Phanerozoic Eon: 542 - 0 Ma

Paleozoic Era: 542 - 252.3 Ma

Period	Range (Ma)
Permian	299 - 252.3
Carboniferous	359 - 299
OR Pennsylvanian	318 - 299
& Mississippian	359 - 318

In traditional non-North American geology the (North American) Mississipian and Pennsylvanian Periods were not recognized; the Mississippian Period is thus the same as the traditional European Early Carboniferous Epoch and the Pennsylvanian Period is equivalent to the traditional European Late Carboniferous Epoch. In the latest international time scale, a compromise was reached, and the Mississippian and Pennsylvanian are "sub-periods" of the Carboniferous (each has three Epochs).

Paleogeography and Geology of the Mississippian:
Tabulate-stromatoporoid reefs disappear; related to switch from calcite seas to aragonite seas:

• When midocean ridges have low activity, high levels of Mg relative to Ca: aragonite seas with reduced reef building activity
• When midocean ridges have high activity, low Mg/Ca ratios: calcite seas with increased reef building activity
• Early Cambrian through mid-Mississippian, calcite seas
• mid-Mississippian through mid Jurassic, aragonite seas
• mid-Jurassic through mid-Cenozoic, calcite seas
• mid-Cenozoic through today, aragonite seas

During Mississippian, no reef complexes of significance.

Huge carbonate banks in most shallow seas: crinoid meadows produce limestones whose main clasts are crinoid columnal elements.

Continued decline of CO₂ -- sucked up into soils, and eventually coals -- lowers greenhouse effect, start of new series of Gondwanan glaciations. Non-reduced oxygen accumulates in the atmosphere.

During Late Mississippian, the Variscan Orogeny:

• Beginning of the collision between the Iberian & Armorican parts of Gondwana and Euramerica, and thus the loss of most of the Rheic Ocean and well on the way to the assembly of Pangaea
• Once the "Hercynian" Orogeny was considered a separate event, but this seems to be part of the same complex

End of Mississippian: major regression, associated with mass extinction of some groups of stalked echinoderms and ammonoids.

Paleogeography and Geology of the Pennsylvanian:
Continued decline of CO₂ (to just about modern levels!). O₂ levels reach their maximum in Earth history (around or above 30%). Since N₂ levels (which represent the majority of the atmosphere) would have likely remained constant, total atmospheric density reaches its highest in the age of blue skies (i.e., since the reducing atmosphere was oxidized).

As a consequence of this, forest fires become EXTREMELY easy to trigger. Also, higher O₂ levels may have been the reason that very large terrestrial arthropods could live in the Carboniferous.

Continued glaciation in Gondwana: southern continents remain largely emergent.

Continued Collision of Gondwana and Euramerica:

• Eastern Laurentia-Gondwana (Africa) collision closes Rheic Ocean producing Alleghanian Orogeny (late Mississippian-Pennsylvanian):
  • The third and last part of the Appalachian Orogeny
  • Resultant mountain range was at least as high as present Himalayas!
• Southern Laurentia-Gondwana (Florida, South America, etc.) collision called Ouachitan Orogeny (Late Pennsylvanian):
  • Deformed large sequence of deep marine passive margin sediments accumulating from Ordovician through Early Pennsylvanian
• Assembly of Laurussia and Gondwana produces most of Pangaea:
  • Only Siberia (which collides with Euramerica to form the Uralian Orogeny in the Pennsylvannian and Permian) and other Asian cratons (which collide in late Permian through Triassic) remain separate continents
• Creation of the Tethys Ocean (a large embayment on the eastern seaboard of Pangaea, with its own typical warm-water fauna)

Alleghanian Orogeny produced the direct "ancestor" of the modern Appalachians: the current mountains are the erosional remnants of the fold-and-thrust belt of the Alleghanian Appalachians.

Unusual cratonic deformation in southwestern Laurentia:

• Formation of the Ancestral Rockies (the Front Range and Umcompahgre uplifts)
• Orogeny not by collisional events per se, but by vertical faulting and uplift
• Uplift may be response to stresses generated by Ouachita orogeny
• Produced huge amount of sediment, deposited nearby throughout southwest

MOST IMPORTANT GEOLOGICAL EVENT OF PENNSYLVANIAN:

• Cyclothems (called "Coal Measures" in Europe)
• Repetitive sequences of transgressions-regressions of nearshore and paludal deposits
• Cover large expanses of North America, Europe, Russia, China: not common in Gondwana
• Contain vast majority of Earth's coal deposits
• Huge amounts of buried vegetation removed vast amounts of CO₂ from Earth's atmosphere: these levels have NEVER returned to pre-Mississippian levels, although did rise again during Late Permian
• The Industrial Revolution ran on Pennsylvanian deposits; the threat of anthropogenic global warming is that we will liberate enough CO₂ to reach mid-Paleozoic levels!
• Coal in great deposists may be helped in Carboniferous for multiple reasons"
  • Physical geology and glacioeustacy (see below)
  • High level of atmospheric oxygen may promote charcoalification (pre-burial burning); some experimental evidence suggests that much coal is actually charcoalified rather than simply buried
  • May have been early enough in history that major tree-decomposing bacteria and fungi had not yet evolved, hence more debris to be buried

Formation of cyclothems

• Northern continents were very low relief during Pennsylvanian
• Extensive coal swamps covered the coastlines
• As Gondwanan glaciers expanded, regressions and swamps moved seaward
• As Gondwana glaciers retreated, transgressions and swamps moved landward
• Repetitions could be VERY rapid (few tens of thousands of years)
• Evidence from charcoal among the coal suggests that some represent huge, high temperature forest fires (not unlikely, given higher O₂ levels during Pennsylvanian)

Some minor reefs formed by sponges, bryozoans, and algae.

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Last modified: 14 January 2011