黔北志留系石牛栏组松坎段上部近岸——远岸的相分异

华南板块上扬子区滇黔桂古陆以北的陆表海缓坡带,因海水深度变化导致志留纪兰多维列世埃隆晚期石牛栏组的岩相—生物相差异,这种差异始于松坎段上部沉积期。本文选择2个剖面,分别代表近岸和远岸带的古地理位置,作松坎段上部灰岩岩相学和沉积环境指标分析。余庆大庄村剖面位于近岸带,可频繁出现薄层泥粒状灰岩和颗粒灰岩,属浅水高能带常见的壳相生屑滩堆积;而桐梓水坝塘剖面位于远岸带,偏深水低能环境中的瘤状灰岩和含泥灰岩透镜体多含灰泥和粉砂屑。这些证据从细节上揭示松坎段上部沉积时,靠近黔中古陆近岸带存在壳相动物大量繁衍的生态位,还伴随有钙质微生物形成的叠层石和核形石,但空间展布上可能是狭窄的,大部分陆表海区的海水深度处于最大风浪暴浪基面以下。

FROM NEAR-SHORE TO OFF-SHORE:FACIES DIFFERENTIATION OF THE UPPER SONGKAN MEMBER(SHINIULAN FORMATION,SILURIAN)IN THE NORTHERN GUIZHOU PROVINCE,CHINA

The northern Guizhou Province was a ramp setting of the Yangtze epicontinental sea,South China Block with the Dian-Qian-Gui Oldland to the south and seaward to the north during the late Aeronian,Llandovery,Silurian.The litho-and biofacies differentiation of the Shiniulan Formation started from the upper Songkan Member due to the depth change of the sea water.Two sections are employed for analyzing the lithofacies and paleoenviromental parameters.Packstones and grainstones are frequently occurred from the near-shore belt of the Dazhuangcun Section,Goupitan,Yuqing County.Thus,its depositional background is considered as a shallow and high-energy shelly bank.In contrast,the coeval sequences from the off-shore belt of the Yangshishan Section at Shuibatang,Tongzi are rich in nodular limestones and argillaceous limestone lenses composed of limemud and silty-sized grains.The present evidences imply that an ecological realm close to the oldland did exist which was idealized for the thriving of shelly biota together with calcimicrobial communities.The limestones herein are rich in stromatolitic and oncolitic components during the depositional interval of the upper Songkan Member.However,such an ecological realm was possibly of a narrow distribution spatially.The depositional sequences were predominately formed below the depth of major wavebase of the epicontinental sea.