July 15. Pennsylvanian paleogeography

By late Pennsylvanian time, the ongoing collision between the southern supercontinent of Gondwana and the northern continent of Laurasia, North America plus Baltica or Europe, was well underway. There was probably still a bit of ocean between the far northwestern part of Gondwana, which is South America today, and what is now southwestern North America. Siberia and other terranes probably hadn’t quite attached themselves to the eastern margin of Baltica, and there were still some continental blocks remaining unattached, including microcontinents that would become parts of China today. But the assembly of Pangea, whose name means “all earth” was clearly approaching its climax. 

Late Pennsylvanian Map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL

We’ve talked about some of the tectonic events in North America, resulting from the collision of Gondwana with what is now eastern and southern United States. The Appalachian-Alleghenian Orogeny had created another high mountain range approximately where the modern Appalachians stand, and the southwestern extension of that collision produced the Ouachita Mountains in what is now Arkansas and Oklahoma, extending as far west as the Marathon Region of West Texas and beyond. Deformation within the western part of the continent broke the crust to create the Ancestral Rocky Mountains (see July 6) Especially across the eastern interior of what is now the United States, Pennsylvania south to Alabama and west to Illinois, the uplifts confined tropical lowlands that became the vast rainforests and swamps whose plants ultimately produced the coal for which the region is famous.

The Pennsylvanian equator ran approximately from modern San Francisco to Nova Scotia and into Britain and France, which were attached to the eastern side of Canada. The huge southern continent, Gondwana, extended from its tropical to sub-tropical attachment to North America and Europe all the way to the south pole, and by late Pennsylvanian glaciers were forming there. The episodic advance and retreat of glaciers probably contributed to sea-level changes that produced the alternations between swamps, during low stands of sea level, and incursions of the sea when the water was deeper as the ice melted. This alternation is what produced the rhythmically alternating layers of coal and sediment in Pennsylvanian or Late Carboniferous strata around the world.

—Richard I. Gibson

Map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL

Another Map