桂西南柳桥地区深水相二叠系-三叠系界线剖面

A UNIQUE DEEP-WATER PERMIAN-TRIASSIC BOUNDARY SECTION FROM THE LIUQIAO REGION IN SOUTHWESTERN GUANGXI, SOUTH CHINA

桂西南柳桥地区上二叠统为厚约84m的浅灰—灰白色块状海绵蓝藻生物灰岩(生物礁),之上为浅灰绿色—暗灰色薄层状含远洋浮游生物硅质岩、含泥质硅质岩、硅质泥岩(大隆组),这套深水相的沉积岩夹有大量浅灰白色的粘土岩。上覆的三叠系罗楼组底部为黄色泥岩,夹多层灰白色粘土岩,往上渐变为灰色薄层状含大量双壳类、菊石化石的钙质泥岩、泥灰岩。柳桥地区晚二叠世末期沉积岩和中基性火山岩的特征显示其是在地壳裂解、快速沉降、相对海平面快速上升过程中迅速淹没台地边缘的生物礁而形成的深水相海盆,造成深水盆地与浅水台地并存,具有多岛洋的地理分布格局。东攀二叠系-三叠系界线剖面的露头完整,对剖面的岩石学特征、生物化石组合特征等方面的详细研究显示,该地区一直处于连续沉积的深水环境中,没有发生沉积间断,界线上下的岩石特征和生物组合特征较清楚。

The Late Permian of the Liuqiao region in southwestern Guangxi consists of about 84m of pale and grey, sponge- and algae-bearing organic limestone （Late Permian organic reef）, that grades upward into 40m of dark thin-beded silicalite, tuff-bearing silicalite, mud-bearing sillicalite, and silica-bearing mudstone （the Dalong Formation） containing an abundant pelagic fauna and interbedded with an abundant pale claystone. The overlying lowermost Triassic consists of yellow mudstone interbedded with pale claystone, that grades upward into thin-beded calcarecous mudstone （the Luolou Formation） containing an abundant bivalves and ammonoids. In the Late Permian, the crusts were broken and dropped in this region characterized by volcanics and sedimentary rock, followed by a rapid rise in relative sea level, and the organic reef on the margin of calcarceous plate were submerged. Thus, there occurred deep-water basin and shallow plate at the same time during the latest Permian in the Liuqiao region, being of archipelagic palaeogeography. The outcrop of the Dongpan Permian-Triassic Boundary section is well exposed and detailedly studied in stratigraphy, paleontology and lithology, indicating continually deposition from the latest Permian to the earliest Triassic. Thus, this section may have included a lot of geologic information and may be one of the best sections for studying the mass extinction, recovery, and the other related geologic events.