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Geology of Hong Kong

Geology of Hong Kong is dominated by Mesozoic igneous rocks which made up 85% of Hong Kong rocks.[1][2] The other 15% left are sedimentary rocks formed during periods before and after the volcanic episode.[1] The geological history of Hong Kong begins with Devonian sedimentary units, followed by Carboniferous, Permian and Jurassic sedimentary periods. In each period, Hong Kong experienced different sedimentary environments. During middle Jurassic, Hong Kong entered a period with significant volcanic activities. Volcanism resulted in a massive area covered by volcanic materials along southeast China coast and Hong Kong. When volcanism ceased, Hong Kong again entered a quiet sedimentary period marked by a reddish rock bed.[1][2]

With massive igneous coverage, quarrying and mining activities existed in the past in Hong Kong.

[add a figure with Hong Kong map + igneous distribution]

[add figure: HK stratigraphic column]

Regional Geology (before 416 million years ago)

Figure showing Yangtze block and Cathaysia block together with Lianhuashan fault zone and Changle-Nanao fault zone. Modified from Sewell R. J. Pre-quaternary Geology of Hong Kong.[1]

South China block comprises two continental blocks: the Yangtze block in the northwest and the Cathaysia block in the southeast. Hong Kong is located in the southeast margin of Cathaysia block. Oldest rocks of the Yangtze block and Cathaysia block were from late Archean Eon(2.5 billion years ago).[3][4][5] However, Archean rocks in Cathaysia block are not exposed to the surface. It is still debatable whether Archean rocks really existed.[5] Widely recognized oldest rocks in Cathaysia block were from Paleoproterozoic (1.8 billion years ago).

1 billion years ago, there was an ocean between Yangtze block and Cathaysia block[3][6]. Zircon dating suggested that Cathaysia block collided with Yangtze block 968 - 800 million years ago during Neoproterozoic. [6] After the collision, the newly combined South China block underwent rifting.[3][7] At that time, southeastern China was in a deep sea setting. Layers of mud and sand turbidites were deposited. Until Devonian, tectonic uplift created an alluvial floodplain environment.[1][3] This marked the start of the geological history of Hong Kong.

Lianhuashan Fault Zone

The east of Cathaysia block contains at least three microcontinental fragments. The suture zones are represented by two major northeast trended fault zones. They are the Changle-Nanao fault zone and the Lianhuashan fault zone.[8] Lianhuashan fault zone is a zone containing northeast trending faults and shear zones. It runs from Shanghai to Hong Kong along today's southeast China coast. The most significant activity happened at the fault zone was a major volcanic eruption period in middle Jurassic.[8] It is believed to be caused by the collision of a microcontinental fragment with the main part of Cathaysia block.[9] Major northeast trending faults in Hong Kong today are considered part of the Lianhuashan Fault Zone.[8]

Early Sedimentary Period (416 - 161 million years ago)

This period is marked by Devonian, Carboniferous and Permian sedimentary rocks. In this period, the Cathaysia block was subsiding and sea level was rising gradually.[1] Hong Kong experienced four distinct sedimentary environments. Rocks of this period cover around 10% of Hong Kong outcrops.[10]

[add map showing locations of exposure]

Devonian (416 - 360 million years ago)

During Devonian, there was a general crustal thickening and uplift towards the southeast of Guangdong. East Guangdong (and Hong Kong) became alluvial plain and west Guangdong became a shallow sea.[1][3] Devonian rocks in Hong Kong are marked by Bluff Head Formation. Outcrops are exposed on the north coast of Tolo Channel and on the Hunch Backs (Ngau Ngak Shan) in Ma On Shan. On the north coast of Tolo Channel, it is marked by reddish brown coloured heavily folded beds. [1]

[add photos of Bluff head folds]

Exposures of Bluff Head formation is divided into three parts: early-mid Devonian exposure, mid-Devonian exposure and late Devonian exposure.[10]

Restoration image of a Placoderm By © Citron /, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14679556

Early-mid Devonian exposure

This part is dated to early to middle Devonian by the discovery of Placoderm fossils. The discovery of Placoderm fossils marked the start of Hong Kong geological history. The early-mid Devonian period is characterized by a deltaic depositional environment. Greyish fine to medium grained sandstones, siltstones and mudstones dominate the exposure. [1] First, calm marine environment led to mud and silt deposition. Then, the emergence of a clastic sediment source deposited coarser sediments. Eventually, a small scale (3m thick) coarsening upward sequence was formed. The environment periodically changed from shallow marine to fluvial environment and back to shallow marine. [10]

Mid-Devonian exposure

This part shows a significant change of depositional environment. Tectonic uplift and mountain building nearby provided a source of clastic sediments. Grains became coarser and more poorly sorted. Sedimentary cycles became thicker (40m). Hong Kong was at mid-Devonian dominated by channels. Conglomerate filled channels (major channels) and sandstone filled channels (minor channels) were both present. This implies the presence of a multichannel braided river system. Thick (20m) cross-bedded coarse-grained sand dunes are evidence of a braided river system. Paleosols and mudcracks found on the exposure indicate a dry climate during the period of deposition.[10]

Late Devonian exposure

The late Devonian period was still dominated by a fluvial environment. Outcrops of this part are exposed on the Hunch Backs in Ma On Shan. There are very few coarse materials in late Devonian exposure. This implies a decrease in the amount of sediment. As the emerged mountains were gradually eroded, slopes became less steep. Rivers flowed slower and fewer sediments could be supplied to the alluvial plain. This resulted in major channels diminishing into minor channels[10].

Carboniferous (360 - 300 million years ago)

In the Carboniferous period, a tropical shallow sea covered the Devonian floodplain. Calcareous remains of marine creatures accumulated in the seabed and became limestones. Limestones were metamorphosed by later igneous rock intrusion and became marble. These marbles were found in Yuen Long and Ma On Shan. They are only seen in boreholes. On top of the marbles is a zone of karst. They often form cavities when acidic rainwater dissolves the material and it causes serious engineering problems in Yuen Long and Ma On Shan new towns.

Yuen Long formation can be subdivided into Long Ping member and Ma Tin member. Long Ping member is comprised of greyish crystalline marble. It was originally a limestone interbedded with calcarecous mudstone. The greyish colour from silt clasts indicates a swamp or tidal flat deposition environment. Ma Tin member is, however, a white marble comprised of >99% of calcium carbonate. The original limestone has been completely recrystallized. Therefore all traces of sediments were destroyed. Since there were no fossils found in Yuen Long Formation, the exact age is unknown. By correlating to similar formations in Shenzhen, Yuen Long formation is believed to be formed at early Carboniferous.

Ma On Shan formation marble is interbedded by dark-coloured metasiltstone. The marble layer is in contact with a granitic intrusion formed during volcanic activities in middle Jurassic. This resulted in the formation of iron ore deposits in Ma On Shan. Mining of magnetite and haematite were active but no longer exist today.[11][12]

[add photos of Ma On Shan marble, magnetite, mine]

In the late Carboniferous, sea level gradually drops and Hong Kong was dominated by a deltaic environment. This is represented by an upward coarsening sequence from siltstone to sandstone to conglomerate in Lok Ma Chau formation. Lok Ma Chau formation can be divided into Mai Po member and Tai Shek Mo member. Mai Po member is featured by phyllite and graphitic schist. Tai Shek Mo member is mainly light coloured sandstone contained the upward coarsening sequence. The conglomerate contains flattened and elongated clasts and quartzite in the sandy matrix showing mild metamorphism.[1]

Permian (299 - 251 million years ago)

In early Permian, Hong Kong was covered by a shallow sea near the coast. It is represented by Lower part of Tolo Harbour formation. Exposures are located around Ma Shi Chau. On Ma Shi Chau, fossils of marine species such as bivalves and corals were found. Calcareous remains of marine creatures together with clastic sediments from the nearby coast were deposited. Therefore, the early Permian part of the formation consists of calcareous siltstone and greyish mudstone interbedded with sandstone. This shows characteristics of a shore tidal plain setting.[13][14]

Later, tectonic uplift led to a drop of sea level. Hong Kong then became a terrestrial plain.[2][14] This period is represented by the upper part of Tolo Harbour formation. It contains a succession of siltstone interbedded black mudstone, cross-bedded pale grey quartz sandstone and quartz-pebble conglomerate. This upward coarsening succession supports an alluvial plain environment. The environment of Hong Kong changed from a shallow sea to a shore tidal plain in early Permian and alluvial plain in late Permian.[1]

Jurassic (201 - 145 million years ago)

During the early Jurassic period, Guangdong was covered by sea and Hong Kong was in a shallow marine environment. This period is represented by Tolo Channel formation. Its outcrops are exposed around Tolo Channel.[2] Ammonite fossils were discovered on Ma Shi Chau and the north coast of Tolo Channel. It remained the only rocks with known age in Hong Kong for nearly 50 years. Various types of fossils including gastropods, ostracods, brachiopods and crinoids were recovered from Tolo Channel formation.[13][15] Tolo Channel formation is mainly greyish laminated siltstones and fossiliferous black mudstone. It indicates a shallow marine low energy deposition environment.[13][16]

In middle Jurassic, Guangdong was uplifted. Hong Kong experienced a terrestrial sedimentation period. Affected by the Yanshanian magmatism northwest of Hong Kong, pyroclastic materials were found in an increasing proportion. Angular quartz in tuffaceous sandstone and tuffs in Lai Chi Chong imply nearby volcanism.[1] This period is represented by Tai O formation. It is exposed along the coast northeast of Tai O village. It contains mainly greyish fine-grained sandstone and sandy siltstone. Sandy siltstones are upward coarsening and with cross-lamination. This indicates an alluvial plain setting.[1][17]

Volcanic period (180 - 140 million years ago)

Figure showing tectonic settings of Hong Kong during different phases in the volcanic period and formation of a back-arc basin

The subduction zone of Paleo-Pacific plate beneath the Eurasian continental plate passed across southeast China from northwest to southeast from middle Jurassic to early Cretaceous. Extensive magmatism had resulted in a massive Mesozoic igneous province in south China. Igneous rocks from this period made up 85% of Hong Kong rocks. [18] Volcanic period of Hong Kong can be divided into one volcanic arc phase and four back arc phases.[2]

Volcanic arc phase (180 million years ago)

In this period, Hong Kong was right at the subduction zone. Stratovolcano gave out andisitic lava and ashes which is now found in Tuen Mun formation. Unlike the other four back arc phases, Tuen Mun formation doesn't have a corresponding pluton. The lower part of the formation contains volcanic breccia, rhyolite and tuff. It is a mixture of andesitic lava and sedimentary country rock. The upper part contains lapilli bearing andesitic lava and crystal tuff. It is formed by mixture of continental material and oceanic material accreted from the subducting crust. [18]

Four back arc phases (165-140 million years ago)

First phase

Further from the subducting front, back-arc volcanism is produced by upward circulation of magma. Extension of continental crust made the crust thinner and formed a back-arc basin (See Fig.3). The first back arc phase is represented by Tsuen Wan Volcanic group. It is exposed in north New Territories. The group is made up of Yim Tin Tsai formation, Shing Mun formation, Tai Mo Shan formation and Sai Lau Kong formation. The related magma cooled down eventually and formed the Lamma Suite granite. [2]

Yim Tin Tsai formation and Tai Mo Shan formation contains mainly crystal tuff with aphanitic grey to black lapilli and volcanic breccia with tuffite interbeds. Both formations are formed in a rapid continuous explosive eruption.

Shing Mun formation, however is very complex. It contains crystal tuff, breccia and a mixture of conglomerate, sandstone, siltstone and mudstone. This suggests an interrupted eruption with quiet intervals.

Figure modified from Hong Kong map showing positions of calderas in Hong Kong

Lastly, Sai Lau Kong formation is dominated by thick layers of viscous lava and tuff-breccia. This indicates an eruptive centre around the formation in northeast New Territories.[1]

Lamma granite suite is the pluton formed by cooling of magma in the first phase. They formed large and widespread irregular magma intrusions which might be related to the extensional tectonic setting. [2]

Second Phase

In the second phase, volcanism and magmatism shifted to the southeast of Hong Kong. It is represented by Lantau Volcanic group. The group is largely affected by northeast-trending faults. Lava flowed into the faults and formed groups of NE trending dyke intrusions. It is called Lantau dyke swarm. The Lantau Caldera was also elongated towards the northeast.[19]

Lantau Volcanic group contains porphyritic rhyolite with a small amount of rhyolitic tuff. High temperature, low viscosity lava was pushed into molten rocks.[1]

Third Phase

In the third phase, volcanism continued to move southeastwards. Repulse Bay Volcanic Group recorded this phase of volcanism. At least two calderas were identified in this phase. The one on Hong Kong Island is marked by Mount Davis formation. It is filled by dark greyish crystal tuff with fewer crystals. The other one in Sai Kung is represented by Long Harbour formation. It produced coarse ash crystal tuff with high crystal content. As the ash gradually cooled down, light-coloured hexagonal cooling columns were formed.[1][20]

Fourth Phase

In the last phase of volcanism, a thick and fused layer of fine ash tuff was formed in Kau Sai Chau Volcanic Group. A large caldera of about 20km diameter was formed in High Island. The caldera was then filled by a 400m-thick layer of viscous, silica-rich ash. The ash cooled down slowly. Then it eventually formed the remarkable High Island hexagonal columns of rhyolitic crystal tuff. [20][2] In 2011, High Island became part of the UNESCO Hong Kong Global Geopark. The hexagonal columns are used as the logo of the Geopark.

[add photos of hexagonal columns]

Late Sedimentary Period (140 - 55 million years ago)

After 40 million years of volcanic activity, subduction finally stopped. Hong Kong was then at a passive continental margin. Sedimentary rocks from this period are mainly formed by erosion of volcanic materials. They are characterised by the reddish colour of the bed which implies an arid climate.

Cretaceous (145 - 66 million years ago)

Cretaceous sediments were mainly formed by erosion of volcanic terranes. This period is represented by Pat Sin Leng formation in the northeast New Territories. Pat Sin Leng formation contains greyish-white thick bedded conglomerate and tuffaceous sandstone. They were deposited in river channels in an alluvial plain setting. The formation is overthrusted by the older volcanic rocks of Tai Mo Shan formation.[1]

In late Cretaceous, reddish brown thick bedded conglomerate and sandstones were formed in an alluvial plain setting. It is marked by Port Island formation. The exposure on Port Island dips about 30 degrees to the south and southeast.[21]

Another cretaceous formation, Kat O formation, is a coarsening upward sequence from greyish white siltstone, sandstone to reddish brown calcareous coarse sandstone and breccia. It implicates an alluvial channel setting where the reddish colour indicates an arid climate.

Paleogene (66 - 23 million years ago)

Around 65 to 55 million years ago, in the early Paleogene, arid climate continued to affect southeast China. Saline lakes and seasonal rivers dominated Hong Kong. Lakes dried out occasionally. Mudcracks and evaporites were formed. This period is represented by Ping Chau formation which is the youngest formation in Hong Kong. The Ping Chau formaion is dominated by reddish brown dolomitic and calcareous siltstones. Terrestrial plant fossils and evporites such as zeolites and calcites were found on Tung Ping Chau. The formation is gently dipping towards the northeast. Beds are well laminated. It is deposited in a terrestrial shallow lake environment of low salinity under tropical climate.[22]

References

  1. ^ a b c d e f g h i j k l m n o p q The Pre-Quaternary Geology of Hong Kong. Sewell, R. J., Hong Kong. Geotechnical Engineering Office. Hong Kong: Hong Kong Geological Survey. [2000]. ISBN 9620202996. OCLC 46968327. {{cite book}}: Check date values in: |date= (help)CS1 maint: others (link)
  2. ^ a b c d e f g h SHAW, R.; TANG, D. L.K.; OWEN, R. B.; SEWELL, R. J. (2010-01). "THE GEOLOGICAL HISTORY OF HONG KONG". Asian Geographer. 27 (1–2): 43–57. doi:10.1080/10225706.2010.9684152. ISSN 1022-5706. {{cite journal}}: Check date values in: |date= (help)
  3. ^ a b c d e Charvet, Jacques (2013-09). "The Neoproterozoic–Early Paleozoic tectonic evolution of the South China Block: An overview". Journal of Asian Earth Sciences. 74: 198–209. doi:10.1016/j.jseaes.2013.02.015. ISSN 1367-9120. {{cite journal}}: Check date values in: |date= (help)
  4. ^ Li, Longming; Lin, Shoufa; Xing, Guangfu; Davis, Donald W.; Jiang, Yang; Davis, William; Zhang, Yanjie (2016-04). "Ca. 830 Ma back-arc type volcanic rocks in the eastern part of the Jiangnan orogen: Implications for the Neoproterozoic tectonic evolution of South China Block". Precambrian Research. 275: 209–224. doi:10.1016/j.precamres.2016.01.016. ISSN 0301-9268. {{cite journal}}: Check date values in: |date= (help)
  5. ^ a b Zheng, J.P.; Griffin, W.L.; Li, L.S.; O’Reilly, Suzanne Y.; Pearson, N.J.; Tang, H.Y.; Liu, G.L.; Zhao, J.H.; Yu, C.M. (2011-01). "Highly evolved Archean basement beneath the western Cathaysia Block, South China". Geochimica et Cosmochimica Acta. 75 (1): 242–255. doi:10.1016/j.gca.2010.09.035. ISSN 0016-7037. {{cite journal}}: Check date values in: |date= (help)
  6. ^ a b Shu, Liang-Shu; Faure, Michel; Yu, Jin-Hai; Jahn, Bor-Ming (2011-06). "Geochronological and geochemical features of the Cathaysia block (South China): New evidence for the Neoproterozoic breakup of Rodinia". Precambrian Research. 187 (3–4): 263–276. doi:10.1016/j.precamres.2011.03.003. ISSN 0301-9268. {{cite journal}}: Check date values in: |date= (help)
  7. ^ Yan, Chaolei; Shu, Liangshu; Santosh, M.; Yao, Jinlong; Li, Jinyi; Li, Cheng (2015-10). "The Precambrian tectonic evolution of the western Jiangnan Orogen and western Cathaysia Block: Evidence from detrital zircon age spectra and geochemistry of clastic rocks". Precambrian Research. 268: 33–60. doi:10.1016/j.precamres.2015.07.002. ISSN 0301-9268. {{cite journal}}: Check date values in: |date= (help)
  8. ^ a b c FLETCHER, C. J. N.; CAMPBELL, S. D. G.; CARRUTHERS, R. M.; BUSBY, J. P.; LAI, K. W. (1997-11). "Regional tectonic setting of Hong Kong: implications of new gravity models". Journal of the Geological Society. 154 (6): 1021–1030. doi:10.1144/gsjgs.154.6.1021. ISSN 0016-7649. {{cite journal}}: Check date values in: |date= (help)
  9. ^ Sewell, Roderick J.; Carter, Andrew; Rittner, Martin (2016-10). "Middle Jurassic collision of an exotic microcontinental fragment: Implications for magmatism across the Southeast China continental margin". Gondwana Research. 38: 304–312. doi:10.1016/j.gr.2016.01.005. ISSN 1342-937X. {{cite journal}}: Check date values in: |date= (help)
  10. ^ a b c d e Jones, N.S.; Fyfe, J.A.; Sewell, R.J.; Lai, K.W.; Lee, C.M. (1997-12). "Devonian fluviodeltaic sedimentation in Hong Kong". Journal of Asian Earth Sciences. 15 (6): 533–545. doi:10.1016/s0743-9547(97)00036-6. ISSN 1367-9120. {{cite journal}}: Check date values in: |date= (help)
  11. ^ Mines., United States. Bureau of. Mineral industry of Hong Kong. OCLC 966859024.
  12. ^ Farmer, Hugh (April 21, 2016). "Ma On Shan Iron Mine 1906-1976 – biggest mine in HK – further information". The Industrial History of Hong Kong Group. {{cite web}}: Cite has empty unknown parameter: |dead-url= (help)
  13. ^ a b c W., Lai, K. (1996). Geology of the Northeastern New Territories : 1:20000 sheets 3 & 4. Geotechnical Engineering Office, Civil Engineering Dept. ISBN 9620202406. OCLC 127695500.{{cite book}}: CS1 maint: multiple names: authors list (link)
  14. ^ a b Wyss., Yim, Wai-shu, (1981). Permian corals in the Tolo Harbour formation, Ma Shi Chau, Hong Kong. Society of Economic Paleontologists and Mineralogists. OCLC 671683633.{{cite book}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
  15. ^ Lee, C. M.; Chan, K. W.; Ho, K. H. (August 2017). "香港的地层序列" [Stratigraphic sequence of Hong Kong]. Journal of Stratigraphy. 21 (3): 161–182 – via China Academic Journals Full-text Database.
  16. ^ Lam, K. C. (1973). "Upper Palaeozoic fossils of the Tolo Harbour formation, Ma Sze Chau, Hong Kong". Bulletin of the Hong Kong Geography Association. 3: 21–27.
  17. ^ Jones, N. S. (1996). "The geology and sedimentology of Upper Palaeozoic and Mesozoic sedimentary successions in Hong Kong". British Geological Servey Technical Report WH/96/130R. 48. British Geological Servey, U. K.
  18. ^ a b Li, Yan Hei Martin; Zhou, Mei-Fu; Lai, Kuen Wai; Chan, Lung Sang; Chen, Wei Terry (2014-01-17). "Geochemical and geochronological constraints on Late Jurassic volcanic rocks at Tuen Mun, Hong Kong, with implications for the Palaeo-Pacific subduction". International Geology Review. 56 (4): 408–429. doi:10.1080/00206814.2013.873355. ISSN 0020-6814.
  19. ^ CAMPBELL, S. D. G.; SEWELL, R. J. (1997-11). "Structural control and tectonic setting of Mesozoic volcanism in Hong Kong". Journal of the Geological Society. 154 (6): 1039–1052. doi:10.1144/gsjgs.154.6.1039. ISSN 0016-7649. {{cite journal}}: Check date values in: |date= (help)
  20. ^ a b Sewell, Roderick J.; Tang, Denise L. K.; Campbell, S. Diarmad G. (2012-01). "Volcanic-plutonic connections in a tilted nested caldera complex in Hong Kong". Geochemistry, Geophysics, Geosystems. 13 (1): n/a–n/a. doi:10.1029/2011gc003865. ISSN 1525-2027. {{cite journal}}: Check date values in: |date= (help)
  21. ^ ZHAO, Longlong; WANG, Lulin; TIAN, Mingzhong; WU, Fadong (2018-06). "The Geological Significance of a Tuffite Interlayer in the Cretaceous Port Island Formation in Northeastern Hong Kong". Acta Geologica Sinica - English Edition. 92 (3): 1041–1052. doi:10.1111/1755-6724.13590. ISSN 1000-9515. {{cite journal}}: Check date values in: |date= (help)
  22. ^ Wang, Lulin (7 February 2015). "Discussion on the sedimentary structure, geochemical characteristics and sedimentary environment of Ping Chau formation at Tung Ping Chau, Hong Kong". Journal of Environmental Biology. 36: 777–788.

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