Effects of grain size on the chemical weathering index: A case study of Neogene fluvial sediments in southwest Japan

Abstract

Grain size variation of sediments and sedimentary rocks generally complicates the assessment of the degree of chemical weathering as measured by the Chemical Index of Alteration (CIA) value. Mineralogical and geochemical research was carried out on Miocene and Pliocene fluvial sediments in southwest Japan in order to evaluate the effects of grain size on the degree of chemical weathering. A strong positive linear trend is usually exhibited between Al2O3/SiO2 ratios and quartz intensities obtained by X-ray powder diffraction, suggesting that the Al2O3/SiO2 ratio may be suitable as a grain size index for semi-consolidated and unconsolidated sediments. Most of the fine-grained sediments, for which Al2O3/SiO2 ratios are over 0.4, have higher CIA values of over 90. In contrast, coarse-grained sediments, for which Al2O3/SiO2 ratios are lower, especially below 0.2, display a wide range of CIA values. The CIA values for intermediate grain sized samples, for which Al2O3/SiO2 ratios range from 0.2 to 0.4, reflect distinctive differences in the degree of chemical weathering between Miocene and Pliocene sediments. Within this range of Al2O3/SiO2 ratios, the Miocene sediments have higher CIA values (86–98) than Pliocene sediments (71–92). To compare the degree of chemical weathering among some sediments or sedimentary rocks with variable grain sizes, it is desirable to evaluate with the Al2O3/SiO2 ratios limited to the intermediate grain size range.