June 22. Late Mississippian Lands and Seas

By late Mississippian time, around 325 million years ago, the extensive warm shallow seas that covered much of North America were becoming restricted. The sedimentary rocks were less often the limestones we heard so much about for most of this month, and more the shales we discussed on June 17. More clastic sediment reflects the onset of uplift of lands around the marine areas.

In North America, in both the east and south-central part of the continent, collision with the northwestern margin of Gondwana was beginning. In the east, this was largely along the same zone where previous collisions, from the Taconic Orogeny at the end of the Ordovician to the Caledonian and Acadian Orogenies in the Silurian and Devonian had produced episodic ongoing uplifts. This is the beginning of the Appalachian or Alleghenian Orogeny, the main continent-continent collision between North America and Gondwana.

Another aspect of that collision was beginning in what is now Oklahoma, Texas, and Arkansas, where it is called the Ouachita Orogeny. The uplifts there led to the deposition of the Barnett Shale in the Fort Worth Basin, which today is a significant source of natural gas production.

The Tethys Ocean between Eurasia and Gondwana was also closing, and aspects of that collision during the latter part of the early Carboniferous, the equivalent of the Mississippian Period, are the Variscan Orogeny in Europe. This was definitely not a simple collision, as there were many isolated microcontinental pieces, oceanic crust, and island arcs involved, so parts of the Tethys remained open while other parts were closing. Parts of the Tethys Ocean still exist today, beneath the Black Sea, the southern Caspian Sea, and in the deeper basins of the Mediterranean.

While the timing of all these events varies by a few tens of millions of years, I think it’s safe to say that the collision between Laurasia – North America plus Eurasia – and Gondwana was definitely underway by late Mississippian time. We’ll hear more about it over the next month or so.

—Richard I. Gibson

Reference: AAPG Memoir 43, Evolution of the Arctic-North Atlantic and the Western Tethys, by Peter A. Ziegler (1988).