THE ROLE OF TIDAL-VELOCITY ASYMMETRIES IN THE DEPOSITION OF SILTY TIDAL RHYTHMITES (CARBONIFEROUS, EASTERN INTERIOR COAL BASIN, USA)

Laminated to thin-bedded siltstones directly overlie Carboniferous coa ls at several localities in the Eastern Interior Coal Basin. Because l amina thicknesses show cyclical variations, the siltstones have been t ermed ''tidal rhythmites''. The rhythmites commonly show a repetitive thick-thin pairing of the laminae, and these two-lamina rhythmites hav e been interpreted as the result of the asymmetry of hood and ebb velo cities that affected sedimentation in a diurnal paleotidal system. Alt ernatively, the pairing has been interpreted to be related to a pronou nced difference in height between successive high tides in a semidiurn al system. This explanation suggests deposition in a mixed, predominat ely semidiurnal system with a marked diurnal inequality. In addition t o the two-lamina rhythmites described originally, subsequent examinati on of drill cores indicates the occurrence of relatively rare, more co mplex three- and four-lamina rhythmites. A variety of mathematical tec hniques are used herein to analyze lamina-thickness data extracted fro m these rhythmites. Processing techniques include: (1) comparing the l inear relationship between the thicknesses of the thinner and thicker laminae and comparing these relationships to modem tidal systems; (2) analyses of thickness periodicities and comparison with modern neap-sp ring tidal cycles; and (3) extraction of lamina-thickness inequalities and comparison to the diurnal inequalities that occur in semidiurnal tidal systems. These tests indicate that reasonable conclusions can be made for the type of originating paleotidal system. For the cases ana lyzed herein, the paleotidal signature suggests a mixed, predominantly semidiurnal system with a marked diurnal inequality.