丹霞山世界地质公园蜂窝状洞穴特征及成因分析

蜂窝状洞穴以其精致的形貌和众说纷纭的成因吸引着科研人员和普通大众的兴趣。丹霞山世界地质公园蜂窝状洞穴主要产于晚白垩世丹霞组二段(锦石岩段)风成砂岩的崖壁上。本文以园区长老峰锦石岩寺和睡美人禄意堂两处蜂窝状洞穴为研究对象,通过野外实地调查,了解洞穴生长环境,定量统计分析其形态特征,并采集砂岩样品进行显微镜下观察,探讨洞穴形成与盐风化作用的关系。研究表明:蜂窝状洞穴所在的岩性主要为中—细粒长石岩屑砂岩,发育大型高角度交错层理,渗透性较好;洞穴开口均向下朝向锦江,有利于来自锦江河流的湿润水汽较长时间地保持在小洞内部;洞穴开口基本服从泰森多边形分布。因此,碎屑颗粒相对比较均匀的风成砂岩为丹霞山蜂窝状洞穴的发育提供了重要的岩石基础,锦江河流为盐风化提供了必要的湿润水汽和盐分。在初始发育阶段,从岩壁上最适合盐风化作用的若干个点开始形成细小的坑洼,逐步向同层位扩张。单个小洞穴由于盐风化作用,洞穴内部风化速率大于洞穴开口边缘,导致蜂窝状洞穴内部空间扩大较快。然后,蜂窝状洞穴的发育突破纹层的限制,逐步加大、变深而呈泰森多边形规则分布。最终,随着洞穴加深,在重力、风力的影响下,蜂窝状洞穴隔壁减薄失稳,洞穴消亡。

Characteristics and Origin of Honeycombs in the Danxiashan Global Geopark in South China

Objectives: Honeycombs are great interests of scientific community and the public alike owing to their delicate morphology and controversial formation processes. The honeycombs in the Danxiashan Global Geopark of South China occur along the cliff faces of the aeolian sandstones, which belong to the Second (Jinshiyan) Member of the Late Cretaceous Danxia Formation. In this study, two sites were chosen to discuss the influences of salt weathering on formation mechanisms of these honeycombs.Methods: Field investigation, size measurement, sandstone sampling and observation under a microscope were performed to obtain lithology, grain composition and fabric, and honeycomb sizes. Moreover, the Voronoi polygons were completed by using ArcGIS 10.2 software to compare with honeycomb openings.Results: The results show that the development locations of the honeycombs are largely selective. The two studied sites are facing downwards the Jinjiang River, which is beneficial for moisture availability and maintenance in them because of the curvature of the rock faces. The bedrock is aeolian sandstones with large scale cross beddings and well permeability. In addition, the openings of honeycombs are basically in accordance with the Voronoi diagrams.Conclusions: The relatively homogeneous thick bedded aeolian sandstones are the crucial bedrock lithology, and the Jinjiang River supplies essential moisture for the development of the honeycombs in the study area. It can be deduced that small pits in the cliffs might generate from a number of points suitable for salt weathering during the initial development stage, and then gradually expands laterally along the same intervals. Owing to the on going salt weathering, the inside back walls are possibly being weathered at higher rates than the side walls of the honeycombs. Then the honeycomb cells cross through laminations, resulting in enlargement and deepening of them. As a result, the honeycomb openings are in agreement with Voronoi polygons at least in part during the mature stage. Eventually, as the honeycomb cells deepen, the side walls become thin and unstable and might suffer sudden collapse owing to the influence of the gravity and wind.