Large scale fades change in the middle Eocene South-Pyrenean foreland basin:The role of tectonics and prelude to Cenozoic ice-ages

摘要

The present study reports a sedimentological analysis of the Guara Limestone Formation deposited during the Lutetian in the Sierras Exteriores, in the South-Pyrenean foreland basin. We provide a detailed fades analysis of the carbonates to precise the paleoenvironmental context during their deposition. We show that those limestones are mainly composed of shallow-water foraminifers and were deposited in relative shallow-water environments (<120 m) during the whole Lutetian (SBZ 13 to SBZ 16). The Guara Limestone Formation represents the last occurrence of carbonate platform in the South-Pyrenean foreland basin and disappeared definitely at the Lutetian to Bartonian transition. The demise of carbonate producers at the end of the Lutetian could be related to an increase of continental erosion, due to tectonic and/or climatic forcing. We illustrate that in the Jaca basin, this event correlates with a marked increase in subsidence rate. However, this deformation event is local and the carbonate systems in the Pyrenean foreland resisted to many deformation events during the whole basin history before. Paleobathymetric reconstructions in the Jaca basin, where shallow marine sections outcrop, suggest an increase of the amplitude of high-frequency sea-level cycles. This increase is contemporaneous with several climatic evidences, which suggest the appearance of early ice-sheets near the Lutetian-Bartonian boundary. The demise of carbonate producers seems, therefore, to be the result of a major environmental shift in the basin accompanying increased subsidence rates, switching from low nutrient oli-gotrophic conditions - favourable for shallow water benthic foraminifers - to eutrophic conditions due to the increase of erosion and terrigenous nutrient input associated with higher-frequency sea-level changes and river destabilization.