The characteristics of the Geological structures of the Tertiary basins on the continental margin in the northern part of the South China Sea

The superimposed assemblages of structural lines trending northeast, east-northeast and west-northwest form the major structural framework of the continental margin in the northern South China Sea. The northeast structural trend represents Mesozoic basement trends, those in early Paleogene sedimentary layers. The east-northeast or west-northwest structural lines developed in post-Eocene-Oligocene sedimentary layers as a result of superimposition of northeast trends. The Tertiary sedimentary basins have been produced and developed in block structures formed by these structural trends. Tertiary deposits at the continental margin of the northern South China Sea reach a maximum thickness of 10 km. Lower Tertiary sequences are usually absent from the high points but are commonly present in the splintered segments, with continental deposits dominating. The Upper Tertiary section is well developed, and extensively overlaps the high points and the splintered segments, with marine deposits dominating. Tertiary sediments can be divided into four subcycles of Paleocene-Early Oligocene, Late Oligocene-Early Miocene, Mid-Miocene-Late Miocene, and Late Miocene-Pliocene ages. The early Pliocene transgression was the greatest since late Oligocene time. During Early-Late Miocene time, regression was most extensive. All or part of the Upper Miocene is absent in many basins. The known productive formations (Eocene Lower Oligocene, Upper Oligocene-Lower Miocene and Mid-Miocene) are all closely related to the middle stages of the sedimentary subcycles. Tertiary basins have different structural positions depending on basement features. Multisite-type faulting depressions are developed within the basin causing splintering. The basins are shaped like grabens and pan-type depressions, and had an initial stage of faulting, a middle stage of depression, and a late stage of overlapping and coverage by sediments. The development of normal faults within the basin, high heat flow, and the lack of compressive folding show that the basins were formed by extension. The development of basins was influenced by four plate movements during Cenozoic time. The Tertiary basins in the north of the South China Sea are related to the gradual disintegration of the northern continental margin or the South China Sea, the frequent movement of the plates around the South China Sea, and the spreading of the South China Sea Basin. Thus basins were formed with different structural attributes but similar sedimentary cycles.