Editing Spratly Islands (section)

== Geology ==
The Spratly Islands consist of islands, reefs, banks and shoals made up of biogenic [[Carbonate rock|carbonate]]. These accumulations of biogenic carbonate lie upon the higher crests of major submarine ridges that are uplifted [[fault block]]s known by geologists as [[Horst (geology)|horsts]].<ref>{{cite book|url=https://books.google.com/books?id=XrICEAAAQBAJ|title=Marine Geology and Geotechnology of the South China Sea and Taiwan Strait|last=Chaney|first=Ronald C.publisher=CRC Press|year=2020|page=[https://books.google.com/books?id=XrICEAAAQBAJ&pg=PA20 20]|publisher=CRC Press |isbn=978-1-000-22217-3}}</ref> These horsts are part of a series of half-grabens and rotated fault-blocks which lie parallel and en echelon. The long axes of the horsts, rotated fault blocks and half-[[grabens]] form well-defined linear trends that lie parallel to [[Magnetic anomaly|magnetic anomalies]] exhibited by the [[oceanic crust]] of the adjacent South China Sea. The horsts, rotated fault blocks, and the rock forming the bottoms of associated grabens consist of stretched and subsided [[continental crust]] that is composed of [[Triassic]], [[Jurassic]], and [[Cretaceous]] [[strata]] that include [[Calc-alkaline magma series|calc-alkalic]] [[extrusive]] [[igneous]] rocks, intermediate to acid [[Intrusion|intrusive]] igneous rocks, [[sandstone]]s, [[siltstone]]s, dark-green [[claystone]]s, and [[metamorphic rocks]] that include [[biotite]]–[[muscovite]]–[[feldspar]]–[[quartz]] [[migmatites]] and [[garnet]]–[[mica]] [[schists]].<ref name="Hutchison2010a">Hutchison, C. S., and V. R. Vijayan, 2010, ''What are the Spratly Islands?'' Journal of Asian Earth Science. vol. 39, no. 5, pp. 371–385.</ref><ref name="Wei2011a">Wei-Weil, D., and L, Jia-Biao, 2011, ''Seismic Stratigraphy, Tectonic Structure and Extension Factors Across the Dangerous Grounds: Evidence from Two Regional Multi-Channel Seismic Profiles.'' Chinese Journal of Geophysics. vol. 54, no. 6, pp. 921–941.</ref><ref name="Zhen2013a">Zhen, S., Z. Zhong-Xian, L. Jia-Biao, Z. Di, and W. Zhang-Wen, 2013, ''Tectonic Analysis of the Breakup and Collision Unconformities in the Nansha Block.'' Chinese Journal of Geophysics. vol. 54, no. 6, pp. 1069–1083.</ref>

The dismemberment and subsidence of continental crust into horsts, rotated fault blocks and half-grabens that underlie the Spratly Islands and surrounding sea bottom occurred in two distinct periods. They occurred as the result of the tectonic stretching of continental crust along underlying deeply rooted detachment faults. During the Late Cretaceous and Early [[Oligocene]], the earliest period of tectonic stretching of continental crust and formation of horsts, half-grabens, and rotated fault-blocks occurred in association with the rifting and later sea-floor spreading that created the South China Sea. During the Late Oligocene-Early [[Miocene]] additional stretching and block faulting of continental crust occurred within the Spratly Islands and adjacent Dangerous Ground. During and after this period of tectonic activity, corals and other marine life colonised the crests of the horsts and other ridges that lay in shallow water. The remains of these organisms accumulated over time as biogenic carbonates that comprise the current day reefs, shoals and cays of the Spratly Islands. Starting with their formation in Late Cretaceous, fine-grained organic-rich marine sediments accumulated within the numerous submarine half-grabens that underlie sea bottom within the Dangerous Ground region.<ref name="Hutchison2010a" /><ref name="Wei2011a" /><ref name="Zhen2013a" />

The geological surveys show localised areas within the Spratly Islands region are favourable for the accumulation of economic oil and gas reserves.<ref name="Owen2012a">Owen, N. A. and C. H. Schofield, 2012, ''Disputed South China Sea hydrocarbons in perspective.'' Marine Policy. vol. 36, no. 3, pp. 809–822.</ref><ref name="BBC 2011-06-13">{{cite news|url=https://www.bbc.com/news/world-asia-pacific-13748349?piano-modal|title=Why is the South China Sea contentious?|date=July 12, 2016|publisher=BBC News}}</ref><ref>{{cite news |title=Q&A: South China Sea dispute |publisher=BBC News |date=13 June 2011 |url=https://www.bbc.co.uk/news/world-asia-pacific-13748349 |access-date=30 October 2013 |archive-url=https://web.archive.org/web/20131017162612/http://www.bbc.co.uk/news/world-asia-pacific-13748349 |archive-date=17 October 2013 |url-status=live}}</ref> They include thick sequences of [[Cenozoic]] sediments east of the Spratly Islands. Southeast and west of them, there also exist thick accumulations of sediments that possibly might contain economic oil and gas reserves, which lie closer to the Spratly Islands.<ref name="Owen2012a" /><ref name="Blanche1997a">Blanche, J. B. and J. D. Blanche, 1997, ''An Overview of the Hydrocarbon Potential of the Spratly Islands Archipelago and its Implications for Regional Development.'' in A. J. Fraser, S. J. Matthews, and R. W. Murphy, eds., pp. 293–310, Petroleum Geology of South East Asia. Special Publication no. 126, The Geological Society, Bath, England 436 pp.</ref>