四川广元中二叠统栖霞组似球粒灰岩微相特征及沉积学意义

Microfacies of Peloidal Limestone of Middle Permian Chihsia Formation at Guangyuan,Sichuan Province and Its Sedimentary Significance

四川广元上寺中二叠统栖霞组中段发育碳酸盐岩浅滩相。露头观察、镜下分析得出,其颗粒类型中,似球粒、包粒十分丰富,占较大比重。研究层段识别出粪球粒、巴哈马似球粒、微生物似球粒三种类型。其中以巴哈马似球粒最为常见,它是由生物碎屑的泥晶化导致的。基于颗粒类型、沉积组构的详细观察和微相分析,得出研究层段总体沉积环境为似球粒为主的浅滩,包括滩体、滩缘、滩间三个亚环境。相对前人得出的生物碎屑滩,似球粒滩形成的水体能量相对较低。而且,这种似球粒滩在华南二叠纪地层中分布广泛。识别这种似球粒滩并对其颗粒组成、形成环境及空间展布进行深入研究,对认识华南二叠纪的沉积古地理格局将具有重要意义。

Carbonate shoal develops in the middle part of Chihsia Formation at Shangsi Section of Guangyuan, Sichuan Province. The outcrop consists predominantly of thick bedded to massive limestone with intercalated dolostone. The rocks are light grey and crystalline, with some fossil fragments. Under microscope,abundant peloids and cortoids were identified. The peloids are classified into three types： fecal pellets, bahamite peloids and microbial peloids. The bahamite peloids are the most common grains observed. They are micritized bioelasts in origin, mainly including red algae, benthic foraminifera, crinoid and minor other shallow water organisms. Fossils are well-preserved in some layers. In light of particle composition and preservation, these particles derived from organic hardparts within carbonate platform, and accumulated in-situ, at least without major transportation. Intergranular porosities and organic cavities are filled mainly with sparry calcite cement with minor carbonate mud. Such a dominant sparry calcite cementation texture indicates the water energy was not strong enough to winnow off all the nmd. Consequently, the peloidal shoal develops in relatively low-medium energy environment. Microfacies types are is grouped into peloid packstone-grainstone,peloid-cortoid grainstone,foraminifera-algae packstone and bioclast wackstone. Based on grain type, sedimentary fabric and microfacies analysis, the peloidal shoal can be subdivided into shoal, shoal fringe and inter-shoal. In addition, such a peloidal shoal is widespread in the Permian strata of South China. Recognition of such a shoal and further study on its grain composition, depositional environment and spatial distrihution will be of great significance on the understanding of Permian palaeogeographic framework of South China.