桌子山中奥陶统公乌素组等积岩的确认及沉积环境

IDENTIFICATION OF CONTOURITES IN MIDDLE ORDOVICIAN GONGWUSHU FORMATION,ZHUOZISHAN,AND DEPOSITIONAL ENVIRONMENT

内蒙桌子山地区中奥陶统公乌素组为一套含浮游生物和丰富遗迹化石的薄层灰岩、砂页岩为主的沉积。本文根据岩性、结构构造、遗迹化石组合等特点，初步确认其上部的砂质岩石为等积岩。由此，并结合古生物类型及其古生态特点等探讨了公乌素组的沉积环境。认为其沉积位置为有等深流活动的大陆斜坡，沉积时水深至少为２００—６００ｍ。

The Middle Ordovician Gongwushu Formation,distributed in the Zhuozishan area,Inner Mongolia,consists chiefly of greycoloured thin-bed limestones interbedded with shales in the lower part and thin-bed siltstones in the upper.It also yields abundant trace fossils,some planktonic organism fossils.In terms of its swhmentary characteristics,trace fossil assemblages and the related body fossils,it is suggested that the upper thin-bed siltstones should be sandy contourites,and its sedimentary characteristics are in accordance with the criteria proposed by Hollister and Heezen,Lovell and Stow.The main grain parameters are:0.45-0.57;SKt:(-) 0.599;Kg,:3.089.Trace fossils which co-exist with the contourites are ichnogenus Helminthopsis and Paleodictyon.Boby fossils which occurred in the formation are graptolites,small thin-shelled brachiopds and deep-coldwater conodents.The sedimentary environment of the formation is belived to be a continental slope with water depth no less than 200-600 metrs during the deposition process.