贵州地区中三叠统垄头组沉积特征及其海平面变化意义

Sedimentary characteristics of the Middle Triassic Longtou Formation in Guizhou Province,South China: Implications for sea-level change

为认识中三叠世拉丁期华南地区碳酸盐岩台地边缘沉积演化及其海平面变化特征,文中主要选取贵州关岭扒子场剖面,以岩石学特征研究为基础,建立中三叠统拉丁阶垄头组的沉积演化序列,来恢复其沉积时期的海平面变化特征。中三叠统垄头组按其岩石类型及组合特征可分为3段:下段主体为核形石灰岩—叠层石沉积旋回,代表潮间带至潮上带环境。中段下亚段由核形石灰岩—泥晶灰岩—玛瑙纹层状灰岩旋回组成,最顶层的玛瑙纹层状灰岩指示地表暴露环境;中段上亚段为核形石灰岩—叠层石旋回,代表潮间带至潮上带环境。上段由代表潮间带至潮下带的核形石灰岩—似球粒灰岩旋回组成。垄头组沉积特征表明,该沉积时期黔西南地区海平面整体表现为持续上升,但在中段沉积时期发生过一次海平面下降事件,导致沉积物持续遭受地表暴露,随后海平面开始上升,又恢复到潮坪沉积环境。与国内外中三叠世拉丁期海平面变化研究结果相比,贵州地区垄头组中段沉积时期的海平面下降很可能是拉丁期大海退的产物,但自中段上亚段至上段沉积时期,海平面逐渐上升,与同时期的黔西南地区和全球海平面变化趋势一致。该结果对于认识玛瑙纹层状灰岩成因及中三叠世晚期生物复苏研究具有重要意义。

To better understand the deposition and sea level change histories of the Ladinian carbonate platform margin in the South China,this paper studied the sedimentary characteristics of the Middle Triassic Longtou Formation at the Pazichang section,Guizhou Province. The Longtou Formation is dividedinto 3 members based on lithofacies and their associations. The Lower Member of Longtou Formation consists mainly of oncoidal limestones and stromatolites,which are indicative of intertidal to supratidal environments. The Middle Member is composed of oncoidal rudstones,mudstones,laminated limestones,and pisoidal rudstones in the lower part,and oncoidal rudstones and stromatolites in the upper part. Subaerial exposure occurs at the uppermost position of the lower part of Middle Member,where the thickest laminated limestones are deposited. In contrast,the upper Middle Member was probably formed in intertidal to supratidal environment. The Upper Member consists of oncoidal rudstones and peloidal packstones,indicating an intertidal-subtidal environment. The reconstructed sea level change curve suggests a general sea level rise occurring during the Ladinian period in Guizhou. During the deposition of the Middle Member of Longtou Formation,however,sea level underwent a rapid fall,which possibly corresponds to the Ladinian Great Sea-level Regression event. Thereafter,sea level rises gradually and the deposition in tidal environment continues. Hence,the sea level change history during the deposition of the upper Middle Member to the Upper Member is consistent with the global sea level change and that in the southwestern Guizhou. The results provide clues to better understanding the origin of the laminated limestones and the latest Middle Triassic biotic recovery.