河南卫辉地区寒武系第三统馒头组二段中的核形石及其沉积特征

Oncoids and their depositional features from the second member of Mantou Formation (Cambrian Series 3),Weihui area,Henan Province

为重建河南卫辉地区寒武纪古环境,揭示微生物岩的成因及演化规律,对河南卫辉地区寒武系第三统馒头组二段中的核形石进行了野外地质调查和室内微观分析。识别出2种不同类型的核形石,其中椭圆形核形石由较小的核心和不规则的明、暗纹层组成,与大量不规则泥质条带伴生,形成于高、低能交替的潮间水道;帽状—圆状核形石发育较大的核心和规则的明、暗纹层,与缓波状叠层石伴生,形成于高、低能交替的潮间—潮下水道。馒头组二段含核形石灰岩垂向序列自下而上呈潮上—潮间带泥、灰坪—潮间—潮下带水道、近岸鲕粒浅滩相的变化规律,反映了海平面向上缓慢上升、水动力逐渐增强的过程。

To rebuild the Precambrian palaeo environment of He＇nan Weihui area and reveal the formation and evolution law of microbial rocks, field geological survey and indoor micro analysis were carried out for oncoids from the second member of the Mantou Formation （Cambrian Series 3）, Weihui area, He＇ nan and two kinds of oncoids have been recognized. The ellipsoidal on- colds are dominated by the small nuclei and irregular dark-light cortices. They are usually interbedded with irregular mud bands and formed in intertidal channel with the alternation of low to high water energy. The catlike and round oncoids are characterized by the big nuclei and regular dark-light cortices. They are interbedded with wavy stromatolites and formed in intertidal-subtidal channel with the alternation of low to high water energy. The vertical sequence of oncoid-bearing limestone in the second member of the Mantou Formation evolves upward from supratidabintertidal mud-lime flats via intertidal-subtidal channels to nearshore oolitic shoals, which reflect the rising sea level and increasing sea water energy gradually upward. The analysis of oncoids and their depositional features from the second member of the Mantou Formation （Cambrian Series 3）, Weihui area, He＇nan can pro vides evidences for the reconstruction of Cambrian palaeo environments of study area and the understanding of genesis and evolu- tion of microbialites.