In 1964, an exploratory petroleum well was drilled through one mile of Gulf Coastal Plain sediments in southeastern Alabama. The retrieved core included Silurian to Devonian sedimentary rocks that surprisingly contain fossils provincial to proto-Africa (i.e., Gondwanan), rather than proto-North America (Laurentian), setting off a scientific debate about how these rocks got there.
At the same time, discoveries made in studies of the ocean floor were being formulated into the sea-floor spreading mechanism that would explain why and how continents drift around the surface of Earth, the missing link that had escaped Alfred Wegner’s conceptualization of continental drift.
The Suwannee terrane, as the Alabama rocks became known, figured prominently in J. Tuzo Wilson’s 1966 article in Nature (“Did the Atlantic close and reopen?”), which led Kevin Burke to later coin the term “Wilson Cycle.”
This orphaned block of Gondwanan crust is now known to extend in the subsurface beneath southern Alabama, Georgia, and South Carolina.
A collage of deformed and metamorphosed fragments of ancient volcanic island arcs, known collectively as Carolinia (i.e., Carolina Superterrane), form the most eastern exposures of the Appalachian orogen and also project to depth beneath the Atlantic coastal plain.
These arc terranes formed in an ancient ocean peripheral to western Gondwanaland prior to its climactic Appalachian collision with Laurentia that consolidated the supercontinent Pangaea.
Due to the lack of fossils and reliable isotopic dates within these terranes, the nature and timing of their docking with Laurentia are controversial; hence, one of the most significant events in Appalachian history is also one of the least understood.
Steltenpohl and others report* new isotopic and structural information on the poorly known Uchee terrane in Alabama that bears on the problem.
The Uchee terrane occupies a particularly critical tectonic position, being sandwiched between Laurentian continental basement exposed in the Pine Mountain window and Gondwanan crust of the overlying, albeit buried, Suwannee terrane.
Steltenpohl and coauthors confirm that the Uchee is an “exotic” peri-Gondwanan arc terrane -- not part of Laurentia as was previously believed -- and thus provide a new puzzle piece that helps to constrain models for plate tectonic development of the Pangaean suture.
*This research was authored by Mark G. Steltenpohl (Auburn University) et al. and published in the February issue of Geosphere, a journal of the Geological Society of America.
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