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Upper Proterozoic Tidal Rhythmites, South Australia: Sedimentary Features, Deposition, and Implications for the Earth’s Paleorotation
Tidal rhythmites of laminated siltstone and very fine grained sandstone from Upper Proterozoic (∼650-800 Ma) formations in South Australia (Reynella Siltstone, Elatina Formation, Chambers Bluff Tillite) record a wide range of paleotidal cycles that provide unique information on the earth’s paleorotation and the past lunar orbit. The rhythmites evidently were built by vertical accretion in littoral settings ranging from estuarine to upper and lower delta slope and distal ebb-tidal delta. Periodic changes in the thickness of semidiurnal and diurnal laminae, and of conspicuous fortnightly groups of such laminae (lamina-cycles), are taken to reflect variations in the amount of fine clastic material entrained and deposited mainly from suspension by tidal currents in response to periodic changes in the velocity and range of paleotides.
Stratigraphic series of lamina and lamina-cycle thickness measurements obtained from drill core of the approximately 10-m thick rhythmite member of the Elatina Formation contain strong periodicities that are evident visually or revealed by Fourier spectral analysis. These data represent an unsurpassed paleotidal record of about 60 years’ continuous deposition; supplemented by observations from the Reynella Siltstone, they allow determination of paleotidal and paleorotational periods with an accuracy previously unobtainable for any geological interval. The Late Proterozoic (∼650 Ma) year contained 13.1±0.1 lunar months and 400±7 solar days, and the Late Proterozoic lunar month 30.5±0.5 solar days. The periods of the lunar apsides (perigee) and lunar nodal cycles were then 9.7±0.1 years and 19.5±0.5 years, respectively. The data clearly establish the synodic character of the paleotidal regimes. The implied low mean rate of lunar recession for the past ∼650 m.y. militates against a close approach of the moon at any time during the Proterozoic.
Overall, the Upper Proterozoic rhythmites in South Australia provide valuable data that support and extend information on the earth’s paleorotation and the past lunar orbit obtained from the growth increments of fossils. Study of sedimentary tidal rhythmites may allow the evolving dynamics of the earth-moon system to be traced over most of geological time.
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