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Downstream Changes in Sand Composition, Roundness, and Gravel Size in a Short-headed, High-Gradient Stream, Northwestern Italy
Earle F. McBride, M. Dane Picard
We assessed changes in grain composition and roundness of the 1-2 and 2-3 size fractions of sand along a 16-km length of the Graveglia River, Liguria, Italy, to its mouth at the Tyrrhenian Sea and then for 1 km along the beach. Rocks exposed in the drainage area include ophiolites, ribbon chert, limestone, shale, sandstone, and Quaternary sands. The erosion of shale and limestone bedrock yields large amounts of sand, but sand grains of these rock types decrease rapidly in abundance downstream, chiefly through dilution rather than abrasion. Carbonate rock fragments that reach the beach survive wave abrasion, but most shale grains do not. Predictably, monocrystalline and polycry talline quartz and chert are the most durable grain types, but serpentine, basalt, and unidentified rock fragments also show significant resistance to both stream and wave abrasion. Polycrystalline quartz, feldspar, basalt, serpentine, sandstone, and shale grains are more abundant in the coarser-sand-size fraction; monocrystalline quartz, carbonate rock fragments, and unidentified rock fragments are more abundant in the finer-sand-size fraction; and chert and plutonic rock fragments have no size preference.
A comparison of the outcrop areas of the different rock types with the corresponding percentages of different grain types present in each of three drainage areas indicates that shale, which breaks down readily to silt- and clay-size particles, and chert, which does not readily break down into sand-size fragments, are underrepresented in the sands. Limestone and serpentine, however, are overrepresented in comparison to outcrop area in certain drainage areas. The overrepresentation of limestone reflects, in part, a sampling bias (sand was sampled adjacent to limestone outcrops), but the overrepresentation of serpentine reflects its resistance to weathering (compared to other aluminosilicates) and its durability.
Shale, limestone, and serpentine increase in roundness downstream and along the beach to a maximum (limiting?) value of 5 rho units. Medium-grained shale clasts attain high rounding in only a few kilometers of transport.
Downstream decrease in gravel size follows an exponential formula that can be expressed in linear form using phi size: size () = 0.1167 distance (km)- 10.29.
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