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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. Geological structures 2. Geological history of Ma Shi Chau 3. Coastal landform General geology of Sai Kung Ma Shi Chau is located in the middle of Tolo Harbour (Figs. 1 and 2). It is an island of an area of 1.5 km x 500m, connected with Yim Tin Tsai with a sand bar. Fig. 1 Geological map of Hong Kong, with Ma Shi Chau indicated in a red box. Tombolo Figure 2 (a) Geological map showing Ma Shi Chau and Yim Tin Tsai. Figure 2 (b) A key indicating the rock types and geochoronological ages of different rock units. Regional setting During Permian period (299-251 Ma), the area of present Ma Shi Chau was part of a shallow water basin on a continental edge. Sediments or rock fragments from surrounding land were transported and deposited in the basin, accumulating layers of sediments. Under process of lithification (compaction and cementation), the sedimentary rocks were formed. Variation of sea level changes depositional environment as well as the current motion. Relative strong current (e.g. river, coastal current) can carry coarser sandy sediments while relative weak current or quiet depositional environment (e.g. delta area, lake) can only transport fine sediments. Gradual increase in grain sizes of the Permian sedimentary rock in Ma Shi Chau from southeastern part to northern part of the island indicates variation of sea level as well as depositional environment in Ma Shi Chau area in the past. During Jurassic period (201-146 Ma), vigorous volcanic activities happened in the northeast of Hong Kong, with volcanic eruptions and magma intrusions in the area (Li, 2007). Granitic rocks (cooled down from magma intrusion) are found in the northern side of Yim Tin Tsai and volcanic sediments are presented on western and northwestern part of the Ma Shi Chau (Li, 2007). Major crustal movement following the intrusions and volcanic activities led to the formations of folds, faults on the island (Li, 2007). Figure 3 A tombolo connecting Yim Tin Tsai and Ma Shi Chau. Figure 4 Permian reddish siltstone in Ma Shi Chau. Tombolo As a result of longshore drift, a tombolo, or a sand bar, is formed, attaching Ma Shi Chau to Yim Tin Tsai (Figures 2 and 3). The tombolo was once under water during high tide and exposed during low tide. For a convenient pathway to Ma Shi Chau, villagers hauled boulders and sediments along the tombolo, resulting in a well-exposed sand-bar even during high tide. Sedimentary rocks of Ma Shi Chau The sedimentary outcrop of Ma Shi Chau was believed to be formed at Permian (Figure 4). Marine fossils such as malluscs, corals, bryozoans, brachiopods, crinoids, and some plant fossils were found in the sedimentary layers and used to determine the approximate age of the sedimentary rocks (Lee et al., 1997). Quartz vein Networks of white veins are found in outcrops of Ma Shi Chau. When fluid passed through the tiny fissures of outcrops, minerals were deposited from the solution along the fissures, forming these vein-like structures. In Ma Shi Chau, most of the veins are quartz veins which are relatively more resistant to weathering (Figure 5). Figure 5 Network of quartz veins in Ma Shi Chau. A fault Ma Shi Chau has experienced complicated tectonic movement, resulting in a series of folds and faults. A fault (Figure 6) is a planar crack in a rock, along which relative displacement of opposite sides of the fault occurs. It is caused by compressional or tensional events during earth movement: Figure 6 A normal fault with the hanging wall (right) sliding down relatively to the footwall (left). Normal fault indicates an extensional event, with the hanging wall sliding down relatively to the footwall (Figure 7a). Reverse fault indicates a compressional event, with the hanging wall moving up relatively to the footwall (Figure 7b). Strike-slip fault shows a (a) (b) (c) (d) parallel displacement of the opposite sides, without vertical motion (Figure 7c-d). Figure 7 Faults and the corresponding motions: (a) A normal fault with horizontal extension. (b) A reverse fault with horizontal compression. (c-d) Strike-slip faults with horizontal parallel motions (Credit: http://cdn.readtapestry.com/stories/lVt8rn40G/index.html). A fold Fold is a form of deformation in which original horizontal layers are bent or curved. Figure 8 shows a fold with one limb parallel to the coastal line while another running towards the sea. The corner at which the limbs change direction is the hinge. The horizontal layers was once folded and later tilted at an angle from the horizon. The hinge line is described to plunge to a direction Hingeline Limb Figure 8 A plunging fold at the coast of Ma Shi Chau. and the fold is thus called plunging fold. Sausage structure This shows the way in which different rock types deform in response to pressure differently. The discontinuous brown layer was originally a single layer, interbedded with the white layers. When these layers were compressed, the brownish harder and more brittle layer broke into elongated and discontinuous blocks, while the whitish softer layer “flowed” and filled the gaps between the blocks. These discontinuous blocks are known as sausage structures. Figure 9 Brownish sausage structures in softer whitish layers. References: Lee, C.M., Chan, K.W., Ho, K.H., 1997. Palaeontology and stratigraphy of Hong Kong vol. 1. Beijing: Science Press, 206 p. plus 57 plates (In Chinese). Li, X.C., 2007. Permian Sedimentary Rocks – Tolo Harbour Formation. In: Hong Kong Geology Guide Book. Hong Kong: Geotechnical Engineering Office Civil Engineering and Development Department. Geological time-scale. Geological Society of Hong Kong. Assignments: 1. Measure 50 dip/srike measurements along the southeast coastal areas of Ma Shi Chau. Make sure that the measurements are at least 2-m apart and at different locations. 2. Mark the measurements on a topographic map (Attachment 1) and interpret the possible structures (e.g. folds, faults) on the map.