海水深度制约黔东北志留系香树园组生物礁滩的分布

DISTRIBUTION OF REEF COMPLEX CONSTRAINED BY DEEPER MARINE-FLOOR:EVIDENCE FROM SILURIAN XIANGSHUYUAN FORMATION IN NORTHEASTERN GUIZHOU PROVINCE,SW CHINA

奥陶纪末的生物冰川灾变事件结束后,华南板块上扬子区进入志留纪初的海进期。黔东北志留纪初期香树园组(兰多维列统鲁丹阶上部-埃隆阶中部)是壳相动物群复苏时沉积的灰岩相地层,具有显著的生物-岩相分异。"白沙型"后生动物化石密集的礁滩沉积仅限于近岸正常浪基面附近的浅海区展布。同期石阡枫香铺沟村剖面远岸带"印江型"香树园组薄层灰岩中壳相动物大化石的生物多样性和丰度低,没有原地绑结岩型的生物礁和粗颗粒堆积的生屑滩,岩相和生物相特征显示各种细颗粒的生屑呈碎片堆积。尽管这里海水清澈度较高,但由于水深偏大且水动力偏低的环境指标抑制了"白沙型"造礁动物向广海区的空间拓展。这一实例凸显了黔东北陆表海区海水深度对香树园组底栖生态组合多样性的控制,导致志留纪最早复苏的生物礁古地理分布的局限性。

The Xiangshuyuan Formation Iimestones, middle Rhuddanian to middle Aeronian （Llandovery, Silurian） in age, was formed during the transgressional interval on the Upper Yangtze Platform, South China Block aftermath the latest Ordovician glacial event. Accordingly, litho- and biofacies of this unit imply shelly fauna recovery; however, their differentiations along seaward of the Qianzhong Land indicate varieties of palaeoenvironmental conditions. Lithologieal and ecological profiles of the Xiangshuyuan Formation are subdivided into the ＂Baisha type＂ of near-shoal belt and coeval ＂Yinjiang type＂ of deeper off-shoal setting spatially. Reef complex of the ＂Baisha type＂ only occurred within depth of normal-wave base. Lithofacies of the thin-bedded limestones at the Pugoucun section, Fengxiang, southeastern Shiqian, Guizhou province were described as a representative of the ＂Yinjiang type＂. Macrofossils of shelly fauna are quite low in biodiversity and abundance herein and they have no potential for the formation of reefal boundstone and coarse-grained bank units. Bioclasts are generally fine in grain-sized. High clarity but deeper and calmer marine-floor of the Pugoucun section was not idealized for the inhibitation of the reef contributor of the ＂Baisha type＂ palaeoecologically. Therefore, the sea water depth took a significant role in configuration of benthic assemblages on the epicontinental sea of the northeast Guizhou resulting a limited spanning of the reef complex during the earliest Silurian.