SOIL GEOMORPHOLOGY IN THE RENO VALLEY, ITALY: INITIAL RESULTS FROM THE RETREAT PROJECT

Well-mapped and dated fluvial terraces along the Reno River, Italy allow for development of a soil chronosequence in fluvial deposits ranging in age from Late Pliestocene to Latest Holocene. Terraces are typically characterized by the presence of a bedrock strath overlain by variable thicknesses (1-7 m) of rounded to sub-rounded gravels that are buried by 1-3 meters of finer sediment. These fine sediments are medium to coarse calcareous sands with occasional pebbles and relatively abundant shells, shell fragments and charcoal. Fine deposits are interpreted to be a mixture and/or an interfingering of fluvial overbank deposits with colluvium originating from higher terrace risers. Cummulic soil profiles on Holocene surfaces high above the modern channel provide evidence for this addition of colluvium to terrace treads. Initial results show that despite the addition of colluvium, there is a consistent progression of soil development evident in Holocene deposits. Calcareous parent materials allow for the development of weak calcic horizons despite the relatively humid Mediterranean climate. The youngest deposits show minimal horizonation and sediments exhibit a strong fiz in reaction to HCl throughout the profile. Increasingly older Holocene soils exhibit increasingly developed Bk horizons up to Stage II-type development (abundant nodules, veins and filaments of CaCO3) in early Holocene profiles. In contrast, CaCO3 is not typically preserved in Pleistocene-age soils. Instead, these soils are dominated by Bt horizons that are sometimes truncated. The lack of CaCo3 in older soils is likely attributable to 1) leaching of CaCO3 from these profiles during past wetter climates and 2) the fact that these surfaces are erosional remnants that have been isolated from higher terrace risers and thus from colluvial additions of calcareous parent materials. These results represent an interesting deviation from the traditional progression of calcic soil horizon development that has been well-documented in more arid environments around the globe.