Comparison of river and beach sand composition with source rocks, Dolomite Alps drainage basins, northeastern Italy
M. Dane Picard, Earle F. McBride, 2007. "Comparison of river and beach sand composition with source rocks, Dolomite Alps drainage basins, northeastern Italy", Sedimentary Provenance and Petrogenesis: Perspectives from Petrography and Geochemistry, José Arribas, Mark J. Johnsson, Salvatore Critelli
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We studied two short, high-gradient river systems draining the Dolomite Alps in northeastern Italy in order to determine which grain types survive transport and to what extent sand grain types reflect source rocks. Grains of all the labile rock types in the source areas survived to lower reaches of the rivers. In one drainage (Boite-Piave), they reached the Adriatic coast. Carbonate grains (largely dolomite) in the Gadera-Rienza Rivers decreased abruptly, largely by dilution, from >50% to trace amounts in 100 km of travel. Percentage of carbonate grains in the lower reaches of these rivers was generally less than one-half the areal percentage of limestone and dolostone exposure in the source areas. However, in the Boite-Piave Rivers (200 km long), enrichment of carbonate grains in beach sand at the expense of polycrystalline quartz and volcanic rock fragments results in dolostone sand at the beach reflecting 78% of its outcrop abundance and limestone (calcite) sand reflecting 68% of its outcrop abundance. Polycrystalline quartz and mafic volcanic rock fragments are less abundant in the beach because of dilution by longshore drift or the breakdown of these grains by wave abrasion. The relative resistance of carbonate textural grain types to abrasion is micrite > spar > mixed micrite/spar.
The results indicate that detritus from dominantly silicic and intermediate volcanic rocks can survive fluvial transport and at least moderate wave abrasion. Metamorphic rock fragments (mostly phyllite) in the Gadera-Rienza Rivers survived transport to the confluence with the Isarco River at Bressanone. In the Boite-Piave river system, metamorphic rock fragments survived fluvial transport to the beach plus some beach abrasion. They did so because the relatively rapid transport down the high-gradient, low-sinuosity streams did not permit extensive chemical weathering. Grains of calcite (micrite and spar), dolomite, and volcanic rock fragments increased in roundness by abrasion in the surf after undergoing only a few kilometers of transport along the coast.