黔东北石阡香树园剖面志留系香树园组生屑滩微相

MICROFACIES OF THE BIOCLASTIC BANKS OF THE SILURIAN XIANGSHUYUAN FORMATION AT THE XIANGSHUYUAN SECTION,SHIQIAN,NORTHEASTERN GUIZHOU PROVINCE,SOUTHWEST CHINA

奥陶纪末赫南特冰期和生物大灭绝事件结束后,扬子陆表海区志留纪兰多维列世早期的海进序列伴随着生物逐渐复苏的过程。在黔中古陆北缘石阡一带鲁丹晚期至晚埃隆早期的香树园组属近岸浅水带灰岩—碎屑岩沉积区。石阡县城北香树园剖面的香树园组以灰岩为主,夹含少量粉砂质、泥质陆源碎屑岩,频繁出现含丰富壳相后生动物化石碎片形成的薄层滩相单元。与大颗粒壳相化石为主形成的各类生物层状礁类似,生屑滩多为动荡流水改造的后生动物碎片近源沉积。滩相具有较高的生物多样性和化石丰度,是奥陶纪末生物礁复苏前浅海暖水区广泛分布的生态单元。此地香树园组缺少粗砂级以上的陆源碎屑颗粒,也无岩屑颗粒,指示沉积物具有较高的成熟度。推测黔中古陆靠近石阡的部位已经夷平,黔东北浅海区在香树园组灰岩沉积期具有偏低的海水浑浊度和碎屑岩沉积速率,生屑滩相灰岩单元是在正常盐度和高清洁度的环境下堆积的,附近有粉砂-泥级颗粒注入,但无大型河流输入丰富的淡水和陆源粗碎屑。

After the glaciation and mass extinction events of the latest Ordovician,transgression sequences of the early Llandovery,Silurian are accompanied with the biotic recovery on the epicontinental sea of the Yangtze region.The Xiangshuyuan Formation of the late Rhuddanian to early late Aeronian covering the near-shore belt close to the Qianzhong Oldland is composed of limestone and terrigenous clastic depositions.The Xiangshuyuan section at the northern Shiqian County Town is dominated by limestones intercalated with less silty and muddy layers.Thin-bedded bioclastic bank units rich in shelly metazoanal fragments are frequently occurred.Biotic components of the bank facies are similar to the biostromal units,which are mainly formed by shelly macrofossils.The grains of the bank facies reformed by turbulent water currents were mostly proximal deposits.The bank facies are high both in biodiversities and fossil abundances,therefore,they are viewed as the typical ecological associations wide-spreading on the shallow and warm marine realms close to the Qianzhong Oldland.Terrigenous clastics are high maturities lacking coarse sandy particle and detritus.Subaerial erosions had led the Qianzhong Oldland of the Shiqian region to a peneplain topographically.Both turbidity and deposition rate of shallow marine belts of northeastern Guizhou were quite low without obvious affection of fresh waters during the intervals of bioclastic limestones formation.Terrigenous clastics are silty and muddy components without coarser debris sourced from big fluvial facies.Therefore,the bank facies were accuminated under a enviroment condition with normal salinity and clarity.