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Sediment Generation in humid Mediterranean setting: Grain-size and Source-rock Control on Sediment Geochemistry and Mineralogy (Sila Massif, Calabria)

von Eynatten, H.; Tolosana-Delgado, R.; Karius, V.; Bachmann, K.; Caracciolo, L.


Grain-size control on sediment composition is investigated in modern proximal sediment from the Sila Massif, where basic to felsic intrusive rocks are exposed in a Mediterranean humidtemperate upland climate. Samples were taken from small creeks and weathering profiles from three areas reflecting different bed rock composition. Samples were separated into eleven grain size fractions from very coarse sand to clay and analyzed by (i) X-ray fluorescence for chemical composition, and (ii) X-ray diffraction and Mineral Liberation Analysis for mineralogical composition. The chemical composition vs. grain size relations were modelled by compositional linear regression. Mineralogical composition from selected samples is used to substantiate the interpretations based on geochemistry. Results reveal a high degree of chemical weathering with chemical index of alteration (CIA) up to 92. High CIA values are restricted to the fine-grained fractions, while sand-sized sediment average at low to moderate CIA values (~ 60). Although strongly weathered, the three sample suites reflecting basic to felsic plutonic bed rock can be effectively discriminated across all grain-size classes using trace elements such as V, Rb, and Sr. Linear trend modelling and mineralogical data reflect similar patterns for all sample suites implying similar processes independent of source rock composition. This includes overall decrease of quartz and K-feldspar over the entire grain-size range from coarse to fine, which is contrasted by overall increase of sheet silicates from coarse to fine. Among the latter, increase of clay minerals strongly outpaces the increase of micas in silt to clay fractions. A more complex behaviour is shown by plagioclase, which is most abundant in intermediate grain-size fractions for all sample suites. This is caused by initial hydrolysis along cleavage planes and subsequent breakage of plagioclase crystals into smaller fragments. Towards finer grain size, intense hydrolysis has destroyed almost all feldspars. In contrast to a similar study in glacial setting Si/Al ratios constantly decrease with decreasing grain size, reflecting the cumulative effects of minor mechanical forces, quartz leaching, and intense hydrolysis.

Keywords: geochemistry; mineralogy; chemical weathering; comminution; provenance; compositional linear regression

  • Sedimentary Geology (2015)


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