江西象山地质公园丹霞地貌成景地层沉积环境分析

doi:10.19657/j.geoscience.1000-8527.2018.02.05

摘要/Abstract

摘要：

随着旅游地学的兴起和地学科普的需求,红层与丹霞地貌正在吸引着越来越多的关注。赣东北贵溪市象山地质公园发育以赤壁丹崖为特色的丹霞地貌景观,成景地层为上白垩统圭峰群河口组以砾岩为主的红色碎屑岩,冲刷侵蚀界面、粒序层理和交错层理等沉积构造发育,这为沉积环境分析提供了良好的判断标志。对该套成景地层进行厘米级实测,另外选择2处砾岩露头进行砾石统计以获取碎屑颗粒粒径、岩性、磨圆度和风化程度等信息。在实测厚度为40余米的地层柱状图上,识别出7种岩相类型,即正粒序砾岩、逆粒序砾岩、无沉积构造砾岩、平行层理砾岩、交错层理砾岩、聚集状砾岩、砂岩。砾石以中砾(2~5 cm)为主,磨圆度低,风化程度中等,成分以凝灰岩为主(58%~66.5%),其次为石英、花岗岩、砂岩。砾石成分和砾向分析结果说明,盆地南部早白垩世凝灰岩提供了主要的碎屑物质。根据野外实测红层剖面和砾石统计分析,认为成景地层为冲积扇沉积体系的产物。河口组红层沉积时期,盆地边缘断层活跃,间歇性强降雨事件导致冲积扇朵叶上洪泛河流搬运的沉积物供应充足,最终在剖面上形成砾岩和砂岩频繁互层及其厚度横向不稳定的现象。在中国东南地区晚白垩世总体干燥的古气候背景下,河口组中河流作用主导的冲积扇沉积体系可能指示了古气候向湿润转变,降水增加,相对丰富的水系将粗碎屑搬运到盆地区发生沉积。

关键词: 丹霞地貌, 沉积环境, 冲积扇沉积体系, 陆相红层, 赣东北

Abstract:

Owing to the rise of tourism geoscience and the requirement of geoscience popularization, the research of redbeds and Danxia landform is attracting more and more attention. The Xiangshan Geopark of Guixi City in northeastern Jiangxi Province is featured by the prominent reddish cliffs. The bedrock of the Danxia landform is the conglomeratic strata of the Late Cretaceous Hekou Formation of the Guifeng Group, which is characterized by erosional bases, graded beddings and cross beddings. The sedimentary structures are useful to interpret depositional environments. A detailed centimeter-scale outcrop section was measured to describe lithofacies. Moreover, two conglomerate outcrops were chosen to perform pebble counting. In the measured 40-meter-thick stratigraphic column of the Hekou Formation, seven lithofacies were distinguished: normal graded conglomerate (facies A), inverse graded conglomerate (facies B), structureless conglomerate (facies C), parallel bedding conglomerate (facies D), cross bedding conglomerate (facies E), concentrated conglomerate (facies F), and sandstone (facies G). Clast sizes are mainly 2 cm to 5 cm, although it can range from 0.5 cm to 12.5 cm. Clasts are poorly rounded and moderately weathered. They are composed dominantly of tuff (58%-66.5%) with minor quartzite, granite, and sandstone, which is indicative of the dominant sediment derivation from the Early Cretaceous volcanic-intrusive complex outcropped along the southern margin of the basin. Based on the measured section and pebble counting, the alluvial fan depositional system is envisaged for the formation of the redbeds of the study area. The sediment increments were produced by the combination of faulting along the basin margin with episodic strong rainfalls and transported by braided streams on the alluvial fan lobes. As a result, the frequent interstratification of the conglomerate and sandstone beds, erosions, prevalent fining-upward units were preserved in the outcrops. In addition, under an overall arid climate regime during Late Cretaceous in Southeastern China, the stream-dominated alluvial fan systems were indicative of a climate change towards greater humidity which possibly resulted in abundant streams transporting coarse sediments to the basin area.

Key words: Danxia landform, depositional environment, alluvial fan depositional system, continental redbed, Northeastern Jiangxi Province

中图分类号:

P512.2

刘鑫, 陈留勤, 李馨敏, 李余亮. 江西象山地质公园丹霞地貌成景地层沉积环境分析[J]. 现代地质, 2018, 32(02): 260-269.

LIU Xin, CHEN Liuqin, LI Xinmin, LI Yuliang. Depositional Environments of the Bedrock of Danxia Landform in Xiangshan Geopark of Jiangxi Province, SE China[J]. Geoscience, 2018, 32(02): 260-269.

图/表 6

图1 江西信江盆地西部地质简图(据文献[23?-25]修编)及象山地质公园河口组红层实测剖面位置

Fig.1 Geological map of the western Xinjiang Basin and the location of the measured section of the Hekou Formation redbeds in the Xiangshan Geopark of Jiangxi Province

图2 象山地质公园丹霞地貌成景地层露头照片 (a)、(b)丹霞地貌宏观照片:(a)近圆丘状山顶和陡峻崖壁;(b)崖壁上发育顺层岩槽、扁平洞穴及穿洞,照片中右下角的人(身高170 cm)为比例尺;(c)发育正粒序层理构造的砾岩,底界面为一个明显的侵蚀面;(d)不发育沉积构造的砾岩层(地质锤所在层)及其上发育槽状交错层理的细砾岩;(e)可分为A、B、C三层,A层为发育逆粒序层理的砾岩,B层(地质锤所在层)为透镜层状粗砂岩,其顶底界面均为岩相突变面,C为发育前积层的砾岩,显示正粒序层理构造;(f)侵蚀面(红色虚线)之上为具有前积层的砾岩,之下为巢状砾石聚集的砾岩(见右下角椭圆形局部放大插图),可见斜立的较大砾石产出于砾岩层的上部

Fig.2 Outcrop photos showing the reddish bedrock of Danxia landforms in the Xiangshan Geopark

表1 象山地质公园河口组红层岩相总结

Table 1 Summary of lithofacies in the redbeds of the Hekou Formation in the Xiangshan Geopark

类型	名称	层厚/cm	沉积构造或特征	成因解释
岩相A	正粒序砾岩	10~60	底侵蚀界面,正粒序层理	反映洪水泛滥事件引起的水流能量由高到低的变化。底侵蚀界面指示了高能水流对早期沉积物的侵蚀作用
岩相B	逆粒序砾岩	5~60	底侵蚀界面,逆粒序层理	在分散压力和动力筛选作用下,颗粒间互相碰撞导致较粗的砾石向上运动
岩相C	无沉积构造砾岩	10~40	不发育沉积构造,砾石可呈线状排列	富砂碎屑流在快速沉积作用下形成的近源沉积,沉积物来不及分选
岩相D	平行层理砾岩	30~110	平行层理	在快速迁移的、宽而浅的河道环境,或是在冲积扇下游的非河道化泛滥作用
岩相E	交错层理砾岩	15~60	槽状、板状交错层理	板状交错层理主要是在低流态条件下,由大的直脊波纹和沙丘迁移形成的,而槽状交错层理是由小型水流波纹迁移形成的
岩相F	聚集状砾岩	10~20	颗粒支撑的砾石,不显示粒序变化	片流或越岸流体将沉积物表层的细粒组分带走,而剩下粗粒碎屑颗粒聚集形成筛状沉积
岩相G	砂岩	< 40	常与下伏砾岩层渐变构成向上变细的沉积旋回,或呈透镜层状夹于砾岩层之间	以粗砂岩为主,少量为中砂岩,多含有砾石。顶界面为被上覆砾岩下切侵蚀而成的冲刷面

图3 象山地质公园河口组红层实测柱状图岩性柱右侧的蓝色折线投点为每个砾岩层中10个最大砾石粒径平均值(MPS)

Fig.3 Stratigraphic section of the Hekou Formation redbeds in the Xiangshan Geopark

图4 象山地质公园砾岩砾石粒径分布

Fig.4 Gravel size distribution of the conglomerates in the Xiangshan Geopark

图5 砾岩层最大砾石粒径(MPS)与砾岩层厚度(BTh)交汇图

Fig.5 The maximum particle size (MPS) versus bed thickness (BTh) plot for the conglomerates

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