Quantifying the oldest tidal record: The 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa

The 3.2 Ga Moodies Group in the Barberton Greenstone Belt, South Africa, co ntains the oldest preserved record of tides. The tidal record is preserved in a tidal sand-wave deposit in the lower Moodies Group as bundles of sands tone foresets separated by mudstone drapes. Detailed analysis of rhythmic f oreset bundles permits quantification of the tidal record and reveals a hie rarchy of diurnal, fortnightly, and monthly tidal periodicities. Thick-thin pairs of foreset bundles reflect deposition from semidiurnal dominant and subordinate flood-tidal currents, respectively. Cyclic variations in forese t bundle thicknesses record longer period changes in strength of the domina nt semidiurnal tidal currents consistent with neap-spring-neap tidal cyclic ity. Alternating thicker and thinner neap-spring-neap cycles are comparable to anomalistic, perigean-apogean tidal signatures. This quantitative recor d of tides in the middle Archean Moodies Group represents, by 2.2 by, the o ldest such documentation. Tidal cyclicity recognized in the Moodies sand-wa ve deposit is comparable to that recorded in modern tidal settings and iden tified in the Carboniferous rock record and is most compatible with a lunar orbital shape similar to that existing today.