40Ar/39Ar geochronological constraints on the evolution of lateritic iron deposits in the Quadrilátero Ferrífero, Minas Gerais, Brazil

Bedrock gorges incising glacial hanging valleys potentially allow measurements of fluvial bedrock incision in mountainous relief. Using digital elevation models, topographic maps, and field reconnaissance, we identified and characterized 30 tributary hanging valleys incised by gorges near their confluence with trunk streams in the Romanche watershed, French Western Alps. Longitudinal profiles of these tributaries are all convex and have abrupt knickpoints at the upper limit of oversteepened gorge reaches. We reconstructed initial glacial profiles from glacially polished bedrock knobs surrounding the gorges in order to quantify the amount of fluvial incision and knickpoint retreat. From morphometric analyses, we find that mean channel gradients and widths, as well as knickpoint retreat rates, display a drainage area dependence modulated by bedrock lithology. However, there appears to be no relation between horizontal retreat and vertical downwearing of knickpoints. Assuming a postglacial origin of these gorges, our results imply high postglacial fluvial incision (0.5–15 mm yr −1 ) and knickpoint retreat (1–200 mm yr −1 ) rates that are, however, consistent with previous estimates. Numerical modeling was used to test the capacity of different fluvial incision models to predict the inferred evolution of the gorges. Results from simple end‐member models suggest transport‐limited behavior of the bedrock gorges. A more sophisticated model including dynamic width adjustment and sediment‐dependent incision rates predicts present‐day channel geometry only if a significant supply of sediment from the gorge sidewalls (∼10 mm yr −1 ) is triggered by gorge deepening, combined with pronounced inhibition of bedrock incision by sediment transport and deposition.

Bedrock

Lithology

10.1029/2008jf001079

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Citations (100)

Geochronological evidence for pervasive Miocene weathering, Minas Gerais, Brazil

Earth Surface Processes and Landforms (2004)

Isabela de Oliveira Carmo Paulo Vasconcelos

Abstract 40 Ar/ 39 Ar laser incremental‐heating analyses of 22 individual grains of supergene cryptomelane from three weathering proﬁles, up to 400 km apart, in the Rio Doce valley and Barbacena regions at Minas Gerais, Brazil, show that the formation of weathering proﬁles in these regions is contemporaneous, suggesting a strong weathering event in the Middle to Late Miocene (10–8 Ma). The preservation of these Miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the Miocene or that a much thicker weathering mantle was present in the region originally. Assuming a constant thickness of weathering proﬁles in the region throughout the Tertiary, we may calculate weathering front propagation rates of 4–8 m Myr −1 during the past 10 Ma. Copyright © 2004 John Wiley & Sons, Ltd.

Supergene (geology)

10.1002/esp.1090

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Citations (48)

Introduction to the petrology of soils and chemical weathering

Daniel Nahon

Introduction to the major geochemical processes of weathering supergene alteration of minerals and rocks - preservation of original structures structural transformations of pedoturbation transfers and accumulations differentiation and evolution of pedological mantles of tropical and subtropical zones.

Supergene (geology)

Soil production function

Parent rock

Source

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Citations (297)

Characterisation of the weathering profile developed over an amphibole schist bedrock in Peninsular Malaysia

Bulletin of the Geological Society of Malaysia (1994)

John Kuna Raj

Five stages of weathering of an amphibole schist bedrock material were distinguished on the basis of differences of colour, texture and the extent of alteration of original mineral grains. These stages of weathering are of a gradational nature except for the abrupt transition between unweathered bedrock material, and bedrock material weathered to the condition of an engineering soil. These stages of weathering are differently distributed within the weathering profile and determination oftheir distribution pattern allowed for the recognition of several morphological horizons. These morphological horizons are considered to represent different stages of weathering of the bedrock on the scale of the mass and can be assigned rock mass weathering grades.

Bedrock

Amphibole

Parent material

Soil production function

Texture (cosmology)

10.7186/bgsm35199413

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Citations (2)

Weathering Of Phosphate Minerals

Springer eBooks (1984)

René Flicoteaux Jacques Lucas

Supergene (geology)