The two most important lateritization cycles in the Amazon region and their paleoecological importance

THE TWO MOST IMPORTANT LATERITIZATION CYCLES IN THE AMAZON REGION AND THEIR PALEOECOLOGICAL IMPORTANCE*

MARCONDES L. DA COSTA

Centro de Geociências, Universidade Federal do Pará, Belém, PA.

Presented by ANTONIO C. ROCHA-CAMPOS

The great part of the Amazon landscape has been developed over lateritic terrain. Geomorphological aspects, horizon toposequence, as well as the textural, mineralogical and chemical characteristics of the laterites leads to recognition of the two most important lateritization cycles in the region. The first one during the Early Tertiary created Mature Laterites, and the second during the Late Tertiary and Quaternary created Immature Laterites. Over the mature laterites developed the plateau landscape, represented for example by the regions of Carriages, Torments, Paragominas, Maicuru and Maraconaí, State of Pará; Pitinga, Seis Lagos, State of Amazonas; and Gurupi-Maracaçumé, states of Pará and Maranhão. Associated are large ore deposits (iron, bauxite, kaolin and manganese, also gold and Al-phosphates). On the other hand, the immature laterites developed on lowland plains, except in areas occupied by Quaternary sediments, even between the plateaus with mature laterites. This means that most of the present Amazon landscape developed over immature laterites. These laterites are barren of bauxite, Al-phosphates and high-grade iron ore but they do contain less important kaolin, manganese and gold deposits.

The mature laterites display well developed vertical profile with thick iron crust. The typical minerals are gibbsite, crandallite group minerals, variscite, wardite, augelite and hematite, which are not found in the immature laterites, except hematite. Mature laterites are strongly leached of SiO₂ and alkalis, but Al₂O₃ and Fe₂O₃ are enriched in comparison to the immature ones. Mature and immature laterites suffered distinct alteration processes after their formation (latosol formation, neotectonic deformation, erosion, bedrock of lake sediments and swamps, podzol formation, etc.).

These laterites and their alteration products demonstrated that the Amazon region experienced several climatic changes during the Tertiary and Quaternary. A long humid and hot to dry climate phase caused formation of the mature laterites followed by very humid phase which formed thick latosols. A second humid to briefly dry climate phase developed the immature laterites. Finally during the Quaternary a very humid climate, with a very short dry hiatus generates widespread latosols, sand podzols, lake sediments, lake and swamp iron carbonates and most recently the lakes, swamps and the dense, extensive drainage system plus the lush rain forest. — ( December 8, 2000 )

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^*

Supported by: CNPq, CAPES and DAAD.

Publication Dates

• Publication in this collection
  05 Oct 2001
• Date of issue
  Sept 2001