Large rivers, megafans and other Quaternary avulsive fluvial systems: A potential “who's who” in the geological record

Abstract An important discussion has recently called the attention of sedimentologists and geomorphologists to the dominant fluvial styles preserved in the geological record. While some authors postulate that distributary (or distributive) fluvial systems (DFS) patterns are the most common systems likely to dominate the present day continental sedimentary basins and the alluvial rock record, others suggest that a variety of fluvial styles including axial fluvial systems, are preserved, thus rejecting the dominance of DFSs. Arguments in favor or against the dominance of DFSs in the geological basins are mainly based on two-dimensional analysis of recent axial rivers and DFSs. However, the Quaternary record of the largest depositional tracts on Earth was not assessed in a comparative way. To fill that void, the most important Quaternary areas of sedimentation in the world, such as the alluvial belts of the largest rivers, the largest megafans and other impressive fluvial dominated wetlands in active tectonic basins are analyzed. These results are based on field work conducted by the author in many of the discussed areas, a literature review and analysis of remotely sensed imagery. Specific examples are examined from the largest axial rivers of the Amazon basin, the Parana River, Yangtze River, among others, and from the large depositional tracts in forelands, and intraplate basins such as the Chaco, the Bananal, Pantanal, and Ucamara, which contain a variety of complex avulsive systems and megafans. The present day occurrence and characteristics of braided, meandering and anabranching rivers are reviewed. It is concluded that the scarcity of purely braided systems in large rivers is notable as they are constrained to small-medium sized rivers, relatively short length piedmont courses, alluvial fans or proximal areas of some megafans. Meandering and mainly a variety of anabranching systems are dominant in large rivers but anabranching systems are the end member for megarivers. Once considered an oddity, anabranching rivers are now considered major and fundamental representatives of the fluvial world. Previous studies related the appearance of anabranching rivers to the diversification of terrestrial vegetation. To that end, the hypothesis that anabranching styles diversified after the emergence of grass in the Tertiary is considered. A quantitative assessment on area coverage and sediment trap capacity of major rivers (Amazon and major tributaries, Parana, Magdalena among others) and megafans (Chaco and Pantanal among others) in South America is used to compare the potential preservation and storage capacity of axial rivers, avulsive rivers, and megafans. Results demonstrate that axial and avulsive rivers cover more area than the largest megafans from Chaco and Pantanal, and they currently trap more sediment. The potential preservation of the fluvial sediments is discussed and it is concluded that a major part of the DFSs recorded in wedge-top depositional zones, fore-arc settings and peripheral forelands are susceptible to short term preservation or destruction (tens of million year scale) by tectonic deformation and erosion. A variety of intraplate basins, aulacogen, and foreland mixed zones (forebulge–backbulge-platforms basins) containing the largest rivers, avulsive systems, and the distal part of gigantic megafans, are more favorable tectonic settings for long-term potential preservation. Finally, it is concluded that megafans, a variety of other avulsive systems, and linear (axial) fluvial belts of major rivers have the potential for preservation in the geological record. Despite findings based on Quaternary records, a major challenge remains to identify characteristic facies assemblages for reconstructing large rivers, avulsive fluvial systems and megafans in the geological archive. The scale-size limitation of the sedimentary structures and the insufficient sedimentological understanding of the architectural characteristics of fluvial landforms and sub-environment complexity of large fluvial systems and megafans are some of the issues that need additional research when looking for analogues in the sedimentary record.