Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The Ar-39-40 Ar dating of supergene K-Mn oxidesReportar como inadecuado

Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The Ar-39-40 Ar dating of supergene K-Mn oxides - Descarga este documento en PDF. Documentación en PDF para descargar gratis. Disponible también para leer online.

1 CEREGE - Centre européen de recherche et d-enseignement de géosciences de l-environnement 2 GR - Géosciences Rennes 3 Institut de Recherches pour le Développement

Abstract : Chemical weathering and mechanical erosion are first-order processes of long-term tropical morphogenesis, which is still poorly deciphered for lack of time constraints. We address this issue by laser probe 39Ar-40Ar dating of generations of cryptomelane K1-2Mn8O16, nH2O from the manganese ore deposit of Tambao in northern Burkina Faso. This Mn deposit results from the supergene weathering of carbonate and silicate Mn protores underneath lateritic palaeolandsurfaces. It consists of an upper cryptomelane-rich domain and a lower domain where pyrolusite b-MnO2 is the dominant Mn oxide. The oldest 39Ar-40Ar ages 59-45 Ma are obtained on surface outcrops while the youngest ones characterize deep oxidation fronts 3.4-2.9 Ma. Apparent correlations of 39Ar-40Ar age groups with d18O and eustatic curves allow definition of the different stages of morphogenesis. Paleocene-Eocene ages 59-45 Ma bracket a greenhouse period propitious to bauxitic weathering. The lack of significant ages between 45 and 29 Ma characterizes a period dominated by mechanical erosion, during which detrital sediments, including lateritic materials, were accumulated in intracratonic basins allowing the exhumation of a new lateritic landsurface. Two major weathering periods separated by a second erosion episode 24-18 Ma are also depicted at the end of Oligocene 29-24 Ma and lower to mid-Miocene 18-11.5 Ma in the upper domain, during which newly shaped land surfaces conspicuously weathered. The shorter-weathering and erosion episodes recorded in the lower domain from 18 to 2.9 Ma led to the final geomorphic changes that were conducive to the formation of glacis. The preservation of old cryptomelane 59-45 Ma in the upper part of the ore deposit indicates a Cenozoic denudation limited to the erosion of previous bauxites, and partly, of ferricretes.

Keywords : laser-probe 39Ar-40Ar dating lateritic paleoland surface Cenozoic denudation

Autor: Anicet Beauvais - Gilles Ruffet - Olivier Hénocque - Fabrice Colin -



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