安徽巢湖平顶山西坡剖面下三叠统殷坑组沉积及地球化学特征

Sedimentary and Geochemical Characteristics of the Early Triassic Yinkeng Formantion at West Pingdingshan Section in Chaohu,Anhui Province

本文在对安徽巢湖平顶山西坡剖面下三叠统殷坑组解剖的基础上,着重对其内部赋存的碳酸盐岩的矿物组成、结构构造、化学成分及碳氧同位素特征等方面做了系统研究,探讨了其沉积环境和成岩环境,以及与海平面变化的关系等。研究表明,下三叠统殷坑组是在海水不断变深的背景下沉积的,经历了较浅水开阔台地、较深水的上斜坡和下斜坡,最后到深水盆地的4个演化过程。瘤状灰岩在殷坑组极其发育,由瘤体和基质两部分组成,瘤体多为灰色、浅灰色或灰白色微晶灰岩、泥晶微晶灰岩和微晶泥晶灰岩;基质颜色较深,以黄色、褐黄色和黄灰色钙质泥岩为主。条带状、断续状和杂乱状瘤状灰岩在剖面上呈规律性分布,与其所处的沉积环境和后期成岩作用强度密切相关,它们分别发育在下斜坡、上斜坡和深水盆地环境,溶蚀作用依次增强。殷坑组碳酸盐岩δ13 C处于负值区,自下而上,由-5.97‰变为-1.17‰,记录了一个明显的δ13 C正向大漂移,幅度达到4.80‰,这可能与生物复苏、有机质大量埋藏,以及海平面上升和沉积环境的改变有关。

In this paper,we discussed the sedimentary and diagenetic environments of the Early Triassic Yinkeng Formation at the west Pingdingshan section,Chaohu,Anhui Province,based on the detailed analyses of sedimentary sequence,mineral composition,sedimentary texture and structure,chemical components,as well as carbon and oxygen isotopes of the carbonates.The results indicate that the Yinkeng Formation contains four sedimentary facies,including the shallow open platform,upper slope,lower slope,and deep basin from bottom up,with the water depth and sea level deepening and rising,respectively.Nodular limestones,which are composed of nodules and matrix,are common in this formation.The nodule is gray or gray white micritic calcite while the matrix occurs predominantly of yellow,brown-yellow,and yellowgray calcareous mudstones.Three types of nodular limestones,namely the banded nodular limestone,discontinuous nodular limestone,and random nodular limestone,are regularly distributed in the section,which may reflect original sedimentary environment from upper slope,lower slope to deep basin,respectively,with dissolution gradually increasing,and following diagenetic intensity.In addition,theδ13C values of the carbonate in the Yinkeng Formation range from-5.97‰ to-1.17‰,displaying an obvious positive excursion with the magnitude up to 4.80‰,which may result from the biotic recovery,burial of organic matters,sea-level rising,and environmental changing.