江西龙虎山地区红层沉积相分析及与丹霞地貌发育关系研究——以仙人城景区为例

FACIES ANALYSIS OF RED BEDS AND ITS RELATIONSHIP WITH THE DEVELOPMENT OF DANXIA LANDFORM IN LONGHUSHAN AREA OF JIANGXI PROVINCE: AN EXAMPLE FROM XIANRENCHENG SCENIC SPOT

江西龙虎山地区以发育老年期丹霞地貌为特色,其成景地层为晚白垩世河口组红层,目前对这套红层的沉积相认识程度还比较低。龙虎山仙人城景区河口组出露厚度达百余米,沉积构造发育,且人工开凿的台阶可以直达山顶,这为沉积相分析提供了便利条件。根据野外露头岩性组成、沉积构造等特点,识别出6种岩相单元:无沉积构造砾岩、正粒序层理砾岩、逆粒序层理砾岩、平行层理砾岩、含砾砂岩、古土壤。砾石统计结果表明,砾石粒径范围主要为3~4 cm,最大可达12.5 cm,成分以紫红色凝灰岩为主,砂岩、花岗岩和石英次之、变质岩(主要是片岩)较少,磨圆度主要为次棱角状,其次为棱角状。在显微镜下,砂岩碎屑颗粒主要呈棱角—次棱角状,分选性较差,粒径为0.05~2.00 mm,主要由石英、长石和岩屑组成,总体具有结构和成分成熟度中等偏低的特点。野外露头宏观和室内显微分析结果表明,仙人城丹霞地貌的成景地层为河流主导的冲积扇沉积体系的产物。在龙虎山地区,丹霞地貌的空间分布与盆地冲积扇沉积相的平面展布具有较好的一致性,盆地边缘冲积扇成因的厚层砾岩为后期丹霞地貌的形成和演化提供了物质基础。

Longhushan area is characterized by late-staged Danxia landforms with Late Cretaceous red beds of Hekou Formation as the scenery layer. Little is known about depositional facies of these redbeds. The outcrop of the Hekou Formation is more than 100 meters thick with abundant sedimentary structures. The man-made stone steps from the bottom to the top of the hill are favourable for performing detailed observation and measurement for facies analysis in Xianrencheng Scenic Spot in Longhushan area. Six lithofacies units are recognized based on the characteristics of lithology and sedimentary structures： structureless conglomerate, normal bedding conglomerate, inverse bedding conglomerate, parallel bedding conglomerate, pebbly sandstone, and paleosol. Pebble counting results show that gravels dominantly range from 3 cm to 4 cm with the largest one of 12.5 cm in diameter, and they are mainly composed of purple tuffs, followed by sandstones, granites, quartzite and minor amounts of metamorphic rocks （mainly schists）. The roundness is mainly subangular, followed by angular. Under a microscope, the detrital grains of the interbedded sandstone samples are mainly angular to subangular, poorly sorted, 0.05～2.00 mm, composed of quartz, feldspar and rock fragments. Overall, the sandstone samples are featured by moderate to low textural and compositional maturities. Both the field macroscopic observation and microscopic analysis indicate that the redbeds were deposited by alluvial fan systems. Moreover, the spatial distribution of Danxia landforms and plane distribution of alluvial fan facies have good consistency. In particular, the coarse-grained sedimentary succession of alluvial fan facies along the basin margin provided the fundamental bedrock for the formation and development of Danxia landforms.