What Brazil was like 100 million years ago

Study establishes chronology of tectonic and climate events in Bauru, Sanfranciscana and Parecis sedimentary basins, central-southern Brazil.

By Peter Moon  |  Agência FAPESP – One hundred and forty million years ago, at the start of the Cretaceous Period, Brazil was covered by a vast desert of dunes. This desert was much larger than the Sahara, but it vanished when it was swallowed up in an ocean of lava disgorged by the greatest magma overflow of the last 500 million years. Seven of the planet’s ten largest volcanic eruptions during this period, including the top three, occurred in southeastern Brazil. The picture of Brazil’s geological origins that researchers are piecing together is startling.

The latest study seeking to join three key pieces of this colossal jigsaw, namely, the three geological basins underlying Brazil’s central-southern region, has been published in the Journal of South American Earth Sciences. One of its authors is geologist Alessandro Batezelli, a researcher affiliated with the Geoscience Institute at the University of Campinas (IG-UNICAMP). The project was supported by FAPESP.

Batezelli’s research focuses on the sedimentary basins of central-southern Brazil, especially the Bauru, Sanfranciscana and Parecis basins. An understanding of how tectonic and climate events interacted in each of these basins in time and space helps establish a chronological sequence.

These events were not discovered by Batezelli or by geographer Francisco Sergio Bernardes Ladeira, co-author of the paper, but the research in which they and other scientists are engaged makes it possible to decipher the geological drama that unfolded in central-southern Brazil between 135 and 60 million years ago.

Breakup of Gondwana

In the Jurassic Period, between 201 and 145 million years ago, South America and Africa were not separated by the Atlantic Ocean, but rather part of a single gigantic land mass known as Gondwana. Moisture-saturated air currents from the ancient Panthalassic Ocean that circled the entire planet were not powerful enough to reach the remote heart of Gondwana. Hence, an immense desert formed, which geologists call the Botucatu Desert. A similar process is occurring today in Central Asia, where a desert climate is created due to the region’s vast distance from the ocean.

Hardly any fossils from the Jurassic have been found in Brazil, probably owing to the inhospitable desert climate of the period and the tendency for fossils to disintegrate when in a dune environment. Although the Botucatu Desert was inhabited, only a few fossilized footprints from mammals and reptiles have been found to date.

When South America and Africa began separating 140 million years ago, they eventually made way for the formation of the South Atlantic. “The phenomenon that caused Gondwana to break up was the emergence of faults in the Earth’s crust,” Batezelli said. Huge upwellings of magma from deep inside the planet poured through these fractures. As the fissures widened and the continents moved apart, the lava flows increased. The process continued for a long time, lasting from 137.4 to 128.7 million years ago.

The epicenter of this massive volcanic eruption, “or more accurately, a massive basalt flood, known as the Paraná-Etendeka Volcanic Province”, Batezelli noted, was southeastern and southern Brazil, then still joined to what is now Namibia in Africa.

The Paraná-Etendeka Volcanic Province was formed from several faults or mega-volcanoes, the largest known to science. They were not explosive volcanoes like those on Earth today. “There were no explosive eruptions. Magma poured continuously from the rifts,” Batezelli said. “From here to Africa, there were vast fissures through which lava overflowed, covering a gigantic area and lasting a very long time.” The upwelling of lava amounted to 2.3 million km³ and totally covered 1.5 million km², equivalent to coating the area of Amazonas, Brazil’s biggest state, with a layer of lava 1.5 km thick.

Origin of Guarani aquifer

All this lava buried the ancient dunes of the Botucatu Desert and built up successive layers, leading to the formation of the Serra Geral mountains, which run through the states of Paraná, Santa Catarina and northern Rio Grande do Sul in Brazil, as well as eastern Paraguay and northern Argentina. The desert sands were cooked to a temperature of 1,200°C and compacted by the weight of the magma. Sand became sandstone, a sufficiently porous rock to store rainwater absorbed by the soil.

The dunes of the Botucatu Desert gave rise to the Guarani aquifer, one of the world’s largest underground freshwater reservoirs, buried beneath central-southwestern Brazil. The Guarani aquifer holds 37,000 km³ of water, equivalent to 1.6 times the volume of Lake Baikal in Siberia, considered the largest freshwater lake by volume.

“In regions where the dunes came into direct contact with the lava, the temperature rose so much that the sediments were literally roasted, forming the harder, more impermeable sandstone used today in Portuguese mosaic sidewalk paving,” Batezelli said. When the lava cooled, it formed basalt, which, after weathering by 100 million years of erosion, became the fertile red soil that helped make São Paulo and Paraná the world’s leading coffee growers in the nineteenth century.

A new desert

When the magma flows ceased 128.7 million years ago, the weight of this gigantic pile of volcanic rock pressed down so hard on part of southeastern Brazil that a new sedimentary basin, the Bauru Basin, was created near the surface. A new dune desert, smaller than the previous one, was formed on top of the basin.

The South Atlantic had only just begun opening up. It was not yet even a small sea; at most, it was a flooded depression into which converged rivers, sediments, and erosion from two continents. In other words, the waters of Panthalassa, the ocean surrounding Pangaea, were still far away, as was its humid breeze. The dry conditions prevailing in central-southern Brazil were not to end for another 60 million years, when the climate was made milder by the South Atlantic, although it was then still only half its present size.

In any event, the depression that was slowly spreading, growing by a few inches each year, already had an effect on the climate. The new dune desert, now called the Caiuá Group, was not as huge as the ancient Botucatu Desert, according to Batezelli. It was arid but dotted here and there with oases infested by several species of terrestrial crocodiles, extinct ancestors of today’s crocodiles.

These prehistoric crocodiles lived on dry land, had long paws, and walked like wolves. Paleontologists have described a dozen species. The most well known is the daunting Baurusuchus, a savage predator. However, there were also bizarre forms with horns, as well as Armadillosuchus, with heavy body armor like an armadillo’s, and even a herbivorous crocodile called Sphagesaurus.

The dunes of Caiuá lasted from 125 to 100 million years ago, when they made way for a new landscape of rivers and lakes. “The climate became much milder, similar to what we see now in the semiarid region of northeastern Brazil called the caatinga,” Batezelli said. This new depression received sediments belonging to the Bauru Group, which existed between 80 and 60 million years ago.

At that point, the titanosaurs proliferated. Most Brazilian species are from this period. Their fossils are known by names such as Uberabatitan and Baurutitan, tributes to the towns nearest to where they were found in São Paulo State and Minas Gerais.

Sanfranciscana Basin

Concurrently with the 60 million years of upheavals in the Bauru Basin, “further north in the Sanfranciscana Basin, very similar phenomena that nevertheless weren’t the same were in progress,” Batezelli continued. The Sanfranciscana Basin comprises western Minas Gerais, Goiás, Tocantins, western Bahia and southern Piauí.

During the Lower Cretaceous, eolian dune fields developed in the Sanfranciscana Basin. Tens of millions of years later, during the Upper Cretaceous, volcanism also occurred. “Various volcanoes were formed right on the boundary between the Bauru and Sanfranciscana Basins,” Batezelli said his research had revealed. “The lava flows from these were smaller than from the volcanoes that gave rise to Serra Geral, but they were responsible for the formation of a higher region between the Bauru and Sanfranciscana Basins. It was as if the crust had swollen because of the heat from magma intrusions.”

The resulting relief can be seen even today in the craters within which the towns of Araxá, Tapira and Poços de Caldas are located. “The large deposits of niobium and other mineral wealth found in southeastern Minas Gerais are associated with this volcanism,” Batezelli said.

Most of the lava that flowed from the volcanoes in the Sanfranciscana Basin less than 100 million years ago advanced over the dunes.

The evolution of the Parecis Basin was similar to that of the Bauru and Sanfranciscana Basins. Moderate volcanism had already occurred in the Parecis during the Upper Jurassic, and in the Upper Cretaceous, 145 million years ago, rivers and lakes were formed in the region between northern Mato Grosso and western Rondônia. Over time, the climate became more arid and the landscape was transformed into a dune field.

In sum, from a comparison of the history of these three sedimentary basins it can be concluded that between the Lower and Upper Cretaceous, a period of more than 60 million years, the dune deserts of Brazil shifted from southeast to northwest.

The article “Stratigraphic framework and evolution of the Cretaceous continental sequences of the Bauru, Sanfranciscana and Parecis Basins, Brazil” by Batezelli and Ladeira, published in the Journal of South American Earth Sciences, can be read at www.sciencedirect.com/science/article/pii/S0895981115300857.