Abstract

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

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