Geomorphology and soils between the Colognati and Trionto River catchments (Calabria, South Italy)

This work deals with an integrated geomorphological, stratigraphical and pedological study of coastal river terraces between the Colognati and Trionto river catchments in NE Calabria (south Italy). In the South, low-relief landscapes occur along the northern flank of the Sila Massif as high as 1000 m a.s.l.; in the North, the Sila slope dips strongly down to a hilly coastal belt attaining a maximum height of 480 m a.s.l. This hilly belt is dissected and separated into two main ridges by streams with braided patterns. The morphology of the ridges was mapped by means of field surveys coupled with topographic map and air-photo analyses. Each ridge is characterized by stepped surfaces, consisting of well-preserved terraces, widely distributed along the river catchments and forming a W-striking staircase geomorphology in the northern ridge front. Terrace mapping was undertaken along the trunk rivers of the major catchments, mainly in their lower reaches, as landsliding and headward stream erosion affecting valley flanks, cause the old landscape to be progressively destroyed. Field mapping of the ridges was integrated by terrace mapping using topographic maps and air-photo analyses, thus allowing either wide-size and small-scale surfaces to be detected. Large-scale topographic maps (1:5000 and 1:10000), air-photos (1:10000) and morphostratigraphic sketches permitted to identify individual surfaces, estimate their heights and interpret them as stepped fluvial terrace surfaces. As a whole, these ridges are interrupted by a flight of five alluvial terraces cutting the folded and faulted Miocene deposits and rarely resting erosively on Plio-Pleistocene marine deposits. These terraces, of Middle to Late Pleistocene age, consist of decimetre to metre-thick fluvial sediments made of subrounded pebble-cobble gravel, sand and silt deposits. The river terraces are characterized by reddish and clay-illuviated surface soils, although some profiles are buried by younger fluvial deposits or soils. Typical surface organic-mineral horizons are often lacking, and both exposed and buried soils appear truncated by erosion. Some profiles are overprinted by deep eluviated tongues or hydromorphic features. Fifteen soil profiles from a soil chronosequence developed on the river terraces were described and sampled in the field for chemical, physical, mineralogical and micromorphological analyses. Most of the examined profiles completely lack of CaCO3 in their matrix, presumably for its complete absence as parent material. Other few profiles have partly leached matrix carbonate associated with secondary precipitation features, suggesting different parent materials sourced from different paleodrainage basins, otherwise conflicting with the terrace setting and related soil ages. The micromorphological observations in thin sections from undisturbed samples showed abundant laminated clay coatings, exhibiting typical postilluvial disturbance and degeneration, and therefore indicating their relict significance. This suggests that the illuvial process is inactive and can be ascribed to climatic conditions with strong seasonal contrast, warmer and more humid than at present (climatic optima of past Quaternary interglacials). Occasional rounded pedorelicts included in some horizons confirm field evidence of erosion and reworking of soil material. Illite and kaolinite (and occasional smectite or vermiculite) represent the main clay minerals, pointing to an overall high degree of weathering of the soil profiles. Their different relative abundance, coupled with iron forms and related weathering indices obtained from selective extraction techniques, the SEM morphoscopic analysis of weathering patterns and degree of primary minerals allowed us to compare the pedogenetic evolution of the soil profiles studied and relate it to their age. This strongly contributed to the reconstruction of Quaternary geomorphic processes and their response to climate changes. 80