Gondwana platform paleoenvironment reconstruction: Insights from Ordovician black-grey shales in NW Iberia

Abstract

Gondwana developed marine platforms at its northern edge with Paleozoic deposits reflecting varied paleoclimatic conditions. In the northwestern (NW) Iberian Peninsula (NW Gondwana), Ordovician black-grey shales and sandstones were deposited in these platforms at southern polar latitudes. The current research contributes to a better understanding of the recycling, climate, and redox conditions during the deposition of black-grey shales in the low/mid-Ordovician period. During the Lower-Middle Ordovician period, the black-grey shales recorded an increase in recycling (Th/Sc, Zr/Sc, Zr/Ti, La/Th), low level of chemical alteration (CIA, Th/U) and relatively low oxygen conditions (Ce/Ce*, Y/Ho). These data indicate arid-cold conditions with a seasonal glacial-periglacial environment, consistent with the location of the NW Iberian Peninsula at low latitudes close to the South Pole. Towards the Middle Ordovician, the black-grey shales recorded a more temperate climate with higher levels of chemical alteration, less recycling, and a relatively more oxygenated environment, what suggests a transition to a moderate climate with no glacial events developed. This climate evolution could have been promoted by the progressive Gondwana drift away from the south pole aided by CO2 input to the atmosphere from alkaline mafic intraplate volcanism linked with a previous felsic LIP event in this area. The rapid decline of this volcanism and the partial alteration of its products (fixing CO2) could have contributed to the development of the Hirnantian glacial conditions during the Upper Ordovician, which brings a new view of the secular climatic evolution of the Earth during the Ordovician.