贵州沫阳剖面二叠纪末浅水沉积特征及其古环境意义

处于大贵州滩南缘的罗甸沫阳剖面在晚二叠世时发育典型的浅水相沉积组构，包括长兴期生物礁、生物灭绝事件界面附近的不连续面以及可能的古岩溶堆积物。其中长兴组为典型的钙质海绵礁，造礁生物主要包括古石孔藻、Tubiphytes和纤维海绵，同时大量的栉壳状胶结物填充礁体孔隙并具有世代性。在沫阳剖面的不连续面，可识别标志包括缝合线和侵蚀面两种，分布于上二叠统顶部泥粒岩-颗粒岩（〖XCTT.TIF〗类、海绵、钙藻以及Tubiphytes为主）与鲕粒灰岩之间。在部分露头上还可见一类特殊的角砾岩:角砾类型既包括生屑团块和泥晶团块，还包括硅质岩碎块，数量众多，大小不一且磨圆很差。大量含巨鲕的鲕粒灰岩直接上覆于角砾岩，以及角砾岩顶部的裂隙之中。除此之外，包括垮塌角砾岩和洞穴堆积物等特征的发现，都指示了沫阳剖面在当时岩溶发育。这类沉积物产生的原因可能是由于晚二叠世时，沫阳地区持续发育的生物礁形成了突出的古地貌单元，随着相对海平面的下降晚二叠世沉积物发生暴露。在大气淡水-渗流成岩作用影响下，暴露地表的突出地形上岩溶作用发育；非突出地形受风化剥蚀及随后海侵过程的双重影响，与上覆地层之间显示侵蚀面或缝合线接触的特征。从野外露头情况来看，沫阳剖面并没有像其他浅水台地一样在生物灭绝事件之后发育微生物岩，而是直接出露一套含核形石和巨鲕的鲕粒灰岩。以鲕粒灰岩为主的“灾难沉积物”紧邻长兴组生物碎屑灰岩的情况在华南比较少见，这对于进一步了解生物灭绝事件起因以及当时特殊的海洋环境状况具有重要意义。

Characteristics and Palaeoenvironmental Significances of Shallow-Marine Sediments in the Latest Permian,Moyang Section,Guizhou

The Moyang section located in the southern margin of the Great Bank of Guizhou developed typical deposits of shallow-marine environments in the Late Permian,including Changhsingian reefs,disconformity surfaces near the mass extinction horizon,and one probably kind of palaeokarst sediments.Calcareous algal-sponge boundstone characterized by Archaeolithoporella,Tubiphytes and winded sponges,and early marine cements constitute the frameworks in the upper of the Changxing Formation.Disconformity surfaces(with the contacts between the lower and upper facies stylolitized or eroded) make a distinction between the latest Permian oolitic limestone and the Changhsingian skeletal packstone-grainstone containing the dominated fusulinids,sphinctozoan sponges,Tubiphytes and other types of calcareous algae.In some outcrops,one special kind of breccias found in the upper part of the Changxing Formation,which overlain or incised by giant-ooid-bearing oolite directly in the topmost,comprises numerous clasts of bioclastic limestone,lime mudstone and silicate,with diverse and angular shapes.In addition,the presence of collapse structures and white cement layers consisting of coarse sparry calcite indicate the possible karstification.Based upon these features,we suggest the fall in relative sea-level of Moyang caused the subaerial exposure of the top of the Changhsingian complex in the latest Permian.Under meteoric vadose diagenesis,karstic processes formed collapse breccia,fissures,solution vugs and speleothem carbonate in the ridgy topography(with persisting growth of the bio-reef).In contrast,erosional surfaces developing in non-ridgy topography probably formed by the process of weathering,and reworking in ensuing rise of sea-level.Not like other sections growing microbialites after the event horizon,the Moyang section developed oolitic limestones bearing oncoids and giant ooids at that moment.This phenomenon implies an unusual phenomenon developing oolite facies overlying top of the Changhsingian bioclastic grainstone directly,and hence,has the significance of helping us understand the origin of the mass extinction and temporal oceanic conditions.