The significance of superimposed dunes in the Amazon River: Implications for how large rivers are identified in the rock record

Author(s)

Galeazzi, Cristiano P. ^[1] ; Almeida, Renato P. ^{[1, 2]} ; Mazoca, Carlos E. M. ^[1] ; Best, Jim L. ^{[3, 4, 5]} ; Freitas, Bernardo T. ^[6] ; Ianniruberto, Marco ^[7] ; Cisneros, Julia ^[8] ; Tamura, Larissa N. ^[1]

Affiliation

^[1] Univ Sao Paulo, Inst Geociencias, Rua Lago 562, Cidade Univ, BR-05508900 Sao Paulo, SP - Brazil

^[2] Univ Sao Paulo, Inst Energia & Ambiente, Ave Prof Luciano Gualberto 1289, Cidade Univ, BR-05508900 Sao Paulo, SP - Brazil

^[3] Univ Illinois, Dept Geol, Ven Te Chow Hydrosyst Lab, 1301 West Green St, Champaign, IL 61801 - USA

^[4] Univ Illinois, Dept Geog, Ven Te Chow Hydrosyst Lab, 1301 West Green St, Champaign, IL 61801 - USA

^[5] Univ Illinois, Dept GIS Mech Sci & Engn, Ven Te Chow Hydrosyst Lab, 1301 West Green St, Champaign, IL 61801 - USA

^[6] Univ Estadual Campinas, Fac Tecnol, R Paschoal Marmo 1888, BR-13484332 Limeira, SP - Brazil

^[7] Univ Brasilia, Inst Geociencias, Campus Univ Darcy Ribeiro, BR-71900000 Brasilia, DF - Brazil

^[8] Univ Illinois, Dept Geol, 1301 West Green St, Champaign, IL 61801 - USA

• Document type: Journal article
• Source: SEDIMENTOLOGY; v. 65, n. 7, p. 2388-2403, DEC 2018.
• Web of Science Citations: 5

Abstract

The recognition of large fluvial channels in the geological record is of great importance for regional palaeohydraulic and palaeogeographical reconstructions, inputs to reservoir modelling, and estimating the input of sediment to sedimentary basins, with consequent larger-scale implications for modelling basin fill. However, available criteria for the interpretation of the scale of ancient fluvial systems are still poorly tested, particularly the widely-adopted assumption that the abundance of large-scale dunes in some deep channels implies that abundant large-scale cross-strata sets will be preserved in similar palaeochannels. To test this hypothesis, high-resolution multibeam echo-sounding imaging of two reaches in the Amazon River where large dunes are common were investigated, yielding an extensive dataset concerning dune geometry, position within the channel and, most importantly, the presence and distribution of smaller superimposed dunes on their lee sides. These results show that despite 90% of the bedforms at water depths >20m being constituted by up to 122m high compound dunes, 94% of the lee sides of these dunes are covered by smaller superimposed dunes. These results suggest that steep avalanche foresets that are several metres in height may be rare in the preserved stratigraphic record of these large channels, which are instead more commonly represented by decimetre-scale cross-stratified cosets formed by superimposed dunes migrating down the lee side of the large-scale host bedforms. This observation thus suggests that the recognition of compound dune cosets is key to the interpretation of river-channel scale, since compound dunes are the principal bedform in most large river channels. Consequently, successions dominated by decimetre-scale thick cross-strata sets, but that show rarer preservation of outsized metre-scale avalanche foresets, and abundant similar-sized cosets near the base of fining-upward cycles are probably the most common bedform record of large-river channels. (AU)

• FAPESP's process: 17/06874-3 - Fluvial dynamics and sedimentary deposits in the Amazon channel: providing tools to decipher the paleogeographical evolution of Amazonia and deposits of large anabranching rivers in the rock record
• Grantee: Cristiano Padalino Galeazzi
• Support Opportunities: Scholarships in Brazil - Doctorate
• 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: 16/19736-5 - Quantitative approaches for big river deposits: integrated field geology and 3D photogrammetric models
• Grantee: Bernardo Tavares Freitas
• Support Opportunities: Regular Research Grants
• 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