Friday, 24 August 2012

How lake-like was Lake Pebas?
What did the long-lived lake complex in (nowadays) western Amazonia look like during the Miocene?
First a bit of context…
The geological history of Amazonia has been profoundly influenced by the uplift of the Andes, which started during the Paleogene, about 65 to 34 million years ago (Ma). When the Andes reached the elevation of approximately 2000 meters, they caused massive rains because they stopped the movement of the clouds from east to west. This huge increase in precipitation resulted in very high erosion rates in the eastern side of the Andes and changed the sedimentation regime in the Andean foreland and in Amazonia. At the same time, subsidence exceeded sediment input creating swamps and lakes.
The thickness of the deposited sediments reached more than 1000 meters. It is not clear if this happened between 23 and 10 Ma, due to the mechanism described above (Hoorn et al. 2010a,b); or between 9 and 4.5 Ma, due to a reduction in the subduction angle of the Nazca plate in the Central Andes that shifted the sedimentation area eastward (Latrubesse et al. 2010). Nowadays these sediments constitute what is known as the Pebas (Solimões in Brazil) formation, which outcrops in several areas of Peru and Brazil. What did Amazonia look like during this period? The most accepted hypothesis suggests that a huge system of lakes and wetlands formed in the foreland basin: the so-called lake Pebas. It would seem that Lake Pebas was connected with the Caribbean Sea. The instability of this connection would contribute to explain the high diversity of fresh water fish with marine ancestors that now live in the Amazon river system (Hubert and Renno, 2006). You can find an in depth analysis of how the geological history of South America shaped modern day Amazonia and its biodiversity in Hoorn et al. (2010a).
Two weeks ago I was in Iquitos-Peru for the Tropical Rivers 2012 conference, where we were taken on a guided excursion along the river Amazon and shown, by Dr. Latrubesse, an outcrop of the Pebas formation located a few hours by boat from Iquitos (fig. 1). This outcrop holds clear indications (such as cross-bedding and climbing ripples) that it was formed in a fluvial depositional environment.

Fig 1 Outcrop of the Pebas formation near Iquitos-Peru

According to Latrubesse, the sediments of the Pebas formation are fluvial and there is no stratigraphic evidence of the thick and widespread lacustrine sediments that should be expected if a huge lake the size of the Mediterranean Sea had been present in Amazonia for 10 million years. Moreover, Latrubesse claims that there is no evidence of tidal influence in the fossil record of the Pebas formation. More on Latrubesse’s reconstruction of lake Pebas can be found in Latrubesse et al. (2010).
My impression is that the discussion about the existence of lake Pebas is more a matter of definitions than a sedimentological problem. As a matter of fact, I cannot see much difference between the “large wetland of shallow lakes and swamps” described by Hoorn et al. (2010a) and the “avulsive rivers associated with megafan systems, flood basins (swamps, lakes, internal deltas and splays) and soils developed on flat dry areas” described by Latrubesse et al. (2010). In both cases, the resulting sedimentary record would be characterized by low energy fluvial deposition and back-swamp lakes. It seems to me that the whole discussion is about how “lake-like” the Pebas lake/wetland actually was. On the other hand, it is not clear that Hoorn and Latrubesse are talking about the same thing when referring to Pebas. Latrubesse et al. say that Pebas was deposited in the late Miocene in a fluvial environment while Hoorn et al. (2010b) say that the Pebas is a Middle Miocene to early Late Miocene (~16 to 11.3 Ma) deposit characterized by lake-embayment and swamp systems. But, Hoorn et al. (2010b) also define a Late Miocene fluvial-tidal-dominated wetland phase (~11.3 to 7 Ma) called “Acre phase” which, again, would be pretty similar to   Latrubesse's reconstruction.
Quite more striking is the disagreement with respect to the connection that the Pebas lake/wetland might have had with the Caribbean Sea. While Latrubesse is quite certain that “there is no evidence of a marine environment in the fossil record” other authors state that the connection with the marine environment is well supported by the paleontological evidence (see references in Hoorn et al. (2010a)).
What do you think? Do you agree with me that the sedimentological record of the scenario proposed by Hoorn et al. (2010a,b) would be pretty much the same as that proposed by Latrubesse et al. 2010? And how would you explain such a discrepancy in the interpretation of the paleontological record? The paleontological record is also key to assess when (Middle or Late Miocene?) the Pebas (Acre?) formation was deposited.
Hoorn et al. (2010a). "Amazonia Through Time: Andean Uplift, Climate Change, Landscape Evolution, and Biodiversity." Science 330(6006): 927-931.
Hoorn et al. (2010b). “The development of the Amazonian mega-wetland (Miocene; Brazil, Colombia, Peru, Bolivia)” in Hoorn and Wessenligh Eds. Amazonia: landscape and species evolution
Hubert and Renno (2006) “Historical biogeography of South American freshwater fishes.” Journal of Biogeography 33: 1414–1436
Latrubesse et al. (2010). “The late Miocene paleogeography of the Amazon Basin and the evolution of the Amazon River system” Earth-Science Reviews 99: 99-124. 

Hoorn et al (2010). Amazonia Through Time: Andean Uplift, Climate Change, Landscape Evolution, and Biodiversity Science DOI: 10.1126/science.1194585
Latrubesse et al (2010). The late Miocene paleogeography of the Amazon Basin and the evolution of the Amazon River system Earth-Science Reviews DOI: 10.1016/j.earscirev.2010.02.005