断陷盆地缓坡带物源转换与沉积响应——以渤海湾盆地饶阳凹陷蠡县斜坡古近系源-汇系统为例

Provenance transformation and sedimentary response of ramp facies in downfaulted basins:A case study on the Paleogene source-to-sink system in Lixian slope,Raoyang Sag,Bohai Bay Basin

寻求盆外物源通量变化与盆内沉积体系的定量耦合响应仍然是源-汇系统研究的薄弱环节,也是断陷湖盆源-汇系统研究框架下的一个重要命题。基于渤海湾盆地饶阳凹陷蠡县斜坡全覆盖的3D地震资料和岩心、测井数据,结合碎屑锆石U-Pb定年技术,对研究区古近系沙(沙河街组)二段与沙一段开展物源体系定量化表征,并通过定量沉积构造解释和定量地震地貌测量,建立了浅水三角洲的定量化表征方法。研究显示,沙一段-沙二段沉积时期,古永定河分支被古大清河袭夺,造成蠡县斜坡A区特征锆石含量由37%下降至26%,对应的A区浅水三角洲面积由144 km2减小至38 km2,平均水下分流河道宽度由104 m减小至47 m;古唐河水系特征锆石含量由31%上升至39%,对应的B区浅水三角洲面积由71 km2增加至94 km2,平均水下分流河道宽度由77 m增加至88 m;古大沙河水系沉积物通量未观察到明显变化,对应的C区浅水三角洲面积由206 km2降低至185 km2,平均水下分流河道宽度由120 m减小至81 m。构造作用、气候变化及湖平面升降等异旋回作用影响范围较大,将会造成沉积体系的同步响应;而物源通量变化将影响特定沉积体系,是研究区浅水三角洲沉积响应的主要驱动机制。

The quantified coupled response of intra-basin sedimentary system to extra-basin sediment flux variation is still a weak link in source-to-sink system research,and also an important topic in the framework of source-to-sink system research in downfaulted basins.Based on the 3D seismic and log data,as well as core observation of the Lixian slope in the Raoyang Sag,combined with detrital zircon U-Pb dating,the study carries out quantitative characterization of the provenance system during the deposition of the second and first members of the Paleogene Shahejie Formation(that is,Sha 2 and Sha 1 members),and establishes a quantitative characterization method for shallow-water delta by means of quantitative sedimentary structure interpretation and quantitative seismic geomorphic measurements.The results show that the sediment flux of the paleo-Yongding River branch was seized by the paleo-Daqing River,resulting in the sediment flux decrease from 37%to 26%in zone A of Lixian slope during the sedimentation of Sha 1 to Sha 2 members,the corresponding area of the delta in zone A declining from 144 km2 to 38 km2 and a reduction of the average subaqueous distributary channel width from 104 m to 47 m.While the sediment flux of the paleo-Tang River increased from 31%to 39%,corresponding to an increase in the area of delta in zone B from 71 km2 to 94 km2 and an increase in the average subaqueous distributary channel width from 77 m to 88 m.No significant changes are observed in the sediment flux of the paleo-Dasha River,and the corresponding area of the delta in zone C decreased from 206 km2 to 185 km2,and the average subaqueous distributary channel width decreased from 120 m to 81 m.Allogenetic forces such as tectonic evolution,climate change,and lake-level fluctuation tend to generate larger scope of influence,causing the synchronous response of sedimentary systems.In contrast,changes in sediment flux will affect specific sedimentary systems and are the primary driving mechanism for the sedimentary response of shallow-water deltas in the study area.