重庆老龙洞二叠系-三叠系界线地层的海平面下降事件

老龙洞的二叠系-三叠系界线地层剖面许多学者做过研究,关于其海平面变化问题的争论一直没有解决。本文的研究首次在该剖面上发现了明显的侵蚀面,说明曾经有海平面下降事件。对沉积环境的解释也有新的认识。该剖面在长兴期钙质海绵灰岩之上沉积了一套开阔台地相的沉积,其上是1.4m 厚的局限台地相沉积,具有斑点状构造。此层顶部出现一个波状起伏的侵蚀面。侵蚀面上下的岩性不同。此面之上是0.8m 厚的浅水的局限台地相沉积,具有树枝状的构造和外貌。此层顶面是一个更加明显的侵蚀面,起伏高差达到0.3m 以上。侵蚀面上下的岩性截然不同,以及其上岩石层理与侵蚀面的斜交关系,以及侵蚀面的形态把侵蚀面同缝合线区分开来。这个侵蚀面代表一次显著海平面下降事件之后的一次较长时间的出露和风化剥蚀。侵蚀面之上是透镜状分布的灰色的层状的小腹足类的颗粒岩,再上是一薄层灰黄色的泥粒岩,含丰富的小双壳类。此层以上是青灰色的薄板状的泥质灰泥岩,舍丰富的0.3～3mm 大小的、内部重结晶的同生角砾。

Sea-level drops in the Permian-Triassic boundary section at Laolongdong, Chongqing, Sichuan province.

Restudy of the Permian-Triassic boundary section at Laolongdong, Chongqing, Sichuan Province, south China shows evidence for sea-level drops. This section comprises a basal Changhsingian calcisponge limestone of normal shallow-water condition. It is overlain by 10. 6 m thick packstone of platform facies. The latter is covered by 1.4 m thick lime mudstone and packstone of restricted platform facies, which is characterized by speckled appearance. The top of this interval is an undulated ancient karstic surface. This surface is coated by a 1 mm thick layer of terrestrial clasts of quartz, feldspar, and clay minerals. The overlying 0.8 m thick packstone and lime mudstone has a characteristic dendroid appearance. This unit has a more remarkable erosional top surface. The height difference of the undulated surface at least reaches 0. 3 m. The lentiform deposits in the lower areas of the surface are bedded grainstone composed of microgastropods cemented by fibrous calcite. The horizontal beddings of the grainstone are intersected with the erosional surface. The grainstone is overlain by a thin layer of packstone of small bivalves. The above rock is caesious tabular marly lime mudstone containing abundant small syndepositional pebbles 0. 3 -3 mm wide, which are internally recrystallined.