Late Quaternary Cuiabá megafan, Brazilian Pantanal: Channel patterns and paleoenvironmental changes

Abstract

The Brazilian Pantanal wetlands comprise some of the largest Quaternary megafans in South America. The origin of these megafans has been linked to climate changes during the Late Quaternary. This study focuses on the Cuiab a megafan, the second largest megafan of the Pantanal, which has not been studied from the perspective of its basic geomorphology, river channel patterns and sediment deposition ages. Analysis of the channel and floodplain morphology as well as optically stimulated luminescence dating enable us to establish the succession of events that built up the Cuiab a megafan since the last glacial period. Abandoned lobes indicate that a phase of fan aggradation occurred between 48 and 19 ka, with a network of distributary paleochannels suggesting that there was relatively low precipitation and reduced vegetation cover. Subsequently, abrupt changes in discharge and sediment supply triggered the channel incision of the upper fan valley, indicating a degradational phase after the Last Glacial Maximum (<19 ka). A new phase of fan aggradation began at the Early Holocene, when meandering and anabranching channels were constrained into the incised valley on the upper fan. In the distal part of the fan, the development of modern depositional lobes has been driven by numerous single-sinuous channels that shift laterally due to successive avulsions. Regional tectonic features suggest that modern Cuiab a lobes are in a subsiding area that provides accommodation space for sediment accumulation and prolonged flooding events. Changes in the channel patterns observed in the Cuiab a megafan are related to changes in the South American monsoon strength, which driven precipitation variation during the Late Quaternary. Periods of monsoon intensification and relatively high precipitation lead to channel incision, while periods of fan aggradation correlate to times of fewer oscillations. Our findings highlighted the diversity of fluvial styles in the Pantanal and its importance as a climate changes archives in the South American lowlands.

Hidden in plain sight: How finding a lake in the Brazilian Pantanal improves understanding of wetland hydrogeomorphology

“…The geological lineaments ( Figure 9) with northeast-southwest trends are aligned with the Transbrasiliano fault system described by Soares et al (1998), and the north-northwest-south-southeast follows the Pantanal southern border basement orientation and normal faults, which separate the Paraguay River plain from the Urucum plateau (Assine et al, 2015b). Geological lineaments and faults are also found in other regions of the Pantanal, mainly in the Taquari, Cuiabá, and São Lourenço megafans and in the Paraguay trunk plain (Paranhos-Filho et al, 2013;Almeida et al, 2015Almeida et al, , 2019Assine et al, 2015b;Pupim et al, 2017;Oliveira et al, 2018). Recent earthquakes registered in both Pantanal and Chaco basins are the results of uniform compressional stresses in the east-west direction (Dias et al, 2016).…”

“…Contrasting river styles and channel arrangements resulting from changes in water discharge, sediment load, sediment type, and gradient are affected by both the tectonics and climate (Schumm et al, 2000). Shifts in channel patterns are constant and widely recorded in the Pantanal wetland, due to climatic variations, tectonic conditioning, or both (Assine and Silva, 2009;Merino et al, 2013;Kuerten et al, 2013;Macedo et al, 2014;Almeida et al, 2015;Pupim et al, 2017). Shifts in channel patterns are constant and widely recorded in the Pantanal wetland, due to climatic variations, tectonic conditioning, or both (Assine and Silva, 2009;Merino et al, 2013;Kuerten et al, 2013;Macedo et al, 2014;Almeida et al, 2015;Pupim et al, 2017).…”

“…The Negro is one of the rivers with the most recognizable changes in floodplain morphology, channel planform, and in hydrological dynamics. Shifts in channel patterns are constant and widely recorded in the Pantanal wetland, due to climatic variations, tectonic conditioning, or both (Assine and Silva, 2009;Merino et al, 2013;Kuerten et al, 2013;Macedo et al, 2014;Almeida et al, 2015;Pupim et al, 2017).…”

“…The river undergoes several poorly understood hydrogeomorphic changes along its course, producing unexpected fluvial landscapes. Recent studies have focused mainly on the dynamics and evolution of megafan systems and in the Paraguay trunk river plain (Assine and Silva, 2009;Assine, 2015;Almeida et al, 2015;Pupim et al, 2017) with very few studies on the interfan systems (e.g. Ab'Saber, 1988;Pupim et al, 2014;Assine, 2015).…”

Compartimentação E Evolução Geomorfológica Da Planície Do Rio Cuiabá, Pantanal Mato-Grossense

“…Os megaleques do Pantanal Mato-Grossense vêm sendo estudados há várias décadas (BRAUN, 1977;AB'SABER, 1988) e tem sido crescente o número de publicações que enfatizam os processos relacionados à sua gênese, principalmente considerando causas climáticas (ASSINE & SOARES, 2004;ASSINE et al, 2005;ZANI, 2008;ZANI e ASSINE, 2009;SILVA, 2010;KUER-TEN, 2010;CORRADINI, 2011;KUERTEN & AS-SINE, 2011;MERINO, 2011;ZANI e ASSINE, 2011;CORRADINI & ASSINE, 2012;PUPIM et al, 2012;PUPIM, 2014;PUPIM et al 2017;KUERTEN et al, 2013;MERINO et al, 2013;MACEDO et al, 2014). Outros estudos vêm colaborando para a compreensão da evolução geológica e geomorfológica dos megaleques fl uviais do Pantanal como, por exemplo, os trabalhos de geofísica e tectônica de Almeida (1964), Shiraiwa (1994), Ussami et al, (1999), Alvarenga et al (2000), Shiraiwa e Ussami (2001) e Fisseha (2003.…”