OLDEST DIRECT EVIDENCE OF LUNAR-SOLAR TIDAL FORCING ENCODED IN SEDIMENTARY RHYTHMITES, PROTEROZOIC BIG COTTONWOOD FORMATION, CENTRAL UTAH

The oldest known tidal rhythmites, identified in the Big Cottonwood Fo rmation, Utah, are Late to Middle Proterozoic in age (800 Ma to 1.0 Ga ), approximately 250 to 400 m.y. older than the previously oldest know n tidal rhythmites. Four tidally forced cycles and one nontidal (seaso nal) cycle control lamina thickness patterns. All of these cycles are recognized in outcrop and core and include cycles associated with dail y, semimonthly (synodic), monthly (anomalistic), semiyearly, and yearl y (seasonal) events. These features form the oldest geological record of lunar-solar tidal forcing and show that the middle to late Precambr ian lunar- and solar-generated tides behaved in a manner very similar to that of today. The analysis also suggests that the Big Cottonwood F ormation may have undergone a seasonal climate.