江西永丰-崇仁盆地晚白垩世沉积体及其演化模式

The Late Cretaceous Depositional Systems and Evolution Model of the Yongfeng- Chongren Basin in Jiangxi Province

华东南地区白垩纪地壳伸展作用形成一系列SW-NE向展布的陆相断陷盆地,一般由粒度较粗的红色碎屑岩地层充填。赣杭构造带西段的江西永丰-崇仁盆地晚白垩世龟峰群角度不整合覆盖于早白垩世鹅湖岭组碎斑熔岩之上,自下而上包括河口组、塘边组和莲荷组。根据野外露头剖面沉积组构特征,可以识别出冲积扇、河流和湖泊等三种沉积体系。对各沉积相沉积物组成、沉积构造、垂向相序进行了探查认为,研究区上白垩统具有粗-细-粗的粒度变化旋回特征,反映了自下而上由冲积扇、河流到河流、湖泊再到冲积扇、河流沉积体系的有序变化。结合区域构造演化资料,初步建立的盆地沉积演化模式表明,河口组和塘边组为连续沉积,代表了断陷盆地开始发育和湖盆扩大阶段,反映了控盆构造由强烈活跃期到间歇宁静期的逐渐过渡。莲荷组再次以冲积扇和辫状河相砾岩为主,其中大量来自下伏红层的砾石成分说明莲荷组沉积时期控盆构造运动加剧。江西永丰-崇仁盆地沉积体系时空上明显的变化过程,可能是赣杭构造带晚白垩世陆相断陷盆地演化主要受区域断裂控制的一个典型代表。

Due to crustal extension in Cretaceous,the continental faulted basins were widely formed in southeastern China,which were generally filled with reddish coarse-grained siliciclastic rocks. There are a lot of Cretaceous volcanic and sedimentary basins in the Gan- Hang tectonic belt. Previous studies have been focused on the volcanicintrusive rocks and related uranium ore deposits,but the Cretaceous red beds have been paid less attention. The red beds are the stratigraphic base of the Longhushan World Geopark in northeastern Jiangxi province. The Yongfeng- Chongren basin is located in the western part of the Gan- Hang tectonic belt,where the Upper Cretaceous red beds are called Guifeng Group. The Guifeng Group rests unconformably on the porphyroclastic lava of the Lower Cretaceous Ehuling Formation,and is covered unconformably by the Quaternary Lianxu Formation. The Guifeng Group is upwards composed of Hekou,Tangbian and Lianhe Formation. Based on sedimentary characteristics at outcrop sections,three kinds of depositional systems（ fluvial fan,river and lacustrine） could be recognized in the Upper Cretaceous Guifeng Group of the Yongfeng- Chongren basin. Sediment components,sedimentary structures and vertical facies sequences are described and summarized for each sedimentary facies. The results show that the Guifeng Group is approximately a coarse-fine-coarse cycle,reflecting a gradual and orderly transformation process from fluvial fan-river to river-lacustrine and then to fluvial fan-river depositional systems. Combined with regional tectonic data,an alternative basin evolution model is constructed. According to the model,from the Hekou Formation to Tangbian Formation,it was a relatively continuous sedimentation process and an initial and extending stage of the basin,which means that the basin-controlling faults might change from intensively active phase to interval quiet phase. The Lianhe Formation is again dominated by layers of coarse conglomerates of fluvial fan and braided river facies. In addition,at Zhoupi section of Yongfeng county,a lot of gravels in the Lianhe Formation conglomerates are interpreted to be from the underlying red beds,which suggests the structural pattern of the basin might have changed from extension to compression. That is to say,there might be an unconformity between the Lianhe Formation and the underlying Tangbian Formation. But the lateral extent of the unconformity and its detailed response to the regional tectonic activities need further study. The obvious spatial and temporal changes of depositional systems in the Yongfeng- Chongren basin could be a typical example of the Late Cretaceous faulted basins which were mainly controlled by regional tectonics in the Gan- Hang tectonic belt.