安徽宿松坐山中二叠统栖霞组碳酸盐岩微相和黏土矿物特征及其对沉积相和古气候的指示

安徽宿松坐山剖面中二叠世地层发育,栖霞组、孤峰组和武穴组连续出露,为下扬子地区中二叠统的典型剖面之一。其中栖霞组主要为碳酸盐岩,其次为硅质岩和碎屑岩。栖霞组碳酸盐岩中共识别出9种微相类型,分别可以与Flügel整理的7种标准微相对比。栖霞组主要为斜坡沉积,其次为台地沉积,而斜坡相又可以进一步分为上斜坡相和下斜坡相。其中,上斜坡相主要出现在臭灰岩段的中部,该层位以产砾屑灰岩为特征,微相类型以MF3为主。其次出现在本部灰岩段的中偏上部,该处微相类型为MF3、MF8和MF9。此外,顶部灰岩上部发育灰岩砾石的层位中的微相类型主要为MF3和MF9,也属于上斜坡相。下斜坡相主要发育于上、下硅质岩段和本部灰岩段的顶部,主要特征为发育薄层硅质岩或长条状硅质结核,微相类型主要为MF4和MF5。台地相主要出现在本部灰岩中下部,其次出现在臭灰岩段的下部和上部及顶部灰岩段的下部,且其微相类型以MF1为主,其次为MF2、MF6和MF7,表明台地相整体属于开阔海台地。微相分析表明栖霞组整体形成于一个海侵过程中,期间经历了4次较明显的海退。黏土矿物分析显示栖霞组碳酸盐岩的黏土矿物以伊利石为主(平均含量为61.97%),高岭石次之(平均含量为27.25%),伊蒙混层(平均含量为9.43%)和绿泥石(平均含量为1.36%)较少。高岭石/(伊利石+绿泥石)的比值与伊利石的相对含量变化共同表明栖霞期由早期的相对干冷变为晚期的相对暖湿,期间发生了3次较为明显的气候变化周期。同时,栖霞期气候整体变暖湿、海平面整体上升的趋势与晚古生代冰川(LPIA)消融的时间对应,则暗示古气候变化导致的冰川消融事件控制了栖霞期的整体海平面变化。而沉积相变化所反映的栖霞期4次相对海平面变化与3次明显的气候波动能够较好的吻合,则显示出气候对海平面变化和沉积记录的控制作用。表现在气候变暖湿导致冰体融化,海平面上升,风化指数(WI)升高,主要发育碳酸盐斜坡相。气候变干冷、冰体增大导致海平面和风化指数(WI)的下降,以碳酸盐台地沉积为主。

The Characteristics of Microfacies and Clay Minerals of the Chihsia Formation Carbonate Rocks of Middle Permian in Susong, Anhui and its Implication for Sedimentary Facies and Paleoclimate

Middle Permian stratum is well-developed in Zuoshan profile of Susong, Anhui, including Chihsia Formation, Kufeng Formation and Wuxue Formation. Among them, Chihsia Formation is dominantly composed of carbonate rocks, and cherts and clastic rocks in minor. This study recognizes nine microfacies, which can compare with seven standard microfacies amended by Flügel, respectively. Chihisia Formation is mainly deposited in slope environment, followed by platform deposition. And slope can be further divided into upper slope and lower slope. The upper slope face appears mainly in the middle part of swine limestone member (SLM), which is characterized by the development of calcirudites and its microfacies is MF3 in dominate. The upper slope face can be also discovered in the upper part of middle limestone member (MLM) and upper limestone member (ULM). The lower slope face is well-exposed in the upper and lower silicon member (USM/LSM), and the upper part of MLM, whose major microfacies is MF4 and MF5, is characterized by the development of thin-bedded cherts or elongated chert nodules. The platform face mainly occurs in the lower to middle part of MLM, and followed by the lower and upper part of SLM and lower part of ULM, whose microfacies is mainly composed of MF1. The microfacies analysis suggests that Chihsia Formation developed in a phase of transgression, during which four regressions happened. The result of clay minerals shows that the composition of clay minerals of the carbonate rocks of Chihsia Formation is dominated by illite (average: 61.97%), kaolinite is the second (average: 27.25%), illite smectite mixed layer (9.43%) and chlorite (1.36%) are less. The ratio of kaolinite/( illite+ chlorite) and the relative content of illite shows that the climate of Chihsia stage changed from cold- dry to warm - wet overall, but during this stage three-fluctuation occured. Finally, we contrast the sea-level fluctuation with palaeoclimate change, and find that the transgression of Chihsia stage was corresponding to the warm- wet climate and every warming and wetting correspond to a transgression. Hence, we draw the conclusion that the transgression- regression cycles were influenced by the fluctuation climate. The warm- wet climate cause ice melt and hence raising of sea-level, as well as the rise of weather index (WI), which corresponding to the deposition of slope. Reversely, during cold and dry period (corresponding to the increasing volume of ice), sea-level and WI declined, and the main sedimentary facies was platform.