南堡凹陷北部始新世米兰科维奇记录及其对沉积响应的指示意义

Eocene Milankovitch Record and Its Indicative Significance of Depositional Response in the Northern Nanpu Sag

古近纪时期的板块运动造就了渤海湾盆地洼隆相间的构造格局,需要从地质年代学角度研究单一湖盆的层序地层格架及其内部的沉积响应。以南堡凹陷高柳地区始新统为例,利用标准井预处理的自然伽马曲线,利用频谱分析、连续小波变换和滑动频谱分析等方法识别蕴藏的米兰科维奇旋回,基于绝对地质年代锚点,建立天文年代标尺,估算沉积速率,重建古湖平面变化和古沉积物供给能力,从区域沉积体系演化的角度讨论米兰科维奇记录对沉积响应的指示意义。结果表明:①始新统沉积序列受控于长偏心率、短偏心率、斜率和岁差周期,共识别出20个长偏心率周期,85个短偏心率周期,累计约8.1 Ma;②Es_(3)^(1)、Es_(2)、Es_(1)^(x)和Es_(1)^(z)亚段顶界面绝对地质年代分别为33.3、31.9、30.5、29.5 Ma,平均沉积速率约124.2 m/Ma;③湖平面于36.6~33.3 Ma大规模上升,于33.3~30.5 Ma持续下降,于30.5~28.5 Ma再次小幅回升,而36.6~28.5 Ma西南庄断层和柏各庄断层持续活动使古燕山沉积物供给能力一直较强,沉积演化对构造信号的响应强于气候信号,造就了高柳地区始新世早期中等规模的扇三角洲沉积体系和中晚期大规模的辫状河三角洲沉积体系。

The plate movement during the Paleogene created the tectonic framework of the Bohai Bay basin with alternating depressions and uplifts.It is necessary to study the sequence stratigraphic framework of a single lake basin and its internal sedimentary response from the perspective of geochronology.Taking the Eocene sedimentary series in the Gaoliu area of the Nanpu sag for example,the preprocessed gamma-ray curve of the standard well was used to identify the Milankovitch cycles using spectral analysis,continuous wavelet transformation and evolutionary spectral analysis.Based on the anchor point of absolute geological age,the astronomical timescale was established and the sedimentation rate was estimated.The paleo-lake level change and the paleo-sediment supply rate were reconstructed.From the perspective of regional sedimentary evolution,the indicative significance of Milankovitch records of depositional response was discussed.The results show these as follows.Eocene sedimentary sequence is controlled by long eccentricity,short eccentricity,obliquity and precession.20 long eccentricity periods and 85 short eccentricity periods were identified,totaling 8.1 Ma.The absolute geological ages of the top boundaries of Es_(3)^(1),Es_(2),Es_(1)^(x)and Es_(1)^(z) are 33.3 Ma,31.9 Ma,30.5 Ma and 29.5 Ma respectively.The average sediment accumulation rate is about 124.2 m/Ma.The lake level rose in a large scale from 36.6 Ma to 33.3 Ma,and continued to fall from 33.3 Ma to 30.5 Ma,and slightly rose again from 30.5 Ma to 28.5 Ma.The continuous activities of the Xinanzhuang fault and the Baigezhuang fault from 36.6 Ma to 28.5 Ma strengthed the sediment supply rate of the ancient Yanshan Mountains.The response of depositional evolution to tectonic signal is stronger than that to climate signal,which leads to the middle-scale fan deltas during the early Eocene and large-scale braided-river deltas during the middle and late Eocene in the Gaoliu area.