Eustacy and pelagic regimes in the Iapetus Ocean during the Ordovician and Silurian

Abstract

During the Ordovician and Early Silurian, coincidence is observed between changes in pelagic sedimentation on the floor of the Iapetus Ocean and major eustatic transgressive and regressive events in shelf sequences on both margins. Prolonged accumulation of radiolarian cherts is followed abruptly in the mid-Ordovician (gracilis Zone) by deposition of black carbonaceous graptolitic mudstones. Following models for similar oceanic anoxic events in the Mesozoic, this is related to augmented organic carbon production in surface waters, and concomitant expansion of the oxygen-minimum layer, following increase in shelf-sea area after the Caradoc transgression. Further eustatic control control of pelagic sedimentation is seen during the Ashgill, when regression associated with glaciation in Gondwanaland caused the widespread deposition of largely non-carbonaceous mudstones on the ocean floor. Large amounts of carbon reappear in the pelagic mudstones immediately after the phase of glaciation, coinciding with the Llandovery transgression.