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.
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.
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.
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