陆相沉积物风化强度与颜色成因探讨——以四川盆地中侏罗统上沙溪庙组为例

Weathering Intensity and Color Genesis of Continental Sediments:A case study from the Shangshaximiao Formation of the Middle Jurassic in the Sichuan Basin

沉积物/岩的新鲜色是其内部成分、组构的直观响应,记载着丰富的气候、环境、构造和成岩改造信息。四川盆地侏罗系中统上沙溪庙组是一套河湖相沉积的碎屑地层,剖面出露好、颜色新鲜且连续多变,是研究沉积物/岩颜色成因及其气候环境指示意义的良好对象。通过对上沙溪庙组灰绿色、灰色、红色等颜色的岩石样品矿物成分、元素含量和Fe化学种的测试和分析,发现上沙溪庙组泥岩的颜色差异是由赤铁矿含量变化直接引起的。红色泥岩,CIA、CIW、Ba指数、退碱指数、黏土化指数、残积系数平均值分别为70.27、82.05、1.14、0.26、0.27、2.38,指示化学风化作用最强;灰色样品相对应的地球化学指标分别为62.65、71.06、2.22、0.75、0.23、1.19,在三种颜色的样品中受到中等化学风化作用;灰绿色样品则分别为58.50、65.11、1.92、0.78、0.26、1.34,受到的化学风化作用最弱。红色样品中代表赤铁矿中的铁含量最高,灰绿色样品以代表黏土矿物中的顺磁性亚铁为主,灰色样品中三种铁元素化学种含量相差不大,总体以三价铁为主。不同颜色样品的铁元素化学种含量变化显示红层样品中形成赤铁矿的Fe元素可能不只来源于源岩风化,还可能来自受二次风化作用的黏土矿物。样品呈现不同颜色的成因目前可确认与源岩受到不同程度的风化作用相关,不能简单归因于炎热干旱的气候条件,需要结合其他指标进行综合判别。

The fresh color of sediment/rock is the intuitive response of its internal composition and fabric and records the rich climate,environment,structure,and diagenetic transformation.The Shangshaximiao Formation of the Middle Jurassic in the Sichuan Basin is a set of clastic strata from river lake facies.The profile is well exposed,fresh,and continuously changeable.It is an excellent object for studying the origin of sediment/rock color and its indicative significance of climate and environment.Through test and analysis of mineral composition,element content,and iron speciation of mudstone samples with gray-green,gray,and red colors in the Shangshaximiao Formation,we found that the change of hematite content directly causes the color difference in the mudstone in Shaximiao Formation.For red mudstone,the average values of chemical index of alteration(CIA),chemical Index of Weathering(CIW),weather eluviation index(Ba),regression index,clayization index,and residual coefficient are 70.27,82.05,1.14,0.26,0.27,and 2.38,respectively,indicating the strongest chemical weathering intensity.The corresponding geochemical indexes of gray samples are 62.65,71.06,2.22,0.75,0.23 and 1.19,respectively.The grey-green samples were 58.50,65.11,1.92,0.78,0.26 and 1.34 respectively,and the chemical weathering was the weakest.The red sample contains the highest hematite iron content.The gray-green sample mainly represents paramagnetic ferrous in clay minerals.The geochemical contents of the three iron elements in the gray sample have a slight difference,mainly trivalent iron.The change of iron chemical species content in samples with different colors shows that the Fe element forming hematite in red bed samples may come from the weathering of source rock and clay minerals subjected to secondary weathering.At present,the causes of the different colors can only be confirmed to be related to the weathering of source rocks to varying degrees,which cannot be attributed to the hot and arid climate conditions.It is necessary to distinguish the climate type in combination with other indicators.