Abstract

Sedimentary rhythmites of siltstone and fine sandstone from late Precambrian (c. 650–800 Ma) glaciogenic formations in South Australia are interpreted as distal ebb-tidal deposits that record variability in the velocity and range of palaeo-ebb tides. Variations in lamina thickness encode a full spectrum of palaeotidal cycles, including semidiurnal, diurnal, fortnightly and monthly tidal cycles as well as the lunar apsides (perigee) and nodal cycles. A half-yearly oscillation is attributable largely to a beat between the fortnightly tidal cycles of luni-solar conjunction and lunar declination; the lunar nodal cycle is discernible as an amplitude modulation of this beat oscillation. The data allow determination of the Earth's palaeorotation and the past dynamics of the Earth-Moon system with an accuracy previously unattainable for the Precambrian. The late Precambrian (c. 650 Ma) year contained 13.1 (±0.5) lunar months and c. 400 (±20) days, and the late Precambrian lunar month c. 30.5 (±1.5) days. These values suggest an average equivalent phase lag near 3° since late Precambrian time rather than the present value of 6°. The period of 19.5 (±0.5) years determined for the lunar nodal cycle c. 650 Ma ago indicates a lunar distance 96.9 (±1.7)% of the present distance. The low rate of lunar recession since late Precambrian time revealed by the rhythmite data militates against a close approach of the Moon during the Proterozoic. Precambrian sedimentary rhythmites may hold a key to the early history of the Earth's rotation.

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