Abstract:

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 σ₁:0.45-0.57;SK₁:(-) 0.599;K_g,: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.