龙门山区北川甘溪剖面泥盆系养马坝组铁质鲕粒成因及其古环境讨论

龙门山区中泥盆统养马坝组底部发育丰富的铁质鲕粒。为分析该铁质鲕粒成因,本文以甘溪石沟里剖面精细实测为基础,通过详细的薄片和扫描电镜等分析,对铁质鲕粒的特征进行了研究,分析结果表明:龙门山区北川甘溪泥盆系养马坝组鲕粒类型多样,鲕粒核心包括石英碎屑颗粒、海百合碎片、黑色赤铁矿和方解石等4种类型,鲕粒圈层可分为明暗相间圈层、颜色均匀圈层和绿泥石圈层等3种类型。根据鲕粒核心和外部圈层的组合,可将甘溪泥盆系养马坝组的铁质鲕粒划分为粉砂质鲕粒、铁化鲕粒和绿泥石薄皮鲕粒等3种类型。龙门山区北川甘溪泥盆系养马坝组铁质鲕粒对古环境具有重要的指示意义,粉砂质鲕粒形成于开放近岸浅海氧化环境,而铁化鲕粒和绿泥石薄皮鲕粒则指示浅海滨岸中相对封闭的泻湖与残积鲕粒滩环境。

Genesis and paleoenvironment of the iron ooids from the Ganxi prolife of the Devonian Yangmaba Formation in Beichuan County, Longmenshan Area

There exist abundant iron ooids at the bottom of the Middle Devonian Yangmaba Formation in the Longmenshan area.In order to analyze the genesis of the iron ooids,their characteristics were studied by means of thin section and scanning electron microscope based on the detailed measurement of the Shiligou Profile in Ganxi village.The analysis results show that there are various types of ooids in Middle Devonian Yangmaba Formation of Longmenshan area.The ooidal cores include four types:quartz clastic particles,crinoid fragments,black hematite and calcite.The outer laminas of ooids can be divided into three types:alternate dark and bright one,uniform-colored one and chlorite-fragmented one.The iron ooids of Middle Devonian Yangmaba Formation in the Longmenshan area have an important significance to the ancient environment.The silty ooids were formed in the open environment nearshore shallow water under oxidation condition,and iron ones and chlorite ones with thin layers were deposited in the relatively closed lagoon and residual ooids beach of shallow seashore.