层序格架制约下的菱铁矿发育特征——以黔西盘关地区晚二叠世煤系为例

Development characteristics of siderite in the constraint of sequence frame: A case study of Late Permian coal measures in Panguan area

黔西盘关地区晚二叠世煤系中发育多层含菱铁矿层,内部菱铁矿以多种微观形态产出,并与研究区高频层序地层格架有着一定对应关系。对研究区钻井的测井数据采用小波分析的方法,对其晚二叠世煤系进行高频层序划分,并基于钻井内含菱铁矿层样品的薄片鉴定及激光微区原位主微量元素分析结果,运用沉积岩石学、沉积地球化学与层序地层学等方法,探讨了各类菱铁矿的古沉积环境以及在层序格架内的发育规律。结果表明,菱铁矿依微观形态差异可划分为凝胶状菱铁矿、微粉晶菱铁矿、球粒状菱铁矿3种类型,其中凝胶状菱铁矿发育于层序内部最大海泛面附近,微粉晶菱铁矿发育于海侵体系域海平面上升期或高位体系域内,球粒状菱铁矿在层序内未见明显发育规律。各类菱铁矿发育受到层序格架的制约作用,层序反映了原始沉积环境与沉积物质组成的旋回性变化,从而导致其内菱铁矿的微观形态存在差异,发育于最大海泛面附近的凝胶状菱铁矿形成于偏海相环境,沉积期溶液中Fe2+与CO32-供应充足,迅速结晶形成隐晶凝胶状菱铁矿集合体;发育于海侵体系域海平面上升期或高位体系域内的微粉晶菱铁矿形成于海陆过渡相沉积环境,陆源铁质充足但CO32-较少,菱铁矿沉淀速率缓慢最终形成亮晶菱铁矿,球粒状菱铁矿的形成与孔隙流体引发的菱铁矿多期次结晶生长有关,不受控于层序格架内部发育位置。

There are multiple siderite-bearing layers in the Late Permian coal measures at the Panguan area in the west of Guizhou.The inner siderite is produced in a variety of microscopic forms and has a corresponding relationship with the high-frequency sequence stratigraphic framework in the study area.The well logging data in the study area was analyzed using wavelet analysis to divide the high-frequency sequence of the Late Permian coal measures.Based on the identification of thin sections of samples containing siderite in the well and the analysis of the main trace elements in-situ in the laser micro-zone,using the sedimentary petrology,sedimentary geochemistry and sequence stratigraphy,the ancient deposits of various siderite environment and developmental rules within the high frequency sequence were discussed.The results show that the siderite can be divided into three types:gelatinite siderite,micro-powder crystal siderite and spheroidal siderite based on the difference of microscopic morphology.The gelatinous siderite was developed near the maximum sea surface of the sequence,micro-powder crystal siderite was developed in the high-level system tract or in the sea level rise during transgressive system tract,and the spherulite siderite had no obvious development law in the sequence.The development of various siderites was restricted by the sequence framework,the sequence reflected the cyclical changes in the original sedimentary environment and the composition of the sedimentary material,resulting in differences in the micromorphology of the siderite in the stratum.The gelatinous siderite developed near the maximum sea surface was formed in the marine environment,during the deposition period,Fe2+and CO32-were well supplied and crystallized rapidly to form the cryptic gelatinous siderite aggregate.The micro-powder crystal siderite developed in the high-level system tract or in the sea level rise during transgressive system tract was formed in the sedimentary environment of the transitional facies between the sea and the land,the source iron was abundant but CO32-was less,and the siderite precipitated slowly and finally formed bright siderite.The spherulite siderite was related to the multi-phase crystal growth of siderite induced by pore fluid,and it was not controlled by the development position inside the sequence frame.