Ma. Chan et al., OLDEST DIRECT EVIDENCE OF LUNAR-SOLAR TIDAL FORCING ENCODED IN SEDIMENTARY RHYTHMITES, PROTEROZOIC BIG COTTONWOOD FORMATION, CENTRAL UTAH, Geology, 22(9), 1994, pp. 791-794
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.