湖南慈利江垭剖面二叠系-三叠系界线层序的沉积微相类型、沉积环境和海平面变化

本文对湖南慈利江垭剖面二叠系-三叠系界线层序(大隆组顶部9.4m和大冶组底部7.5m)进行了详细的沉积微相分析,划分出八种微相类型,并结合露头和光面上的沉积特征,对每种微相的成因和沉积环境进行了分析和讨论。在此基础上,研究了界线附近的沉积环境和相对海平面变化。大隆组顶部层序沉积于相对海平面持续上升阶段,随着相对海平面的上升,沉积环境逐渐由盆地边缘向盆地内部迁移。在大隆组最顶部,相对海平面有一快速的大幅度下降,沉积环境由深水盆地突然转变为浅水台地。之后自大冶组底部向上,相对海平面又逐渐上升,沉积盆地依次经历了滞流盆地、半循环盆地和循环盆地的转换;在距离大冶组底部约7m处,相对海平面开始下降,气候变得极为干燥,沉积盆地转变为蒸发盆地。值得指出的是,大隆组-大冶组界线处的快速海退面正好对应于二叠纪末生物灭绝面,从而表明大海退很可能是造成二叠纪末生物大灭绝的重要原因。

Microfacies, sedimentary environment and sea-level changes of the Permian-Triassic boundary succession in the Jiangya section, Cili County, Hunan Province

This paper focuses on the microfacies features of the Permian-Triassic boundary succession (including the uppermost 9.4m of the Dalong Formation and the lowermost 7.5m of the Daye Formation) in the Jiangya section, Cili County, Hunan Province in order to understand the depositional environments and relative sea-level changes during this critical time interval. Based on the detailed microfacies analysis in combination with sedimentary features observed on outcrops and polished slabs, the uppermost part (9.4m thick) of the Dalong Formation was deposited in gradually deepening settings from basin margin to the center of the basin. At the top surface of the Dalong Formation, relative sea level dramatically dropped, and the depositional environment shifted abruptly from a deep-water basin to a shallow-water platform. From the base of the Daye Formation above, relative sea level rose slowly, causing changes in depositional environments from an isolated basin, to a semi-isolated basin, and to a circulated basin. At the horizon of about 7m above the base of the Daye Formation, relative sea level began to fall again, and the climate became extremely arid, which resulted in an evaporative basin in the Jiangya area during the Earliest Triassic. It is worth noticing that the great regression across the boundary of the Dalong and Daye formations exactly coincided with the End-Permian mass extinction horizon, which may indicate that the rapid regression might have been one of the major causes to the End-Permian mass extinction.