扬子地台与华南南盘江盆地大贵州滩三叠系沉积演化史

扬子地台是一个横跨华南地块的以浅海沉积为主的大型碳酸盐岩台地,南盘江盆地是发育在扬子地台碳酸盐岩台地背景之上的一个沉积盆地,从晚元古代到晚三叠世长期海相沉积演化历史中,扬子地台-南盘江盆地体系经历了多次重要的构造演化阶段.扬子地台从晚元古代到早三叠世末期一直保持为一个稳定的碳酸盐岩台地,在中三叠世末期扬子地块整体抬升,海平面下降,形成了遍及扬子主体的拉丁期大海退,从而使扬子地块大部分地区抬升为陆.南盘江盆地位于华南地块南缘,从晚元古代到晚三叠世沉积了一套厚度巨大的海相碳酸盐岩,晚三叠世发育了一套硅质碎屑的浊流沉积,区域沉积也由此转化为河流相沉积.二叠纪和三叠纪碳酸盐岩地层记录了碳酸盐岩台地长期演化历史及其特征多样的沉积建造和沉积环境,而硅质碎屑流和构造沉降速率的变化反映了盆地在三叠纪期间经历的聚合构造和前陆盆地发展过程.在三叠纪时期扬子地台沿西南-北东方向从云南围绕南盘江盆地向贵州延伸,在南盘江盆地中发育了几个孤立的碳酸盐岩台地,包括位于贵州南部和广西境内的大贵州滩和崇左-平果台地.南盘江盆地在晚二叠世发生过一次区域性的海侵事件,早三叠世时期扬子地台和几个孤立台地为由鲕粒边滩组成的低角度斜坡,中三叠世(安尼期)变为由Tubiphytes边礁组成的陡倾斜坡.盆地范围内斜坡变陡激发了Tubiphytes礁和其它的生物体发育,而且它们组成了稳定碳酸盐岩台地的边缘.位于扬子地台西部地区的关林和贞丰一带与最北部的孤立台地(大贵州滩)在安尼期发育了陡倾的边礁.在拉丁期,扬子地台在关林一带进积并与盆地碎屑沉积互层穿插沉积,而位于贞丰的台地边缘出现了由断层控制的地貌特征.与此同时,扬子地台东部(贵阳)由侵蚀滑塌边缘变为进积边缘,向盆地内部进积充填形成了超过600 m的碎屑沉积.但是,与扬子地台不同,位于最北部的孤立台地(大贵州滩)在拉丁期由加积边缘礁变为起伏明显的侵蚀陡崖和饥饿盆地边缘.晚三叠世(卡尼期)扬子地台西部下沉并被晚三叠世浊流沉积埋藏,而扬子地台东部地区在被硅质碎屑沉积埋藏之前持续沉积了一套浅水碳酸盐岩沉积.孤立台地为从南到北逐渐变陡的边缘沉积,而且发育了多个小丘,其中南部地区早期沉降后来被硅质碎屑沉积埋藏,而北部地区到后期下沉.与扬子地台西部一样,最北部的孤立台地(大贵州滩)在晚三叠世下降被碎屑沉积埋藏.以上这种差异源于华南地块南缘因构造聚合作用导致的盆地南部地区不同沉降速率.大贵州滩是盆地中演化历史最长的孤立台地,穿过大贵州滩孤立台地内部和边缘的两条正交剖面显示出了一个被断层切断的向斜构造,这样就很容易识别其沉积建造特征及演化历史.大贵州滩发育的整合的二叠系-三叠系界线剖面以及从早三叠世到中三叠世生物复苏阶段连续的巨厚沉积,使其成为一个研究二叠世末期生物大绝灭期间的海相环境以及生物生态条件最为理想的地区.

Triassic Depositional History of the Yangtze Platform and Great Bank of Guizhou in the Nanpanjiang Basin of South China

The Nanpanjiang basin occurs in the southern margin of the south China plate. Marine sedimentation dominated from the Late Proterozoic to the Late Triassic when siliciclastic turbidites filled the basin and sedimentation regionally shifted to fluvial deposition. Permian and Triassic carbonate strata record a long history of platform evolution and include diverse architectures and evolutionary histories that reflect the impact of local depositional environments, rates of siliciclastic flux and accelerating tectonic subsidence as the basin experienced tectonic convergence and foreland basin development in the Triassic. The Triassic margin of Yangtze platform that rims the basin extends in a sigmoidal SW/NE trend fromYunnan through Guizhou.Several isolated platforms, including the Great Bank of Guizhou (GBG) and Chongzuo-Pingguo platform, occur within the basin in southern Guizhou and Guangxi.The basin expanded in the Late Permian during a regional transgression. The Yangtze platform and isolated platforms evolved from low-angle ramps with oolite margins in the Early Triassic to steepening Tubiphytes reef margins in the Middle Triassic (Anisian). Basin-wide shift from ramp to steepening-margins was stimulated by the evolution of Tubiphytes and other organisms that stabilized platform margins.The western Yangtze platform (Guanling and Zhenfeng) and northernmost isolated platform (GBG) aggraded and developed steep reef rimmed margins in the Anisian. During the Ladinian the Yangtze platform at Guanling aggraded and intertongued with basin filling clastics, while the margin at Zhenfeng developed a tectonically backstepped morphology controlled by faults.At the same time the eastern sector of the Yangtze platform (Guiyang) evolved from an erosionally collapsed margin to a progradational margin that advanced basinward at least 600 m over basin filling clastics. Unlike the Yangtze platform the northernmost isolated platform (GBG) evolved during the Ladinian from an aggradational reef rim to a high-relief erosional escarpment with a starved basin margin.The western Yangtze platform was drowned and buried by turbidites in the Late Triassic (Carnian) whereas shallow-water carbonate sedimentation continued until burial by siliciclastics in the eastern sector.The isolated platforms exhibit a south to north pattern of step-backed margins and pinnacle development and earlier drowning and burial by siliciclastics in the south versus greater longevity and later drowning in the north. Like the western sector of Yangtze platform, the northernmost isolated platform (GBG) drowned and was buried by clastic sediments in the Later Triassic (Carnian).These differences resulted from faster subsidence rates in the southern part of the basin caused by tectonic convergence along the southern margin of the south China plate. GBG has the longest history of isolated carbonate platforms in the basin. A faulted syncline exposes a continuous two-dimensional cross section through the platform interior and margins, thus facilitating a detailed assessment of its architecture and depositional history. Conformable Permian-Triassic boundary sections, and thick, continuously exposed sections through the Early to Middle Triassic biotic recovery interval make this platform an ideal area for evaluating the marine environments and biotic conditions that operated during the end-Permian extinction and its aftermath.