The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Thursday, November 13, 2014

November 13. The South Atlantic opens



Base from NOAA (annotated by Gibson)

It was the obvious good fit of the coastlines of Africa and South America that helped lead Alfred Wegener to his theory of continental drift back in 1915. The South Atlantic has a different history from the North Atlantic, not least in being rather younger than the northern portion.

The geometry of the coasts reflects differences in the way parts of the South Atlantic opened. The near east-west margin of West Africa turns to north-south at the Gulf of Guinea, the corner where the northeastern tip of South America used to be attached. In fact, that corner is probably really a triple junction, a tectonic location where three relatively distinct rifts began, radiating away from the location of the Niger Delta today.

The combined South America-Africa continent, which we’ve referred to previously as West Gondwana, began to separate in several places during the Jurassic. In the north, the irregular boundary between West Gondwana and North America left a piece of West Gondwana attached to North America – Florida. And the zone between West Africa and northern South America began to crack, too. In the south, also during the Jurassic, we talked last month about the separation of East Gondwana, and that also put the beginnings of a rift between southern Africa and southern South America. But the middles of what are now Africa and South America were still attached to each other until the Cretaceous.

At the corner, today’s Gulf of Guinea, two pull-apart rifts started. One ultimately became the rift that makes up most of the South Atlantic, with the north-south margin of Africa from Gabon down to South Africa on one side and Brazil on the other. The other rift was within the African continent, extending northeast from today’s Niger Delta, practically all the way across Africa, through Chad and Sudan to the Red Sea. In places this was a pull-apart zone, especially in Nigeria, where it is called the Benue Rift, but further into Africa it is a tear or shear zone, with the northern and southern parts of Africa moving alongside each other. If this rift had not failed, we would have two continents today instead of one single African continent.

Another shear zone developed in the third branch of the triple junction. This branch ran west from the Niger Delta today, along the east-west trending coast of West Africa, from Nigeria to Ghana to the Cote D’Ivoire to Liberia. On the southern side of this zone, the northern coast of Brazil, from the tip at Cabo San Roque up to the mouth of the Amazon, that section was sliding to the west. So the tectonic boundary between South America and Africa was mostly strike-slip faults, also called transform faults, where the two big blocks slid alongside each other and ultimately parted, leaving the Middle Atlantic Ocean in their wake. The southern Atlantic Ocean was formed in a more straightforward way, with the two sides pulling apart more or less perpendicularly from the spreading center at the Mid-Atlantic Ridge.

But as usual, there was plenty of variety within those overall general parameters. In the southern, relatively simple rifted system, things were complicated by two hotspots that poured basalt and other volcanics into the widening ocean and on the adjacent continents as well. We talked about one of them, the Tristan Hotspot that produced the Parana Basalts and the Rio Grande-Walvis Seamount Chain, the other day. (November 11) The second one was further north, and today it is reflected in active volcanoes like Mt. Cameroon in Africa and St. Helena, the volcanic island near the Mid-Atlantic Ridge. Between them under the ocean, the St. Helena seamount chain represents the movement of the African Plate over the hotspot as the South Atlantic opened. And there is a conjugate submarine ridge on the South American side too.

The big deal about these two volcanic centers, the Tristan and St. Helena hotspots, is that as the South Atlantic was opening, the volcanics erupted from these centers blocked off, segmented, the widening ocean basin. Instead of a long, narrow oceanic seaway, we ended up with long, narrow segments with differing history. In particular, the central section between the hotspots was periodically cut off from the open sea. You won’t be surprised that in that low-lying part of the rift thick evaporites developed as salty marine waters came in and evaporated. Those evaporite beds are hugely important to the economics of Brazil, and across the ocean, to Angola, Congo, and Gabon. Not for the salt, but for the oil that the salt helps trap beneath it.

Early in the formation of the South Atlantic rift, when it was still part of the combined continent of West Gondwana, the region that would ultimately break the continent apart was a low-lying, down-faulted zone. Think of it like the Triassic grabens that we talked about a lot in Eastern North America as the North Atlantic was beginning to form. In West Gondwana, the low-lying, tropical basins held extensive lakes, and the organic rich material that was deposited in those lakes became an excellent oil source rock. As the region pulled apart more and more, the volcanic centers to north and south restricted the basin allowing for the salt deposits to form and serve as the seal to keep the oil from escaping to the surface.

There is some argument about the origin of the salt, and it may be more complicated than simple incursions of marine waters and evaporation; some of the mineralization may have come from hydrothermal sources. The salt was deposited mostly during the Aptian Stage of the Early Cretaceous, about 125 to 115 million years ago. Eventually, about 112 million years ago, the South Atlantic became wide enough that we had an open ocean between the two continents, and the salt basins were split into two – one now offshore Brazil, and the other offshore (and in a few places, onshore) Angola to Gabon. These two areas are among the most prolific oil provinces in the world. Almost all of Brazil’s oil production comes from these sources, more than 2 million barrels a day, from fields that probably contain at least 13 billion barrels of oil. Brazil’s oil also comes from some of the deepest water depths ever drilled, with the sea floor around 7,000 feet below the sea surface. On the opposite side, the African coastal offshore from Nigeria down to Angola produces around 4 million barrels a day.

There’s plenty more to explore about the opening of the South Atlantic, and I have a handful of links and references below if you are interested in more.

* * *

The volcano Nevado del Ruiz erupted in Colombia on November 13, 1985. The eruption melted glaciers on the mountain summit, which is more then 5,300 meters, or 17,400 feet above sea level. The eruption was really quite small, only about 3% of the volume erupted from Mt. St. Helens in 1980, but the glacial melting proved catastrophic. The volcanic debris mixed with the glacial meltwater, loose rock, and surface soil on the mountain flank to produce a pyroclastic flow, also called a lahar, which increased in volume as it came down the slopes. The flow wiped out the town of Armero and several villages, with a total death toll of about 23,000, the fourth-deadliest volcanic eruption in recorded history. Tectonically, Nevado del Ruiz is a typical volcano in the Andes, the result of the subduction of the Pacific Oceanic Plate beneath the South American Continental Plate.
—Richard I. Gibson

Links and References:
A new scheme for opening of the South Atlantic

Aptian evaporites (2012)

Tectonic evolution of South Atlantic (2000)

South Atlantic 

Brazil’s offshore oil 

Evaporites Through Space and Time, edited by B. Charlotte Schreiber, S. Lugli, M. Ba̜bel (The Geological Society, 2007)

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