July 31. End of the Pennsylvanian

The end of the Carboniferous, and the end of its second sub-period, the Pennsylvanian, is placed at about 299 million years ago. The supercontinent of Pangaea was mostly but not completely formed, and the glacial period that typified the following Permian Period was underway. The global climate was cooler and drier than it had been during most of the Pennsylvanian.  

There is no global mass extinction to mark the end of the Carboniferous. The rainforest collapse was about 305 million years ago, six million years or so before the end of the Period.

So how is the end of the Pennsylvanian, the end of the Carboniferous, defined? This is another boundary like the end of the Silurian that’s in the category of “not with a bang.” Officially, the base of the Permian, and therefore the top of the Carboniferous, is placed below the first appearance of a particular conodont species. The internationally accepted location that defines it is in the Ural Mountains of Russia, where Permian rocks are well exposed.

All this is not exactly satisfying – there’s a tendency to want the major periods of earth history to be separated by major events. It might be a little more fun if the boundary was at the end of the rainforest collapse episode. But it’s not.

Despite the international agreement on the timing of the Pennsylvanian-Permian boundary, you will definitely see a diversity in references to the age of that boundary, sometimes indicating it’s at 280 million years ago and other times as well. Remember that geologic events occur at different times at different places – so even with international agreement, it’s reasonable to assign things like these boundaries to somewhat different times depending on where you are in the world. Saying the Pennsylvanian “ended” at 298.9 million years ago is just a human construction anyway.

—Richard I. Gibson

July 30. Flysch

On July 21, when I talked about the Marathon Orogeny in West Texas, I mentioned in passing the relatively deep, narrow seaway between the colliding continents. The Pennsylvanian sediments that were dumped into that trough, and eventually got caught up in the collision, made a sedimentary sequence called flysch.

Flysch sediments are called syn-tectonic or syn-orogenic, meaning that they are deposited contemporaneously with the mountain building. Often the basin in which they are laid down may be a foreland basin, the result of loading the crust and depressing it so more and more sediment can go into the basin, or sometimes flysch may be deposited in a remnant bit of ocean basin or something more complex.   

Zumaiako Flysch, Spain. Photo by Torpe via Wikipedia

Flysch deposits are cyclic, reflecting pulses of mountain building. When mountains are higher, coarser sediments are often dumped into the adjacent basin, and as the mountainous relief reduces through erosion, sediments become finer and finer. So a typical flysch sequence is one where the grain size of the rocks becomes finer upward, as the rocks get younger. The whole package can be repeated many times as the mountainous sources for the sediments are uplifted and eroded down. The cycles are usually clastic rocks – clastic means broken, and the broken fragments can be as coarse as pebbles to make conglomerate, but often it’s just various grain sizes in sandstones, siltstones, and muds.

Because the flysch deposits are often dumped into narrowing ocean basins during collisional tectonics, and because the oceanic crust underlying the flysch is often being subducted beneath one of the colliding continents, flysch deposits are often highly deformed, tightly folded and faulted. There are spectacular examples of tilted and folded flysch rocks in the Alps and other collision zones, made all the more evident because the alternating layers are often quite thin, so the structures are pretty obvious.

The word is something of a misnomer, since it was applied to rocks that were thought to be deposited by rivers. It comes from German meaning to flow, as in flowing rivers. But flysch deposits were generally laid down in deep oceanic water – sometimes, by flowing subsea currents called turbidite flows, which took sediments great distances out into oceanic basins.

Eventually, a basin begins to fill up, or the tectonism reduces in intensity, and shallow-water sediments typically overlie deep-water flysch deposits. Those shallow-water or even terrestrial deposits are called molasse. That’s a French word meaning “soft,” since the molasse sediments are often less well cemented than the underlying flysch deposits.

—Richard I. Gibson

Zumaiako Flysch, Spain. Photo by Torpe via Wikipedia under Creative Commons license

July 29. Oxygen levels

On July 14 when we talked about the giant insects of the Pennsylvanian, I mentioned the significantly greater oxygen content of the Pennsylvanian atmosphere as a possible contributor to the large size of those bugs. The extra oxygen could have provided more energy for growth as well as better support for winged insects.  

Oxygen levels in the Pennsylvanian are estimated at as much as 31% to 35%, versus today’s 21% oxygen. The cause is probably complex, but it seems reasonable to expect that the abundance of plants must have had something to do with it, and they could have also helped reduce carbon dioxide in the atmosphere as well, as I mentioned a few days ago. Plants take in CO2 and give off oxygen, and especially if the carbon is sequestered in coal in the ground, this could have a profound impact on the atmosphere.

The high in atmospheric oxygen during the Pennsylvanian and early Permian was probably the highest it has ever been, at least during Phanerozoic time, the past 550 million years. And it was followed in the Permian and Triassic by one of the lowest lows – around 15% oxygen. That crash might have been related to the crash in the Pennsylvanian rainforest ecosystem – many fewer plants, much less oxygen, and that in turn might be connected in part to the growing glaciers in Gondwana, which we’ll talk more about next month, during the Permian. But whatever the causes, these dramatic shifts in atmospheric oxygen content must have had dramatic impacts on life, positive for some groups and negative for others.

How do we know the concentration of oxygen in ancient atmospheres? It’s estimated mostly using geochemical models that integrate fluctuations in the carbon cycle with measurements of oxygen isotope ratios in rocks and fossils, as well as erosion and sedimentation rates and other expectable chemical reactions, all of which give us indications of ancient concentrations of atmospheric gasses. The oldest known amber, fossil resin from plants, is from the Carboniferous, and while some younger amber specimens have trapped air bubbles that can be analyzed, I haven’t found any reports that that has been done for Carboniferous amber. So it’s pretty much a combination of observations such as abundant plants and giant insects plus detailed chemical modeling. The nature of Carboniferous plants – thick corky outer layers that could have served as protection from fire, as well as some modern analogs that suggest Carboniferous plants thrived in an ecosystem that had abundant fires, also points toward the likelihood of significantly more atmospheric oxygen then, to fuel such fires.

It seems to be pretty much accepted today that Carboniferous atmospheric oxygen was at least 31% or as much as 35% of the total, much greater than today’s 21% oxygen content.

—Richard I. Gibson

Links and References:
Historical CO2 levels

Atmospheric oxygen over Phanerozoic time (Berner, 1999) 

Geochemical modeling 

A new model for atmospheric oxygen over Phanerozoic time (Berner, 1989)

PHANEROZOIC ATMOSPHERIC OXYGEN (Berner et al., 2003): Ann. Rev. Earth Planet. Sci. 2003, 31:105.

Oxygen balance 

Insect growth in various oxygen concentrations

July 28. Kazakh-Siberia-Baltica Collisions

I’m sure I’ve mentioned Siberia from time to time. It was a discrete continental block, like Baltica or Europe, out there kind of doing its own thing. By the late Pennsylvanian, or late Carboniferous as it would be called in Europe and Asia, Siberia was beginning to reach the edge of Europe. But it wasn’t alone. 

There’s a smaller – but still large – continental block occupied by much of Kazakhstan today. It contains Precambrian rocks, like Siberia and Baltica and North America, but it probably formed by a complex assemblage of smaller Precambrian microcontinents, island arcs, and oceanic terranes. It may not have been assembled into something we’d call a continent until about Ordovician time. It’s such a complex mess, geologically, that the details, and even the general aspects of its history are still being worked out.

By late Carboniferous, about 300 million years ago, the Kazakh Block was beginning to impact the southern part of Baltica, the core of Europe. The ocean between the two was closing, and the typical continent-continent collision was starting to create a mountain range – the Ural Mountains. At least the southern and probably central part of today’s Ural Mountains.

Tectonic Map of Soviet Union (above) from “Atlas SSSR,” Moscow 1984. Annotated by Gibson. KAZ = Kazakh Craton
Boundaries are highly generalized.

The collision was between a passive margin – like the present-day east coast of North America – on the Baltica side and an active subducting margin, like western South America today, on the Kazakh side. Both sides were deformed, and there’s a 2,000-kilometer-long fault, called the Main Ural Fault, which is taken to represent the suture, the line of joining, between the two continental blocks. Parts of the oceanic crust that had been between the two got caught up in the collision as well, and some were even pushed up and over the rocks of the Baltica craton. The term for oceanic crustal rocks that are found on the continents today is ophiolite, and they are fairly common in these sorts of collision zones.

Depending on which papers you read, the Kazakh-Baltica collision might have begun as long ago as early Carboniferous, perhaps 340 million years ago, but I believe the consensus is that it culminated with the two blocks combining near the end of the Carboniferous.

The other side of the Kazakh Craton, where today it is attached to Siberia, is even less clear. The interaction between the Kazakh Craton and the Siberian Craton appears to have been long – many millions of years – and complex, ranging from a typical subduction interaction to multiple long strike-slip faults like the San Andreas today, and the fault zone along the Queen Charlotte Islands off western British Columbia. Most evidence seems to indicate that the Kazakh Block and the Siberian Craton were assembled at least a bit later than the Kazakh-Baltica collision, probably during the Permian or later, and that the northern Ural Mountains formed when the combined Kazakh-Siberia continent finally collided with Baltica. But I’m definitely not sure about that, even though I spent a year working on an interpretation of the magnetic map of the Soviet Union, to create a tectonic framework for oil exploration back in 1990. It’s complicated, for sure.

* * *

July 28, 1840, was the birthday of Edward Drinker Cope in Philadelphia. Cope was a paleontologist and fossil collector, focusing extensively on dinosaur and giant mammal fossils of the western U.S. His personal feud with O.C. Marsh is called the “Bone Wars” today.

Today is also the anniversary of the Tangshan Earthquake, in eastern China in 1976. The estimated death toll, 242,000, probably makes it the most deadly earthquake of the 20th century. The site in northeastern China is near the tectonic boundaries of several smaller blocks that make up China today. These blocks are still moving relative to each other, but the ultimate push that probably caused the Tangshan Earthquake is most likely the collision between India and Eurasia. Even though that collision is taking place thousands of kilometers from Tangshan, its impacts are felt across eastern Eurasia as old weak zones and boundaries are exploited to relieve the compressive stress caused by the collision of India.

—Richard I. Gibson

Geodynamics of East Kazakhstan 

Tectonic History of the Urals

Tectonic Map of Soviet Union (above) from “Atlas SSSR,” Moscow 1984. Annotated by Gibson. KAZ = Kazakh Craton

July 27. Lansing Formation

Throughout the Pennsylvanian Period, I’ve focused a lot on the coal swamps that characterized the time, as well as mountain uplifts. But there were plenty of regions still covered by shallow seas like those that were typical of so much of the earlier Paleozoic Era.  

Take Kansas, for example. Many of the Pennsylvanian rocks there are shallow-water limestones similar to those that covered much of the interior United States for much of the previous 200 million years. One difference is that those Kansas rocks do also reflect the alternations in sea level that produced the coal cyclothems further east. Cyclothems are alternating layers of coal and sandy sediment, reflecting high stands and low stands of the sea. In the Lansing Limestone of Kansas, the same cycles are shown by alternations between limestone and shale in the Pennsylvanian strata.

It’s pretty clear that the limestones were formed in very shallow water. A lot of the rock is made up of broken shells, which must have been fragmented by wave action. They were piled up into shoals, just like sand banks at or near the water line or at least above wave base. When sea level rose just a bit, fine-grained deeper-water shaly sediments could wash over those carbonate banks. The skeletal shell fragments in the limestone made for some excellent porosity in the limestone – commonly as much as 15% of the rock, and the tight, fine-grained shale made for a nice impermeable barrier – both necessary conditions for oil and gas accumulations. All that we need is a source rock with time to mature – and we have that too, in the same shales that washed organic material into the sea along with the fine sediment.

Drawing of shoal environments of Lansing Limestone (from Harbaugh, 1960)

The Lansing and Kansas City Group of limestones has produced something like a billion barrels of oil over many decades, and that adds up to at least a fifth of all the oil Kansas has produced, and Kansas is one of the leading states in terms of total cumulative production. About 25% of all U.S. oil production comes from rocks of Pennsylvanian age. The Lansing Formation and its equivalents extend into Colorado and Oklahoma too. They were laid down about 210 million years ago, contemporaneous with the coal swamps in Pennsylvania and West Virginia and elsewhere.

—Richard I. Gibson

Links:
Petrology of Marine Bank Limestones of Lansing Group (Pennsylvanian), Southeast Kansas, by John W. Harbaugh - Originally published in 1960 as Kansas Geological Survey Bulletin 142, part 5

Oil & Gas

Oil Potential  

July 26. The African Suture

Earlier this year, 2014, there was a flurry of news reports in the popular science press about the Brunswick Magnetic Anomaly in southern Georgia as the expression of the old boundary between Africa and North America when they came together in Pennsylvanian and Permian time.  

That’s fascinating, but it isn’t news. We’ve known this pretty much since the early 1980s or longer. There have been some new studies addressing the details of what’s going on there, but the fundamental nature of this linear zone has been known for years.

A magnetic anomaly is a departure from the broad general magnetic field of the earth. The magnetic anomalies that geologists like me care about represent geology, because they represent differences in magnetite content that can allow us to infer things about the subsurface. Most of my career, since 1975, has been focused on studying magnetic anomalies as well as anomalies in the earth’s gravity field, and trying to figure out what they mean. I made an interpretation of the magnetic map of the U.S. including the Brunswick anomaly as the suture zone between Gondwana and North America back in 1988.

Part of Magnetic Map of North America (USGS) showing Brunswick Anomaly (curving blue zone)

Across southern Georgia, and extending east into the Atlantic Ocean and west across Alabama, the Brunswick magnetic anomaly is a long, curving magnetic low, representing a strong contrast in magnetic material deep in the earth’s crust. There is plenty of discussion about its exact nature – is it a fault zone, a string of intrusive igneous rocks, simpler changes in rock type, or something else. But fundamentally, it represents the zone along which the leading edge of Gondwana collided with the southeastern margin of North America. We call this zone of amalgamation a suture zone, where continents or other blocks have been attached to each other, like a huge medical suture using a lot of different ways to do the attaching.

You usually see this part of the U.S. referred to as part of Africa, but the zone that includes all of Florida, some of the Florida Shelf, and the southern parts of Georgia and Alabama as well as possibly the Bahamas and points to the southeast were really part of Gondwana, occupying a triangular zone between what is now Senegal to Liberia on the African coast and Venezuela and the Guianas down to northeastern Brazil on the South American coast. This little bit of Gondwana had been part of the supercontinent for at least a few hundred million years, and it was at the forefront of part of the Allegheny-Appalachian collision in Pennsylvanian time that welded Gondwana to North America. They remained attached for close to 100 million years, forming part of the supercontinent of Pangaea.

White line is a highly generalized boundary of the terrane that was once part of Gondwana, but is now part of North America.

It wasn’t until the Jurassic Period, which we will get to next October, that Pangaea began to break apart again. As you might expect, it broke apart along its weakest zones, and some of the weak zones were the highly faulted and deformed areas where the continents had come together in Pennsylvanian-Permian time. Africa rifted away from North America approximately along the old suture between them – but not exactly. The rift split along different lines in what is now the southeastern United States, and Florida and southern Georgia and southern Alabama and the adjacent continental shelves got left behind. A bit of Gondwana had become part of North America.

—Richard I. Gibson

Links:
http://www.livescience.com/44633-brunswick-magnetic-anomaly-explained.html
http://www.geosociety.org/news/pr/2014/14-22.htm

1988 COCORP study
Brunswick anomaly

July 25. Fossil spores

With all the ferns and other spore-bearing plants around during the Pennsylvanian, you’d expect that there would be a lot of fossil spores, too. You’d be right. The science of palynology is the study of fossil spores and pollen. Because they’re microscopic, assemblages of spores and pollen make really good markers for particular time periods – often pretty short time periods, so they’re useful in biostratigraphy. They help explorers for oil and natural gas pin down pretty exactly where you are in the stratigraphic section, even when the information comes from tiny cuttings in wells. 

The word palynology comes from a Greek word meaning “strewn,” or “sprinkle,” so it’s the study of things that are scattered or strewn.

Pennsylvanian spore Reinschospora magnifica, about 70 microns across.
From Kosanke, 1950, Illinois State Geol. Survey Bulletin 74.

The smallest pollen grains are around 6 micrometers across – it would take more than 100 to span a millimeter. Spores are similar in size, but they can range up to at least 90 micrometers across. Fossil spores come in a wild array of shapes, just as modern pollen and spores do. They’ve been studied scientifically since the 1800s but modern palynology really took off in the 1950s and 1960s. The American Association of Stratigraphic Palynologists was founded in 1967, and today under the name The Palynological Society the organization has around 500 members worldwide.

—Richard I. Gibson

Pennsylvanian spore Reinschospora magnifica, about 70 microns across. From Kosanke, 1950, Illinois State Geol. Survey Bulletin 74.

July 24. The end of coal

One possible explanation for the decline in carbon dioxide in the atmosphere toward the end of the Pennsylvanian, which we talked about yesterday, is that so much of the carbon was being tied up in coal and therefore not available to the atmosphere. Plants take CO2 in – and if they died and became coal rather than decomposing more completely, the carbon would be sequestered in the earth, in coal beds.

Some recent research by scientists at Clark University and the US Department of Energy, and elsewhere, published in 2012, suggests that one reason for all the plants and all the coal in the Pennsylvanian was a sort of arms race between plants and the enemies that could destroy them – specifically, fungi.

When plants evolved a biopolymer called lignin, it allowed them to better transfer fluids around their bodies. Lignin gives wood both strength and functionality for plants, and a vast amount of the world’s carbon is contained in lignin and related chemicals in plants. It may have been the evolution of lignin that allowed plants to grow to large sizes, beyond the tiny stems of Cooksonia back in the Silurian.

Having lignin may have given plants another advantage of sorts – protection against attack by some other organisms. The recent work suggests that the abundance of coal in the Pennsylvanian is because there was nothing around to attack dead woody tissue, because nothing had evolved the necessary chemicals to break it down. Dead wood simply accumulated and altered under physical weathering conditions, together with the acids formed by the plants themselves. Nothing was eating the wood, so it became coal.

Toward the end of the Carboniferous, a fungus evolved that had the chemicals needed to decompose lignin. Was this enough to mean that following periods yielded less coal because more wood was broken down, with carbon returning to the atmosphere more than to the earth? The authors of the recent work think so.

White-rot fungus photo by Sten Porse
via Wikipedia under Creative Commons license.

This has some significant consequences for earth history as well as for present-day energy studies. The Department of Energy was involved because they were studying ways of degrading wood to make better biofuels. But geologically, this might help account for the low point in atmospheric carbon dioxide at the end of the Pennsylvanian and early Permian Period. Later, the additional input due to fungal decomposition of wood could have brought CO2 levels up again. Never discount the power of plants over time – remember that the oxygenation of earth’s atmosphere was probably due to the action of algae and cyanobacteria back in the Precambrian.

So, there is apparently a coincidence in timing between the evolution of genes in these fungi that could degrade lignin and the end of abundant coal, toward the end of the Pennsylvanian. But there are probably other factors in the decline of rainforests, such as we discussed yesterday, and while a coincidence in timing is interesting, it isn’t proof. And the work was based on genome studies – there are not many fungi in the fossil record. Probably more telling is the fact that there is hardly anything other than the enzymes from funguses that can really attack lignin and break it down. So the evolution of those chemicals must have indeed been an important event in the history of plant life, and in the evolution of the carbon cycle on earth. It’s definitely an intriguing idea.

—Richard I. Gibson

White-rot fungus photo by Sten Porse via Wikipedia under Creative Commons license.   

Wood-decay fungus 

Links (all to reports on the 2012 study):

http://science.energy.gov/ber/highlights/2012/ber-2012-06-a/

http://www.sciencemag.org/content/336/6089/1715.short

http://www.sciencedaily.com/releases/2012/06/120628181721.htm

July 23. Collapse of the Pennsylvanian Rainforest

About 305 million years ago, five or six million years before the end of the Pennsylvanian Period, the rainforests and coal swamps came to a relatively abrupt end. This was an extinction event that seems to have caused a step-wise decimation of the plant life that was so abundant earlier in the Period.

The cause of this rapid decline in the tropical rainforests is not clear. One good candidate is the increasing glaciation in the southern hemisphere, in Gondwana. A short but intense pulse of glacier formation would have lowered sea level, perhaps enough to affect the swamps as well. And it could have led to cooler overall climates that would be challenging for the warm-weather adapted forests. But it may have been the variability as much as anything – rapid warming following a short glacial episode. Forests acclimated to a pretty standard, unchanging environment might not have been able to cope with the variations. The overall climate appears to have become considerably drier at the same time it cooled, and that too would have impacted rainforests negatively. 

Some evidence seems to suggest that the collapse of rainforests may have taken only a few thousand years, and that variable climates followed. There were survivors, but they occupied ecological islands where diversity was restricted for the older rainforest flora. The amount of carbon dioxide in the atmosphere had been declining through the Pennsylvanian, and reached some of the lowest levels in the past 600 million years toward the end of the Pennsylvanian and into the Permian Period. Was that a cause of the decline of the rainforests - less CO2 for them to use? Or was the decline in CO2 a consequence of the abundance of plants, taking it out of the atmosphere? That’s not clear, but the CO2 decline was a long, gradual affair, while the crash of the rainforest ecosystem was quite rapid.

There is a well-known periodicity to extinctions in earth history, at about 26 to 27 million years. It’s suspected to relate to some cosmic-scale phenomenon such as the solar system’s position within the Milky Way Galaxy, but the ultimate cause of cyclic extinction events is not known with certainty. But the Carboniferous collapse of the rainforest ecosystem does fall within one of the maxima of the 26-million-year cycle.

What was a disaster for the ferns and scale trees of the Pennsylvanian swamps may have been a blessing for the tetrapods, the vertebrates that were establishing themselves on land during Mississippian and Pennsylvanian time. A 2010 study by scientists at the University of Bristol and University of London in the U.K. found that following an initial decimation that coincided with the rainforest collapse, land animals diversified rapidly into the new ecological niches created by the plant extinctions. Amphibians were hard hit by the changes, and while they survived, they were outpaced by the animals that were adapted to drier conditions – namely, the ancestors of the reptiles and mammals. Reptiles and mammals did not rely on water to reproduce, and they had skins that allowed for better management of both temperature and moisture. The Pennsylvanian rainforest collapse may have created the circumstances that led to reptilian and mammalian diversification, leading eventually to the dinosaurs, and to us humans too.

—Richard I. Gibson

Links:
Rainforest collapse triggers tetrapod diversification


Floodplain evolution 

The Carboniferous Crisis

July 22. Ferns

Earlier this month we talked about the tree-sized plants that dominated the swamps and forests of the Pennsylvanian – at least the tropical Pennsylvanian, which spanned much of North America and Europe. Some of the common trees such as sigillaria and lepidodendron are sometimes called “tree ferns” but they were really more closely related to club mosses and rushes.

True ferns evolved by the very late Devonian or Mississippian Period and certainly contributed to the biomass that produced the Pennsylvanian coal. Some grew to tree-like proportions, including a group historically named Megaphyton or Aristophyton that was probably as much as 50 feet tall.

Ferns have neither seeds nor flowers, and they reproduce by spores. The dense forests of the Pennsylvanian were completely without flowers – flowering plants didn’t evolve until the Cretaceous Period, about 175 million years after the end of the Pennsylvanian. Ferns apparently evolved leaves, or fronds, independently from other groups of plants, possibly by developing photosynthetic webbing between branches.

A group separate from true ferns, called seed ferns, also flourished during the Pennsylvanian, and their leaves form some of the most common fossils in coal beds and concretions. Seed ferns are not really very closely related to true ferns, so the name is a misnomer. The were probably a little more close to cycads and ginkoes than to ferns, but the name “seed fern” is still commonly used. Their fronds look an awful lot like ferns, and some of the fronds were as much as 6 feet long. Two types that are common in Pennsylvanian coal beds and concretions in the U.S. are called Neuropteris and Alethopteris. Neuropteris means “nerve-wing” for the complex vein system in its leaves. They are all extinct, with most of them disappearing at the end of the Permian but some may have survived as late as the Eocene, about 55 million years ago. It’s not completely agreed that the possible survivors should be classified as seed ferns or not.

* * *

Today’s birthday is Kirk Bryan’s, born July 22, 1888, in Albuquerque, New Mexico. Bryan was a geomorphologist, a student of landforms, especially those in arid lands, as well as glacial geology. The Geological Society of America gives the Kirk Bryan award annually for distinctive work in the areas of science that he pioneered.

—Richard I. Gibson

Tree fern drawing by Dawson (from an old textbook; public domain)

July 21. Marathon Orogeny

Allegheny-Ouachita-Marathon orogenic belt. From Thomas (1983) via this link.

 
The Marathon Orogeny in West Texas wasn’t really a discrete, separate mountain-building event, but just another aspect of the complex collision between Gondwana and North America.  In the east, from New York to Alabama, it was more or less the head-on interaction between what is now West Africa and eastern North America. Further south and west, the geometry of the North American margin together with complexities in northwestern Gondwana made for a less straightforward collision.

Folds in Caballos Novaculite, Marathon region, West Texas

The heart of the eastern and southern margins of North America was built up in the Precambrian, during the Grenville Orogeny, when a long relatively narrow strip of continental material was accreted, added, to the ancient Archean core of the continent. That included somewhat discrete blocks such as the Nashville Dome in and around central Tennessee, the Ozark Dome in Missouri and Arkansas, and the Llano Uplift in what is now central Texas. These blocks served as buttresses to the impinging parts of Gondwana.

As a result, there are two big salients, or places where the big push worked its way further into the continent. One formed the Ouachita Mountains in Arkansas and Oklahoma, where the tectonic push worked its way between the Nashville and Ozark Domes on one side and the Llano Uplift on the other. On the far southwestern side of the Llano Uplift, another salient developed where rocks were pushed further into the continent. This became the Marathon Orogeny.

Think of the Llano Uplift as a big round chunk of concrete sitting in the sand on a wave-washed beach. As the waves come in, pushing sand with them, they break around the concrete block and push sand around the flanks of it. The zones on either side, where sand and shells and other debris push furthest up the beach are the analogs to the Ouachita and Marathon Orogenies.

As the South American corner of Gondwana approached North America, it approached somewhat obliquely. I think it’s fair to think of this part of the collision as something like a zipper closing the ocean between the two continents – two continents with edges that were definitely NOT straight lines. The zipper closed from east to west, so the Marathon region was one of the later aspects of this complex collision.

The push caught up the older rocks – everything older than Pennsylvanian, and even including some Pennsylvanian rocks that had been laid down in the seaway, a fairly deep trough between the continents, a depression probably caused by subduction associated with the onset of the collision. The subduction zone probably plunged to the south, beneath Gondwana, where a volcanic belt developed. The volcanic zone erupted into the deep trough and even it was eventually caught up in the collision. Everything was pushed to the northwest in modern coordinates, up and over the margin of North America in what is now West Texas. Pennsylvanian rocks on the shallower slope were pushed over the deep-water Pennsylvanian rocks in the trough. This happened more or less in late Pennsylvanian time and into the following Permian Period.

Complex folds and faults in Marathon region, west Texas. From King (1937). Purple (Dc) is Devonian Caballos Novaculite.

The push and collision were enough to break the North American continental crust far beyond the margin. We talked about the Ancestral Rockies of Colorado and adjacent areas a few days ago, but they must be a consequence of this collision.

The Appalachian-Alleghenian-Ouachita-Marathon Orogeny continues into Mexico where it is called the Sonoran Orogeny. It may or may not have been shifted into its present location Mexico along a long complex shear zone similar to the San Andreas Fault, some time after the Marathon-Sonora fold belt was formed. The hypothesis of the Mojave-Sonora Megashear, as it’s called, is still somewhat controversial, and there is evidence on both sides. I have a link on the blog to a Geological Society of America Special Paper on the topic. However the tectonic energy was transferred, the impacts of the collision of Gondwana were definitely far-reaching, and even they have been changed over later time.

All of these orogenies, mountain-building events, from the Allegheny to the Sonora, were part of the assembly of the huge supercontinent called Pangaea, which means “all earth.” We’ll talk more about it next month, in the Permian, when its final assembly was complete.

Thanks to my friend Pat Dickerson for one of the images above. Pat has worked extensively on the geology of West Texas and its former connections to South America.

—Richard I. Gibson

References and links:

Ouachita-Marathon Orogeny 

Structural Style of the Marathon Fold Belt (Hickman et al.) 

Geology of the Marathon Region, P.B. King, 1937, USGS Prof. Paper 187 (1937)

Ouachita-Marathon and Ancestral Rockies

The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives: edited by Thomas Howard Anderson and others; Geol. Soc. Amer. Special Paper 393, 2005.

July 20. Alice Springs Orogeny

Gondwana

It’s easy to visualize Gondwana as one big unified block with not much going on except around the margins. But in at least some places, there was plenty of action, even though we don’t necessarily understand it too well. Deformation, mountain building, within a tectonic plate is sometimes enigmatic. We know about collisions and subduction and such and what they can do to the earth, but the causes and consequences of intraplate deformation are harder to understand. Today, in central United States in southeastern Missouri and adjacent states, intraplate deformation is occurring, producing the well-known earthquakes in the New Madrid Zone. But the ultimate plate tectonic cause is challenging to pin down with certainty.

During the Pennsylvanian, a long-lived intraplate orogeny was culminating in what is now Central Australia. Australia was firmly attached to the eastern side of Gondwana, but mountain uplifts had been occurring there, well within the tectonic plate, since Devonian time or earlier. It’s called the Alice Springs Orogeny, which appears to represent north-south compression (in modern coordinates) and shortening that rejuvenated some old faults whose heritage dated back to the Cambrian or longer. One of the rejuvenated zones, called the Redbank Shear Zone, may be a really major break that extends all the way to the crust-mantle boundary. It is marked by the most intense gravity gradient in the world, a reflection of the huge contrast in density between the mantle and the crust. Such a prominent weak zone would be the sort of thing that could be activated relatively easily, more easily than a fresh break through unbroken crust. 

It seems that it wasn’t simply a matter of reactivating old faults, though. Central Australia had subsided over time, and the load of sediment eroded into the subsiding basins may have had a role in the tectonic activity as well. The tectonism broke the old basin into several sub-basins.

What was causing the compression? By Pennsylvanian time, Gondwana was rotating in a more-or-less clockwise manner, so that its western parts were moving north to collide with North America. Australia, to the east, would have been moving south. It’s possible that the highly variable thickness of the crust in Australia could have led to differential motion, sort of like one section catching up with another to provide squeezing. Whatever the cause, it seems pretty certain that there was no typical continent-continent collision, so the ultimate cause remains pretty enigmatic.

—Richard I. Gibson

References and Links:
Martin Hand’s abstract

Chris Klootwijk’s “heretical view” 

Modeling the Alice Springs Orogeny 

Cartwright, Buick, Foster, and Lambert (1999), Alice Springs age shear zones from the southeastern Reynolds Range, central Australia: Australian Journal of Earth Sciences: Geological Society of Australia, 46:3, 355-363

July 19. Spiders

Pennsylvanian spider

The basic form of the spider must be a good one, because it has remained nearly unchanged since Pennsylvanian time, about 300 million years. There are plenty of variations on the theme, of course, and today more than 40,000 species have been described.

Spiders are arthropods, a group whose name means jointed leg and which includes insects, scorpions, centipedes, and trilobites as well as spiders.  

The first true spiders probably evolved from Devonian spider-like arachnids called Attercopus, found in the Devonian Gilboa Fossil Forest of New York which is dated to about 380 million years ago. That ancestor could make silk, but recent work by Paul Selden and his colleagues suggests that it didn’t have spinnerets and did not weave webs. This means that as far as we know, the first true spiders developed by the Pennsylvanian Period, 300 million years ago or so. You may recall that there is a pretty dramatic gap in the fossil record of insects during Mississippian time, 320 to 360 million years ago, and that includes spiders as well. We don’t have much evidence for spider evolution during that time.

But there is abundant evidence for spiders in Pennsylvanian coal beds and the concretions like those we talked about at Mazon Creek, Illinois, on July 11. Many Pennsylvanian spiders, at least at a glance, are practically indistinguishable from modern varieties.  

—Richard I. Gibson

Papers by Paul Selden

Pennsylvanian spider drawing by James Dwight Dana (public domain). The original specimen is about an inch across.

July 18. Coal

Coal is known from most geologic periods, even the Precambrian, when it probably derived from algal accumulations. But the Carboniferous, especially the later part of the period called Pennsylvanian in the United States, was probably the time when more coal formed than any other period. That was pretty much directly proportional to the widespread environments that encouraged the growth of plants in settings that led to their burial and alteration to coal.  

Depending on how much the original plant matter is compressed and heated up, you get various grades of coal. The least compacted type with the most impurities is called lignite, which is essentially compressed peat, a loose accumulation of plant matter. Lignite has only about 30% carbon content and high moisture. The next grade is called bituminous, around 70% carbon and fewer impurities and lower moisture than lignite. The word bituminous means “containing bitumen,” and bitumen was the name of the tarry substance used as pitch or glue in Roman times and earlier. Lignite comes from a Latin word meaning wood.

The highest grade of coal, anthracite, can have carbon content as high as 95% or more. It is essentially a low-grade metamorphic rock, low in moisture and volatiles and other impurities. The grades of coal are proportional to their heat content when burned, and consequently the price of coal follows its grade, with anthracite most expensive. Anthracite’s name comes from the Greek word for coal.

The biggest deposits of anthracite in the world are in the Pennsylvanian rocks of Pennsylvania, but today more anthracite is mined in China than any other country. China is also the world’s largest consumer of coal. Anthracite accounts for only about 1% of all the coal reserves known, but its value means about 9% of total coal production is anthracite.

Bituminous coal is the bulk of coal mined and burned for fuel and energy generation. About a third of the world’s energy comes from coal, with 40% of electricity generated by burning coal. And about 70% of all the steel made relies on coal to fuel the blast furnaces and refineries where steel is fabricated.

Today the leading producers of coal, in order, are China, with almost half of the total world production, followed by the US, India, Indonesia, and Australia. Close to eight billon tons of coal is mined worldwide every year.

While burning coal is the primary end use, there are lots of products that come from coal and the coal tars derived from coal. Heat-resistant black plastics, aspirin pills, dyes, mothballs, and more were ultimately made from chemicals distilled from coal. There’s a fair amount about these topics in my other book, What Things Are Made Of.

—Richard I. Gibson

July 17. Fusulinids

Foraminifera, or forams, are small single-celled animals that produce shells of calcite, calcium carbonate. The name foraminifera means “hole-bearing” because the shells have tiny perforations in them. The shells can be quite complex, ranging from snail-like spirals and assemblages to blob-like forms that reflect the amoeba-like shapes of the animals. Some float in water, but many live in the mud on the sea floor. 



Forams are known from the Cambrian and they exist today, totaling more than a quarter million species both living and extinct. Most forams were microscopic, but during the Pennsylvanian one group called fusulinids reached sizes greater than a quarter inch long. Fusulinid means “spindle like” and their diverse shells range in appearance from rice grains to tiny footballs. Fusulinids began during the Silurian and really thrived during the Pennsylvanian, but they disappeared in the end-Permian extinction. They are excellent index fossils, and because they are small they are often used in biostratigraphic studies of sedimentary rocks of the late Paleozoic Era. 

Fusulina cylindrica

Fusulinids were mostly marine, and they are common in the limestones that formed in the remaining shallow seas of Mississippian and Pennsylvanian time. In some places, they were so abundant that they make up most of the rock over short time spans.

—Richard I. Gibson

Upper photo by Wilson44691 under Creative Commons license.

Fusulina cylindrica from Naco Limestone, Arizona. Largest individual is ¼ inch long. Photo from USGS Prof. Paper 21, Geology and Ore Deposits of the Bisbee Quadrangle, Arizona, by F.L. Ransome, 1904 (public domain).

July 16. Stone Mountain, Georgia

The southeastern part of the United States bore the brunt of the collision with Gondwana. What is now the westernmost part of Africa, around Senegal and Mauritania, collided with the Carolinas and Georgia. This was a true continent-continent collision, similar to that between India and Eurasia, and the mountain uplift squeezed between the continents was a huge one. 

When continents collide, everything doesn’t just go up. Plenty of material is forced down, too. Far enough down for temperatures and pressures to change the rock, to metamorphose the rock into radically different forms. A sandstone with quartz and feldspar and iron oxide cement and other grains can be changed into a banded rock called gneiss, with the chemicals in the original rock reorganized into entirely different minerals, often arranged in thin layers whose geometry is related to the orientation of the pressure regime.

Under such conditions, melting can also happen, and molten rock, magma, is typically associated with collision zones. Because of the chemistry of the rocks involved, it’s actually pretty easy to see the differences between oceanic subduction, with a slab of oceanic crust diving down beneath a continent and giving rise to a volcanic mountain range like the Andes in western South America, and the melting of complex continental crust which tends to be more silica rich, more granitic in composition.

Photo by Kyleandmelissa22 via Wikipedia, public domain.

In Georgia, Stone Mountain outside Atlanta is a solidified granitic body that formed as a result of the collision of Gondwana and North America about 300 million years ago, near the end of the Pennsylvanian Period. Technically the rock is called quartz monzonite, but it amounts to granite with a lower percentage of quartz than most granite. Petrologists have dozens of terms for rocks to give them a way to talk about specific compositions more clearly.

The granitic rock at Stone Mountain solidified well within the earth’s crust, probably 5 to 10 miles down. The difference between chemically similar deep-seated or plutonic rocks (from the realm of Pluto) and those that solidify on the surface, such as lava flows, is their grain size. At the surface, molten rock cools and solidifies quickly, so individual crystals don’t have time to grow very large. Insulated within the earth, granites and other plutonic rocks solidify over many thousands or even millions of years, so their crystal grains are relatively large, sometimes several centimeters across.

Over time, with more uplift and more erosion, the solid granitic rock was brought to the earth’s surface. It is more resistant than the surrounding rock, so it eventually eroded into the prominent dome-like feature that stands outside Atlanta today. There is a huge carving of Confederate Generals on one side of the mountain.

—Richard I. Gibson

Reference:
Age and origin of the Stone Mountain Granite, Lithonia district, Georgia (Whitney, Jones, and Walker, 1976)


Photo by Kyleandmelissa22 via Wikipedia, public domain.

July 15. Pennsylvanian paleogeography

By late Pennsylvanian time, the ongoing collision between the southern supercontinent of Gondwana and the northern continent of Laurasia, North America plus Baltica or Europe, was well underway. There was probably still a bit of ocean between the far northwestern part of Gondwana, which is South America today, and what is now southwestern North America. Siberia and other terranes probably hadn’t quite attached themselves to the eastern margin of Baltica, and there were still some continental blocks remaining unattached, including microcontinents that would become parts of China today. But the assembly of Pangea, whose name means “all earth” was clearly approaching its climax. 

Late Pennsylvanian Map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL

We’ve talked about some of the tectonic events in North America, resulting from the collision of Gondwana with what is now eastern and southern United States. The Appalachian-Alleghenian Orogeny had created another high mountain range approximately where the modern Appalachians stand, and the southwestern extension of that collision produced the Ouachita Mountains in what is now Arkansas and Oklahoma, extending as far west as the Marathon Region of West Texas and beyond. Deformation within the western part of the continent broke the crust to create the Ancestral Rocky Mountains (see July 6) Especially across the eastern interior of what is now the United States, Pennsylvania south to Alabama and west to Illinois, the uplifts confined tropical lowlands that became the vast rainforests and swamps whose plants ultimately produced the coal for which the region is famous.

The Pennsylvanian equator ran approximately from modern San Francisco to Nova Scotia and into Britain and France, which were attached to the eastern side of Canada. The huge southern continent, Gondwana, extended from its tropical to sub-tropical attachment to North America and Europe all the way to the south pole, and by late Pennsylvanian glaciers were forming there. The episodic advance and retreat of glaciers probably contributed to sea-level changes that produced the alternations between swamps, during low stands of sea level, and incursions of the sea when the water was deeper as the ice melted. This alternation is what produced the rhythmically alternating layers of coal and sediment in Pennsylvanian or Late Carboniferous strata around the world.

—Richard I. Gibson

Map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL

Another Map

July 14. Insects

Based on the abundance of insects in the fossil record and analogies with modern settings, Pennsylvanian coal swamps teemed with insects and spiders. Many were delicately preserved in concretions in fine-grained sediments, like those at Mazon Creek we mentioned a few days ago. Even details such as wings and their veins are often preserved. The warm, moist climate in at least the tropical and sub-tropical Pennsylvanian world might have stimulated the evolution of insects, or it might have been more a co-evolution with the plants that dominated those swamps.  

Did the presence of insects as food promote the development of land life? Or did the pressure from evolving land predators stimulate the diversification of insects? Most likely, it was a combination of both and more. There is actually quite a gap in the fossil record between the first insects in the Devonian and the proliferation of insects in the Pennsylvanian, so their evolutionary history is not all that well known. Insects had evolved the ability to fly by Pennsylvanian time, but the details of that evolution are obscure.

Meganeura - Dragonfly photo by Hcrepin via Wikipedia under GDFL.

Pennsylvanian swamps had cockroaches four inches long, and the air buzzed with relatives of dragonflies with wingspreads of 25 inches or more. Gerarus was a bug with a wingspan of 10 centimeters – 4 inches – and a body covered with spikes.

It’s not certain why Pennsylvanian insects grew so large. A common idea is that there was more oxygen in the Pennsylvanian atmosphere, giving both more energy for growth and more lift to winged insects. There was more oxygen – possibly as much as 35% vs. today’s 21%, probably largely because of all the plants taking up carbon dioxide and giving off oxygen. But one problem with that is that gigantic insects survived and even became larger into the Permian, when oxygen levels were considerably lower than in the Pennsylvanian. It might have been that large insects were better able to manage oxygen usage than small ones.  It has also been suggested that the gigantism was a result of evolutionary pressure, an arms race, among competitors for food resources.

* * *

Florence Bascomb was born July 14, 1862, in Williamstown, Massachusetts. She was educated at the University of Wisconsin and Johns Hopkins, where she earned her PhD in geology in 1893 – the second American woman to do so. She was the first woman hired as a professional by the U.S. Geological Survey, and she also established the geology department at Bryn Mawr College. Her research focused on the igneous and metamorphic rocks of the mid-Atlantic states, including both seminal field work and state-of-the-art petrographic analysis.

Today is also Woody Guthrie’s birthday.

—Richard I. Gibson

Links:
Insect fossils in Kansas 
Gerarus
Oxygen and giant insects 
Oxygen and giant insects – another view 

Dragonfly photo by Hcrepin via Wikipedia under GDFL.

July 13. Pennsylvanian scale trees

Scale trees – named for the scale-like symmetrical arrangement of close-spaced leaf scars on the trunks – were common and abundant in the forests of the Pennsylvanian Period. The classic example is lepidodendron – its name means “scale tree” – which grew to over 30 meters or 100 feet tall. Some trunks were a meter in diameter.  

Lepidodendron

Lepidodendrons are related to modern club mosses and quillworts and probably evolved from the earliest vascular plants including the Silurian Cooksonia that we talked about April 25. Together with many other plants large and small, lepidodendrons formed world-wide rain forests during the Pennsylvanian, and ultimately contributed to much of the coal that formed when they died. Lepidodendron fossils are common in coal beds.


They didn’t have bark like modern trees but rather had photosynthetic skin covered with leaves and pores. The skin was tough, like bark, and protected the internal structures of the plants that delivered water and nutrients to the plant’s extremities. The leaves fell off as the tree grew, leaving a tall pole-like plant with branching fronds only at the top. They formed huge stands in some places, with hundreds or even thousands of individuals per acre.

Sigillaria (public domain)

Sigillaria was a related tree-like plant common in the Pennsylvanian coal swamps. Unlike lepidodendrons, sigillaria sometimes grew into branched trees. Otherwise, the main difference between them in fossils is the geometry of the leaf scars. In lepidodendron, the leaves spiraled around the trunk, while sigillaria leaves were arranged in vertical rows up the trunk. Sigillaria fossils look a lot like tire treads.

Both lepidodendron and sigillaria reproduced with spores that were encapsulated, but were not true seeds. The spore capsules were carried in cone-like structures at the ends of the leafy branches at the top of the plants.

Both these plants declined precipitously in late Pennsylvania time when the rainforest ecosystem collapsed, and both were extinct by sometime in the Permian.

—Richard I. Gibson

Lepidodendron photograph taken by Mark A. Wilson (Department of Geology, The College of Wooster) public domain.

July 12. Proto-reptiles and proto-mammals

A couple days ago I mentioned the first reptile, a fossil found in a fossil tree in the Joggins formation of Nova Scotia. Before we get into that a little more, let’s define how a reptile differs from an amphibian. Amphibians, as you know, have their feet in both the world of water and the world of land; the name itself means “life of both kinds.” While some amphibians are mostly aquatic and some live largely terrestrial lives, they must return to water to breed. 

Reptiles have broken the tie to the water by evolving a way of containing the water world within a package kept on land – the reptilian egg. The word “reptile” is from Latin meaning “creeping.”

Reptiles evolved from amphibians that had some characteristics of reptiles, including strong legs and skins that resisted the loss of moisture. The oldest unquestionable reptile is an animal named Hylonomus, whose name means “forest dweller” or “forest mouse.” The first fossils were found at Joggins, Nova Scotia, with the animals found within fossilized tree stumps, where they had presumably gotten trapped and died.

They were small, maybe 20 centimeters or 8 inches long, and looked a lot like modern lizards. They lived about 312 to 315 million years ago, during the early part of the Pennsylvanian Period. It had a solid skull without the openings typical of amphibians and impressions and footprints that are probably from Hylonomus show it had protective scales rather than smooth, water-permeable skins. It was probably an insectivore.

Hylonomus - Drawing by Nobu Tamura (http://spinops.blogspot.com) – via Wikipedia under GFDL

The earliest synapsids, once considered to be “mammal-like reptiles,” also developed at about the same time as Hylonomus. They are now considered to be the ancestors of mammals, and it’s thought that reptiles on one hand and mammals on the other evolved at about the same time from an ancestor that laid its eggs on land. The first synapsids may have developed as long ago as 324 million years, putting them into the very late Mississippian Period, but they began to have more mammal-like adaptations in late Pennsylvanian time, about 306 to 312 million years ago, pretty much the same time the first true reptiles were evolving. So rather than thinking it was a smooth progression from amphibians to reptiles to mammals, as I was taught, it seems that the current thinking would have amphibians developing into animals that could lay eggs on land, and those animals fairly quickly branched into the varieties that would lead, respectively, to both reptiles and mammals. The earliest amniotes, animals that lay eggs on land, may be as old as 340 million years, middle Mississippian time. That interpretation is based on a single poor specimen called Casineria from Scotland, but it may represent an animal that was ancestral to all reptiles, mammals, and birds.

Conditions during the Pennsylvanian clearly must have favored the development of land life. The diversification of amphibians into both reptiles and mammals may have been fostered by the abundance of plants in the extensive swamps, a situation that would have created innumerable niches for both small and large animals to thrive. And there were also lots of bugs for those insect-eating early reptiles and mammals to eat.

—Richard I. Gibson

Drawing by Nobu Tamura (http://spinops.blogspot.com) – via Wikipedia under GFDL

July 11. Mazon Creek

Pennsylvanian shrimp Acanthotelson stimpsoni,
from Dugger Formation, Indiana.

Another remarkable Pennsylvanian fossil assemblage is found at Mazon Creek, Illinois. It’s a lagerstatte, one of those natural collections that’s remarkable in its state of preservation. The Pennsylvanian rock unit that these fossils are found in is a shale – but the fossils aren’t in shale but rather are in concretions within the shale. The concretions are called ironstone, oval to circular concentrations of iron in the form of siderite, iron carbonate that seem to have preferentially focused on the fossils, encasing and entombing them. Possibly the bacteria that began to decompose plant and animal parts generated carbon dioxide which combined with iron – which must have been anomalously high in concentration in the water to make iron carbonate. The concretion is still mostly shale or siltstone, but the iron dissolved from the rock makes the concretion enclosing a fossil harder than the general rock, and they weather out nicely. Collectors split the concretions to reveal plant and animal fossils in wild diversity. 

The overall setting for the Mazon Creek rocks was probably a large river delta, with sediment and iron eroding off the uplifting Allegheny and Appalachian Mountains to the east. It was a tropical environment.

Over 400 species of plants and more than 300 species of animals have been found in Mazon Creek concretions. Leaves such as parts of fern fronds are common, as well as seeds and cones from plants. The animals include jellyfish, worms, shrimp, snails, and fish, plus centipedes, insects, spiders, and beetles – the oldest known beetle is from Mazon Creek, and it was described in 2009.

Tully Monster, collection of Mike Hamilton, photo by Steve Henderson

One interesting animal, found only at Mazon Creek, is called the Tully Monster. It was an invertebrate with no hard parts that ranged in size from about 8 to 35 centimeters, or three to 14 inches. It had a long proboscis with teeth at the end, with which it presumably fed on small animals or debris in muddy water, and a linear bar with possible eyes on each end. We flat-out do not know what the Tully Monster is. We don’t know its phylum or its affinities, beyond a suggestion that it might be some variation on some of the worm-like themes seen in the Burgess Shale of Cambrian age.

—Richard I. Gibson

Oldest beetle
Tully Monster 

Pennsylvanian shrimp Acanthotelson stimpsoni, from Dugger Formation, Indiana. Collected by Richard Gibson. Photo by Steve Henderson. One half of this fossil is in Montana, the other half is in Georgia. Both photos used by permission from Steve Henderson.

July 10. Joggins Formation, Nova Scotia

With the collision of Gondwana and the combined North America-Europe, a pretty significant mountain range was formed. The deformation was mostly the Alleghenian Orogeny in eastern North America and the Variscan in southern Europe, and while it made some new mountains it probably rejuvenated some old mountains as well, including parts of the Caledonian mountain belt between Maritime Canada – Nova Scotia and Newfoundland – and Greenland on one side and the British Isles and Scandinavia on the other.  

Within the mountain belts, quite a variety of environments formed, including extensive swampy basins in which abundant plant life lived, and died to make the coal that gives the Carboniferous its name. One such swampy basin was in what is now western Nova Scotia, where a low-lying area received sediments that became the Joggins Formation. 

Photo of upright Pennsylvanian tree fossil in
Joggins Formation by Michael C. Rygel.

The tectonic deformation that was associated with the continental collisions wasn’t all just a big crunch, a big squeeze. In places, because of the angle of collision, actual extension could occur, leading to down-dropped areas that became basins. In some places including parts of what is now Nova Scotia, older salt deposits might have been dissolved and flowed so that there was very rapid subsidence and infill of sediments. This is one explanation, suggested by geologists John Waldron and Michael Rygel, for the many upright tree fossils that are found in the Joggins Formation. The trees could have been buried during life by incursions of sediment. The sediment was brought from the Caledonian highlands by complex river systems, resulting in the alternating coal swamps and the sands and silts that buried them, pretty much the standard example of Pennsylvanian coal deposits.

The package of rocks is one of the best examples of Pennsylvanian coal strata in the world. At least Charles Lyell thought so. Lyell was the author of the books entitled the Principles of Geology, which arguably made geology into a modern science, and Lyell wrote that these were the finest examples of coal-age rocks on earth.

The upright fossil trees at Joggins contain animal fossils as well, including the oldest known reptile. We’ll talk more about them in a couple days.

The fossiliferous cliffs at Joggins are a World Heritage Site, designated in 2008.

—Richard I. Gibson

References:
Joggins Formation 
Alluvial sedimentology and basin analysis of Carboniferous strata near Joggins, Nova Scotia, Atlantic Canada, by Michael C. Rygel, 2005
Five more articles
Historical perspective on the Joggins cliffs geology 
Jogins Fossil Cliffs

Photo of upright Pennsylvanian tree fossil in Joggins Formation by Michael C. Rygel.

July 9. Labyrinthodonts

Labyrinthodont tooth cross-section

Labyrinthodonts’ name means maze-tooth for the incredibly convoluted infoldings of their tooth enamel. They were tetrapods, which had evolved from the lobe-finned fishes, and while they began during the Devonian, it appears that they evolved rapidly during the Mississippian and Pennsylvanian. During the Pennsylvanian, they developed better and better adaptations to living on land.  

Broadly speaking labyrinthodonts were amphibians, which return to water to reproduce. But their subgroups include variations that were more fish-like and perhaps entirely aquatic, as well as some that are more reptile-like, with stronger legs, branched toes, thick skulls, and maybe even skin that helped retain water during excursions on land.

Labyrinthodonts came in many sizes, ranging from salamander-like animals a few centimeters long to 9-meter monsters similar in size and shape to crocodiles, but they were still amphibians. Most of them were carnivorous, attacking prey in the shallows and swampy areas that were abundant during Pennsylvanian time. They reached their peak of diversity in Pennsylvanian and Permian time, especially following the collapse of the Pennsylvanian rainforest ecosystem which we’ll talk about later this month. But the dry climate that developed during the Permian was unfavorable to amphibians, and most groups of labyrinthodonts declined before the end-Permian extinction event. The few survivors made it all the way to the Cretaceous before all labyrinthodonts became extinct.

Labyrinthodonts are the ancestors of modern amphibians, but it’s definitely not clear exactly how the descent worked. There is controversy as to which of the sub-groups of labyrinthodonts gave rise to modern amphibians such as frogs, toads, and salamanders. It’s not even completely agreed whether modern amphibians derive from one lineage of labyrinthodont or two. The fossil record is sparse for these animals during the Permian and Triassic, when the change to modern forms was taking place.

* * *

On this day, July 9, 1812, Mammoth Cave in Kentucky was sold three times for its saltpeter resources. Saltpeter is potassium nitrate and forms in caves by alteration of bat guano. Saltpeter is one of the important components of gunpowder, and the sales of the cave were a response to the start of the War of 1812. The value of the saltpeter is reflected in the three selling prices that day: $116.67, $400, and finally $3,000 late that afternoon.

—Richard I. Gibson

Drawing (public domain) from Wikipedia