August 27. North Caspian Basin

The Caspian Sea today is a salt lake that lies below sea level in Russia, Kazakhstan, and other former Soviet republics. Geologically, it has three sections that are incredibly different from each other. The small South Caspian area is a remnant bit of oceanic crust, trapped in the collision between some small continental blocks rifted off Gondwana and the southern margin of Eurasia. The middle Caspian is more or less a fold belt related to the same collision. And the North Caspian Basin seems like a gigantic hole where the bottom dropped out.  

Several linear rift systems that break the Eurasian continent intersect at the North Caspian Basin. It’s pretty close to circular, with those rifts branching away from it like the points of a star, away from a central circle. The deepest part of the basin is around 20 kilometers down – exceptionally deep for a sedimentary basin, and it is likely that the intersecting rifts broke the crust enough that there was actually some oceanic crust formed in the floor of the North Caspian Basin. Why it’s circular is problematic, since most intersecting rifts tend to create angular bends in otherwise more or less linear pull-apart basins. Some have speculated that the circular geometry of the North Caspian represents a giant impact feature, but other than its circularity, there’s no evidence for that at all. Most likely it is just an unusual variation in the general rift scheme. Possibly it began as a normal, elongate rift, and the intersection of other rifts later shaped it into the generally circular form it has today.

Sediments began to fill the basin probably at least by Ordovician time, continuing into the Devonian and Carboniferous. During the Devonian, the basin was much like the Permian Basin we spent the past four days with – a deep central basin, with shallow margins where reefs and atolls developed. But these were of Devonian age, rather than the Permian when the Capitan Reef formed in New Mexico and Texas.

By the late part of Early Permian time, called the Kungurian stage, collisions, including those that assembled the Kazakstan Continent to Baltica or Europe, as well as uniting it with the Siberian craton, were lifting up regions around this deep basin so that it became isolated from the main Tethys Ocean. And like the Delaware Basin in West Texas, it became a restricted salty sea. Unlike the Delaware Basin, most of the evaporites deposited in the North Caspian were salt, the mineral halite, sodium chloride. And it was deposited in thick, massive beds rather than the tiny thin annual layers of West Texas. The Kungurian salt in the Caspian Basin is 4 to 5 kilometers thick. That’s a lot of salt.

Once the salt began to be buried by later sediments, it began to mobilize. Salt is considerably lower in density than most other sedimentary rocks, so the pressure squeeze and heat of burial also helped, so that the salt rose in gigantic cylinders called salt domes – some as much as 10 kilometers high, but all down deep within the subsurface.

The Permian salt lay over the Devonian reef complexes, forming a perfect seal to prevent any hydrocarbons from migrating to the surface and escaping. Those Devonian reefs contain some of the richest oil fields on earth. Tengiz Field, in Kazakhstan on the eastern flank of the North Caspian Basin, had an estimated 26 billion barrels of oil in place when it was discovered in 1979, making it the 6th largest oil field in the world. That’s double the oil in Prudhoe Bay, North America’s largest oil field. In 2012, plans were in place to bring production at Tengiz to 500,000 barrels a day.

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We have two geological birthdays today, and one anniversary. Robert Schrock was born August 27, 1904, in Wawpecong, Indiana. He was on the faculty at MIT for 38 years, and was noted for categorizing index fossils as well as for contributing to the development of standard classifications of sedimentary rocks. Tanya Atwater was born in Los Angeles August 27, 1942. Among her significant contributions to the growing field of plate tectonics in the early 1970s were studies of the transform plate boundary represented by the San Andreas Fault Zone in California, and its implications for tectonic history.

On August 27, 1883, Krakatau, a small island in the Sunda Strait between Java and Sumatra in Indonesia, exploded cataclysmically. The ensuing tsunami killed at least 36,000 people, and the sound of the explosion was heard 3,000 miles away. The eruption was related to the subduction of the Indian Ocean plate beneath the southern extension of Eurasia. All of western and southern Indonesia is part of the volcanic arc produced by this interaction. There’s a vast literature about Krakatau, but the book I recommend both for its readability and scientific accuracy is Simon Winchester’s Krakatoa: The day the world exploded, published in 2003 by HarperCollins.

—Richard I. Gibson

North Caspian Basin oil 1) http://pubs.usgs.gov/bul/2201/B/  2) http://pubs.usgs.gov/fs/2010/3094/pdf/FS10-3094.pdf