Weathering, erosion and sediment composition in a high-gradient river, Calabria, Italy

Source rock lithology and immediate modifying processes, such as chemical w eathering and mechanical erosion, are primary controls on fluvial sediment supply. Sand composition and Chemical Index of Alteration (CIA) of parent r ocks, soil and fluvial sand of the Savuto River watershed, Calabria (Italy) , were used to evaluate the modifications of source rocks through different sections of the basin, characterized by different geomorphic processes, in a sub-humid Mediterranean climate. The headwaters, with gentle topography, produce a coarse-grained sediment load derived from deeply weathered gneis s, having sand of quartzofeldspathic composition, compositionally very diff erent from in situ degraded bedrock. Maximum estimated CIA values suggest t hat source rock has been affected significantly by weathering, and it testi fies to a climatic threshold on the destruction of the bedrock. The mid-course has steeper slopes and a deeply incised valley; bedrock cons ists of mica-schist and phyllite with a very thin regolith, which provides large cobble to very coarse sand sediments to the main channel. Slope insta bility, with an areal incidence of over 40 per cent, largely supplies detri tus to the main channel. Sand-sized detritus of soil and fluvial sand is li thic. Estimated CIA value testifies to a significant weathering of the bedr ock too, even if in this part of the drainage basin steeper slopes allow er osion to exceed chemical weathering. The lower course has a braided pattern and sediment load is coarse to mediu m-fine grained. The river cuts across Palaeozoic crystalline rocks and Mioc ene siliciclastic deposits. Sand-sized detritus, contributed from these roc ks and homogenized by transport processes, has been found in the quartzolit hic distal samples. Field and laboratory evidence indicates that landscape development was the result of extensive weathering during the last postglacial temperature maxi mum in the headwaters, and of mass-failure and fluvial erosional processes in the mid- and low course. Copyright (C) 2000 John Wiley & Sons, Ltd.