Middle-Late Ordovician Deep-water Bottom Current Sedimentary System And Its Evolution In The Western North China Craton

The Ordovician is generally considered to be a period of enhanced ocean circulation.However,the deep-water bottom current activity and the deep-sea circulation system in the Ordovician are relatively poorly understood due to the lack of direct sedimentological evidence and reliable characterization indicators.In addition,reported ancient bottom current deposits in deep-water settings are scarce and most of them remain contentious.In this study,based on the five Middle–Upper Ordovician sections in the western North China craton(NCC),we focus on the Ordovician deep-water bottom current deposits.The identification marks,sedimentary process,sedimentary evolution,main controlling factors,direction of bottom current activity and deep-water circulation pattern of the Ordovician are systematically studied by methods of the anisotropy of magnetic susceptibility(AMS),sedimentary fabrics,microfacies and facies associations at different scales.On the one hand,the identification marks and facies models of contourites lacking bioturbation are proposed,which make up for the shortcomings of the identification marks and facies models of classical contourites.On the other hand,the direction and sedimentary evolution process of the Ordovician deep-water bottom current activity in the western NCC were ascertained,and the Ordovician deep-sea circulation model was preliminarily established.This not only provides reliable sedimentological evidence for the Ordovician bottom current activity,but also contributes to a better understanding of the deep-water bottom current deposition process and the nature of the Ordovician palaeoocean circulation,as well as to provide some new evidence and enlightenment for exploring the relationship between the diversity evolution of marine organisms and the ocean circulation under the background of the Great Ordovician Biodiversification Event(GOBE).