South American cratons and Neoproterozoic  cover and fold belts (after Misi et al. 2007).

One of the most puzzling questions in the geological record is the cause of the major Cambrian diversification of the animals, the so-called “Cambrian Explosion”.

Some ideas have been formulated to explain why only after four billion years, since the origin of the life in the Earth, this animal diversification occurred. One is the Snowball Earth hypothesis, which argues that the planet had been completely frozen once or more before diversification could take off.

Researchers have been looking for an answer in the numerous records of the period before the Precambrian-Cambrian boundary. One important record has been found in central South America at the Brazilian, Paraguayan and Bolivian boundary in the southeast part of the Amazon Craton.

Beside the amazing Pantanal plain, this area has caught the interest of many researchers, due to the occurrence of Precambian animal fossils and excellent rock preservation. Detailed geochemical investigations of carbonate rocks can be used to interpret the chemical condition of the water, and understand what happened in that enigmatic period of the Earth’s history.

The Pantanal wetland surrounding Precambrian rock outcrops. The high modern biodiversity contrasts with stratigraphic units with the record anticipating the rise of animal diversification.

Geological sketch of the area (Alvarenga et al. 2009). The South Paraguay Belt is located on the eastside of the Rio Apa Block, in the Southern region of the Pantanal.

 

 

 

The carbonate succession in the Bodoquena-Pantanal region is represented by the Corumbá Group in Brazil and the corresponding Itapucumi Group in Paraguay. This sedimentary unit was deposited after deposition of a glacial diamictite (Puga Formation), whose age has not yet been properly defined. These units in the north continue in the Paraguay Mobile Belt.

 

Dating the Precambrian glacial event in this area is important because of its relationship with huge manganese and iron deposits in the Urucum Massif. Most of  the Neoproterozoic occurrence of iron formation in the world are related to the old Neoproterozoic glaciation (the Sturtian), but there is some evidence that puts the iron formations of Urucum as late Cryogenian (Marinoan); although there is no geochronological data supporting which glacial period it is associated.

 

 

 

 

 

 

 

The Urucum Massif, where economic manganese and iron formation has been economically exploited (photo courtesy of ore company Vale)

Detail of the iron formation in the Urucum Massif (Jacadigo Group).

The iron formation with isolated granite blocks (see detail below).

The isolated granite block in iron formation  could be related to glacial  deposition, but the lack of deformation at the base would be evidence of landslide, as originally pointed out by Trompette et al (1998).

Seen from above, the Urucum Massif reminds one of a monkey. Because of this, the hills are named after  body parts of a monkey.

There are two cap carbonates over the diamictites of the Puga Formation- one in the Corumbá Group context, and another in the north of the Paraguay Mobile Belt related to Araras Group.

 

Location of the cap carbonate occurrences of the Corumbá Group and the Araras Group.

The Mirassol do Oeste Cap Carbonate in the context of the Araras Group. The carbonate rocks lie directly on top of the diamictites, where soft deformation indicates contemporaneous sedimentation (Nogueira et al. 2003).

The Corumbá Group is composed of two carbonate units, one of dolomites rich in microbialites (Bocaina Formation), and another characterized by black limestones (Tamengo Formation).

 

Outcrop of stromatolites from the Bocaina Formation.

Tamengo Formation animal fossils (Walde et al. 2007, Warren et al. 2012, Pacheco et al.,2012)

The main exposure of carbonate rocks of the Corumbá Group is in southern Paraguay’s Mobile Belt in the Serra da Bodoquena, where a well-developed carst relief is present with caves and tufas deposits.

 

General view of the inner part of the Serra da Bodoquena, with a concentration of carbonate rock in the national park (Juca-Ygarapé photo).

Canyon of the Rio Perdido River inside the national park (Juca-Ygarapé photo).

Rivers in the Serra da Bodoquena are characterized by bicarbonate water and extremely clean water (Paulo Robson de Sousa photo).

Numerous tufa deposits develop in rivers of the Serra da Bodoquena.

The Tufa waterfall of the Aquidaban River located in the western part of the Serra da Bodoquena (Juca-Ygarapé photo).

Detail of the Aquidaban Tufa waterfall (Juca-Ygarapé photo).

River of extremely clean water in the Serra da Bodoquena (Juca-Ygarapé photo).

For the divulgation of the local geology and paleontology, a Geopark has been organized in the region to promote educational projects and increase tourism.

 

View of the Urucum Massif which is the symbol of the Geopark Bodoquena – Pantanal has been inspiride Nivaldo Vitorino to create the label below.

 

References:

Alvarenga, C. J. S. et al. 2009. The Amazonian Palaeocontinet. In: Gaucher, C.; Sial, A.N.; Frimmel, H.; Halverson, G.P.. (Org.). Neoproterozoic-Cambrian Tectonics, Global Changes and Evolution: A focus on Southwestern Gondwana. Neoproterozoic-Cambrian Tectonics, Global Changes and Evolution: A focus on Southwestern Gondwana. Elsevier, v. 16, p. 15-28.

Boggiani, P. C. ; Gaucher, Claudio ; Sial, A. N. ; Babinski, M. ; Simon, Cynthia M. ; Riccomini, Claudio ; Ferreira, V. P. ; Fairchild, Thomas R. 2010. Chemostratigraphy of the Tamengo Formation (Corumbá Group, Brazil): a contribution to the calibration of the Ediacaran carbon-isotope curve. Precambrian Research, 182: 382-401.

Walde, D.H.G.; Leonardos, O.H.; Hahn, G.; Hahn, R.; Pflug, H. 1982. The first Precambrian megafossil from South America, Corumbella werneri. Anais da Academia Brasileira de Ciências, 54(2):461. 

Nogueira, A. C. R. ; RiccominI, C. ; Sial, A. N. ; Moura, C. A. V. ; Fairchild, T. R. 2003. Soft-sediment deformation at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon craton, Brazil): Confirmation of rapid icehouse-greenhouse transition in snowball earth. Geology, 31:613-616.

Trompette, R.; Alvarenga, C.J.S. de; Walde, D. 1998. Geological evolution of the Neoproterozoic Corumbá graben system (Brazil). Depositional context of the stratified Fe and Mn ores of the Jacadigo Group. Jorunal of South America Earth Sciences, 11(6):587-597.

Freitas, Bernardo T. ; Warren, Lucas V. ; Boggiani, Paulo C. ; De Almeida, R. P. ; Piacentini, T. 2011. . Tectono-sedimentary evolution of the Neoproterozoic BIF-bearing Jacadigo Group, SW-Brazil. Sedimentary Geology, 238: 48-70.

Walde, D.H.G., Hagemann, S.G.E., 2007. The Neoproterozoic Urucum/Mutún Fe and Mn deposits in W-Brazil/SE-Bolivia: assessment of ore deposit models. Zentralblatt dt. Geologische Geowissenschaften, 158 (1), 45–55.