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

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.