Holocene megafan dynamics in the southernmost Amazon basin
Abstract
The Río Grande (Guapay) catchment in the Central Andes of Eastern Bolivia constitutes the southernmost extension of the Amazonian drainage system and feeds one of the largest megafans in South America. Today, the Río Grande flows into the Río Mamoré, marking the transition from a distributive to tributive channel network. A rich record of preserved paleochannels has been in a variety of remote sensing imagery, and reflects the dynamic history of large-scale channel shifts across the >55.000 km2 large Grande system. However, virtually no chronological information exists with regard to the temporal and spatial scales of these megafan processes, precluding the reconstruction of late Quaternary and Holocene megafan dynamics, and significantly limiting our insight into potential mechanisms and controls on megafan evolution. Here, we (i) present the results of an in-depth geomorphic study of megafan geomorphology, and (ii) provide first stratigraphic and chronological data from four different transects across the Río Grande and Mamoré megafans between ~19°S and ~13°S. Our study shows comparatively rapid and impulsive channel shifts along the proximal Río Grande megafan during the mid- to late Holocene, establishing the connection between the Grande and the Mamoré system along the distal megafan, and thus representing a major reorganization in the drainage network of the Bolivian Amazon basin. The corresponding increase in discharge and sediment load is readily expressed in downstream paleochannel and channel geometry, and may have triggered a late Holocene avulsion of the Río Mamoré meander-belt observed in our data. In combination with the available information from regional paleoclimate proxy records, these results point to climate and environmental change as a dominant control on megafan dynamics during the Holocene, and ultimately contribute to an improved understanding of megafan processes and their relevance for landscape evolution on all Quaternary timescales.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMEP21B0706M
- Keywords:
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- 5419 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Hydrology and fluvial processes