Morphological, mineralogical and chemical characterization of the alteration levels of Marília Formation (Serra da Galga member) from the Neo Cretaceous of Bauru Basin, in the Triângulo Mineiro (MG)

Abstract

In the flat plateaus of the Triângulo Mineiro, the sediments of Marília Formation (Neo Cretaceous) are intensively altered and pedogenised, developing thick profiles, strongly clayey with high concentrations of kaolinite and gibbsite. The objective of this paper is to characterize the major changes of petrography (optical microscopy), mineralogical (XRD and DTA-TG) and geochemical (XRF Geochemical Transport and function) in an alteration profile involving the sandstone, the saprolite and the superficial plinthic horizons and with aluminous clays. The results have shown that the kaolinite and gibbsite are the dominant secondary paragenesis of the profile. The concentrations are accentuated in the saprolite on account of the weathering of feldspars and micaceous minerals, and it is most present in the plinthite and in the aluminous clay. In the superficial horizons, the gibbsite concentration is higher than the kaolinite. The geochemical data and the transport function have shown that there is an increase of iron oxides in saprolite (1.45%) and in the plinthite (2.66%) due to the absolute concentration in the nodules. In saprolite there are absolute gains of Si, Al and Fe. There was loss of Si in the plinthite and in the aluminous clay. Regarding the Al, it is noticed gains in the saprolite, plinthite and aluminous clay. There is a relative increase in the proportion of kaolinite and gibbsite from the base to the top of the profile, from ~ 72% of kaolinite and ~ 3% of gibbsite in the sandstone and from to ~ 28% of kaolinite and ~ 71% of gibbsite in aluminous clay, indicating strong loss of silica in the soil solution. The results obtained show strong similarity between the analyzed samples, suggesting a possible mineralogical relationship among them.