Composizione dei sedimenti sabbiosi nelle perforazioni lungo il tracciato ferroviario ad alta velocità: indicazioni preliminari sull'evoluzione sedimentaria della media pianura modenese

The Modena alluvial plain has been geologically investigated in great detail and is characterized by a spectacular abundance of archaeological sites of various age. For this reasons the area may be considered a natural laboratory for the reconstruction of the recent sedimentary evolution of the Po Plain. The 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 of 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 lithic fragments. The study of sand sediments indicates that the compositional fields have not varied significantly since the Neolithic. 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 by comparing ancient with modern fluvial sand compositions.The drilling of numerous wells along the new high speed train tract (TAV) provided us with new insight on the sedimentary evolution of the plain through time. The samples recovered from 6 wells reaching depths up to -50 m show that the sand sediments older than 10-12 kyr, have a significant shift in composition from the modern ones. This compositional change consists in a marked overall increase of quartz and feldspar components. The compositional variations can be explained by the combination of various factors: a) significant change in the bedrock lithology through time induced by tectonics and/or change in the local drainage pattern, and b) recycling of older fluvial sediments enriched in feldspar. Because the sands older than 10-12 kyr compared to the overlaying olocenic and modern sediments are enriched in feldspar, which is considered a particularly alterable component, the compositional differentiation can not be related to effects induced to glacial-interglacial climate changes and post-depositional diagenetic processes.The considerable change in sand composition and the stratigraphy of the deposits suggest dramatic geographical, environmental and depositional variations supporting the observation that a regional unconformity separates the Late Pleistocene from the Olocene sediments.