows a long and narrow waterway trending in a WNW-ESE direction with rivers entering at right angles, reflecting a rectangular drainage system developed along existing geological structures. Fig. 1 3D view of Sai Kung area derived from SRTM data and processed with Global mapper The High Island area in Sai Kung Peninsula is occupied predominantly by volcanic rock formations formed during the Jurassic-Cretaceous time. During this ‘Yenshanian Movement’, northward subduction of a plate under South China formed an island arc in the coastal province of South China, causing extensive intrusion and extrusion of acid rocks. Some of the volcanic rocks formed by such violent eruption can be seen at Sai Kung. Fig. 2 High Island area in 1964 before construction of reservoir Sai Kung area was probably a caldera, a volcanic depression formed by subsidence of the ground as the magma chamber below was drained, during the Jurassic Period. Volcanic centers were present around the caldera. Thick layers of volcanic ashes, lavas and tephra were formed on the surface during several eruption episodes. These were ‘Plinian’ type eruptions, which can last up to a few weeks. A sequence of ash over 100 m thick was deposited within the caldera. The major rock types in Sai Kung are pyroclastic rocks including tuff and volcanic breccias, rhyolite, and sedimentary rocks such conglomerate, sandstone and mudstone. Fig. 3 Convergent tectonic setting of Hong Kong area during the Jurassic time Fig. 4 Jointed columns with mafic dike intruded along a kind-band in east dam area Fig. 5 Toppled rock columns near Pak Lap Spectacular columnar joints and kink bands were formed within the ash layer, seemingly only shortly after the volcanic eruption. These columns are best exposed in the East Dam area, Ninepin Islands and on sea cliffs on several outlying islands. The jointed columns vary in geometry and size, with the majority of the columns being 5- or 6-sided, the ideal geometry for regularly spaced cooling centers. A careful examination of the jointed columns shows that they are occasionally bent, with ‘kink-bands’ inclining consistent at about 40o to the southeast (Fig. 4). The formation of the joint patterns and the significance of the kink-banding will be discussed on the trip. Toppling failures are commonly observed in the jointed columns (Fig. 5). The last phase of the volcanic activities is represented by intrusion of mafic dikes, sometimes along pre-existing kink bands in the jointed rocks. The post-Cretaceous tectonics is mainly characterized by faulting events. We now know that during the Cenozoic, the coastal province of South China underwent several phases of extension including at least a phase of N-S extension and a phase of NE-SW extension. The multiple sets of fault and lineaments found in the Sai Kung area are associated with these extensional events. The current tectonic movement is controlled by a E-W directed compression, associated with the collision of the Philippines with Eurasia along Taiwan. Coastal Landforms Remarkable erosional landforms can be observed along the coast of Sai Kung Peninsula. The distribution of beaches and headlands reflects the energy conditions. Erosional landforms including sea caves, arches and sea stacks often develop along pre-existing lines of weakness. Depositional features are confined to coves and small bays. The many islands in Sai Kung Sea were once peaks of a hilly region during the last Ice Age. Global climatic warming since about 12000 year ago caused the sea level to rise, drowning the lowlying areas. The irregular and jagged inlets and bays in Sai Kung were conceivably once a valley system during the Ice Ages. The great water depths around the islands and rivers without fully developed lower course morphology are evidence for the drowned landscape. The islands in Sai Kung are marked by the presence of numerous sea caves and arches. Among the more picturesque and notable ones are the Tiu Chung Chau Twin Cave, Basalt Island Cave, Sha Tong Hou Cave and Muk Min Cave. Most of these sea caves and arches developed along pre-existing fractures and faults, along which wave erosion and removal of material is intensified in rocks weakened by the fractures and faults. The best example is illustrated in Muk Min Fig. 6 Two faults can be observed to control the Cave where two faults controlling the development of Muk Min Cave near Pak Lap development of the cave can be readily observed (Fig. 6). A sand bar, called tombolo, connecting the small island to Sharp Island formed as water current flow is retarded; the reduction in current velocity causes sediments in the currents to be deposited. The tombolo, like several others in Sai Kung area, is probably part of a ridge connecting the two peaks. High Island Reservoir Project The construction of the High Island Reservoir commenced in 1971 in an attempt to alleviate the water supply problem of Hong Kong. The reservoir has a capacity of 280 million cubic meters. Major geotechnical works to build the reservoir included the construction of the east main dam, now standing at 64m above sea level, the cofferdam and the outer layer of concrete dolosse to attenuate wave action. A few villages, Lan Nai Wan in particular, were submerged as the reservoir was filled. Resources Hong Kong Geological Survey http://www.cedd.gov.hk/eng/about/organisation/int.htm Water Supplies Department http://www.wsd.gov.hk/en/text/edu/fish/hi_rev.htm Recommended stops for the field trip: STOP 1 STOP 2 STOP 3 STOP 4 West Dam & Yuen Ng Fan East Dam Trail towards Mok Min Cave from Pak Lap Muk Min Cave Major legends QHH Quaternary marine mud ms Quaternary marine sand JHI JurassicCretaceous fine ash tuff JCB JurassicCretaceous trachydacite & rhyolite lava 1 2 Entrance to Pak Lap 3 4 Assignments: 1. Are all the polygonal surfaces are in hexagonal shapes? If no, why? 2. Measure the diameters of the polygonal surfaces at Stops 2 and 4 and summarize the measurement in a table. 3. What do the difference of size and shape tell you about the cooling history of these volcanic rocks in Sai Kung? 4. Describe different kinds of dams built at Stop 1 (West Dam) and Stop 2 (East Dam). What are the purposes of these dams? Summarize their features in a table.