Pleistocene calcareous tufa from the Ellera basin (Umbria, central Italy) as a key for an integrated paleoenvironmental and tectonic reconstruction

Continental carbonates (calcareous tufa and associated deposits) represent a natural archive of paleo-environmental and tectonic information that can be fully disclosed only by the application of a multi-disciplinary approach. The aim of this paper is to describe the geological features of the Ellera basin (Umbria, central Italy) by integrating physical (stratigraphy, sedimentology, geochemistry, tectonics) and biological (paleontology, palynology) data. The study of the Ellera basin represents a starting point for the improvement of a methodology that could be applied to similar sedimentary systems, regardless of their actual location, extension and age. Two stratigraphic units were recognized in the Ellera basin, both characterized by multiple events of calcareous tufa deposition during the Pleistocene. In the lower San Biagio Unit, the concurrent finding of Hippopotamus (?H. antiquus) and Leptobos remains, associated with Late Villafranchian molluscs, suggests an attribution to the Early Pleistocene (Tasso or Farneta Faunal Unit, about 1.8-1.5 Ma). Both sedimentological and paleontological data indicate a periodically flooded alluvial plain depositional environment, and isotopic values of delta C-13 and delta O-18 register the establishment of a wet paleoclimate. The overlying Santa Sabina Unit lies with an angular unconformity on the San Biagio Unit, and was dated to 115,000 +/- 8000 years by the application of the U-Th method. The age of the Santa Sabina Unit falls within a period of warm and humid climate (MIS 5d-e), and paleoclimatic inferences are supported by the isotopic record (delta C-13 and delta O-18). The unit deposited in a well-vegetated fluvio-lacustrine and fluvio-palustrine environment locally dominated by herbaceous taxa, as revealed by palynological analysis. Despite the 1.5 Ma time lag between the two units, their depositional evolution shared the presence of calcareous tufa, whose deposition was controlled by similar paleoenvironmental and paleoclimatic conditions (hot and humid interglacial conditions with high rainfalls characterized both periods), as well as by the local tectonic setting and activity. In both units the maximum thickness/abundance of the calcareous tufa occurs at the intersection between two regional tectonic lineaments, controlling both the basin subsidence and the ascent of CO2-rich water.