August 31. The Great Dying

We’ve reached the end of the Permian Period, and the end of the Paleozoic Era, which we’ve been plodding through since the first of February. This ending is marked by the most devastating extinction event in the entire history of the earth. It’s the end of the world as we knew it, and lots of species don’t feel fine.  

Most estimates say that at least 90%, maybe 95%, of all marine species and 70% of all land animals did not survive this mass extinction. Trilobites, fusulinids, blastoids, rugose corals, and eurypterids were gone for good. Every other marine group was decimated – bryozoa, crinoids, ammonites, brachiopods were all hard hit. On land, most of the large amphibians disappeared, and even the large trees that made forests were gone. Even insects were impacted. And most of the early therapsids, the mammal-like animals including the dimetrodons, also disappeared, although some of their descendents survived.

Source: Wikipedia

Before we look at what may have caused the extinction, let’s look at what happened. In the oceans, water became seriously depleted in oxygen. The anoxia alone would have affected many organisms. We know about this because of abundant organic-rich shales, deposited in conditions where oxygen was not around to decompose the debris. Pyrite, iron sulfide, in those shales suggests a lot of sulfur, which in turn implies acidification of ocean waters. Overall, temperatures rose dramatically at the end of the Permian.

So what caused all that? After the “smoking gun” of a catastrophic impact was identified and finally accepted as the event that put an end to the dinosaurs, at the end of the Cretaceous Period, it became popular to look for impact events to relate to other extinctions. There are some at about the end of the Permian, notably the Bedout feature offshore northwestern Australia. Its age is essentially identical to the end-Permian extinction, but it’s not entirely accepted as an impact structure. Also, there’s geophysical evidence in Wilkes Land, Antarctica, that can be interpreted as a really big crater. It got a lot of attention in the popular press a few years ago as the potential cause of the Permian extinction. But it’s under the ice – we have no rocks from it – and its age is pinned down to only 100 million to 500 million years ago – hardly a close match for the Permian extinction. Also, there’s no conclusive geochemical evidence, such as the famous iridium layer associated with the end-Cretaceous impact, to support a huge Permian impact. I really think that for now, impact hypotheses for this event have no weight.

Much more interesting is the close correlation in time with the vast Siberian basalt flows that we talked about yesterday. Such volcanism could put acid aerosols into the atmosphere – consider Laki, the volcano that erupted in Iceland in 1783, whose aerosols put poisonous fogs as far away as France. Volcanoes also introduce ash into the atmosphere, which might create a brief nuclear winter, a quick cool-down, but they also inject carbon dioxide into the atmosphere, which would soon lead to greenhouse conditions and the warming that we observe. Warmer conditions could have led to melting of methane hydrates in the sea floor, and releasing methane would add to the greenhouse effect.

Earlier this year, the popular press was inundated with headlines like “Bug farts nearly killed us” – a reference to an idea that methane-generating organisms, archaea, microorganisms that were once considered to be bacteria, were the culprits. They are now classified at the same level as the kingdoms of plants and animals. The idea is that these critters evolved a new way of growth, just about at the end of the Permian, that led to exponential reproduction and an associated generation of huge amounts of methane, producing the greenhouse conditions and acidification and anoxic waters that we know were present. There are some aspects of carbon isotope ratios that this idea explains better than the concept of volcanism as the cause of the extinction, but the jury is still out on this, I think.

I think that the Siberian basalt flows probably have the strongest support as the cause for the Permian extinction, but even that isn’t really a smoking gun. At least not yet.

As with so much in geology – and this is what keeps it interesting – it’s not completely certain that this was even one “event.” You’ve heard me refer to mountain-building “events” that lasted tens of millions of years. Geologists tend to play pretty loosely with our time scales, even though we recognize the value of accurate age dating. Often, we just don’t have it. So consequently there’s some debate about the nature of the Permian extinction. There is really no doubt that the final blow, at about 251 million years ago, was very much a “spike” in terms of geologic time. But such a spike could represent most of a million years, or as few as 200,000 years, and some scientists suggest that most of the Great Dying took place within a span as short as 20,000 years – and that really would be essentially instantaneously, geologically speaking. The other view would have the final blow coming as the culmination of a 15-million-year period of climate change that was punctuated by other more short-lived episodes. There isn’t really any doubt that there was another specific event near the end of the Middle Permian, at about 260 million years ago – we mentioned that the other day in connection with the last trilobites – and there seems to be a decent correlation with it and a glacial pulse that lowered sea level. At this point we could almost call that a “conventional” cause for extinction. But was it an isolated event or was it, and the big one at the end, all part of some more general set of changes? We don’t really know, but obviously everything from glacial advances to aerosols from volcanoes to ocean circulation patterns caused by Pangaea’s size and shape and position – all those things could be part of the cause for the greatest extinction the world has ever seen.

As you can imagine there is a vast amount of work that has been done on this topic, so please look at this as just the barest summary of possibilities. 

* * *

On a far smaller scale than the Permian extinction, on August 31, 1886, a strong earthquake struck a very unexpected place – Charleston, South Carolina. The magnitude is estimated at 7.3 – huge by any estimation. It was felt as far away as Boston, where the shaking rang church bells. Almost every building in Charleston was severely damaged or destroyed, and at least 60 people were killed. The area had a long history of low-level seismic activity, just as it does today. But why is there any seismicity? It’s far from plate margins and volcanic zones. It’s really challenging to explain these earthquakes that occur far from areas of active fault motion, but I think the current explanation for the Charleston quake is that there was significant faulting in this area when the Atlantic Ocean opened, and that the 1886 quake was a rejuvenation of that faulting related to the ongoing pull-apart of the Atlantic. Right now in our calendar, we’re at the end of the Permian, and South Carolina is still firmly attached to Africa. It didn’t begin to break apart until the Jurassic, which we’ll get to in October.

Tomorrow, the Triassic Period and the Mesozoic Era begin.

—Richard I. Gibson

References:
Graphic from Wikipedia under GFDL 

Palaeos mass extinctions 

Methane-generating bugs

August 30. Siberian basalts

Original extent of Siberian traps (see below for credit)

At just about the end of the Permian Period, one of the most voluminous volcanic eruptions of the past 500 million years occurred, in what is now Siberia. The rocks are largely basalts, relatively iron-rich, dark, fine-grained igneous rocks that erupted from many vents and perhaps some fissures scattered through the area. The eruptions spanned about a million years, 251 to 250 million years ago, and ultimately covered as much as 7 million square kilometers, about 2,700,000 square miles – about the area of the conterminous United States excluding Texas and California. Probably less than a third of the original extent is still preserved; the rest has been eroded away.

Flow after flow was erupted, along with explosive eruption of ash and other volcanic products. The flows are usually called “flood basalts” because they flooded out of the vents and fissures to cover vast areas. And the whole package is often referred to as the Siberian Traps – the word “trap” in geology comes from a Swedish word meaning “step,” because the piles of flows typically create a stair-step landscape.

There have been several huge eruptions of flood basalts through earth history, including 11 of various sizes in the past 250 million years. The Columbia Plateau in Washington and Oregon, the Deccan in India, and the Parana basalts in Brazil, are all huge, but the Siberian traps were the largest volcanic flows in the past half billion years.

What caused it? The ultimate cause of all the flood basalts is not completely certain. Ideas include active rifting, pulling apart of the crust, under special circumstances such as unusually thin crust. A mantle plume, a hot spot like those beneath Yellowstone and Iceland today, could have made it happen, especially if associated with rifting. It’s been suggested that an impact might have blasted the crust to the point that vast volumes of magma would erupt through the crater, but there isn’t much support for that idea.

The age of the eruptions has been pinned down quite accurately. The eruptions do have a moderately wide range, taken together, from about 256 to 246 million years ago, but the age dates really do strongly cluster at 251 to 250 million years ago. This is right at the Permian-Triassic boundary, the end of the Permian, when the greatest mass extinction in the history of life on earth occurred. It’s pretty much inescapable, given the extremely close correlation in time between the vast Siberian eruptions and the mass extinction event, that there was probably a connection between the two. But there’s more to it than a simple one-for-one correlation, and we’ll explore that in more depth tomorrow.

* * *

Today is the birthday of William Stephens Twenhofel, August 30, 1918, in Madison, Wisconsin. His father, William Henry Twenhofel, has been called the patriarch of sedimentary geology: he pioneered the study of sedimentation as a subdivision of geologic science. The son, William Stephens, was a geologist with the U.S. Geological Survey who worked on ore deposits in Alaska, among many other things.

—Richard I. Gibson

Links and references:
Siberian Traps (Cowan)  
Siberian Traps ages 
Siberian Flood Basalts 
Multiple eruptions – not one
Extinction connection

Map by Jo Weber, under Creative Commons license.

August 29. The Last Trilobites

We haven’t talked about trilobites for a while. From their incredible abundance, and even dominance in Cambrian and Ordovician time, trilobites declined in both numbers and diversity. They were still really quite abundant and diverse in Silurian and Devonian time, but the extinction at the end of the Devonian had a pretty significant impact on them. The decline continued, until at the start of the Permian, there were only three families of trilobites in existence. Earlier in the Paleozoic, there were dozens of trilobite families. 

Permian trilobite from Kansas, about 1.5 cm long. See below for image credit.

The Permian trilobite families were divided into about 30 subdivisions at the genus level. They suffered two blows during the Permian – first, in the Middle Permian, about 266 million years ago, when more than half the living genera were wiped out. Combined with earlier losses, that meant that there were only five trilobite genera that survived until the end of the Permian, when they too were destroyed in the mass extinction that eliminated more than 90% of all the species on earth. We’ll talk about that extinction the day after tomorrow, when the Permian ends.

Trilobites survived in one form or another for almost 300 million years, making them one of the most successful animal groups in all of earth history. Their decline and ultimate extinction was probably the result of many factors, including competition from other kinds of organisms, more predatory organisms, climate change, and loss of habitat, the typical reasons any plant or animal goes extinct. There’s some speculation that poor and inconsistent molting, combined with increases in animals that could – and did – eat trilobites while they were in the vulnerable molting state contributed to their demise over a period of millions of years. But ultimately, I don’t think we have a really good handle on the reasons trilobites declined. Their final extinction, at the end of the Permian, is no surprise, since that event caused wholesale destruction. But their slow decline until that final blow probably represents a combination of various factors.

—Richard I. Gibson

The Last Trilobites

Nice drawing showing the expansion and decline of trilobites through the Paleozoic 

Poor molting style

Photo by Dwergenpaartje under Creative Commons license : proetid trilobite Ditomopyge decurtata from Permian of Kansas

August 28. Cimmerian continent

I hope you remember Tethys, the huge triangular-shaped bay in the eastern side of Pangaea after Gondwana had collided with North America and Europe. The Tethys coast of Gondwana extended from the connected North Africa, to Arabia, to India, to Australia. But not quite the modern margins of those continents.  

The Cimmerian Continent as it rifted away from the Gondwana portion of Pangaea in Late Permian time, about 250-260 million years ago. See below for source.

There was a long, linear zone along that Tethyan shore that was attached to Gondwana at the start of Permian time, but by sometime in Late Permian time, the zone began to rift away from Gondwana. Technically the ocean was the Paleo-Tethys, the confusing ocean we talked about August 5, and the rifting we are talking about today created a new ocean, Neo-Tethys or just plain Tethys. Visualize this something like East Africa today, where the East African Rift system is breaking a zone away from the core of Africa, and will open up a new ocean in the process. The ocean has already started, in the Red Sea between Africa and Arabia, which were formerly part and parcel with each other. The ocean will eventually split East Africa away from the main part of Africa, with a narrow continent drifting across the Indian Ocean, and a new, narrow ocean forming between it and Africa.

That’s what was happening in Late Permian time in the northeastern part of Gondwana. The narrow strip that rifted away wasn’t necessarily uniformly long and narrow – and for sure it didn’t stay that way. Individual blocks, much like Madagascar today, drifted more or less in tandem, but not necessarily as a continuous, connected continent. Those blocks ultimately became what we know today as Turkey, Iran, Afghanistan, probably several blocks that make up Tibet, and some bits of Indochina and Malaya. Taken together, these blocks are called the Cimmerian Continent, or Cimmeria.

It’s possible that the continental fragments that comprise the Iberian and Italian Peninsulas today originated in a similar manner to Cimmeria, but they did have a different history and were becoming amalgamated to Europe earlier. It is likely, however, that the Carpathian Mountains in Romania and nearby have a heritage related to a Cimmerian block, as do parts of the Caucasus. The Cimmerians were an ancient people living north of the Black Sea and Caucasus Mountains, areas affected by the Cimmerian Orogeny.

After the Cimmerian blocks finished their drift across the Paleo-Tethys Ocean, they collided with the southern margin of Eurasia, creating mountain uplifts and the Cimmerian Orogeny, and closing the Paleo-Tethys Ocean while opening the Neo-Tethys Ocean. Those collisions would not occur for many millions of years after the rifting began during the Permian, so we’ll talk about them later in the year.

—Richard I. Gibson

Cimmeride Orogenic System

Tethys globe view from PhD thesis of Pierre Dèzes (1999; Institut de Mineralogie et Petrographie, Université de Lausanne) from http://www-sst.unil.ch/research/plate_tecto/alp_tet.htm via Wikimedia commons 

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.

* * *

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

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.

* * *

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 24. Capitan Reef

El Capitan. Photo by Richard Gibson (1980)

That huge reef that developed around the margins of the Delaware Basin portion of the Permian Basin is called the Capitan Reef. It’s named that for outcrops in the Guadalupe Mountains, specifically for El Capitan, a prominent mountain there which is the core of the Permian reef. El Capitan stands about 2,000 feet above the adjacent valley floor today – just about the same elevation that the ancient reef stood above the adjacent deep sea basin. The overall geometry of the reef was a long, narrow ribbon encircling the oval Delaware Basin. There were some breaks in it, but its total length was about 450 miles.

Source: National Park Service

The reef was constructed in the standard way, by the skeletons of tiny calcite-secreting organisms including algae, sponges, bryozoans, and some corals, but not mostly corals. Other life, including brachiopods and fusulinids, contributed their shells to the reef complex as well, so that it became a huge edifice nearly a half-mile high. The top would have been just about at the water line, the warm, aerated shallow zone where life lived, as in the modern Great Barrier Reef of Australia or the reefs that make atolls around tropical islands.

The critters were sustained by an influx of nutrients from all directions. The back reef, the shallow lagoonal area on the shelf, was much like the shallow seas we heard so much about throughout the Paleozoic. Lots of life – crinoids, even more fusulinids, the large single-celled organisms we talked about a few days ago, plus gastropods (snails) and more. The forereef area, the deep ocean in front of the high reef itself, also contained abundant nutrients that would flow up the slope to feed the reef organisms. Organic matter that settled into the deep forereef basin also helped create some of the rich hydrocarbon source rocks that have matured and migrated into the oil and gas fields we exploit in this area today.

Source: Texas Water Development Board

The area was tropical to subtropical, with the Permian equator running approximately from what is now northern California to Newfoundland, making the region similar to the Great Barrier Reef in terms of latitude.

The modern El Capitan and Guadalupe Mountains are exposed and high-standing because of much later tectonics and erosion that exposed the reef core. In arid country like West Texas, carbonates tend to be resistant. They dissolve better in rainy country because of the weak carbonic acid produced by the interaction of rainwater and carbon dioxide in the atmosphere. The shales and sands in the forereef area, a broad plain today, are less resistant and end up eroding away more easily. This is not to say that there was no dissolution of these limestones. There was, in spectacular fashion – and we’ll talk more about that tomorrow. 

* * *

The eruption of Mt. Vesuvius in Italy on this date, August 24, 79 A.D., destroyed the cities of Pompeii and Herculaneum. Most of the estimated death toll of 16,000 were buried under ash falls or pyroclastic flows. Vesuvius has erupted many times, including 17 since 1700. The last major eruption was in 1944. The volcanism in southern Italy and Sicily is ultimately related to subduction caused by the collision of Africa with the Italian Peninsula, but it’s pretty complicated because there are multiple small blocks involved in the collision. Italy itself is pushing like a finger into Europe, raising up the Alps. Oceanic crust beneath the Tyrrhenian Sea north of Sicily and west of southern Italy is probably subducting or being overridden by small continental blocks, resulting in the volcanism.

—Richard I. Gibson

Links and references
Guadalupe Mountains 
Permian Reef 
Capitan Reef  (TX Water Development Board – source of cross-section)

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.   

* * *

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.

* * *

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) 

August 22. New Red Sandstone

If you’ve been with us since May 3, maybe you remember the Old Red Sandstone. Those were the sands laid down in largely terrestrial environments, in basins scattered through the Caledonian Mountain Range that resulted from Baltica – Europe, especially the British Isles and Scandinavia – colliding with North America, specifically Greenland, Labrador, and Newfoundland. The early European geologists recognized two red sandstones, and applying the law of superposition, also recognized that one was very definitely older than the other. Hence Old Red and New Red Sandstones.  

The Old Red became assigned to the Devonian Period as the early and middle Paleozoic strata were sorted out. The New Red was above, and therefore younger than the distinctive coal measures of the Carboniferous. Ultimately the rocks of the New Red Sandstone were recognized as being of Permian and Triassic age. In Britain, much of the package is quartz sandstone cemented by hematite, iron oxide, which gives it the red color. As we’ve indicated several times, oxidized iron usually indicates that the rock was alternately under water and exposed to the atmosphere for the oxidation to happen. This gives us some good clues about the environment, and like the Old Red, the New Red was laid down in largely terrestrial environments, including rivers, deltas, and ephemeral seas and lakes.

East of Britain, beneath the waters of today’s North Sea, and extending into the Netherlands, Germany, and Poland, the later New Red equivalents include the Zechstein salt that we talked about a few days ago. Salt and other evaporites, including gypsum and sulfates, indicate a restricted sea or large lake in a relatively hot, arid environment. Sea level changes would alternately flood the land or recede leaving mud flats and sandy delta plains exposed to the atmosphere so the iron in the sediment could oxidize and the salts could precipitate out. Today this environment is called a sabkha, an Arabic word for salt flat.

The Zechstein formed in Late Permian time, and the older equivalents of Britain’s New Red are called the Rotliegend Group in Germany, the Netherlands, and in the subsurface of the North Sea. Rotliegend means “the underlying red,” for their presence beneath the distinctive Zechstein evaporites. The basin containing the New Red Sandstone and the Rotliegend Group was surrounded by the mountains uplifted when Gondwana collided with central Europe, so the setting was very similar to that of the Old Red Sandstone – just with a somewhat different geography of mountain ranges.

Carboniferous-Rotliegend oil and gas fields (from USGS Bulletin 2211)

There are quite a few oil and gas fields in the North Sea whose reservoirs are in the Permian Rotliegend rocks. The complex changing environments during the deposition of the Zechstein also make for abundant source and reservoir rocks, and the salt can flow and move in ways that help create significant traps for oil and gas. The Rotliegend strata probably spanned a considerable time during the early to middle Permian, perhaps as much as 30 million years. Zechstein deposition was more constrained, maybe only 5 to 7 million years, very near the end of the Permian Period. It’s possible that the incursion of salt water into the Rotliegend basin represents the very first pulse of the break-up of Pangaea. Some reconstructions show a narrow rift beginning between Greenland and Scandinavia by Late Permian Zechstein time, but alternatively, the sea could have come in through a trough extending southeast from Poland to the Tethys Ocean.

* * *

Laurence McKinley Gould was born August 22, 1896, at Lacota, Michigan. He was a geological explorer of the Arctic and Antarctic, working as Richard Byrd’s chief scientist on Byrd’s first trip to the Antarctic. Gould was on the faculty at Carelton College in Minnesota for 30 years.

—Richard I. Gibson

References:

• Permian and Triassic red beds   p.11, 17
• Petroleum Geology of the North Sea, K.W. Glennie, ed. (1998, Blackwell Science)
• Evolution of the Arctic-North Atlantic and the Western Tethys, by Peter Ziegler (1988, AAPG Memoir 43)
• Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary by Donald Gautier, USGS Bulletin 2211

August 21. Therapsids

Therapsids were a group of synapsids, the tetrapods that sort of bridge the gap between ancestral reptiles and ancestral mammals. They evolved from the pelycosaurs, which included dimetrodons and their relatives. Dimetrodons had dominated the early Permian, but by later in the period therapsids were diversifying and finally became the dominant land vertebrates in late Permian time. Their skeletal structures were more mammal-like than reptilian, including the geometry of their legs and feet which were located more directly under the animal. This gave them a somewhat more upright posture than the sprawling, crocodilian stance of the reptiles. Their teeth were also evolving into the canines and incisors more typical of mammals.    

Permian therapsid drawn by Nobu Tamura (http://spinops.blogspot.com) used under GFDL

The therapsids occupied many different ecological niches during the Permian, and included both herbivores and carnivores. They ranged in size from monsters that weighed at least a ton down to tiny critters scuttling through the undergrowth.

One of the key features that separates modern mammals from reptiles is that mammals are warm-blooded. They don’t rely on the sun’s direct rays for warmth, but can maintain and manage their internal temperatures. Cynodonts – a name that means “dog teeth” – appeared in Late Permian time, and managed to survive the extinction at the end of the Permian. It appears that the cynodonts gave rise to true mammals during the Triassic Period. Cynodonts had a lot of mammalian characteristics, and many researchers think they were warm-blooded, so they might have been classified as mammals if we had one to look at. You’ll see them called half-mammal, half-reptile, missing links, and more, but I think the best way to think of the early cynodonts is as pre-mammals. No longer really reptiles, but perhaps lacking some of the traits we use to define mammals, such as hair and milk production. Or maybe they did have those things. I don’t think there are any Permian cynodont fossils with hair preserved, so we aren’t sure – but we’re definitely on the mammalian track.

Most of the known cynodonts from the Permian were small, the size of a rat or a cat.

—Richard I. Gibson

Permian therapsid drawn by Nobu Tamura (http://spinops.blogspot.com) used under GFDL 

August 20. Appalachian folding

Map of fold belt in central Pennsylvania from Pennsylvania Geological Survey

We talked about the Appalachian-Ouachita orogeny a lot last month, as Gondwana collided with what is now eastern North America in one of the main acts of the assembly of Pangaea. Collision was certainly underway during the Pennsylvanian Period, last month, but it also certainly continued into the Permian. Even after the land masses were attached, compression continued. Consider India and Eurasia – they’ve been colliding for at least the past 30 million years, and the consequences of the ongoing collision are seen in earthquakes throughout the Himalayan region today, and well beyond. The Appalachian orogeny was a similar event. 

It seems that especially the deformation, folding and faulting, that extended well into the continent and beyond the zone of active collision took place in the early to middle Permian Period. Some of the broad folds that dominate central and western Pennsylvania today were probably formed during the Permian. They still control the topography, 275 million years later, as erosion preferentially focuses on less resistant rock layers, leaving the more resistant beds as long high-standing ridges. You can get such mountain ridges even in a syncline, a down-folded zone in the rock, because of alternating high- and low-resistant beds.

The geologic map of Pennsylvania shows a beautiful zig-sag pattern in the rock units, because the folds, anticlines and synclines, have been tilted. Visualize a sheet of paper, squeezed so that it forms a scoop-like bend. Then point the end away from you down. Then, cut the paper off along a line parallel to the floor. The edge of the paper will make a broad U or V shape, depending on how tightly you bent the paper. That’s the zig-sag effect of folded rocks that have been tilted so that they plunge down into the earth.

I think the consensus is that most of the deformation in the Appalachian-Ouachita belt of North America was pretty much over by late Permian time. At that point, Pangaea with a major mountain range running through part of it was pretty much just sitting there. I’m not saying there were no earthquakes – I bet there were plenty. But the intense folding and faulting that resulted from two continents impinging on each other was pretty much done. And actually, as we’ll hear in a few days, it’s possible that the supercontinent was already beginning to break apart as early as the late Permian. Nothing lasts forever!

* * *

Today’s birthday is Eduard Suess. His name came up a couple weeks ago as the Austrian geologist who used the widespread nature of glossopteris fossils to suggest the existence of the supercontinent of Gondwana. He was born August 20, 1831.

—Richard I. Gibson

Appalachian folding 

Map of fold belt in central Pennsylvania from Pennsylvania Geological Survey

August 19. Dimetrodon

If there was one animal that was iconic for the Permian, it would be the fin-backed Dimetrodon, discovered in 1878. When I was in college, dimetrodon was considered to be a reptile, and my historical geology textbook said “the reason for such extraordinary specialization is entirely problematical,” meaning of course, we didn’t know. But they were often called mammal-like reptiles, for some skeletal characteristics that appeared to put them closer to mammals than reptiles. 

Dimetrodon drawing by DiBgd at en.wikipedia used under GFDL CC-BY-2.5

Today dimetrodons and their relatives, pelecosaurs, a term that isn’t used much anymore, are considered to be synapsids, the group that includes mammals. I’ve seen synapsids given as a class of the tetrapods or four-limbed animals. We’re familiar with the other classes of tetrapods – amphibians, reptiles, birds, and mammals. Synapsids might be better considered to be a group that includes primitive ancestral forms that probably would not be classed as mammals today, together with modern mammals themselves.

Dimetrodons are classed as synapsids. Not reptiles, but not quite mammals either. In early Permian time they became the largest land vertebrates, up to 15 feet long. Their fossils have come almost entirely from Texas and Oklahoma, where they lived in lowland deltaic wetlands, but there are dimetrodons from Germany as well. There are at least 12 species of fin-backed dimetrodons. The spines on dimetrodons’ backs extend from their spinal vertebrae, making a sail-like fin as much as 10 feet high. The speculation that they used the sail for temperature regulation – warming the blood circulating through it, and radiating heat to cool the animal – dates back to 1940, but the question isn’t settled yet. Plenty of work has been done, but with little other than bones to go by, such a characteristic is really difficult to prove. An alternative explanation is that dimetrodons exhibited sexual dimorphism – males and females had consistently different body features – and the fin might have been related to some kind of mating display.

Dimetrodons are often thought of as early dinosaurs, but they were extinct at least 40 million years before the dinosaurs appeared. The didn’t even make it to the Great Dying at the end of the Permian; dimetrodons were extinct by the middle Permian, about 272 million years ago.

—Richard I. Gibson

Dimetrodon drawing by DiBgd at en.wikipedia used under GFDL CC-BY-2.5.

August 18. Permian reptiles

Bradysaurus, a Permian reptile

Reptiles were better suited to the dry, arid climates that were common during Permian time than amphibians, and reptiles proliferated and diversified during this time. The Pareiasaurs were large herbivores, two to nine feet long, with bony plates armoring their bodies. Reconstructions look rather like big horned toads. In some varieties the bony plates have grown together, suggesting that this group may be ancestors of modern turtles, but the group was extinct at the end of the Permian and it is not certain that they gave rise to descendents that became turtles. It may be that the coalescing bony plates, somewhat like turtles’ shells, might have developed independently in both lineages.  

Mesosaurs, of early Permian age, were among the first reptiles to return to the water. They were clearly adapted to an aquatic life, with webbed feet and a long, streamlined body. Its leg joints – wrists and ankles – were designed in a way that would have made it impossible for them to walk on land, but they might have waddled ashore to lay eggs as modern sea turtles do, but embryo fossils of mesosaurs are not associated with egg shells, so an alternative interpretation is that they bore their young alive. If so, they are among the first animals to do so.

A lot of the known Permian reptiles are lizard-like, several inches to a foot or so long, and many are presumed to have been insectivores, filling the ecological niche that similar reptiles do today. The basic body plan of these animals seems to have been well established by the Permian. One lizard-like Permian reptile called Eudibamus was described in 2000 from a fossil found in Germany. It is possibly the first bipedal reptile. And another, known from Madagascar, had a wide skin layer between its ribs that probably allowed it to glide like modern flying squirrels. All of these adaptations make it clear that the Permian was a time of experimentation and expansion for the reptiles.

* * *

On the night of August 17-18, 1959, the strongest earthquake recorded in the Northern Rocky Mountains struck the upper Madison River Valley near Hebgen Lake west of Yellowstone National Park. It measured about 7.4 on the moment magnitude scale and it triggered a huge landslide in Madison Canyon. The slide buried a campground, killing at least 26 people and damming the Madison River, which backed up to form a new lake, Quake Lake. The earthquake re-set the rhythms of geysers in Yellowstone Park and damaged buildings as far away as Butte and Bozeman. The fault scarps were as much as 19 feet high and can still be seen 50 years later. The faults were normal faults, dropping the Hebgen Lake basin down relative to the adjacent mountains.

—Richard I. Gibson

Drawing by Nobu Tamura (http://spinops.blogspot.com) used under GFDL

Reference:
Road Log for the Hebgen Lake Earthquake Area, by Michael Stickney, Tobacco Root Geological Society Guidebook (2012), p. 71

August 17. Glaciers and Coal

Today’s episode is a response to a listener’s question about the close juxtaposition of glacial deposits and coal beds in Australia.

Despite the abundance of coal in the Carboniferous, especially in the northern hemisphere, and despite the changing climate that meant coal formation there largely ended with the end of the Carboniferous, there’s plenty of Permian coal too. Most of it is in the former Gondwana – Australia, South Africa, India, South America, and Antarctica, but there is a lot of Permian coal in Russia as well. In Gondwana, the coal is pretty closely associated with glacial deposits.

We might expect that glacial deposits and coal swamps would reflect two very different environments, but so far as I can tell, they pretty much co-existed in Permian time at least in quite a few places.

In Australia and elsewhere, the coal-bearing rocks and glacially deposited layers actually interfinger. So at best, we might have had some relatively rapid changes in climate to switch from glacial times to warmer, coal-swamp times, and from what I read there were at least 8 specific glacial periods in the late Carboniferous and early Permian.

But the alternative explanation, and from what I can gather it seems to be the preferred one, is that these areas were on the margin of the ice, and plant life actually thrived there. The keys to making coal are 1) lots of plants and 2) rapid burial of the plant matter so it does not have time to decay. Our typical vision of warm swampy areas with low oxygen to prevent decay is just one way to do that. A cold climate, with plants buried by glacial debris, would work just as well, if not better.

Gondwana base map from Du Toit (1937, Our Wandering Continents). Blue line is highly generalized margin of glacial area; solid black are highly generalized coal deposits (based on Langford, 1992).

I think that while Australia was certainly part of the glaciation in the southern polar part of Pangaea, it was probably far enough away from the pole (which was more or less in South Africa, but there are coal deposits there, too) that the climate might not have been like modern Antarctica, but perhaps more like modern Patagonia but with glaciers. So abundant plant life could have been growing, even thriving, near the glacial margin. A modern analogy would be the peat bogs of temperate and even arctic climates. When glaciers receded, forests and peat on the tundra would advance. When glaciers advanced, the deposits the glaciers carried would have buried the forests. This would be a good way to get the interfingering of glacial and coal deposits that we do observe.

This would not strictly be cyclothems, which represent rises and falls of sea level, alternately allowing swamps to form and then burying them in river sediment, but they would be cyclic nonetheless, like cyclothems. I do not know if the coal-glacial sediment packages follow the 8 known glacial periods or are (likely) something more complex, but if they do it would be on a periodicity of a few million years. Standard cyclothems can show alternations that may represent changes on scales of a few tens of thousands of years, or even fewer, as well as the longer periods of millions of years. If the coal results from glacier-margin plant life, as I infer it does, then the alternations would not reflect sea-level changes as cyclothems do, but more directly would reflect changes in position of the glaciers (together with the dumping of sediment to bury the forests or other vegetation).

—Richard I. Gibson

Links:
Permian of Australia
Permian coal in South Africa 

Langford, 1992 - Gondwana’s Permian coal 

August 16. Kaibab formation

The rim rocks at the Grand Canyon are the Kaibab Formation of Permian age. It’s a package of rocks dominated by limestones, reaching as much as 400 feet thick. Limestones are resistant in arid country, so the upper layers at the Grand Canyon are prominent cliff formers. The rocks that were once above it, younger than the Kaibab, have been eroded away, leaving the broad, relatively flat plateaus that run up to the rims of the canyon.

It’s not all limestone – and the variations, including sand and silt, tell us that the Permian in the Grand Canyon area was a place of fluctuating sea levels, with alternating influx of sandy sediment from lands and chemical precipitates like limestone in shallow water offshore. The age of the Kaibab is late early Permian or early middle Permian, just when glaciers were coming and going repeatedly in the southern hemisphere, so it’s pretty easy to explain these sea level changes in terms of glacial periods.

The Kaibab sedimentation was near the shore at times, but the shallow shelf where the sediments accumulated was as much as 200 miles wide in places and at times, so there was plenty of room for diverse types of sediment to be laid down. Many of the fossils, including brachiopods, corals, crinoids, and mollusks suggest that the Kaibab was in the aerated nutrient-rich intertidal zone around much of what is now the Grand Canyon.

One obvious conclusion you can draw from the Kaibab following the Coconino, which we talked about yesterday, is that when I say things like “The Permian was arid” it’s quite a vast simplification. Even if it was arid at some places and at some times, the situation could change in a few million years.

—Richard I. Gibson

Photo by Richard Gibson

See also The Earth Story - Kaibab

August 15. Coconino Formation

By early Permian time, about 260 million years ago, what is now southwestern United States, around the Grand Canyon area today, was becoming pretty arid. The Coconino sandstone represents an extensive dune terrane, essentially a Permian desert that formed there. Wind-borne or eolian sand in dunes forms sloping dune faces, and when these are preserved in the sandstone, the sloping forms are called cross-beds, angular curving beds within a single package of sand. We can infer wind direction from the orientation of cross-beds.

The Coconino is typically a white, almost pure sandstone 60 to 100 feet thick. It is so resistant that it forms near vertical cliffs in places, and makes for some of the most difficult passages down into the Grand Canyon today. I climbed through it on a route in the western part of the canyon back in 1987 – a 70-foot section that required the use of ropes to descend.

The Coconino is extensive and forms prominent landscapes across much of southern Utah as well as northern Arizona.

* * *

On August 15, 1950, an 8.6-magnitude earthquake in Assam, eastern India, reportedly killed more than 30,000 people, but other estimates give much smaller death tolls. This quake was definitely related to the ongoing collision between India and Eurasia, pushing up the Himalayas. There are other consequences to that collision, and this location, in northeastern India and adjacent Tibet, is essentially at the corner of the collision, so mountain belts and fault zones change direction here from about east-west to more north-south.

—Richard I. Gibson

See also The Earth Story's report on the Coconino

Photo by Richard Gibson

August 14. Permian ammonites

Cephalopods are the group that includes the octopuses, squids, and chambered nautilus. The extinct ammonites were spiral cephalopods that thrived in the Paleozoic and Mesozoic. Ammonites diversified during the Permian Period, but many varieties including the goniatites were killed off in the end Permian extinction.

Most Permian ammonites were relatively small, a few inches across. Many ammonites are associated with another fossil, called aptychus, which was thought for years to be the shells of a clam-like mollusk. Now it is thought to be part of the ammonite animal, but exactly what isn’t settled yet. It might be a trap-door-like mechanism, an operculum, that sealed the animal inside its shell when it was inactive. Snails have things like that. But it’s also possible that it may be some kind of jaw apparatus that helped the animal munch its prey. Or just maybe, it served both purposes. Paleontologists are still working on that little question.

—Richard I. Gibson

Drawing of Permian ammonite from an old text (public domain)

August 13. Permian salt

I’ve indicated a couple times that the Earth’s climate was changing pretty dramatically from the Carboniferous into the Permian. This was partly a result of (or a cause of) the end of the Carboniferous rainforests, and partly a result of increased glaciation in the southern hemisphere. It was also to at least some extent a consequence of the land masses combining into the supercontinent of Pangaea – vast areas were distant from shores, and depending on wind circulation patterns, vast areas were also in the rain shadows of some pretty large mountain ranges. By the middle part of the Permian, arid conditions were common, and despite the glaciers in the southern hemisphere, the tropics were pretty hot.

One result of the hot arid climate was extensive deposits of salt. In what is now Kansas and Oklahoma, more than 500 feet of Permian salt lies in the subsurface. Salt mines around Hutchinson, Kansas, led to its nickname, The Salt City. The salt in Kansas and Oklahoma, while more than 500 feet thick in total, is really a package of individual beds that are a few to many tens of feet thick, separated by zones of mudstone and shale. It appears that the area was a shallow, evaporative sea, probably an arm of the restricted basins of West Texas and New Mexico that we’ll talk about later this month.

To an extent, the shallow salt sea in Kansas during the Permian represents the last bit of sea, left over from the seaway that extended through what is now the Great Plains for much of the Paleozoic Era.

Zechstein salt in northern Europe

In northern Europe, a similar basin formed in which the Zechstein salt was deposited. The restricted basin extended from what is now Poland, west across northern Germany, the Netherlands, and beneath extensive areas of today’s North Sea. It formed fairly late in Permian time, about 270 to 260 million years ago. Because it was later in the period, it’s possible that it began at least in part as a result of deglaciation in the southern hemisphere – melting ice flooded shallow areas in the tropics, where evaporation exceeded the rate of rise so salts precipitated out of the water. The Zechstein salt serves as the impermeable seal for some significant oil accumulations in older Permian rocks in the North Sea area. In some places, the buried salt has risen buoyantly to form salt domes that help trap oil and natural gas as well.

Permian salt is found all over the world in the former tropical areas, including northern South America, central Russia, and smaller basins in Australia and Eurasia. Most of southern South America, Africa and Antarctica appear to have been well within or near the Antarctic Circle. 

—Richard I. Gibson

Zechstein salt

Map by San Jose and Drdoht at the German language Wikipedia used under GFDL 

August 12. Permian amphibians

Although amphibians suffered in the collapse of the Carboniferous rainforest ecosystem just before the end of the Carboniferous, they did adapt enough to survive into the Permian.  Since the primary information about fossil vertebrates comes from their bones, it can be difficult to separate the early amphibians, reptiles, and mammals from each other based on fossil evidence, but with more and more discoveries, the branches are becoming clearer. Some groups that were formerly considered to be reptiles are now thought of as amphibians. 

Eryops, a terrestrial-adapted temnospondyl amphibian about 5 feet long.

One such group, the Temnospondyls, have been found in enough diversity that we consider them today to be amphibians. These guys were diverse, often relatively large animals pretty much like crocodiles in body form, but they lived in environments ranging from fully aquatic to fully terrestrial. Temnospondyls got their start in the late Carboniferous and during the Permian became fully adapted to life on land. As things became drier during the course of the Permian, the terrestrial varieties seem to have declined, but aquatic types thrived, and in fact the largest amphibian known is from the Permian. It’s called Prionosuchus and its fossils were found in Brazil, where it lived in tropical lagoons and river systems. It was as much as 9 meters or 30 feet long, and looked much like a crocodile. That particular genus was extinct by the end of the Permian, but Temnospondyls generally survived the Permo-Triassic extinction and continued into the Cretaceous. Evolutionary branches of the temnospondyls probably gave rise to modern toads and frogs sometime during the Triassic, but that history isn’t certain.

There are quite a few amphibians known from the Permian, but most are from the early Permian. A lot of them were little lizard-like critters, and some were almost snake like. The Aïstopods – a name meaning “no visible feet” – had a couple hundred vertebrae but some were only a few inches long. Some aquatic Permian amphibians had wide heads shaped like boomerangs that might have helped them swim, though to be honest it seems to me that we don’t really understand these head ornamentations. Microsaurs, whose name means “small lizards,” were around by Permian time, and they may be the ancestors of modern salamanders and newts. There’s really quite a large diversity of amphibians from the Permian, but many of them are known only from one specimen – so figuring out their interrelationships can be pretty challenging.

By mid- to late Permian, conditions were no longer favorable for amphibians, or perhaps conditions were just not favorable for preservation of their fossils. Or some combination of both. Amphibians obviously have survived to the present, so they must have found some niches where they were able to thrive, or at least hang on, for millions of years.

—Richard I. Gibson

Eryops, a terrestrial-adapted temnospondyl about 5 feet long. Photo by Daderot (public domain) 

August 11. Permian fusulinids

Permian fusulinid from Iowa about 12 mm long (Mark Wilson photo)

Fusulinids, one-celled animals that made shells, had proliferated in the Carboniferous and they continued to expand during the Permian. These critters were really large compared to most single-celled organisms, and some Permian types were more than a centimeter long. Their cross-sections reveal complex internal compartments where the animal lived, and some had coiled spiral chambers. Their shells were calcite, not silica like diatoms and radiolarians, and sometimes they were so abundant that they are the principal component of limestones. 

Fusulinids were an order within the phylum of foraminifera, amoeba-like single-celled organisms that made shells with tiny holes in them. Most of them probably lived within the mud on the sea floor, but some probably were floaters. Either way, they had thin filament-like pseudopods that they extended from the shell to catch food. 

Although there are many living kinds of foraminifera, fusulinids were among the many forms of life that became extinct at the end of the Permian in the great mass extinction event then. The had a run of about 190 million years though – pretty successful for a little animal.

* * *

Today’s geological birthday is Robert Thomas Hill, born August 11, 1858, in Nashville, Tennessee. Hill was a pioneer in the study of Texas geology, and he taught some of the first geology courses at the University of Texas.

—Richard I. Gibson

Permian fusulinids 

Photo by Mark Wilson (public domain) Permian fusulinid from Iowa about 12 mm long