江西石城国家地质公园龟裂地貌特征及成因分析

江西石城国家地质公园的龟裂地貌主要产于晚白垩世河口组红色砂岩中,造型奇特、多样,形成许多象形景观,如龟寿石、松果石、鳄鱼石、仙人犁田等,引人注目。但是,对这些龟裂地貌的微观研究不足,导致对其成因的解释比较模糊。本次在野外用罗盘测量裂隙走向,同时采集砂岩样品进行偏光显微镜和扫描电子显微镜实验观察,分析龟裂地貌形成的原因。野外观察发现,龟裂地貌主要发育在颗粒相对均匀的厚层细粒砂岩中,可产在砂岩层面或其它暴露面上,向下延伸一般不超过15 cm。在偏光显微镜下,砂岩样品的碎屑颗粒主要为次棱角状-次圆状,分选性较好,填隙物中含有方解石胶结物。在扫描电镜下,颗粒胶结疏松,颗粒表面的溶蚀小孔和裂隙比较常见。结合国内外类似研究,本文认为岩性是龟裂地貌发育的基本要素,裂隙走向的玫瑰花图显示,断裂构造控制了一部分龟裂的形成,但并不是唯一的原因。在风化作用过程中,由于硅、铁、锰氧化物沉淀而在砂岩表层形成薄层结壳,后由于应力拉张使结壳破裂,形成细小的裂隙,流水沿着裂隙侵蚀使其扩大,最终形成龟裂地貌。最后,提出龟裂地貌的四个阶段演化模式。

Characteristics and Origin of Polygonal Cracks in the Shicheng National Geopark of Jiangxi Province

Polygonal cracks in the Shicheng National Geopark of Jiangxi Province are generally observed along the red sandstone surfaces of the Late Cretaceous Hekou Formation. They are characterized by various vivid shapes and have formed many pictorial landforms, such as Guishoushi (huge turtle-like rock), Songguoshi (huge pine cone-like rock), and Xianrenlitian (rectangular polygons). However, microscopic studies of these polygonal cracks are insufficient. In consequence, their formation mechanism has been yet poorly understood. In this study, a compass was used to measure the orientations of the cracks in the field. Sandstone samples were collected for experimental analysis by a polarizing microscope and scanning electron microscope. The results show that the polygonal cracks are mainly developed in thick-bedded fine-grained sandstones with relatively uniform granules, which can be observed along the sandstone beddings or other exposed rock surfaces, and generally extend downwards for less than 15 cm. Under the polarizing microscope, the particles of the sandstone samples are mostly sub-angular to sub-rounded and well-sorted. The interstitial material dominantly consists of calcite cement. Under a scanning electron microscope, the grains are loosely cemented, and the pores and cracks on the surface of quartz and feldspar particles are commonly observed. Based on similar studies both in China and abroad, it is held that the lithology should be fundamental for the development of polygonal cracks. The rose diagrams of the cracking show that the regional fault systems control the formation of some cracks to some extent, but this is not the only mechanism. Owing to the precipitation of silicon, iron and manganese oxides, a thin layer of crust is formed within the surface of the sandstones during the weathering processes. The crust would be broken to form tiny cracks after the stress is stretched. These small cracks would become enlarged particularly because of the water flows along them. Then, the cracking features are formed. Finally, a four-stage evolution model of cracking landforms is proposed.