As he passed through the area around what is now Porto Seguro, in the southern part of the Brazilian state of Bahia, aboard the flagship commanded by Pedro Álvares Cabral, Pero Vaz de Caminha marveled at the size of the coastline of the island of Vera Cruz, the first name given to the recently-discovered Brazil. He also wrote about eye-catching cliffs that rose along the shore, nestled against the Atlantic: “In some parts there are great banks, along by the shore, some of which are red and some white; inland it is all flat and very full of large woods.” The Portuguese writer was looking at a section of what is now called the Barreiras Formation, which consists of layers of sand and clay, generally a few dozen meters thick, that stretch for over 5,000 kilometers along the Brazilian coastline between the states of Amapá and Rio de Janeiro.
Geologists believe that these cliffs, mentioned in the first description of a geological unit in Brazil, tell a story that is much older than the saga of the discovery. They are witness to the last great rise in the Atlantic sea level that occurred along stretches of the Brazilian coastline, particularly in the North and Northeast, between 25 and 16 million years ago in the late Oligocene and middle Miocene epochs. The cliffs were largely formed by the action of tidal currents along the coast that carried sediments into the continental interior as a result of this significant rise in sea level. According to some studies, during the Miocene the oceans rose as much as 180 meters above their current levels in some parts of the world. In Brazil, the average elevation was more modest, generally on the order of 60 meters, with peaks of up to 140 meters along the coast of the states of Sergipe and Alagoas, according to an extensive study of the Barreiras Formation published in the August 2013 issue of the scientific journal Earth-Science Reviews.
But that may not have been the most surprising bit of data revealed in the article, which was written by three Brazilian geologists. The paper points out that, following this period marked by episodes during which the Atlantic rose along sections of the northern and northeastern coastline between 25 and 16 million years ago, the sea level along the Brazilian coastline sank between 15 and 10 million years ago. Paradoxically, this drop in the Atlantic sea level along the Brazilian coast occurred at the same time as the level of the oceans reached its maximum height in other parts of the planet. Why didn’t the period of high Atlantic sea level in the North and Northeast correspond to the elevation of the oceans worldwide? “It was probably due to land movements caused by tectonic activity along stretches of the Brazilian coast,” says geologist Dilce de Fátima Rossetti of the National Institute for Space Research (INPE), the article’s first author, who has been studying the Barreiras Formation for more than two decades.
According to the geologists’ data, the land along much of the Brazilian coastline was submerged under several dozen meters of water between 25 and 16 million years ago as a result of tectonic movements. Even though the oceans during that period had not reached their worldwide peak elevation, the lower level along sections of the northern and northeastern coast paved the way for material from the ocean to come ashore. It created basins that were conducive to receiving and storing sediments carried in by the Atlantic. Thus, the rise in sea level along parts of the Brazilian coast resulted in sediment deposition that created the Barreiras Formation, as well as the Pirabas Formation, the latter slightly older and not as long.
DILCE ROSSETTIBetween 15 and 10 million years ago, during the worldwide peak elevation of the oceans, seismic activity produced exactly the opposite effect on the contours of the Brazilian coast. “During that period, the land stabilized or even lifted. This offset the rise in the global sea level and left no sedimentary record in the region,” Rossetti explains. “At that point, a wide strip along the northern and northeastern coast of Brazil, which had been covered by the ocean, emerged, became exposed to erosion and turned into a place amenable to vegetation growth.”
The researchers’ hypothesis was based on a finding that has gained momentum over the past two decades. Contrary to what scientists had always believed, the Brazilian coast was not situated in a region that was entirely geologically stable. Although all of Brazil’s national territory lies in the middle of the South American tectonic plate—a feature that keeps it free of major earthquakes—medium seismic disturbances and significant changes in land elevation occur with some frequency. “The topography is dynamic,” says Francisco Hilário Rego Bezerra, a geologist at the Federal University of Rio Grande do Norte (UFRN), a co-author of the article. “Our coastline was thought to be stable, but our work shows that not to be the case. Evidence indicates that there are many tectonic faults in that region, and that they were reactivated during the Miocene. In fact, there is evidence that they are probably active to this day.”
The simple explanation is that three major factors can affect the sea level along a stretch of coastline: the worldwide level of all the oceans; the local stability of the land (whether it is sinking or rising as a result of tectonic movements); and the occurrence of erosive processes that wear away the surface, or sediment deposition that add layers to the soil. Depending on the weight of each of these factors, the overall tendency for the sea level to rise or fall may be amplified, mitigated or even nullified on a local or regional scale. It seems like a contradiction, but it is not. Because of the interaction of these factors, the sea may rise at only one or a few points along a coast, as happened in the North and the Northeast during the Miocene, while the level of the oceans on much of the planet falls or stabilizes. “The continents do not remain fixed on the vertical plane,” explains marine geology expert José Dominguez of the Federal University of Bahia (UFBA), another co-author of the study. “They are always moving around.”
The eastern edge of South America is associated with a geologically calmer, more stable scenario than what we see in the West. The Pacific coast, molded during the Miocene by the uplifting of the Andes when the Nazca and Antarctic tectonic plates collided with the western edge of the South American continental plate, is frequently shaken today by highly powerful tectonic movements. “The Andes rose about four kilometers during the Miocene,” says Rossetti, whose research is funded in part by FAPESP. The last major event that molded the topography of the region was the separation of South America from the African continent, which began more than 100 million years ago. The fracture that separated the two blocks of land was filled in by the waters of the Atlantic. But, as the work of the three geologists suggests, this does not mean there have not been tremors and earth movements along the Brazilian coast since then.
One of the difficulties involved in showing that the cliffs in the North and Northeast could be an indicator of how high the sea level rose locally some 20 million years ago was related to uncertainties about the age and nature of the Barreiras Formation. The studies confirmed that the sediments of this formation were deposited no earlier than five million years ago, hence dating back to the Pliocene, the geological epoch immediately following the Miocene. It has now been proven that the clay and sand of the Barreiras Formation were deposited well before that time. Another gray area were the places where the sediments of that formation accumulated. Until a few years ago, most authors said that the sediments were deposited in continental areas, mainly river and lake environments. In the past two decades, however, several studies—many of them by Dilce Rossetti—have revealed the true source of the material that lies at the base of many cliffs along the Brazilian coast: the invasion of certain sections of the continent by Atlantic seawater that transported marine sediments onto its eastern fringe.
Owing to its carbonate-poor composition, the Barreiras Formation tends not to preserve a good fossil record of animals that lived there in the distant past. Finding direct traces of a marine organism preserved in these sediments is practically impossible. Weathering in the present day—and in the past—also contributed to the destruction of any fossils in that geological unit. These particular features meant that associating this formation with a marine origin was a challenge from the very beginning.
In recent years, meanwhile, the researchers have been able to gather a number of signs that the formation was largely created along coastal areas, influenced in particular by tidal currents. Tidal cycles carry the sediments back and forth, and these oscillations create signatures in the sediments that reveal their origin and are unlike any other agent of sedimentation. Marks created by tidal currents can be seen in abundance in the Barreiras Formation. Although the actual bodies of animals from the Atlantic are not preserved in this geological unit, the presence of that type of organism in these sedimentary deposits is confirmed by the existence of ichnofossils typical of marine or saline environments. Ichnofossils are indirect evidence of the presence of organisms, such as tracks, burrows or footprints, preserved in geological strata. “We also have records of plant pollens from the Miocene in the Barreiras Formation,” Rossetti says.
All of these signs are invisible to the uninformed layman’s eye, as may have been the case for Pero Vaz de Caminha and many present-day tourists traveling along the northern and northeastern coasts in search of beautiful landscapes, some of them sculpted by the encounter between the cliffs of the Barreiras Formation and the sea. But not to the trained eye of geologists who use this evidence to reveal ocean level fluctuations over time.
Post-rift tectonosedimentary evolution of the Paraíba basin, Northeastern Brazil (2012/06010-5); Grant mechanism Regular Line of Research Project Grant; Coord. Dilce de Fátima Rossetti (INPE); Investment R$109,710.00 (FAPESP).
ROSSETTI, D. F. et al. Late Oligocene–Miocene transgressions along the equatorial and eastern margins of Brazil. Earth-Science Reviews. v. 123, p. 87-112. Aug. 2013.