ABSTRACT

The grain-size distributions of the jointed and weathered source rocks and the river-mouth sediments of 19 rivers in an 1800 km² area of Calabria, South Italy, were investigated. The primary distributions of the untransported source rocks are unimodally distributed and very positively skewed; the average sand fraction is only 4%. Only 6% of the distributions are lognormal; 94% follow the Rosin distribution, which is typical for products of technical breakage and milling processes.

Twelve of the 19 river-mouth sediments are bimodally distributed with a primary mode in the gravel range, a marked valley in the 2 mm range, and a secondary mode in the sand range, The seven rivers with unimodal sediments also have their modes in the gravel range. Both unimodal and bimodal sediments consist of, on the average, 74% boulder and gravel, 25% sand, and 1% silt and clay.

Surprisingly, 77% of the gravel modes of the river sediments also follow the Rosin distribution. Only the sand fractions of both the unimodal and bimodal sediments are lognormally distributed.

The Rosin distribution of the Calabrian river gravel is interpreted as a source-specific primary feature; the lognormality of the sand mode, in contrast, as a transport-specific secondary feature. Since the jointed and weathered source rocks contain only 4% sand they cannot be the only source of the 25% sand in the river sediments. The sand must also come from other sources such as soil building and transport wear.