Various paleotidal periods can be extracted from analyses of cyclic tidal rhythmites, which are common in upper Carboniferous (~300 Ma) rocks of the eastern and midcontinental United States. These periods provide details of the paleotidal regime that controlled sedimentation during deposition of the rhythmites. In addition to shorter-term diurnal/semidiurnal periods and longer-term neap-spring tidal periods, some rhythmites have unusual intermediate periods of approximately 50 hours. These intermediate periods are not readily explainable by any known completely tidal period. It is hypothesized that this 50-hour period could be the product of a basinal resonance, or seiche. Herein estimates are made regarding the required basin depths and widths necessary to produce a seiche of this period. These estimates are based upon paleogeographic reconstructions of the global paleocean, Panthalassa, that surrounded Pangea. These reconstructions provide basinal paleowidth estimates; subsequently, paleodepths can be extrapolated from the depth-to-width relationships in modern oceans. Combining such fine-scale tidal-rhythmite periodicities with large-scale paleogeographic information provides many useful constraints for modeling of Pangean tidal systems. Forward modeling of global tides, based upon very large scale paleogeographic controls, when combined with reverse modeling, based upon fine-scale, detailed rhythmite-period analysis, allows detailed calibration of the paleotidal-systems response. On the basis of available fine- and large-scale information, the unusual 50-hour period that occurs within cyclic rhythmites is most likely related to a global paleocean seiche. ¿ American Geophysical Union 1996