Review of Amazonian Depositional Systems

  1. Michael D. Blum2,
  2. Susan B. Marriott3 and
  3. Suzanne F. Leclair4
  1. Allen W. Archer

Published Online: 17 MAR 2009

DOI: 10.1002/9781444304350.ch2

Fluvial Sedimentology VII

Fluvial Sedimentology VII

How to Cite

Archer, A. W. (2009) Review of Amazonian Depositional Systems, in Fluvial Sedimentology VII (eds M. D. Blum, S. B. Marriott and S. F. Leclair), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304350.ch2

Editor Information

  1. 2

    Baton Rouge, Louisiana, USA

  2. 3

    School of Geography and Environmental Management, University of the West of England, Bristol BS16 1QY, UK

  3. 4

    Department of Earth and Environmental Sciences, Tulane University, Dimwiddie Hall, New Orleans, LA 70118, USA

Author Information

  1. Department of Geology, Kansas State University, Manhattan, KS 66506, USA

Publication History

  1. Published Online: 17 MAR 2009
  2. Published Print: 15 FEB 2005

Book Series:

  1. Special Publication Number 35 of the International Association of Sedimentologists

Book Series Editors:

  1. Ian Jarvis

Series Editor Information

  1. School of Earth Sciences and Geography, Centre for Earth and Environmental Science Research, Kingston University, Penrhyn Road, Kingston-upon-Thames KT1 2EE, UK

ISBN Information

Print ISBN: 9781405126519

Online ISBN: 9781444304350

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Keywords:

  • Amazonian depositional systems;
  • types of Amazonian rivers;
  • sequence stratigraphy considerations;
  • geological evolution of Amazon drainage basin;
  • water-level fluctuation;
  • coastal estuarine environments;
  • erosional headlands and estuaries;
  • coastal and shelf settings

Summary

Many types of depositional system exist within the Amazon River basin and surrounding areas. These areas provide a number of valuable analogues for large-scale and tropical palaeoriver systems. Only rarely, however, are such important analogues invoked because of a lack of published information. Herein, major components are summarized among the fluvial, estuarine and coastal depositional environments of the Amazon River basin. Of particular importance in the Amazon system are the recurring depositional cyclicities that affect sedimentation. In the upper reaches of the system, yearly water-level fluctuations are related to seasonal variations in rainfall. In some areas these fluctuations exceed 10 m. The resultant flooding of vast areas of rainforest greatly affects sedimentation. In the lower reaches of the system, the dominant water-level fluctuations are related to tides. Tidal ranges are as high as 6 m at the mouth of the Amazon and tidal influences extend more than 800 km up-river. The result is a vast area of tidally influenced, freshwater environments, which are a poorly documented, but important, depositional setting.

There is a variety of river types within the Amazonian system, and the depositional environments along each are greatly affected by their differences in sediment load. Some rivers, which include the tributaries that drain the Andes, are rich in suspended sediment and have alluvial valleys containing features typical of meandering rivers, such as levees, floodplain lakes and nutrient-rich floodplains. Other rivers, with drainages entirely within extensive areas of rainforest, drain deeply weathered terrains with highly leached soils. These rivers lack suspended sediment, but have small amounts of bedload, and do not produce alluvial valleys or levees. From a sequence stratigraphy perspective, the differences among rivers have resulted in considerable variation of response to glacioeustatic-induced sea-level fluctuations and the ensuing inland changes in base level. During low stands, the Amazon deeply incised its valley for thousands of kilometres inland from the present position of the mouth. During transgression, rivers with a high sediment load vertically aggraded and kept pace with base-level rise. At the same time, the valleys of sediment-poor rivers flooded and were transformed into river-mouth lakes. These flooded valleys are not restricted to coast-proximal settings, but can be found at distances more than 1000 km inland from the modern coast.

Sediment supply has also differentially affected coastal settings along the Atlantic north and south of the Amazon mouth. The plume of turbid Amazonian water moves north-westward along the coast because of predominant winds and currents. The longshore drift of this sediment-rich plume has resulted in a low-slope, mud-dominated, prograding coast. South of the Amazon mouth, however, the lack of sediment influx has resulted in a complexly embayed erosional coastline. This southern coast consists of sea cliffs and headlands comprised of Mesozoic and younger rocks, which separate numerous small-scale macrotidal estuaries.