The Evolution Of Sedimentary Paleoenvironment And Provenance In The Deepwater Area Of The Qiongdongnan Basin

The deepwater area of the Qiongdongnan basin (QDNB), which is located in the continental margin of the northwestern South China Sea, is a typical Cenozoic rifted continental marginal basin. The basin, with lots of sags and great sedimentary thickness, has experienced several tectonic evolution stages, such as faulting, depressing and post-rifting subsidence, has received abundant organic-rich terrigenous sediments in the evolutionary process of the South China Sea, and is one of the main objectives for the oil and gas exploration in the deepwater area of the northern South China Sea.At present, the multi-discipline study on sedimentary paleoenvironment and provenance based on micropaleontology, heavy mineralogy, geochemistry and geophysics has become an important part of oil and gas exploration and marine geology research. In this paper, oil and gas drillings LS33a and LS22a located in the deepwater area of the QDNB are selected as main study objects and systematic analyses of drilling cuttings from the respects of micropalaeontology, heavy mineralogy and element geochemistry are performed. Based on the analysed results, combined with seismic sections and grain size data, supplemented by heavy mineral data from another nine drillings in the shallowwater area, the evolution of sedimentary paleoenvironment and provenance in the deepwater area of the QDNB is synthetically analysed and discussed, which is expected to be rewarding for the sedimentary evolution research and oil and gas exploration in the deepwater area of the northern South China Sea. The main results are as follows:1. According to the environmental information inherent in fossil assemblages of benthic foraminiferas and calcareous nannonfossils, combined with the heavy mineral and grain size data, the evolutionary history of sedimentary paleoenvironment since the Oligocene in the study area is reconstructed.The study area began to undergo transgressions during early Oligocene and was fully affected by transgressions to late Oligocene, with a generally increasing tendency in water depth. The sedimentary environment evolved roughly from terrestrial-marine transitional to littoral-inner neritic (Yacheng Formation, Early Oligocene)â†'outer neritic to upper-middle-lower bathyal (Lingshui Formation, Late Oligocene)â†'middle-lower-middle bathyal (Sanya Formation. Early Miocene)â†' middle-lower bathyal (Meishan Formation, Middle Miocene)â†'bathyal-abysmal transitional (Huangliu Formation, Late Miocene)â†'bathyal (water depth>800m, Yinggehai Formation, Pliocene)â†'bathyal to outer neritic (Ledong Formation, Quaternary). During this period, the bathyal environment first appeared during the middle Late Oligocene and stabilized during the late Late Oligocene.2. Based on the heavy mineral, element geochemical and seismic data, the sediment provenance and its evolutionary history are analyzed, and possible influences of tectonic events are discussed.Since the Oligocene, with changes in sedimentary environment, provenances of the strata (from the Yacheng Formation to the Yinggehai Formation) showed principal characteristics of multi-sources, evolving from autochthonous source, short source to distant source step by step. During the Early Oligocene, the sediments were mainly proximal basaltic pyroclastic source and adjacent terrigenous clastic source, afterwards were becoming distant terrigenous clastic sources, including Hainan Uplift on the north, Yongle Uplift on the south, Shenhu Uplift on the northeast, the Red River System on the northwest and Indochina Peninsula on the southwest and so on. The Hainan Island provenance began to develop during the Early Oligocene and has become a main provenance in the QDNB since the Middle Miocene. The provenances from Yongle Uplift and Shenhu Uplift most developed from the Late Oligocene to the Early Miocene and gradually subsided during the Middle Miocene. During the Late Miocene, the Red River System provenance added to the central canyon area in the QDNB massively, whose impact terminated at the end of the Pliocene. The western Yinggehai basin provenance derived from Indochina Peninsula had developed from the Pliocene on to the Pleistocene.At the depth of 4207m (aged 33.9-31.5Ma) and dating 31.5Ma,28.4Ma,25.5Ma, 23Ma and 16Ma, there exist abrupt changes of element geochemical parameters, reflecting the impacts of tectonic events, including early expansion movement of the South China Sea (34-25Ma BP), Baiyun movement (23Ma BP.) and late expansion movement of the South China Sea (23.5～16.5Ma BP.), and so on, on the sedimentary evolution (including provenance and sedimentary environment) of the QDNB.3. Combining with the seismic, element geochemical, lithological, heavy mineral and paleontological data, the sedimentary system of the central canyon is studied to explore the development history of the western part of the canyon and the evolution of provenance and sedimentary paleoenvironment of the filled sediments.The western part of the central canyon was developed from the early Late Miocene (11.6Ma BP.) to the early Pliocene (4.2Ma BP.) and was most developed during the Late Miocene (11.6～5.5Ma BP). The fillings of the canyon have sedimentary characteristics of multi-phases and multi-sources. During the late Middle Miocene (13.4-11.6Ma BP.), namely before the formation of the canyon, the sediments in the canyon were dominated by the Hainan Island silty provenance. During the late Miocene (11.6～5.5Ma BP.), namely at the main developmental phase of the canyon, the Red River sandy provenance began to develop and gradually became predominant, which was a major contribution to the sandy sediments in the canyon, while the contribution of the Hainan Island provenance decreased correspondingly. The massively adding of the Red River materials was probably influenced by the large-scale uplift of the Tibetan Plateau and the massive regression in the QDNB caused by the D.ongsha movement. During the early Pliocene (5.5-4.2Ma BP.). namely at the last developmental phase of the canyon, the influence of the Red River provenance gradually weakened while the Hainan Island provenance returned to be dominated. Materials form Hainan Island and Red River might be respectively the weathering products of intermediate-acidic intrusive rocks in Hainan Island and uplifted materials in Tibetan Plateau. Before and after the formation of the canyon, the region around the central canyon was in a stable bathyal sedimentary environment.