Paleosurfaces from northeastern Amazonia as a key for reconstructing paleolandscapes and understanding weathering products

Abstract

The characterization and mapping of paleoweathering surfaces, as well as their associated deposits, throughout the northeastern Amazonia can be a useful tool for inter-basin stratigraphic correlations and for establishing the evolution of landscapes and climates through time. This type of investigation has led to the recognition of five weathering surfaces developed from the Upper Cretaceous through the Quaternary, designated here as S1 to S5. Surface S1 occurs at the top of Late Cretaceous deposits, represented by the Ipixuna/Cujupe succession, as well as on older Precambrian and Paleozoic rocks. Surface S2 marks the top of an indurated, massive, semi-flint 1 1 The term semi-flint is applied here for a flint-like fire clay consisting of indurated kaolinite, which develops no plasticity when grounds up. unit. For cases in which surfaces S1 and S2 are amalgamated, a thick, lateritic paleosol displaying hexagonal, branched concretions is present. Surface S3 occurs at the top of the Pirabas and lower portion of the Barreiras Formations of Late Oligocene/Early Miocene age, and it is also associated with paleosol, but has no lateritization. When surface S3 is amalgamated with surfaces S1 and S2, bauxite is present in association with the underlying laterites. Surface S4 bounds the top of the middle to upper Barreiras Formation of Middle Miocene age, as well as correlatable deposits known as the Belterra Clay. This surface is marked by a lateritic paleosol having columnar concretions. Surface S5 occurs between a lower unit consisting of massive, light red sands of the Plio-Pleistocene Post-Barreiras 1 sediments and an upper unit of yellow, eolian sands of the Holocene Post-Barreiras 2 Sediments. The paleoweathering surfaces present in the northeastern Amazonia enable the reconstruction of several depositional episodes represented by thin (only few meters thick) sedimentary units from the Late Cretaceous to the Quaternary. The creation of new accommodation space on subsiding areas gave rise to sediment accumulation during transgressions. Intervening weathering surfaces formed during significant falls in relative sea-level, associated with tectonically stable episodes. The overprinting of weathering surfaces formed during different times, and thus under different climatic conditions, seems to have been crucial for the development of the various weathering products observed throughout the region, which include economically important laterites, bauxites, semi-flint beds and kaolin deposits. This paper shows that the genesis of these deposits was controlled by the complex interplay of tectonics, climate, sediment deposition and erosion, resulting in a complex evolution of paleolandscapes.