The Carboniferous
The Great Coal Swamps
Growing up in Illinois, I lived above the ancient inland seabeds of the Carboniferous period.
And occasionally, my father would take me to the edges and deltas of this ghost sea. We'd walk around the Mazon Creek area and collect nodules – which we'd break open to see what was enclosed.
Here's a picture of an open one.
Figure 1.
These nodules are called siderite concretions, iron carbonate, and they usually split along the middle of their sides.
Also, all around this area, we'd see the strip mines bringing up the coal, and those massive, magnificent shovels.
Figure 2.
Notice on the back wall you can see alternating layers of coal and sedimentary rocks, and the current gray sediment layer that the shovel is stripping away to get to the black coal underneath.
The strip mines were used because the coal was so close to the surface.
Here's a better picture of the seams of coal and layers of sediments.
Figure 3.
These seams were the result of an ancient process.
Side Trip – The Process of Coal
When plants and animals die, they normally decay and are converted to carbon dioxide, water, and other elements that leach away. The result: a few bits and a few bones.
However, sometimes a different form of decay occurs. The bodies of dead plants and animals undergo only partial decay.
Here's an example.
A plant dies in a swamp and is quickly covered with water, silt, sand, and other sediments. This material prevents the plant from reacting with oxygen and decomposing to carbon dioxide and water – the normal process.
A type of bacteria not requiring oxygen begins to convert the plant debris to pure carbon and simple compounds of hydrocarbons (hydrogen and carbon).
This type of bacteria is called anaerobic (an-nuh-Robe-ik).
This anaerobic process first converts the decaying plant to a soft, woody material known as peat. Peat can be burned, but produces smoke and does not release much heat. It's a simple form of fossil fuel.
If the peat remains in the ground for long periods of time, it eventually compacts, especially if layers of sediment cover it.
The pressure and heat of this sediment gradually converts the peat into a form of coal known as lignite or brown coal.
If the process of compaction continues, the lignite is converted into bituminous (soft) coal. And with more time and pressure, this soft coal is further converted into hard anthracite coal.
Here's an illustration from the Kentucky Geological Survey of the process.
Figure 4.
Today, coal formed by these processes is often found layered between sedimentary rock. Sedimentary rock is formed when sand, silt, clay, or similar materials like mud are packed together under heavy pressure. You can see such layers in Figures 2 and 3 above.
Where I grew up in Illinois, the coal layers were close to the Earth's surface, and so strip mines were used to gather it up.
So, let's look at the time of these great coal swamps.
Mississippian & Pennsylvanian Coal Swamps
Coal has been formed at many times in the past, but most abundantly during the Carboniferous Age and again during the Upper Cretaceous Age (about 100 million years ago).
The Carboniferous period (360 to 286 million years ago) is known in North America as the
Figure 5.
The Carboniferous forests produced tremendous biomass which, when buried, eventually turned into massive coal deposits.
Figure 6. A lump of Pittsburgh vein bituminous coal. Courtesy of the University of Pittsburgh (PA) Geology Dept.
In the process discussed above, this biomass eventually became a sedimentary rock composed of the plant and animal debris deposited in the carboniferous bogs or swamps (coal).
One of the key facets of creating coal was the layers of other sediments that covered and cut-off the biomass from oxygen. This came about through the fluctuating sea-levels during the Carboniferous.
So, let's look back at this environment.
The Carboniferous World
The Carboniferous, as seen through North American deposits, is really two distinct sets of conditions:
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First, a Marine dominated environment, characterized by shallow seas,
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Followed, by a terrestrial "coal-swamp" environment.
The super-continent of Pangaea was assembled during this time, causing the uplift of sea floor as continental land masses collided to build the Appalachian and other mountains.
We can see this in a United States map, giving a closer view of the inland sea during the Pennsylvanian:
Figure 7.
As you can see, most of the U.S. was wet: swamps, a large shallow sea, along with some western deep water.
If we take an even closer view, focusing on Illinois and Indiana, we see a river system which flowed to the southwest, across a swampy lowland, and which carried mud and sand from the highlands on the ocean.
Figure 8.
This river system formed thin but widespread deltas, which slowly developed into a vast coastal plain or lowland and built out into the shallow sea that covered the rest of Illinois.
A Carboniferous Extinction Cycle
Another aspect that played a part in the development of massive coal beds was the role of glaciation.
Glaciation in Gondwana during the late Carboniferous contributed to a decline in marine environments and to fluctuating sea levels. These fluctuating levels alternately formed and destroyed coastal swamp environments.
This helped to cover the biomass of the swamps with sediments, thus creating “over burden” layers above the seams of decaying plants.
As the record of coal seam to sediment layer shows us, there was a continual extinction cycle throughout the Carboniferous, one of fluctuating sea levels and one of forming coastal swamps.
Let's look at the plants that formed these swamps.
Life in The Great Swamps
Plants
In the Carboniferous, plants and animals evolved to the point where true forests developed in the lowland, coastal sites. Most of these forests are characterized as swamps, and they were dominated by plants and animals that were quite different, in many ways, from todays swampy lowlands.
Lycopods
I have these in my backyard.
Figure 9. A modern lycopods, Lycopodium
Modern lycopods are generally only a few inches tall (maximum of 1 meter). They are characterized by small, pointed leaves that cover a branched stem and its branches in a dense spiral.
Spore-bearing cones, located at the ends of the main branches, form the basis for a fern-like free-sporing life cycle.
While the Carboniferous also had these smaller lycopods, during this period, the most impressive plants were several types of tree-sized lycopods, such as the Lepidodendron.
Figure 10. From Book 15 of the 4th edition of Meyers Konversationslexikon (1885-90). Lepidodendron is the tall tree on the left.
Lepidodendrons reached nearly 40 meters in height, with trunks nearly 2 meters in diameter. The trees branched extensively and produced a large number of leaves, that, when they fell from the branches, produced characteristic leaf scars, a diamond-shaped "snakeskin."
Figure 11.
On the left is a lower magnification view of this type of pattern, showing the general features of many of these trees. On the right is a higher magnification photo showing the details of the leaf scars.
Here what this tree would look like fallen over on the forest floor.
Figure 12. Lepidodendron close up
Also, part of the family of lycopods in the giant swamps, was Sigillaria – a tall, tree-like plant, occasionally with forked trunk. Support for this plant came from a layer of closely packed leaf bases just below the surface of the trunk, while the center was filled with pith.
Figure 13.
Here's a close up of its trunk. Notice the distinctive pattern.
Figure 14.
Horsetails
Figure 15. A modern horsetail, Equisetum, (the only living genus of the group).
© J R Crellin 2005
Horsetails were another important component of the Carboniferous plant communities.
Modern horsetails reach a height of just about 1-2 meters. They are easily recognized by the division of the stems and branches into definite nodes, with both leaves and branches occurring in whorls at these nodes.
Figure 16. The leaf whorls of an extinct horsetail, Asterophyllites equisetiformis.
During the Carboniferous, the largest of this group were trees called Calamites.
Figure 17.
Numerous leaf fossils, such as the specimen of Annularia shown below, have been associated with these trees.
Figure 18.
Ferns
Modern ferns exhibit a wide range of growth forms from small, under-story plants to large, tree-ferns, such as these from New Zealand (Cyathea).
Figure 19.
During the Carboniferous period, large tree ferns, known as Psaronius, were the second largest plants, second to the large arborescent Lycopods.
Figure 20.
Reconstruction of Psaronius by Morgan (1959)
a, b, c and d are drawings of the wood kernel of the trunk.
Growing to a height of 20 meters, with large, compound fronds, the plants would have been even more spectacular than the living Cyatheas (modern tree fern) illustrated above.
The forests also contained a wide range of smaller ferns, including epiphytes found attached to the Psaronius trunks.
Ferns were well represented in the coal swamps. Numerous fern leaf fossils have been found. Sometimes, because they are so well preserved, they are quite beautiful.
Ferns ranged from small, shrubby forms similar to those we see in many places today, to towering tree-ferns.
Figure 21. Pecopteris sp. is the foliage of a tree fern.
Pecopteris grew on Psaronius which was one of the most common fossil tree ferns.
Seed Ferns
In addition to true ferns, the Carboniferous forest also provided habitat for a group of early seed plants known as seed ferns or pteridosperms, such as this Diplopteridium.
Figure 22.
Reconstruction of an Early Carboniferous lagenostomalean pteridosperm Diplopteridium. Based on Rowe (1988b, figure 35).
Seed ferns had fern-like leaves but a seed-type life cycle, often with walnut-sized seeds produced directly on the foliage.
Isolated leaf fossils are difficult to distinguish between ferns and seed ferns. Often seed ferns had the leaf fossil known as Neuropteris associated with the seeds.
Figure 23. Neuropteris sp. is the name given to the foliage of a seed fern.
Seed ferns (Pteridospermales) are an extinct group of gymnosperms. Although their foliage resembled that of modern ferns, they reproduced by means of seeds. Modern ferns reproduce by means of spores.
Early Conifers
The first conifers appear in the late Carboniferous but from a most unlikely-looking ancestor.
The plants that gave rise to the conifers were Cordaites, a gymnosperm with long, strap-like leaves.
Figure 24. Reconstruction of Late Carboniferous cordaitaleans:
(A) an arborescent form found in the palaeoequatorial swamp-forests;
(B) a smaller form. From Thomas and Spicer (1987, figure 11.1; after D.H. Scott, and Rothwell and Warner).
These plants ranged from large shrubs to very small trees and many probably grew like modern mangroves, living on mud flats in brackish water.
The first true conifers had branches and needles that strongly resemble this modern Araucaria.
Figure 25.
Here's a Mazon Creek fossil of a cordaite leaf:
Figure 26. From the Illinois State Museum.
The pictures and illustration that you have seen are just a bit of the huge and dense forests of these early plants, plants developed in the extensive lowland swamps around the lakes, rivers, and edges of Carboniferous period.
Figure 27. Illustration by Mary Parrish © Smithsonian Institution:
These swamps existed in a continuous cycle of growth and death.
The build-up of huge mats of rotting vegetation followed by periodic flooding which capped the layers of peat with layers of sand.
When the seas withdrew the dense forested areas grew again... and the cycle repeated for tens of millions of years.
As we have seen, the thick bands of peat hardened into coal seams, and the over-capping sediments into sandstones and shales.
Terms
As with other earth events and features, the unique composition of swamps can teach us about minerals, how they interact to produce structure (such as nodules), and ultimately the world we live on.
Definition of Anaerobic (an-nuh-Robe-ik)
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adj. Living or active in the absence of free oxygen. "Anaerobic bacteria"
Definition of Arborescent
- Resembling a tree (applied to non-woody plants attaining tree height and to shrubs tending to become tree-like in size). Compare: dendroid. (09 Oct 1997)
www.earthhistory.org.uk
Definition of Glaciation
- n. The condition of being covered with glaciers or masses of ice; the result of glacial action. "Agassiz recognized marks of glaciation all over northern Europe"
www.geology.wisc.edu
Definition of Gondwana
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Southern supercontinent, formed after the break up of Pangaea before the Triassic Period, including most of the landmasses which make up today's continents of the southern hemisphere.
Definition of Pangaea
- A supercontinent that included all the landmasses of the earth before the Triassic period and that broke up into Laurasia and Gondwana.
en.wiktionary.org
Definition of Gymnosperm
- n. A plant that bears naked seeds (i. e., seeds not enclosed in an ovary), such as the common pine and hemlock. Cf. Angiosperm.
http://www.nilesbio.com/images/categories/C170.jpg
Definition of Siderite, (iron carbonate)
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Carbonate of iron, an important ore of iron occurring generally in cleavable masses, but also in rhombohedral crystals. It is of a light yellowish brown color. Called also sparry iron, spathic iron.
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A meteorite consisting solely of metallic iron. An indigo-blue variety of quartz. Formerly, magnetic iron ore, or loadstone.
- Any plant of the genus Sideritis; ironwort. Origin: L. Sideritis loadstone, Gr, of iron, from iron. Source: Websters Dictionary (01 Mar 1998)
www.universmineral.com
Links
These links will give you more information about the Carboniferous, or about interesting facts, fossils, and how they were found.
For example, to read about how the above illustration was made, the study of the fossils, the reconstruction of each feature of each fossil – and then to puzzle the pieces together, go here:
paleobiology.si.edu
The Smithsonian's “Reconstructing an ancient environment” really gives you insight to how scientists study and re-create ancient environment to help all of us understand our history:
paleobiology.si.edu
Here's a link to the Smithsonian, and one of the largest plant fossils ever collected - 3.9 m (13 ft) long and 3.7 m (12 ft) high, and weighing more than 16 tons: A giant scale tree.
www.mnh.si.edu
To read about the “Tully Monster,” click here:
www.fieldmuseum.org
© The Field Museum.
It’s the official Illinois state fossil.
This site examines the Tree Fern, Psaronius:
www.xs4all.nl
There are a lot of species of Pecopteris: probably all were leaves of Psaronius trees.
Here's information about the Horsetail Tree, Calamites:
www.xs4all.nl
This is from the University of Munster; a excellent site about the History of Palaeozoic Forests:
www.uni-muenster.de
And it has some great fossil photographs.
Figures & Acknowledgments
Figures
Figure 1. commons.wikimedia.org
Figure 2. www.coalcampmemories.com
Figure 3. www.scienceclarified.com
Figure 4. www.emc.maricopa.edu
Figure 5. www.emc.maricopa.edu
Figure 6. www.appaltree.net
Figure 7. www.library.ndsu.edu
Figure 8. www.isgs.uiuc.edu
Figure 9. herbal-medicine.us
Figure 10. en.wikipedia.org
Figure 11. lsvl.la.asu.edu
Figure 12. www.emc.maricopa.edu
Figure 13. folk.uio.no
Figure 14. commons.wikimedia.org
Figure 15. www.floralimages.co.uk
Figure 16. commons.wikimedia.org
Figure 17. delta-intkey.com
Figure 18. www.uky.edu
Figure 19. taggart.glg.msu.edu
Figure 20. www.xs4all.nl
Figure 21. www.museum.state.il.us
Figure 22. www.thegcr.org.uk
Figure 23. www.museum.state.il.us
Figure 24. www.thegcr.org.uk
Figure 25. www.atreeaday.com
Figure 26. www.museum.state.il.us
Figure 27. rst.gsfc.nasa.gov
References
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Baird, G.C. 1990. Mazon Creek. In D.E.G. Briggs and P.R. Crowther (eds.), Palaeobiology: a synthesis, Blackwell Scientific Publications, pp. 279-282.
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Baird, G.C., Shabica, C.W., Anderson, J.L. & Richardson, E.S. 1985. Biota of a Pennsylvanian muddy coast:habitats within the Mazonian delta complex, Northeastern Illinois. Journal of Paleontology, 59, 253-281.
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Baird, G.C., Sroka, S.D., Shabica, C.W. & Kuecher, G.J. 1986. Taphonomy of Middle Pennsylvanian Mazon Creek Area Fossil Localities, Northeast Illinois: Significance of Exceptional Fossil Preservation in Syngenetic Concretions. Palaios, 1, 271-285.
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Briggs, D.E.G. 2001. Lagerstätten. In D.E.G. Briggs and P.R. Crowther (eds.), Palaeobiology II, Blackwell Science Ltd., pp. 328-332.
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Kuecher, G.J., Woodland, B.G, Broadhurst, F.M. 1990. Evidence of deposition from individual tides and of tidal cycles from the Francis Creek Shale (host rock to the Mazon Creek Biota), Westphalian D (Pennsylvanian), northeast Illinois. Sedimentary Geology, 68, 211-221.
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Seilacher, A. 1990. Taphonomy of Fossil-Lagerstätten. In D.E.G. Briggs and P.R. Crowther (eds.), Palaeobiology: a synthesis, Blackwell Scientific Publications, pp. 266-270.
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- Earth Science Club of Northern Illinois
1990, Keys to Identify Pennsylvanian Fossil Plants of the Mazon Creek Area (revised edition). Downers Grove, Ill. (P.O. Box 321, Downers Grove 60515): Earth Science Club of Northern Illinois, 60pp.
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- Hay, A.
1989, Keys to Identify Pennsylvanian Fossil Animals of the Mazon Creek Area. Downers Grove, Ill. (P.O. Box 321, Downers Grove 60515): Earth Science Club of Northern Illinois, 125pp.
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- J.R. Jennings
1990, Guide to Pennsylvanian Fossil Plants of Illinois. Illinois State Geological Survey, Educational Series, 13: 1-75.
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- G. Langford
1958, The Wilmington Coal Flora From a Pennsylvanian Deposit in Will County, Illinois, Downers Grove, IL: Esconi Associates, 366pp.
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- G. Langford
1963, The Wilmington Coal Fauna and Additions to the Wilmington Coal Flora From a Pennsylvanian Deposit in Will County, Illinois, Downers Grove, IL: Esconi Associates, 280pp.
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- M.H. Nitecki (editor)
1979, Mazon Creek Fossils, New York: Academic Press, 581pp. ISBN 0-12-519650-4
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- C.W. Shabica and A.A. Hay (editors)
1997, The Fossil Fauna of Mazon Creek, Chicago: Northeastern Illinois University, 308pp. ISBN 0-925065-21-8
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