Granulometry of sediments

Granulometric analysis method

The granulometric analyses of the sediment samples were carried out by the diffractometry method, using the granulometric analyzer “Mastersizer 2000E Ver. 5.20” – Malvern. This diffractometer measures the dimensional classes in 0.0001 – 1.0 mm interval.

            The separation of the granulometric classes (sand, silt, clay) and the fractions within each class are in accordance with the logarithmic scale (Udden, 1914, Wentworth, 1922), completed with the detailing of three fractions, at intervals of 1φ, in the clay field. The classification of the sediments was made according to the Shepard diagram (1954). Shell debris and plastic particles bigger than 2 mm was removed from the overall sample amount. It should be noted the presence in some samples of plant remains that the diffractometer considers sand and which, upon analysis, some of them decompose, making it difficult to interpret the results.

            Based on the primary data, the following granulometric parameters were calculated: Median (Md=Ø50, Inman, 1952), graphical mean Mz, standard deviation σ, asymmetry coefficient SkI and kurtosis KG (graphic sharpness). These parameters were calculated according to the Folk & Ward (1957) original formulas, using the Gradistat program (Blott & Pye, 2001):

Mz  =  (Ø16+Ø50+Ø84)

             3

σ  =   (Ø84-Ø16)   +   (Ø95-Ø5)   

                 4                      6,6

SkI  =  Ø16+ Ø84-2(Ø50)   +   Ø5+Ø95-2(Ø50)

                2(Ø 84-Ø16)                 2(Ø95-Ø5)

KG  =        Ø95-Ø5

           2.44(Ø75-Ø25)