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

Le perforazioni effettuate lungo il tratto modenese del tracciato ferroviario ad Alta Velocità hanno fornito numerosi campioni di sabbieprelevate in profondità fino a 40-50 m dal piano campagna, per un intervallo temporale che presumibilmente copre il Pleistocene sommitalee l’Olocene. L’esame composizionale di 28 campioni di sabbie prelevati da 6 perforazioni ha permesso di evidenziare come lasedimentazione fluviale nella media pianura modenese sia drasticamente mutata a partire da circa 10-12 ka. La composizione deisedimenti sabbiosi più recenti sembra riconducibile a quella dei corsi d’acqua minori che attraversano oggi la pianura nel tratto compresotra i fiumi Secchia e Panaro. I sedimenti sabbiosi più antichi presentano invece composizioni diverse, particolarmente arricchitein feldspati. Tali differenze composizionali possono riferirsi a combinazioni di vari fattori quali: a) variazioni nella litologia del substratoper fenomeni tettonici e/o per variazioni nello sviluppo del reticolo idrografico, b) fenomeni di riciclo di sedimenti fluviali antichi, c)effetti di alterazione per variazioni climatiche. I dati composizionali acquisiti, uniti a dati di tipo stratigrafico, suggeriscono che tra isedimenti tardo-pleistocenici e quelli olocenici sia presente una significativa discontinuità stratigrafica di importanza regionale.

The Modena alluvial plain has been geologically investigated in great detail and is characterized by a spectacular abundance ofarchaeological sites of various age. For this reasons the area may be considered a natural laboratory for the reconstruction of therecent sedimentary evolution of the Po Plain. The alluvial plain area examined for this study has an approximate extent of 150 km2 andis located at the northern side of the Northern Apennines thrust- and fold-belt, where streams draining the chain flow toward the northeastinto the Po river. Detailed modal analyses by point-counting of thin sections show that modern stream sands in the Modena plainshow similar overall compositional fields, but can be discriminated on the base of key-components, such as quartz, feldspar, carbonateand lithic fragments. The study of sand sediments indicates that the compositional fields have not varied significantly since theNeolithic. The only major diagenetic process is the formation of carbonate concretions (caliche), which can be easily recognized assecondary particles during point counting of thin sections. These results indicate that the reconstruction of the recent evolution of thelocal 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 ofthe 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 increaseof quartz and feldspar components. The compositional variations can be explained by the combination of various factors: a) significantchange in the bedrock lithology through time induced by tectonics and/or change in the local drainage pattern, and b) recycling ofolder fluvial sediments enriched in feldspar. Because the sands older than 10-12 kyr compared to the overlaying olocenic and modernsediments are enriched in feldspar, which is considered a particularly alterable component, the compositional differentiation can berelated to effects induced to glacial-interglacial climate changes but not to post-depositional diagenetic processes.The considerable change in sand composition and the stratigraphy of the deposits suggest dramatic geographical, environmental anddepositional variations supporting the observation that a regional unconformity separates the Late Pleistocene from the Olocene sediments.