March 31. End of the Ordovician mass extinction

It’s the end of March, and the end of the Ordovician Period. If you’ve been following along, you know that this time, 443 million years ago, is marked by a huge mass extinction – by some estimates, second only to the Great Dying at the end of the Permian in terms of its decimation of life on earth.

More than 100 families of marine animals went extinct. Families are groups of genera which are groups of species – so this was a LOT of diverse fauna. More than half the species of bryozoans, which had just appeared early in the Ordovician, were destroyed. Trilobites, brachiopods, and nautiloids were severely impacted. Something like 60% of all marine species died.

Ordovician life

As near as we can tell, it seems that the extinction event was worse among animals that lived in shallow tropical seas, which supports the idea that the glaciation going on at the same time was an important factor. Cooler conditions, cooler water, fewer tropical animals. Also, when water is tied up in ice on land, sea levels drop, so the shallow seas that predominated over much of North America retreated. There were many fewer niches for life to occupy. And in fact there are two phases to the extinction, which seem to be related to these two different but related causes.

We talked about some of the factors that may have led to the glacial period. The position of the supercontinent of Gondwana over the south pole, the possibility that life on land was removing CO2 from the atmosphere, reducing greenhouse conditions, and the cooling effect of some of the greatest volcanic episodes known in earth history.

There isn’t much doubt that whatever caused the glaciation, the consequences for life were pretty dire, and generally, it’s the cooler conditions and lower sea level that probably did the killing of marine life. All of the phyla of life survived and came back after the glacial period, in the Silurian that starts tomorrow.

I think it’s important to note that this is the only glacial period that is closely connected with a mass extinction, unless the modern glacial period has been a factor in the fairly recent destruction of a great many species. The jury is still out on whether or not it was climate change or human activity that eliminated things like mastodons, giant sloths, and saber-toothed tigers.

The end Ordovician glacial epoch was relatively short-lived, spanning just a few million years, and the extinction event at 443.4 million years ago is very much a sharp spike in terms of species loss. I think it’s fairly well accepted that cooling and loss of shallow-water niches were the ultimate primary causes of the extinction, and those things were caused by the glaciation. There are lots of potential factors that might have caused the glaciation, as we discussed March 28.

A recent look at gamma-ray bursts in the Milky Way Galaxy suggests the possibility that such an event might have been a factor in at least one extinction in the past 500 or so million years, so it is a possible cause for the Ordovician extinction. 

Tomorrow, the Silurian begins.

* * *

March 31, 1850, is the birthday of Charles Doolittle Walcott, in New York Mills, New York. Walcott was the geologist who discovered and described the Burgess Shale in British Columbia back in 1909. You can hear more about Walcott in the podcast for February 12.

—Richard I. Gibson

Photo by Ryan Somma under Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)  

March 30. Stonehenge

When oceans close, and force island arcs, oceanic floor, and what ever else is out there to collide with continents, as happened in the Taconic Orogeny, there’s usually enough subduction or partial subduction that some materials reach temperatures and pressures at which they melt.

The molten rock generated by tectonic activity is what makes the volcanoes in a volcanic island arc – which might not be a string of islands; the Andes is a fine example of a volcanic arc related to subduction that is not islands.

Remember that we can have magma flowing on the surface as volcanic lava flows and ash falls, but we can also have molten material down inside the earth, never reaching the surface. We called such rocks plutonic the other day, because they form at depth, down in the realm of Pluto. They can also be injected through and between pre-existing rocks, cutting across them. Then, they’re called intrusive – a fairly straightforward word meaning that the magma, the molten material, is forced into other, solid rocks. Think of the neck of a volcano, or better, the deeper roots where magma is pushing here and there, wherever it can find a weak zone to send a narrow, vein-like shoot through. Those weak zones might be bedding planes in a sedimentary rock, and intrusive igneous rocks that are parallel to bedding in sedimentary rocks make a structure called a sill. If the igneous rocks cut across the older rocks, the feature is called a dike.

Dikes and sills were forming at depth during the Taconic Orogeny, on both sides of the closing ocean. On the side where parts of Great Britain were found, the microcontinent of Avalonia, the same thing was happening.

Some of the intrusive rocks there formed dikes made of dolerite. That’s the British term; Americans call it diabase, and it’s a fine-grained version of gabbro – or a coarse-grained version of basalt, depending on how you look at it. It’s the intermediate variety of those rocks that are relatively low in silica, as compared to silica-rich granite, and relatively rich in iron and magnesium.

After they were formed in the Ordovician mountain-building event about 450 million years ago, the dolerites of Wales sat there – tossed around by later tectonic activity, but still there, until erosion exposed them. Today, they crop out in the Preseli Hills of Wales, a bluish rock. By using careful geochemical analysis, those outcrops have now been identified as the source of the bluestones at Stonehenge, 250 miles away. The bluestones at Stonehenge are not all dolerite, and some 20 different rock types have been identified, but all are from the Ordovician outcrops in Wales. Some are volcanic equivalents of the dolerites, and the location they came from was not pinpointed until 2011. They’re from the same general area of the Preseli Hills in Pembrokeshire, in far southwestern Wales.

It isn’t clear whether the stones were transported from Pembrokeshire to the Salisbury Plain by the builders of Stonehenge, or by glacial movements thousands of years earlier.

—Richard I. Gibson

Photo by Richard Gibson

References and further reading:

The Earth Story’s nice essay with good links. 

Another piece in Stonehenge rock source puzzle 

Stonehenge mystery

March 29. Taconic Orogeny

The ends of the periods of geologic time tend to be marked by major events – extinctions, glaciations, and mountain-building, at least in Britain and the United States, where most of the big packages of geologic time were first described. The Ordovician is no exception.

The Taconic Orogeny is the mountain-building event – that’s what orogeny means – that began toward the end of the Ordovician Period in eastern North America. This was the first significant pulse in the construction of the Appalachian Mountain system, a process that took many millions of years.


The Taconic Mountains today lie along the eastern border of New York, and in western Vermont, Massachusetts, and Connecticut, but the Taconic Orogeny really extends south into Georgia. The collision that caused the mountain uplift involved a complex mess of masses, including some small continental blocks, an extensive island arc system, maybe some slices of oceanic crust, and the sediments that were among all those pieces – but most simplistically, it was probably a volcanic island arc. I’ve used the analogy of the modern western Pacific, and I think that’s a reasonable view. Something like Kamchatka-Japan-Taiwan-Philippines or Indonesia, slamming into a rigid North American continental block, over a period of several million years. And not at the same time, and not in the same way everywhere along its length.

There’s evidence for the beginning of the convergence even in Cambrian and early Ordovician times, in the types of sedimentary rocks we find, but the effects really started to be felt in late Ordovician time. The culmination of the Taconic Orogeny, the uplift of a significant mountain range along the east side of North America, was near the end of the Ordovician, something like 455 to 445 million years ago. The evidence is in the modern Taconic Mountains and in the subsurface elsewhere, where we see the deeply eroded roots of that Ordovician mountain range.

The Queenston Delta, which we discussed on March 21, was one of the most significant consequences of the Taconic Orogeny, at least in North America.

Ebenezer Emmons, a geologist in New York in the 1840s, defined the Taconic Orogeny. He had already described the Potsdam Sandstone and other specific elements of early Paleozoic geology. Emmons ascribed his Taconic System of rocks to the Cambrian – a view vehemently opposed by James Hall, the State Geologist of New York. Hall felt that the rocks belonged to the Silurian. This bitter controversy – called the greatest in American geology – was analogous to the debate going on in Britain, between supporters of Sedgwick and Murchison over the same part of the geologic section. In New York, Emmons was banned from practicing geology; he sued Hall for slander and libel, but lost, and moved to North Carolina. In 1888, 25 years after Emmons’ death, he was ultimately vindicated, as the new Ordovician Period was defined, putting Emmons’ rocks into the Cambrian as he had originally claimed. But it wasn’t until 1903 that the U.S. Geological Survey formally accepted the Ordovician Period.

The present-day Taconic Mountains are mountainous not because of the Taconic Orogeny. The Ordovician mountains were long since eroded to low hills, or less. Later events have pushed the rocks there back up. Building the Appalachians was a long process which we will touch on for many months – many million years – to come.

—Richard I. Gibson

Callan Bentley’s outstanding animation

Another useful link from Paleontological Research Association


Emmons image – public domain
Cross sections from USGS – public domain

March 28. Late Ordovician Glaciation

I’ve mentioned the late Ordovician glaciation several times in recent posts, suggesting things that might have been factors causing it. One possible factor is the fact that the largest continent, Gondwana, which included most of Africa, South America, India, Australia, and Antarctica, was located over the south pole. That alone wouldn’t do it unless the climate was cold enough.

Upsala Glacier, Argentina

During most of the Ordovician, planet Earth was in greenhouse conditions, with high concentrations of carbon dioxide in the atmosphere. We mentioned two things that might have affected that – the presence of life on land, taking more and more CO2 out of the atmosphere, and the tectonic activity that made a high mountain range in what is now eastern North America. Erosion of that mountain range, to create the immense Queenston Delta, might have produced enough sediment to have an effect on the atmosphere so that CO2 was reduced. Both of those ideas are in the “might have” category, but something certainly did affect CO2 concentrations.

And then we talked about the humongous volcanic eruptions, the Deicke and Millbrig and others near the beginning of the Late Ordovician epoch, around 455 to 457 million years ago. Cooling because of high concentrations of dust in the atmosphere was certainly likely, and the timing is good with respect to the glaciation.

The glaciation seems to have been at its height about 440-455 million years ago, which includes the first part of the Silurian – the boundary with the Ordovician is put at 443 million years ago. There’s some evidence that the glaciation might have started as long ago as 460 million years.

The evidence about CO2 values isn’t speculative. There is abundant evidence in carbon isotopes to indicate significant changes in water temperature – a pretty significant disruption of the carbon cycle, enough to imply cooling and a reduction of the greenhouse effect. In addition to the ideas that life and erosion helped remove CO2 from the atmosphere, another idea is that volcanism helped. But wait, you say – I thought volcanoes added CO2 to the atmosphere. Well, they do. But earlier in the Ordovician, extensive basaltic eruptions – when they were finished – created large expanses of solid basalt, which reacts and erodes relatively quickly, and might have been a factor in CO2 removal. That’s another pretty speculative possibility.

Glacially-deposited rocks of Late Ordovician age are common across what is now the Sahara Desert, from Morocco to Ghana to Libya, and they’ve also been found in South Africa, Brazil, Arabia, Germany, Nova Scotia, and Newfoundland. The ages of all those deposits are not absolutely determined, but they are all of Late Ordovician or Early Silurian age, coinciding nicely with and defining the glacial period.

There’s evidence that this glacial event ended quite abruptly, for reasons that seem to me highly speculative and hardly able to explain it. One suggestion is that once the ice sheets reached their limit, they essentially collapsed – which to me doesn’t adequately explain why they then retreated to practically nothing. CO2 levels increased in the Silurian, but I haven’t seen a good explanation for why.

The Ordovician-Silurian glaciation is the only one associated with a mass extinction event – one of the largest in earth’s history. We’ll talk more about that at the end of the Ordovician, in a few days.

—Richard I. Gibson

Further reading:
The carbon cycle
Photo by longhorndave via Wikipedia, under cc-by-2.0

March 27. Kukersite

Yesterday we talked about some common rock types, granite and basalt and some others. Today, let’s talk about a very special rock type, but one that’s specific to the Ordovician.

Kukersite is oil shale – one of the richest oil shales in the world, with as much as 40% organic material in it. It’s found in Estonia.

First, let’s distinguish between oil shale and shale oil. Shale is a very fine grained rock, solidified from mud. Oil shale is a rock – not liquid – in which a lot of organic material, called kerogen, has been incorporated with the sediment. Shale oil, like the famous Bakken in North Dakota, is entirely different – it’s liquid oil, trapped in very tiny, very poorly interconnected spaces, in contrast to a good oil reservoir like a sandstone with lots of open pore space and plenty of interconnectedness to allow the oil to flow. Some of the Bakken isn’t even in shale, but it in dolomite. The main difference is that oil shale is a rock that does not contain ANY liquid oil. To get the organic matter out, you have to mine the rock, then cook it. The organic stuff might burn directly, like it does in peat or coal, or you can use a distillation system to convert the solid kerogen to liquid oil. See also this post.

The geology of the Ordovician kukersite in Estonia suggests that there were episodic periods when the waters there were anoxic – lacking in oxygen. The alternation with periods of more oxygenated water resulted in maybe 50 separate beds of kukersite, separated from each other by layers of limestone and other rock. The individual kukersite layers are typically around 20-30 centimeters thick, but some are a couple meters thick.

The setting was probably something like a low, flat tidal zone along the sea coast that was periodically cut off from circulation, so that stagnant, swampy conditions developed. What was in those swamps? This was the Ordovician, so don’t visualize the Okeefenokee Swamp, with big trees, lily pads, and such. The life that put its organic matter into these sediments was algae. In fact, algae and other microbial plants are the greatest contributors to the source rocks that make all oil and natural gas. The volume of dead animals was usually far too small to add significantly to the stuff that would become oil – so forget that attractive idea of dinosaurs in your gas tank. It was algae.

The Estonian kukersite – a rock that burns – was known as long ago as 1700, or even earlier, but its commercial development began in 1918 in the face of fuel shortages resulting from World War I and the Russian Revolution. Production has continued pretty much up to this day, with both open-pit mines and underground mines. It’s an important energy resource for Estonia. In 2011, Estonia manufactured about 11,000 barrels of oil per day from its oil shale – a bit less than half of its needs. 90% of Estonia’s electricity comes from plants fueled with oil derived from oil shale.

It’s important as a local resource, but producing oil from oil shale is expensive, given that you have to mine it, like you mine coal, then you have to input a lot of energy to heat it up to distill the oil out of it. We know how to do this technologically; the issue is the cost. Building a high-volume oil shale reduction plant might cost something comparable to an oil refinery – maybe several billion dollars or more, with many, many years before you get a real return on your investment. Conventional crude oil’s energy return on investment is 20 to 40 to one, while oil shale’s return is more like 2 to 1. You don’t make much money, and it takes a lot longer to make it. For an oil shale plant to be economic, I’ve read figures all over the map for a sustained price of oil, from $80 per barrel to $150 per barrel. I think something like a cost to produce of $90 to $100 per barrel for oil shale is reasonable. If that’s the production cost, then you have to add taxes, royalties, and so on, plus a reasonable 10% profit margin – and you end up with oil from oil shale being worth it when the price of oil is something like $130 per barrel.

So, yes, there is a LOT of oil tied up in oil shale – estimates of as much as a trillion barrels are common.  But it will be expensive to produce it.

* * *

March 27 is the anniversary of the earthquake in Alaska in 1964 that killed 139 people. Here's a link to a couple good videos.

—Richard I. Gibson

Map from USGS

Further reading:

http://www.kirj.ee/public/oilshale/3_lille_2003_3.pdf
http://pubs.usgs.gov/sir/2005/5294/pdf/sir5294_508.pdf
http://www.kirj.ee/public/oilshale/oil-2006-3-2.pdf

March 26. Granite versus basalt

Today, we take a break from the chronology of earth history to talk a little about rocks types: igneous, derived from molten magma, sedimentary, deposited as tiny broken fragments of other rocks, as sand, silt, or mud, or precipitated chemically, like limestone, and metamorphic rocks, which are the changed forms derived from anything else that’s been subjected to high temperatures or pressures or both.

Let’s expand a little on igneous rocks. The word is from Latin, ignis, meaning fire, and it’s the same root that gives us words like ignition. These rocks come from fire, from volcanoes, but there’s more to it than that.


When magma – molten rock – flows out on the surface of the earth as lava, it cools quickly. “Quickly” is relative, of course – in geological terms that could mean a million years, but when we talk about cooling of rocks, I’d day that generally means anything from hours to years, maybe even hundreds of years. When magma cools quickly, the stuff, the minerals, that are crystallizing from the magma don’t have much time to grow, so they are usually pretty small. Sometimes molten rock can cool so quickly that it solidifies into a glass – obsidian – where the crystals are so tiny that they are invisible. In fact, in natural glass, obsidian, it isn’t really crystalline at all. The stuff just solidified, but didn’t crystallize. Give it a little more time, say months or more, and tiny crystals can grow, like salt forming around a pan of evaporating salty water. They might still be microscopic, but they’re there, as crystals. Give it a long, long time – say, a few hundred thousand years – and the crystals have time to grow to be pretty big, visible to the naked eye, or maybe even a few inches long. How can it take a hundred thousand years for molten rock to solidify? Well, generally, it would have to be pretty well insulated to retain heat that long, so we’re talking about magma that is NOT erupted onto the surface of the earth, where it cools quickly, but magma that’s down inside the earth – maybe miles down, where it cools very very slowly.

Small crystals, quick cooling. Big crystals, long time cooling. The size of the crystals is part of the texture of the rock, one of the most important things in giving it a name and figuring out its story.

granite

The other important thing about igneous rocks is their composition. You can mix up the chemicals in hundreds of different ways, depending on the abundance of the elements in the mix. But only a relatively few common minerals will form from whatever elements are present. So, if you have a lot of silicon in the melt, you’re probably going to get a lot of quartz, silicon dioxide, in the solidified rock. Quartz is the most common mineral in the earth’s crust, but there are rocks that have none. They’d often be rocks that solidified from magma with more iron and magnesium, but don’t visualize a simple one-to-one relationship between silica and iron. There’s way too much variety for that.

But, as a first pass, we can think of two common igneous rock types – granite and basalt. Granite is silica rich, and it’s made of a lot of quartz and feldspar, another silica rich mineral that includes aluminum and other things. Basalt is silica poor, and contains more iron-rich minerals than granite. Granite is coarse-grained, telling us it formed down inside the earth and cooled over a long period of time. Basalt is fine-grained, telling us it cooled relatively quickly.

basalt

But you can have the same composition as granite that cools quickly, such as in lava flows. That rock, fine-grained but silica rich like granite, is called rhyolite. And likewise, you can have magma of basaltic composition that cools over a long time within the earth. We call that rock gabbro.

The two other terms I’m sure I’ll use in these presentations are volcanic – you know what that means – and plutonic, which means the rock formed inside the earth, in the realm of Pluto, so it took a long time to cool. Both granite and gabbro are plutonic, usually.

I think that’s enough jargon for today! Just remember that molten rock has various compositions, and cools to either fine-grained or coarse-grained rocks depending on how long it took to solidify.

—Richard I. Gibson

Granite photo by Friman, via Wikipedia, under GNU free documentation license.

Basalt photo by USGS (public domain) 

March 25. Trenton oil and gas field

In 1883 pioneers of oil and gas exploration drilled more than 1,000 feet – a deep well in those days – to reach the Middle Ordovician Trenton limestone in northeastern Ohio, encountering a significant flow of natural gas. Within a few years, an extensive area of Trenton oil and gas production had been established in northwestern Ohio and eastern Indiana. It was one of the largest accumulations discovered in the United States before 1900.

Most of the Ordovician production in this area has now been depleted, but in 1887 a group of small companies, threatened by John D. Rockefeller’s Standard Oil, based in Cleveland, joined forces in Findlay, Ohio, to form the Ohio Oil Company, known today as Marathon.

Just two years after its founding, the Ohio Oil Company was gobbled up by Standard, in 1889, and remained part of the Standard Oil Trust until the breakup of Standard Oil in 1911. In 1930, the Ohio Oil Company acquired a small marketer, Transcontinental Oil Company, and also acquired Transcontinental’s brand, Marathon, which ultimately became the trademark of the original company. US Steel bought Marathon in 1982, but the combined company spun off the steel business in 2001 and now it’s Marathon Oil Company again, an important multinational oil company.

The middle Ordovician limestones of Ohio and Indiana that held the hydrocarbons, oil and natural gas, that made Marathon, are part of the quiescent, warm, shallow-water deposition that dominated Ordovician times in what is now North America – at least until the action started in late Ordovician time, over in what is now the Appalachian Mountains.

Production from the Trenton Oil and Gas Field peaked long ago, about 1902 to 1905. The estimated total production over time is a trillion cubic feet of natural gas and 105 million barrels of oil. For context, while that was a huge boom back in its day, 105 million barrels of oil is just five and a half days worth of US oil consumption today. Winter natural gas consumption in the United States is about 3 trillion cubic feet per month, so the Trenton Field would have provided three months’ supply at today’s rates.

By no means was all of the oil and gas removed – it never is – and newer technology and higher prices have driven some rejuvenation of production in this area today. But the production is small, and expensive compared to that of the late 19th century.

An interesting side light to the natural gas boom in Indiana in the 1880s was the development of a flourishing stained glass industry around Kokomo. The Opalescent Glass Company was established there in 1888 because of the abundant natural gas in the area, which served as a ready fuel for the glass kilns. They’re still in business today.

—Richard I. Gibson

Photo of gas flares from Trenton Field, Jan, 18, 1889, Leslie’s Illustrated Magazine (public domain)

Oil & gas statistics from EIA  

March 24. Ordovician explosive volcanism

Across much of the eastern United States, from Minnesota to Georgia to New York, there are several thick layers in the Ordovician rocks that are bentonite. Bentonite – specifically, potassium bentonite – is a rock that’s the altered form of a volcanic ash fall. Such things are really pretty common in the rock record, given that there have been probably hundreds of thousands of volcanic eruptions over geologic time. What makes the Deicke bentonite – pronounced "dickie" – special is that in lots of places it’s around a meter thick. Volcanic ash does tend to erode easily, and it also compresses – so to have a meter-thick zone after 450 million years is remarkable, unless it was right next to the volcanic vent. So that fact that we have this kind of thickness spread out over thousands of square miles makes it doubly remarkable.

Mt. Pinatubo's 1991 eruption was vastly smaller
than the Ordovician eruptions discussed here.

Given all the tectonic events that have happened since, it won’t surprise you to hear that this is NOT one continuous sheet of bentonite today. It’s broken up, tilted, faulted, folded, eroded. So it took a lot of careful study, including painstaking geochemical work, to figure out that it really was all one sheet. One BIG sheet of volcanic ash.

There are actually two major and several minor bentonites close to each other in the Upper Ordovician, and as many as 16 others not to far away. The second-largest one is called the Millbrig. And if you need even more amazement, the probable equivalents of these layers are found in Europe as well, in England, Scandinavia, and Russia.

Together, they probably represent two of the largest – if not the largest – volcanic eruptions in at least the past 600 million years and probably quite a bit longer. The nature of the rocks, and their chemistry, suggests that it really was one or two events – erupted in a time span of days or weeks or months. That’s essentially instantaneous, geologically speaking. They’ve been estimated at volumes of 5,000 times the ash that came out of Mt. St. Helens in 1980 – or more.

The possible volcanic island arc discussed in the text is not shown on this map. It would lie between Laurentia (the core of North America) and Avalonia, which includes terranes that today are in New England, maritime Canada, Newfoundland, and Great Britain. Avalonia is also a possible source for the Ordovician volcanism.

Where did they come from? No one is certain. Since they thicken across the United States to the southeast, it’s likely that the source, the volcano, was somewhere off the coast of what is now Georgia. Back in the Ordovician, that was the volcanic island arc that was just about to collide with North America to start the Taconic Orogeny. Beyond that was another complex terrane that we mentioned a few days ago – Avalonia, which included bits and pieces that are now attached to North America in New England, Nova Scotia, and Newfoundland, as well as in Great Britain and Ireland. Maybe the source was in that terrane. Last week I compared Avalonia to the western Pacific – Kamchatka, Japan, the Philippines. Plenty of big-time volcanoes there. Or you could think of it as similar to Indonesia – Sumatra, Java, and Borneo, which include both continental fragments as well as major volcanic zones. Sumatra has the remnants of a volcano at Toba, which exploded 75,000 years ago and has been suggested to have reduced global human populations to a few thousand. Then there’s Krakatau, whose explosion in 1883 was heard 3,000 miles away, and which affected sunsets around the world for years. And Indonesia also harbors Tambora. Its eruption in 1815 caused the famous “Year Without a Summer,” when it snowed in Washington, D.C., in June, and made the weather in Europe so miserable that a depressed Mary Shelly wrote her most famous novel, Frankenstein.

There’s some reason to think that Avalonia was the host of the Deicke and Millbrig volcanoes. In England’s Lake District, the Borrowdale volcanics are lava flows of essentially the same age as the bentonites. That could make them the lava flows that came from the vents that put the ash all over. As it happens, I talked about the Borrowdale volcanics in a completely different context just a few weeks ago, in my first YouTube presentation based on my other book, What Things Are Made Of. The graphite that formed the basis for Europe’s pencil business in the 1700s is found in those rocks. The video is embedded at the bottom of this post.

There’s also another area, in New Brunswick today, that might have been the source of the volcanic eruptions.

It’s almost impossible to imagine the impact the Ordovician Deicke and Millbrig explosions would have had. Life on land would have certainly suffered – but remember, there was hardly any life on land yet, mostly just those moss-like plants we talked about on March 9.   All that ash would have affected global temperatures, and might even have changed water chemistry, which in turn would have affected marine life.

We know the timing of the Deicke event really accurately, because the ash includes zircons, those tough little mineral grains that contain radioactive trace elements which give us ages based on their decay rates. So we know that the Deicke eruption was 457.1 million years ago, plus or minus 1 million years – really accurate for that long ago. This date was reported by Ryan Mathur in 2011 as well as by Samson and others in 1989. The Millbrig bentonite overlies the Deicke in the United States, but it is essentially the same age. They may represent episodes of the same event, but in any case they are almost certainly related to the same overall volcanic system.

If you’ve been going through these blogs and podcasts sequentially, you know that I’ve mentioned a couple of possible causes for the glaciation at the end of the Ordovician, which probably was a major factor in the end Ordovician mass extinction. I doubt if you’ll be surprised that we can now add another possible factor in both the glaciation and the extinction: unprecedented explosive volcanism during the Late Ordovician.

Besides this interesting story, what good is bentonite? In the United States, about a fifth of it is used in muds for oil and gas well drilling. Bentonite is mostly a mixture of clays, which can take up the fluids used in oil exploration, and it helps control underground pressures and strengthens the drill hole wall. Almost half the world’s commercial bentonite production comes from the United States, mostly from Wyoming and Montana where the stuff is much younger, much less consolidated than the Ordovician bentonites of the east. Bentonite is also used as absorbents like kitty litter, and to help pelletize iron ore for smelting. All told, the U.S. uses about a million tons of bentonite every year.


Thanks to Steve Henderson for pointing this topic out to me.

* * *

As if that volcanic story’s not enough, today is also a pretty cool geological birthday – John Wesley Powell, the one-armed veteran of the Civil War who took the first exploring expedition by boat through the Grand Canyon, was born on this day in 1834. He became the second director of the U.S. Geological Survey.

—Richard I. Gibson

Selected references and further reading
Ryan Mathur’s abstract

USGS reference descriptions

Huff et al., 2010, Ordovician Explosive Volcanism, Geological Society of America Special Paper 466.

Samson et al., 1989, Origin and tectonic setting of Ordovician bentonites in North America: Isotopic and age constraints: Geo. Soc. America Bulletin, v. 101, p. 1175.

Did intense volcanism trigger the first Late Ordovician icehouse? Werner Buggisch et al., Geozentrum Nordbayern, Universität Erlangen Nürnberg, Schlossgarten 5, D-91054 Erlangen, Germany. Pages 327-330.

Map by Ron Blakey, via Wikipedia, public domain.

March 23. Appalachian basin

When blocks of the earth’s crust collide, several different things can happen. When good dense oceanic crust impinges on relatively light continental crust, the denser one, oceanic crust, usually goes down under the lighter one. This process is called subduction, and it’s going on all over the world today. The continental crust above isn’t immune to effects – it can get uplifted, depressed, scrunched and broken, and volcanoes can and do pop up through the continental crust. Probably the best example of this today is the Andes Mountains along the west coast of South America, where the oceanic plate underlying the Pacific Ocean is diving down beneath the South American continental plate.


If two relatively low density blocks – two continents, or a continent and something like an island arc – collide, then neither is likely to really descend beneath the other. This is a true head-on collision, and it can make some of the highest mountain ranges. This is what’s happening to day where the Indian continental plate collides with the Eurasian Plate. The Himalayas form.

An obvious consequence of uplifted mountains is erosion – it starts as soon as rocks are above sea level. The Queenston Delta that we talked about the other day is the evidence of that kind of erosion from an uplifting mountain range. Sometimes there is so much erosion that the weight of the sediment is enough to bow down the crust itself, starting a trough-like depression along the mountain front. It can become a self-perpetuating thing, a depression, a basin, into which more and more sediment pours, and all that sediment keeps pushing the crust further and further down…. And so on. The weight of the stuff that’s colliding and being pushed up over the edge of the continent helps, too – all adding up to a physically low area to receive sediments. It’s called a foreland basin, because it’s in the foreland, adjacent to a rising mountain uplift.

That’s what happened in eastern North America, where the Appalachian Mountains are today. We’ll be talking about the Appalachian Mountains for months – millions of years – as various things happen over time to contribute to their formation. But that’s getting started now, toward the end of the Ordovician, as the Taconic Orogeny gets started.

During the Cambrian and early Ordovician, most of eastern North America was pretty stable, accumulating relatively thin uniform packages of sediment over pretty large areas. In the late Ordovician, as collisions began to bow down the crust, and lift up mountains to be eroded, changes began. Packages of sediment thicken noticeably to the east or southeast (see cross section above - the colored part is the Upper Ordovician), closer to the source area. More sediment is dumped close to the mountains than is carried hundreds of miles away from the mountain front, even though those far-flung sediments are part of the process as well. The mountains may have been in New England, extending down to Tennessee and Georgia, but the eroded mud made its way as far northwest as Ohio and beyond.

The foreland basin that began during late Ordovician time is called the Appalachian Basin, and like I said, it will be months before we’ve heard the end of it.

—Richard I. Gibson

See also this excellent SmartFigure by Callan Bentley. One of the best visualizations of the tectonic development of the Appalachians that I've seen.

Cross section from Harris & Milici, 1977, USGS Prof. Paper 1018.
Map from USGS

March 22. Vermont marble

As we approach the end of March, we’re getting later and later in the Ordovician, and the collisions along the east coast of North America are getting more and more intense. We’re building up to the Taconic Orogeny, and yesterday we talked about one of the consequences of that mountain-building event, the eroded sediment that created the Queenston Delta.

Rutland marble quarry

Closer to the action, where blocks of continental crust, piles of volcanic rocks, slices of oceanic crust, and whatever else was out there were actually colliding to raise up those mountains, things got pretty hot and heavy. All those nice tropical shallow seas earlier in the Ordovician made nice limestones. But when you put a limestone under high pressure and add some heat, the calcite crystals in it recrystallize – it’s still calcite, but instead of gently interlocking and usually tiny crystals, they grow and intergrow more tightly, producing a metamorphic rock called marble.

Some of the best marble in the United States was created during the Taconic Orogeny, cooking those nice Ordovician limestones. Especially in Vermont, but also in Tennessee and Georgia, Ordovician marbles have been mined for centuries as building stone and for monuments. The Amphitheater of Arlington National Cemetery is made of Ordovician Vermont marble – one of a great many structures that are. The quarries near Rutland, Vermont, yield white marble, but with plenty of variations in the original limestone, you can get all sorts of colors in marble, from red to black. The oldest marble quarry in Vermont, at Isle la Motte, was opened by Ichabod Fisk in 1664. Vermont quarries also produce verde antique, a metamorphic rock containing green serpentine and similar minerals, used for countertops, tiles, and facades.

* * *

Today, March 22, is the birthday of Adam Sedgwick. You must remember him, the geologist who defined the Cambrian and lost a friend over the details of Cambrian-Silurian stratigraphy. He was born March 22, 1785, in Dent’s Town, Yorkshire, England.

—Richard I. Gibson

Photo by C.W. Nichols, Rutland, VT marble quarry c. 1870. Public domain, via NY Public Library and Wikipedia.

Reference
Vermont Marble Industry

March 21. Queenston delta

Red rocks of the Queenston Formation
in Ontario

You recall from our discussion of the Bighorn Dolomite a few days ago that much of western North America was a tropical shallow sea into the Late Ordovician. To the east, in Illinois, Michigan, Indiana, Ohio, and Kentucky, things were changing because of the onset of a mountain-building event, the Taconic Orogeny, even further east. A string of volcanic islands – an island arc, probably with other sorts of things in it, something like Indonesia today – began to collide with the ancestral core of North America, and a mountain range formed in what is now central New England and points to the north and south.

What happens as soon as you lift up a mountain range? It starts to erode. The stuff eroded off this mountain range was carried by large river systems to the west, especially into what is now eastern Ontario and western New York and Pennsylvania. The sediments were dumped into the shallow sea that was there, forming a huge delta, probably quite a bit larger in area than today’s Mississippi Delta. The pile of sediment is called a clastic wedge, because it contains clastics – broken pieces of rock, worn down to gravel, sand, silt and mud, and it’s a wedge because it’s shaped like a doorstop – thickest toward the mountains, and thinning off to the far west. It’s called the Queenston Delta and the rocks in it are called the Queenston Formation.

Yellow=sandy sediments of Queenston Delta.
Mud across Ohio is part of the system, too.,

The Queenston Formation – it’s called the Juniata Formation and other names in some places –  was laid down over a period of several million years, about 451 to 446 million years ago. In places it’s close to 1,000 feet, 300 meters, thick. A lot of it contains iron oxide cement, making the rock reddish in places, and suggesting the deltaic nature of the sediments – sometimes underwater, sometimes exposed to air.

The Queenston Delta system was so vast, and contained so much sediment, that it may have contributed to a reduction in atmospheric CO2 which could have reacted with all that exposed sediment. Maybe even enough to reduce the ongoing Ordovician greenhouse situation and to contribute to the glaciation that’s coming at the end of the period.

—Richard I. Gibson

Photo by Ian Muttoo licensed under the terms of the cc-by-sa-2.0   

Map from Ohio Geological Survey, Ohio Geology, Fall 1997 (PDF)

March 20. Ordovician trilobites

Ordovician trilobite

Are you familiar with pill bugs? Roll-up bugs? I haven’t seen one in decades – maybe they don’t live here in the arid west, or heaven forbid, maybe I’ve lost my curiosity at what might be under a rock. If that’s it I’ll have to remedy that. But I recall well as a child in Michigan finding these little gray multi-legged critters in the garden. If you touched them, they’d roll up into a tight little ball. Their soft underbellies were protected by their relatively hard carapaces.

Pill bugs are arthropods like insects, centipedes, and spiders, but they aren’t closely related to them. They are actually crustaceans, isopods, more closely related to shrimp and lobsters. And of course, trilobites were arthropods too, and they shared with pill bugs the ability to roll themselves up into a defensive posture.

Flexicalymene, enrolled

Trilobites may have actually peaked during the Cambrian Period, but they certainly participated in the Ordovician diversification, with lots of new species appearing. They seem to get a little fancier in their ornamentation and development of spines and they clearly became adept at enrolling. Trilobites could definitely roll themselves into tight defensive balls early in the Cambrian, but for some time it was thought that the earliest trilobites couldn’t do it. But in 2013, a team from the University of Cambridge described some olenellids – early trilobites – from about 510 or so million years ago that could and did enroll themselves. So this is not strictly an Ordovician trilobite thing. But some Ordovician trilobites, such as Felexicalymene from the Ordovician of Ohio, Kentucky, and Indiana, made themselves famous by doing it.

—Richard I. Gibson

Ordovician trilobite photo by Vassil, under GNU free documentation license. Flexicalymene photo by Steve Henderson, used by permission.

Further reading:
http://www.trilobites.info/enrollment.htm
http://www.sci-news.com/paleontology/science-enrollment-trilobites-01420.html
http://www.livescience.com/39920-trilobites-curled-like-pill-bugs.html

March 19. Ordovician Bighorn Dolomite

Much of what is now North America – especially the western part – was under a warm, shallow sea during most of the Ordovician. The continent was near the equator, and all that warm, shallow water undoubtedly contributed to the proliferation of life that we see in the Ordovician. In the west, toward the end of the Ordovician, a wide carbonate bank developed, similar in some ways to the shallow waters off the west side of Florida today. We call the rocks that lithified from those sediments the Bighorn Dolomite for prominent outcrops in the Big Horn Mountains of Wyoming. The carbonate platform where the Bighorn and equivalent rocks were deposited was much larger than the Florida coast – it extended from what is now Yukon Territory in Canada to northern Mexico. All of that area was pretty much tropical to sub-tropical during the Ordovician.

Wide, flat, shallow water zones are sensitive to subtle changes in sea level. As planet earth approached the end of the Ordovician, polar ice caps were growing, on the way to a major glacial epoch that probably contributed to the mass extinction at the end of the period. Details of cycles in the sediments of the Bighorn Dolomite coincide quite well with the sea level changes related to the initiation of this ice age.

The Bighorn Dolomite is pretty pure dolomite. Dolomite, you recall, is calcium magnesium carbonate – almost the same as calcite, calcium carbonate, which is the mineral that makes limestone. Dolomite has that added magnesium, which can be added during lithification, the process that turns soft sediment into hard stone. Adding magnesium to the molecular structure makes the rock more porous, and the Bighorn is a useful aquifer in places.

Out here in the arid west, limestones and dolomites are resistant rocks. In rainy country, like the Midwest and eastern parts of the United States, such rocks tend to dissolve in the weakly acidic rain water – and that’s not really modern acid rain, but a very weak acid, carbonic acid, created when rain falls through the atmosphere and reacts with carbon dioxide. Acid, even weak acid, dissolves carbonates eventually – and that takes a lot longer where there isn’t much rain. So here in Montana and Wyoming, carbonates make prominent ridges, and the Bighorn Dolomite is no exception. In places like the Tensleep Canyon on the west flank of the Big Horn Mountains along highway 16, the Bighorn is a near vertical cliff that adds to the scenic beauty of that drive – a drive that I recommend highly. In some places, the Bighorn is as much as 400 feet thick – but in many locations, the Bighorn Dolomite is not present – not because it was not deposited, but more likely because it was eroded away during later times when the land was uplifted above sea level by mountain-building events.

* * * 

The mineral dolomite, calcium magnesium carbonate, which comprises the rock dolomite or dolostone, was named for Déodat Gui Sylvain Tancrède Gratet de Dolomieu, who lived from 1750 to 1801. He was a Knight of Malta, Professor of Mineralogy, and geologist to Napoleon. He wasn’t quite the first to recognize the mineral dolomite, but he got the credit and the mineral was named for him during his lifetime. He found his specimens in the Dolomites, a part of the Alps also named for him. In politics, he helped engineer the surrender of the island of Malta to Napoleon, which pissed off the Grand Master of the Knights of Malta. Dolomieu was imprisoned for 21 months in solitary confinement. He was freed, but in broken health, died at the young age of 51.

—Richard I. Gibson

Further reading
http://jsedres.geoscienceworld.org/content/82/8/599.abstract

March 18. The oldest starfish

A couple weeks ago we talked about cystoids, an extinct class of the echinoderms. Today let’s focus on some echinoderms that you’ll be more familiar with: starfish. The problem with studying ancient starfish is that they tend to fall apart – they don’t fossilize well. Professor Tony Martin’s blog, which I linked in the post on trace fossils on March 11, has a photo of a great trace fossil, the resting mark of an Ordovician starfish, so we know they were around at least that long ago.

Stenaster huxleyi, from the Ordovician of Newfoundland,
drawn by Elkanah Billings in 1865.
Animal is about 4” across.

I checked quite a bit and so far as I can tell, there are no known Cambrian starfish. So the echinoderms form another group, a phylum, that was established during the Cambrian explosion but diversified greatly during the Ordovician. Starfish were part of that diversification. Starfish were described from the Tremadocian, the lower Ordovician, by W.K. Spencer in 1951, published in the transactions of the British Royal Society, and I think no older starfish have been found so far.

It appears to me that specialists in echinoderms really don’t know the ancestry of starfish. Maybe it was a soft-bodied critter originally, and evolved to produce a skeleton of sorts during the Ordovician, but it doesn’t seem clear to me that we know what starfish evolved from. Perhaps it was a quick evolution, something like the development of the trilobite eye, which seems to be almost instantaneous in the fossil record.

However they came about, starfish have survived for almost 500 million years. They’ve changed some – the Silurian Period, which we’ll take on next month, saw the development of starfish with more than five arms – and those multi-armed sea stars survive to this day as well.

Will they continue to survive? Who knows? Last fall, 2013, the popular science press was full of a mystery story of millions of starfish dying. Along parts of the west coast of North America, as much as 95% of the starfish population has vanished. It appears to be some disease that makes a starfish turn to goo – quoting one news story. It affects a dozen different species – that’s unusual – and as of a month ago, February 2014, scientists were still trying to figure it out. It’s a big deal, because starfish are the main thing keeping opportunistic organisms like mussels in check. Stay tuned. And for the record, while radiation from Japan’s Fukushima nuclear plant hasn’t been ruled out entirely as a cause, it’s considered to be quite unlikely. See the links below.

—Richard I. Gibson

Starfish deaths 
Starfish epidemic 
Oldest multi-armed starfish (and lots more)

March 17. Ordovician cephalopods

Cephalopods – the name means head-foot, because their heads typically have tentacles, which seem like feet – cephalopods today are represented by octopuses, squids, and cuttlefish, plus the chambered nautilus.

nautiloid Trilacinoceras

Like so many other groups, cephalopods saw a huge diversification and even a period of dominance during the Ordovician. The most common type from that time is the nautiloids.

Nautiloids lived in a shell, but unlike a snail or clam, where the animal lives in essentially a single hollow space, even if it is complex, nautiloids’ shells had multiple chambers in which the animal lived, with the segments interconnected by a thin tissue called a siphuncle. As the animal grows, more segments are added, and each one is separated by a distinct layer called a septum. The boundary layers, the septa, eventually became incredibly complex, with a fractal-like appearance.

Ordovician nautiloid from Kentucky

They grew to be pretty big – Endoceras, an Ordovician type, has been measured at 11 feet long – you can imagine that with a mass of grasping tentacles at the front –  and it’s possible that there were even longer nautiloids. Earlier specimens tended to be straight, but some from the Ordovician are curved as well. They were predators, and probably amounted to the terror of the seas during much of the Paleozoic Era.

Nautiloids survived multiple mass extinctions until the Late Cenozoic, only about 10 or 20 million years ago. They’ve declined to the point that there are only six species today, compared to 2,500 fossil species known.

—Richard I. Gibson

Photo by Mark Wilson, public domain. Ordovician of Kentucky; an internal mold showing siphuncle and half-filled camerae, both encrusted.

Photograph of the fossil nautiloid Trilacinoceras taken by Dlloyd, used under GNU free documentation license.

March 16. Ordovician paleogeography

Plate tectonics, the process driven by heat convection down in the mantle, has operated pretty much continuously at least since the late Precambrian, and probably even longer than that. During the Ordovician, it seems that there may have been a little more “action” than at some other times – probably the normal variation in the intensity of tectonic activity.

Most of the supercontinent of Gondwana, which we first discussed on February 9, was finally assembled during the Ordovician, and it would remain together for the next 200 million years or more. At least by the late part of the Ordovician, Gondwana was situated near the south pole.

But apart from Gondwana, several important continental blocks, including Laurentia, which is the core of North America, Baltica, the heart of northern Europe, and Siberia, were still moving around pretty much independently. And moving continents means subduction zones, volcanic island arcs, and small continental blocks fragmenting and colliding. Those three major continental blocks, Laurentia, Baltica, and Siberia, were relatively close to each other, but there were also some complex island chains not too far away. One of the most important of those is called Avalonia, named for the Avalon Peninsula of Newfoundland.

The long, linear Avalon terrane today is found in Massachusetts, Maritime Canada, Newfoundland, southern Ireland and Britain, and maybe some bits of northwestern Europe. But during the Ordovician, it was separate from the major continental blocks. I think of Avalonia as something like the western Pacific today – from the Kamchatka Peninsula to Japan to the Philippines. A discontinuous string of continental fragments, volcanic islands, oceanic crust, and more – a real mess.  This string began to collide with North America during the Ordovician Period, probably causing the buckling of the crust that formed the Cincinnati Arch and Michigan Basin, which we talked about earlier this month. We’ll talk about the culmination of this collision, called the Taconic Orogeny, at the end of March.

You can imagine that such a complex strip of diverse geological settings wasn’t necessarily just sitting there idly, even before it collided. We’ll talk about some of the consequences of the plate tectonic events related to Avalonia in a few days.

—Richard I. Gibson

Ordovician 450 million years ago, map by Ron Blakey, via Wikipedia, public domain.

See also http://jan.ucc.nau.edu/~rcb7/global_history.html

March 15. Ostracoderms

The first critters that you’d recognize as primitive fish appeared during the Ordovician. They were armored with bony plates, and their general informal name, ostracoderms, means shell-skinned. For the earliest varieties, we only know them from fossils of these individual scaly plates. They didn’t have a rigid internal skeleton, so they generally fell apart when the animal died.

The Ordovician ostracoderms, thelodonts, which means “nipple teeth,” didn’t have jaws, but they did have scales much like fish today – but the scales were tiny, only a millimeter or two long. They appeared probably during the Middle Ordovician, around 470 million years ago or a bit earlier. Since they were without jaws, and their mouths were on the bottom of the head, we think that these early fish were probably sediment bottom feeders, sucking stuff into their mouths and filtering out food.

I talked about conodonts on March 3, and indicated that once we finally found the conodont animal, it was seen to be a small, eel-like animal. Eels are fish, and the conodont animal was indeed a primitive fish. But ostracoderms were the first ones that really had a fishy look to them. And they were widespread – they’ve been found in Ordovician rocks all over the world.

Neil Shubin, in Your Inner Fish, a book I’ve recommended previously, tells us that the bony plates on ostracoderms’ heads were made of material – calcium phosphate, the mineral apatite – and have structures that are essentially teeth – teeth fused together and on the outside of the animal, but teeth nonetheless, in evolutionary terms. So, Shubin argues, the first hard parts in chordates, the group that includes us and the other vertebrates, were teeth in conodonts, the better to eat you with, and the second hard parts were teeth that evolved into armor – protection from those other gnashing tooth-filled mouths. It’s really a cool story that hangs together quite well, and if you’re interested in this sort of thing, I recommend – again – Shubin’s book, Your Inner Fish.

The entire body of ostracodems was covered in scales, like modern fish, but the head area was more strongly armored by the fused-together plates into a more bony shield. 

If you have comments about the podcast, please leave a review on iTunes or a comment on the blog.

* * *

Today, March 15, is the birthday of Wallace Pratt, born in 1885 in Phillipsburg, Kansas. Pratt was a pioneer in petroleum exploration geology. In 1918 he became the first geologist hired by Humble Oil & Refining – a company that would eventually evolve into the giant corporation we know as Exxon today. One of his major contributions was fostering the use of geophysical instruments in oil exploration, and he was also a founding member of the American Association of Petroleum Geologists. He donated 23 square miles of land in West Texas, where he had a ranch, to the National Park Service, forming the core of what today is Guadalupe Mountains National Park. He died in 1981.

—Richard I. Gibson

Drawing of reconstructed ostracoderms by Philippe Janvier under CC-by-A license. The black and white drawing is from an old textbook.

March 14. Bayan Obo

Today, by request, the podcast has some background information about me as a geologist, and the March 14 episode focuses on the rare-earth mineral deposit at Bayan Obo, which was formed at least partly during the Ordovician.

The image below, of the Bayan Obo mine complex, is from the NASA earth observatory’s photo of the day. 

March 13. Bryozoans

Three or four times now, I’ve mentioned that all the modern phyla of animals were established during the Cambrian, except one. It’s time to talk about the one, the bryozoans. They began during the Ordovician, so far as we know.

Ordovician bryozoa

If you saw a bryozoan fossil, you might think it was a relatively delicate kind of coral. They have lots of diverse appearances, like corals, and their colonies are usually calcareous, made of calcium carbonate, like corals, though some bryozoans apparently had phosphatic colonies. Some bryozoans are stubby little pillars, some are lacy branches, and others are tubular, branched like staghorns, or form tiny thin hair-like crusts on other fossils such as brachiopods.

Some bryozoans that grow into flat, fan-like branches with numerous small rectangular openings in the overall structure are called fenestrate bryozoans, for the window-like openings in the colony. Fenestrate means window-like. 

Encrusting types are probably the most common, at least in living species. and there are plenty in the fossil record as well. They grow on other animals, on rocks, on modern seaweed, and one colony might have two million or more individual zooids cooperating to make a colonial structure half a meter long. These encrustations sometimes look like moss, and modern bryozoans are sometimes called “moss animals” for that reason. They were encrusters pretty much from their beginning in the Ordovician, and they are encrusters today – to the extent that they can become nuisances on ships’ hulls and dock pilings.

What are they? They do still exist – more than 4,000 modern species are known, along with 15,000 fossil species – and they are colonial animals, like corals or graptolites, meaning that the individual zooids can’t survive on their own, away from the colony. The zooids are tiny, maybe a half millimeter long, but the whole colony can be many centimeters across, even a meter in some varieties. They are filter feeders, taking nutrients out of the water the live in, and they make that happen with the help of lophophores – the tentacle-like features that make brachiopods different from clams and other mollusks. 

The oldest mineralized colonial bryozoan known is from the Lower Ordovician. Since everything else got started in the Cambrian or earlier, there’s been an intense search for Cambrian bryozoa. One candidate, described from the Upper Cambrian of Mexico in 2010, was thought to fill the bill, but it has since, just last year, 2013, been reclassified as a kind of coral. You can imagine that it’s a little challenging to be certain about these things, when the fossil remains – the structure that held the colony – might not include anything of the original animals, the zooids. It’s not as if lophophores, soft structures a tiny fraction of a millimeter long, are easy to preserve for 460 million years, but many of the modern orders of bryozoa were established by that time in the Ordovician.

It is likely, though, that bryozoans did exist during the Cambrian – but that they were late to the game of secreting calcium carbonate to make a hard skeleton, the colony. Why were they the only phylum of animals that didn’t figure out how to do that during the Cambrian explosion? There’s an enigma waiting for someone to explore.

Bryozoans contain interesting chemicals – some that cause serious skin diseases in fishermen, as well as some that show potential against Alzheimer’s disease.

—Richard I. Gibson

Photo by Mark Wilson via Wikipedia (public domain)

Further Reading
Cambrian bryozoans? Not yet.

March 12. Michigan Basin

The Michigan Basin is a bull’s eye on the lower peninsula of Michigan – a nearly circular target painted on the geologic map of North America. It’s about 250 kilometers wide, and 5 kilometers deep. Basins like the Michigan Basin are important because they often contain important resources such as oil and natural gas, so understanding how they form helps us explore for such resources.

In some of the Ordovician rocks, called the Prairie du Chien Group, porosities are great enough to serve as natural gas reservoirs, and more than 5 billion cubic feet of natural gas has been produced from that part of the section. Not too shabby, but not too much in the grand scheme – and in fact the United States today consumes almost 100 billion cubic feet of natural gas per day, so that total historic production of 5 billion cubic feet from the Prairie du Chien of Michigan amounts to about 80 minutes’ worth of natural gas consumption today.  We’ll talk more about the Michigan Basin next month in connection with its mineral resources.

The problem is, we really aren’t sure how the Michigan Basin formed. It’s shaped like a big bowl, and clearly there was subsidence in the basin to allow for the 5 kilometers of sediment to fill it. And fill it they did – the layers of rock are thicker in the center than on the flanks.

One possible mechanism for formation suggests that the earth’s crust or upper mantle was weaker, or thinner, or of different composition, so that broad stretching on a crustal scale might have allowed this area to sink more than other areas, becoming the bowl in which the sediments were deposited. It’s a fact that a branch of the Mid-Continent Rift, the pull-apart zone that affected this region about 1.1 billion years ago – we talked about it on January 26 — but that zone was clearly very linear, oriented north-south. I suppose it might have controlled the subsiding, and the Michigan Basin is somewhat oval shaped, with the longer axis north-south, but honestly this seems to me to be a stretch. Possible, or possibly some degree of affect to the whole process, but hard to see as the one and only cause.

Some mechanisms call on thermal subsidence as the basis for the Michigan Basin. In this scenario, a relatively small portion of the upper mantle cools more than adjacent areas, and when it cools, it contracts, it shrinks, and that smaller volume is also a physically lower place, a basin in which sediments can be deposited. This is a reasonable theoretical idea, but I don’t know of any good solid evidence for it in Michigan. 

You can also get subsidence of the crust when you have an upwelling of the mantle down below. It pretty much stretches the crust above the upwelling hot mantle, and the stretched crust forms a neck, like when you pull silly putty apart – or partly apart. This has almost certainly happened in the Mississippi Salt Basin, near the Gulf Coast, but there, we have good geophysical evidence for that process which we don’t find in Michigan.

And because the basin is so symmetrical, so nearly circular, it’s been suggested that it represents a huge impact crater. But beyond the circularity – and it’s really oval, not circular – there’s no evidence for an impact.

Maps is from Devonian time;
Michigan Basin began to subside in
Late Cambrian and Ordovician time.

Back in 1990, geologist Paul Howell and his colleagues at the University of Michigan studied the sequences of sedimentation in the Michigan Basin in detail, and found a good correlation in time between the deposition and tectonic events along the east coast of North America. I mentioned this idea in connection with the Cincinnati Arch the other day. The really good coincidence of several different mountain building events – collisions – on the east coast with pulses of subsidence in Michigan suggests a causative relationship, and it does put the development of the Michigan Basin into a plate-tectonic context, rather than a simple, isolated basin subsiding a lot, but for no obvious reason.

I think that idea is most likely, that the basin is a reaction within the continent to big-time collisions happening a few hundred kilometers to the east. Push it down on the east coast, it bows up along the Cincinnati Arch, and sags beyond the arch in the Michigan and Illinois Basins. I don’t think you can quite take that to the bank, yet, but it’s an idea that works pretty well with what we see in the rocks. Howell paper proposing this tectonic mechanism is linked below.

—Richard I. Gibson

Images from Michigan State University and USGS. Devonian paleogeographic map by Ron Blakey, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Link to paper by Howell et al. (1990) PDF