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| Titel |
Cenozoic burial and exhumation of the interior highlands of Brazil: Palaeo-thermal evidence from the Eocene Fonseca Formation, Minas Gerais |
| VerfasserIn |
Peter Japsen, Paul F. Green, Peter R. Cobbold, Lucy G. Sant'Anna, Johan M. Bonow |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250058203
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| Zusammenfassung |
| A rare insight into the Cenozoic development of the highlands in the interior of Brazil is
possible due to an outlier of the Eocene Fonseca Formation, near Belo Horizonte in the state
of Minas Gerais. This sediment is in a small, graben-like basement structure c. 800 m a.s.l.,
and is unconformably overlain by undated, ironstone conglomerate of the Chapada de Canga
Formation (Sant’Anna et al., 1997).
Organic material from the Fonseca Formation has yielded a Tmax value of 427°C. This is
equivalent to a vitrinite reflectance of 0.4 to 0.5%, indicating a maximum palaeotemperature
between 66°C and 83°C. AFTA data from a basement sample near the Fonseca outcrop
reveal three cooling events, (1) from >110°C, beginning at around 150 Ma, (2) from 100°C to
85°C, beginning between 145 Ma and 70 Ma and (3) from 80°C to 60°C, beginning between
50 Ma and 10 Ma. The peak palaeotemperature in the most recent event is highly consistent
with the Tmax value, confirming that the youngest event post-dates deposition of the Fonseca
Formation.
The estimated post-Fonseca palaeotemperatures are equivalent to burial under 1.5–2.0 km
of cover, for a palaeogeothermal gradient of 25°C/km, or 0.7–1.0 km of cover,
for a gradient of 60°C/km. Because the Fonseca Formation was deposited by a
meandering river system (Sant’Anna et al., 1997), more extensive areas must have been
covered by these thick deposits of Eocene and younger sediment. A likely scenario
is thus that even the highest summit in the area, Pico do Sol (2.1 km a.s.l.), was
once under this cover. Furthermore, predominantly early Eocene ages (Spier et al.,
2006), for the formation of laterite less than 100 km from the outcrop of the Fonseca
Formation, most likely reflect exposure prior to deposition of (and burial by) the Fonseca
Formation.
Sant’Anna, L.G., Schorscher, H.D. and Riccomini, C., 1997. Cenozoic tectonics of the
Fonseca Basin region, eastern Quadrilatero Ferrifero, MG, Brazil. Journal of South American
Earth Sciences, 10(3-4), 275-284.
Spier, C.A., Vasconcelos, P.M. and Oliveira, S.M.B., 2006. Ar-40/Ar-39 geochronological
constraints on the evolution of lateritic iron deposits in the Quadrilatero Ferrifero, Minas
Gerais, Brazil. Chemical Geology, 234(1-2), 79-104. |
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