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Stranded landscapes in the humid tropics: Earth's oldest land surfaces

Vasconcelos, Paulo M. and Farley, Kenneth A. and Stone, John and Piacentini, Thiago and Fifield, L. Keith (2019) Stranded landscapes in the humid tropics: Earth's oldest land surfaces. Earth and Planetary Science Letters, 519 . pp. 152-164. ISSN 0012-821X. doi:10.1016/j.epsl.2019.04.014.

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Defying common expectations, some of the oldest landscapes on Earth are found in the humid tropics. ^(40)Ar/^(39)Ar dating shows that Fe-rich regolith that armors the ∼700–1000 m elevation banded iron-formation plateaus at Urucum, Mato Grosso do Sul, Brazil, results from chemical weathering that began at least 70 million years ago. Cosmogenic ^3He concentrations in residual hematite clasts and the underlying saprolite on one plateau imply an extraordinarily slow surface lowering rate of 0.07–0.10 m Ma^(−1) maintained over the last ∼17–70 Ma. Cosmogenic ^(10)Be and ^(26)Al concentrations in a quartz vein in the same plateau suggest a similarly low surface erosion rate of ∼0.11–0.18 m Ma−1 over the past few million years. Cosmogenic ^3He concentrations in goethite cementing a ∼200 m elevation indurated pediment ∼3 km northwest of the plateau escarpment yields a minimum exposure age of ∼2.6 Ma, revealing that some of the low-lying areas had already been incised and locally stabilized by goethite cementation by then. In contrast, ^(36)Cl measurements on limestone outcrops at ∼170 m elevation and ∼6.5 km northwest of the Urucum plateau reveal short-term erosion rates of 7.1 ± 0.4 and 10.1 ± 0.5 m Ma^(−1), showing that areas not stabilized by goethite cementation are undergoing erosion at a relatively fast pace. The ∼700 m of relief at Urucum is consistent with differential denudation over ∼70 Ma controlled by lithological differences in chemical and physical rates of weathering and erosion. Effective chemical weathering and protracted low rates of erosion for the high-elevation surface result in armoring by a resilient residue of hematite blocks that are relatively insoluble and too heavy to transport. Armoring by the surface lag of large chemically resilient hematite blocks and bedded hematite saprolite focuses meteoric water flow into the subsurface and along faults. Preferential subsurface flow along the permeable pathways between insoluble hematite blocks, combined with segmentation of the Urucum Surface into isolated catchments too small to host active drainage systems, brings physical erosion to a stand-still. The only effective erosive processes on the Urucum plateau are subsurface chemical erosion and scarp retreat. Non-steady-state landscapes in which elevation differences result from prolonged differential denudation are a common feature of Earth's southern hemisphere cratons, and they inherit their present form from underlying lithologies coupled with geomorphic processes active since the Mesozoic.

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Farley, Kenneth A.0000-0002-7846-7546
Additional Information:© 2019 Elsevier B.V. Received 13 December 2018, Accepted 9 April 2019, Available online 23 May 2019. We thank Arildo Oliveira for facilitating access to the Urucum Mine; the Australian Research Council for partially funding the ^(40)Ar/^(39)Ar laboratory at UQ (ARC Equipment Grant A39531815); and the CNPq PhD scholarship grant GDE200895/2009-5 to T. Piacentini. Kurt Krigbaum and Joy Laydbak assisted with sample preparation in the UW Cosmogenic Nuclide Lab. UW work on this project supported by National Science Foundation award EAR 9805132.
Funding AgencyGrant Number
Australian Research CouncilA39531815
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)GDE200895/2009-5
Subject Keywords:40Ar/39Ar Mn oxide geochronology; hematite cosmogenic 3He; quartz cosmogenic 10Be and 26Al; calcite cosmogenic 36Cl; cratonal landscape; Neoproterozoic banded iron-formation
Record Number:CaltechAUTHORS:20190523-103853641
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Official Citation:Paulo M. Vasconcelos, Kenneth A. Farley, John Stone, Thiago Piacentini, L. Keith Fifield, Stranded landscapes in the humid tropics: Earth's oldest land surfaces, Earth and Planetary Science Letters, Volume 519, 2019, Pages 152-164, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95734
Deposited By: Tony Diaz
Deposited On:23 May 2019 17:50
Last Modified:16 Nov 2021 17:15

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