鄂尔多斯盆地中上三叠统延长组长7段碳酸盐结核成因讨论

A discussion on the genesis of carbonate concretions in the Chang 7 Member of the Middle-Upper Triassic Yanchang Formation of Ordos basin

鄂尔多斯盆地南部铜川地区露头区中三叠统延长组长7段细砂岩、粉砂岩、泥质粉砂岩、粉砂质泥岩层序中发育有大量圆球状、椭球状、扁球状碳酸盐岩结核,对于其成因存在争议。对铜川地区多个剖面的结核开展宏观描述,在微观薄片观察基础上开展岩石矿物组成、稳定同位素、簇同位素以及微区同位素等分析,特别是选取一个结核开展了详细的解剖。结果显示结核成分以方解石为主,少量白云石;内部结构有差异,在球体横切面上明显分为两个圈层;碳同位素值正偏,由内部圈层向外部圈层降低;氧同位素与碳同位素变化趋势一致,计算结核形成古温度变化区间为28.6~76.5℃;簇同位素揭示结核中部形成温度为42.9℃。认为该类结核为早期发酵带内细菌参与甲烷生成作用形成,甲烷菌活动引起有机质碳同位素分馏,碳酸盐结核中方解石和白云石圆球粒可能是蓝细菌细胞方解石化或白云石化的结果。根据埋藏史,认为该类结核形成于成岩作用早期,后期在成岩过程中叠加生长,同心环层结构明显。

There are a large number of spherical,ellipsoidal and oblate carbonate concretions in the sequence of fine sandstone,siltstone,argillaceous siltstone and silty mudstone in the Chang 7 Member of the Middle Triassic Yanchang Formation in the Tongchuan area,south of Ordos Basin.Macroscopic description was carried out for concretions in several sections in the Tongchuan area.On the basis of thin section observation,rock and mineral composition,stable isotopes,cluster isotopes and micro-area isotopes were analyzed.In particular,one concretion was selected for detailed anatomical studies.The results showed that the concretion was composed mainly of calcite and a little dolomite.The inner structure is different,which is divided into two circles on the cross section of the sphere.The carbon isotope value was positive and decreased from the inner layer to the outer layer.The variation trend of oxygen isotope is consistent with that of carbon isotope,and the variation range of the concretion formation palaeo-temperature range was 28.6~76.5℃.The formation temperature of cluster isotope in the middle of parts was 42.9℃.It is believed that such nodules are formed by the early fermentation zone bacteria participating in methanogenesis,and the activity of methanogens causes the organic carbon isotope fractionation,and the calcite and dolomite spherules in carbonate nodules may be the result of calcite or dolomitization of cyanobacteria cells.According to the burial history,it is believed that the nodules were formed in the early stage of diagenesis and grew superimposed in the later stage of diagenesis,with obvious concentric ring structure.