The carbon that gives its name to the Carboniferous is found in abundant coal beds from around the world. Coal is a sedimentary rock composed almost entirely of plant matter. Some older coal beds exist, but it wasn’t until the Pennsylvanian Period that plants were abundant enough to form extensive deposits. And it wasn’t just a matter of piles of wood and leaves – a stagnant, low-oxygen environment was needed to prevent complete decomposition by oxidation. Such a setting was found in the swamps that were common – and huge – during Pennsylvanian time around the world.
Partially decayed plant matter initially produces peat. As the pile is buried by later sediment, higher temperatures drive off water and coal forms.
Mark Twain’s 1903 description of coal measures is really pretty accurate. “In the first place,” Twain wrote, “a coal bed is a slow and troublesome and tiresome thing to construct. You have to grow prodigious forests of tree-ferns and reeds and calamites and such things in a marshy region; then you have to sink them under, out of sight and let them rot; then you have to turn the streams on them, so as to bury them under several feet of sediment, and the sediment must have time to harden and turn to rock; next you must grow another forest on top, then sink it an put on another layer of sediment and harden it; then more forest and more rock, layer upon layer, three miles thick – ah, indeed, it is a sickening slow job to build a coal-measure and do it right!”
Twain was describing sequences of rock known today as cyclothems, alternating layers of coal and sediment that reflect alternations in the environment, from coal swamps to river lowlands and mud flats and even occasional incursions by the sea. These alternating environments produced rhythmically layered coal, shale, sandstone and occasional limestone called cyclothems. The individual layers, representing distinct environments, can be as thin as a few inches, but the total package might be 5 to 10 feet or more thick, and the pile of packages reaches many hundreds to thousands of feet in places. It’s possible to recognize specific erosion surfaces – unconformities – that may not represent the vast amounts of time we’ve talked about previously with unconformities, but that may be regionally extensive even if they represent time spans of less than a million years.
The alternating environments most likely indicate rhythmic variations in sea level, and those variations in turn were probably caused by episodes of glaciation in the southern hemisphere that locked up water to lower sea level, then ended to allow sea level to rise again.
Predictable variation in the kinds of rock in a sequence is a great tool for understanding not only where you should expect to find coal, but also for understanding the variations in rock type that can point to oil source rocks and reservoir rocks. This study is an important aspect of oil exploration, where it’s given the general name sequence stratigraphy.
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Today’s geological birthday is Charles Schuchert, born July 3, 1858, in Cincinnati, Ohio. Schuchert was a brachiopod specialist and spent much of his career at Yale University, but he’s probably best known as one of the first geoscientists to make comprehensive paleogrographic maps – maps that show the distribution of lands and seas and other environments at various points in the geologic past.
—Richard I. Gibson
Coal swamp image from 4th edition of Meyers Konversationslexikon (1885–90, public domain)