Sequence Architecture, Depositional Evolution and Controlling Processes of Continental Slope in Pearl River Mouth Basin, Northern South China Sea
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摘要: 南海盆地是东南亚陆缘最大的、含有丰富油气等资源的边缘海盆地.对于南海大陆斜坡带的发育、沉积演化与南海盆地构造作用及动力学过程的响应关系等方面缺乏深入认识.依据地震、测井及岩心等丰富资料，对南海珠江口盆地东南部陆架边缘斜坡带的层序地层、沉积-地貌演化及其对构造、海平面和沉积物供应变化的响应关系开展了系统性的研究.研究表明盆地的沉积充填可划分为由区域性不整合所限定的7个复合（二级）层序（CS1-CS7）.复合层序CS3-CS7（上渐新统-第四系）均由区域性的海侵-海退旋回构成；其内可进一步划分出由局部不整合或水退-水进的转换面为界的20个次级层序（三级）.研究识别出包括外陆架至陆架边缘三角洲、前三角洲-斜坡扇、陆架边缘前积体、单向迁移的横向底流-斜坡重力流复合水道、大型斜坡下切峡谷、泥质斜坡扇、斜坡滑塌泥石流复合体以及大规模软沉积物变形体等沉积体系或沉积复合体，它们在不同层序具有特定的时空分布，构成多种沉积样式.短周期（三级）的沉积旋回变化与Haq的海平面变化曲线总体上可对比，但长周期的海侵和海退则明显不同，受到了构造隆升和沉降等的控制.陆架边缘沉积演化可划分出裂后早期海底扩张沉积（破裂层序）、裂后晚期海底扩张沉积、后海底扩张等构造-沉积演化阶段.裂谷作用晚期的热隆起、构造差异沉降、裂后热衰减沉降以及上新世以后的东侧碰撞等对主要不整合的形成和海侵-海退产生了重要的影响.晚渐新世至中中新世发育的复合层序（CS3和CS4）记录了裂后海底扩张到停止的大陆斜坡沉积过程；而裂后早期的沉积层序（CS3）为破裂层序，以发育大型的陆架边缘三角洲-前三角洲斜坡扇体系构成的前积层为特征.气候变化和季风加强可能增强了晚渐新世-早中新世和更新世沉积期的沉积物供应，为大规模陆架边缘三角洲体系的发育提供了充足的物源供给.发育于陆架边缘的三角洲-滨岸碎屑体系和共生的前三角洲斜坡扇体系构成区内最重要的油气勘探对象.Abstract: The South China Sea (SCS) Basin is the largest marginal sea basin with abundant petroleum resources on the continental shelf of Southeast Asia. However, there is a lack of research on the relationship between the development and sedimentary evolution of South China Sea continental slope belt and the tectonics and dynamic processes of the South China Sea Basin. Based on integrated analyses of seismic, well-logging and core data, the sequence architecture, depositional-geomorphological evolution and controlling processes of the continental slope in the Pearl River Mouth Basin of the northern SCS have been systematically documented. The sedimentary infill of the marginal sea basin can be divided into seven composite sequences (CS1-CS7) that are bounded by regional unconformities. Each of the composite sequences CS3 to CS7 (Upper Oligocene to Quaternary) comprises of generally a regional transgressive-regressive cycle. The CS3 to CS7 can be further divided into 20 sequences that are defined by local unconformities and correlative conformities. Depositional systems recognized in the continental slope deposits mainly include outer shelf to shelf-edge deltas, prodelta-slope fans, shelf-margin slope clinoforms, unidirectionally-migrating bottom current-gravity flow composite slope channels, large-scale incised slope valleys, muddy slope fans, slope slump-debris-flow complexes and large-scale soft-sediment deformed beds. They were arranged with distinguishing patterns in different sequences, forming various styles of depositional architectures. The study shows that the short-term sea level changes (sequences) are generally comparable with those of the Haq global sea level curve, whereas long-term sea level changes (composite sequences) were apparently controlled or enhanced by tectonic uplift and subsidence. The depositional evolution of the continental shelf margin can be divided into (1) the early post-rift seafloor spreading (breakup sequence), (2) the late post-rift seafloor spreading, and (3) the post-seafloor spreading tectonic-depositional stages. The thermal uplift, tectonic differential subsidence, post-rift thermal subsidence and the eastern collision after Pliocene are all regarded to have played an important role in the formation of major unconformities and regional transgressive-regressive cycles. The composite sequences (CS3 and CS4) developed during the Late Oligocene to Middle Miocene recorded the depositional successions of the continental slope formed from the beginning to the stopping of the seafloor spreading. The composite sequence (CS3) deposited during early post-rift stage is regarded as a breakup sequences. It is characterized by the development of large-scale continental shelf edge delta and prodelta slope fan systems. Climate change and monsoon strengthening may have enhanced the sediment supply during the Late Oligocene to Early Miocene and the Pleistocene, which may be responsible for the sufficient sediment supply for the development of large-scale continental shelf marginal delta systems during these periods. The shelf margin delta-shoreline clastic systems and the associated slope fan systems on the continental margin have proven to be the most important targets for oil and gas exploration.
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图 1 南海北部大陆边缘珠江口盆地构造单元划分
Fig. 1. Tectonic unit division of the Pearl River Mouth Basin on the northern continental margin of the South China Sea
图 2 珠江口盆地北部陆架边缘斜坡新生代沉积演化、层序划分及构造演化
Fig. 2. Depositional evolution, sequence classification and tectonic evolution of the Cenozoic shelf edge-slope in the northern Pearl River Mouth Basin
图 3 横跨研究区东南部的地震剖面显示珠海组(CS3-3和CS3-4)进积到陆架边缘的三角洲体系和前三角洲斜坡扇沉积(a)和珠江组下部(CS4-1)三角洲体系均方根振幅切片(b)
Fig. 3. Interpreted seismic profile across the southeastern part of the study area showing the shelf-edge delta and prodelta slope fan deposits in the Zhuhai Formation(a), and seismic root mean square amplitude slice of a deltaic system in the Lower Zhujiang Formation (b)
图 4 地震剖面显示粤海组至第四系(CS6-CS7)的单向迁移斜坡水道复合体和陆架边缘三角洲体系的层序和沉积结构特征(a)，复合层序CS6中的单向迁移斜坡水道复合体和CS5内的斜坡扇体系(b)及斜坡扇体系均方根振幅切片(c)
USSC.单向迁移斜坡复合体; ASP.沿斜坡底流前积充填; SL.滑塌堆积体; GFF.水道底浊积或泥石流充填; LSF.下斜坡泥质扇沉积; SCF.斜坡峡谷充填；IC.峡谷斜坡沉积; SLF.下斜坡扇沉积; SED.陆架边缘三角洲；SCC.单向迁移斜坡水道复合体; HST.高位体系域；LST.低位体系域；FST.下降体系域；TST.海侵体系域；MSF.最大海泛面；SF.斜坡扇
Fig. 4. Interpreted seismic profile showing the sequence and depositional architecture of unidirectionally migrating slope-channel complexes and shelf-edge delta systems of CS6 and CS7 (a); unidirectionally migrating slope-channel complexes of CS6 and slope fan systems of CS5 (b); and slice of root mean square amplitude of slope fan systems (c)
图 5 通过回剥技术恢复的大陆斜坡带从渐新世到第四纪的沉降曲线(a)和沉降速率(b)
Fig. 5. Subsidence curve (a) and subsidence rate (b) of the continental slope reconstructed by backstripping techniques from the Oligocene to Quaternary
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