Direct evidence of the late Paleozoic glaciation of Gondwana from glacial deposits suggests that geographically extensive continental glaciation began some time in the early Namurian (Latest Mississippian). However, the timing and characteristics of the onset of glaciation are poorly understood because of a lack of reliable paleontological control and reworking of initial glacial deposits by subsequent glacial advances. Indirect evidence of glaciation preserved in unconformity-bounded, low-latitude ramp sequences in the Illinois Basin, suggests that geographically extensive continental glaciation of Gondwana actually began in the late Visean. An abrupt change from carbonate-dominated sequences bounded by disconformities with little evidence of erosion to mixed carbonate-siliciclastic sequences bounded by unconformities with deep incised valleys was likely produced by a three-fold increase in the magnitude of eustatic sea-level fluctuations. The increase in the magnitude of sea-level fluctuations was likely driven by an equally abrupt increase in ice volume and marks the onset of the geographically extensive late Paleozoic glaciation of Gondwana.

A possible explanation for the rapid onset of glaciation is the closing of the equatorial seaway between Laurussia and Gondwana. The subsidence history of the Appalachian Foreland Basin to the east suggests that the two landmasses first collided sometime in the late Visean. Closing of this seaway would have led to an abrupt change in oceanic and atmospheric circulation patters that could have initiated major continental glaciation in the Southern Hemisphere.

The three-fold increase in the amplitude of sea-level fluctuations should affect all same-aged marine strata and serve as an effective global stratigraphic marker in settings where incised valleys are preserved. Future research could include more precise correlation with same-aged strata in other parts of the world, computer modeling of the effect closure of the equatorial seaway would have on atmospheric and oceanic circulation and further use of high-resolution sequence stratigraphy to learn more about this and other ice ages.