Age and origin of Mississippian lydites: Examples from the Pyrénées, southern France

Abstract

Mississippian biogenic siliceous deposits (known as 'lydites' or 'lydian rocks') intercalated within limestone successions are studied to characterize the signature of this 'siliceous period' and its significance, particularly in the Hercynian history of Europe. Therefore, a multidisciplinary approach was conducted, using analysis of conodont faunas, lithologic and microfacies analyses, and the study of inorganic geochemistry, from outcrops in the French Pyrénées. Biostratigraphical study of conodont faunas has established the age of the lydites, as occurring between the lower crenulata and anchoralis-latus zones. The sedimentological study shows that the sediments were deposited in a deep-water shelf environment. The lithological succession shows a gradual deepening prior to the siliceous sedimentation. However, it appears that the radiolarites have not been deposited in a deeper water basin, but in a more proximal environment, on an external continental shelf, at a depth of less than 300 m. Analysis of major and trace elements does not show particular trends and there is no discernible change in the ocean water chemistry to explain the change of sediment type. However, the disappearance of limestone deposits and the formation of lydites are interpreted as a combination of several factors including a transgression that changes the oceanic circulation, and eutrophication linked to the circulation of nutrient-rich cold currents. Copyright © 2007 John Wiley & Sons, Ltd.