Large barchanoid dunes in the Amazon River and the rock record: Implications for interpreting large river systems

Author(s)

de Almeida, Renato Paes ; Galeazzi, Cristiano Padalino ; Freitas, Bernardo Tavares ; Janikian, Liliane ; Lanniruberto, Marco ; Marconato, Andre

• Document type: Journal article
• Source: Earth and Planetary Science Letters; v. 454, p. 92-102, NOV 15 2016.
• Web of Science Citations: 8

Abstract

The interpretation of large river deposits from the rock record is hampered by the scarcity of direct observations of active large river systems. That is particularly true for deep-channel environments, where tens of meters deep flows dominate. These conditions are extremely different from what is found in smaller systems, from which current facies models were derived. MBES and shallow seismic surveys in a selected area of the Upper Amazonas River in Northern Brazil revealed the presence of large compound barchanoid dunes along the channel thalweg. The dunes are characterized by V-shaped, concave-downstream crest lines and convex-up longitudinal profiles, hundreds of meters wide, up to 300 m in wavelength and several meters high. Based on the morphology of compound dunes, expected preserved sedimentary structures are broad, large-scale, low-angle, concave up and downstream cross strata, passing laterally and downstream to inclined cosets. Examples of such structures from large river deposits in the rock record are described in the Silurian Serra Grande Group and the Cretaceous Sao Sebastiao and Marizal formations in Northeastern Brazil, as well as in Triassic Hawkesburry Sandstone in Southeastern Australia and the Plio-Pleistocene Ica Formation in the western Amazon. All these sedimentary structures are found near channel base surfaces and are somewhat coarser than the overlying fluvial deposits, favoring the interpretation of thalweg depositional settings. The recognition of large barchanoid dunes as bedforms restricted to river thalwegs and probably to large river systems brings the possibility of establishing new criteria for the interpretation of fluvial system scale in the rock record. Sedimentary structures compatible with the morphological characteristics of these bedforms seem to be relatively common in large river deposits, given their initial recognition in five different fluvial successions in Brazil and Australia, potentially enabling substantial improvements in facies models for large rivers. (C) 2016 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 13/01825-3 - Relationship between facies and microscale heterogeneities in analogues of fluvial system reservoirs from the Camaquã (RS) and Tucano Central (BA) basins
• Grantee: Liliane Janikian Paes de Almeida
• Support Opportunities: Regular Research Grants
• FAPESP's process: 14/16739-8 - Developing facies models for large river systems: processes and products in active bars in the Brazilian Amazon and implications for the paleogeographic reconstructions of the Neogene Amazon and the Mesozoic of Gondwana in NE-Brazil and E-Australia
• Grantee: Renato Paes de Almeida
• Support Opportunities: Regular Research Grants
• FAPESP's process: 14/09800-2 - Large river deposits in the Solimões River area of the Brazilian Amazon: Neogene evolution and role as biogeographical barriers
• Grantee: André Marconato
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 13/02114-3 - Ediacaran to Cambrian paleosols of the Camaquã Basin (Southern Brazil)
• Grantee: André Marconato
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate
• FAPESP's process: 12/50260-6 - Structure and evolution of the Amazonian biota and its environment: an integrative approach
• Grantee: Lúcia Garcez Lohmann
• Support Opportunities: BIOTA-FAPESP Program - Thematic Grants