A multidisciplinary study of denudation processes and landforms in the Vrica area (Calabria, South Italy)

This study focuses on the analysis of denudation processes and related landforms in a hilly area characterized by a coastal Mediterranean climate, with low mean annual rainfall (700 mm), mean annual temperatures around 15°C, high evapotranspiration rates and summer water deficit. The study area is located close to the town of Crotone, in eastern Calabria (southern Italy), and is of primary importance within the worldwide scientific community for the presence of the Vrica geological section and its recent institution as a geosite. This section represents the Pliocene-Pleistocene boundary stratotype, recorded in a gently-inclined, monoclinal stratigraphic succession, made of deep-sea, grey marly silty clays with arkosic sand and organic-rich sapropel interlayers. The top reaching about 160 m a.s.l. consists of gravels and sands intercalated with partly cemented bioclastic calcarenites (uncoformably overlying the clayey succession), which form wide marine terraces of late Middle Pleistocene age. The specific geological and geomorphological context, coupled with the climatic and environmental conditions, make it very susceptible to desertification and soil erosion. Our study was performed with a multidisciplinary approach, ranging from a geomorphological and pedological survey in the field to aerophoto interpretation, GIS-aided mapping and morphometric measurements, up to mineralogical, geochemical and geotechnical laboratory analyses of key samples. The landscape appears intensely dissected by surface water runoff and affected by mass-wasting processes. The main landforms and morphodynamic process described in the field and mapped are related to sheet/rill wash (dominating on terraced surfaces and gentle slopes) and gullying (as far as slope steepness increases) and especially badland development in the clay succession. The summit gravel-sand and calcarenites undergo rockfall phenomena and soil erosion, clearly testified by complete absence or severe truncation of topsoil organic horizons (A) and exposure at surface of typical deep, illuvial ones (Bt or Bk). The badland morphologies are dominated by sharp knife-edged to smoothshaped “calanchi” and extremely rare, small hummocky “biancana” clusters. Calanchi prevail on 20-40° inclined and eastern-facing slopes. Conversely, mass movements dominate on western and northern slopes with the main scars developed around 10-30°. An important control seems to be played by the monoclinal setting of the clayey strata contributing to an asymmetrical pattern of valley sides. Both calanchi and biancana landforms occasionally exhibit metre- to decimetre-wide pipe holes at different height along the slopes, as well as a millimetre- to centimetre-thick, microcracked surface crust overlying an inner massive and compacted portion. Selected samples were collected from these different portions for laboratory investigations: analysis of clay minerals using X-ray diffraction techniques, total chemical composition (major and trace elements) by fluorescence spectrometry (XRF), particle size distribution, Atterberg limits, pH, total carbonate by calcimetry, soluble salts in ionic chromatography and calculation of the SAR (sodium adsorption ratio) dispersivity index, total microporosity, permeability, bulk and apparent (skeletal) density using a mercury porosimeter. An overall qualitative and quantitative homogeneity was found for different properties, such as the particle size distribution (all samples representing clayey silt fractions), the clay mineralogy (including smectite, illite, kaolinite and subordinate chlorite), pH values attesting around a moderately alkaline reaction, SAR > 10, total CaCO3 (on average equal to 19%), microporosity (mean value around 30%). Among the most interesting results there is the evidence that pipes are triggered in or above coarser-textured, more porous and more dispersive layers, presumably enhancing drainage intensity and erosive efficacy of subsurface water through-flow.