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GEOMORPHOLOGY

The supply town of Redenção looking west towards the Gradaús range, a continuous inselberg ridge.
Cattle Herd
The supply town of Redenção looking west towards the Gradaús range, a continuous inselberg ridge.

The study region is underlain by the Brazilian Shield, a Precambrian crystalline basement that began drifting apart from the once-contiguous Guiana Shield to the north roughly 600 million years ago, opening the intercratonic depression which now comprises the alluvial Amazon Basin. Shield bedrock is composed of metamorphic and igneous materials—gneisses, schists, andesites, granites, basalts—which, in their more weather-resistant forms, crop out extensively as isolate or aggregate inselbergs (‘island mountains’) at intervals tens of kilometers wide. According to Klammer (1984), the most widespread relief type on the crystalline Shields is a “high-level erosion plain with inselbergs of all sizes and a basin and swell topography in which the basins are mostly leveled out by sediment from the ranges” (p. 55). This description aptly describes the study region, where scattered inselbergs rise 50–200 m above the essentially flat surrounding plain which is itself 200–300 m above sea level. Shield bedrock commonly protrudes on high ground where soils thin markedly, either as massive concretions or in extensive boulderfields.

Landscape cleared for pasture with the palm babaçu (Attalea speciosa) and boulder outcrops. The Gradaús range rises in the background to the south.
Cattle Herd
Landscape cleared for pasture with the palm babaçu (Attalea speciosa) and boulder outcrops. The Gradaús range rises in the background to the south.

The Brazilian Shield is tectonically stable. The last orogenic cycle to affect it occurred > 600 million years ago. Two major ranges of outcropping materials which flank the study region, the northern Carajás and southern Gradaús Ranges, are granite-greenstone belts of volcanic origin intruded by Archean granitoids dated at nearly three billion years. Denudation processes—weathering from rainfall at high year-round temperatures decomposing bedrock into soils, with loss of solid materials and nutrient cations to drainage—have shaped topographic relief since the last orogenesis.

Flat Brazilian Sheild landscape with occasional inselbergs. Here, 3 inselbergs north of Marajoara with pasture.
Cattle Herd
Flat Brazilian Sheild landscape with occasional inselbergs. Here, 3 inselbergs north of Marajoara with pasture.

This terrain’s crystalline nature translates into structural stability insofar as stream and river channels are effectively permanent within narrow limits on lateral migration. This contrasts with the more fluid nature of landscape dynamics in the western alluvial Amazon Basin. The Brazilian Shield’s great antiquity means that, since bedrock-derived soils are highly weathered, with primary materials receded far below the biotic (surface) zone except where bedrock stands exposed, nutrient inputs must be largely atmospheric through precipitation.




Those same 3 inselbergs from the air. The Corral Redondo forest with mahogany trees is located between the left & center inselbergs.
Cattle Herd
Those same 3 inselbergs from the air. The Corral Redondo forest with mahogany trees is located between the left & center inselbergs.
The Rio Xingu approximately 200 km northwest of Marajoara in the dry season, looking north. Brazilian Shield bedrock can be seen in the foreground.
Pastures
The Rio Xingu approximately 200 km northwest of Marajoara in the dry season, looking north. Brazilian Shield bedrock can be seen in the foreground.
Extensive stretches of Brazilian Shield bedrock is exposed in the Rio Xingu as water levels fall during the dry season.
Pastures
Extensive stretches of Brazilian Shield bedrock is exposed in the Rio Xingu as water levels fall during the dry season.

SELECTED SOURCES

Bigarella JJ & Ferreira AMM (1985) Amazonian geology and the Pleistocene and the Cenozoic environments and paleoclimates. In: Prance GE & Lovejoy TE (Eds.), Key Environments: Amazonia, pp. 49-71. Pergamon, Oxford, UK.

Clapperton C (1993) Quaternary Geology and Geomorphology of South America. Elsevier Science Publishers BV, Amsterdam, the Netherlands.

Irion G (1984) Clay minerals of Amazonian soils. In: Sioli H. (Ed.), The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River, pp. 537-579. Dr. WJ Junk Publishers, Boston, MA, USA.

Kalliola R, Salo J, Puhakka M, Rajasilta M, Häme T, Neller RJ, Räsänen ME & Danjoy Arias WA (1992) Upper Amazon channel migration. Naturwissenschaften 79: 75-79.

Klammer G (1984) The relief of the extra-Andean Amazon Basin. In: Sioli H. (Ed.), The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River, pp. 47-83. Dr. WJ Junk Publishers, Boston, MA, USA.

Macambira MJB & Lancelot JR (1996) Time constraints for the formation of the archean Rio Maria crust, southeastern Amazonian Craton, Brazil. International Geology Review 38: 1134-1142.

Putzer H (1984) The geological evolution of the Amazon Basin and its mineral resources. In: Sioli H. (Ed.), The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River, pp. 15-46. Dr. WJ Junk Publishers, Boston, MA, USA.

Salo J, Kalliola R, Häkkinen I, Mäkinen Y, Niemelä P, Puhakka M & Coley PD (1986) River dynamics and the diversity of Amazon lowland forest. Nature 322: 254-258.

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