Organic-carbon deposition and coastal upwelling at mid-latitude during the Upper Ordovician (Late Katian): A case study from the Welsh Basin, UK

Abstract

A lack of stratigraphical, sedimentological and geochemical data for sediment accumulation rates and indicators of productivity and anoxia means that causative models for ancient black shales are largely inferred from modern settings. Coastal upwelling has been suggested as a general hypothesis for Ordovician black shale deposition within the Iapetus Ocean, but has not been directly tested. Despite anchizone metamorphism we utilize a suite of geological and geochemical environmental proxy data (TOC wt.%, δ 13C org, Ba/Al, P) to elucidate the mechanism for the origin of a single grey-black shale cycle within the upper Katian succession of the Welsh Basin. Here we interpret organic carbon (OC)-rich deposition to be suggestive of a period of high photic productivity (higher TOC wt.%), with 12C and Ba enrichment, comparable to high productivity events in modern coastal upwelling systems. Productivity proxy values are consistent with those from margins where sedimentation rates are high (e.g. Gulf of California). Inter-bedded grey shales have low TOC wt.% more positive δ 13C org and marginally lower Ba and are interpreted as low productivity events. Thalassinoides, Planolites and Chondrites ichnofacies indicate changing seafloor oxygen levels. These show predominantly dysoxic conditions even during the deposition of OC-poor grey shales. During an OC-rich laminated hemipelagite event, oxygen levels declined at the sea floor before the deposition of the OC-rich layers. Full anoxia was established early on in the deposition of OC-rich layers and the return to more oxic conditions was rapid though fluctuating and coincident with the return of grey shale deposition. This pattern suggests OC accumulation at the seafloor resulted from a complex interaction of productivity and preservation. None of the widely used trace element redox proxies are reliable in anchizone metamorphic rocks. Climate sensitive detrital proxies (K/Al and Ti/Al) indicate arid–temperate conditions in the basin hinterland during the deposition of the OC-rich layers. The Welsh Basin was situated on the southern margin of the Iapetus Ocean in (30°S) beneath the prevailing SE trade winds. We interpret the occurrence of OC-rich laminated hemipelagite events to represent the intensification of upwelling, prior to the Hirnantian glaciation, possibly having resulted from a strengthening of the trade winds, associated with stepped changes in ice volume, and a more arid local climate.