Dados da Submissão

Título

Holocene floodplain development and contamination history in the northernmost portion of the Araguaia Belt, southeastern Amazonia

Texto do resumo

This study aimed to investigate the factors that contributed to the development of floodplains and evolution of the landscape in a Precambrian Shield region in southeastern Amazonia. The area has undergone significant changes during the Quaternary period, resulting in a denudational system with strong structural control. This led to the formation of valley-side slopes between the Araguaia Belt (AB) and the topmost Parnaíba basin, and narrow floodplains in the lower Vermelho River. The floodplains were formed between approximately 11215-11410 cal yr BP due to the deposition of mud on weathered basement rocks of the Couto Magalhães Formation in the AB (CMF) long before sea-level stabilization on the Amazon coast. The muds were derived from the CMF-AB, as evidenced by the chondrite- and upper continental crust-normalized patterns, and Al/Ti and La/Th ratios. The study also found that Aluminous clay phases and a similar transport process played a significant role in the transport of trace and rare earth elements (REEs). The Vermelho River's overbank flows resulted in sand deposition and vertical accretion from the fluvial channel over the floodplain, forming marginal levees after 4520-4710 cal yr BP. The accreted sands, which were dominated by Si, Zr, and Hf and depleted of most of the major and trace elements, were likely associated with quartz enrichment and strong sorting of heavy minerals. This was further supported by Al2O3/SiO2 vs Fe2O3/SiO2. The study found that the tributaries of the lower Vermelho River were active until 3890-4095 cal yr BP, following which the channel was abandoned due to the filling process. At approximately 3440-3560 cal yr BP, the floodplain was influenced by prolonged and severe droughts that fully exposed the muddy deposits and caused the enrichment of Fe-Mnoxyoxide phases. The study revealed that most of the potentially toxic elements had an affinity with Al, which suggests that these floodplain deposits can serve as a natural record for geochemical background studies.

Palavras Chave

Holocene; Stratigraphy; Geochemistry; Araguaia Belt; Carajás