太行山南部前寒武纪红层的岩石磁学特征及沉积环境探析

摘要

Due to time and environmental proxies limitations, understanding of Precambrian environment is limited.A thick layer of Precambrian strata in the southern Taihang Mountains red deposition was considered as littoral-shallow marine deposition, according to its widespread ripple marks, mud cracks and bedding and other features.However, the understanding is in contradiction with the formation of red sediment which forms in a well permeable oxidizing environment.So, we conducted a detailed field investigation and sampling system of Precambrian red beds in Huixian, Henan province.Several regular magnetic parameters were measured including room temperature magnetism and high temperature magnetism, and some typical samples have also been measured by X ray diffraction.Results showed that the main magnetic minerals of red sandstone and purplish red clay are fine-grained and hard magnetic hematite and the grayish yellow clay rock mainly contains paramagnetic minerals and small amount of magnetite.Overall, hematite content in Taihang Mountains is much higher than that in paleosol S5 which is the strongest level of oxidation.Therefore, the red beds in Taihang Mountains formed mainly in good condition of permeable surface oxidizing environment.It is also in the process of formation and development of mud cracks and ripples structure, indicating that the effect of water on the deposition process.But the sediment is still showing red, and hematite content is higher than the Quaternary paleosols, which indicates that the water effect of these structures is short.We draw the following conclusions: In the lower part of the profile, it was quite arid and fluctuated greatly with less influence by water.In the middle of the profile, it might become moist, indicated by frequent influence of water (e.g.ripple marks) whereas it was still in oxidizing environment.In the upper profile of clay layer, it was characteristic of alternation of arid and moist environments, forming interlayered purple red and yellow mudstone.%由于年代久远及环境代用指标的局限性,目前对前寒武纪古环境的认识还相当有限.太行山南部沉积了厚层前寒武纪红色地层,根据其普遍存在的波痕、泥裂和层理等特征,前人将其认为是滨海-浅海环境沉积.然而,这一认识与红色沉积物形成于透水较好的氧化环境相矛盾.为此,对位于河南辉县郭亮村典型的前寒武纪沉积红层剖面进行详细的野外考察和系统采样,开展常温磁学、高温磁学和部分样品X射线衍射等实验.结果显示,红色砂岩和紫红色黏土岩主要磁性矿物是细粒硬磁性的赤铁矿,紫红色黏土岩中赤铁矿含量较高,灰黄色黏土岩以顺磁性矿物为主,含少量磁铁矿.因此认为,太行山中元古代红色地层主要形成于透水条件较好的地表氧化沉积环境.它在形成过程中同时还发育泥裂和波痕构造,指示着沉积过程中受到水的作用,但是沉积物总体仍呈现原生红色,表明波痕和泥裂形成过程中所经历水的作用过程是短暂的.通过磁学参数的变化特征推断,砂岩剖面下部相对干燥,气候波动较大,受水作用影响很小;砂岩剖面下部相对湿润,受水作用影响频繁,波痕出现频率增多,但仍以氧化环境为主;黏土层剖面以干湿交替环境为主,形成紫红色和灰黄色黏土岩互层.