August 26. Castile Formation

The last stage of Permian Time is called the Ochoan Epoch. At that time, as the Delaware Basin portion of the Permian Basin filled with sediments, the fringing reefs grew higher and higher – but ultimately the basin was cut off from open ocean circulation. This may have been caused in part by the rising reefs, blocking channels, but there must have been some sea-level fall as well, probably due to an advance of glaciers in the southern continent. When the basin was essentially completely landlocked, like the modern Caspian Sea, concentrations of dissolved salts increased, finally reaching the point where they precipitated out. The Castile Formation represents the period when these evaporites formed.  

I just said “essentially completely landlocked,” but there must have been ways for sea water to episodically enter the basin, since it had to come in in order to evaporate. So alternating influx of sea water with periods of arid evaporation are the more likely scenarios, rather than simply a big lake that evaporated down to nothing.

The beginning of evaporite deposition coincided closely with the end of reef growth, but we aren’t really sure if one development caused the other – did the salty conditions kill the reef?  Did the reef constrain the basin so evaporites could form? Or if the two events are more or less coincidental. But it was a dramatic change in the environment no matter what the cause.

Thin alternating bands in Castile Formation. US quarter for scale. Photo by Richard Gibson.

Much of the Castile is thin alternating couplets of anhydrite, calcium sulfate, and calcite, calcium carbonate. Anhydrite, whose name means “without water” is chemically the same as gypsum, but gypsum’s crystal structure has two water molecules bonded to the calcium sulfate. The alternating anhydrite-calcite layers are typically only one to two millimeters thick, and they are pretty evident because the anhydrite layers are white and the calcite layers include enough organic matter to make them darker. They extend vertically through the Castile Formation, which has a maximum thickness of 2,000 feet (600 meters), and individual laminations can be correlated as far as 70 miles laterally.

Believe it or not, the tiny laminations have been counted – and there are at least 260,000 anhydrite-calcite cycles. They are thought to be annual cycles, with the anhydrite deposited during hotter, dry summer seasons, and the calcite with organic material representing more humid annual periods. They could represent annual freshening of the water and associated algal blooms (see Peter Scholle's online article).

While the Castile Formation is mostly within the Delaware Basin, the next formation up, the Salado Formation, extends beyond the marginal reef and far onto the shallow shelf. It’s got lots of evaporites in it too, and the Salado is mined in places for potash for fertilizer. The potash occurs as the mineral sylvite – potassium chloride, the potassium variety of sodium chloride, which is common salt.

The last of the Permian deposits in this area, above the Salado, are pretty much terrestrial deposits of river-borne red sands and silts.

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Today’s birthday is Laurence Louis Sloss, born August 26, 1913. Larry Sloss was a professor at Northwestern University, and was a pioneer in the field of sequence stratigraphy, recognizing packages of sediments from small to large and their implications for earth history. He was co-author, with William Krumbein, of one of the most-used college textbooks, Stratigraphy and Sedimentation, used from 1951 when it was published, into the 1970s.

—Richard I. Gibson

Castile Formation

Photo by Richard Gibson

August 25. Carlsbad Caverns

The Capitan Reef that we discussed yesterday is not all the prominent, high-standing Guadalupe Mountains. Parts of it are in the subsurface, and like any limestone, given the proper conditions of water and climate, limestone can be dissolved by water percolating and flowing through the rock. That’s the process that makes caves.

In addition to the limestones in the reef, as the Permian Basin became more and more restricted toward the end of the Permian Period, sea water evaporated and salts precipitated out. Halite, common table salt, was one common precipitate, as well as gypsum, calcium sulfate. The petroleum that formed from the source rocks in the forereef are also contained sulfur. When sulfur reacts with water, sulfuric acid forms. We’re not talking about huge hissing pools of acid – just enough to make the groundwater a bit on the acidic side. Enough to actively dissolve some of the limestone. This makes the caves in this area, in particular Carlsbad Caverns, different from most caves, which are dissolved by the weak carbonic acid that forms when rainwater reacts with carbon dioxide in the atmosphere.

The dissolution of limestone to produce Carlsbad Caverns took place a few million years ago – most estimates say 4 to 6 million years, but the process could have begun a few million years before then. That was a time when New Mexico and West Texas were much more humid and rainfall was more plentiful than today. When rainwater mixed with the sulfuric groundwater, it provided a great agent for dissolving the rock.

Carlsbad Caverns. Photo by Eric Guinther licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Eventually, probably within the past one million years or so, the country became more arid, and the water table began to fall, leaving the upper caves dry. Parts of the caves collapsed, allowing for surface water to get in – there’s still water around, just not as much, and the caves themselves are mostly empty of water today. That water, relatively modern surface and groundwater resulting from glacial climates, modified the cave by dissolving some limestone and redepositing it in the kinds of features we associate with caves today – stalagmites, stalactites, draperies, flowstones, and much more. Because of the presence of sulfur, many of the cave features in Carlsbad Caverns are composed of gypsum, calcium sulfate, as well as the more common calcite, calcium carbonate, the same as the limestone rock that was dissolved.

Much of the modern cave activity – the formation of stalactites and such – has more or less ended at Carlsbad Caverns today, because the climate today is so arid. But many of the formations were probably pretty actively growing as recently as 12,000 years ago, when the last ice age ended and the climate changed.

Carlsbad contains some of the largest caverns known on earth, including one that’s nearly 4,000 feet long and 225 feet high. And Carlsbad Caverns is just one of many caves in the Permian reef of southeastern New Mexico. Nearby Lechugilla Cave, explored in 1986, is the fifth longest cave in the world, with more than 136 miles of mapped passages and a total depth of more than 1600 feet. The gypsum cave formations in Lechugilla are even more spectacular than those of Carlsbad.

—Richard I. Gibson

Photo by Eric Guinther licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.   

August 23. Permian Basin, West Texas

First, just a moment to thank you for listening to the podcast or reading the blog. I appreciate your interest and support very much. As short as these things are, it takes quite a bit of work to put them out on a daily basis, so I’m grateful to have you as an interested audience.   

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We’ll be spending the next four days in West Texas and New Mexico, where the thickest sections of Permian rocks in the world are found. They occupy a complex basin called, appropriately enough, the Permian Basin. It extends over a broad area, but the human history is focused on oil exploration, which was and is centered in Midland and Odessa, Texas. One of Odessa’s high schools is Permian High.

For much of Paleozoic time, until the Pennsylvanian Period, West Texas and New Mexico were part of the broad realm of shallow seas that covered much of western and central North America. Lots of limestones and occasional sands and shales were laid down. As we discussed last month, with the beginning of the collision of the South American corner of Gondwana, things began to get more complicated. The Marathon Fold Belt developed. In addition to the Ouachita-Marathon Mountains, some areas were also subsiding, creating large troughs for deposition. And some old lines of weakness were broken again when the Ancestral Rockies were uplifted to the northwest, in Colorado, but there may have been some impacts in West Texas too, somewhat segmenting the developing basin.

By Permian time the basin was both segmented and restricted on its margins – it was essentially two wide, relatively deep oceanic bays called the Delaware and Midland Basins. They were partially isolated from the open ocean by the rising Marathon Mountains to the south and separated from each other by a high-standing fault-bounded uplift called the Central Basin Platform, a feature with an ancient heritage that was reactivated by the compression during the Pennsylvanian continental collision. To the north and northeast of the basins was a broad shallow shelf.

The boundary between the deep basins and the shallow shelf was fairly sharp, and along parts of it, especially in the southeastern corner of what is now New Mexico and adjacent parts of Texas, a huge reef developed. We’ll talk more about that tomorrow.

The multiple phases of collision with Gondwana, to the south of the Permian Basin, resulted in at least three significant pulses of sediment distribution and subsidence in the basin. This subsidence was partly the result of the mass of sediment dumped into the basin, and partly the tectonic uplift of the margins of the basin as well as the Central Basin Platform. Taken together, they allowed for deposition of a pile of Permian rocks more than 12,000 feet thick in places.

The basin pretty much began to fill up, and parts of it became isolated from the circulation patterns of the ocean and bays, so that evaporation on a large scale took place. And we'll talk about the results of that evaporation in a few days. 

The diversity of environments in these basins also provided for the right circumstances for the preservation of organic matter that make the Permian Basin one of the most prolific producers of oil and natural gas in the world. Oil was first produced in the Permian Basin in 1921, and since then something like 30 billion barrels of oil and 80 trillion cubic feet of natural gas have been produced – and it’s not over. There has been a recent surge in production in the basin, taking total basin oil production from about 850,000 barrels per day in 2007 to about 1,400,000 barrels per day in 2014. That’s about 7% of the total U.S. oil consumption of about 20,000,000 barrels per day, and 17% of the U.S. crude oil production total, which was 8,400,000 barrels per day in May 2014.

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Georges Cuvier was born August 23, 1769, at Montbéliard, France. As a pioneering geologist, he established many of the tenets of stratigraphy, but he is probably best known for studies of comparative anatomy that laid the groundwork for the field of vertebrate paleontology. He was among the first scientists to suggest that reptiles had once dominated the earth, and he also brought the concept of extinction into the realm of scientific acceptance.

—Richard I. Gibson

References and Links
Permian Basin tectonics 

Tectonics of Central Basin Platform

Colored map from US Department of Energy (public domain)

Permian Basin province

Surge in oil production (2014)