Tuesday, 3 September 2013

The rectangular and oriented lakes in the Bolivian Amazon are not tectonic, and now what?

Our latest paper has been published a few days ago in Geomorphology. The title is: "The origin of oriented lakes: Evidence from the Bolivian Amazon". Here goes a very short version of it.
The presence of hundreds of rectangular and oriented lakes is one of the most striking characteristics of the Llanos de Moxos landscape (Fig. 1). Many different mechanisms have been proposed for their formation, including subsidence resulting from the propagation of bedrock faults through the foreland sediments, scouring caused by large-scale flooding, paleo deflation combined with wind/wave action and human agency. Nevertheless, amid this diversity of hypothesis, the most commonly accepted cause of lake formation to date has been tectonics.
Figure 1. Landsat image of oriented and rectangular lakes in the Llanos de Moxos
Plafker’s tectonic model (Fig. 2) has never been tested. If faulting is involved, the displacement should be visible and measurable through sediment profiling. The only element needed is a stratigraphic marker that allows the measurement of the vertical displacement.
Figure 2. Tectonic model for lake formation (Plafker, 1967). According to Plafker, the lakes' rectangular shape results from the propagation of bedrock fractures through unconsolidated sediments.
Thanks to our recent discovery of a paleosol below mid-Holocene fluvial sediments in the south-eastern LM (Lombardo et al., 2012), where several lakes are found, it is now possible to test the tectonic hypothesis. If lakes were formed by local subsidence induced by bedrock faults, we should find the paleosol at a greater depth below the lake than in the area surrounding it. 

This is how we cored the lakes
Stratigraphic profiles from transects that cut across the borders of three lakes show otherwise (Fig. 3): the depth of the paleosol is the same. Hence, tectonics, as the mechanism behind the formation of the lakes, can be ruled out. The origin of the Moxos rectangular and oriented lakes is still very much unresolved. A more detailed discussion about the possible mechanisms behind the lakes' formation can be found in Lombardo & Veit (In Press)
Figure 3. Stratigraphic transects from the outside to the inside of the lakes. Dotted white lines define the lakes’ basins. The early to mid-Holocene paleosol acts as a stratigraphic marker (see Fig. 2). Cores 52, 63, 81, 170, 205 and 210 provide the reference depth of the paleosol outside the lakes; cores 77 and 204 have been performed in areas of the original lakes’ basins that have been infilled; cores 78, 169, 171 and 209_b come from inside the lakes. Continuous black lines reconstruct the original lake bottom (previous to lacustrine infilling); dashed black lines connect the paleosol. Source of digital images: Google earth.
Lombardo, U., May, J.-H., & Veit, H. (2012). Mid- to late-Holocene fluvial activity behind pre-Columbian social complexity in the southwestern Amazon basin The Holocene DOI: 10.1177/0959683612437872
Lombardo, U., & Veit, H. (2013). The origin of oriented lakes: Evidence from the Bolivian Amazon Geomorphology DOI: 10.1016/j.geomorph.2013.08.029
Plafker, G. (1964). Oriented Lakes and Lineaments of Northeastern Bolivia Geological Society of America Bulletin DOI: 10.1130/0016-7606(1964)75[503:OLALON]2.0.CO;2