陆相断陷湖盆浅水三角洲沉积特征——以高邮凹陷刘五舍次凹戴南组一段为例

Depositional characteristics of a shallow-water delta in a continental faulted basin:a case study of the first member of Dainan Formation,Liuwushe Subsag,Gaoyou Sag,North Jiangsu Basin

苏北盆地高邮凹陷刘五舍次凹是新生代陆相断陷湖盆,其主力层系始新统戴南组一段现今呈(东)南陡、北缓、中深凹的箕状格局。埋藏史恢复表明,戴一段沉积时期刘五舍次凹地势平缓,且沉降中心并非现今深凹带,古地貌远比现今构造格局平缓。物源分析显示,研究区戴一段砂体基本来自北部斜坡带,南部陡坡带供源极少。岩心观察与测、录井资料指示了研究区浅水环境下河控作用为主的沉积特征,认为断陷湖盆在其边界断层活动强度减弱阶段亦可发育浅水三角洲沉积。井震结合预测了砂岩与含砾砂岩空间展布,指出在断陷湖盆浅水三角洲沉积体系中,陡坡带亦可阶段性出现大量重力流沉积。

The Liuwushe Subsag,located in the east of Gaoyou Sag,North Jiangsu Basin,is a Cenozoic continental faulted basin formed at the end of Late Cretaceous.The present structural pattern of the first member of Dainan Formation is dustpan shaped,with slope zone in the north,steep slope zone in the south and deep sag in the middle.However,the restoration of the burial history indicated that the paleotopography of Liuwushe Subsag was limited when the first member of Dainan Formation was deposited.The depocenter during that period was located east of the present deep sag with a much smaller range.Provenance analyses indicated that most sand bodies of the first member of Dainan Formation in the Liuwushe Subsag came from the northern slope zone rather than the southern steep slope zone.Core observation and seismic and logging data indicated that a great many underwater channel sand bodies extended from north to south in the study area.Shallow-water delta deposits also developed in a faulted basin when the activity intensity of the basin boundary fault was weak.Well logging and seismic data were used to predict the spatial distribution of sandstone or pebbly sandstone.A large number of gravity flow deposits associated with the boundary fault remobilization occurred in the steep slope zone,which was identified as one difference between the shallow-water delta system of the fault basin and that of the depression basin.