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| Titel |
Modes, tempo and spatial variability of Cenozoic cratonic denudation: morphoclimatic constraints from West Africa |
| VerfasserIn |
Anicet Beauvais, Dominique Chardon |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250033126
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| Zusammenfassung |
| After the onset of Gondwana break-up in the Early Mesozoic, the emerged part of the African
plate underwent long Greenhouse effect climatic periods and epeirogeny. The last
Greenhouse effect period in the Early Cenozoic and the alternation of wet and dry climatic
periods since the Eocene enhanced episodes of rock chemical weathering and laterite
production, forming bauxites and ferricretes, interrupted by drier periods of dominantly
mechanical denudation, shaping glacis [1]. In Sub-Saharan West Africa, this evolution
resulted in pulsate and essentially climatically-forced denudation that has shaped an
ubiquitous sequence of five stepped lateritic paleosurfaces that synchronously developed over
Cenozoic times. The modes, timing and spatial variability of continental denudation of
the region are investigated by combining geomorphologic and geochronological
data sets. The geomorphologic data set comprises the altitudinal distribution of the
lateritic paleosurfaces relicts and their differential elevation from 42 locations in
Sub-Saharan West Africa where the sequence (or part of it) has been documented. The
geochronological data set consists in the age ranges of each paleosurface tackled by
radiometric 39Ar-40Ar dating of the neoformed oxy-hydroxides (i.e., cryptomelane,
K1-2Mn8O16, nH2O, [4]) carried by their laterites at the Tambao reference site, Burkina
Faso [1, 3].
Five groups of 39Ar-40Ar ages, ~ 59 - 45 Ma, ~ 29 - 24 Ma, ~ 18 - 11.5 Ma, ~ 7.2 - 5.8
Ma, and ~ 3.4 - 2.9 Ma, characterize periods of chemical weathering whereas the time laps
between these groups of ages correspond to episodes of mechanical denudation that reflect
physical shaping of the paleosurfaces. For the last 45 Ma, the denudation rate estimates (3 to
8 m Ma-1) are comparable with those derived on shorter time scale (103 to 106 y.) in the
same region by the cosmogenic radionuclide method [2]. Combined with the geomorphologic
data set, these age ranges allow the visualization of the regional variability in the estimates of
local relief and denudation rates for several time spans defined between selected
paleosurfaces in the sequence. Denudation rates, ranging from ~ 4 m to ~ 25 m Ma-1,
reflect overall acceleration of erosion rates in the Neogene. The observed space-time
variability of the denudation rates suggest the interplay of (1) duration and intensity of
climatically driven physical erosion periods, (2) absolute elevation and position of
the considered sites with respect to the main continental divides, and (3) potential
reorganization of the large-scale drainage. The results provide a new perspective for the
detection, dating and quantification of subtle epeirogenic movements in West Africa,
once combined with the sedimentary record of Cenozoic intracratonic and coastal
basins.
[1] Beauvais, A., Ruffet, G., Hénocque, O., Colin, F., 2008. Chemical and physical
erosion rhythms of the West African Cenozoic morphogenesis: The 39Ar-40Ar
dating of supergene K-Mn oxides. J. Geophys. Res. Earth Surface 113, F04007,
doi :10.1029/2008JF000996.
[2] Brown, E.T., Bourlès, D.L., Colin, F., Sanfo, Z., Raisbeck, G.M., Yiou, F.,
1994. The development of iron crust lateritic systems in Burkina Faso, West Africa
examined with in-situ-produced cosmogenic nuclides. Earth Planet. Sci. Letters 124,
19-33.
[3] Colin, F., Beauvais, A., Ruffet, G., Hénocque, O., 2005. First 40Ar/39Ar
geochronology of lateritic manganiferous pisolites: Implications for the palaeogene history of
a West African landscape. Earth Planet. Sci. letters 238, 172-188.
[4] Vasconcelos, P.M., 1999. K-Ar and 40Ar/39Ar geochronology of weathering
processes. Ann. Rev. Earth Planet. Sci. 27, 183-229. |
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