Sand production by chemical and mechanical weathering of well lithified siliciclastic turbidites.

The production of sand by chemical and mechanical weathering in a temperate humid climate and in a middle altitude mountain area has been investigated by comparing the composition of the sandy fraction of ancient and recent soils with the composition of parent rocks and stream sands. We examined the drainage basins of two high-gradient streams in the northern Apennines (Ospitale and Fellicarolo) and a short portion of the Leo stream derived from their confluence. Bedrock is made of a single rock type, the siliciclastic arkosic turbidites of the Cervarola Formation, and only in the lower portion of the basin the Leo stream cuts carbonate and pelitic lithotypes of Ligurian units. River bed sands have a lithic composition and are very rich in sedimentary rock fragments such as shales, silstones and fine-grained arenites derived from the Cervarola tubidites beds. The variations observed in the quartz/lithics ratio is primarily explained with weathering conditions (in particular soil type and steepness of slope), and only subordinately seems to be related with the river transport. The change in outcropping rocks in the lower part of the basin is immediately reflected in the lithic fraction of stream sands, marked by micritic fragments indicating contribution from carbonate units.Soils sands show different compositions. Recent soils from the highest watershed areas have a lithic composition. The more developed older dark soils capping red soils, and the chemically more active black swampy soils, show compositions gradually richer in quartz and feldspars. Pedogenic reactions initially reduce the lithic fragments, leading to an indirect enrichment of the more resistant quartz and feldspar grains. It is likely that the quartz-feldspar rich composition of the black swampy soils is partially related to the coarser-grained arenites of the bedrock.Results from this study indicate that in a drainage basin cutting well lithified sedimentary rocks, rich in shale and siltstones, the chemical activity and duration of pedogenetic processes, and the steepness of valley slopes, which controls the intensity of physical breackage of outcropping rocks, are the major factors controlling the production and composition of sand sediments. In a temperate humid climate these two factors seem to work some what independently and give an imprint to the produced sediment which changes as a function of different topographic conditions. Physical breackage prevails on steep slopes, thus the sediments are rich in unstable lithic grains. Even in very acidic soils, alteration of feldspar seems to be not significant, probably because the time from the last glaciation has been not long enough to produce visible effects on feldspar grains.