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1 CIVL 270 Geology Field Trips Guide Field Trip 1. Oct. 9, 2010 HKUST Campus High Island Reservoir Site Map

Mesozoic volcanism in Hong Kong (165-140 ma)

yr ago) High Island Reservoir: High Island

2 Mesozoic volcanism in Hong Kong ( ma) Inferred locations of the Mesozoic volcanic centres or calderas Local Geology HKUST: Che Kwu Shan Formation (142 mil. yr ago) High Island Reservoir: High Island Formation (141 ma) Che Kwu Shan Formation High Island Formation

3 Schematic representation of caldera development and related subvolcanic intrusions between 143 to 142 million years ago

4 Time Scale of Solid Geology of Hong Kong Time Scale of Solid Geology of Hong Kong (Continued) Age (millions of years) Rock Unit Age (millions of years) Rock Unit CENOZOIC MESOZOIC TERTIARY QUATERNARY CRETACEOUS Kau Sai Chau Volcanic Group Repulse Bay Volcanic Group Superficial Deposits Sedimentary Rocks Rhyolitic Subgroup Trachytic Subgroup Volcanic Rocks Ping Chau Formation Kat O Formation Port Island Formation Pat Sin Leng Formation High Island Formation Clear Water Bay Formation Undifferentiated Mount Davis Formation Long Harbour Formation Pan Long Wan Formation Mang Kung Uk Formation Che Kwu Shan Formation Ap Lei Chau Formation Ngo Mei Chau Formation Granitoid Rocks Mount Butler Granite Po Toi Granite Kowloon Granite Fan Lau Granite Sok Kwu Wan Granite Tei Tong Tsui Quartz Monzonite Tong Fuk Quartz Monzonite D Aguilar Quartz Monzonite Luk Keng Quartz Monzonite Shan Tei Tong Rhyodacite Chi Ma Wan Granite Shui Chuen O Granite Granitic Subsuite Monzonitic Subsuite Lion Rock Suite Cheung Chau Suite MESOZOIC PALAEOZOIC JURASSIC TRIASSIC PERMIAN CARBONI- FEROUS DEVONIAN San Tin Group Tsuen Wan Volcanic Group Lantau Volcanic Group Lai Chi Chong Formation Undifferentiated Sai Lau Kong Formation Sedimentary Rocks Volcanic Rocks Granitoid Roc ks Tai Mo Shan Formation Shing Mun Formation Yim Tin Tsai Formation Tuen Mun Formation Tai O Formation Tolo Channel Formation Tolo Harbour Formation Lok Ma Chau Formation Yuen Long Formation Bluff Head Formation Sha Tin Granite East Lantau Rhyolite East Lantau Rhyodacite Needle Hill Granite Sham Chung Rhyolite South Lamma Granite Hok Tsui Rhyolite Tai Lam Granite Tsing Shan Granite Chek Lap Kok Granite Chek Mun Rhyolite Lantau Granite Tai Po Granodiorite A-type Subsuite I-type Subsuite Kwai Chung Suite Lamma Suite

5 Silverstrand Bay / HKUST, Sai Kung, Eastern New Territories Chapter 13. Eutaxitic Tuff and Tuff-Breccia Che Kwu Shan Formation Introduction The term volcanic rock is used for any rock that is formed by extrusive magmatic activity, that is when hot magma reaches the earth s surface in or around volcanoes. The characteristics of volcanic rocks are determined by magma composition and eruption processes. Rocks formed during different phases of an eruption, or at different locations on a volcano, may vary significantly in their structures and textures. The Clear Water Bay Peninsula is underlain predominantly by rocks of the Repulse Bay Volcanic Group. Within this Group, the Che Kwu Shan Formation is the most extensively exposed formation in the area. The Formation comprises mainly welded crystal-bearing fine ash vitric tuff and tuff breccia. In addition to the type locality of Che Kwu Shan, the formation also crops out around the Hong Kong University of Science and Technology (HKUST), near Pak Shui Wun, and to the east of Silverstrand Beach. Coastal areas provide the best exposures of the Formation, as well as displaying illustrative volcanic rock landforms. Geological map of the Clear Water Bay Peninsula (from HKGS 1:100,000 HGM100) How to Get There The most convenient access to the volcanic rocks in the area is along the southeastern section of Silverstrand Beach. Silverstrand Beach is well-served by public transport. The bus routes include, No. 91 (Diamond Hill MTR Station to Sai Kung), No. 91M 86

6 (Diamond Hill MTR Station to Hang Hau), No. 298 (Lam Tin MTR Station to HKUST) or No. 792 (Tseung Kwan O to Sai Kung). Alight at the bus stop near the junction of Clear Water Bay Road and Silverstrand Beach Road, and then walk about 1 km to Silverstrand Beach. At the beach, walk eastwards for about 150 m to the Chek Kwu Shan Formation outcrop. Travelling by private car, turn into Silverstrand Beach Road from the Clear Water Bay Road. Access to Silverstrand Bay and HKUST The Che Kwu Shan Formation is also well exposed on the coast at Pak Shui Wan near the HKUST. From near the bus terminus at the gates of HKUST, a well-established footpath leads directly to the seashore at Pak Shui Wan. Outline of the Geology Rocks of the Repulse Bay Volcanic Group are divided into two sub-groups, the lower trachytic sub-group and the upper rhyolitic sub-group. The trachytic sub-group is characterised by well-developed welding structures, whereas the rhyolitic subgroup is dominated by coarse ash crystal tuff. 87

7 Welded tuff, eastern side of Silverstrand Beach Unweathered welded tuff Weathered surface of welded tuff Fresh surface of volcanic lava. Note the tiny vesicles nd finae quartz crystals Tuff breccia, Pak Shui Wun Mafic veins in tuff breccia, Pak Shui Wun 88

8 The Che Kwu Shan Formation is the dominant component of the lower trachytic subgroup. Rocks of the Formation are characterised by a streaked appearance that results from the flattening and welding of the many included pumiceous rock fragments (fiamme) within the tuff. This structure is termed eutaxitic foliation. The Formation is similar in appearance to the Ap Lei Chau Formation. However, whole-rock geochemical analyses have confirmed that rocks of the Che Kwu Shan Formation are generally composed of less than 70% SiO 2, whereas rocks of the Ap Lei Chau Formation contain more than 75% SiO 2. The groundmass of the welded tuff comprises mainly quartz and feldspar crystal fragments set within a fine ash vitric matrix. Upon weathering, the pumiceous rock fragments may be removed, resulting in the array of voids that can be seen in many outcrops. Interbeds of lava occur locally in the Che Kwu Shan Formation. Guide to Field Observations 1) Summarise the main characteristics of volcanic rocks in the area. This is the type locality of the Che Kwu Shan Formation, from which the name of the formation is derived, and all the rocks exposed in this area are representative of the Che Kwu Shan Formation. 2) Determine the composition of the rocks. Try to determine the size, mineralogy, and relative percentages of the different types of crystal fragments. 3) Observe the structures of the rocks. Try to distinguish between welded tuff structures (eutaxitic foliation) and breccia, and consider their modes of occurrence. 4) Note the included rock fragments. Determine if they are flattened, indicating flowbanding, and what the structures reveal about the characteristics of the volcanic processes. 5) Note the variation in rock types, and observe if there are any interbeds of lava and tuff. What can the occurrence of lava reveal about changes in the nature of the volcanic activity? 89

9 East New Territories, High Island Reservoir 16. Columnar Jointed Tuff High Island Formation Introduction The geological landscape in the area between High Island Reservoir and the Ninepin Group of islands in the eastern New Territories is particularly spectacular, the outcrops of volcanic rocks comprising rows of giant hexagonal or polygonal columns separated by uniform vertical or sub-vertical joints. How to Get There High Island Reservoir provides a relatively convenient location for examining the spectacular outcrops of columnar-jointed tuff. High Island Reservoir is located in the Sai Kung East Country Park. Access is on foot from the Country Park Visitor s Centre at Pak Tam Chung, which is on the route of public bus no. 94 from Sai Kung to Wong Shek Pier, and no. 96R from Diamond Hill MTR Station to Wong Shek Pier (services only on weekends and public holidays). Alight by the car park in front of Country Park Visitor s Centre, then walk southeast along the Tai Mong Tsai Road for about 1.5 km to the shelter at the High Island Reservoir. At this point, Tai Mong Tsai Road branches. The left branch is Sai Kung Sai Wan Road, which runs along the north side of the reservoir, and the right branch is Sai Kung Man Yee Road, which runs along the south side of the reservoir. Take the right branch, and walk southeast for about 9 km, crossing the West Dam, until the East Dam of High Island Reservoir is reached. The columnar jointed rock is visible at the far side of the dam. To reach the Ninepin Group of islands, it is necessary to hire a boat because there are no public ferry services available. The East Dam of High Island Reservoir from Po Pin Chau 103

Access to the East Dam of High Island Reservoir Outline of the Geology The columnar jointed volcanic rock exposed in the area from High Island Reservoir to the Ninepin Group of islands comprises

These rocks belong to the Early Cretaceous High Island Formation of the Kau Sai Chau Volcanic Group. The Formation is estimated to have a minimum thickness of 400 m.

10 Access to the East Dam of High Island Reservoir Outline of the Geology The columnar jointed volcanic rock exposed in the area from High Island Reservoir to the Ninepin Group of islands comprises rhyolitic to dacitic ash flow deposits. The rock is relatively homogenous, containing abundant large crystals of potassium feldspar (K-feldspar) and quartz. These rocks belong to the Early Cretaceous High Island Formation of the Kau Sai Chau Volcanic Group. The Formation is estimated to have a minimum thickness of 400 m. Collapsed rhyolitic-dacitic volcanic rock column at north Ninepin Island The spectacular columns of volcanic rock formed about 140 million years ago as a result of catastrophic volcanic eruption. Voluminous ash flows were produced during the eruption, covering the surrounding ground to a great thickness. Multiple ash flows were probably generated during the eruption that built up successive layers of tuff. Upon cessation of the eruption, the tuff cooled relatively slowly, gradually consolidating to form rock. Hexagonal columns developed in the tuff as a result of contraction during cooling. The cooling joints within the tuff can extend vertically for up to 30 metres, leading to the development of spectacular scenery. 104

The wide distribution of the tuff surrounding the High Island Reservoir and

Landscape near the East Dam of High Island Reservoir Guide to Field

dam, provide extremely good cross-sections through the columns.

11 The wide distribution of the tuff surrounding the High Island Reservoir and beyond suggests that it formed within a complex, fault-bounded caldera. Landscape near the East Dam of High Island Reservoir Guide to Field Observations Columnar jointed volcanic tuffs are well-exposed at the East Dam of High Island Reservoir. The rock slopes at the site, which were excavated during construction of the dam, provide extremely good cross-sections through the columns. The rock slopes can be accessed easily and safely, making them ideal for detailed observations. Tightly interlocked columnar joints Scarp in thick layer of welded tuff 105

1) At the base of many of the rock slopes are scattered blocks of columnar-jointed tuff. Note the almost regular hexagonal shape of these blocks, which vary from about 0.8 to 2.0 metres across.

These cracks may be gently inclined to the east or southeast.

12 1) At the base of many of the rock slopes are scattered blocks of columnar-jointed tuff. Note the almost regular hexagonal shape of these blocks, which vary from about 0.8 to 2.0 metres across. The interlocking columns fit together like the pieces of a jig-saw. 2) In some places, continuous, subhorizontal cracks can be seen extending into the rock face. These cracks may be gently inclined to the east or southeast. They could possibly represent the boundary between different pulses of ash flow produced during the volcanic eruption although, because the rocks form a single, continuous cooling unit, they are more likely to be stress release joints within the rock mass. 3) Carefully examine the rock type and the characteristics of the columnar jointing. Abundant large crystals of potassium feldspar and quartz can be seen by the naked eye in the fresh rock. The potassium feldspar crystals are pink in colour and are well-shaped, with sizes in the range of 1.5 to 2.0 mm. Quartz crystals are a glassy grey, spherical in shape, and about 1.0 mm in size. Potassium feldspar crystals usually weather to a pink colour, or may even become white, making them difficult to distinguish in hand specimen. Sometimes, only quartz crystals are visible in the weathered rock. 4) Gentle S-shaped flexures of the columns can be seen in some places. This may be due to gravitational settling of the mass of cooling ash when it was still in a plastic state. The original slip direction can be determined from the direction of the bending. Pink potassium feldspar and dark grey quartz phenocrysts in fresh rhyolitic tuff Banding in rhyolitic tuff 106

13 5) Mafic dykes are common in the area. These are later intrusions of magma that probably exploited tension cracks within the tuff. Toppling failure of the columns can be seen near the top of some of the rock faces, with the columns leaning out of the face. This mechanism is important from a slope engineering perspective. 6) Further examples of spectacular scenery formed in columnar-jointed rocks can be seen by climbing over the mountain to Po Pin Chau, Pak Lap Tsai, or by visiting the coast around Pak Lap Wan. Dark coloured basic dyke intruded along joint in columnar tuff C omments The development of columnar joints in volcanic rocks is not an uncommon phenomenon in other areas of the world. Indeed, they are a feature of many national parks. However, in most cases, the cooling columns formed in basaltic lava flows, which are relatively fluid and pond easily. The development of columnar joints in volcanic ash, particularly of rhyolitic composition, is less common. Also, rarely are columns seen developed to the size and extent of those that are exposed at High Island Reservoir. R eferences Allen, P. M. & Stephens, E. A Report on the Geological Survey of Hong Kong. Government Printer, 107 p. plus 2 maps. Campbell, S.D.G., Parry, S. & Liu, Y.s Hong Kong s Classic Geology No. 1: Deformation and slope failure modes in columnar jointed tuffs of the High Island Formation, Hong Kong. Hong Kong Geologist, v. 5, pp Davis, S. G The geology of Hong Kong. Government Printer, Hong Kong. 231 p. 107

14 plus 14 plates & 3 maps. Fisher, R.V. & Schminke, H.U Pyroclastic rocks. Springer Verlag, New York, 472 p. Ruxton, B.P The Geology of Hong Kong. Quarterly Journal of Geological Society of London, v. 115, pp Smith, R.L Ash-flow magmatism. Geological Society of America Special Paper No. 180, pp Strange, P.J., Shaw, R. & Addison, R Geology of Sai Kung and Clear Water Bay. Hong Kong Geological Survey Memoir No. 4, Geotechnical Control Office, Hong Kong, 111 p. Tam, S.W. & Chan, Y.M Late Jurassic ash-flow tuffs in the eastern part of Hong Kong, South China. Asian Geographer, v. 2, no. 1, pp Tam, S.W Landform in Sai Kung Peninsula and adjacent islands. Unpublished MA Thesis, Hong Kong University, 240 p. 108

15 侏罗纪火山对香港的影响大家對 1 億 4 千至 1 億 6 千萬年前的歷史可能有點陌生香港在這個時期經歷了一次猛烈的火山爆發, 起因是太平洋板塊俯衝插入歐亞板塊內, 後來經過多年的風化及幾次地殼的變動, 形成了西貢 Sai Kung 沿海一帶多個有趣的地質現象在 1 億 6 千 4 百萬年至 1 億 4 千萬年前的侏羅紀時期 (Jurassic Era), 香港發生了多次強烈的火山爆發, 每次噴出數十立方千米的熔岩微細火山灰將廣泛地面掩埋, 並在火山口慢慢冷卻, 形成呈六邊形的長石柱現時果洲群島和萬宜水庫均有此奇觀, 而香港約有一半 (50%) 的岩石是由火山灰和岩漿所形成的未有噴發到地面的熔岩在地底深處慢慢冷卻凝固, 形成大顆粒的花崗岩 (Granite) 由於地殼隆起和侵蝕所致, 三成 (30%) 的香港地表有這些帶有粗礦物顆粒的岩石外露白堊紀時期 (the Cretaceous Period) 距今 1 億 4 千萬至 6 千 5 百萬年前, 恐龍逐漸絕跡, 被子植物漸露頭角香港重要的地質活動有 : 地殼褶皺和斷裂約 1 億年前的白堊紀時期, 香港是一片荒漠, 內地火山帶的河道偶爾出現洪流, 把礫石沙粒和軟泥沖積到荒漠上的平原現時香港東北部的赤洲和烏蛟騰一帶的山嶺, 都可發現這類紅色的岩石層可能由於這一帶非常乾燥和荒蕪, 岩石裡並無古生物的痕跡, 但從廣東發現的恐龍化石, 顯示白堊紀時期香港附近可能曾有恐龍出沒約於 8 千萬年前至 5 千萬年前的白堊紀時期 Cretaceous 和第三紀 Tertiary 早期, 由於氣候變得潮濕, 香港地區逐漸形成一個遼闊的淺湖每當雨季的來臨, 湖底便會積聚一層很薄的軟泥到了乾旱的季節, 湖水蒸發後, 鹽份會凝結於軟泥上如是者年復年的循環累積, 至今仍可在東平洲的岩石見到數不清的粉砂岩 (Siltstone) 薄層湖泊周圍滿佈各種植物, 並有大量昆蟲棲居因此, 今日的粉砂岩中仍保留著樹葉花粉植物孢子和昆蟲的化石粉砂岩層上也有由淺水中的波浪所造成的岩石漣漪紋, 及因湖泊乾涸而產生的裂紋第三紀時期 (the Tertiary Period) 距今 6 千 5 百萬至 2 百萬年前, 草原出現哺乳動物及大型鳥類香港重要活動有 : 地殼上升和岩脈侵入活動陸地的隆起發生於約 5 千萬年前的第三紀時期, 其後的侵蝕開始塑造今日的地形景觀當時的氣候跟現時一般的溫暖潮濕, 風化作用侵蝕岩石表層, 形成軟黏土和土壤, 導致今日一些結合力弱和風化嚴重的斜坡不斷受到山泥傾瀉的威脅第四紀, 距今 2 百萬年前至現在, 氣候變化, 經歷冰河時期, 人類開始在非洲大陸出現香港重要的地質活動有 : 地表沉積開始於 2 百萬年前的第三紀時期, 大片冰原由兩極向前推進, 全球海平面大幅下降當 1 萬 8 千年前最後一次主要冰期達到高峰時, 海面比現時低 130 米, 海濱線在現今香港以南 120 公里處, 令香港地貌新添了一些山谷全球氣候和暖和冰原消退後, 海平面回升, 新形成的山谷遭海水掩沒, 造成現今香港迂迴曲折的海岸線, 以及無數的海灣, 小港和 260 多個島嶼 * 資料節錄自香港歷史博物館

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$(Microsoft Word - 01 \255\273\264\344\252\272\246a\262z\273P\246a\275\350\276\372\245v_1 by cc.doc)$ 圖香米港的山地 2 拔 100 李曉池香港的地理與地質歷史 (Geography and Geological History of Hong Kong) Li Xiaochi 香港的自然地理和地形 (Geography & Natural Topography of Hong Kong) 香港位於中國南方珠江口的東南