Characteristics of S40 Boundary and Its Significance in Qiongdongnan Basin, Northern Continental Margin of South China Sea
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摘要: S40界面是琼东南盆地新近系裂后期的重要地质界面, 发育时代为晚中新世末期(约11.6Ma), 是裂后期充填序列中梅山组和黄流组的分界面.通过对琼东南盆地大量二维测线的精细解释, 结合钻井、生物等资料, 从海平面变化、地震-沉积相跃迁、沉降速率等方面总结了该界面的特征, 指出S40界面在盆地东西部特征差异明显: 在盆地西北部浅水区, 该界面具有明显的角度不整合特征, “下削上超”特征明显, 在局部地区梅山组一段整套地层都被削蚀; 在盆地东南部半深水-深水区域, S40界面上下反射具明显差异, 界面之上为一组中等连续性、中-强振幅的地震反射同相轴, 界面之下地震反射多呈强振幅、中等连续-丘状反射, 甚至局部出现前积反射.研究认为, S40界面的东西差异性可能与该界面在盆地东西部受到的控制因素差异相关: 盆地西部主要受区域性海平面大规模下降的影响, S40界面表现出对下伏地层的强烈削蚀; 盆地东部则可能是西沙海槽在晚中新世以来的加速沉降在琼东南盆地的具体表征.受这次区域性构造活动的影响, 盆地东部水体突然加深, 接受大规模的沉降, 界面上下表现为相的突变和跃迁, 此时盆地已经具有“东深西浅”的地貌特征, 这种结构一直延续至今.S40界面的性质厘定及其地质意义的重新认识有可能为中国南海新生代盆地形成演化及其动力学背景研究提供新的信息.Abstract: Based on the integrated study of 2D seismic data, well data and biostratigraphy in the Qiongdongnan basin (QDNB), the Late Miocene unconformity (S40) at about 11.6 Ma which separated the underlying Meishan and overlying Huangliu Formation is interpreted.The characters of S40 including sea level changes, seismic-sedimentary facies and subsidence rate show significant differences in its eastern and western areas as follows: (1) In the littoral-neritic area of the northwestern QDNB, it is angular with intense erosion of the underlying strata, and even erosion of the whole Meishan Formation. (2) In the bathyal-abyssal area of southeastern QDNB, Meishan Formation is characterized by higher amplitude, generally continuous-hummocky and even progradational reflection, whereas Huangliu Formation is featured with low-amplitude, continuous facies.We suggest that the S40 interface displaying different characteristics was controlled by different mechanism factors, the western basin was strongly eroded by the underlying strata under the influence of massive sea-level fall; whereas the eastern basin with great subsidence occurred due to the Xisha trough activity since Late Miocene.This tectonic activity caused the eastern QDNB to change abruptly from shallow water to semi-deep or deep water environment, consequently the basin exhibits deep and shallow landforms in the east and west respectively.This topography remains until the present day.The interpretation of S40 is helpful not only to explain the South China Sea evolution but also to predict potential reservoir target.
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Key words:
- Qiongdongnan basin /
- S40 boundary /
- tectonics /
- subsidence /
- Miocene /
- marine geology
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图 1 琼东南盆地区域位置及构造划分(黑色粗实线为剖面线)
Fig. 1. The location and structural units of Qiongdongnan basin
图 3 ODP1148站位20 Ma以来元素含量比、沉积速率、粒度及有孔虫碳氧同位素分布特征
其中全球海平面变化来自Haq et al., 1987;同位素资料源自赵泉鸿等, 2001;其余资料源自邵磊等, 2004
Fig. 3. The characteristics of elements rates, sedimentary rate, grain size and oxygen-carbon isotopes in ODP Site 1148
图 4 琼东南盆地西部(a) 和东部(b) S40界面地震反射特征(剖面位置见图 1)
Fig. 4. The seismic reflection character of boundary S40 in western (a) and eastern (b) of Qiongdongnan basin
图 5 琼东南盆地东部松南-宝岛凹陷裂后期沉降特征(剖面位置见图 1)
Fig. 5. The characteristics of post-rift subsidence in Songnan-Baodao depression in eastern of Qiongdongnan basin
图 6 琼东南盆地东部长昌凹陷裂后期沉降特征(剖面位置见图 1)
Fig. 6. The characteristics of post-rift subsidence in Changchang depression in eastern of Qiongdongnan basin
图 7 琼东南盆地东部单井S40界面特征(井位见图 1)
Fig. 7. The characteristic of boundary S40 in boreholes in the eastern of Qiongdongnan basin
图 8 琼东南盆地西部S40界面与储层分布之间的关系(剖面位置见图 1)
Fig. 8. The relationship between reservoirs and S40 boundary in western of Qiongdongnan basin
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