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Geochemical and grain-size variations within the overbank sediment from the Sava river, Croatia

Four overbank profiles (S7, S8, S9, S10) of the Sava river alluvium downstream from Samobor to Sisak were sampled over 10- to 20-cm intervals. Grain-size analysis was carried out by the combination of wet sieving (> 0.063 mm) and sedigraph (< 0.063 mm). Results of the chemical and the grain-size analyses were processed by means of statistical techniques to reveal the profile patterns of textural and chemical data, as well as correlations of selected elements in the different sediment types. Geochemical composition was determined in dry-sieved fraction < 0.125 mm by ICP-MS after the four acid digestion. The minerals identified by XRD were calcite, dolomite, plagioclase, quartz, mica and chlorite. The previous study (PAVLOVIĆ et al., 2004) revealed that the bottom parts of the overbank sediment had decreased Al, Na, K, etc. values and increased Ca and Mg values, whereas the opposite trend was found for the overlying strata. It was concluded that the lower-lying carbonate (CA) and the overlying silicate (SI) population resulted from the intensive erosive action of melting glaciers exerted on limestones and dolomites in the alpine region, followed by local inputs of chemical weathering products mainly of silicate composition. Generally, newly accomplished grain-size analysis has confirmed this finding since the lower parts of the profiles S8, S9 and S10 are dominated by sandy fractions, while the upper ones are characterized by mud which is dominated by silt fractions. Unlike these three, there are two intervals in the upper part of the profile S7 (30-50 cm and 80-100 cm) showing coarsening trend, mostly due to the gravel fraction, and to a lesser extent due to the sand fraction. This coarsening is in accordance with the previous study (PAVLOVIĆ et al., 2005), corresponding to the two sharp peaks of the 137Cs curve. The two caesium maxima were interpreted in the context of the grain-size distribution and sedimentation history, what has been confirmed with new results of the grain-size analysis in this work. The study (PAVLOVIĆ et al., 2005) also found that the sandiest profile was S8 which is the oldest sequence according to gamma-ray measurements. The Kendall’s Tau correlation matrix was calculated for selected elements with respect to the grain-size fraction < 0.063 mm. With the exception of the profile S7, the results (Table 1) demonstrate a strong positive correlation for the pair Al-<0.063 mm as well as strong negative correlations among particle size and Ca and Mg, what is consistent with the bimodal lithology of overbank strata previously defined as carbonate and silicate populations (PAVLOVIĆ et al., 2004). Positive correlations among La, Ce and Zr, and the < 0.063 mm fraction in the profiles S9 and S10 could be attributed to mechanical transport during sedimentary processes (TAYLOR & McLENNAN, 1985). Figure 1 presents downcore variations of selected variables in the profile S9 which are approximately similar to those recorded for the other three profiles. It can be seen that Al, La, Ce, Zr and the two grain-size fractions show concordant patterns, whereas the opposite trend is observed for Ca. This finding is in agreement with above statements regarding geochemical patterns of overbank sediment controlled by the grain-size distribution and sedimentary processes in the upstream catchment area.

Sava river alluvium; grain-size composition; trace elements; correlation; calcite; quartz

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