南海北部IODP U1499站位砾石层的古沉积环境及构造意义
Discussion on the Palaeo-Sedimentary Environment and Tectonic Significance of Gravel Layers at IODP Site U1499 in Northern South China Sea

南海北部IODP 367航次深海钻遇砾石层，因前期地球物理资料预测不足，导致出现多种成因猜测。利用南海北部大量高精度地震、油气钻井和IODP资料，从构造地层学、层序地层学和沉积充填动力学等角度，恢复研究区砾石层沉积时的古地貌特征。研究结果表明，砾石层形成于晚始新世－早渐新世，发育于小型断陷陆相湖盆周缘，以近源三角洲相沉积为主，其物源主要来自湖盆周缘的东沙和礼乐中生代地层剥蚀区；砾石层的顶界面对应于T70与T60重合面，其中T70为南海北部海底渐进式扩张形成的破裂不整合面，年龄介于33.0~28.4 Ma，具有东早西晚的特征。此外，认为南海北部洋陆转换带的位置并非目前认为的3 000 m水深附近，而应该是水深更深的一系列小型箕状断陷的南部边缘。
Gravel layers were drilled at IODP 367 Site U1499 in northern South China Sea. A variety of possible genetic mechanism for gravel layers, which are incorrect or uncertain predictions from observations in geophysical data in advance, have been proposed through the lithostratigraphic units in the cored sediment at site U1499. Using tectonostratigraphy and sediment-filling dynamics, this study restores the palaeo-geomorphology during the gravel layers sedimentation period based on a high-resolution seismic database and IODP site data from the northern margin of the South China Sea. The results indicate that the gravel layers should be deposited in proximal delta sedimentation environment along Late Eocene-Early Oligocene lacustrine basin peripheral, whose provenance is mainly from Mesozoic erosion areas of Dongsha and Liyue. The top of the gravel layers corresponds to a coincidence boundary of T70 and T60 in seismic profiles. T70 unconformity is likely directly related to the continental breakup during the initial opening of the northern South China Sea. T70 boundary age is estimated to be at 33.0–228.4 Ma and is earlier in the east than that of the west. Besides, this paper suggests that the position of ocean-continent transition zone may be near the southern edge of a series of small extensional half-grabens in the northern South China Sea rather than a geomorphic abrupt domain with a water depth of 3 000 m