area, are: The total area of the archipelago's naturally occurring islands is and with reclaimed land. Geology The Spratly Islands consist of islands, reefs, banks and shoals made up of biogenic carbonate. These accumulations of biogenic carbonate lie upon the higher crests of major submarine ridges that are uplifted fault blocks known by geologists as horsts. These horsts are part of a series of parallel and en echelon, half-grabens and rotated fault-blocks. The long axes of the horsts, rotated fault blocks and half-grabens form well-defined linear trends that lie parallel to 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-alkalic extrusive igneous rocks, intermediate to acid intrusive igneous rocks, sandstones, siltstones, dark-green claystones, and metamorphic rocks that include biotite–muscovite–feldspar–quartz migmatites and garnet–mica schists. 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. The geological surveys show localised areas within the Spratly Islands region are favourable for the accumulation of economic oil and gas reserves. 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