珠江口盆地中中新世SQ14.8层序‒沉积演化及其地质意义

The Mid-Miocene Stratigraphic-depositional Evolution Recorded by the SQ14.8 Layer in Pearl River Mouth Basin and its Geological Significances

中中新世,位于南海北部陆缘的珠江口盆地发生了构造、沉积和古气候格局的骤然变迁,而对古珠江沉积体系层序、沉积的研究为重塑该变革提供了良好契机。本文利用珠江口盆地三维地震和钻井资料,选取中中新世SQ14.8三级层序开展了系统的层序地层和沉积解剖,通过厘定关键层序、体系域界面,建立了高精度区域四分体系域格架,并在此基础上重建陆架‒陆坡区主要沉积体系的宏观展布特征和演化过程。研究发现,SQ14.8三级层序记录了珠江口盆地新近纪以来最大规模的一次相对海平面下降,引发沉积滨线跨越陆架长距离迁移到陆架坡折,发育了从低位到强制海退完整的体系域单元。古珠江三角洲作为陆架区主要的沉积体系,在相对海平面升降旋回的驱动下,其发育位置从内陆架到陆架边缘的迁移过程中,发育主控因素也由河流作用为主逐渐变为河流和海洋水动力共同控制。强制海退体系域的识别及沉积展布研究为预测深水储层提供了依据,该时期发育的陆架边缘三角洲下方陆坡区是寻找富砂重力流储集体的有利地区,而陆架边缘三角洲侧翼砂质沉积较少,下方的扇体以泥质扇为主。综合分析认为中中新世变冷事件是造成13.8 Ma相对海平面强烈下降的主要原因,在珠江口盆地促成SQ14.8层序典型强制海退体系域及陆架边缘三角洲的发育,进一步证明该事件是一次全球性的气候事件,其响应不仅可以在大洋钻孔获取的O同位素中找到证据,在边缘海盆的沉积记录中也有明显反映。

Located on the northern margin of the South China Sea,the Pearl River Mouth Basin experienced an abrupt change in tectonics,sedimentation,and paleoclimate conditions during the Middle Miocene.Research of the sequence stratigraphy and sedimentation of the ancient Pearl River sedimentary system is critical to decode this important transition.The SQ14.8 layer deposited during the Mid-Miocene was studied by utilizing 3D seismic and borehole data.Through systematic study of the sequence stratigraphy and the key stratigraphic boundaries,we established a high-resolution,four-order sequence stratigraphic framework for the SQ14.8 layer,and mapped the shelf to slope distribution and evolution of major depositional systems.Our results revealed that the SQ14.8 layer possibly recorded the largest sea level drop since the Neogene for the entire Pearl River Mouth Basin.This sea level drop triggered a long-distance migration of the depositional shorelines across the shelf to the shelf edge,forming a set of lowstand to forced-regressive deposition.Along with the progradation from the inner shelf to the shelf margin,the sedimentary process of the ancient Pearl River Delta which dominated the shelf region,had changed from river-dominated to wave-or wave-current dominated types,driven by the relative sea-level fall.It should be noted that the recognition of the forced regressive units provided valuable insights into the prediction of deep-water sandy reservoirs.For instance,it will be much easier to pinpoint deep-water sands beyond the region with the presence of shelf-edge deltas associated with the forced regression.In contrast,only muddy submarine fans tended to form within the flank region of the same shelf-edge delta.After comprehensive analysis,it is believed that that the Mid-Miocene cooling event was the most important driver for the relative sea level fall at 13.8 Ma,which final led to the development of the typical forced regressive units as well as the shelf-edge deltas.Our results also demonstrated that the cooling event should be a global one,and its profound responses can be found not only in the oxygen isotopes from cores in major oceans,but also in the depositional records from the marginal sea basins.