Phanerozoic/Pennsylvanian Period

The Pennsylvanian Period - 318 to 299 million years ago (MYA)

The Pennsylvanian Period is most notable for its role in the creation of most of the world's coal. Also in this period, the first reptiles were formed. The term Pennsylvanian is a U.S. coinage that is based on the frequency of rocks of this period in the state of Pennsylvania. Internationally, the terms late Carboniferous Period or Silesian Period are preferred.

Subdivisions following the international timescale: (stages, ages)

Gzhellian
Kasimovian
Moscovian
Bashkirian

North American subdivisions:

Morrowan
Atokan
Desmoinesian
Missourian
Virgilian

Climate

Evidence in the form of well-developed tree rings, less diverse fossil floras and faunas, and glacial deposits indicates that temperate and glacial conditions were common in non-equatorial climatic belts during Pennsylvanian time. Climatic fluctuations during the period caused significant increases and decreases in the amount of water that was temporarily stored in the glaciers in Gondwana and contributed to eustatic changes of sea level. Although most artist's conceptions of the Pennsylvanian Period emphasize its prolific swamps, these were characteristic only of the equatorial regions. The Southern Hemisphere, which was dominated by the huge continent Gondwana, underwent a series of ice ages during this period. These ice ages sequestered water in times of ice growth and released it in times of melting, causing the ocean to cyclically regress (uncover coastal lands) and transgress (cover coastal lands) around the world. Repeating sequences of sedimentary rock layers record these changes in sea level.

Geology

From the bottom up, a typical sequence is sandstone , shale, coal, limestone , and sandstone again. Each such unit is termed a cyclothem and was formed as follows: (1) As ice melted in Gondwana, seas rose globally. Rivers and streams deposited sand and gravel in the coastal lowlands as they sought new equilibrium profiles (i.e., stable altitude-vs.-distance cross-sections). This sand layer eventually became sandstone. Although the coastal zones where sandstone deposition was taking place at any one time were narrow, large areas were blanketed by these sediments as the seas rose and coastlines swept slowly inland. (2) As the rising sea neared a given location, a lush coastal swamp developed. This deposited a thick layer of dead leaves, tree trunks, and other organic material rich in carbon that would eventually form coal. (3) When the sea finally submerged the swamp, a shallow marine environment appeared. The remains of shelly marine animals built up on the sea floor and eventually became limestone. (4) Ice began to build again in Gondwana, and sea levels began to drop in a new phase of regression. (5) Erosion of re-exposed coastal lands scraped off the topmost sediments left by the last transgression, including some of the limestone layer. (6) Ice began to melt again in Gondwana, triggering a fresh cycle of transgression.

Fauna (Animals)

This is when the first reptiles were formed. During this period the land was abundant with amphibian life. Also there were spiders, millipedes, land snails, scorpions, enormous dragonflies (with wingspans of approximately 2.5ft), and more than 800 kinds of cockroaches. The inland waters were inhabited by fish, clams, and various crustaceans. The oceans were inhabited by mollusks, crinoids, sea urchins and one-celled foraminifera. One of the greatest evolutionary innovations of the Carboniferous was the amniote egg, which allowed for the further exploitation of the land by certain tetrapods.

Flora (Plants)

The plants that were present during this period includes ferns and fernlike trees; giant horsetails, called calamites; club mosses, or lycopods, such as Lepidodendron and Sigillaria; seed ferns; and cordaites, or primitive conifers. Three basic floras inhabited the climatic zones of the time: the northern Angaran temperate flora, the southern Gondwanan temperate flora, and the Euramerican tropical flora. Each of these floristic regions was distinct in terms of its plant composition.

Paleogeography

Pennsylvanian paleogeography changed significantly during the period as the supercontinent Pangaea gradually was formed by the joining together of Gondwana and Laurasia. North America and northern Europe, which had been combined into the continent Laurasia since the late Silurian, and South America and northwestern Africa, which formed the northern part of the continent of Gondwana, came together along the Ouachita–Southern Appalachian–Hercynian geosyncline. The result was an extensive orogeny, or mountain-building episode, which supplied the vast amounts of sediments that make up most of the Pennsylvanian strata in the eastern and midwestern parts of the United States.

Orogeny

Click to enlarge

In North America, the Pennsylvanian Period was characterized by the progressive growth and enlargement of the Alleghenian-Ouachita-Marathon orogenic belt, which formed as the northwestern parts of the large continent Gondwana (mainly northwestern Africa, the area that is now Florida, and northern South America) collided against and deformed the eastern and southern parts of the North American continent.

Carbon (coal creation)

The stratigraphy of the Lower Carboniferous can be easily distinguished from that of the Upper Carboniferous. The environment of the Lower Carboniferous in North America was heavily marine, when seas covered parts of the continents. As a result, most of the mineral found in Lower Carboniferous is limestone, which are composed of the remains of crinoids, lime-encrusted green algae, or calcium carbonate shaped by waves. The North American Upper Carboniferous environment was alternately terrestrialand marine, with the transgression and regression of the seas caused by glaciation. These environmental conditions, with the vast amount of plant material provided by the extensive coal forests, allowed for the production of coal. Plant material did not decay when the seas covered them and pressure and heat eventually built up over the millions of years to transform the plant material to coal.