SEDIMENTOLOGY AND EVOLUTION OF A LATE PLEISTOCENE-HOLOCENE HUMID MEGAFAN IN

NORTHERN BRAZIL

Dilce F. Rossetti

Hiran Zani

Marcelo C. L. Cohen

Édipo H. Cremon

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MEGAFAN MODEL

Okavango Megafan (Gumbricht & McCarthy, 2001)

Tectono and climatic setting for the formation of megafans (Nichols & Fisher, 2007)

Nichols & Fisher 2007

Taquari (Assine, 2005)

Kosi (Chakraborty et al. 2010)

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Escudo das Guianas

Fan-shaped deposits of the Pantanal Setentrional Basin

Viruá

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Morphology of the Viruá megafan

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Morphology of the Viruá megafan

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Chronology of the Viruá megafan

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Viruá megafan

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Viruá megafan

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Viruá megafan

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MODEL FOR A MEGAFAN IN A HUMID TROPICAL REGION

- Tropical areas with high temperature and high precipitation

-Modern tributaries

AMAZONIAN REGION

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MEGAFANS IN THE PANTANAL SENTENTRIONAL BASIN

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TECTONIC INFLUENCE IN THE PANTANAL SENTENTRIONAL BASIN

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ORIGIN OF THE VIRUÁ MEGAFAN

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VIRUÁ MEGAFAN- TECTONIC INFLUENCE

VIRUÁ MEGAFAN: TECTONICS + MONSOONAL-LIKE CLIMATE

Assine 2005

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- Late Pleistocene-Holocene deposits are much more widespread inthe Amazonian wetlands than previously recognized;

-They occur as numerous, large, fan-like morphologies in the PSB, with the Viruá fan in Roraima being one of them;

-This feature displays morphological (i.e., large-scale fan geometry, divergent drainage networks, gently-dipping slopes, concave-up and convex-up longitudinal and transversal profiles) and sedimentological (dominantly sandy lithologies and fining and coarsening upward units related to active channels, overbank sand/sheets and terminal fan lobe, abandoned channel, floodplain, debris flow deposits and lake/pond muds) characteristics compatible with residual deposits of a megafan;

-The establishment of a megafan in this humid tropical region wasprobably the due to the creation of new space for sediment accommodation by tectonic reactivation added to a favorable, monsoonal-like climate, with high summer temperatures and drought contrasting with high water discharge during wet winters.

CONCLUSIONS

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THANKS!THANKS!