Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39Ar‐40Ar dating of supergene K‐Mn oxides

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 (β‐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 δ18O 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.