Chemical and physical erosion rhythms of the West African Cenozoic 2 morphogenesis : 39 Ar - 40 Ar dating of supergene K - Mn oxides

24 25 Chemical weathering and mechanical erosion are first order processes of long-term 26 tropical morphogenesis, which is still poorly deciphered for lack of time constraints. We 27 address this issue by laser-probe 39 Ar40 Ar dating of generations of cryptomelane [K128 2Mn8O16, nH2O] from the manganese ore deposit of Tambao in northern Burkina Faso. This 29 Mn deposit results from the supergene weathering of carbonate and silicate Mn-protores 30 underneath lateritic palaeolandsurfaces. It consists of an upper cryptomelane-rich domain, and 31 a lower domain where pyrolusite (!-MnO2) is the dominant Mn-oxide. The oldest 39 Ar40 Ar 32 ages (59-45 Ma) are obtained on surface outcrops while the youngest ones characterize deep 33 oxidation fronts (3.4-2.9 Ma). Apparent correlations of 39 Ar40 Ar age groups with ! 18 O and 34 eustatic curves allow definition of the different stages of morphogenesis. Paleocene-Eocene 35 ages (59-45 Ma), bracket a greenhouse period propitious to bauxitic weathering. The lack of 36 significant ages between ca. 45 Ma and 29 Ma characterizes a period dominated by 37 mechanical erosion, during which detrital sediments including lateritic materials were 38 accumulated in intracratonic basins allowing the exhumation of a new lateritic landsurface. 39 Two major weathering periods separated by a second erosion episode (24-18 Ma) are also 40 depicted at the end of Oligocene (29-24 Ma) and lower to mid-Miocene (18-11.5 Ma) in the 41 upper domain, during which newly shaped landsurfaces conspicuously weathered. The shorter 42 weathering and erosion episodes recorded in the lower domain from ca 18 Ma to ca 2.9 Ma 43 led to the final geomorphic changes that were conducive to the formation of glacis. The 44 preservation of old cryptomelane (59-45 Ma) in the upper part of the ore deposit indicates a 45 Cenozoic denudation limited to the erosion of previous bauxites, and partly, of ferricretes. 46 47