京西雾迷山组第4段强迫型海退过程中的层序旋回特征及其构造古地理意义

摘要

The sequence cycle of the transgressive stage of lower to middle Wumishan Formation in the YanshanLiaoning area is characterized by symmetrical facies sequences and fabrics,but no detailed study has been paid on the sequence cycle of the regressive process of the upper Wumishan Formation.For this purpose,a systematic research about sedimentary microfacies,sedimentary facies sequences,sequences stratigraphy and chemical stratigraphy of the fourth member of the Wumishan Formation at Zhuanghuwa section,Yongding river valley,western Beijing,was carried out to restore sedimentary facies and classify parasequence type.Carbon and oxygon isotopic compositions and relevant data analyses were also performed to verify the evolution process of the ancient environment.Nine lithofacies have been identified in the Zhuanghuwa section,indicating different depths of water,topography,and biological conditions,which are also confirmed by corresponding δ13C and δ18O values.Generally,with the paleotemperature T increasing,the values of δ18O and paleosalinity Z decrease,reflecting sea level rise;conversely,reflecting sea level drop.Our research shows the sequences cycle of the regressive process is characterized by asymmetrical facies sequences and fabrics,its parasequence fabrics becomes gradually simple from supratidal zone to subtidal zone,and the minisequence of superimposed patterns evolves from 1∶ 4 to 1∶2.Therefore,facies sequences and fabrics of the fourth member of Wumishan Formation is distinctively different from the typical "Sequence Cycle of Wumishan Formation",and this is probably related to the tectonic evolution from aulacogen to epeiric sea at the north margin of North China Platform in the Mesoproterozoic.%燕辽地区雾迷山组下、中部主体属海进阶段的层序旋回,以发育对称型相序组构为特征,但上部表现为强迫型海退过程,层序旋回特征未有精细的研究.为此,笔者等选择京西永定河庄户洼剖面的雾迷山组第4段开展系统的沉积微相、沉积相序和层序地层学、化学地层学研究,用以恢复沉积相和划分副层序类型,并通过碳、氧同位素组成和相关数据分析的方法验证古环境的演变过程.庄户洼剖面中共识别出九种岩相,不同岩相反映了特定的水深、地形和生物条件,其δ13C值和δ18O值也会有相应的体现.一般情况下,古温度T值增大,δ18O值和古盐度Z值减小,反映海平面上升;反之则指示海平面下降.研究表明,强迫型海退过程中的层序旋回为不对称相序组构,从潮上带至潮下带其副层序结构由复杂趋于简单,微层序的主要叠加方式也由1∶4向1∶2演变.因此,雾迷山组第4段层序旋回的相序组构与典型的“雾迷山组旋回”差异巨大,这可能与华北古陆北缘中元古代的构造演化转折相关.