Geochemical and mineralogical variations as indicators of provenance changes in Late Quaternary deposits of SE Po Plain

Abstract

Combined geochemical and mineralogical characterisation of mudstone-prone successions can provide suitable indicators of sediment provenance. A case study from the Late Quaternary depositional sequence of southeastern Po Plain shows that major changes in geochemical and mineralogical composition can be ascribed to changes in sediment dispersal patterns occurring during the last 30 ka in response to fluctuating sea level. The Po River, with a mixed contribution from the Alpine and Apenninic chains and its southern tributaries flowing from the Apennines, acted as the major feeders for the study area during the Late Quaternary. Chromium (and nickel) among trace elements, and serpentine (and dolomite) among minerals appear as the major provenance indicators for the study succession, allowing distinction of an Apenninic (chromium- and serpentine-poor) province from a mixed Alpine–Apenninic, Po-related (chromium- and serpentine-rich) province. The relatively low Cr/Al2O3 and serpentine/silicate ratios recorded in sediments of late Pleistocene age (<30 ka BP), which accumulated at the last glacial maximum, suggest that during lowstand times and early stages of transgression, the southeastern Po Plain was beyond the Po River influence, being fed uniquely by rivers of Apenninic provenance. Increasing Cr/Al2O3 and serpentine/silicate ratios, recorded within back-barrier transgressive deposits (8–10 ka BP) in the Comacchio and Ravenna sectors, reflect a major change in sediment supply taking place during the Holocene, when partially barred environments largely connected with the open sea became established in the study area, and Po-derived detritus was delivered to estuaries and lagoons by littoral drift. Maximum chromium and serpentine values are recorded at peak transgression and during the following sea-level highstand (6 ka BP–Present) when sediment supply exceeded the rate at which new accommodation space was created, leading to extensive Po-delta progradation. The drop in Cr/Al2O3 and serpentine/silicate ratios recorded in some cores at top of the Holocene succession reflects the local reestablishment, behind the prograding Po-delta complexes, of an alluvial plain drained by rivers of Apenninic provenance as a result of distributary channel switching and delta lobe abandonment. Geochemical and mineralogical analyses show negligible compositional variations between Pleistocene and Holocene deposits landward of the line of maximum marine incursion especially close to the basin margin. In these instances, sample composition throughout the entire stratigraphic succession clearly suggests an ongoing supply from the adjacent Apenninic chain. Comparison with sand compositional analyses from major sand bodies of the same stratigraphic succession highlights the complementary but fundamental role of geochemical and mineralogical characterisation of clays for provenance research.