南海西北次海盆构造演化的沉积响应

Sedimentary Response to Tectonic Evolution of the Northwest Sub-basin,South China Sea

通过对南海西北次海盆新获得的地震资料进行综合解释和层序地层分析,揭示了海盆中的沉积对构造演化阶段的响应。始新世-早渐新世陆缘裂陷期,盆地以对称裂谷形式,发育地堑裂谷层序,沉积以近物源为特征,相变大,发育了冲积扇-扇三角洲-湖相沉积,沉积体系的配置受同沉积断裂控制明显,快速沉降和充分的物源供给决定了沉积体系的构成特征。晚渐新世海底扩张期,岩石圈破裂,陆缘进一步拉开并开始海底扩张,出现海相沉积,来自陆坡的陆架边缘三角洲越过陆坡进入海盆,在海盆内沉积了一套向海盆中部逐渐减薄的楔状地层,并伴有大量的火山碎屑沉积物。早-中新世以来热沉降期,随着构造沉降增大,相对海平面总体不断上升,进入深水盆地,形成陆架陆坡体系,大量的碎屑物质以重力流、深水底流等深水作用方式进入海盆;沉降晚期陆架-陆坡物源供应减弱,琼东南中央峡谷成为其主要的物质供应来源通道,在此期间二次海平面下降、回升的综合作用下,海盆内发育了多期以下切水道为特征的低水位域沉积体系。

The evolution of basin filling sequence is the response to the basin dynamic processes. The sedimentary response to tectonic evolution of the Northwest Sub-basin in South China Sea was revealed by comprehensive interpreting of the seismic data and analyzing of the sequence stratigraphy. During the continental rifting stage from Eocene to Early Oligocene, the continental margin experienced sysmmetrical rifting and accepted garben sequence. The sedimentary filling was distinguished by proximal provenance and the large change of the sedimentary facies. The depositional system was dominated by the major syn-depositional faults. The rate of subsidence and the supply of provenance determine the composition of sedimentary system. During the Late Oligocene, the continental margin was stretched further and sea-floor spreading began, and consequently marine facies sedimentation prevailed. The shelf-margin delta crossed the slope from the continental shelf into the basin. It developed a sphenoid marine strata with lots of pyroclastic sediments, while the thickness of the sediments gradually decreased toward the center of the ocean basin. At thermal subsidence stage since Early Miocene, the ocean basin changed to deep-water sedimentary basin. Under the background of the increasing of the tectonic subsidence and the rising of the relative sea-level, the north shelf-slope was still the main source of the provenance, detrital materials were transit into the basin by the deepwater mechanism such as gravity flow and deep-water bottom current and finally deposited in the basin. In the late stage of thermal subsidence, the capacity of provenance supply reduced gradually from the slope direction, and the Qiongdongnan central canyon provenance systems become the main source, the sea-level drop and rise resulted in multi-stage lowstand systems tract deposits with characteristics of incised valley.