Neotectonics, flooding patterns and landscape evolution in southern Amazonia

U Lombardo

doi:10.5194/esurf-2-493-2014

Abstract

Abstract. The paper examines the role of neotectonic activity in the evolution of the landscape in southern Amazonia during the Holocene. It uses both new and published data based on the analysis of remote sensing imagery and extensive field work in the Llanos de Moxos, Bolivian Amazon. The study of the region's modern and palaeorivers, ria lakes, palaeosols and topography provides a strong case in favour of the thesis that the northern part of the Llanos de Moxos constitutes the southern margin of the Fitzcarrald Arch and that it has experienced uplift during the Holocene. The paper assesses the extent and timing of the neotectonic activity in light of the new data and reconstructs the evolution of the landscape since the late Pleistocene. The evidence suggests that at least two uplift events took place: a first uplift in the late Pleistocene, which caused the formation of Lake Oceano, and a second uplift during the mid-Holocene, which formed Lake Rogaguado. These two uplifts appear to be linked to the knickpoints observed close to the towns of Guayaramerín and Puerto Siles respectively. The backwater effect due to these uplifts transformed the region's major rivers in seasonal ria lakes, causing the deposition of thick organic clay layers along the Beni, Mamoré and Madre de Dios river banks. I argue that neotectonic episodes could have dramatically changed the drainage of the Llanos de Moxos, determining its flooding regime, soil properties and forest–savannah ecotone. These results stress the need for geomorphologists, palaeo-ecologists and archaeologists to take into account neotectonics when reconstructing the region's past.

Highlights

Palaeo-ecological reconstructions are a fundamental step in order to understand the origins of modern landscapes and the potential changes they could undergo due to climate change
This study suggests that the uplifting Fitzcarrald Arch is the most likely mechanism behind the observed uplift along the Linea Bala–Rogaguado, the general topography of the Llanos de Moxos (LM), the outcropping of the basement in the northern LM, the presence of underfit rivers crossing the savannahs in the transitional zone, the meandering pattern of the Mamoré River and the general shape of the fluvial network
This paper has reviewed the published evidence of neotectonic activity in southern Amazonia and integrated it with new data from the analysis of remote sensing imagery and extensive field work in the LM

Summary

Introduction

Palaeo-ecological reconstructions are a fundamental step in order to understand the origins of modern landscapes and the potential changes they could undergo due to climate change. At the same time, understanding the processes behind the formation of modern landscapes and ecosystems can help assess to what extent these are “natural” or anthropogenic, eventually contributing to better informed conservation policies (Angermeier, 2000; Willis and Birks, 2006; Vegas-Vilarrúbia et al, 2011) These considerations are true for the Llanos de Moxos (LM) (Fig. 1), which cover most of the savannah of southern Amazonia. The LM, located between the central Andes and the Brazilian Shield, is one of the largest seasonally flooded savannahs of South America This region covers most of the southern part of the Amazon Basin and represents about 10 % of the wetlands of South America (Junk, 2013). In addition to its biodiversity, the LM is of great palaeo-

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