Provenance Evolution Since Neogene In The Yinggehai And Qiongdongnan Basins

The Yinggehai and Qiongdongnan Basins, Cenozoic sedimentary basins in the northwestern South China Sea, are located at the conjunction of Eurasian, the Indio-Australian, the Philippine and the Pacific Plates. The basins preserve important sedimentary records of Cenozoic Tibetan Plateau uplift, adjacent plate tectonic activity and East Asian monsoon formation, thus their sediment accumulation and structural evolution have become current topics in the geological study of the South China Sea. Meanwhile, with the development of oil and gas exploration in recent years, a number of large-scale gas fields and hydrocarbon structures have been identified in the Yinggehai and Qiongdongnan Basins, indicating a good exploration prospects in this area.Provenance analysis, namely the research of source-sink system, is a major component of basin analysis, and its research content mainly include the location of source area, lithologies of source rocks, path of transportation and evolution process. As an independent research direction in the field of sedimentology, provenance analysis is widely used in various geological problems, which not only affords abundant information for the restoration of paleogeography, paleostructure and paleoclimate, but also is of significance to the research of sediment routing and reservoir evaluation. Although a series of analytical methods (e.g. seismology, geochemistry, mineralogy and isotopic geochronology) have been carried out in Yinggehai and Qiongdongnan Basins and surrounding source areas, there is still no clear understanding about provenance evolution. In this paper, on the basis of abundant research achievements and analytical data, an integrated analyses of rare earth elements, heavy mineral and zircon U-Pb dating were conducted to river sands from eastern Hainan Island and Neogene basin sediments. Consequently, the characteristics of several source areas were illustrated and the evolution model of source-sink system since Neogene was established. Also, certain factors (e.g. lithologies of source rocks. tectonic activities) that influence provenance evolution were discussed, providing geological bases for the hydrocarbon exploration. The main results of this thesis are shown as follows:1. This study was based on collection and utilization of previous research achievements, and supplementary samples were further analyzed, consequently indicating distinctions and discriminant characteristics of source rocks and sediments from Hainan Island, Red River and eastern Vietnam.1) The lithologies of Hainan Island are dominated by the Hereynian-Indosinian and the Yanshanian granitoids, with locally Paleozoic and Mesozoic sedimentary rocks exposed. The content of source rocks’rare earth element show substantial fluctuation, with the enrichment of light rare earth element. The U-Pb age distributions of detrital zircons from river sands share a certain degree of similarity, which mainly reflects two phase intrusive activities with peaks around100Ma and250Ma respectively. By contrast, great differences exist in heavy mineral assemblages of eastern and western Hainan Island, showing diverse contributions from granitoids and metasedimentary rocks to rivers.2) The source rocks of eastern Vietnam are mainly Paleozoic sedimentary rocks, and partly acid igneous rocks. The U-Pb ages of detrital zircons from river sands largely consist of the Indosinian period (250Ma) and a small amount of the Caledonian period (360-620Ma). However, heavy mineral assemblages are relatively various, with acid igneous rocks-indicating group (e.g. zircon, rutile and apatite) and high-grade metamorphic rocks-indicating group (e.g. garnet and epidote) co-existing.3) The lithological compositions of along Red River drainage area are rather complex, but the group of garnet, epidote and hornblende suggests large contribution of high-grade metamorphic rocks to river sediment. Detrital zircons display a wide range of U-Pb ages, which feature population dated at660-820Ma,26-35Ma and sporadic1600-2000Ma except the Indosinian period and the Yanshanian period.2. Based on massive amount of evidence from geochemistry, mineralogy and zircon geochronology of borehole cores and cuttings, the characteristics of provenance evolution since Neogene in the Yinggehai and Qiongdongnan Basins were illustrated.1) During Early Miocene, Hainan Island accounted for major sedimentary contribution to the basins, which was exemplified by high rare earth element content and the heavy mineral group of zircon, tourmaline, hematite, limonite and ilmenite of offshore sediments. Its influence areas in Qiongdongnan Basin could extend to the Central Depression, whereas provenance characteristics of Red River and eastern Vietnam were not clear due to scarce analytical data. In addition, local source might exist in the basins, such as metamorphic provenance at Shenhu Uplift and mafic volcanism around the Songnan-Baodao Depression.2) When it came to Middle Miocene, on the basis of previous source-sink system, provenance of Red River was reflected from sediments of Lingao Low Uplift and northern Yingdong Slope, which featured relatively low content of rare earth element and characteristic mineral group of staurolite and garnet. Despite the lack of analytical data from the southwestern Yinggehai Basin, the results of zircon U-Pb dating at the conjunction of Yinggehai and Qiongdongnan Basins revealed that this area was not controlled by provenance of Red River and eastern Vietnam.3) In the Late Miocene, sediment supply from Vietnamese Truong Son Uplift considerably strengthened, resulting in low content of rare earth element, zircon and very high content of ilmenite at the Dongfang area of Yinggehai Basin. Also, integrated analyses of rare earth element, heavy mineral and zircon U-Pb ages drew the consistent conclusion that sediments at the Central Depression of Yinggehai Basin could migrate to the deep-water area of Qiongdongnan Basin through canyon and channel. Meanwhile, the source of sediments at Shenhu Uplift was more complex and probably affected by local mafic volcanism.4) Since Pliocene, local provenance in the basins had been disappeared. Provenance contribution from Vietnam, especially Red River, further reinforced, leading to extensive distribution of epidote and staurolite in the sediments of whole basins.3. According to the basic principle of sedimentology and geological setting, major controlling factors of provenance evolution in the Yinggehai and Qiongdongnan Basins since Neogene were discussed.1) The source rock lithology was the primary factor that controlled the provenance evolution. The lithological diversities among Hainan Island, Red River and eastern Vietnam leaded to zone distributions of sediments composition. For example, provenance of Hainan Island, dominated by acid igneous rocks, continuously controlled sediment supply of offshore shelf, whereas provenance of Red River, dominated by high-grade metamorphic rocks, mainly influenced deposition of the northern Yinggehai Basin.2) The impact of tectonic activities on provenance evolution was mainly manifested by intense structural inversion of the northern Yinggehai Basin during Late Miocene. Consequently, the extension of Red River provenance in the basins was limited by Lingao Low Uplift, and the Central Depression of Yinggehai Basin mainly accepted sediments from eastern Vietnam.3) Although Cenozoic volcanism was very limited in time and space in the northwestern South China Sea, magmatism intensified in the southeastern part of Indochina during Late Miocene. Subsequently extensive uplift and accelerated denudation affected the local climate and environment, which together augmented offshore sediment supply from eastern Vietnam. Besides, Shenhu Uplift and Songnan-Baodao Depression of the Qiongdongnan Basin might be affected by local volcanism.4) The modification of diagenesis to sediment composition was also evident. For example, mica and hornblende, abundant in modern sediments from source areas though, generally displayed rather low content in the basins, which resulted in relative enrichment of stable minerals (e.g. zircon, tourmaline and ilmenite).