Millions of scientific words have been written about the Gulf of Mexico, mostly for one big reason: it’s a humongous oil and gas province. As much as it’s been studied, you might think its story would be completely understood by now, but that’s not true – even now, competing theories about its formation and the details of its geologic history are still being worked out. Many of the different views of the Gulf of Mexico depend on what you look at for data – my own work there has focused on using maps of the gravity and magnetic fields to infer structural features, and to use those in turn to come to conclusions about the basin’s history. So take this episode as a very general overview, and only focusing on the Jurassic part of the story.
In broad terms, what happened here is pretty clear. Gondwana collided with North America. In what is now the Gulf Coast area, it was the northwestern prong of Gondwana, largely northwestern South America today, plus some other pieces, which collided, pushing up the Ouachita Mountains in what are now Oklahoma and Arkansas, as well as the Marathon Mountains in West Texas.
Then, by late Triassic time, Gondwana began to break away from North America, generally but not exactly along the same line as the original suture formed by the collision. We talked about this pull-apart extensively last month and again this month – the breaks formed the Triassic grabens of eastern North America, and where the ultimate rifting took place to generate the Atlantic Ocean, it left parts of Gondwana attached to North America. Florida and nearby areas were the most important such piece left back on the Atlantic Seaboard. But today, our focus will be on Yucatan.
Yucatan was part of Gondwana, the leading edge pretty much, when the collision with North America happened. When the break-up got going strong by Jurassic time, Yucatan seems to have become a relatively independent microcontinental block – breaking away from both its parent, northwestern South America, and from the continent it had become attached to, North America.
In detail, the way Yucatan broke away from what is now south-central United States was somewhat different from the formation of the North Atlantic Ocean. It seems that a broad sag, a lowland developed first, and by late Middle Jurassic time it was low enough to allow sea water to enter that low basin. In the eastern United States, we had all those fault-bounded basins forming, but ultimately one relatively sharp, distinctive rift began that became the Atlantic, like a break in a piece of peanut brittle. Yucatan’s pull-apart seems to have stretched the crust, more like caramel or taffy. There are lots of ideas for why the crust behaved differently in the Gulf of Mexico region. It might have been thinner, or hotter, or its base might have been higher. That’s one of the aspects of the formation of the Gulf that isn’t totally figured out.
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| Image courtesy of Gulf of Mexico 2002, NOAA/OER |
That’s what happened in the Callovian age of the Jurassic in the Gulf of Mexico region, about 165 million years ago. Thick deposits of salt formed. It’s called the Louann Salt, and it underlies not only much of the Gulf itself but also much of the coastal plain from Texas to Florida. There is a second, similar salt basin on the Yucatan side of the Gulf, which probably represents the southern margin of a single salt basin that was rifted apart as the Gulf became a small oceanic basin, with oceanic crust, but it’s possible that the two basins formed independently along the two sides of the axial spreading center.
The Louann Salt accumulated to thicknesses of around 5,000 feet before the late Jurassic when the basin had pulled apart enough that it was interconnected to the open ocean, and circulation prevented extreme evaporation and salt deposition. The two sides of the developing Gulf of Mexico basin were still receiving sediments from the adjacent land masses – sediments that added up to tens of thousands of feet during the Cretaceous and up to the present. Think of all the sediment the Mississippi River is dumping into the Gulf near New Orleans, then multiply that by tens of millions of years. That’s a lot of sediment that was burying the Louann Salt.
When you bury salt, it gets warmer and it’s under considerable pressure – enough so that salt can flow as a plastic solid, sort of like silly putty. The pressure over time squeezed the salt into huge, domal uplifts, which rose as cylinders of salt as much as two miles or more high. Some of them are a few miles across. As they rose, they punched through surrounding rocks, bowing them upward and creating arches over the salt columns, making salt domes. In some places, there’s a surface expression of a salt dome, but in many, there’s no evidence on the surface at all. One that did make a low, circular hill called Spindletop sits along the Gulf Coast south of Beaumont, Texas. There, in 1901, the first oil associated with a salt dome was discovered. The blowout yielded more than 100,000 barrels per day for many days – far more than any other oil field to that time. For comparison, in the US today the average is 10 barrels per day per well, and in Saudi Arabia today, 6,000 barrels a day is excellent. The discovery of Spindletop touched off the Texas oil boom, and its timing – just as the automobile was about to create a huge demand for gasoline – helped intertwine the oil and auto industries in a linkage that exists to this day.
That’s all I’m going to say today about the Gulf of Mexico in the Jurassic, but I do have a lot of links below to further information if you are interested.
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Everette DeGolyer was born October 9, 1886, in Greensburg, Kansas. He’s known as the father of exploration geophysics because of his work in the 1920s and later using gravity and seismic techniques to explore for oil and gas. His oil company is credited with the first oil discovery using geophysics, at Nash, Texas, in 1924, where a salt dome was delineated using gravity measurements. This is actually fairly simple to do because salt domes, huge vertical cylinders of salt, are significantly lower in density than most other sedimentary rocks, so the gravity map of a salt dome is a circular bull’s-eye, a sharp gravity low. The company he founded, Geophysical Research Corporation, ultimately gave rise to Geophysical Service Incorporated and Texas Instruments. He also founded Core Laboratories, another major oilfield service company. He was the Director of the American Petroleum Institute for 20 years.
—Richard I. Gibson
LINKS:
Gulf of Mexico Tectonics
Jurassic evolution of Gulf of Mexico salt basin
Jurassic back-arc basin
Gulf of Mexico and Louann Salt
Salt domes
Pillars of Salt
Louann Salt (thesis)
Image courtesy of Gulf of Mexico 2002, NOAA/OER
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