Reconstructing the sedimentation history of an alluvial plain by sand composition investigations: the Modena case study

The Modena alluvial plain area examined for this study has an approximate extent of 150 km2 and is located at the northern side of the Northern Apennines thrust- and fold-belt, where streams draining the chain flow toward the north-east into the Po river. Detailed modal analyses by point-counting in thin sections show that modern stream sands in the Modena plain show similar overall compositional fields, but can be discriminated on the base of key-components, such as quartz, feldspar, carbonate and ophiolitic fragments.The spectacular abundance of archaeological sites of various age along the main rivers allowed us to reconstruct the variation through time of the sand composition. The study of sand sediments that buried Neolithic, Iron Age, Bronze Age, Roman and Longobardian sites indicates that the compositional fields have not varied significantly since the Neolithic. These results clearly indicate that the bedrock lithology of the source area has not changed in the considered time interval and that diagenesis has not obliterated the sediment provenance signature in such a relatively short time span. The only major diagenetic process is the formation of carbonate concretions (caliche), which can be easily recognized as secondary particles during point counting of thin sections.These results indicate that the reconstruction of the recent evolution of the local drainage system is possible simply by comparing ancient with modern fluvial sand compositions.On the other, the sand sediments older than the Neolithic, and approximately older than 10.000 years, show a significant shift in composition from the modern ones, with an overall increase of quartz and feldspar components. These compositional variations can be explained by the combination of various factors: a significant change in the bedrock lithology (neotectonic effect), climate changes related to glacial-interglacial phases which induced a variation in bedrock alteration, erosion and sand deposition rates (climate effect), and the post-depositional changes that may have varied the sand composition (diagenetic effect). Further studies, now in progress, will permit a better characterization of the sediment supply through space and time, possibly allowing the reconstruction of the sand composition in the older local drainage system as response and interplay among neotectonics, climate changes and diagenesis.