ONE IF BY LAND, TWO IF BY SEA: PERMIAN OF THE U.S. SOUTH-CENTRAL REGION

The Permian, the third longest and final period of the Paleozoic Era, encompasses just under 50 million years of geologic time. Named after the region of Perm, Russia, and consisting of the Cisuralian (lower), Guadalupian (middle), and Lopingian (upper) series, the Permian represents a dynamic time in Earth history. Following collision of Hercynian Europe and Laurussia with Gondwana to form the western half of Pangea, the supercontinent drifted slowly northward astride the equator. Global ice-age conditions during the early Permian transitioned to warmer, arid conditions by late Permian time, providing the most recent record of icehouse to greenhouse climate change. Expansion of Late Paleozoic epicontinental seaways reached its ultimate extent during the early Permian, but sea-level lowered steadily throughout middle and late Permian time, concurrent with initial opening of the Neo-Tethys sea and closure of Paleo-Tethys in eastern Pangea. Terrestrial and marine life was highly diverse during the Permian, including expansion of Carboniferous plant groups, the first appearance of large tetrapods on land, and an abundance of mollusks, brachiopods, sponges, and larger benthic foraminifera in the sea. The period witnessed significant extinction events in the middle Permian, followed by the end-Permian mass extinction, the largest extinction event in Earth history. Environmental stress from large igneous province eruptions may have been a contributing factor in driving these extinctions.

Voluminous Permian sediment accumulated in the south-central U.S. region. A narrow arm of the Panthalassic Ocean extended from west Texas to the Dakotas, fringed by eastward- and westward-facing landmasses. Early and middle Permian deep-water marine shales and sands were deposited in the central portion of the seaway (Permian and Anadarko basins), transitioning to restricted shallow-water marine and terrestrial facies along the seaway margins. Rapid expansion of marine evaporites and terrestrial red beds during the late Permian record progressive lowering of sea-level coupled with increasingly hot, dry climate conditions. Outcropping and subsurface data from the south-central region continue to add to our knowledge of this important period of Earth history.