October 10. Sierra Nevada Batholith

If you visualize North America and Africa pulling apart along the new-born Atlantic Ocean, it’s obvious that in relative terms, Africa is moving east or southeast and North America is moving west or northwest. If there is anything out there beyond the continents, it could get caught in a collision with those two big masses. 

That’s what happened in western North America during the Jurassic. We’d been having collisions out there earlier, as long ago as the Devonian, when the Antler Orogeny happened, and last month, on September 15, we talked about the Sonoma Orogeny that added some terranes to what is now northwestern Nevada. During the Jurassic, things got serious. 

This scenario has pretty much been going on since the Jurassic. The east coast of North America is still moving west, to make way for the new oceanic crust produced at the Mid-Atlantic Ridge, and the Atlantic Ocean is still getting wider. In the west, one thing after another has been colliding.

Subduction (from USGS)

By mid-Jurassic time, something like 175 million years ago, active subduction was taking place in what is now eastern California. It had probably begun by late Triassic time, about the same time as extension in eastern North America began to produce the Newark Grabens, and it continued through the Jurassic and into the Cretaceous. The area must have been much like the modern Andes, with a large slab of oceanic crust plunging beneath the western leading edge of North America as it plowed westward. The standard picture of subduction is dense oceanic crust sinking beneath more buoyant continental crust, reaching depths where temperatures are hot enough for some partial melting. But the process is driven by water and other volatiles that are driven off and migrate upward, reducing the melting temperature of the rocks enough to begin melting, which then rise like blobs in a lava lamp toward the surface. If they reach the surface, they can erupt in volcanoes. If they solidify deep inside the earth, in the roots of the volcanoes, they can form huge bodies of igneous rocks. If the molten material derived from mostly uncontaminated oceanic crust, the material will be basalt, such as erupts in Hawaii and Iceland. If the uprising material goes through continental crust, it will melt more silica-rich materials and will likely end up with a granitic composition.

Mesozoic Batholiths in red

These bodies of granite are called batholiths, a word that means “deep rock” for the fact that they crystallize at great depth. The blobs of granitic magma in eastern California rose for millions of years, well into the Cretaceous Period. They ultimately formed the Sierra Nevada Batholith, a complex of granitic bodies 400 miles long and 100 miles wide. If there were volcanoes above these deep-seated granites, most of the rock record of them has been eroded away. The granite in the Sierra Nevada is now exposed at the surface because of much later uplift and erosion – miles of rock had to be removed to get down to where the granite was. So the modern mountain range is the result of very different kinds of tectonic forces than the ancient, probably volcanic chain that was there during Jurassic and Cretaceous time.

The white granite rocks of Yosemite National Park, including Half Dome, as well as Mount Whitney and most of the entire length of the Sierra Nevada are parts of the Jurassic Sierra Nevada Batholith.

The subducting oceanic plate that gave rise to the Sierra Nevada Batholith is called the Farallon Plate. Its early interaction with North America, during the Jurassic, was probably relatively straightforward, that standard view of subduction I discussed earlier. But it got complicated over time, and we’ll talk about some of those complications over the next couple months. Most of the Farallon Plate is gone – subducted into the earth, its materials recycled into the crust and mantle. One large remnant is offshore Washington, Oregon, and far northern California, where it is still subducting and giving rise to the Cascade Volcanoes. It’s probably fair to think of eastern California during the Jurassic as something like the Cascades today, although the extent and volume of the now-exposed granites might suggest that it was more like Cascades on steroids – a huge and long-lived subduction system. 

The mountain-building event associated with this subduction and the intrusion of the Sierra Nevada Batholith is called the Nevadan Orogeny. Although there had been earlier accretions that added small terranes to western North America, this was really the first major pulse of mountain construction in the ongoing creation of the Cordillera, the western mountain ranges of North America. Their development continues to this day, so we’ll certainly talk more about them over the rest of our trip through earth history. And even during the Jurassic, the uplift of mountains in the far west had consequences much further east, which we’ll talk about later this month.

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Today’s geologic birthday is Hugh Miller, born October 10, 1802, in Cromarty, Scotland. He’s probably best known as the author of The Old Red Sandstone, first published in 1841. He is considered to be a pioneer in the paleontology of Scotland and Britain.

—Richard I. Gibson

LINKS:
More about the batholith

And more 

Geologic History of California


Geologic map of California

October 9. Opening the Gulf of Mexico

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.

Image courtesy of Gulf of Mexico 2002, NOAA/OER

When the sea invaded the low-lying basin in Jurassic time, the sea waters were largely restricted from oceanic circulation patterns. You can probably guess what comes next, since we’ve seen it repeatedly, in the Michigan Basin, the Permian Basin of West Texas, the North Caspian Basin, and elsewhere. A sharply restricted marine basin – if climatic conditions are right – results in evaporation that leads to deposition of salt.

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

October 8. Navajo Sandstone and Rainbow Bridge

First, a quick pointer to another blog I recently encountered, called Triassica – its focus is on the Triassic and there is a lot about the Triassic-Jurassic boundary and the poorly understood extinction event at that time. The link is https://triassica.wordpress.com/

Today’s episode takes us to southwestern United States in early Jurassic time.


The arid, desert conditions of the Triassic continued into the Jurassic, at least in some areas. In what is now southwestern United States such conditions resulted in the deposition of wind-blown dunes that solidified into the thick Navajo Sandstone. It forms massive cliffs in many of the parklands of southern Utah, including in Zion, Capitol Reef, and Canyonlands National Parks. It’s thickest around Zion National Park, where the Navajo is more than 2000 feet thick in places.

Checkerboard mesa photo by J.R. Gill, USGS

The Navajo is similar to other packages of desert sands in this region, such as the Wingate Formation that we discussed last month, but the Navajo Sandstone is often not quite so red. When it is reddish, or pink, the color may come from hematite, iron oxide, in the rock itself, or it may be stained by overlying red beds like the Redwall Limestone in the Grand Canyon. In places the iron is concentrated into little deep red or purple concretions similar to the “blueberries” of hematite discovered on Mars by the Opportunity rover. But often enough, the Navajo is white in color.

Although the region must have been pretty much a broad desert sand sea like parts of the modern Sahara Desert, it wasn’t barren of life. Some vertebrate bones and tracks have been found, mostly archosaurs that were probably early examples of the bird line of dinosaurs. It’s likely that seasonal rains, a monsoon, brought precipitation to the desert.

The Navajo sands were laid down mostly in Early Jurassic time, about 175 to 185 million years ago. One of the most spectacular examples of the Navajo is the feature at Zion called Checkerboard Mesa. The cross-bedded sand layers, which are essentially the lithified sloping faces of migrating sand dunes, are cut by vertical cracks, which are called joints. The intersecting pattern of bedding planes and joints gives the outcrop a rectilinear look similar to a checkerboard.

Rainbow Bridge Photographer: en:User:BoNoMoJo,
used under Creative Commons license

Another spectacular structure cut in the Navajo Sandstone is Borohoini – the Paiute word for “the rainbow,” also known as Rainbow Bridge. This natural arch stands 290 feet above the canyon floor, making it one of the largest arches in the world.

Rainbow Bridge, Checkerboard Mesa, and all such features are basically ephemeral in terms of geologic time – they will last only a few tens of thousands to hundreds of thousands of years at most. Erosion in arid country like southern Utah is typically very slow compared to humid, rainy climates – but in arid country, when erosion does happen, it tends to be catastrophic, in flash floods, landslides, and dramatic runoff events. And collapsing arches.

—Richard I. Gibson

Checkerboard mesa photo by J.R. Gill, USGS (public domain)
Rainbow Bridge Photographer: en:User:BoNoMoJo, used under Creative Commons license.