Late Carboniferous Separation of the Eastern Interior Seaway from the Rheic Ocean as Indicated by Tidal Rhythmites in the Illinois Basin, U.S.A.

Erik P. Kvale
Indiana Geological Survey, Bloomington, IN

The assembly of Pangaea and the closure of the Rheic Ocean in early Pennsylvanian corresponds to a significant change in tides experienced by the cratonic seas that covered the Illinois Basin of the eastern U.S. Specifically, there is a change in the type of neap-spring tidal cycles recorded by Carboniferous tidal rhythmites.

Most geologists regard all neap-spring cycles as having their origins in the phase changes of the Moon (“synodically driven neap-spring cycles” or “S-cycles”) that occur over a 29.53-day period.

Neap-spring cycles in the Gulf of Mexico, however, and in large areas in the Pacific and Indian oceans occur independent of the phases of the Moon. Rather, these cycles respond to the Moon's position relative to the Earth's equator in its 27.32-day orbit (“tropically driven neap-spring cycles” or “T-cycles”). Neap tides driven by T-cycles occur when the Moon is over the Earth's equator and the corresponding spring tides occur when the Moon is at its maximum orbital declinations. Both S- and T-cycles are distinguishable in the rock record and from modern tide data.

Tidal rhythmites exhibiting neap-spring cycles are a relatively common facies in the Carboniferous rock record of the Illinois Basin. Tidalites older than early Atokan preserve the S-cycles, whereas younger tidalites exhibit T-cycles. The timing of this change within the tidalite record suggests that it correlates with the cut-off of the cratonic North American seaway from the Rheic Ocean and its extension westward through New Mexico. This trend is consistent with tidal rhythmite packages in the Appalachian Basin.