High-frequency depositional cycles within the late Quaternary alluvial succession of Reno River (northern Italy)

Luigi Bruno (*), Alessandro Amorosi (*), Paolo Severi (**) & Paolo Bartolomei (***)
(*) Dipartimento di Scienze Biologiche, Geologiche ed Ambientali, Università di Bologna, Via Zamboni, 67 - 40126 Bologna, Italy. Tel: +39 3494663443; email: luigi.bruno4@unibo.it
(**) Servizio Geologico, Sismico e dei Suoli della Regione Emilia-Romagna. Viale della Fiera, 8 - 40127 Bologna, Italy.
(***) Enea, Agenzia nazionale per le nuove tecnologie, l'energia e lo sviluppo economico sostenibile, Centro Ricerche di Bologna. Via Martiri Monte Sole, 4 - 40129 Bologna, Italy.

Palaeosol-based correlations within the Late Pleistocene-Holocene alluvial succession along the Reno River, in the southern Po Plain, enabled the identification of depositional cycles falling in the sub-Milankovitch band. Each cycle, composed of overbank and fluvial facies capped by poorly to weakly developed palaeosols, is correlatable upstream to a single fluvial terrace in the Reno River valley and to an individual channel belt close to the valley outlet. Four cycles, dated to about 15-10 (c1), 10-5.5 (c2), 5.5-1.5 (c3) and <1.5 (c4) cal ky BP, respectively, were identified within the Ravenna subsynthem (AES8), an unconformity-bounded unit of the Geological Map of Italy to scale 1:50,000, corresponding to the post-Last Glacial Maximum deposits. This unit, typically wedge-shaped in coastal areas, where it consists of retrogradational (coastal plain and estuarine) deposits overlain by progradational (deltaic) facies, at the basin margin is a mud-dominated alluvial succession deposited atop laterally extensive fluvial-channel complexes. The base of AES8, correlatable to the transgressive surface identified in the coastal area, is a palaeosol dated to about 18-15 ky BP. The bounding surfaces of the high-frequency cycles are diachronous along the Reno longitudinal profile, and not necessary associated to remarkable lithological contrasts, but can be detected even in mud-dominated successions. Climate change likely exerted a major control in triggering alternating phases of river aggradation and degradation, with an increasing contribution of anthropogenic factors since the middle-late Holocene. Based on the correlation of 34 core logs and 33 well descriptions, with the aid of 71 radiocarbon dates, this study highlights to what extent palaeosols can represent powerful stratigraphic tools to identify cyclic patterns in alluvial successions, even at the millennial time scale

alluvial deposits, palaeosol stratigraphy, depositional cycles, late Quaternary, Po Plain, Apennines.