Composition and spatiotemporal evolution of the mixed turbidite-contourite systems from the Middle Ordovician, in western margin of the North China Craton

Abstract

Accurately identifying contourites and analysing the interaction between turbidity flows and bottom currents from the mixed turbidite-contourite systems (MTCSs) in the deep-sea environments are still challenging. Reports on ancient MTCSs are scarce and most of them remain contentious. This study describes at various scales the sedimentological and sedimentary geological characteristics of the Middle Ordovician Yingtaogou Formation, which is identified as a MTCS, from the western margin of the North China Craton (NCC). Twelve typical lithofacies are recognised in the Yingtaogou Formation, which can be grouped into four facies associations interpreted to reflect various deposition from turbidity flow, contour current, debris flow, and hemipelagic settling in the slope–abyssal plain environment. The evidence for contourites is four-fold. Firstly, the palaeocurrent direction based on flute casts on turbidite soles is almost perpendicular to that indicated by traction current structures. Secondly, it is possible to observe a variety of traction current structures (large-scale cross-bedding, cross-lamination, wavy bedding, lenticular bedding, parallel bedding, and horizontal lamination), which are commonly found in reverse and bi-gradational sedimentary sequences at various scales. Thirdly, internal erosive surfaces are widely developed, which reflect the fluid winnowing and velocity fluctuations. Fourthly, these contourites are characterised by an abundance of reverse grading and bi-gradational sequences at various scales. In addition, the contourites identified in this study generally lack bioturbational structures, which is probably related to the late migration and radiation of the Ordovician deep-sea benthos. The sedimentary processes, products, and controlling factors of the interacting down- and along-slope currents were further identified, and a bioturbation-free facies model of contourites and a depositional model of the MTCS were established. This study may contribute to establishing the sedimentological criteria for recognition of ancient contourites and a better understanding of the interaction and spatiotemporal evolution of down- and along-slope sedimentary processes, as well as to a deeper comprehension of the Middle Ordovician palaeoceanographical environment in the western margin of the NCC.