THE TRACE ELEMENTS GEOCHEMISTRY AND DEPOSITIONAL ENVIRONMENT CHANGES RECORDED IN THE CORE OF WELL LS33-1-1 IN DEEPWATER AREA OF QIONGDONGNAN BASIN

In order to reveal the sedimentary environment and provenance of the deepwater deposits in the Qiongdongnan Basin(QDNB), trace elements geochemistry of the core samples of Well LS33-1-1 in the deepwater area of QDNB is studied in this paper. Since Oligocene, the study area has been suffered severe changes in depositional environment, and the provenance of sediments was complicated. In the early stage of the Third Member of the Yacheng Formation, the sediments were mainly from basaltic pyroclastic rocks, which might be the contribution from magmatic activity resulted from the expansion of the South China Sea. Since the late stage of the Third Member of the Yacheng Formation, however, the sediments were mainly from terrigenous and marine deposits,with a considerable proportion of weathering product of volcanic rocks. The trace elements geochemistry of the study area shows drastic variation at 31.5 MaBP, 28.4 MaBP, 25.5 MaBP, 23 MaBP, 16 MaBP, 8.2 MaBP, 5.5 MaBP, 2.7 MaBP respectively, indicating the significant changes in sedimentary environment and provenance, as the results of the tectonic movement. The change in geochemical indicators at the bottom (about 4 207 m) of the Third Member of the Yacheng Formation suggests a strong tectonic movement in QDNB, leading to the transition of sediment provenance from the basic pyroclastic rocks into the terrigenous deposits. Close to the Oligocene-Miocene boundary (23 MaBP), all indices show significant variation, indicating that the Baiyun Movement, whicvh was discovered in the ODP1148 Station and the Pearl River Mouth Basin, also affected the QDNB deepwater area.