October 5. Karoo volcanics

When we talked about the Karoo Supergroup of rocks in South Africa on September 28, I indicated that parts of it extended up into the Jurassic. The Jurassic part of the Karoo rocks includes the Drakensberg Volcanics, lava flows from about 180 million years ago that originally may have covered as much as 300,000 square miles. The flows extended at least into South America and Antarctica and beyond, all of which were still attached to South Africa at this time. 

Map of Karro-Ferrar and related volcanics from: The links between large igneous provinces, and continental break-up: evidence reviewed from Antarctica, by Bryan C Storey, Alan P M Vaughan, and Teal R Riley

The origin of the Drakensberg Volcanics is similar to the lavas and sills in the Newark Group and the Palisades, which we talked about yesterday. Eastern Gondwana, Antarctica, India, and Australia, was beginning to rift away from the western part of Gondwana, Africa and South America combined. The initial pulse of this rifting included cracks in the earth that resulted in magma reaching the surface.

The Drakensberg Escarpment is a huge, long mountain front, the erosional edge of the South African plateau. The resistant cap of the escarpment contains various rocks along its 1,000-kilometer length, including hard sandstones and other rocks of the Karoo Supergroup, including the Drakensberg Volcanics in Lesotho and eastern Cape Province. 

In places the Drakensburg lavas are more than 1,600 meters thick – just about a mile of uninterrupted lava. The outpourings appear to have lasted for a couple million years. Based on what we’ve said about massive volcanic eruptions, you should be wondering, “where’s the associated extinction?” Well, there is a likely contender for that extinction. It’s called the Toarcian event, for the Toarcian age of the early Jurassic when it occurred. It wasn’t really huge, although it was global in extent. It seems to have mostly affected marine life, especially ammonites.

The volcanics extend to both the Atlantic and Indian Ocean sides of South Africa, suggesting that the initial rifting that would ultimately form both the Indian Ocean and the South Atlantic was beginning. But this wasn’t a smooth, continuous event, and the “action” on the South Atlantic side especially wouldn’t really take off for a long time yet. It’s suspected that a mantle plume, an upwelling of heat from deep in the earth, may have contributed to the initiation of these rifts that were about to help dismember Pangaea, but as you may recall from previous episodes, the ultimate causes of continental break-up are still debated.

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M. King Hubbert was born October 5, 1903, in San Saba, Texas. He lad a long geological career in academia and with Shell Oil Company. He’s probably most remembered for creating the Hubbert Peak Theory, which says that for any geographic area, from a single field to the whole planet, the oil production rate tends to follow a bell-shaped curve. This is one of the fundamental concepts in the idea of Peak Oil, that we are near or at the peak of that bell-shaped curve, which would mean that half the oil that can be produced has been produced, and half remains to be produced. Hubbert predicted in 1956 that the peak of oil production in the United States would occur about 1970. He was correct, much to almost everyone’s surprise. The theory cannot accommodate things like major changes in technology, both in exploration, such as horizontal drilling, and in things like refining processes.

—Richard I. Gibson

Map of Karro-Ferrar and related volcanics from: The links between large igneous provinces, and continental break-up: evidence reviewed from Antarctica, by Bryan C Storey, Alan P M Vaughan, and Teal R Riley

Karoo-Ferrar Igneous Province

September 28. Karoo Supergroup

Map of the Karoo Supergroup in South Africa by
Oggmus, used under Creative Commons license.
Most of the Beaufort and Stormberg Groups are Triassic in age. 

The Karoo Supergroup is an extensive package of rocks in South Africa whose age extends from the Carboniferous into the Jurassic. As you probably recall, South Africa during most of that time was well inside the supercontinent of Gondwana and later Pangaea, distant from the sea, so it is no surprise that most of the rocks of the Karoo are non-marine, deposited in river systems, flood plains, lakes, deserts, and alluvial fans in uplands. The Karoo has some outstanding Triassic fossils.

Much of the late Triassic Stormberg group was deposited by a vast braided stream system that was home to abundant life by late Triassic time. Cycads and other gymnosperms including confers created diverse woodland habitats, from riparian along the rivers to marshes and meadows. The Molteno Formation in the Stormberg group is the primary coal producer in the eastern part of South Africa’s Cape Province – so we had clearly left the early Triassic coal gap behind.  

The coal-bearing sands of the Molteno Formation are overlain by younger red mudstones of the Eliot Formation, which spans the time from very late Triassic into the very early Jurassic, about 210 to 190 million years ago, with the Triassic-Jurassic boundary at 199 million years ago. The Eliot strata contain fossils of more than 40 species of dinosaurs, ranging from theropods like coelophysis which we mentioned a few days ago, to the multi-ton bipedal herbivore, plateosaurus.

The Stormberg group of the Karoo Supergroup also contains fossils of some of those almost-mammals that we discussed the other day, along with a remarkable array of fossil insects.

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Triassic Lystrosaurus  drawing by Dmitry Bogdanov,
used under Creative Commons license

Edwin Harris Colbert was born September 28, 1905, in Clarinda, Iowa. He was a vertebrate paleontologist who discovered and described the coelophysis dinosaurs in New Mexico, and his discovery of Lystrosaurus in Antarctica in 1969 contributed to the acceptance of the theory of continental drift. The dicynodont Lystrosaurus, a mammalian ancestor, was the most common terrestrial vertebrate of the early Triassic. It’s abundant in the early Triassic part of the Karoo sediments.

—Richard I. Gibson

Image sources:
Map of the Karoo Supergroup in South Africa by Oggmus, used under Creative Commons license. Most of the Beaufort and Stormberg Groups are Triassic in age. 

Triassic Lystrosaurus  drawing by Dmitry Bogdanov, used under Creative Commons license.