Abstract

The 3.2 Ga Moodies Group in the Barberton Greenstone Belt, South Africa, contains 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 sandstone foresets separated by mudstone drapes. Detailed analysis of rhythmic foreset bundles permits quantification of the tidal record and reveals a hierarchy 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 foreset bundle thicknesses record longer period changes in strength of the dominant semidiurnal tidal currents consistent with neap-spring-neap tidal cyclicity. Alternating thicker and thinner neap-spring-neap cycles are comparable to anomalistic, perigean-apogean tidal signatures. This quantitative record of tides in the middle Archean Moodies Group represents, by 2.2 b.y., the oldest such documentation. Tidal cyclicity recognized in the Moodies sand-wave deposit is comparable to that recorded in modern tidal settings and identified in the Carboniferous rock record and is most compatible with a lunar orbital shape similar to that existing today.

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