南海琼东南盆地西区晚更新世陆架边缘层序结构及差异机制

Variability in the Shelf-edge Sequence Architecture and Its Controlling Factors in the Western Part of the Late-Pleistocene Qiongdongnan Basin,South China Sea

层序地层学前期研究注重顺物源方向的二维剖面解释,垂直物源方向的层序结构差异性是当前研究的热点和难点。以南海琼东南盆地西区晚更新世(0.125 Ma至今)陆架边缘地层序列为例,通过典型地层终止关系、地层叠置样式和陆架边缘迁移轨迹方法,识别了体系域单元内部结构和组合特征,自下而上划分为低位体系域、海侵体系域、高位体系域和下降体系域。其中下降体系域内部分界面可将下降体系域分为早、晚两期,该界面对应陆架边缘迁移轨迹角由正到负和地层叠置样式由进积到降积的转换面。琼东南盆地西区上更新统发育稳定型和滑塌型两类陆架边缘层序结构,随着相对海平面的变化,稳定型陆架边缘主要发育多期次陆架边缘前积体和深水扇沉积,滑塌型陆架边缘则主要发育大规模峡谷和块体搬运沉积体系;响应于“快速海侵而缓慢下降”的不对称海平面升降特征,晚更新世陆架边缘层序表现为较薄或者不发育的低位—海侵体系域,厚的强制海退楔单元,而外陆架区活动性断裂增加了高位体系域组分在层序中的占比。先存的坡折地形、断裂活动及非对称海平面升降旋回差异共同导致了研究区层序结构多样性体系,高频层序地层驱动机制的定量化探索是更新世层序地层学未来的发展趋势,为层序地层学标准化实践和侧向层序结构差异研究提供了一定的参考。

Previous sequence stratigraphy research has mainly focused on two-dimensional seismic interpretation in the dipping direction,while variations in sequence architecture in the vertical provenance direction are the focus and difficulty of current research.This study considers the Late Pleistocene(0.125 Ma to the present)shelf-margin stratigraphic successions of the Qiongdongnan Basin as a typical example.The internal structure and combination characteristics of the system tract units were established and identified according to typical stratal termination,stratal stacking patterns,and shelf-edge migration trajectories.The systems tracts in the studied sequence were classified from bottom to top,including the Lowstand Systems Tract(LST),Transgressive Systems Tract(TST),Highstand Systems Tract(HST),and Falling-Stage Systems Tract(FSST).The surface of Within Systems Tract Surface(WSTS)divides the FSST into early and late phases.The WSTS interface is the transitional surface for the positive-to-negative angle of the migration trajectory of the shelf edge and for the stratal stacking transition from progradation to degradation.Stabilized and collapsed shelf-edge sequence architectures developed during the Upper Pleistocene in the western part of the eastern Qiongdongnan Basin.With the change in relative sea level,the stabilized shelf edge mainly developed multi-phase shelf-margin clinoforms and deep-water fan deposits,whereas the collapsed shelf edge mainly developed large-scale canyons and mass transport deposits.In response to the short sea-level rise but prominent falling cycle,the late Pleistocene shelf-edge sequences were composed of thin or undeveloped LST and TST units and a thick FSST unit,whereas the active faults in the outer shelf locations increased the proportion of the HST unit in the sequences.The preexisting slope-break geomorphology,fault activities,and asymmetric sea-level fluctuations coevally led to the diverse sequence architectures in the study area.The quantitative exploration of high-frequency sequence stratigraphic driving mechanisms is a future development trend in Pleistocene stratigraphy,and this study provides a potential reference for the standardization of 3D sequence stratigraphic investigations.