November 15. Shark’s teeth

You probably recall that sharks have been around since the Silurian, about 420 million years ago, or possibly even longer ago. They became abundant in the Devonian and expanded in diversity during the Carboniferous. It’s not true that sharks have remained unchanged in all that time, but the basic plan and structure of sharks has been generally about the same. The second major radiation of shark varieties, including stingrays and their kin, began during the Jurassic and continued in the Cretaceous, and most of the modern families of sharks date to the Cretaceous.

Photo of Cretaceous shark’s tooth from Morocco by
Parent Géry, used under Creative Commons license.  

Modern sharks and rays are in the group called neoselachians, which date to Permian time. They expanded in diversity during the late Mesozoic, possibly because of a contemporary expansion in small bony fishes which must have constituted much of the prey of Cretaceous sharks.  

Marine reptiles such as ichthyosaurs and plesiosaurs undoubtedly competed with sharks for the role of top predator in the Jurassic and Cretaceous oceans, but the marine reptiles went extinct at the end of the Cretaceous, while sharks survived. 

Because sharks have cartilaginous skeletons rather than the hard calcium phosphate of bones, body fossils are quite rare. Shark’s teeth are by far the most numerous fossils representing sharks, because they are harder and more resistant – the calcium phosphate mineral, apatite, like human bones and teeth. Some individual sharks grew hundreds or even thousands or tens of thousands of replaceable teeth over their lifetimes, so they are abundant in the fossil record in many places. In the United States, the deposits of the Cretaceous Interior Seaway in the west and Great Plains area, as well as the Cretaceous rocks of the Atlantic and Gulf Coastal Plains, contain many sharks’ teeth. A nearly complete, articulated specimen of a shark has been found in the Cretaceous of Kansas.   

—Richard I. Gibson

Shark evolution 

Rise of modern sharks 

Hammerheads at the optometrist 

Photo of Cretaceous shark’s tooth from Morocco by Parent Géry, used under Creative Commons license.  

November 14. The Atlantic and Gulf Coastal Plains

Today, November 14, is the day the Precambrian would have ended if this calendar were at a proper time scale. And everything we have covered since February 1 would have to be squeezed into the next 6 weeks. That’s why I chose to organize the original book and these podcasts as I have, so I could give more time to each of the more recent periods. But geologic time, deep time, is really hard to grasp. I hope this helps a bit with that.

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Geologic map of the US (from USGS) showing the Atlantic and Gulf Coastal Plains south and east of the heavy black line. The Cretaceous rocks that crop out in the most inland part of the plains (green colors) underlie the Cenozoic (Tertiary) rocks in orange and yellow, and the Cretaceous rocks continue in the subsurface far offshore as well.

Our Cretaceous topic today is the Atlantic and Gulf Coastal Plans of the United States. These relatively flat lands are underlain at depth by all sorts of complicated geology, reflecting the various orogenies that were parts of the assembly of Pangaea, as well as the break-up to produce the present continents. But the rocks closer to the surface of these plains have been virtually undisturbed since they were laid down, beginning in the Cretaceous.

The sedimentary rocks of the coastal plains are from times when sea level was higher. In Late Cretaceous time, the shoreline was located well inland from where it is today, crossing through east-central North Carolina, central South Carolina and Georgia, and central to northwestern Alabama. Almost all of what is now the state of Mississippi was under water, and there was a huge embayment that extended up what is now the Mississippi River valley all the way to southern Illinois. Southeast Arkansas, all of Louisiana, and a wide swath of eastern Texas were also under the Cretaceous sea. The positions of the shoreline in the Cretaceous, and later into the Cenozoic era, shifted many times. In our earlier discussions, that kind of transgression and regression of the sea was often attributed to glacial periods, alternately locking up water into ice sheets and melting. There’s no evidence for Cretaceous glaciation, so we have to call on some other rationale for the changing sea levels. Probably the most popular possible cause relates to changes in ocean water level because of increases and decreases in tectonic activity. More high-standing mid-ocean ridges would displace more water, raising sea level. But once established, the mid-ocean ridges were more or less still there over time, so that explanation doesn’t work too well.

Sedimentation on the coastal plains and the adjacent continental shelves of eastern and southern United States has been more or less continuous for 100,000,000 years or more, making for a huge pile of sediment – as much as 30,000 feet in places. The sediments are mostly clastics, sand, silt, mud, and gravel, washed off the continent into the adjacent sea. This is continuing to this day, with the Mississippi River the leading source of sediment into the Gulf of Mexico.

Throughout the US East Coast, a topographic break marks the boundary between the coastal plain sediments and the complex igneous and metamorphic rocks of the Appalachian Orogenic belt. It’s called the fall line, because the escarpment the boundary forms results in waterfalls in many places. The Great Falls of the Potomac River is one such waterfall. During colonial times, the fall line was typically the head of navigation for the coastal rivers, so cities like Richmond, Virginia, and Columbia, South Carolina, sprang up along it.

All that material washing into the Gulf of Mexico, especially the porous sand, created a great many reservoirs for oil and natural gas. Both the Cretaceous and Tertiary sediments in the Gulf of Mexico hold many oil and gas fields. Although the general setting was similar on the east coast, there are no oil and gas fields there – probably because of differences in organic content of the source rocks, or in the thermal maturation of the sediments, but the reason for the paucity of oil on the east coast is not completely clear.

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Charles Lyell was born November 14, 1797, in Kinnordy, north of Dundee, Scotland. He is best known as the author of the Principles of Geology, first published in the early 1830s. The multiple volumes of the Principles were among the most influential books in the early development of the geological sciences. It was published continuously, in 12 revised editions, until 1872. Lyell effectively popularized the geological concepts of James Hutton, including uniformitarianism.

—Richard I. Gibson

Atlantic coastal plain

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.

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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)