Selected age-diagnostic datums of ODP Site 184-1148

The 853 m thick sediment sequence recovered at ODP Site 1148 provides an unprecedented record of tectonic and paleoceanographic evolution in the South China Sea over the past 33 Ma. Litho-, bio-, and chemo-stratigraphic studies helped identify six periods of changes marking the major steps of the South China Sea geohistory. Rapid deposition with sedimentation rates of 60 m/Ma or more characterized the early Oligocene rifting. Several unconformities from the slumped unit between 457 and 495 mcd together erased about 3 Ma late Oligocene record, providing solid evidence of tectonic transition from rifting/slow spreading to rapid spreading in the South China Sea. Slow sedimentation of ~20-30 m/Ma signifies stable seafloor spreading in the early Miocene. Dissolution may have affected the completeness of Miocene-Pleistocene succession with short-term hiatuses beyond current biostratigraphical resolution. Five major dissolution events, D-1 to D-5, characterize the stepwise development of deep water masses in close association to post-Oligocene South China Sea basin transformation. The concurrence of local and global dissolution events in the Miocene and Pliocene suggests climatic forcing as the main mechanism causing deep water circulation changes concomitantly in world oceans and in marginal seas. A return of high sedimentation rate of 60 m/Ma to the late Pliocene and Pleistocene South China Sea was caused by intensified down-slope transport due to frequent sea level fluctuations and exposure of a large shelf area during sea level low-stands. The six paleoceanographic stages, respectively corresponding to rifting (~33-28.5 Ma), changing spreading southward (28.5-23 Ma), stable spreading to end of spreading (23-15 Ma), post-spreading balance (15-9 Ma), further modification and monsoon influence (9-5 Ma), and glacial prevalence (5-0 Ma), had transformed the South China Sea from a series of deep grabens to a rapidly expanding open gulf and finally to a semi-enclosed marginal sea in the past 33 Ma.

在大洋钻探计划（Ocean Drilling Program, ODP）1148站位获取的853米厚沉积序列，为南海过去33百万年（Ma）以来的构造与古海洋演化历史提供了前所未有的记录。岩石地层、生物地层与化学地层研究识别出六个演化阶段，标志着南海地质历史的主要转折节点。渐新世早期以沉积速率不低于60米/百万年的快速沉积为特征，对应裂谷作用阶段。457至495复合深度米（meters composite depth, mcd）区间内的滑塌单元中存在多处不整合面，抹去了约3百万年的晚渐新世沉积记录，为南海从裂谷/慢速扩张向快速扩张的构造转型提供了确凿证据。中新世早期沉积速率约为20-30米/百万年，反映了稳定的海底扩张过程。溶解作用可能影响了中新世-更新世沉积序列的完整性，造成了超出当前生物地层分辨率的短期沉积间断。D-1至D-5五大主要溶解事件，表征了深水水团的逐步演化过程，这与渐新世后南海盆地的转型密切相关。中新世与上新世期间局部与全球溶解事件的同步发生，表明气候强迫是引发全球大洋与边缘海深水环流同步变化的主要机制。上新世晚期至更新世，南海沉积速率重回60米/百万年的高水平，这是由于海平面频繁波动引发的斜坡搬运作用增强，以及海平面低水位期大面积陆架出露所致。这六个古海洋学阶段依次对应：裂谷期（约33-28.5 Ma）、扩张作用向南迁移期（28.5-23 Ma）、稳定扩张至扩张结束期（23-15 Ma）、扩张后平衡期（15-9 Ma）、进一步改造与季风影响期（9-5 Ma）以及冰期盛行期（5-0 Ma）；在过去33百万年间，它们将南海从一系列深地堑改造为快速扩张的开阔海湾，最终演变为半封闭的边缘海。