There is little mystery behind why the west coast of Africa and the east coast of South America fit together so nicely, a fact that has caught the attention of scientists for more than a century.
The existence of a single supercontinent was first suggested in the 19th century by Edward Suess, an Austrian geologist, on the basis of comparative geological evidence. He called the area that was made up of what are now Africa, South America, Australia, and India “Gondwanaland”, after the Upper Paleozoic and Mesozoic formations in the Gondwana district of central India.
The dawning of a theory
The rift valleys are often flanked by volcanoes such as Kilimanjaro in Africa and large areas can be covered lavas. Dinosaurs replace giraffes in the Jurassic western Gondwana rifting. Photo: Tom SmithThe German meteorologist Alfred Wegener was also intrigued by the close fit of continents. Similarities across the oceans such as fossils of identical plants and animals and the continuation of distinctive rock strata when the continents are brought back together added more supporting evidence for his theory of continental drift. In 1915, he wrote The Origin of Continents and Oceans where he claimed the continents formed a single mass that has since split into pieces.
We now know that Antarctica was also joined with the other continents (possibly as long as 650 million years ago) and was around for about 520 million years. The convergence and collision of smaller continental landmasses that dominated the Paleozoic Era formed this super continent. These continental collisions were responsible for mountain building events, much like the collision of the Indian subcontinent with Asia that is forming the Himalayan Mountains today.
The roots of one of these ancient mountain ranges can be seen in southeastern Brazil, along the Santos and Campos basins. These Late Precambrian-Early Paleozoic granites form some of the spectacular scenery that makes Rio de Janeiro and the surrounding areas so special (see also Geotourism in this issue).
From the Carboniferous to Middle Jurassic, Gondwana was joined with another large landmass called Laurasia to form the supercontinent of Pangaea. At this time, the earth’s landmass constituted a single, very large continent (Pangaea), which was surrounded by the world ocean (Panthalassa). The shifting of the Earth’s lithosphere caused by internal currents in the mantle (a tenet based on plate tectonics concepts) started to tear this supercontinent apart. Laurasia broke off in the middle Mesozoic Period and shortly afterward Gondwana began to break apart, and the newly formed landmasses ultimately started their journey, drifting to their present day locations. They are still moving, as indicated by satellite measurements of the increasing distance between the American and African plates.
Incipient rifting
The lithosphere is that solid, uppermost 80 to 100 km of the earth’s crust and upper mantle that is broken into big and small plates. These plates move a few centimeters each year riding on the earth’s ductile or semi-fluid lower mantle called the asthenosphere.
A common way to develop rifting, where continental crust is extended and thinned, is above upwelling convection cells (mantle plumes) in the asthenosphere. The extensional stresses are induced by the outflow at the base of the lithosphere from one or several zones of upwelling.
The extensional sedimentary basins start as intracratonic, down-thrown blocks that are filled with lacustrine sediments, as is happening today with the East African Rift, forming large, rather deep lakes. The continued rifting results in the break-up of continental plates and the creation of oceanic crust, which is usually heralded by massive outpouring of lava flows, erupted from volcanoes and fissures.
Evolution of the Atlantic Margin
Most of this section has been taken from papers authored by Webster Mohriak and others. One recent paper discussing the breakup of the South Atlantic continental margin was published as a chapter in the book edited by Pankhurst et al., Geological Society, London, Special Publication 2008; v. 294. Dr. Mohriak, an exploration geologist with Petrobras and expert on salt tectonics and rifted continental margins, kindly edited this article. Photo: Webster Mohriak
The current Brazil margin from the continential slope to oceanic crust showing all the pre-rift, rifting, and post rifting elements. ©Webster Morhiak and PetrobrasThe Rift Phase in the South Atlantic started with crustal extension in the Late Jurassic to Early Cretaceous. The first phases (Stage 1) were characterized by minor uplift and thinning of the continental crust and upper mantle. As uplift and extension continued (Stage 2), flood basalts filled the early phases of subsidence along the southernmost part of the South American Plate, particularly onshore in the Parana basin and along the incipient rifts that formed the southernmost Atlantic marginal basins (Pelotas, Santos and Campos). The northern rift segment lacks this magmatism.
Extensional forces at this time were concentrated along the continental margin (that area between the shoreline and the abyssal plain) forming a series of elongate and deep lakes parallel to the coast, due to extensive normal faulting. These eventually evolved into the present day sedimentary basins located on both sides of the South Atlantic margin. The continental lacustrine sediments deposited at this time are one of the key pieces to the developing petroleum systems, namely the source rocks.
The Transitional phase (Stage 3) is characterized by diminishing activity of the large faults associated with rift blocks and local volcanism, fault reactivation, and erosion. A regional unconformity (Break-up Unconformity) separates continental lacustrine sediments in rift blocks from the overlying sediments. This Lower Cretaceous (Aptian) sequence was less affected by normal faults, and was deposited in transitional to marine environments, forming substantial thicknesses of clastic and carbonate rocks well developed offshore of Brazil and Angola, as part of the ‘sag basin fill’.
With the first marine incursions into the forming Brazilian-West African gulf, a shallow-water evaporite basin was developed along both margins. A second key piece of this petroleum system occurred with the deposition of high quality reservoir rocks in shallow water around bathymetric highs formed during the rifting episode. More source rocks were also locally deposited at the time preceding the evaporite deposition. With an extremely arid climate and episodic marine influx, a thick evaporite sequence was deposited over most of the gulf sediments in the Late Aptian. The gulf formed by rifting was now more than 1,000 km long and locally separated by igneous intrusions and volcanic highs.
During the early post-rift phase (Stage 4) the crust was ruptured by oceaning propagators (such as a mid-ocean ridge or spreading center) that advanced from south to north. One of those is known in the northern Pelotas-southern Santos basin as the Abimael Ridge, puncturing the rifted margin with igneous intrusions, as is observed today in the region between the Gulf of Aden and the Afar Triangle. With full development of the mid-Atlantic Ridge (spreading center) and opening of the South Atlantic Ocean, the continental margin development is primarily associated with thermal subsidence typical of divergent margins. Carbonate deposition predominates during the early stages of the post-salt deposition, indicating a shallow water environment that was progressively deepening and invaded by marine waters.
For the late post-rift phase (Stage 5) Lower Cretaceous (Albian) to the Recent, water depths along the margins continued to deepen and progradation episodes resulted in the accumulation of several kilometers of sediments. Extensional tectonics and the weight of prograding clastic wedges initiated a period of widespread salt tectonics.
“It is the salt movement during this post-rift phase that has controlled much of the structures and petroleum plays along the eastern Brazilian margin,” says Dr. Webster Mohriak.
“Several magmatic events occurred in the southeastern margin basins during the Late Cretaceous to Early Tertiary, particularly in the Cabo Frio region, which separates the Santos and the Campos basins, and in the Abrolhos region, north of the Espírito Santo basin. These three basins have been the main focus of petroleum exploration in the South Atlantic because of the successful interplay between tectonics and sedimentation that occurred during and after rifting. Excellent source rock and adequate maturation, presence of several reservoir intervals in the stratigraphic column (both above and below the salt layer), and efficient oil migration and trapping have made these basins and their counterparts across the Atlantic into very productive petroleum systems.”
“It is the rifting activity and consequent lacustrine formations that resulted in the deposition and preservation of organic matter during anoxic events. The subsequent “capping” by the salt formation created the huge potential for oil development that we are now discovering in the ultra-deep water provinces along the Brazilian Atlantic margin. The magnitude of potential in these structures has recently been proved by pre-salt supergiant discoveries, and the promise of analogous discoveries in other areas in the South Atlantic is certain to be tremendous,” concludes Dr. Webster Mohriak
